source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2278

Last change on this file since 2278 was 2278, checked in by fielding@…, 8 years ago

(editorial) clarify that name delegation for http and https URIs depend on TCP and TLS/TCP, respectively; addresses #447

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 230.2 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 "May">
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-expires         "<xref target='Part6' x:rel='#header.expires' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY header-last-modified   "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY header-mime-version    "<xref target='Part2' x:rel='#mime-version' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40  <!ENTITY header-pragma          "<xref target='Part6' x:rel='#header.pragma' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41  <!ENTITY header-proxy-authenticate  "<xref target='Part7' x:rel='#header.proxy-authenticate' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
42  <!ENTITY header-proxy-authorization "<xref target='Part7' x:rel='#header.proxy-authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
43  <!ENTITY header-server          "<xref target='Part2' x:rel='#header.server' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
44  <!ENTITY header-warning         "<xref target='Part6' x:rel='#header.warning' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
45  <!ENTITY idempotent-methods     "<xref target='Part2' x:rel='#idempotent.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
46  <!ENTITY safe-methods           "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
47  <!ENTITY methods                "<xref target='Part2' x:rel='#methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
48  <!ENTITY OPTIONS                "<xref target='Part2' x:rel='#OPTIONS' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
49  <!ENTITY qvalue                 "<xref target='Part2' x:rel='#quality.values' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
50  <!ENTITY resource               "<xref target='Part2' x:rel='#resources' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
51  <!ENTITY status-codes           "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
52  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
53  <!ENTITY status-203             "<xref target='Part2' x:rel='#status.203' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
54  <!ENTITY status-3xx             "<xref target='Part2' x:rel='#status.3xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
55  <!ENTITY status-304             "<xref target='Part4' x:rel='#status.304' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
56  <!ENTITY status-4xx             "<xref target='Part2' x:rel='#status.4xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
57  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
58  <!ENTITY iana-header-registry   "<xref target='Part2' x:rel='#header.field.registry' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
59]>
60<?rfc toc="yes" ?>
61<?rfc symrefs="yes" ?>
62<?rfc sortrefs="yes" ?>
63<?rfc compact="yes"?>
64<?rfc subcompact="no" ?>
65<?rfc linkmailto="no" ?>
66<?rfc editing="no" ?>
67<?rfc comments="yes"?>
68<?rfc inline="yes"?>
69<?rfc rfcedstyle="yes"?>
70<?rfc-ext allow-markup-in-artwork="yes" ?>
71<?rfc-ext include-references-in-index="yes" ?>
72<rfc obsoletes="2145,2616" updates="2817,2818" category="std" x:maturity-level="proposed"
73     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
74     xmlns:x='http://purl.org/net/xml2rfc/ext'>
75<x:link rel="next" basename="p2-semantics"/>
76<x:feedback template="mailto:ietf-http-wg@w3.org?subject={docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
77<front>
78
79  <title abbrev="HTTP/1.1 Message Syntax and Routing">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
80
81  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
82    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
83    <address>
84      <postal>
85        <street>345 Park Ave</street>
86        <city>San Jose</city>
87        <region>CA</region>
88        <code>95110</code>
89        <country>USA</country>
90      </postal>
91      <email>fielding@gbiv.com</email>
92      <uri>http://roy.gbiv.com/</uri>
93    </address>
94  </author>
95
96  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
97    <organization abbrev="greenbytes">greenbytes GmbH</organization>
98    <address>
99      <postal>
100        <street>Hafenweg 16</street>
101        <city>Muenster</city><region>NW</region><code>48155</code>
102        <country>Germany</country>
103      </postal>
104      <email>julian.reschke@greenbytes.de</email>
105      <uri>http://greenbytes.de/tech/webdav/</uri>
106    </address>
107  </author>
108
109  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
110  <workgroup>HTTPbis Working Group</workgroup>
111
112<abstract>
113<t>
114   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
115   distributed, collaborative, hypertext information systems. HTTP has been in
116   use by the World Wide Web global information initiative since 1990.
117   This document provides an overview of HTTP architecture and its associated
118   terminology, defines the "http" and "https" Uniform Resource Identifier
119   (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements,
120   and describes general security concerns for implementations.
121</t>   
122</abstract>
123
124<note title="Editorial Note (To be removed by RFC Editor)">
125  <t>
126    Discussion of this draft takes place on the HTTPBIS working group
127    mailing list (ietf-http-wg@w3.org), which is archived at
128    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
129  </t>
130  <t>
131    The current issues list is at
132    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
133    documents (including fancy diffs) can be found at
134    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
135  </t>
136  <t>
137    The changes in this draft are summarized in <xref target="changes.since.22"/>.
138  </t>
139</note>
140</front>
141<middle>
142<section title="Introduction" anchor="introduction">
143<t>
144   The Hypertext Transfer Protocol (HTTP) is an application-level
145   request/response protocol that uses extensible semantics and self-descriptive
146   message payloads for flexible interaction with network-based hypertext
147   information systems. This document is the first in a series of documents
148   that collectively form the HTTP/1.1 specification:
149   <list style="empty">
150    <t>RFC xxx1: Message Syntax and Routing</t>
151    <t><xref target="Part2" x:fmt="none">RFC xxx2</xref>: Semantics and Content</t>
152    <t><xref target="Part4" x:fmt="none">RFC xxx3</xref>: Conditional Requests</t>
153    <t><xref target="Part5" x:fmt="none">RFC xxx4</xref>: Range Requests</t>
154    <t><xref target="Part6" x:fmt="none">RFC xxx5</xref>: Caching</t>
155    <t><xref target="Part7" x:fmt="none">RFC xxx6</xref>: Authentication</t>
156   </list>
157</t>
158<t>
159   This HTTP/1.1 specification obsoletes and moves to historic status
160   <xref target="RFC2616" x:fmt="none">RFC 2616</xref>, its predecessor
161   <xref target="RFC2068" x:fmt="none">RFC 2068</xref>, and
162   <xref target="RFC2145" x:fmt="none">RFC 2145</xref> (on HTTP versioning).
163   This specification also updates the use of CONNECT to establish a tunnel,
164   previously defined in <xref target="RFC2817" x:fmt="none">RFC 2817</xref>,
165   and defines the "https" URI scheme that was described informally in
166   <xref target="RFC2818" x:fmt="none">RFC 2818</xref>.
167</t>
168<t>
169   HTTP is a generic interface protocol for information systems. It is
170   designed to hide the details of how a service is implemented by presenting
171   a uniform interface to clients that is independent of the types of
172   resources provided. Likewise, servers do not need to be aware of each
173   client's purpose: an HTTP request can be considered in isolation rather
174   than being associated with a specific type of client or a predetermined
175   sequence of application steps. The result is a protocol that can be used
176   effectively in many different contexts and for which implementations can
177   evolve independently over time.
178</t>
179<t>
180   HTTP is also designed for use as an intermediation protocol for translating
181   communication to and from non-HTTP information systems.
182   HTTP proxies and gateways can provide access to alternative information
183   services by translating their diverse protocols into a hypertext
184   format that can be viewed and manipulated by clients in the same way
185   as HTTP services.
186</t>
187<t>
188   One consequence of this flexibility is that the protocol cannot be
189   defined in terms of what occurs behind the interface. Instead, we
190   are limited to defining the syntax of communication, the intent
191   of received communication, and the expected behavior of recipients.
192   If the communication is considered in isolation, then successful
193   actions ought to be reflected in corresponding changes to the
194   observable interface provided by servers. However, since multiple
195   clients might act in parallel and perhaps at cross-purposes, we
196   cannot require that such changes be observable beyond the scope
197   of a single response.
198</t>
199<t>
200   This document describes the architectural elements that are used or
201   referred to in HTTP, defines the "http" and "https" URI schemes,
202   describes overall network operation and connection management,
203   and defines HTTP message framing and forwarding requirements.
204   Our goal is to define all of the mechanisms necessary for HTTP message
205   handling that are independent of message semantics, thereby defining the
206   complete set of requirements for message parsers and
207   message-forwarding intermediaries.
208</t>
209
210
211<section title="Requirement Notation" anchor="intro.requirements">
212<t>
213   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
214   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
215   document are to be interpreted as described in <xref target="RFC2119"/>.
216</t>
217<t>
218   Conformance criteria and considerations regarding error handling
219   are defined in <xref target="conformance"/>.
220</t>
221</section>
222
223<section title="Syntax Notation" anchor="notation">
224<iref primary="true" item="Grammar" subitem="ALPHA"/>
225<iref primary="true" item="Grammar" subitem="CR"/>
226<iref primary="true" item="Grammar" subitem="CRLF"/>
227<iref primary="true" item="Grammar" subitem="CTL"/>
228<iref primary="true" item="Grammar" subitem="DIGIT"/>
229<iref primary="true" item="Grammar" subitem="DQUOTE"/>
230<iref primary="true" item="Grammar" subitem="HEXDIG"/>
231<iref primary="true" item="Grammar" subitem="HTAB"/>
232<iref primary="true" item="Grammar" subitem="LF"/>
233<iref primary="true" item="Grammar" subitem="OCTET"/>
234<iref primary="true" item="Grammar" subitem="SP"/>
235<iref primary="true" item="Grammar" subitem="VCHAR"/>
236<t>
237   This specification uses the Augmented Backus-Naur Form (ABNF) notation
238   of <xref target="RFC5234"/> with the list rule extension defined in
239   <xref target="abnf.extension"/>.  <xref target="collected.abnf"/> shows
240   the collected ABNF with the list rule expanded.
241</t>
242<t anchor="core.rules">
243  <x:anchor-alias value="ALPHA"/>
244  <x:anchor-alias value="CTL"/>
245  <x:anchor-alias value="CR"/>
246  <x:anchor-alias value="CRLF"/>
247  <x:anchor-alias value="DIGIT"/>
248  <x:anchor-alias value="DQUOTE"/>
249  <x:anchor-alias value="HEXDIG"/>
250  <x:anchor-alias value="HTAB"/>
251  <x:anchor-alias value="LF"/>
252  <x:anchor-alias value="OCTET"/>
253  <x:anchor-alias value="SP"/>
254  <x:anchor-alias value="VCHAR"/>
255   The following core rules are included by
256   reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
257   ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
258   DIGIT (decimal 0-9), DQUOTE (double quote),
259   HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed),
260   OCTET (any 8-bit sequence of data), SP (space), and
261   VCHAR (any visible <xref target="USASCII"/> character).
262</t>
263<t>
264   As a convention, ABNF rule names prefixed with "obs-" denote
265   "obsolete" grammar rules that appear for historical reasons.
266</t>
267</section>
268</section>
269
270<section title="Architecture" anchor="architecture">
271<t>
272   HTTP was created for the World Wide Web architecture
273   and has evolved over time to support the scalability needs of a worldwide
274   hypertext system. Much of that architecture is reflected in the terminology
275   and syntax productions used to define HTTP.
276</t>
277
278<section title="Client/Server Messaging" anchor="operation">
279<iref primary="true" item="client"/>
280<iref primary="true" item="server"/>
281<iref primary="true" item="connection"/>
282<t>
283   HTTP is a stateless request/response protocol that operates by exchanging
284   <x:dfn>messages</x:dfn> (<xref target="http.message"/>) across a reliable
285   transport or session-layer
286   "<x:dfn>connection</x:dfn>" (<xref target="connection.management"/>).
287   An HTTP "<x:dfn>client</x:dfn>" is a program that establishes a connection
288   to a server for the purpose of sending one or more HTTP requests.
289   An HTTP "<x:dfn>server</x:dfn>" is a program that accepts connections
290   in order to service HTTP requests by sending HTTP responses.
291</t>
292<iref primary="true" item="user agent"/>
293<iref primary="true" item="origin server"/>
294<iref primary="true" item="browser"/>
295<iref primary="true" item="spider"/>
296<iref primary="true" item="sender"/>
297<iref primary="true" item="recipient"/>
298<t>
299   The terms client and server refer only to the roles that
300   these programs perform for a particular connection.  The same program
301   might act as a client on some connections and a server on others.
302   We use the term "<x:dfn>user agent</x:dfn>" to refer to any of the various
303   client programs that initiate a request, including (but not limited to)
304   browsers, spiders (web-based robots), command-line tools, native
305   applications, and mobile apps.  The term "<x:dfn>origin server</x:dfn>" is
306   used to refer to the program that can originate authoritative responses to
307   a request. For general requirements, we use the terms
308   "<x:dfn>sender</x:dfn>" and "<x:dfn>recipient</x:dfn>" to refer to any
309   component that sends or receives, respectively, a given message.
310</t>
311<t>
312   HTTP relies upon the Uniform Resource Identifier (URI)
313   standard <xref target="RFC3986"/> to indicate the target resource
314   (<xref target="target-resource"/>) and relationships between resources.
315   Messages are passed in a format similar to that used by Internet mail
316   <xref target="RFC5322"/> and the Multipurpose Internet Mail Extensions
317   (MIME) <xref target="RFC2045"/> (see &diff-mime; for the differences
318   between HTTP and MIME messages).
319</t>
320<t>
321   Most HTTP communication consists of a retrieval request (GET) for
322   a representation of some resource identified by a URI.  In the
323   simplest case, this might be accomplished via a single bidirectional
324   connection (===) between the user agent (UA) and the origin server (O).
325</t>
326<figure><artwork type="drawing">
327         request   &gt;
328    <x:highlight>UA</x:highlight> ======================================= <x:highlight>O</x:highlight>
329                                &lt;   response
330</artwork></figure>
331<iref primary="true" item="message"/>
332<iref primary="true" item="request"/>
333<iref primary="true" item="response"/>
334<t>
335   A client sends an HTTP request to a server in the form of a <x:dfn>request</x:dfn>
336   message, beginning with a request-line that includes a method, URI, and
337   protocol version (<xref target="request.line"/>),
338   followed by header fields containing
339   request modifiers, client information, and representation metadata
340   (<xref target="header.fields"/>),
341   an empty line to indicate the end of the header section, and finally
342   a message body containing the payload body (if any,
343   <xref target="message.body"/>).
344</t>
345<t>
346   A server responds to a client's request by sending one or more HTTP
347   <x:dfn>response</x:dfn>
348   messages, each beginning with a status line that
349   includes the protocol version, a success or error code, and textual
350   reason phrase (<xref target="status.line"/>),
351   possibly followed by header fields containing server
352   information, resource metadata, and representation metadata
353   (<xref target="header.fields"/>),
354   an empty line to indicate the end of the header section, and finally
355   a message body containing the payload body (if any,
356   <xref target="message.body"/>).
357</t>
358<t>
359   A connection might be used for multiple request/response exchanges,
360   as defined in <xref target="persistent.connections"/>.
361</t>
362<t>
363   The following example illustrates a typical message exchange for a
364   GET request on the URI "http://www.example.com/hello.txt":
365</t>
366<figure><preamble>
367client request:
368</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
369GET /hello.txt HTTP/1.1
370User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
371Host: www.example.com
372Accept-Language: en, mi
373
374</artwork></figure>
375<figure><preamble>
376server response:
377</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
378HTTP/1.1 200 OK
379Date: Mon, 27 Jul 2009 12:28:53 GMT
380Server: Apache
381Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
382ETag: "34aa387-d-1568eb00"
383Accept-Ranges: bytes
384Content-Length: <x:length-of target="exbody"/>
385Vary: Accept-Encoding
386Content-Type: text/plain
387
388<x:span anchor="exbody">Hello World! My payload includes a trailing CRLF.
389</x:span></artwork>
390</figure>
391</section>
392
393<section title="Implementation Diversity" anchor="implementation-diversity">
394<t>
395   When considering the design of HTTP, it is easy to fall into a trap of
396   thinking that all user agents are general-purpose browsers and all origin
397   servers are large public websites. That is not the case in practice.
398   Common HTTP user agents include household appliances, stereos, scales,
399   firmware update scripts, command-line programs, mobile apps,
400   and communication devices in a multitude of shapes and sizes.  Likewise,
401   common HTTP origin servers include home automation units, configurable
402   networking components, office machines, autonomous robots, news feeds,
403   traffic cameras, ad selectors, and video delivery platforms.
404</t>
405<t>
406   The term "user agent" does not imply that there is a human user directly
407   interacting with the software agent at the time of a request. In many
408   cases, a user agent is installed or configured to run in the background
409   and save its results for later inspection (or save only a subset of those
410   results that might be interesting or erroneous). Spiders, for example, are
411   typically given a start URI and configured to follow certain behavior while
412   crawling the Web as a hypertext graph.
413</t>
414<t>
415   The implementation diversity of HTTP means that we cannot assume the
416   user agent can make interactive suggestions to a user or provide adequate
417   warning for security or privacy options.  In the few cases where this
418   specification requires reporting of errors to the user, it is acceptable
419   for such reporting to only be observable in an error console or log file.
420   Likewise, requirements that an automated action be confirmed by the user
421   before proceeding might be met via advance configuration choices,
422   run-time options, or simple avoidance of the unsafe action; confirmation
423   does not imply any specific user interface or interruption of normal
424   processing if the user has already made that choice.
425</t>
426</section>
427
428<section title="Intermediaries" anchor="intermediaries">
429<iref primary="true" item="intermediary"/>
430<t>
431   HTTP enables the use of intermediaries to satisfy requests through
432   a chain of connections.  There are three common forms of HTTP
433   <x:dfn>intermediary</x:dfn>: proxy, gateway, and tunnel.  In some cases,
434   a single intermediary might act as an origin server, proxy, gateway,
435   or tunnel, switching behavior based on the nature of each request.
436</t>
437<figure><artwork type="drawing">
438         &gt;             &gt;             &gt;             &gt;
439    <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>
440               &lt;             &lt;             &lt;             &lt;
441</artwork></figure>
442<t>
443   The figure above shows three intermediaries (A, B, and C) between the
444   user agent and origin server. A request or response message that
445   travels the whole chain will pass through four separate connections.
446   Some HTTP communication options
447   might apply only to the connection with the nearest, non-tunnel
448   neighbor, only to the end-points of the chain, or to all connections
449   along the chain. Although the diagram is linear, each participant might
450   be engaged in multiple, simultaneous communications. For example, B
451   might be receiving requests from many clients other than A, and/or
452   forwarding requests to servers other than C, at the same time that it
453   is handling A's request. Likewise, later requests might be sent through a
454   different path of connections, often based on dynamic configuration for
455   load balancing.   
456</t>
457<t>
458<iref primary="true" item="upstream"/><iref primary="true" item="downstream"/>
459<iref primary="true" item="inbound"/><iref primary="true" item="outbound"/>
460   We use the terms "<x:dfn>upstream</x:dfn>" and "<x:dfn>downstream</x:dfn>"
461   to describe various requirements in relation to the directional flow of a
462   message: all messages flow from upstream to downstream.
463   Likewise, we use the terms inbound and outbound to refer to
464   directions in relation to the request path:
465   "<x:dfn>inbound</x:dfn>" means toward the origin server and
466   "<x:dfn>outbound</x:dfn>" means toward the user agent.
467</t>
468<t><iref primary="true" item="proxy"/>
469   A "<x:dfn>proxy</x:dfn>" is a message forwarding agent that is selected by the
470   client, usually via local configuration rules, to receive requests
471   for some type(s) of absolute URI and attempt to satisfy those
472   requests via translation through the HTTP interface.  Some translations
473   are minimal, such as for proxy requests for "http" URIs, whereas
474   other requests might require translation to and from entirely different
475   application-level protocols. Proxies are often used to group an
476   organization's HTTP requests through a common intermediary for the
477   sake of security, annotation services, or shared caching.
478</t>
479<t>
480<iref primary="true" item="transforming proxy"/>
481<iref primary="true" item="non-transforming proxy"/>
482   An HTTP-to-HTTP proxy is called a "<x:dfn>transforming proxy</x:dfn>" if it is designed
483   or configured to modify request or response messages in a semantically
484   meaningful way (i.e., modifications, beyond those required by normal
485   HTTP processing, that change the message in a way that would be
486   significant to the original sender or potentially significant to
487   downstream recipients).  For example, a transforming proxy might be
488   acting as a shared annotation server (modifying responses to include
489   references to a local annotation database), a malware filter, a
490   format transcoder, or an intranet-to-Internet privacy filter.  Such
491   transformations are presumed to be desired by the client (or client
492   organization) that selected the proxy and are beyond the scope of
493   this specification.  However, when a proxy is not intended to transform
494   a given message, we use the term "<x:dfn>non-transforming proxy</x:dfn>" to target
495   requirements that preserve HTTP message semantics. See &status-203; and
496   &header-warning; for status and warning codes related to transformations.
497</t>
498<t><iref primary="true" item="gateway"/><iref primary="true" item="reverse proxy"/>
499<iref primary="true" item="accelerator"/>
500   A "<x:dfn>gateway</x:dfn>" (a.k.a., "<x:dfn>reverse proxy</x:dfn>") is an
501   intermediary that acts as an origin server for the outbound connection, but
502   translates received requests and forwards them inbound to another server or
503   servers. Gateways are often used to encapsulate legacy or untrusted
504   information services, to improve server performance through
505   "<x:dfn>accelerator</x:dfn>" caching, and to enable partitioning or load
506   balancing of HTTP services across multiple machines.
507</t>
508<t>
509   All HTTP requirements applicable to an origin server
510   also apply to the outbound communication of a gateway.
511   A gateway communicates with inbound servers using any protocol that
512   it desires, including private extensions to HTTP that are outside
513   the scope of this specification.  However, an HTTP-to-HTTP gateway
514   that wishes to interoperate with third-party HTTP servers &MUST;
515   conform to HTTP user agent requirements on the gateway's inbound
516   connection and &MUST; implement the <x:ref>Connection</x:ref>
517   (<xref target="header.connection"/>) and <x:ref>Via</x:ref>
518   (<xref target="header.via"/>) header fields for both inbound and outbound
519   connections.
520</t>
521<t><iref primary="true" item="tunnel"/>
522   A "<x:dfn>tunnel</x:dfn>" acts as a blind relay between two connections
523   without changing the messages. Once active, a tunnel is not
524   considered a party to the HTTP communication, though the tunnel might
525   have been initiated by an HTTP request. A tunnel ceases to exist when
526   both ends of the relayed connection are closed. Tunnels are used to
527   extend a virtual connection through an intermediary, such as when
528   Transport Layer Security (TLS, <xref target="RFC5246"/>) is used to
529   establish confidential communication through a shared firewall proxy.
530</t>
531<t><iref primary="true" item="interception proxy"/>
532<iref primary="true" item="transparent proxy"/>
533<iref primary="true" item="captive portal"/>
534   The above categories for intermediary only consider those acting as
535   participants in the HTTP communication.  There are also intermediaries
536   that can act on lower layers of the network protocol stack, filtering or
537   redirecting HTTP traffic without the knowledge or permission of message
538   senders. Network intermediaries often introduce security flaws or
539   interoperability problems by violating HTTP semantics.  For example, an
540   "<x:dfn>interception proxy</x:dfn>" <xref target="RFC3040"/> (also commonly
541   known as a "<x:dfn>transparent proxy</x:dfn>" <xref target="RFC1919"/> or
542   "<x:dfn>captive portal</x:dfn>")
543   differs from an HTTP proxy because it is not selected by the client.
544   Instead, an interception proxy filters or redirects outgoing TCP port 80
545   packets (and occasionally other common port traffic).
546   Interception proxies are commonly found on public network access points,
547   as a means of enforcing account subscription prior to allowing use of
548   non-local Internet services, and within corporate firewalls to enforce
549   network usage policies.
550   They are indistinguishable from a man-in-the-middle attack.
551</t>
552<t>
553   HTTP is defined as a stateless protocol, meaning that each request message
554   can be understood in isolation.  Many implementations depend on HTTP's
555   stateless design in order to reuse proxied connections or dynamically
556   load-balance requests across multiple servers.  Hence, servers &MUST-NOT;
557   assume that two requests on the same connection are from the same user
558   agent unless the connection is secured and specific to that agent.
559   Some non-standard HTTP extensions (e.g., <xref target="RFC4559"/>) have
560   been known to violate this requirement, resulting in security and
561   interoperability problems.
562</t>
563</section>
564
565<section title="Caches" anchor="caches">
566<iref primary="true" item="cache"/>
567<t>
568   A "<x:dfn>cache</x:dfn>" is a local store of previous response messages and the
569   subsystem that controls its message storage, retrieval, and deletion.
570   A cache stores cacheable responses in order to reduce the response
571   time and network bandwidth consumption on future, equivalent
572   requests. Any client or server &MAY; employ a cache, though a cache
573   cannot be used by a server while it is acting as a tunnel.
574</t>
575<t>
576   The effect of a cache is that the request/response chain is shortened
577   if one of the participants along the chain has a cached response
578   applicable to that request. The following illustrates the resulting
579   chain if B has a cached copy of an earlier response from O (via C)
580   for a request that has not been cached by UA or A.
581</t>
582<figure><artwork type="drawing">
583            &gt;             &gt;
584       <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>
585                  &lt;             &lt;
586</artwork></figure>
587<t><iref primary="true" item="cacheable"/>
588   A response is "<x:dfn>cacheable</x:dfn>" if a cache is allowed to store a copy of
589   the response message for use in answering subsequent requests.
