source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2276

Last change on this file since 2276 was 2276, checked in by julian.reschke@…, 7 years ago

update acks (#219)

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 229.6 KB
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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "May">
16  <!ENTITY ID-YEAR "2013">
17  <!ENTITY mdash "&#8212;">
18  <!ENTITY Note "<x:h xmlns:x='http://purl.org/net/xml2rfc/ext'>Note:</x:h>">
19  <!ENTITY caching-overview       "<xref target='Part6' x:rel='#caching.overview' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY cache-incomplete       "<xref target='Part6' x:rel='#response.cacheability' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY payload                "<xref target='Part2' x:rel='#payload' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY media-type            "<xref target='Part2' x:rel='#media.type' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY content-codings        "<xref target='Part2' x:rel='#content.codings' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY CONNECT                "<xref target='Part2' x:rel='#CONNECT' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY content.negotiation    "<xref target='Part2' x:rel='#content.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY diff-mime              "<xref target='Part2' x:rel='#differences.between.http.and.mime' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY representation         "<xref target='Part2' x:rel='#representations' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY HEAD                   "<xref target='Part2' x:rel='#HEAD' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-allow           "<xref target='Part2' x:rel='#header.allow' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-cache-control   "<xref target='Part6' x:rel='#header.cache-control' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY header-content-encoding    "<xref target='Part2' x:rel='#header.content-encoding' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY header-content-location    "<xref target='Part2' x:rel='#header.content-location' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY header-content-range   "<xref target='Part5' x:rel='#header.content-range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY header-content-type    "<xref target='Part2' x:rel='#header.content-type' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY header-date            "<xref target='Part2' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY header-etag            "<xref target='Part4' x:rel='#header.etag' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY header-expires         "<xref target='Part6' x:rel='#header.expires' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY header-last-modified   "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY header-mime-version    "<xref target='Part2' x:rel='#mime-version' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40  <!ENTITY header-pragma          "<xref target='Part6' x:rel='#header.pragma' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41  <!ENTITY header-proxy-authenticate  "<xref target='Part7' x:rel='#header.proxy-authenticate' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
42  <!ENTITY header-proxy-authorization "<xref target='Part7' x:rel='#header.proxy-authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
43  <!ENTITY header-server          "<xref target='Part2' x:rel='#header.server' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
44  <!ENTITY header-warning         "<xref target='Part6' x:rel='#header.warning' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
45  <!ENTITY idempotent-methods     "<xref target='Part2' x:rel='#idempotent.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
46  <!ENTITY safe-methods           "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
47  <!ENTITY methods                "<xref target='Part2' x:rel='#methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
48  <!ENTITY OPTIONS                "<xref target='Part2' x:rel='#OPTIONS' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
49  <!ENTITY qvalue                 "<xref target='Part2' x:rel='#quality.values' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
50  <!ENTITY resource               "<xref target='Part2' x:rel='#resources' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
51  <!ENTITY status-codes           "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
52  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
53  <!ENTITY status-203             "<xref target='Part2' x:rel='#status.203' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
54  <!ENTITY status-3xx             "<xref target='Part2' x:rel='#status.3xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
55  <!ENTITY status-304             "<xref target='Part4' x:rel='#status.304' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
56  <!ENTITY status-4xx             "<xref target='Part2' x:rel='#status.4xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
57  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
58  <!ENTITY iana-header-registry   "<xref target='Part2' x:rel='#header.field.registry' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
59]>
60<?rfc toc="yes" ?>
61<?rfc symrefs="yes" ?>
62<?rfc sortrefs="yes" ?>
63<?rfc compact="yes"?>
64<?rfc subcompact="no" ?>
65<?rfc linkmailto="no" ?>
66<?rfc editing="no" ?>
67<?rfc comments="yes"?>
68<?rfc inline="yes"?>
69<?rfc rfcedstyle="yes"?>
70<?rfc-ext allow-markup-in-artwork="yes" ?>
71<?rfc-ext include-references-in-index="yes" ?>
72<rfc obsoletes="2145,2616" updates="2817,2818" category="std" x:maturity-level="proposed"
73     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
74     xmlns:x='http://purl.org/net/xml2rfc/ext'>
75<x:link rel="next" basename="p2-semantics"/>
76<x:feedback template="mailto:ietf-http-wg@w3.org?subject={docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
77<front>
78
79  <title abbrev="HTTP/1.1 Message Syntax and Routing">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
80
81  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
82    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
83    <address>
84      <postal>
85        <street>345 Park Ave</street>
86        <city>San Jose</city>
87        <region>CA</region>
88        <code>95110</code>
89        <country>USA</country>
90      </postal>
91      <email>fielding@gbiv.com</email>
92      <uri>http://roy.gbiv.com/</uri>
93    </address>
94  </author>
95
96  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
97    <organization abbrev="greenbytes">greenbytes GmbH</organization>
98    <address>
99      <postal>
100        <street>Hafenweg 16</street>
101        <city>Muenster</city><region>NW</region><code>48155</code>
102        <country>Germany</country>
103      </postal>
104      <email>julian.reschke@greenbytes.de</email>
105      <uri>http://greenbytes.de/tech/webdav/</uri>
106    </address>
107  </author>
108
109  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
110  <workgroup>HTTPbis Working Group</workgroup>
111
112<abstract>
113<t>
114   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
115   distributed, collaborative, hypertext information systems. HTTP has been in
116   use by the World Wide Web global information initiative since 1990.
117   This document provides an overview of HTTP architecture and its associated
118   terminology, defines the "http" and "https" Uniform Resource Identifier
119   (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements,
120   and describes general security concerns for implementations.
121</t>   
122</abstract>
123
124<note title="Editorial Note (To be removed by RFC Editor)">
125  <t>
126    Discussion of this draft takes place on the HTTPBIS working group
127    mailing list (ietf-http-wg@w3.org), which is archived at
128    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
129  </t>
130  <t>
131    The current issues list is at
132    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
133    documents (including fancy diffs) can be found at
134    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
135  </t>
136  <t>
137    The changes in this draft are summarized in <xref target="changes.since.22"/>.
138  </t>
139</note>
140</front>
141<middle>
142<section title="Introduction" anchor="introduction">
143<t>
144   The Hypertext Transfer Protocol (HTTP) is an application-level
145   request/response protocol that uses extensible semantics and self-descriptive
146   message payloads for flexible interaction with network-based hypertext
147   information systems. This document is the first in a series of documents
148   that collectively form the HTTP/1.1 specification:
149   <list style="empty">
150    <t>RFC xxx1: Message Syntax and Routing</t>
151    <t><xref target="Part2" x:fmt="none">RFC xxx2</xref>: Semantics and Content</t>
152    <t><xref target="Part4" x:fmt="none">RFC xxx3</xref>: Conditional Requests</t>
153    <t><xref target="Part5" x:fmt="none">RFC xxx4</xref>: Range Requests</t>
154    <t><xref target="Part6" x:fmt="none">RFC xxx5</xref>: Caching</t>
155    <t><xref target="Part7" x:fmt="none">RFC xxx6</xref>: Authentication</t>
156   </list>
157</t>
158<t>
159   This HTTP/1.1 specification obsoletes and moves to historic status
160   <xref target="RFC2616" x:fmt="none">RFC 2616</xref>, its predecessor
161   <xref target="RFC2068" x:fmt="none">RFC 2068</xref>, and
162   <xref target="RFC2145" x:fmt="none">RFC 2145</xref> (on HTTP versioning).
163   This specification also updates the use of CONNECT to establish a tunnel,
164   previously defined in <xref target="RFC2817" x:fmt="none">RFC 2817</xref>,
165   and defines the "https" URI scheme that was described informally in
166   <xref target="RFC2818" x:fmt="none">RFC 2818</xref>.
167</t>
168<t>
169   HTTP is a generic interface protocol for information systems. It is
170   designed to hide the details of how a service is implemented by presenting
171   a uniform interface to clients that is independent of the types of
172   resources provided. Likewise, servers do not need to be aware of each
173   client's purpose: an HTTP request can be considered in isolation rather
174   than being associated with a specific type of client or a predetermined
175   sequence of application steps. The result is a protocol that can be used
176   effectively in many different contexts and for which implementations can
177   evolve independently over time.
178</t>
179<t>
180   HTTP is also designed for use as an intermediation protocol for translating
181   communication to and from non-HTTP information systems.
182   HTTP proxies and gateways can provide access to alternative information
183   services by translating their diverse protocols into a hypertext
184   format that can be viewed and manipulated by clients in the same way
185   as HTTP services.
186</t>
187<t>
188   One consequence of this flexibility is that the protocol cannot be
189   defined in terms of what occurs behind the interface. Instead, we
190   are limited to defining the syntax of communication, the intent
191   of received communication, and the expected behavior of recipients.
192   If the communication is considered in isolation, then successful
193   actions ought to be reflected in corresponding changes to the
194   observable interface provided by servers. However, since multiple
195   clients might act in parallel and perhaps at cross-purposes, we
196   cannot require that such changes be observable beyond the scope
197   of a single response.
198</t>
199<t>
200   This document describes the architectural elements that are used or
201   referred to in HTTP, defines the "http" and "https" URI schemes,
202   describes overall network operation and connection management,
203   and defines HTTP message framing and forwarding requirements.
204   Our goal is to define all of the mechanisms necessary for HTTP message
205   handling that are independent of message semantics, thereby defining the
206   complete set of requirements for message parsers and
207   message-forwarding intermediaries.
208</t>
209
210
211<section title="Requirement Notation" anchor="intro.requirements">
212<t>
213   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
214   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
215   document are to be interpreted as described in <xref target="RFC2119"/>.
