source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2277

Last change on this file since 2277 was 2277, checked in by fielding@…, 7 years ago

Clarify that a recipient SHOULD process received messages in higher minor versions as if they were the highest minor version to wwhich the recipient is conformant; Clarify that 505 is for major version rejection; addresses #449

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