590   Even when a response is cacheable, there might be additional
591   constraints placed by the client or by the origin server on when
592   that cached response can be used for a particular request. HTTP
593   requirements for cache behavior and cacheable responses are
594   defined in &caching-overview;. 
595</t>
596<t>
597   There are a wide variety of architectures and configurations
598   of caches deployed across the World Wide Web and
599   inside large organizations. These include national hierarchies
600   of proxy caches to save transoceanic bandwidth, collaborative systems that
601   broadcast or multicast cache entries, archives of pre-fetched cache
602   entries for use in off-line or high-latency environments, and so on.
603</t>
604</section>
605
606<section title="Conformance and Error Handling" anchor="conformance">
607<t>
608   This specification targets conformance criteria according to the role of
609   a participant in HTTP communication.  Hence, HTTP requirements are placed
610   on senders, recipients, clients, servers, user agents, intermediaries,
611   origin servers, proxies, gateways, or caches, depending on what behavior
612   is being constrained by the requirement. Additional (social) requirements
613   are placed on implementations, resource owners, and protocol element
614   registrations when they apply beyond the scope of a single communication.
615</t>
616<t>
617   The verb "generate" is used instead of "send" where a requirement
618   differentiates between creating a protocol element and merely forwarding a
619   received element downstream.
620</t>
621<t>
622   An implementation is considered conformant if it complies with all of the
623   requirements associated with the roles it partakes in HTTP.
624</t>
625<t>
626   Conformance applies to both the syntax and semantics of HTTP protocol
627   elements. A sender &MUST-NOT; generate protocol elements that convey a
628   meaning that is known by that sender to be false. A sender &MUST-NOT;
629   generate protocol elements that do not match the grammar defined by the
630   ABNF rules for those protocol elements that are applicable to the sender's
631   role. If a received protocol element is processed, the recipient &MUST; be
632   able to parse any value that would match the ABNF rules for that protocol
633   element, excluding only those rules not applicable to the recipient's role.
634</t>
635<t>
636   Unless noted otherwise, a recipient &MAY; attempt to recover a usable
637   protocol element from an invalid construct.  HTTP does not define
638   specific error handling mechanisms except when they have a direct impact
639   on security, since different applications of the protocol require
640   different error handling strategies.  For example, a Web browser might
641   wish to transparently recover from a response where the
642   <x:ref>Location</x:ref> header field doesn't parse according to the ABNF,
643   whereas a systems control client might consider any form of error recovery
644   to be dangerous.
645</t>
646</section>
647
648<section title="Protocol Versioning" anchor="http.version">
649  <x:anchor-alias value="HTTP-version"/>
650  <x:anchor-alias value="HTTP-name"/>
651<t>
652   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate
653   versions of the protocol. This specification defines version "1.1".
654   The protocol version as a whole indicates the sender's conformance
655   with the set of requirements laid out in that version's corresponding
656   specification of HTTP.
657</t>
658<t>
659   The version of an HTTP message is indicated by an HTTP-version field
660   in the first line of the message. HTTP-version is case-sensitive.
661</t>
662<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-version"/><iref primary="true" item="Grammar" subitem="HTTP-name"/>
663  <x:ref>HTTP-version</x:ref>  = <x:ref>HTTP-name</x:ref> "/" <x:ref>DIGIT</x:ref> "." <x:ref>DIGIT</x:ref>
664  <x:ref>HTTP-name</x:ref>     = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
665</artwork></figure>
666<t>
667   The HTTP version number consists of two decimal digits separated by a "."
668   (period or decimal point).  The first digit ("major version") indicates the
669   HTTP messaging syntax, whereas the second digit ("minor version") indicates
670   the highest minor version within that major version to which the sender is
671   conformant and able to understand for future communication.  The minor
672   version advertises the sender's communication capabilities even when the
673   sender is only using a backwards-compatible subset of the protocol,
674   thereby letting the recipient know that more advanced features can
675   be used in response (by servers) or in future requests (by clients).
676</t>
677<t>
678   When an HTTP/1.1 message is sent to an HTTP/1.0 recipient
679   <xref target="RFC1945"/> or a recipient whose version is unknown,
680   the HTTP/1.1 message is constructed such that it can be interpreted
681   as a valid HTTP/1.0 message if all of the newer features are ignored.
682   This specification places recipient-version requirements on some
683   new features so that a conformant sender will only use compatible
684   features until it has determined, through configuration or the
685   receipt of a message, that the recipient supports HTTP/1.1.
686</t>
687<t>
688   The interpretation of a header field does not change between minor
689   versions of the same major HTTP version, though the default
690   behavior of a recipient in the absence of such a field can change.
691   Unless specified otherwise, header fields defined in HTTP/1.1 are
692   defined for all versions of HTTP/1.x.  In particular, the <x:ref>Host</x:ref>
693   and <x:ref>Connection</x:ref> header fields ought to be implemented by all
694   HTTP/1.x implementations whether or not they advertise conformance with
695   HTTP/1.1.
696</t>
697<t>
698   New header fields can be defined such that, when they are
699   understood by a recipient, they might override or enhance the
700   interpretation of previously defined header fields.  When an
701   implementation receives an unrecognized header field, the recipient
702   &MUST; ignore that header field for local processing regardless of
703   the message's HTTP version.  An unrecognized header field received
704   by a proxy &MUST; be forwarded downstream unless the header field's
705   field-name is listed in the message's <x:ref>Connection</x:ref> header field
706   (see <xref target="header.connection"/>).
707   These requirements allow HTTP's functionality to be enhanced without
708   requiring prior update of deployed intermediaries.
709</t>
710<t>
711   Intermediaries that process HTTP messages (i.e., all intermediaries
712   other than those acting as tunnels) &MUST; send their own HTTP-version
713   in forwarded messages.  In other words, they &MUST-NOT; blindly
714   forward the first line of an HTTP message without ensuring that the
715   protocol version in that message matches a version to which that
716   intermediary is conformant for both the receiving and
717   sending of messages.  Forwarding an HTTP message without rewriting
718   the HTTP-version might result in communication errors when downstream
719   recipients use the message sender's version to determine what features
720   are safe to use for later communication with that sender.
721</t>
722<t>
723   A client &SHOULD; send a request version equal to the highest
724   version to which the client is conformant and
725   whose major version is no higher than the highest version supported
726   by the server, if this is known.  A client &MUST-NOT; send a
727   version to which it is not conformant.
728</t>
729<t>
730   A client &MAY; send a lower request version if it is known that
731   the server incorrectly implements the HTTP specification, but only
732   after the client has attempted at least one normal request and determined
733   from the response status or header fields (e.g., <x:ref>Server</x:ref>) that
734   the server improperly handles higher request versions.
735</t>
736<t>
737   A server &SHOULD; send a response version equal to the highest
738   version to which the server is conformant and
739   whose major version is less than or equal to the one received in the
740   request.  A server &MUST-NOT; send a version to which it is not
741   conformant.  A server &MAY; send a <x:ref>505 (HTTP Version Not
742   Supported)</x:ref> response if it cannot send a response using the
743   major version used in the client's request.
744</t>
745<t>
746   A server &MAY; send an HTTP/1.0 response to a request
747   if it is known or suspected that the client incorrectly implements the
748   HTTP specification and is incapable of correctly processing later
749   version responses, such as when a client fails to parse the version
750   number correctly or when an intermediary is known to blindly forward
751   the HTTP-version even when it doesn't conform to the given minor
752   version of the protocol. Such protocol downgrades &SHOULD-NOT; be
753   performed unless triggered by specific client attributes, such as when
754   one or more of the request header fields (e.g., <x:ref>User-Agent</x:ref>)
755   uniquely match the values sent by a client known to be in error.
756</t>
757<t>
758   The intention of HTTP's versioning design is that the major number
759   will only be incremented if an incompatible message syntax is
760   introduced, and that the minor number will only be incremented when
761   changes made to the protocol have the effect of adding to the message
762   semantics or implying additional capabilities of the sender.  However,
763   the minor version was not incremented for the changes introduced between
764   <xref target="RFC2068"/> and <xref target="RFC2616"/>, and this revision
765   has specifically avoided any such changes to the protocol.
766</t>
767<t>
768   When an HTTP message is received with a major version number that the
769   recipient implements, but a higher minor version number than what the
770   recipient implements, the recipient &SHOULD; process the message as if it
771   were in the highest minor version within that major version to which the
772   recipient is conformant. A recipient can assume that a message with a
773   higher minor version, when sent to a recipient that has not yet indicated
774   support for that higher version, is sufficiently backwards-compatible to be
775   safely processed by any implementation of the same major version.
776</t>
777</section>
778
779<section title="Uniform Resource Identifiers" anchor="uri">
780<iref primary="true" item="resource"/>
781<t>
782   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used
783   throughout HTTP as the means for identifying resources (&resource;).
784   URI references are used to target requests, indicate redirects, and define
785   relationships.
786</t>
787  <x:anchor-alias value="URI-reference"/>
788  <x:anchor-alias value="absolute-URI"/>
789  <x:anchor-alias value="relative-part"/>
790  <x:anchor-alias value="authority"/>
791  <x:anchor-alias value="path-abempty"/>
792  <x:anchor-alias value="port"/>
793  <x:anchor-alias value="query"/>
794  <x:anchor-alias value="segment"/>
795  <x:anchor-alias value="uri-host"/>
796  <x:anchor-alias value="absolute-path"/>
797  <x:anchor-alias value="partial-URI"/>
798<t>
799   This specification adopts the definitions of "URI-reference",
800   "absolute-URI", "relative-part", "port", "host",
801   "path-abempty", "query", "segment", and "authority" from the
802   URI generic syntax.
803   In addition, we define an "absolute-path" rule (that differs from
804   RFC 3986's "path-absolute" in that it allows a leading "//")
805   and a "partial-URI" rule for protocol elements
806   that allow a relative URI but not a fragment.
807</t>
808<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="segment"/><iref primary="true" item="Grammar" subitem="uri-host"/><iref primary="true" item="Grammar" subitem="partial-URI"><!--exported production--></iref>
809  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
810  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
811  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
812  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
813  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
814  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
815  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
816  <x:ref>segment</x:ref>       = &lt;segment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
817  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
818 
819  <x:ref>absolute-path</x:ref> = 1*( "/" segment )
820  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
821</artwork></figure>
822<t>
823   Each protocol element in HTTP that allows a URI reference will indicate
824   in its ABNF production whether the element allows any form of reference
825   (URI-reference), only a URI in absolute form (absolute-URI), only the
826   path and optional query components, or some combination of the above.
827   Unless otherwise indicated, URI references are parsed
828   relative to the effective request URI
829   (<xref target="effective.request.uri"/>).
830</t>
831
832<section title="http URI scheme" anchor="http.uri">
833  <x:anchor-alias value="http-URI"/>
834  <iref item="http URI scheme" primary="true"/>
835  <iref item="URI scheme" subitem="http" primary="true"/>
836<t>
837   The "http" URI scheme is hereby defined for the purpose of minting
838   identifiers according to their association with the hierarchical
839   namespace governed by a potential HTTP origin server listening for
840   TCP (<xref target="RFC0793"/>) connections on a given port.
841</t>
842<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"><!--terminal production--></iref>
843  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
844</artwork></figure>
845<t>
846   The HTTP origin server is identified by the generic syntax's
847   <x:ref>authority</x:ref> component, which includes a host identifier
848   and optional TCP port (<xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>).
849   The remainder of the URI, consisting of both the hierarchical path
850   component and optional query component, serves as an identifier for
851   a potential resource within that origin server's name space.
852</t>
853<t>
854   If the host identifier is provided as an IP address,
855   then the origin server is any listener on the indicated TCP port at
856   that IP address. If host is a registered name, then that name is
857   considered an indirect identifier and the recipient might use a name
858   resolution service, such as DNS, to find the address of a listener
859   for that host.
860   The host &MUST-NOT; be empty; if an "http" URI is received with an
861   empty host, then it &MUST; be rejected as invalid.
862   If the port subcomponent is empty or not given, then TCP port 80 is
863   assumed (the default reserved port for WWW services).
864</t>
865<t>
866   Regardless of the form of host identifier, access to that host is not
867   implied by the mere presence of its name or address. The host might or might
868   not exist and, even when it does exist, might or might not be running an
869   HTTP server or listening to the indicated port. The "http" URI scheme
870   makes use of the delegated nature of Internet names and addresses to
871   establish a naming authority (whatever entity has the ability to place
872   an HTTP server at that Internet name or address) and allows that
873   authority to determine which names are valid and how they might be used.
874</t>
875<t>
876   When an "http" URI is used within a context that calls for access to the
877   indicated resource, a client &MAY; attempt access by resolving
878   the host to an IP address, establishing a TCP connection to that address
879   on the indicated port, and sending an HTTP request message
880   (<xref target="http.message"/>) containing the URI's identifying data
881   (<xref target="message.routing"/>) to the server.
882   If the server responds to that request with a non-interim HTTP response
883   message, as described in &status-codes;, then that response
884   is considered an authoritative answer to the client's request.
885</t>
886<t>
887   Although HTTP is independent of the transport protocol, the "http"
888   scheme is specific to TCP-based services because the name delegation
889   process depends on TCP for establishing authority.
890   An HTTP service based on some other underlying connection protocol
891   would presumably be identified using a different URI scheme, just as
892   the "https" scheme (below) is used for resources that require an
893   end-to-end secured connection. Other protocols might also be used to
894   provide access to "http" identified resources &mdash; it is only the
895   authoritative interface that is specific to TCP.
896</t>
897<t>
898   The URI generic syntax for authority also includes a deprecated
899   userinfo subcomponent (<xref target="RFC3986" x:fmt="," x:sec="3.2.1"/>)
900   for including user authentication information in the URI.  Some
901   implementations make use of the userinfo component for internal
902   configuration of authentication information, such as within command
903   invocation options, configuration files, or bookmark lists, even
904   though such usage might expose a user identifier or password.
905   Senders &MUST; exclude the userinfo subcomponent (and its "@"
906   delimiter) when an "http" URI is transmitted within a message as a
907   request target or header field value.
908   Recipients of an "http" URI reference &SHOULD; parse for userinfo and
909   treat its presence as an error, since it is likely being used to obscure
910   the authority for the sake of phishing attacks.
911</t>
912</section>
913
914<section title="https URI scheme" anchor="https.uri">
915   <x:anchor-alias value="https-URI"/>
916   <iref item="https URI scheme"/>
917   <iref item="URI scheme" subitem="https"/>
918<t>
919   The "https" URI scheme is hereby defined for the purpose of minting
920   identifiers according to their association with the hierarchical
921   namespace governed by a potential HTTP origin server listening to a
922   given TCP port for TLS-secured connections
923   (<xref target="RFC0793"/>, <xref target="RFC5246"/>).
924</t>
925<t>
926   All of the requirements listed above for the "http" scheme are also
927   requirements for the "https" scheme, except that a default TCP port
928   of 443 is assumed if the port subcomponent is empty or not given,
929   and the TCP connection &MUST; be secured, end-to-end, through the
930   use of strong encryption prior to sending the first HTTP request.
931</t>
932<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="https-URI"><!--terminal production--></iref>
933  <x:ref>https-URI</x:ref> = "https:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
934</artwork></figure>
935<t>
936   Note that the "https" URI scheme depends on both TLS and TCP for
937   establishing authority.
938   Resources made available via the "https" scheme have no shared
939   identity with the "http" scheme even if their resource identifiers
940   indicate the same authority (the same host listening to the same
941   TCP port).  They are distinct name spaces and are considered to be
942   distinct origin servers.  However, an extension to HTTP that is
943   defined to apply to entire host domains, such as the Cookie protocol
944   <xref target="RFC6265"/>, can allow information
945   set by one service to impact communication with other services
946   within a matching group of host domains.
947</t>
948<t>
949   The process for authoritative access to an "https" identified
950   resource is defined in <xref target="RFC2818"/>.
951</t>
952</section>
953
954<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
955<t>
956   Since the "http" and "https" schemes conform to the URI generic syntax,
957   such URIs are normalized and compared according to the algorithm defined
958   in <xref target="RFC3986" x:fmt="," x:sec="6"/>, using the defaults
959   described above for each scheme.
960</t>
961<t>
962   If the port is equal to the default port for a scheme, the normal form is
963   to elide the port subcomponent. When not being used in absolute form as the
964   request target of an OPTIONS request, an empty path component is equivalent
965   to an absolute path of "/", so the normal form is to provide a path of "/"
966   instead. The scheme and host are case-insensitive and normally provided in
967   lowercase; all other components are compared in a case-sensitive manner.
968   Characters other than those in the "reserved" set are equivalent to their
969   percent-encoded octets (see <xref target="RFC3986" x:fmt=","
970   x:sec="2.1"/>): the normal form is to not encode them.
971</t>
972<t>
973   For example, the following three URIs are equivalent:
974</t>
975<figure><artwork type="example">
976   http://example.com:80/~smith/home.html
977   http://EXAMPLE.com/%7Esmith/home.html
978   http://EXAMPLE.com:/%7esmith/home.html
979</artwork></figure>
980</section>
981</section>
982</section>
983
984<section title="Message Format" anchor="http.message">
985<x:anchor-alias value="generic-message"/>
986<x:anchor-alias value="message.types"/>
987<x:anchor-alias value="HTTP-message"/>
988<x:anchor-alias value="start-line"/>
989<iref item="header section"/>
990<iref item="headers"/>
991<iref item="header field"/>
992<t>
993   All HTTP/1.1 messages consist of a start-line followed by a sequence of
994   octets in a format similar to the Internet Message Format
995   <xref target="RFC5322"/>: zero or more header fields (collectively
996   referred to as the "headers" or the "header section"), an empty line
997   indicating the end of the header section, and an optional message body.
998</t>
999<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"><!--terminal production--></iref>
1000  <x:ref>HTTP-message</x:ref>   = <x:ref>start-line</x:ref>
1001                   *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1002                   <x:ref>CRLF</x:ref>
1003                   [ <x:ref>message-body</x:ref> ]
1004</artwork></figure>
1005<t>
1006   The normal procedure for parsing an HTTP message is to read the
1007   start-line into a structure, read each header field into a hash
1008   table by field name until the empty line, and then use the parsed
1009   data to determine if a message body is expected.  If a message body
1010   has been indicated, then it is read as a stream until an amount
1011   of octets equal to the message body length is read or the connection
1012   is closed.
1013</t>
1014<t>
1015   Recipients &MUST; parse an HTTP message as a sequence of octets in an
1016   encoding that is a superset of US-ASCII <xref target="USASCII"/>.
1017   Parsing an HTTP message as a stream of Unicode characters, without regard
1018   for the specific encoding, creates security vulnerabilities due to the
1019   varying ways that string processing libraries handle invalid multibyte
1020   character sequences that contain the octet LF (%x0A).  String-based
1021   parsers can only be safely used within protocol elements after the element
1022   has been extracted from the message, such as within a header field-value
1023   after message parsing has delineated the individual fields.
1024</t>
1025<t>
1026   An HTTP message can be parsed as a stream for incremental processing or
1027   forwarding downstream.  However, recipients cannot rely on incremental
1028   delivery of partial messages, since some implementations will buffer or
1029   delay message forwarding for the sake of network efficiency, security
1030   checks, or payload transformations.
1031</t>
1032<t>
1033   A sender &MUST-NOT; send whitespace between the start-line and
1034   the first header field.
1035   A recipient that receives whitespace between the start-line and
1036   the first header field &MUST; either reject the message as invalid or
1037   consume each whitespace-preceded line without further processing of it
1038   (i.e., ignore the entire line, along with any subsequent lines preceded
1039   by whitespace, until a properly formed header field is received or the
1040   header block is terminated).
1041</t>
1042<t>
1043   The presence of such whitespace in a request
1044   might be an attempt to trick a server into ignoring that field or
1045   processing the line after it as a new request, either of which might
1046   result in a security vulnerability if other implementations within
1047   the request chain interpret the same message differently.
1048   Likewise, the presence of such whitespace in a response might be
1049   ignored by some clients or cause others to cease parsing.
1050</t>
1051
1052<section title="Start Line" anchor="start.line">
1053  <x:anchor-alias value="Start-Line"/>
1054<t>
1055   An HTTP message can either be a request from client to server or a
1056   response from server to client.  Syntactically, the two types of message
1057   differ only in the start-line, which is either a request-line (for requests)
1058   or a status-line (for responses), and in the algorithm for determining
1059   the length of the message body (<xref target="message.body"/>).
1060</t>
1061<t>
1062   In theory, a client could receive requests and a server could receive
1063   responses, distinguishing them by their different start-line formats,
1064   but in practice servers are implemented to only expect a request
1065   (a response is interpreted as an unknown or invalid request method)
1066   and clients are implemented to only expect a response.
1067</t>
1068<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="start-line"/>
1069  <x:ref>start-line</x:ref>     = <x:ref>request-line</x:ref> / <x:ref>status-line</x:ref>
1070</artwork></figure>
1071
1072<section title="Request Line" anchor="request.line">
1073  <x:anchor-alias value="Request"/>
1074  <x:anchor-alias value="request-line"/>
1075<t>
1076   A request-line begins with a method token, followed by a single
1077   space (SP), the request-target, another single space (SP), the
1078   protocol version, and ending with CRLF.
1079</t>
1080<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1081  <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>
1082</artwork></figure>
1083<iref primary="true" item="method"/>
1084<t anchor="method">
1085   The method token indicates the request method to be performed on the
1086   target resource. The request method is case-sensitive.
1087</t>
1088<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1089  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1090</artwork></figure>
1091<t>
1092   The methods defined by this specification can be found in
1093   &methods;, along with information regarding the HTTP method registry
1094   and considerations for defining new methods.
1095</t>
1096<iref item="request-target"/>
1097<t>
1098   The request-target identifies the target resource upon which to apply
1099   the request, as defined in <xref target="request-target"/>.
1100</t>
1101<t>
1102   Recipients typically parse the request-line into its component parts by
1103   splitting on whitespace (see <xref target="message.robustness"/>), since
1104   no whitespace is allowed in the three components.
1105   Unfortunately, some user agents fail to properly encode or exclude
1106   whitespace found in hypertext references, resulting in those disallowed
1107   characters being sent in a request-target.
1108</t>
1109<t>
1110   Recipients of an invalid request-line &SHOULD; respond with either a
1111   <x:ref>400 (Bad Request)</x:ref> error or a <x:ref>301 (Moved Permanently)</x:ref>
1112   redirect with the request-target properly encoded.  Recipients &SHOULD-NOT;
1113   attempt to autocorrect and then process the request without a redirect,
1114   since the invalid request-line might be deliberately crafted to bypass
1115   security filters along the request chain.
1116</t>
1117<t>
1118   HTTP does not place a pre-defined limit on the length of a request-line.
1119   A server that receives a method longer than any that it implements
1120   &SHOULD; respond with a <x:ref>501 (Not Implemented)</x:ref> status code.
1121   A server &MUST; be prepared to receive URIs of unbounded length and
1122   respond with the <x:ref>414 (URI Too Long)</x:ref> status code if the received
1123   request-target would be longer than the server wishes to handle
1124   (see &status-414;).
1125</t>
1126<t>
1127   Various ad-hoc limitations on request-line length are found in practice.
1128   It is &RECOMMENDED; that all HTTP senders and recipients support, at a
1129   minimum, request-line lengths of 8000 octets.
1130</t>
1131</section>
1132
1133<section title="Status Line" anchor="status.line">
1134  <x:anchor-alias value="response"/>
1135  <x:anchor-alias value="status-line"/>
1136  <x:anchor-alias value="status-code"/>
1137  <x:anchor-alias value="reason-phrase"/>
1138<t>
1139   The first line of a response message is the status-line, consisting
1140   of the protocol version, a space (SP), the status code, another space,
1141   a possibly-empty textual phrase describing the status code, and
1142   ending with CRLF.
1143</t>
1144<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-line"/>
1145  <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>
1146</artwork></figure>
1147<t>
1148   The status-code element is a 3-digit integer code describing the
1149   result of the server's attempt to understand and satisfy the client's
1150   corresponding request. The rest of the response message is to be
1151   interpreted in light of the semantics defined for that status code.
1152   See &status-codes; for information about the semantics of status codes,
1153   including the classes of status code (indicated by the first digit),
1154   the status codes defined by this specification, considerations for the
1155   definition of new status codes, and the IANA registry.
1156</t>
1157<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-code"/>
1158  <x:ref>status-code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1159</artwork></figure>
1160<t>  
1161   The reason-phrase element exists for the sole purpose of providing a
1162   textual description associated with the numeric status code, mostly
1163   out of deference to earlier Internet application protocols that were more
1164   frequently used with interactive text clients. A client &SHOULD; ignore
1165   the reason-phrase content.
1166</t>
1167<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
1168  <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> )
1169</artwork></figure>
1170</section>
1171</section>
1172
1173<section title="Header Fields" anchor="header.fields">
1174  <x:anchor-alias value="header-field"/>
1175  <x:anchor-alias value="field-content"/>
1176  <x:anchor-alias value="field-name"/>
1177  <x:anchor-alias value="field-value"/>
1178  <x:anchor-alias value="obs-fold"/>
1179<t>
1180   Each HTTP header field consists of a case-insensitive field name
1181   followed by a colon (":"), optional leading whitespace, the field value,
1182   and optional trailing whitespace.