216</t>
217<t>
218   Conformance criteria and considerations regarding error handling
219   are defined in <xref target="conformance"/>.
220</t>
221</section>
222
223<section title="Syntax Notation" anchor="notation">
224<iref primary="true" item="Grammar" subitem="ALPHA"/>
225<iref primary="true" item="Grammar" subitem="CR"/>
226<iref primary="true" item="Grammar" subitem="CRLF"/>
227<iref primary="true" item="Grammar" subitem="CTL"/>
228<iref primary="true" item="Grammar" subitem="DIGIT"/>
229<iref primary="true" item="Grammar" subitem="DQUOTE"/>
230<iref primary="true" item="Grammar" subitem="HEXDIG"/>
231<iref primary="true" item="Grammar" subitem="HTAB"/>
232<iref primary="true" item="Grammar" subitem="LF"/>
233<iref primary="true" item="Grammar" subitem="OCTET"/>
234<iref primary="true" item="Grammar" subitem="SP"/>
235<iref primary="true" item="Grammar" subitem="VCHAR"/>
236<t>
237   This specification uses the Augmented Backus-Naur Form (ABNF) notation
238   of <xref target="RFC5234"/> with the list rule extension defined in
239   <xref target="abnf.extension"/>.  <xref target="collected.abnf"/> shows
240   the collected ABNF with the list rule expanded.
241</t>
242<t anchor="core.rules">
243  <x:anchor-alias value="ALPHA"/>
244  <x:anchor-alias value="CTL"/>
245  <x:anchor-alias value="CR"/>
246  <x:anchor-alias value="CRLF"/>
247  <x:anchor-alias value="DIGIT"/>
248  <x:anchor-alias value="DQUOTE"/>
249  <x:anchor-alias value="HEXDIG"/>
250  <x:anchor-alias value="HTAB"/>
251  <x:anchor-alias value="LF"/>
252  <x:anchor-alias value="OCTET"/>
253  <x:anchor-alias value="SP"/>
254  <x:anchor-alias value="VCHAR"/>
255   The following core rules are included by
256   reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
257   ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
258   DIGIT (decimal 0-9), DQUOTE (double quote),
259   HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed),
260   OCTET (any 8-bit sequence of data), SP (space), and
261   VCHAR (any visible <xref target="USASCII"/> character).
262</t>
263<t>
264   As a convention, ABNF rule names prefixed with "obs-" denote
265   "obsolete" grammar rules that appear for historical reasons.
266</t>
267</section>
268</section>
269
270<section title="Architecture" anchor="architecture">
271<t>
272   HTTP was created for the World Wide Web architecture
273   and has evolved over time to support the scalability needs of a worldwide
274   hypertext system. Much of that architecture is reflected in the terminology
275   and syntax productions used to define HTTP.
276</t>
277
278<section title="Client/Server Messaging" anchor="operation">
279<iref primary="true" item="client"/>
280<iref primary="true" item="server"/>
281<iref primary="true" item="connection"/>
282<t>
283   HTTP is a stateless request/response protocol that operates by exchanging
284   <x:dfn>messages</x:dfn> (<xref target="http.message"/>) across a reliable
285   transport or session-layer
286   "<x:dfn>connection</x:dfn>" (<xref target="connection.management"/>).
287   An HTTP "<x:dfn>client</x:dfn>" is a program that establishes a connection
288   to a server for the purpose of sending one or more HTTP requests.
289   An HTTP "<x:dfn>server</x:dfn>" is a program that accepts connections
290   in order to service HTTP requests by sending HTTP responses.
291</t>
292<iref primary="true" item="user agent"/>
293<iref primary="true" item="origin server"/>
294<iref primary="true" item="browser"/>
295<iref primary="true" item="spider"/>
296<iref primary="true" item="sender"/>
297<iref primary="true" item="recipient"/>
298<t>
299   The terms client and server refer only to the roles that
300   these programs perform for a particular connection.  The same program
301   might act as a client on some connections and a server on others.
302   We use the term "<x:dfn>user agent</x:dfn>" to refer to any of the various
303   client programs that initiate a request, including (but not limited to)
304   browsers, spiders (web-based robots), command-line tools, native
305   applications, and mobile apps.  The term "<x:dfn>origin server</x:dfn>" is
306   used to refer to the program that can originate authoritative responses to
307   a request. For general requirements, we use the terms
308   "<x:dfn>sender</x:dfn>" and "<x:dfn>recipient</x:dfn>" to refer to any
309   component that sends or receives, respectively, a given message.
310</t>
311<t>
312   HTTP relies upon the Uniform Resource Identifier (URI)
313   standard <xref target="RFC3986"/> to indicate the target resource
314   (<xref target="target-resource"/>) and relationships between resources.
315   Messages are passed in a format similar to that used by Internet mail
316   <xref target="RFC5322"/> and the Multipurpose Internet Mail Extensions
317   (MIME) <xref target="RFC2045"/> (see &diff-mime; for the differences
318   between HTTP and MIME messages).
319</t>
320<t>
321   Most HTTP communication consists of a retrieval request (GET) for
322   a representation of some resource identified by a URI.  In the
323   simplest case, this might be accomplished via a single bidirectional
324   connection (===) between the user agent (UA) and the origin server (O).
325</t>
326<figure><artwork type="drawing">
327         request   &gt;
328    <x:highlight>UA</x:highlight> ======================================= <x:highlight>O</x:highlight>
329                                &lt;   response
330</artwork></figure>
331<iref primary="true" item="message"/>
332<iref primary="true" item="request"/>
333<iref primary="true" item="response"/>
334<t>
335   A client sends an HTTP request to a server in the form of a <x:dfn>request</x:dfn>
336   message, beginning with a request-line that includes a method, URI, and
337   protocol version (<xref target="request.line"/>),
338   followed by header fields containing
339   request modifiers, client information, and representation metadata
340   (<xref target="header.fields"/>),
341   an empty line to indicate the end of the header section, and finally
342   a message body containing the payload body (if any,
343   <xref target="message.body"/>).
344</t>
345<t>
346   A server responds to a client's request by sending one or more HTTP
347   <x:dfn>response</x:dfn>
348   messages, each beginning with a status line that
349   includes the protocol version, a success or error code, and textual
350   reason phrase (<xref target="status.line"/>),
351   possibly followed by header fields containing server
352   information, resource metadata, and representation metadata
353   (<xref target="header.fields"/>),
354   an empty line to indicate the end of the header section, and finally
355   a message body containing the payload body (if any,
356   <xref target="message.body"/>).
357</t>
358<t>
359   A connection might be used for multiple request/response exchanges,
360   as defined in <xref target="persistent.connections"/>.
361</t>
362<t>
363   The following example illustrates a typical message exchange for a
364   GET request on the URI "http://www.example.com/hello.txt":
365</t>
366<figure><preamble>
367client request:
368</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
369GET /hello.txt HTTP/1.1
370User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
371Host: www.example.com
372Accept-Language: en, mi
373
374</artwork></figure>
375<figure><preamble>
376server response:
377</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
378HTTP/1.1 200 OK
379Date: Mon, 27 Jul 2009 12:28:53 GMT
380Server: Apache
381Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
382ETag: "34aa387-d-1568eb00"
383Accept-Ranges: bytes
384Content-Length: <x:length-of target="exbody"/>
385Vary: Accept-Encoding
386Content-Type: text/plain
387
388<x:span anchor="exbody">Hello World! My payload includes a trailing CRLF.
389</x:span></artwork>
390</figure>
391</section>
392
393<section title="Implementation Diversity" anchor="implementation-diversity">
394<t>
395   When considering the design of HTTP, it is easy to fall into a trap of
396   thinking that all user agents are general-purpose browsers and all origin
397   servers are large public websites. That is not the case in practice.
398   Common HTTP user agents include household appliances, stereos, scales,
399   firmware update scripts, command-line programs, mobile apps,
400   and communication devices in a multitude of shapes and sizes.  Likewise,
401   common HTTP origin servers include home automation units, configurable
402   networking components, office machines, autonomous robots, news feeds,
403   traffic cameras, ad selectors, and video delivery platforms.
404</t>
405<t>
406   The term "user agent" does not imply that there is a human user directly
407   interacting with the software agent at the time of a request. In many
408   cases, a user agent is installed or configured to run in the background
409   and save its results for later inspection (or save only a subset of those
410   results that might be interesting or erroneous). Spiders, for example, are
411   typically given a start URI and configured to follow certain behavior while
412   crawling the Web as a hypertext graph.
413</t>
414<t>
415   The implementation diversity of HTTP means that we cannot assume the
416   user agent can make interactive suggestions to a user or provide adequate
417   warning for security or privacy options.  In the few cases where this
418   specification requires reporting of errors to the user, it is acceptable
419   for such reporting to only be observable in an error console or log file.
420   Likewise, requirements that an automated action be confirmed by the user
421   before proceeding might be met via advance configuration choices,
422   run-time options, or simple avoidance of the unsafe action; confirmation
423   does not imply any specific user interface or interruption of normal
424   processing if the user has already made that choice.
425</t>
426</section>
427
428<section title="Intermediaries" anchor="intermediaries">
429<iref primary="true" item="intermediary"/>
430<t>
431   HTTP enables the use of intermediaries to satisfy requests through
432   a chain of connections.  There are three common forms of HTTP
433   <x:dfn>intermediary</x:dfn>: proxy, gateway, and tunnel.  In some cases,
434   a single intermediary might act as an origin server, proxy, gateway,
435   or tunnel, switching behavior based on the nature of each request.
