source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2521

Last change on this file since 2521 was 2521, checked in by julian.reschke@…, 7 years ago

change tracking for [2519] (see #541)

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 235.0 KB
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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "December">
16  <!ENTITY ID-YEAR "2013">
17  <!ENTITY mdash "&#8212;">
18  <!ENTITY Note "<x:h xmlns:x='http://purl.org/net/xml2rfc/ext'>Note:</x:h>">
19  <!ENTITY caching-overview       "<xref target='Part6' x:rel='#caching.overview' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY cache-incomplete       "<xref target='Part6' x:rel='#response.cacheability' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY payload                "<xref target='Part2' x:rel='#payload' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY media-type            "<xref target='Part2' x:rel='#media.type' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY content-codings        "<xref target='Part2' x:rel='#content.codings' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY CONNECT                "<xref target='Part2' x:rel='#CONNECT' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY content.negotiation    "<xref target='Part2' x:rel='#content.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY diff-mime              "<xref target='Part2' x:rel='#differences.between.http.and.mime' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY representation         "<xref target='Part2' x:rel='#representations' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY HEAD                   "<xref target='Part2' x:rel='#HEAD' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-allow           "<xref target='Part2' x:rel='#header.allow' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-cache-control   "<xref target='Part6' x:rel='#header.cache-control' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY header-content-encoding    "<xref target='Part2' x:rel='#header.content-encoding' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY header-content-location    "<xref target='Part2' x:rel='#header.content-location' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY header-content-range   "<xref target='Part5' x:rel='#header.content-range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY header-content-type    "<xref target='Part2' x:rel='#header.content-type' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY header-date            "<xref target='Part2' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY header-etag            "<xref target='Part4' x:rel='#header.etag' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY header-expect          "<xref target='Part2' x:rel='#header.expect' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY header-expires         "<xref target='Part6' x:rel='#header.expires' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY header-last-modified   "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40  <!ENTITY header-mime-version    "<xref target='Part2' x:rel='#mime-version' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41  <!ENTITY header-pragma          "<xref target='Part6' x:rel='#header.pragma' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
42  <!ENTITY header-proxy-authenticate  "<xref target='Part7' x:rel='#header.proxy-authenticate' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
43  <!ENTITY header-proxy-authorization "<xref target='Part7' x:rel='#header.proxy-authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
44  <!ENTITY header-server          "<xref target='Part2' x:rel='#header.server' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
45  <!ENTITY header-warning         "<xref target='Part6' x:rel='#header.warning' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
46  <!ENTITY idempotent-methods     "<xref target='Part2' x:rel='#idempotent.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
47  <!ENTITY safe-methods           "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
48  <!ENTITY methods                "<xref target='Part2' x:rel='#methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
49  <!ENTITY OPTIONS                "<xref target='Part2' x:rel='#OPTIONS' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
50  <!ENTITY qvalue                 "<xref target='Part2' x:rel='#quality.values' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
51  <!ENTITY resource               "<xref target='Part2' x:rel='#resources' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
52  <!ENTITY status-codes           "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
53  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
54  <!ENTITY status-203             "<xref target='Part2' x:rel='#status.203' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
55  <!ENTITY status-3xx             "<xref target='Part2' x:rel='#status.3xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
56  <!ENTITY status-304             "<xref target='Part4' x:rel='#status.304' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
57  <!ENTITY status-4xx             "<xref target='Part2' x:rel='#status.4xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
58  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
59  <!ENTITY iana-header-registry   "<xref target='Part2' x:rel='#header.field.registry' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
60]>
61<?rfc toc="yes" ?>
62<?rfc symrefs="yes" ?>
63<?rfc sortrefs="yes" ?>
64<?rfc compact="yes"?>
65<?rfc subcompact="no" ?>
66<?rfc linkmailto="no" ?>
67<?rfc editing="no" ?>
68<?rfc comments="yes"?>
69<?rfc inline="yes"?>
70<?rfc rfcedstyle="yes"?>
71<?rfc-ext allow-markup-in-artwork="yes" ?>
72<?rfc-ext include-references-in-index="yes" ?>
73<rfc obsoletes="2145,2616" updates="2817,2818" category="std" x:maturity-level="proposed"
74     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
75     xmlns:x='http://purl.org/net/xml2rfc/ext'>
76<x:link rel="next" basename="p2-semantics"/>
77<x:feedback template="mailto:ietf-http-wg@w3.org?subject={docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
78<front>
79
80  <title abbrev="HTTP/1.1 Message Syntax and Routing">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
81
82  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
83    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
84    <address>
85      <postal>
86        <street>345 Park Ave</street>
87        <city>San Jose</city>
88        <region>CA</region>
89        <code>95110</code>
90        <country>USA</country>
91      </postal>
92      <email>fielding@gbiv.com</email>
93      <uri>http://roy.gbiv.com/</uri>
94    </address>
95  </author>
96
97  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
98    <organization abbrev="greenbytes">greenbytes GmbH</organization>
99    <address>
100      <postal>
101        <street>Hafenweg 16</street>
102        <city>Muenster</city><region>NW</region><code>48155</code>
103        <country>Germany</country>
104      </postal>
105      <email>julian.reschke@greenbytes.de</email>
106      <uri>http://greenbytes.de/tech/webdav/</uri>
107    </address>
108  </author>
109
110  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
111  <workgroup>HTTPbis Working Group</workgroup>
112
113<abstract>
114<t>
115   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
116   distributed, collaborative, hypertext information systems. HTTP has been in
117   use by the World Wide Web global information initiative since 1990.
118   This document provides an overview of HTTP architecture and its associated
119   terminology, defines the "http" and "https" Uniform Resource Identifier
120   (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements,
121   and describes general security concerns for implementations.
122</t>   
123</abstract>
124
125<note title="Editorial Note (To be removed by RFC Editor)">
126  <t>
127    Discussion of this draft takes place on the HTTPBIS working group
128    mailing list (ietf-http-wg@w3.org), which is archived at
129    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
130  </t>
131  <t>
132    The current issues list is at
133    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
134    documents (including fancy diffs) can be found at
135    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
136  </t>
137  <t>
138    The changes in this draft are summarized in <xref target="changes.since.25"/>.
139  </t>
140</note>
141</front>
142<middle>
143<section title="Introduction" anchor="introduction">
144<t>
145   The Hypertext Transfer Protocol (HTTP) is an application-level
146   request/response protocol that uses extensible semantics and self-descriptive
147   message payloads for flexible interaction with network-based hypertext
148   information systems. This document is the first in a series of documents
149   that collectively form the HTTP/1.1 specification:
150   <list style="empty">
151    <t>RFC xxx1: Message Syntax and Routing</t>
152    <t><xref target="Part2" x:fmt="none">RFC xxx2</xref>: Semantics and Content</t>
153    <t><xref target="Part4" x:fmt="none">RFC xxx3</xref>: Conditional Requests</t>
154    <t><xref target="Part5" x:fmt="none">RFC xxx4</xref>: Range Requests</t>
155    <t><xref target="Part6" x:fmt="none">RFC xxx5</xref>: Caching</t>
156    <t><xref target="Part7" x:fmt="none">RFC xxx6</xref>: Authentication</t>
157   </list>
158</t>
159<t>
160   This HTTP/1.1 specification obsoletes and moves to historic status
161   <xref target="RFC2616" x:fmt="none">RFC 2616</xref>, its predecessor
162   <xref target="RFC2068" x:fmt="none">RFC 2068</xref>, and
163   <xref target="RFC2145" x:fmt="none">RFC 2145</xref> (on HTTP versioning).
164   This specification also updates the use of CONNECT to establish a tunnel,
165   previously defined in <xref target="RFC2817" x:fmt="none">RFC 2817</xref>,
166   and defines the "https" URI scheme that was described informally in
167   <xref target="RFC2818" x:fmt="none">RFC 2818</xref>.
168</t>
169<t>
170   HTTP is a generic interface protocol for information systems. It is
171   designed to hide the details of how a service is implemented by presenting
172   a uniform interface to clients that is independent of the types of
173   resources provided. Likewise, servers do not need to be aware of each
174   client's purpose: an HTTP request can be considered in isolation rather
175   than being associated with a specific type of client or a predetermined
176   sequence of application steps. The result is a protocol that can be used
177   effectively in many different contexts and for which implementations can
178   evolve independently over time.
179</t>
180<t>
181   HTTP is also designed for use as an intermediation protocol for translating
182   communication to and from non-HTTP information systems.
183   HTTP proxies and gateways can provide access to alternative information
184   services by translating their diverse protocols into a hypertext
185   format that can be viewed and manipulated by clients in the same way
186   as HTTP services.
187</t>
188<t>
189   One consequence of this flexibility is that the protocol cannot be
190   defined in terms of what occurs behind the interface. Instead, we
191   are limited to defining the syntax of communication, the intent
192   of received communication, and the expected behavior of recipients.
193   If the communication is considered in isolation, then successful
194   actions ought to be reflected in corresponding changes to the
195   observable interface provided by servers. However, since multiple
196   clients might act in parallel and perhaps at cross-purposes, we
197   cannot require that such changes be observable beyond the scope
198   of a single response.
199</t>
200<t>
201   This document describes the architectural elements that are used or
202   referred to in HTTP, defines the "http" and "https" URI schemes,
203   describes overall network operation and connection management,
204   and defines HTTP message framing and forwarding requirements.
205   Our goal is to define all of the mechanisms necessary for HTTP message
206   handling that are independent of message semantics, thereby defining the
207   complete set of requirements for message parsers and
208   message-forwarding intermediaries.
209</t>
210
211
212<section title="Requirement Notation" anchor="intro.requirements">
213<t>
214   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
215   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
216   document are to be interpreted as described in <xref target="RFC2119"/>.
217</t>
218<t>
219   Conformance criteria and considerations regarding error handling
220   are defined in <xref target="conformance"/>.
221</t>
222</section>
223
224<section title="Syntax Notation" anchor="notation">
225<iref primary="true" item="Grammar" subitem="ALPHA"/>
226<iref primary="true" item="Grammar" subitem="CR"/>
227<iref primary="true" item="Grammar" subitem="CRLF"/>
228<iref primary="true" item="Grammar" subitem="CTL"/>
229<iref primary="true" item="Grammar" subitem="DIGIT"/>
230<iref primary="true" item="Grammar" subitem="DQUOTE"/>
231<iref primary="true" item="Grammar" subitem="HEXDIG"/>
232<iref primary="true" item="Grammar" subitem="HTAB"/>
233<iref primary="true" item="Grammar" subitem="LF"/>
234<iref primary="true" item="Grammar" subitem="OCTET"/>
235<iref primary="true" item="Grammar" subitem="SP"/>
236<iref primary="true" item="Grammar" subitem="VCHAR"/>
237<t>
238   This specification uses the Augmented Backus-Naur Form (ABNF) notation
239   of <xref target="RFC5234"/> with the list rule extension defined in
240   <xref target="abnf.extension"/>.  <xref target="collected.abnf"/> shows
241   the collected ABNF with the list rule expanded.
242</t>
243<t anchor="core.rules">
244  <x:anchor-alias value="ALPHA"/>
245  <x:anchor-alias value="CTL"/>
246  <x:anchor-alias value="CR"/>
247  <x:anchor-alias value="CRLF"/>
248  <x:anchor-alias value="DIGIT"/>
249  <x:anchor-alias value="DQUOTE"/>
250  <x:anchor-alias value="HEXDIG"/>
251  <x:anchor-alias value="HTAB"/>
252  <x:anchor-alias value="LF"/>
253  <x:anchor-alias value="OCTET"/>
254  <x:anchor-alias value="SP"/>
255  <x:anchor-alias value="VCHAR"/>
256   The following core rules are included by
257   reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
258   ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
259   DIGIT (decimal 0-9), DQUOTE (double quote),
260   HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed),
261   OCTET (any 8-bit sequence of data), SP (space), and
262   VCHAR (any visible <xref target="USASCII"/> character).
263</t>
264<t>
265   As a convention, ABNF rule names prefixed with "obs-" denote
266   "obsolete" grammar rules that appear for historical reasons.
267</t>
268</section>
269</section>
270
271<section title="Architecture" anchor="architecture">
272<t>
273   HTTP was created for the World Wide Web architecture
274   and has evolved over time to support the scalability needs of a worldwide
275   hypertext system. Much of that architecture is reflected in the terminology
276   and syntax productions used to define HTTP.
277</t>
278
279<section title="Client/Server Messaging" anchor="operation">
280<iref primary="true" item="client"/>
281<iref primary="true" item="server"/>
282<iref primary="true" item="connection"/>
283<t>
284   HTTP is a stateless request/response protocol that operates by exchanging
285   <x:dfn>messages</x:dfn> (<xref target="http.message"/>) across a reliable
286   transport or session-layer
287   "<x:dfn>connection</x:dfn>" (<xref target="connection.management"/>).
288   An HTTP "<x:dfn>client</x:dfn>" is a program that establishes a connection
289   to a server for the purpose of sending one or more HTTP requests.
290   An HTTP "<x:dfn>server</x:dfn>" is a program that accepts connections
291   in order to service HTTP requests by sending HTTP responses.
292</t>
293<iref primary="true" item="user agent"/>
294<iref primary="true" item="origin server"/>
295<iref primary="true" item="browser"/>
296<iref primary="true" item="spider"/>
297<iref primary="true" item="sender"/>
298<iref primary="true" item="recipient"/>
299<t>
300   The terms client and server refer only to the roles that
301   these programs perform for a particular connection.  The same program
302   might act as a client on some connections and a server on others.
303   We use the term "<x:dfn>user agent</x:dfn>" to refer to any of the various
304   client programs that initiate a request, including (but not limited to)
305   browsers, spiders (web-based robots), command-line tools, native
306   applications, and mobile apps.  The term "<x:dfn>origin server</x:dfn>" is
307   used to refer to the program that can originate authoritative responses to
308   a request. For general requirements, we use the terms
309   "<x:dfn>sender</x:dfn>" and "<x:dfn>recipient</x:dfn>" to refer to any
310   component that sends or receives, respectively, a given message.
311</t>
312<t>
313   HTTP relies upon the Uniform Resource Identifier (URI)
314   standard <xref target="RFC3986"/> to indicate the target resource
315   (<xref target="target-resource"/>) and relationships between resources.
316   Messages are passed in a format similar to that used by Internet mail
317   <xref target="RFC5322"/> and the Multipurpose Internet Mail Extensions
318   (MIME) <xref target="RFC2045"/> (see &diff-mime; for the differences
319   between HTTP and MIME messages).
320</t>
321<t>
322   Most HTTP communication consists of a retrieval request (GET) for
323   a representation of some resource identified by a URI.  In the
324   simplest case, this might be accomplished via a single bidirectional
325   connection (===) between the user agent (UA) and the origin server (O).
326</t>
327<figure><artwork type="drawing">
328         request   &gt;
329    <x:highlight>UA</x:highlight> ======================================= <x:highlight>O</x:highlight>
330                                &lt;   response
331</artwork></figure>
332<iref primary="true" item="message"/>
333<iref primary="true" item="request"/>
334<iref primary="true" item="response"/>
335<t>
336   A client sends an HTTP request to a server in the form of a <x:dfn>request</x:dfn>
337   message, beginning with a request-line that includes a method, URI, and
338   protocol version (<xref target="request.line"/>),
339   followed by header fields containing
340   request modifiers, client information, and representation metadata
341   (<xref target="header.fields"/>),
342   an empty line to indicate the end of the header section, and finally
343   a message body containing the payload body (if any,
344   <xref target="message.body"/>).
345</t>
346<t>
347   A server responds to a client's request by sending one or more HTTP
348   <x:dfn>response</x:dfn>
349   messages, each beginning with a status line that
350   includes the protocol version, a success or error code, and textual
351   reason phrase (<xref target="status.line"/>),
352   possibly followed by header fields containing server
353   information, resource metadata, and representation metadata
354   (<xref target="header.fields"/>),
355   an empty line to indicate the end of the header section, and finally
356   a message body containing the payload body (if any,
357   <xref target="message.body"/>).
358</t>
359<t>
360   A connection might be used for multiple request/response exchanges,
361   as defined in <xref target="persistent.connections"/>.
362</t>
363<t>
364   The following example illustrates a typical message exchange for a
365   GET request on the URI "http://www.example.com/hello.txt":
366</t>
367<figure><preamble>
368Client request:
369</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
370GET /hello.txt HTTP/1.1
371User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
372Host: www.example.com
373Accept-Language: en, mi
374
375</artwork></figure>
376<figure><preamble>
377Server response:
378</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
379HTTP/1.1 200 OK
380Date: Mon, 27 Jul 2009 12:28:53 GMT
381Server: Apache
382Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
383ETag: "34aa387-d-1568eb00"
384Accept-Ranges: bytes
385Content-Length: <x:length-of target="exbody"/>
386Vary: Accept-Encoding
387Content-Type: text/plain
388
389<x:span anchor="exbody">Hello World! My payload includes a trailing CRLF.
390</x:span></artwork>
391</figure>
392</section>
393
394<section title="Implementation Diversity" anchor="implementation-diversity">
395<t>
396   When considering the design of HTTP, it is easy to fall into a trap of
397   thinking that all user agents are general-purpose browsers and all origin
398   servers are large public websites. That is not the case in practice.
399   Common HTTP user agents include household appliances, stereos, scales,
400   firmware update scripts, command-line programs, mobile apps,
401   and communication devices in a multitude of shapes and sizes.  Likewise,
402   common HTTP origin servers include home automation units, configurable
403   networking components, office machines, autonomous robots, news feeds,
404   traffic cameras, ad selectors, and video delivery platforms.
405</t>
406<t>
407   The term "user agent" does not imply that there is a human user directly
408   interacting with the software agent at the time of a request. In many
409   cases, a user agent is installed or configured to run in the background
410   and save its results for later inspection (or save only a subset of those
411   results that might be interesting or erroneous). Spiders, for example, are
412   typically given a start URI and configured to follow certain behavior while
413   crawling the Web as a hypertext graph.
414</t>
415<t>
416   The implementation diversity of HTTP means that we cannot assume the
417   user agent can make interactive suggestions to a user or provide adequate
418   warning for security or privacy options.  In the few cases where this
419   specification requires reporting of errors to the user, it is acceptable
420   for such reporting to only be observable in an error console or log file.
421   Likewise, requirements that an automated action be confirmed by the user
422   before proceeding might be met via advance configuration choices,
423   run-time options, or simple avoidance of the unsafe action; confirmation
424   does not imply any specific user interface or interruption of normal
425   processing if the user has already made that choice.
426</t>
427</section>
428
429<section title="Intermediaries" anchor="intermediaries">
430<iref primary="true" item="intermediary"/>
431<t>
432   HTTP enables the use of intermediaries to satisfy requests through
433   a chain of connections.  There are three common forms of HTTP
434   <x:dfn>intermediary</x:dfn>: proxy, gateway, and tunnel.  In some cases,
435   a single intermediary might act as an origin server, proxy, gateway,
436   or tunnel, switching behavior based on the nature of each request.
437</t>
438<figure><artwork type="drawing">
439         &gt;             &gt;             &gt;             &gt;
440    <x:highlight>UA</x:highlight> =========== <x:highlight>A</x:highlight> =========== <x:highlight>B</x:highlight> =========== <x:highlight>C</x:highlight> =========== <x:highlight>O</x:highlight>
441               &lt;             &lt;             &lt;             &lt;
442</artwork></figure>
443<t>
444   The figure above shows three intermediaries (A, B, and C) between the
445   user agent and origin server. A request or response message that
446   travels the whole chain will pass through four separate connections.
447   Some HTTP communication options
448   might apply only to the connection with the nearest, non-tunnel
449   neighbor, only to the end-points of the chain, or to all connections
450   along the chain. Although the diagram is linear, each participant might
451   be engaged in multiple, simultaneous communications. For example, B
452   might be receiving requests from many clients other than A, and/or
453   forwarding requests to servers other than C, at the same time that it
454   is handling A's request. Likewise, later requests might be sent through a
455   different path of connections, often based on dynamic configuration for
456   load balancing.   
457</t>
458<t>
459<iref primary="true" item="upstream"/><iref primary="true" item="downstream"/>
460<iref primary="true" item="inbound"/><iref primary="true" item="outbound"/>
461   We use the terms "<x:dfn>upstream</x:dfn>" and "<x:dfn>downstream</x:dfn>"
462   to describe various requirements in relation to the directional flow of a
463   message: all messages flow from upstream to downstream.
464   Likewise, we use the terms inbound and outbound to refer to
465   directions in relation to the request path:
466   "<x:dfn>inbound</x:dfn>" means toward the origin server and
467   "<x:dfn>outbound</x:dfn>" means toward the user agent.
468</t>
469<t><iref primary="true" item="proxy"/>
470   A "<x:dfn>proxy</x:dfn>" is a message forwarding agent that is selected by the
471   client, usually via local configuration rules, to receive requests
472   for some type(s) of absolute URI and attempt to satisfy those
473   requests via translation through the HTTP interface.  Some translations
474   are minimal, such as for proxy requests for "http" URIs, whereas
475   other requests might require translation to and from entirely different
476   application-level protocols. Proxies are often used to group an
477   organization's HTTP requests through a common intermediary for the
478   sake of security, annotation services, or shared caching.
479</t>
480<t>
481<iref primary="true" item="transforming proxy"/>
482<iref primary="true" item="non-transforming proxy"/>
483   An HTTP-to-HTTP proxy is called a "<x:dfn>transforming proxy</x:dfn>" if it is designed
484   or configured to modify request or response messages in a semantically
485   meaningful way (i.e., modifications, beyond those required by normal
486   HTTP processing, that change the message in a way that would be
487   significant to the original sender or potentially significant to
488   downstream recipients).  For example, a transforming proxy might be
489   acting as a shared annotation server (modifying responses to include
490   references to a local annotation database), a malware filter, a
491   format transcoder, or an intranet-to-Internet privacy filter.  Such
492   transformations are presumed to be desired by the client (or client
493   organization) that selected the proxy and are beyond the scope of
494   this specification.  However, when a proxy is not intended to transform
495   a given message, we use the term "<x:dfn>non-transforming proxy</x:dfn>" to target
496   requirements that preserve HTTP message semantics. See &status-203; and
497   &header-warning; for status and warning codes related to transformations.
498</t>
499<t><iref primary="true" item="gateway"/><iref primary="true" item="reverse proxy"/>
500<iref primary="true" item="accelerator"/>
501   A "<x:dfn>gateway</x:dfn>" (a.k.a., "<x:dfn>reverse proxy</x:dfn>") is an
502   intermediary that acts as an origin server for the outbound connection, but
503   translates received requests and forwards them inbound to another server or
504   servers. Gateways are often used to encapsulate legacy or untrusted
505   information services, to improve server performance through
506   "<x:dfn>accelerator</x:dfn>" caching, and to enable partitioning or load
507   balancing of HTTP services across multiple machines.
508</t>
509<t>
510   All HTTP requirements applicable to an origin server
511   also apply to the outbound communication of a gateway.
512   A gateway communicates with inbound servers using any protocol that
513   it desires, including private extensions to HTTP that are outside
514   the scope of this specification.  However, an HTTP-to-HTTP gateway
515   that wishes to interoperate with third-party HTTP servers ought to conform
516   to user agent requirements on the gateway's inbound connection.
517</t>
518<t><iref primary="true" item="tunnel"/>
519   A "<x:dfn>tunnel</x:dfn>" acts as a blind relay between two connections
520   without changing the messages. Once active, a tunnel is not
521   considered a party to the HTTP communication, though the tunnel might
522   have been initiated by an HTTP request. A tunnel ceases to exist when
523   both ends of the relayed connection are closed. Tunnels are used to
524   extend a virtual connection through an intermediary, such as when
525   Transport Layer Security (TLS, <xref target="RFC5246"/>) is used to
526   establish confidential communication through a shared firewall proxy.
527</t>
528<t><iref primary="true" item="interception proxy"/>
529<iref primary="true" item="transparent proxy"/>
530<iref primary="true" item="captive portal"/>
531   The above categories for intermediary only consider those acting as
532   participants in the HTTP communication.  There are also intermediaries
533   that can act on lower layers of the network protocol stack, filtering or
534   redirecting HTTP traffic without the knowledge or permission of message
535   senders. Network intermediaries often introduce security flaws or
536   interoperability problems by violating HTTP semantics.  For example, an
537   "<x:dfn>interception proxy</x:dfn>" <xref target="RFC3040"/> (also commonly
538   known as a "<x:dfn>transparent proxy</x:dfn>" <xref target="RFC1919"/> or
539   "<x:dfn>captive portal</x:dfn>")
540   differs from an HTTP proxy because it is not selected by the client.
541   Instead, an interception proxy filters or redirects outgoing TCP port 80
542   packets (and occasionally other common port traffic).
543   Interception proxies are commonly found on public network access points,
544   as a means of enforcing account subscription prior to allowing use of
545   non-local Internet services, and within corporate firewalls to enforce
546   network usage policies.
547   They are indistinguishable from a man-in-the-middle attack.
548</t>
549<t>
550   HTTP is defined as a stateless protocol, meaning that each request message
551   can be understood in isolation.  Many implementations depend on HTTP's
552   stateless design in order to reuse proxied connections or dynamically
553   load-balance requests across multiple servers.  Hence, a server &MUST-NOT;
554   assume that two requests on the same connection are from the same user
555   agent unless the connection is secured and specific to that agent.
556   Some non-standard HTTP extensions (e.g., <xref target="RFC4559"/>) have
557   been known to violate this requirement, resulting in security and
558   interoperability problems.
559</t>
560</section>
561
562<section title="Caches" anchor="caches">
563<iref primary="true" item="cache"/>
564<t>
565   A "<x:dfn>cache</x:dfn>" is a local store of previous response messages and the
566   subsystem that controls its message storage, retrieval, and deletion.
567   A cache stores cacheable responses in order to reduce the response
568   time and network bandwidth consumption on future, equivalent
569   requests. Any client or server &MAY; employ a cache, though a cache
570   cannot be used by a server while it is acting as a tunnel.
571</t>
572<t>
573   The effect of a cache is that the request/response chain is shortened
574   if one of the participants along the chain has a cached response
575   applicable to that request. The following illustrates the resulting
576   chain if B has a cached copy of an earlier response from O (via C)
577   for a request that has not been cached by UA or A.