1183</t>
1184<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"/>
1185  <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>
1186  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1187  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>obs-fold</x:ref> )
1188  <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> )
1189  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref> ( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1190                 ; obsolete line folding
1191                 ; see <xref target="field.parsing"/>
1192</artwork></figure>
1193<t>
1194   The field-name token labels the corresponding field-value as having the
1195   semantics defined by that header field.  For example, the <x:ref>Date</x:ref>
1196   header field is defined in &header-date; as containing the origination
1197   timestamp for the message in which it appears.
1198</t>
1199
1200<section title="Field Extensibility" anchor="field.extensibility">
1201<t>
1202   HTTP header fields are fully extensible: there is no limit on the
1203   introduction of new field names, each presumably defining new semantics,
1204   nor on the number of header fields used in a given message.  Existing
1205   fields are defined in each part of this specification and in many other
1206   specifications outside the core standard.
1207   New header fields can be introduced without changing the protocol version
1208   if their defined semantics allow them to be safely ignored by recipients
1209   that do not recognize them.
1210</t>
1211<t>
1212   New HTTP header fields ought to be registered with IANA in the
1213   Message Header Field Registry, as described in &iana-header-registry;.
1214   A proxy &MUST; forward unrecognized header fields unless the
1215   field-name is listed in the <x:ref>Connection</x:ref> header field
1216   (<xref target="header.connection"/>) or the proxy is specifically
1217   configured to block, or otherwise transform, such fields.
1218   Other recipients &SHOULD; ignore unrecognized header fields.
1219</t>
1220</section>
1221
1222<section title="Field Order" anchor="field.order">
1223<t>
1224   The order in which header fields with differing field names are
1225   received is not significant. However, it is "good practice" to send
1226   header fields that contain control data first, such as <x:ref>Host</x:ref>
1227   on requests and <x:ref>Date</x:ref> on responses, so that implementations
1228   can decide when not to handle a message as early as possible.  A server
1229   &MUST; wait until the entire header section is received before interpreting
1230   a request message, since later header fields might include conditionals,
1231   authentication credentials, or deliberately misleading duplicate
1232   header fields that would impact request processing.
1233</t>
1234<t>
1235   A sender &MUST-NOT; generate multiple header fields with the same field
1236   name in a message unless either the entire field value for that
1237   header field is defined as a comma-separated list [i.e., #(values)]
1238   or the header field is a well-known exception (as noted below).
1239</t>
1240<t>
1241   Multiple header fields with the same field name can be combined into
1242   one "field-name: field-value" pair, without changing the semantics of the
1243   message, by appending each subsequent field value to the combined
1244   field value in order, separated by a comma. The order in which
1245   header fields with the same field name are received is therefore
1246   significant to the interpretation of the combined field value;
1247   a proxy &MUST-NOT; change the order of these field values when
1248   forwarding a message.
1249</t>
1250<x:note>
1251  <t>
1252   &Note; In practice, the "Set-Cookie" header field (<xref target="RFC6265"/>)
1253   often appears multiple times in a response message and does not use the
1254   list syntax, violating the above requirements on multiple header fields
1255   with the same name. Since it cannot be combined into a single field-value,
1256   recipients ought to handle "Set-Cookie" as a special case while processing
1257   header fields. (See Appendix A.2.3 of <xref target="Kri2001"/> for details.)
1258  </t>
1259</x:note>
1260</section>
1261
1262<section title="Whitespace" anchor="whitespace">
1263<t anchor="rule.LWS">
1264   This specification uses three rules to denote the use of linear
1265   whitespace: OWS (optional whitespace), RWS (required whitespace), and
1266   BWS ("bad" whitespace).
1267</t>
1268<t anchor="rule.OWS">
1269   The OWS rule is used where zero or more linear whitespace octets might
1270   appear. For protocol elements where optional whitespace is preferred to
1271   improve readability, a sender &SHOULD; generate the optional whitespace
1272   as a single SP; otherwise, a sender &SHOULD-NOT; generate optional
1273   whitespace except as needed to white-out invalid or unwanted protocol
1274   elements during in-place message filtering.
1275</t>
1276<t anchor="rule.RWS">
1277   The RWS rule is used when at least one linear whitespace octet is required
1278   to separate field tokens. A sender &SHOULD; generate RWS as a single SP.
1279</t>
1280<t anchor="rule.BWS">
1281   The BWS rule is used where the grammar allows optional whitespace only for
1282   historical reasons. A sender &MUST-NOT; generate BWS in messages.
1283   A recipient &MUST; parse for such bad whitespace and remove it before
1284   interpreting the protocol element.
1285</t>
1286<t anchor="rule.whitespace">
1287  <x:anchor-alias value="BWS"/>
1288  <x:anchor-alias value="OWS"/>
1289  <x:anchor-alias value="RWS"/>
1290</t>
1291<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"/>
1292  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1293                 ; optional whitespace
1294  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1295                 ; required whitespace
1296  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1297                 ; "bad" whitespace
1298</artwork></figure>
1299</section>
1300
1301<section title="Field Parsing" anchor="field.parsing">
1302<t>
1303   No whitespace is allowed between the header field-name and colon.
1304   In the past, differences in the handling of such whitespace have led to
1305   security vulnerabilities in request routing and response handling.
1306   A server &MUST; reject any received request message that contains
1307   whitespace between a header field-name and colon with a response code of
1308   <x:ref>400 (Bad Request)</x:ref>. A proxy &MUST; remove any such whitespace
1309   from a response message before forwarding the message downstream.
1310</t>
1311<t>
1312   A field value is preceded by optional whitespace (OWS); a single SP is
1313   preferred. The field value does not include any leading or trailing white
1314   space: OWS occurring before the first non-whitespace octet of the field
1315   value or after the last non-whitespace octet of the field value ought to be
1316   excluded by parsers when extracting the field value from a header field.
1317</t>
1318<t>
1319   A recipient of field-content containing multiple sequential octets of
1320   optional (OWS) or required (RWS) whitespace &SHOULD; either replace the
1321   sequence with a single SP or transform any non-SP octets in the sequence to
1322   SP octets before interpreting the field value or forwarding the message
1323   downstream.
1324</t>
1325<t>
1326   Historically, HTTP header field values could be extended over multiple
1327   lines by preceding each extra line with at least one space or horizontal
1328   tab (obs-fold). This specification deprecates such line folding except
1329   within the message/http media type
1330   (<xref target="internet.media.type.message.http"/>).
1331   Senders &MUST-NOT; generate messages that include line folding
1332   (i.e., that contain any field-value that contains a match to the
1333   <x:ref>obs-fold</x:ref> rule) unless the message is intended for packaging
1334   within the message/http media type.
1335</t>
1336<t>
1337   A server that receives an <x:ref>obs-fold</x:ref> in a request message that
1338   is not within a message/http container &MUST; either reject the message by
1339   sending a <x:ref>400 (Bad Request)</x:ref>, preferably with a
1340   representation explaining that obsolete line folding is unacceptable, or
1341   replace each received <x:ref>obs-fold</x:ref> with one or more
1342   <x:ref>SP</x:ref> octets prior to interpreting the field value or
1343   forwarding the message downstream.
1344</t>
1345<t>
1346   A proxy or gateway that receives an <x:ref>obs-fold</x:ref> in a response
1347   message that is not within a message/http container &MUST; either discard
1348   the message and replace it with a <x:ref>502 (Bad Gateway)</x:ref>
1349   response, preferably with a representation explaining that unacceptable
1350   line folding was received, or replace each received <x:ref>obs-fold</x:ref>
1351   with one or more <x:ref>SP</x:ref> octets prior to interpreting the field
1352   value or forwarding the message downstream.
1353</t>
1354<t>
1355   A user agent that receives an <x:ref>obs-fold</x:ref> in a response message
1356   that is not within a message/http container &MUST; replace each received
1357   <x:ref>obs-fold</x:ref> with one or more <x:ref>SP</x:ref> octets prior to
1358   interpreting the field value.
1359</t>
1360<t>
1361   Historically, HTTP has allowed field content with text in the ISO-8859-1
1362   <xref target="ISO-8859-1"/> charset, supporting other charsets only
1363   through use of <xref target="RFC2047"/> encoding.
1364   In practice, most HTTP header field values use only a subset of the
1365   US-ASCII charset <xref target="USASCII"/>. Newly defined
1366   header fields &SHOULD; limit their field values to US-ASCII octets.
1367   Recipients &SHOULD; treat other octets in field content (obs-text) as
1368   opaque data.
1369</t>
1370</section>
1371
1372<section title="Field Limits" anchor="field.limits">
1373<t>
1374   HTTP does not place a pre-defined limit on the length of each header field
1375   or on the length of the header block as a whole.  Various ad-hoc
1376   limitations on individual header field length are found in practice,
1377   often depending on the specific field semantics.
1378</t>
1379<t>
1380   A server &MUST; be prepared to receive request header fields of unbounded
1381   length and respond with an appropriate <x:ref>4xx (Client Error)</x:ref>
1382   status code if the received header field(s) are larger than the server
1383   wishes to process.
1384</t>
1385<t>
1386   A client &MUST; be prepared to receive response header fields of unbounded
1387   length. A client &MAY; discard or truncate received header fields that are
1388   larger than the client wishes to process if the field semantics are such
1389   that the dropped value(s) can be safely ignored without changing the
1390   response semantics.
1391</t>
1392</section>
1393
1394<section title="Field value components" anchor="field.components">
1395<t anchor="rule.token.separators">
1396  <x:anchor-alias value="tchar"/>
1397  <x:anchor-alias value="token"/>
1398  <x:anchor-alias value="special"/>
1399  <x:anchor-alias value="word"/>
1400   Many HTTP header field values consist of words (token or quoted-string)
1401   separated by whitespace or special characters. These special characters
1402   &MUST; be in a quoted string to be used within a parameter value (as defined
1403   in <xref target="transfer.codings"/>).
1404</t>
1405<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>
1406  <x:ref>word</x:ref>           = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1407
1408  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1409<!--
1410  IMPORTANT: when editing "tchar" make sure that "special" is updated accordingly!!!
1411 -->
1412  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1413                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1414                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1415                 ; any <x:ref>VCHAR</x:ref>, except <x:ref>special</x:ref>
1416
1417  <x:ref>special</x:ref>        = "(" / ")" / "&lt;" / ">" / "@" / ","
1418                 / ";" / ":" / "\" / DQUOTE / "/" / "["
1419                 / "]" / "?" / "=" / "{" / "}"
1420</artwork></figure>
1421<t anchor="rule.quoted-string">
1422  <x:anchor-alias value="quoted-string"/>
1423  <x:anchor-alias value="qdtext"/>
1424  <x:anchor-alias value="obs-text"/>
1425   A string of text is parsed as a single word if it is quoted using
1426   double-quote marks.
1427</t>
1428<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"/>
1429  <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>
1430  <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>
1431  <x:ref>obs-text</x:ref>       = %x80-FF
1432</artwork></figure>
1433<t anchor="rule.quoted-pair">
1434  <x:anchor-alias value="quoted-pair"/>
1435   The backslash octet ("\") can be used as a single-octet
1436   quoting mechanism within quoted-string constructs:
1437</t>
1438<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1439  <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> )
1440</artwork></figure>
1441<t>
1442   Recipients that process the value of a quoted-string &MUST; handle a
1443   quoted-pair as if it were replaced by the octet following the backslash.
1444</t>
1445<t>
1446   Senders &SHOULD-NOT; generate a quoted-pair in a quoted-string except where
1447   necessary to quote DQUOTE and backslash octets occurring within that string.
1448</t>
1449<t anchor="rule.comment">
1450  <x:anchor-alias value="comment"/>
1451  <x:anchor-alias value="ctext"/>
1452   Comments can be included in some HTTP header fields by surrounding
1453   the comment text with parentheses. Comments are only allowed in
1454   fields containing "comment" as part of their field value definition.
1455</t>
1456<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1457  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-cpair</x:ref> / <x:ref>comment</x:ref> ) ")"
1458  <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>
1459</artwork></figure>
1460<t anchor="rule.quoted-cpair">
1461  <x:anchor-alias value="quoted-cpair"/>
1462   The backslash octet ("\") can be used as a single-octet
1463   quoting mechanism within comment constructs:
1464</t>
1465<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-cpair"/>
1466  <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> )
1467</artwork></figure>
1468<t>
1469   Senders &SHOULD-NOT; escape octets in comments that do not require escaping
1470   (i.e., other than the backslash octet "\" and the parentheses "(" and ")").
1471</t>
1472</section>
1473
1474</section>
1475
1476<section title="Message Body" anchor="message.body">
1477  <x:anchor-alias value="message-body"/>
1478<t>
1479   The message body (if any) of an HTTP message is used to carry the
1480   payload body of that request or response.  The message body is
1481   identical to the payload body unless a transfer coding has been
1482   applied, as described in <xref target="header.transfer-encoding"/>.
1483</t>
1484<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1485  <x:ref>message-body</x:ref> = *OCTET
1486</artwork></figure>
1487<t>
1488   The rules for when a message body is allowed in a message differ for
1489   requests and responses.
1490</t>
1491<t>
1492   The presence of a message body in a request is signaled by a
1493   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1494   field. Request message framing is independent of method semantics,
1495   even if the method does not define any use for a message body.
1496</t>
1497<t>
1498   The presence of a message body in a response depends on both
1499   the request method to which it is responding and the response
1500   status code (<xref target="status.line"/>).
1501   Responses to the HEAD request method never include a message body
1502   because the associated response header fields (e.g.,
1503   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1504   if present, indicate only what their values would have been if the request
1505   method had been GET (&HEAD;).
1506   <x:ref>2xx (Successful)</x:ref> responses to CONNECT switch to tunnel
1507   mode instead of having a message body (&CONNECT;).
1508   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1509   <x:ref>304 (Not Modified)</x:ref> responses do not include a message body.
1510   All other responses do include a message body, although the body
1511   might be of zero length.
1512</t>
1513
1514<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1515  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1516  <iref item="chunked (Coding Format)"/>
1517  <x:anchor-alias value="Transfer-Encoding"/>
1518<t>
1519   The Transfer-Encoding header field lists the transfer coding names
1520   corresponding to the sequence of transfer codings that have been
1521   (or will be) applied to the payload body in order to form the message body.
1522   Transfer codings are defined in <xref target="transfer.codings"/>.
1523</t>
1524<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1525  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1526</artwork></figure>
1527<t>
1528   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1529   MIME, which was designed to enable safe transport of binary data over a
1530   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1531   However, safe transport has a different focus for an 8bit-clean transfer
1532   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1533   accurately delimit a dynamically generated payload and to distinguish
1534   payload encodings that are only applied for transport efficiency or
1535   security from those that are characteristics of the selected resource.
1536</t>
1537<t>
1538   All HTTP/1.1 recipients &MUST; implement the chunked transfer coding
1539   (<xref target="chunked.encoding"/>) because it plays a crucial role in
1540   framing messages when the payload body size is not known in advance.
1541   If chunked is applied to a payload body, the sender &MUST-NOT; apply
1542   chunked more than once (i.e., chunking an already chunked message is not
1543   allowed).
1544   If any transfer coding is applied to a request payload body, the
1545   sender &MUST; apply chunked as the final transfer coding to ensure that
1546   the message is properly framed.
1547   If any transfer coding is applied to a response payload body, the
1548   sender &MUST; either apply chunked as the final transfer coding or
1549   terminate the message by closing the connection.
1550</t>
1551<figure><preamble>
1552   For example,
1553</preamble><artwork type="example">
1554  Transfer-Encoding: gzip, chunked
1555</artwork><postamble>
1556   indicates that the payload body has been compressed using the gzip
1557   coding and then chunked using the chunked coding while forming the
1558   message body.
1559</postamble></figure>
1560<t>
1561   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1562   Transfer-Encoding is a property of the message, not of the representation, and
1563   any recipient along the request/response chain &MAY; decode the received
1564   transfer coding(s) or apply additional transfer coding(s) to the message
1565   body, assuming that corresponding changes are made to the Transfer-Encoding
1566   field-value. Additional information about the encoding parameters &MAY; be
1567   provided by other header fields not defined by this specification.
1568</t>
1569<t>
1570   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1571   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1572   neither of which includes a message body,
1573   to indicate that the origin server would have applied a transfer coding
1574   to the message body if the request had been an unconditional GET.
1575   This indication is not required, however, because any recipient on
1576   the response chain (including the origin server) can remove transfer
1577   codings when they are not needed.
1578</t>
1579<t>
1580   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1581   implementations advertising only HTTP/1.0 support will not understand
1582   how to process a transfer-encoded payload.
1583   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1584   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1585   might be in the form of specific user configuration or by remembering the
1586   version of a prior received response.
1587   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1588   the corresponding request indicates HTTP/1.1 (or later).
1589</t>
1590<t>
1591   A server that receives a request message with a transfer coding it does
1592   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref>.
1593</t>
1594</section>
1595
1596<section title="Content-Length" anchor="header.content-length">
1597  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1598  <x:anchor-alias value="Content-Length"/>
1599<t>
1600   When a message does not have a <x:ref>Transfer-Encoding</x:ref> header
1601   field, a Content-Length header field can provide the anticipated size,
1602   as a decimal number of octets, for a potential payload body.
1603   For messages that do include a payload body, the Content-Length field-value
1604   provides the framing information necessary for determining where the body
1605   (and message) ends.  For messages that do not include a payload body, the
1606   Content-Length indicates the size of the selected representation
1607   (&representation;).
1608</t>
1609<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1610  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1611</artwork></figure>
1612<t>
1613   An example is
1614</t>
1615<figure><artwork type="example">
1616  Content-Length: 3495
1617</artwork></figure>
1618<t>
1619   A sender &MUST-NOT; send a Content-Length header field in any message that
1620   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1621</t>
1622<t>
1623   A user agent &SHOULD; send a Content-Length in a request message when no
1624   <x:ref>Transfer-Encoding</x:ref> is sent and the request method defines
1625   a meaning for an enclosed payload body. For example, a Content-Length
1626   header field is normally sent in a POST request even when the value is
1627   0 (indicating an empty payload body).  A user agent &SHOULD-NOT; send a
1628   Content-Length header field when the request message does not contain a
1629   payload body and the method semantics do not anticipate such a body.
1630</t>
1631<t>
1632   A server &MAY; send a Content-Length header field in a response to a HEAD
1633   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1634   response unless its field-value equals the decimal number of octets that
1635   would have been sent in the payload body of a response if the same
1636   request had used the GET method.
1637</t>
1638<t>
1639   A server &MAY; send a Content-Length header field in a
1640   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1641   (&status-304;); a server &MUST-NOT; send Content-Length in such a
1642   response unless its field-value equals the decimal number of octets that
1643   would have been sent in the payload body of a <x:ref>200 (OK)</x:ref>
1644   response to the same request.
1645</t>
1646<t>
1647   A server &MUST-NOT; send a Content-Length header field in any response
1648   with a status code of
1649   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1650   A server &SHOULD-NOT; send a Content-Length header field in any
1651   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1652</t>
1653<t>
1654   Aside from the cases defined above, in the absence of Transfer-Encoding,
1655   an origin server &SHOULD; send a Content-Length header field when the
1656   payload body size is known prior to sending the complete header block.
1657   This will allow downstream recipients to measure transfer progress,
1658   know when a received message is complete, and potentially reuse the
1659   connection for additional requests.
1660</t>
1661<t>
1662   Any Content-Length field value greater than or equal to zero is valid.
1663   Since there is no predefined limit to the length of a payload,
1664   recipients &SHOULD; anticipate potentially large decimal numerals and
1665   prevent parsing errors due to integer conversion overflows
1666   (<xref target="attack.protocol.element.size.overflows"/>).
1667</t>
1668<t>
1669   If a message is received that has multiple Content-Length header fields
1670   with field-values consisting of the same decimal value, or a single
1671   Content-Length header field with a field value containing a list of
1672   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1673   duplicate Content-Length header fields have been generated or combined by an
1674   upstream message processor, then the recipient &MUST; either reject the
1675   message as invalid or replace the duplicated field-values with a single
1676   valid Content-Length field containing that decimal value prior to
1677   determining the message body length.
1678</t>
1679<x:note>
1680  <t>
1681   &Note; HTTP's use of Content-Length for message framing differs
1682   significantly from the same field's use in MIME, where it is an optional
1683   field used only within the "message/external-body" media-type.
1684  </t>
1685</x:note>
1686</section>
1687
1688<section title="Message Body Length" anchor="message.body.length">
1689  <iref item="chunked (Coding Format)"/>
1690<t>
1691   The length of a message body is determined by one of the following
1692   (in order of precedence):
1693</t>
1694<t>
1695  <list style="numbers">
1696    <x:lt><t>
1697     Any response to a HEAD request and any response with a
1698     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1699     <x:ref>304 (Not Modified)</x:ref> status code is always
1700     terminated by the first empty line after the header fields, regardless of
1701     the header fields present in the message, and thus cannot contain a
1702     message body.
1703    </t></x:lt>
1704    <x:lt><t>
1705     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1706     connection will become a tunnel immediately after the empty line that
1707     concludes the header fields.  A client &MUST; ignore any
1708     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1709     fields received in such a message.
1710    </t></x:lt>
1711    <x:lt><t>
1712     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1713     and the chunked transfer coding (<xref target="chunked.encoding"/>)
1714     is the final encoding, the message body length is determined by reading
1715     and decoding the chunked data until the transfer coding indicates the
1716     data is complete.
1717    </t>
1718    <t>
1719     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1720     response and the chunked transfer coding is not the final encoding, the
1721     message body length is determined by reading the connection until it is
1722     closed by the server.
1723     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a request and the
1724     chunked transfer coding is not the final encoding, the message body
1725     length cannot be determined reliably; the server &MUST; respond with
1726     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1727    </t>
1728    <t>
1729     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1730     and a <x:ref>Content-Length</x:ref> header field, the Transfer-Encoding
1731     overrides the Content-Length. Such a message might indicate an attempt
1732     to perform request or response smuggling (bypass of security-related
1733     checks on message routing or content) and thus ought to be handled as
1734     an error.  A sender &MUST; remove the received Content-Length field
1735     prior to forwarding such a message downstream.
1736    </t></x:lt>
1737    <x:lt><t>
1738     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1739     either multiple <x:ref>Content-Length</x:ref> header fields having
1740     differing field-values or a single Content-Length header field having an
1741     invalid value, then the message framing is invalid and &MUST; be treated
1742     as an error to prevent request or response smuggling.
1743     If this is a request message, the server &MUST; respond with
1744     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1745     If this is a response message received by a proxy, the proxy
1746     &MUST; discard the received response, send a <x:ref>502 (Bad Gateway)</x:ref>
1747     status code as its downstream response, and then close the connection.
1748     If this is a response message received by a user agent, it &MUST; be
1749     treated as an error by discarding the message and closing the connection.
1750    </t></x:lt>
1751    <x:lt><t>
1752     If a valid <x:ref>Content-Length</x:ref> header field is present without
1753     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1754     expected message body length in octets.
1755     If the sender closes the connection or the recipient times out before the
1756     indicated number of octets are received, the recipient &MUST; consider
1757     the message to be incomplete and close the connection.
1758    </t></x:lt>
1759    <x:lt><t>
1760     If this is a request message and none of the above are true, then the
1761     message body length is zero (no message body is present).
1762    </t></x:lt>
1763    <x:lt><t>
1764     Otherwise, this is a response message without a declared message body
1765     length, so the message body length is determined by the number of octets
1766     received prior to the server closing the connection.
1767    </t></x:lt>
1768  </list>
1769</t>
1770<t>
1771   Since there is no way to distinguish a successfully completed,
1772   close-delimited message from a partially-received message interrupted
1773   by network failure, a server &SHOULD; use encoding or
1774   length-delimited messages whenever possible.  The close-delimiting
1775   feature exists primarily for backwards compatibility with HTTP/1.0.
1776</t>
1777<t>
1778   A server &MAY; reject a request that contains a message body but
1779   not a <x:ref>Content-Length</x:ref> by responding with
1780   <x:ref>411 (Length Required)</x:ref>.
1781</t>
1782<t>
1783   Unless a transfer coding other than chunked has been applied,
1784   a client that sends a request containing a message body &SHOULD;
1785   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1786   length is known in advance, rather than the chunked transfer coding, since some
1787   existing services respond to chunked with a <x:ref>411 (Length Required)</x:ref>
1788   status code even though they understand the chunked transfer coding.  This
1789   is typically because such services are implemented via a gateway that
1790   requires a content-length in advance of being called and the server
1791   is unable or unwilling to buffer the entire request before processing.
1792</t>
1793<t>
1794   A user agent that sends a request containing a message body &MUST; send a
1795   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1796   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1797   the form of specific user configuration or by remembering the version of a
1798   prior received response.