436</t>
437<figure><artwork type="drawing">
438         &gt;             &gt;             &gt;             &gt;
439    <x:highlight>UA</x:highlight> =========== <x:highlight>A</x:highlight> =========== <x:highlight>B</x:highlight> =========== <x:highlight>C</x:highlight> =========== <x:highlight>O</x:highlight>
440               &lt;             &lt;             &lt;             &lt;
441</artwork></figure>
442<t>
443   The figure above shows three intermediaries (A, B, and C) between the
444   user agent and origin server. A request or response message that
445   travels the whole chain will pass through four separate connections.
446   Some HTTP communication options
447   might apply only to the connection with the nearest, non-tunnel
448   neighbor, only to the end-points of the chain, or to all connections
449   along the chain. Although the diagram is linear, each participant might
450   be engaged in multiple, simultaneous communications. For example, B
451   might be receiving requests from many clients other than A, and/or
452   forwarding requests to servers other than C, at the same time that it
453   is handling A's request. Likewise, later requests might be sent through a
454   different path of connections, often based on dynamic configuration for
455   load balancing.   
456</t>
457<t>
458<iref primary="true" item="upstream"/><iref primary="true" item="downstream"/>
459<iref primary="true" item="inbound"/><iref primary="true" item="outbound"/>
460   We use the terms "<x:dfn>upstream</x:dfn>" and "<x:dfn>downstream</x:dfn>"
461   to describe various requirements in relation to the directional flow of a
462   message: all messages flow from upstream to downstream.
463   Likewise, we use the terms inbound and outbound to refer to
464   directions in relation to the request path:
465   "<x:dfn>inbound</x:dfn>" means toward the origin server and
466   "<x:dfn>outbound</x:dfn>" means toward the user agent.
467</t>
468<t><iref primary="true" item="proxy"/>
469   A "<x:dfn>proxy</x:dfn>" is a message forwarding agent that is selected by the
470   client, usually via local configuration rules, to receive requests
471   for some type(s) of absolute URI and attempt to satisfy those
472   requests via translation through the HTTP interface.  Some translations
473   are minimal, such as for proxy requests for "http" URIs, whereas
474   other requests might require translation to and from entirely different
475   application-level protocols. Proxies are often used to group an
476   organization's HTTP requests through a common intermediary for the
477   sake of security, annotation services, or shared caching.
478</t>
479<t>
480<iref primary="true" item="transforming proxy"/>
481<iref primary="true" item="non-transforming proxy"/>
482   An HTTP-to-HTTP proxy is called a "<x:dfn>transforming proxy</x:dfn>" if it is designed
483   or configured to modify request or response messages in a semantically
484   meaningful way (i.e., modifications, beyond those required by normal
485   HTTP processing, that change the message in a way that would be
486   significant to the original sender or potentially significant to
487   downstream recipients).  For example, a transforming proxy might be
488   acting as a shared annotation server (modifying responses to include
489   references to a local annotation database), a malware filter, a
490   format transcoder, or an intranet-to-Internet privacy filter.  Such
491   transformations are presumed to be desired by the client (or client
492   organization) that selected the proxy and are beyond the scope of
493   this specification.  However, when a proxy is not intended to transform
494   a given message, we use the term "<x:dfn>non-transforming proxy</x:dfn>" to target
495   requirements that preserve HTTP message semantics. See &status-203; and
496   &header-warning; for status and warning codes related to transformations.
497</t>
498<t><iref primary="true" item="gateway"/><iref primary="true" item="reverse proxy"/>
499<iref primary="true" item="accelerator"/>
500   A "<x:dfn>gateway</x:dfn>" (a.k.a., "<x:dfn>reverse proxy</x:dfn>") is an
501   intermediary that acts as an origin server for the outbound connection, but
502   translates received requests and forwards them inbound to another server or
503   servers. Gateways are often used to encapsulate legacy or untrusted
504   information services, to improve server performance through
505   "<x:dfn>accelerator</x:dfn>" caching, and to enable partitioning or load
506   balancing of HTTP services across multiple machines.
507</t>
508<t>
509   All HTTP requirements applicable to an origin server
510   also apply to the outbound communication of a gateway.
511   A gateway communicates with inbound servers using any protocol that
512   it desires, including private extensions to HTTP that are outside
513   the scope of this specification.  However, an HTTP-to-HTTP gateway
514   that wishes to interoperate with third-party HTTP servers &MUST;
515   conform to HTTP user agent requirements on the gateway's inbound
516   connection and &MUST; implement the <x:ref>Connection</x:ref>
517   (<xref target="header.connection"/>) and <x:ref>Via</x:ref>
518   (<xref target="header.via"/>) header fields for both inbound and outbound
519   connections.
520</t>
521<t><iref primary="true" item="tunnel"/>
522   A "<x:dfn>tunnel</x:dfn>" acts as a blind relay between two connections
523   without changing the messages. Once active, a tunnel is not
524   considered a party to the HTTP communication, though the tunnel might
525   have been initiated by an HTTP request. A tunnel ceases to exist when
526   both ends of the relayed connection are closed. Tunnels are used to
527   extend a virtual connection through an intermediary, such as when
528   Transport Layer Security (TLS, <xref target="RFC5246"/>) is used to
529   establish confidential communication through a shared firewall proxy.
530</t>
531<t><iref primary="true" item="interception proxy"/>
532<iref primary="true" item="transparent proxy"/>
533<iref primary="true" item="captive portal"/>
534   The above categories for intermediary only consider those acting as
535   participants in the HTTP communication.  There are also intermediaries
536   that can act on lower layers of the network protocol stack, filtering or
537   redirecting HTTP traffic without the knowledge or permission of message
538   senders. Network intermediaries often introduce security flaws or
539   interoperability problems by violating HTTP semantics.  For example, an
540   "<x:dfn>interception proxy</x:dfn>" <xref target="RFC3040"/> (also commonly
541   known as a "<x:dfn>transparent proxy</x:dfn>" <xref target="RFC1919"/> or
542   "<x:dfn>captive portal</x:dfn>")
543   differs from an HTTP proxy because it is not selected by the client.
544   Instead, an interception proxy filters or redirects outgoing TCP port 80
545   packets (and occasionally other common port traffic).
546   Interception proxies are commonly found on public network access points,
547   as a means of enforcing account subscription prior to allowing use of
548   non-local Internet services, and within corporate firewalls to enforce
549   network usage policies.
550   They are indistinguishable from a man-in-the-middle attack.
551</t>
552<t>
553   HTTP is defined as a stateless protocol, meaning that each request message
554   can be understood in isolation.  Many implementations depend on HTTP's
555   stateless design in order to reuse proxied connections or dynamically
556   load-balance requests across multiple servers.  Hence, servers &MUST-NOT;
557   assume that two requests on the same connection are from the same user
558   agent unless the connection is secured and specific to that agent.
559   Some non-standard HTTP extensions (e.g., <xref target="RFC4559"/>) have
560   been known to violate this requirement, resulting in security and
561   interoperability problems.
562</t>
563</section>
564
565<section title="Caches" anchor="caches">
566<iref primary="true" item="cache"/>
567<t>
568   A "<x:dfn>cache</x:dfn>" is a local store of previous response messages and the
569   subsystem that controls its message storage, retrieval, and deletion.
570   A cache stores cacheable responses in order to reduce the response
571   time and network bandwidth consumption on future, equivalent
572   requests. Any client or server &MAY; employ a cache, though a cache
573   cannot be used by a server while it is acting as a tunnel.
574</t>
575<t>
576   The effect of a cache is that the request/response chain is shortened
577   if one of the participants along the chain has a cached response
578   applicable to that request. The following illustrates the resulting
579   chain if B has a cached copy of an earlier response from O (via C)
580   for a request that has not been cached by UA or A.
581</t>
582<figure><artwork type="drawing">
583            &gt;             &gt;
584       <x:highlight>UA</x:highlight> =========== <x:highlight>A</x:highlight> =========== <x:highlight>B</x:highlight> - - - - - - <x:highlight>C</x:highlight> - - - - - - <x:highlight>O</x:highlight>
585                  &lt;             &lt;
586</artwork></figure>
587<t><iref primary="true" item="cacheable"/>
588   A response is "<x:dfn>cacheable</x:dfn>" if a cache is allowed to store a copy of
589   the response message for use in answering subsequent requests.
590   Even when a response is cacheable, there might be additional
591   constraints placed by the client or by the origin server on when
592   that cached response can be used for a particular request. HTTP
593   requirements for cache behavior and cacheable responses are
594   defined in &caching-overview;. 
595</t>
596<t>
597   There are a wide variety of architectures and configurations
598   of caches deployed across the World Wide Web and
599   inside large organizations. These include national hierarchies
600   of proxy caches to save transoceanic bandwidth, collaborative systems that
601   broadcast or multicast cache entries, archives of pre-fetched cache
602   entries for use in off-line or high-latency environments, and so on.
603</t>
604</section>
605
606<section title="Conformance and Error Handling" anchor="conformance">
607<t>
608   This specification targets conformance criteria according to the role of
609   a participant in HTTP communication.  Hence, HTTP requirements are placed
610   on senders, recipients, clients, servers, user agents, intermediaries,
611   origin servers, proxies, gateways, or caches, depending on what behavior
612   is being constrained by the requirement. Additional (social) requirements
613   are placed on implementations, resource owners, and protocol element
614   registrations when they apply beyond the scope of a single communication.
615</t>
616<t>
617   The verb "generate" is used instead of "send" where a requirement
618   differentiates between creating a protocol element and merely forwarding a
619   received element downstream.
620</t>
621<t>
622   An implementation is considered conformant if it complies with all of the
623   requirements associated with the roles it partakes in HTTP.
624</t>
625<t>
626   Conformance applies to both the syntax and semantics of HTTP protocol
627   elements. A sender &MUST-NOT; generate protocol elements that convey a
628   meaning that is known by that sender to be false. A sender &MUST-NOT;
629   generate protocol elements that do not match the grammar defined by the
630   ABNF rules for those protocol elements that are applicable to the sender's
631   role. If a received protocol element is processed, the recipient &MUST; be
632   able to parse any value that would match the ABNF rules for that protocol
633   element, excluding only those rules not applicable to the recipient's role.
