source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2281

Last change on this file since 2281 was 2280, checked in by julian.reschke@…, 7 years ago

update to latest version of rfc2629.xslt, bump up document dates

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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 "June">
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="uri-host"/>
792  <x:anchor-alias value="port"/>
793  <x:anchor-alias value="path-abempty"/>
794  <x:anchor-alias value="segment"/>
795  <x:anchor-alias value="query"/>
796  <x:anchor-alias value="fragment"/>
797  <x:anchor-alias value="absolute-path"/>
798  <x:anchor-alias value="partial-URI"/>
799<t>
800   This specification adopts the definitions of "URI-reference",
801   "absolute-URI", "relative-part", "authority", "port", "host",
802   "path-abempty", "segment", "query", and "fragment" from the
803   URI generic syntax.
804   In addition, we define an "absolute-path" rule (that differs from
805   RFC 3986's "path-absolute" in that it allows a leading "//")
806   and a "partial-URI" rule for protocol elements
807   that allow a relative URI but not a fragment.
808</t>
809<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="URI-reference"><!--exported production--></iref><iref primary="true" item="Grammar" subitem="absolute-URI"/><iref primary="true" item="Grammar" subitem="authority"/><iref primary="true" item="Grammar" subitem="absolute-path"/><iref primary="true" item="Grammar" subitem="port"/><iref primary="true" item="Grammar" subitem="query"/><iref primary="true" item="Grammar" subitem="fragment"/><iref primary="true" item="Grammar" subitem="segment"/><iref primary="true" item="Grammar" subitem="uri-host"/><iref primary="true" item="Grammar" subitem="partial-URI"><!--exported production--></iref>
810  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
811  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
812  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
813  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
814  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
815  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
816  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
817  <x:ref>segment</x:ref>       = &lt;segment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
818  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
819  <x:ref>fragment</x:ref>      = &lt;fragment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.5"/>&gt;
820 
821  <x:ref>absolute-path</x:ref> = 1*( "/" segment )
822  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
823</artwork></figure>
824<t>
825   Each protocol element in HTTP that allows a URI reference will indicate
826   in its ABNF production whether the element allows any form of reference
827   (URI-reference), only a URI in absolute form (absolute-URI), only the
828   path and optional query components, or some combination of the above.
829   Unless otherwise indicated, URI references are parsed
830   relative to the effective request URI
831   (<xref target="effective.request.uri"/>).
832</t>
833
834<section title="http URI scheme" anchor="http.uri">
835  <x:anchor-alias value="http-URI"/>
836  <iref item="http URI scheme" primary="true"/>
837  <iref item="URI scheme" subitem="http" primary="true"/>
838<t>
839   The "http" URI scheme is hereby defined for the purpose of minting
840   identifiers according to their association with the hierarchical
841   namespace governed by a potential HTTP origin server listening for
842   TCP (<xref target="RFC0793"/>) connections on a given port.
843</t>
844<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"><!--terminal production--></iref>
845  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
846             [ "#" <x:ref>fragment</x:ref> ]
847</artwork></figure>
848<t>
849   The HTTP origin server is identified by the generic syntax's
850   <x:ref>authority</x:ref> component, which includes a host identifier
851   and optional TCP port (<xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>).
852   The remainder of the URI, consisting of both the hierarchical path
853   component and optional query component, serves as an identifier for
854   a potential resource within that origin server's name space.
855</t>
856<t>
857   If the host identifier is provided as an IP address,
858   then the origin server is any listener on the indicated TCP port at
859   that IP address. If host is a registered name, then that name is
860   considered an indirect identifier and the recipient might use a name
861   resolution service, such as DNS, to find the address of a listener
862   for that host.
863   The host &MUST-NOT; be empty; if an "http" URI is received with an
864   empty host, then it &MUST; be rejected as invalid.
865   If the port subcomponent is empty or not given, then TCP port 80 is
866   assumed (the default reserved port for WWW services).
867</t>
868<t>
869   Regardless of the form of host identifier, access to that host is not
870   implied by the mere presence of its name or address. The host might or might
871   not exist and, even when it does exist, might or might not be running an
872   HTTP server or listening to the indicated port. The "http" URI scheme
873   makes use of the delegated nature of Internet names and addresses to
874   establish a naming authority (whatever entity has the ability to place
875   an HTTP server at that Internet name or address) and allows that
876   authority to determine which names are valid and how they might be used.
877</t>
878<t>
879   When an "http" URI is used within a context that calls for access to the
880   indicated resource, a client &MAY; attempt access by resolving
881   the host to an IP address, establishing a TCP connection to that address
882   on the indicated port, and sending an HTTP request message
883   (<xref target="http.message"/>) containing the URI's identifying data
884   (<xref target="message.routing"/>) to the server.
885   If the server responds to that request with a non-interim HTTP response
886   message, as described in &status-codes;, then that response
887   is considered an authoritative answer to the client's request.
888</t>
889<t>
890   Although HTTP is independent of the transport protocol, the "http"
891   scheme is specific to TCP-based services because the name delegation
892   process depends on TCP for establishing authority.
893   An HTTP service based on some other underlying connection protocol
894   would presumably be identified using a different URI scheme, just as
895   the "https" scheme (below) is used for resources that require an
896   end-to-end secured connection. Other protocols might also be used to
897   provide access to "http" identified resources &mdash; it is only the
898   authoritative interface that is specific to TCP.
899</t>
900<t>
901   The URI generic syntax for authority also includes a deprecated
902   userinfo subcomponent (<xref target="RFC3986" x:fmt="," x:sec="3.2.1"/>)
903   for including user authentication information in the URI.  Some
904   implementations make use of the userinfo component for internal
905   configuration of authentication information, such as within command
906   invocation options, configuration files, or bookmark lists, even
907   though such usage might expose a user identifier or password.
908   Senders &MUST; exclude the userinfo subcomponent (and its "@"
909   delimiter) when an "http" URI is transmitted within a message as a
910   request target or header field value.
911   Recipients of an "http" URI reference &SHOULD; parse for userinfo and
912   treat its presence as an error, since it is likely being used to obscure
913   the authority for the sake of phishing attacks.
914</t>
915</section>
916
917<section title="https URI scheme" anchor="https.uri">
918   <x:anchor-alias value="https-URI"/>
919   <iref item="https URI scheme"/>
920   <iref item="URI scheme" subitem="https"/>
921<t>
922   The "https" URI scheme is hereby defined for the purpose of minting
923   identifiers according to their association with the hierarchical
924   namespace governed by a potential HTTP origin server listening to a
925   given TCP port for TLS-secured connections
926   (<xref target="RFC0793"/>, <xref target="RFC5246"/>).
927</t>
928<t>
929   All of the requirements listed above for the "http" scheme are also
930   requirements for the "https" scheme, except that a default TCP port
931   of 443 is assumed if the port subcomponent is empty or not given,
932   and the TCP connection &MUST; be secured, end-to-end, through the
933   use of strong encryption prior to sending the first HTTP request.
934</t>
935<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="https-URI"><!--terminal production--></iref>
936  <x:ref>https-URI</x:ref> = "https:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
937              [ "#" <x:ref>fragment</x:ref> ]
938</artwork></figure>
939<t>
940   Note that the "https" URI scheme depends on both TLS and TCP for
941   establishing authority.
942   Resources made available via the "https" scheme have no shared
943   identity with the "http" scheme even if their resource identifiers
944   indicate the same authority (the same host listening to the same
945   TCP port).  They are distinct name spaces and are considered to be
946   distinct origin servers.  However, an extension to HTTP that is
947   defined to apply to entire host domains, such as the Cookie protocol
948   <xref target="RFC6265"/>, can allow information
949   set by one service to impact communication with other services
950   within a matching group of host domains.
951</t>
952<t>
953   The process for authoritative access to an "https" identified
954   resource is defined in <xref target="RFC2818"/>.
955</t>
956</section>
957
958<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
959<t>
960   Since the "http" and "https" schemes conform to the URI generic syntax,
961   such URIs are normalized and compared according to the algorithm defined
962   in <xref target="RFC3986" x:fmt="," x:sec="6"/>, using the defaults
963   described above for each scheme.
964</t>
965<t>
966   If the port is equal to the default port for a scheme, the normal form is
967   to elide the port subcomponent. When not being used in absolute form as the
968   request target of an OPTIONS request, an empty path component is equivalent
969   to an absolute path of "/", so the normal form is to provide a path of "/"
970   instead. The scheme and host are case-insensitive and normally provided in
971   lowercase; all other components are compared in a case-sensitive manner.
972   Characters other than those in the "reserved" set are equivalent to their
973   percent-encoded octets (see <xref target="RFC3986" x:fmt=","
974   x:sec="2.1"/>): the normal form is to not encode them.
975</t>
976<t>
977   For example, the following three URIs are equivalent:
978</t>
979<figure><artwork type="example">
980   http://example.com:80/~smith/home.html
981   http://EXAMPLE.com/%7Esmith/home.html
982   http://EXAMPLE.com:/%7esmith/home.html
983</artwork></figure>
984</section>
985</section>
986</section>
987
988<section title="Message Format" anchor="http.message">
989<x:anchor-alias value="generic-message"/>
990<x:anchor-alias value="message.types"/>
991<x:anchor-alias value="HTTP-message"/>
992<x:anchor-alias value="start-line"/>
993<iref item="header section"/>
994<iref item="headers"/>
995<iref item="header field"/>
996<t>
997   All HTTP/1.1 messages consist of a start-line followed by a sequence of
998   octets in a format similar to the Internet Message Format
999   <xref target="RFC5322"/>: zero or more header fields (collectively
1000   referred to as the "headers" or the "header section"), an empty line
1001   indicating the end of the header section, and an optional message body.
1002</t>
1003<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"><!--terminal production--></iref>
1004  <x:ref>HTTP-message</x:ref>   = <x:ref>start-line</x:ref>
1005                   *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1006                   <x:ref>CRLF</x:ref>
1007                   [ <x:ref>message-body</x:ref> ]
1008</artwork></figure>
1009<t>
1010   The normal procedure for parsing an HTTP message is to read the
1011   start-line into a structure, read each header field into a hash
1012   table by field name until the empty line, and then use the parsed
1013   data to determine if a message body is expected.  If a message body
1014   has been indicated, then it is read as a stream until an amount
1015   of octets equal to the message body length is read or the connection
1016   is closed.
1017</t>
1018<t>
1019   Recipients &MUST; parse an HTTP message as a sequence of octets in an
1020   encoding that is a superset of US-ASCII <xref target="USASCII"/>.
1021   Parsing an HTTP message as a stream of Unicode characters, without regard
1022   for the specific encoding, creates security vulnerabilities due to the
1023   varying ways that string processing libraries handle invalid multibyte
1024   character sequences that contain the octet LF (%x0A).  String-based
1025   parsers can only be safely used within protocol elements after the element
1026   has been extracted from the message, such as within a header field-value
1027   after message parsing has delineated the individual fields.
1028</t>
1029<t>
1030   An HTTP message can be parsed as a stream for incremental processing or
1031   forwarding downstream.  However, recipients cannot rely on incremental
1032   delivery of partial messages, since some implementations will buffer or
1033   delay message forwarding for the sake of network efficiency, security
1034   checks, or payload transformations.
1035</t>
1036<t>
1037   A sender &MUST-NOT; send whitespace between the start-line and
1038   the first header field.
1039   A recipient that receives whitespace between the start-line and
1040   the first header field &MUST; either reject the message as invalid or
1041   consume each whitespace-preceded line without further processing of it
1042   (i.e., ignore the entire line, along with any subsequent lines preceded
1043   by whitespace, until a properly formed header field is received or the
1044   header block is terminated).
1045</t>
1046<t>
1047   The presence of such whitespace in a request
1048   might be an attempt to trick a server into ignoring that field or
1049   processing the line after it as a new request, either of which might
1050   result in a security vulnerability if other implementations within
1051   the request chain interpret the same message differently.
1052   Likewise, the presence of such whitespace in a response might be
1053   ignored by some clients or cause others to cease parsing.
1054</t>
1055
1056<section title="Start Line" anchor="start.line">
1057  <x:anchor-alias value="Start-Line"/>
1058<t>
1059   An HTTP message can either be a request from client to server or a
1060   response from server to client.  Syntactically, the two types of message
1061   differ only in the start-line, which is either a request-line (for requests)
1062   or a status-line (for responses), and in the algorithm for determining
1063   the length of the message body (<xref target="message.body"/>).
1064</t>
1065<t>
1066   In theory, a client could receive requests and a server could receive
1067   responses, distinguishing them by their different start-line formats,
1068   but in practice servers are implemented to only expect a request
1069   (a response is interpreted as an unknown or invalid request method)
1070   and clients are implemented to only expect a response.
1071</t>
1072<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="start-line"/>
1073  <x:ref>start-line</x:ref>     = <x:ref>request-line</x:ref> / <x:ref>status-line</x:ref>
1074</artwork></figure>
1075
1076<section title="Request Line" anchor="request.line">
1077  <x:anchor-alias value="Request"/>
1078  <x:anchor-alias value="request-line"/>
1079<t>
1080   A request-line begins with a method token, followed by a single
1081   space (SP), the request-target, another single space (SP), the
1082   protocol version, and ending with CRLF.
1083</t>
1084<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1085  <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>
1086</artwork></figure>
1087<iref primary="true" item="method"/>
1088<t anchor="method">
1089   The method token indicates the request method to be performed on the
1090   target resource. The request method is case-sensitive.
1091</t>
1092<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1093  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1094</artwork></figure>
1095<t>
1096   The methods defined by this specification can be found in
1097   &methods;, along with information regarding the HTTP method registry
1098   and considerations for defining new methods.
1099</t>
1100<iref item="request-target"/>
1101<t>
1102   The request-target identifies the target resource upon which to apply
1103   the request, as defined in <xref target="request-target"/>.
1104</t>
1105<t>
1106   Recipients typically parse the request-line into its component parts by
1107   splitting on whitespace (see <xref target="message.robustness"/>), since
1108   no whitespace is allowed in the three components.
1109   Unfortunately, some user agents fail to properly encode or exclude
1110   whitespace found in hypertext references, resulting in those disallowed
1111   characters being sent in a request-target.
1112</t>
1113<t>
1114   Recipients of an invalid request-line &SHOULD; respond with either a
1115   <x:ref>400 (Bad Request)</x:ref> error or a <x:ref>301 (Moved Permanently)</x:ref>
1116   redirect with the request-target properly encoded.  Recipients &SHOULD-NOT;
1117   attempt to autocorrect and then process the request without a redirect,
1118   since the invalid request-line might be deliberately crafted to bypass
1119   security filters along the request chain.
1120</t>
1121<t>
1122   HTTP does not place a pre-defined limit on the length of a request-line.
1123   A server that receives a method longer than any that it implements
1124   &SHOULD; respond with a <x:ref>501 (Not Implemented)</x:ref> status code.
1125   A server &MUST; be prepared to receive URIs of unbounded length and
1126   respond with the <x:ref>414 (URI Too Long)</x:ref> status code if the received
1127   request-target would be longer than the server wishes to handle
1128   (see &status-414;).
1129</t>
1130<t>
1131   Various ad-hoc limitations on request-line length are found in practice.
1132   It is &RECOMMENDED; that all HTTP senders and recipients support, at a
1133   minimum, request-line lengths of 8000 octets.
1134</t>
1135</section>
1136
1137<section title="Status Line" anchor="status.line">
1138  <x:anchor-alias value="response"/>
1139  <x:anchor-alias value="status-line"/>
1140  <x:anchor-alias value="status-code"/>
1141  <x:anchor-alias value="reason-phrase"/>
1142<t>
1143   The first line of a response message is the status-line, consisting
1144   of the protocol version, a space (SP), the status code, another space,
1145   a possibly-empty textual phrase describing the status code, and
1146   ending with CRLF.
1147</t>
1148<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-line"/>
1149  <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>
1150</artwork></figure>
1151<t>
1152   The status-code element is a 3-digit integer code describing the
1153   result of the server's attempt to understand and satisfy the client's
1154   corresponding request. The rest of the response message is to be
1155   interpreted in light of the semantics defined for that status code.
1156   See &status-codes; for information about the semantics of status codes,
1157   including the classes of status code (indicated by the first digit),
1158   the status codes defined by this specification, considerations for the
1159   definition of new status codes, and the IANA registry.
1160</t>
1161<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-code"/>
1162  <x:ref>status-code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1163</artwork></figure>
1164<t>   
1165   The reason-phrase element exists for the sole purpose of providing a
1166   textual description associated with the numeric status code, mostly
1167   out of deference to earlier Internet application protocols that were more
1168   frequently used with interactive text clients. A client &SHOULD; ignore
1169   the reason-phrase content.
1170</t>
1171<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
1172  <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> )
1173</artwork></figure>
1174</section>
1175</section>
1176
1177<section title="Header Fields" anchor="header.fields">
1178  <x:anchor-alias value="header-field"/>
1179  <x:anchor-alias value="field-content"/>
1180  <x:anchor-alias value="field-name"/>
1181  <x:anchor-alias value="field-value"/>
1182  <x:anchor-alias value="obs-fold"/>
1183<t>
1184   Each HTTP header field consists of a case-insensitive field name
1185   followed by a colon (":"), optional leading whitespace, the field value,
1186   and optional trailing whitespace.
1187</t>
1188<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"/>
1189  <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>
1190  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1191  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>obs-fold</x:ref> )
1192  <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> )
1193  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref> ( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1194                 ; obsolete line folding
1195                 ; see <xref target="field.parsing"/>
1196</artwork></figure>
1197<t>
1198   The field-name token labels the corresponding field-value as having the
1199   semantics defined by that header field.  For example, the <x:ref>Date</x:ref>
1200   header field is defined in &header-date; as containing the origination
1201   timestamp for the message in which it appears.
1202</t>
1203
1204<section title="Field Extensibility" anchor="field.extensibility">
1205<t>
1206   HTTP header fields are fully extensible: there is no limit on the
1207   introduction of new field names, each presumably defining new semantics,
1208   nor on the number of header fields used in a given message.  Existing
1209   fields are defined in each part of this specification and in many other
1210   specifications outside the core standard.
1211   New header fields can be introduced without changing the protocol version
1212   if their defined semantics allow them to be safely ignored by recipients
1213   that do not recognize them.
1214</t>
1215<t>
1216   New HTTP header fields ought to be registered with IANA in the
1217   Message Header Field Registry, as described in &iana-header-registry;.
1218   A proxy &MUST; forward unrecognized header fields unless the
1219   field-name is listed in the <x:ref>Connection</x:ref> header field
1220   (<xref target="header.connection"/>) or the proxy is specifically
1221   configured to block, or otherwise transform, such fields.
1222   Other recipients &SHOULD; ignore unrecognized header fields.
1223</t>
1224</section>
1225
1226<section title="Field Order" anchor="field.order">
1227<t>
1228   The order in which header fields with differing field names are
1229   received is not significant. However, it is "good practice" to send
1230   header fields that contain control data first, such as <x:ref>Host</x:ref>
1231   on requests and <x:ref>Date</x:ref> on responses, so that implementations
1232   can decide when not to handle a message as early as possible.  A server
1233   &MUST; wait until the entire header section is received before interpreting
1234   a request message, since later header fields might include conditionals,
1235   authentication credentials, or deliberately misleading duplicate
1236   header fields that would impact request processing.
1237</t>
1238<t>
1239   A sender &MUST-NOT; generate multiple header fields with the same field
1240   name in a message unless either the entire field value for that
1241   header field is defined as a comma-separated list [i.e., #(values)]
1242   or the header field is a well-known exception (as noted below).
1243</t>
1244<t>
1245   Multiple header fields with the same field name can be combined into
1246   one "field-name: field-value" pair, without changing the semantics of the
1247   message, by appending each subsequent field value to the combined
1248   field value in order, separated by a comma. The order in which
1249   header fields with the same field name are received is therefore
1250   significant to the interpretation of the combined field value;
1251   a proxy &MUST-NOT; change the order of these field values when
1252   forwarding a message.
1253</t>
1254<x:note>
1255  <t>
1256   &Note; In practice, the "Set-Cookie" header field (<xref target="RFC6265"/>)
1257   often appears multiple times in a response message and does not use the
1258   list syntax, violating the above requirements on multiple header fields
1259   with the same name. Since it cannot be combined into a single field-value,
1260   recipients ought to handle "Set-Cookie" as a special case while processing
1261   header fields. (See Appendix A.2.3 of <xref target="Kri2001"/> for details.)
1262  </t>
1263</x:note>
1264</section>
1265
1266<section title="Whitespace" anchor="whitespace">
1267<t anchor="rule.LWS">
1268   This specification uses three rules to denote the use of linear
1269   whitespace: OWS (optional whitespace), RWS (required whitespace), and
1270   BWS ("bad" whitespace).
1271</t>
1272<t anchor="rule.OWS">
1273   The OWS rule is used where zero or more linear whitespace octets might
1274   appear. For protocol elements where optional whitespace is preferred to
1275   improve readability, a sender &SHOULD; generate the optional whitespace
1276   as a single SP; otherwise, a sender &SHOULD-NOT; generate optional
1277   whitespace except as needed to white-out invalid or unwanted protocol
1278   elements during in-place message filtering.
