source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2480

Last change on this file since 2480 was 2480, checked in by julian.reschke@…, 7 years ago

clarify integer parsing requirement (see #507)

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 235.7 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 "November">
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.24"/>.
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 HTTP 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  <x:anchor-alias value="word"/>
1445   Many HTTP header field values consist of words (token or quoted-string)
1446   separated by whitespace or special characters.
1447</t>
1448<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="word"/><iref primary="true" item="Grammar" subitem="token"/><iref primary="true" item="Grammar" subitem="tchar"/><iref primary="true" item="Grammar" subitem="special"><!--unused production--></iref>
1449  <x:ref>word</x:ref>           = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1450
1451  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1452<!--
1453  IMPORTANT: when editing "tchar" make sure that "special" is updated accordingly!!!
1454 -->
1455  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1456                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1457                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1458                 ; any <x:ref>VCHAR</x:ref>, except <x:ref>special</x:ref>
1459
1460  <x:ref>special</x:ref>        = "(" / ")" / "&lt;" / ">" / "@" / ","
1461                 / ";" / ":" / "\" / DQUOTE / "/" / "["
1462                 / "]" / "?" / "=" / "{" / "}"
1463</artwork></figure>
1464<t anchor="rule.quoted-string">
1465  <x:anchor-alias value="quoted-string"/>
1466  <x:anchor-alias value="qdtext"/>
1467  <x:anchor-alias value="obs-text"/>
1468   A string of text is parsed as a single word if it is quoted using
1469   double-quote marks.
1470</t>
1471<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"/>
1472  <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>
1473  <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>
1474  <x:ref>obs-text</x:ref>       = %x80-FF
1475</artwork></figure>
1476<t anchor="rule.quoted-pair">
1477  <x:anchor-alias value="quoted-pair"/>
1478   The backslash octet ("\") can be used as a single-octet
1479   quoting mechanism within quoted-string constructs:
1480</t>
1481<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1482  <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> )
1483</artwork></figure>
1484<t>
1485   Recipients that process the value of a quoted-string &MUST; handle a
1486   quoted-pair as if it were replaced by the octet following the backslash.
1487</t>
1488<t>
1489   A sender &SHOULD-NOT; generate a quoted-pair in a quoted-string except where
1490   necessary to quote DQUOTE and backslash octets occurring within that string.
1491</t>
1492<t anchor="rule.comment">
1493  <x:anchor-alias value="comment"/>
1494  <x:anchor-alias value="ctext"/>
1495   Comments can be included in some HTTP header fields by surrounding
1496   the comment text with parentheses. Comments are only allowed in
1497   fields containing "comment" as part of their field value definition.
1498</t>
1499<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1500  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-cpair</x:ref> / <x:ref>comment</x:ref> ) ")"
1501  <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>
1502</artwork></figure>
1503<t anchor="rule.quoted-cpair">
1504  <x:anchor-alias value="quoted-cpair"/>
1505   The backslash octet ("\") can be used as a single-octet
1506   quoting mechanism within comment constructs:
1507</t>
1508<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-cpair"/>
1509  <x:ref>quoted-cpair</x:ref>   = "\" ( <x:ref>HTAB</x:ref> / <x:ref>SP</x:ref> / <x:ref>VCHAR</x:ref> / <x:ref>obs-text</x:ref> )
1510</artwork></figure>
1511<t>
1512   A sender &SHOULD-NOT; escape octets in comments that do not require escaping
1513   (i.e., other than the backslash octet "\" and the parentheses "(" and ")").
1514</t>
1515</section>
1516
1517</section>
1518
1519<section title="Message Body" anchor="message.body">
1520  <x:anchor-alias value="message-body"/>
1521<t>
1522   The message body (if any) of an HTTP message is used to carry the
1523   payload body of that request or response.  The message body is
1524   identical to the payload body unless a transfer coding has been
1525   applied, as described in <xref target="header.transfer-encoding"/>.
1526</t>
1527<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1528  <x:ref>message-body</x:ref> = *OCTET
1529</artwork></figure>
1530<t>
1531   The rules for when a message body is allowed in a message differ for
1532   requests and responses.
1533</t>
1534<t>
1535   The presence of a message body in a request is signaled by a
1536   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1537   field. Request message framing is independent of method semantics,
1538   even if the method does not define any use for a message body.
1539</t>
1540<t>
1541   The presence of a message body in a response depends on both
1542   the request method to which it is responding and the response
1543   status code (<xref target="status.line"/>).
1544   Responses to the HEAD request method never include a message body
1545   because the associated response header fields (e.g.,
1546   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1547   if present, indicate only what their values would have been if the request
1548   method had been GET (&HEAD;).
1549   <x:ref>2xx (Successful)</x:ref> responses to CONNECT switch to tunnel
1550   mode instead of having a message body (&CONNECT;).
1551   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1552   <x:ref>304 (Not Modified)</x:ref> responses do not include a message body.
1553   All other responses do include a message body, although the body
1554   might be of zero length.
1555</t>
1556
1557<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1558  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1559  <iref item="chunked (Coding Format)"/>
1560  <x:anchor-alias value="Transfer-Encoding"/>
1561<t>
1562   The Transfer-Encoding header field lists the transfer coding names
1563   corresponding to the sequence of transfer codings that have been
1564   (or will be) applied to the payload body in order to form the message body.
1565   Transfer codings are defined in <xref target="transfer.codings"/>.
1566</t>
1567<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1568  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1569</artwork></figure>
1570<t>
1571   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1572   MIME, which was designed to enable safe transport of binary data over a
1573   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1574   However, safe transport has a different focus for an 8bit-clean transfer
1575   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1576   accurately delimit a dynamically generated payload and to distinguish
1577   payload encodings that are only applied for transport efficiency or
1578   security from those that are characteristics of the selected resource.
1579</t>
1580<t>
1581   A recipient &MUST; be able to parse the chunked transfer coding
1582   (<xref target="chunked.encoding"/>) because it plays a crucial role in
1583   framing messages when the payload body size is not known in advance.
1584   A sender &MUST-NOT; apply chunked more than once to a message body
1585   (i.e., chunking an already chunked message is not allowed).
1586   If any transfer coding other than chunked is applied to a request payload
1587   body, the sender &MUST; apply chunked as the final transfer coding to
1588   ensure that the message is properly framed.
1589   If any transfer coding other than chunked is applied to a response payload
1590   body, the sender &MUST; either apply chunked as the final transfer coding
1591   or terminate the message by closing the connection.
1592</t>
1593<figure><preamble>
1594   For example,
1595</preamble><artwork type="example">
1596  Transfer-Encoding: gzip, chunked
1597</artwork><postamble>
1598   indicates that the payload body has been compressed using the gzip
1599   coding and then chunked using the chunked coding while forming the
1600   message body.
1601</postamble></figure>
1602<t>
1603   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1604   Transfer-Encoding is a property of the message, not of the representation, and
1605   any recipient along the request/response chain &MAY; decode the received
1606   transfer coding(s) or apply additional transfer coding(s) to the message
1607   body, assuming that corresponding changes are made to the Transfer-Encoding
1608   field-value. Additional information about the encoding parameters &MAY; be
1609   provided by other header fields not defined by this specification.
1610</t>
1611<t>
1612   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1613   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1614   neither of which includes a message body,
1615   to indicate that the origin server would have applied a transfer coding
1616   to the message body if the request had been an unconditional GET.
1617   This indication is not required, however, because any recipient on
1618   the response chain (including the origin server) can remove transfer
1619   codings when they are not needed.
1620</t>
1621<t>
1622   A server &MUST-NOT; send a Transfer-Encoding header field in any response
1623   with a status code of
1624   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1625   A server &MUST-NOT; send a Transfer-Encoding header field in any
1626   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1627</t>
1628<t>
1629   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1630   implementations advertising only HTTP/1.0 support will not understand
1631   how to process a transfer-encoded payload.
1632   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1633   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1634   might be in the form of specific user configuration or by remembering the
1635   version of a prior received response.
1636   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1637   the corresponding request indicates HTTP/1.1 (or later).
1638</t>
1639<t>
1640   A server that receives a request message with a transfer coding it does
1641   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref>.
1642</t>
1643</section>
1644
1645<section title="Content-Length" anchor="header.content-length">
1646  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1647  <x:anchor-alias value="Content-Length"/>
1648<t>
1649   When a message does not have a <x:ref>Transfer-Encoding</x:ref> header
1650   field, a Content-Length header field can provide the anticipated size,
1651   as a decimal number of octets, for a potential payload body.
1652   For messages that do include a payload body, the Content-Length field-value
1653   provides the framing information necessary for determining where the body
1654   (and message) ends.  For messages that do not include a payload body, the
1655   Content-Length indicates the size of the selected representation
1656   (&representation;).
1657</t>
1658<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1659  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1660</artwork></figure>
1661<t>
1662   An example is
1663</t>
1664<figure><artwork type="example">
1665  Content-Length: 3495
1666</artwork></figure>
1667<t>
1668   A sender &MUST-NOT; send a Content-Length header field in any message that
1669   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1670</t>
1671<t>
1672   A user agent &SHOULD; send a Content-Length in a request message when no
1673   <x:ref>Transfer-Encoding</x:ref> is sent and the request method defines
1674   a meaning for an enclosed payload body. For example, a Content-Length
1675   header field is normally sent in a POST request even when the value is
1676   0 (indicating an empty payload body).  A user agent &SHOULD-NOT; send a
1677   Content-Length header field when the request message does not contain a
1678   payload body and the method semantics do not anticipate such a body.
1679</t>
1680<t>
1681   A server &MAY; send a Content-Length header field in a response to a HEAD
1682   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1683   response unless its field-value equals the decimal number of octets that
1684   would have been sent in the payload body of a response if the same
1685   request had used the GET method.
1686</t>
1687<t>
1688   A server &MAY; send a Content-Length header field in a
1689   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1690   (&status-304;); a server &MUST-NOT; send Content-Length in such a
1691   response unless its field-value equals the decimal number of octets that
1692   would have been sent in the payload body of a <x:ref>200 (OK)</x:ref>
1693   response to the same request.
1694</t>
1695<t>
1696   A server &MUST-NOT; send a Content-Length header field in any response
1697   with a status code of
1698   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1699   A server &MUST-NOT; send a Content-Length header field in any
1700   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1701</t>
1702<t>
1703   Aside from the cases defined above, in the absence of Transfer-Encoding,
1704   an origin server &SHOULD; send a Content-Length header field when the
1705   payload body size is known prior to sending the complete header section.
1706   This will allow downstream recipients to measure transfer progress,
1707   know when a received message is complete, and potentially reuse the
1708   connection for additional requests.
1709</t>
1710<t>
1711   Any Content-Length field value greater than or equal to zero is valid.
1712   Since there is no predefined limit to the length of a payload, a
1713   recipient &MUST; anticipate potentially large decimal numerals and
1714   prevent parsing errors due to integer conversion overflows
1715   (<xref target="attack.protocol.element.size.overflows"/>).
1716</t>
1717<t>
1718   If a message is received that has multiple Content-Length header fields
1719   with field-values consisting of the same decimal value, or a single
1720   Content-Length header field with a field value containing a list of
1721   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1722   duplicate Content-Length header fields have been generated or combined by an
1723   upstream message processor, then the recipient &MUST; either reject the
1724   message as invalid or replace the duplicated field-values with a single
1725   valid Content-Length field containing that decimal value prior to
1726   determining the message body length or forwarding the message.
1727</t>
1728<x:note>
1729  <t>
1730   &Note; HTTP's use of Content-Length for message framing differs
1731   significantly from the same field's use in MIME, where it is an optional
1732   field used only within the "message/external-body" media-type.
1733  </t>
1734</x:note>
1735</section>
1736
1737<section title="Message Body Length" anchor="message.body.length">
1738  <iref item="chunked (Coding Format)"/>
1739<t>
1740   The length of a message body is determined by one of the following
1741   (in order of precedence):
1742</t>
1743<t>
1744  <list style="numbers">
1745    <x:lt><t>
1746     Any response to a HEAD request and any response with a
1747     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1748     <x:ref>304 (Not Modified)</x:ref> status code is always
1749     terminated by the first empty line after the header fields, regardless of
1750     the header fields present in the message, and thus cannot contain a
1751     message body.
1752    </t></x:lt>
1753    <x:lt><t>
1754     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1755     connection will become a tunnel immediately after the empty line that
1756     concludes the header fields.  A client &MUST; ignore any
1757     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1758     fields received in such a message.
1759    </t></x:lt>
1760    <x:lt><t>
1761     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1762     and the chunked transfer coding (<xref target="chunked.encoding"/>)
1763     is the final encoding, the message body length is determined by reading
1764     and decoding the chunked data until the transfer coding indicates the
1765     data is complete.
1766    </t>
1767    <t>
1768     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1769     response and the chunked transfer coding is not the final encoding, the
1770     message body length is determined by reading the connection until it is
1771     closed by the server.
1772     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a request and the
1773     chunked transfer coding is not the final encoding, the message body
1774     length cannot be determined reliably; the server &MUST; respond with
1775     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1776    </t>
1777    <t>
1778     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1779     and a <x:ref>Content-Length</x:ref> header field, the Transfer-Encoding
1780     overrides the Content-Length. Such a message might indicate an attempt
1781     to perform request or response smuggling (bypass of security-related
1782     checks on message routing or content) and thus ought to be handled as
1783     an error.  A sender &MUST; remove the received Content-Length field
1784     prior to forwarding such a message downstream.
1785    </t></x:lt>
1786    <x:lt><t>
1787     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1788     either multiple <x:ref>Content-Length</x:ref> header fields having
1789     differing field-values or a single Content-Length header field having an
1790     invalid value, then the message framing is invalid and
1791     the recipient &MUST; treat it as an unrecoverable error to prevent
1792     request or response smuggling.
1793     If this is a request message, the server &MUST; respond with
1794     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1795     If this is a response message received by a proxy,
1796     the proxy &MUST; close the connection to the server, discard the received
1797     response, and send a <x:ref>502 (Bad Gateway)</x:ref> response to the
1798     client.
1799     If this is a response message received by a user agent,
1800     the user agent &MUST; close the connection to the server and discard the
1801     received response.
1802    </t></x:lt>
1803    <x:lt><t>
1804     If a valid <x:ref>Content-Length</x:ref> header field is present without
1805     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1806     expected message body length in octets.
1807     If the sender closes the connection or the recipient times out before the
1808     indicated number of octets are received, the recipient &MUST; consider
1809     the message to be incomplete and close the connection.
1810    </t></x:lt>
1811    <x:lt><t>
1812     If this is a request message and none of the above are true, then the
1813     message body length is zero (no message body is present).
1814    </t></x:lt>
1815    <x:lt><t>
1816     Otherwise, this is a response message without a declared message body
1817     length, so the message body length is determined by the number of octets
1818     received prior to the server closing the connection.
1819    </t></x:lt>
1820  </list>
1821</t>
1822<t>
1823   Since there is no way to distinguish a successfully completed,
1824   close-delimited message from a partially-received message interrupted
1825   by network failure, a server &SHOULD; generate encoding or
1826   length-delimited messages whenever possible.  The close-delimiting
1827   feature exists primarily for backwards compatibility with HTTP/1.0.
1828</t>
1829<t>
1830   A server &MAY; reject a request that contains a message body but
1831   not a <x:ref>Content-Length</x:ref> by responding with
1832   <x:ref>411 (Length Required)</x:ref>.
1833</t>
1834<t>
1835   Unless a transfer coding other than chunked has been applied,
1836   a client that sends a request containing a message body &SHOULD;
1837   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1838   length is known in advance, rather than the chunked transfer coding, since some
1839   existing services respond to chunked with a <x:ref>411 (Length Required)</x:ref>
1840   status code even though they understand the chunked transfer coding.  This
1841   is typically because such services are implemented via a gateway that
1842   requires a content-length in advance of being called and the server
1843   is unable or unwilling to buffer the entire request before processing.
1844</t>
1845<t>
1846   A user agent that sends a request containing a message body &MUST; send a
1847   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1848   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1849   the form of specific user configuration or by remembering the version of a
1850   prior received response.
1851</t>
1852<t>
1853   If the final response to the last request on a connection has been
1854   completely received and there remains additional data to read, a user agent
1855   &MAY; discard the remaining data or attempt to determine if that data
1856   belongs as part of the prior response body, which might be the case if the
1857   prior message's Content-Length value is incorrect. A client &MUST-NOT;
1858   process, cache, or forward such extra data as a separate response, since
1859   such behavior would be vulnerable to cache poisoning.
1860</t>
1861</section>
1862</section>
1863
1864<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1865<t>
1866   A server that receives an incomplete request message, usually due to a
1867   canceled request or a triggered time-out exception, &MAY; send an error
1868   response prior to closing the connection.
1869</t>
1870<t>
1871   A client that receives an incomplete response message, which can occur
1872   when a connection is closed prematurely or when decoding a supposedly
1873   chunked transfer coding fails, &MUST; record the message as incomplete.
1874   Cache requirements for incomplete responses are defined in
1875   &cache-incomplete;.
1876</t>
1877<t>
1878   If a response terminates in the middle of the header section (before the
1879   empty line is received) and the status code might rely on header fields to
1880   convey the full meaning of the response, then the client cannot assume
1881   that meaning has been conveyed; the client might need to repeat the
1882   request in order to determine what action to take next.
1883</t>
1884<t>
1885   A message body that uses the chunked transfer coding is
1886   incomplete if the zero-sized chunk that terminates the encoding has not
1887   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1888   incomplete if the size of the message body received (in octets) is less than
1889   the value given by Content-Length.  A response that has neither chunked
1890   transfer coding nor Content-Length is terminated by closure of the
1891   connection, and thus is considered complete regardless of the number of
1892   message body octets received, provided that the header section was received
1893   intact.
1894</t>
1895</section>
1896
1897<section title="Message Parsing Robustness" anchor="message.robustness">
1898<t>
1899   Older HTTP/1.0 user agent implementations might send an extra CRLF
1900   after a POST request as a workaround for some early server
1901   applications that failed to read message body content that was
1902   not terminated by a line-ending. An HTTP/1.1 user agent &MUST-NOT;
1903   preface or follow a request with an extra CRLF.  If terminating
1904   the request message body with a line-ending is desired, then the
1905   user agent &MUST; count the terminating CRLF octets as part of the
1906   message body length.