1799</t>
1800<t>
1801   If the final response to the last request on a connection has been
1802   completely received and there remains additional data to read, a user agent
1803   &MAY; discard the remaining data or attempt to determine if that data
1804   belongs as part of the prior response body, which might be the case if the
1805   prior message's Content-Length value is incorrect. A client &MUST-NOT;
1806   process, cache, or forward such extra data as a separate response, since
1807   such behavior would be vulnerable to cache poisoning.
1808</t>
1809</section>
1810</section>
1811
1812<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1813<t>
1814   A server that receives an incomplete request message, usually due to a
1815   canceled request or a triggered time-out exception, &MAY; send an error
1816   response prior to closing the connection.
1817</t>
1818<t>
1819   A client that receives an incomplete response message, which can occur
1820   when a connection is closed prematurely or when decoding a supposedly
1821   chunked transfer coding fails, &MUST; record the message as incomplete.
1822   Cache requirements for incomplete responses are defined in
1823   &cache-incomplete;.
1824</t>
1825<t>
1826   If a response terminates in the middle of the header block (before the
1827   empty line is received) and the status code might rely on header fields to
1828   convey the full meaning of the response, then the client cannot assume
1829   that meaning has been conveyed; the client might need to repeat the
1830   request in order to determine what action to take next.
1831</t>
1832<t>
1833   A message body that uses the chunked transfer coding is
1834   incomplete if the zero-sized chunk that terminates the encoding has not
1835   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1836   incomplete if the size of the message body received (in octets) is less than
1837   the value given by Content-Length.  A response that has neither chunked
1838   transfer coding nor Content-Length is terminated by closure of the
1839   connection, and thus is considered complete regardless of the number of
1840   message body octets received, provided that the header block was received
1841   intact.
1842</t>
1843</section>
1844
1845<section title="Message Parsing Robustness" anchor="message.robustness">
1846<t>
1847   Older HTTP/1.0 user agent implementations might send an extra CRLF
1848   after a POST request as a workaround for some early server
1849   applications that failed to read message body content that was
1850   not terminated by a line-ending. An HTTP/1.1 user agent &MUST-NOT;
1851   preface or follow a request with an extra CRLF.  If terminating
1852   the request message body with a line-ending is desired, then the
1853   user agent &MUST; count the terminating CRLF octets as part of the
1854   message body length.
1855</t>
1856<t>
1857   In the interest of robustness, servers &SHOULD; ignore at least one
1858   empty line received where a request-line is expected. In other words, if
1859   a server is reading the protocol stream at the beginning of a
1860   message and receives a CRLF first, the server &SHOULD; ignore the CRLF.
1861</t>
1862<t>
1863   Although the line terminator for the start-line and header
1864   fields is the sequence CRLF, recipients &MAY; recognize a
1865   single LF as a line terminator and ignore any preceding CR.
1866</t>
1867<t>
1868   Although the request-line and status-line grammar rules require that each
1869   of the component elements be separated by a single SP octet, recipients
1870   &MAY; instead parse on whitespace-delimited word boundaries and, aside
1871   from the CRLF terminator, treat any form of whitespace as the SP separator
1872   while ignoring preceding or trailing whitespace;
1873   such whitespace includes one or more of the following octets:
1874   SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
1875</t>
1876<t>
1877   When a server listening only for HTTP request messages, or processing
1878   what appears from the start-line to be an HTTP request message,
1879   receives a sequence of octets that does not match the HTTP-message
1880   grammar aside from the robustness exceptions listed above, the
1881   server &SHOULD; respond with a <x:ref>400 (Bad Request)</x:ref> response. 
1882</t>
1883</section>
1884</section>
1885
1886<section title="Transfer Codings" anchor="transfer.codings">
1887  <x:anchor-alias value="transfer-coding"/>
1888  <x:anchor-alias value="transfer-extension"/>
1889<t>
1890   Transfer coding names are used to indicate an encoding
1891   transformation that has been, can be, or might need to be applied to a
1892   payload body in order to ensure "safe transport" through the network.
1893   This differs from a content coding in that the transfer coding is a
1894   property of the message rather than a property of the representation
1895   that is being transferred.
1896</t>
1897<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1898  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1899                     / "compress" ; <xref target="compress.coding"/>
1900                     / "deflate" ; <xref target="deflate.coding"/>
1901                     / "gzip" ; <xref target="gzip.coding"/>
1902                     / <x:ref>transfer-extension</x:ref>
1903  <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> )
1904</artwork></figure>
1905<t anchor="rule.parameter">
1906  <x:anchor-alias value="attribute"/>
1907  <x:anchor-alias value="transfer-parameter"/>
1908  <x:anchor-alias value="value"/>
1909   Parameters are in the form of attribute/value pairs.
1910</t>
1911<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"/>
1912  <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>
1913  <x:ref>attribute</x:ref>          = <x:ref>token</x:ref>
1914  <x:ref>value</x:ref>              = <x:ref>word</x:ref>
1915</artwork></figure>
1916<t>
1917   All transfer-coding names are case-insensitive and ought to be registered
1918   within the HTTP Transfer Coding registry, as defined in
1919   <xref target="transfer.coding.registry"/>.
1920   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1921   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1922   header fields.
1923</t>
1924
1925<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1926  <iref primary="true" item="chunked (Coding Format)"/>
1927  <x:anchor-alias value="chunk"/>
1928  <x:anchor-alias value="chunked-body"/>
1929  <x:anchor-alias value="chunk-data"/>
1930  <x:anchor-alias value="chunk-ext"/>
1931  <x:anchor-alias value="chunk-ext-name"/>
1932  <x:anchor-alias value="chunk-ext-val"/>
1933  <x:anchor-alias value="chunk-size"/>
1934  <x:anchor-alias value="last-chunk"/>
1935  <x:anchor-alias value="trailer-part"/>
1936  <x:anchor-alias value="quoted-str-nf"/>
1937  <x:anchor-alias value="qdtext-nf"/>
1938<t>
1939   The chunked transfer coding modifies the body of a message in order to
1940   transfer it as a series of chunks, each with its own size indicator,
1941   followed by an &OPTIONAL; trailer containing header fields. This
1942   allows dynamically generated content to be transferred along with the
1943   information necessary for the recipient to verify that it has
1944   received the full message.
1945</t>
1946<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="true" item="Grammar" subitem="trailer-part"/><iref primary="true" item="Grammar" subitem="quoted-str-nf"/><iref primary="true" item="Grammar" subitem="qdtext-nf"/>
1947  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1948                   <x:ref>last-chunk</x:ref>
1949                   <x:ref>trailer-part</x:ref>
1950                   <x:ref>CRLF</x:ref>
1951 
1952  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1953                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1954  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1955  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1956 
1957  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
1958  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
1959  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-str-nf</x:ref>
1960  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1961  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1962 
1963  <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>
1964                 ; like <x:ref>quoted-string</x:ref>, but disallowing line folding
1965  <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>
1966</artwork></figure>
1967<t>
1968   Chunk extensions within the chunked transfer coding are deprecated.
1969   Senders &SHOULD-NOT; send chunk-ext.
1970   Definition of new chunk extensions is discouraged.
1971</t>
1972<t>
1973   The chunk-size field is a string of hex digits indicating the size of
1974   the chunk-data in octets. The chunked transfer coding is complete when a
1975   chunk with a chunk-size of zero is received, possibly followed by a
1976   trailer, and finally terminated by an empty line.
1977</t>
1978
1979<section title="Trailer" anchor="header.trailer">
1980  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
1981  <x:anchor-alias value="Trailer"/>
1982<t>
1983   A trailer allows the sender to include additional fields at the end of a
1984   chunked message in order to supply metadata that might be dynamically
1985   generated while the message body is sent, such as a message integrity
1986   check, digital signature, or post-processing status.
1987   The trailer &MUST-NOT; contain fields that need to be known before a
1988   recipient processes the body, such as <x:ref>Transfer-Encoding</x:ref>,
1989   <x:ref>Content-Length</x:ref>, and <x:ref>Trailer</x:ref>.
1990</t>
1991<t>
1992   When a message includes a message body encoded with the chunked
1993   transfer coding and the sender desires to send metadata in the form of
1994   trailer fields at the end of the message, the sender &SHOULD; send a
1995   <x:ref>Trailer</x:ref> header field before the message body to indicate
1996   which fields will be present in the trailers. This allows the recipient
1997   to prepare for receipt of that metadata before it starts processing the body,
1998   which is useful if the message is being streamed and the recipient wishes
1999   to confirm an integrity check on the fly.
2000</t>
2001<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
2002  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
2003</artwork></figure>
2004<t>
2005   If no <x:ref>Trailer</x:ref> header field is present, the sender of a
2006   chunked message body &SHOULD; send an empty trailer.
2007</t>
2008<t>
2009   A server &MUST; send an empty trailer with the chunked transfer coding
2010   unless at least one of the following is true:
2011  <list style="numbers">
2012    <t>the request included a <x:ref>TE</x:ref> header field that indicates
2013    "trailers" is acceptable in the transfer coding of the response, as
2014    described in <xref target="header.te"/>; or,</t>
2015     
2016    <t>the trailer fields consist entirely of optional metadata and the
2017    recipient could use the message (in a manner acceptable to the server where
2018    the field originated) without receiving that metadata. In other words,
2019    the server that generated the header field is willing to accept the
2020    possibility that the trailer fields might be silently discarded along
2021    the path to the client.</t>
2022  </list>
2023</t>
2024<t>
2025   The above requirement prevents the need for an infinite buffer when a
2026   message is being received by an HTTP/1.1 (or later) proxy and forwarded to
2027   an HTTP/1.0 recipient.
2028</t>
2029</section>
2030
2031<section title="Decoding chunked" anchor="decoding.chunked">
2032<t>
2033   A process for decoding the chunked transfer coding
2034   can be represented in pseudo-code as:
2035</t>
2036<figure><artwork type="code">
2037  length := 0
2038  read chunk-size, chunk-ext (if any), and CRLF
2039  while (chunk-size &gt; 0) {
2040     read chunk-data and CRLF
2041     append chunk-data to decoded-body
2042     length := length + chunk-size
2043     read chunk-size, chunk-ext (if any), and CRLF
2044  }
2045  read header-field
2046  while (header-field not empty) {
2047     append header-field to existing header fields
2048     read header-field
2049  }
2050  Content-Length := length
2051  Remove "chunked" from Transfer-Encoding
2052  Remove Trailer from existing header fields
2053</artwork></figure>
2054<t>
2055   All recipients &MUST; be able to receive and decode the
2056   chunked transfer coding and &MUST; ignore chunk-ext extensions
2057   they do not understand.
2058</t>
2059</section>
2060</section>
2061
2062<section title="Compression Codings" anchor="compression.codings">
2063<t>
2064   The codings defined below can be used to compress the payload of a
2065   message.
2066</t>
2067
2068<section title="Compress Coding" anchor="compress.coding">
2069<iref item="compress (Coding Format)"/>
2070<t>
2071   The "compress" format is produced by the common UNIX file compression
2072   program "compress". This format is an adaptive Lempel-Ziv-Welch
2073   coding (LZW). Recipients &SHOULD; consider "x-compress" to be
2074   equivalent to "compress".
2075</t>
2076</section>
2077
2078<section title="Deflate Coding" anchor="deflate.coding">
2079<iref item="deflate (Coding Format)"/>
2080<t>
2081   The "deflate" format is defined as the "deflate" compression mechanism
2082   (described in <xref target="RFC1951"/>) used inside the "zlib"
2083   data format (<xref target="RFC1950"/>).
2084</t>
2085<x:note>
2086  <t>
2087    &Note; Some incorrect implementations send the "deflate"
2088    compressed data without the zlib wrapper.
2089   </t>
2090</x:note>
2091</section>
2092
2093<section title="Gzip Coding" anchor="gzip.coding">
2094<iref item="gzip (Coding Format)"/>
2095<t>
2096   The "gzip" format is produced by the file compression program
2097   "gzip" (GNU zip), as described in <xref target="RFC1952"/>. This format is a
2098   Lempel-Ziv coding (LZ77) with a 32 bit CRC.
2099   Recipients &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2100</t>
2101</section>
2102
2103</section>
2104
2105<section title="TE" anchor="header.te">
2106  <iref primary="true" item="TE header field" x:for-anchor=""/>
2107  <x:anchor-alias value="TE"/>
2108  <x:anchor-alias value="t-codings"/>
2109  <x:anchor-alias value="t-ranking"/>
2110  <x:anchor-alias value="rank"/>
2111<t>
2112   The "TE" header field in a request indicates what transfer codings,
2113   besides chunked, the client is willing to accept in response, and
2114   whether or not the client is willing to accept trailer fields in a
2115   chunked transfer coding.
2116</t>
2117<t>
2118   The TE field-value consists of a comma-separated list of transfer coding
2119   names, each allowing for optional parameters (as described in
2120   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2121   Clients &MUST-NOT; send the chunked transfer coding name in TE;
2122   chunked is always acceptable for HTTP/1.1 recipients.
2123</t>
2124<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"/>
2125  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2126  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2127  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2128  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2129             / ( "1" [ "." 0*3("0") ] )
2130</artwork></figure>
2131<t>
2132   Three examples of TE use are below.
2133</t>
2134<figure><artwork type="example">
2135  TE: deflate
2136  TE:
2137  TE: trailers, deflate;q=0.5
2138</artwork></figure>
2139<t>
2140   The presence of the keyword "trailers" indicates that the client is willing
2141   to accept trailer fields in a chunked transfer coding, as defined in
2142   <xref target="chunked.encoding"/>, on behalf of itself and any downstream
2143   clients. For requests from an intermediary, this implies that either:
2144   (a) all downstream clients are willing to accept trailer fields in the
2145   forwarded response; or,
2146   (b) the intermediary will attempt to buffer the response on behalf of
2147   downstream recipients.
2148   Note that HTTP/1.1 does not define any means to limit the size of a
2149   chunked response such that an intermediary can be assured of buffering the
2150   entire response.
2151</t>
2152<t>
2153   When multiple transfer codings are acceptable, the client &MAY; rank the
2154   codings by preference using a case-insensitive "q" parameter (similar to
2155   the qvalues used in content negotiation fields, &qvalue;). The rank value
2156   is a real number in the range 0 through 1, where 0.001 is the least
2157   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2158</t>
2159<t>
2160   If the TE field-value is empty or if no TE field is present, the only
2161   acceptable transfer coding is chunked. A message with no transfer coding
2162   is always acceptable.
2163</t>
2164<t>
2165   Since the TE header field only applies to the immediate connection,
2166   a sender of TE &MUST; also send a "TE" connection option within the
2167   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2168   in order to prevent the TE field from being forwarded by intermediaries
2169   that do not support its semantics.
2170</t>
2171</section>
2172</section>
2173
2174<section title="Message Routing" anchor="message.routing">
2175<t>
2176   HTTP request message routing is determined by each client based on the
2177   target resource, the client's proxy configuration, and
2178   establishment or reuse of an inbound connection.  The corresponding
2179   response routing follows the same connection chain back to the client.
2180</t>
2181
2182<section title="Identifying a Target Resource" anchor="target-resource">
2183  <iref primary="true" item="target resource"/>
2184  <iref primary="true" item="target URI"/>
2185  <x:anchor-alias value="target resource"/>
2186  <x:anchor-alias value="target URI"/>
2187<t>
2188   HTTP is used in a wide variety of applications, ranging from
2189   general-purpose computers to home appliances.  In some cases,
2190   communication options are hard-coded in a client's configuration.
2191   However, most HTTP clients rely on the same resource identification
2192   mechanism and configuration techniques as general-purpose Web browsers.
2193</t>
2194<t>
2195   HTTP communication is initiated by a user agent for some purpose.
2196   The purpose is a combination of request semantics, which are defined in
2197   <xref target="Part2"/>, and a target resource upon which to apply those
2198   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2199   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2200   would resolve to its absolute form in order to obtain the
2201   "<x:dfn>target URI</x:dfn>".  The target URI
2202   excludes the reference's fragment identifier component, if any,
2203   since fragment identifiers are reserved for client-side processing
2204   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2205</t>
2206</section>
2207
2208<section title="Connecting Inbound" anchor="connecting.inbound">
2209<t>
2210   Once the target URI is determined, a client needs to decide whether
2211   a network request is necessary to accomplish the desired semantics and,
2212   if so, where that request is to be directed.
2213</t>
2214<t>
2215   If the client has a cache <xref target="Part6"/> and the request can be
2216   satisfied by it, then the request is
2217   usually directed there first.
2218</t>
2219<t>
2220   If the request is not satisfied by a cache, then a typical client will
2221   check its configuration to determine whether a proxy is to be used to
2222   satisfy the request.  Proxy configuration is implementation-dependent,
2223   but is often based on URI prefix matching, selective authority matching,
2224   or both, and the proxy itself is usually identified by an "http" or
2225   "https" URI.  If a proxy is applicable, the client connects inbound by
2226   establishing (or reusing) a connection to that proxy.
2227</t>
2228<t>
2229   If no proxy is applicable, a typical client will invoke a handler routine,
2230   usually specific to the target URI's scheme, to connect directly
2231   to an authority for the target resource.  How that is accomplished is
2232   dependent on the target URI scheme and defined by its associated
2233   specification, similar to how this specification defines origin server
2234   access for resolution of the "http" (<xref target="http.uri"/>) and
2235   "https" (<xref target="https.uri"/>) schemes.
2236</t>
2237<t>
2238   HTTP requirements regarding connection management are defined in
2239   <xref target="connection.management"/>.
2240</t>
2241</section>
2242
2243<section title="Request Target" anchor="request-target">
2244<t>
2245   Once an inbound connection is obtained,
2246   the client sends an HTTP request message (<xref target="http.message"/>)
2247   with a request-target derived from the target URI.
2248   There are four distinct formats for the request-target, depending on both
2249   the method being requested and whether the request is to a proxy.
2250</t>   
2251<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"/>
2252  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2253                 / <x:ref>absolute-form</x:ref>
2254                 / <x:ref>authority-form</x:ref>
2255                 / <x:ref>asterisk-form</x:ref>
2256
2257  <x:ref>origin-form</x:ref>    = <x:ref>absolute-path</x:ref> [ "?" <x:ref>query</x:ref> ]
2258  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2259  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2260  <x:ref>asterisk-form</x:ref>  = "*"
2261</artwork></figure>
2262<t anchor="origin-form"><iref item="origin-form (of request-target)"/>
2263  <x:h>origin-form</x:h>
2264</t>
2265<t>
2266   The most common form of request-target is the <x:dfn>origin-form</x:dfn>.
2267   When making a request directly to an origin server, other than a CONNECT
2268   or server-wide OPTIONS request (as detailed below),
2269   a client &MUST; send only the absolute path and query components of
2270   the target URI as the request-target.
2271   If the target URI's path component is empty, then the client &MUST; send
2272   "/" as the path within the origin-form of request-target.
2273   A <x:ref>Host</x:ref> header field is also sent, as defined in
2274   <xref target="header.host"/>, containing the target URI's
2275   authority component (excluding any userinfo).
2276</t>
2277<t>
2278   For example, a client wishing to retrieve a representation of the resource
2279   identified as
2280</t>
2281<figure><artwork x:indent-with="  " type="example">
2282http://www.example.org/where?q=now
2283</artwork></figure>
2284<t>
2285   directly from the origin server would open (or reuse) a TCP connection
2286   to port 80 of the host "www.example.org" and send the lines:
2287</t>
2288<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2289GET /where?q=now HTTP/1.1
2290Host: www.example.org
2291</artwork></figure>
2292<t>
2293   followed by the remainder of the request message.
2294</t>
2295<t anchor="absolute-form"><iref item="absolute-form (of request-target)"/>
2296  <x:h>absolute-form</x:h>
2297</t>
2298<t>
2299   When making a request to a proxy, other than a CONNECT or server-wide
2300   OPTIONS request (as detailed below), a client &MUST; send the target URI
2301   in <x:dfn>absolute-form</x:dfn> as the request-target.
2302   The proxy is requested to either service that request from a valid cache,
2303   if possible, or make the same request on the client's behalf to either
2304   the next inbound proxy server or directly to the origin server indicated
2305   by the request-target.  Requirements on such "forwarding" of messages are
2306   defined in <xref target="message.forwarding"/>.
2307</t>
2308<t>
2309   An example absolute-form of request-line would be:
2310</t>
2311<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2312GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
2313</artwork></figure>
2314<t>
2315   To allow for transition to the absolute-form for all requests in some
2316   future version of HTTP, HTTP/1.1 servers &MUST; accept the absolute-form
2317   in requests, even though HTTP/1.1 clients will only send them in requests
2318   to proxies.
2319</t>
2320<t anchor="authority-form"><iref item="authority-form (of request-target)"/>
2321  <x:h>authority-form</x:h>
2322</t>
2323<t>
2324   The <x:dfn>authority-form</x:dfn> of request-target is only used for CONNECT requests
2325   (&CONNECT;).  When making a CONNECT request to establish a tunnel through
2326   one or more proxies, a client &MUST; send only the target URI's
2327   authority component (excluding any userinfo) as the request-target.
2328   For example,
2329</t>
2330<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2331CONNECT www.example.com:80 HTTP/1.1
2332</artwork></figure>
2333<t anchor="asterisk-form"><iref item="asterisk-form (of request-target)"/>
2334  <x:h>asterisk-form</x:h>
2335</t>
2336<t>
2337   The <x:dfn>asterisk-form</x:dfn> of request-target is only used for a server-wide
2338   OPTIONS request (&OPTIONS;).  When a client wishes to request OPTIONS
2339   for the server as a whole, as opposed to a specific named resource of
2340   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2341   For example,
2342</t>
2343<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2344OPTIONS * HTTP/1.1
2345</artwork></figure>
2346<t>
2347   If a proxy receives an OPTIONS request with an absolute-form of
2348   request-target in which the URI has an empty path and no query component,
2349   then the last proxy on the request chain &MUST; send a request-target
2350   of "*" when it forwards the request to the indicated origin server.
2351</t>
2352<figure><preamble>  
2353   For example, the request
2354</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2355OPTIONS http://www.example.org:8001 HTTP/1.1
2356</artwork></figure>
2357<figure><preamble>  
2358  would be forwarded by the final proxy as
2359</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2360OPTIONS * HTTP/1.1
2361Host: www.example.org:8001
2362</artwork>
2363<postamble>
2364   after connecting to port 8001 of host "www.example.org".
2365</postamble>
2366</figure>
2367</section>
2368
2369<section title="Host" anchor="header.host">
2370  <iref primary="true" item="Host header field" x:for-anchor=""/>
2371  <x:anchor-alias value="Host"/>
2372<t>
2373   The "Host" header field in a request provides the host and port
2374   information from the target URI, enabling the origin
2375   server to distinguish among resources while servicing requests
2376   for multiple host names on a single IP address.  Since the Host
2377   field-value is critical information for handling a request, it
2378   &SHOULD; be sent as the first header field following the request-line.
2379</t>
2380<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2381  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2382</artwork></figure>
2383<t>
2384   A client &MUST; send a Host header field in all HTTP/1.1 request
2385   messages.  If the target URI includes an authority component, then
2386   the Host field-value &MUST; be identical to that authority component
2387   after excluding any userinfo (<xref target="http.uri"/>).
2388   If the authority component is missing or undefined for the target URI,
2389   then the Host header field &MUST; be sent with an empty field-value.
2390</t>
2391<t>
2392   For example, a GET request to the origin server for
2393   &lt;http://www.example.org/pub/WWW/&gt; would begin with:
2394</t>
2395<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2396GET /pub/WWW/ HTTP/1.1
2397Host: www.example.org
2398</artwork></figure>
2399<t>
2400   The Host header field &MUST; be sent in an HTTP/1.1 request even
2401   if the request-target is in the absolute-form, since this
2402   allows the Host information to be forwarded through ancient HTTP/1.0
2403   proxies that might not have implemented Host.
2404</t>
2405<t>
2406   When a proxy receives a request with an absolute-form of
2407   request-target, the proxy &MUST; ignore the received
2408   Host header field (if any) and instead replace it with the host
2409   information of the request-target.  If the proxy forwards the request,
2410   it &MUST; generate a new Host field-value based on the received
2411   request-target rather than forward the received Host field-value.
2412</t>
2413<t>
2414   Since the Host header field acts as an application-level routing
2415   mechanism, it is a frequent target for malware seeking to poison
2416   a shared cache or redirect a request to an unintended server.
2417   An interception proxy is particularly vulnerable if it relies on
2418   the Host field-value for redirecting requests to internal
2419   servers, or for use as a cache key in a shared cache, without
2420   first verifying that the intercepted connection is targeting a
2421   valid IP address for that host.
2422</t>
2423<t>
2424   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2425   to any HTTP/1.1 request message that lacks a Host header field and
2426   to any request message that contains more than one Host header field
2427   or a Host header field with an invalid field-value.
2428</t>
2429</section>
2430
2431<section title="Effective Request URI" anchor="effective.request.uri">
2432  <iref primary="true" item="effective request URI"/>
2433  <x:anchor-alias value="effective request URI"/>
2434<t>
2435   A server that receives an HTTP request message &MUST; reconstruct
2436   the user agent's original target URI, based on the pieces of information
2437   learned from the request-target, <x:ref>Host</x:ref> header field, and
2438   connection context, in order to identify the intended target resource and
2439   properly service the request. The URI derived from this reconstruction
2440   process is referred to as the "<x:dfn>effective request URI</x:dfn>".