1279</t>
1280<t anchor="rule.RWS">
1281   The RWS rule is used when at least one linear whitespace octet is required
1282   to separate field tokens. A sender &SHOULD; generate RWS as a single SP.
1283</t>
1284<t anchor="rule.BWS">
1285   The BWS rule is used where the grammar allows optional whitespace only for
1286   historical reasons. A sender &MUST-NOT; generate BWS in messages.
1287   A recipient &MUST; parse for such bad whitespace and remove it before
1288   interpreting the protocol element.
1289</t>
1290<t anchor="rule.whitespace">
1291  <x:anchor-alias value="BWS"/>
1292  <x:anchor-alias value="OWS"/>
1293  <x:anchor-alias value="RWS"/>
1294</t>
1295<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"/>
1296  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1297                 ; optional whitespace
1298  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1299                 ; required whitespace
1300  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1301                 ; "bad" whitespace
1302</artwork></figure>
1303</section>
1304
1305<section title="Field Parsing" anchor="field.parsing">
1306<t>
1307   No whitespace is allowed between the header field-name and colon.
1308   In the past, differences in the handling of such whitespace have led to
1309   security vulnerabilities in request routing and response handling.
1310   A server &MUST; reject any received request message that contains
1311   whitespace between a header field-name and colon with a response code of
1312   <x:ref>400 (Bad Request)</x:ref>. A proxy &MUST; remove any such whitespace
1313   from a response message before forwarding the message downstream.
1314</t>
1315<t>
1316   A field value is preceded by optional whitespace (OWS); a single SP is
1317   preferred. The field value does not include any leading or trailing white
1318   space: OWS occurring before the first non-whitespace octet of the field
1319   value or after the last non-whitespace octet of the field value ought to be
1320   excluded by parsers when extracting the field value from a header field.
1321</t>
1322<t>
1323   A recipient of field-content containing multiple sequential octets of
1324   optional (OWS) or required (RWS) whitespace &SHOULD; either replace the
1325   sequence with a single SP or transform any non-SP octets in the sequence to
1326   SP octets before interpreting the field value or forwarding the message
1327   downstream.
1328</t>
1329<t>
1330   Historically, HTTP header field values could be extended over multiple
1331   lines by preceding each extra line with at least one space or horizontal
1332   tab (obs-fold). This specification deprecates such line folding except
1333   within the message/http media type
1334   (<xref target="internet.media.type.message.http"/>).
1335   Senders &MUST-NOT; generate messages that include line folding
1336   (i.e., that contain any field-value that contains a match to the
1337   <x:ref>obs-fold</x:ref> rule) unless the message is intended for packaging
1338   within the message/http media type.
1339</t>
1340<t>
1341   A server that receives an <x:ref>obs-fold</x:ref> in a request message that
1342   is not within a message/http container &MUST; either reject the message by
1343   sending a <x:ref>400 (Bad Request)</x:ref>, preferably with a
1344   representation explaining that obsolete line folding is unacceptable, or
1345   replace each received <x:ref>obs-fold</x:ref> with one or more
1346   <x:ref>SP</x:ref> octets prior to interpreting the field value or
1347   forwarding the message downstream.
1348</t>
1349<t>
1350   A proxy or gateway that receives an <x:ref>obs-fold</x:ref> in a response
1351   message that is not within a message/http container &MUST; either discard
1352   the message and replace it with a <x:ref>502 (Bad Gateway)</x:ref>
1353   response, preferably with a representation explaining that unacceptable
1354   line folding was received, or replace each received <x:ref>obs-fold</x:ref>
1355   with one or more <x:ref>SP</x:ref> octets prior to interpreting the field
1356   value or forwarding the message downstream.
1357</t>
1358<t>
1359   A user agent that receives an <x:ref>obs-fold</x:ref> in a response message
1360   that is not within a message/http container &MUST; replace each received
1361   <x:ref>obs-fold</x:ref> with one or more <x:ref>SP</x:ref> octets prior to
1362   interpreting the field value.
1363</t>
1364<t>
1365   Historically, HTTP has allowed field content with text in the ISO-8859-1
1366   <xref target="ISO-8859-1"/> charset, supporting other charsets only
1367   through use of <xref target="RFC2047"/> encoding.
1368   In practice, most HTTP header field values use only a subset of the
1369   US-ASCII charset <xref target="USASCII"/>. Newly defined
1370   header fields &SHOULD; limit their field values to US-ASCII octets.
1371   Recipients &SHOULD; treat other octets in field content (obs-text) as
1372   opaque data.
1373</t>
1374</section>
1375
1376<section title="Field Limits" anchor="field.limits">
1377<t>
1378   HTTP does not place a pre-defined limit on the length of each header field
1379   or on the length of the header block as a whole.  Various ad-hoc
1380   limitations on individual header field length are found in practice,
1381   often depending on the specific field semantics.
1382</t>
1383<t>
1384   A server &MUST; be prepared to receive request header fields of unbounded
1385   length and respond with an appropriate <x:ref>4xx (Client Error)</x:ref>
1386   status code if the received header field(s) are larger than the server
1387   wishes to process.
1388</t>
1389<t>
1390   A client &MUST; be prepared to receive response header fields of unbounded
1391   length. A client &MAY; discard or truncate received header fields that are
1392   larger than the client wishes to process if the field semantics are such
1393   that the dropped value(s) can be safely ignored without changing the
1394   response semantics.
1395</t>
1396</section>
1397
1398<section title="Field value components" anchor="field.components">
1399<t anchor="rule.token.separators">
1400  <x:anchor-alias value="tchar"/>
1401  <x:anchor-alias value="token"/>
1402  <x:anchor-alias value="special"/>
1403  <x:anchor-alias value="word"/>
1404   Many HTTP header field values consist of words (token or quoted-string)
1405   separated by whitespace or special characters. These special characters
1406   &MUST; be in a quoted string to be used within a parameter value (as defined
1407   in <xref target="transfer.codings"/>).
1408</t>
1409<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>
1410  <x:ref>word</x:ref>           = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1411
1412  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1413<!--
1414  IMPORTANT: when editing "tchar" make sure that "special" is updated accordingly!!!
1415 -->
1416  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1417                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1418                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1419                 ; any <x:ref>VCHAR</x:ref>, except <x:ref>special</x:ref>
1420
1421  <x:ref>special</x:ref>        = "(" / ")" / "&lt;" / ">" / "@" / ","
1422                 / ";" / ":" / "\" / DQUOTE / "/" / "["
1423                 / "]" / "?" / "=" / "{" / "}"
1424</artwork></figure>
1425<t anchor="rule.quoted-string">
1426  <x:anchor-alias value="quoted-string"/>
1427  <x:anchor-alias value="qdtext"/>
1428  <x:anchor-alias value="obs-text"/>
1429   A string of text is parsed as a single word if it is quoted using
1430   double-quote marks.
1431</t>
1432<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"/>
1433  <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>
1434  <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>
1435  <x:ref>obs-text</x:ref>       = %x80-FF
1436</artwork></figure>
1437<t anchor="rule.quoted-pair">
1438  <x:anchor-alias value="quoted-pair"/>
1439   The backslash octet ("\") can be used as a single-octet
1440   quoting mechanism within quoted-string constructs:
1441</t>
1442<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1443  <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> )
1444</artwork></figure>
1445<t>
1446   Recipients that process the value of a quoted-string &MUST; handle a
1447   quoted-pair as if it were replaced by the octet following the backslash.
1448</t>
1449<t>
1450   Senders &SHOULD-NOT; generate a quoted-pair in a quoted-string except where
1451   necessary to quote DQUOTE and backslash octets occurring within that string.
1452</t>
1453<t anchor="rule.comment">
1454  <x:anchor-alias value="comment"/>
1455  <x:anchor-alias value="ctext"/>
1456   Comments can be included in some HTTP header fields by surrounding
1457   the comment text with parentheses. Comments are only allowed in
1458   fields containing "comment" as part of their field value definition.
1459</t>
1460<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1461  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-cpair</x:ref> / <x:ref>comment</x:ref> ) ")"
1462  <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>
1463</artwork></figure>
1464<t anchor="rule.quoted-cpair">
1465  <x:anchor-alias value="quoted-cpair"/>
1466   The backslash octet ("\") can be used as a single-octet
1467   quoting mechanism within comment constructs:
1468</t>
1469<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-cpair"/>
1470  <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> )
1471</artwork></figure>
1472<t>
1473   Senders &SHOULD-NOT; escape octets in comments that do not require escaping
1474   (i.e., other than the backslash octet "\" and the parentheses "(" and ")").
1475</t>
1476</section>
1477
1478</section>
1479
1480<section title="Message Body" anchor="message.body">
1481  <x:anchor-alias value="message-body"/>
1482<t>
1483   The message body (if any) of an HTTP message is used to carry the
1484   payload body of that request or response.  The message body is
1485   identical to the payload body unless a transfer coding has been
1486   applied, as described in <xref target="header.transfer-encoding"/>.
1487</t>
1488<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1489  <x:ref>message-body</x:ref> = *OCTET
1490</artwork></figure>
1491<t>
1492   The rules for when a message body is allowed in a message differ for
1493   requests and responses.
1494</t>
1495<t>
1496   The presence of a message body in a request is signaled by a
1497   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1498   field. Request message framing is independent of method semantics,
1499   even if the method does not define any use for a message body.
1500</t>
1501<t>
1502   The presence of a message body in a response depends on both
1503   the request method to which it is responding and the response
1504   status code (<xref target="status.line"/>).
1505   Responses to the HEAD request method never include a message body
1506   because the associated response header fields (e.g.,
1507   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1508   if present, indicate only what their values would have been if the request
1509   method had been GET (&HEAD;).
1510   <x:ref>2xx (Successful)</x:ref> responses to CONNECT switch to tunnel
1511   mode instead of having a message body (&CONNECT;).
1512   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1513   <x:ref>304 (Not Modified)</x:ref> responses do not include a message body.
1514   All other responses do include a message body, although the body
1515   might be of zero length.
1516</t>
1517
1518<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1519  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1520  <iref item="chunked (Coding Format)"/>
1521  <x:anchor-alias value="Transfer-Encoding"/>
1522<t>
1523   The Transfer-Encoding header field lists the transfer coding names
1524   corresponding to the sequence of transfer codings that have been
1525   (or will be) applied to the payload body in order to form the message body.
1526   Transfer codings are defined in <xref target="transfer.codings"/>.
1527</t>
1528<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1529  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1530</artwork></figure>
1531<t>
1532   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1533   MIME, which was designed to enable safe transport of binary data over a
1534   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1535   However, safe transport has a different focus for an 8bit-clean transfer
1536   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1537   accurately delimit a dynamically generated payload and to distinguish
1538   payload encodings that are only applied for transport efficiency or
1539   security from those that are characteristics of the selected resource.
1540</t>
1541<t>
1542   All HTTP/1.1 recipients &MUST; implement the chunked transfer coding
1543   (<xref target="chunked.encoding"/>) because it plays a crucial role in
1544   framing messages when the payload body size is not known in advance.
1545   If chunked is applied to a payload body, the sender &MUST-NOT; apply
1546   chunked more than once (i.e., chunking an already chunked message is not
1547   allowed).
1548   If any transfer coding is applied to a request payload body, the
1549   sender &MUST; apply chunked as the final transfer coding to ensure that
1550   the message is properly framed.
1551   If any transfer coding is applied to a response payload body, the
1552   sender &MUST; either apply chunked as the final transfer coding or
1553   terminate the message by closing the connection.
1554</t>
1555<figure><preamble>
1556   For example,
1557</preamble><artwork type="example">
1558  Transfer-Encoding: gzip, chunked
1559</artwork><postamble>
1560   indicates that the payload body has been compressed using the gzip
1561   coding and then chunked using the chunked coding while forming the
1562   message body.
1563</postamble></figure>
1564<t>
1565   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1566   Transfer-Encoding is a property of the message, not of the representation, and
1567   any recipient along the request/response chain &MAY; decode the received
1568   transfer coding(s) or apply additional transfer coding(s) to the message
1569   body, assuming that corresponding changes are made to the Transfer-Encoding
1570   field-value. Additional information about the encoding parameters &MAY; be
1571   provided by other header fields not defined by this specification.
1572</t>
1573<t>
1574   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1575   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1576   neither of which includes a message body,
1577   to indicate that the origin server would have applied a transfer coding
1578   to the message body if the request had been an unconditional GET.
1579   This indication is not required, however, because any recipient on
1580   the response chain (including the origin server) can remove transfer
1581   codings when they are not needed.
1582</t>
1583<t>
1584   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1585   implementations advertising only HTTP/1.0 support will not understand
1586   how to process a transfer-encoded payload.
1587   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1588   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1589   might be in the form of specific user configuration or by remembering the
1590   version of a prior received response.
1591   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1592   the corresponding request indicates HTTP/1.1 (or later).
1593</t>
1594<t>
1595   A server that receives a request message with a transfer coding it does
1596   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref>.
1597</t>
1598</section>
1599
1600<section title="Content-Length" anchor="header.content-length">
1601  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1602  <x:anchor-alias value="Content-Length"/>
1603<t>
1604   When a message does not have a <x:ref>Transfer-Encoding</x:ref> header
1605   field, a Content-Length header field can provide the anticipated size,
1606   as a decimal number of octets, for a potential payload body.
1607   For messages that do include a payload body, the Content-Length field-value
1608   provides the framing information necessary for determining where the body
1609   (and message) ends.  For messages that do not include a payload body, the
1610   Content-Length indicates the size of the selected representation
1611   (&representation;).
1612</t>
1613<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1614  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1615</artwork></figure>
1616<t>
1617   An example is
1618</t>
1619<figure><artwork type="example">
1620  Content-Length: 3495
1621</artwork></figure>
1622<t>
1623   A sender &MUST-NOT; send a Content-Length header field in any message that
1624   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1625</t>
1626<t>
1627   A user agent &SHOULD; send a Content-Length in a request message when no
1628   <x:ref>Transfer-Encoding</x:ref> is sent and the request method defines
1629   a meaning for an enclosed payload body. For example, a Content-Length
1630   header field is normally sent in a POST request even when the value is
1631   0 (indicating an empty payload body).  A user agent &SHOULD-NOT; send a
1632   Content-Length header field when the request message does not contain a
1633   payload body and the method semantics do not anticipate such a body.
1634</t>
1635<t>
1636   A server &MAY; send a Content-Length header field in a response to a HEAD
1637   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1638   response unless its field-value equals the decimal number of octets that
1639   would have been sent in the payload body of a response if the same
1640   request had used the GET method.
1641</t>
1642<t>
1643   A server &MAY; send a Content-Length header field in a
1644   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1645   (&status-304;); a server &MUST-NOT; send Content-Length in such a
1646   response unless its field-value equals the decimal number of octets that
1647   would have been sent in the payload body of a <x:ref>200 (OK)</x:ref>
1648   response to the same request.
1649</t>
1650<t>
1651   A server &MUST-NOT; send a Content-Length header field in any response
1652   with a status code of
1653   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1654   A server &SHOULD-NOT; send a Content-Length header field in any
1655   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1656</t>
1657<t>
1658   Aside from the cases defined above, in the absence of Transfer-Encoding,
1659   an origin server &SHOULD; send a Content-Length header field when the
1660   payload body size is known prior to sending the complete header block.
1661   This will allow downstream recipients to measure transfer progress,
1662   know when a received message is complete, and potentially reuse the
1663   connection for additional requests.
1664</t>
1665<t>
1666   Any Content-Length field value greater than or equal to zero is valid.
1667   Since there is no predefined limit to the length of a payload,
1668   recipients &SHOULD; anticipate potentially large decimal numerals and
1669   prevent parsing errors due to integer conversion overflows
1670   (<xref target="attack.protocol.element.size.overflows"/>).
1671</t>
1672<t>
1673   If a message is received that has multiple Content-Length header fields
1674   with field-values consisting of the same decimal value, or a single
1675   Content-Length header field with a field value containing a list of
1676   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1677   duplicate Content-Length header fields have been generated or combined by an
1678   upstream message processor, then the recipient &MUST; either reject the
1679   message as invalid or replace the duplicated field-values with a single
1680   valid Content-Length field containing that decimal value prior to
1681   determining the message body length.
1682</t>
1683<x:note>
1684  <t>
1685   &Note; HTTP's use of Content-Length for message framing differs
1686   significantly from the same field's use in MIME, where it is an optional
1687   field used only within the "message/external-body" media-type.
1688  </t>
1689</x:note>
1690</section>
1691
1692<section title="Message Body Length" anchor="message.body.length">
1693  <iref item="chunked (Coding Format)"/>
1694<t>
1695   The length of a message body is determined by one of the following
1696   (in order of precedence):
1697</t>
1698<t>
1699  <list style="numbers">
1700    <x:lt><t>
1701     Any response to a HEAD request and any response with a
1702     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1703     <x:ref>304 (Not Modified)</x:ref> status code is always
1704     terminated by the first empty line after the header fields, regardless of
1705     the header fields present in the message, and thus cannot contain a
1706     message body.
1707    </t></x:lt>
1708    <x:lt><t>
1709     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1710     connection will become a tunnel immediately after the empty line that
1711     concludes the header fields.  A client &MUST; ignore any
1712     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1713     fields received in such a message.
1714    </t></x:lt>
1715    <x:lt><t>
1716     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1717     and the chunked transfer coding (<xref target="chunked.encoding"/>)
1718     is the final encoding, the message body length is determined by reading
1719     and decoding the chunked data until the transfer coding indicates the
1720     data is complete.
1721    </t>
1722    <t>
1723     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1724     response and the chunked transfer coding is not the final encoding, the
1725     message body length is determined by reading the connection until it is
1726     closed by the server.
1727     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a request and the
1728     chunked transfer coding is not the final encoding, the message body
1729     length cannot be determined reliably; the server &MUST; respond with
1730     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1731    </t>
1732    <t>
1733     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1734     and a <x:ref>Content-Length</x:ref> header field, the Transfer-Encoding
1735     overrides the Content-Length. Such a message might indicate an attempt
1736     to perform request or response smuggling (bypass of security-related
1737     checks on message routing or content) and thus ought to be handled as
1738     an error.  A sender &MUST; remove the received Content-Length field
1739     prior to forwarding such a message downstream.
1740    </t></x:lt>
1741    <x:lt><t>
1742     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1743     either multiple <x:ref>Content-Length</x:ref> header fields having
1744     differing field-values or a single Content-Length header field having an
1745     invalid value, then the message framing is invalid and &MUST; be treated
1746     as an error to prevent request or response smuggling.
1747     If this is a request message, the server &MUST; respond with
1748     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1749     If this is a response message received by a proxy, the proxy
1750     &MUST; discard the received response, send a <x:ref>502 (Bad Gateway)</x:ref>
1751     status code as its downstream response, and then close the connection.
1752     If this is a response message received by a user agent, it &MUST; be
1753     treated as an error by discarding the message and closing the connection.
1754    </t></x:lt>
1755    <x:lt><t>
1756     If a valid <x:ref>Content-Length</x:ref> header field is present without
1757     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1758     expected message body length in octets.
1759     If the sender closes the connection or the recipient times out before the
1760     indicated number of octets are received, the recipient &MUST; consider
1761     the message to be incomplete and close the connection.
1762    </t></x:lt>
1763    <x:lt><t>
1764     If this is a request message and none of the above are true, then the
1765     message body length is zero (no message body is present).
1766    </t></x:lt>
1767    <x:lt><t>
1768     Otherwise, this is a response message without a declared message body
1769     length, so the message body length is determined by the number of octets
1770     received prior to the server closing the connection.
1771    </t></x:lt>
1772  </list>
1773</t>
1774<t>
1775   Since there is no way to distinguish a successfully completed,
1776   close-delimited message from a partially-received message interrupted
1777   by network failure, a server &SHOULD; use encoding or
1778   length-delimited messages whenever possible.  The close-delimiting
1779   feature exists primarily for backwards compatibility with HTTP/1.0.
1780</t>
1781<t>
1782   A server &MAY; reject a request that contains a message body but
1783   not a <x:ref>Content-Length</x:ref> by responding with
1784   <x:ref>411 (Length Required)</x:ref>.
1785</t>
1786<t>
1787   Unless a transfer coding other than chunked has been applied,
1788   a client that sends a request containing a message body &SHOULD;
1789   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1790   length is known in advance, rather than the chunked transfer coding, since some
1791   existing services respond to chunked with a <x:ref>411 (Length Required)</x:ref>
1792   status code even though they understand the chunked transfer coding.  This
1793   is typically because such services are implemented via a gateway that
1794   requires a content-length in advance of being called and the server
1795   is unable or unwilling to buffer the entire request before processing.
1796</t>
1797<t>
1798   A user agent that sends a request containing a message body &MUST; send a
1799   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1800   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1801   the form of specific user configuration or by remembering the version of a
1802   prior received response.