1907</t>
1908<t>
1909   In the interest of robustness, a server that is expecting to receive and
1910   parse a request-line &SHOULD; ignore at least one empty line (CRLF)
1911   received prior to the request-line.
1912</t>
1913<t>
1914   Although the line terminator for the start-line and header
1915   fields is the sequence CRLF, a recipient &MAY; recognize a
1916   single LF as a line terminator and ignore any preceding CR.
1917</t>
1918<t>
1919   Although the request-line and status-line grammar rules require that each
1920   of the component elements be separated by a single SP octet, recipients
1921   &MAY; instead parse on whitespace-delimited word boundaries and, aside
1922   from the CRLF terminator, treat any form of whitespace as the SP separator
1923   while ignoring preceding or trailing whitespace;
1924   such whitespace includes one or more of the following octets:
1925   SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
1926</t>
1927<t>
1928   When a server listening only for HTTP request messages, or processing
1929   what appears from the start-line to be an HTTP request message,
1930   receives a sequence of octets that does not match the HTTP-message
1931   grammar aside from the robustness exceptions listed above, the
1932   server &SHOULD; respond with a <x:ref>400 (Bad Request)</x:ref> response. 
1933</t>
1934</section>
1935</section>
1936
1937<section title="Transfer Codings" anchor="transfer.codings">
1938  <x:anchor-alias value="transfer-coding"/>
1939  <x:anchor-alias value="transfer-extension"/>
1940<t>
1941   Transfer coding names are used to indicate an encoding
1942   transformation that has been, can be, or might need to be applied to a
1943   payload body in order to ensure "safe transport" through the network.
1944   This differs from a content coding in that the transfer coding is a
1945   property of the message rather than a property of the representation
1946   that is being transferred.
1947</t>
1948<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1949  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1950                     / "compress" ; <xref target="compress.coding"/>
1951                     / "deflate" ; <xref target="deflate.coding"/>
1952                     / "gzip" ; <xref target="gzip.coding"/>
1953                     / <x:ref>transfer-extension</x:ref>
1954  <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> )
1955</artwork></figure>
1956<t anchor="rule.parameter">
1957  <x:anchor-alias value="attribute"/>
1958  <x:anchor-alias value="transfer-parameter"/>
1959  <x:anchor-alias value="value"/>
1960   Parameters are in the form of attribute/value pairs.
1961</t>
1962<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-parameter"/><iref primary="true" item="Grammar" subitem="attribute"/><iref primary="true" item="Grammar" subitem="value"/><iref primary="true" item="Grammar" subitem="date2"/><iref primary="true" item="Grammar" subitem="date3"/>
1963  <x:ref>transfer-parameter</x:ref> = <x:ref>attribute</x:ref> <x:ref>BWS</x:ref> "=" <x:ref>BWS</x:ref> <x:ref>value</x:ref>
1964  <x:ref>attribute</x:ref>          = <x:ref>token</x:ref>
1965  <x:ref>value</x:ref>              = <x:ref>word</x:ref>
1966</artwork></figure>
1967<t>
1968   All transfer-coding names are case-insensitive and ought to be registered
1969   within the HTTP Transfer Coding registry, as defined in
1970   <xref target="transfer.coding.registry"/>.
1971   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1972   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1973   header fields.
1974</t>
1975
1976<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1977  <iref primary="true" item="chunked (Coding Format)"/>
1978  <x:anchor-alias value="chunk"/>
1979  <x:anchor-alias value="chunked-body"/>
1980  <x:anchor-alias value="chunk-data"/>
1981  <x:anchor-alias value="chunk-size"/>
1982  <x:anchor-alias value="last-chunk"/>
1983<t>
1984   The chunked transfer coding wraps the payload body in order to transfer it
1985   as a series of chunks, each with its own size indicator, followed by an
1986   &OPTIONAL; trailer containing header fields. Chunked enables content
1987   streams of unknown size to be transferred as a sequence of length-delimited
1988   buffers, which enables the sender to retain connection persistence and the
1989   recipient to know when it has received the entire message.
1990</t>
1991<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"/>
1992  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1993                   <x:ref>last-chunk</x:ref>
1994                   <x:ref>trailer-part</x:ref>
1995                   <x:ref>CRLF</x:ref>
1996 
1997  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1998                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1999  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
2000  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
2001 
2002  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
2003</artwork></figure>
2004<t>
2005   The chunk-size field is a string of hex digits indicating the size of
2006   the chunk-data in octets. The chunked transfer coding is complete when a
2007   chunk with a chunk-size of zero is received, possibly followed by a
2008   trailer, and finally terminated by an empty line.
2009</t>
2010<t>
2011   A recipient &MUST; be able to parse and decode the chunked transfer coding.
2012</t>
2013
2014<section title="Chunk Extensions" anchor="chunked.extension">
2015  <x:anchor-alias value="chunk-ext"/>
2016  <x:anchor-alias value="chunk-ext-name"/>
2017  <x:anchor-alias value="chunk-ext-val"/>
2018  <x:anchor-alias value="quoted-str-nf"/>
2019  <x:anchor-alias value="qdtext-nf"/>
2020<t>
2021   The chunked encoding allows each chunk to include zero or more chunk
2022   extensions, immediately following the <x:ref>chunk-size</x:ref>, for the
2023   sake of supplying per-chunk metadata (such as a signature or hash),
2024   mid-message control information, or randomization of message body size.
2025</t>
2026<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"/>
2027  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
2028
2029  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
2030  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-str-nf</x:ref>
2031 
2032  <x:ref>quoted-str-nf</x:ref>  = <x:ref>DQUOTE</x:ref> *( <x:ref>qdtext-nf</x:ref> / <x:ref>quoted-pair</x:ref> ) <x:ref>DQUOTE</x:ref>
2033                 ; like <x:ref>quoted-string</x:ref>, but disallowing line folding
2034  <x:ref>qdtext-nf</x:ref>      = <x:ref>HTAB</x:ref> / <x:ref>SP</x:ref> / %x21 / %x23-5B / %x5D-7E / <x:ref>obs-text</x:ref>
2035</artwork></figure>
2036<t>
2037   The chunked encoding is specific to each connection and is likely to be
2038   removed or recoded by each recipient (including intermediaries) before any
2039   higher-level application would have a chance to inspect the extensions.
2040   Hence, use of chunk extensions is generally limited to specialized HTTP
2041   services such as "long polling" (where client and server can have shared
2042   expectations regarding the use of chunk extensions) or for padding within
2043   an end-to-end secured connection.
2044</t>
2045<t>
2046   A recipient &MUST; ignore unrecognized chunk extensions.
2047   A server ought to limit the total length of chunk extensions received in a
2048   request to an amount reasonable for the services provided, in the same way
2049   that it applies length limitations and timeouts for other parts of a
2050   message, and generate an appropriate <x:ref>4xx (Client Error)</x:ref>
2051   response if that amount is exceeded.
2052</t>
2053</section>
2054
2055<section title="Chunked Trailer Part" anchor="chunked.trailer.part">
2056  <x:anchor-alias value="trailer-part"/>
2057<t>
2058   A trailer allows the sender to include additional fields at the end of a
2059   chunked message in order to supply metadata that might be dynamically
2060   generated while the message body is sent, such as a message integrity
2061   check, digital signature, or post-processing status. The trailer fields are
2062   identical to header fields, except they are sent in a chunked trailer
2063   instead of the message's header section.
2064</t>
2065<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="trailer-part"/>
2066  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
2067</artwork></figure>
2068<t>
2069   A sender &MUST-NOT; generate a trailer that contains a field which needs to
2070   be known by the recipient before it can begin processing the message body.
2071   For example, most recipients need to know the values of
2072   <x:ref>Content-Encoding</x:ref> and <x:ref>Content-Type</x:ref> in order to
2073   select a content handler, so placing those fields in a trailer would force
2074   the recipient to buffer the entire body before it could begin, greatly
2075   increasing user-perceived latency and defeating one of the main advantages
2076   of using chunked to send data streams of unknown length.
2077   A sender &MUST-NOT; generate a trailer containing a
2078   <x:ref>Transfer-Encoding</x:ref>,
2079   <x:ref>Content-Length</x:ref>, or
2080   <x:ref>Trailer</x:ref> field.
2081</t>
2082<t>
2083   A server &MUST; generate an empty trailer with the chunked transfer coding
2084   unless at least one of the following is true:
2085  <list style="numbers">
2086    <t>the request included a <x:ref>TE</x:ref> header field that indicates
2087    "trailers" is acceptable in the transfer coding of the response, as
2088    described in <xref target="header.te"/>; or,</t>
2089     
2090    <t>the trailer fields consist entirely of optional metadata and the
2091    recipient could use the message (in a manner acceptable to the generating
2092    server) without receiving that metadata. In other words, the generating
2093    server is willing to accept the possibility that the trailer fields might
2094    be silently discarded along the path to the client.</t>
2095  </list>
2096</t>
2097<t>
2098   The above requirement prevents the need for an infinite buffer when a
2099   message is being received by an HTTP/1.1 (or later) proxy and forwarded to
2100   an HTTP/1.0 recipient.
2101</t>
2102</section>
2103
2104<section title="Decoding Chunked" anchor="decoding.chunked">
2105<t>
2106   A process for decoding the chunked transfer coding
2107   can be represented in pseudo-code as:
2108</t>
2109<figure><artwork type="code">
2110  length := 0
2111  read chunk-size, chunk-ext (if any), and CRLF
2112  while (chunk-size &gt; 0) {
2113     read chunk-data and CRLF
2114     append chunk-data to decoded-body
2115     length := length + chunk-size
2116     read chunk-size, chunk-ext (if any), and CRLF
2117  }
2118  read header-field
2119  while (header-field not empty) {
2120     append header-field to existing header fields
2121     read header-field
2122  }
2123  Content-Length := length
2124  Remove "chunked" from Transfer-Encoding
2125  Remove Trailer from existing header fields
2126</artwork></figure>
2127</section>
2128</section>
2129
2130<section title="Compression Codings" anchor="compression.codings">
2131<t>
2132   The codings defined below can be used to compress the payload of a
2133   message.
2134</t>
2135
2136<section title="Compress Coding" anchor="compress.coding">
2137<iref item="compress (Coding Format)"/>
2138<t>
2139   The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding
2140   <xref target="Welch"/> that is commonly produced by the UNIX file
2141   compression program "compress".
2142   A recipient &SHOULD; consider "x-compress" to be equivalent to "compress".
2143</t>
2144</section>
2145
2146<section title="Deflate Coding" anchor="deflate.coding">
2147<iref item="deflate (Coding Format)"/>
2148<t>
2149   The "deflate" coding is a "zlib" data format <xref target="RFC1950"/>
2150   containing a "deflate" compressed data stream <xref target="RFC1951"/>
2151   that uses a combination of the Lempel-Ziv (LZ77) compression algorithm and
2152   Huffman coding.
2153</t>
2154<x:note>
2155  <t>
2156    &Note; Some incorrect implementations send the "deflate"
2157    compressed data without the zlib wrapper.
2158   </t>
2159</x:note>
2160</section>
2161
2162<section title="Gzip Coding" anchor="gzip.coding">
2163<iref item="gzip (Coding Format)"/>
2164<t>
2165   The "gzip" coding is an LZ77 coding with a 32 bit CRC that is commonly
2166   produced by the gzip file compression program <xref target="RFC1952"/>.
2167   A recipient &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2168</t>
2169</section>
2170
2171</section>
2172
2173<section title="TE" anchor="header.te">
2174  <iref primary="true" item="TE header field" x:for-anchor=""/>
2175  <x:anchor-alias value="TE"/>
2176  <x:anchor-alias value="t-codings"/>
2177  <x:anchor-alias value="t-ranking"/>
2178  <x:anchor-alias value="rank"/>
2179<t>
2180   The "TE" header field in a request indicates what transfer codings,
2181   besides chunked, the client is willing to accept in response, and
2182   whether or not the client is willing to accept trailer fields in a
2183   chunked transfer coding.
2184</t>
2185<t>
2186   The TE field-value consists of a comma-separated list of transfer coding
2187   names, each allowing for optional parameters (as described in
2188   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2189   A client &MUST-NOT; send the chunked transfer coding name in TE;
2190   chunked is always acceptable for HTTP/1.1 recipients.
2191</t>
2192<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"/>
2193  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2194  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2195  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2196  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2197             / ( "1" [ "." 0*3("0") ] )
2198</artwork></figure>
2199<t>
2200   Three examples of TE use are below.
2201</t>
2202<figure><artwork type="example">
2203  TE: deflate
2204  TE:
2205  TE: trailers, deflate;q=0.5
2206</artwork></figure>
2207<t>
2208   The presence of the keyword "trailers" indicates that the client is willing
2209   to accept trailer fields in a chunked transfer coding, as defined in
2210   <xref target="chunked.trailer.part"/>, on behalf of itself and any downstream
2211   clients. For requests from an intermediary, this implies that either:
2212   (a) all downstream clients are willing to accept trailer fields in the
2213   forwarded response; or,
2214   (b) the intermediary will attempt to buffer the response on behalf of
2215   downstream recipients.
2216   Note that HTTP/1.1 does not define any means to limit the size of a
2217   chunked response such that an intermediary can be assured of buffering the
2218   entire response.
2219</t>
2220<t>
2221   When multiple transfer codings are acceptable, the client &MAY; rank the
2222   codings by preference using a case-insensitive "q" parameter (similar to
2223   the qvalues used in content negotiation fields, &qvalue;). The rank value
2224   is a real number in the range 0 through 1, where 0.001 is the least
2225   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2226</t>
2227<t>
2228   If the TE field-value is empty or if no TE field is present, the only
2229   acceptable transfer coding is chunked. A message with no transfer coding
2230   is always acceptable.
2231</t>
2232<t>
2233   Since the TE header field only applies to the immediate connection,
2234   a sender of TE &MUST; also send a "TE" connection option within the
2235   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2236   in order to prevent the TE field from being forwarded by intermediaries
2237   that do not support its semantics.
2238</t>
2239</section>
2240
2241<section title="Trailer" anchor="header.trailer">
2242  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
2243  <x:anchor-alias value="Trailer"/>
2244<t>
2245   When a message includes a message body encoded with the chunked
2246   transfer coding and the sender desires to send metadata in the form of
2247   trailer fields at the end of the message, the sender &SHOULD; generate a
2248   <x:ref>Trailer</x:ref> header field before the message body to indicate
2249   which fields will be present in the trailers. This allows the recipient
2250   to prepare for receipt of that metadata before it starts processing the body,
2251   which is useful if the message is being streamed and the recipient wishes
2252   to confirm an integrity check on the fly.
2253</t>
2254<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
2255  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
2256</artwork></figure>
2257</section>
2258</section>
2259
2260<section title="Message Routing" anchor="message.routing">
2261<t>
2262   HTTP request message routing is determined by each client based on the
2263   target resource, the client's proxy configuration, and
2264   establishment or reuse of an inbound connection.  The corresponding
2265   response routing follows the same connection chain back to the client.
2266</t>
2267
2268<section title="Identifying a Target Resource" anchor="target-resource">
2269  <iref primary="true" item="target resource"/>
2270  <iref primary="true" item="target URI"/>
2271  <x:anchor-alias value="target resource"/>
2272  <x:anchor-alias value="target URI"/>
2273<t>
2274   HTTP is used in a wide variety of applications, ranging from
2275   general-purpose computers to home appliances.  In some cases,
2276   communication options are hard-coded in a client's configuration.
2277   However, most HTTP clients rely on the same resource identification
2278   mechanism and configuration techniques as general-purpose Web browsers.
2279</t>
2280<t>
2281   HTTP communication is initiated by a user agent for some purpose.
2282   The purpose is a combination of request semantics, which are defined in
2283   <xref target="Part2"/>, and a target resource upon which to apply those
2284   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2285   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2286   would resolve to its absolute form in order to obtain the
2287   "<x:dfn>target URI</x:dfn>".  The target URI
2288   excludes the reference's fragment component, if any,
2289   since fragment identifiers are reserved for client-side processing
2290   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2291</t>
2292</section>
2293
2294<section title="Connecting Inbound" anchor="connecting.inbound">
2295<t>
2296   Once the target URI is determined, a client needs to decide whether
2297   a network request is necessary to accomplish the desired semantics and,
2298   if so, where that request is to be directed.
2299</t>
2300<t>
2301   If the client has a cache <xref target="Part6"/> and the request can be
2302   satisfied by it, then the request is
2303   usually directed there first.
2304</t>
2305<t>
2306   If the request is not satisfied by a cache, then a typical client will
2307   check its configuration to determine whether a proxy is to be used to
2308   satisfy the request.  Proxy configuration is implementation-dependent,
2309   but is often based on URI prefix matching, selective authority matching,
2310   or both, and the proxy itself is usually identified by an "http" or
2311   "https" URI.  If a proxy is applicable, the client connects inbound by
2312   establishing (or reusing) a connection to that proxy.
2313</t>
2314<t>
2315   If no proxy is applicable, a typical client will invoke a handler routine,
2316   usually specific to the target URI's scheme, to connect directly
2317   to an authority for the target resource.  How that is accomplished is
2318   dependent on the target URI scheme and defined by its associated
2319   specification, similar to how this specification defines origin server
2320   access for resolution of the "http" (<xref target="http.uri"/>) and
2321   "https" (<xref target="https.uri"/>) schemes.
2322</t>
2323<t>
2324   HTTP requirements regarding connection management are defined in
2325   <xref target="connection.management"/>.
2326</t>
2327</section>
2328
2329<section title="Request Target" anchor="request-target">
2330<t>
2331   Once an inbound connection is obtained,
2332   the client sends an HTTP request message (<xref target="http.message"/>)
2333   with a request-target derived from the target URI.
2334   There are four distinct formats for the request-target, depending on both
2335   the method being requested and whether the request is to a proxy.