578</t>
579<figure><artwork type="drawing">
580            &gt;             &gt;
581       <x:highlight>UA</x:highlight> =========== <x:highlight>A</x:highlight> =========== <x:highlight>B</x:highlight> - - - - - - <x:highlight>C</x:highlight> - - - - - - <x:highlight>O</x:highlight>
582                  &lt;             &lt;
583</artwork></figure>
584<t><iref primary="true" item="cacheable"/>
585   A response is "<x:dfn>cacheable</x:dfn>" if a cache is allowed to store a copy of
586   the response message for use in answering subsequent requests.
587   Even when a response is cacheable, there might be additional
588   constraints placed by the client or by the origin server on when
589   that cached response can be used for a particular request. HTTP
590   requirements for cache behavior and cacheable responses are
591   defined in &caching-overview;. 
592</t>
593<t>
594   There are a wide variety of architectures and configurations
595   of caches deployed across the World Wide Web and
596   inside large organizations. These include national hierarchies
597   of proxy caches to save transoceanic bandwidth, collaborative systems that
598   broadcast or multicast cache entries, archives of pre-fetched cache
599   entries for use in off-line or high-latency environments, and so on.
600</t>
601</section>
602
603<section title="Conformance and Error Handling" anchor="conformance">
604<t>
605   This specification targets conformance criteria according to the role of
606   a participant in HTTP communication.  Hence, HTTP requirements are placed
607   on senders, recipients, clients, servers, user agents, intermediaries,
608   origin servers, proxies, gateways, or caches, depending on what behavior
609   is being constrained by the requirement. Additional (social) requirements
610   are placed on implementations, resource owners, and protocol element
611   registrations when they apply beyond the scope of a single communication.
612</t>
613<t>
614   The verb "generate" is used instead of "send" where a requirement
615   differentiates between creating a protocol element and merely forwarding a
616   received element downstream.
617</t>
618<t>
619   An implementation is considered conformant if it complies with all of the
620   requirements associated with the roles it partakes in HTTP.
621</t>
622<t>
623   Conformance includes both the syntax and semantics of protocol
624   elements. A sender &MUST-NOT; generate protocol elements that convey a
625   meaning that is known by that sender to be false. A sender &MUST-NOT;
626   generate protocol elements that do not match the grammar defined by the
627   corresponding ABNF rules. Within a given message, a sender &MUST-NOT;
628   generate protocol elements or syntax alternatives that are only allowed to
629   be generated by participants in other roles (i.e., a role that the sender
630   does not have for that message).
631</t>
632<t>
633   When a received protocol element is parsed, the recipient &MUST; be able to
634   parse any value of reasonable length that is applicable to the recipient's
635   role and matches the grammar defined by the corresponding ABNF rules.
636   Note, however, that some received protocol elements might not be parsed.
637   For example, an intermediary forwarding a message might parse a
638   header-field into generic field-name and field-value components, but then
639   forward the header field without further parsing inside the field-value.
640</t>
641<t>
642   HTTP does not have specific length limitations for many of its protocol
643   elements because the lengths that might be appropriate will vary widely,
644   depending on the deployment context and purpose of the implementation.
645   Hence, interoperability between senders and recipients depends on shared
646   expectations regarding what is a reasonable length for each protocol
647   element. Furthermore, what is commonly understood to be a reasonable length
648   for some protocol elements has changed over the course of the past two
649   decades of HTTP use, and is expected to continue changing in the future.
650</t>
651<t>
652   At a minimum, a recipient &MUST; be able to parse and process protocol
653   element lengths that are at least as long as the values that it generates
654   for those same protocol elements in other messages. For example, an origin
655   server that publishes very long URI references to its own resources needs
656   to be able to parse and process those same references when received as a
657   request target.
658</t>
659<t>
660   A recipient &MUST; interpret a received protocol element according to the
661   semantics defined for it by this specification, including extensions to
662   this specification, unless the recipient has determined (through experience
663   or configuration) that the sender incorrectly implements what is implied by
664   those semantics.
665   For example, an origin server might disregard the contents of a received
666   <x:ref>Accept-Encoding</x:ref> header field if inspection of the
667   <x:ref>User-Agent</x:ref> header field indicates a specific implementation
668   version that is known to fail on receipt of certain content codings.
669</t>
670<t>
671   Unless noted otherwise, a recipient &MAY; attempt to recover a usable
672   protocol element from an invalid construct.  HTTP does not define
673   specific error handling mechanisms except when they have a direct impact
674   on security, since different applications of the protocol require
675   different error handling strategies.  For example, a Web browser might
676   wish to transparently recover from a response where the
677   <x:ref>Location</x:ref> header field doesn't parse according to the ABNF,
678   whereas a systems control client might consider any form of error recovery
679   to be dangerous.
680</t>
681</section>
682
683<section title="Protocol Versioning" anchor="http.version">
684  <x:anchor-alias value="HTTP-version"/>
685  <x:anchor-alias value="HTTP-name"/>
686<t>
687   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate
688   versions of the protocol. This specification defines version "1.1".
689   The protocol version as a whole indicates the sender's conformance
690   with the set of requirements laid out in that version's corresponding
691   specification of HTTP.
692</t>
693<t>
694   The version of an HTTP message is indicated by an HTTP-version field
695   in the first line of the message. HTTP-version is case-sensitive.
696</t>
697<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-version"/><iref primary="true" item="Grammar" subitem="HTTP-name"/>
698  <x:ref>HTTP-version</x:ref>  = <x:ref>HTTP-name</x:ref> "/" <x:ref>DIGIT</x:ref> "." <x:ref>DIGIT</x:ref>
699  <x:ref>HTTP-name</x:ref>     = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
700</artwork></figure>
701<t>
702   The HTTP version number consists of two decimal digits separated by a "."
703   (period or decimal point).  The first digit ("major version") indicates the
704   HTTP messaging syntax, whereas the second digit ("minor version") indicates
705   the highest minor version within that major version to which the sender is
706   conformant and able to understand for future communication.  The minor
707   version advertises the sender's communication capabilities even when the
708   sender is only using a backwards-compatible subset of the protocol,
709   thereby letting the recipient know that more advanced features can
710   be used in response (by servers) or in future requests (by clients).
711</t>
712<t>
713   When an HTTP/1.1 message is sent to an HTTP/1.0 recipient
714   <xref target="RFC1945"/> or a recipient whose version is unknown,
715   the HTTP/1.1 message is constructed such that it can be interpreted
716   as a valid HTTP/1.0 message if all of the newer features are ignored.
717   This specification places recipient-version requirements on some
718   new features so that a conformant sender will only use compatible
719   features until it has determined, through configuration or the
720   receipt of a message, that the recipient supports HTTP/1.1.
721</t>
722<t>
723   The interpretation of a header field does not change between minor
724   versions of the same major HTTP version, though the default
725   behavior of a recipient in the absence of such a field can change.
726   Unless specified otherwise, header fields defined in HTTP/1.1 are
727   defined for all versions of HTTP/1.x.  In particular, the <x:ref>Host</x:ref>
728   and <x:ref>Connection</x:ref> header fields ought to be implemented by all
729   HTTP/1.x implementations whether or not they advertise conformance with
730   HTTP/1.1.
731</t>
732<t>
733   New header fields can be introduced without changing the protocol version
734   if their defined semantics allow them to be safely ignored by recipients
735   that do not recognize them. Header field extensibility is discussed in
736   <xref target="field.extensibility"/>.
737</t>
738<t>
739   Intermediaries that process HTTP messages (i.e., all intermediaries
740   other than those acting as tunnels) &MUST; send their own HTTP-version
741   in forwarded messages.  In other words, they are not allowed to blindly
742   forward the first line of an HTTP message without ensuring that the
743   protocol version in that message matches a version to which that
744   intermediary is conformant for both the receiving and
745   sending of messages.  Forwarding an HTTP message without rewriting
746   the HTTP-version might result in communication errors when downstream
747   recipients use the message sender's version to determine what features
748   are safe to use for later communication with that sender.
749</t>
750<t>
751   A client &SHOULD; send a request version equal to the highest
752   version to which the client is conformant and
753   whose major version is no higher than the highest version supported
754   by the server, if this is known.  A client &MUST-NOT; send a
755   version to which it is not conformant.
756</t>
757<t>
758   A client &MAY; send a lower request version if it is known that
759   the server incorrectly implements the HTTP specification, but only
760   after the client has attempted at least one normal request and determined
761   from the response status code or header fields (e.g., <x:ref>Server</x:ref>) that
762   the server improperly handles higher request versions.
763</t>
764<t>
765   A server &SHOULD; send a response version equal to the highest version to
766   which the server is conformant that has a major version less than or equal
767   to the one received in the request.
768   A server &MUST-NOT; send a version to which it is not conformant.
769   A server can send a <x:ref>505 (HTTP Version Not Supported)</x:ref>
770   response if it wishes, for any reason, to refuse service of the client's
771   major protocol version.
772</t>
773<t>
774   A server &MAY; send an HTTP/1.0 response to a request
775   if it is known or suspected that the client incorrectly implements the
776   HTTP specification and is incapable of correctly processing later
777   version responses, such as when a client fails to parse the version
778   number correctly or when an intermediary is known to blindly forward
779   the HTTP-version even when it doesn't conform to the given minor
780   version of the protocol. Such protocol downgrades &SHOULD-NOT; be
781   performed unless triggered by specific client attributes, such as when
782   one or more of the request header fields (e.g., <x:ref>User-Agent</x:ref>)
783   uniquely match the values sent by a client known to be in error.
784</t>
785<t>
786   The intention of HTTP's versioning design is that the major number
787   will only be incremented if an incompatible message syntax is
788   introduced, and that the minor number will only be incremented when
789   changes made to the protocol have the effect of adding to the message
790   semantics or implying additional capabilities of the sender.  However,
791   the minor version was not incremented for the changes introduced between
792   <xref target="RFC2068"/> and <xref target="RFC2616"/>, and this revision
793   has specifically avoided any such changes to the protocol.
794</t>
795<t>
796   When an HTTP message is received with a major version number that the
797   recipient implements, but a higher minor version number than what the
798   recipient implements, the recipient &SHOULD; process the message as if it
799   were in the highest minor version within that major version to which the
800   recipient is conformant. A recipient can assume that a message with a
801   higher minor version, when sent to a recipient that has not yet indicated
802   support for that higher version, is sufficiently backwards-compatible to be
803   safely processed by any implementation of the same major version.
804</t>
805</section>
806
807<section title="Uniform Resource Identifiers" anchor="uri">
808<iref primary="true" item="resource"/>
809<t>
810   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used
811   throughout HTTP as the means for identifying resources (&resource;).
812   URI references are used to target requests, indicate redirects, and define
813   relationships.
814</t>
815  <x:anchor-alias value="URI-reference"/>
816  <x:anchor-alias value="absolute-URI"/>
817  <x:anchor-alias value="relative-part"/>
818  <x:anchor-alias value="authority"/>
819  <x:anchor-alias value="uri-host"/>
820  <x:anchor-alias value="port"/>
821  <x:anchor-alias value="path-abempty"/>
822  <x:anchor-alias value="segment"/>
823  <x:anchor-alias value="query"/>
824  <x:anchor-alias value="fragment"/>
825  <x:anchor-alias value="absolute-path"/>
826  <x:anchor-alias value="partial-URI"/>
827<t>
828   This specification adopts the definitions of "URI-reference",
829   "absolute-URI", "relative-part", "authority", "port", "host",
830   "path-abempty", "segment", "query", and "fragment" from the
831   URI generic syntax.
832   In addition, we define an "absolute-path" rule (that differs from
833   RFC 3986's "path-absolute" in that it allows a leading "//")
834   and a "partial-URI" rule for protocol elements
835   that allow a relative URI but not a fragment.
836</t>
837<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="URI-reference"><!--exported production--></iref><iref primary="true" item="Grammar" subitem="absolute-URI"/><iref primary="true" item="Grammar" subitem="authority"/><iref primary="true" item="Grammar" subitem="absolute-path"/><iref primary="true" item="Grammar" subitem="port"/><iref primary="true" item="Grammar" subitem="query"/><iref primary="true" item="Grammar" subitem="fragment"/><iref primary="true" item="Grammar" subitem="segment"/><iref primary="true" item="Grammar" subitem="uri-host"/><iref primary="true" item="Grammar" subitem="partial-URI"><!--exported production--></iref>
838  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
839  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
840  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
841  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
842  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
843  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
844  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
845  <x:ref>segment</x:ref>       = &lt;segment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
846  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
847  <x:ref>fragment</x:ref>      = &lt;fragment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.5"/>&gt;
848 
849  <x:ref>absolute-path</x:ref> = 1*( "/" segment )
850  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
851</artwork></figure>
852<t>
853   Each protocol element in HTTP that allows a URI reference will indicate
854   in its ABNF production whether the element allows any form of reference
855   (URI-reference), only a URI in absolute form (absolute-URI), only the
856   path and optional query components, or some combination of the above.
857   Unless otherwise indicated, URI references are parsed
858   relative to the effective request URI
859   (<xref target="effective.request.uri"/>).
860</t>
861
862<section title="http URI scheme" anchor="http.uri">
863  <x:anchor-alias value="http-URI"/>
864  <iref item="http URI scheme" primary="true"/>
865  <iref item="URI scheme" subitem="http" primary="true"/>
866<t>
867   The "http" URI scheme is hereby defined for the purpose of minting
868   identifiers according to their association with the hierarchical
869   namespace governed by a potential HTTP origin server listening for
870   TCP (<xref target="RFC0793"/>) connections on a given port.
871</t>
872<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"><!--terminal production--></iref>
873  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
874             [ "#" <x:ref>fragment</x:ref> ]
875</artwork></figure>
876<t>
877   The HTTP origin server is identified by the generic syntax's
878   <x:ref>authority</x:ref> component, which includes a host identifier
879   and optional TCP port (<xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>).
880   The remainder of the URI, consisting of both the hierarchical path
881   component and optional query component, serves as an identifier for
882   a potential resource within that origin server's name space.
883</t>
884<t>
885   A sender &MUST-NOT; generate an "http" URI with an empty host identifier.
886   A recipient that processes such a URI reference &MUST; reject it as invalid.
887</t>
888<t>
889   If the host identifier is provided as an IP address,
890   then the origin server is any listener on the indicated TCP port at
891   that IP address. If host is a registered name, then that name is
892   considered an indirect identifier and the recipient might use a name
893   resolution service, such as DNS, to find the address of a listener
894   for that host.
895   If the port subcomponent is empty or not given, then TCP port 80 is
896   assumed (the default reserved port for WWW services).
897</t>
898<t>
899   Regardless of the form of host identifier, access to that host is not
900   implied by the mere presence of its name or address. The host might or might
901   not exist and, even when it does exist, might or might not be running an
902   HTTP server or listening to the indicated port. The "http" URI scheme
903   makes use of the delegated nature of Internet names and addresses to
904   establish a naming authority (whatever entity has the ability to place
905   an HTTP server at that Internet name or address) and allows that
906   authority to determine which names are valid and how they might be used.
907</t>
908<t>
909   When an "http" URI is used within a context that calls for access to the
910   indicated resource, a client &MAY; attempt access by resolving
911   the host to an IP address, establishing a TCP connection to that address
912   on the indicated port, and sending an HTTP request message
913   (<xref target="http.message"/>) containing the URI's identifying data
914   (<xref target="message.routing"/>) to the server.
915   If the server responds to that request with a non-interim HTTP response
916   message, as described in &status-codes;, then that response
917   is considered an authoritative answer to the client's request.
918</t>
919<t>
920   Although HTTP is independent of the transport protocol, the "http"
921   scheme is specific to TCP-based services because the name delegation
922   process depends on TCP for establishing authority.
923   An HTTP service based on some other underlying connection protocol
924   would presumably be identified using a different URI scheme, just as
925   the "https" scheme (below) is used for resources that require an
926   end-to-end secured connection. Other protocols might also be used to
927   provide access to "http" identified resources &mdash; it is only the
928   authoritative interface that is specific to TCP.
929</t>
930<t>
931   The URI generic syntax for authority also includes a deprecated
932   userinfo subcomponent (<xref target="RFC3986" x:fmt="," x:sec="3.2.1"/>)
933   for including user authentication information in the URI.  Some
934   implementations make use of the userinfo component for internal
935   configuration of authentication information, such as within command
936   invocation options, configuration files, or bookmark lists, even
937   though such usage might expose a user identifier or password.
938   A sender &MUST-NOT; generate the userinfo subcomponent (and its "@"
939   delimiter) when an "http" URI reference is generated within a message as a
940   request target or header field value.
941   Before making use of an "http" URI reference received from an untrusted
942   source, a recipient ought to parse for userinfo and treat its presence as
943   an error; it is likely being used to obscure the authority for the sake of
944   phishing attacks.
945</t>
946</section>
947
948<section title="https URI scheme" anchor="https.uri">
949   <x:anchor-alias value="https-URI"/>
950   <iref item="https URI scheme"/>
951   <iref item="URI scheme" subitem="https"/>
952<t>
953   The "https" URI scheme is hereby defined for the purpose of minting
954   identifiers according to their association with the hierarchical
955   namespace governed by a potential HTTP origin server listening to a
956   given TCP port for TLS-secured connections
957   (<xref target="RFC0793"/>, <xref target="RFC5246"/>).
958</t>
959<t>
960   All of the requirements listed above for the "http" scheme are also
961   requirements for the "https" scheme, except that a default TCP port
962   of 443 is assumed if the port subcomponent is empty or not given,
963   and the user agent &MUST; ensure that its connection to the origin
964   server is secured through the use of strong encryption, end-to-end,
965   prior to sending the first HTTP request.
966</t>
967<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="https-URI"><!--terminal production--></iref>
968  <x:ref>https-URI</x:ref> = "https:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
969              [ "#" <x:ref>fragment</x:ref> ]
970</artwork></figure>
971<t>
972   Note that the "https" URI scheme depends on both TLS and TCP for
973   establishing authority.
974   Resources made available via the "https" scheme have no shared
975   identity with the "http" scheme even if their resource identifiers
976   indicate the same authority (the same host listening to the same
977   TCP port).  They are distinct name spaces and are considered to be
978   distinct origin servers.  However, an extension to HTTP that is
979   defined to apply to entire host domains, such as the Cookie protocol
980   <xref target="RFC6265"/>, can allow information
981   set by one service to impact communication with other services
982   within a matching group of host domains.
983</t>
984<t>
985   The process for authoritative access to an "https" identified
986   resource is defined in <xref target="RFC2818"/>.
987</t>
988</section>
989
990<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
991<t>
992   Since the "http" and "https" schemes conform to the URI generic syntax,
993   such URIs are normalized and compared according to the algorithm defined
994   in <xref target="RFC3986" x:fmt="," x:sec="6"/>, using the defaults
995   described above for each scheme.
996</t>
997<t>
998   If the port is equal to the default port for a scheme, the normal form is
999   to omit the port subcomponent. When not being used in absolute form as the
1000   request target of an OPTIONS request, an empty path component is equivalent
1001   to an absolute path of "/", so the normal form is to provide a path of "/"
1002   instead. The scheme and host are case-insensitive and normally provided in
1003   lowercase; all other components are compared in a case-sensitive manner.
1004   Characters other than those in the "reserved" set are equivalent to their
1005   percent-encoded octets (see <xref target="RFC3986" x:fmt=","
1006   x:sec="2.1"/>): the normal form is to not encode them.
1007</t>
1008<t>
1009   For example, the following three URIs are equivalent:
1010</t>
1011<figure><artwork type="example">
1012   http://example.com:80/~smith/home.html
1013   http://EXAMPLE.com/%7Esmith/home.html
1014   http://EXAMPLE.com:/%7esmith/home.html
1015</artwork></figure>
1016</section>
1017</section>
1018</section>
1019
1020<section title="Message Format" anchor="http.message">
1021<x:anchor-alias value="generic-message"/>
1022<x:anchor-alias value="message.types"/>
1023<x:anchor-alias value="HTTP-message"/>
1024<x:anchor-alias value="start-line"/>
1025<iref item="header section"/>
1026<iref item="headers"/>
1027<iref item="header field"/>
1028<t>
1029   All HTTP/1.1 messages consist of a start-line followed by a sequence of
1030   octets in a format similar to the Internet Message Format
1031   <xref target="RFC5322"/>: zero or more header fields (collectively
1032   referred to as the "headers" or the "header section"), an empty line
1033   indicating the end of the header section, and an optional message body.
1034</t>
1035<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"><!--terminal production--></iref>
1036  <x:ref>HTTP-message</x:ref>   = <x:ref>start-line</x:ref>
1037                   *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1038                   <x:ref>CRLF</x:ref>
1039                   [ <x:ref>message-body</x:ref> ]
1040</artwork></figure>
1041<t>
1042   The normal procedure for parsing an HTTP message is to read the
1043   start-line into a structure, read each header field into a hash
1044   table by field name until the empty line, and then use the parsed
1045   data to determine if a message body is expected.  If a message body
1046   has been indicated, then it is read as a stream until an amount
1047   of octets equal to the message body length is read or the connection
1048   is closed.
1049</t>
1050<t>
1051   A recipient &MUST; parse an HTTP message as a sequence of octets in an
1052   encoding that is a superset of US-ASCII <xref target="USASCII"/>.
1053   Parsing an HTTP message as a stream of Unicode characters, without regard
1054   for the specific encoding, creates security vulnerabilities due to the
1055   varying ways that string processing libraries handle invalid multibyte
1056   character sequences that contain the octet LF (%x0A).  String-based
1057   parsers can only be safely used within protocol elements after the element
1058   has been extracted from the message, such as within a header field-value
1059   after message parsing has delineated the individual fields.
1060</t>
1061<t>
1062   An HTTP message can be parsed as a stream for incremental processing or
1063   forwarding downstream.  However, recipients cannot rely on incremental
1064   delivery of partial messages, since some implementations will buffer or
1065   delay message forwarding for the sake of network efficiency, security
1066   checks, or payload transformations.
1067</t>
1068<t>
1069   A sender &MUST-NOT; send whitespace between the start-line and
1070   the first header field.
1071   A recipient that receives whitespace between the start-line and
1072   the first header field &MUST; either reject the message as invalid or
1073   consume each whitespace-preceded line without further processing of it
1074   (i.e., ignore the entire line, along with any subsequent lines preceded
1075   by whitespace, until a properly formed header field is received or the
1076   header section is terminated).
1077</t>
1078<t>
1079   The presence of such whitespace in a request
1080   might be an attempt to trick a server into ignoring that field or
1081   processing the line after it as a new request, either of which might
1082   result in a security vulnerability if other implementations within
1083   the request chain interpret the same message differently.
1084   Likewise, the presence of such whitespace in a response might be
1085   ignored by some clients or cause others to cease parsing.
1086</t>
1087
1088<section title="Start Line" anchor="start.line">
1089  <x:anchor-alias value="Start-Line"/>
1090<t>
1091   An HTTP message can either be a request from client to server or a
1092   response from server to client.  Syntactically, the two types of message
1093   differ only in the start-line, which is either a request-line (for requests)
1094   or a status-line (for responses), and in the algorithm for determining
1095   the length of the message body (<xref target="message.body"/>).
1096</t>
1097<t>
1098   In theory, a client could receive requests and a server could receive
1099   responses, distinguishing them by their different start-line formats,
1100   but in practice servers are implemented to only expect a request
1101   (a response is interpreted as an unknown or invalid request method)
1102   and clients are implemented to only expect a response.
1103</t>
1104<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="start-line"/>
1105  <x:ref>start-line</x:ref>     = <x:ref>request-line</x:ref> / <x:ref>status-line</x:ref>
1106</artwork></figure>
1107
1108<section title="Request Line" anchor="request.line">
1109  <x:anchor-alias value="Request"/>
1110  <x:anchor-alias value="request-line"/>
1111<t>
1112   A request-line begins with a method token, followed by a single
1113   space (SP), the request-target, another single space (SP), the
1114   protocol version, and ending with CRLF.
1115</t>
1116<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1117  <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>
1118</artwork></figure>
1119<iref primary="true" item="method"/>
1120<t anchor="method">
1121   The method token indicates the request method to be performed on the
1122   target resource. The request method is case-sensitive.
1123</t>
1124<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1125  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1126</artwork></figure>
1127<t>
1128   The request methods defined by this specification can be found in
1129   &methods;, along with information regarding the HTTP method registry
1130   and considerations for defining new methods.
1131</t>
1132<iref item="request-target"/>
1133<t>
1134   The request-target identifies the target resource upon which to apply
1135   the request, as defined in <xref target="request-target"/>.
1136</t>
1137<t>
1138   Recipients typically parse the request-line into its component parts by
1139   splitting on whitespace (see <xref target="message.robustness"/>), since
1140   no whitespace is allowed in the three components.
1141   Unfortunately, some user agents fail to properly encode or exclude
1142   whitespace found in hypertext references, resulting in those disallowed
1143   characters being sent in a request-target.
1144</t>
1145<t>
1146   Recipients of an invalid request-line &SHOULD; respond with either a
1147   <x:ref>400 (Bad Request)</x:ref> error or a <x:ref>301 (Moved Permanently)</x:ref>
1148   redirect with the request-target properly encoded.  A recipient &SHOULD-NOT;
1149   attempt to autocorrect and then process the request without a redirect,
1150   since the invalid request-line might be deliberately crafted to bypass
1151   security filters along the request chain.
1152</t>
1153<t>
1154   HTTP does not place a pre-defined limit on the length of a request-line.
1155   A server that receives a method longer than any that it implements
1156   &SHOULD; respond with a <x:ref>501 (Not Implemented)</x:ref> status code.
1157   A server ought to be prepared to receive URIs of unbounded length, as
1158   described in <xref target="conformance"/>, and &MUST; respond with a
1159   <x:ref>414 (URI Too Long)</x:ref> status code if the received
1160   request-target is longer than the server wishes to parse (see &status-414;).
1161</t>
1162<t>
1163   Various ad-hoc limitations on request-line length are found in practice.
1164   It is &RECOMMENDED; that all HTTP senders and recipients support, at a
1165   minimum, request-line lengths of 8000 octets.
1166</t>
1167</section>
1168
1169<section title="Status Line" anchor="status.line">
1170  <x:anchor-alias value="response"/>
1171  <x:anchor-alias value="status-line"/>
1172  <x:anchor-alias value="status-code"/>
1173  <x:anchor-alias value="reason-phrase"/>
1174<t>
1175   The first line of a response message is the status-line, consisting
1176   of the protocol version, a space (SP), the status code, another space,
1177   a possibly-empty textual phrase describing the status code, and
1178   ending with CRLF.