634</t>
635<t>
636   Unless noted otherwise, a recipient &MAY; attempt to recover a usable
637   protocol element from an invalid construct.  HTTP does not define
638   specific error handling mechanisms except when they have a direct impact
639   on security, since different applications of the protocol require
640   different error handling strategies.  For example, a Web browser might
641   wish to transparently recover from a response where the
642   <x:ref>Location</x:ref> header field doesn't parse according to the ABNF,
643   whereas a systems control client might consider any form of error recovery
644   to be dangerous.
645</t>
646</section>
647
648<section title="Protocol Versioning" anchor="http.version">
649  <x:anchor-alias value="HTTP-version"/>
650  <x:anchor-alias value="HTTP-name"/>
651<t>
652   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate
653   versions of the protocol. This specification defines version "1.1".
654   The protocol version as a whole indicates the sender's conformance
655   with the set of requirements laid out in that version's corresponding
656   specification of HTTP.
657</t>
658<t>
659   The version of an HTTP message is indicated by an HTTP-version field
660   in the first line of the message. HTTP-version is case-sensitive.
661</t>
662<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-version"/><iref primary="true" item="Grammar" subitem="HTTP-name"/>
663  <x:ref>HTTP-version</x:ref>  = <x:ref>HTTP-name</x:ref> "/" <x:ref>DIGIT</x:ref> "." <x:ref>DIGIT</x:ref>
664  <x:ref>HTTP-name</x:ref>     = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
665</artwork></figure>
666<t>
667   The HTTP version number consists of two decimal digits separated by a "."
668   (period or decimal point).  The first digit ("major version") indicates the
669   HTTP messaging syntax, whereas the second digit ("minor version") indicates
670   the highest minor version 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   An HTTP 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.  An HTTP client &MUST-NOT; send a
727   version to which it is not conformant.
728</t>
729<t>
730   An HTTP 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   An HTTP 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.  An HTTP 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   An HTTP server &MAY; send an HTTP/1.0 response to an HTTP/1.0 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</section>
768
769<section title="Uniform Resource Identifiers" anchor="uri">
770<iref primary="true" item="resource"/>
771<t>
772   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used
773   throughout HTTP as the means for identifying resources (&resource;).
774   URI references are used to target requests, indicate redirects, and define
775   relationships.
776</t>
777  <x:anchor-alias value="URI-reference"/>
778  <x:anchor-alias value="absolute-URI"/>
779  <x:anchor-alias value="relative-part"/>
780  <x:anchor-alias value="authority"/>
781  <x:anchor-alias value="path-abempty"/>
782  <x:anchor-alias value="port"/>
783  <x:anchor-alias value="query"/>
784  <x:anchor-alias value="segment"/>
785  <x:anchor-alias value="uri-host"/>
786  <x:anchor-alias value="absolute-path"/>
787  <x:anchor-alias value="partial-URI"/>
788<t>
789   This specification adopts the definitions of "URI-reference",
790   "absolute-URI", "relative-part", "port", "host",
791   "path-abempty", "query", "segment", and "authority" from the
792   URI generic syntax.
793   In addition, we define an "absolute-path" rule (that differs from
794   RFC 3986's "path-absolute" in that it allows a leading "//")
795   and a "partial-URI" rule for protocol elements
796   that allow a relative URI but not a fragment.
797</t>
798<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="URI-reference"><!--exported production--></iref><iref primary="true" item="Grammar" subitem="absolute-URI"/><iref primary="true" item="Grammar" subitem="authority"/><iref primary="true" item="Grammar" subitem="absolute-path"/><iref primary="true" item="Grammar" subitem="port"/><iref primary="true" item="Grammar" subitem="query"/><iref primary="true" item="Grammar" subitem="segment"/><iref primary="true" item="Grammar" subitem="uri-host"/><iref primary="true" item="Grammar" subitem="partial-URI"><!--exported production--></iref>
799  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
800  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
801  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
802  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
803  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
804  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
805  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
806  <x:ref>segment</x:ref>       = &lt;segment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
807  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
808 
809  <x:ref>absolute-path</x:ref> = 1*( "/" segment )
810  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
811</artwork></figure>
812<t>
813   Each protocol element in HTTP that allows a URI reference will indicate
814   in its ABNF production whether the element allows any form of reference
815   (URI-reference), only a URI in absolute form (absolute-URI), only the
816   path and optional query components, or some combination of the above.
817   Unless otherwise indicated, URI references are parsed
818   relative to the effective request URI
819   (<xref target="effective.request.uri"/>).
820</t>
821
822<section title="http URI scheme" anchor="http.uri">
823  <x:anchor-alias value="http-URI"/>
824  <iref item="http URI scheme" primary="true"/>
825  <iref item="URI scheme" subitem="http" primary="true"/>
826<t>
827   The "http" URI scheme is hereby defined for the purpose of minting
828   identifiers according to their association with the hierarchical
829   namespace governed by a potential HTTP origin server listening for
830   TCP (<xref target="RFC0793"/>) connections on a given port.
831</t>
832<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"><!--terminal production--></iref>
833  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
834</artwork></figure>
835<t>
836   The HTTP origin server is identified by the generic syntax's
837   <x:ref>authority</x:ref> component, which includes a host identifier
838   and optional TCP port (<xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>).
839   The remainder of the URI, consisting of both the hierarchical path
840   component and optional query component, serves as an identifier for
841   a potential resource within that origin server's name space.
842</t>
843<t>
844   If the host identifier is provided as an IP address,
845   then the origin server is any listener on the indicated TCP port at
846   that IP address. If host is a registered name, then that name is
847   considered an indirect identifier and the recipient might use a name
848   resolution service, such as DNS, to find the address of a listener
849   for that host.
850   The host &MUST-NOT; be empty; if an "http" URI is received with an
851   empty host, then it &MUST; be rejected as invalid.
852   If the port subcomponent is empty or not given, then TCP port 80 is
853   assumed (the default reserved port for WWW services).
854</t>
855<t>
856   Regardless of the form of host identifier, access to that host is not
857   implied by the mere presence of its name or address. The host might or might
858   not exist and, even when it does exist, might or might not be running an
859   HTTP server or listening to the indicated port. The "http" URI scheme
860   makes use of the delegated nature of Internet names and addresses to
861   establish a naming authority (whatever entity has the ability to place
862   an HTTP server at that Internet name or address) and allows that
863   authority to determine which names are valid and how they might be used.
864</t>
865<t>
866   When an "http" URI is used within a context that calls for access to the
867   indicated resource, a client &MAY; attempt access by resolving
868   the host to an IP address, establishing a TCP connection to that address
869   on the indicated port, and sending an HTTP request message
870   (<xref target="http.message"/>) containing the URI's identifying data
871   (<xref target="message.routing"/>) to the server.
872   If the server responds to that request with a non-interim HTTP response
873   message, as described in &status-codes;, then that response
874   is considered an authoritative answer to the client's request.
875</t>
876<t>
877   Although HTTP is independent of the transport protocol, the "http"
878   scheme is specific to TCP-based services because the name delegation
879   process depends on TCP for establishing authority.
880   An HTTP service based on some other underlying connection protocol
881   would presumably be identified using a different URI scheme, just as
882   the "https" scheme (below) is used for resources that require an
883   end-to-end secured connection. Other protocols might also be used to
884   provide access to "http" identified resources &mdash; it is only the
885   authoritative interface used for mapping the namespace that is
886   specific to TCP.
887</t>
888<t>
889   The URI generic syntax for authority also includes a deprecated
890   userinfo subcomponent (<xref target="RFC3986" x:fmt="," x:sec="3.2.1"/>)
891   for including user authentication information in the URI.  Some
892   implementations make use of the userinfo component for internal
893   configuration of authentication information, such as within command
894   invocation options, configuration files, or bookmark lists, even
895   though such usage might expose a user identifier or password.
896   Senders &MUST; exclude the userinfo subcomponent (and its "@"
897   delimiter) when an "http" URI is transmitted within a message as a
898   request target or header field value.
899   Recipients of an "http" URI reference &SHOULD; parse for userinfo and
900   treat its presence as an error, since it is likely being used to obscure
901   the authority for the sake of phishing attacks.
902</t>
903</section>
904
905<section title="https URI scheme" anchor="https.uri">
906   <x:anchor-alias value="https-URI"/>
907   <iref item="https URI scheme"/>
908   <iref item="URI scheme" subitem="https"/>
909<t>
910   The "https" URI scheme is hereby defined for the purpose of minting
911   identifiers according to their association with the hierarchical
912   namespace governed by a potential HTTP origin server listening to a
913   given TCP port for TLS-secured connections
914   (<xref target="RFC0793"/>, <xref target="RFC5246"/>).
915</t>
916<t>
917   All of the requirements listed above for the "http" scheme are also
918   requirements for the "https" scheme, except that a default TCP port
919   of 443 is assumed if the port subcomponent is empty or not given,
920   and the TCP connection &MUST; be secured, end-to-end, through the
921   use of strong encryption prior to sending the first HTTP request.
922</t>
923<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="https-URI"><!--terminal production--></iref>
924  <x:ref>https-URI</x:ref> = "https:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
925</artwork></figure>
926<t>
927   Resources made available via the "https" scheme have no shared
928   identity with the "http" scheme even if their resource identifiers
929   indicate the same authority (the same host listening to the same
930   TCP port).  They are distinct name spaces and are considered to be
931   distinct origin servers.  However, an extension to HTTP that is
932   defined to apply to entire host domains, such as the Cookie protocol
933   <xref target="RFC6265"/>, can allow information
934   set by one service to impact communication with other services
935   within a matching group of host domains.