2336</t>   
2337<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"/>
2338  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2339                 / <x:ref>absolute-form</x:ref>
2340                 / <x:ref>authority-form</x:ref>
2341                 / <x:ref>asterisk-form</x:ref>
2342
2343  <x:ref>origin-form</x:ref>    = <x:ref>absolute-path</x:ref> [ "?" <x:ref>query</x:ref> ]
2344  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2345  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2346  <x:ref>asterisk-form</x:ref>  = "*"
2347</artwork></figure>
2348<t anchor="origin-form"><iref item="origin-form (of request-target)"/>
2349  <x:h>origin-form</x:h>
2350</t>
2351<t>
2352   The most common form of request-target is the <x:dfn>origin-form</x:dfn>.
2353   When making a request directly to an origin server, other than a CONNECT
2354   or server-wide OPTIONS request (as detailed below),
2355   a client &MUST; send only the absolute path and query components of
2356   the target URI as the request-target.
2357   If the target URI's path component is empty, then the client &MUST; send
2358   "/" as the path within the origin-form of request-target.
2359   A <x:ref>Host</x:ref> header field is also sent, as defined in
2360   <xref target="header.host"/>.
2361</t>
2362<t>
2363   For example, a client wishing to retrieve a representation of the resource
2364   identified as
2365</t>
2366<figure><artwork x:indent-with="  " type="example">
2367http://www.example.org/where?q=now
2368</artwork></figure>
2369<t>
2370   directly from the origin server would open (or reuse) a TCP connection
2371   to port 80 of the host "www.example.org" and send the lines:
2372</t>
2373<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2374GET /where?q=now HTTP/1.1
2375Host: www.example.org
2376</artwork></figure>
2377<t>
2378   followed by the remainder of the request message.
2379</t>
2380<t anchor="absolute-form"><iref item="absolute-form (of request-target)"/>
2381  <x:h>absolute-form</x:h>
2382</t>
2383<t>
2384   When making a request to a proxy, other than a CONNECT or server-wide
2385   OPTIONS request (as detailed below), a client &MUST; send the target URI
2386   in <x:dfn>absolute-form</x:dfn> as the request-target.
2387   The proxy is requested to either service that request from a valid cache,
2388   if possible, or make the same request on the client's behalf to either
2389   the next inbound proxy server or directly to the origin server indicated
2390   by the request-target.  Requirements on such "forwarding" of messages are
2391   defined in <xref target="message.forwarding"/>.
2392</t>
2393<t>
2394   An example absolute-form of request-line would be:
2395</t>
2396<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2397GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
2398</artwork></figure>
2399<t>
2400   To allow for transition to the absolute-form for all requests in some
2401   future version of HTTP, a server &MUST; accept the absolute-form
2402   in requests, even though HTTP/1.1 clients will only send them in requests
2403   to proxies.
2404</t>
2405<t anchor="authority-form"><iref item="authority-form (of request-target)"/>
2406  <x:h>authority-form</x:h>
2407</t>
2408<t>
2409   The <x:dfn>authority-form</x:dfn> of request-target is only used for
2410   CONNECT requests (&CONNECT;). When making a CONNECT request to establish a
2411   tunnel through one or more proxies, a client &MUST; send only the target
2412   URI's authority component (excluding any userinfo and its "@" delimiter) as
2413   the request-target. For example,
2414</t>
2415<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2416CONNECT www.example.com:80 HTTP/1.1
2417</artwork></figure>
2418<t anchor="asterisk-form"><iref item="asterisk-form (of request-target)"/>
2419  <x:h>asterisk-form</x:h>
2420</t>
2421<t>
2422   The <x:dfn>asterisk-form</x:dfn> of request-target is only used for a server-wide
2423   OPTIONS request (&OPTIONS;).  When a client wishes to request OPTIONS
2424   for the server as a whole, as opposed to a specific named resource of
2425   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2426   For example,
2427</t>
2428<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2429OPTIONS * HTTP/1.1
2430</artwork></figure>
2431<t>
2432   If a proxy receives an OPTIONS request with an absolute-form of
2433   request-target in which the URI has an empty path and no query component,
2434   then the last proxy on the request chain &MUST; send a request-target
2435   of "*" when it forwards the request to the indicated origin server.
2436</t>
2437<figure><preamble>  
2438   For example, the request
2439</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2440OPTIONS http://www.example.org:8001 HTTP/1.1
2441</artwork></figure>
2442<figure><preamble>  
2443  would be forwarded by the final proxy as
2444</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2445OPTIONS * HTTP/1.1
2446Host: www.example.org:8001
2447</artwork>
2448<postamble>
2449   after connecting to port 8001 of host "www.example.org".
2450</postamble>
2451</figure>
2452</section>
2453
2454<section title="Host" anchor="header.host">
2455  <iref primary="true" item="Host header field" x:for-anchor=""/>
2456  <x:anchor-alias value="Host"/>
2457<t>
2458   The "Host" header field in a request provides the host and port
2459   information from the target URI, enabling the origin
2460   server to distinguish among resources while servicing requests
2461   for multiple host names on a single IP address.
2462</t>
2463<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2464  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2465</artwork></figure>
2466<t>
2467   A client &MUST; send a Host header field in all HTTP/1.1 request messages.
2468   If the target URI includes an authority component, then a client &MUST;
2469   send a field-value for Host that is identical to that authority
2470   component, excluding any userinfo subcomponent and its "@" delimiter
2471   (<xref target="http.uri"/>).
2472   If the authority component is missing or undefined for the target URI,
2473   then a client &MUST; send a Host header field with an empty field-value.
2474</t>
2475<t>
2476   Since the Host field-value is critical information for handling a request,
2477   a user agent &SHOULD; generate Host as the first header field following the
2478   request-line.
2479</t>
2480<t>
2481   For example, a GET request to the origin server for
2482   &lt;http://www.example.org/pub/WWW/&gt; would begin with:
2483</t>
2484<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2485GET /pub/WWW/ HTTP/1.1
2486Host: www.example.org
2487</artwork></figure>
2488<t>
2489   A client &MUST; send a Host header field in an HTTP/1.1 request even
2490   if the request-target is in the absolute-form, since this
2491   allows the Host information to be forwarded through ancient HTTP/1.0
2492   proxies that might not have implemented Host.
2493</t>
2494<t>
2495   When a proxy receives a request with an absolute-form of
2496   request-target, the proxy &MUST; ignore the received
2497   Host header field (if any) and instead replace it with the host
2498   information of the request-target.  A proxy that forwards such a request
2499   &MUST; generate a new Host field-value based on the received
2500   request-target rather than forward the received Host field-value.
2501</t>
2502<t>
2503   Since the Host header field acts as an application-level routing
2504   mechanism, it is a frequent target for malware seeking to poison
2505   a shared cache or redirect a request to an unintended server.
2506   An interception proxy is particularly vulnerable if it relies on
2507   the Host field-value for redirecting requests to internal
2508   servers, or for use as a cache key in a shared cache, without
2509   first verifying that the intercepted connection is targeting a
2510   valid IP address for that host.
2511</t>
2512<t>
2513   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2514   to any HTTP/1.1 request message that lacks a Host header field and
2515   to any request message that contains more than one Host header field
2516   or a Host header field with an invalid field-value.
2517</t>
2518</section>
2519
2520<section title="Effective Request URI" anchor="effective.request.uri">
2521  <iref primary="true" item="effective request URI"/>
2522  <x:anchor-alias value="effective request URI"/>
2523<t>
2524   A server that receives an HTTP request message &MUST; reconstruct
2525   the user agent's original target URI, based on the pieces of information
2526   learned from the request-target, <x:ref>Host</x:ref> header field, and
2527   connection context, in order to identify the intended target resource and
2528   properly service the request. The URI derived from this reconstruction
2529   process is referred to as the "<x:dfn>effective request URI</x:dfn>".
2530</t>
2531<t>
2532   For a user agent, the effective request URI is the target URI.
2533</t>
2534<t>
2535   If the request-target is in absolute-form, then the effective request URI
2536   is the same as the request-target.  Otherwise, the effective request URI
2537   is constructed as follows.
2538</t>
2539<t>
2540   If the request is received over a TLS-secured TCP connection,
2541   then the effective request URI's scheme is "https"; otherwise, the
2542   scheme is "http".
2543</t>
2544<t>
2545   If the request-target is in authority-form, then the effective
2546   request URI's authority component is the same as the request-target.
2547   Otherwise, if a <x:ref>Host</x:ref> header field is supplied with a
2548   non-empty field-value, then the authority component is the same as the
2549   Host field-value. Otherwise, the authority component is the concatenation of
2550   the default host name configured for the server, a colon (":"), and the
2551   connection's incoming TCP port number in decimal form.
2552</t>
2553<t>
2554   If the request-target is in authority-form or asterisk-form, then the
2555   effective request URI's combined path and query component is empty.
2556   Otherwise, the combined path and query component is the same as the
2557   request-target.
2558</t>
2559<t>
2560   The components of the effective request URI, once determined as above,
2561   can be combined into absolute-URI form by concatenating the scheme,
2562   "://", authority, and combined path and query component.
2563</t>
2564<figure>
2565<preamble>
2566   Example 1: the following message received over an insecure TCP connection
2567</preamble>
2568<artwork type="example" x:indent-with="  ">
2569GET /pub/WWW/TheProject.html HTTP/1.1
2570Host: www.example.org:8080
2571</artwork>
2572</figure>
2573<figure>
2574<preamble>
2575  has an effective request URI of
2576</preamble>
2577<artwork type="example" x:indent-with="  ">
2578http://www.example.org:8080/pub/WWW/TheProject.html
2579</artwork>
2580</figure>
2581<figure>
2582<preamble>
2583   Example 2: the following message received over a TLS-secured TCP connection
2584</preamble>
2585<artwork type="example" x:indent-with="  ">
2586OPTIONS * HTTP/1.1
2587Host: www.example.org
2588</artwork>
2589</figure>
2590<figure>
2591<preamble>
2592  has an effective request URI of
2593</preamble>
2594<artwork type="example" x:indent-with="  ">
2595https://www.example.org
2596</artwork>
2597</figure>
2598<t>
2599   An origin server that does not allow resources to differ by requested
2600   host &MAY; ignore the <x:ref>Host</x:ref> field-value and instead replace it
2601   with a configured server name when constructing the effective request URI.
2602</t>
2603<t>
2604   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2605   field &MAY; attempt to use heuristics (e.g., examination of the URI path for
2606   something unique to a particular host) in order to guess the
2607   effective request URI's authority component.
2608</t>
2609</section>
2610
2611<section title="Associating a Response to a Request" anchor="associating.response.to.request">
2612<t>
2613   HTTP does not include a request identifier for associating a given
2614   request message with its corresponding one or more response messages.
2615   Hence, it relies on the order of response arrival to correspond exactly
2616   to the order in which requests are made on the same connection.
2617   More than one response message per request only occurs when one or more
2618   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a
2619   final response to the same request.
2620</t>
2621<t>
2622   A client that has more than one outstanding request on a connection &MUST;
2623   maintain a list of outstanding requests in the order sent and &MUST;
2624   associate each received response message on that connection to the highest
2625   ordered request that has not yet received a final (non-<x:ref>1xx</x:ref>)
2626   response.
2627</t>
2628</section>
2629
2630<section title="Message Forwarding" anchor="message.forwarding">
2631<t>
2632   As described in <xref target="intermediaries"/>, intermediaries can serve
2633   a variety of roles in the processing of HTTP requests and responses.
2634   Some intermediaries are used to improve performance or availability.
2635   Others are used for access control or to filter content.
2636   Since an HTTP stream has characteristics similar to a pipe-and-filter
2637   architecture, there are no inherent limits to the extent an intermediary
2638   can enhance (or interfere) with either direction of the stream.
2639</t>
2640<t>
2641   An intermediary not acting as a tunnel &MUST; implement the
2642   <x:ref>Connection</x:ref> header field, as specified in
2643   <xref target="header.connection"/>, and exclude fields from being forwarded
2644   that are only intended for the incoming connection.
2645</t>
2646<t>
2647   An intermediary &MUST-NOT; forward a message to itself unless it is
2648   protected from an infinite request loop. In general, an intermediary ought
2649   to recognize its own server names, including any aliases, local variations,
2650   or literal IP addresses, and respond to such requests directly.
2651</t>
2652
2653<section title="Via" anchor="header.via">
2654  <iref primary="true" item="Via header field" x:for-anchor=""/>
2655  <x:anchor-alias value="pseudonym"/>
2656  <x:anchor-alias value="received-by"/>
2657  <x:anchor-alias value="received-protocol"/>
2658  <x:anchor-alias value="Via"/>
2659<t>
2660   The "Via" header field indicates the presence of intermediate protocols and
2661   recipients between the user agent and the server (on requests) or between
2662   the origin server and the client (on responses), similar to the
2663   "Received" header field in email
2664   (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2665   Via can be used for tracking message forwards,
2666   avoiding request loops, and identifying the protocol capabilities of
2667   senders along the request/response chain.
2668</t>
2669<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"/>
2670  <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> ] )
2671
2672  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2673                      ; see <xref target="header.upgrade"/>
2674  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2675  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2676</artwork></figure>
2677<t>
2678   Multiple Via field values represent each proxy or gateway that has
2679   forwarded the message. Each intermediary appends its own information
2680   about how the message was received, such that the end result is ordered
2681   according to the sequence of forwarding recipients.
2682</t>
2683<t>
2684   A proxy &MUST; send an appropriate Via header field, as described below, in
2685   each message that it forwards.
2686   An HTTP-to-HTTP gateway &MUST; send an appropriate Via header field in
2687   each inbound request message and &MAY; send a Via header field in
2688   forwarded response messages.
2689</t>
2690<t>
2691   For each intermediary, the received-protocol indicates the protocol and
2692   protocol version used by the upstream sender of the message. Hence, the
2693   Via field value records the advertised protocol capabilities of the
2694   request/response chain such that they remain visible to downstream
2695   recipients; this can be useful for determining what backwards-incompatible
2696   features might be safe to use in response, or within a later request, as
2697   described in <xref target="http.version"/>. For brevity, the protocol-name
2698   is omitted when the received protocol is HTTP.
2699</t>
2700<t>
2701   The received-by field is normally the host and optional port number of a
2702   recipient server or client that subsequently forwarded the message.
2703   However, if the real host is considered to be sensitive information, a
2704   sender &MAY; replace it with a pseudonym. If a port is not provided,
2705   a recipient &MAY; interpret that as meaning it was received on the default
2706   TCP port, if any, for the received-protocol.
2707</t>
2708<t>
2709   A sender &MAY; generate comments in the Via header field to identify the
2710   software of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2711   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2712   are optional and a recipient &MAY; remove them prior to forwarding the
2713   message.
2714</t>
2715<t>
2716   For example, a request message could be sent from an HTTP/1.0 user
2717   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2718   forward the request to a public proxy at p.example.net, which completes
2719   the request by forwarding it to the origin server at www.example.com.
2720   The request received by www.example.com would then have the following
2721   Via header field:
2722</t>
2723<figure><artwork type="example">
2724  Via: 1.0 fred, 1.1 p.example.net
2725</artwork></figure>
2726<t>
2727   An intermediary used as a portal through a network firewall
2728   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2729   region unless it is explicitly enabled to do so. If not enabled, such an
2730   intermediary &SHOULD; replace each received-by host of any host behind the
2731   firewall by an appropriate pseudonym for that host.
2732</t>
2733<t>
2734   An intermediary &MAY; combine an ordered subsequence of Via header
2735   field entries into a single such entry if the entries have identical
2736   received-protocol values. For example,
2737</t>
2738<figure><artwork type="example">
2739  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2740</artwork></figure>
2741<t>
2742  could be collapsed to
2743</t>
2744<figure><artwork type="example">
2745  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2746</artwork></figure>
2747<t>
2748   A sender &SHOULD-NOT; combine multiple entries unless they are all
2749   under the same organizational control and the hosts have already been
2750   replaced by pseudonyms. A sender &MUST-NOT; combine entries that
2751   have different received-protocol values.
2752</t>
2753</section>
2754
2755<section title="Transformations" anchor="message.transformations">
2756<t>
2757   Some intermediaries include features for transforming messages and their
2758   payloads.  A transforming proxy might, for example, convert between image
2759   formats in order to save cache space or to reduce the amount of traffic on
2760   a slow link. However, operational problems might occur when these
2761   transformations are applied to payloads intended for critical applications,
2762   such as medical imaging or scientific data analysis, particularly when
2763   integrity checks or digital signatures are used to ensure that the payload
2764   received is identical to the original.
2765</t>
2766<t>
2767   If a proxy receives a request-target with a host name that is not a
2768   fully qualified domain name, it &MAY; add its own domain to the host name
2769   it received when forwarding the request.  A proxy &MUST-NOT; change the
2770   host name if it is a fully qualified domain name.
2771</t>
2772<t>
2773   A proxy &MUST-NOT; modify the "absolute-path" and "query" parts of the
2774   received request-target when forwarding it to the next inbound server,
2775   except as noted above to replace an empty path with "/" or "*".
2776</t>
2777<t>
2778   A proxy &MUST-NOT; modify header fields that provide information about the
2779   end points of the communication chain, the resource state, or the selected
2780   representation. A proxy &MAY; change the message body through application
2781   or removal of a transfer coding (<xref target="transfer.codings"/>).
2782</t>
2783<t>
2784   A non-transforming proxy &MUST-NOT; modify the message payload (&payload;).
2785   A transforming proxy &MUST-NOT; modify the payload of a message that
2786   contains the no-transform cache-control directive.
2787</t>
2788<t>
2789   A transforming proxy &MAY; transform the payload of a message
2790   that does not contain the no-transform cache-control directive;
2791   if the payload is transformed, the transforming proxy &MUST; add a
2792   Warning header field with the warn-code of 214 ("Transformation Applied")
2793   if one does not already appear in the message (see &header-warning;).
2794   If the payload of a <x:ref>200 (OK)</x:ref> response is transformed, the
2795   transforming proxy can also inform downstream recipients that a
2796   transformation has been applied by changing the response status code to
2797   <x:ref>203 (Non-Authoritative Information)</x:ref> (&status-203;).
2798</t>
2799</section>
2800</section>
2801</section>
2802
2803<section title="Connection Management" anchor="connection.management">
2804<t>
2805   HTTP messaging is independent of the underlying transport or
2806   session-layer connection protocol(s).  HTTP only presumes a reliable
2807   transport with in-order delivery of requests and the corresponding
2808   in-order delivery of responses.  The mapping of HTTP request and
2809   response structures onto the data units of an underlying transport
2810   protocol is outside the scope of this specification.