1803</t>
1804<t>
1805   If the final response to the last request on a connection has been
1806   completely received and there remains additional data to read, a user agent
1807   &MAY; discard the remaining data or attempt to determine if that data
1808   belongs as part of the prior response body, which might be the case if the
1809   prior message's Content-Length value is incorrect. A client &MUST-NOT;
1810   process, cache, or forward such extra data as a separate response, since
1811   such behavior would be vulnerable to cache poisoning.
1812</t>
1813</section>
1814</section>
1815
1816<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1817<t>
1818   A server that receives an incomplete request message, usually due to a
1819   canceled request or a triggered time-out exception, &MAY; send an error
1820   response prior to closing the connection.
1821</t>
1822<t>
1823   A client that receives an incomplete response message, which can occur
1824   when a connection is closed prematurely or when decoding a supposedly
1825   chunked transfer coding fails, &MUST; record the message as incomplete.
1826   Cache requirements for incomplete responses are defined in
1827   &cache-incomplete;.
1828</t>
1829<t>
1830   If a response terminates in the middle of the header block (before the
1831   empty line is received) and the status code might rely on header fields to
1832   convey the full meaning of the response, then the client cannot assume
1833   that meaning has been conveyed; the client might need to repeat the
1834   request in order to determine what action to take next.
1835</t>
1836<t>
1837   A message body that uses the chunked transfer coding is
1838   incomplete if the zero-sized chunk that terminates the encoding has not
1839   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1840   incomplete if the size of the message body received (in octets) is less than
1841   the value given by Content-Length.  A response that has neither chunked
1842   transfer coding nor Content-Length is terminated by closure of the
1843   connection, and thus is considered complete regardless of the number of
1844   message body octets received, provided that the header block was received
1845   intact.
1846</t>
1847</section>
1848
1849<section title="Message Parsing Robustness" anchor="message.robustness">
1850<t>
1851   Older HTTP/1.0 user agent implementations might send an extra CRLF
1852   after a POST request as a workaround for some early server
1853   applications that failed to read message body content that was
1854   not terminated by a line-ending. An HTTP/1.1 user agent &MUST-NOT;
1855   preface or follow a request with an extra CRLF.  If terminating
1856   the request message body with a line-ending is desired, then the
1857   user agent &MUST; count the terminating CRLF octets as part of the
1858   message body length.
1859</t>
1860<t>
1861   In the interest of robustness, servers &SHOULD; ignore at least one
1862   empty line received where a request-line is expected. In other words, if
1863   a server is reading the protocol stream at the beginning of a
1864   message and receives a CRLF first, the server &SHOULD; ignore the CRLF.
1865</t>
1866<t>
1867   Although the line terminator for the start-line and header
1868   fields is the sequence CRLF, recipients &MAY; recognize a
1869   single LF as a line terminator and ignore any preceding CR.
1870</t>
1871<t>
1872   Although the request-line and status-line grammar rules require that each
1873   of the component elements be separated by a single SP octet, recipients
1874   &MAY; instead parse on whitespace-delimited word boundaries and, aside
1875   from the CRLF terminator, treat any form of whitespace as the SP separator
1876   while ignoring preceding or trailing whitespace;
1877   such whitespace includes one or more of the following octets:
1878   SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
1879</t>
1880<t>
1881   When a server listening only for HTTP request messages, or processing
1882   what appears from the start-line to be an HTTP request message,
1883   receives a sequence of octets that does not match the HTTP-message
1884   grammar aside from the robustness exceptions listed above, the
1885   server &SHOULD; respond with a <x:ref>400 (Bad Request)</x:ref> response. 
1886</t>
1887</section>
1888</section>
1889
1890<section title="Transfer Codings" anchor="transfer.codings">
1891  <x:anchor-alias value="transfer-coding"/>
1892  <x:anchor-alias value="transfer-extension"/>
1893<t>
1894   Transfer coding names are used to indicate an encoding
1895   transformation that has been, can be, or might need to be applied to a
1896   payload body in order to ensure "safe transport" through the network.
1897   This differs from a content coding in that the transfer coding is a
1898   property of the message rather than a property of the representation
1899   that is being transferred.
1900</t>
1901<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1902  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1903                     / "compress" ; <xref target="compress.coding"/>
1904                     / "deflate" ; <xref target="deflate.coding"/>
1905                     / "gzip" ; <xref target="gzip.coding"/>
1906                     / <x:ref>transfer-extension</x:ref>
1907  <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> )
1908</artwork></figure>
1909<t anchor="rule.parameter">
1910  <x:anchor-alias value="attribute"/>
1911  <x:anchor-alias value="transfer-parameter"/>
1912  <x:anchor-alias value="value"/>
1913   Parameters are in the form of attribute/value pairs.
1914</t>
1915<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"/>
1916  <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>
1917  <x:ref>attribute</x:ref>          = <x:ref>token</x:ref>
1918  <x:ref>value</x:ref>              = <x:ref>word</x:ref>
1919</artwork></figure>
1920<t>
1921   All transfer-coding names are case-insensitive and ought to be registered
1922   within the HTTP Transfer Coding registry, as defined in
1923   <xref target="transfer.coding.registry"/>.
1924   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1925   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1926   header fields.
1927</t>
1928
1929<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1930  <iref primary="true" item="chunked (Coding Format)"/>
1931  <x:anchor-alias value="chunk"/>
1932  <x:anchor-alias value="chunked-body"/>
1933  <x:anchor-alias value="chunk-data"/>
1934  <x:anchor-alias value="chunk-ext"/>
1935  <x:anchor-alias value="chunk-ext-name"/>
1936  <x:anchor-alias value="chunk-ext-val"/>
1937  <x:anchor-alias value="chunk-size"/>
1938  <x:anchor-alias value="last-chunk"/>
1939  <x:anchor-alias value="trailer-part"/>
1940  <x:anchor-alias value="quoted-str-nf"/>
1941  <x:anchor-alias value="qdtext-nf"/>
1942<t>
1943   The chunked transfer coding modifies the body of a message in order to
1944   transfer it as a series of chunks, each with its own size indicator,
1945   followed by an &OPTIONAL; trailer containing header fields. This
1946   allows dynamically generated content to be transferred along with the
1947   information necessary for the recipient to verify that it has
1948   received the full message.
1949</t>
1950<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"/>
1951  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1952                   <x:ref>last-chunk</x:ref>
1953                   <x:ref>trailer-part</x:ref>
1954                   <x:ref>CRLF</x:ref>
1955 
1956  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1957                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1958  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1959  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1960 
1961  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
1962  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
1963  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-str-nf</x:ref>
1964  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1965  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1966 
1967  <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>
1968                 ; like <x:ref>quoted-string</x:ref>, but disallowing line folding
1969  <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>
1970</artwork></figure>
1971<t>
1972   Chunk extensions within the chunked transfer coding are deprecated.
1973   Senders &SHOULD-NOT; send chunk-ext.
1974   Definition of new chunk extensions is discouraged.
1975</t>
1976<t>
1977   The chunk-size field is a string of hex digits indicating the size of
1978   the chunk-data in octets. The chunked transfer coding is complete when a
1979   chunk with a chunk-size of zero is received, possibly followed by a
1980   trailer, and finally terminated by an empty line.
1981</t>
1982
1983<section title="Trailer" anchor="header.trailer">
1984  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
1985  <x:anchor-alias value="Trailer"/>
1986<t>
1987   A trailer allows the sender to include additional fields at the end of a
1988   chunked message in order to supply metadata that might be dynamically
1989   generated while the message body is sent, such as a message integrity
1990   check, digital signature, or post-processing status.
1991   The trailer &MUST-NOT; contain fields that need to be known before a
1992   recipient processes the body, such as <x:ref>Transfer-Encoding</x:ref>,
1993   <x:ref>Content-Length</x:ref>, and <x:ref>Trailer</x:ref>.
1994</t>
1995<t>
1996   When a message includes a message body encoded with the chunked
1997   transfer coding and the sender desires to send metadata in the form of
1998   trailer fields at the end of the message, the sender &SHOULD; send a
1999   <x:ref>Trailer</x:ref> header field before the message body to indicate
2000   which fields will be present in the trailers. This allows the recipient
2001   to prepare for receipt of that metadata before it starts processing the body,
2002   which is useful if the message is being streamed and the recipient wishes
2003   to confirm an integrity check on the fly.
2004</t>
2005<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
2006  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
2007</artwork></figure>
2008<t>
2009   If no <x:ref>Trailer</x:ref> header field is present, the sender of a
2010   chunked message body &SHOULD; send an empty trailer.
2011</t>
2012<t>
2013   A server &MUST; send an empty trailer with the chunked transfer coding
2014   unless at least one of the following is true:
2015  <list style="numbers">
2016    <t>the request included a <x:ref>TE</x:ref> header field that indicates
2017    "trailers" is acceptable in the transfer coding of the response, as
2018    described in <xref target="header.te"/>; or,</t>
2019     
2020    <t>the trailer fields consist entirely of optional metadata and the
2021    recipient could use the message (in a manner acceptable to the server where
2022    the field originated) without receiving that metadata. In other words,
2023    the server that generated the header field is willing to accept the
2024    possibility that the trailer fields might be silently discarded along
2025    the path to the client.</t>
2026  </list>
2027</t>
2028<t>
2029   The above requirement prevents the need for an infinite buffer when a
2030   message is being received by an HTTP/1.1 (or later) proxy and forwarded to
2031   an HTTP/1.0 recipient.
2032</t>
2033</section>
2034
2035<section title="Decoding chunked" anchor="decoding.chunked">
2036<t>
2037   A process for decoding the chunked transfer coding
2038   can be represented in pseudo-code as:
2039</t>
2040<figure><artwork type="code">
2041  length := 0
2042  read chunk-size, chunk-ext (if any), and CRLF
2043  while (chunk-size &gt; 0) {
2044     read chunk-data and CRLF
2045     append chunk-data to decoded-body
2046     length := length + chunk-size
2047     read chunk-size, chunk-ext (if any), and CRLF
2048  }
2049  read header-field
2050  while (header-field not empty) {
2051     append header-field to existing header fields
2052     read header-field
2053  }
2054  Content-Length := length
2055  Remove "chunked" from Transfer-Encoding
2056  Remove Trailer from existing header fields
2057</artwork></figure>
2058<t>
2059   All recipients &MUST; be able to receive and decode the
2060   chunked transfer coding and &MUST; ignore chunk-ext extensions
2061   they do not understand.
2062</t>
2063</section>
2064</section>
2065
2066<section title="Compression Codings" anchor="compression.codings">
2067<t>
2068   The codings defined below can be used to compress the payload of a
2069   message.
2070</t>
2071
2072<section title="Compress Coding" anchor="compress.coding">
2073<iref item="compress (Coding Format)"/>
2074<t>
2075   The "compress" format is produced by the common UNIX file compression
2076   program "compress". This format is an adaptive Lempel-Ziv-Welch
2077   coding (LZW). Recipients &SHOULD; consider "x-compress" to be
2078   equivalent to "compress".
2079</t>
2080</section>
2081
2082<section title="Deflate Coding" anchor="deflate.coding">
2083<iref item="deflate (Coding Format)"/>
2084<t>
2085   The "deflate" format is defined as the "deflate" compression mechanism
2086   (described in <xref target="RFC1951"/>) used inside the "zlib"
2087   data format (<xref target="RFC1950"/>).
2088</t>
2089<x:note>
2090  <t>
2091    &Note; Some incorrect implementations send the "deflate"
2092    compressed data without the zlib wrapper.
2093   </t>
2094</x:note>
2095</section>
2096
2097<section title="Gzip Coding" anchor="gzip.coding">
2098<iref item="gzip (Coding Format)"/>
2099<t>
2100   The "gzip" format is produced by the file compression program
2101   "gzip" (GNU zip), as described in <xref target="RFC1952"/>. This format is a
2102   Lempel-Ziv coding (LZ77) with a 32 bit CRC.
2103   Recipients &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2104</t>
2105</section>
2106
2107</section>
2108
2109<section title="TE" anchor="header.te">
2110  <iref primary="true" item="TE header field" x:for-anchor=""/>
2111  <x:anchor-alias value="TE"/>
2112  <x:anchor-alias value="t-codings"/>
2113  <x:anchor-alias value="t-ranking"/>
2114  <x:anchor-alias value="rank"/>
2115<t>
2116   The "TE" header field in a request indicates what transfer codings,
2117   besides chunked, the client is willing to accept in response, and
2118   whether or not the client is willing to accept trailer fields in a
2119   chunked transfer coding.
2120</t>
2121<t>
2122   The TE field-value consists of a comma-separated list of transfer coding
2123   names, each allowing for optional parameters (as described in
2124   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2125   Clients &MUST-NOT; send the chunked transfer coding name in TE;
2126   chunked is always acceptable for HTTP/1.1 recipients.
2127</t>
2128<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"/>
2129  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2130  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2131  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2132  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2133             / ( "1" [ "." 0*3("0") ] )
2134</artwork></figure>
2135<t>
2136   Three examples of TE use are below.
2137</t>
2138<figure><artwork type="example">
2139  TE: deflate
2140  TE:
2141  TE: trailers, deflate;q=0.5
2142</artwork></figure>
2143<t>
2144   The presence of the keyword "trailers" indicates that the client is willing
2145   to accept trailer fields in a chunked transfer coding, as defined in
2146   <xref target="chunked.encoding"/>, on behalf of itself and any downstream
2147   clients. For requests from an intermediary, this implies that either:
2148   (a) all downstream clients are willing to accept trailer fields in the
2149   forwarded response; or,
2150   (b) the intermediary will attempt to buffer the response on behalf of
2151   downstream recipients.
2152   Note that HTTP/1.1 does not define any means to limit the size of a
2153   chunked response such that an intermediary can be assured of buffering the
2154   entire response.
2155</t>
2156<t>
2157   When multiple transfer codings are acceptable, the client &MAY; rank the
2158   codings by preference using a case-insensitive "q" parameter (similar to
2159   the qvalues used in content negotiation fields, &qvalue;). The rank value
2160   is a real number in the range 0 through 1, where 0.001 is the least
2161   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2162</t>
2163<t>
2164   If the TE field-value is empty or if no TE field is present, the only
2165   acceptable transfer coding is chunked. A message with no transfer coding
2166   is always acceptable.
2167</t>
2168<t>
2169   Since the TE header field only applies to the immediate connection,
2170   a sender of TE &MUST; also send a "TE" connection option within the
2171   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2172   in order to prevent the TE field from being forwarded by intermediaries
2173   that do not support its semantics.
2174</t>
2175</section>
2176</section>
2177
2178<section title="Message Routing" anchor="message.routing">
2179<t>
2180   HTTP request message routing is determined by each client based on the
2181   target resource, the client's proxy configuration, and
2182   establishment or reuse of an inbound connection.  The corresponding
2183   response routing follows the same connection chain back to the client.
2184</t>
2185
2186<section title="Identifying a Target Resource" anchor="target-resource">
2187  <iref primary="true" item="target resource"/>
2188  <iref primary="true" item="target URI"/>
2189  <x:anchor-alias value="target resource"/>
2190  <x:anchor-alias value="target URI"/>
2191<t>
2192   HTTP is used in a wide variety of applications, ranging from
2193   general-purpose computers to home appliances.  In some cases,
2194   communication options are hard-coded in a client's configuration.
2195   However, most HTTP clients rely on the same resource identification
2196   mechanism and configuration techniques as general-purpose Web browsers.
2197</t>
2198<t>
2199   HTTP communication is initiated by a user agent for some purpose.
2200   The purpose is a combination of request semantics, which are defined in
2201   <xref target="Part2"/>, and a target resource upon which to apply those
2202   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2203   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2204   would resolve to its absolute form in order to obtain the
2205   "<x:dfn>target URI</x:dfn>".  The target URI
2206   excludes the reference's fragment component, if any,
2207   since fragment identifiers are reserved for client-side processing
2208   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2209</t>
2210</section>
2211
2212<section title="Connecting Inbound" anchor="connecting.inbound">
2213<t>
2214   Once the target URI is determined, a client needs to decide whether
2215   a network request is necessary to accomplish the desired semantics and,
2216   if so, where that request is to be directed.
2217</t>
2218<t>
2219   If the client has a cache <xref target="Part6"/> and the request can be
2220   satisfied by it, then the request is
2221   usually directed there first.
2222</t>
2223<t>
2224   If the request is not satisfied by a cache, then a typical client will
2225   check its configuration to determine whether a proxy is to be used to
2226   satisfy the request.  Proxy configuration is implementation-dependent,
2227   but is often based on URI prefix matching, selective authority matching,
2228   or both, and the proxy itself is usually identified by an "http" or
2229   "https" URI.  If a proxy is applicable, the client connects inbound by
2230   establishing (or reusing) a connection to that proxy.
2231</t>
2232<t>
2233   If no proxy is applicable, a typical client will invoke a handler routine,
2234   usually specific to the target URI's scheme, to connect directly
2235   to an authority for the target resource.  How that is accomplished is
2236   dependent on the target URI scheme and defined by its associated
2237   specification, similar to how this specification defines origin server
2238   access for resolution of the "http" (<xref target="http.uri"/>) and
2239   "https" (<xref target="https.uri"/>) schemes.
2240</t>
2241<t>
2242   HTTP requirements regarding connection management are defined in
2243   <xref target="connection.management"/>.
2244</t>
2245</section>
2246
2247<section title="Request Target" anchor="request-target">
2248<t>
2249   Once an inbound connection is obtained,
2250   the client sends an HTTP request message (<xref target="http.message"/>)
2251   with a request-target derived from the target URI.
2252   There are four distinct formats for the request-target, depending on both
2253   the method being requested and whether the request is to a proxy.
2254</t>   
2255<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"/>
2256  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2257                 / <x:ref>absolute-form</x:ref>
2258                 / <x:ref>authority-form</x:ref>
2259                 / <x:ref>asterisk-form</x:ref>
2260
2261  <x:ref>origin-form</x:ref>    = <x:ref>absolute-path</x:ref> [ "?" <x:ref>query</x:ref> ]
2262  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2263  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2264  <x:ref>asterisk-form</x:ref>  = "*"
2265</artwork></figure>
2266<t anchor="origin-form"><iref item="origin-form (of request-target)"/>
2267  <x:h>origin-form</x:h>
2268</t>
2269<t>
2270   The most common form of request-target is the <x:dfn>origin-form</x:dfn>.
2271   When making a request directly to an origin server, other than a CONNECT
2272   or server-wide OPTIONS request (as detailed below),
2273   a client &MUST; send only the absolute path and query components of
2274   the target URI as the request-target.
2275   If the target URI's path component is empty, then the client &MUST; send
2276   "/" as the path within the origin-form of request-target.
2277   A <x:ref>Host</x:ref> header field is also sent, as defined in
2278   <xref target="header.host"/>, containing the target URI's
2279   authority component (excluding any userinfo).
2280</t>
2281<t>
2282   For example, a client wishing to retrieve a representation of the resource
2283   identified as
2284</t>
2285<figure><artwork x:indent-with="  " type="example">
2286http://www.example.org/where?q=now
2287</artwork></figure>
2288<t>
2289   directly from the origin server would open (or reuse) a TCP connection
2290   to port 80 of the host "www.example.org" and send the lines:
2291</t>
2292<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2293GET /where?q=now HTTP/1.1
2294Host: www.example.org
2295</artwork></figure>
2296<t>
2297   followed by the remainder of the request message.
2298</t>
2299<t anchor="absolute-form"><iref item="absolute-form (of request-target)"/>
2300  <x:h>absolute-form</x:h>
2301</t>
2302<t>
2303   When making a request to a proxy, other than a CONNECT or server-wide
2304   OPTIONS request (as detailed below), a client &MUST; send the target URI
2305   in <x:dfn>absolute-form</x:dfn> as the request-target.
2306   The proxy is requested to either service that request from a valid cache,
2307   if possible, or make the same request on the client's behalf to either
2308   the next inbound proxy server or directly to the origin server indicated
2309   by the request-target.  Requirements on such "forwarding" of messages are
2310   defined in <xref target="message.forwarding"/>.
2311</t>
2312<t>
2313   An example absolute-form of request-line would be:
2314</t>
2315<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2316GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
2317</artwork></figure>
2318<t>
2319   To allow for transition to the absolute-form for all requests in some
2320   future version of HTTP, HTTP/1.1 servers &MUST; accept the absolute-form
2321   in requests, even though HTTP/1.1 clients will only send them in requests
2322   to proxies.
2323</t>
2324<t anchor="authority-form"><iref item="authority-form (of request-target)"/>
2325  <x:h>authority-form</x:h>
2326</t>
2327<t>
2328   The <x:dfn>authority-form</x:dfn> of request-target is only used for CONNECT requests
2329   (&CONNECT;).  When making a CONNECT request to establish a tunnel through
2330   one or more proxies, a client &MUST; send only the target URI's
2331   authority component (excluding any userinfo) as the request-target.