1179</t>
1180<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-line"/>
1181  <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>
1182</artwork></figure>
1183<t>
1184   The status-code element is a 3-digit integer code describing the
1185   result of the server's attempt to understand and satisfy the client's
1186   corresponding request. The rest of the response message is to be
1187   interpreted in light of the semantics defined for that status code.
1188   See &status-codes; for information about the semantics of status codes,
1189   including the classes of status code (indicated by the first digit),
1190   the status codes defined by this specification, considerations for the
1191   definition of new status codes, and the IANA registry.
1192</t>
1193<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-code"/>
1194  <x:ref>status-code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1195</artwork></figure>
1196<t>  
1197   The reason-phrase element exists for the sole purpose of providing a
1198   textual description associated with the numeric status code, mostly
1199   out of deference to earlier Internet application protocols that were more
1200   frequently used with interactive text clients. A client &SHOULD; ignore
1201   the reason-phrase content.
1202</t>
1203<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
1204  <x:ref>reason-phrase</x:ref>  = *( <x:ref>HTAB</x:ref> / <x:ref>SP</x:ref> / <x:ref>VCHAR</x:ref> / <x:ref>obs-text</x:ref> )
1205</artwork></figure>
1206</section>
1207</section>
1208
1209<section title="Header Fields" anchor="header.fields">
1210  <x:anchor-alias value="header-field"/>
1211  <x:anchor-alias value="field-content"/>
1212  <x:anchor-alias value="field-name"/>
1213  <x:anchor-alias value="field-value"/>
1214  <x:anchor-alias value="obs-fold"/>
1215<t>
1216   Each HTTP header field consists of a case-insensitive field name
1217   followed by a colon (":"), optional leading whitespace, the field value,
1218   and optional trailing whitespace.
1219</t>
1220<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="header-field"/><iref primary="true" item="Grammar" subitem="field-name"/><iref primary="true" item="Grammar" subitem="field-value"/><iref primary="true" item="Grammar" subitem="field-content"/><iref primary="true" item="Grammar" subitem="obs-fold"/>
1221  <x:ref>header-field</x:ref>   = <x:ref>field-name</x:ref> ":" <x:ref>OWS</x:ref> <x:ref>field-value</x:ref> <x:ref>OWS</x:ref>
1222  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1223  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>obs-fold</x:ref> )
1224  <x:ref>field-content</x:ref>  = *( <x:ref>HTAB</x:ref> / <x:ref>SP</x:ref> / <x:ref>VCHAR</x:ref> / <x:ref>obs-text</x:ref> )
1225  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref> ( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1226                 ; obsolete line folding
1227                 ; see <xref target="field.parsing"/>
1228</artwork></figure>
1229<t>
1230   The field-name token labels the corresponding field-value as having the
1231   semantics defined by that header field.  For example, the <x:ref>Date</x:ref>
1232   header field is defined in &header-date; as containing the origination
1233   timestamp for the message in which it appears.
1234</t>
1235
1236<section title="Field Extensibility" anchor="field.extensibility">
1237<t>
1238   Header fields are fully extensible: there is no limit on the
1239   introduction of new field names, each presumably defining new semantics,
1240   nor on the number of header fields used in a given message.  Existing
1241   fields are defined in each part of this specification and in many other
1242   specifications outside the core standard.
1243</t>
1244<t>
1245   New header fields can be defined such that, when they are understood by a
1246   recipient, they might override or enhance the interpretation of previously
1247   defined header fields, define preconditions on request evaluation, or
1248   refine the meaning of responses.
1249</t>
1250<t>
1251   A proxy &MUST; forward unrecognized header fields unless the
1252   field-name is listed in the <x:ref>Connection</x:ref> header field
1253   (<xref target="header.connection"/>) or the proxy is specifically
1254   configured to block, or otherwise transform, such fields.
1255   Other recipients &SHOULD; ignore unrecognized header fields.
1256   These requirements allow HTTP's functionality to be enhanced without
1257   requiring prior update of deployed intermediaries.
1258</t>
1259<t>
1260   All defined header fields ought to be registered with IANA in the
1261   Message Header Field Registry, as described in &iana-header-registry;.
1262</t>
1263</section>
1264
1265<section title="Field Order" anchor="field.order">
1266<t>
1267   The order in which header fields with differing field names are
1268   received is not significant. However, it is "good practice" to send
1269   header fields that contain control data first, such as <x:ref>Host</x:ref>
1270   on requests and <x:ref>Date</x:ref> on responses, so that implementations
1271   can decide when not to handle a message as early as possible.  A server
1272   &MUST; wait until the entire header section is received before interpreting
1273   a request message, since later header fields might include conditionals,
1274   authentication credentials, or deliberately misleading duplicate
1275   header fields that would impact request processing.
1276</t>
1277<t>
1278   A sender &MUST-NOT; generate multiple header fields with the same field
1279   name in a message unless either the entire field value for that
1280   header field is defined as a comma-separated list [i.e., #(values)]
1281   or the header field is a well-known exception (as noted below).
1282</t>
1283<t>
1284   A recipient &MAY; combine multiple header fields with the same field name
1285   into one "field-name: field-value" pair, without changing the semantics of
1286   the message, by appending each subsequent field value to the combined
1287   field value in order, separated by a comma. The order in which
1288   header fields with the same field name are received is therefore
1289   significant to the interpretation of the combined field value;
1290   a proxy &MUST-NOT; change the order of these field values when
1291   forwarding a message.
1292</t>
1293<x:note>
1294  <t>
1295   &Note; In practice, the "Set-Cookie" header field (<xref target="RFC6265"/>)
1296   often appears multiple times in a response message and does not use the
1297   list syntax, violating the above requirements on multiple header fields
1298   with the same name. Since it cannot be combined into a single field-value,
1299   recipients ought to handle "Set-Cookie" as a special case while processing
1300   header fields. (See Appendix A.2.3 of <xref target="Kri2001"/> for details.)
1301  </t>
1302</x:note>
1303</section>
1304
1305<section title="Whitespace" anchor="whitespace">
1306<t anchor="rule.LWS">
1307   This specification uses three rules to denote the use of linear
1308   whitespace: OWS (optional whitespace), RWS (required whitespace), and
1309   BWS ("bad" whitespace).
1310</t>
1311<t anchor="rule.OWS">
1312   The OWS rule is used where zero or more linear whitespace octets might
1313   appear. For protocol elements where optional whitespace is preferred to
1314   improve readability, a sender &SHOULD; generate the optional whitespace
1315   as a single SP; otherwise, a sender &SHOULD-NOT; generate optional
1316   whitespace except as needed to white-out invalid or unwanted protocol
1317   elements during in-place message filtering.
1318</t>
1319<t anchor="rule.RWS">
1320   The RWS rule is used when at least one linear whitespace octet is required
1321   to separate field tokens. A sender &SHOULD; generate RWS as a single SP.
1322</t>
1323<t anchor="rule.BWS">
1324   The BWS rule is used where the grammar allows optional whitespace only for
1325   historical reasons. A sender &MUST-NOT; generate BWS in messages.
1326   A recipient &MUST; parse for such bad whitespace and remove it before
1327   interpreting the protocol element.
1328</t>
1329<t anchor="rule.whitespace">
1330  <x:anchor-alias value="BWS"/>
1331  <x:anchor-alias value="OWS"/>
1332  <x:anchor-alias value="RWS"/>
1333</t>
1334<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"/>
1335  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1336                 ; optional whitespace
1337  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1338                 ; required whitespace
1339  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1340                 ; "bad" whitespace
1341</artwork></figure>
1342</section>
1343
1344<section title="Field Parsing" anchor="field.parsing">
1345<t>
1346   No whitespace is allowed between the header field-name and colon.
1347   In the past, differences in the handling of such whitespace have led to
1348   security vulnerabilities in request routing and response handling.
1349   A server &MUST; reject any received request message that contains
1350   whitespace between a header field-name and colon with a response code of
1351   <x:ref>400 (Bad Request)</x:ref>. A proxy &MUST; remove any such whitespace
1352   from a response message before forwarding the message downstream.
1353</t>
1354<t>
1355   A field value is preceded by optional whitespace (OWS); a single SP is
1356   preferred. The field value does not include any leading or trailing white
1357   space: OWS occurring before the first non-whitespace octet of the field
1358   value or after the last non-whitespace octet of the field value ought to be
1359   excluded by parsers when extracting the field value from a header field.
1360</t>
1361<t>
1362   A recipient of field-content containing multiple sequential octets of
1363   optional (OWS) or required (RWS) whitespace &SHOULD; either replace the
1364   sequence with a single SP or transform any non-SP octets in the sequence to
1365   SP octets before interpreting the field value or forwarding the message
1366   downstream.
1367</t>
1368<t>
1369   Historically, HTTP header field values could be extended over multiple
1370   lines by preceding each extra line with at least one space or horizontal
1371   tab (obs-fold). This specification deprecates such line folding except
1372   within the message/http media type
1373   (<xref target="internet.media.type.message.http"/>).
1374   A sender &MUST-NOT; generate a message that includes line folding
1375   (i.e., that has any field-value that contains a match to the
1376   <x:ref>obs-fold</x:ref> rule) unless the message is intended for packaging
1377   within the message/http media type.
1378</t>
1379<t>
1380   A server that receives an <x:ref>obs-fold</x:ref> in a request message that
1381   is not within a message/http container &MUST; either reject the message by
1382   sending a <x:ref>400 (Bad Request)</x:ref>, preferably with a
1383   representation explaining that obsolete line folding is unacceptable, or
1384   replace each received <x:ref>obs-fold</x:ref> with one or more
1385   <x:ref>SP</x:ref> octets prior to interpreting the field value or
1386   forwarding the message downstream.
1387</t>
1388<t>
1389   A proxy or gateway that receives an <x:ref>obs-fold</x:ref> in a response
1390   message that is not within a message/http container &MUST; either discard
1391   the message and replace it with a <x:ref>502 (Bad Gateway)</x:ref>
1392   response, preferably with a representation explaining that unacceptable
1393   line folding was received, or replace each received <x:ref>obs-fold</x:ref>
1394   with one or more <x:ref>SP</x:ref> octets prior to interpreting the field
1395   value or forwarding the message downstream.
1396</t>
1397<t>
1398   A user agent that receives an <x:ref>obs-fold</x:ref> in a response message
1399   that is not within a message/http container &MUST; replace each received
1400   <x:ref>obs-fold</x:ref> with one or more <x:ref>SP</x:ref> octets prior to
1401   interpreting the field value.
1402</t>
1403<t>
1404   Historically, HTTP has allowed field content with text in the ISO-8859-1
1405   <xref target="ISO-8859-1"/> charset, supporting other charsets only
1406   through use of <xref target="RFC2047"/> encoding.
1407   In practice, most HTTP header field values use only a subset of the
1408   US-ASCII charset <xref target="USASCII"/>. Newly defined
1409   header fields &SHOULD; limit their field values to US-ASCII octets.
1410   A recipient &SHOULD; treat other octets in field content (obs-text) as
1411   opaque data.
1412</t>
1413</section>
1414
1415<section title="Field Limits" anchor="field.limits">
1416<t>
1417   HTTP does not place a pre-defined limit on the length of each header field
1418   or on the length of the header section as a whole, as described in
1419   <xref target="conformance"/>. Various ad-hoc limitations on individual
1420   header field length are found in practice, often depending on the specific
1421   field semantics.
1422</t>
1423<t>
1424   A server ought to be prepared to receive request header fields of unbounded
1425   length and &MUST; respond with an appropriate
1426   <x:ref>4xx (Client Error)</x:ref> status code if the received header
1427   field(s) are larger than the server wishes to process.
1428</t>
1429<t>
1430   A client ought to be prepared to receive response header fields of
1431   unbounded length.
1432   A client &MAY; discard or truncate received header fields that are larger
1433   than the client wishes to process if the field semantics are such that the
1434   dropped value(s) can be safely ignored without changing the
1435   message framing or response semantics.
1436</t>
1437</section>
1438
1439<section title="Field value components" anchor="field.components">
1440<t anchor="rule.token.separators">
1441  <x:anchor-alias value="tchar"/>
1442  <x:anchor-alias value="token"/>
1443  <x:anchor-alias value="special"/>
1444   Most HTTP header field values are defined using common syntax components
1445   (token, quoted-string, and comment) separated by whitespace or specific
1446   delimiting characters. Delimiters are chosen from the set of US-ASCII
1447   visual characters not allowed in a token ({VCHAR - tchar}).
1448</t>
1449<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="token"/><iref primary="true" item="Grammar" subitem="tchar"/>
1450  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1451<!--
1452  IMPORTANT: when editing "tchar" make sure that "special" is updated accordingly!!!
1453 -->
1454  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1455                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1456                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1457                 ; any <x:ref>VCHAR</x:ref>, except <x:ref>special</x:ref>
1458
1459  <x:ref>special</x:ref>        = "(" / ")" / "&lt;" / ">" / "@" / ","
1460                 / ";" / ":" / "\" / DQUOTE / "/" / "["
1461                 / "]" / "?" / "=" / "{" / "}"
1462</artwork></figure>
1463<t anchor="rule.quoted-string">
1464  <x:anchor-alias value="quoted-string"/>
1465  <x:anchor-alias value="qdtext"/>
1466  <x:anchor-alias value="obs-text"/>
1467   A string of text is parsed as a single value if it is quoted using
1468   double-quote marks.
1469</t>
1470<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"/>
1471  <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>
1472  <x:ref>qdtext</x:ref>         = <x:ref>HTAB</x:ref> / <x:ref>SP</x:ref> /%x21 / %x23-5B / %x5D-7E / <x:ref>obs-text</x:ref>
1473  <x:ref>obs-text</x:ref>       = %x80-FF
1474</artwork></figure>
1475<t anchor="rule.comment">
1476  <x:anchor-alias value="comment"/>
1477  <x:anchor-alias value="ctext"/>
1478   Comments can be included in some HTTP header fields by surrounding
1479   the comment text with parentheses. Comments are only allowed in
1480   fields containing "comment" as part of their field value definition.
1481</t>
1482<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1483  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-pair</x:ref> / <x:ref>comment</x:ref> ) ")"
1484  <x:ref>ctext</x:ref>          = <x:ref>HTAB</x:ref> / <x:ref>SP</x:ref> / %x21-27 / %x2A-5B / %x5D-7E / <x:ref>obs-text</x:ref>
1485</artwork></figure>
1486<t anchor="rule.quoted-pair">
1487  <x:anchor-alias value="quoted-pair"/>
1488   The backslash octet ("\") can be used as a single-octet
1489   quoting mechanism within quoted-string and comment constructs.
1490   Recipients that process the value of a quoted-string &MUST; handle a
1491   quoted-pair as if it were replaced by the octet following the backslash.
1492</t>
1493<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1494  <x:ref>quoted-pair</x:ref>    = "\" ( <x:ref>HTAB</x:ref> / <x:ref>SP</x:ref> / <x:ref>VCHAR</x:ref> / <x:ref>obs-text</x:ref> )
1495</artwork></figure>
1496<t>
1497   A sender &SHOULD-NOT; generate a quoted-pair in a quoted-string except
1498   where necessary to quote DQUOTE and backslash octets occurring within that
1499   string.
1500   A sender &SHOULD-NOT; generate a quoted-pair in a comment except
1501   where necessary to quote parentheses ["(" and ")"] and backslash octets
1502   occurring within that comment.
1503</t>
1504</section>
1505
1506</section>
1507
1508<section title="Message Body" anchor="message.body">
1509  <x:anchor-alias value="message-body"/>
1510<t>
1511   The message body (if any) of an HTTP message is used to carry the
1512   payload body of that request or response.  The message body is
1513   identical to the payload body unless a transfer coding has been
1514   applied, as described in <xref target="header.transfer-encoding"/>.
1515</t>
1516<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1517  <x:ref>message-body</x:ref> = *OCTET
1518</artwork></figure>
1519<t>
1520   The rules for when a message body is allowed in a message differ for
1521   requests and responses.
1522</t>
1523<t>
1524   The presence of a message body in a request is signaled by a
1525   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1526   field. Request message framing is independent of method semantics,
1527   even if the method does not define any use for a message body.
1528</t>
1529<t>
1530   The presence of a message body in a response depends on both
1531   the request method to which it is responding and the response
1532   status code (<xref target="status.line"/>).
1533   Responses to the HEAD request method never include a message body
1534   because the associated response header fields (e.g.,
1535   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1536   if present, indicate only what their values would have been if the request
1537   method had been GET (&HEAD;).
1538   <x:ref>2xx (Successful)</x:ref> responses to CONNECT switch to tunnel
1539   mode instead of having a message body (&CONNECT;).
1540   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1541   <x:ref>304 (Not Modified)</x:ref> responses do not include a message body.
1542   All other responses do include a message body, although the body
1543   might be of zero length.
1544</t>
1545
1546<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1547  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1548  <iref item="chunked (Coding Format)"/>
1549  <x:anchor-alias value="Transfer-Encoding"/>
1550<t>
1551   The Transfer-Encoding header field lists the transfer coding names
1552   corresponding to the sequence of transfer codings that have been
1553   (or will be) applied to the payload body in order to form the message body.
1554   Transfer codings are defined in <xref target="transfer.codings"/>.
1555</t>
1556<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1557  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1558</artwork></figure>
1559<t>
1560   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1561   MIME, which was designed to enable safe transport of binary data over a
1562   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1563   However, safe transport has a different focus for an 8bit-clean transfer
1564   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1565   accurately delimit a dynamically generated payload and to distinguish
1566   payload encodings that are only applied for transport efficiency or
1567   security from those that are characteristics of the selected resource.
1568</t>
1569<t>
1570   A recipient &MUST; be able to parse the chunked transfer coding
1571   (<xref target="chunked.encoding"/>) because it plays a crucial role in
1572   framing messages when the payload body size is not known in advance.
1573   A sender &MUST-NOT; apply chunked more than once to a message body
1574   (i.e., chunking an already chunked message is not allowed).
1575   If any transfer coding other than chunked is applied to a request payload
1576   body, the sender &MUST; apply chunked as the final transfer coding to
1577   ensure that the message is properly framed.
1578   If any transfer coding other than chunked is applied to a response payload
1579   body, the sender &MUST; either apply chunked as the final transfer coding
1580   or terminate the message by closing the connection.
1581</t>
1582<figure><preamble>
1583   For example,
1584</preamble><artwork type="example">
1585  Transfer-Encoding: gzip, chunked
1586</artwork><postamble>
1587   indicates that the payload body has been compressed using the gzip
1588   coding and then chunked using the chunked coding while forming the
1589   message body.
1590</postamble></figure>
1591<t>
1592   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1593   Transfer-Encoding is a property of the message, not of the representation, and
1594   any recipient along the request/response chain &MAY; decode the received
1595   transfer coding(s) or apply additional transfer coding(s) to the message
1596   body, assuming that corresponding changes are made to the Transfer-Encoding
1597   field-value. Additional information about the encoding parameters &MAY; be
1598   provided by other header fields not defined by this specification.
1599</t>
1600<t>
1601   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1602   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1603   neither of which includes a message body,
1604   to indicate that the origin server would have applied a transfer coding
1605   to the message body if the request had been an unconditional GET.
1606   This indication is not required, however, because any recipient on
1607   the response chain (including the origin server) can remove transfer
1608   codings when they are not needed.
1609</t>
1610<t>
1611   A server &MUST-NOT; send a Transfer-Encoding header field in any response
1612   with a status code of
1613   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1614   A server &MUST-NOT; send a Transfer-Encoding header field in any
1615   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1616</t>
1617<t>
1618   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1619   implementations advertising only HTTP/1.0 support will not understand
1620   how to process a transfer-encoded payload.
1621   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1622   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1623   might be in the form of specific user configuration or by remembering the
1624   version of a prior received response.
1625   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1626   the corresponding request indicates HTTP/1.1 (or later).
1627</t>
1628<t>
1629   A server that receives a request message with a transfer coding it does
1630   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref>.
1631</t>
1632</section>
1633
1634<section title="Content-Length" anchor="header.content-length">
1635  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1636  <x:anchor-alias value="Content-Length"/>
1637<t>
1638   When a message does not have a <x:ref>Transfer-Encoding</x:ref> header
1639   field, a Content-Length header field can provide the anticipated size,
1640   as a decimal number of octets, for a potential payload body.
1641   For messages that do include a payload body, the Content-Length field-value
1642   provides the framing information necessary for determining where the body
1643   (and message) ends.  For messages that do not include a payload body, the
1644   Content-Length indicates the size of the selected representation
1645   (&representation;).
1646</t>
1647<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1648  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1649</artwork></figure>
1650<t>
1651   An example is
1652</t>
1653<figure><artwork type="example">
1654  Content-Length: 3495
1655</artwork></figure>
1656<t>
1657   A sender &MUST-NOT; send a Content-Length header field in any message that
1658   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1659</t>
1660<t>
1661   A user agent &SHOULD; send a Content-Length in a request message when no
1662   <x:ref>Transfer-Encoding</x:ref> is sent and the request method defines
1663   a meaning for an enclosed payload body. For example, a Content-Length
1664   header field is normally sent in a POST request even when the value is
1665   0 (indicating an empty payload body).  A user agent &SHOULD-NOT; send a
1666   Content-Length header field when the request message does not contain a
1667   payload body and the method semantics do not anticipate such a body.
1668</t>
1669<t>
1670   A server &MAY; send a Content-Length header field in a response to a HEAD
1671   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1672   response unless its field-value equals the decimal number of octets that
1673   would have been sent in the payload body of a response if the same
1674   request had used the GET method.
1675</t>
1676<t>
1677   A server &MAY; send a Content-Length header field in a
1678   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1679   (&status-304;); a server &MUST-NOT; send Content-Length in such a
1680   response unless its field-value equals the decimal number of octets that
1681   would have been sent in the payload body of a <x:ref>200 (OK)</x:ref>
1682   response to the same request.
1683</t>
1684<t>
1685   A server &MUST-NOT; send a Content-Length header field in any response
1686   with a status code of
1687   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1688   A server &MUST-NOT; send a Content-Length header field in any
1689   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1690</t>
1691<t>
1692   Aside from the cases defined above, in the absence of Transfer-Encoding,
1693   an origin server &SHOULD; send a Content-Length header field when the
1694   payload body size is known prior to sending the complete header section.
1695   This will allow downstream recipients to measure transfer progress,
1696   know when a received message is complete, and potentially reuse the
1697   connection for additional requests.
1698</t>
1699<t>
1700   Any Content-Length field value greater than or equal to zero is valid.
1701   Since there is no predefined limit to the length of a payload, a
1702   recipient &MUST; anticipate potentially large decimal numerals and
1703   prevent parsing errors due to integer conversion overflows
1704   (<xref target="attack.protocol.element.size.overflows"/>).
1705</t>
1706<t>
1707   If a message is received that has multiple Content-Length header fields
1708   with field-values consisting of the same decimal value, or a single
1709   Content-Length header field with a field value containing a list of
1710   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1711   duplicate Content-Length header fields have been generated or combined by an
1712   upstream message processor, then the recipient &MUST; either reject the
1713   message as invalid or replace the duplicated field-values with a single
1714   valid Content-Length field containing that decimal value prior to
1715   determining the message body length or forwarding the message.
1716</t>
1717<x:note>
1718  <t>
1719   &Note; HTTP's use of Content-Length for message framing differs
1720   significantly from the same field's use in MIME, where it is an optional
1721   field used only within the "message/external-body" media-type.
1722  </t>
1723</x:note>
1724</section>
1725
1726<section title="Message Body Length" anchor="message.body.length">
1727  <iref item="chunked (Coding Format)"/>
1728<t>
1729   The length of a message body is determined by one of the following
1730   (in order of precedence):
1731</t>
1732<t>
1733  <list style="numbers">
1734    <x:lt><t>
1735     Any response to a HEAD request and any response with a
1736     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1737     <x:ref>304 (Not Modified)</x:ref> status code is always
1738     terminated by the first empty line after the header fields, regardless of
1739     the header fields present in the message, and thus cannot contain a
1740     message body.
1741    </t></x:lt>
1742    <x:lt><t>
1743     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1744     connection will become a tunnel immediately after the empty line that
1745     concludes the header fields.  A client &MUST; ignore any
1746     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1747     fields received in such a message.
1748    </t></x:lt>
1749    <x:lt><t>
1750     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1751     and the chunked transfer coding (<xref target="chunked.encoding"/>)
1752     is the final encoding, the message body length is determined by reading
1753     and decoding the chunked data until the transfer coding indicates the
1754     data is complete.
1755    </t>
1756    <t>
1757     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1758     response and the chunked transfer coding is not the final encoding, the
1759     message body length is determined by reading the connection until it is
1760     closed by the server.
1761     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a request and the
1762     chunked transfer coding is not the final encoding, the message body
1763     length cannot be determined reliably; the server &MUST; respond with
1764     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1765    </t>
1766    <t>
1767     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1768     and a <x:ref>Content-Length</x:ref> header field, the Transfer-Encoding
1769     overrides the Content-Length. Such a message might indicate an attempt
1770     to perform request or response smuggling (bypass of security-related
1771     checks on message routing or content) and thus ought to be handled as
1772     an error.  A sender &MUST; remove the received Content-Length field
1773     prior to forwarding such a message downstream.
1774    </t></x:lt>
1775    <x:lt><t>
1776     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1777     either multiple <x:ref>Content-Length</x:ref> header fields having
1778     differing field-values or a single Content-Length header field having an
1779     invalid value, then the message framing is invalid and
1780     the recipient &MUST; treat it as an unrecoverable error to prevent
1781     request or response smuggling.
1782     If this is a request message, the server &MUST; respond with
1783     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1784     If this is a response message received by a proxy,
1785     the proxy &MUST; close the connection to the server, discard the received
1786     response, and send a <x:ref>502 (Bad Gateway)</x:ref> response to the
1787     client.
1788     If this is a response message received by a user agent,
1789     the user agent &MUST; close the connection to the server and discard the
1790     received response.
1791    </t></x:lt>
1792    <x:lt><t>
1793     If a valid <x:ref>Content-Length</x:ref> header field is present without
1794     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1795     expected message body length in octets.
1796     If the sender closes the connection or the recipient times out before the
1797     indicated number of octets are received, the recipient &MUST; consider
1798     the message to be incomplete and close the connection.