2811</t>
2812<t>
2813   As described in <xref target="connecting.inbound"/>, the specific
2814   connection protocols to be used for an HTTP interaction are determined by
2815   client configuration and the <x:ref>target URI</x:ref>.
2816   For example, the "http" URI scheme
2817   (<xref target="http.uri"/>) indicates a default connection of TCP
2818   over IP, with a default TCP port of 80, but the client might be
2819   configured to use a proxy via some other connection, port, or protocol.
2820</t>
2821<t>
2822   HTTP implementations are expected to engage in connection management,
2823   which includes maintaining the state of current connections,
2824   establishing a new connection or reusing an existing connection,
2825   processing messages received on a connection, detecting connection
2826   failures, and closing each connection.
2827   Most clients maintain multiple connections in parallel, including
2828   more than one connection per server endpoint.
2829   Most servers are designed to maintain thousands of concurrent connections,
2830   while controlling request queues to enable fair use and detect
2831   denial of service attacks.
2832</t>
2833
2834<section title="Connection" anchor="header.connection">
2835  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2836  <iref primary="true" item="close" x:for-anchor=""/>
2837  <x:anchor-alias value="Connection"/>
2838  <x:anchor-alias value="connection-option"/>
2839  <x:anchor-alias value="close"/>
2840<t>
2841   The "Connection" header field allows the sender to indicate desired
2842   control options for the current connection.  In order to avoid confusing
2843   downstream recipients, a proxy or gateway &MUST; remove or replace any
2844   received connection options before forwarding the message.
2845</t>
2846<t>
2847   When a header field aside from Connection is used to supply control
2848   information for or about the current connection, the sender &MUST; list
2849   the corresponding field-name within the "Connection" header field.
2850   A proxy or gateway &MUST; parse a received Connection
2851   header field before a message is forwarded and, for each
2852   connection-option in this field, remove any header field(s) from
2853   the message with the same name as the connection-option, and then
2854   remove the Connection header field itself (or replace it with the
2855   intermediary's own connection options for the forwarded message).
2856</t>
2857<t>
2858   Hence, the Connection header field provides a declarative way of
2859   distinguishing header fields that are only intended for the
2860   immediate recipient ("hop-by-hop") from those fields that are
2861   intended for all recipients on the chain ("end-to-end"), enabling the
2862   message to be self-descriptive and allowing future connection-specific
2863   extensions to be deployed without fear that they will be blindly
2864   forwarded by older intermediaries.
2865</t>
2866<t>
2867   The Connection header field's value has the following grammar:
2868</t>
2869<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2870  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2871  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2872</artwork></figure>
2873<t>
2874   Connection options are case-insensitive.
2875</t>
2876<t>
2877   A sender &MUST-NOT; send a connection option corresponding to a header
2878   field that is intended for all recipients of the payload.
2879   For example, <x:ref>Cache-Control</x:ref> is never appropriate as a
2880   connection option (&header-cache-control;).
2881</t>
2882<t>
2883   The connection options do not always correspond to a header field
2884   present in the message, since a connection-specific header field
2885   might not be needed if there are no parameters associated with a
2886   connection option. In contrast, a connection-specific header field that
2887   is received without a corresponding connection option usually indicates
2888   that the field has been improperly forwarded by an intermediary and
2889   ought to be ignored by the recipient.
2890</t>
2891<t>
2892   When defining new connection options, specification authors ought to survey
2893   existing header field names and ensure that the new connection option does
2894   not share the same name as an already deployed header field.
2895   Defining a new connection option essentially reserves that potential
2896   field-name for carrying additional information related to the
2897   connection option, since it would be unwise for senders to use
2898   that field-name for anything else.
2899</t>
2900<t>
2901   The "<x:dfn>close</x:dfn>" connection option is defined for a
2902   sender to signal that this connection will be closed after completion of
2903   the response. For example,
2904</t>
2905<figure><artwork type="example">
2906  Connection: close
2907</artwork></figure>
2908<t>
2909   in either the request or the response header fields indicates that the
2910   sender is going to close the connection after the current request/response
2911   is complete (<xref target="persistent.tear-down"/>).
2912</t>
2913<t>
2914   A client that does not support <x:ref>persistent connections</x:ref> &MUST;
2915   send the "close" connection option in every request message.
2916</t>
2917<t>
2918   A server that does not support <x:ref>persistent connections</x:ref> &MUST;
2919   send the "close" connection option in every response message that
2920   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2921</t>
2922</section>
2923
2924<section title="Establishment" anchor="persistent.establishment">
2925<t>
2926   It is beyond the scope of this specification to describe how connections
2927   are established via various transport or session-layer protocols.
2928   Each connection applies to only one transport link.
2929</t>
2930</section>
2931
2932<section title="Persistence" anchor="persistent.connections">
2933   <x:anchor-alias value="persistent connections"/>
2934<t>
2935   HTTP/1.1 defaults to the use of "<x:dfn>persistent connections</x:dfn>",
2936   allowing multiple requests and responses to be carried over a single
2937   connection. The "<x:ref>close</x:ref>" connection-option is used to signal
2938   that a connection will not persist after the current request/response.
2939   HTTP implementations &SHOULD; support persistent connections.
2940</t>
2941<t>
2942   A recipient determines whether a connection is persistent or not based on
2943   the most recently received message's protocol version and
2944   <x:ref>Connection</x:ref> header field (if any):
2945   <list style="symbols">
2946     <t>If the <x:ref>close</x:ref> connection option is present, the
2947        connection will not persist after the current response; else,</t>
2948     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2949        persist after the current response; else,</t>
2950     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2951        connection option is present, the recipient is not a proxy, and
2952        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2953        the connection will persist after the current response; otherwise,</t>
2954     <t>The connection will close after the current response.</t>
2955   </list>
2956</t>
2957<t>
2958   A server &MAY; assume that an HTTP/1.1 client intends to maintain a
2959   persistent connection until a <x:ref>close</x:ref> connection option
2960   is received in a request.
2961</t>
2962<t>
2963   A client &MAY; reuse a persistent connection until it sends or receives
2964   a <x:ref>close</x:ref> connection option or receives an HTTP/1.0 response
2965   without a "keep-alive" connection option.
2966</t>
2967<t>
2968   In order to remain persistent, all messages on a connection need to
2969   have a self-defined message length (i.e., one not defined by closure
2970   of the connection), as described in <xref target="message.body"/>.
2971   A server &MUST; read the entire request message body or close
2972   the connection after sending its response, since otherwise the
2973   remaining data on a persistent connection would be misinterpreted
2974   as the next request.  Likewise,
2975   a client &MUST; read the entire response message body if it intends
2976   to reuse the same connection for a subsequent request.
2977</t>
2978<t>
2979   A proxy server &MUST-NOT; maintain a persistent connection with an
2980   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
2981   information and discussion of the problems with the Keep-Alive header field
2982   implemented by many HTTP/1.0 clients).
2983</t>
2984<t>
2985   Clients and servers &SHOULD-NOT; assume that a persistent connection is
2986   maintained for HTTP versions less than 1.1 unless it is explicitly
2987   signaled.
2988   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
2989   for more information on backward compatibility with HTTP/1.0 clients.
2990</t>
2991
2992<section title="Retrying Requests" anchor="persistent.retrying.requests">
2993<t>
2994   Connections can be closed at any time, with or without intention.
2995   Implementations ought to anticipate the need to recover
2996   from asynchronous close events.
2997</t>
2998<t>
2999   When an inbound connection is closed prematurely, a client &MAY; open a new
3000   connection and automatically retransmit an aborted sequence of requests if
3001   all of those requests have idempotent methods (&idempotent-methods;).
3002   A proxy &MUST-NOT; automatically retry non-idempotent requests.
3003</t>
3004<t>
3005   A user agent &MUST-NOT; automatically retry a request with a non-idempotent
3006   method unless it has some means to know that the request semantics are
3007   actually idempotent, regardless of the method, or some means to detect that
3008   the original request was never applied. For example, a user agent that
3009   knows (through design or configuration) that a POST request to a given
3010   resource is safe can repeat that request automatically.
3011   Likewise, a user agent designed specifically to operate on a version
3012   control repository might be able to recover from partial failure conditions
3013   by checking the target resource revision(s) after a failed connection,
3014   reverting or fixing any changes that were partially applied, and then
3015   automatically retrying the requests that failed.
3016</t>
3017<t>
3018   A client &SHOULD-NOT; automatically retry a failed automatic retry.
3019</t>
3020</section>
3021
3022<section title="Pipelining" anchor="pipelining">
3023   <x:anchor-alias value="pipeline"/>
3024<t>
3025   A client that supports persistent connections &MAY; "<x:dfn>pipeline</x:dfn>"
3026   its requests (i.e., send multiple requests without waiting for each
3027   response). A server &MAY; process a sequence of pipelined requests in
3028   parallel if they all have safe methods (&safe-methods;), but &MUST; send
3029   the corresponding responses in the same order that the requests were
3030   received.
3031</t>
3032<t>
3033   A client that pipelines requests &SHOULD; retry unanswered requests if the
3034   connection closes before it receives all of the corresponding responses.
3035   When retrying pipelined requests after a failed connection (a connection
3036   not explicitly closed by the server in its last complete response), a
3037   client &MUST-NOT; pipeline immediately after connection establishment,
3038   since the first remaining request in the prior pipeline might have caused
3039   an error response that can be lost again if multiple requests are sent on a
3040   prematurely closed connection (see the TCP reset problem described in
3041   <xref target="persistent.tear-down"/>).
3042</t>
3043<t>
3044   Idempotent methods (&idempotent-methods;) are significant to pipelining
3045   because they can be automatically retried after a connection failure.
3046   A user agent &SHOULD-NOT; pipeline requests after a non-idempotent method,
3047   until the final response status code for that method has been received,
3048   unless the user agent has a means to detect and recover from partial
3049   failure conditions involving the pipelined sequence.
3050</t>
3051<t>
3052   An intermediary that receives pipelined requests &MAY; pipeline those
3053   requests when forwarding them inbound, since it can rely on the outbound
3054   user agent(s) to determine what requests can be safely pipelined. If the
3055   inbound connection fails before receiving a response, the pipelining
3056   intermediary &MAY; attempt to retry a sequence of requests that have yet
3057   to receive a response if the requests all have idempotent methods;
3058   otherwise, the pipelining intermediary &SHOULD; forward any received
3059   responses and then close the corresponding outbound connection(s) so that
3060   the outbound user agent(s) can recover accordingly.
3061</t>
3062</section>
3063</section>
3064   
3065<section title="Concurrency" anchor="persistent.concurrency">
3066<t>
3067   A client &SHOULD; limit the number of simultaneous open
3068   connections that it maintains to a given server.
3069</t>
3070<t>
3071   Previous revisions of HTTP gave a specific number of connections as a
3072   ceiling, but this was found to be impractical for many applications. As a
3073   result, this specification does not mandate a particular maximum number of
3074   connections, but instead encourages clients to be conservative when opening
3075   multiple connections.
3076</t>
3077<t>
3078   Multiple connections are typically used to avoid the "head-of-line
3079   blocking" problem, wherein a request that takes significant server-side
3080   processing and/or has a large payload blocks subsequent requests on the
3081   same connection. However, each connection consumes server resources.
3082   Furthermore, using multiple connections can cause undesirable side effects
3083   in congested networks.
3084</t>
3085<t>
3086   Note that servers might reject traffic that they deem abusive, including an
3087   excessive number of connections from a client.
3088</t>
3089</section>
3090
3091<section title="Failures and Time-outs" anchor="persistent.failures">
3092<t>
3093   Servers will usually have some time-out value beyond which they will
3094   no longer maintain an inactive connection. Proxy servers might make
3095   this a higher value since it is likely that the client will be making
3096   more connections through the same server. The use of persistent
3097   connections places no requirements on the length (or existence) of
3098   this time-out for either the client or the server.
3099</t>
3100<t>
3101   A client or server that wishes to time-out &SHOULD; issue a graceful close
3102   on the connection. Implementations &SHOULD; constantly monitor open
3103   connections for a received closure signal and respond to it as appropriate,
3104   since prompt closure of both sides of a connection enables allocated system
3105   resources to be reclaimed.
3106</t>
3107<t>
3108   A client, server, or proxy &MAY; close the transport connection at any
3109   time. For example, a client might have started to send a new request
3110   at the same time that the server has decided to close the "idle"
3111   connection. From the server's point of view, the connection is being
3112   closed while it was idle, but from the client's point of view, a
3113   request is in progress.
3114</t>
3115<t>
3116   A server &SHOULD; sustain persistent connections, when possible, and allow
3117   the underlying
3118   transport's flow control mechanisms to resolve temporary overloads, rather
3119   than terminate connections with the expectation that clients will retry.
3120   The latter technique can exacerbate network congestion.
3121</t>
3122<t>
3123   A client sending a message body &SHOULD; monitor
3124   the network connection for an error response while it is transmitting
3125   the request. If the client sees a response that indicates the server does
3126   not wish to receive the message body and is closing the connection, the
3127   client &SHOULD; immediately cease transmitting the body and close its side
3128   of the connection.
3129</t>
3130</section>
3131   
3132<section title="Tear-down" anchor="persistent.tear-down">
3133  <iref primary="false" item="Connection header field" x:for-anchor=""/>
3134  <iref primary="false" item="close" x:for-anchor=""/>
3135<t>
3136   The <x:ref>Connection</x:ref> header field
3137   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
3138   connection option that a sender &SHOULD; send when it wishes to close
3139   the connection after the current request/response pair.
3140</t>
3141<t>
3142   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
3143   send further requests on that connection (after the one containing
3144   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
3145   final response message corresponding to this request.
3146</t>
3147<t>
3148   A server that receives a <x:ref>close</x:ref> connection option &MUST;
3149   initiate a close of the connection (see below) after it sends the
3150   final response to the request that contained <x:ref>close</x:ref>.
3151   The server &SHOULD; send a <x:ref>close</x:ref> connection option
3152   in its final response on that connection. The server &MUST-NOT; process
3153   any further requests received on that connection.
3154</t>
3155<t>
3156   A server that sends a <x:ref>close</x:ref> connection option &MUST;
3157   initiate a close of the connection (see below) after it sends the
3158   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
3159   any further requests received on that connection.
3160</t>
3161<t>
3162   A client that receives a <x:ref>close</x:ref> connection option &MUST;
3163   cease sending requests on that connection and close the connection
3164   after reading the response message containing the close; if additional
3165   pipelined requests had been sent on the connection, the client &SHOULD-NOT;
3166   assume that they will be processed by the server.
3167</t>
3168<t>
3169   If a server performs an immediate close of a TCP connection, there is a
3170   significant risk that the client will not be able to read the last HTTP
3171   response.  If the server receives additional data from the client on a
3172   fully-closed connection, such as another request that was sent by the
3173   client before receiving the server's response, the server's TCP stack will
3174   send a reset packet to the client; unfortunately, the reset packet might
3175   erase the client's unacknowledged input buffers before they can be read
3176   and interpreted by the client's HTTP parser.
3177</t>
3178<t>
3179   To avoid the TCP reset problem, servers typically close a connection in
3180   stages. First, the server performs a half-close by closing only the write
3181   side of the read/write connection. The server then continues to read from
3182   the connection until it receives a corresponding close by the client, or
3183   until the server is reasonably certain that its own TCP stack has received
3184   the client's acknowledgement of the packet(s) containing the server's last
3185   response. Finally, the server fully closes the connection.
3186</t>
3187<t>
3188   It is unknown whether the reset problem is exclusive to TCP or might also
3189   be found in other transport connection protocols.
3190</t>
3191</section>
3192
3193<section title="Upgrade" anchor="header.upgrade">
3194  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3195  <x:anchor-alias value="Upgrade"/>
3196  <x:anchor-alias value="protocol"/>
3197  <x:anchor-alias value="protocol-name"/>
3198  <x:anchor-alias value="protocol-version"/>
3199<t>
3200   The "Upgrade" header field is intended to provide a simple mechanism
3201   for transitioning from HTTP/1.1 to some other protocol on the same
3202   connection.  A client &MAY; send a list of protocols in the Upgrade
3203   header field of a request to invite the server to switch to one or
3204   more of those protocols, in order of descending preference, before sending
3205   the final response. A server &MAY; ignore a received Upgrade header field
3206   if it wishes to continue using the current protocol on that connection.
3207</t>
3208<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3209  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3210
3211  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3212  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3213  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3214</artwork></figure>
3215<t>
3216   A server that sends a <x:ref>101 (Switching Protocols)</x:ref> response
3217   &MUST; send an Upgrade header field to indicate the new protocol(s) to
3218   which the connection is being switched; if multiple protocol layers are
3219   being switched, the sender &MUST; list the protocols in layer-ascending
3220   order. A server &MUST-NOT; switch to a protocol that was not indicated by
3221   the client in the corresponding request's Upgrade header field.
3222   A server &MAY; choose to ignore the order of preference indicated by the
3223   client and select the new protocol(s) based on other factors, such as the
3224   nature of the request or the current load on the server.
3225</t>
3226<t>
3227   A server that sends a <x:ref>426 (Upgrade Required)</x:ref> response
3228   &MUST; send an Upgrade header field to indicate the acceptable protocols,
3229   in order of descending preference.
3230</t>
3231<t>
3232   A server &MAY; send an Upgrade header field in any other response to
3233   advertise that it implements support for upgrading to the listed protocols,
3234   in order of descending preference, when appropriate for a future request.
3235</t>
3236<figure><preamble>
3237   The following is a hypothetical example sent by a client:
3238</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
3239GET /hello.txt HTTP/1.1
3240Host: www.example.com
3241Connection: upgrade
3242Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3243
3244</artwork></figure>
3245<t>
3246   Upgrade cannot be used to insist on a protocol change; its acceptance and
3247   use by the server is optional. The capabilities and nature of the
3248   application-level communication after the protocol change is entirely
3249   dependent upon the new protocol(s) chosen. However, immediately after
3250   sending the 101 response, the server is expected to continue responding to
3251   the original request as if it had received its equivalent within the new
3252   protocol (i.e., the server still has an outstanding request to satisfy
3253   after the protocol has been changed, and is expected to do so without
3254   requiring the request to be repeated).