2332   For example,
2333</t>
2334<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2335CONNECT www.example.com:80 HTTP/1.1
2336</artwork></figure>
2337<t anchor="asterisk-form"><iref item="asterisk-form (of request-target)"/>
2338  <x:h>asterisk-form</x:h>
2339</t>
2340<t>
2341   The <x:dfn>asterisk-form</x:dfn> of request-target is only used for a server-wide
2342   OPTIONS request (&OPTIONS;).  When a client wishes to request OPTIONS
2343   for the server as a whole, as opposed to a specific named resource of
2344   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2345   For example,
2346</t>
2347<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2348OPTIONS * HTTP/1.1
2349</artwork></figure>
2350<t>
2351   If a proxy receives an OPTIONS request with an absolute-form of
2352   request-target in which the URI has an empty path and no query component,
2353   then the last proxy on the request chain &MUST; send a request-target
2354   of "*" when it forwards the request to the indicated origin server.
2355</t>
2356<figure><preamble>   
2357   For example, the request
2358</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2359OPTIONS http://www.example.org:8001 HTTP/1.1
2360</artwork></figure>
2361<figure><preamble>   
2362  would be forwarded by the final proxy as
2363</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2364OPTIONS * HTTP/1.1
2365Host: www.example.org:8001
2366</artwork>
2367<postamble>
2368   after connecting to port 8001 of host "www.example.org".
2369</postamble>
2370</figure>
2371</section>
2372
2373<section title="Host" anchor="header.host">
2374  <iref primary="true" item="Host header field" x:for-anchor=""/>
2375  <x:anchor-alias value="Host"/>
2376<t>
2377   The "Host" header field in a request provides the host and port
2378   information from the target URI, enabling the origin
2379   server to distinguish among resources while servicing requests
2380   for multiple host names on a single IP address.  Since the Host
2381   field-value is critical information for handling a request, it
2382   &SHOULD; be sent as the first header field following the request-line.
2383</t>
2384<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2385  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2386</artwork></figure>
2387<t>
2388   A client &MUST; send a Host header field in all HTTP/1.1 request
2389   messages.  If the target URI includes an authority component, then
2390   the Host field-value &MUST; be identical to that authority component
2391   after excluding any userinfo (<xref target="http.uri"/>).
2392   If the authority component is missing or undefined for the target URI,
2393   then the Host header field &MUST; be sent with an empty field-value.
2394</t>
2395<t>
2396   For example, a GET request to the origin server for
2397   &lt;http://www.example.org/pub/WWW/&gt; would begin with:
2398</t>
2399<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2400GET /pub/WWW/ HTTP/1.1
2401Host: www.example.org
2402</artwork></figure>
2403<t>
2404   The Host header field &MUST; be sent in an HTTP/1.1 request even
2405   if the request-target is in the absolute-form, since this
2406   allows the Host information to be forwarded through ancient HTTP/1.0
2407   proxies that might not have implemented Host.
2408</t>
2409<t>
2410   When a proxy receives a request with an absolute-form of
2411   request-target, the proxy &MUST; ignore the received
2412   Host header field (if any) and instead replace it with the host
2413   information of the request-target.  If the proxy forwards the request,
2414   it &MUST; generate a new Host field-value based on the received
2415   request-target rather than forward the received Host field-value.
2416</t>
2417<t>
2418   Since the Host header field acts as an application-level routing
2419   mechanism, it is a frequent target for malware seeking to poison
2420   a shared cache or redirect a request to an unintended server.
2421   An interception proxy is particularly vulnerable if it relies on
2422   the Host field-value for redirecting requests to internal
2423   servers, or for use as a cache key in a shared cache, without
2424   first verifying that the intercepted connection is targeting a
2425   valid IP address for that host.
2426</t>
2427<t>
2428   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2429   to any HTTP/1.1 request message that lacks a Host header field and
2430   to any request message that contains more than one Host header field
2431   or a Host header field with an invalid field-value.
2432</t>
2433</section>
2434
2435<section title="Effective Request URI" anchor="effective.request.uri">
2436  <iref primary="true" item="effective request URI"/>
2437  <x:anchor-alias value="effective request URI"/>
2438<t>
2439   A server that receives an HTTP request message &MUST; reconstruct
2440   the user agent's original target URI, based on the pieces of information
2441   learned from the request-target, <x:ref>Host</x:ref> header field, and
2442   connection context, in order to identify the intended target resource and
2443   properly service the request. The URI derived from this reconstruction
2444   process is referred to as the "<x:dfn>effective request URI</x:dfn>".
2445</t>
2446<t>
2447   For a user agent, the effective request URI is the target URI.
2448</t>
2449<t>
2450   If the request-target is in absolute-form, then the effective request URI
2451   is the same as the request-target.  Otherwise, the effective request URI
2452   is constructed as follows.
2453</t>
2454<t>
2455   If the request is received over a TLS-secured TCP connection,
2456   then the effective request URI's scheme is "https"; otherwise, the
2457   scheme is "http".
2458</t>
2459<t>
2460   If the request-target is in authority-form, then the effective
2461   request URI's authority component is the same as the request-target.
2462   Otherwise, if a <x:ref>Host</x:ref> header field is supplied with a
2463   non-empty field-value, then the authority component is the same as the
2464   Host field-value. Otherwise, the authority component is the concatenation of
2465   the default host name configured for the server, a colon (":"), and the
2466   connection's incoming TCP port number in decimal form.
2467</t>
2468<t>
2469   If the request-target is in authority-form or asterisk-form, then the
2470   effective request URI's combined path and query component is empty.
2471   Otherwise, the combined path and query component is the same as the
2472   request-target.
2473</t>
2474<t>
2475   The components of the effective request URI, once determined as above,
2476   can be combined into absolute-URI form by concatenating the scheme,
2477   "://", authority, and combined path and query component.
2478</t>
2479<figure>
2480<preamble>
2481   Example 1: the following message received over an insecure TCP connection
2482</preamble> 
2483<artwork type="example" x:indent-with="  ">
2484GET /pub/WWW/TheProject.html HTTP/1.1
2485Host: www.example.org:8080
2486</artwork>
2487</figure>
2488<figure>
2489<preamble>
2490  has an effective request URI of
2491</preamble>
2492<artwork type="example" x:indent-with="  ">
2493http://www.example.org:8080/pub/WWW/TheProject.html
2494</artwork>
2495</figure>
2496<figure>
2497<preamble>
2498   Example 2: the following message received over a TLS-secured TCP connection
2499</preamble> 
2500<artwork type="example" x:indent-with="  ">
2501OPTIONS * HTTP/1.1
2502Host: www.example.org
2503</artwork>
2504</figure>
2505<figure>
2506<preamble>
2507  has an effective request URI of
2508</preamble>
2509<artwork type="example" x:indent-with="  ">
2510https://www.example.org
2511</artwork>
2512</figure>
2513<t>
2514   An origin server that does not allow resources to differ by requested
2515   host &MAY; ignore the <x:ref>Host</x:ref> field-value and instead replace it
2516   with a configured server name when constructing the effective request URI.
2517</t>
2518<t>
2519   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2520   field &MAY; attempt to use heuristics (e.g., examination of the URI path for
2521   something unique to a particular host) in order to guess the
2522   effective request URI's authority component.
2523</t>
2524</section>
2525
2526<section title="Associating a Response to a Request" anchor="associating.response.to.request">
2527<t>
2528   HTTP does not include a request identifier for associating a given
2529   request message with its corresponding one or more response messages.
2530   Hence, it relies on the order of response arrival to correspond exactly
2531   to the order in which requests are made on the same connection.
2532   More than one response message per request only occurs when one or more
2533   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a
2534   final response to the same request.
2535</t>
2536<t>
2537   A client that has more than one outstanding request on a connection &MUST;
2538   maintain a list of outstanding requests in the order sent and &MUST;
2539   associate each received response message on that connection to the highest
2540   ordered request that has not yet received a final (non-<x:ref>1xx</x:ref>)
2541   response.
2542</t>
2543</section>
2544
2545<section title="Message Forwarding" anchor="message.forwarding">
2546<t>
2547   As described in <xref target="intermediaries"/>, intermediaries can serve
2548   a variety of roles in the processing of HTTP requests and responses.
2549   Some intermediaries are used to improve performance or availability.
2550   Others are used for access control or to filter content.
2551   Since an HTTP stream has characteristics similar to a pipe-and-filter
2552   architecture, there are no inherent limits to the extent an intermediary
2553   can enhance (or interfere) with either direction of the stream.
2554</t>
2555<t>
2556   Intermediaries that forward a message &MUST; implement the
2557   <x:ref>Connection</x:ref> header field, as specified in
2558   <xref target="header.connection"/>, to exclude fields that are only
2559   intended for the incoming connection.
2560</t>
2561<t>
2562   In order to avoid request loops, a proxy that forwards requests to other
2563   proxies &MUST; be able to recognize and exclude all of its own server
2564   names, including any aliases, local variations, or literal IP addresses.
2565</t>
2566
2567<section title="Via" anchor="header.via">
2568  <iref primary="true" item="Via header field" x:for-anchor=""/>
2569  <x:anchor-alias value="pseudonym"/>
2570  <x:anchor-alias value="received-by"/>
2571  <x:anchor-alias value="received-protocol"/>
2572  <x:anchor-alias value="Via"/>
2573<t>
2574   The "Via" header field &MUST; be sent by a proxy or gateway in forwarded
2575   messages to indicate the intermediate protocols and recipients between the
2576   user agent and the server on requests, and between the origin server and
2577   the client on responses. It is analogous to the "Received" field
2578   used by email systems (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2579   Via is used in HTTP for tracking message forwards,
2580   avoiding request loops, and identifying the protocol capabilities of
2581   all senders along the request/response chain.
2582</t>
2583<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"/>
2584  <x:ref>Via</x:ref>               = 1#( <x:ref>received-protocol</x:ref> <x:ref>RWS</x:ref> <x:ref>received-by</x:ref>
2585                          [ <x:ref>RWS</x:ref> <x:ref>comment</x:ref> ] )
2586  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2587                      ; see <xref target="header.upgrade"/>
2588  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2589  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2590</artwork></figure>
2591<t>
2592   The received-protocol indicates the protocol version of the message
2593   received by the server or client along each segment of the
2594   request/response chain. The received-protocol version is appended to
2595   the Via field value when the message is forwarded so that information
2596   about the protocol capabilities of upstream applications remains
2597   visible to all recipients.
2598</t>
2599<t>
2600   The protocol-name is excluded if and only if it would be "HTTP". The
2601   received-by field is normally the host and optional port number of a
2602   recipient server or client that subsequently forwarded the message.
2603   However, if the real host is considered to be sensitive information,
2604   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2605   be assumed to be the default port of the received-protocol.
2606</t>
2607<t>
2608   Multiple Via field values represent each proxy or gateway that has
2609   forwarded the message. Each recipient &MUST; append its information
2610   such that the end result is ordered according to the sequence of
2611   forwarding applications.
2612</t>
2613<t>
2614   Comments &MAY; be used in the Via header field to identify the software
2615   of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2616   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2617   are optional and &MAY; be removed by any recipient prior to forwarding the
2618   message.
2619</t>
2620<t>
2621   For example, a request message could be sent from an HTTP/1.0 user
2622   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2623   forward the request to a public proxy at p.example.net, which completes
2624   the request by forwarding it to the origin server at www.example.com.
2625   The request received by www.example.com would then have the following
2626   Via header field:
2627</t>
2628<figure><artwork type="example">
2629  Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
2630</artwork></figure>
2631<t>
2632   A proxy or gateway used as a portal through a network firewall
2633   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2634   region unless it is explicitly enabled to do so. If not enabled, the
2635   received-by host of any host behind the firewall &SHOULD; be replaced
2636   by an appropriate pseudonym for that host.
2637</t>
2638<t>
2639   A proxy or gateway &MAY; combine an ordered subsequence of Via header
2640   field entries into a single such entry if the entries have identical
2641   received-protocol values. For example,
2642</t>
2643<figure><artwork type="example">
2644  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2645</artwork></figure>
2646<t>
2647  could be collapsed to
2648</t>
2649<figure><artwork type="example">
2650  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2651</artwork></figure>
2652<t>
2653   Senders &SHOULD-NOT; combine multiple entries unless they are all
2654   under the same organizational control and the hosts have already been
2655   replaced by pseudonyms. Senders &MUST-NOT; combine entries that
2656   have different received-protocol values.
2657</t>
2658</section>
2659
2660<section title="Transformations" anchor="message.transformations">
2661<t>
2662   Some intermediaries include features for transforming messages and their
2663   payloads.  A transforming proxy might, for example, convert between image
2664   formats in order to save cache space or to reduce the amount of traffic on
2665   a slow link. However, operational problems might occur when these
2666   transformations are applied to payloads intended for critical applications,
2667   such as medical imaging or scientific data analysis, particularly when
2668   integrity checks or digital signatures are used to ensure that the payload
2669   received is identical to the original.
2670</t>
2671<t>
2672   If a proxy receives a request-target with a host name that is not a
2673   fully qualified domain name, it &MAY; add its own domain to the host name
2674   it received when forwarding the request.  A proxy &MUST-NOT; change the
2675   host name if it is a fully qualified domain name.
2676</t>
2677<t>
2678   A proxy &MUST-NOT; modify the "absolute-path" and "query" parts of the
2679   received request-target when forwarding it to the next inbound server,
2680   except as noted above to replace an empty path with "/" or "*".
2681</t>
2682<t>
2683   A proxy &MUST-NOT; modify header fields that provide information about the
2684   end points of the communication chain, the resource state, or the selected
2685   representation. A proxy &MAY; change the message body through application
2686   or removal of a transfer coding (<xref target="transfer.codings"/>).
2687</t>
2688<t>
2689   A non-transforming proxy &MUST-NOT; modify the message payload (&payload;).
2690   A transforming proxy &MUST-NOT; modify the payload of a message that
2691   contains the no-transform cache-control directive.
2692</t>
2693<t>
2694   A transforming proxy &MAY; transform the payload of a message
2695   that does not contain the no-transform cache-control directive;
2696   if the payload is transformed, the transforming proxy &MUST; add a
2697   Warning 214 (Transformation applied) header field if one does not
2698   already appear in the message (see &header-warning;).
2699</t>
2700</section>
2701</section>
2702</section>
2703
2704<section title="Connection Management" anchor="connection.management">
2705<t>
2706   HTTP messaging is independent of the underlying transport or
2707   session-layer connection protocol(s).  HTTP only presumes a reliable
2708   transport with in-order delivery of requests and the corresponding
2709   in-order delivery of responses.  The mapping of HTTP request and
2710   response structures onto the data units of an underlying transport
2711   protocol is outside the scope of this specification.
2712</t>
2713<t>
2714   As described in <xref target="connecting.inbound"/>, the specific
2715   connection protocols to be used for an HTTP interaction are determined by
2716   client configuration and the <x:ref>target URI</x:ref>.
2717   For example, the "http" URI scheme
2718   (<xref target="http.uri"/>) indicates a default connection of TCP
2719   over IP, with a default TCP port of 80, but the client might be
2720   configured to use a proxy via some other connection, port, or protocol.
2721</t>
2722<t>
2723   HTTP implementations are expected to engage in connection management,
2724   which includes maintaining the state of current connections,
2725   establishing a new connection or reusing an existing connection,
2726   processing messages received on a connection, detecting connection
2727   failures, and closing each connection.
2728   Most clients maintain multiple connections in parallel, including
2729   more than one connection per server endpoint.
2730   Most servers are designed to maintain thousands of concurrent connections,
2731   while controlling request queues to enable fair use and detect
2732   denial of service attacks.
2733</t>
2734
2735<section title="Connection" anchor="header.connection">
2736  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2737  <iref primary="true" item="close" x:for-anchor=""/>
2738  <x:anchor-alias value="Connection"/>
2739  <x:anchor-alias value="connection-option"/>
2740  <x:anchor-alias value="close"/>
2741<t>
2742   The "Connection" header field allows the sender to indicate desired
2743   control options for the current connection.  In order to avoid confusing
2744   downstream recipients, a proxy or gateway &MUST; remove or replace any
2745   received connection options before forwarding the message.
2746</t>
2747<t>
2748   When a header field aside from Connection is used to supply control
2749   information for or about the current connection, the sender &MUST; list
2750   the corresponding field-name within the "Connection" header field.
2751   A proxy or gateway &MUST; parse a received Connection
2752   header field before a message is forwarded and, for each
2753   connection-option in this field, remove any header field(s) from
2754   the message with the same name as the connection-option, and then
2755   remove the Connection header field itself (or replace it with the
2756   intermediary's own connection options for the forwarded message).
2757</t>
2758<t>
2759   Hence, the Connection header field provides a declarative way of
2760   distinguishing header fields that are only intended for the
2761   immediate recipient ("hop-by-hop") from those fields that are
2762   intended for all recipients on the chain ("end-to-end"), enabling the
2763   message to be self-descriptive and allowing future connection-specific
2764   extensions to be deployed without fear that they will be blindly
2765   forwarded by older intermediaries.
2766</t>
2767<t>
2768   The Connection header field's value has the following grammar:
2769</t>
2770<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2771  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2772  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2773</artwork></figure>
2774<t>
2775   Connection options are case-insensitive.
2776</t>
2777<t>
2778   A sender &MUST-NOT; send a connection option corresponding to a header
2779   field that is intended for all recipients of the payload.
2780   For example, <x:ref>Cache-Control</x:ref> is never appropriate as a
2781   connection option (&header-cache-control;).
2782</t>
2783<t>
2784   The connection options do not have to correspond to a header field
2785   present in the message, since a connection-specific header field
2786   might not be needed if there are no parameters associated with that
2787   connection option.  Recipients that trigger certain connection
2788   behavior based on the presence of connection options &MUST; do so
2789   based on the presence of the connection-option rather than only the
2790   presence of the optional header field.  In other words, if the
2791   connection option is received as a header field but not indicated
2792   within the Connection field-value, then the recipient &MUST; ignore
2793   the connection-specific header field because it has likely been
2794   forwarded by an intermediary that is only partially conformant.
2795</t>
2796<t>
2797   When defining new connection options, specifications ought to
2798   carefully consider existing deployed header fields and ensure
2799   that the new connection option does not share the same name as
2800   an unrelated header field that might already be deployed.
2801   Defining a new connection option essentially reserves that potential
2802   field-name for carrying additional information related to the
2803   connection option, since it would be unwise for senders to use
2804   that field-name for anything else.
2805</t>
2806<t>
2807   The "<x:dfn>close</x:dfn>" connection option is defined for a
2808   sender to signal that this connection will be closed after completion of
2809   the response. For example,
2810</t>
2811<figure><artwork type="example">
2812  Connection: close
2813</artwork></figure>
2814<t>
2815   in either the request or the response header fields indicates that
2816   the connection &MUST; be closed after the current request/response
2817   is complete (<xref target="persistent.tear-down"/>).
2818</t>
2819<t>
2820   A client that does not support <x:ref>persistent connections</x:ref> &MUST;
2821   send the "close" connection option in every request message.
2822</t>
2823<t>
2824   A server that does not support <x:ref>persistent connections</x:ref> &MUST;
2825   send the "close" connection option in every response message that
2826   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2827</t>
2828</section>
2829
2830<section title="Establishment" anchor="persistent.establishment">
2831<t>
2832   It is beyond the scope of this specification to describe how connections
2833   are established via various transport or session-layer protocols.
2834   Each connection applies to only one transport link.
2835</t>
2836</section>
2837
2838<section title="Persistence" anchor="persistent.connections">
2839   <x:anchor-alias value="persistent connections"/>
2840<t>
2841   HTTP/1.1 defaults to the use of "<x:dfn>persistent connections</x:dfn>",
2842   allowing multiple requests and responses to be carried over a single
2843   connection. The "<x:ref>close</x:ref>" connection-option is used to signal
2844   that a connection will not persist after the current request/response.
2845   HTTP implementations &SHOULD; support persistent connections.
2846</t>
2847<t>
2848   A recipient determines whether a connection is persistent or not based on
2849   the most recently received message's protocol version and
2850   <x:ref>Connection</x:ref> header field (if any):
2851   <list style="symbols">
2852     <t>If the <x:ref>close</x:ref> connection option is present, the
2853        connection will not persist after the current response; else,</t>
2854     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2855        persist after the current response; else,</t>
2856     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2857        connection option is present, the recipient is not a proxy, and
2858        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2859        the connection will persist after the current response; otherwise,</t>
2860     <t>The connection will close after the current response.</t>
2861   </list>
2862</t>
2863<t>
2864   A server &MAY; assume that an HTTP/1.1 client intends to maintain a
2865   persistent connection until a <x:ref>close</x:ref> connection option
2866   is received in a request.
2867</t>
2868<t>
2869   A client &MAY; reuse a persistent connection until it sends or receives
2870   a <x:ref>close</x:ref> connection option or receives an HTTP/1.0 response
2871   without a "keep-alive" connection option.
2872</t>
2873<t>
2874   In order to remain persistent, all messages on a connection &MUST;
2875   have a self-defined message length (i.e., one not defined by closure
2876   of the connection), as described in <xref target="message.body"/>.