1799    </t></x:lt>
1800    <x:lt><t>
1801     If this is a request message and none of the above are true, then the
1802     message body length is zero (no message body is present).
1803    </t></x:lt>
1804    <x:lt><t>
1805     Otherwise, this is a response message without a declared message body
1806     length, so the message body length is determined by the number of octets
1807     received prior to the server closing the connection.
1808    </t></x:lt>
1809  </list>
1810</t>
1811<t>
1812   Since there is no way to distinguish a successfully completed,
1813   close-delimited message from a partially-received message interrupted
1814   by network failure, a server &SHOULD; generate encoding or
1815   length-delimited messages whenever possible.  The close-delimiting
1816   feature exists primarily for backwards compatibility with HTTP/1.0.
1817</t>
1818<t>
1819   A server &MAY; reject a request that contains a message body but
1820   not a <x:ref>Content-Length</x:ref> by responding with
1821   <x:ref>411 (Length Required)</x:ref>.
1822</t>
1823<t>
1824   Unless a transfer coding other than chunked has been applied,
1825   a client that sends a request containing a message body &SHOULD;
1826   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1827   length is known in advance, rather than the chunked transfer coding, since some
1828   existing services respond to chunked with a <x:ref>411 (Length Required)</x:ref>
1829   status code even though they understand the chunked transfer coding.  This
1830   is typically because such services are implemented via a gateway that
1831   requires a content-length in advance of being called and the server
1832   is unable or unwilling to buffer the entire request before processing.
1833</t>
1834<t>
1835   A user agent that sends a request containing a message body &MUST; send a
1836   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1837   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1838   the form of specific user configuration or by remembering the version of a
1839   prior received response.
1840</t>
1841<t>
1842   If the final response to the last request on a connection has been
1843   completely received and there remains additional data to read, a user agent
1844   &MAY; discard the remaining data or attempt to determine if that data
1845   belongs as part of the prior response body, which might be the case if the
1846   prior message's Content-Length value is incorrect. A client &MUST-NOT;
1847   process, cache, or forward such extra data as a separate response, since
1848   such behavior would be vulnerable to cache poisoning.
1849</t>
1850</section>
1851</section>
1852
1853<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1854<t>
1855   A server that receives an incomplete request message, usually due to a
1856   canceled request or a triggered time-out exception, &MAY; send an error
1857   response prior to closing the connection.
1858</t>
1859<t>
1860   A client that receives an incomplete response message, which can occur
1861   when a connection is closed prematurely or when decoding a supposedly
1862   chunked transfer coding fails, &MUST; record the message as incomplete.
1863   Cache requirements for incomplete responses are defined in
1864   &cache-incomplete;.
1865</t>
1866<t>
1867   If a response terminates in the middle of the header section (before the
1868   empty line is received) and the status code might rely on header fields to
1869   convey the full meaning of the response, then the client cannot assume
1870   that meaning has been conveyed; the client might need to repeat the
1871   request in order to determine what action to take next.
1872</t>
1873<t>
1874   A message body that uses the chunked transfer coding is
1875   incomplete if the zero-sized chunk that terminates the encoding has not
1876   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1877   incomplete if the size of the message body received (in octets) is less than
1878   the value given by Content-Length.  A response that has neither chunked
1879   transfer coding nor Content-Length is terminated by closure of the
1880   connection, and thus is considered complete regardless of the number of
1881   message body octets received, provided that the header section was received
1882   intact.
1883</t>
1884</section>
1885
1886<section title="Message Parsing Robustness" anchor="message.robustness">
1887<t>
1888   Older HTTP/1.0 user agent implementations might send an extra CRLF
1889   after a POST request as a workaround for some early server
1890   applications that failed to read message body content that was
1891   not terminated by a line-ending. An HTTP/1.1 user agent &MUST-NOT;
1892   preface or follow a request with an extra CRLF.  If terminating
1893   the request message body with a line-ending is desired, then the
1894   user agent &MUST; count the terminating CRLF octets as part of the
1895   message body length.
1896</t>
1897<t>
1898   In the interest of robustness, a server that is expecting to receive and
1899   parse a request-line &SHOULD; ignore at least one empty line (CRLF)
1900   received prior to the request-line.
1901</t>
1902<t>
1903   Although the line terminator for the start-line and header
1904   fields is the sequence CRLF, a recipient &MAY; recognize a
1905   single LF as a line terminator and ignore any preceding CR.
1906</t>
1907<t>
1908   Although the request-line and status-line grammar rules require that each
1909   of the component elements be separated by a single SP octet, recipients
1910   &MAY; instead parse on whitespace-delimited word boundaries and, aside
1911   from the CRLF terminator, treat any form of whitespace as the SP separator
1912   while ignoring preceding or trailing whitespace;
1913   such whitespace includes one or more of the following octets:
1914   SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
1915</t>
1916<t>
1917   When a server listening only for HTTP request messages, or processing
1918   what appears from the start-line to be an HTTP request message,
1919   receives a sequence of octets that does not match the HTTP-message
1920   grammar aside from the robustness exceptions listed above, the
1921   server &SHOULD; respond with a <x:ref>400 (Bad Request)</x:ref> response. 
1922</t>
1923</section>
1924</section>
1925
1926<section title="Transfer Codings" anchor="transfer.codings">
1927  <x:anchor-alias value="transfer-coding"/>
1928  <x:anchor-alias value="transfer-extension"/>
1929<t>
1930   Transfer coding names are used to indicate an encoding
1931   transformation that has been, can be, or might need to be applied to a
1932   payload body in order to ensure "safe transport" through the network.
1933   This differs from a content coding in that the transfer coding is a
1934   property of the message rather than a property of the representation
1935   that is being transferred.
1936</t>
1937<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1938  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1939                     / "compress" ; <xref target="compress.coding"/>
1940                     / "deflate" ; <xref target="deflate.coding"/>
1941                     / "gzip" ; <xref target="gzip.coding"/>
1942                     / <x:ref>transfer-extension</x:ref>
1943  <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> )
1944</artwork></figure>
1945<t anchor="rule.parameter">
1946  <x:anchor-alias value="transfer-parameter"/>
1947   Parameters are in the form of a name or name=value pair.
1948</t>
1949<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-parameter"/>
1950  <x:ref>transfer-parameter</x:ref> = <x:ref>token</x:ref> <x:ref>BWS</x:ref> "=" <x:ref>BWS</x:ref> ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> )
1951</artwork></figure>
1952<t>
1953   All transfer-coding names are case-insensitive and ought to be registered
1954   within the HTTP Transfer Coding registry, as defined in
1955   <xref target="transfer.coding.registry"/>.
1956   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1957   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1958   header fields.
1959</t>
1960
1961<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1962  <iref primary="true" item="chunked (Coding Format)"/>
1963  <x:anchor-alias value="chunk"/>
1964  <x:anchor-alias value="chunked-body"/>
1965  <x:anchor-alias value="chunk-data"/>
1966  <x:anchor-alias value="chunk-size"/>
1967  <x:anchor-alias value="last-chunk"/>
1968<t>
1969   The chunked transfer coding wraps the payload body in order to transfer it
1970   as a series of chunks, each with its own size indicator, followed by an
1971   &OPTIONAL; trailer containing header fields. Chunked enables content
1972   streams of unknown size to be transferred as a sequence of length-delimited
1973   buffers, which enables the sender to retain connection persistence and the
1974   recipient to know when it has received the entire message.
1975</t>
1976<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="chunked-body"><!--terminal production--></iref><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="false" item="Grammar" subitem="trailer-part"/>
1977  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1978                   <x:ref>last-chunk</x:ref>
1979                   <x:ref>trailer-part</x:ref>
1980                   <x:ref>CRLF</x:ref>
1981 
1982  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1983                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1984  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1985  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1986 
1987  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1988</artwork></figure>
1989<t>
1990   The chunk-size field is a string of hex digits indicating the size of
1991   the chunk-data in octets. The chunked transfer coding is complete when a
1992   chunk with a chunk-size of zero is received, possibly followed by a
1993   trailer, and finally terminated by an empty line.
1994</t>
1995<t>
1996   A recipient &MUST; be able to parse and decode the chunked transfer coding.
1997</t>
1998
1999<section title="Chunk Extensions" anchor="chunked.extension">
2000  <x:anchor-alias value="chunk-ext"/>
2001  <x:anchor-alias value="chunk-ext-name"/>
2002  <x:anchor-alias value="chunk-ext-val"/>
2003<t>
2004   The chunked encoding allows each chunk to include zero or more chunk
2005   extensions, immediately following the <x:ref>chunk-size</x:ref>, for the
2006   sake of supplying per-chunk metadata (such as a signature or hash),
2007   mid-message control information, or randomization of message body size.
2008</t>
2009<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="chunked-body"><!--terminal production--></iref><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="false" item="Grammar" subitem="trailer-part"/><iref primary="true" item="Grammar" subitem="quoted-str-nf"/><iref primary="true" item="Grammar" subitem="qdtext-nf"/>
2010  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
2011
2012  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
2013  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
2014</artwork></figure>
2015<t>
2016   The chunked encoding is specific to each connection and is likely to be
2017   removed or recoded by each recipient (including intermediaries) before any
2018   higher-level application would have a chance to inspect the extensions.
2019   Hence, use of chunk extensions is generally limited to specialized HTTP
2020   services such as "long polling" (where client and server can have shared
2021   expectations regarding the use of chunk extensions) or for padding within
2022   an end-to-end secured connection.
2023</t>
2024<t>
2025   A recipient &MUST; ignore unrecognized chunk extensions.
2026   A server ought to limit the total length of chunk extensions received in a
2027   request to an amount reasonable for the services provided, in the same way
2028   that it applies length limitations and timeouts for other parts of a
2029   message, and generate an appropriate <x:ref>4xx (Client Error)</x:ref>
2030   response if that amount is exceeded.
2031</t>
2032</section>
2033
2034<section title="Chunked Trailer Part" anchor="chunked.trailer.part">
2035  <x:anchor-alias value="trailer-part"/>
2036<t>
2037   A trailer allows the sender to include additional fields at the end of a
2038   chunked message in order to supply metadata that might be dynamically
2039   generated while the message body is sent, such as a message integrity
2040   check, digital signature, or post-processing status. The trailer fields are
2041   identical to header fields, except they are sent in a chunked trailer
2042   instead of the message's header section.
2043</t>
2044<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="trailer-part"/>
2045  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
2046</artwork></figure>
2047<t>
2048   A sender &MUST-NOT; generate a trailer that contains a field which needs to
2049   be known by the recipient before it can begin processing the message body.
2050   For example, most recipients need to know the values of
2051   <x:ref>Content-Encoding</x:ref> and <x:ref>Content-Type</x:ref> in order to
2052   select a content handler, so placing those fields in a trailer would force
2053   the recipient to buffer the entire body before it could begin, greatly
2054   increasing user-perceived latency and defeating one of the main advantages
2055   of using chunked to send data streams of unknown length.
2056   A sender &MUST-NOT; generate a trailer containing a
2057   <x:ref>Transfer-Encoding</x:ref>,
2058   <x:ref>Content-Length</x:ref>, or
2059   <x:ref>Trailer</x:ref> field.
2060</t>
2061<t>
2062   A server &MUST; generate an empty trailer with the chunked transfer coding
2063   unless at least one of the following is true:
2064  <list style="numbers">
2065    <t>the request included a <x:ref>TE</x:ref> header field that indicates
2066    "trailers" is acceptable in the transfer coding of the response, as
2067    described in <xref target="header.te"/>; or,</t>
2068     
2069    <t>the trailer fields consist entirely of optional metadata and the
2070    recipient could use the message (in a manner acceptable to the generating
2071    server) without receiving that metadata. In other words, the generating
2072    server is willing to accept the possibility that the trailer fields might
2073    be silently discarded along the path to the client.</t>
2074  </list>
2075</t>
2076<t>
2077   The above requirement prevents the need for an infinite buffer when a
2078   message is being received by an HTTP/1.1 (or later) proxy and forwarded to
2079   an HTTP/1.0 recipient.
2080</t>
2081</section>
2082
2083<section title="Decoding Chunked" anchor="decoding.chunked">
2084<t>
2085   A process for decoding the chunked transfer coding
2086   can be represented in pseudo-code as:
2087</t>
2088<figure><artwork type="code">
2089  length := 0
2090  read chunk-size, chunk-ext (if any), and CRLF
2091  while (chunk-size &gt; 0) {
2092     read chunk-data and CRLF
2093     append chunk-data to decoded-body
2094     length := length + chunk-size
2095     read chunk-size, chunk-ext (if any), and CRLF
2096  }
2097  read header-field
2098  while (header-field not empty) {
2099     append header-field to existing header fields
2100     read header-field
2101  }
2102  Content-Length := length
2103  Remove "chunked" from Transfer-Encoding
2104  Remove Trailer from existing header fields
2105</artwork></figure>
2106</section>
2107</section>
2108
2109<section title="Compression Codings" anchor="compression.codings">
2110<t>
2111   The codings defined below can be used to compress the payload of a
2112   message.
2113</t>
2114
2115<section title="Compress Coding" anchor="compress.coding">
2116<iref item="compress (Coding Format)"/>
2117<t>
2118   The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding
2119   <xref target="Welch"/> that is commonly produced by the UNIX file
2120   compression program "compress".
2121   A recipient &SHOULD; consider "x-compress" to be equivalent to "compress".
2122</t>
2123</section>
2124
2125<section title="Deflate Coding" anchor="deflate.coding">
2126<iref item="deflate (Coding Format)"/>
2127<t>
2128   The "deflate" coding is a "zlib" data format <xref target="RFC1950"/>
2129   containing a "deflate" compressed data stream <xref target="RFC1951"/>
2130   that uses a combination of the Lempel-Ziv (LZ77) compression algorithm and
2131   Huffman coding.
2132</t>
2133<x:note>
2134  <t>
2135    &Note; Some incorrect implementations send the "deflate"
2136    compressed data without the zlib wrapper.
2137   </t>
2138</x:note>
2139</section>
2140
2141<section title="Gzip Coding" anchor="gzip.coding">
2142<iref item="gzip (Coding Format)"/>
2143<t>
2144   The "gzip" coding is an LZ77 coding with a 32 bit CRC that is commonly
2145   produced by the gzip file compression program <xref target="RFC1952"/>.
2146   A recipient &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2147</t>
2148</section>
2149
2150</section>
2151
2152<section title="TE" anchor="header.te">
2153  <iref primary="true" item="TE header field" x:for-anchor=""/>
2154  <x:anchor-alias value="TE"/>
2155  <x:anchor-alias value="t-codings"/>
2156  <x:anchor-alias value="t-ranking"/>
2157  <x:anchor-alias value="rank"/>
2158<t>
2159   The "TE" header field in a request indicates what transfer codings,
2160   besides chunked, the client is willing to accept in response, and
2161   whether or not the client is willing to accept trailer fields in a
2162   chunked transfer coding.
2163</t>
2164<t>
2165   The TE field-value consists of a comma-separated list of transfer coding
2166   names, each allowing for optional parameters (as described in
2167   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2168   A client &MUST-NOT; send the chunked transfer coding name in TE;
2169   chunked is always acceptable for HTTP/1.1 recipients.
2170</t>
2171<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="TE"/><iref primary="true" item="Grammar" subitem="t-codings"/><iref primary="true" item="Grammar" subitem="t-ranking"/><iref primary="true" item="Grammar" subitem="rank"/>
2172  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2173  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2174  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2175  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2176             / ( "1" [ "." 0*3("0") ] )
2177</artwork></figure>
2178<t>
2179   Three examples of TE use are below.
2180</t>
2181<figure><artwork type="example">
2182  TE: deflate
2183  TE:
2184  TE: trailers, deflate;q=0.5
2185</artwork></figure>
2186<t>
2187   The presence of the keyword "trailers" indicates that the client is willing
2188   to accept trailer fields in a chunked transfer coding, as defined in
2189   <xref target="chunked.trailer.part"/>, on behalf of itself and any downstream
2190   clients. For requests from an intermediary, this implies that either:
2191   (a) all downstream clients are willing to accept trailer fields in the
2192   forwarded response; or,
2193   (b) the intermediary will attempt to buffer the response on behalf of
2194   downstream recipients.
2195   Note that HTTP/1.1 does not define any means to limit the size of a
2196   chunked response such that an intermediary can be assured of buffering the
2197   entire response.
2198</t>
2199<t>
2200   When multiple transfer codings are acceptable, the client &MAY; rank the
2201   codings by preference using a case-insensitive "q" parameter (similar to
2202   the qvalues used in content negotiation fields, &qvalue;). The rank value
2203   is a real number in the range 0 through 1, where 0.001 is the least
2204   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2205</t>
2206<t>
2207   If the TE field-value is empty or if no TE field is present, the only
2208   acceptable transfer coding is chunked. A message with no transfer coding
2209   is always acceptable.
2210</t>
2211<t>
2212   Since the TE header field only applies to the immediate connection,
2213   a sender of TE &MUST; also send a "TE" connection option within the
2214   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2215   in order to prevent the TE field from being forwarded by intermediaries
2216   that do not support its semantics.
2217</t>
2218</section>
2219
2220<section title="Trailer" anchor="header.trailer">
2221  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
2222  <x:anchor-alias value="Trailer"/>
2223<t>
2224   When a message includes a message body encoded with the chunked
2225   transfer coding and the sender desires to send metadata in the form of
2226   trailer fields at the end of the message, the sender &SHOULD; generate a
2227   <x:ref>Trailer</x:ref> header field before the message body to indicate
2228   which fields will be present in the trailers. This allows the recipient
2229   to prepare for receipt of that metadata before it starts processing the body,
2230   which is useful if the message is being streamed and the recipient wishes
2231   to confirm an integrity check on the fly.
2232</t>
2233<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
2234  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
2235</artwork></figure>
2236</section>
2237</section>
2238
2239<section title="Message Routing" anchor="message.routing">
2240<t>
2241   HTTP request message routing is determined by each client based on the
2242   target resource, the client's proxy configuration, and
2243   establishment or reuse of an inbound connection.  The corresponding
2244   response routing follows the same connection chain back to the client.
2245</t>
2246
2247<section title="Identifying a Target Resource" anchor="target-resource">
2248  <iref primary="true" item="target resource"/>
2249  <iref primary="true" item="target URI"/>
2250  <x:anchor-alias value="target resource"/>
2251  <x:anchor-alias value="target URI"/>
2252<t>
2253   HTTP is used in a wide variety of applications, ranging from
2254   general-purpose computers to home appliances.  In some cases,
2255   communication options are hard-coded in a client's configuration.
2256   However, most HTTP clients rely on the same resource identification
2257   mechanism and configuration techniques as general-purpose Web browsers.
2258</t>
2259<t>
2260   HTTP communication is initiated by a user agent for some purpose.
2261   The purpose is a combination of request semantics, which are defined in
2262   <xref target="Part2"/>, and a target resource upon which to apply those
2263   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2264   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2265   would resolve to its absolute form in order to obtain the
2266   "<x:dfn>target URI</x:dfn>".  The target URI
2267   excludes the reference's fragment component, if any,
2268   since fragment identifiers are reserved for client-side processing
2269   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2270</t>
2271</section>
2272
2273<section title="Connecting Inbound" anchor="connecting.inbound">
2274<t>
2275   Once the target URI is determined, a client needs to decide whether
2276   a network request is necessary to accomplish the desired semantics and,
2277   if so, where that request is to be directed.
2278</t>
2279<t>
2280   If the client has a cache <xref target="Part6"/> and the request can be
2281   satisfied by it, then the request is
2282   usually directed there first.
2283</t>
2284<t>
2285   If the request is not satisfied by a cache, then a typical client will
2286   check its configuration to determine whether a proxy is to be used to
2287   satisfy the request.  Proxy configuration is implementation-dependent,
2288   but is often based on URI prefix matching, selective authority matching,
2289   or both, and the proxy itself is usually identified by an "http" or
2290   "https" URI.  If a proxy is applicable, the client connects inbound by
2291   establishing (or reusing) a connection to that proxy.
2292</t>
2293<t>
2294   If no proxy is applicable, a typical client will invoke a handler routine,
2295   usually specific to the target URI's scheme, to connect directly
2296   to an authority for the target resource.  How that is accomplished is
2297   dependent on the target URI scheme and defined by its associated
2298   specification, similar to how this specification defines origin server
2299   access for resolution of the "http" (<xref target="http.uri"/>) and
2300   "https" (<xref target="https.uri"/>) schemes.
2301</t>
2302<t>
2303   HTTP requirements regarding connection management are defined in
2304   <xref target="connection.management"/>.
2305</t>
2306</section>
2307
2308<section title="Request Target" anchor="request-target">
2309<t>
2310   Once an inbound connection is obtained,
2311   the client sends an HTTP request message (<xref target="http.message"/>)
2312   with a request-target derived from the target URI.
2313   There are four distinct formats for the request-target, depending on both
2314   the method being requested and whether the request is to a proxy.
2315</t>   
2316<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-target"/><iref primary="true" item="Grammar" subitem="origin-form"/><iref primary="true" item="Grammar" subitem="absolute-form"/><iref primary="true" item="Grammar" subitem="authority-form"/><iref primary="true" item="Grammar" subitem="asterisk-form"/>
2317  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2318                 / <x:ref>absolute-form</x:ref>
2319                 / <x:ref>authority-form</x:ref>
2320                 / <x:ref>asterisk-form</x:ref>
2321
2322  <x:ref>origin-form</x:ref>    = <x:ref>absolute-path</x:ref> [ "?" <x:ref>query</x:ref> ]
2323  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2324  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2325  <x:ref>asterisk-form</x:ref>  = "*"
2326</artwork></figure>
2327<t anchor="origin-form"><iref item="origin-form (of request-target)"/>
2328  <x:h>origin-form</x:h>
2329</t>
2330<t>
2331   The most common form of request-target is the <x:dfn>origin-form</x:dfn>.
2332   When making a request directly to an origin server, other than a CONNECT
2333   or server-wide OPTIONS request (as detailed below),
2334   a client &MUST; send only the absolute path and query components of
2335   the target URI as the request-target.
2336   If the target URI's path component is empty, then the client &MUST; send
2337   "/" as the path within the origin-form of request-target.
2338   A <x:ref>Host</x:ref> header field is also sent, as defined in
2339   <xref target="header.host"/>.
2340</t>
2341<t>
2342   For example, a client wishing to retrieve a representation of the resource
2343   identified as
2344</t>
2345<figure><artwork x:indent-with="  " type="example">
2346http://www.example.org/where?q=now
2347</artwork></figure>
2348<t>
2349   directly from the origin server would open (or reuse) a TCP connection
2350   to port 80 of the host "www.example.org" and send the lines:
2351</t>
2352<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2353GET /where?q=now HTTP/1.1
2354Host: www.example.org
2355</artwork></figure>
2356<t>
2357   followed by the remainder of the request message.
2358</t>
2359<t anchor="absolute-form"><iref item="absolute-form (of request-target)"/>
2360  <x:h>absolute-form</x:h>
2361</t>
2362<t>
2363   When making a request to a proxy, other than a CONNECT or server-wide
2364   OPTIONS request (as detailed below), a client &MUST; send the target URI
2365   in <x:dfn>absolute-form</x:dfn> as the request-target.
2366   The proxy is requested to either service that request from a valid cache,
2367   if possible, or make the same request on the client's behalf to either
2368   the next inbound proxy server or directly to the origin server indicated
2369   by the request-target.  Requirements on such "forwarding" of messages are
2370   defined in <xref target="message.forwarding"/>.
2371</t>
2372<t>
2373   An example absolute-form of request-line would be:
2374</t>
2375<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2376GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
2377</artwork></figure>
2378<t>
2379   To allow for transition to the absolute-form for all requests in some
2380   future version of HTTP, a server &MUST; accept the absolute-form
2381   in requests, even though HTTP/1.1 clients will only send them in requests
2382   to proxies.
2383</t>
2384<t anchor="authority-form"><iref item="authority-form (of request-target)"/>
2385  <x:h>authority-form</x:h>
2386</t>
2387<t>
2388   The <x:dfn>authority-form</x:dfn> of request-target is only used for
2389   CONNECT requests (&CONNECT;). When making a CONNECT request to establish a
2390   tunnel through one or more proxies, a client &MUST; send only the target
2391   URI's authority component (excluding any userinfo and its "@" delimiter) as
2392   the request-target. For example,
2393</t>
2394<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2395CONNECT www.example.com:80 HTTP/1.1
2396</artwork></figure>
2397<t anchor="asterisk-form"><iref item="asterisk-form (of request-target)"/>
2398  <x:h>asterisk-form</x:h>
2399</t>
2400<t>
2401   The <x:dfn>asterisk-form</x:dfn> of request-target is only used for a server-wide
2402   OPTIONS request (&OPTIONS;).  When a client wishes to request OPTIONS
2403   for the server as a whole, as opposed to a specific named resource of
2404   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2405   For example,
2406</t>
2407<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2408OPTIONS * HTTP/1.1
2409</artwork></figure>
2410<t>
2411   If a proxy receives an OPTIONS request with an absolute-form of
2412   request-target in which the URI has an empty path and no query component,
2413   then the last proxy on the request chain &MUST; send a request-target
2414   of "*" when it forwards the request to the indicated origin server.
2415</t>
2416<figure><preamble>  
2417   For example, the request
2418</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2419OPTIONS http://www.example.org:8001 HTTP/1.1
2420</artwork></figure>
2421<figure><preamble>  
2422  would be forwarded by the final proxy as
2423</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2424OPTIONS * HTTP/1.1
2425Host: www.example.org:8001
2426</artwork>
2427<postamble>
2428   after connecting to port 8001 of host "www.example.org".
2429</postamble>
2430</figure>
2431</section>
2432
2433<section title="Host" anchor="header.host">
2434  <iref primary="true" item="Host header field" x:for-anchor=""/>
2435  <x:anchor-alias value="Host"/>
2436<t>
2437   The "Host" header field in a request provides the host and port
2438   information from the target URI, enabling the origin
2439   server to distinguish among resources while servicing requests
2440   for multiple host names on a single IP address.
2441</t>
2442<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2443  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2444</artwork></figure>
2445<t>
2446   A client &MUST; send a Host header field in all HTTP/1.1 request messages.
2447   If the target URI includes an authority component, then a client &MUST;
2448   send a field-value for Host that is identical to that authority
2449   component, excluding any userinfo subcomponent and its "@" delimiter
2450   (<xref target="http.uri"/>).