3255</t>
3256<t>
3257   For example, if the Upgrade header field is received in a GET request
3258   and the server decides to switch protocols, it first responds
3259   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3260   then immediately follows that with the new protocol's equivalent of a
3261   response to a GET on the target resource.  This allows a connection to be
3262   upgraded to protocols with the same semantics as HTTP without the
3263   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3264   protocols unless the received message semantics can be honored by the new
3265   protocol; an OPTIONS request can be honored by any protocol.
3266</t>
3267<figure><preamble>
3268   The following is an example response to the above hypothetical request:
3269</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
3270HTTP/1.1 101 Switching Protocols
3271Connection: upgrade
3272Upgrade: HTTP/2.0
3273
3274[... data stream switches to HTTP/2.0 with an appropriate response
3275(as defined by new protocol) to the "GET /hello.txt" request ...]
3276</artwork></figure>
3277<t>
3278   When Upgrade is sent, the sender &MUST; also send a
3279   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3280   that contains an "upgrade" connection option, in order to prevent Upgrade
3281   from being accidentally forwarded by intermediaries that might not implement
3282   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3283   is received in an HTTP/1.0 request.
3284</t>
3285<t>
3286   A client cannot begin using an upgraded protocol on the connection until
3287   it has completely sent the request message (i.e., the client can't change
3288   the protocol it is sending in the middle of a message).
3289   If a server receives both Upgrade and an <x:ref>Expect</x:ref> header field
3290   with the "100-continue" expectation (&header-expect;), the
3291   server &MUST; send a <x:ref>100 (Continue)</x:ref> response before sending
3292   a <x:ref>101 (Switching Protocols)</x:ref> response.
3293</t>
3294<t>
3295   The Upgrade header field only applies to switching protocols on top of the
3296   existing connection; it cannot be used to switch the underlying connection
3297   (transport) protocol, nor to switch the existing communication to a
3298   different connection. For those purposes, it is more appropriate to use a
3299   <x:ref>3xx (Redirection)</x:ref> response (&status-3xx;).
3300</t>
3301<t>
3302   This specification only defines the protocol name "HTTP" for use by
3303   the family of Hypertext Transfer Protocols, as defined by the HTTP
3304   version rules of <xref target="http.version"/> and future updates to this
3305   specification. Additional tokens ought to be registered with IANA using the
3306   registration procedure defined in <xref target="upgrade.token.registry"/>.
3307</t>
3308</section>
3309</section>
3310
3311<section title="ABNF list extension: #rule" anchor="abnf.extension">
3312<t>
3313  A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
3314  improve readability in the definitions of some header field values.
3315</t>
3316<t>
3317  A construct "#" is defined, similar to "*", for defining comma-delimited
3318  lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
3319  at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
3320  comma (",") and optional whitespace (OWS).   
3321</t>
3322<figure><preamble>
3323  Thus, a sender &MUST; expand the list construct as follows:
3324</preamble><artwork type="example">
3325  1#element =&gt; element *( OWS "," OWS element )
3326</artwork></figure>
3327<figure><preamble>
3328  and:
3329</preamble><artwork type="example">
3330  #element =&gt; [ 1#element ]
3331</artwork></figure>
3332<figure><preamble>
3333  and for n &gt;= 1 and m &gt; 1:
3334</preamble><artwork type="example">
3335  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
3336</artwork></figure>
3337<t>
3338  For compatibility with legacy list rules, a recipient &MUST; parse and ignore
3339  a reasonable number of empty list elements: enough to handle common mistakes
3340  by senders that merge values, but not so much that they could be used as a
3341  denial of service mechanism. In other words, a recipient &MUST; expand the
3342  list construct as follows:
3343</t>
3344<figure><artwork type="example">
3345  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
3346 
3347  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
3348</artwork></figure>
3349<t>
3350  Empty elements do not contribute to the count of elements present.
3351  For example, given these ABNF productions:
3352</t>
3353<figure><artwork type="example">
3354  example-list      = 1#example-list-elmt
3355  example-list-elmt = token ; see <xref target="field.components"/>
3356</artwork></figure>
3357<t>
3358  Then the following are valid values for example-list (not including the
3359  double quotes, which are present for delimitation only):
3360</t>
3361<figure><artwork type="example">
3362  "foo,bar"
3363  "foo ,bar,"
3364  "foo , ,bar,charlie   "
3365</artwork></figure>
3366<t>
3367  In contrast, the following values would be invalid, since at least one
3368  non-empty element is required by the example-list production:
3369</t>
3370<figure><artwork type="example">
3371  ""
3372  ","
3373  ",   ,"
3374</artwork></figure>
3375<t>
3376  <xref target="collected.abnf"/> shows the collected ABNF after the list
3377  constructs have been expanded, as described above, for recipients.
3378</t>
3379</section>
3380
3381<section title="IANA Considerations" anchor="IANA.considerations">
3382
3383<section title="Header Field Registration" anchor="header.field.registration">
3384<t>
3385   HTTP header fields are registered within the Message Header Field Registry
3386   maintained at
3387   <eref target="http://www.iana.org/assignments/message-headers/"/>.
3388</t>
3389<t>
3390   This document defines the following HTTP header fields, so their
3391   associated registry entries shall be updated according to the permanent
3392   registrations below (see <xref target="BCP90"/>):
3393</t>
3394<?BEGININC p1-messaging.iana-headers ?>
3395<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3396<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3397   <ttcol>Header Field Name</ttcol>
3398   <ttcol>Protocol</ttcol>
3399   <ttcol>Status</ttcol>
3400   <ttcol>Reference</ttcol>
3401
3402   <c>Connection</c>
3403   <c>http</c>
3404   <c>standard</c>
3405   <c>
3406      <xref target="header.connection"/>
3407   </c>
3408   <c>Content-Length</c>
3409   <c>http</c>
3410   <c>standard</c>
3411   <c>
3412      <xref target="header.content-length"/>
3413   </c>
3414   <c>Host</c>
3415   <c>http</c>
3416   <c>standard</c>
3417   <c>
3418      <xref target="header.host"/>
3419   </c>
3420   <c>TE</c>
3421   <c>http</c>
3422   <c>standard</c>
3423   <c>
3424      <xref target="header.te"/>
3425   </c>
3426   <c>Trailer</c>
3427   <c>http</c>
3428   <c>standard</c>
3429   <c>
3430      <xref target="header.trailer"/>
3431   </c>
3432   <c>Transfer-Encoding</c>
3433   <c>http</c>
3434   <c>standard</c>
3435   <c>
3436      <xref target="header.transfer-encoding"/>
3437   </c>
3438   <c>Upgrade</c>
3439   <c>http</c>
3440   <c>standard</c>
3441   <c>
3442      <xref target="header.upgrade"/>
3443   </c>
3444   <c>Via</c>
3445   <c>http</c>
3446   <c>standard</c>
3447   <c>
3448      <xref target="header.via"/>
3449   </c>
3450</texttable>
3451<!--(END)-->
3452<?ENDINC p1-messaging.iana-headers ?>
3453<t>
3454   Furthermore, the header field-name "Close" shall be registered as
3455   "reserved", since using that name as an HTTP header field might
3456   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3457   header field (<xref target="header.connection"/>).
3458</t>
3459<texttable align="left" suppress-title="true">
3460   <ttcol>Header Field Name</ttcol>
3461   <ttcol>Protocol</ttcol>
3462   <ttcol>Status</ttcol>
3463   <ttcol>Reference</ttcol>
3464
3465   <c>Close</c>
3466   <c>http</c>
3467   <c>reserved</c>
3468   <c>
3469      <xref target="header.field.registration"/>
3470   </c>
3471</texttable>
3472<t>
3473   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3474</t>
3475</section>
3476
3477<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3478<t>
3479   IANA maintains the registry of URI Schemes <xref target="BCP115"/> at
3480   <eref target="http://www.iana.org/assignments/uri-schemes/"/>.
3481</t>
3482<t>
3483   This document defines the following URI schemes, so their
3484   associated registry entries shall be updated according to the permanent
3485   registrations below:
3486</t>
3487<texttable align="left" suppress-title="true">
3488   <ttcol>URI Scheme</ttcol>
3489   <ttcol>Description</ttcol>
3490   <ttcol>Reference</ttcol>
3491
3492   <c>http</c>
3493   <c>Hypertext Transfer Protocol</c>
3494   <c><xref target="http.uri"/></c>
3495
3496   <c>https</c>
3497   <c>Hypertext Transfer Protocol Secure</c>
3498   <c><xref target="https.uri"/></c>
3499</texttable>
3500</section>
3501
3502<section title="Internet Media Type Registration" anchor="internet.media.type.http">
3503<t>
3504   IANA maintains the registry of Internet media types <xref target="BCP13"/> at
3505   <eref target="http://www.iana.org/assignments/media-types"/>.
3506</t>
3507<t>
3508   This document serves as the specification for the Internet media types
3509   "message/http" and "application/http". The following is to be registered with
3510   IANA.
3511</t>
3512<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
3513<iref item="Media Type" subitem="message/http" primary="true"/>
3514<iref item="message/http Media Type" primary="true"/>
3515<t>
3516   The message/http type can be used to enclose a single HTTP request or
3517   response message, provided that it obeys the MIME restrictions for all
3518   "message" types regarding line length and encodings.
3519</t>
3520<t>
3521  <list style="hanging" x:indent="12em">
3522    <t hangText="Type name:">
3523      message
3524    </t>
3525    <t hangText="Subtype name:">
3526      http
3527    </t>
3528    <t hangText="Required parameters:">
3529      none
3530    </t>
3531    <t hangText="Optional parameters:">
3532      version, msgtype
3533      <list style="hanging">
3534        <t hangText="version:">
3535          The HTTP-version number of the enclosed message
3536          (e.g., "1.1"). If not present, the version can be
3537          determined from the first line of the body.
3538        </t>
3539        <t hangText="msgtype:">
3540          The message type &mdash; "request" or "response". If not
3541          present, the type can be determined from the first
3542          line of the body.
3543        </t>
3544      </list>
3545    </t>
3546    <t hangText="Encoding considerations:">
3547      only "7bit", "8bit", or "binary" are permitted
3548    </t>
3549    <t hangText="Security considerations:">
3550      none
3551    </t>
3552    <t hangText="Interoperability considerations:">
3553      none
3554    </t>
3555    <t hangText="Published specification:">
3556      This specification (see <xref target="internet.media.type.message.http"/>).
3557    </t>
3558    <t hangText="Applications that use this media type:">
3559    </t>
3560    <t hangText="Additional information:">
3561      <list style="hanging">
3562        <t hangText="Magic number(s):">none</t>
3563        <t hangText="File extension(s):">none</t>
3564        <t hangText="Macintosh file type code(s):">none</t>
3565      </list>
3566    </t>
3567    <t hangText="Person and email address to contact for further information:">
3568      See Authors Section.
3569    </t>
3570    <t hangText="Intended usage:">
3571      COMMON
3572    </t>
3573    <t hangText="Restrictions on usage:">
3574      none
3575    </t>
3576    <t hangText="Author:">
3577      See Authors Section.
3578    </t>
3579    <t hangText="Change controller:">
3580      IESG
3581    </t>
3582  </list>
3583</t>
3584</section>
3585<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
3586<iref item="Media Type" subitem="application/http" primary="true"/>
3587<iref item="application/http Media Type" primary="true"/>
3588<t>
3589   The application/http type can be used to enclose a pipeline of one or more
3590   HTTP request or response messages (not intermixed).
3591</t>
3592<t>
3593  <list style="hanging" x:indent="12em">
3594    <t hangText="Type name:">
3595      application
3596    </t>
3597    <t hangText="Subtype name:">
3598      http
3599    </t>
3600    <t hangText="Required parameters:">
3601      none
3602    </t>
3603    <t hangText="Optional parameters:">
3604      version, msgtype
3605      <list style="hanging">
3606        <t hangText="version:">
3607          The HTTP-version number of the enclosed messages
3608          (e.g., "1.1"). If not present, the version can be
3609          determined from the first line of the body.
3610        </t>
3611        <t hangText="msgtype:">
3612          The message type &mdash; "request" or "response". If not
3613          present, the type can be determined from the first
3614          line of the body.
3615        </t>
3616      </list>
3617    </t>
3618    <t hangText="Encoding considerations:">
3619      HTTP messages enclosed by this type
3620      are in "binary" format; use of an appropriate
3621      Content-Transfer-Encoding is required when
3622      transmitted via E-mail.
3623    </t>
3624    <t hangText="Security considerations:">
3625      none
3626    </t>
3627    <t hangText="Interoperability considerations:">
3628      none
3629    </t>
3630    <t hangText="Published specification:">
3631      This specification (see <xref target="internet.media.type.application.http"/>).
3632    </t>
3633    <t hangText="Applications that use this media type:">
3634    </t>
3635    <t hangText="Additional information:">
3636      <list style="hanging">
3637        <t hangText="Magic number(s):">none</t>
3638        <t hangText="File extension(s):">none</t>
3639        <t hangText="Macintosh file type code(s):">none</t>
3640      </list>
3641    </t>
3642    <t hangText="Person and email address to contact for further information:">
3643      See Authors Section.
3644    </t>
3645    <t hangText="Intended usage:">
3646      COMMON
3647    </t>
3648    <t hangText="Restrictions on usage:">
3649      none
3650    </t>
3651    <t hangText="Author:">
3652      See Authors Section.
3653    </t>
3654    <t hangText="Change controller:">
3655      IESG
3656    </t>
3657  </list>
3658</t>
3659</section>
3660</section>
3661
3662<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3663<t>
3664   The HTTP Transfer Coding Registry defines the name space for transfer
3665   coding names. It is maintained at <eref target="http://www.iana.org/assignments/http-parameters"/>.
3666</t>
3667
3668<section title="Procedure" anchor="transfer.coding.registry.procedure">
3669<t>
3670   Registrations &MUST; include the following fields:
3671   <list style="symbols">
3672     <t>Name</t>
3673     <t>Description</t>
3674     <t>Pointer to specification text</t>
3675   </list>
3676</t>
3677<t>
3678   Names of transfer codings &MUST-NOT; overlap with names of content codings
3679   (&content-codings;) unless the encoding transformation is identical, as
3680   is the case for the compression codings defined in
3681   <xref target="compression.codings"/>.
3682</t>
3683<t>
3684   Values to be added to this name space require IETF Review (see
3685   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3686   conform to the purpose of transfer coding defined in this specification.
3687</t>
3688<t>
3689   Use of program names for the identification of encoding formats
3690   is not desirable and is discouraged for future encodings.
3691</t>
3692</section>
3693
3694<section title="Registration" anchor="transfer.coding.registration">
3695<t>
3696   The HTTP Transfer Coding Registry shall be updated with the registrations
3697   below:
3698</t>
3699<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3700   <ttcol>Name</ttcol>
3701   <ttcol>Description</ttcol>
3702   <ttcol>Reference</ttcol>
3703   <c>chunked</c>
3704   <c>Transfer in a series of chunks</c>
3705   <c>
3706      <xref target="chunked.encoding"/>
3707   </c>
3708   <c>compress</c>
3709   <c>UNIX "compress" data format <xref target="Welch"/></c>
3710   <c>
3711      <xref target="compress.coding"/>
3712   </c>
3713   <c>deflate</c>
3714   <c>"deflate" compressed data (<xref target="RFC1951"/>) inside
3715   the "zlib" data format (<xref target="RFC1950"/>)
3716   </c>
3717   <c>
3718      <xref target="deflate.coding"/>
3719   </c>
3720   <c>gzip</c>
3721   <c>GZIP file format <xref target="RFC1952"/></c>
3722   <c>
3723      <xref target="gzip.coding"/>
3724   </c>
3725   <c>x-compress</c>
3726   <c>Deprecated (alias for compress)</c>
3727   <c>
3728      <xref target="compress.coding"/>
3729   </c>
3730   <c>x-gzip</c>
3731   <c>Deprecated (alias for gzip)</c>
3732   <c>
3733      <xref target="gzip.coding"/>
3734   </c>
3735</texttable>
3736</section>
3737</section>
3738
3739<section title="Content Coding Registration" anchor="content.coding.registration">
3740<t>
3741   IANA maintains the registry of HTTP Content Codings at
3742   <eref target="http://www.iana.org/assignments/http-parameters"/>.
3743</t>
3744<t>
3745   The HTTP Content Codings Registry shall be updated with the registrations
3746   below:
3747</t>
3748<texttable align="left" suppress-title="true" anchor="iana.content.coding.registration.table">
3749   <ttcol>Name</ttcol>
3750   <ttcol>Description</ttcol>
3751   <ttcol>Reference</ttcol>
3752   <c>compress</c>
3753   <c>UNIX "compress" data format <xref target="Welch"/></c>
3754   <c>
3755      <xref target="compress.coding"/>
3756   </c>
3757   <c>deflate</c>
3758   <c>"deflate" compressed data (<xref target="RFC1951"/>) inside
3759   the "zlib" data format (<xref target="RFC1950"/>)</c>
3760   <c>
3761      <xref target="deflate.coding"/>
3762   </c>
3763   <c>gzip</c>
3764   <c>GZIP file format <xref target="RFC1952"/></c>
3765   <c>
3766      <xref target="gzip.coding"/>
3767   </c>
3768   <c>x-compress</c>
3769   <c>Deprecated (alias for compress)</c>
3770   <c>
3771      <xref target="compress.coding"/>
3772   </c>
3773   <c>x-gzip</c>
3774   <c>Deprecated (alias for gzip)</c>
3775   <c>
3776      <xref target="gzip.coding"/>
3777   </c>
3778</texttable>
3779</section>
3780
3781<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3782<t>
3783   The HTTP Upgrade Token Registry defines the name space for protocol-name
3784   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3785   field. The registry is maintained at <eref target="http://www.iana.org/assignments/http-upgrade-tokens"/>.
3786</t>
3787
3788<section title="Procedure" anchor="upgrade.token.registry.procedure">  
3789<t>
3790   Each registered protocol name is associated with contact information
3791   and an optional set of specifications that details how the connection
3792   will be processed after it has been upgraded.