2877   A server &MUST; read the entire request message body or close
2878   the connection after sending its response, since otherwise the
2879   remaining data on a persistent connection would be misinterpreted
2880   as the next request.  Likewise,
2881   a client &MUST; read the entire response message body if it intends
2882   to reuse the same connection for a subsequent request.
2883</t>
2884<t>
2885   A proxy server &MUST-NOT; maintain a persistent connection with an
2886   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
2887   information and discussion of the problems with the Keep-Alive header field
2888   implemented by many HTTP/1.0 clients).
2889</t>
2890<t>
2891   Clients and servers &SHOULD-NOT; assume that a persistent connection is
2892   maintained for HTTP versions less than 1.1 unless it is explicitly
2893   signaled.
2894   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
2895   for more information on backward compatibility with HTTP/1.0 clients.
2896</t>
2897
2898<section title="Retrying Requests" anchor="persistent.retrying.requests">
2899<t>
2900   Connections can be closed at any time, with or without intention.
2901   Implementations ought to anticipate the need to recover
2902   from asynchronous close events.
2903</t>
2904<t>
2905   When an inbound connection is closed prematurely, a client &MAY; open a new
2906   connection and automatically retransmit an aborted sequence of requests if
2907   all of those requests have idempotent methods (&idempotent-methods;).
2908   A proxy &MUST-NOT; automatically retry non-idempotent requests.
2909</t>
2910<t>
2911   A user agent &MUST-NOT; automatically retry a request with a non-idempotent
2912   method unless it has some means to know that the request semantics are
2913   actually idempotent, regardless of the method, or some means to detect that
2914   the original request was never applied. For example, a user agent that
2915   knows (through design or configuration) that a POST request to a given
2916   resource is safe can repeat that request automatically.
2917   Likewise, a user agent designed specifically to operate on a version
2918   control repository might be able to recover from partial failure conditions
2919   by checking the target resource revision(s) after a failed connection,
2920   reverting or fixing any changes that were partially applied, and then
2921   automatically retrying the requests that failed.
2922</t>
2923<t>
2924   An automatic retry &SHOULD-NOT; be repeated if it fails.
2925</t>
2926</section>
2927
2928<section title="Pipelining" anchor="pipelining">
2929   <x:anchor-alias value="pipeline"/>
2930<t>
2931   A client that supports persistent connections &MAY; "<x:dfn>pipeline</x:dfn>"
2932   its requests (i.e., send multiple requests without waiting for each
2933   response). A server &MAY; process a sequence of pipelined requests in
2934   parallel if they all have safe methods (&safe-methods;), but &MUST; send
2935   the corresponding responses in the same order that the requests were
2936   received.
2937</t>
2938<t>
2939   A client that pipelines requests &MUST; be prepared to retry those
2940   requests if the connection closes before it receives all of the
2941   corresponding responses. A client that assumes a persistent connection and
2942   pipelines immediately after connection establishment &MUST-NOT; pipeline
2943   on a retry connection until it knows the connection is persistent.
2944</t>
2945<t>
2946   Idempotent methods (&idempotent-methods;) are significant to pipelining
2947   because they can be automatically retried after a connection failure.
2948   A user agent &SHOULD-NOT; pipeline requests after a non-idempotent method
2949   until the final response status code for that method has been received,
2950   unless the user agent has a means to detect and recover from partial
2951   failure conditions involving the pipelined sequence.
2952</t>
2953<t>
2954   An intermediary that receives pipelined requests &MAY; pipeline those
2955   requests when forwarding them inbound, since it can rely on the outbound
2956   user agent(s) to determine what requests can be safely pipelined. If the
2957   inbound connection fails before receiving a response, the pipelining
2958   intermediary &MAY; attempt to retry a sequence of requests that have yet
2959   to receive a response if the requests all have idempotent methods;
2960   otherwise, the pipelining intermediary &SHOULD; forward any received
2961   responses and then close the corresponding outbound connection(s) so that
2962   the outbound user agent(s) can recover accordingly.
2963</t>
2964</section>
2965</section>
2966   
2967<section title="Concurrency" anchor="persistent.concurrency">
2968<t>
2969   Clients &SHOULD; limit the number of simultaneous
2970   connections that they maintain to a given server.
2971</t>
2972<t>
2973   Previous revisions of HTTP gave a specific number of connections as a
2974   ceiling, but this was found to be impractical for many applications. As a
2975   result, this specification does not mandate a particular maximum number of
2976   connections, but instead encourages clients to be conservative when opening
2977   multiple connections.
2978</t>
2979<t>
2980   Multiple connections are typically used to avoid the "head-of-line
2981   blocking" problem, wherein a request that takes significant server-side
2982   processing and/or has a large payload blocks subsequent requests on the
2983   same connection. However, each connection consumes server resources.
2984   Furthermore, using multiple connections can cause undesirable side effects
2985   in congested networks.
2986</t>
2987<t>
2988   Note that servers might reject traffic that they deem abusive, including an
2989   excessive number of connections from a client.
2990</t>
2991</section>
2992
2993<section title="Failures and Time-outs" anchor="persistent.failures">
2994<t>
2995   Servers will usually have some time-out value beyond which they will
2996   no longer maintain an inactive connection. Proxy servers might make
2997   this a higher value since it is likely that the client will be making
2998   more connections through the same server. The use of persistent
2999   connections places no requirements on the length (or existence) of
3000   this time-out for either the client or the server.
3001</t>
3002<t>
3003   When a client or server wishes to time-out it &SHOULD; issue a graceful
3004   close on the transport connection. Clients and servers &SHOULD; both
3005   constantly watch for the other side of the transport close, and
3006   respond to it as appropriate. If a client or server does not detect
3007   the other side's close promptly it could cause unnecessary resource
3008   drain on the network.
3009</t>
3010<t>
3011   A client, server, or proxy &MAY; close the transport connection at any
3012   time. For example, a client might have started to send a new request
3013   at the same time that the server has decided to close the "idle"
3014   connection. From the server's point of view, the connection is being
3015   closed while it was idle, but from the client's point of view, a
3016   request is in progress.
3017</t>
3018<t>
3019   Servers &SHOULD; maintain persistent connections and allow the underlying
3020   transport's flow control mechanisms to resolve temporary overloads, rather
3021   than terminate connections with the expectation that clients will retry.
3022   The latter technique can exacerbate network congestion.
3023</t>
3024<t>
3025   A client sending a message body &SHOULD; monitor
3026   the network connection for an error response while it is transmitting
3027   the request. If the client sees an error response, it &SHOULD;
3028   immediately cease transmitting the body and close the connection.
3029</t>
3030</section>
3031   
3032<section title="Tear-down" anchor="persistent.tear-down">
3033  <iref primary="false" item="Connection header field" x:for-anchor=""/>
3034  <iref primary="false" item="close" x:for-anchor=""/>
3035<t>
3036   The <x:ref>Connection</x:ref> header field
3037   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
3038   connection option that a sender &SHOULD; send when it wishes to close
3039   the connection after the current request/response pair.
3040</t>
3041<t>
3042   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
3043   send further requests on that connection (after the one containing
3044   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
3045   final response message corresponding to this request.
3046</t>
3047<t>
3048   A server that receives a <x:ref>close</x:ref> connection option &MUST;
3049   initiate a close of the connection (see below) after it sends the
3050   final response to the request that contained <x:ref>close</x:ref>.
3051   The server &SHOULD; send a <x:ref>close</x:ref> connection option
3052   in its final response on that connection. The server &MUST-NOT; process
3053   any further requests received on that connection.
3054</t>
3055<t>
3056   A server that sends a <x:ref>close</x:ref> connection option &MUST;
3057   initiate a close of the connection (see below) after it sends the
3058   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
3059   any further requests received on that connection.
3060</t>
3061<t>
3062   A client that receives a <x:ref>close</x:ref> connection option &MUST;
3063   cease sending requests on that connection and close the connection
3064   after reading the response message containing the close; if additional
3065   pipelined requests had been sent on the connection, the client &SHOULD-NOT;
3066   assume that they will be processed by the server.
3067</t>
3068<t>
3069   If a server performs an immediate close of a TCP connection, there is a
3070   significant risk that the client will not be able to read the last HTTP
3071   response.  If the server receives additional data from the client on a
3072   fully-closed connection, such as another request that was sent by the
3073   client before receiving the server's response, the server's TCP stack will
3074   send a reset packet to the client; unfortunately, the reset packet might
3075   erase the client's unacknowledged input buffers before they can be read
3076   and interpreted by the client's HTTP parser.
3077</t>
3078<t>
3079   To avoid the TCP reset problem, servers typically close a connection in
3080   stages. First, the server performs a half-close by closing only the write
3081   side of the read/write connection. The server then continues to read from
3082   the connection until it receives a corresponding close by the client, or
3083   until the server is reasonably certain that its own TCP stack has received
3084   the client's acknowledgement of the packet(s) containing the server's last
3085   response. Finally, the server fully closes the connection.
3086</t>
3087<t>
3088   It is unknown whether the reset problem is exclusive to TCP or might also
3089   be found in other transport connection protocols.
3090</t>
3091</section>
3092
3093<section title="Upgrade" anchor="header.upgrade">
3094  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3095  <x:anchor-alias value="Upgrade"/>
3096  <x:anchor-alias value="protocol"/>
3097  <x:anchor-alias value="protocol-name"/>
3098  <x:anchor-alias value="protocol-version"/>
3099<t>
3100   The "Upgrade" header field is intended to provide a simple mechanism
3101   for transitioning from HTTP/1.1 to some other protocol on the same
3102   connection.  A client &MAY; send a list of protocols in the Upgrade
3103   header field of a request to invite the server to switch to one or
3104   more of those protocols, in order of descending preference, before sending
3105   the final response. A server &MAY; ignore a received Upgrade header field
3106   if it wishes to continue using the current protocol on that connection.
3107</t>
3108<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3109  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3110
3111  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3112  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3113  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3114</artwork></figure>
3115<t>
3116   A server that sends a <x:ref>101 (Switching Protocols)</x:ref> response
3117   &MUST; send an Upgrade header field to indicate the new protocol(s) to
3118   which the connection is being switched; if multiple protocol layers are
3119   being switched, the new protocols &MUST; be listed in layer-ascending
3120   order. A server &MUST-NOT; switch to a protocol that was not indicated by
3121   the client in the corresponding request's Upgrade header field.
3122   A server &MAY; choose to ignore the order of preference indicated by the
3123   client and select the new protocol(s) based on other factors, such as the
3124   nature of the request or the current load on the server.
3125</t>
3126<t>
3127   A server that sends a <x:ref>426 (Upgrade Required)</x:ref> response
3128   &MUST; send an Upgrade header field to indicate the acceptable protocols,
3129   in order of descending preference.
3130</t>
3131<t>
3132   A server &MAY; send an Upgrade header field in any other response to
3133   advertise that it implements support for upgrading to the listed protocols,
3134   in order of descending preference, when appropriate for a future request.
3135</t>
3136<figure><preamble>
3137   The following is a hypothetical example sent by a client:
3138</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
3139GET /hello.txt HTTP/1.1
3140Host: www.example.com
3141Connection: upgrade
3142Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3143
3144</artwork></figure>
3145<t>
3146   Upgrade cannot be used to insist on a protocol change; its acceptance and
3147   use by the server is optional. The capabilities and nature of the
3148   application-level communication after the protocol change is entirely
3149   dependent upon the new protocol(s) chosen, although the first action
3150   after changing the protocol &MUST; be a response to the initial HTTP
3151   request that contained the Upgrade header field.
3152</t>
3153<t>
3154   For example, if the Upgrade header field is received in a GET request
3155   and the server decides to switch protocols, it first responds
3156   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3157   then immediately follows that with the new protocol's equivalent of a
3158   response to a GET on the target resource.  This allows a connection to be
3159   upgraded to protocols with the same semantics as HTTP without the
3160   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3161   protocols unless the received message semantics can be honored by the new
3162   protocol; an OPTIONS request can be honored by any protocol.
3163</t>
3164<figure><preamble>
3165   The following is an example response to the above hypothetical request:
3166</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
3167HTTP/1.1 101 Switching Protocols
3168Connection: upgrade
3169Upgrade: HTTP/2.0
3170
3171[... data stream switches to HTTP/2.0 with an appropriate response
3172(as defined by new protocol) to the "GET /hello.txt" request ...]
3173</artwork></figure>
3174<t>
3175   When Upgrade is sent, the sender &MUST; also send a
3176   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3177   that contains an "upgrade" connection option, in order to prevent Upgrade
3178   from being accidentally forwarded by intermediaries that might not implement
3179   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3180   is received in an HTTP/1.0 request.
3181</t>
3182<t>
3183   The Upgrade header field only applies to switching protocols on top of the
3184   existing connection; it cannot be used to switch the underlying connection
3185   (transport) protocol, nor to switch the existing communication to a
3186   different connection. For those purposes, it is more appropriate to use a
3187   <x:ref>3xx (Redirection)</x:ref> response (&status-3xx;).
3188</t>
3189<t>
3190   This specification only defines the protocol name "HTTP" for use by
3191   the family of Hypertext Transfer Protocols, as defined by the HTTP
3192   version rules of <xref target="http.version"/> and future updates to this
3193   specification. Additional tokens ought to be registered with IANA using the
3194   registration procedure defined in <xref target="upgrade.token.registry"/>.
3195</t>
3196</section>
3197</section>
3198
3199<section title="IANA Considerations" anchor="IANA.considerations">
3200
3201<section title="Header Field Registration" anchor="header.field.registration">
3202<t>
3203   HTTP header fields are registered within the Message Header Field Registry
3204   maintained at
3205   <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
3206</t>
3207<t>
3208   This document defines the following HTTP header fields, so their
3209   associated registry entries shall be updated according to the permanent
3210   registrations below (see <xref target="BCP90"/>):
3211</t>
3212<?BEGININC p1-messaging.iana-headers ?>
3213<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3214<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3215   <ttcol>Header Field Name</ttcol>
3216   <ttcol>Protocol</ttcol>
3217   <ttcol>Status</ttcol>
3218   <ttcol>Reference</ttcol>
3219
3220   <c>Connection</c>
3221   <c>http</c>
3222   <c>standard</c>
3223   <c>
3224      <xref target="header.connection"/>
3225   </c>
3226   <c>Content-Length</c>
3227   <c>http</c>
3228   <c>standard</c>
3229   <c>
3230      <xref target="header.content-length"/>
3231   </c>
3232   <c>Host</c>
3233   <c>http</c>
3234   <c>standard</c>
3235   <c>
3236      <xref target="header.host"/>
3237   </c>
3238   <c>TE</c>
3239   <c>http</c>
3240   <c>standard</c>
3241   <c>
3242      <xref target="header.te"/>
3243   </c>
3244   <c>Trailer</c>
3245   <c>http</c>
3246   <c>standard</c>
3247   <c>
3248      <xref target="header.trailer"/>
3249   </c>
3250   <c>Transfer-Encoding</c>
3251   <c>http</c>
3252   <c>standard</c>
3253   <c>
3254      <xref target="header.transfer-encoding"/>
3255   </c>
3256   <c>Upgrade</c>
3257   <c>http</c>
3258   <c>standard</c>
3259   <c>
3260      <xref target="header.upgrade"/>
3261   </c>
3262   <c>Via</c>
3263   <c>http</c>
3264   <c>standard</c>
3265   <c>
3266      <xref target="header.via"/>
3267   </c>
3268</texttable>
3269<!--(END)-->
3270<?ENDINC p1-messaging.iana-headers ?>
3271<t>
3272   Furthermore, the header field-name "Close" shall be registered as
3273   "reserved", since using that name as an HTTP header field might
3274   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3275   header field (<xref target="header.connection"/>).
3276</t>
3277<texttable align="left" suppress-title="true">
3278   <ttcol>Header Field Name</ttcol>
3279   <ttcol>Protocol</ttcol>
3280   <ttcol>Status</ttcol>
3281   <ttcol>Reference</ttcol>
3282
3283   <c>Close</c>
3284   <c>http</c>
3285   <c>reserved</c>
3286   <c>
3287      <xref target="header.field.registration"/>
3288   </c>
3289</texttable>
3290<t>
3291   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3292</t>
3293</section>
3294
3295<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3296<t>
3297   IANA maintains the registry of URI Schemes <xref target="BCP115"/> at
3298   <eref target="http://www.iana.org/assignments/uri-schemes.html"/>.
3299</t>
3300<t>
3301   This document defines the following URI schemes, so their
3302   associated registry entries shall be updated according to the permanent
3303   registrations below:
3304</t>
3305<texttable align="left" suppress-title="true">
3306   <ttcol>URI Scheme</ttcol>
3307   <ttcol>Description</ttcol>
3308   <ttcol>Reference</ttcol>
3309
3310   <c>http</c>
3311   <c>Hypertext Transfer Protocol</c>
3312   <c><xref target="http.uri"/></c>
3313
3314   <c>https</c>
3315   <c>Hypertext Transfer Protocol Secure</c>
3316   <c><xref target="https.uri"/></c>
3317</texttable>
3318</section>
3319
3320<section title="Internet Media Type Registration" anchor="internet.media.type.http">
3321<t>
3322   This document serves as the specification for the Internet media types
3323   "message/http" and "application/http". The following is to be registered with
3324   IANA (see <xref target="BCP13"/>).
3325</t>
3326<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
3327<iref item="Media Type" subitem="message/http" primary="true"/>
3328<iref item="message/http Media Type" primary="true"/>
3329<t>
3330   The message/http type can be used to enclose a single HTTP request or
3331   response message, provided that it obeys the MIME restrictions for all
3332   "message" types regarding line length and encodings.
3333</t>
3334<t>
3335  <list style="hanging" x:indent="12em">
3336    <t hangText="Type name:">
3337      message
3338    </t>
3339    <t hangText="Subtype name:">
3340      http
3341    </t>
3342    <t hangText="Required parameters:">
3343      none
3344    </t>
3345    <t hangText="Optional parameters:">
3346      version, msgtype
3347      <list style="hanging">
3348        <t hangText="version:">
3349          The HTTP-version number of the enclosed message
3350          (e.g., "1.1"). If not present, the version can be
3351          determined from the first line of the body.
3352        </t>
3353        <t hangText="msgtype:">
3354          The message type &mdash; "request" or "response". If not
3355          present, the type can be determined from the first
3356          line of the body.
3357        </t>
3358      </list>
3359    </t>
3360    <t hangText="Encoding considerations:">
3361      only "7bit", "8bit", or "binary" are permitted
3362    </t>
3363    <t hangText="Security considerations:">
3364      none
3365    </t>
3366    <t hangText="Interoperability considerations:">
3367      none
3368    </t>
3369    <t hangText="Published specification:">
3370      This specification (see <xref target="internet.media.type.message.http"/>).
3371    </t>
3372    <t hangText="Applications that use this media type:">
3373    </t>
3374    <t hangText="Additional information:">
3375      <list style="hanging">
3376        <t hangText="Magic number(s):">none</t>
3377        <t hangText="File extension(s):">none</t>
3378        <t hangText="Macintosh file type code(s):">none</t>
3379      </list>
3380    </t>
3381    <t hangText="Person and email address to contact for further information:">
3382      See Authors Section.
3383    </t>
3384    <t hangText="Intended usage:">
3385      COMMON
3386    </t>
3387    <t hangText="Restrictions on usage:">
3388      none
3389    </t>
3390    <t hangText="Author:">
3391      See Authors Section.
3392    </t>
3393    <t hangText="Change controller:">
3394      IESG
3395    </t>
3396  </list>
3397</t>
3398</section>
3399<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
3400<iref item="Media Type" subitem="application/http" primary="true"/>
3401<iref item="application/http Media Type" primary="true"/>
3402<t>
3403   The application/http type can be used to enclose a pipeline of one or more
3404   HTTP request or response messages (not intermixed).
3405</t>
3406<t>
3407  <list style="hanging" x:indent="12em">
3408    <t hangText="Type name:">
3409      application
3410    </t>
3411    <t hangText="Subtype name:">
3412      http
3413    </t>
3414    <t hangText="Required parameters:">
3415      none
3416    </t>
3417    <t hangText="Optional parameters:">
3418      version, msgtype
3419      <list style="hanging">
3420        <t hangText="version:">
3421          The HTTP-version number of the enclosed messages
3422          (e.g., "1.1"). If not present, the version can be
3423          determined from the first line of the body.
3424        </t>
3425        <t hangText="msgtype:">
3426          The message type &mdash; "request" or "response". If not
3427          present, the type can be determined from the first
3428          line of the body.
3429        </t>
3430      </list>
3431    </t>
3432    <t hangText="Encoding considerations:">
3433      HTTP messages enclosed by this type
3434      are in "binary" format; use of an appropriate
3435      Content-Transfer-Encoding is required when
3436      transmitted via E-mail.