2441</t>
2442<t>
2443   For a user agent, the effective request URI is the target URI.
2444</t>
2445<t>
2446   If the request-target is in absolute-form, then the effective request URI
2447   is the same as the request-target.  Otherwise, the effective request URI
2448   is constructed as follows.
2449</t>
2450<t>
2451   If the request is received over a TLS-secured TCP connection,
2452   then the effective request URI's scheme is "https"; otherwise, the
2453   scheme is "http".
2454</t>
2455<t>
2456   If the request-target is in authority-form, then the effective
2457   request URI's authority component is the same as the request-target.
2458   Otherwise, if a <x:ref>Host</x:ref> header field is supplied with a
2459   non-empty field-value, then the authority component is the same as the
2460   Host field-value. Otherwise, the authority component is the concatenation of
2461   the default host name configured for the server, a colon (":"), and the
2462   connection's incoming TCP port number in decimal form.
2463</t>
2464<t>
2465   If the request-target is in authority-form or asterisk-form, then the
2466   effective request URI's combined path and query component is empty.
2467   Otherwise, the combined path and query component is the same as the
2468   request-target.
2469</t>
2470<t>
2471   The components of the effective request URI, once determined as above,
2472   can be combined into absolute-URI form by concatenating the scheme,
2473   "://", authority, and combined path and query component.
2474</t>
2475<figure>
2476<preamble>
2477   Example 1: the following message received over an insecure TCP connection
2478</preamble>
2479<artwork type="example" x:indent-with="  ">
2480GET /pub/WWW/TheProject.html HTTP/1.1
2481Host: www.example.org:8080
2482</artwork>
2483</figure>
2484<figure>
2485<preamble>
2486  has an effective request URI of
2487</preamble>
2488<artwork type="example" x:indent-with="  ">
2489http://www.example.org:8080/pub/WWW/TheProject.html
2490</artwork>
2491</figure>
2492<figure>
2493<preamble>
2494   Example 2: the following message received over a TLS-secured TCP connection
2495</preamble>
2496<artwork type="example" x:indent-with="  ">
2497OPTIONS * HTTP/1.1
2498Host: www.example.org
2499</artwork>
2500</figure>
2501<figure>
2502<preamble>
2503  has an effective request URI of
2504</preamble>
2505<artwork type="example" x:indent-with="  ">
2506https://www.example.org
2507</artwork>
2508</figure>
2509<t>
2510   An origin server that does not allow resources to differ by requested
2511   host &MAY; ignore the <x:ref>Host</x:ref> field-value and instead replace it
2512   with a configured server name when constructing the effective request URI.
2513</t>
2514<t>
2515   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2516   field &MAY; attempt to use heuristics (e.g., examination of the URI path for
2517   something unique to a particular host) in order to guess the
2518   effective request URI's authority component.
2519</t>
2520</section>
2521
2522<section title="Associating a Response to a Request" anchor="associating.response.to.request">
2523<t>
2524   HTTP does not include a request identifier for associating a given
2525   request message with its corresponding one or more response messages.
2526   Hence, it relies on the order of response arrival to correspond exactly
2527   to the order in which requests are made on the same connection.
2528   More than one response message per request only occurs when one or more
2529   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a
2530   final response to the same request.
2531</t>
2532<t>
2533   A client that has more than one outstanding request on a connection &MUST;
2534   maintain a list of outstanding requests in the order sent and &MUST;
2535   associate each received response message on that connection to the highest
2536   ordered request that has not yet received a final (non-<x:ref>1xx</x:ref>)
2537   response.
2538</t>
2539</section>
2540
2541<section title="Message Forwarding" anchor="message.forwarding">
2542<t>
2543   As described in <xref target="intermediaries"/>, intermediaries can serve
2544   a variety of roles in the processing of HTTP requests and responses.
2545   Some intermediaries are used to improve performance or availability.
2546   Others are used for access control or to filter content.
2547   Since an HTTP stream has characteristics similar to a pipe-and-filter
2548   architecture, there are no inherent limits to the extent an intermediary
2549   can enhance (or interfere) with either direction of the stream.
2550</t>
2551<t>
2552   Intermediaries that forward a message &MUST; implement the
2553   <x:ref>Connection</x:ref> header field, as specified in
2554   <xref target="header.connection"/>, to exclude fields that are only
2555   intended for the incoming connection.
2556</t>
2557<t>
2558   In order to avoid request loops, a proxy that forwards requests to other
2559   proxies &MUST; be able to recognize and exclude all of its own server
2560   names, including any aliases, local variations, or literal IP addresses.
2561</t>
2562
2563<section title="Via" anchor="header.via">
2564  <iref primary="true" item="Via header field" x:for-anchor=""/>
2565  <x:anchor-alias value="pseudonym"/>
2566  <x:anchor-alias value="received-by"/>
2567  <x:anchor-alias value="received-protocol"/>
2568  <x:anchor-alias value="Via"/>
2569<t>
2570   The "Via" header field &MUST; be sent by a proxy or gateway in forwarded
2571   messages to indicate the intermediate protocols and recipients between the
2572   user agent and the server on requests, and between the origin server and
2573   the client on responses. It is analogous to the "Received" field
2574   used by email systems (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2575   Via is used in HTTP for tracking message forwards,
2576   avoiding request loops, and identifying the protocol capabilities of
2577   all senders along the request/response chain.
2578</t>
2579<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"/>
2580  <x:ref>Via</x:ref>               = 1#( <x:ref>received-protocol</x:ref> <x:ref>RWS</x:ref> <x:ref>received-by</x:ref>
2581                          [ <x:ref>RWS</x:ref> <x:ref>comment</x:ref> ] )
2582  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2583                      ; see <xref target="header.upgrade"/>
2584  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2585  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2586</artwork></figure>
2587<t>
2588   The received-protocol indicates the protocol version of the message
2589   received by the server or client along each segment of the
2590   request/response chain. The received-protocol version is appended to
2591   the Via field value when the message is forwarded so that information
2592   about the protocol capabilities of upstream applications remains
2593   visible to all recipients.
2594</t>
2595<t>
2596   The protocol-name is excluded if and only if it would be "HTTP". The
2597   received-by field is normally the host and optional port number of a
2598   recipient server or client that subsequently forwarded the message.
2599   However, if the real host is considered to be sensitive information,
2600   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2601   be assumed to be the default port of the received-protocol.
2602</t>
2603<t>
2604   Multiple Via field values represent each proxy or gateway that has
2605   forwarded the message. Each recipient &MUST; append its information
2606   such that the end result is ordered according to the sequence of
2607   forwarding applications.
2608</t>
2609<t>
2610   Comments &MAY; be used in the Via header field to identify the software
2611   of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2612   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2613   are optional and &MAY; be removed by any recipient prior to forwarding the
2614   message.
2615</t>
2616<t>
2617   For example, a request message could be sent from an HTTP/1.0 user
2618   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2619   forward the request to a public proxy at p.example.net, which completes
2620   the request by forwarding it to the origin server at www.example.com.
2621   The request received by www.example.com would then have the following
2622   Via header field:
2623</t>
2624<figure><artwork type="example">
2625  Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
2626</artwork></figure>
2627<t>
2628   A proxy or gateway used as a portal through a network firewall
2629   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2630   region unless it is explicitly enabled to do so. If not enabled, the
2631   received-by host of any host behind the firewall &SHOULD; be replaced
2632   by an appropriate pseudonym for that host.
2633</t>
2634<t>
2635   A proxy or gateway &MAY; combine an ordered subsequence of Via header
2636   field entries into a single such entry if the entries have identical
2637   received-protocol values. For example,
2638</t>
2639<figure><artwork type="example">
2640  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2641</artwork></figure>
2642<t>
2643  could be collapsed to
2644</t>
2645<figure><artwork type="example">
2646  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2647</artwork></figure>
2648<t>
2649   Senders &SHOULD-NOT; combine multiple entries unless they are all
2650   under the same organizational control and the hosts have already been
2651   replaced by pseudonyms. Senders &MUST-NOT; combine entries that
2652   have different received-protocol values.
2653</t>
2654</section>
2655
2656<section title="Transformations" anchor="message.transformations">
2657<t>
2658   Some intermediaries include features for transforming messages and their
2659   payloads.  A transforming proxy might, for example, convert between image
2660   formats in order to save cache space or to reduce the amount of traffic on
2661   a slow link. However, operational problems might occur when these
2662   transformations are applied to payloads intended for critical applications,
2663   such as medical imaging or scientific data analysis, particularly when
2664   integrity checks or digital signatures are used to ensure that the payload
2665   received is identical to the original.
2666</t>
2667<t>
2668   If a proxy receives a request-target with a host name that is not a
2669   fully qualified domain name, it &MAY; add its own domain to the host name
2670   it received when forwarding the request.  A proxy &MUST-NOT; change the
2671   host name if it is a fully qualified domain name.
2672</t>
2673<t>
2674   A proxy &MUST-NOT; modify the "absolute-path" and "query" parts of the
2675   received request-target when forwarding it to the next inbound server,
2676   except as noted above to replace an empty path with "/" or "*".
2677</t>
2678<t>
2679   A proxy &MUST-NOT; modify header fields that provide information about the
2680   end points of the communication chain, the resource state, or the selected
2681   representation. A proxy &MAY; change the message body through application
2682   or removal of a transfer coding (<xref target="transfer.codings"/>).
2683</t>
2684<t>
2685   A non-transforming proxy &MUST-NOT; modify the message payload (&payload;).
2686   A transforming proxy &MUST-NOT; modify the payload of a message that
2687   contains the no-transform cache-control directive.
2688</t>
2689<t>
2690   A transforming proxy &MAY; transform the payload of a message
2691   that does not contain the no-transform cache-control directive;
2692   if the payload is transformed, the transforming proxy &MUST; add a
2693   Warning 214 (Transformation applied) header field if one does not
2694   already appear in the message (see &header-warning;).
2695</t>
2696</section>
2697</section>
2698</section>
2699
2700<section title="Connection Management" anchor="connection.management">
2701<t>
2702   HTTP messaging is independent of the underlying transport or
2703   session-layer connection protocol(s).  HTTP only presumes a reliable
2704   transport with in-order delivery of requests and the corresponding
2705   in-order delivery of responses.  The mapping of HTTP request and
2706   response structures onto the data units of an underlying transport
2707   protocol is outside the scope of this specification.
2708</t>
2709<t>
2710   As described in <xref target="connecting.inbound"/>, the specific
2711   connection protocols to be used for an HTTP interaction are determined by
2712   client configuration and the <x:ref>target URI</x:ref>.
2713   For example, the "http" URI scheme
2714   (<xref target="http.uri"/>) indicates a default connection of TCP
2715   over IP, with a default TCP port of 80, but the client might be
2716   configured to use a proxy via some other connection, port, or protocol.
2717</t>
2718<t>
2719   HTTP implementations are expected to engage in connection management,
2720   which includes maintaining the state of current connections,
2721   establishing a new connection or reusing an existing connection,
2722   processing messages received on a connection, detecting connection
2723   failures, and closing each connection.
2724   Most clients maintain multiple connections in parallel, including
2725   more than one connection per server endpoint.
2726   Most servers are designed to maintain thousands of concurrent connections,
2727   while controlling request queues to enable fair use and detect
2728   denial of service attacks.
2729</t>
2730
2731<section title="Connection" anchor="header.connection">
2732  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2733  <iref primary="true" item="close" x:for-anchor=""/>
2734  <x:anchor-alias value="Connection"/>
2735  <x:anchor-alias value="connection-option"/>
2736  <x:anchor-alias value="close"/>
2737<t>
2738   The "Connection" header field allows the sender to indicate desired
2739   control options for the current connection.  In order to avoid confusing
2740   downstream recipients, a proxy or gateway &MUST; remove or replace any
2741   received connection options before forwarding the message.
2742</t>
2743<t>
2744   When a header field aside from Connection is used to supply control
2745   information for or about the current connection, the sender &MUST; list
2746   the corresponding field-name within the "Connection" header field.
2747   A proxy or gateway &MUST; parse a received Connection
2748   header field before a message is forwarded and, for each
2749   connection-option in this field, remove any header field(s) from
2750   the message with the same name as the connection-option, and then
2751   remove the Connection header field itself (or replace it with the
2752   intermediary's own connection options for the forwarded message).
2753</t>
2754<t>
2755   Hence, the Connection header field provides a declarative way of
2756   distinguishing header fields that are only intended for the
2757   immediate recipient ("hop-by-hop") from those fields that are
2758   intended for all recipients on the chain ("end-to-end"), enabling the
2759   message to be self-descriptive and allowing future connection-specific
2760   extensions to be deployed without fear that they will be blindly
2761   forwarded by older intermediaries.
2762</t>
2763<t>
2764   The Connection header field's value has the following grammar:
2765</t>
2766<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2767  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2768  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2769</artwork></figure>
2770<t>
2771   Connection options are case-insensitive.
2772</t>
2773<t>
2774   A sender &MUST-NOT; send a connection option corresponding to a header
2775   field that is intended for all recipients of the payload.
2776   For example, <x:ref>Cache-Control</x:ref> is never appropriate as a
2777   connection option (&header-cache-control;).
2778</t>
2779<t>
2780   The connection options do not have to correspond to a header field
2781   present in the message, since a connection-specific header field
2782   might not be needed if there are no parameters associated with that
2783   connection option.  Recipients that trigger certain connection
2784   behavior based on the presence of connection options &MUST; do so
2785   based on the presence of the connection-option rather than only the
2786   presence of the optional header field.  In other words, if the
2787   connection option is received as a header field but not indicated
2788   within the Connection field-value, then the recipient &MUST; ignore
2789   the connection-specific header field because it has likely been
2790   forwarded by an intermediary that is only partially conformant.
2791</t>
2792<t>
2793   When defining new connection options, specifications ought to
2794   carefully consider existing deployed header fields and ensure
2795   that the new connection option does not share the same name as
2796   an unrelated header field that might already be deployed.
2797   Defining a new connection option essentially reserves that potential
2798   field-name for carrying additional information related to the
2799   connection option, since it would be unwise for senders to use
2800   that field-name for anything else.
2801</t>
2802<t>
2803   The "<x:dfn>close</x:dfn>" connection option is defined for a
2804   sender to signal that this connection will be closed after completion of
2805   the response. For example,
2806</t>
2807<figure><artwork type="example">
2808  Connection: close
2809</artwork></figure>
2810<t>
2811   in either the request or the response header fields indicates that
2812   the connection &MUST; be closed after the current request/response
2813   is complete (<xref target="persistent.tear-down"/>).
2814</t>
2815<t>
2816   A client that does not support <x:ref>persistent connections</x:ref> &MUST;
2817   send the "close" connection option in every request message.
2818</t>
2819<t>
2820   A server that does not support <x:ref>persistent connections</x:ref> &MUST;
2821   send the "close" connection option in every response message that
2822   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2823</t>
2824</section>
2825
2826<section title="Establishment" anchor="persistent.establishment">
2827<t>
2828   It is beyond the scope of this specification to describe how connections
2829   are established via various transport or session-layer protocols.
2830   Each connection applies to only one transport link.
2831</t>
2832</section>
2833
2834<section title="Persistence" anchor="persistent.connections">
2835   <x:anchor-alias value="persistent connections"/>
2836<t>
2837   HTTP/1.1 defaults to the use of "<x:dfn>persistent connections</x:dfn>",
2838   allowing multiple requests and responses to be carried over a single
2839   connection. The "<x:ref>close</x:ref>" connection-option is used to signal
2840   that a connection will not persist after the current request/response.
2841   HTTP implementations &SHOULD; support persistent connections.
2842</t>
2843<t>
2844   A recipient determines whether a connection is persistent or not based on
2845   the most recently received message's protocol version and
2846   <x:ref>Connection</x:ref> header field (if any):
2847   <list style="symbols">
2848     <t>If the <x:ref>close</x:ref> connection option is present, the
2849        connection will not persist after the current response; else,</t>
2850     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2851        persist after the current response; else,</t>
2852     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2853        connection option is present, the recipient is not a proxy, and
2854        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2855        the connection will persist after the current response; otherwise,</t>
2856     <t>The connection will close after the current response.</t>
2857   </list>
2858</t>
2859<t>
2860   A server &MAY; assume that an HTTP/1.1 client intends to maintain a
2861   persistent connection until a <x:ref>close</x:ref> connection option
2862   is received in a request.
2863</t>
2864<t>
2865   A client &MAY; reuse a persistent connection until it sends or receives
2866   a <x:ref>close</x:ref> connection option or receives an HTTP/1.0 response
2867   without a "keep-alive" connection option.
2868</t>
2869<t>
2870   In order to remain persistent, all messages on a connection &MUST;
2871   have a self-defined message length (i.e., one not defined by closure
2872   of the connection), as described in <xref target="message.body"/>.
2873   A server &MUST; read the entire request message body or close
2874   the connection after sending its response, since otherwise the
2875   remaining data on a persistent connection would be misinterpreted
2876   as the next request.  Likewise,
2877   a client &MUST; read the entire response message body if it intends
2878   to reuse the same connection for a subsequent request.
2879</t>
2880<t>
2881   A proxy server &MUST-NOT; maintain a persistent connection with an
2882   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
2883   information and discussion of the problems with the Keep-Alive header field
2884   implemented by many HTTP/1.0 clients).
2885</t>
2886<t>
2887   Clients and servers &SHOULD-NOT; assume that a persistent connection is
2888   maintained for HTTP versions less than 1.1 unless it is explicitly
2889   signaled.
2890   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
2891   for more information on backward compatibility with HTTP/1.0 clients.
2892</t>
2893
2894<section title="Retrying Requests" anchor="persistent.retrying.requests">
2895<t>
2896   Connections can be closed at any time, with or without intention.
2897   Implementations ought to anticipate the need to recover
2898   from asynchronous close events.
2899</t>
2900<t>
2901   When an inbound connection is closed prematurely, a client &MAY; open a new
2902   connection and automatically retransmit an aborted sequence of requests if
2903   all of those requests have idempotent methods (&idempotent-methods;).
2904   A proxy &MUST-NOT; automatically retry non-idempotent requests.
2905</t>
2906<t>
2907   A user agent &MUST-NOT; automatically retry a request with a non-idempotent
2908   method unless it has some means to know that the request semantics are
2909   actually idempotent, regardless of the method, or some means to detect that
2910   the original request was never applied. For example, a user agent that
2911   knows (through design or configuration) that a POST request to a given
2912   resource is safe can repeat that request automatically.
2913   Likewise, a user agent designed specifically to operate on a version
2914   control repository might be able to recover from partial failure conditions
2915   by checking the target resource revision(s) after a failed connection,
2916   reverting or fixing any changes that were partially applied, and then
2917   automatically retrying the requests that failed.
2918</t>
2919<t>
2920   An automatic retry &SHOULD-NOT; be repeated if it fails.
2921</t>
2922</section>
2923
2924<section title="Pipelining" anchor="pipelining">
2925   <x:anchor-alias value="pipeline"/>
2926<t>
2927   A client that supports persistent connections &MAY; "<x:dfn>pipeline</x:dfn>"
2928   its requests (i.e., send multiple requests without waiting for each
2929   response). A server &MAY; process a sequence of pipelined requests in
2930   parallel if they all have safe methods (&safe-methods;), but &MUST; send
2931   the corresponding responses in the same order that the requests were
2932   received.
2933</t>
2934<t>
2935   A client that pipelines requests &MUST; be prepared to retry those
2936   requests if the connection closes before it receives all of the
2937   corresponding responses. A client that assumes a persistent connection and
2938   pipelines immediately after connection establishment &MUST-NOT; pipeline
2939   on a retry connection until it knows the connection is persistent.
2940</t>
2941<t>
2942   Idempotent methods (&idempotent-methods;) are significant to pipelining
2943   because they can be automatically retried after a connection failure.
2944   A user agent &SHOULD-NOT; pipeline requests after a non-idempotent method
2945   until the final response status code for that method has been received,
2946   unless the user agent has a means to detect and recover from partial
2947   failure conditions involving the pipelined sequence.
2948</t>
2949<t>
2950   An intermediary that receives pipelined requests &MAY; pipeline those
2951   requests when forwarding them inbound, since it can rely on the outbound
2952   user agent(s) to determine what requests can be safely pipelined. If the
2953   inbound connection fails before receiving a response, the pipelining
2954   intermediary &MAY; attempt to retry a sequence of requests that have yet
2955   to receive a response if the requests all have idempotent methods;
2956   otherwise, the pipelining intermediary &SHOULD; forward any received
2957   responses and then close the corresponding outbound connection(s) so that
2958   the outbound user agent(s) can recover accordingly.
2959</t>
2960</section>
2961</section>
2962   
2963<section title="Concurrency" anchor="persistent.concurrency">
2964<t>
2965   Clients &SHOULD; limit the number of simultaneous
2966   connections that they maintain to a given server.
2967</t>
2968<t>
2969   Previous revisions of HTTP gave a specific number of connections as a
2970   ceiling, but this was found to be impractical for many applications. As a
2971   result, this specification does not mandate a particular maximum number of
2972   connections, but instead encourages clients to be conservative when opening
2973   multiple connections.
2974</t>
2975<t>
2976   Multiple connections are typically used to avoid the "head-of-line
2977   blocking" problem, wherein a request that takes significant server-side
2978   processing and/or has a large payload blocks subsequent requests on the
2979   same connection. However, each connection consumes server resources.
2980   Furthermore, using multiple connections can cause undesirable side effects
2981   in congested networks.
2982</t>
2983<t>
2984   Note that servers might reject traffic that they deem abusive, including an
2985   excessive number of connections from a client.
2986</t>
2987</section>
2988
2989<section title="Failures and Time-outs" anchor="persistent.failures">
2990<t>
2991   Servers will usually have some time-out value beyond which they will
2992   no longer maintain an inactive connection. Proxy servers might make
2993   this a higher value since it is likely that the client will be making
2994   more connections through the same server. The use of persistent
2995   connections places no requirements on the length (or existence) of
2996   this time-out for either the client or the server.
2997</t>
2998<t>
2999   When a client or server wishes to time-out it &SHOULD; issue a graceful
3000   close on the transport connection. Clients and servers &SHOULD; both
3001   constantly watch for the other side of the transport close, and
3002   respond to it as appropriate. If a client or server does not detect
3003   the other side's close promptly it could cause unnecessary resource
3004   drain on the network.
3005</t>
3006<t>
3007   A client, server, or proxy &MAY; close the transport connection at any
3008   time. For example, a client might have started to send a new request
3009   at the same time that the server has decided to close the "idle"
3010   connection. From the server's point of view, the connection is being
3011   closed while it was idle, but from the client's point of view, a
3012   request is in progress.
3013</t>
3014<t>
3015   Servers &SHOULD; maintain persistent connections and allow the underlying
3016   transport's flow control mechanisms to resolve temporary overloads, rather
3017   than terminate connections with the expectation that clients will retry.
3018   The latter technique can exacerbate network congestion.
3019</t>
3020<t>
3021   A client sending a message body &SHOULD; monitor
3022   the network connection for an error response while it is transmitting
3023   the request. If the client sees an error response, it &SHOULD;
3024   immediately cease transmitting the body and close the connection.
3025</t>
3026</section>
3027   
3028<section title="Tear-down" anchor="persistent.tear-down">
3029  <iref primary="false" item="Connection header field" x:for-anchor=""/>
3030  <iref primary="false" item="close" x:for-anchor=""/>
3031<t>
3032   The <x:ref>Connection</x:ref> header field
3033   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
3034   connection option that a sender &SHOULD; send when it wishes to close
3035   the connection after the current request/response pair.
3036</t>
3037<t>
3038   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
3039   send further requests on that connection (after the one containing
3040   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
3041   final response message corresponding to this request.
3042</t>
3043<t>
3044   A server that receives a <x:ref>close</x:ref> connection option &MUST;
3045   initiate a close of the connection (see below) after it sends the
3046   final response to the request that contained <x:ref>close</x:ref>.
3047   The server &SHOULD; send a <x:ref>close</x:ref> connection option
3048   in its final response on that connection. The server &MUST-NOT; process
3049   any further requests received on that connection.
3050</t>
3051<t>
3052   A server that sends a <x:ref>close</x:ref> connection option &MUST;
3053   initiate a close of the connection (see below) after it sends the
3054   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
3055   any further requests received on that connection.
3056</t>
3057<t>
3058   A client that receives a <x:ref>close</x:ref> connection option &MUST;
3059   cease sending requests on that connection and close the connection
3060   after reading the response message containing the close; if additional
3061   pipelined requests had been sent on the connection, the client &SHOULD-NOT;
3062   assume that they will be processed by the server.
3063</t>
3064<t>
3065   If a server performs an immediate close of a TCP connection, there is a
3066   significant risk that the client will not be able to read the last HTTP
3067   response.  If the server receives additional data from the client on a
3068   fully-closed connection, such as another request that was sent by the
3069   client before receiving the server's response, the server's TCP stack will
3070   send a reset packet to the client; unfortunately, the reset packet might
3071   erase the client's unacknowledged input buffers before they can be read
3072   and interpreted by the client's HTTP parser.