2451   If the authority component is missing or undefined for the target URI,
2452   then a client &MUST; send a Host header field with an empty field-value.
2453</t>
2454<t>
2455   Since the Host field-value is critical information for handling a request,
2456   a user agent &SHOULD; generate Host as the first header field following the
2457   request-line.
2458</t>
2459<t>
2460   For example, a GET request to the origin server for
2461   &lt;http://www.example.org/pub/WWW/&gt; would begin with:
2462</t>
2463<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2464GET /pub/WWW/ HTTP/1.1
2465Host: www.example.org
2466</artwork></figure>
2467<t>
2468   A client &MUST; send a Host header field in an HTTP/1.1 request even
2469   if the request-target is in the absolute-form, since this
2470   allows the Host information to be forwarded through ancient HTTP/1.0
2471   proxies that might not have implemented Host.
2472</t>
2473<t>
2474   When a proxy receives a request with an absolute-form of
2475   request-target, the proxy &MUST; ignore the received
2476   Host header field (if any) and instead replace it with the host
2477   information of the request-target.  A proxy that forwards such a request
2478   &MUST; generate a new Host field-value based on the received
2479   request-target rather than forward the received Host field-value.
2480</t>
2481<t>
2482   Since the Host header field acts as an application-level routing
2483   mechanism, it is a frequent target for malware seeking to poison
2484   a shared cache or redirect a request to an unintended server.
2485   An interception proxy is particularly vulnerable if it relies on
2486   the Host field-value for redirecting requests to internal
2487   servers, or for use as a cache key in a shared cache, without
2488   first verifying that the intercepted connection is targeting a
2489   valid IP address for that host.
2490</t>
2491<t>
2492   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2493   to any HTTP/1.1 request message that lacks a Host header field and
2494   to any request message that contains more than one Host header field
2495   or a Host header field with an invalid field-value.
2496</t>
2497</section>
2498
2499<section title="Effective Request URI" anchor="effective.request.uri">
2500  <iref primary="true" item="effective request URI"/>
2501  <x:anchor-alias value="effective request URI"/>
2502<t>
2503   A server that receives an HTTP request message &MUST; reconstruct
2504   the user agent's original target URI, based on the pieces of information
2505   learned from the request-target, <x:ref>Host</x:ref> header field, and
2506   connection context, in order to identify the intended target resource and
2507   properly service the request. The URI derived from this reconstruction
2508   process is referred to as the "<x:dfn>effective request URI</x:dfn>".
2509</t>
2510<t>
2511   For a user agent, the effective request URI is the target URI.
2512</t>
2513<t>
2514   If the request-target is in absolute-form, then the effective request URI
2515   is the same as the request-target.  Otherwise, the effective request URI
2516   is constructed as follows.
2517</t>
2518<t>
2519   If the request is received over a TLS-secured TCP connection,
2520   then the effective request URI's scheme is "https"; otherwise, the
2521   scheme is "http".
2522</t>
2523<t>
2524   If the request-target is in authority-form, then the effective
2525   request URI's authority component is the same as the request-target.
2526   Otherwise, if a <x:ref>Host</x:ref> header field is supplied with a
2527   non-empty field-value, then the authority component is the same as the
2528   Host field-value. Otherwise, the authority component is the concatenation of
2529   the default host name configured for the server, a colon (":"), and the
2530   connection's incoming TCP port number in decimal form.
2531</t>
2532<t>
2533   If the request-target is in authority-form or asterisk-form, then the
2534   effective request URI's combined path and query component is empty.
2535   Otherwise, the combined path and query component is the same as the
2536   request-target.
2537</t>
2538<t>
2539   The components of the effective request URI, once determined as above,
2540   can be combined into absolute-URI form by concatenating the scheme,
2541   "://", authority, and combined path and query component.
2542</t>
2543<figure>
2544<preamble>
2545   Example 1: the following message received over an insecure TCP connection
2546</preamble>
2547<artwork type="example" x:indent-with="  ">
2548GET /pub/WWW/TheProject.html HTTP/1.1
2549Host: www.example.org:8080
2550</artwork>
2551</figure>
2552<figure>
2553<preamble>
2554  has an effective request URI of
2555</preamble>
2556<artwork type="example" x:indent-with="  ">
2557http://www.example.org:8080/pub/WWW/TheProject.html
2558</artwork>
2559</figure>
2560<figure>
2561<preamble>
2562   Example 2: the following message received over a TLS-secured TCP connection
2563</preamble>
2564<artwork type="example" x:indent-with="  ">
2565OPTIONS * HTTP/1.1
2566Host: www.example.org
2567</artwork>
2568</figure>
2569<figure>
2570<preamble>
2571  has an effective request URI of
2572</preamble>
2573<artwork type="example" x:indent-with="  ">
2574https://www.example.org
2575</artwork>
2576</figure>
2577<t>
2578   An origin server that does not allow resources to differ by requested
2579   host &MAY; ignore the <x:ref>Host</x:ref> field-value and instead replace it
2580   with a configured server name when constructing the effective request URI.
2581</t>
2582<t>
2583   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2584   field &MAY; attempt to use heuristics (e.g., examination of the URI path for
2585   something unique to a particular host) in order to guess the
2586   effective request URI's authority component.
2587</t>
2588</section>
2589
2590<section title="Associating a Response to a Request" anchor="associating.response.to.request">
2591<t>
2592   HTTP does not include a request identifier for associating a given
2593   request message with its corresponding one or more response messages.
2594   Hence, it relies on the order of response arrival to correspond exactly
2595   to the order in which requests are made on the same connection.
2596   More than one response message per request only occurs when one or more
2597   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a
2598   final response to the same request.
2599</t>
2600<t>
2601   A client that has more than one outstanding request on a connection &MUST;
2602   maintain a list of outstanding requests in the order sent and &MUST;
2603   associate each received response message on that connection to the highest
2604   ordered request that has not yet received a final (non-<x:ref>1xx</x:ref>)
2605   response.
2606</t>
2607</section>
2608
2609<section title="Message Forwarding" anchor="message.forwarding">
2610<t>
2611   As described in <xref target="intermediaries"/>, intermediaries can serve
2612   a variety of roles in the processing of HTTP requests and responses.
2613   Some intermediaries are used to improve performance or availability.
2614   Others are used for access control or to filter content.
2615   Since an HTTP stream has characteristics similar to a pipe-and-filter
2616   architecture, there are no inherent limits to the extent an intermediary
2617   can enhance (or interfere) with either direction of the stream.
2618</t>
2619<t>
2620   An intermediary not acting as a tunnel &MUST; implement the
2621   <x:ref>Connection</x:ref> header field, as specified in
2622   <xref target="header.connection"/>, and exclude fields from being forwarded
2623   that are only intended for the incoming connection.
2624</t>
2625<t>
2626   An intermediary &MUST-NOT; forward a message to itself unless it is
2627   protected from an infinite request loop. In general, an intermediary ought
2628   to recognize its own server names, including any aliases, local variations,
2629   or literal IP addresses, and respond to such requests directly.
2630</t>
2631
2632<section title="Via" anchor="header.via">
2633  <iref primary="true" item="Via header field" x:for-anchor=""/>
2634  <x:anchor-alias value="pseudonym"/>
2635  <x:anchor-alias value="received-by"/>
2636  <x:anchor-alias value="received-protocol"/>
2637  <x:anchor-alias value="Via"/>
2638<t>
2639   The "Via" header field indicates the presence of intermediate protocols and
2640   recipients between the user agent and the server (on requests) or between
2641   the origin server and the client (on responses), similar to the
2642   "Received" header field in email
2643   (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2644   Via can be used for tracking message forwards,
2645   avoiding request loops, and identifying the protocol capabilities of
2646   senders along the request/response chain.
2647</t>
2648<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Via"/><iref primary="true" item="Grammar" subitem="received-protocol"/><iref primary="true" item="Grammar" subitem="protocol-name"/><iref primary="true" item="Grammar" subitem="protocol-version"/><iref primary="true" item="Grammar" subitem="received-by"/><iref primary="true" item="Grammar" subitem="pseudonym"/>
2649  <x:ref>Via</x:ref> = 1#( <x:ref>received-protocol</x:ref> <x:ref>RWS</x:ref> <x:ref>received-by</x:ref> [ <x:ref>RWS</x:ref> <x:ref>comment</x:ref> ] )
2650
2651  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2652                      ; see <xref target="header.upgrade"/>
2653  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2654  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2655</artwork></figure>
2656<t>
2657   Multiple Via field values represent each proxy or gateway that has
2658   forwarded the message. Each intermediary appends its own information
2659   about how the message was received, such that the end result is ordered
2660   according to the sequence of forwarding recipients.
2661</t>
2662<t>
2663   A proxy &MUST; send an appropriate Via header field, as described below, in
2664   each message that it forwards.
2665   An HTTP-to-HTTP gateway &MUST; send an appropriate Via header field in
2666   each inbound request message and &MAY; send a Via header field in
2667   forwarded response messages.
2668</t>
2669<t>
2670   For each intermediary, the received-protocol indicates the protocol and
2671   protocol version used by the upstream sender of the message. Hence, the
2672   Via field value records the advertised protocol capabilities of the
2673   request/response chain such that they remain visible to downstream
2674   recipients; this can be useful for determining what backwards-incompatible
2675   features might be safe to use in response, or within a later request, as
2676   described in <xref target="http.version"/>. For brevity, the protocol-name
2677   is omitted when the received protocol is HTTP.
2678</t>
2679<t>
2680   The received-by field is normally the host and optional port number of a
2681   recipient server or client that subsequently forwarded the message.
2682   However, if the real host is considered to be sensitive information, a
2683   sender &MAY; replace it with a pseudonym. If a port is not provided,
2684   a recipient &MAY; interpret that as meaning it was received on the default
2685   TCP port, if any, for the received-protocol.
2686</t>
2687<t>
2688   A sender &MAY; generate comments in the Via header field to identify the
2689   software of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2690   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2691   are optional and a recipient &MAY; remove them prior to forwarding the
2692   message.
2693</t>
2694<t>
2695   For example, a request message could be sent from an HTTP/1.0 user
2696   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2697   forward the request to a public proxy at p.example.net, which completes
2698   the request by forwarding it to the origin server at www.example.com.
2699   The request received by www.example.com would then have the following
2700   Via header field:
2701</t>
2702<figure><artwork type="example">
2703  Via: 1.0 fred, 1.1 p.example.net
2704</artwork></figure>
2705<t>
2706   An intermediary used as a portal through a network firewall
2707   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2708   region unless it is explicitly enabled to do so. If not enabled, such an
2709   intermediary &SHOULD; replace each received-by host of any host behind the
2710   firewall by an appropriate pseudonym for that host.
2711</t>
2712<t>
2713   An intermediary &MAY; combine an ordered subsequence of Via header
2714   field entries into a single such entry if the entries have identical
2715   received-protocol values. For example,
2716</t>
2717<figure><artwork type="example">
2718  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2719</artwork></figure>
2720<t>
2721  could be collapsed to
2722</t>
2723<figure><artwork type="example">
2724  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2725</artwork></figure>
2726<t>
2727   A sender &SHOULD-NOT; combine multiple entries unless they are all
2728   under the same organizational control and the hosts have already been
2729   replaced by pseudonyms. A sender &MUST-NOT; combine entries that
2730   have different received-protocol values.
2731</t>
2732</section>
2733
2734<section title="Transformations" anchor="message.transformations">
2735<t>
2736   Some intermediaries include features for transforming messages and their
2737   payloads.  A transforming proxy might, for example, convert between image
2738   formats in order to save cache space or to reduce the amount of traffic on
2739   a slow link. However, operational problems might occur when these
2740   transformations are applied to payloads intended for critical applications,
2741   such as medical imaging or scientific data analysis, particularly when
2742   integrity checks or digital signatures are used to ensure that the payload
2743   received is identical to the original.
2744</t>
2745<t>
2746   If a proxy receives a request-target with a host name that is not a
2747   fully qualified domain name, it &MAY; add its own domain to the host name
2748   it received when forwarding the request.  A proxy &MUST-NOT; change the
2749   host name if it is a fully qualified domain name.
2750</t>
2751<t>
2752   A proxy &MUST-NOT; modify the "absolute-path" and "query" parts of the
2753   received request-target when forwarding it to the next inbound server,
2754   except as noted above to replace an empty path with "/" or "*".
2755</t>
2756<t>
2757   A proxy &MUST-NOT; modify header fields that provide information about the
2758   end points of the communication chain, the resource state, or the selected
2759   representation. A proxy &MAY; change the message body through application
2760   or removal of a transfer coding (<xref target="transfer.codings"/>).
2761</t>
2762<t>
2763   A non-transforming proxy &MUST-NOT; modify the message payload (&payload;).
2764   A transforming proxy &MUST-NOT; modify the payload of a message that
2765   contains the no-transform cache-control directive.
2766</t>
2767<t>
2768   A transforming proxy &MAY; transform the payload of a message
2769   that does not contain the no-transform cache-control directive;
2770   if the payload is transformed, the transforming proxy &MUST; add a
2771   Warning header field with the warn-code of 214 ("Transformation Applied")
2772   if one does not already appear in the message (see &header-warning;).
2773   If the payload of a <x:ref>200 (OK)</x:ref> response is transformed, the
2774   transforming proxy can also inform downstream recipients that a
2775   transformation has been applied by changing the response status code to
2776   <x:ref>203 (Non-Authoritative Information)</x:ref> (&status-203;).
2777</t>
2778</section>
2779</section>
2780</section>
2781
2782<section title="Connection Management" anchor="connection.management">
2783<t>
2784   HTTP messaging is independent of the underlying transport or
2785   session-layer connection protocol(s).  HTTP only presumes a reliable
2786   transport with in-order delivery of requests and the corresponding
2787   in-order delivery of responses.  The mapping of HTTP request and
2788   response structures onto the data units of an underlying transport
2789   protocol is outside the scope of this specification.
2790</t>
2791<t>
2792   As described in <xref target="connecting.inbound"/>, the specific
2793   connection protocols to be used for an HTTP interaction are determined by
2794   client configuration and the <x:ref>target URI</x:ref>.
2795   For example, the "http" URI scheme
2796   (<xref target="http.uri"/>) indicates a default connection of TCP
2797   over IP, with a default TCP port of 80, but the client might be
2798   configured to use a proxy via some other connection, port, or protocol.
2799</t>
2800<t>
2801   HTTP implementations are expected to engage in connection management,
2802   which includes maintaining the state of current connections,
2803   establishing a new connection or reusing an existing connection,
2804   processing messages received on a connection, detecting connection
2805   failures, and closing each connection.
2806   Most clients maintain multiple connections in parallel, including
2807   more than one connection per server endpoint.
2808   Most servers are designed to maintain thousands of concurrent connections,
2809   while controlling request queues to enable fair use and detect
2810   denial of service attacks.
2811</t>
2812
2813<section title="Connection" anchor="header.connection">
2814  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2815  <iref primary="true" item="close" x:for-anchor=""/>
2816  <x:anchor-alias value="Connection"/>
2817  <x:anchor-alias value="connection-option"/>
2818  <x:anchor-alias value="close"/>
2819<t>
2820   The "Connection" header field allows the sender to indicate desired
2821   control options for the current connection.  In order to avoid confusing
2822   downstream recipients, a proxy or gateway &MUST; remove or replace any
2823   received connection options before forwarding the message.
2824</t>
2825<t>
2826   When a header field aside from Connection is used to supply control
2827   information for or about the current connection, the sender &MUST; list
2828   the corresponding field-name within the "Connection" header field.
2829   A proxy or gateway &MUST; parse a received Connection
2830   header field before a message is forwarded and, for each
2831   connection-option in this field, remove any header field(s) from
2832   the message with the same name as the connection-option, and then
2833   remove the Connection header field itself (or replace it with the
2834   intermediary's own connection options for the forwarded message).
2835</t>
2836<t>
2837   Hence, the Connection header field provides a declarative way of
2838   distinguishing header fields that are only intended for the
2839   immediate recipient ("hop-by-hop") from those fields that are
2840   intended for all recipients on the chain ("end-to-end"), enabling the
2841   message to be self-descriptive and allowing future connection-specific
2842   extensions to be deployed without fear that they will be blindly
2843   forwarded by older intermediaries.
2844</t>
2845<t>
2846   The Connection header field's value has the following grammar:
2847</t>
2848<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2849  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2850  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2851</artwork></figure>
2852<t>
2853   Connection options are case-insensitive.
2854</t>
2855<t>
2856   A sender &MUST-NOT; send a connection option corresponding to a header
2857   field that is intended for all recipients of the payload.
2858   For example, <x:ref>Cache-Control</x:ref> is never appropriate as a
2859   connection option (&header-cache-control;).
2860</t>
2861<t>
2862   The connection options do not always correspond to a header field
2863   present in the message, since a connection-specific header field
2864   might not be needed if there are no parameters associated with a
2865   connection option. In contrast, a connection-specific header field that
2866   is received without a corresponding connection option usually indicates
2867   that the field has been improperly forwarded by an intermediary and
2868   ought to be ignored by the recipient.
2869</t>
2870<t>
2871   When defining new connection options, specification authors ought to survey
2872   existing header field names and ensure that the new connection option does
2873   not share the same name as an already deployed header field.
2874   Defining a new connection option essentially reserves that potential
2875   field-name for carrying additional information related to the
2876   connection option, since it would be unwise for senders to use
2877   that field-name for anything else.
2878</t>
2879<t>
2880   The "<x:dfn>close</x:dfn>" connection option is defined for a
2881   sender to signal that this connection will be closed after completion of
2882   the response. For example,
2883</t>
2884<figure><artwork type="example">
2885  Connection: close
2886</artwork></figure>
2887<t>
2888   in either the request or the response header fields indicates that the
2889   sender is going to close the connection after the current request/response
2890   is complete (<xref target="persistent.tear-down"/>).
2891</t>
2892<t>
2893   A client that does not support <x:ref>persistent connections</x:ref> &MUST;
2894   send the "close" connection option in every request message.
2895</t>
2896<t>
2897   A server that does not support <x:ref>persistent connections</x:ref> &MUST;
2898   send the "close" connection option in every response message that
2899   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2900</t>
2901</section>
2902
2903<section title="Establishment" anchor="persistent.establishment">
2904<t>
2905   It is beyond the scope of this specification to describe how connections
2906   are established via various transport or session-layer protocols.
2907   Each connection applies to only one transport link.
2908</t>
2909</section>
2910
2911<section title="Persistence" anchor="persistent.connections">
2912   <x:anchor-alias value="persistent connections"/>
2913<t>
2914   HTTP/1.1 defaults to the use of "<x:dfn>persistent connections</x:dfn>",
2915   allowing multiple requests and responses to be carried over a single
2916   connection. The "<x:ref>close</x:ref>" connection-option is used to signal
2917   that a connection will not persist after the current request/response.
2918   HTTP implementations &SHOULD; support persistent connections.
2919</t>
2920<t>
2921   A recipient determines whether a connection is persistent or not based on
2922   the most recently received message's protocol version and
2923   <x:ref>Connection</x:ref> header field (if any):
2924   <list style="symbols">
2925     <t>If the <x:ref>close</x:ref> connection option is present, the
2926        connection will not persist after the current response; else,</t>
2927     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2928        persist after the current response; else,</t>
2929     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2930        connection option is present, the recipient is not a proxy, and
2931        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2932        the connection will persist after the current response; otherwise,</t>
2933     <t>The connection will close after the current response.</t>
2934   </list>
2935</t>
2936<t>
2937   A server &MAY; assume that an HTTP/1.1 client intends to maintain a
2938   persistent connection until a <x:ref>close</x:ref> connection option
2939   is received in a request.
2940</t>
2941<t>
2942   A client &MAY; reuse a persistent connection until it sends or receives
2943   a <x:ref>close</x:ref> connection option or receives an HTTP/1.0 response
2944   without a "keep-alive" connection option.
2945</t>
2946<t>
2947   In order to remain persistent, all messages on a connection need to
2948   have a self-defined message length (i.e., one not defined by closure
2949   of the connection), as described in <xref target="message.body"/>.
2950   A server &MUST; read the entire request message body or close
2951   the connection after sending its response, since otherwise the
2952   remaining data on a persistent connection would be misinterpreted
2953   as the next request.  Likewise,
2954   a client &MUST; read the entire response message body if it intends
2955   to reuse the same connection for a subsequent request.
2956</t>
2957<t>
2958   A proxy server &MUST-NOT; maintain a persistent connection with an
2959   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
2960   information and discussion of the problems with the Keep-Alive header field
2961   implemented by many HTTP/1.0 clients).
2962</t>
2963<t>
2964   Clients and servers &SHOULD-NOT; assume that a persistent connection is
2965   maintained for HTTP versions less than 1.1 unless it is explicitly
2966   signaled.
2967   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
2968   for more information on backward compatibility with HTTP/1.0 clients.
2969</t>
2970
2971<section title="Retrying Requests" anchor="persistent.retrying.requests">
2972<t>
2973   Connections can be closed at any time, with or without intention.
2974   Implementations ought to anticipate the need to recover
2975   from asynchronous close events.
2976</t>
2977<t>
2978   When an inbound connection is closed prematurely, a client &MAY; open a new
2979   connection and automatically retransmit an aborted sequence of requests if
2980   all of those requests have idempotent methods (&idempotent-methods;).
2981   A proxy &MUST-NOT; automatically retry non-idempotent requests.
2982</t>
2983<t>
2984   A user agent &MUST-NOT; automatically retry a request with a non-idempotent
2985   method unless it has some means to know that the request semantics are
2986   actually idempotent, regardless of the method, or some means to detect that
2987   the original request was never applied. For example, a user agent that
2988   knows (through design or configuration) that a POST request to a given
2989   resource is safe can repeat that request automatically.
2990   Likewise, a user agent designed specifically to operate on a version
2991   control repository might be able to recover from partial failure conditions
2992   by checking the target resource revision(s) after a failed connection,
2993   reverting or fixing any changes that were partially applied, and then
2994   automatically retrying the requests that failed.
2995</t>
2996<t>
2997   A client &SHOULD-NOT; automatically retry a failed automatic retry.
2998</t>
2999</section>
3000
3001<section title="Pipelining" anchor="pipelining">
3002   <x:anchor-alias value="pipeline"/>
3003<t>
3004   A client that supports persistent connections &MAY; "<x:dfn>pipeline</x:dfn>"
3005   its requests (i.e., send multiple requests without waiting for each
3006   response). A server &MAY; process a sequence of pipelined requests in
3007   parallel if they all have safe methods (&safe-methods;), but &MUST; send
3008   the corresponding responses in the same order that the requests were
3009   received.
3010</t>
3011<t>
3012   A client that pipelines requests &SHOULD; retry unanswered requests if the
3013   connection closes before it receives all of the corresponding responses.
3014   When retrying pipelined requests after a failed connection (a connection
3015   not explicitly closed by the server in its last complete response), a
3016   client &MUST-NOT; pipeline immediately after connection establishment,
3017   since the first remaining request in the prior pipeline might have caused
3018   an error response that can be lost again if multiple requests are sent on a
3019   prematurely closed connection (see the TCP reset problem described in
3020   <xref target="persistent.tear-down"/>).
3021</t>
3022<t>
3023   Idempotent methods (&idempotent-methods;) are significant to pipelining
3024   because they can be automatically retried after a connection failure.
3025   A user agent &SHOULD-NOT; pipeline requests after a non-idempotent method,
3026   until the final response status code for that method has been received,
3027   unless the user agent has a means to detect and recover from partial
3028   failure conditions involving the pipelined sequence.
3029</t>
3030<t>
3031   An intermediary that receives pipelined requests &MAY; pipeline those
3032   requests when forwarding them inbound, since it can rely on the outbound
3033   user agent(s) to determine what requests can be safely pipelined. If the
3034   inbound connection fails before receiving a response, the pipelining
3035   intermediary &MAY; attempt to retry a sequence of requests that have yet
3036   to receive a response if the requests all have idempotent methods;
3037   otherwise, the pipelining intermediary &SHOULD; forward any received
3038   responses and then close the corresponding outbound connection(s) so that
3039   the outbound user agent(s) can recover accordingly.
3040</t>
3041</section>
3042</section>
3043   
3044<section title="Concurrency" anchor="persistent.concurrency">
3045<t>
3046   A client &SHOULD; limit the number of simultaneous open
3047   connections that it maintains to a given server.
3048</t>
3049<t>
3050   Previous revisions of HTTP gave a specific number of connections as a
3051   ceiling, but this was found to be impractical for many applications. As a
3052   result, this specification does not mandate a particular maximum number of
3053   connections, but instead encourages clients to be conservative when opening
3054   multiple connections.
3055</t>
3056<t>
3057   Multiple connections are typically used to avoid the "head-of-line
3058   blocking" problem, wherein a request that takes significant server-side
3059   processing and/or has a large payload blocks subsequent requests on the
3060   same connection. However, each connection consumes server resources.
3061   Furthermore, using multiple connections can cause undesirable side effects
3062   in congested networks.
3063</t>
3064<t>
3065   Note that servers might reject traffic that they deem abusive, including an
3066   excessive number of connections from a client.
3067</t>
3068</section>
3069
3070<section title="Failures and Time-outs" anchor="persistent.failures">
3071<t>
3072   Servers will usually have some time-out value beyond which they will
3073   no longer maintain an inactive connection. Proxy servers might make
3074   this a higher value since it is likely that the client will be making
3075   more connections through the same proxy server. The use of persistent
3076   connections places no requirements on the length (or existence) of
3077   this time-out for either the client or the server.
3078</t>
3079<t>
3080   A client or server that wishes to time-out &SHOULD; issue a graceful close
3081   on the connection. Implementations &SHOULD; constantly monitor open
3082   connections for a received closure signal and respond to it as appropriate,
3083   since prompt closure of both sides of a connection enables allocated system
3084   resources to be reclaimed.
3085</t>
3086<t>
3087   A client, server, or proxy &MAY; close the transport connection at any
3088   time. For example, a client might have started to send a new request
3089   at the same time that the server has decided to close the "idle"
3090   connection. From the server's point of view, the connection is being
3091   closed while it was idle, but from the client's point of view, a
3092   request is in progress.
3093</t>
3094<t>
3095   A server &SHOULD; sustain persistent connections, when possible, and allow
3096   the underlying
3097   transport's flow control mechanisms to resolve temporary overloads, rather
3098   than terminate connections with the expectation that clients will retry.