3437    </t>
3438    <t hangText="Security considerations:">
3439      none
3440    </t>
3441    <t hangText="Interoperability considerations:">
3442      none
3443    </t>
3444    <t hangText="Published specification:">
3445      This specification (see <xref target="internet.media.type.application.http"/>).
3446    </t>
3447    <t hangText="Applications that use this media type:">
3448    </t>
3449    <t hangText="Additional information:">
3450      <list style="hanging">
3451        <t hangText="Magic number(s):">none</t>
3452        <t hangText="File extension(s):">none</t>
3453        <t hangText="Macintosh file type code(s):">none</t>
3454      </list>
3455    </t>
3456    <t hangText="Person and email address to contact for further information:">
3457      See Authors Section.
3458    </t>
3459    <t hangText="Intended usage:">
3460      COMMON
3461    </t>
3462    <t hangText="Restrictions on usage:">
3463      none
3464    </t>
3465    <t hangText="Author:">
3466      See Authors Section.
3467    </t>
3468    <t hangText="Change controller:">
3469      IESG
3470    </t>
3471  </list>
3472</t>
3473</section>
3474</section>
3475
3476<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3477<t>
3478   The HTTP Transfer Coding Registry defines the name space for transfer
3479   coding names. It is maintained at <eref target="http://www.iana.org/assignments/http-parameters"/>.
3480</t>
3481
3482<section title="Procedure" anchor="transfer.coding.registry.procedure">
3483<t>
3484   Registrations &MUST; include the following fields:
3485   <list style="symbols">
3486     <t>Name</t>
3487     <t>Description</t>
3488     <t>Pointer to specification text</t>
3489   </list>
3490</t>
3491<t>
3492   Names of transfer codings &MUST-NOT; overlap with names of content codings
3493   (&content-codings;) unless the encoding transformation is identical, as
3494   is the case for the compression codings defined in
3495   <xref target="compression.codings"/>.
3496</t>
3497<t>
3498   Values to be added to this name space require IETF Review (see
3499   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3500   conform to the purpose of transfer coding defined in this specification.
3501</t>
3502<t>
3503   Use of program names for the identification of encoding formats
3504   is not desirable and is discouraged for future encodings.
3505</t>
3506</section>
3507
3508<section title="Registration" anchor="transfer.coding.registration">
3509<t>
3510   The HTTP Transfer Coding Registry shall be updated with the registrations
3511   below:
3512</t>
3513<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3514   <ttcol>Name</ttcol>
3515   <ttcol>Description</ttcol>
3516   <ttcol>Reference</ttcol>
3517   <c>chunked</c>
3518   <c>Transfer in a series of chunks</c>
3519   <c>
3520      <xref target="chunked.encoding"/>
3521   </c>
3522   <c>compress</c>
3523   <c>UNIX "compress" program method</c>
3524   <c>
3525      <xref target="compress.coding"/>
3526   </c>
3527   <c>deflate</c>
3528   <c>"deflate" compression mechanism (<xref target="RFC1951"/>) used inside
3529   the "zlib" data format (<xref target="RFC1950"/>)
3530   </c>
3531   <c>
3532      <xref target="deflate.coding"/>
3533   </c>
3534   <c>gzip</c>
3535   <c>Same as GNU zip <xref target="RFC1952"/></c>
3536   <c>
3537      <xref target="gzip.coding"/>
3538   </c>
3539</texttable>
3540</section>
3541</section>
3542
3543<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3544<t>
3545   The HTTP Upgrade Token Registry defines the name space for protocol-name
3546   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3547   field. The registry is maintained at <eref target="http://www.iana.org/assignments/http-upgrade-tokens"/>.
3548</t>
3549
3550<section title="Procedure" anchor="upgrade.token.registry.procedure">   
3551<t>
3552   Each registered protocol name is associated with contact information
3553   and an optional set of specifications that details how the connection
3554   will be processed after it has been upgraded.
3555</t>
3556<t>
3557   Registrations happen on a "First Come First Served" basis (see
3558   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3559   following rules:
3560  <list style="numbers">
3561    <t>A protocol-name token, once registered, stays registered forever.</t>
3562    <t>The registration &MUST; name a responsible party for the
3563       registration.</t>
3564    <t>The registration &MUST; name a point of contact.</t>
3565    <t>The registration &MAY; name a set of specifications associated with
3566       that token. Such specifications need not be publicly available.</t>
3567    <t>The registration &SHOULD; name a set of expected "protocol-version"
3568       tokens associated with that token at the time of registration.</t>
3569    <t>The responsible party &MAY; change the registration at any time.
3570       The IANA will keep a record of all such changes, and make them
3571       available upon request.</t>
3572    <t>The IESG &MAY; reassign responsibility for a protocol token.
3573       This will normally only be used in the case when a
3574       responsible party cannot be contacted.</t>
3575  </list>
3576</t>
3577<t>
3578   This registration procedure for HTTP Upgrade Tokens replaces that
3579   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3580</t>
3581</section>
3582
3583<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3584<t>
3585   The HTTP Upgrade Token Registry shall be updated with the registration
3586   below:
3587</t>
3588<texttable align="left" suppress-title="true">
3589   <ttcol>Value</ttcol>
3590   <ttcol>Description</ttcol>
3591   <ttcol>Expected Version Tokens</ttcol>
3592   <ttcol>Reference</ttcol>
3593
3594   <c>HTTP</c>
3595   <c>Hypertext Transfer Protocol</c>
3596   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3597   <c><xref target="http.version"/></c>
3598</texttable>
3599<t>
3600   The responsible party is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3601</t>
3602</section>
3603</section>
3604
3605</section>
3606
3607<section title="Security Considerations" anchor="security.considerations">
3608<t>
3609   This section is meant to inform developers, information providers, and
3610   users of known security concerns relevant to HTTP/1.1 message syntax,
3611   parsing, and routing.
3612</t>
3613
3614<section title="DNS-related Attacks" anchor="dns.related.attacks">
3615<t>
3616   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3617   generally prone to security attacks based on the deliberate misassociation
3618   of IP addresses and DNS names not protected by DNSSEC. Clients need to be
3619   cautious in assuming the validity of an IP number/DNS name association unless
3620   the response is protected by DNSSEC (<xref target="RFC4033"/>).
3621</t>
3622</section>
3623
3624<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3625<t>
3626   By their very nature, HTTP intermediaries are men-in-the-middle, and
3627   represent an opportunity for man-in-the-middle attacks. Compromise of
3628   the systems on which the intermediaries run can result in serious security
3629   and privacy problems. Intermediaries have access to security-related
3630   information, personal information about individual users and
3631   organizations, and proprietary information belonging to users and
3632   content providers. A compromised intermediary, or an intermediary
3633   implemented or configured without regard to security and privacy
3634   considerations, might be used in the commission of a wide range of
3635   potential attacks.
3636</t>
3637<t>
3638   Intermediaries that contain a shared cache are especially vulnerable
3639   to cache poisoning attacks.
3640</t>
3641<t>
3642   Implementers need to consider the privacy and security
3643   implications of their design and coding decisions, and of the
3644   configuration options they provide to operators (especially the
3645   default configuration).
3646</t>
3647<t>
3648   Users need to be aware that intermediaries are no more trustworthy than
3649   the people who run them; HTTP itself cannot solve this problem.
3650</t>
3651</section>
3652
3653<section title="Buffer Overflows" anchor="attack.protocol.element.size.overflows">
3654<t>
3655   Because HTTP uses mostly textual, character-delimited fields, attackers can
3656   overflow buffers in implementations, and/or perform a Denial of Service
3657   against implementations that accept fields with unlimited lengths.
3658</t>
3659<t>
3660   To promote interoperability, this specification makes specific
3661   recommendations for minimum size limits on request-line
3662   (<xref target="request.line"/>)
3663   and blocks of header fields (<xref target="header.fields"/>). These are
3664   minimum recommendations, chosen to be supportable even by implementations
3665   with limited resources; it is expected that most implementations will
3666   choose substantially higher limits.
3667</t>
3668<t>
3669   This specification also provides a way for servers to reject messages that
3670   have request-targets that are too long (&status-414;) or request entities
3671   that are too large (&status-4xx;). Additional status codes related to
3672   capacity limits have been defined by extensions to HTTP
3673   <xref target="RFC6585"/>.
3674</t>
3675<t>
3676   Recipients &SHOULD; carefully limit the extent to which they read other
3677   fields, including (but not limited to) request methods, response status
3678   phrases, header field-names, and body chunks, so as to avoid denial of
3679   service attacks without impeding interoperability.
3680</t>
3681</section>
3682
3683<section title="Message Integrity" anchor="message.integrity">
3684<t>
3685   HTTP does not define a specific mechanism for ensuring message integrity,
3686   instead relying on the error-detection ability of underlying transport
3687   protocols and the use of length or chunk-delimited framing to detect
3688   completeness. Additional integrity mechanisms, such as hash functions or
3689   digital signatures applied to the content, can be selectively added to
3690   messages via extensible metadata header fields. Historically, the lack of
3691   a single integrity mechanism has been justified by the informal nature of
3692   most HTTP communication.  However, the prevalence of HTTP as an information
3693   access mechanism has resulted in its increasing use within environments
3694   where verification of message integrity is crucial.
3695</t>
3696<t>
3697   User agents are encouraged to implement configurable means for detecting
3698   and reporting failures of message integrity such that those means can be
3699   enabled within environments for which integrity is necessary. For example,
3700   a browser being used to view medical history or drug interaction
3701   information needs to indicate to the user when such information is detected
3702   by the protocol to be incomplete, expired, or corrupted during transfer.
3703   Such mechanisms might be selectively enabled via user agent extensions or
3704   the presence of message integrity metadata in a response.
3705   At a minimum, user agents ought to provide some indication that allows a
3706   user to distinguish between a complete and incomplete response message
3707   (<xref target="incomplete.messages"/>) when such verification is desired.
3708</t>
3709</section>
3710
3711<section title="Server Log Information" anchor="abuse.of.server.log.information">
3712<t>
3713   A server is in the position to save personal data about a user's requests
3714   over time, which might identify their reading patterns or subjects of
3715   interest.  In particular, log information gathered at an intermediary
3716   often contains a history of user agent interaction, across a multitude
3717   of sites, that can be traced to individual users.
3718</t>
3719<t>
3720   HTTP log information is confidential in nature; its handling is often
3721   constrained by laws and regulations.  Log information needs to be securely
3722   stored and appropriate guidelines followed for its analysis.
3723   Anonymization of personal information within individual entries helps,
3724   but is generally not sufficient to prevent real log traces from being
3725   re-identified based on correlation with other access characteristics.
3726   As such, access traces that are keyed to a specific client should not
3727   be published even if the key is pseudonymous.
3728</t>
3729<t>
3730   To minimize the risk of theft or accidental publication, log information
3731   should be purged of personally identifiable information, including
3732   user identifiers, IP addresses, and user-provided query parameters,
3733   as soon as that information is no longer necessary to support operational
3734   needs for security, auditing, or fraud control.
3735</t>
3736</section>
3737</section>
3738
3739<section title="Acknowledgments" anchor="acks">
3740<t>
3741   This edition of HTTP/1.1 builds on the many contributions that went into
3742   <xref target="RFC1945" format="none">RFC 1945</xref>,
3743   <xref target="RFC2068" format="none">RFC 2068</xref>,
3744   <xref target="RFC2145" format="none">RFC 2145</xref>, and
3745   <xref target="RFC2616" format="none">RFC 2616</xref>, including
3746   substantial contributions made by the previous authors, editors, and
3747   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
3748   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
3749   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
3750</t>
3751<t>
3752   Since 1999, the following contributors have helped improve the HTTP
3753   specification by reporting bugs, asking smart questions, drafting or
3754   reviewing text, and evaluating open issues:
3755</t>
3756<?BEGININC acks ?>
3757<t>Adam Barth,
3758Adam Roach,
3759Addison Phillips,
3760Adrian Chadd,
3761Adrien W. de Croy,
3762Alan Ford,
3763Alan Ruttenberg,
3764Albert Lunde,
3765Alek Storm,
3766Alex Rousskov,
3767Alexandre Morgaut,
3768Alexey Melnikov,
3769Alisha Smith,
3770Amichai Rothman,
3771Amit Klein,
3772Amos Jeffries,
3773Andreas Maier,
3774Andreas Petersson,
3775Anil Sharma,
3776Anne van Kesteren,
3777Anthony Bryan,
3778Asbjorn Ulsberg,
3779Ashok Kumar,
3780Balachander Krishnamurthy,
3781Barry Leiba,
3782Ben Laurie,
3783Benjamin Carlyle,
3784Benjamin Niven-Jenkins,
3785Bil Corry,
3786Bill Burke,
3787Bjoern Hoehrmann,
3788Bob Scheifler,
3789Boris Zbarsky,
3790Brett Slatkin,
3791Brian Kell,
3792Brian McBarron,
3793Brian Pane,
3794Brian Raymor,
3795Brian Smith,
3796Bryce Nesbitt,
3797Cameron Heavon-Jones,
3798Carl Kugler,
3799Carsten Bormann,
3800Charles Fry,
3801Chris Newman,
3802Cyrus Daboo,
3803Dale Robert Anderson,
3804Dan Wing,
3805Dan Winship,
3806Daniel Stenberg,
3807Darrel Miller,
3808Dave Cridland,
3809Dave Crocker,
3810Dave Kristol,
3811Dave Thaler,
3812David Booth,
3813David Singer,
3814David W. Morris,
3815Diwakar Shetty,
3816Dmitry Kurochkin,
3817Drummond Reed,
3818Duane Wessels,
3819Edward Lee,
3820Eitan Adler,
3821Eliot Lear,
3822Eran Hammer-Lahav,
3823Eric D. Williams,
3824Eric J. Bowman,
3825Eric Lawrence,
3826Eric Rescorla,
3827Erik Aronesty,
3828Evan Prodromou,
3829Felix Geisendoerfer,
3830Florian Weimer,
3831Frank Ellermann,
3832Fred Bohle,
3833Frederic Kayser,
3834Gabriel Montenegro,
3835Geoffrey Sneddon,
3836Gervase Markham,
3837Grahame Grieve,
3838Greg Wilkins,
3839Grzegorz Calkowski,
3840Harald Tveit Alvestrand,
3841Harry Halpin,
3842Helge Hess,
3843Henrik Nordstrom,
3844Henry S. Thompson,
3845Henry Story,
3846Herbert van de Sompel,
3847Herve Ruellan,
3848Howard Melman,
3849Hugo Haas,
3850Ian Fette,
3851Ian Hickson,
3852Ido Safruti,
3853Ilari Liusvaara,
3854Ilya Grigorik,
3855Ingo Struck,
3856J. Ross Nicoll,
3857James Cloos,
3858James H. Manger,
3859James Lacey,
3860James M. Snell,
3861Jamie Lokier,
3862Jan Algermissen,
3863Jeff Hodges (who came up with the term 'effective Request-URI'),
3864Jeff Pinner,
3865Jeff Walden,
3866Jim Luther,
3867Jitu Padhye,
3868Joe D. Williams,
3869Joe Gregorio,
3870Joe Orton,
3871John C. Klensin,
3872John C. Mallery,
3873John Cowan,
3874John Kemp,
3875John Panzer,
3876John Schneider,
3877John Stracke,
3878John Sullivan,
3879Jonas Sicking,
3880Jonathan A. Rees,
3881Jonathan Billington,
3882Jonathan Moore,
3883Jonathan Silvera,
3884Jordi Ros,
3885Joris Dobbelsteen,
3886Josh Cohen,
3887Julien Pierre,
3888Jungshik Shin,
3889Justin Chapweske,
3890Justin Erenkrantz,
3891Justin James,
3892Kalvinder Singh,
3893Karl Dubost,
3894Keith Hoffman,
3895Keith Moore,
3896Ken Murchison,
3897Koen Holtman,
3898Konstantin Voronkov,
3899Kris Zyp,
3900Lisa Dusseault,
3901Maciej Stachowiak,
3902Manu Sporny,
3903Marc Schneider,
3904Marc Slemko,
3905Mark Baker,
3906Mark Pauley,
3907Mark Watson,
3908Markus Isomaki,
3909Markus Lanthaler,
3910Martin J. Duerst,
3911Martin Musatov,
3912Martin Nilsson,
3913Martin Thomson,
3914Matt Lynch,
3915Matthew Cox,
3916Max Clark,
3917Michael Burrows,
3918Michael Hausenblas,
3919Mike Amundsen,
3920Mike Belshe,
3921Mike Kelly,
3922Mike Schinkel,
3923Miles Sabin,
3924Murray S. Kucherawy,
3925Mykyta Yevstifeyev,
3926Nathan Rixham,
3927Nicholas Shanks,
3928Nico Williams,
3929Nicolas Alvarez,
3930Nicolas Mailhot,
3931Noah Slater,
3932Osama Mazahir,
3933Pablo Castro,
3934Pat Hayes,
3935Patrick R. McManus,
3936Paul E. Jones,
3937Paul Hoffman,
3938Paul Marquess,
3939Peter Lepeska,
3940Peter Occil,
3941Peter Saint-Andre,
3942Peter Watkins,
3943Phil Archer,
3944Philippe Mougin,
3945Phillip Hallam-Baker,
3946Piotr Dobrogost,
3947Poul-Henning Kamp,
3948Preethi Natarajan,
3949Rajeev Bector,
3950Ray Polk,
3951Reto Bachmann-Gmuer,
3952Richard Cyganiak,
3953Robby Simpson,
3954Robert Brewer,
3955Robert Collins,
3956Robert Mattson,
3957Robert O'Callahan,
3958Robert Olofsson,
3959Robert Sayre,
3960Robert Siemer,
3961Robert de Wilde,
3962Roberto Javier Godoy,
3963Roberto Peon,
3964Roland Zink,
3965Ronny Widjaja,
3966S. Mike Dierken,
3967Salvatore Loreto,
3968Sam Johnston,
3969Sam Ruby,
3970Scott Lawrence (who maintained the original issues list),
3971Sean B. Palmer,
3972Shane McCarron,
3973Stefan Eissing,
3974Stefan Tilkov,
3975Stefanos Harhalakis,
3976Stephane Bortzmeyer,
3977Stephen Farrell,
3978Stephen Ludin,
3979Stuart Williams,
3980Subbu Allamaraju,
3981Sylvain Hellegouarch,
3982Tapan Divekar,
3983Tatsuya Hayashi,
3984Ted Hardie,
3985Thomas Broyer,
3986Thomas Fossati,
3987Thomas Maslen,
3988Thomas Nordin,
3989Thomas Roessler,
3990Tim Bray,
3991Tim Morgan,
3992Tim Olsen,
3993Tom Zhou,
3994Travis Snoozy,
3995Tyler Close,
3996Vincent Murphy,
3997Wenbo Zhu,
3998Werner Baumann,
3999Wilbur Streett,
4000Wilfredo Sanchez Vega,
4001William A. Rowe Jr.,
4002William Chan,
4003Willy Tarreau,
4004Xiaoshu Wang,
4005Yaron Goland,
4006Yngve Nysaeter Pettersen,
4007Yoav Nir,
4008Yogesh Bang,
4009Yutaka Oiwa,
4010Yves Lafon (long-time member of the editor team),
4011Zed A. Shaw, and
4012Zhong Yu.
4013</t>
4014<?ENDINC acks ?>
4015<t>
4016   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
4017   acknowledgements from prior revisions.