936</t>
937<t>
938   The process for authoritative access to an "https" identified
939   resource is defined in <xref target="RFC2818"/>.
940</t>
941</section>
942
943<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
944<t>
945   Since the "http" and "https" schemes conform to the URI generic syntax,
946   such URIs are normalized and compared according to the algorithm defined
947   in <xref target="RFC3986" x:fmt="," x:sec="6"/>, using the defaults
948   described above for each scheme.
949</t>
950<t>
951   If the port is equal to the default port for a scheme, the normal form is
952   to elide the port subcomponent. When not being used in absolute form as the
953   request target of an OPTIONS request, an empty path component is equivalent
954   to an absolute path of "/", so the normal form is to provide a path of "/"
955   instead. The scheme and host are case-insensitive and normally provided in
956   lowercase; all other components are compared in a case-sensitive manner.
957   Characters other than those in the "reserved" set are equivalent to their
958   percent-encoded octets (see <xref target="RFC3986" x:fmt=","
959   x:sec="2.1"/>): the normal form is to not encode them.
960</t>
961<t>
962   For example, the following three URIs are equivalent:
963</t>
964<figure><artwork type="example">
965   http://example.com:80/~smith/home.html
966   http://EXAMPLE.com/%7Esmith/home.html
967   http://EXAMPLE.com:/%7esmith/home.html
968</artwork></figure>
969</section>
970</section>
971</section>
972
973<section title="Message Format" anchor="http.message">
974<x:anchor-alias value="generic-message"/>
975<x:anchor-alias value="message.types"/>
976<x:anchor-alias value="HTTP-message"/>
977<x:anchor-alias value="start-line"/>
978<iref item="header section"/>
979<iref item="headers"/>
980<iref item="header field"/>
981<t>
982   All HTTP/1.1 messages consist of a start-line followed by a sequence of
983   octets in a format similar to the Internet Message Format
984   <xref target="RFC5322"/>: zero or more header fields (collectively
985   referred to as the "headers" or the "header section"), an empty line
986   indicating the end of the header section, and an optional message body.
987</t>
988<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"><!--terminal production--></iref>
989  <x:ref>HTTP-message</x:ref>   = <x:ref>start-line</x:ref>
990                   *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
991                   <x:ref>CRLF</x:ref>
992                   [ <x:ref>message-body</x:ref> ]
993</artwork></figure>
994<t>
995   The normal procedure for parsing an HTTP message is to read the
996   start-line into a structure, read each header field into a hash
997   table by field name until the empty line, and then use the parsed
998   data to determine if a message body is expected.  If a message body
999   has been indicated, then it is read as a stream until an amount
1000   of octets equal to the message body length is read or the connection
1001   is closed.
1002</t>
1003<t>
1004   Recipients &MUST; parse an HTTP message as a sequence of octets in an
1005   encoding that is a superset of US-ASCII <xref target="USASCII"/>.
1006   Parsing an HTTP message as a stream of Unicode characters, without regard
1007   for the specific encoding, creates security vulnerabilities due to the
1008   varying ways that string processing libraries handle invalid multibyte
1009   character sequences that contain the octet LF (%x0A).  String-based
1010   parsers can only be safely used within protocol elements after the element
1011   has been extracted from the message, such as within a header field-value
1012   after message parsing has delineated the individual fields.
1013</t>
1014<t>
1015   An HTTP message can be parsed as a stream for incremental processing or
1016   forwarding downstream.  However, recipients cannot rely on incremental
1017   delivery of partial messages, since some implementations will buffer or
1018   delay message forwarding for the sake of network efficiency, security
1019   checks, or payload transformations.
1020</t>
1021<t>
1022   A sender &MUST-NOT; send whitespace between the start-line and
1023   the first header field.
1024   A recipient that receives whitespace between the start-line and
1025   the first header field &MUST; either reject the message as invalid or
1026   consume each whitespace-preceded line without further processing of it
1027   (i.e., ignore the entire line, along with any subsequent lines preceded
1028   by whitespace, until a properly formed header field is received or the
1029   header block is terminated).
1030</t>
1031<t>
1032   The presence of such whitespace in a request
1033   might be an attempt to trick a server into ignoring that field or
1034   processing the line after it as a new request, either of which might
1035   result in a security vulnerability if other implementations within
1036   the request chain interpret the same message differently.
1037   Likewise, the presence of such whitespace in a response might be
1038   ignored by some clients or cause others to cease parsing.
1039</t>
1040
1041<section title="Start Line" anchor="start.line">
1042  <x:anchor-alias value="Start-Line"/>
1043<t>
1044   An HTTP message can either be a request from client to server or a
1045   response from server to client.  Syntactically, the two types of message
1046   differ only in the start-line, which is either a request-line (for requests)
1047   or a status-line (for responses), and in the algorithm for determining
1048   the length of the message body (<xref target="message.body"/>).
1049</t>
1050<t>
1051   In theory, a client could receive requests and a server could receive
1052   responses, distinguishing them by their different start-line formats,
1053   but in practice servers are implemented to only expect a request
1054   (a response is interpreted as an unknown or invalid request method)
1055   and clients are implemented to only expect a response.
1056</t>
1057<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="start-line"/>
1058  <x:ref>start-line</x:ref>     = <x:ref>request-line</x:ref> / <x:ref>status-line</x:ref>
1059</artwork></figure>
1060
1061<section title="Request Line" anchor="request.line">
1062  <x:anchor-alias value="Request"/>
1063  <x:anchor-alias value="request-line"/>
1064<t>
1065   A request-line begins with a method token, followed by a single
1066   space (SP), the request-target, another single space (SP), the
1067   protocol version, and ending with CRLF.
1068</t>
1069<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1070  <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>
1071</artwork></figure>
1072<iref primary="true" item="method"/>
1073<t anchor="method">
1074   The method token indicates the request method to be performed on the
1075   target resource. The request method is case-sensitive.
1076</t>
1077<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1078  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1079</artwork></figure>
1080<t>
1081   The methods defined by this specification can be found in
1082   &methods;, along with information regarding the HTTP method registry
1083   and considerations for defining new methods.
1084</t>
1085<iref item="request-target"/>
1086<t>
1087   The request-target identifies the target resource upon which to apply
1088   the request, as defined in <xref target="request-target"/>.
1089</t>
1090<t>
1091   Recipients typically parse the request-line into its component parts by
1092   splitting on whitespace (see <xref target="message.robustness"/>), since
1093   no whitespace is allowed in the three components.
1094   Unfortunately, some user agents fail to properly encode or exclude
1095   whitespace found in hypertext references, resulting in those disallowed
1096   characters being sent in a request-target.
1097</t>
1098<t>
1099   Recipients of an invalid request-line &SHOULD; respond with either a
1100   <x:ref>400 (Bad Request)</x:ref> error or a <x:ref>301 (Moved Permanently)</x:ref>
1101   redirect with the request-target properly encoded.  Recipients &SHOULD-NOT;
1102   attempt to autocorrect and then process the request without a redirect,
1103   since the invalid request-line might be deliberately crafted to bypass
1104   security filters along the request chain.
1105</t>
1106<t>
1107   HTTP does not place a pre-defined limit on the length of a request-line.
1108   A server that receives a method longer than any that it implements
1109   &SHOULD; respond with a <x:ref>501 (Not Implemented)</x:ref> status code.
1110   A server &MUST; be prepared to receive URIs of unbounded length and
1111   respond with the <x:ref>414 (URI Too Long)</x:ref> status code if the received
1112   request-target would be longer than the server wishes to handle
1113   (see &status-414;).
1114</t>
1115<t>
1116   Various ad-hoc limitations on request-line length are found in practice.
1117   It is &RECOMMENDED; that all HTTP senders and recipients support, at a
1118   minimum, request-line lengths of 8000 octets.
1119</t>
1120</section>
1121
1122<section title="Status Line" anchor="status.line">
1123  <x:anchor-alias value="response"/>
1124  <x:anchor-alias value="status-line"/>
1125  <x:anchor-alias value="status-code"/>
1126  <x:anchor-alias value="reason-phrase"/>
1127<t>
1128   The first line of a response message is the status-line, consisting
1129   of the protocol version, a space (SP), the status code, another space,
1130   a possibly-empty textual phrase describing the status code, and
1131   ending with CRLF.
1132</t>
1133<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-line"/>
1134  <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>
1135</artwork></figure>
1136<t>
1137   The status-code element is a 3-digit integer code describing the
1138   result of the server's attempt to understand and satisfy the client's
1139   corresponding request. The rest of the response message is to be
1140   interpreted in light of the semantics defined for that status code.
1141   See &status-codes; for information about the semantics of status codes,
1142   including the classes of status code (indicated by the first digit),
1143   the status codes defined by this specification, considerations for the
1144   definition of new status codes, and the IANA registry.
1145</t>
1146<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-code"/>
1147  <x:ref>status-code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1148</artwork></figure>
1149<t>  
1150   The reason-phrase element exists for the sole purpose of providing a
1151   textual description associated with the numeric status code, mostly
1152   out of deference to earlier Internet application protocols that were more
1153   frequently used with interactive text clients. A client &SHOULD; ignore
1154   the reason-phrase content.
1155</t>
1156<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
1157  <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> )
1158</artwork></figure>
1159</section>
1160</section>
1161
1162<section title="Header Fields" anchor="header.fields">
1163  <x:anchor-alias value="header-field"/>
1164  <x:anchor-alias value="field-content"/>
1165  <x:anchor-alias value="field-name"/>
1166  <x:anchor-alias value="field-value"/>
1167  <x:anchor-alias value="obs-fold"/>
1168<t>
1169   Each HTTP header field consists of a case-insensitive field name
1170   followed by a colon (":"), optional leading whitespace, the field value,
1171   and optional trailing whitespace.