3073</t>
3074<t>
3075   To avoid the TCP reset problem, servers typically close a connection in
3076   stages. First, the server performs a half-close by closing only the write
3077   side of the read/write connection. The server then continues to read from
3078   the connection until it receives a corresponding close by the client, or
3079   until the server is reasonably certain that its own TCP stack has received
3080   the client's acknowledgement of the packet(s) containing the server's last
3081   response. Finally, the server fully closes the connection.
3082</t>
3083<t>
3084   It is unknown whether the reset problem is exclusive to TCP or might also
3085   be found in other transport connection protocols.
3086</t>
3087</section>
3088
3089<section title="Upgrade" anchor="header.upgrade">
3090  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3091  <x:anchor-alias value="Upgrade"/>
3092  <x:anchor-alias value="protocol"/>
3093  <x:anchor-alias value="protocol-name"/>
3094  <x:anchor-alias value="protocol-version"/>
3095<t>
3096   The "Upgrade" header field is intended to provide a simple mechanism
3097   for transitioning from HTTP/1.1 to some other protocol on the same
3098   connection.  A client &MAY; send a list of protocols in the Upgrade
3099   header field of a request to invite the server to switch to one or
3100   more of those protocols, in order of descending preference, before sending
3101   the final response. A server &MAY; ignore a received Upgrade header field
3102   if it wishes to continue using the current protocol on that connection.
3103</t>
3104<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3105  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3106
3107  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3108  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3109  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3110</artwork></figure>
3111<t>
3112   A server that sends a <x:ref>101 (Switching Protocols)</x:ref> response
3113   &MUST; send an Upgrade header field to indicate the new protocol(s) to
3114   which the connection is being switched; if multiple protocol layers are
3115   being switched, the new protocols &MUST; be listed in layer-ascending
3116   order. A server &MUST-NOT; switch to a protocol that was not indicated by
3117   the client in the corresponding request's Upgrade header field.
3118   A server &MAY; choose to ignore the order of preference indicated by the
3119   client and select the new protocol(s) based on other factors, such as the
3120   nature of the request or the current load on the server.
3121</t>
3122<t>
3123   A server that sends a <x:ref>426 (Upgrade Required)</x:ref> response
3124   &MUST; send an Upgrade header field to indicate the acceptable protocols,
3125   in order of descending preference.
3126</t>
3127<t>
3128   A server &MAY; send an Upgrade header field in any other response to
3129   advertise that it implements support for upgrading to the listed protocols,
3130   in order of descending preference, when appropriate for a future request.
3131</t>
3132<figure><preamble>
3133   The following is a hypothetical example sent by a client:
3134</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
3135GET /hello.txt HTTP/1.1
3136Host: www.example.com
3137Connection: upgrade
3138Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3139
3140</artwork></figure>
3141<t>
3142   Upgrade cannot be used to insist on a protocol change; its acceptance and
3143   use by the server is optional. The capabilities and nature of the
3144   application-level communication after the protocol change is entirely
3145   dependent upon the new protocol(s) chosen, although the first action
3146   after changing the protocol &MUST; be a response to the initial HTTP
3147   request that contained the Upgrade header field.
3148</t>
3149<t>
3150   For example, if the Upgrade header field is received in a GET request
3151   and the server decides to switch protocols, it first responds
3152   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3153   then immediately follows that with the new protocol's equivalent of a
3154   response to a GET on the target resource.  This allows a connection to be
3155   upgraded to protocols with the same semantics as HTTP without the
3156   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3157   protocols unless the received message semantics can be honored by the new
3158   protocol; an OPTIONS request can be honored by any protocol.
3159</t>
3160<figure><preamble>
3161   The following is an example response to the above hypothetical request:
3162</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
3163HTTP/1.1 101 Switching Protocols
3164Connection: upgrade
3165Upgrade: HTTP/2.0
3166
3167[... data stream switches to HTTP/2.0 with an appropriate response
3168(as defined by new protocol) to the "GET /hello.txt" request ...]
3169</artwork></figure>
3170<t>
3171   When Upgrade is sent, the sender &MUST; also send a
3172   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3173   that contains an "upgrade" connection option, in order to prevent Upgrade
3174   from being accidentally forwarded by intermediaries that might not implement
3175   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3176   is received in an HTTP/1.0 request.
3177</t>
3178<t>
3179   The Upgrade header field only applies to switching protocols on top of the
3180   existing connection; it cannot be used to switch the underlying connection
3181   (transport) protocol, nor to switch the existing communication to a
3182   different connection. For those purposes, it is more appropriate to use a
3183   <x:ref>3xx (Redirection)</x:ref> response (&status-3xx;).
3184</t>
3185<t>
3186   This specification only defines the protocol name "HTTP" for use by
3187   the family of Hypertext Transfer Protocols, as defined by the HTTP
3188   version rules of <xref target="http.version"/> and future updates to this
3189   specification. Additional tokens ought to be registered with IANA using the
3190   registration procedure defined in <xref target="upgrade.token.registry"/>.
3191</t>
3192</section>
3193</section>
3194
3195<section title="IANA Considerations" anchor="IANA.considerations">
3196
3197<section title="Header Field Registration" anchor="header.field.registration">
3198<t>
3199   HTTP header fields are registered within the Message Header Field Registry
3200   maintained at
3201   <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
3202</t>
3203<t>
3204   This document defines the following HTTP header fields, so their
3205   associated registry entries shall be updated according to the permanent
3206   registrations below (see <xref target="BCP90"/>):
3207</t>
3208<?BEGININC p1-messaging.iana-headers ?>
3209<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3210<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3211   <ttcol>Header Field Name</ttcol>
3212   <ttcol>Protocol</ttcol>
3213   <ttcol>Status</ttcol>
3214   <ttcol>Reference</ttcol>
3215
3216   <c>Connection</c>
3217   <c>http</c>
3218   <c>standard</c>
3219   <c>
3220      <xref target="header.connection"/>
3221   </c>
3222   <c>Content-Length</c>
3223   <c>http</c>
3224   <c>standard</c>
3225   <c>
3226      <xref target="header.content-length"/>
3227   </c>
3228   <c>Host</c>
3229   <c>http</c>
3230   <c>standard</c>
3231   <c>
3232      <xref target="header.host"/>
3233   </c>
3234   <c>TE</c>
3235   <c>http</c>
3236   <c>standard</c>
3237   <c>
3238      <xref target="header.te"/>
3239   </c>
3240   <c>Trailer</c>
3241   <c>http</c>
3242   <c>standard</c>
3243   <c>
3244      <xref target="header.trailer"/>
3245   </c>
3246   <c>Transfer-Encoding</c>
3247   <c>http</c>
3248   <c>standard</c>
3249   <c>
3250      <xref target="header.transfer-encoding"/>
3251   </c>
3252   <c>Upgrade</c>
3253   <c>http</c>
3254   <c>standard</c>
3255   <c>
3256      <xref target="header.upgrade"/>
3257   </c>
3258   <c>Via</c>
3259   <c>http</c>
3260   <c>standard</c>
3261   <c>
3262      <xref target="header.via"/>
3263   </c>
3264</texttable>
3265<!--(END)-->
3266<?ENDINC p1-messaging.iana-headers ?>
3267<t>
3268   Furthermore, the header field-name "Close" shall be registered as
3269   "reserved", since using that name as an HTTP header field might
3270   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3271   header field (<xref target="header.connection"/>).
3272</t>
3273<texttable align="left" suppress-title="true">
3274   <ttcol>Header Field Name</ttcol>
3275   <ttcol>Protocol</ttcol>
3276   <ttcol>Status</ttcol>
3277   <ttcol>Reference</ttcol>
3278
3279   <c>Close</c>
3280   <c>http</c>
3281   <c>reserved</c>
3282   <c>
3283      <xref target="header.field.registration"/>
3284   </c>
3285</texttable>
3286<t>
3287   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3288</t>
3289</section>
3290
3291<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3292<t>
3293   IANA maintains the registry of URI Schemes <xref target="BCP115"/> at
3294   <eref target="http://www.iana.org/assignments/uri-schemes.html"/>.
3295</t>
3296<t>
3297   This document defines the following URI schemes, so their
3298   associated registry entries shall be updated according to the permanent
3299   registrations below:
3300</t>
3301<texttable align="left" suppress-title="true">
3302   <ttcol>URI Scheme</ttcol>
3303   <ttcol>Description</ttcol>
3304   <ttcol>Reference</ttcol>
3305
3306   <c>http</c>
3307   <c>Hypertext Transfer Protocol</c>
3308   <c><xref target="http.uri"/></c>
3309
3310   <c>https</c>
3311   <c>Hypertext Transfer Protocol Secure</c>
3312   <c><xref target="https.uri"/></c>
3313</texttable>
3314</section>
3315
3316<section title="Internet Media Type Registration" anchor="internet.media.type.http">
3317<t>
3318   This document serves as the specification for the Internet media types
3319   "message/http" and "application/http". The following is to be registered with
3320   IANA (see <xref target="BCP13"/>).
3321</t>
3322<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
3323<iref item="Media Type" subitem="message/http" primary="true"/>
3324<iref item="message/http Media Type" primary="true"/>
3325<t>
3326   The message/http type can be used to enclose a single HTTP request or
3327   response message, provided that it obeys the MIME restrictions for all
3328   "message" types regarding line length and encodings.
3329</t>
3330<t>
3331  <list style="hanging" x:indent="12em">
3332    <t hangText="Type name:">
3333      message
3334    </t>
3335    <t hangText="Subtype name:">
3336      http
3337    </t>
3338    <t hangText="Required parameters:">
3339      none
3340    </t>
3341    <t hangText="Optional parameters:">
3342      version, msgtype
3343      <list style="hanging">
3344        <t hangText="version:">
3345          The HTTP-version number of the enclosed message
3346          (e.g., "1.1"). If not present, the version can be
3347          determined from the first line of the body.
3348        </t>
3349        <t hangText="msgtype:">
3350          The message type &mdash; "request" or "response". If not
3351          present, the type can be determined from the first
3352          line of the body.
3353        </t>
3354      </list>
3355    </t>
3356    <t hangText="Encoding considerations:">
3357      only "7bit", "8bit", or "binary" are permitted
3358    </t>
3359    <t hangText="Security considerations:">
3360      none
3361    </t>
3362    <t hangText="Interoperability considerations:">
3363      none
3364    </t>
3365    <t hangText="Published specification:">
3366      This specification (see <xref target="internet.media.type.message.http"/>).
3367    </t>
3368    <t hangText="Applications that use this media type:">
3369    </t>
3370    <t hangText="Additional information:">
3371      <list style="hanging">
3372        <t hangText="Magic number(s):">none</t>
3373        <t hangText="File extension(s):">none</t>
3374        <t hangText="Macintosh file type code(s):">none</t>
3375      </list>
3376    </t>
3377    <t hangText="Person and email address to contact for further information:">
3378      See Authors Section.
3379    </t>
3380    <t hangText="Intended usage:">
3381      COMMON
3382    </t>
3383    <t hangText="Restrictions on usage:">
3384      none
3385    </t>
3386    <t hangText="Author:">
3387      See Authors Section.
3388    </t>
3389    <t hangText="Change controller:">
3390      IESG
3391    </t>
3392  </list>
3393</t>
3394</section>
3395<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
3396<iref item="Media Type" subitem="application/http" primary="true"/>
3397<iref item="application/http Media Type" primary="true"/>
3398<t>
3399   The application/http type can be used to enclose a pipeline of one or more
3400   HTTP request or response messages (not intermixed).
3401</t>
3402<t>
3403  <list style="hanging" x:indent="12em">
3404    <t hangText="Type name:">
3405      application
3406    </t>
3407    <t hangText="Subtype name:">
3408      http
3409    </t>
3410    <t hangText="Required parameters:">
3411      none
3412    </t>
3413    <t hangText="Optional parameters:">
3414      version, msgtype
3415      <list style="hanging">
3416        <t hangText="version:">
3417          The HTTP-version number of the enclosed messages
3418          (e.g., "1.1"). If not present, the version can be
3419          determined from the first line of the body.
3420        </t>
3421        <t hangText="msgtype:">
3422          The message type &mdash; "request" or "response". If not
3423          present, the type can be determined from the first
3424          line of the body.
3425        </t>
3426      </list>
3427    </t>
3428    <t hangText="Encoding considerations:">
3429      HTTP messages enclosed by this type
3430      are in "binary" format; use of an appropriate
3431      Content-Transfer-Encoding is required when
3432      transmitted via E-mail.
3433    </t>
3434    <t hangText="Security considerations:">
3435      none
3436    </t>
3437    <t hangText="Interoperability considerations:">
3438      none
3439    </t>
3440    <t hangText="Published specification:">
3441      This specification (see <xref target="internet.media.type.application.http"/>).
3442    </t>
3443    <t hangText="Applications that use this media type:">
3444    </t>
3445    <t hangText="Additional information:">
3446      <list style="hanging">
3447        <t hangText="Magic number(s):">none</t>
3448        <t hangText="File extension(s):">none</t>
3449        <t hangText="Macintosh file type code(s):">none</t>
3450      </list>
3451    </t>
3452    <t hangText="Person and email address to contact for further information:">
3453      See Authors Section.
3454    </t>
3455    <t hangText="Intended usage:">
3456      COMMON
3457    </t>
3458    <t hangText="Restrictions on usage:">
3459      none
3460    </t>
3461    <t hangText="Author:">
3462      See Authors Section.
3463    </t>
3464    <t hangText="Change controller:">
3465      IESG
3466    </t>
3467  </list>
3468</t>
3469</section>
3470</section>
3471
3472<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3473<t>
3474   The HTTP Transfer Coding Registry defines the name space for transfer
3475   coding names. It is maintained at <eref target="http://www.iana.org/assignments/http-parameters"/>.
3476</t>
3477
3478<section title="Procedure" anchor="transfer.coding.registry.procedure">
3479<t>
3480   Registrations &MUST; include the following fields:
3481   <list style="symbols">
3482     <t>Name</t>
3483     <t>Description</t>
3484     <t>Pointer to specification text</t>
3485   </list>
3486</t>
3487<t>
3488   Names of transfer codings &MUST-NOT; overlap with names of content codings
3489   (&content-codings;) unless the encoding transformation is identical, as
3490   is the case for the compression codings defined in
3491   <xref target="compression.codings"/>.
3492</t>
3493<t>
3494   Values to be added to this name space require IETF Review (see
3495   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3496   conform to the purpose of transfer coding defined in this specification.
3497</t>
3498<t>
3499   Use of program names for the identification of encoding formats
3500   is not desirable and is discouraged for future encodings.
3501</t>
3502</section>
3503
3504<section title="Registration" anchor="transfer.coding.registration">
3505<t>
3506   The HTTP Transfer Coding Registry shall be updated with the registrations
3507   below:
3508</t>
3509<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3510   <ttcol>Name</ttcol>
3511   <ttcol>Description</ttcol>
3512   <ttcol>Reference</ttcol>
3513   <c>chunked</c>
3514   <c>Transfer in a series of chunks</c>
3515   <c>
3516      <xref target="chunked.encoding"/>
3517   </c>
3518   <c>compress</c>
3519   <c>UNIX "compress" program method</c>
3520   <c>
3521      <xref target="compress.coding"/>
3522   </c>
3523   <c>deflate</c>
3524   <c>"deflate" compression mechanism (<xref target="RFC1951"/>) used inside
3525   the "zlib" data format (<xref target="RFC1950"/>)
3526   </c>
3527   <c>
3528      <xref target="deflate.coding"/>
3529   </c>
3530   <c>gzip</c>
3531   <c>Same as GNU zip <xref target="RFC1952"/></c>
3532   <c>
3533      <xref target="gzip.coding"/>
3534   </c>
3535</texttable>
3536</section>
3537</section>
3538
3539<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3540<t>
3541   The HTTP Upgrade Token Registry defines the name space for protocol-name
3542   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3543   field. The registry is maintained at <eref target="http://www.iana.org/assignments/http-upgrade-tokens"/>.
3544</t>
3545
3546<section title="Procedure" anchor="upgrade.token.registry.procedure">  
3547<t>
3548   Each registered protocol name is associated with contact information
3549   and an optional set of specifications that details how the connection
3550   will be processed after it has been upgraded.
3551</t>
3552<t>
3553   Registrations happen on a "First Come First Served" basis (see
3554   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3555   following rules:
3556  <list style="numbers">
3557    <t>A protocol-name token, once registered, stays registered forever.</t>
3558    <t>The registration &MUST; name a responsible party for the
3559       registration.</t>
3560    <t>The registration &MUST; name a point of contact.</t>
3561    <t>The registration &MAY; name a set of specifications associated with
3562       that token. Such specifications need not be publicly available.</t>
3563    <t>The registration &SHOULD; name a set of expected "protocol-version"
3564       tokens associated with that token at the time of registration.</t>
3565    <t>The responsible party &MAY; change the registration at any time.
3566       The IANA will keep a record of all such changes, and make them
3567       available upon request.</t>
3568    <t>The IESG &MAY; reassign responsibility for a protocol token.
3569       This will normally only be used in the case when a
3570       responsible party cannot be contacted.</t>
3571  </list>
3572</t>
3573<t>
3574   This registration procedure for HTTP Upgrade Tokens replaces that
3575   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3576</t>
3577</section>
3578
3579<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3580<t>
3581   The HTTP Upgrade Token Registry shall be updated with the registration
3582   below:
3583</t>
3584<texttable align="left" suppress-title="true">
3585   <ttcol>Value</ttcol>
3586   <ttcol>Description</ttcol>
3587   <ttcol>Expected Version Tokens</ttcol>
3588   <ttcol>Reference</ttcol>
3589
3590   <c>HTTP</c>
3591   <c>Hypertext Transfer Protocol</c>
3592   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3593   <c><xref target="http.version"/></c>
3594</texttable>
3595<t>
3596   The responsible party is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3597</t>
3598</section>
3599</section>
3600
3601</section>
3602
3603<section title="Security Considerations" anchor="security.considerations">
3604<t>
3605   This section is meant to inform developers, information providers, and
3606   users of known security concerns relevant to HTTP/1.1 message syntax,
3607   parsing, and routing.
3608</t>
3609
3610<section title="DNS-related Attacks" anchor="dns.related.attacks">
3611<t>
3612   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3613   generally prone to security attacks based on the deliberate misassociation
3614   of IP addresses and DNS names not protected by DNSSEC. Clients need to be
3615   cautious in assuming the validity of an IP number/DNS name association unless
3616   the response is protected by DNSSEC (<xref target="RFC4033"/>).
3617</t>
3618</section>
3619
3620<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3621<t>
3622   By their very nature, HTTP intermediaries are men-in-the-middle, and
3623   represent an opportunity for man-in-the-middle attacks. Compromise of
3624   the systems on which the intermediaries run can result in serious security
3625   and privacy problems. Intermediaries have access to security-related
3626   information, personal information about individual users and
3627   organizations, and proprietary information belonging to users and
3628   content providers. A compromised intermediary, or an intermediary
3629   implemented or configured without regard to security and privacy
3630   considerations, might be used in the commission of a wide range of
3631   potential attacks.
3632</t>
3633<t>
3634   Intermediaries that contain a shared cache are especially vulnerable
3635   to cache poisoning attacks.
3636</t>
3637<t>
3638   Implementers need to consider the privacy and security
3639   implications of their design and coding decisions, and of the
3640   configuration options they provide to operators (especially the
3641   default configuration).
3642</t>
3643<t>
3644   Users need to be aware that intermediaries are no more trustworthy than
3645   the people who run them; HTTP itself cannot solve this problem.
3646</t>
3647</section>
3648
3649<section title="Buffer Overflows" anchor="attack.protocol.element.size.overflows">
3650<t>
3651   Because HTTP uses mostly textual, character-delimited fields, attackers can
3652   overflow buffers in implementations, and/or perform a Denial of Service
3653   against implementations that accept fields with unlimited lengths.
3654</t>
3655<t>
3656   To promote interoperability, this specification makes specific
3657   recommendations for minimum size limits on request-line
3658   (<xref target="request.line"/>)
3659   and blocks of header fields (<xref target="header.fields"/>). These are
3660   minimum recommendations, chosen to be supportable even by implementations
3661   with limited resources; it is expected that most implementations will
3662   choose substantially higher limits.
3663</t>
3664<t>
3665   This specification also provides a way for servers to reject messages that
3666   have request-targets that are too long (&status-414;) or request entities
3667   that are too large (&status-4xx;). Additional status codes related to
3668   capacity limits have been defined by extensions to HTTP
3669   <xref target="RFC6585"/>.
3670</t>
3671<t>
3672   Recipients &SHOULD; carefully limit the extent to which they read other
3673   fields, including (but not limited to) request methods, response status
3674   phrases, header field-names, and body chunks, so as to avoid denial of
3675   service attacks without impeding interoperability.
3676</t>
3677</section>
3678
3679<section title="Message Integrity" anchor="message.integrity">
3680<t>
3681   HTTP does not define a specific mechanism for ensuring message integrity,
3682   instead relying on the error-detection ability of underlying transport
3683   protocols and the use of length or chunk-delimited framing to detect
3684   completeness. Additional integrity mechanisms, such as hash functions or
3685   digital signatures applied to the content, can be selectively added to
3686   messages via extensible metadata header fields. Historically, the lack of
3687   a single integrity mechanism has been justified by the informal nature of
3688   most HTTP communication.  However, the prevalence of HTTP as an information
3689   access mechanism has resulted in its increasing use within environments
3690   where verification of message integrity is crucial.
3691</t>
3692<t>
3693   User agents are encouraged to implement configurable means for detecting
3694   and reporting failures of message integrity such that those means can be
3695   enabled within environments for which integrity is necessary. For example,
3696   a browser being used to view medical history or drug interaction
3697   information needs to indicate to the user when such information is detected
3698   by the protocol to be incomplete, expired, or corrupted during transfer.
3699   Such mechanisms might be selectively enabled via user agent extensions or
3700   the presence of message integrity metadata in a response.
3701   At a minimum, user agents ought to provide some indication that allows a
3702   user to distinguish between a complete and incomplete response message
3703   (<xref target="incomplete.messages"/>) when such verification is desired.
3704</t>
3705</section>
3706
3707<section title="Server Log Information" anchor="abuse.of.server.log.information">
3708<t>
3709   A server is in the position to save personal data about a user's requests
3710   over time, which might identify their reading patterns or subjects of
3711   interest.  In particular, log information gathered at an intermediary
3712   often contains a history of user agent interaction, across a multitude
3713   of sites, that can be traced to individual users.
3714</t>
3715<t>
3716   HTTP log information is confidential in nature; its handling is often
3717   constrained by laws and regulations.  Log information needs to be securely
3718   stored and appropriate guidelines followed for its analysis.
3719   Anonymization of personal information within individual entries helps,
3720   but is generally not sufficient to prevent real log traces from being
3721   re-identified based on correlation with other access characteristics.
3722   As such, access traces that are keyed to a specific client should not
3723   be published even if the key is pseudonymous.
3724</t>
3725<t>
3726   To minimize the risk of theft or accidental publication, log information
3727   should be purged of personally identifiable information, including
3728   user identifiers, IP addresses, and user-provided query parameters,
3729   as soon as that information is no longer necessary to support operational
3730   needs for security, auditing, or fraud control.