4018</t>
4019</section>
4020
4021</middle>
4022<back>
4023
4024<references title="Normative References">
4025
4026<reference anchor="Part2">
4027  <front>
4028    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
4029    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4030      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4031      <address><email>fielding@gbiv.com</email></address>
4032    </author>
4033    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4034      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4035      <address><email>julian.reschke@greenbytes.de</email></address>
4036    </author>
4037    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4038  </front>
4039  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
4040  <x:source href="p2-semantics.xml" basename="p2-semantics">
4041    <x:defines>1xx (Informational)</x:defines>
4042    <x:defines>1xx</x:defines>
4043    <x:defines>100 (Continue)</x:defines>
4044    <x:defines>101 (Switching Protocols)</x:defines>
4045    <x:defines>2xx (Successful)</x:defines>
4046    <x:defines>2xx</x:defines>
4047    <x:defines>200 (OK)</x:defines>
4048    <x:defines>204 (No Content)</x:defines>
4049    <x:defines>3xx (Redirection)</x:defines>
4050    <x:defines>3xx</x:defines>
4051    <x:defines>301 (Moved Permanently)</x:defines>
4052    <x:defines>4xx (Client Error)</x:defines>
4053    <x:defines>4xx</x:defines>
4054    <x:defines>400 (Bad Request)</x:defines>
4055    <x:defines>411 (Length Required)</x:defines>
4056    <x:defines>414 (URI Too Long)</x:defines>
4057    <x:defines>417 (Expectation Failed)</x:defines>
4058    <x:defines>426 (Upgrade Required)</x:defines>
4059    <x:defines>501 (Not Implemented)</x:defines>
4060    <x:defines>502 (Bad Gateway)</x:defines>
4061    <x:defines>505 (HTTP Version Not Supported)</x:defines>
4062    <x:defines>Allow</x:defines>
4063    <x:defines>Content-Encoding</x:defines>
4064    <x:defines>Content-Location</x:defines>
4065    <x:defines>Content-Type</x:defines>
4066    <x:defines>Date</x:defines>
4067    <x:defines>Expect</x:defines>
4068    <x:defines>Location</x:defines>
4069    <x:defines>Server</x:defines>
4070    <x:defines>User-Agent</x:defines>
4071  </x:source>
4072</reference>
4073
4074<reference anchor="Part4">
4075  <front>
4076    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
4077    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
4078      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4079      <address><email>fielding@gbiv.com</email></address>
4080    </author>
4081    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
4082      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4083      <address><email>julian.reschke@greenbytes.de</email></address>
4084    </author>
4085    <date month="&ID-MONTH;" year="&ID-YEAR;" />
4086  </front>
4087  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
4088  <x:source basename="p4-conditional" href="p4-conditional.xml">
4089    <x:defines>304 (Not Modified)</x:defines>
4090    <x:defines>ETag</x:defines>
4091    <x:defines>Last-Modified</x:defines>
4092  </x:source>
4093</reference>
4094
4095<reference anchor="Part5">
4096  <front>
4097    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
4098    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4099      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4100      <address><email>fielding@gbiv.com</email></address>
4101    </author>
4102    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
4103      <organization abbrev="W3C">World Wide Web Consortium</organization>
4104      <address><email>ylafon@w3.org</email></address>
4105    </author>
4106    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4107      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4108      <address><email>julian.reschke@greenbytes.de</email></address>
4109    </author>
4110    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4111  </front>
4112  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4113  <x:source href="p5-range.xml" basename="p5-range">
4114    <x:defines>Content-Range</x:defines>
4115  </x:source>
4116</reference>
4117
4118<reference anchor="Part6">
4119  <front>
4120    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4121    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4122      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4123      <address><email>fielding@gbiv.com</email></address>
4124    </author>
4125    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4126      <organization>Akamai</organization>
4127      <address><email>mnot@mnot.net</email></address>
4128    </author>
4129    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4130      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4131      <address><email>julian.reschke@greenbytes.de</email></address>
4132    </author>
4133    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4134  </front>
4135  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4136  <x:source href="p6-cache.xml" basename="p6-cache">
4137    <x:defines>Cache-Control</x:defines>
4138    <x:defines>Expires</x:defines>
4139  </x:source>
4140</reference>
4141
4142<reference anchor="Part7">
4143  <front>
4144    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4145    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4146      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4147      <address><email>fielding@gbiv.com</email></address>
4148    </author>
4149    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4150      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4151      <address><email>julian.reschke@greenbytes.de</email></address>
4152    </author>
4153    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4154  </front>
4155  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4156  <x:source href="p7-auth.xml" basename="p7-auth">
4157    <x:defines>Proxy-Authenticate</x:defines>
4158    <x:defines>Proxy-Authorization</x:defines>
4159  </x:source>
4160</reference>
4161
4162<reference anchor="RFC5234">
4163  <front>
4164    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4165    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4166      <organization>Brandenburg InternetWorking</organization>
4167      <address>
4168        <email>dcrocker@bbiw.net</email>
4169      </address> 
4170    </author>
4171    <author initials="P." surname="Overell" fullname="Paul Overell">
4172      <organization>THUS plc.</organization>
4173      <address>
4174        <email>paul.overell@thus.net</email>
4175      </address>
4176    </author>
4177    <date month="January" year="2008"/>
4178  </front>
4179  <seriesInfo name="STD" value="68"/>
4180  <seriesInfo name="RFC" value="5234"/>
4181</reference>
4182
4183<reference anchor="RFC2119">
4184  <front>
4185    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4186    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4187      <organization>Harvard University</organization>
4188      <address><email>sob@harvard.edu</email></address>
4189    </author>
4190    <date month="March" year="1997"/>
4191  </front>
4192  <seriesInfo name="BCP" value="14"/>
4193  <seriesInfo name="RFC" value="2119"/>
4194</reference>
4195
4196<reference anchor="RFC3986">
4197 <front>
4198  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4199  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4200    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4201    <address>
4202       <email>timbl@w3.org</email>
4203       <uri>http://www.w3.org/People/Berners-Lee/</uri>
4204    </address>
4205  </author>
4206  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4207    <organization abbrev="Day Software">Day Software</organization>
4208    <address>
4209      <email>fielding@gbiv.com</email>
4210      <uri>http://roy.gbiv.com/</uri>
4211    </address>
4212  </author>
4213  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4214    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4215    <address>
4216      <email>LMM@acm.org</email>
4217      <uri>http://larry.masinter.net/</uri>
4218    </address>
4219  </author>
4220  <date month='January' year='2005'></date>
4221 </front>
4222 <seriesInfo name="STD" value="66"/>
4223 <seriesInfo name="RFC" value="3986"/>
4224</reference>
4225
4226<reference anchor="RFC0793">
4227  <front>
4228    <title>Transmission Control Protocol</title>
4229    <author initials='J.' surname='Postel' fullname='Jon Postel'>
4230      <organization>University of Southern California (USC)/Information Sciences Institute</organization>
4231    </author>
4232    <date year='1981' month='September' />
4233  </front>
4234  <seriesInfo name='STD' value='7' />
4235  <seriesInfo name='RFC' value='793' />
4236</reference>
4237
4238<reference anchor="USASCII">
4239  <front>
4240    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4241    <author>
4242      <organization>American National Standards Institute</organization>
4243    </author>
4244    <date year="1986"/>
4245  </front>
4246  <seriesInfo name="ANSI" value="X3.4"/>
4247</reference>
4248
4249<reference anchor="RFC1950">
4250  <front>
4251    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4252    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4253      <organization>Aladdin Enterprises</organization>
4254      <address><email>ghost@aladdin.com</email></address>
4255    </author>
4256    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4257    <date month="May" year="1996"/>
4258  </front>
4259  <seriesInfo name="RFC" value="1950"/>
4260  <!--<annotation>
4261    RFC 1950 is an Informational RFC, thus it might be less stable than
4262    this specification. On the other hand, this downward reference was
4263    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4264    therefore it is unlikely to cause problems in practice. See also
4265    <xref target="BCP97"/>.
4266  </annotation>-->
4267</reference>
4268
4269<reference anchor="RFC1951">
4270  <front>
4271    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4272    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4273      <organization>Aladdin Enterprises</organization>
4274      <address><email>ghost@aladdin.com</email></address>
4275    </author>
4276    <date month="May" year="1996"/>
4277  </front>
4278  <seriesInfo name="RFC" value="1951"/>
4279  <!--<annotation>
4280    RFC 1951 is an Informational RFC, thus it might be less stable than
4281    this specification. On the other hand, this downward reference was
4282    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4283    therefore it is unlikely to cause problems in practice. See also
4284    <xref target="BCP97"/>.
4285  </annotation>-->
4286</reference>
4287
4288<reference anchor="RFC1952">
4289  <front>
4290    <title>GZIP file format specification version 4.3</title>
4291    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4292      <organization>Aladdin Enterprises</organization>
4293      <address><email>ghost@aladdin.com</email></address>
4294    </author>
4295    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4296      <address><email>gzip@prep.ai.mit.edu</email></address>
4297    </author>
4298    <author initials="M." surname="Adler" fullname="Mark Adler">
4299      <address><email>madler@alumni.caltech.edu</email></address>
4300    </author>
4301    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4302      <address><email>ghost@aladdin.com</email></address>
4303    </author>
4304    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4305      <address><email>randeg@alumni.rpi.edu</email></address>
4306    </author>
4307    <date month="May" year="1996"/>
4308  </front>
4309  <seriesInfo name="RFC" value="1952"/>
4310  <!--<annotation>
4311    RFC 1952 is an Informational RFC, thus it might be less stable than
4312    this specification. On the other hand, this downward reference was
4313    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4314    therefore it is unlikely to cause problems in practice. See also
4315    <xref target="BCP97"/>.
4316  </annotation>-->
4317</reference>
4318
4319</references>
4320
4321<references title="Informative References">
4322
4323<reference anchor="ISO-8859-1">
4324  <front>
4325    <title>
4326     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4327    </title>
4328    <author>
4329      <organization>International Organization for Standardization</organization>
4330    </author>
4331    <date year="1998"/>
4332  </front>
4333  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4334</reference>
4335
4336<reference anchor='RFC1919'>
4337  <front>
4338    <title>Classical versus Transparent IP Proxies</title>
4339    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4340      <address><email>mchatel@pax.eunet.ch</email></address>
4341    </author>
4342    <date year='1996' month='March' />
4343  </front>
4344  <seriesInfo name='RFC' value='1919' />
4345</reference>
4346
4347<reference anchor="RFC1945">
4348  <front>
4349    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4350    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4351      <organization>MIT, Laboratory for Computer Science</organization>
4352      <address><email>timbl@w3.org</email></address>
4353    </author>
4354    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4355      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4356      <address><email>fielding@ics.uci.edu</email></address>
4357    </author>
4358    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4359      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4360      <address><email>frystyk@w3.org</email></address>
4361    </author>
4362    <date month="May" year="1996"/>
4363  </front>
4364  <seriesInfo name="RFC" value="1945"/>
4365</reference>
4366
4367<reference anchor="RFC2045">
4368  <front>
4369    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4370    <author initials="N." surname="Freed" fullname="Ned Freed">
4371      <organization>Innosoft International, Inc.</organization>
4372      <address><email>ned@innosoft.com</email></address>
4373    </author>
4374    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4375      <organization>First Virtual Holdings</organization>
4376      <address><email>nsb@nsb.fv.com</email></address>
4377    </author>
4378    <date month="November" year="1996"/>
4379  </front>
4380  <seriesInfo name="RFC" value="2045"/>
4381</reference>
4382
4383<reference anchor="RFC2047">
4384  <front>
4385    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4386    <author initials="K." surname="Moore" fullname="Keith Moore">
4387      <organization>University of Tennessee</organization>
4388      <address><email>moore@cs.utk.edu</email></address>
4389    </author>
4390    <date month="November" year="1996"/>
4391  </front>
4392  <seriesInfo name="RFC" value="2047"/>
4393</reference>
4394
4395<reference anchor="RFC2068">
4396  <front>
4397    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4398    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
4399      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4400      <address><email>fielding@ics.uci.edu</email></address>
4401    </author>
4402    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4403      <organization>MIT Laboratory for Computer Science</organization>
4404      <address><email>jg@w3.org</email></address>
4405    </author>
4406    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4407      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4408      <address><email>mogul@wrl.dec.com</email></address>
4409    </author>
4410    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4411      <organization>MIT Laboratory for Computer Science</organization>
4412      <address><email>frystyk@w3.org</email></address>
4413    </author>
4414    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4415      <organization>MIT Laboratory for Computer Science</organization>
4416      <address><email>timbl@w3.org</email></address>
4417    </author>
4418    <date month="January" year="1997"/>
4419  </front>
4420  <seriesInfo name="RFC" value="2068"/>
4421</reference>
4422
4423<reference anchor="RFC2145">
4424  <front>
4425    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4426    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4427      <organization>Western Research Laboratory</organization>
4428      <address><email>mogul@wrl.dec.com</email></address>
4429    </author>
4430    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4431      <organization>Department of Information and Computer Science</organization>
4432      <address><email>fielding@ics.uci.edu</email></address>
4433    </author>
4434    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4435      <organization>MIT Laboratory for Computer Science</organization>
4436      <address><email>jg@w3.org</email></address>
4437    </author>
4438    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4439      <organization>W3 Consortium</organization>
4440      <address><email>frystyk@w3.org</email></address>
4441    </author>
4442    <date month="May" year="1997"/>
4443  </front>
4444  <seriesInfo name="RFC" value="2145"/>
4445</reference>
4446
4447<reference anchor="RFC2616">
4448  <front>
4449    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4450    <author initials="R." surname="Fielding" fullname="R. Fielding">
4451      <organization>University of California, Irvine</organization>
4452      <address><email>fielding@ics.uci.edu</email></address>
4453    </author>
4454    <author initials="J." surname="Gettys" fullname="J. Gettys">
4455      <organization>W3C</organization>
4456      <address><email>jg@w3.org</email></address>
4457    </author>
4458    <author initials="J." surname="Mogul" fullname="J. Mogul">
4459      <organization>Compaq Computer Corporation</organization>
4460      <address><email>mogul@wrl.dec.com</email></address>
4461    </author>
4462    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4463      <organization>MIT Laboratory for Computer Science</organization>
4464      <address><email>frystyk@w3.org</email></address>
4465    </author>
4466    <author initials="L." surname="Masinter" fullname="L. Masinter">
4467      <organization>Xerox Corporation</organization>
4468      <address><email>masinter@parc.xerox.com</email></address>
4469    </author>
4470    <author initials="P." surname="Leach" fullname="P. Leach">
4471      <organization>Microsoft Corporation</organization>
4472      <address><email>paulle@microsoft.com</email></address>
4473    </author>
4474    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4475      <organization>W3C</organization>
4476      <address><email>timbl@w3.org</email></address>
4477    </author>
4478    <date month="June" year="1999"/>
4479  </front>
4480  <seriesInfo name="RFC" value="2616"/>
4481</reference>
4482
4483<reference anchor='RFC2817'>
4484  <front>
4485    <title>Upgrading to TLS Within HTTP/1.1</title>
4486    <author initials='R.' surname='Khare' fullname='R. Khare'>
4487      <organization>4K Associates / UC Irvine</organization>
4488      <address><email>rohit@4K-associates.com</email></address>
4489    </author>
4490    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4491      <organization>Agranat Systems, Inc.</organization>
4492      <address><email>lawrence@agranat.com</email></address>
4493    </author>
4494    <date year='2000' month='May' />
4495  </front>
4496  <seriesInfo name='RFC' value='2817' />
4497</reference>
4498
4499<reference anchor='RFC2818'>
4500  <front>
4501    <title>HTTP Over TLS</title>
4502    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4503      <organization>RTFM, Inc.</organization>
4504      <address><email>ekr@rtfm.com</email></address>
4505    </author>
4506    <date year='2000' month='May' />
4507  </front>
4508  <seriesInfo name='RFC' value='2818' />
4509</reference>
4510
4511<reference anchor='RFC3040'>
4512  <front>
4513    <title>Internet Web Replication and Caching Taxonomy</title>
4514    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4515      <organization>Equinix, Inc.</organization>
4516    </author>
4517    <author initials='I.' surname='Melve' fullname='I. Melve'>
4518      <organization>UNINETT</organization>
4519    </author>
4520    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4521      <organization>CacheFlow Inc.</organization>
4522    </author>
4523    <date year='2001' month='January' />
4524  </front>
4525  <seriesInfo name='RFC' value='3040' />
4526</reference>
4527
4528<reference anchor='BCP90'>
4529  <front>
4530    <title>Registration Procedures for Message Header Fields</title>
4531    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4532      <organization>Nine by Nine</organization>
4533      <address><email>GK-IETF@ninebynine.org</email></address>
4534    </author>
4535    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4536      <organization>BEA Systems</organization>
4537      <address><email>mnot@pobox.com</email></address>
4538    </author>
4539    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4540      <organization>HP Labs</organization>
4541      <address><email>JeffMogul@acm.org</email></address>
4542    </author>
4543    <date year='2004' month='September' />
4544  </front>
4545  <seriesInfo name='BCP' value='90' />
4546  <seriesInfo name='RFC' value='3864' />
4547</reference>
4548
4549<reference anchor='RFC4033'>
4550  <front>
4551    <title>DNS Security Introduction and Requirements</title>
4552    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4553    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4554    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4555    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4556    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4557    <date year='2005' month='March' />
4558  </front>
4559  <seriesInfo name='RFC' value='4033' />
4560</reference>
4561
4562<reference anchor="BCP13">
4563  <front>
4564    <title>Media Type Specifications and Registration Procedures</title>
4565    <author initials="N." surname="Freed" fullname="Ned Freed">
4566      <organization>Oracle</organization>
4567      <address>
4568        <email>ned+ietf@mrochek.com</email>
4569      </address>
4570    </author>
4571    <author initials="J." surname="Klensin" fullname="John C. Klensin">
4572      <address>
4573        <email>john+ietf@jck.com</email>
4574      </address>
4575    </author>
4576    <author initials="T." surname="Hansen" fullname="Tony Hansen">
4577      <organization>AT&amp;T Laboratories</organization>
4578      <address>
4579        <email>tony+mtsuffix@maillennium.att.com</email>
4580      </address>
4581    </author>
4582    <date year="2013" month="January"/>
4583  </front>
4584  <seriesInfo name="BCP" value="13"/>
4585  <seriesInfo name="RFC" value="6838"/>
4586</reference>
4587
4588<reference anchor='BCP115'>
4589  <front>
4590    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4591    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4592      <organization>AT&amp;T Laboratories</organization>
4593      <address>
4594        <email>tony+urireg@maillennium.att.com</email>
4595      </address>
4596    </author>
4597    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4598      <organization>Qualcomm, Inc.</organization>
4599      <address>
4600        <email>hardie@qualcomm.com</email>
4601      </address>
4602    </author>
4603    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4604      <organization>Adobe Systems</organization>
4605      <address>
4606        <email>LMM@acm.org</email>
4607      </address>
4608    </author>
4609    <date year='2006' month='February' />
4610  </front>
4611  <seriesInfo name='BCP' value='115' />
4612  <seriesInfo name='RFC' value='4395' />
4613</reference>
4614
4615<reference anchor='RFC4559'>
4616  <front>
4617    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4618    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4619    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4620    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4621    <date year='2006' month='June' />
4622  </front>
4623  <seriesInfo name='RFC' value='4559' />
4624</reference>
4625
4626<reference anchor='RFC5226'>
4627  <front>
4628    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4629    <author initials='T.' surname='Narten' fullname='T. Narten'>
4630      <organization>IBM</organization>
4631      <address><email>narten@us.ibm.com</email></address>
4632    </author>
4633    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4634      <organization>Google</organization>
4635      <address><email>Harald@Alvestrand.no</email></address>
4636    </author>
4637    <date year='2008' month='May' />
4638  </front>
4639  <seriesInfo name='BCP' value='26' />
4640  <seriesInfo name='RFC' value='5226' />
4641</reference>
4642
4643<reference anchor='RFC5246'>
4644   <front>
4645      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4646      <author initials='T.' surname='Dierks' fullname='T. Dierks'>
4647         <organization />
4648      </author>
4649      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4650         <organization>RTFM, Inc.</organization>
4651      </author>
4652      <date year='2008' month='August' />
4653   </front>
4654   <seriesInfo name='RFC' value='5246' />
4655</reference>
4656
4657<reference anchor="RFC5322">
4658  <front>
4659    <title>Internet Message Format</title>
4660    <author initials="P." surname="Resnick" fullname="P. Resnick">
4661      <organization>Qualcomm Incorporated</organization>
4662    </author>
4663    <date year="2008" month="October"/>
4664  </front> 
4665  <seriesInfo name="RFC" value="5322"/>
4666</reference>
4667
4668<reference anchor="RFC6265">
4669  <front>
4670    <title>HTTP State Management Mechanism</title>
4671    <author initials="A." surname="Barth" fullname="Adam Barth">
4672      <organization abbrev="U.C. Berkeley">
4673        University of California, Berkeley
4674      </organization>
4675      <address><email>abarth@eecs.berkeley.edu</email></address>
4676    </author>
4677    <date year="2011" month="April" />
4678  </front>
4679  <seriesInfo name="RFC" value="6265"/>
4680</reference>
4681
4682<reference anchor='RFC6585'>
4683  <front>
4684    <title>Additional HTTP Status Codes</title>
4685    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4686      <organization>Rackspace</organization>
4687    </author>
4688    <author initials='R.' surname='Fielding' fullname='R. Fielding'>
4689      <organization>Adobe</organization>
4690    </author>
4691    <date year='2012' month='April' />
4692   </front>
4693   <seriesInfo name='RFC' value='6585' />
4694</reference>
4695
4696<!--<reference anchor='BCP97'>
4697  <front>
4698    <title>Handling Normative References to Standards-Track Documents</title>
4699    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
4700      <address>
4701        <email>klensin+ietf@jck.com</email>
4702      </address>
4703    </author>
4704    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
4705      <organization>MIT</organization>
4706      <address>
4707        <email>hartmans-ietf@mit.edu</email>
4708      </address>
4709    </author>
4710    <date year='2007' month='June' />
4711  </front>
4712  <seriesInfo name='BCP' value='97' />
4713  <seriesInfo name='RFC' value='4897' />
4714</reference>-->
4715
4716<reference anchor="Kri2001" target="http://arxiv.org/abs/cs.SE/0105018">
4717  <front>
4718    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
4719    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
4720    <date year="2001" month="November"/>
4721  </front>
4722  <seriesInfo name="ACM Transactions on Internet Technology" value="Vol. 1, #2"/>
4723</reference>
4724
4725</references>
4726
4727
4728<section title="HTTP Version History" anchor="compatibility">
4729<t>
4730   HTTP has been in use by the World-Wide Web global information initiative
4731   since 1990. The first version of HTTP, later referred to as HTTP/0.9,
4732   was a simple protocol for hypertext data transfer across the Internet
4733   with only a single request method (GET) and no metadata.