3099   The latter technique can exacerbate network congestion.
3100</t>
3101<t>
3102   A client sending a message body &SHOULD; monitor
3103   the network connection for an error response while it is transmitting
3104   the request. If the client sees a response that indicates the server does
3105   not wish to receive the message body and is closing the connection, the
3106   client &SHOULD; immediately cease transmitting the body and close its side
3107   of the connection.
3108</t>
3109</section>
3110   
3111<section title="Tear-down" anchor="persistent.tear-down">
3112  <iref primary="false" item="Connection header field" x:for-anchor=""/>
3113  <iref primary="false" item="close" x:for-anchor=""/>
3114<t>
3115   The <x:ref>Connection</x:ref> header field
3116   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
3117   connection option that a sender &SHOULD; send when it wishes to close
3118   the connection after the current request/response pair.
3119</t>
3120<t>
3121   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
3122   send further requests on that connection (after the one containing
3123   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
3124   final response message corresponding to this request.
3125</t>
3126<t>
3127   A server that receives a <x:ref>close</x:ref> connection option &MUST;
3128   initiate a close of the connection (see below) after it sends the
3129   final response to the request that contained <x:ref>close</x:ref>.
3130   The server &SHOULD; send a <x:ref>close</x:ref> connection option
3131   in its final response on that connection. The server &MUST-NOT; process
3132   any further requests received on that connection.
3133</t>
3134<t>
3135   A server that sends a <x:ref>close</x:ref> connection option &MUST;
3136   initiate a close of the connection (see below) after it sends the
3137   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
3138   any further requests received on that connection.
3139</t>
3140<t>
3141   A client that receives a <x:ref>close</x:ref> connection option &MUST;
3142   cease sending requests on that connection and close the connection
3143   after reading the response message containing the close; if additional
3144   pipelined requests had been sent on the connection, the client &SHOULD-NOT;
3145   assume that they will be processed by the server.
3146</t>
3147<t>
3148   If a server performs an immediate close of a TCP connection, there is a
3149   significant risk that the client will not be able to read the last HTTP
3150   response.  If the server receives additional data from the client on a
3151   fully-closed connection, such as another request that was sent by the
3152   client before receiving the server's response, the server's TCP stack will
3153   send a reset packet to the client; unfortunately, the reset packet might
3154   erase the client's unacknowledged input buffers before they can be read
3155   and interpreted by the client's HTTP parser.
3156</t>
3157<t>
3158   To avoid the TCP reset problem, servers typically close a connection in
3159   stages. First, the server performs a half-close by closing only the write
3160   side of the read/write connection. The server then continues to read from
3161   the connection until it receives a corresponding close by the client, or
3162   until the server is reasonably certain that its own TCP stack has received
3163   the client's acknowledgement of the packet(s) containing the server's last
3164   response. Finally, the server fully closes the connection.
3165</t>
3166<t>
3167   It is unknown whether the reset problem is exclusive to TCP or might also
3168   be found in other transport connection protocols.
3169</t>
3170</section>
3171
3172<section title="Upgrade" anchor="header.upgrade">
3173  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3174  <x:anchor-alias value="Upgrade"/>
3175  <x:anchor-alias value="protocol"/>
3176  <x:anchor-alias value="protocol-name"/>
3177  <x:anchor-alias value="protocol-version"/>
3178<t>
3179   The "Upgrade" header field is intended to provide a simple mechanism
3180   for transitioning from HTTP/1.1 to some other protocol on the same
3181   connection.  A client &MAY; send a list of protocols in the Upgrade
3182   header field of a request to invite the server to switch to one or
3183   more of those protocols, in order of descending preference, before sending
3184   the final response. A server &MAY; ignore a received Upgrade header field
3185   if it wishes to continue using the current protocol on that connection.
3186</t>
3187<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3188  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3189
3190  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3191  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3192  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3193</artwork></figure>
3194<t>
3195   A server that sends a <x:ref>101 (Switching Protocols)</x:ref> response
3196   &MUST; send an Upgrade header field to indicate the new protocol(s) to
3197   which the connection is being switched; if multiple protocol layers are
3198   being switched, the sender &MUST; list the protocols in layer-ascending
3199   order. A server &MUST-NOT; switch to a protocol that was not indicated by
3200   the client in the corresponding request's Upgrade header field.
3201   A server &MAY; choose to ignore the order of preference indicated by the
3202   client and select the new protocol(s) based on other factors, such as the
3203   nature of the request or the current load on the server.
3204</t>
3205<t>
3206   A server that sends a <x:ref>426 (Upgrade Required)</x:ref> response
3207   &MUST; send an Upgrade header field to indicate the acceptable protocols,
3208   in order of descending preference.
3209</t>
3210<t>
3211   A server &MAY; send an Upgrade header field in any other response to
3212   advertise that it implements support for upgrading to the listed protocols,
3213   in order of descending preference, when appropriate for a future request.
3214</t>
3215<figure><preamble>
3216   The following is a hypothetical example sent by a client:
3217</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
3218GET /hello.txt HTTP/1.1
3219Host: www.example.com
3220Connection: upgrade
3221Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3222
3223</artwork></figure>
3224<t>
3225   Upgrade cannot be used to insist on a protocol change; its acceptance and
3226   use by the server is optional. The capabilities and nature of the
3227   application-level communication after the protocol change is entirely
3228   dependent upon the new protocol(s) chosen. However, immediately after
3229   sending the 101 response, the server is expected to continue responding to
3230   the original request as if it had received its equivalent within the new
3231   protocol (i.e., the server still has an outstanding request to satisfy
3232   after the protocol has been changed, and is expected to do so without
3233   requiring the request to be repeated).
3234</t>
3235<t>
3236   For example, if the Upgrade header field is received in a GET request
3237   and the server decides to switch protocols, it first responds
3238   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3239   then immediately follows that with the new protocol's equivalent of a
3240   response to a GET on the target resource.  This allows a connection to be
3241   upgraded to protocols with the same semantics as HTTP without the
3242   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3243   protocols unless the received message semantics can be honored by the new
3244   protocol; an OPTIONS request can be honored by any protocol.
3245</t>
3246<figure><preamble>
3247   The following is an example response to the above hypothetical request:
3248</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
3249HTTP/1.1 101 Switching Protocols
3250Connection: upgrade
3251Upgrade: HTTP/2.0
3252
3253[... data stream switches to HTTP/2.0 with an appropriate response
3254(as defined by new protocol) to the "GET /hello.txt" request ...]
3255</artwork></figure>
3256<t>
3257   When Upgrade is sent, the sender &MUST; also send a
3258   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3259   that contains an "upgrade" connection option, in order to prevent Upgrade
3260   from being accidentally forwarded by intermediaries that might not implement
3261   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3262   is received in an HTTP/1.0 request.
3263</t>
3264<t>
3265   A client cannot begin using an upgraded protocol on the connection until
3266   it has completely sent the request message (i.e., the client can't change
3267   the protocol it is sending in the middle of a message).
3268   If a server receives both Upgrade and an <x:ref>Expect</x:ref> header field
3269   with the "100-continue" expectation (&header-expect;), the
3270   server &MUST; send a <x:ref>100 (Continue)</x:ref> response before sending
3271   a <x:ref>101 (Switching Protocols)</x:ref> response.
3272</t>
3273<t>
3274   The Upgrade header field only applies to switching protocols on top of the
3275   existing connection; it cannot be used to switch the underlying connection
3276   (transport) protocol, nor to switch the existing communication to a
3277   different connection. For those purposes, it is more appropriate to use a
3278   <x:ref>3xx (Redirection)</x:ref> response (&status-3xx;).
3279</t>
3280<t>
3281   This specification only defines the protocol name "HTTP" for use by
3282   the family of Hypertext Transfer Protocols, as defined by the HTTP
3283   version rules of <xref target="http.version"/> and future updates to this
3284   specification. Additional tokens ought to be registered with IANA using the
3285   registration procedure defined in <xref target="upgrade.token.registry"/>.
3286</t>
3287</section>
3288</section>
3289
3290<section title="ABNF list extension: #rule" anchor="abnf.extension">
3291<t>
3292  A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
3293  improve readability in the definitions of some header field values.
3294</t>
3295<t>
3296  A construct "#" is defined, similar to "*", for defining comma-delimited
3297  lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
3298  at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
3299  comma (",") and optional whitespace (OWS).   
3300</t>
3301<figure><preamble>
3302  Thus, a sender &MUST; expand the list construct as follows:
3303</preamble><artwork type="example">
3304  1#element =&gt; element *( OWS "," OWS element )
3305</artwork></figure>
3306<figure><preamble>
3307  and:
3308</preamble><artwork type="example">
3309  #element =&gt; [ 1#element ]
3310</artwork></figure>
3311<figure><preamble>
3312  and for n &gt;= 1 and m &gt; 1:
3313</preamble><artwork type="example">
3314  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
3315</artwork></figure>
3316<t>
3317  For compatibility with legacy list rules, a recipient &MUST; parse and ignore
3318  a reasonable number of empty list elements: enough to handle common mistakes
3319  by senders that merge values, but not so much that they could be used as a
3320  denial of service mechanism. In other words, a recipient &MUST; expand the
3321  list construct as follows:
3322</t>
3323<figure><artwork type="example">
3324  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
3325 
3326  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
3327</artwork></figure>
3328<t>
3329  Empty elements do not contribute to the count of elements present.
3330  For example, given these ABNF productions:
3331</t>
3332<figure><artwork type="example">
3333  example-list      = 1#example-list-elmt
3334  example-list-elmt = token ; see <xref target="field.components"/>
3335</artwork></figure>
3336<t>
3337  Then the following are valid values for example-list (not including the
3338  double quotes, which are present for delimitation only):
3339</t>
3340<figure><artwork type="example">
3341  "foo,bar"
3342  "foo ,bar,"
3343  "foo , ,bar,charlie   "
3344</artwork></figure>
3345<t>
3346  In contrast, the following values would be invalid, since at least one
3347  non-empty element is required by the example-list production:
3348</t>
3349<figure><artwork type="example">
3350  ""
3351  ","
3352  ",   ,"
3353</artwork></figure>
3354<t>
3355  <xref target="collected.abnf"/> shows the collected ABNF after the list
3356  constructs have been expanded, as described above, for recipients.
3357</t>
3358</section>
3359
3360<section title="IANA Considerations" anchor="IANA.considerations">
3361
3362<section title="Header Field Registration" anchor="header.field.registration">
3363<t>
3364   HTTP header fields are registered within the Message Header Field Registry
3365   maintained at
3366   <eref target="http://www.iana.org/assignments/message-headers/"/>.
3367</t>
3368<t>
3369   This document defines the following HTTP header fields, so their
3370   associated registry entries shall be updated according to the permanent
3371   registrations below (see <xref target="BCP90"/>):
3372</t>
3373<?BEGININC p1-messaging.iana-headers ?>
3374<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3375<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3376   <ttcol>Header Field Name</ttcol>
3377   <ttcol>Protocol</ttcol>
3378   <ttcol>Status</ttcol>
3379   <ttcol>Reference</ttcol>
3380
3381   <c>Connection</c>
3382   <c>http</c>
3383   <c>standard</c>
3384   <c>
3385      <xref target="header.connection"/>
3386   </c>
3387   <c>Content-Length</c>
3388   <c>http</c>
3389   <c>standard</c>
3390   <c>
3391      <xref target="header.content-length"/>
3392   </c>
3393   <c>Host</c>
3394   <c>http</c>
3395   <c>standard</c>
3396   <c>
3397      <xref target="header.host"/>
3398   </c>
3399   <c>TE</c>
3400   <c>http</c>
3401   <c>standard</c>
3402   <c>
3403      <xref target="header.te"/>
3404   </c>
3405   <c>Trailer</c>
3406   <c>http</c>
3407   <c>standard</c>
3408   <c>
3409      <xref target="header.trailer"/>
3410   </c>
3411   <c>Transfer-Encoding</c>
3412   <c>http</c>
3413   <c>standard</c>
3414   <c>
3415      <xref target="header.transfer-encoding"/>
3416   </c>
3417   <c>Upgrade</c>
3418   <c>http</c>
3419   <c>standard</c>
3420   <c>
3421      <xref target="header.upgrade"/>
3422   </c>
3423   <c>Via</c>
3424   <c>http</c>
3425   <c>standard</c>
3426   <c>
3427      <xref target="header.via"/>
3428   </c>
3429</texttable>
3430<!--(END)-->
3431<?ENDINC p1-messaging.iana-headers ?>
3432<t>
3433   Furthermore, the header field-name "Close" shall be registered as
3434   "reserved", since using that name as an HTTP header field might
3435   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3436   header field (<xref target="header.connection"/>).
3437</t>
3438<texttable align="left" suppress-title="true">
3439   <ttcol>Header Field Name</ttcol>
3440   <ttcol>Protocol</ttcol>
3441   <ttcol>Status</ttcol>
3442   <ttcol>Reference</ttcol>
3443
3444   <c>Close</c>
3445   <c>http</c>
3446   <c>reserved</c>
3447   <c>
3448      <xref target="header.field.registration"/>
3449   </c>
3450</texttable>
3451<t>
3452   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3453</t>
3454</section>
3455
3456<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3457<t>
3458   IANA maintains the registry of URI Schemes <xref target="BCP115"/> at
3459   <eref target="http://www.iana.org/assignments/uri-schemes/"/>.
3460</t>
3461<t>
3462   This document defines the following URI schemes, so their
3463   associated registry entries shall be updated according to the permanent
3464   registrations below:
3465</t>
3466<texttable align="left" suppress-title="true">
3467   <ttcol>URI Scheme</ttcol>
3468   <ttcol>Description</ttcol>
3469   <ttcol>Reference</ttcol>
3470
3471   <c>http</c>
3472   <c>Hypertext Transfer Protocol</c>
3473   <c><xref target="http.uri"/></c>
3474
3475   <c>https</c>
3476   <c>Hypertext Transfer Protocol Secure</c>
3477   <c><xref target="https.uri"/></c>
3478</texttable>
3479</section>
3480
3481<section title="Internet Media Type Registration" anchor="internet.media.type.http">
3482<t>
3483   IANA maintains the registry of Internet media types <xref target="BCP13"/> at
3484   <eref target="http://www.iana.org/assignments/media-types"/>.
3485</t>
3486<t>
3487   This document serves as the specification for the Internet media types
3488   "message/http" and "application/http". The following is to be registered with
3489   IANA.
3490</t>
3491<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
3492<iref item="Media Type" subitem="message/http" primary="true"/>
3493<iref item="message/http Media Type" primary="true"/>
3494<t>
3495   The message/http type can be used to enclose a single HTTP request or
3496   response message, provided that it obeys the MIME restrictions for all
3497   "message" types regarding line length and encodings.
3498</t>
3499<t>
3500  <list style="hanging" x:indent="12em">
3501    <t hangText="Type name:">
3502      message
3503    </t>
3504    <t hangText="Subtype name:">
3505      http
3506    </t>
3507    <t hangText="Required parameters:">
3508      N/A
3509    </t>
3510    <t hangText="Optional parameters:">
3511      version, msgtype
3512      <list style="hanging">
3513        <t hangText="version:">
3514          The HTTP-version number of the enclosed message
3515          (e.g., "1.1"). If not present, the version can be
3516          determined from the first line of the body.
3517        </t>
3518        <t hangText="msgtype:">
3519          The message type &mdash; "request" or "response". If not
3520          present, the type can be determined from the first
3521          line of the body.
3522        </t>
3523      </list>
3524    </t>
3525    <t hangText="Encoding considerations:">
3526      only "7bit", "8bit", or "binary" are permitted
3527    </t>
3528    <t hangText="Security considerations:">
3529      see <xref target="security.considerations"/>
3530    </t>
3531    <t hangText="Interoperability considerations:">
3532      N/A
3533    </t>
3534    <t hangText="Published specification:">
3535      This specification (see <xref target="internet.media.type.message.http"/>).
3536    </t>
3537    <t hangText="Applications that use this media type:">
3538      N/A
3539    </t>
3540    <t hangText="Fragment identifier considerations:">
3541      N/A
3542    </t>
3543    <t hangText="Additional information:">
3544      <list style="hanging">
3545        <t hangText="Magic number(s):">N/A</t>
3546        <t hangText="Deprecated alias names for this type:">N/A</t>
3547        <t hangText="File extension(s):">N/A</t>
3548        <t hangText="Macintosh file type code(s):">N/A</t>
3549      </list>
3550    </t>
3551    <t hangText="Person and email address to contact for further information:">
3552      See Authors Section.
3553    </t>
3554    <t hangText="Intended usage:">
3555      COMMON
3556    </t>
3557    <t hangText="Restrictions on usage:">
3558      N/A
3559    </t>
3560    <t hangText="Author:">
3561      See Authors Section.
3562    </t>
3563    <t hangText="Change controller:">
3564      IESG
3565    </t>
3566  </list>
3567</t>
3568</section>
3569<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
3570<iref item="Media Type" subitem="application/http" primary="true"/>
3571<iref item="application/http Media Type" primary="true"/>
3572<t>
3573   The application/http type can be used to enclose a pipeline of one or more
3574   HTTP request or response messages (not intermixed).
3575</t>
3576<t>
3577  <list style="hanging" x:indent="12em">
3578    <t hangText="Type name:">
3579      application
3580    </t>
3581    <t hangText="Subtype name:">
3582      http
3583    </t>
3584    <t hangText="Required parameters:">
3585      N/A
3586    </t>
3587    <t hangText="Optional parameters:">
3588      version, msgtype
3589      <list style="hanging">
3590        <t hangText="version:">
3591          The HTTP-version number of the enclosed messages
3592          (e.g., "1.1"). If not present, the version can be
3593          determined from the first line of the body.
3594        </t>
3595        <t hangText="msgtype:">
3596          The message type &mdash; "request" or "response". If not
3597          present, the type can be determined from the first
3598          line of the body.
3599        </t>
3600      </list>
3601    </t>
3602    <t hangText="Encoding considerations:">
3603      HTTP messages enclosed by this type
3604      are in "binary" format; use of an appropriate
3605      Content-Transfer-Encoding is required when
3606      transmitted via E-mail.
3607    </t>
3608    <t hangText="Security considerations:">
3609      see <xref target="security.considerations"/>
3610    </t>
3611    <t hangText="Interoperability considerations:">
3612      N/A
3613    </t>
3614    <t hangText="Published specification:">
3615      This specification (see <xref target="internet.media.type.application.http"/>).
3616    </t>
3617    <t hangText="Applications that use this media type:">
3618      N/A
3619    </t>
3620    <t hangText="Fragment identifier considerations:">
3621      N/A
3622    </t>
3623    <t hangText="Additional information:">
3624      <list style="hanging">
3625        <t hangText="Deprecated alias names for this type:">N/A</t>
3626        <t hangText="Magic number(s):">N/A</t>
3627        <t hangText="File extension(s):">N/A</t>
3628        <t hangText="Macintosh file type code(s):">N/A</t>
3629      </list>
3630    </t>
3631    <t hangText="Person and email address to contact for further information:">
3632      See Authors Section.
3633    </t>
3634    <t hangText="Intended usage:">
3635      COMMON
3636    </t>
3637    <t hangText="Restrictions on usage:">
3638      N/A
3639    </t>
3640    <t hangText="Author:">
3641      See Authors Section.
3642    </t>
3643    <t hangText="Change controller:">
3644      IESG
3645    </t>
3646  </list>
3647</t>
3648</section>
3649</section>
3650
3651<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3652<t>
3653   The HTTP Transfer Coding Registry defines the name space for transfer
3654   coding names. It is maintained at <eref target="http://www.iana.org/assignments/http-parameters"/>.
3655</t>
3656
3657<section title="Procedure" anchor="transfer.coding.registry.procedure">
3658<t>
3659   Registrations &MUST; include the following fields:
3660   <list style="symbols">
3661     <t>Name</t>
3662     <t>Description</t>
3663     <t>Pointer to specification text</t>
3664   </list>
3665</t>
3666<t>
3667   Names of transfer codings &MUST-NOT; overlap with names of content codings
3668   (&content-codings;) unless the encoding transformation is identical, as
3669   is the case for the compression codings defined in
3670   <xref target="compression.codings"/>.
3671</t>
3672<t>
3673   Values to be added to this name space require IETF Review (see
3674   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3675   conform to the purpose of transfer coding defined in this specification.
3676</t>
3677<t>
3678   Use of program names for the identification of encoding formats
3679   is not desirable and is discouraged for future encodings.
3680</t>
3681</section>
3682
3683<section title="Registration" anchor="transfer.coding.registration">
3684<t>
3685   The HTTP Transfer Coding Registry shall be updated with the registrations
3686   below:
3687</t>
3688<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3689   <ttcol>Name</ttcol>
3690   <ttcol>Description</ttcol>
3691   <ttcol>Reference</ttcol>
3692   <c>chunked</c>
3693   <c>Transfer in a series of chunks</c>
3694   <c>
3695      <xref target="chunked.encoding"/>
3696   </c>
3697   <c>compress</c>
3698   <c>UNIX "compress" data format <xref target="Welch"/></c>
3699   <c>
3700      <xref target="compress.coding"/>
3701   </c>
3702   <c>deflate</c>
3703   <c>"deflate" compressed data (<xref target="RFC1951"/>) inside
3704   the "zlib" data format (<xref target="RFC1950"/>)
3705   </c>
3706   <c>
3707      <xref target="deflate.coding"/>
3708   </c>
3709   <c>gzip</c>
3710   <c>GZIP file format <xref target="RFC1952"/></c>
3711   <c>
3712      <xref target="gzip.coding"/>
3713   </c>
3714   <c>x-compress</c>
3715   <c>Deprecated (alias for compress)</c>
3716   <c>
3717      <xref target="compress.coding"/>
3718   </c>
3719   <c>x-gzip</c>
3720   <c>Deprecated (alias for gzip)</c>
3721   <c>
3722      <xref target="gzip.coding"/>
3723   </c>
3724</texttable>
3725</section>
3726</section>
3727
3728<section title="Content Coding Registration" anchor="content.coding.registration">
3729<t>
3730   IANA maintains the registry of HTTP Content Codings at
3731   <eref target="http://www.iana.org/assignments/http-parameters"/>.
3732</t>
3733<t>
3734   The HTTP Content Codings Registry shall be updated with the registrations
3735   below:
3736</t>
3737<texttable align="left" suppress-title="true" anchor="iana.content.coding.registration.table">
3738   <ttcol>Name</ttcol>
3739   <ttcol>Description</ttcol>
3740   <ttcol>Reference</ttcol>
3741   <c>compress</c>
3742   <c>UNIX "compress" data format <xref target="Welch"/></c>
3743   <c>
3744      <xref target="compress.coding"/>
3745   </c>
3746   <c>deflate</c>
3747   <c>"deflate" compressed data (<xref target="RFC1951"/>) inside
3748   the "zlib" data format (<xref target="RFC1950"/>)</c>
3749   <c>
3750      <xref target="deflate.coding"/>
3751   </c>
3752   <c>gzip</c>
3753   <c>GZIP file format <xref target="RFC1952"/></c>
3754   <c>
3755      <xref target="gzip.coding"/>
3756   </c>
3757   <c>x-compress</c>
3758   <c>Deprecated (alias for compress)</c>
3759   <c>
3760      <xref target="compress.coding"/>
3761   </c>
3762   <c>x-gzip</c>
3763   <c>Deprecated (alias for gzip)</c>
3764   <c>
3765      <xref target="gzip.coding"/>
3766   </c>
3767</texttable>
3768</section>
3769
3770<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3771<t>
3772   The HTTP Upgrade Token Registry defines the name space for protocol-name
3773   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3774   field. The registry is maintained at <eref target="http://www.iana.org/assignments/http-upgrade-tokens"/>.
3775</t>
3776
3777<section title="Procedure" anchor="upgrade.token.registry.procedure">  
3778<t>
3779   Each registered protocol name is associated with contact information
3780   and an optional set of specifications that details how the connection
3781   will be processed after it has been upgraded.
3782</t>
3783<t>
3784   Registrations happen on a "First Come First Served" basis (see
3785   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3786   following rules:
3787  <list style="numbers">
3788    <t>A protocol-name token, once registered, stays registered forever.</t>
3789    <t>The registration &MUST; name a responsible party for the
3790       registration.</t>
3791    <t>The registration &MUST; name a point of contact.</t>
3792    <t>The registration &MAY; name a set of specifications associated with
3793       that token. Such specifications need not be publicly available.</t>
3794    <t>The registration &SHOULD; name a set of expected "protocol-version"
3795       tokens associated with that token at the time of registration.</t>
3796    <t>The responsible party &MAY; change the registration at any time.
3797       The IANA will keep a record of all such changes, and make them
3798       available upon request.</t>
3799    <t>The IESG &MAY; reassign responsibility for a protocol token.
3800       This will normally only be used in the case when a
3801       responsible party cannot be contacted.</t>
3802  </list>
3803</t>
3804<t>
3805   This registration procedure for HTTP Upgrade Tokens replaces that
3806   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3807</t>
3808</section>
3809
3810<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3811<t>
3812   The "HTTP" entry in the HTTP Upgrade Token Registry shall be updated with
3813   the registration below:
3814</t>
3815<texttable align="left" suppress-title="true">
3816   <ttcol>Value</ttcol>
3817   <ttcol>Description</ttcol>
3818   <ttcol>Expected Version Tokens</ttcol>
3819   <ttcol>Reference</ttcol>
3820
3821   <c>HTTP</c>
3822   <c>Hypertext Transfer Protocol</c>
3823   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3824   <c><xref target="http.version"/></c>
3825</texttable>
3826<t>
3827   The responsible party is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3828</t>
3829</section>
3830</section>
3831
3832</section>
3833
3834<section title="Security Considerations" anchor="security.considerations">
3835<t>
3836   This section is meant to inform developers, information providers, and
3837   users of known security concerns relevant to HTTP/1.1 message syntax,
3838   parsing, and routing.
3839</t>
3840
3841<section title="DNS-related Attacks" anchor="dns.related.attacks">
3842<t>
3843   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3844   generally prone to security attacks based on the deliberate misassociation
3845   of IP addresses and DNS names not protected by DNSSEC. Clients need to be
3846   cautious in assuming the validity of an IP number/DNS name association unless
3847   the response is protected by DNSSEC (<xref target="RFC4033"/>).