3793</t>
3794<t>
3795   Registrations happen on a "First Come First Served" basis (see
3796   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3797   following rules:
3798  <list style="numbers">
3799    <t>A protocol-name token, once registered, stays registered forever.</t>
3800    <t>The registration &MUST; name a responsible party for the
3801       registration.</t>
3802    <t>The registration &MUST; name a point of contact.</t>
3803    <t>The registration &MAY; name a set of specifications associated with
3804       that token. Such specifications need not be publicly available.</t>
3805    <t>The registration &SHOULD; name a set of expected "protocol-version"
3806       tokens associated with that token at the time of registration.</t>
3807    <t>The responsible party &MAY; change the registration at any time.
3808       The IANA will keep a record of all such changes, and make them
3809       available upon request.</t>
3810    <t>The IESG &MAY; reassign responsibility for a protocol token.
3811       This will normally only be used in the case when a
3812       responsible party cannot be contacted.</t>
3813  </list>
3814</t>
3815<t>
3816   This registration procedure for HTTP Upgrade Tokens replaces that
3817   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3818</t>
3819</section>
3820
3821<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3822<t>
3823   The "HTTP" entry in the HTTP Upgrade Token Registry shall be updated with
3824   the registration below:
3825</t>
3826<texttable align="left" suppress-title="true">
3827   <ttcol>Value</ttcol>
3828   <ttcol>Description</ttcol>
3829   <ttcol>Expected Version Tokens</ttcol>
3830   <ttcol>Reference</ttcol>
3831
3832   <c>HTTP</c>
3833   <c>Hypertext Transfer Protocol</c>
3834   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3835   <c><xref target="http.version"/></c>
3836</texttable>
3837<t>
3838   The responsible party is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3839</t>
3840</section>
3841</section>
3842
3843</section>
3844
3845<section title="Security Considerations" anchor="security.considerations">
3846<t>
3847   This section is meant to inform developers, information providers, and
3848   users of known security concerns relevant to HTTP/1.1 message syntax,
3849   parsing, and routing.
3850</t>
3851
3852<section title="DNS-related Attacks" anchor="dns.related.attacks">
3853<t>
3854   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3855   generally prone to security attacks based on the deliberate misassociation
3856   of IP addresses and DNS names not protected by DNSSEC. Clients need to be
3857   cautious in assuming the validity of an IP number/DNS name association unless
3858   the response is protected by DNSSEC (<xref target="RFC4033"/>).
3859</t>
3860</section>
3861
3862<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3863<t>
3864   By their very nature, HTTP intermediaries are men-in-the-middle, and
3865   represent an opportunity for man-in-the-middle attacks. Compromise of
3866   the systems on which the intermediaries run can result in serious security
3867   and privacy problems. Intermediaries have access to security-related
3868   information, personal information about individual users and
3869   organizations, and proprietary information belonging to users and
3870   content providers. A compromised intermediary, or an intermediary
3871   implemented or configured without regard to security and privacy
3872   considerations, might be used in the commission of a wide range of
3873   potential attacks.
3874</t>
3875<t>
3876   Intermediaries that contain a shared cache are especially vulnerable
3877   to cache poisoning attacks.
3878</t>
3879<t>
3880   Implementers need to consider the privacy and security
3881   implications of their design and coding decisions, and of the
3882   configuration options they provide to operators (especially the
3883   default configuration).
3884</t>
3885<t>
3886   Users need to be aware that intermediaries are no more trustworthy than
3887   the people who run them; HTTP itself cannot solve this problem.
3888</t>
3889</section>
3890
3891<section title="Buffer Overflows" anchor="attack.protocol.element.size.overflows">
3892<t>
3893   Because HTTP uses mostly textual, character-delimited fields, attackers can
3894   overflow buffers in implementations, and/or perform a Denial of Service
3895   against implementations that accept fields with unlimited lengths.
3896</t>
3897<t>
3898   To promote interoperability, this specification makes specific
3899   recommendations for minimum size limits on request-line
3900   (<xref target="request.line"/>)
3901   and header fields (<xref target="header.fields"/>). These are
3902   minimum recommendations, chosen to be supportable even by implementations
3903   with limited resources; it is expected that most implementations will
3904   choose substantially higher limits.
3905</t>
3906<t>
3907   This specification also provides a way for servers to reject messages that
3908   have request-targets that are too long (&status-414;) or request entities
3909   that are too large (&status-4xx;). Additional status codes related to
3910   capacity limits have been defined by extensions to HTTP
3911   <xref target="RFC6585"/>.
3912</t>
3913<t>
3914   Recipients ought to carefully limit the extent to which they read other
3915   fields, including (but not limited to) request methods, response status
3916   phrases, header field-names, and body chunks, so as to avoid denial of
3917   service attacks without impeding interoperability.
3918</t>
3919</section>
3920
3921<section title="Message Integrity" anchor="message.integrity">
3922<t>
3923   HTTP does not define a specific mechanism for ensuring message integrity,
3924   instead relying on the error-detection ability of underlying transport
3925   protocols and the use of length or chunk-delimited framing to detect
3926   completeness. Additional integrity mechanisms, such as hash functions or
3927   digital signatures applied to the content, can be selectively added to
3928   messages via extensible metadata header fields. Historically, the lack of
3929   a single integrity mechanism has been justified by the informal nature of
3930   most HTTP communication.  However, the prevalence of HTTP as an information
3931   access mechanism has resulted in its increasing use within environments
3932   where verification of message integrity is crucial.
3933</t>
3934<t>
3935   User agents are encouraged to implement configurable means for detecting
3936   and reporting failures of message integrity such that those means can be
3937   enabled within environments for which integrity is necessary. For example,
3938   a browser being used to view medical history or drug interaction
3939   information needs to indicate to the user when such information is detected
3940   by the protocol to be incomplete, expired, or corrupted during transfer.
3941   Such mechanisms might be selectively enabled via user agent extensions or
3942   the presence of message integrity metadata in a response.
3943   At a minimum, user agents ought to provide some indication that allows a
3944   user to distinguish between a complete and incomplete response message
3945   (<xref target="incomplete.messages"/>) when such verification is desired.
3946</t>
3947</section>
3948
3949<section title="Server Log Information" anchor="abuse.of.server.log.information">
3950<t>
3951   A server is in the position to save personal data about a user's requests
3952   over time, which might identify their reading patterns or subjects of
3953   interest.  In particular, log information gathered at an intermediary
3954   often contains a history of user agent interaction, across a multitude
3955   of sites, that can be traced to individual users.
3956</t>
3957<t>
3958   HTTP log information is confidential in nature; its handling is often
3959   constrained by laws and regulations.  Log information needs to be securely
3960   stored and appropriate guidelines followed for its analysis.
3961   Anonymization of personal information within individual entries helps,
3962   but is generally not sufficient to prevent real log traces from being
3963   re-identified based on correlation with other access characteristics.
3964   As such, access traces that are keyed to a specific client are unsafe to
3965   publish even if the key is pseudonymous.
3966</t>
3967<t>
3968   To minimize the risk of theft or accidental publication, log information
3969   ought to be purged of personally identifiable information, including
3970   user identifiers, IP addresses, and user-provided query parameters,
3971   as soon as that information is no longer necessary to support operational
3972   needs for security, auditing, or fraud control.
3973</t>
3974</section>
3975</section>
3976
3977<section title="Acknowledgments" anchor="acks">
3978<t>
3979   This edition of HTTP/1.1 builds on the many contributions that went into
3980   <xref target="RFC1945" format="none">RFC 1945</xref>,
3981   <xref target="RFC2068" format="none">RFC 2068</xref>,
3982   <xref target="RFC2145" format="none">RFC 2145</xref>, and
3983   <xref target="RFC2616" format="none">RFC 2616</xref>, including
3984   substantial contributions made by the previous authors, editors, and
3985   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
3986   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
3987   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
3988</t>
3989<t>
3990   Since 1999, the following contributors have helped improve the HTTP
3991   specification by reporting bugs, asking smart questions, drafting or
3992   reviewing text, and evaluating open issues:
3993</t>
3994<?BEGININC acks ?>
3995<t>Adam Barth,
3996Adam Roach,
3997Addison Phillips,
3998Adrian Chadd,
3999Adrien W. de Croy,
4000Alan Ford,
4001Alan Ruttenberg,
4002Albert Lunde,
4003Alek Storm,
4004Alex Rousskov,
4005Alexandre Morgaut,
4006Alexey Melnikov,
4007Alisha Smith,
4008Amichai Rothman,
4009Amit Klein,
4010Amos Jeffries,
4011Andreas Maier,
4012Andreas Petersson,
4013Andrei Popov,
4014Anil Sharma,
4015Anne van Kesteren,
4016Anthony Bryan,
4017Asbjorn Ulsberg,
4018Ashok Kumar,
4019Balachander Krishnamurthy,
4020Barry Leiba,
4021Ben Laurie,
4022Benjamin Carlyle,
4023Benjamin Niven-Jenkins,
4024Bil Corry,
4025Bill Burke,
4026Bjoern Hoehrmann,
4027Bob Scheifler,
4028Boris Zbarsky,
4029Brett Slatkin,
4030Brian Kell,
4031Brian McBarron,
4032Brian Pane,
4033Brian Raymor,
4034Brian Smith,
4035Bryce Nesbitt,
4036Cameron Heavon-Jones,
4037Carl Kugler,
4038Carsten Bormann,
4039Charles Fry,
4040Chris Newman,
4041Cyrus Daboo,
4042Dale Robert Anderson,
4043Dan Wing,
4044Dan Winship,
4045Daniel Stenberg,
4046Darrel Miller,
4047Dave Cridland,
4048Dave Crocker,
4049Dave Kristol,
4050Dave Thaler,
4051David Booth,
4052David Singer,
4053David W. Morris,
4054Diwakar Shetty,
4055Dmitry Kurochkin,
4056Drummond Reed,
4057Duane Wessels,
4058Edward Lee,
4059Eitan Adler,
4060Eliot Lear,
4061Emile Stephan,
4062Eran Hammer-Lahav,
4063Eric D. Williams,
4064Eric J. Bowman,
4065Eric Lawrence,
4066Eric Rescorla,
4067Erik Aronesty,
4068EungJun Yi,
4069Evan Prodromou,
4070Felix Geisendoerfer,
4071Florian Weimer,
4072Frank Ellermann,
4073Fred Akalin,
4074Fred Bohle,
4075Frederic Kayser,
4076Gabor Molnar,
4077Gabriel Montenegro,
4078Geoffrey Sneddon,
4079Gervase Markham,
4080Gili Tzabari,
4081Grahame Grieve,
4082Greg Wilkins,
4083Grzegorz Calkowski,
4084Harald Tveit Alvestrand,
4085Harry Halpin,
4086Helge Hess,
4087Henrik Nordstrom,
4088Henry S. Thompson,
4089Henry Story,
4090Herbert van de Sompel,
4091Herve Ruellan,
4092Howard Melman,
4093Hugo Haas,
4094Ian Fette,
4095Ian Hickson,
4096Ido Safruti,
4097Ilari Liusvaara,
4098Ilya Grigorik,
4099Ingo Struck,
4100J. Ross Nicoll,
4101James Cloos,
4102James H. Manger,
4103James Lacey,
4104James M. Snell,
4105Jamie Lokier,
4106Jan Algermissen,
4107Jeff Hodges (who came up with the term 'effective Request-URI'),
4108Jeff Pinner,
4109Jeff Walden,
4110Jim Luther,
4111Jitu Padhye,
4112Joe D. Williams,
4113Joe Gregorio,
4114Joe Orton,
4115John C. Klensin,
4116John C. Mallery,
4117John Cowan,
4118John Kemp,
4119John Panzer,
4120John Schneider,
4121John Stracke,
4122John Sullivan,
4123Jonas Sicking,
4124Jonathan A. Rees,
4125Jonathan Billington,
4126Jonathan Moore,
4127Jonathan Silvera,
4128Jordi Ros,
4129Joris Dobbelsteen,
4130Josh Cohen,
4131Julien Pierre,
4132Jungshik Shin,
4133Justin Chapweske,
4134Justin Erenkrantz,
4135Justin James,
4136Kalvinder Singh,
4137Karl Dubost,
4138Keith Hoffman,
4139Keith Moore,
4140Ken Murchison,
4141Koen Holtman,
4142Konstantin Voronkov,
4143Kris Zyp,
4144Leif Hedstrom,
4145Lisa Dusseault,
4146Maciej Stachowiak,
4147Manu Sporny,
4148Marc Schneider,
4149Marc Slemko,
4150Mark Baker,
4151Mark Pauley,
4152Mark Watson,
4153Markus Isomaki,
4154Markus Lanthaler,
4155Martin J. Duerst,
4156Martin Musatov,
4157Martin Nilsson,
4158Martin Thomson,
4159Matt Lynch,
4160Matthew Cox,
4161Max Clark,
4162Michael Burrows,
4163Michael Hausenblas,
4164Michael Scharf,
4165Michael Sweet,
4166Michael Tuexen,
4167Michael Welzl,
4168Mike Amundsen,
4169Mike Belshe,
4170Mike Bishop,
4171Mike Kelly,
4172Mike Schinkel,
4173Miles Sabin,
4174Murray S. Kucherawy,
4175Mykyta Yevstifeyev,
4176Nathan Rixham,
4177Nicholas Shanks,
4178Nico Williams,
4179Nicolas Alvarez,
4180Nicolas Mailhot,
4181Noah Slater,
4182Osama Mazahir,
4183Pablo Castro,
4184Pat Hayes,
4185Patrick R. McManus,
4186Paul E. Jones,
4187Paul Hoffman,
4188Paul Marquess,
4189Peter Lepeska,
4190Peter Occil,
4191Peter Saint-Andre,
4192Peter Watkins,
4193Phil Archer,
4194Philippe Mougin,
4195Phillip Hallam-Baker,
4196Piotr Dobrogost,
4197Poul-Henning Kamp,
4198Preethi Natarajan,
4199Rajeev Bector,
4200Ray Polk,
4201Reto Bachmann-Gmuer,
4202Richard Cyganiak,
4203Robby Simpson,
4204Robert Brewer,
4205Robert Collins,
4206Robert Mattson,
4207Robert O'Callahan,
4208Robert Olofsson,
4209Robert Sayre,
4210Robert Siemer,
4211Robert de Wilde,
4212Roberto Javier Godoy,
4213Roberto Peon,
4214Roland Zink,
4215Ronny Widjaja,
4216Ryan Hamilton,
4217S. Mike Dierken,
4218Salvatore Loreto,
4219Sam Johnston,
4220Sam Pullara,
4221Sam Ruby,
4222Saurabh Kulkarni,
4223Scott Lawrence (who maintained the original issues list),
4224Sean B. Palmer,
4225Sebastien Barnoud,
4226Shane McCarron,
4227Shigeki Ohtsu,
4228Stefan Eissing,
4229Stefan Tilkov,
4230Stefanos Harhalakis,
4231Stephane Bortzmeyer,
4232Stephen Farrell,
4233Stephen Ludin,
4234Stuart Williams,
4235Subbu Allamaraju,
4236Subramanian Moonesamy,
4237Sylvain Hellegouarch,
4238Tapan Divekar,
4239Tatsuhiro Tsujikawa,
4240Tatsuya Hayashi,
4241Ted Hardie,
4242Thomas Broyer,
4243Thomas Fossati,
4244Thomas Maslen,
4245Thomas Nordin,
4246Thomas Roessler,
4247Tim Bray,
4248Tim Morgan,
4249Tim Olsen,
4250Tom Zhou,
4251Travis Snoozy,
4252Tyler Close,
4253Vincent Murphy,
4254Wenbo Zhu,
4255Werner Baumann,
4256Wilbur Streett,
4257Wilfredo Sanchez Vega,
4258William A. Rowe Jr.,
4259William Chan,
4260Willy Tarreau,
4261Xiaoshu Wang,
4262Yaron Goland,
4263Yngve Nysaeter Pettersen,
4264Yoav Nir,
4265Yogesh Bang,
4266Yuchung Cheng,
4267Yutaka Oiwa,
4268Yves Lafon (long-time member of the editor team),
4269Zed A. Shaw, and
4270Zhong Yu.
4271</t>
4272<?ENDINC acks ?>
4273<t>
4274   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
4275   acknowledgements from prior revisions.