4734   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
4735   methods and MIME-like messaging that could include metadata about the data
4736   transferred and modifiers on the request/response semantics. However,
4737   HTTP/1.0 did not sufficiently take into consideration the effects of
4738   hierarchical proxies, caching, the need for persistent connections, or
4739   name-based virtual hosts. The proliferation of incompletely-implemented
4740   applications calling themselves "HTTP/1.0" further necessitated a
4741   protocol version change in order for two communicating applications
4742   to determine each other's true capabilities.
4743</t>
4744<t>
4745   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
4746   requirements that enable reliable implementations, adding only
4747   those new features that will either be safely ignored by an HTTP/1.0
4748   recipient or only sent when communicating with a party advertising
4749   conformance with HTTP/1.1.
4750</t>
4751<t>
4752   It is beyond the scope of a protocol specification to mandate
4753   conformance with previous versions. HTTP/1.1 was deliberately
4754   designed, however, to make supporting previous versions easy.
4755   We would expect a general-purpose HTTP/1.1 server to understand
4756   any valid request in the format of HTTP/1.0 and respond appropriately
4757   with an HTTP/1.1 message that only uses features understood (or
4758   safely ignored) by HTTP/1.0 clients.  Likewise, we would expect
4759   an HTTP/1.1 client to understand any valid HTTP/1.0 response.
4760</t>
4761<t>
4762   Since HTTP/0.9 did not support header fields in a request,
4763   there is no mechanism for it to support name-based virtual
4764   hosts (selection of resource by inspection of the <x:ref>Host</x:ref> header
4765   field).  Any server that implements name-based virtual hosts
4766   ought to disable support for HTTP/0.9.  Most requests that
4767   appear to be HTTP/0.9 are, in fact, badly constructed HTTP/1.x
4768   requests wherein a buggy client failed to properly encode
4769   linear whitespace found in a URI reference and placed in
4770   the request-target.
4771</t>
4772
4773<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
4774<t>
4775   This section summarizes major differences between versions HTTP/1.0
4776   and HTTP/1.1.
4777</t>
4778
4779<section title="Multi-homed Web Servers" anchor="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">
4780<t>
4781   The requirements that clients and servers support the <x:ref>Host</x:ref>
4782   header field (<xref target="header.host"/>), report an error if it is
4783   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
4784   are among the most important changes defined by HTTP/1.1.
4785</t>
4786<t>
4787   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4788   addresses and servers; there was no other established mechanism for
4789   distinguishing the intended server of a request than the IP address
4790   to which that request was directed. The <x:ref>Host</x:ref> header field was
4791   introduced during the development of HTTP/1.1 and, though it was
4792   quickly implemented by most HTTP/1.0 browsers, additional requirements
4793   were placed on all HTTP/1.1 requests in order to ensure complete
4794   adoption.  At the time of this writing, most HTTP-based services
4795   are dependent upon the Host header field for targeting requests.
4796</t>
4797</section>
4798
4799<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4800<t>
4801   In HTTP/1.0, each connection is established by the client prior to the
4802   request and closed by the server after sending the response. However, some
4803   implementations implement the explicitly negotiated ("Keep-Alive") version
4804   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
4805   target="RFC2068"/>.
4806</t>
4807<t>
4808   Some clients and servers might wish to be compatible with these previous
4809   approaches to persistent connections, by explicitly negotiating for them
4810   with a "Connection: keep-alive" request header field. However, some
4811   experimental implementations of HTTP/1.0 persistent connections are faulty;
4812   for example, if an HTTP/1.0 proxy server doesn't understand
4813   <x:ref>Connection</x:ref>, it will erroneously forward that header field
4814   to the next inbound server, which would result in a hung connection.
4815</t>
4816<t>
4817   One attempted solution was the introduction of a Proxy-Connection header
4818   field, targeted specifically at proxies. In practice, this was also
4819   unworkable, because proxies are often deployed in multiple layers, bringing
4820   about the same problem discussed above.
4821</t>
4822<t>
4823   As a result, clients are encouraged not to send the Proxy-Connection header
4824   field in any requests.
4825</t>
4826<t>
4827   Clients are also encouraged to consider the use of Connection: keep-alive
4828   in requests carefully; while they can enable persistent connections with
4829   HTTP/1.0 servers, clients using them will need to monitor the
4830   connection for "hung" requests (which indicate that the client ought stop
4831   sending the header field), and this mechanism ought not be used by clients
4832   at all when a proxy is being used.
4833</t>
4834</section>
4835
4836<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
4837<t>
4838   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
4839   (<xref target="header.transfer-encoding"/>).
4840   Transfer codings need to be decoded prior to forwarding an HTTP message
4841   over a MIME-compliant protocol.
4842</t>
4843</section>
4844
4845</section>
4846
4847<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4848<t>
4849  HTTP's approach to error handling has been explained.
4850  (<xref target="conformance"/>)
4851</t>
4852<t>
4853  The expectation to support HTTP/0.9 requests has been removed.
4854</t>
4855<t>
4856  The term "Effective Request URI" has been introduced.
4857  (<xref target="effective.request.uri" />)
4858</t>
4859<t>
4860  HTTP messages can be (and often are) buffered by implementations; despite
4861  it sometimes being available as a stream, HTTP is fundamentally a
4862  message-oriented protocol.
4863  (<xref target="http.message" />)
4864</t>
4865<t>
4866  Minimum supported sizes for various protocol elements have been
4867  suggested, to improve interoperability.
4868</t>
4869<t>
4870  Header fields that span multiple lines ("line folding") are deprecated.
4871  (<xref target="field.parsing" />)
4872</t>
4873<t>
4874  The HTTP-version ABNF production has been clarified to be case-sensitive.
4875  Additionally, version numbers has been restricted to single digits, due
4876  to the fact that implementations are known to handle multi-digit version
4877  numbers incorrectly.
4878  (<xref target="http.version"/>)
4879</t>
4880<t>
4881  The HTTPS URI scheme is now defined by this specification; previously,
4882  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
4883  (<xref target="https.uri"/>)
4884</t>
4885<t>
4886  The HTTPS URI scheme implies end-to-end security.
4887  (<xref target="https.uri"/>)
4888</t>
4889<t>
4890  Userinfo (i.e., username and password) are now disallowed in HTTP and
4891  HTTPS URIs, because of security issues related to their transmission on the
4892  wire.
4893  (<xref target="http.uri" />)
4894</t>
4895<t>
4896  Invalid whitespace around field-names is now required to be rejected,
4897  because accepting it represents a security vulnerability.
4898  (<xref target="header.fields"/>)
4899</t>
4900<t>
4901  The ABNF productions defining header fields now only list the field value.
4902  (<xref target="header.fields"/>)
4903</t>
4904<t>
4905  Rules about implicit linear whitespace between certain grammar productions
4906  have been removed; now whitespace is only allowed where specifically
4907  defined in the ABNF.
4908  (<xref target="whitespace"/>)
4909</t>
4910<t> 
4911  The NUL octet is no longer allowed in comment and quoted-string text, and
4912  handling of backslash-escaping in them has been clarified.
4913  (<xref target="field.components"/>)
4914</t> 
4915<t>
4916  The quoted-pair rule no longer allows escaping control characters other than
4917  HTAB.
4918  (<xref target="field.components"/>)
4919</t>
4920<t>
4921  Non-ASCII content in header fields and the reason phrase has been obsoleted
4922  and made opaque (the TEXT rule was removed).
4923  (<xref target="field.components"/>)
4924</t>
4925<t>
4926  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
4927  handled as errors by recipients.
4928  (<xref target="header.content-length"/>)
4929</t>
4930<t>
4931  The "identity" transfer coding token has been removed.
4932  (Sections <xref format="counter" target="message.body"/> and
4933  <xref format="counter" target="transfer.codings"/>)
4934</t>
4935<t>
4936  The algorithm for determining the message body length has been clarified
4937  to indicate all of the special cases (e.g., driven by methods or status
4938  codes) that affect it, and that new protocol elements cannot define such
4939  special cases.
4940  (<xref target="message.body.length"/>)
4941</t>
4942<t>
4943  "multipart/byteranges" is no longer a way of determining message body length
4944  detection.
4945  (<xref target="message.body.length"/>)
4946</t>
4947<t>
4948  CONNECT is a new, special case in determining message body length.
4949  (<xref target="message.body.length"/>)
4950</t>
4951<t>
4952  Chunk length does not include the count of the octets in the
4953  chunk header and trailer.
4954  (<xref target="chunked.encoding"/>)
4955</t>
4956<t>
4957  Use of chunk extensions is deprecated, and line folding in them is
4958  disallowed.
4959  (<xref target="chunked.encoding"/>)
4960</t>
4961<t>
4962  The segment + query components of RFC3986 have been used to define the
4963  request-target, instead of abs_path from RFC 1808.
4964  (<xref target="request-target"/>)
4965</t>
4966<t>
4967  The asterisk form of the request-target is only allowed in the OPTIONS
4968  method.
4969  (<xref target="request-target"/>)
4970</t>
4971<t>
4972  Exactly when "close" connection options have to be sent has been clarified.
4973  (<xref target="header.connection"/>)
4974</t>
4975<t> 
4976  "hop-by-hop" header fields are required to appear in the Connection header
4977  field; just because they're defined as hop-by-hop in this specification
4978  doesn't exempt them.
4979  (<xref target="header.connection"/>)
4980</t>
4981<t>
4982  The limit of two connections per server has been removed.
4983  (<xref target="persistent.connections"/>)
4984</t>
4985<t>
4986  An idempotent sequence of requests is no longer required to be retried.
4987  (<xref target="persistent.connections"/>)
4988</t>
4989<t>
4990  The requirement to retry requests under certain circumstances when the
4991  server prematurely closes the connection has been removed.
4992  (<xref target="persistent.connections"/>)
4993</t>
4994<t>
4995  Some extraneous requirements about when servers are allowed to close
4996  connections prematurely have been removed.
4997  (<xref target="persistent.connections"/>)
4998</t>
4999<t>
5000  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
5001  responses other than 101 (this was incorporated from <xref
5002  target="RFC2817"/>).
5003  (<xref target="header.upgrade"/>)
5004</t>
5005<t>
5006  Registration of Transfer Codings now requires IETF Review
5007  (<xref target="transfer.coding.registry"/>)
5008</t>
5009<t>
5010  The meaning of the "deflate" content coding has been clarified.
5011  (<xref target="deflate.coding" />)
5012</t>
5013<t>
5014  This specification now defines the Upgrade Token Registry, previously
5015  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
5016  (<xref target="upgrade.token.registry"/>)
5017</t>
5018<t>
5019  Issues with the Keep-Alive and Proxy-Connection header fields in requests
5020  are pointed out, with use of the latter being discouraged altogether.
5021  (<xref target="compatibility.with.http.1.0.persistent.connections" />)
5022</t>
5023<t>
5024  Empty list elements in list productions (e.g., a list header field containing
5025  ", ,") have been deprecated.
5026  (<xref target="abnf.extension"/>)
5027</t>
5028</section>
5029</section>
5030
5031<section title="ABNF list extension: #rule" anchor="abnf.extension">
5032<t>
5033  A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
5034  improve readability in the definitions of some header field values.
5035</t>
5036<t>
5037  A construct "#" is defined, similar to "*", for defining comma-delimited
5038  lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
5039  at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
5040  comma (",") and optional whitespace (OWS).   
5041</t>
5042<figure><preamble>
5043  Thus,
5044</preamble><artwork type="example">
5045  1#element =&gt; element *( OWS "," OWS element )
5046</artwork></figure>
5047<figure><preamble>
5048  and:
5049</preamble><artwork type="example">
5050  #element =&gt; [ 1#element ]
5051</artwork></figure>
5052<figure><preamble>
5053  and for n &gt;= 1 and m &gt; 1:
5054</preamble><artwork type="example">
5055  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
5056</artwork></figure>
5057<t>
5058  For compatibility with legacy list rules, recipients &SHOULD; accept empty
5059  list elements. In other words, consumers would follow the list productions:
5060</t>
5061<figure><artwork type="example">
5062  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
5063 
5064  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
5065</artwork></figure>
5066<t>
5067  Note that empty elements do not contribute to the count of elements present,
5068  though.
5069</t>
5070<t>
5071  For example, given these ABNF productions:
5072</t>
5073<figure><artwork type="example">
5074  example-list      = 1#example-list-elmt
5075  example-list-elmt = token ; see <xref target="field.components"/> 
5076</artwork></figure>
5077<t>
5078  Then these are valid values for example-list (not including the double
5079  quotes, which are present for delimitation only):
5080</t>
5081<figure><artwork type="example">
5082  "foo,bar"
5083  "foo ,bar,"
5084  "foo , ,bar,charlie   "
5085</artwork></figure>
5086<t>
5087  But these values would be invalid, as at least one non-empty element is
5088  required:
5089</t>
5090<figure><artwork type="example">
5091  ""
5092  ","
5093  ",   ,"
5094</artwork></figure>
5095<t>
5096  <xref target="collected.abnf"/> shows the collected ABNF, with the list rules
5097  expanded as explained above.
5098</t>
5099</section>
5100
5101<?BEGININC p1-messaging.abnf-appendix ?>
5102<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
5103<figure>
5104<artwork type="abnf" name="p1-messaging.parsed-abnf">
5105<x:ref>BWS</x:ref> = OWS
5106
5107<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
5108 connection-option ] )
5109<x:ref>Content-Length</x:ref> = 1*DIGIT
5110
5111<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
5112 ]
5113<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
5114<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
5115<x:ref>Host</x:ref> = uri-host [ ":" port ]
5116
5117<x:ref>OWS</x:ref> = *( SP / HTAB )
5118
5119<x:ref>RWS</x:ref> = 1*( SP / HTAB )
5120
5121<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
5122<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
5123<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
5124 transfer-coding ] )
5125
5126<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
5127<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
5128
5129<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
5130 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
5131 comment ] ) ] )
5132
5133<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
5134<x:ref>absolute-form</x:ref> = absolute-URI
5135<x:ref>absolute-path</x:ref> = 1*( "/" segment )
5136<x:ref>asterisk-form</x:ref> = "*"
5137<x:ref>attribute</x:ref> = token
5138<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
5139<x:ref>authority-form</x:ref> = authority
5140
5141<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
5142<x:ref>chunk-data</x:ref> = 1*OCTET
5143<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
5144<x:ref>chunk-ext-name</x:ref> = token
5145<x:ref>chunk-ext-val</x:ref> = token / quoted-str-nf
5146<x:ref>chunk-size</x:ref> = 1*HEXDIG
5147<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
5148<x:ref>comment</x:ref> = "(" *( ctext / quoted-cpair / comment ) ")"
5149<x:ref>connection-option</x:ref> = token
5150<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
5151 / %x2A-5B ; '*'-'['
5152 / %x5D-7E ; ']'-'~'
5153 / obs-text
5154
5155<x:ref>field-content</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5156<x:ref>field-name</x:ref> = token
5157<x:ref>field-value</x:ref> = *( field-content / obs-fold )
5158<x:ref>fragment</x:ref> = &lt;fragment, defined in [RFC3986], Section 3.5&gt;
5159
5160<x:ref>header-field</x:ref> = field-name ":" OWS field-value OWS
5161<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ] [ "#"
5162 fragment ]
5163<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ] [ "#"
5164 fragment ]
5165
5166<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
5167
5168<x:ref>message-body</x:ref> = *OCTET
5169<x:ref>method</x:ref> = token
5170
5171<x:ref>obs-fold</x:ref> = CRLF ( SP / HTAB )
5172<x:ref>obs-text</x:ref> = %x80-FF
5173<x:ref>origin-form</x:ref> = absolute-path [ "?" query ]
5174
5175<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
5176<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
5177<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
5178<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
5179<x:ref>protocol-name</x:ref> = token
5180<x:ref>protocol-version</x:ref> = token
5181<x:ref>pseudonym</x:ref> = token
5182
5183<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5184 / %x5D-7E ; ']'-'~'
5185 / obs-text
5186<x:ref>qdtext-nf</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5187 / %x5D-7E ; ']'-'~'
5188 / obs-text
5189<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5190<x:ref>quoted-cpair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5191<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5192<x:ref>quoted-str-nf</x:ref> = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
5193<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5194
5195<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5196<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5197<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5198<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5199<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5200<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5201<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5202 asterisk-form
5203
5204<x:ref>segment</x:ref> = &lt;segment, defined in [RFC3986], Section 3.3&gt;
5205<x:ref>special</x:ref> = "(" / ")" / "&lt;" / "&gt;" / "@" / "," / ";" / ":" / "\" /
5206 DQUOTE / "/" / "[" / "]" / "?" / "=" / "{" / "}"
5207<x:ref>start-line</x:ref> = request-line / status-line
5208<x:ref>status-code</x:ref> = 3DIGIT
5209<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5210
5211<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5212<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5213<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5214 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5215<x:ref>token</x:ref> = 1*tchar
5216<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5217<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5218 transfer-extension
5219<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5220<x:ref>transfer-parameter</x:ref> = attribute BWS "=" BWS value
5221
5222<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5223
5224<x:ref>value</x:ref> = word
5225
5226<x:ref>word</x:ref> = token / quoted-string
5227</artwork>
5228</figure>
5229</section>
5230<?ENDINC p1-messaging.abnf-appendix ?>
5231
5232<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5233
5234<section title="Since RFC 2616">
5235<t>
5236  Changes up to the first Working Group Last Call draft are summarized
5237  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-21#appendix-D"/>.
5238</t>
5239</section>
5240
5241<section title="Since draft-ietf-httpbis-p1-messaging-21" anchor="changes.since.21">
5242<t>
5243  Closed issues:
5244  <list style="symbols">
5245    <t>
5246      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/128"/>:
5247      "Cite HTTPS URI scheme definition" (the spec now includes the HTTPs
5248      scheme definition and thus updates RFC 2818)
5249    </t>
5250    <t>
5251      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/389"/>:
5252      "mention of 'proxies' in section about caches"
5253    </t>
5254    <t>
5255      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/390"/>:
5256      "use of ABNF terms from RFC 3986"
5257    </t>
5258    <t>
5259      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/391"/>:
5260      "transferring URIs with userinfo in payload"
5261    </t>
5262    <t>
5263      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/392"/>:
5264      "editorial improvements to message length definition"
5265    </t>
5266    <t>
5267      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/395"/>:
5268      "Connection header field MUST vs SHOULD"
5269    </t>
5270    <t>
5271      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/396"/>:
5272      "editorial improvements to persistent connections section"
5273    </t>
5274    <t>
5275      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/397"/>:
5276      "URI normalization vs empty path"
5277    </t>
5278    <t>
5279      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/408"/>:
5280      "p1 feedback"
5281    </t>
5282    <t>
5283      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/409"/>:
5284      "is parsing OBS-FOLD mandatory?"
5285    </t>
5286    <t>
5287      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/410"/>:
5288      "HTTPS and Shared Caching"
5289    </t>
5290    <t>
5291      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/411"/>:
5292      "Requirements for recipients of ws between start-line and first header field"
5293    </t>
5294    <t>
5295      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/412"/>:
5296      "SP and HT when being tolerant"
5297    </t>
5298    <t>
5299      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/414"/>:
5300      "Message Parsing Strictness"
5301    </t>
5302    <t>
5303      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/415"/>:
5304      "'Render'"
5305    </t>
5306    <t>
5307      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/418"/>:
5308      "No-Transform"
5309    </t>
5310    <t>
5311      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/419"/>:
5312      "p2 editorial feedback"
5313    </t>
5314    <t>
5315      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/420"/>:
5316      "Content-Length SHOULD be sent"
5317    </t>
5318    <t>
5319      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/431"/>:
5320      "origin-form does not allow path starting with "//""
5321    </t>
5322    <t>
5323      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/433"/>:
5324      "ambiguity in part 1 example"
5325    </t>
5326  </list>
5327</t>
5328</section>
5329
5330<section title="Since draft-ietf-httpbis-p1-messaging-22" anchor="changes.since.22">
5331<t>
5332  Closed issues:
5333  <list style="symbols">
5334    <t>
5335      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/435"/>:
5336      "Part1 should have a reference to TCP (RFC 793)"
5337    </t>
5338    <t>
5339      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/438"/>:
5340      "media type registration template issues"
5341    </t>
5342    <t>
5343      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/442"/>:
5344      "BWS" (vs conformance)
5345    </t>
5346    <t>
5347      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/444"/>:
5348      "obs-fold language"
5349    </t>
5350    <t>
5351      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/445"/>:
5352      "Ordering in Upgrade"
5353    </t>
5354    <t>
5355      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/446"/>:
5356      "p1 editorial feedback"
5357    </t>
5358    <t>
5359      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/476"/>:
5360      "SHOULD and conformance"
5361    </t>
5362    <t>
5363      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/477"/>:
5364      "Pipelining language"
5365    </t>
5366  </list>
5367</t>
5368</section>
5369</section>
5370
5371</back>
5372</rfc>
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