1172</t>
1173<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"/>
1174  <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>
1175  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1176  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>obs-fold</x:ref> )
1177  <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> )
1178  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref> ( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1179                 ; obsolete line folding
1180                 ; see <xref target="field.parsing"/>
1181</artwork></figure>
1182<t>
1183   The field-name token labels the corresponding field-value as having the
1184   semantics defined by that header field.  For example, the <x:ref>Date</x:ref>
1185   header field is defined in &header-date; as containing the origination
1186   timestamp for the message in which it appears.
1187</t>
1188
1189<section title="Field Extensibility" anchor="field.extensibility">
1190<t>
1191   HTTP header fields are fully extensible: there is no limit on the
1192   introduction of new field names, each presumably defining new semantics,
1193   nor on the number of header fields used in a given message.  Existing
1194   fields are defined in each part of this specification and in many other
1195   specifications outside the core standard.
1196   New header fields can be introduced without changing the protocol version
1197   if their defined semantics allow them to be safely ignored by recipients
1198   that do not recognize them.
1199</t>
1200<t>
1201   New HTTP header fields ought to be registered with IANA in the
1202   Message Header Field Registry, as described in &iana-header-registry;.
1203   A proxy &MUST; forward unrecognized header fields unless the
1204   field-name is listed in the <x:ref>Connection</x:ref> header field
1205   (<xref target="header.connection"/>) or the proxy is specifically
1206   configured to block, or otherwise transform, such fields.
1207   Other recipients &SHOULD; ignore unrecognized header fields.
1208</t>
1209</section>
1210
1211<section title="Field Order" anchor="field.order">
1212<t>
1213   The order in which header fields with differing field names are
1214   received is not significant. However, it is "good practice" to send
1215   header fields that contain control data first, such as <x:ref>Host</x:ref>
1216   on requests and <x:ref>Date</x:ref> on responses, so that implementations
1217   can decide when not to handle a message as early as possible.  A server
1218   &MUST; wait until the entire header section is received before interpreting
1219   a request message, since later header fields might include conditionals,
1220   authentication credentials, or deliberately misleading duplicate
1221   header fields that would impact request processing.
1222</t>
1223<t>
1224   A sender &MUST-NOT; generate multiple header fields with the same field
1225   name in a message unless either the entire field value for that
1226   header field is defined as a comma-separated list [i.e., #(values)]
1227   or the header field is a well-known exception (as noted below).
1228</t>
1229<t>
1230   Multiple header fields with the same field name can be combined into
1231   one "field-name: field-value" pair, without changing the semantics of the
1232   message, by appending each subsequent field value to the combined
1233   field value in order, separated by a comma. The order in which
1234   header fields with the same field name are received is therefore
1235   significant to the interpretation of the combined field value;
1236   a proxy &MUST-NOT; change the order of these field values when
1237   forwarding a message.
1238</t>
1239<x:note>
1240  <t>
1241   &Note; In practice, the "Set-Cookie" header field (<xref target="RFC6265"/>)
1242   often appears multiple times in a response message and does not use the
1243   list syntax, violating the above requirements on multiple header fields
1244   with the same name. Since it cannot be combined into a single field-value,
1245   recipients ought to handle "Set-Cookie" as a special case while processing
1246   header fields. (See Appendix A.2.3 of <xref target="Kri2001"/> for details.)
1247  </t>
1248</x:note>
1249</section>
1250
1251<section title="Whitespace" anchor="whitespace">
1252<t anchor="rule.LWS">
1253   This specification uses three rules to denote the use of linear
1254   whitespace: OWS (optional whitespace), RWS (required whitespace), and
1255   BWS ("bad" whitespace).
1256</t>
1257<t anchor="rule.OWS">
1258   The OWS rule is used where zero or more linear whitespace octets might
1259   appear. For protocol elements where optional whitespace is preferred to
1260   improve readability, a sender &SHOULD; generate the optional whitespace
1261   as a single SP; otherwise, a sender &SHOULD-NOT; generate optional
1262   whitespace except as needed to white-out invalid or unwanted protocol
1263   elements during in-place message filtering.
1264</t>
1265<t anchor="rule.RWS">
1266   The RWS rule is used when at least one linear whitespace octet is required
1267   to separate field tokens. A sender &SHOULD; generate RWS as a single SP.
1268</t>
1269<t anchor="rule.BWS">
1270   The BWS rule is used where the grammar allows optional whitespace only for
1271   historical reasons. A sender &MUST-NOT; generate BWS in messages.
1272   A recipient &MUST; parse for such bad whitespace and remove it before
1273   interpreting the protocol element.
1274</t>
1275<t anchor="rule.whitespace">
1276  <x:anchor-alias value="BWS"/>
1277  <x:anchor-alias value="OWS"/>
1278  <x:anchor-alias value="RWS"/>
1279</t>
1280<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"/>
1281  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1282                 ; optional whitespace
1283  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1284                 ; required whitespace
1285  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1286                 ; "bad" whitespace
1287</artwork></figure>
1288</section>
1289
1290<section title="Field Parsing" anchor="field.parsing">
1291<t>
1292   No whitespace is allowed between the header field-name and colon.
1293   In the past, differences in the handling of such whitespace have led to
1294   security vulnerabilities in request routing and response handling.
1295   A server &MUST; reject any received request message that contains
1296   whitespace between a header field-name and colon with a response code of
1297   <x:ref>400 (Bad Request)</x:ref>. A proxy &MUST; remove any such whitespace
1298   from a response message before forwarding the message downstream.
1299</t>
1300<t>
1301   A field value is preceded by optional whitespace (OWS); a single SP is
1302   preferred. The field value does not include any leading or trailing white
1303   space: OWS occurring before the first non-whitespace octet of the field
1304   value or after the last non-whitespace octet of the field value ought to be
1305   excluded by parsers when extracting the field value from a header field.
1306</t>
1307<t>
1308   A recipient of field-content containing multiple sequential octets of
1309   optional (OWS) or required (RWS) whitespace &SHOULD; either replace the
1310   sequence with a single SP or transform any non-SP octets in the sequence to
1311   SP octets before interpreting the field value or forwarding the message
1312   downstream.
1313</t>
1314<t>
1315   Historically, HTTP header field values could be extended over multiple
1316   lines by preceding each extra line with at least one space or horizontal
1317   tab (obs-fold). This specification deprecates such line folding except
1318   within the message/http media type
1319   (<xref target="internet.media.type.message.http"/>).
1320   Senders &MUST-NOT; generate messages that include line folding
1321   (i.e., that contain any field-value that contains a match to the
1322   <x:ref>obs-fold</x:ref> rule) unless the message is intended for packaging
1323   within the message/http media type.
1324</t>
1325<t>
1326   A server that receives an <x:ref>obs-fold</x:ref> in a request message that
1327   is not within a message/http container &MUST; either reject the message by
1328   sending a <x:ref>400 (Bad Request)</x:ref>, preferably with a
1329   representation explaining that obsolete line folding is unacceptable, or
1330   replace each received <x:ref>obs-fold</x:ref> with one or more
1331   <x:ref>SP</x:ref> octets prior to interpreting the field value or
1332   forwarding the message downstream.
1333</t>
1334<t>
1335   A proxy or gateway that receives an <x:ref>obs-fold</x:ref> in a response
1336   message that is not within a message/http container &MUST; either discard
1337   the message and replace it with a <x:ref>502 (Bad Gateway)</x:ref>
1338   response, preferably with a representation explaining that unacceptable
1339   line folding was received, or replace each received <x:ref>obs-fold</x:ref>
1340   with one or more <x:ref>SP</x:ref> octets prior to interpreting the field
1341   value or forwarding the message downstream.
1342</t>
1343<t>
1344   A user agent that receives an <x:ref>obs-fold</x:ref> in a response message
1345   that is not within a message/http container &MUST; replace each received
1346   <x:ref>obs-fold</x:ref> with one or more <x:ref>SP</x:ref> octets prior to
1347   interpreting the field value.
1348</t>
1349<t>
1350   Historically, HTTP has allowed field content with text in the ISO-8859-1
1351   <xref target="ISO-8859-1"/> charset, supporting other charsets only
1352   through use of <xref target="RFC2047"/> encoding.
1353   In practice, most HTTP header field values use only a subset of the
1354   US-ASCII charset <xref target="USASCII"/>. Newly defined
1355   header fields &SHOULD; limit their field values to US-ASCII octets.
1356   Recipients &SHOULD; treat other octets in field content (obs-text) as
1357   opaque data.
1358</t>
1359</section>
1360
1361<section title="Field Limits" anchor="field.limits">
1362<t>
1363   HTTP does not place a pre-defined limit on the length of each header field
1364   or on the length of the header block as a whole.  Various ad-hoc
1365   limitations on individual header field length are found in practice,
1366   often depending on the specific field semantics.
1367</t>
1368<t>
1369   A server &MUST; be prepared to receive request header fields of unbounded
1370   length and respond with an appropriate <x:ref>4xx (Client Error)</x:ref>
1371   status code if the received header field(s) are larger than the server
1372   wishes to process.
1373</t>
1374<t>
1375   A client &MUST; be prepared to receive response header fields of unbounded
1376   length. A client &MAY; discard or truncate received header fields that are
1377   larger than the client wishes to process if the field semantics are such
1378   that the dropped value(s) can be safely ignored without changing the
1379   response semantics.