3731</t>
3732</section>
3733</section>
3734
3735<section title="Acknowledgments" anchor="acks">
3736<t>
3737   This edition of HTTP/1.1 builds on the many contributions that went into
3738   <xref target="RFC1945" format="none">RFC 1945</xref>,
3739   <xref target="RFC2068" format="none">RFC 2068</xref>,
3740   <xref target="RFC2145" format="none">RFC 2145</xref>, and
3741   <xref target="RFC2616" format="none">RFC 2616</xref>, including
3742   substantial contributions made by the previous authors, editors, and
3743   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
3744   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
3745   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
3746</t>
3747<t>
3748   Since 1999, the following contributors have helped improve the HTTP
3749   specification by reporting bugs, asking smart questions, drafting or
3750   reviewing text, and evaluating open issues:
3751</t>
3752<?BEGININC acks ?>
3753<t>Adam Barth,
3754Adam Roach,
3755Addison Phillips,
3756Adrian Chadd,
3757Adrien W. de Croy,
3758Alan Ford,
3759Alan Ruttenberg,
3760Albert Lunde,
3761Alek Storm,
3762Alex Rousskov,
3763Alexandre Morgaut,
3764Alexey Melnikov,
3765Alisha Smith,
3766Amichai Rothman,
3767Amit Klein,
3768Amos Jeffries,
3769Andreas Maier,
3770Andreas Petersson,
3771Anil Sharma,
3772Anne van Kesteren,
3773Anthony Bryan,
3774Asbjorn Ulsberg,
3775Ashok Kumar,
3776Balachander Krishnamurthy,
3777Barry Leiba,
3778Ben Laurie,
3779Benjamin Carlyle,
3780Benjamin Niven-Jenkins,
3781Bil Corry,
3782Bill Burke,
3783Bjoern Hoehrmann,
3784Bob Scheifler,
3785Boris Zbarsky,
3786Brett Slatkin,
3787Brian Kell,
3788Brian McBarron,
3789Brian Pane,
3790Brian Raymor,
3791Brian Smith,
3792Bryce Nesbitt,
3793Cameron Heavon-Jones,
3794Carl Kugler,
3795Carsten Bormann,
3796Charles Fry,
3797Chris Newman,
3798Cyrus Daboo,
3799Dale Robert Anderson,
3800Dan Wing,
3801Dan Winship,
3802Daniel Stenberg,
3803Darrel Miller,
3804Dave Cridland,
3805Dave Crocker,
3806Dave Kristol,
3807Dave Thaler,
3808David Booth,
3809David Singer,
3810David W. Morris,
3811Diwakar Shetty,
3812Dmitry Kurochkin,
3813Drummond Reed,
3814Duane Wessels,
3815Edward Lee,
3816Eitan Adler,
3817Eliot Lear,
3818Eran Hammer-Lahav,
3819Eric D. Williams,
3820Eric J. Bowman,
3821Eric Lawrence,
3822Eric Rescorla,
3823Erik Aronesty,
3824Evan Prodromou,
3825Felix Geisendoerfer,
3826Florian Weimer,
3827Frank Ellermann,
3828Fred Bohle,
3829Frederic Kayser,
3830Gabriel Montenegro,
3831Geoffrey Sneddon,
3832Gervase Markham,
3833Grahame Grieve,
3834Greg Wilkins,
3835Grzegorz Calkowski,
3836Harald Tveit Alvestrand,
3837Harry Halpin,
3838Helge Hess,
3839Henrik Nordstrom,
3840Henry S. Thompson,
3841Henry Story,
3842Herbert van de Sompel,
3843Herve Ruellan,
3844Howard Melman,
3845Hugo Haas,
3846Ian Fette,
3847Ian Hickson,
3848Ido Safruti,
3849Ilari Liusvaara,
3850Ilya Grigorik,
3851Ingo Struck,
3852J. Ross Nicoll,
3853James Cloos,
3854James H. Manger,
3855James Lacey,
3856James M. Snell,
3857Jamie Lokier,
3858Jan Algermissen,
3859Jeff Hodges (who came up with the term 'effective Request-URI'),
3860Jeff Pinner,
3861Jeff Walden,
3862Jim Luther,
3863Jitu Padhye,
3864Joe D. Williams,
3865Joe Gregorio,
3866Joe Orton,
3867John C. Klensin,
3868John C. Mallery,
3869John Cowan,
3870John Kemp,
3871John Panzer,
3872John Schneider,
3873John Stracke,
3874John Sullivan,
3875Jonas Sicking,
3876Jonathan A. Rees,
3877Jonathan Billington,
3878Jonathan Moore,
3879Jonathan Silvera,
3880Jordi Ros,
3881Joris Dobbelsteen,
3882Josh Cohen,
3883Julien Pierre,
3884Jungshik Shin,
3885Justin Chapweske,
3886Justin Erenkrantz,
3887Justin James,
3888Kalvinder Singh,
3889Karl Dubost,
3890Keith Hoffman,
3891Keith Moore,
3892Ken Murchison,
3893Koen Holtman,
3894Konstantin Voronkov,
3895Kris Zyp,
3896Lisa Dusseault,
3897Maciej Stachowiak,
3898Manu Sporny,
3899Marc Schneider,
3900Marc Slemko,
3901Mark Baker,
3902Mark Pauley,
3903Mark Watson,
3904Markus Isomaki,
3905Markus Lanthaler,
3906Martin J. Duerst,
3907Martin Musatov,
3908Martin Nilsson,
3909Martin Thomson,
3910Matt Lynch,
3911Matthew Cox,
3912Max Clark,
3913Michael Burrows,
3914Michael Hausenblas,
3915Mike Amundsen,
3916Mike Belshe,
3917Mike Kelly,
3918Mike Schinkel,
3919Miles Sabin,
3920Murray S. Kucherawy,
3921Mykyta Yevstifeyev,
3922Nathan Rixham,
3923Nicholas Shanks,
3924Nico Williams,
3925Nicolas Alvarez,
3926Nicolas Mailhot,
3927Noah Slater,
3928Osama Mazahir,
3929Pablo Castro,
3930Pat Hayes,
3931Patrick R. McManus,
3932Paul E. Jones,
3933Paul Hoffman,
3934Paul Marquess,
3935Peter Lepeska,
3936Peter Occil,
3937Peter Saint-Andre,
3938Peter Watkins,
3939Phil Archer,
3940Philippe Mougin,
3941Phillip Hallam-Baker,
3942Piotr Dobrogost,
3943Poul-Henning Kamp,
3944Preethi Natarajan,
3945Rajeev Bector,
3946Ray Polk,
3947Reto Bachmann-Gmuer,
3948Richard Cyganiak,
3949Robby Simpson,
3950Robert Brewer,
3951Robert Collins,
3952Robert Mattson,
3953Robert O'Callahan,
3954Robert Olofsson,
3955Robert Sayre,
3956Robert Siemer,
3957Robert de Wilde,
3958Roberto Javier Godoy,
3959Roberto Peon,
3960Roland Zink,
3961Ronny Widjaja,
3962S. Mike Dierken,
3963Salvatore Loreto,
3964Sam Johnston,
3965Sam Ruby,
3966Scott Lawrence (who maintained the original issues list),
3967Sean B. Palmer,
3968Shane McCarron,
3969Stefan Eissing,
3970Stefan Tilkov,
3971Stefanos Harhalakis,
3972Stephane Bortzmeyer,
3973Stephen Farrell,
3974Stephen Ludin,
3975Stuart Williams,
3976Subbu Allamaraju,
3977Sylvain Hellegouarch,
3978Tapan Divekar,
3979Tatsuya Hayashi,
3980Ted Hardie,
3981Thomas Broyer,
3982Thomas Fossati,
3983Thomas Maslen,
3984Thomas Nordin,
3985Thomas Roessler,
3986Tim Bray,
3987Tim Morgan,
3988Tim Olsen,
3989Tom Zhou,
3990Travis Snoozy,
3991Tyler Close,
3992Vincent Murphy,
3993Wenbo Zhu,
3994Werner Baumann,
3995Wilbur Streett,
3996Wilfredo Sanchez Vega,
3997William A. Rowe Jr.,
3998William Chan,
3999Willy Tarreau,
4000Xiaoshu Wang,
4001Yaron Goland,
4002Yngve Nysaeter Pettersen,
4003Yoav Nir,
4004Yogesh Bang,
4005Yutaka Oiwa,
4006Yves Lafon (long-time member of the editor team),
4007Zed A. Shaw, and
4008Zhong Yu.
4009</t>
4010<?ENDINC acks ?>
4011<t>
4012   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
4013   acknowledgements from prior revisions.
4014</t>
4015</section>
4016
4017</middle>
4018<back>
4019
4020<references title="Normative References">
4021
4022<reference anchor="Part2">
4023  <front>
4024    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
4025    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4026      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4027      <address><email>fielding@gbiv.com</email></address>
4028    </author>
4029    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4030      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4031      <address><email>julian.reschke@greenbytes.de</email></address>
4032    </author>
4033    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4034  </front>
4035  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
4036  <x:source href="p2-semantics.xml" basename="p2-semantics">
4037    <x:defines>1xx (Informational)</x:defines>
4038    <x:defines>1xx</x:defines>
4039    <x:defines>100 (Continue)</x:defines>
4040    <x:defines>101 (Switching Protocols)</x:defines>
4041    <x:defines>2xx (Successful)</x:defines>
4042    <x:defines>2xx</x:defines>
4043    <x:defines>200 (OK)</x:defines>
4044    <x:defines>204 (No Content)</x:defines>
4045    <x:defines>3xx (Redirection)</x:defines>
4046    <x:defines>3xx</x:defines>
4047    <x:defines>301 (Moved Permanently)</x:defines>
4048    <x:defines>4xx (Client Error)</x:defines>
4049    <x:defines>4xx</x:defines>
4050    <x:defines>400 (Bad Request)</x:defines>
4051    <x:defines>411 (Length Required)</x:defines>
4052    <x:defines>414 (URI Too Long)</x:defines>
4053    <x:defines>417 (Expectation Failed)</x:defines>
4054    <x:defines>426 (Upgrade Required)</x:defines>
4055    <x:defines>501 (Not Implemented)</x:defines>
4056    <x:defines>502 (Bad Gateway)</x:defines>
4057    <x:defines>505 (HTTP Version Not Supported)</x:defines>
4058    <x:defines>Allow</x:defines>
4059    <x:defines>Content-Encoding</x:defines>
4060    <x:defines>Content-Location</x:defines>
4061    <x:defines>Content-Type</x:defines>
4062    <x:defines>Date</x:defines>
4063    <x:defines>Expect</x:defines>
4064    <x:defines>Location</x:defines>
4065    <x:defines>Server</x:defines>
4066    <x:defines>User-Agent</x:defines>
4067  </x:source>
4068</reference>
4069
4070<reference anchor="Part4">
4071  <front>
4072    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
4073    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
4074      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4075      <address><email>fielding@gbiv.com</email></address>
4076    </author>
4077    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
4078      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4079      <address><email>julian.reschke@greenbytes.de</email></address>
4080    </author>
4081    <date month="&ID-MONTH;" year="&ID-YEAR;" />
4082  </front>
4083  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
4084  <x:source basename="p4-conditional" href="p4-conditional.xml">
4085    <x:defines>304 (Not Modified)</x:defines>
4086    <x:defines>ETag</x:defines>
4087    <x:defines>Last-Modified</x:defines>
4088  </x:source>
4089</reference>
4090
4091<reference anchor="Part5">
4092  <front>
4093    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
4094    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4095      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4096      <address><email>fielding@gbiv.com</email></address>
4097    </author>
4098    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
4099      <organization abbrev="W3C">World Wide Web Consortium</organization>
4100      <address><email>ylafon@w3.org</email></address>
4101    </author>
4102    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4103      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4104      <address><email>julian.reschke@greenbytes.de</email></address>
4105    </author>
4106    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4107  </front>
4108  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4109  <x:source href="p5-range.xml" basename="p5-range">
4110    <x:defines>Content-Range</x:defines>
4111  </x:source>
4112</reference>
4113
4114<reference anchor="Part6">
4115  <front>
4116    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4117    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4118      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4119      <address><email>fielding@gbiv.com</email></address>
4120    </author>
4121    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4122      <organization>Akamai</organization>
4123      <address><email>mnot@mnot.net</email></address>
4124    </author>
4125    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4126      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4127      <address><email>julian.reschke@greenbytes.de</email></address>
4128    </author>
4129    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4130  </front>
4131  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4132  <x:source href="p6-cache.xml" basename="p6-cache">
4133    <x:defines>Cache-Control</x:defines>
4134    <x:defines>Expires</x:defines>
4135  </x:source>
4136</reference>
4137
4138<reference anchor="Part7">
4139  <front>
4140    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4141    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4142      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4143      <address><email>fielding@gbiv.com</email></address>
4144    </author>
4145    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4146      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4147      <address><email>julian.reschke@greenbytes.de</email></address>
4148    </author>
4149    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4150  </front>
4151  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4152  <x:source href="p7-auth.xml" basename="p7-auth">
4153    <x:defines>Proxy-Authenticate</x:defines>
4154    <x:defines>Proxy-Authorization</x:defines>
4155  </x:source>
4156</reference>
4157
4158<reference anchor="RFC5234">
4159  <front>
4160    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4161    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4162      <organization>Brandenburg InternetWorking</organization>
4163      <address>
4164        <email>dcrocker@bbiw.net</email>
4165      </address> 
4166    </author>
4167    <author initials="P." surname="Overell" fullname="Paul Overell">
4168      <organization>THUS plc.</organization>
4169      <address>
4170        <email>paul.overell@thus.net</email>
4171      </address>
4172    </author>
4173    <date month="January" year="2008"/>
4174  </front>
4175  <seriesInfo name="STD" value="68"/>
4176  <seriesInfo name="RFC" value="5234"/>
4177</reference>
4178
4179<reference anchor="RFC2119">
4180  <front>
4181    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4182    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4183      <organization>Harvard University</organization>
4184      <address><email>sob@harvard.edu</email></address>
4185    </author>
4186    <date month="March" year="1997"/>
4187  </front>
4188  <seriesInfo name="BCP" value="14"/>
4189  <seriesInfo name="RFC" value="2119"/>
4190</reference>
4191
4192<reference anchor="RFC3986">
4193 <front>
4194  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4195  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4196    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4197    <address>
4198       <email>timbl@w3.org</email>
4199       <uri>http://www.w3.org/People/Berners-Lee/</uri>
4200    </address>
4201  </author>
4202  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4203    <organization abbrev="Day Software">Day Software</organization>
4204    <address>
4205      <email>fielding@gbiv.com</email>
4206      <uri>http://roy.gbiv.com/</uri>
4207    </address>
4208  </author>
4209  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4210    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4211    <address>
4212      <email>LMM@acm.org</email>
4213      <uri>http://larry.masinter.net/</uri>
4214    </address>
4215  </author>
4216  <date month='January' year='2005'></date>
4217 </front>
4218 <seriesInfo name="STD" value="66"/>
4219 <seriesInfo name="RFC" value="3986"/>
4220</reference>
4221
4222<reference anchor="RFC0793">
4223  <front>
4224    <title>Transmission Control Protocol</title>
4225    <author initials='J.' surname='Postel' fullname='Jon Postel'>
4226      <organization>University of Southern California (USC)/Information Sciences Institute</organization>
4227    </author>
4228    <date year='1981' month='September' />
4229  </front>
4230  <seriesInfo name='STD' value='7' />
4231  <seriesInfo name='RFC' value='793' />
4232</reference>
4233
4234<reference anchor="USASCII">
4235  <front>
4236    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4237    <author>
4238      <organization>American National Standards Institute</organization>
4239    </author>
4240    <date year="1986"/>
4241  </front>
4242  <seriesInfo name="ANSI" value="X3.4"/>
4243</reference>
4244
4245<reference anchor="RFC1950">
4246  <front>
4247    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4248    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4249      <organization>Aladdin Enterprises</organization>
4250      <address><email>ghost@aladdin.com</email></address>
4251    </author>
4252    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4253    <date month="May" year="1996"/>
4254  </front>
4255  <seriesInfo name="RFC" value="1950"/>
4256  <!--<annotation>
4257    RFC 1950 is an Informational RFC, thus it might be less stable than
4258    this specification. On the other hand, this downward reference was
4259    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4260    therefore it is unlikely to cause problems in practice. See also
4261    <xref target="BCP97"/>.
4262  </annotation>-->
4263</reference>
4264
4265<reference anchor="RFC1951">
4266  <front>
4267    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4268    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4269      <organization>Aladdin Enterprises</organization>
4270      <address><email>ghost@aladdin.com</email></address>
4271    </author>
4272    <date month="May" year="1996"/>
4273  </front>
4274  <seriesInfo name="RFC" value="1951"/>
4275  <!--<annotation>
4276    RFC 1951 is an Informational RFC, thus it might be less stable than
4277    this specification. On the other hand, this downward reference was
4278    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4279    therefore it is unlikely to cause problems in practice. See also
4280    <xref target="BCP97"/>.
4281  </annotation>-->
4282</reference>
4283
4284<reference anchor="RFC1952">
4285  <front>
4286    <title>GZIP file format specification version 4.3</title>
4287    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4288      <organization>Aladdin Enterprises</organization>
4289      <address><email>ghost@aladdin.com</email></address>
4290    </author>
4291    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4292      <address><email>gzip@prep.ai.mit.edu</email></address>
4293    </author>
4294    <author initials="M." surname="Adler" fullname="Mark Adler">
4295      <address><email>madler@alumni.caltech.edu</email></address>
4296    </author>
4297    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4298      <address><email>ghost@aladdin.com</email></address>
4299    </author>
4300    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4301      <address><email>randeg@alumni.rpi.edu</email></address>
4302    </author>
4303    <date month="May" year="1996"/>
4304  </front>
4305  <seriesInfo name="RFC" value="1952"/>
4306  <!--<annotation>
4307    RFC 1952 is an Informational RFC, thus it might be less stable than
4308    this specification. On the other hand, this downward reference was
4309    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4310    therefore it is unlikely to cause problems in practice. See also
4311    <xref target="BCP97"/>.
4312  </annotation>-->
4313</reference>
4314
4315</references>
4316
4317<references title="Informative References">
4318
4319<reference anchor="ISO-8859-1">
4320  <front>
4321    <title>
4322     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4323    </title>
4324    <author>
4325      <organization>International Organization for Standardization</organization>
4326    </author>
4327    <date year="1998"/>
4328  </front>
4329  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4330</reference>
4331
4332<reference anchor='RFC1919'>
4333  <front>
4334    <title>Classical versus Transparent IP Proxies</title>
4335    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4336      <address><email>mchatel@pax.eunet.ch</email></address>
4337    </author>
4338    <date year='1996' month='March' />
4339  </front>
4340  <seriesInfo name='RFC' value='1919' />
4341</reference>
4342
4343<reference anchor="RFC1945">
4344  <front>
4345    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4346    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4347      <organization>MIT, Laboratory for Computer Science</organization>
4348      <address><email>timbl@w3.org</email></address>
4349    </author>
4350    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4351      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4352      <address><email>fielding@ics.uci.edu</email></address>
4353    </author>
4354    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4355      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4356      <address><email>frystyk@w3.org</email></address>
4357    </author>
4358    <date month="May" year="1996"/>
4359  </front>
4360  <seriesInfo name="RFC" value="1945"/>
4361</reference>
4362
4363<reference anchor="RFC2045">
4364  <front>
4365    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4366    <author initials="N." surname="Freed" fullname="Ned Freed">
4367      <organization>Innosoft International, Inc.</organization>
4368      <address><email>ned@innosoft.com</email></address>
4369    </author>
4370    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4371      <organization>First Virtual Holdings</organization>
4372      <address><email>nsb@nsb.fv.com</email></address>
4373    </author>
4374    <date month="November" year="1996"/>
4375  </front>
4376  <seriesInfo name="RFC" value="2045"/>
4377</reference>
4378
4379<reference anchor="RFC2047">
4380  <front>
4381    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4382    <author initials="K." surname="Moore" fullname="Keith Moore">
4383      <organization>University of Tennessee</organization>
4384      <address><email>moore@cs.utk.edu</email></address>
4385    </author>
4386    <date month="November" year="1996"/>
4387  </front>
4388  <seriesInfo name="RFC" value="2047"/>
4389</reference>
4390
4391<reference anchor="RFC2068">
4392  <front>
4393    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4394    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
4395      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4396      <address><email>fielding@ics.uci.edu</email></address>
4397    </author>
4398    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4399      <organization>MIT Laboratory for Computer Science</organization>
4400      <address><email>jg@w3.org</email></address>
4401    </author>
4402    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4403      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4404      <address><email>mogul@wrl.dec.com</email></address>
4405    </author>
4406    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4407      <organization>MIT Laboratory for Computer Science</organization>
4408      <address><email>frystyk@w3.org</email></address>
4409    </author>
4410    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4411      <organization>MIT Laboratory for Computer Science</organization>
4412      <address><email>timbl@w3.org</email></address>
4413    </author>
4414    <date month="January" year="1997"/>
4415  </front>
4416  <seriesInfo name="RFC" value="2068"/>
4417</reference>
4418
4419<reference anchor="RFC2145">
4420  <front>
4421    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4422    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4423      <organization>Western Research Laboratory</organization>
4424      <address><email>mogul@wrl.dec.com</email></address>
4425    </author>
4426    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4427      <organization>Department of Information and Computer Science</organization>
4428      <address><email>fielding@ics.uci.edu</email></address>
4429    </author>
4430    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4431      <organization>MIT Laboratory for Computer Science</organization>
4432      <address><email>jg@w3.org</email></address>
4433    </author>
4434    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4435      <organization>W3 Consortium</organization>
4436      <address><email>frystyk@w3.org</email></address>
4437    </author>
4438    <date month="May" year="1997"/>
4439  </front>
4440  <seriesInfo name="RFC" value="2145"/>
4441</reference>
4442
4443<reference anchor="RFC2616">
4444  <front>
4445    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4446    <author initials="R." surname="Fielding" fullname="R. Fielding">
4447      <organization>University of California, Irvine</organization>
4448      <address><email>fielding@ics.uci.edu</email></address>
4449    </author>
4450    <author initials="J." surname="Gettys" fullname="J. Gettys">
4451      <organization>W3C</organization>
4452      <address><email>jg@w3.org</email></address>
4453    </author>
4454    <author initials="J." surname="Mogul" fullname="J. Mogul">
4455      <organization>Compaq Computer Corporation</organization>
4456      <address><email>mogul@wrl.dec.com</email></address>
4457    </author>
4458    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4459      <organization>MIT Laboratory for Computer Science</organization>
4460      <address><email>frystyk@w3.org</email></address>
4461    </author>
4462    <author initials="L." surname="Masinter" fullname="L. Masinter">
4463      <organization>Xerox Corporation</organization>
4464      <address><email>masinter@parc.xerox.com</email></address>
4465    </author>
4466    <author initials="P." surname="Leach" fullname="P. Leach">
4467      <organization>Microsoft Corporation</organization>
4468      <address><email>paulle@microsoft.com</email></address>
4469    </author>
4470    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4471      <organization>W3C</organization>
4472      <address><email>timbl@w3.org</email></address>
4473    </author>
4474    <date month="June" year="1999"/>
4475  </front>
4476  <seriesInfo name="RFC" value="2616"/>
4477</reference>
4478
4479<reference anchor='RFC2817'>
4480  <front>
4481    <title>Upgrading to TLS Within HTTP/1.1</title>
4482    <author initials='R.' surname='Khare' fullname='R. Khare'>
4483      <organization>4K Associates / UC Irvine</organization>
4484      <address><email>rohit@4K-associates.com</email></address>
4485    </author>
4486    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4487      <organization>Agranat Systems, Inc.</organization>
4488      <address><email>lawrence@agranat.com</email></address>
4489    </author>
4490    <date year='2000' month='May' />
4491  </front>
4492  <seriesInfo name='RFC' value='2817' />
4493</reference>
4494
4495<reference anchor='RFC2818'>
4496  <front>
4497    <title>HTTP Over TLS</title>
4498    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4499      <organization>RTFM, Inc.</organization>
4500      <address><email>ekr@rtfm.com</email></address>
4501    </author>
4502    <date year='2000' month='May' />
4503  </front>
4504  <seriesInfo name='RFC' value='2818' />
4505</reference>
4506
4507<reference anchor='RFC3040'>
4508  <front>
4509    <title>Internet Web Replication and Caching Taxonomy</title>
4510    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4511      <organization>Equinix, Inc.</organization>
4512    </author>
4513    <author initials='I.' surname='Melve' fullname='I. Melve'>
4514      <organization>UNINETT</organization>
4515    </author>
4516    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4517      <organization>CacheFlow Inc.</organization>
4518    </author>
4519    <date year='2001' month='January' />
4520  </front>
4521  <seriesInfo name='RFC' value='3040' />
4522</reference>
4523
4524<reference anchor='BCP90'>
4525  <front>
4526    <title>Registration Procedures for Message Header Fields</title>
4527    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4528      <organization>Nine by Nine</organization>
4529      <address><email>GK-IETF@ninebynine.org</email></address>
4530    </author>
4531    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4532      <organization>BEA Systems</organization>
4533      <address><email>mnot@pobox.com</email></address>
4534    </author>
4535    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4536      <organization>HP Labs</organization>
4537      <address><email>JeffMogul@acm.org</email></address>
4538    </author>
4539    <date year='2004' month='September' />
4540  </front>
4541  <seriesInfo name='BCP' value='90' />
4542  <seriesInfo name='RFC' value='3864' />
4543</reference>
4544
4545<reference anchor='RFC4033'>
4546  <front>
4547    <title>DNS Security Introduction and Requirements</title>
4548    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4549    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4550    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4551    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4552    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4553    <date year='2005' month='March' />
4554  </front>
4555  <seriesInfo name='RFC' value='4033' />
4556</reference>
4557
4558<reference anchor="BCP13">
4559  <front>
4560    <title>Media Type Specifications and Registration Procedures</title>
4561    <author initials="N." surname="Freed" fullname="Ned Freed">
4562      <organization>Oracle</organization>
4563      <address>
4564        <email>ned+ietf@mrochek.com</email>
4565      </address>
4566    </author>
4567    <author initials="J." surname="Klensin" fullname="John C. Klensin">
4568      <address>
4569        <email>john+ietf@jck.com</email>
4570      </address>
4571    </author>
4572    <author initials="T." surname="Hansen" fullname="Tony Hansen">
4573      <organization>AT&amp;T Laboratories</organization>
4574      <address>
4575        <email>tony+mtsuffix@maillennium.att.com</email>
4576      </address>
4577    </author>
4578    <date year="2013" month="January"/>
4579  </front>
4580  <seriesInfo name="BCP" value="13"/>
4581  <seriesInfo name="RFC" value="6838"/>
4582</reference>
4583
4584<reference anchor='BCP115'>
4585  <front>
4586    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4587    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4588      <organization>AT&amp;T Laboratories</organization>
4589      <address>
4590        <email>tony+urireg@maillennium.att.com</email>
4591      </address>
4592    </author>
4593    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4594      <organization>Qualcomm, Inc.</organization>
4595      <address>
4596        <email>hardie@qualcomm.com</email>
4597      </address>
4598    </author>
4599    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4600      <organization>Adobe Systems</organization>
4601      <address>
4602        <email>LMM@acm.org</email>
4603      </address>
4604    </author>
4605    <date year='2006' month='February' />
4606  </front>
4607  <seriesInfo name='BCP' value='115' />
4608  <seriesInfo name='RFC' value='4395' />
4609</reference>
4610
4611<reference anchor='RFC4559'>
4612  <front>
4613    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4614    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4615    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4616    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4617    <date year='2006' month='June' />
4618  </front>
4619  <seriesInfo name='RFC' value='4559' />
4620</reference>
4621
4622<reference anchor='RFC5226'>
4623  <front>
4624    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4625    <author initials='T.' surname='Narten' fullname='T. Narten'>
4626      <organization>IBM</organization>
4627      <address><email>narten@us.ibm.com</email></address>
4628    </author>
4629    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4630      <organization>Google</organization>
4631      <address><email>Harald@Alvestrand.no</email></address>
4632    </author>
4633    <date year='2008' month='May' />
4634  </front>
4635  <seriesInfo name='BCP' value='26' />
4636  <seriesInfo name='RFC' value='5226' />
4637</reference>
4638
4639<reference anchor='RFC5246'>
4640   <front>
4641      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4642      <author initials='T.' surname='Dierks' fullname='T. Dierks'>
4643         <organization />
4644      </author>
4645      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4646         <organization>RTFM, Inc.</organization>
4647      </author>
4648      <date year='2008' month='August' />
4649   </front>
4650   <seriesInfo name='RFC' value='5246' />
4651</reference>
4652
4653<reference anchor="RFC5322">
4654  <front>
4655    <title>Internet Message Format</title>
4656    <author initials="P." surname="Resnick" fullname="P. Resnick">
4657      <organization>Qualcomm Incorporated</organization>
4658    </author>
4659    <date year="2008" month="October"/>
4660  </front>
4661  <seriesInfo name="RFC" value="5322"/>
4662</reference>
4663
4664<reference anchor="RFC6265">
4665  <front>
4666    <title>HTTP State Management Mechanism</title>
4667    <author initials="A." surname="Barth" fullname="Adam Barth">
4668      <organization abbrev="U.C. Berkeley">
4669        University of California, Berkeley
4670      </organization>
4671      <address><email>abarth@eecs.berkeley.edu</email></address>
4672    </author>
4673    <date year="2011" month="April" />
4674  </front>
4675  <seriesInfo name="RFC" value="6265"/>
4676</reference>
4677
4678<reference anchor='RFC6585'>
4679  <front>
4680    <title>Additional HTTP Status Codes</title>
4681    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4682      <organization>Rackspace</organization>
4683    </author>
4684    <author initials='R.' surname='Fielding' fullname='R. Fielding'>
4685      <organization>Adobe</organization>
4686    </author>
4687    <date year='2012' month='April' />
4688   </front>
4689   <seriesInfo name='RFC' value='6585' />
4690</reference>
4691
4692<!--<reference anchor='BCP97'>
4693  <front>
4694    <title>Handling Normative References to Standards-Track Documents</title>
4695    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
4696      <address>
4697        <email>klensin+ietf@jck.com</email>
4698      </address>
4699    </author>
4700    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
4701      <organization>MIT</organization>
4702      <address>
4703        <email>hartmans-ietf@mit.edu</email>
4704      </address>
4705    </author>
4706    <date year='2007' month='June' />
4707  </front>
4708  <seriesInfo name='BCP' value='97' />
4709  <seriesInfo name='RFC' value='4897' />
4710</reference>-->
4711
4712<reference anchor="Kri2001" target="http://arxiv.org/abs/cs.SE/0105018">
4713  <front>
4714    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
4715    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
4716    <date year="2001" month="November"/>
4717  </front>
4718  <seriesInfo name="ACM Transactions on Internet Technology" value="Vol. 1, #2"/>
4719</reference>
4720
4721</references>
4722
4723
4724<section title="HTTP Version History" anchor="compatibility">
4725<t>
4726   HTTP has been in use by the World-Wide Web global information initiative
4727   since 1990. The first version of HTTP, later referred to as HTTP/0.9,
4728   was a simple protocol for hypertext data transfer across the Internet
4729   with only a single request method (GET) and no metadata.