4276</t>
4277</section>
4278
4279</middle>
4280<back>
4281
4282<references title="Normative References">
4283
4284<reference anchor="Part2">
4285  <front>
4286    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
4287    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4288      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4289      <address><email>fielding@gbiv.com</email></address>
4290    </author>
4291    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4292      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4293      <address><email>julian.reschke@greenbytes.de</email></address>
4294    </author>
4295    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4296  </front>
4297  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
4298  <x:source href="p2-semantics.xml" basename="p2-semantics">
4299    <x:defines>1xx (Informational)</x:defines>
4300    <x:defines>1xx</x:defines>
4301    <x:defines>100 (Continue)</x:defines>
4302    <x:defines>101 (Switching Protocols)</x:defines>
4303    <x:defines>2xx (Successful)</x:defines>
4304    <x:defines>2xx</x:defines>
4305    <x:defines>200 (OK)</x:defines>
4306    <x:defines>203 (Non-Authoritative Information)</x:defines>
4307    <x:defines>204 (No Content)</x:defines>
4308    <x:defines>3xx (Redirection)</x:defines>
4309    <x:defines>3xx</x:defines>
4310    <x:defines>301 (Moved Permanently)</x:defines>
4311    <x:defines>4xx (Client Error)</x:defines>
4312    <x:defines>4xx</x:defines>
4313    <x:defines>400 (Bad Request)</x:defines>
4314    <x:defines>411 (Length Required)</x:defines>
4315    <x:defines>414 (URI Too Long)</x:defines>
4316    <x:defines>417 (Expectation Failed)</x:defines>
4317    <x:defines>426 (Upgrade Required)</x:defines>
4318    <x:defines>501 (Not Implemented)</x:defines>
4319    <x:defines>502 (Bad Gateway)</x:defines>
4320    <x:defines>505 (HTTP Version Not Supported)</x:defines>
4321    <x:defines>Accept-Encoding</x:defines>
4322    <x:defines>Allow</x:defines>
4323    <x:defines>Content-Encoding</x:defines>
4324    <x:defines>Content-Location</x:defines>
4325    <x:defines>Content-Type</x:defines>
4326    <x:defines>Date</x:defines>
4327    <x:defines>Expect</x:defines>
4328    <x:defines>Location</x:defines>
4329    <x:defines>Server</x:defines>
4330    <x:defines>User-Agent</x:defines>
4331  </x:source>
4332</reference>
4333
4334<reference anchor="Part4">
4335  <front>
4336    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
4337    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
4338      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4339      <address><email>fielding@gbiv.com</email></address>
4340    </author>
4341    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
4342      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4343      <address><email>julian.reschke@greenbytes.de</email></address>
4344    </author>
4345    <date month="&ID-MONTH;" year="&ID-YEAR;" />
4346  </front>
4347  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
4348  <x:source basename="p4-conditional" href="p4-conditional.xml">
4349    <x:defines>304 (Not Modified)</x:defines>
4350    <x:defines>ETag</x:defines>
4351    <x:defines>Last-Modified</x:defines>
4352  </x:source>
4353</reference>
4354
4355<reference anchor="Part5">
4356  <front>
4357    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
4358    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4359      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4360      <address><email>fielding@gbiv.com</email></address>
4361    </author>
4362    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
4363      <organization abbrev="W3C">World Wide Web Consortium</organization>
4364      <address><email>ylafon@w3.org</email></address>
4365    </author>
4366    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4367      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4368      <address><email>julian.reschke@greenbytes.de</email></address>
4369    </author>
4370    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4371  </front>
4372  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4373  <x:source href="p5-range.xml" basename="p5-range">
4374    <x:defines>Content-Range</x:defines>
4375  </x:source>
4376</reference>
4377
4378<reference anchor="Part6">
4379  <front>
4380    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4381    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4382      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4383      <address><email>fielding@gbiv.com</email></address>
4384    </author>
4385    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4386      <organization>Akamai</organization>
4387      <address><email>mnot@mnot.net</email></address>
4388    </author>
4389    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4390      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4391      <address><email>julian.reschke@greenbytes.de</email></address>
4392    </author>
4393    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4394  </front>
4395  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4396  <x:source href="p6-cache.xml" basename="p6-cache">
4397    <x:defines>Cache-Control</x:defines>
4398    <x:defines>Expires</x:defines>
4399  </x:source>
4400</reference>
4401
4402<reference anchor="Part7">
4403  <front>
4404    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4405    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4406      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4407      <address><email>fielding@gbiv.com</email></address>
4408    </author>
4409    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4410      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4411      <address><email>julian.reschke@greenbytes.de</email></address>
4412    </author>
4413    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4414  </front>
4415  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4416  <x:source href="p7-auth.xml" basename="p7-auth">
4417    <x:defines>Proxy-Authenticate</x:defines>
4418    <x:defines>Proxy-Authorization</x:defines>
4419  </x:source>
4420</reference>
4421
4422<reference anchor="RFC5234">
4423  <front>
4424    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4425    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4426      <organization>Brandenburg InternetWorking</organization>
4427      <address>
4428        <email>dcrocker@bbiw.net</email>
4429      </address> 
4430    </author>
4431    <author initials="P." surname="Overell" fullname="Paul Overell">
4432      <organization>THUS plc.</organization>
4433      <address>
4434        <email>paul.overell@thus.net</email>
4435      </address>
4436    </author>
4437    <date month="January" year="2008"/>
4438  </front>
4439  <seriesInfo name="STD" value="68"/>
4440  <seriesInfo name="RFC" value="5234"/>
4441</reference>
4442
4443<reference anchor="RFC2119">
4444  <front>
4445    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4446    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4447      <organization>Harvard University</organization>
4448      <address><email>sob@harvard.edu</email></address>
4449    </author>
4450    <date month="March" year="1997"/>
4451  </front>
4452  <seriesInfo name="BCP" value="14"/>
4453  <seriesInfo name="RFC" value="2119"/>
4454</reference>
4455
4456<reference anchor="RFC3986">
4457 <front>
4458  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4459  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4460    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4461    <address>
4462       <email>timbl@w3.org</email>
4463       <uri>http://www.w3.org/People/Berners-Lee/</uri>
4464    </address>
4465  </author>
4466  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4467    <organization abbrev="Day Software">Day Software</organization>
4468    <address>
4469      <email>fielding@gbiv.com</email>
4470      <uri>http://roy.gbiv.com/</uri>
4471    </address>
4472  </author>
4473  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4474    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4475    <address>
4476      <email>LMM@acm.org</email>
4477      <uri>http://larry.masinter.net/</uri>
4478    </address>
4479  </author>
4480  <date month='January' year='2005'></date>
4481 </front>
4482 <seriesInfo name="STD" value="66"/>
4483 <seriesInfo name="RFC" value="3986"/>
4484</reference>
4485
4486<reference anchor="RFC0793">
4487  <front>
4488    <title>Transmission Control Protocol</title>
4489    <author initials='J.' surname='Postel' fullname='Jon Postel'>
4490      <organization>University of Southern California (USC)/Information Sciences Institute</organization>
4491    </author>
4492    <date year='1981' month='September' />
4493  </front>
4494  <seriesInfo name='STD' value='7' />
4495  <seriesInfo name='RFC' value='793' />
4496</reference>
4497
4498<reference anchor="USASCII">
4499  <front>
4500    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4501    <author>
4502      <organization>American National Standards Institute</organization>
4503    </author>
4504    <date year="1986"/>
4505  </front>
4506  <seriesInfo name="ANSI" value="X3.4"/>
4507</reference>
4508
4509<reference anchor="RFC1950">
4510  <front>
4511    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4512    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4513      <organization>Aladdin Enterprises</organization>
4514      <address><email>ghost@aladdin.com</email></address>
4515    </author>
4516    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4517    <date month="May" year="1996"/>
4518  </front>
4519  <seriesInfo name="RFC" value="1950"/>
4520  <!--<annotation>
4521    RFC 1950 is an Informational RFC, thus it might be less stable than
4522    this specification. On the other hand, this downward reference was
4523    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4524    therefore it is unlikely to cause problems in practice. See also
4525    <xref target="BCP97"/>.
4526  </annotation>-->
4527</reference>
4528
4529<reference anchor="RFC1951">
4530  <front>
4531    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4532    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4533      <organization>Aladdin Enterprises</organization>
4534      <address><email>ghost@aladdin.com</email></address>
4535    </author>
4536    <date month="May" year="1996"/>
4537  </front>
4538  <seriesInfo name="RFC" value="1951"/>
4539  <!--<annotation>
4540    RFC 1951 is an Informational RFC, thus it might be less stable than
4541    this specification. On the other hand, this downward reference was
4542    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4543    therefore it is unlikely to cause problems in practice. See also
4544    <xref target="BCP97"/>.
4545  </annotation>-->
4546</reference>
4547
4548<reference anchor="RFC1952">
4549  <front>
4550    <title>GZIP file format specification version 4.3</title>
4551    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4552      <organization>Aladdin Enterprises</organization>
4553      <address><email>ghost@aladdin.com</email></address>
4554    </author>
4555    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4556      <address><email>gzip@prep.ai.mit.edu</email></address>
4557    </author>
4558    <author initials="M." surname="Adler" fullname="Mark Adler">
4559      <address><email>madler@alumni.caltech.edu</email></address>
4560    </author>
4561    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4562      <address><email>ghost@aladdin.com</email></address>
4563    </author>
4564    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4565      <address><email>randeg@alumni.rpi.edu</email></address>
4566    </author>
4567    <date month="May" year="1996"/>
4568  </front>
4569  <seriesInfo name="RFC" value="1952"/>
4570  <!--<annotation>
4571    RFC 1952 is an Informational RFC, thus it might be less stable than
4572    this specification. On the other hand, this downward reference was
4573    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4574    therefore it is unlikely to cause problems in practice. See also
4575    <xref target="BCP97"/>.
4576  </annotation>-->
4577</reference>
4578
4579<reference anchor="Welch">
4580  <front>
4581    <title>A Technique for High Performance Data Compression</title>
4582    <author initials="T.A." surname="Welch" fullname="Terry A. Welch"/>
4583    <date month="June" year="1984"/>
4584  </front>
4585  <seriesInfo name="IEEE Computer" value="17(6)"/>
4586</reference>
4587
4588</references>
4589
4590<references title="Informative References">
4591
4592<reference anchor="ISO-8859-1">
4593  <front>
4594    <title>
4595     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4596    </title>
4597    <author>
4598      <organization>International Organization for Standardization</organization>
4599    </author>
4600    <date year="1998"/>
4601  </front>
4602  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4603</reference>
4604
4605<reference anchor='RFC1919'>
4606  <front>
4607    <title>Classical versus Transparent IP Proxies</title>
4608    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4609      <address><email>mchatel@pax.eunet.ch</email></address>
4610    </author>
4611    <date year='1996' month='March' />
4612  </front>
4613  <seriesInfo name='RFC' value='1919' />
4614</reference>
4615
4616<reference anchor="RFC1945">
4617  <front>
4618    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4619    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4620      <organization>MIT, Laboratory for Computer Science</organization>
4621      <address><email>timbl@w3.org</email></address>
4622    </author>
4623    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4624      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4625      <address><email>fielding@ics.uci.edu</email></address>
4626    </author>
4627    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4628      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4629      <address><email>frystyk@w3.org</email></address>
4630    </author>
4631    <date month="May" year="1996"/>
4632  </front>
4633  <seriesInfo name="RFC" value="1945"/>
4634</reference>
4635
4636<reference anchor="RFC2045">
4637  <front>
4638    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4639    <author initials="N." surname="Freed" fullname="Ned Freed">
4640      <organization>Innosoft International, Inc.</organization>
4641      <address><email>ned@innosoft.com</email></address>
4642    </author>
4643    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4644      <organization>First Virtual Holdings</organization>
4645      <address><email>nsb@nsb.fv.com</email></address>
4646    </author>
4647    <date month="November" year="1996"/>
4648  </front>
4649  <seriesInfo name="RFC" value="2045"/>
4650</reference>
4651
4652<reference anchor="RFC2047">
4653  <front>
4654    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4655    <author initials="K." surname="Moore" fullname="Keith Moore">
4656      <organization>University of Tennessee</organization>
4657      <address><email>moore@cs.utk.edu</email></address>
4658    </author>
4659    <date month="November" year="1996"/>
4660  </front>
4661  <seriesInfo name="RFC" value="2047"/>
4662</reference>
4663
4664<reference anchor="RFC2068">
4665  <front>
4666    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4667    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
4668      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4669      <address><email>fielding@ics.uci.edu</email></address>
4670    </author>
4671    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4672      <organization>MIT Laboratory for Computer Science</organization>
4673      <address><email>jg@w3.org</email></address>
4674    </author>
4675    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4676      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4677      <address><email>mogul@wrl.dec.com</email></address>
4678    </author>
4679    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4680      <organization>MIT Laboratory for Computer Science</organization>
4681      <address><email>frystyk@w3.org</email></address>
4682    </author>
4683    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4684      <organization>MIT Laboratory for Computer Science</organization>
4685      <address><email>timbl@w3.org</email></address>
4686    </author>
4687    <date month="January" year="1997"/>
4688  </front>
4689  <seriesInfo name="RFC" value="2068"/>
4690</reference>
4691
4692<reference anchor="RFC2145">
4693  <front>
4694    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4695    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4696      <organization>Western Research Laboratory</organization>
4697      <address><email>mogul@wrl.dec.com</email></address>
4698    </author>
4699    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4700      <organization>Department of Information and Computer Science</organization>
4701      <address><email>fielding@ics.uci.edu</email></address>
4702    </author>
4703    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4704      <organization>MIT Laboratory for Computer Science</organization>
4705      <address><email>jg@w3.org</email></address>
4706    </author>
4707    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4708      <organization>W3 Consortium</organization>
4709      <address><email>frystyk@w3.org</email></address>
4710    </author>
4711    <date month="May" year="1997"/>
4712  </front>
4713  <seriesInfo name="RFC" value="2145"/>
4714</reference>
4715
4716<reference anchor="RFC2616">
4717  <front>
4718    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4719    <author initials="R." surname="Fielding" fullname="R. Fielding">
4720      <organization>University of California, Irvine</organization>
4721      <address><email>fielding@ics.uci.edu</email></address>
4722    </author>
4723    <author initials="J." surname="Gettys" fullname="J. Gettys">
4724      <organization>W3C</organization>
4725      <address><email>jg@w3.org</email></address>
4726    </author>
4727    <author initials="J." surname="Mogul" fullname="J. Mogul">
4728      <organization>Compaq Computer Corporation</organization>
4729      <address><email>mogul@wrl.dec.com</email></address>
4730    </author>
4731    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4732      <organization>MIT Laboratory for Computer Science</organization>
4733      <address><email>frystyk@w3.org</email></address>
4734    </author>
4735    <author initials="L." surname="Masinter" fullname="L. Masinter">
4736      <organization>Xerox Corporation</organization>
4737      <address><email>masinter@parc.xerox.com</email></address>
4738    </author>
4739    <author initials="P." surname="Leach" fullname="P. Leach">
4740      <organization>Microsoft Corporation</organization>
4741      <address><email>paulle@microsoft.com</email></address>
4742    </author>
4743    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4744      <organization>W3C</organization>
4745      <address><email>timbl@w3.org</email></address>
4746    </author>
4747    <date month="June" year="1999"/>
4748  </front>
4749  <seriesInfo name="RFC" value="2616"/>
4750</reference>
4751
4752<reference anchor='RFC2817'>
4753  <front>
4754    <title>Upgrading to TLS Within HTTP/1.1</title>
4755    <author initials='R.' surname='Khare' fullname='R. Khare'>
4756      <organization>4K Associates / UC Irvine</organization>
4757      <address><email>rohit@4K-associates.com</email></address>
4758    </author>
4759    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4760      <organization>Agranat Systems, Inc.</organization>
4761      <address><email>lawrence@agranat.com</email></address>
4762    </author>
4763    <date year='2000' month='May' />
4764  </front>
4765  <seriesInfo name='RFC' value='2817' />
4766</reference>
4767
4768<reference anchor='RFC2818'>
4769  <front>
4770    <title>HTTP Over TLS</title>
4771    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4772      <organization>RTFM, Inc.</organization>
4773      <address><email>ekr@rtfm.com</email></address>
4774    </author>
4775    <date year='2000' month='May' />
4776  </front>
4777  <seriesInfo name='RFC' value='2818' />
4778</reference>
4779
4780<reference anchor='RFC3040'>
4781  <front>
4782    <title>Internet Web Replication and Caching Taxonomy</title>
4783    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4784      <organization>Equinix, Inc.</organization>
4785    </author>
4786    <author initials='I.' surname='Melve' fullname='I. Melve'>
4787      <organization>UNINETT</organization>
4788    </author>
4789    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4790      <organization>CacheFlow Inc.</organization>
4791    </author>
4792    <date year='2001' month='January' />
4793  </front>
4794  <seriesInfo name='RFC' value='3040' />
4795</reference>
4796
4797<reference anchor='BCP90'>
4798  <front>
4799    <title>Registration Procedures for Message Header Fields</title>
4800    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4801      <organization>Nine by Nine</organization>
4802      <address><email>GK-IETF@ninebynine.org</email></address>
4803    </author>
4804    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4805      <organization>BEA Systems</organization>
4806      <address><email>mnot@pobox.com</email></address>
4807    </author>
4808    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4809      <organization>HP Labs</organization>
4810      <address><email>JeffMogul@acm.org</email></address>
4811    </author>
4812    <date year='2004' month='September' />
4813  </front>
4814  <seriesInfo name='BCP' value='90' />
4815  <seriesInfo name='RFC' value='3864' />
4816</reference>
4817
4818<reference anchor='RFC4033'>
4819  <front>
4820    <title>DNS Security Introduction and Requirements</title>
4821    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4822    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4823    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4824    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4825    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4826    <date year='2005' month='March' />
4827  </front>
4828  <seriesInfo name='RFC' value='4033' />
4829</reference>
4830
4831<reference anchor="BCP13">
4832  <front>
4833    <title>Media Type Specifications and Registration Procedures</title>
4834    <author initials="N." surname="Freed" fullname="Ned Freed">
4835      <organization>Oracle</organization>
4836      <address>
4837        <email>ned+ietf@mrochek.com</email>
4838      </address>
4839    </author>
4840    <author initials="J." surname="Klensin" fullname="John C. Klensin">
4841      <address>
4842        <email>john+ietf@jck.com</email>
4843      </address>
4844    </author>
4845    <author initials="T." surname="Hansen" fullname="Tony Hansen">
4846      <organization>AT&amp;T Laboratories</organization>
4847      <address>
4848        <email>tony+mtsuffix@maillennium.att.com</email>
4849      </address>
4850    </author>
4851    <date year="2013" month="January"/>
4852  </front>
4853  <seriesInfo name="BCP" value="13"/>
4854  <seriesInfo name="RFC" value="6838"/>
4855</reference>
4856
4857<reference anchor='BCP115'>
4858  <front>
4859    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4860    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4861      <organization>AT&amp;T Laboratories</organization>
4862      <address>
4863        <email>tony+urireg@maillennium.att.com</email>
4864      </address>
4865    </author>
4866    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4867      <organization>Qualcomm, Inc.</organization>
4868      <address>
4869        <email>hardie@qualcomm.com</email>
4870      </address>
4871    </author>
4872    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4873      <organization>Adobe Systems</organization>
4874      <address>
4875        <email>LMM@acm.org</email>
4876      </address>
4877    </author>
4878    <date year='2006' month='February' />
4879  </front>
4880  <seriesInfo name='BCP' value='115' />
4881  <seriesInfo name='RFC' value='4395' />
4882</reference>
4883
4884<reference anchor='RFC4559'>
4885  <front>
4886    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4887    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4888    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4889    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4890    <date year='2006' month='June' />
4891  </front>
4892  <seriesInfo name='RFC' value='4559' />
4893</reference>
4894
4895<reference anchor='RFC5226'>
4896  <front>
4897    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4898    <author initials='T.' surname='Narten' fullname='T. Narten'>
4899      <organization>IBM</organization>
4900      <address><email>narten@us.ibm.com</email></address>
4901    </author>
4902    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4903      <organization>Google</organization>
4904      <address><email>Harald@Alvestrand.no</email></address>
4905    </author>
4906    <date year='2008' month='May' />
4907  </front>
4908  <seriesInfo name='BCP' value='26' />
4909  <seriesInfo name='RFC' value='5226' />
4910</reference>
4911
4912<reference anchor='RFC5246'>
4913   <front>
4914      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4915      <author initials='T.' surname='Dierks' fullname='T. Dierks'>
4916         <organization />
4917      </author>
4918      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4919         <organization>RTFM, Inc.</organization>
4920      </author>
4921      <date year='2008' month='August' />
4922   </front>
4923   <seriesInfo name='RFC' value='5246' />
4924</reference>
4925
4926<reference anchor="RFC5322">
4927  <front>
4928    <title>Internet Message Format</title>
4929    <author initials="P." surname="Resnick" fullname="P. Resnick">
4930      <organization>Qualcomm Incorporated</organization>
4931    </author>
4932    <date year="2008" month="October"/>
4933  </front>
4934  <seriesInfo name="RFC" value="5322"/>
4935</reference>
4936
4937<reference anchor="RFC6265">
4938  <front>
4939    <title>HTTP State Management Mechanism</title>
4940    <author initials="A." surname="Barth" fullname="Adam Barth">
4941      <organization abbrev="U.C. Berkeley">
4942        University of California, Berkeley
4943      </organization>
4944      <address><email>abarth@eecs.berkeley.edu</email></address>
4945    </author>
4946    <date year="2011" month="April" />
4947  </front>
4948  <seriesInfo name="RFC" value="6265"/>
4949</reference>
4950
4951<reference anchor='RFC6585'>
4952  <front>
4953    <title>Additional HTTP Status Codes</title>
4954    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4955      <organization>Rackspace</organization>
4956    </author>
4957    <author initials='R.' surname='Fielding' fullname='R. Fielding'>
4958      <organization>Adobe</organization>
4959    </author>
4960    <date year='2012' month='April' />
4961   </front>
4962   <seriesInfo name='RFC' value='6585' />
4963</reference>
4964
4965<!--<reference anchor='BCP97'>
4966  <front>
4967    <title>Handling Normative References to Standards-Track Documents</title>
4968    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
4969      <address>
4970        <email>klensin+ietf@jck.com</email>
4971      </address>
4972    </author>
4973    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
4974      <organization>MIT</organization>
4975      <address>
4976        <email>hartmans-ietf@mit.edu</email>
4977      </address>
4978    </author>
4979    <date year='2007' month='June' />
4980  </front>
4981  <seriesInfo name='BCP' value='97' />
4982  <seriesInfo name='RFC' value='4897' />
4983</reference>-->
4984
4985<reference anchor="Kri2001" target="http://arxiv.org/abs/cs.SE/0105018">
4986  <front>
4987    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
4988    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
4989    <date year="2001" month="November"/>
4990  </front>
4991  <seriesInfo name="ACM Transactions on Internet Technology" value="1(2)"/>
4992</reference>
4993
4994</references>
4995
4996
4997<section title="HTTP Version History" anchor="compatibility">
4998<t>
4999   HTTP has been in use by the World-Wide Web global information initiative
5000   since 1990. The first version of HTTP, later referred to as HTTP/0.9,
5001   was a simple protocol for hypertext data transfer across the Internet
5002   with only a single request method (GET) and no metadata.