1380</t>
1381</section>
1382
1383<section title="Field value components" anchor="field.components">
1384<t anchor="rule.token.separators">
1385  <x:anchor-alias value="tchar"/>
1386  <x:anchor-alias value="token"/>
1387  <x:anchor-alias value="special"/>
1388  <x:anchor-alias value="word"/>
1389   Many HTTP header field values consist of words (token or quoted-string)
1390   separated by whitespace or special characters. These special characters
1391   &MUST; be in a quoted string to be used within a parameter value (as defined
1392   in <xref target="transfer.codings"/>).
1393</t>
1394<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>
1395  <x:ref>word</x:ref>           = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1396
1397  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1398<!--
1399  IMPORTANT: when editing "tchar" make sure that "special" is updated accordingly!!!
1400 -->
1401  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1402                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1403                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1404                 ; any <x:ref>VCHAR</x:ref>, except <x:ref>special</x:ref>
1405
1406  <x:ref>special</x:ref>        = "(" / ")" / "&lt;" / ">" / "@" / ","
1407                 / ";" / ":" / "\" / DQUOTE / "/" / "["
1408                 / "]" / "?" / "=" / "{" / "}"
1409</artwork></figure>
1410<t anchor="rule.quoted-string">
1411  <x:anchor-alias value="quoted-string"/>
1412  <x:anchor-alias value="qdtext"/>
1413  <x:anchor-alias value="obs-text"/>
1414   A string of text is parsed as a single word if it is quoted using
1415   double-quote marks.
1416</t>
1417<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"/>
1418  <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>
1419  <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>
1420  <x:ref>obs-text</x:ref>       = %x80-FF
1421</artwork></figure>
1422<t anchor="rule.quoted-pair">
1423  <x:anchor-alias value="quoted-pair"/>
1424   The backslash octet ("\") can be used as a single-octet
1425   quoting mechanism within quoted-string constructs:
1426</t>
1427<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1428  <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> )
1429</artwork></figure>
1430<t>
1431   Recipients that process the value of a quoted-string &MUST; handle a
1432   quoted-pair as if it were replaced by the octet following the backslash.
1433</t>
1434<t>
1435   Senders &SHOULD-NOT; generate a quoted-pair in a quoted-string except where
1436   necessary to quote DQUOTE and backslash octets occurring within that string.
1437</t>
1438<t anchor="rule.comment">
1439  <x:anchor-alias value="comment"/>
1440  <x:anchor-alias value="ctext"/>
1441   Comments can be included in some HTTP header fields by surrounding
1442   the comment text with parentheses. Comments are only allowed in
1443   fields containing "comment" as part of their field value definition.
1444</t>
1445<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1446  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-cpair</x:ref> / <x:ref>comment</x:ref> ) ")"
1447  <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>
1448</artwork></figure>
1449<t anchor="rule.quoted-cpair">
1450  <x:anchor-alias value="quoted-cpair"/>
1451   The backslash octet ("\") can be used as a single-octet
1452   quoting mechanism within comment constructs:
1453</t>
1454<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-cpair"/>
1455  <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> )
1456</artwork></figure>
1457<t>
1458   Senders &SHOULD-NOT; escape octets in comments that do not require escaping
1459   (i.e., other than the backslash octet "\" and the parentheses "(" and ")").
1460</t>
1461</section>
1462
1463</section>
1464
1465<section title="Message Body" anchor="message.body">
1466  <x:anchor-alias value="message-body"/>
1467<t>
1468   The message body (if any) of an HTTP message is used to carry the
1469   payload body of that request or response.  The message body is
1470   identical to the payload body unless a transfer coding has been
1471   applied, as described in <xref target="header.transfer-encoding"/>.
1472</t>
1473<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1474  <x:ref>message-body</x:ref> = *OCTET
1475</artwork></figure>
1476<t>
1477   The rules for when a message body is allowed in a message differ for
1478   requests and responses.
1479</t>
1480<t>
1481   The presence of a message body in a request is signaled by a
1482   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1483   field. Request message framing is independent of method semantics,
1484   even if the method does not define any use for a message body.
1485</t>
1486<t>
1487   The presence of a message body in a response depends on both
1488   the request method to which it is responding and the response
1489   status code (<xref target="status.line"/>).
1490   Responses to the HEAD request method never include a message body
1491   because the associated response header fields (e.g.,
1492   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1493   if present, indicate only what their values would have been if the request
1494   method had been GET (&HEAD;).
1495   <x:ref>2xx (Successful)</x:ref> responses to CONNECT switch to tunnel
1496   mode instead of having a message body (&CONNECT;).
1497   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1498   <x:ref>304 (Not Modified)</x:ref> responses do not include a message body.
1499   All other responses do include a message body, although the body
1500   might be of zero length.
1501</t>
1502
1503<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1504  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1505  <iref item="chunked (Coding Format)"/>
1506  <x:anchor-alias value="Transfer-Encoding"/>
1507<t>
1508   The Transfer-Encoding header field lists the transfer coding names
1509   corresponding to the sequence of transfer codings that have been
1510   (or will be) applied to the payload body in order to form the message body.
1511   Transfer codings are defined in <xref target="transfer.codings"/>.
1512</t>
1513<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1514  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1515</artwork></figure>
1516<t>
1517   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1518   MIME, which was designed to enable safe transport of binary data over a
1519   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1520   However, safe transport has a different focus for an 8bit-clean transfer
1521   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1522   accurately delimit a dynamically generated payload and to distinguish
1523   payload encodings that are only applied for transport efficiency or
1524   security from those that are characteristics of the selected resource.
1525</t>
1526<t>
1527   All HTTP/1.1 recipients &MUST; implement the chunked transfer coding
1528   (<xref target="chunked.encoding"/>) because it plays a crucial role in
1529   framing messages when the payload body size is not known in advance.
1530   If chunked is applied to a payload body, the sender &MUST-NOT; apply
1531   chunked more than once (i.e., chunking an already chunked message is not
1532   allowed).
1533   If any transfer coding is applied to a request payload body, the
1534   sender &MUST; apply chunked as the final transfer coding to ensure that
1535   the message is properly framed.
1536   If any transfer coding is applied to a response payload body, the
1537   sender &MUST; either apply chunked as the final transfer coding or
1538   terminate the message by closing the connection.
1539</t>
1540<figure><preamble>
1541   For example,
1542</preamble><artwork type="example">
1543  Transfer-Encoding: gzip, chunked
1544</artwork><postamble>
1545   indicates that the payload body has been compressed using the gzip
1546   coding and then chunked using the chunked coding while forming the
1547   message body.
1548</postamble></figure>
1549<t>
1550   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1551   Transfer-Encoding is a property of the message, not of the representation, and
1552   any recipient along the request/response chain &MAY; decode the received
1553   transfer coding(s) or apply additional transfer coding(s) to the message
1554   body, assuming that corresponding changes are made to the Transfer-Encoding
1555   field-value. Additional information about the encoding parameters &MAY; be
1556   provided by other header fields not defined by this specification.
1557</t>
1558<t>
1559   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1560   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1561   neither of which includes a message body,
1562   to indicate that the origin server would have applied a transfer coding
1563   to the message body if the request had been an unconditional GET.
1564   This indication is not required, however, because any recipient on
1565   the response chain (including the origin server) can remove transfer
1566   codings when they are not needed.
1567</t>
1568<t>
1569   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1570   implementations advertising only HTTP/1.0 support will not understand
1571   how to process a transfer-encoded payload.
1572   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1573   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1574   might be in the form of specific user configuration or by remembering the
1575   version of a prior received response.
1576   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1577   the corresponding request indicates HTTP/1.1 (or later).
1578</t>
1579<t>
1580   A server that receives a request message with a transfer coding it does
1581   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref>.
1582</t>
1583</section>
1584
1585<section title="Content-Length" anchor="header.content-length">
1586  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1587  <x:anchor-alias value="Content-Length"/>
1588<t>
1589   When a message does not have a <x:ref>Transfer-Encoding</x:ref> header
1590   field, a Content-Length header field can provide the anticipated size,
1591   as a decimal number of octets, for a potential payload body.
1592   For messages that do include a payload body, the Content-Length field-value
1593   provides the framing information necessary for determining where the body
1594   (and message) ends.  For messages that do not include a payload body, the
1595   Content-Length indicates the size of the selected representation
1596   (&representation;).
1597</t>
1598<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1599  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1600</artwork></figure>
1601<t>
1602   An example is
1603</t>
1604<figure><artwork type="example">
1605  Content-Length: 3495
1606</artwork></figure>
1607<t>
1608   A sender &MUST-NOT; send a Content-Length header field in any message that
1609   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1610</t>
1611<t>
1612   A user agent &SHOULD; send a Content-Length in a request message when no
1613   <x:ref>Transfer-Encoding</x:ref> is sent and the request method defines
1614   a meaning for an enclosed payload body. For example, a Content-Length
1615   header field is normally sent in a POST request even when the value is
1616   0 (indicating an empty payload body).  A user agent &SHOULD-NOT; send a
1617   Content-Length header field when the request message does not contain a
1618   payload body and the method semantics do not anticipate such a body.
1619</t>
1620<t>
1621   A server &MAY; send a Content-Length header field in a response to a HEAD
1622   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1623   response unless its field-value equals the decimal number of octets that
1624   would have been sent in the payload body of a response if the same
1625   request had used the GET method.
1626</t>
1627<t>
1628   A server &MAY; send a Content-Length header field in a
1629   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1630   (&status-304;); a server &MUST-NOT; send Content-Length in such a
1631   response unless its field-value equals the decimal number of octets that
1632   would have been sent in the payload body of a <x:ref>200 (OK)</x:ref>
1633   response to the same request.
1634</t>
1635<t>
1636   A server &MUST-NOT; send a Content-Length header field in any response
1637   with a status code of
1638   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1639   A server &SHOULD-NOT; send a Content-Length header field in any
1640   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1641</t>
1642<t>
1643   Aside from the cases defined above, in the absence of Transfer-Encoding,
1644   an origin server &SHOULD; send a Content-Length header field when the
1645   payload body size is known prior to sending the complete header block.