4730   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
4731   methods and MIME-like messaging that could include metadata about the data
4732   transferred and modifiers on the request/response semantics. However,
4733   HTTP/1.0 did not sufficiently take into consideration the effects of
4734   hierarchical proxies, caching, the need for persistent connections, or
4735   name-based virtual hosts. The proliferation of incompletely-implemented
4736   applications calling themselves "HTTP/1.0" further necessitated a
4737   protocol version change in order for two communicating applications
4738   to determine each other's true capabilities.
4739</t>
4740<t>
4741   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
4742   requirements that enable reliable implementations, adding only
4743   those new features that will either be safely ignored by an HTTP/1.0
4744   recipient or only sent when communicating with a party advertising
4745   conformance with HTTP/1.1.
4746</t>
4747<t>
4748   It is beyond the scope of a protocol specification to mandate
4749   conformance with previous versions. HTTP/1.1 was deliberately
4750   designed, however, to make supporting previous versions easy.
4751   We would expect a general-purpose HTTP/1.1 server to understand
4752   any valid request in the format of HTTP/1.0 and respond appropriately
4753   with an HTTP/1.1 message that only uses features understood (or
4754   safely ignored) by HTTP/1.0 clients.  Likewise, we would expect
4755   an HTTP/1.1 client to understand any valid HTTP/1.0 response.
4756</t>
4757<t>
4758   Since HTTP/0.9 did not support header fields in a request,
4759   there is no mechanism for it to support name-based virtual
4760   hosts (selection of resource by inspection of the <x:ref>Host</x:ref> header
4761   field).  Any server that implements name-based virtual hosts
4762   ought to disable support for HTTP/0.9.  Most requests that
4763   appear to be HTTP/0.9 are, in fact, badly constructed HTTP/1.x
4764   requests wherein a buggy client failed to properly encode
4765   linear whitespace found in a URI reference and placed in
4766   the request-target.
4767</t>
4768
4769<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
4770<t>
4771   This section summarizes major differences between versions HTTP/1.0
4772   and HTTP/1.1.
4773</t>
4774
4775<section title="Multi-homed Web Servers" anchor="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">
4776<t>
4777   The requirements that clients and servers support the <x:ref>Host</x:ref>
4778   header field (<xref target="header.host"/>), report an error if it is
4779   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
4780   are among the most important changes defined by HTTP/1.1.
4781</t>
4782<t>
4783   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4784   addresses and servers; there was no other established mechanism for
4785   distinguishing the intended server of a request than the IP address
4786   to which that request was directed. The <x:ref>Host</x:ref> header field was
4787   introduced during the development of HTTP/1.1 and, though it was
4788   quickly implemented by most HTTP/1.0 browsers, additional requirements
4789   were placed on all HTTP/1.1 requests in order to ensure complete
4790   adoption.  At the time of this writing, most HTTP-based services
4791   are dependent upon the Host header field for targeting requests.
4792</t>
4793</section>
4794
4795<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4796<t>
4797   In HTTP/1.0, each connection is established by the client prior to the
4798   request and closed by the server after sending the response. However, some
4799   implementations implement the explicitly negotiated ("Keep-Alive") version
4800   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
4801   target="RFC2068"/>.
4802</t>
4803<t>
4804   Some clients and servers might wish to be compatible with these previous
4805   approaches to persistent connections, by explicitly negotiating for them
4806   with a "Connection: keep-alive" request header field. However, some
4807   experimental implementations of HTTP/1.0 persistent connections are faulty;
4808   for example, if an HTTP/1.0 proxy server doesn't understand
4809   <x:ref>Connection</x:ref>, it will erroneously forward that header field
4810   to the next inbound server, which would result in a hung connection.
4811</t>
4812<t>
4813   One attempted solution was the introduction of a Proxy-Connection header
4814   field, targeted specifically at proxies. In practice, this was also
4815   unworkable, because proxies are often deployed in multiple layers, bringing
4816   about the same problem discussed above.
4817</t>
4818<t>
4819   As a result, clients are encouraged not to send the Proxy-Connection header
4820   field in any requests.
4821</t>
4822<t>
4823   Clients are also encouraged to consider the use of Connection: keep-alive
4824   in requests carefully; while they can enable persistent connections with
4825   HTTP/1.0 servers, clients using them will need to monitor the
4826   connection for "hung" requests (which indicate that the client ought stop
4827   sending the header field), and this mechanism ought not be used by clients
4828   at all when a proxy is being used.
4829</t>
4830</section>
4831
4832<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
4833<t>
4834   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
4835   (<xref target="header.transfer-encoding"/>).
4836   Transfer codings need to be decoded prior to forwarding an HTTP message
4837   over a MIME-compliant protocol.
4838</t>
4839</section>
4840
4841</section>
4842
4843<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4844<t>
4845  HTTP's approach to error handling has been explained.
4846  (<xref target="conformance"/>)
4847</t>
4848<t>
4849  The expectation to support HTTP/0.9 requests has been removed.
4850</t>
4851<t>
4852  The term "Effective Request URI" has been introduced.
4853  (<xref target="effective.request.uri" />)
4854</t>
4855<t>
4856  HTTP messages can be (and often are) buffered by implementations; despite
4857  it sometimes being available as a stream, HTTP is fundamentally a
4858  message-oriented protocol.
4859  (<xref target="http.message" />)
4860</t>
4861<t>
4862  Minimum supported sizes for various protocol elements have been
4863  suggested, to improve interoperability.
4864</t>
4865<t>
4866  Header fields that span multiple lines ("line folding") are deprecated.
4867  (<xref target="field.parsing" />)
4868</t>
4869<t>
4870  The HTTP-version ABNF production has been clarified to be case-sensitive.
4871  Additionally, version numbers has been restricted to single digits, due
4872  to the fact that implementations are known to handle multi-digit version
4873  numbers incorrectly.
4874  (<xref target="http.version"/>)
4875</t>
4876<t>
4877  The HTTPS URI scheme is now defined by this specification; previously,
4878  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
4879  (<xref target="https.uri"/>)
4880</t>
4881<t>
4882  The HTTPS URI scheme implies end-to-end security.
4883  (<xref target="https.uri"/>)
4884</t>
4885<t>
4886  Userinfo (i.e., username and password) are now disallowed in HTTP and
4887  HTTPS URIs, because of security issues related to their transmission on the
4888  wire.
4889  (<xref target="http.uri" />)
4890</t>
4891<t>
4892  Invalid whitespace around field-names is now required to be rejected,
4893  because accepting it represents a security vulnerability.
4894  (<xref target="header.fields"/>)
4895</t>
4896<t>
4897  The ABNF productions defining header fields now only list the field value.
4898  (<xref target="header.fields"/>)
4899</t>
4900<t>
4901  Rules about implicit linear whitespace between certain grammar productions
4902  have been removed; now whitespace is only allowed where specifically
4903  defined in the ABNF.
4904  (<xref target="whitespace"/>)
4905</t>
4906<t> 
4907  The NUL octet is no longer allowed in comment and quoted-string text, and
4908  handling of backslash-escaping in them has been clarified.
4909  (<xref target="field.components"/>)
4910</t> 
4911<t>
4912  The quoted-pair rule no longer allows escaping control characters other than
4913  HTAB.
4914  (<xref target="field.components"/>)
4915</t>
4916<t>
4917  Non-ASCII content in header fields and the reason phrase has been obsoleted
4918  and made opaque (the TEXT rule was removed).
4919  (<xref target="field.components"/>)
4920</t>
4921<t>
4922  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
4923  handled as errors by recipients.
4924  (<xref target="header.content-length"/>)
4925</t>
4926<t>
4927  The "identity" transfer coding token has been removed.
4928  (Sections <xref format="counter" target="message.body"/> and
4929  <xref format="counter" target="transfer.codings"/>)
4930</t>
4931<t>
4932  The algorithm for determining the message body length has been clarified
4933  to indicate all of the special cases (e.g., driven by methods or status
4934  codes) that affect it, and that new protocol elements cannot define such
4935  special cases.
4936  (<xref target="message.body.length"/>)
4937</t>
4938<t>
4939  "multipart/byteranges" is no longer a way of determining message body length
4940  detection.
4941  (<xref target="message.body.length"/>)
4942</t>
4943<t>
4944  CONNECT is a new, special case in determining message body length.
4945  (<xref target="message.body.length"/>)
4946</t>
4947<t>
4948  Chunk length does not include the count of the octets in the
4949  chunk header and trailer.
4950  (<xref target="chunked.encoding"/>)
4951</t>
4952<t>
4953  Use of chunk extensions is deprecated, and line folding in them is
4954  disallowed.
4955  (<xref target="chunked.encoding"/>)
4956</t>
4957<t>
4958  The segment + query components of RFC3986 have been used to define the
4959  request-target, instead of abs_path from RFC 1808.
4960  (<xref target="request-target"/>)
4961</t>
4962<t>
4963  The asterisk form of the request-target is only allowed in the OPTIONS
4964  method.
4965  (<xref target="request-target"/>)
4966</t>
4967<t>
4968  Exactly when "close" connection options have to be sent has been clarified.
4969  (<xref target="header.connection"/>)
4970</t>
4971<t> 
4972  "hop-by-hop" header fields are required to appear in the Connection header
4973  field; just because they're defined as hop-by-hop in this specification
4974  doesn't exempt them.
4975  (<xref target="header.connection"/>)
4976</t>
4977<t>
4978  The limit of two connections per server has been removed.
4979  (<xref target="persistent.connections"/>)
4980</t>
4981<t>
4982  An idempotent sequence of requests is no longer required to be retried.
4983  (<xref target="persistent.connections"/>)
4984</t>
4985<t>
4986  The requirement to retry requests under certain circumstances when the
4987  server prematurely closes the connection has been removed.
4988  (<xref target="persistent.connections"/>)
4989</t>
4990<t>
4991  Some extraneous requirements about when servers are allowed to close
4992  connections prematurely have been removed.
4993  (<xref target="persistent.connections"/>)
4994</t>
4995<t>
4996  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
4997  responses other than 101 (this was incorporated from <xref
4998  target="RFC2817"/>).
4999  (<xref target="header.upgrade"/>)
5000</t>
5001<t>
5002  Registration of Transfer Codings now requires IETF Review
5003  (<xref target="transfer.coding.registry"/>)
5004</t>
5005<t>
5006  The meaning of the "deflate" content coding has been clarified.
5007  (<xref target="deflate.coding" />)
5008</t>
5009<t>
5010  This specification now defines the Upgrade Token Registry, previously
5011  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
5012  (<xref target="upgrade.token.registry"/>)
5013</t>
5014<t>
5015  Issues with the Keep-Alive and Proxy-Connection header fields in requests
5016  are pointed out, with use of the latter being discouraged altogether.
5017  (<xref target="compatibility.with.http.1.0.persistent.connections" />)
5018</t>
5019<t>
5020  Empty list elements in list productions (e.g., a list header field containing
5021  ", ,") have been deprecated.
5022  (<xref target="abnf.extension"/>)
5023</t>
5024</section>
5025</section>
5026
5027<section title="ABNF list extension: #rule" anchor="abnf.extension">
5028<t>
5029  A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
5030  improve readability in the definitions of some header field values.
5031</t>
5032<t>
5033  A construct "#" is defined, similar to "*", for defining comma-delimited
5034  lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
5035  at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
5036  comma (",") and optional whitespace (OWS).   
5037</t>
5038<figure><preamble>
5039  Thus,
5040</preamble><artwork type="example">
5041  1#element =&gt; element *( OWS "," OWS element )
5042</artwork></figure>
5043<figure><preamble>
5044  and:
5045</preamble><artwork type="example">
5046  #element =&gt; [ 1#element ]
5047</artwork></figure>
5048<figure><preamble>
5049  and for n &gt;= 1 and m &gt; 1:
5050</preamble><artwork type="example">
5051  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
5052</artwork></figure>
5053<t>
5054  For compatibility with legacy list rules, recipients &SHOULD; accept empty
5055  list elements. In other words, consumers would follow the list productions:
5056</t>
5057<figure><artwork type="example">
5058  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
5059 
5060  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
5061</artwork></figure>
5062<t>
5063  Note that empty elements do not contribute to the count of elements present,
5064  though.
5065</t>
5066<t>
5067  For example, given these ABNF productions:
5068</t>
5069<figure><artwork type="example">
5070  example-list      = 1#example-list-elmt
5071  example-list-elmt = token ; see <xref target="field.components"/>
5072</artwork></figure>
5073<t>
5074  Then these are valid values for example-list (not including the double
5075  quotes, which are present for delimitation only):
5076</t>
5077<figure><artwork type="example">
5078  "foo,bar"
5079  "foo ,bar,"
5080  "foo , ,bar,charlie   "
5081</artwork></figure>
5082<t>
5083  But these values would be invalid, as at least one non-empty element is
5084  required:
5085</t>
5086<figure><artwork type="example">
5087  ""
5088  ","
5089  ",   ,"
5090</artwork></figure>
5091<t>
5092  <xref target="collected.abnf"/> shows the collected ABNF, with the list rules
5093  expanded as explained above.
5094</t>
5095</section>
5096
5097<?BEGININC p1-messaging.abnf-appendix ?>
5098<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
5099<figure>
5100<artwork type="abnf" name="p1-messaging.parsed-abnf">
5101<x:ref>BWS</x:ref> = OWS
5102
5103<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
5104 connection-option ] )
5105<x:ref>Content-Length</x:ref> = 1*DIGIT
5106
5107<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
5108 ]
5109<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
5110<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
5111<x:ref>Host</x:ref> = uri-host [ ":" port ]
5112
5113<x:ref>OWS</x:ref> = *( SP / HTAB )
5114
5115<x:ref>RWS</x:ref> = 1*( SP / HTAB )
5116
5117<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
5118<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
5119<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
5120 transfer-coding ] )
5121
5122<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
5123<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
5124
5125<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
5126 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
5127 comment ] ) ] )
5128
5129<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
5130<x:ref>absolute-form</x:ref> = absolute-URI
5131<x:ref>absolute-path</x:ref> = 1*( "/" segment )
5132<x:ref>asterisk-form</x:ref> = "*"
5133<x:ref>attribute</x:ref> = token
5134<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
5135<x:ref>authority-form</x:ref> = authority
5136
5137<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
5138<x:ref>chunk-data</x:ref> = 1*OCTET
5139<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
5140<x:ref>chunk-ext-name</x:ref> = token
5141<x:ref>chunk-ext-val</x:ref> = token / quoted-str-nf
5142<x:ref>chunk-size</x:ref> = 1*HEXDIG
5143<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
5144<x:ref>comment</x:ref> = "(" *( ctext / quoted-cpair / comment ) ")"
5145<x:ref>connection-option</x:ref> = token
5146<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
5147 / %x2A-5B ; '*'-'['
5148 / %x5D-7E ; ']'-'~'
5149 / obs-text
5150
5151<x:ref>field-content</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5152<x:ref>field-name</x:ref> = token
5153<x:ref>field-value</x:ref> = *( field-content / obs-fold )
5154
5155<x:ref>header-field</x:ref> = field-name ":" OWS field-value OWS
5156<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ]
5157<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ]
5158
5159<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
5160
5161<x:ref>message-body</x:ref> = *OCTET
5162<x:ref>method</x:ref> = token
5163
5164<x:ref>obs-fold</x:ref> = CRLF ( SP / HTAB )
5165<x:ref>obs-text</x:ref> = %x80-FF
5166<x:ref>origin-form</x:ref> = absolute-path [ "?" query ]
5167
5168<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
5169<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
5170<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
5171<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
5172<x:ref>protocol-name</x:ref> = token
5173<x:ref>protocol-version</x:ref> = token
5174<x:ref>pseudonym</x:ref> = token
5175
5176<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5177 / %x5D-7E ; ']'-'~'
5178 / obs-text
5179<x:ref>qdtext-nf</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5180 / %x5D-7E ; ']'-'~'
5181 / obs-text
5182<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5183<x:ref>quoted-cpair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5184<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5185<x:ref>quoted-str-nf</x:ref> = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
5186<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5187
5188<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5189<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5190<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5191<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5192<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5193<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5194<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5195 asterisk-form
5196
5197<x:ref>segment</x:ref> = &lt;segment, defined in [RFC3986], Section 3.3&gt;
5198<x:ref>special</x:ref> = "(" / ")" / "&lt;" / "&gt;" / "@" / "," / ";" / ":" / "\" /
5199 DQUOTE / "/" / "[" / "]" / "?" / "=" / "{" / "}"
5200<x:ref>start-line</x:ref> = request-line / status-line
5201<x:ref>status-code</x:ref> = 3DIGIT
5202<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5203
5204<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5205<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5206<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5207 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5208<x:ref>token</x:ref> = 1*tchar
5209<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5210<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5211 transfer-extension
5212<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5213<x:ref>transfer-parameter</x:ref> = attribute BWS "=" BWS value
5214
5215<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5216
5217<x:ref>value</x:ref> = word
5218
5219<x:ref>word</x:ref> = token / quoted-string
5220</artwork>
5221</figure>
5222</section>
5223<?ENDINC p1-messaging.abnf-appendix ?>
5224
5225<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5226
5227<section title="Since RFC 2616">
5228<t>
5229  Changes up to the first Working Group Last Call draft are summarized
5230  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-21#appendix-D"/>.
5231</t>
5232</section>
5233
5234<section title="Since draft-ietf-httpbis-p1-messaging-21" anchor="changes.since.21">
5235<t>
5236  Closed issues:
5237  <list style="symbols">
5238    <t>
5239      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/128"/>:
5240      "Cite HTTPS URI scheme definition" (the spec now includes the HTTPs
5241      scheme definition and thus updates RFC 2818)
5242    </t>
5243    <t>
5244      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/389"/>:
5245      "mention of 'proxies' in section about caches"
5246    </t>
5247    <t>
5248      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/390"/>:
5249      "use of ABNF terms from RFC 3986"
5250    </t>
5251    <t>
5252      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/391"/>:
5253      "transferring URIs with userinfo in payload"
5254    </t>
5255    <t>
5256      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/392"/>:
5257      "editorial improvements to message length definition"
5258    </t>
5259    <t>
5260      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/395"/>:
5261      "Connection header field MUST vs SHOULD"
5262    </t>
5263    <t>
5264      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/396"/>:
5265      "editorial improvements to persistent connections section"
5266    </t>
5267    <t>
5268      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/397"/>:
5269      "URI normalization vs empty path"
5270    </t>
5271    <t>
5272      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/408"/>:
5273      "p1 feedback"
5274    </t>
5275    <t>
5276      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/409"/>:
5277      "is parsing OBS-FOLD mandatory?"
5278    </t>
5279    <t>
5280      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/410"/>:
5281      "HTTPS and Shared Caching"
5282    </t>
5283    <t>
5284      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/411"/>:
5285      "Requirements for recipients of ws between start-line and first header field"
5286    </t>
5287    <t>
5288      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/412"/>:
5289      "SP and HT when being tolerant"
5290    </t>
5291    <t>
5292      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/414"/>:
5293      "Message Parsing Strictness"
5294    </t>
5295    <t>
5296      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/415"/>:
5297      "'Render'"
5298    </t>
5299    <t>
5300      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/418"/>:
5301      "No-Transform"
5302    </t>
5303    <t>
5304      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/419"/>:
5305      "p2 editorial feedback"
5306    </t>
5307    <t>
5308      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/420"/>:
5309      "Content-Length SHOULD be sent"
5310    </t>
5311    <t>
5312      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/431"/>:
5313      "origin-form does not allow path starting with "//""
5314    </t>
5315    <t>
5316      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/433"/>:
5317      "ambiguity in part 1 example"
5318    </t>
5319  </list>
5320</t>
5321</section>
5322
5323<section title="Since draft-ietf-httpbis-p1-messaging-22" anchor="changes.since.22">
5324<t>
5325  Closed issues:
5326  <list style="symbols">
5327    <t>
5328      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/435"/>:
5329      "Part1 should have a reference to TCP (RFC 793)"
5330    </t>
5331    <t>
5332      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/438"/>:
5333      "media type registration template issues"
5334    </t>
5335    <t>
5336      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/442"/>:
5337      "BWS" (vs conformance)
5338    </t>
5339    <t>
5340      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/444"/>:
5341      "obs-fold language"
5342    </t>
5343    <t>
5344      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/445"/>:
5345      "Ordering in Upgrade"
5346    </t>
5347    <t>
5348      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/446"/>:
5349      "p1 editorial feedback"
5350    </t>
5351    <t>
5352      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/476"/>:
5353      "SHOULD and conformance"
5354    </t>
5355    <t>
5356      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/477"/>:
5357      "Pipelining language"
5358    </t>
5359  </list>
5360</t>
5361</section>
5362</section>
5363
5364</back>
5365</rfc>
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