5003   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
5004   methods and MIME-like messaging that could include metadata about the data
5005   transferred and modifiers on the request/response semantics. However,
5006   HTTP/1.0 did not sufficiently take into consideration the effects of
5007   hierarchical proxies, caching, the need for persistent connections, or
5008   name-based virtual hosts. The proliferation of incompletely-implemented
5009   applications calling themselves "HTTP/1.0" further necessitated a
5010   protocol version change in order for two communicating applications
5011   to determine each other's true capabilities.
5012</t>
5013<t>
5014   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
5015   requirements that enable reliable implementations, adding only
5016   those new features that will either be safely ignored by an HTTP/1.0
5017   recipient or only sent when communicating with a party advertising
5018   conformance with HTTP/1.1.
5019</t>
5020<t>
5021   It is beyond the scope of a protocol specification to mandate
5022   conformance with previous versions. HTTP/1.1 was deliberately
5023   designed, however, to make supporting previous versions easy.
5024   We would expect a general-purpose HTTP/1.1 server to understand
5025   any valid request in the format of HTTP/1.0 and respond appropriately
5026   with an HTTP/1.1 message that only uses features understood (or
5027   safely ignored) by HTTP/1.0 clients.  Likewise, we would expect
5028   an HTTP/1.1 client to understand any valid HTTP/1.0 response.
5029</t>
5030<t>
5031   Since HTTP/0.9 did not support header fields in a request,
5032   there is no mechanism for it to support name-based virtual
5033   hosts (selection of resource by inspection of the <x:ref>Host</x:ref> header
5034   field).  Any server that implements name-based virtual hosts
5035   ought to disable support for HTTP/0.9.  Most requests that
5036   appear to be HTTP/0.9 are, in fact, badly constructed HTTP/1.x
5037   requests wherein a buggy client failed to properly encode
5038   linear whitespace found in a URI reference and placed in
5039   the request-target.
5040</t>
5041
5042<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
5043<t>
5044   This section summarizes major differences between versions HTTP/1.0
5045   and HTTP/1.1.
5046</t>
5047
5048<section title="Multi-homed Web Servers" anchor="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">
5049<t>
5050   The requirements that clients and servers support the <x:ref>Host</x:ref>
5051   header field (<xref target="header.host"/>), report an error if it is
5052   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
5053   are among the most important changes defined by HTTP/1.1.
5054</t>
5055<t>
5056   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
5057   addresses and servers; there was no other established mechanism for
5058   distinguishing the intended server of a request than the IP address
5059   to which that request was directed. The <x:ref>Host</x:ref> header field was
5060   introduced during the development of HTTP/1.1 and, though it was
5061   quickly implemented by most HTTP/1.0 browsers, additional requirements
5062   were placed on all HTTP/1.1 requests in order to ensure complete
5063   adoption.  At the time of this writing, most HTTP-based services
5064   are dependent upon the Host header field for targeting requests.
5065</t>
5066</section>
5067
5068<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
5069<t>
5070   In HTTP/1.0, each connection is established by the client prior to the
5071   request and closed by the server after sending the response. However, some
5072   implementations implement the explicitly negotiated ("Keep-Alive") version
5073   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
5074   target="RFC2068"/>.
5075</t>
5076<t>
5077   Some clients and servers might wish to be compatible with these previous
5078   approaches to persistent connections, by explicitly negotiating for them
5079   with a "Connection: keep-alive" request header field. However, some
5080   experimental implementations of HTTP/1.0 persistent connections are faulty;
5081   for example, if an HTTP/1.0 proxy server doesn't understand
5082   <x:ref>Connection</x:ref>, it will erroneously forward that header field
5083   to the next inbound server, which would result in a hung connection.
5084</t>
5085<t>
5086   One attempted solution was the introduction of a Proxy-Connection header
5087   field, targeted specifically at proxies. In practice, this was also
5088   unworkable, because proxies are often deployed in multiple layers, bringing
5089   about the same problem discussed above.
5090</t>
5091<t>
5092   As a result, clients are encouraged not to send the Proxy-Connection header
5093   field in any requests.
5094</t>
5095<t>
5096   Clients are also encouraged to consider the use of Connection: keep-alive
5097   in requests carefully; while they can enable persistent connections with
5098   HTTP/1.0 servers, clients using them will need to monitor the
5099   connection for "hung" requests (which indicate that the client ought stop
5100   sending the header field), and this mechanism ought not be used by clients
5101   at all when a proxy is being used.
5102</t>
5103</section>
5104
5105<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
5106<t>
5107   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
5108   (<xref target="header.transfer-encoding"/>).
5109   Transfer codings need to be decoded prior to forwarding an HTTP message
5110   over a MIME-compliant protocol.
5111</t>
5112</section>
5113
5114</section>
5115
5116<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
5117<t>
5118  HTTP's approach to error handling has been explained.
5119  (<xref target="conformance" />)
5120</t>
5121<t>
5122  The HTTP-version ABNF production has been clarified to be case-sensitive.
5123  Additionally, version numbers has been restricted to single digits, due
5124  to the fact that implementations are known to handle multi-digit version
5125  numbers incorrectly.
5126  (<xref target="http.version"/>)
5127</t>
5128<t>
5129  Userinfo (i.e., username and password) are now disallowed in HTTP and
5130  HTTPS URIs, because of security issues related to their transmission on the
5131  wire.
5132  (<xref target="http.uri" />)
5133</t>
5134<t>
5135  The HTTPS URI scheme is now defined by this specification; previously,
5136  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
5137  Furthermore, it implies end-to-end security.
5138  (<xref target="https.uri"/>)
5139</t>
5140<t>
5141  HTTP messages can be (and often are) buffered by implementations; despite
5142  it sometimes being available as a stream, HTTP is fundamentally a
5143  message-oriented protocol.
5144  Minimum supported sizes for various protocol elements have been
5145  suggested, to improve interoperability.
5146  (<xref target="http.message" />)
5147</t>
5148<t>
5149  Invalid whitespace around field-names is now required to be rejected,
5150  because accepting it represents a security vulnerability.
5151  The ABNF productions defining header fields now only list the field value.
5152  (<xref target="header.fields"/>)
5153</t>
5154<t>
5155  Rules about implicit linear whitespace between certain grammar productions
5156  have been removed; now whitespace is only allowed where specifically
5157  defined in the ABNF.
5158  (<xref target="whitespace"/>)
5159</t>
5160<t>
5161  Header fields that span multiple lines ("line folding") are deprecated.
5162  (<xref target="field.parsing" />)
5163</t>
5164<t> 
5165  The NUL octet is no longer allowed in comment and quoted-string text, and
5166  handling of backslash-escaping in them has been clarified.
5167  The quoted-pair rule no longer allows escaping control characters other than
5168  HTAB.
5169  Non-ASCII content in header fields and the reason phrase has been obsoleted
5170  and made opaque (the TEXT rule was removed).
5171  (<xref target="field.components"/>)
5172</t> 
5173<t>
5174  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
5175  handled as errors by recipients.
5176  (<xref target="header.content-length"/>)
5177</t>
5178<t>
5179  The algorithm for determining the message body length has been clarified
5180  to indicate all of the special cases (e.g., driven by methods or status
5181  codes) that affect it, and that new protocol elements cannot define such
5182  special cases.
5183  CONNECT is a new, special case in determining message body length.
5184  "multipart/byteranges" is no longer a way of determining message body length
5185  detection.
5186  (<xref target="message.body.length"/>)
5187</t>
5188<t>
5189  The "identity" transfer coding token has been removed.
5190  (Sections <xref format="counter" target="message.body"/> and
5191  <xref format="counter" target="transfer.codings"/>)
5192</t>
5193<t>
5194  Chunk length does not include the count of the octets in the
5195  chunk header and trailer.
5196  Line folding in chunk extensions is  disallowed.
5197  (<xref target="chunked.encoding"/>)
5198</t>
5199<t>
5200  The meaning of the "deflate" content coding has been clarified.
5201  (<xref target="deflate.coding" />)
5202</t>
5203<t>
5204  The segment + query components of RFC 3986 have been used to define the
5205  request-target, instead of abs_path from RFC 1808.
5206  The asterisk-form of the request-target is only allowed with the OPTIONS
5207  method.
5208  (<xref target="request-target"/>)
5209</t>
5210<t>
5211  The term "Effective Request URI" has been introduced.
5212  (<xref target="effective.request.uri" />)
5213</t>
5214<t>
5215  Gateways do not need to generate <x:ref>Via</x:ref> header fields anymore.
5216  (<xref target="header.via"/>)
5217</t>
5218<t>
5219  Exactly when "close" connection options have to be sent has been clarified.
5220  Also, "hop-by-hop" header fields are required to appear in the Connection header
5221  field; just because they're defined as hop-by-hop in this specification
5222  doesn't exempt them.
5223  (<xref target="header.connection"/>)
5224</t>
5225<t>
5226  The limit of two connections per server has been removed.
5227  An idempotent sequence of requests is no longer required to be retried.
5228  The requirement to retry requests under certain circumstances when the
5229  server prematurely closes the connection has been removed.
5230  Also, some extraneous requirements about when servers are allowed to close
5231  connections prematurely have been removed.
5232  (<xref target="persistent.connections"/>)
5233</t>
5234<t>
5235  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
5236  responses other than 101 (this was incorporated from <xref
5237  target="RFC2817"/>). Furthermore, the ordering in the field value is now
5238  significant.
5239  (<xref target="header.upgrade"/>)
5240</t>
5241<t>
5242  Empty list elements in list productions (e.g., a list header field containing
5243  ", ,") have been deprecated.
5244  (<xref target="abnf.extension"/>)
5245</t>
5246<t>
5247  Registration of Transfer Codings now requires IETF Review
5248  (<xref target="transfer.coding.registry"/>)
5249</t>
5250<t>
5251  This specification now defines the Upgrade Token Registry, previously
5252  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
5253  (<xref target="upgrade.token.registry"/>)
5254</t>
5255<t>
5256  The expectation to support HTTP/0.9 requests has been removed.
5257  (<xref target="compatibility"/>)
5258</t>
5259<t>
5260  Issues with the Keep-Alive and Proxy-Connection header fields in requests
5261  are pointed out, with use of the latter being discouraged altogether.
5262  (<xref target="compatibility.with.http.1.0.persistent.connections" />)
5263</t>
5264</section>
5265</section>
5266
5267<?BEGININC p1-messaging.abnf-appendix ?>
5268<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
5269<figure>
5270<artwork type="abnf" name="p1-messaging.parsed-abnf">
5271<x:ref>BWS</x:ref> = OWS
5272
5273<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
5274 connection-option ] )
5275<x:ref>Content-Length</x:ref> = 1*DIGIT
5276
5277<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
5278 ]
5279<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
5280<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
5281<x:ref>Host</x:ref> = uri-host [ ":" port ]
5282
5283<x:ref>OWS</x:ref> = *( SP / HTAB )
5284
5285<x:ref>RWS</x:ref> = 1*( SP / HTAB )
5286
5287<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
5288<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
5289<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
5290 transfer-coding ] )
5291
5292<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
5293<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
5294
5295<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
5296 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
5297 comment ] ) ] )
5298
5299<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
5300<x:ref>absolute-form</x:ref> = absolute-URI
5301<x:ref>absolute-path</x:ref> = 1*( "/" segment )
5302<x:ref>asterisk-form</x:ref> = "*"
5303<x:ref>attribute</x:ref> = token
5304<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
5305<x:ref>authority-form</x:ref> = authority
5306
5307<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
5308<x:ref>chunk-data</x:ref> = 1*OCTET
5309<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
5310<x:ref>chunk-ext-name</x:ref> = token
5311<x:ref>chunk-ext-val</x:ref> = token / quoted-str-nf
5312<x:ref>chunk-size</x:ref> = 1*HEXDIG
5313<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
5314<x:ref>comment</x:ref> = "(" *( ctext / quoted-cpair / comment ) ")"
5315<x:ref>connection-option</x:ref> = token
5316<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
5317 / %x2A-5B ; '*'-'['
5318 / %x5D-7E ; ']'-'~'
5319 / obs-text
5320
5321<x:ref>field-content</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5322<x:ref>field-name</x:ref> = token
5323<x:ref>field-value</x:ref> = *( field-content / obs-fold )
5324<x:ref>fragment</x:ref> = &lt;fragment, defined in [RFC3986], Section 3.5&gt;
5325
5326<x:ref>header-field</x:ref> = field-name ":" OWS field-value OWS
5327<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ] [ "#"
5328 fragment ]
5329<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ] [ "#"
5330 fragment ]
5331
5332<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
5333
5334<x:ref>message-body</x:ref> = *OCTET
5335<x:ref>method</x:ref> = token
5336
5337<x:ref>obs-fold</x:ref> = CRLF ( SP / HTAB )
5338<x:ref>obs-text</x:ref> = %x80-FF
5339<x:ref>origin-form</x:ref> = absolute-path [ "?" query ]
5340
5341<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
5342<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
5343<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
5344<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
5345<x:ref>protocol-name</x:ref> = token
5346<x:ref>protocol-version</x:ref> = token
5347<x:ref>pseudonym</x:ref> = token
5348
5349<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5350 / %x5D-7E ; ']'-'~'
5351 / obs-text
5352<x:ref>qdtext-nf</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5353 / %x5D-7E ; ']'-'~'
5354 / obs-text
5355<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5356<x:ref>quoted-cpair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5357<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5358<x:ref>quoted-str-nf</x:ref> = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
5359<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5360
5361<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5362<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5363<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5364<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5365<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5366<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5367<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5368 asterisk-form
5369
5370<x:ref>segment</x:ref> = &lt;segment, defined in [RFC3986], Section 3.3&gt;
5371<x:ref>special</x:ref> = "(" / ")" / "&lt;" / "&gt;" / "@" / "," / ";" / ":" / "\" /
5372 DQUOTE / "/" / "[" / "]" / "?" / "=" / "{" / "}"
5373<x:ref>start-line</x:ref> = request-line / status-line
5374<x:ref>status-code</x:ref> = 3DIGIT
5375<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5376
5377<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5378<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5379<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5380 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5381<x:ref>token</x:ref> = 1*tchar
5382<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5383<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5384 transfer-extension
5385<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5386<x:ref>transfer-parameter</x:ref> = attribute BWS "=" BWS value
5387
5388<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5389
5390<x:ref>value</x:ref> = word
5391
5392<x:ref>word</x:ref> = token / quoted-string
5393</artwork>
5394</figure>
5395</section>
5396<?ENDINC p1-messaging.abnf-appendix ?>
5397
5398<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5399
5400<section title="Since RFC 2616">
5401<t>
5402  Changes up to the IETF Last Call draft are summarized
5403  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-24#appendix-C"/>.
5404</t>
5405</section>
5406
5407<section title="Since draft-ietf-httpbis-p1-messaging-24" anchor="changes.since.24">
5408<t>
5409  Closed issues:
5410  <list style="symbols">
5411    <t>
5412      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/502"/>:
5413      "APPSDIR review of draft-ietf-httpbis-p1-messaging-24"
5414    </t>
5415    <t>
5416      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/507"/>:
5417      "integer value parsing"
5418    </t>
5419    <t>
5420      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/517"/>:
5421      "move IANA registrations to correct draft"
5422    </t>
5423  </list>
5424</t>
5425</section>
5426</section>
5427
5428</back>
5429</rfc>
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