3848</t>
3849</section>
3850
3851<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3852<t>
3853   By their very nature, HTTP intermediaries are men-in-the-middle, and
3854   represent an opportunity for man-in-the-middle attacks. Compromise of
3855   the systems on which the intermediaries run can result in serious security
3856   and privacy problems. Intermediaries have access to security-related
3857   information, personal information about individual users and
3858   organizations, and proprietary information belonging to users and
3859   content providers. A compromised intermediary, or an intermediary
3860   implemented or configured without regard to security and privacy
3861   considerations, might be used in the commission of a wide range of
3862   potential attacks.
3863</t>
3864<t>
3865   Intermediaries that contain a shared cache are especially vulnerable
3866   to cache poisoning attacks.
3867</t>
3868<t>
3869   Implementers need to consider the privacy and security
3870   implications of their design and coding decisions, and of the
3871   configuration options they provide to operators (especially the
3872   default configuration).
3873</t>
3874<t>
3875   Users need to be aware that intermediaries are no more trustworthy than
3876   the people who run them; HTTP itself cannot solve this problem.
3877</t>
3878</section>
3879
3880<section title="Buffer Overflows" anchor="attack.protocol.element.size.overflows">
3881<t>
3882   Because HTTP uses mostly textual, character-delimited fields, attackers can
3883   overflow buffers in implementations, and/or perform a Denial of Service
3884   against implementations that accept fields with unlimited lengths.
3885</t>
3886<t>
3887   To promote interoperability, this specification makes specific
3888   recommendations for minimum size limits on request-line
3889   (<xref target="request.line"/>)
3890   and header fields (<xref target="header.fields"/>). These are
3891   minimum recommendations, chosen to be supportable even by implementations
3892   with limited resources; it is expected that most implementations will
3893   choose substantially higher limits.
3894</t>
3895<t>
3896   This specification also provides a way for servers to reject messages that
3897   have request-targets that are too long (&status-414;) or request entities
3898   that are too large (&status-4xx;). Additional status codes related to
3899   capacity limits have been defined by extensions to HTTP
3900   <xref target="RFC6585"/>.
3901</t>
3902<t>
3903   Recipients ought to carefully limit the extent to which they read other
3904   fields, including (but not limited to) request methods, response status
3905   phrases, header field-names, and body chunks, so as to avoid denial of
3906   service attacks without impeding interoperability.
3907</t>
3908</section>
3909
3910<section title="Message Integrity" anchor="message.integrity">
3911<t>
3912   HTTP does not define a specific mechanism for ensuring message integrity,
3913   instead relying on the error-detection ability of underlying transport
3914   protocols and the use of length or chunk-delimited framing to detect
3915   completeness. Additional integrity mechanisms, such as hash functions or
3916   digital signatures applied to the content, can be selectively added to
3917   messages via extensible metadata header fields. Historically, the lack of
3918   a single integrity mechanism has been justified by the informal nature of
3919   most HTTP communication.  However, the prevalence of HTTP as an information
3920   access mechanism has resulted in its increasing use within environments
3921   where verification of message integrity is crucial.
3922</t>
3923<t>
3924   User agents are encouraged to implement configurable means for detecting
3925   and reporting failures of message integrity such that those means can be
3926   enabled within environments for which integrity is necessary. For example,
3927   a browser being used to view medical history or drug interaction
3928   information needs to indicate to the user when such information is detected
3929   by the protocol to be incomplete, expired, or corrupted during transfer.
3930   Such mechanisms might be selectively enabled via user agent extensions or
3931   the presence of message integrity metadata in a response.
3932   At a minimum, user agents ought to provide some indication that allows a
3933   user to distinguish between a complete and incomplete response message
3934   (<xref target="incomplete.messages"/>) when such verification is desired.
3935</t>
3936</section>
3937
3938<section title="Server Log Information" anchor="abuse.of.server.log.information">
3939<t>
3940   A server is in the position to save personal data about a user's requests
3941   over time, which might identify their reading patterns or subjects of
3942   interest.  In particular, log information gathered at an intermediary
3943   often contains a history of user agent interaction, across a multitude
3944   of sites, that can be traced to individual users.
3945</t>
3946<t>
3947   HTTP log information is confidential in nature; its handling is often
3948   constrained by laws and regulations.  Log information needs to be securely
3949   stored and appropriate guidelines followed for its analysis.
3950   Anonymization of personal information within individual entries helps,
3951   but is generally not sufficient to prevent real log traces from being
3952   re-identified based on correlation with other access characteristics.
3953   As such, access traces that are keyed to a specific client are unsafe to
3954   publish even if the key is pseudonymous.
3955</t>
3956<t>
3957   To minimize the risk of theft or accidental publication, log information
3958   ought to be purged of personally identifiable information, including
3959   user identifiers, IP addresses, and user-provided query parameters,
3960   as soon as that information is no longer necessary to support operational
3961   needs for security, auditing, or fraud control.
3962</t>
3963</section>
3964</section>
3965
3966<section title="Acknowledgments" anchor="acks">
3967<t>
3968   This edition of HTTP/1.1 builds on the many contributions that went into
3969   <xref target="RFC1945" format="none">RFC 1945</xref>,
3970   <xref target="RFC2068" format="none">RFC 2068</xref>,
3971   <xref target="RFC2145" format="none">RFC 2145</xref>, and
3972   <xref target="RFC2616" format="none">RFC 2616</xref>, including
3973   substantial contributions made by the previous authors, editors, and
3974   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
3975   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
3976   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
3977</t>
3978<t>
3979   Since 1999, the following contributors have helped improve the HTTP
3980   specification by reporting bugs, asking smart questions, drafting or
3981   reviewing text, and evaluating open issues:
3982</t>
3983<?BEGININC acks ?>
3984<t>Adam Barth,
3985Adam Roach,
3986Addison Phillips,
3987Adrian Chadd,
3988Adrien W. de Croy,
3989Alan Ford,
3990Alan Ruttenberg,
3991Albert Lunde,
3992Alek Storm,
3993Alex Rousskov,
3994Alexandre Morgaut,
3995Alexey Melnikov,
3996Alisha Smith,
3997Amichai Rothman,
3998Amit Klein,
3999Amos Jeffries,
4000Andreas Maier,
4001Andreas Petersson,
4002Andrei Popov,
4003Anil Sharma,
4004Anne van Kesteren,
4005Anthony Bryan,
4006Asbjorn Ulsberg,
4007Ashok Kumar,
4008Balachander Krishnamurthy,
4009Barry Leiba,
4010Ben Laurie,
4011Benjamin Carlyle,
4012Benjamin Niven-Jenkins,
4013Bil Corry,
4014Bill Burke,
4015Bjoern Hoehrmann,
4016Bob Scheifler,
4017Boris Zbarsky,
4018Brett Slatkin,
4019Brian Kell,
4020Brian McBarron,
4021Brian Pane,
4022Brian Raymor,
4023Brian Smith,
4024Bruce Perens,
4025Bryce Nesbitt,
4026Cameron Heavon-Jones,
4027Carl Kugler,
4028Carsten Bormann,
4029Charles Fry,
4030Chris Burdess,
4031Chris Newman,
4032Cyrus Daboo,
4033Dale Robert Anderson,
4034Dan Wing,
4035Dan Winship,
4036Daniel Stenberg,
4037Darrel Miller,
4038Dave Cridland,
4039Dave Crocker,
4040Dave Kristol,
4041Dave Thaler,
4042David Booth,
4043David Singer,
4044David W. Morris,
4045Diwakar Shetty,
4046Dmitry Kurochkin,
4047Drummond Reed,
4048Duane Wessels,
4049Edward Lee,
4050Eitan Adler,
4051Eliot Lear,
4052Emile Stephan,
4053Eran Hammer-Lahav,
4054Eric D. Williams,
4055Eric J. Bowman,
4056Eric Lawrence,
4057Eric Rescorla,
4058Erik Aronesty,
4059EungJun Yi,
4060Evan Prodromou,
4061Felix Geisendoerfer,
4062Florian Weimer,
4063Frank Ellermann,
4064Fred Akalin,
4065Fred Bohle,
4066Frederic Kayser,
4067Gabor Molnar,
4068Gabriel Montenegro,
4069Geoffrey Sneddon,
4070Gervase Markham,
4071Gili Tzabari,
4072Grahame Grieve,
4073Greg Slepak,
4074Greg Wilkins,
4075Grzegorz Calkowski,
4076Harald Tveit Alvestrand,
4077Harry Halpin,
4078Helge Hess,
4079Henrik Nordstrom,
4080Henry S. Thompson,
4081Henry Story,
4082Herbert van de Sompel,
4083Herve Ruellan,
4084Howard Melman,
4085Hugo Haas,
4086Ian Fette,
4087Ian Hickson,
4088Ido Safruti,
4089Ilari Liusvaara,
4090Ilya Grigorik,
4091Ingo Struck,
4092J. Ross Nicoll,
4093James Cloos,
4094James H. Manger,
4095James Lacey,
4096James M. Snell,
4097Jamie Lokier,
4098Jan Algermissen,
4099Jeff Hodges (who came up with the term 'effective Request-URI'),
4100Jeff Pinner,
4101Jeff Walden,
4102Jim Luther,
4103Jitu Padhye,
4104Joe D. Williams,
4105Joe Gregorio,
4106Joe Orton,
4107John C. Klensin,
4108John C. Mallery,
4109John Cowan,
4110John Kemp,
4111John Panzer,
4112John Schneider,
4113John Stracke,
4114John Sullivan,
4115Jonas Sicking,
4116Jonathan A. Rees,
4117Jonathan Billington,
4118Jonathan Moore,
4119Jonathan Silvera,
4120Jordi Ros,
4121Joris Dobbelsteen,
4122Josh Cohen,
4123Julien Pierre,
4124Jungshik Shin,
4125Justin Chapweske,
4126Justin Erenkrantz,
4127Justin James,
4128Kalvinder Singh,
4129Karl Dubost,
4130Keith Hoffman,
4131Keith Moore,
4132Ken Murchison,
4133Koen Holtman,
4134Konstantin Voronkov,
4135Kris Zyp,
4136Leif Hedstrom,
4137Lisa Dusseault,
4138Maciej Stachowiak,
4139Manu Sporny,
4140Marc Schneider,
4141Marc Slemko,
4142Mark Baker,
4143Mark Pauley,
4144Mark Watson,
4145Markus Isomaki,
4146Markus Lanthaler,
4147Martin J. Duerst,
4148Martin Musatov,
4149Martin Nilsson,
4150Martin Thomson,
4151Matt Lynch,
4152Matthew Cox,
4153Matthew Kerwin,
4154Max Clark,
4155Michael Burrows,
4156Michael Hausenblas,
4157Michael Scharf,
4158Michael Sweet,
4159Michael Tuexen,
4160Michael Welzl,
4161Mike Amundsen,
4162Mike Belshe,
4163Mike Bishop,
4164Mike Kelly,
4165Mike Schinkel,
4166Miles Sabin,
4167Murray S. Kucherawy,
4168Mykyta Yevstifeyev,
4169Nathan Rixham,
4170Nicholas Shanks,
4171Nico Williams,
4172Nicolas Alvarez,
4173Nicolas Mailhot,
4174Noah Slater,
4175Osama Mazahir,
4176Pablo Castro,
4177Pat Hayes,
4178Patrick R. McManus,
4179Paul E. Jones,
4180Paul Hoffman,
4181Paul Marquess,
4182Peter Lepeska,
4183Peter Occil,
4184Peter Saint-Andre,
4185Peter Watkins,
4186Phil Archer,
4187Philippe Mougin,
4188Phillip Hallam-Baker,
4189Piotr Dobrogost,
4190Poul-Henning Kamp,
4191Preethi Natarajan,
4192Rajeev Bector,
4193Ray Polk,
4194Reto Bachmann-Gmuer,
4195Richard Cyganiak,
4196Robby Simpson,
4197Robert Brewer,
4198Robert Collins,
4199Robert Mattson,
4200Robert O'Callahan,
4201Robert Olofsson,
4202Robert Sayre,
4203Robert Siemer,
4204Robert de Wilde,
4205Roberto Javier Godoy,
4206Roberto Peon,
4207Roland Zink,
4208Ronny Widjaja,
4209Ryan Hamilton,
4210S. Mike Dierken,
4211Salvatore Loreto,
4212Sam Johnston,
4213Sam Pullara,
4214Sam Ruby,
4215Saurabh Kulkarni,
4216Scott Lawrence (who maintained the original issues list),
4217Sean B. Palmer,
4218Sebastien Barnoud,
4219Shane McCarron,
4220Shigeki Ohtsu,
4221Simon Yarde,
4222Stefan Eissing,
4223Stefan Tilkov,
4224Stefanos Harhalakis,
4225Stephane Bortzmeyer,
4226Stephen Farrell,
4227Stephen Ludin,
4228Stuart Williams,
4229Subbu Allamaraju,
4230Subramanian Moonesamy,
4231Sylvain Hellegouarch,
4232Tapan Divekar,
4233Tatsuhiro Tsujikawa,
4234Tatsuya Hayashi,
4235Ted Hardie,
4236Thomas Broyer,
4237Thomas Fossati,
4238Thomas Maslen,
4239Thomas Nordin,
4240Thomas Roessler,
4241Tim Bray,
4242Tim Morgan,
4243Tim Olsen,
4244Tom Zhou,
4245Travis Snoozy,
4246Tyler Close,
4247Vincent Murphy,
4248Wenbo Zhu,
4249Werner Baumann,
4250Wilbur Streett,
4251Wilfredo Sanchez Vega,
4252William A. Rowe Jr.,
4253William Chan,
4254Willy Tarreau,
4255Xiaoshu Wang,
4256Yaron Goland,
4257Yngve Nysaeter Pettersen,
4258Yoav Nir,
4259Yogesh Bang,
4260Yuchung Cheng,
4261Yutaka Oiwa,
4262Yves Lafon (long-time member of the editor team),
4263Zed A. Shaw, and
4264Zhong Yu.
4265</t>
4266<?ENDINC acks ?>
4267<t>
4268   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
4269   acknowledgements from prior revisions.
4270</t>
4271</section>
4272
4273</middle>
4274<back>
4275
4276<references title="Normative References">
4277
4278<reference anchor="Part2">
4279  <front>
4280    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
4281    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4282      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4283      <address><email>fielding@gbiv.com</email></address>
4284    </author>
4285    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4286      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4287      <address><email>julian.reschke@greenbytes.de</email></address>
4288    </author>
4289    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4290  </front>
4291  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
4292  <x:source href="p2-semantics.xml" basename="p2-semantics">
4293    <x:defines>1xx (Informational)</x:defines>
4294    <x:defines>1xx</x:defines>
4295    <x:defines>100 (Continue)</x:defines>
4296    <x:defines>101 (Switching Protocols)</x:defines>
4297    <x:defines>2xx (Successful)</x:defines>
4298    <x:defines>2xx</x:defines>
4299    <x:defines>200 (OK)</x:defines>
4300    <x:defines>203 (Non-Authoritative Information)</x:defines>
4301    <x:defines>204 (No Content)</x:defines>
4302    <x:defines>3xx (Redirection)</x:defines>
4303    <x:defines>3xx</x:defines>
4304    <x:defines>301 (Moved Permanently)</x:defines>
4305    <x:defines>4xx (Client Error)</x:defines>
4306    <x:defines>4xx</x:defines>
4307    <x:defines>400 (Bad Request)</x:defines>
4308    <x:defines>411 (Length Required)</x:defines>
4309    <x:defines>414 (URI Too Long)</x:defines>
4310    <x:defines>417 (Expectation Failed)</x:defines>
4311    <x:defines>426 (Upgrade Required)</x:defines>
4312    <x:defines>501 (Not Implemented)</x:defines>
4313    <x:defines>502 (Bad Gateway)</x:defines>
4314    <x:defines>505 (HTTP Version Not Supported)</x:defines>
4315    <x:defines>Accept-Encoding</x:defines>
4316    <x:defines>Allow</x:defines>
4317    <x:defines>Content-Encoding</x:defines>
4318    <x:defines>Content-Location</x:defines>
4319    <x:defines>Content-Type</x:defines>
4320    <x:defines>Date</x:defines>
4321    <x:defines>Expect</x:defines>
4322    <x:defines>Location</x:defines>
4323    <x:defines>Server</x:defines>
4324    <x:defines>User-Agent</x:defines>
4325  </x:source>
4326</reference>
4327
4328<reference anchor="Part4">
4329  <front>
4330    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
4331    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
4332      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4333      <address><email>fielding@gbiv.com</email></address>
4334    </author>
4335    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
4336      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4337      <address><email>julian.reschke@greenbytes.de</email></address>
4338    </author>
4339    <date month="&ID-MONTH;" year="&ID-YEAR;" />
4340  </front>
4341  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
4342  <x:source basename="p4-conditional" href="p4-conditional.xml">
4343    <x:defines>304 (Not Modified)</x:defines>
4344    <x:defines>ETag</x:defines>
4345    <x:defines>Last-Modified</x:defines>
4346  </x:source>
4347</reference>
4348
4349<reference anchor="Part5">
4350  <front>
4351    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
4352    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4353      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4354      <address><email>fielding@gbiv.com</email></address>
4355    </author>
4356    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
4357      <organization abbrev="W3C">World Wide Web Consortium</organization>
4358      <address><email>ylafon@w3.org</email></address>
4359    </author>
4360    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4361      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4362      <address><email>julian.reschke@greenbytes.de</email></address>
4363    </author>
4364    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4365  </front>
4366  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4367  <x:source href="p5-range.xml" basename="p5-range">
4368    <x:defines>Content-Range</x:defines>
4369  </x:source>
4370</reference>
4371
4372<reference anchor="Part6">
4373  <front>
4374    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4375    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4376      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4377      <address><email>fielding@gbiv.com</email></address>
4378    </author>
4379    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4380      <organization>Akamai</organization>
4381      <address><email>mnot@mnot.net</email></address>
4382    </author>
4383    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4384      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4385      <address><email>julian.reschke@greenbytes.de</email></address>
4386    </author>
4387    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4388  </front>
4389  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4390  <x:source href="p6-cache.xml" basename="p6-cache">
4391    <x:defines>Cache-Control</x:defines>
4392    <x:defines>Expires</x:defines>
4393  </x:source>
4394</reference>
4395
4396<reference anchor="Part7">
4397  <front>
4398    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4399    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4400      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4401      <address><email>fielding@gbiv.com</email></address>
4402    </author>
4403    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4404      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4405      <address><email>julian.reschke@greenbytes.de</email></address>
4406    </author>
4407    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4408  </front>
4409  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4410  <x:source href="p7-auth.xml" basename="p7-auth">
4411    <x:defines>Proxy-Authenticate</x:defines>
4412    <x:defines>Proxy-Authorization</x:defines>
4413  </x:source>
4414</reference>
4415
4416<reference anchor="RFC5234">
4417  <front>
4418    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4419    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4420      <organization>Brandenburg InternetWorking</organization>
4421      <address>
4422        <email>dcrocker@bbiw.net</email>
4423      </address> 
4424    </author>
4425    <author initials="P." surname="Overell" fullname="Paul Overell">
4426      <organization>THUS plc.</organization>
4427      <address>
4428        <email>paul.overell@thus.net</email>
4429      </address>
4430    </author>
4431    <date month="January" year="2008"/>
4432  </front>
4433  <seriesInfo name="STD" value="68"/>
4434  <seriesInfo name="RFC" value="5234"/>
4435</reference>
4436
4437<reference anchor="RFC2119">
4438  <front>
4439    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4440    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4441      <organization>Harvard University</organization>
4442      <address><email>sob@harvard.edu</email></address>
4443    </author>
4444    <date month="March" year="1997"/>
4445  </front>
4446  <seriesInfo name="BCP" value="14"/>
4447  <seriesInfo name="RFC" value="2119"/>
4448</reference>
4449
4450<reference anchor="RFC3986">
4451 <front>
4452  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4453  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4454    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4455    <address>
4456       <email>timbl@w3.org</email>
4457       <uri>http://www.w3.org/People/Berners-Lee/</uri>
4458    </address>
4459  </author>
4460  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4461    <organization abbrev="Day Software">Day Software</organization>
4462    <address>
4463      <email>fielding@gbiv.com</email>
4464      <uri>http://roy.gbiv.com/</uri>
4465    </address>
4466  </author>
4467  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4468    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4469    <address>
4470      <email>LMM@acm.org</email>
4471      <uri>http://larry.masinter.net/</uri>
4472    </address>
4473  </author>
4474  <date month='January' year='2005'></date>
4475 </front>
4476 <seriesInfo name="STD" value="66"/>
4477 <seriesInfo name="RFC" value="3986"/>
4478</reference>
4479
4480<reference anchor="RFC0793">
4481  <front>
4482    <title>Transmission Control Protocol</title>
4483    <author initials='J.' surname='Postel' fullname='Jon Postel'>
4484      <organization>University of Southern California (USC)/Information Sciences Institute</organization>
4485    </author>
4486    <date year='1981' month='September' />
4487  </front>
4488  <seriesInfo name='STD' value='7' />
4489  <seriesInfo name='RFC' value='793' />
4490</reference>
4491
4492<reference anchor="USASCII">
4493  <front>
4494    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4495    <author>
4496      <organization>American National Standards Institute</organization>
4497    </author>
4498    <date year="1986"/>
4499  </front>
4500  <seriesInfo name="ANSI" value="X3.4"/>
4501</reference>
4502
4503<reference anchor="RFC1950">
4504  <front>
4505    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4506    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4507      <organization>Aladdin Enterprises</organization>
4508      <address><email>ghost@aladdin.com</email></address>
4509    </author>
4510    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4511    <date month="May" year="1996"/>
4512  </front>
4513  <seriesInfo name="RFC" value="1950"/>
4514  <!--<annotation>
4515    RFC 1950 is an Informational RFC, thus it might be less stable than
4516    this specification. On the other hand, this downward reference was
4517    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4518    therefore it is unlikely to cause problems in practice. See also
4519    <xref target="BCP97"/>.
4520  </annotation>-->
4521</reference>
4522
4523<reference anchor="RFC1951">
4524  <front>
4525    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4526    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4527      <organization>Aladdin Enterprises</organization>
4528      <address><email>ghost@aladdin.com</email></address>
4529    </author>
4530    <date month="May" year="1996"/>
4531  </front>
4532  <seriesInfo name="RFC" value="1951"/>
4533  <!--<annotation>
4534    RFC 1951 is an Informational RFC, thus it might be less stable than
4535    this specification. On the other hand, this downward reference was
4536    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4537    therefore it is unlikely to cause problems in practice. See also
4538    <xref target="BCP97"/>.
4539  </annotation>-->
4540</reference>
4541
4542<reference anchor="RFC1952">
4543  <front>
4544    <title>GZIP file format specification version 4.3</title>
4545    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4546      <organization>Aladdin Enterprises</organization>
4547      <address><email>ghost@aladdin.com</email></address>
4548    </author>
4549    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4550      <address><email>gzip@prep.ai.mit.edu</email></address>
4551    </author>
4552    <author initials="M." surname="Adler" fullname="Mark Adler">
4553      <address><email>madler@alumni.caltech.edu</email></address>
4554    </author>
4555    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4556      <address><email>ghost@aladdin.com</email></address>
4557    </author>
4558    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4559      <address><email>randeg@alumni.rpi.edu</email></address>
4560    </author>
4561    <date month="May" year="1996"/>
4562  </front>
4563  <seriesInfo name="RFC" value="1952"/>
4564  <!--<annotation>
4565    RFC 1952 is an Informational RFC, thus it might be less stable than
4566    this specification. On the other hand, this downward reference was
4567    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4568    therefore it is unlikely to cause problems in practice. See also
4569    <xref target="BCP97"/>.
4570  </annotation>-->
4571</reference>
4572
4573<reference anchor="Welch">
4574  <front>
4575    <title>A Technique for High Performance Data Compression</title>
4576    <author initials="T.A." surname="Welch" fullname="Terry A. Welch"/>
4577    <date month="June" year="1984"/>
4578  </front>
4579  <seriesInfo name="IEEE Computer" value="17(6)"/>
4580</reference>
4581
4582</references>
4583
4584<references title="Informative References">
4585
4586<reference anchor="ISO-8859-1">
4587  <front>
4588    <title>
4589     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4590    </title>
4591    <author>
4592      <organization>International Organization for Standardization</organization>
4593    </author>
4594    <date year="1998"/>
4595  </front>
4596  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4597</reference>
4598
4599<reference anchor='RFC1919'>
4600  <front>
4601    <title>Classical versus Transparent IP Proxies</title>
4602    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4603      <address><email>mchatel@pax.eunet.ch</email></address>
4604    </author>
4605    <date year='1996' month='March' />
4606  </front>
4607  <seriesInfo name='RFC' value='1919' />
4608</reference>
4609
4610<reference anchor="RFC1945">
4611  <front>
4612    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4613    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4614      <organization>MIT, Laboratory for Computer Science</organization>
4615      <address><email>timbl@w3.org</email></address>
4616    </author>
4617    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4618      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4619      <address><email>fielding@ics.uci.edu</email></address>
4620    </author>
4621    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4622      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4623      <address><email>frystyk@w3.org</email></address>
4624    </author>
4625    <date month="May" year="1996"/>
4626  </front>
4627  <seriesInfo name="RFC" value="1945"/>
4628</reference>
4629
4630<reference anchor="RFC2045">
4631  <front>
4632    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4633    <author initials="N." surname="Freed" fullname="Ned Freed">
4634      <organization>Innosoft International, Inc.</organization>
4635      <address><email>ned@innosoft.com</email></address>
4636    </author>
4637    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4638      <organization>First Virtual Holdings</organization>
4639      <address><email>nsb@nsb.fv.com</email></address>
4640    </author>
4641    <date month="November" year="1996"/>
4642  </front>
4643  <seriesInfo name="RFC" value="2045"/>
4644</reference>
4645
4646<reference anchor="RFC2047">
4647  <front>
4648    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4649    <author initials="K." surname="Moore" fullname="Keith Moore">
4650      <organization>University of Tennessee</organization>
4651      <address><email>moore@cs.utk.edu</email></address>
4652    </author>
4653    <date month="November" year="1996"/>
4654  </front>
4655  <seriesInfo name="RFC" value="2047"/>
4656</reference>
4657
4658<reference anchor="RFC2068">
4659  <front>
4660    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4661    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
4662      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4663      <address><email>fielding@ics.uci.edu</email></address>
4664    </author>
4665    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4666      <organization>MIT Laboratory for Computer Science</organization>
4667      <address><email>jg@w3.org</email></address>
4668    </author>
4669    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4670      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4671      <address><email>mogul@wrl.dec.com</email></address>
4672    </author>
4673    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4674      <organization>MIT Laboratory for Computer Science</organization>
4675      <address><email>frystyk@w3.org</email></address>
4676    </author>
4677    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4678      <organization>MIT Laboratory for Computer Science</organization>
4679      <address><email>timbl@w3.org</email></address>
4680    </author>
4681    <date month="January" year="1997"/>
4682  </front>
4683  <seriesInfo name="RFC" value="2068"/>
4684</reference>
4685
4686<reference anchor="RFC2145">
4687  <front>
4688    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4689    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4690      <organization>Western Research Laboratory</organization>
4691      <address><email>mogul@wrl.dec.com</email></address>
4692    </author>
4693    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4694      <organization>Department of Information and Computer Science</organization>
4695      <address><email>fielding@ics.uci.edu</email></address>
4696    </author>
4697    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4698      <organization>MIT Laboratory for Computer Science</organization>
4699      <address><email>jg@w3.org</email></address>
4700    </author>
4701    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4702      <organization>W3 Consortium</organization>
4703      <address><email>frystyk@w3.org</email></address>
4704    </author>
4705    <date month="May" year="1997"/>
4706  </front>
4707  <seriesInfo name="RFC" value="2145"/>
4708</reference>
4709
4710<reference anchor="RFC2616">
4711  <front>
4712    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4713    <author initials="R." surname="Fielding" fullname="R. Fielding">
4714      <organization>University of California, Irvine</organization>
4715      <address><email>fielding@ics.uci.edu</email></address>
4716    </author>
4717    <author initials="J." surname="Gettys" fullname="J. Gettys">
4718      <organization>W3C</organization>
4719      <address><email>jg@w3.org</email></address>
4720    </author>
4721    <author initials="J." surname="Mogul" fullname="J. Mogul">
4722      <organization>Compaq Computer Corporation</organization>
4723      <address><email>mogul@wrl.dec.com</email></address>
4724    </author>
4725    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4726      <organization>MIT Laboratory for Computer Science</organization>
4727      <address><email>frystyk@w3.org</email></address>
4728    </author>
4729    <author initials="L." surname="Masinter" fullname="L. Masinter">
4730      <organization>Xerox Corporation</organization>
4731      <address><email>masinter@parc.xerox.com</email></address>
4732    </author>
4733    <author initials="P." surname="Leach" fullname="P. Leach">
4734      <organization>Microsoft Corporation</organization>
4735      <address><email>paulle@microsoft.com</email></address>
4736    </author>
4737    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4738      <organization>W3C</organization>
4739      <address><email>timbl@w3.org</email></address>
4740    </author>
4741    <date month="June" year="1999"/>
4742  </front>
4743  <seriesInfo name="RFC" value="2616"/>
4744</reference>
4745
4746<reference anchor='RFC2817'>
4747  <front>
4748    <title>Upgrading to TLS Within HTTP/1.1</title>
4749    <author initials='R.' surname='Khare' fullname='R. Khare'>
4750      <organization>4K Associates / UC Irvine</organization>
4751      <address><email>rohit@4K-associates.com</email></address>
4752    </author>
4753    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4754      <organization>Agranat Systems, Inc.</organization>
4755      <address><email>lawrence@agranat.com</email></address>
4756    </author>
4757    <date year='2000' month='May' />
4758  </front>
4759  <seriesInfo name='RFC' value='2817' />
4760</reference>
4761
4762<reference anchor='RFC2818'>
4763  <front>
4764    <title>HTTP Over TLS</title>
4765    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4766      <organization>RTFM, Inc.</organization>
4767      <address><email>ekr@rtfm.com</email></address>
4768    </author>
4769    <date year='2000' month='May' />
4770  </front>
4771  <seriesInfo name='RFC' value='2818' />
4772</reference>
4773
4774<reference anchor='RFC3040'>
4775  <front>
4776    <title>Internet Web Replication and Caching Taxonomy</title>
4777    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4778      <organization>Equinix, Inc.</organization>
4779    </author>
4780    <author initials='I.' surname='Melve' fullname='I. Melve'>
4781      <organization>UNINETT</organization>
4782    </author>
4783    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4784      <organization>CacheFlow Inc.</organization>
4785    </author>
4786    <date year='2001' month='January' />
4787  </front>
4788  <seriesInfo name='RFC' value='3040' />
4789</reference>
4790
4791<reference anchor='BCP90'>
4792  <front>
4793    <title>Registration Procedures for Message Header Fields</title>
4794    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4795      <organization>Nine by Nine</organization>
4796      <address><email>GK-IETF@ninebynine.org</email></address>
4797    </author>
4798    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4799      <organization>BEA Systems</organization>
4800      <address><email>mnot@pobox.com</email></address>
4801    </author>
4802    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4803      <organization>HP Labs</organization>
4804      <address><email>JeffMogul@acm.org</email></address>
4805    </author>
4806    <date year='2004' month='September' />
4807  </front>
4808  <seriesInfo name='BCP' value='90' />
4809  <seriesInfo name='RFC' value='3864' />
4810</reference>
4811
4812<reference anchor='RFC4033'>
4813  <front>
4814    <title>DNS Security Introduction and Requirements</title>
4815    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4816    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4817    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4818    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4819    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4820    <date year='2005' month='March' />
4821  </front>
4822  <seriesInfo name='RFC' value='4033' />
4823</reference>
4824
4825<reference anchor="BCP13">
4826  <front>
4827    <title>Media Type Specifications and Registration Procedures</title>
4828    <author initials="N." surname="Freed" fullname="Ned Freed">
4829      <organization>Oracle</organization>
4830      <address>
4831        <email>ned+ietf@mrochek.com</email>
4832      </address>
4833    </author>
4834    <author initials="J." surname="Klensin" fullname="John C. Klensin">
4835      <address>
4836        <email>john+ietf@jck.com</email>
4837      </address>
4838    </author>
4839    <author initials="T." surname="Hansen" fullname="Tony Hansen">
4840      <organization>AT&amp;T Laboratories</organization>
4841      <address>
4842        <email>tony+mtsuffix@maillennium.att.com</email>
4843      </address>
4844    </author>
4845    <date year="2013" month="January"/>
4846  </front>
4847  <seriesInfo name="BCP" value="13"/>
4848  <seriesInfo name="RFC" value="6838"/>
4849</reference>
4850
4851<reference anchor='BCP115'>
4852  <front>
4853    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4854    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4855      <organization>AT&amp;T Laboratories</organization>
4856      <address>
4857        <email>tony+urireg@maillennium.att.com</email>
4858      </address>
4859    </author>
4860    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4861      <organization>Qualcomm, Inc.</organization>
4862      <address>
4863        <email>hardie@qualcomm.com</email>
4864      </address>
4865    </author>
4866    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4867      <organization>Adobe Systems</organization>
4868      <address>
4869        <email>LMM@acm.org</email>
4870      </address>
4871    </author>
4872    <date year='2006' month='February' />
4873  </front>
4874  <seriesInfo name='BCP' value='115' />
4875  <seriesInfo name='RFC' value='4395' />
4876</reference>
4877
4878<reference anchor='RFC4559'>
4879  <front>
4880    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4881    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4882    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4883    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4884    <date year='2006' month='June' />
4885  </front>
4886  <seriesInfo name='RFC' value='4559' />
4887</reference>
4888
4889<reference anchor='RFC5226'>
4890  <front>
4891    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4892    <author initials='T.' surname='Narten' fullname='T. Narten'>
4893      <organization>IBM</organization>
4894      <address><email>narten@us.ibm.com</email></address>
4895    </author>
4896    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4897      <organization>Google</organization>
4898      <address><email>Harald@Alvestrand.no</email></address>
4899    </author>
4900    <date year='2008' month='May' />
4901  </front>
4902  <seriesInfo name='BCP' value='26' />
4903  <seriesInfo name='RFC' value='5226' />
4904</reference>
4905
4906<reference anchor='RFC5246'>
4907   <front>
4908      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4909      <author initials='T.' surname='Dierks' fullname='T. Dierks'>
4910         <organization />
4911      </author>
4912      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4913         <organization>RTFM, Inc.</organization>
4914      </author>
4915      <date year='2008' month='August' />
4916   </front>
4917   <seriesInfo name='RFC' value='5246' />
4918</reference>
4919
4920<reference anchor="RFC5322">
4921  <front>
4922    <title>Internet Message Format</title>
4923    <author initials="P." surname="Resnick" fullname="P. Resnick">
4924      <organization>Qualcomm Incorporated</organization>
4925    </author>
4926    <date year="2008" month="October"/>
4927  </front>
4928  <seriesInfo name="RFC" value="5322"/>
4929</reference>
4930
4931<reference anchor="RFC6265">
4932  <front>
4933    <title>HTTP State Management Mechanism</title>
4934    <author initials="A." surname="Barth" fullname="Adam Barth">
4935      <organization abbrev="U.C. Berkeley">
4936        University of California, Berkeley
4937      </organization>
4938      <address><email>abarth@eecs.berkeley.edu</email></address>
4939    </author>
4940    <date year="2011" month="April" />
4941  </front>
4942  <seriesInfo name="RFC" value="6265"/>
4943</reference>
4944
4945<reference anchor='RFC6585'>
4946  <front>
4947    <title>Additional HTTP Status Codes</title>
4948    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4949      <organization>Rackspace</organization>
4950    </author>
4951    <author initials='R.' surname='Fielding' fullname='R. Fielding'>
4952      <organization>Adobe</organization>
4953    </author>
4954    <date year='2012' month='April' />
4955   </front>
4956   <seriesInfo name='RFC' value='6585' />
4957</reference>
4958
4959<!--<reference anchor='BCP97'>
4960  <front>
4961    <title>Handling Normative References to Standards-Track Documents</title>
4962    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
4963      <address>
4964        <email>klensin+ietf@jck.com</email>
4965      </address>
4966    </author>
4967    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
4968      <organization>MIT</organization>
4969      <address>
4970        <email>hartmans-ietf@mit.edu</email>
4971      </address>
4972    </author>
4973    <date year='2007' month='June' />
4974  </front>
4975  <seriesInfo name='BCP' value='97' />
4976  <seriesInfo name='RFC' value='4897' />
4977</reference>-->
4978
4979<reference anchor="Kri2001" target="http://arxiv.org/abs/cs.SE/0105018">
4980  <front>
4981    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
4982    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
4983    <date year="2001" month="November"/>
4984  </front>
4985  <seriesInfo name="ACM Transactions on Internet Technology" value="1(2)"/>
4986</reference>
4987
4988</references>
4989
4990
4991<section title="HTTP Version History" anchor="compatibility">
4992<t>
4993   HTTP has been in use by the World-Wide Web global information initiative
4994   since 1990. The first version of HTTP, later referred to as HTTP/0.9,
4995   was a simple protocol for hypertext data transfer across the Internet
4996   with only a single request method (GET) and no metadata.
4997   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
4998   methods and MIME-like messaging that could include metadata about the data
4999   transferred and modifiers on the request/response semantics. However,
5000   HTTP/1.0 did not sufficiently take into consideration the effects of
5001   hierarchical proxies, caching, the need for persistent connections, or
5002   name-based virtual hosts. The proliferation of incompletely-implemented
5003   applications calling themselves "HTTP/1.0" further necessitated a
5004   protocol version change in order for two communicating applications
5005   to determine each other's true capabilities.
5006</t>
5007<t>
5008   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
5009   requirements that enable reliable implementations, adding only
5010   those new features that will either be safely ignored by an HTTP/1.0
5011   recipient or only sent when communicating with a party advertising
5012   conformance with HTTP/1.1.
5013</t>
5014<t>
5015   It is beyond the scope of a protocol specification to mandate
5016   conformance with previous versions. HTTP/1.1 was deliberately
5017   designed, however, to make supporting previous versions easy.
5018   We would expect a general-purpose HTTP/1.1 server to understand
5019   any valid request in the format of HTTP/1.0 and respond appropriately
5020   with an HTTP/1.1 message that only uses features understood (or
5021   safely ignored) by HTTP/1.0 clients.  Likewise, we would expect
5022   an HTTP/1.1 client to understand any valid HTTP/1.0 response.
5023</t>
5024<t>
5025   Since HTTP/0.9 did not support header fields in a request,
5026   there is no mechanism for it to support name-based virtual
5027   hosts (selection of resource by inspection of the <x:ref>Host</x:ref> header
5028   field).  Any server that implements name-based virtual hosts
5029   ought to disable support for HTTP/0.9.  Most requests that
5030   appear to be HTTP/0.9 are, in fact, badly constructed HTTP/1.x
5031   requests wherein a buggy client failed to properly encode
5032   linear whitespace found in a URI reference and placed in
5033   the request-target.
5034</t>
5035
5036<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
5037<t>
5038   This section summarizes major differences between versions HTTP/1.0
5039   and HTTP/1.1.
5040</t>
5041
5042<section title="Multi-homed Web Servers" anchor="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">
5043<t>
5044   The requirements that clients and servers support the <x:ref>Host</x:ref>
5045   header field (<xref target="header.host"/>), report an error if it is
5046   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
5047   are among the most important changes defined by HTTP/1.1.
5048</t>
5049<t>
5050   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
5051   addresses and servers; there was no other established mechanism for
5052   distinguishing the intended server of a request than the IP address
5053   to which that request was directed. The <x:ref>Host</x:ref> header field was
5054   introduced during the development of HTTP/1.1 and, though it was
5055   quickly implemented by most HTTP/1.0 browsers, additional requirements
5056   were placed on all HTTP/1.1 requests in order to ensure complete
5057   adoption.  At the time of this writing, most HTTP-based services
5058   are dependent upon the Host header field for targeting requests.
5059</t>
5060</section>
5061
5062<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
5063<t>
5064   In HTTP/1.0, each connection is established by the client prior to the
5065   request and closed by the server after sending the response. However, some
5066   implementations implement the explicitly negotiated ("Keep-Alive") version
5067   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
5068   target="RFC2068"/>.
5069</t>
5070<t>
5071   Some clients and servers might wish to be compatible with these previous
5072   approaches to persistent connections, by explicitly negotiating for them
5073   with a "Connection: keep-alive" request header field. However, some
5074   experimental implementations of HTTP/1.0 persistent connections are faulty;
5075   for example, if an HTTP/1.0 proxy server doesn't understand
5076   <x:ref>Connection</x:ref>, it will erroneously forward that header field
5077   to the next inbound server, which would result in a hung connection.
5078</t>
5079<t>
5080   One attempted solution was the introduction of a Proxy-Connection header
5081   field, targeted specifically at proxies. In practice, this was also
5082   unworkable, because proxies are often deployed in multiple layers, bringing
5083   about the same problem discussed above.
5084</t>
5085<t>
5086   As a result, clients are encouraged not to send the Proxy-Connection header
5087   field in any requests.
5088</t>
5089<t>
5090   Clients are also encouraged to consider the use of Connection: keep-alive
5091   in requests carefully; while they can enable persistent connections with
5092   HTTP/1.0 servers, clients using them will need to monitor the
5093   connection for "hung" requests (which indicate that the client ought stop
5094   sending the header field), and this mechanism ought not be used by clients
5095   at all when a proxy is being used.
5096</t>
5097</section>
5098
5099<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
5100<t>
5101   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
5102   (<xref target="header.transfer-encoding"/>).
5103   Transfer codings need to be decoded prior to forwarding an HTTP message
5104   over a MIME-compliant protocol.
5105</t>
5106</section>
5107
5108</section>
5109
5110<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
5111<t>
5112  HTTP's approach to error handling has been explained.
5113  (<xref target="conformance" />)
5114</t>
5115<t>
5116  The HTTP-version ABNF production has been clarified to be case-sensitive.
5117  Additionally, version numbers has been restricted to single digits, due
5118  to the fact that implementations are known to handle multi-digit version
5119  numbers incorrectly.
5120  (<xref target="http.version"/>)
5121</t>
5122<t>
5123  Userinfo (i.e., username and password) are now disallowed in HTTP and
5124  HTTPS URIs, because of security issues related to their transmission on the
5125  wire.
5126  (<xref target="http.uri" />)
5127</t>
5128<t>
5129  The HTTPS URI scheme is now defined by this specification; previously,
5130  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
5131  Furthermore, it implies end-to-end security.
5132  (<xref target="https.uri"/>)
5133</t>
5134<t>
5135  HTTP messages can be (and often are) buffered by implementations; despite
5136  it sometimes being available as a stream, HTTP is fundamentally a
5137  message-oriented protocol.
5138  Minimum supported sizes for various protocol elements have been
5139  suggested, to improve interoperability.
5140  (<xref target="http.message" />)
5141</t>
5142<t>
5143  Invalid whitespace around field-names is now required to be rejected,
5144  because accepting it represents a security vulnerability.
5145  The ABNF productions defining header fields now only list the field value.
5146  (<xref target="header.fields"/>)
5147</t>
5148<t>
5149  Rules about implicit linear whitespace between certain grammar productions
5150  have been removed; now whitespace is only allowed where specifically
5151  defined in the ABNF.
5152  (<xref target="whitespace"/>)
5153</t>
5154<t>
5155  Header fields that span multiple lines ("line folding") are deprecated.
5156  (<xref target="field.parsing" />)
5157</t>
5158<t> 
5159  The NUL octet is no longer allowed in comment and quoted-string text, and
5160  handling of backslash-escaping in them has been clarified.
5161  The quoted-pair rule no longer allows escaping control characters other than
5162  HTAB.
5163  Non-ASCII content in header fields and the reason phrase has been obsoleted
5164  and made opaque (the TEXT rule was removed).
5165  (<xref target="field.components"/>)
5166</t> 
5167<t>
5168  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
5169  handled as errors by recipients.
5170  (<xref target="header.content-length"/>)
5171</t>
5172<t>
5173  The algorithm for determining the message body length has been clarified
5174  to indicate all of the special cases (e.g., driven by methods or status
5175  codes) that affect it, and that new protocol elements cannot define such
5176  special cases.
5177  CONNECT is a new, special case in determining message body length.
5178  "multipart/byteranges" is no longer a way of determining message body length
5179  detection.
5180  (<xref target="message.body.length"/>)
5181</t>
5182<t>
5183  The "identity" transfer coding token has been removed.
5184  (Sections <xref format="counter" target="message.body"/> and
5185  <xref format="counter" target="transfer.codings"/>)
5186</t>
5187<t>
5188  Chunk length does not include the count of the octets in the
5189  chunk header and trailer.
5190  Line folding in chunk extensions is  disallowed.
5191  (<xref target="chunked.encoding"/>)
5192</t>
5193<t>
5194  The meaning of the "deflate" content coding has been clarified.
5195  (<xref target="deflate.coding" />)
5196</t>
5197<t>
5198  The segment + query components of RFC 3986 have been used to define the
5199  request-target, instead of abs_path from RFC 1808.
5200  The asterisk-form of the request-target is only allowed with the OPTIONS
5201  method.
5202  (<xref target="request-target"/>)
5203</t>
5204<t>
5205  The term "Effective Request URI" has been introduced.
5206  (<xref target="effective.request.uri" />)
5207</t>
5208<t>
5209  Gateways do not need to generate <x:ref>Via</x:ref> header fields anymore.
5210  (<xref target="header.via"/>)
5211</t>
5212<t>
5213  Exactly when "close" connection options have to be sent has been clarified.
5214  Also, "hop-by-hop" header fields are required to appear in the Connection header
5215  field; just because they're defined as hop-by-hop in this specification
5216  doesn't exempt them.
5217  (<xref target="header.connection"/>)
5218</t>
5219<t>
5220  The limit of two connections per server has been removed.
5221  An idempotent sequence of requests is no longer required to be retried.
5222  The requirement to retry requests under certain circumstances when the
5223  server prematurely closes the connection has been removed.
5224  Also, some extraneous requirements about when servers are allowed to close
5225  connections prematurely have been removed.
5226  (<xref target="persistent.connections"/>)
5227</t>
5228<t>
5229  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
5230  responses other than 101 (this was incorporated from <xref
5231  target="RFC2817"/>). Furthermore, the ordering in the field value is now
5232  significant.
5233  (<xref target="header.upgrade"/>)
5234</t>
5235<t>
5236  Empty list elements in list productions (e.g., a list header field containing
5237  ", ,") have been deprecated.
5238  (<xref target="abnf.extension"/>)
5239</t>
5240<t>
5241  Registration of Transfer Codings now requires IETF Review
5242  (<xref target="transfer.coding.registry"/>)
5243</t>
5244<t>
5245  This specification now defines the Upgrade Token Registry, previously
5246  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
5247  (<xref target="upgrade.token.registry"/>)
5248</t>
5249<t>
5250  The expectation to support HTTP/0.9 requests has been removed.
5251  (<xref target="compatibility"/>)
5252</t>
5253<t>
5254  Issues with the Keep-Alive and Proxy-Connection header fields in requests
5255  are pointed out, with use of the latter being discouraged altogether.
5256  (<xref target="compatibility.with.http.1.0.persistent.connections" />)
5257</t>
5258</section>
5259</section>
5260
5261<?BEGININC p1-messaging.abnf-appendix ?>
5262<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
5263<figure>
5264<artwork type="abnf" name="p1-messaging.parsed-abnf">
5265<x:ref>BWS</x:ref> = OWS
5266
5267<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
5268 connection-option ] )
5269<x:ref>Content-Length</x:ref> = 1*DIGIT
5270
5271<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
5272 ]
5273<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
5274<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
5275<x:ref>Host</x:ref> = uri-host [ ":" port ]
5276
5277<x:ref>OWS</x:ref> = *( SP / HTAB )
5278
5279<x:ref>RWS</x:ref> = 1*( SP / HTAB )
5280
5281<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
5282<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
5283<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
5284 transfer-coding ] )
5285
5286<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
5287<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
5288
5289<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
5290 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
5291 comment ] ) ] )
5292
5293<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
5294<x:ref>absolute-form</x:ref> = absolute-URI
5295<x:ref>absolute-path</x:ref> = 1*( "/" segment )
5296<x:ref>asterisk-form</x:ref> = "*"
5297<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
5298<x:ref>authority-form</x:ref> = authority
5299
5300<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
5301<x:ref>chunk-data</x:ref> = 1*OCTET
5302<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
5303<x:ref>chunk-ext-name</x:ref> = token
5304<x:ref>chunk-ext-val</x:ref> = token / quoted-string
5305<x:ref>chunk-size</x:ref> = 1*HEXDIG
5306<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
5307<x:ref>comment</x:ref> = "(" *( ctext / quoted-pair / comment ) ")"
5308<x:ref>connection-option</x:ref> = token
5309<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
5310 / %x2A-5B ; '*'-'['
5311 / %x5D-7E ; ']'-'~'
5312 / obs-text
5313
5314<x:ref>field-content</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5315<x:ref>field-name</x:ref> = token
5316<x:ref>field-value</x:ref> = *( field-content / obs-fold )
5317<x:ref>fragment</x:ref> = &lt;fragment, defined in [RFC3986], Section 3.5&gt;
5318
5319<x:ref>header-field</x:ref> = field-name ":" OWS field-value OWS
5320<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ] [ "#"
5321 fragment ]
5322<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ] [ "#"
5323 fragment ]
5324
5325<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
5326
5327<x:ref>message-body</x:ref> = *OCTET
5328<x:ref>method</x:ref> = token
5329
5330<x:ref>obs-fold</x:ref> = CRLF ( SP / HTAB )
5331<x:ref>obs-text</x:ref> = %x80-FF
5332<x:ref>origin-form</x:ref> = absolute-path [ "?" query ]
5333
5334<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
5335<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
5336<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
5337<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
5338<x:ref>protocol-name</x:ref> = token
5339<x:ref>protocol-version</x:ref> = token
5340<x:ref>pseudonym</x:ref> = token
5341
5342<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5343 / %x5D-7E ; ']'-'~'
5344 / obs-text
5345<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5346<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5347<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5348
5349<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5350<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5351<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5352<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5353<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5354<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5355<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5356 asterisk-form
5357
5358<x:ref>segment</x:ref> = &lt;segment, defined in [RFC3986], Section 3.3&gt;
5359<x:ref>special</x:ref> = "(" / ")" / "&lt;" / "&gt;" / "@" / "," / ";" / ":" / "\" /
5360 DQUOTE / "/" / "[" / "]" / "?" / "=" / "{" / "}"
5361<x:ref>start-line</x:ref> = request-line / status-line
5362<x:ref>status-code</x:ref> = 3DIGIT
5363<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5364
5365<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5366<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5367<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5368 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5369<x:ref>token</x:ref> = 1*tchar
5370<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5371<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5372 transfer-extension
5373<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5374<x:ref>transfer-parameter</x:ref> = token BWS "=" BWS ( token / quoted-string )
5375
5376<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5377</artwork>
5378</figure>
5379</section>
5380<?ENDINC p1-messaging.abnf-appendix ?>
5381
5382<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5383
5384<section title="Since RFC 2616">
5385<t>
5386  Changes up to the IETF Last Call draft are summarized
5387  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-24#appendix-C"/>.
5388</t>
5389</section>
5390
5391<section title="Since draft-ietf-httpbis-p1-messaging-24" anchor="changes.since.24">
5392<t>
5393  Closed issues:
5394  <list style="symbols">
5395    <t>
5396      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/502"/>:
5397      "APPSDIR review of draft-ietf-httpbis-p1-messaging-24"
5398    </t>
5399    <t>
5400      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/507"/>:
5401      "integer value parsing"
5402    </t>
5403    <t>
5404      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/517"/>:
5405      "move IANA registrations to correct draft"
5406    </t>
5407  </list>
5408</t>
5409</section>
5410
5411<section title="Since draft-ietf-httpbis-p1-messaging-25" anchor="changes.since.25">
5412<t>
5413  Closed issues:
5414  <list style="symbols">
5415    <t>
5416      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/526"/>:
5417      "check media type registration templates"
5418    </t>
5419    <t>
5420      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/528"/>:
5421      "Redundant rule quoted-str-nf"
5422    </t>
5423    <t>
5424      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/541"/>:
5425      "use of 'word' ABNF production"
5426    </t>
5427  </list>
5428</t>
5429</section>
5430</section>
5431
5432</back>
5433</rfc>
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