1646   This will allow downstream recipients to measure transfer progress,
1647   know when a received message is complete, and potentially reuse the
1648   connection for additional requests.
1649</t>
1650<t>
1651   Any Content-Length field value greater than or equal to zero is valid.
1652   Since there is no predefined limit to the length of a payload,
1653   recipients &SHOULD; anticipate potentially large decimal numerals and
1654   prevent parsing errors due to integer conversion overflows
1655   (<xref target="attack.protocol.element.size.overflows"/>).
1656</t>
1657<t>
1658   If a message is received that has multiple Content-Length header fields
1659   with field-values consisting of the same decimal value, or a single
1660   Content-Length header field with a field value containing a list of
1661   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1662   duplicate Content-Length header fields have been generated or combined by an
1663   upstream message processor, then the recipient &MUST; either reject the
1664   message as invalid or replace the duplicated field-values with a single
1665   valid Content-Length field containing that decimal value prior to
1666   determining the message body length.
1667</t>
1668<x:note>
1669  <t>
1670   &Note; HTTP's use of Content-Length for message framing differs
1671   significantly from the same field's use in MIME, where it is an optional
1672   field used only within the "message/external-body" media-type.
1673  </t>
1674</x:note>
1675</section>
1676
1677<section title="Message Body Length" anchor="message.body.length">
1678  <iref item="chunked (Coding Format)"/>
1679<t>
1680   The length of a message body is determined by one of the following
1681   (in order of precedence):
1682</t>
1683<t>
1684  <list style="numbers">
1685    <x:lt><t>
1686     Any response to a HEAD request and any response with a
1687     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1688     <x:ref>304 (Not Modified)</x:ref> status code is always
1689     terminated by the first empty line after the header fields, regardless of
1690     the header fields present in the message, and thus cannot contain a
1691     message body.
1692    </t></x:lt>
1693    <x:lt><t>
1694     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1695     connection will become a tunnel immediately after the empty line that
1696     concludes the header fields.  A client &MUST; ignore any
1697     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1698     fields received in such a message.
1699    </t></x:lt>
1700    <x:lt><t>
1701     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1702     and the chunked transfer coding (<xref target="chunked.encoding"/>)
1703     is the final encoding, the message body length is determined by reading
1704     and decoding the chunked data until the transfer coding indicates the
1705     data is complete.
1706    </t>
1707    <t>
1708     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1709     response and the chunked transfer coding is not the final encoding, the
1710     message body length is determined by reading the connection until it is
1711     closed by the server.
1712     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a request and the
1713     chunked transfer coding is not the final encoding, the message body
1714     length cannot be determined reliably; the server &MUST; respond with
1715     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1716    </t>
1717    <t>
1718     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1719     and a <x:ref>Content-Length</x:ref> header field, the Transfer-Encoding
1720     overrides the Content-Length. Such a message might indicate an attempt
1721     to perform request or response smuggling (bypass of security-related
1722     checks on message routing or content) and thus ought to be handled as
1723     an error.  A sender &MUST; remove the received Content-Length field
1724     prior to forwarding such a message downstream.
1725    </t></x:lt>
1726    <x:lt><t>
1727     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1728     either multiple <x:ref>Content-Length</x:ref> header fields having
1729     differing field-values or a single Content-Length header field having an
1730     invalid value, then the message framing is invalid and &MUST; be treated
1731     as an error to prevent request or response smuggling.
1732     If this is a request message, the server &MUST; respond with
1733     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1734     If this is a response message received by a proxy, the proxy
1735     &MUST; discard the received response, send a <x:ref>502 (Bad Gateway)</x:ref>
1736     status code as its downstream response, and then close the connection.
1737     If this is a response message received by a user agent, it &MUST; be
1738     treated as an error by discarding the message and closing the connection.
1739    </t></x:lt>
1740    <x:lt><t>
1741     If a valid <x:ref>Content-Length</x:ref> header field is present without
1742     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1743     expected message body length in octets.
1744     If the sender closes the connection or the recipient times out before the
1745     indicated number of octets are received, the recipient &MUST; consider
1746     the message to be incomplete and close the connection.
1747    </t></x:lt>
1748    <x:lt><t>
1749     If this is a request message and none of the above are true, then the
1750     message body length is zero (no message body is present).
1751    </t></x:lt>
1752    <x:lt><t>
1753     Otherwise, this is a response message without a declared message body
1754     length, so the message body length is determined by the number of octets
1755     received prior to the server closing the connection.
1756    </t></x:lt>
1757  </list>
1758</t>
1759<t>
1760   Since there is no way to distinguish a successfully completed,
1761   close-delimited message from a partially-received message interrupted
1762   by network failure, a server &SHOULD; use encoding or
1763   length-delimited messages whenever possible.  The close-delimiting
1764   feature exists primarily for backwards compatibility with HTTP/1.0.
1765</t>
1766<t>
1767   A server &MAY; reject a request that contains a message body but
1768   not a <x:ref>Content-Length</x:ref> by responding with
1769   <x:ref>411 (Length Required)</x:ref>.
1770</t>
1771<t>
1772   Unless a transfer coding other than chunked has been applied,
1773   a client that sends a request containing a message body &SHOULD;
1774   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1775   length is known in advance, rather than the chunked transfer coding, since some
1776   existing services respond to chunked with a <x:ref>411 (Length Required)</x:ref>
1777   status code even though they understand the chunked transfer coding.  This
1778   is typically because such services are implemented via a gateway that
1779   requires a content-length in advance of being called and the server
1780   is unable or unwilling to buffer the entire request before processing.
1781</t>
1782<t>
1783   A user agent that sends a request containing a message body &MUST; send a
1784   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1785   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1786   the form of specific user configuration or by remembering the version of a
1787   prior received response.
1788</t>
1789<t>
1790   If the final response to the last request on a connection has been
1791   completely received and there remains additional data to read, a user agent
1792   &MAY; discard the remaining data or attempt to determine if that data
1793   belongs as part of the prior response body, which might be the case if the
1794   prior message's Content-Length value is incorrect. A client &MUST-NOT;
1795   process, cache, or forward such extra data as a separate response, since
1796   such behavior would be vulnerable to cache poisoning.
1797</t>
1798</section>
1799</section>
1800
1801<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1802<t>
1803   A server that receives an incomplete request message, usually due to a
1804   canceled request or a triggered time-out exception, &MAY; send an error
1805   response prior to closing the connection.
1806</t>
1807<t>
1808   A client that receives an incomplete response message, which can occur
1809   when a connection is closed prematurely or when decoding a supposedly
1810   chunked transfer coding fails, &MUST; record the message as incomplete.
1811   Cache requirements for incomplete responses are defined in
1812   &cache-incomplete;.
1813</t>
1814<t>
1815   If a response terminates in the middle of the header block (before the
1816   empty line is received) and the status code might rely on header fields to
1817   convey the full meaning of the response, then the client cannot assume
1818   that meaning has been conveyed; the client might need to repeat the
1819   request in order to determine what action to take next.
1820</t>
1821<t>
1822   A message body that uses the chunked transfer coding is
1823   incomplete if the zero-sized chunk that terminates the encoding has not
1824   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1825   incomplete if the size of the message body received (in octets) is less than
1826   the value given by Content-Length.  A response that has neither chunked
1827   transfer coding nor Content-Length is terminated by closure of the
1828   connection, and thus is considered complete regardless of the number of
1829   message body octets received, provided that the header block was received
1830   intact.
1831</t>
1832</section>
1833
1834<section title="Message Parsing Robustness" anchor="message.robustness">
1835<t>
1836   Older HTTP/1.0 user agent implementations might send an extra CRLF
1837   after a POST request as a workaround for some early server
1838   applications that failed to read message body content that was
1839   not terminated by a line-ending. An HTTP/1.1 user agent &MUST-NOT;
1840   preface or follow a request with an extra CRLF.  If terminating
1841   the request message body with a line-ending is desired, then the
1842   user agent &MUST; count the terminating CRLF octets as part of the
1843   message body length.
1844</t>
1845<t>
1846   In the interest of robustness, servers &SHOULD; ignore at least one
1847   empty line received where a request-line is expected. In other words, if
1848   a server is reading the protocol stream at the beginning of a
1849   message and receives a CRLF first, the server &SHOULD; ignore the CRLF.
1850</t>
1851<t>
1852   Although the line terminator for the start-line and header
1853   fields is the sequence CRLF, recipients &MAY; recognize a
1854   single LF as a line terminator and ignore any preceding CR.
1855</t>
1856<t>
1857   Although the request-line and status-line grammar rules require that each
1858   of the component elements be separated by a single SP octet, recipients
1859   &MAY; instead parse on whitespace-delimited word boundaries and, aside
1860   from the CRLF terminator, treat any form of whitespace as the SP separator
1861   while ignoring preceding or trailing whitespace;
1862   such whitespace includes one or more of the following octets:
1863   SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
1864</t>
1865<t>
1866   When a server listening only for HTTP request messages, or processing
1867   what appears from the start-line to be an HTTP request message,
1868   receives a sequence of octets that does not match the HTTP-message
1869   grammar aside from the robustness exceptions listed above, the
1870   server &SHOULD; respond with a <x:ref>400 (Bad Request)</x:ref> response. 
1871</t>
1872</section>
1873</section>
1874
1875<section title="Transfer Codings" anchor="transfer.codings">
1876  <x:anchor-alias value="transfer-coding"/>
1877  <x:anchor-alias value="transfer-extension"/>
1878<t>
1879   Transfer coding names are used to indicate an encoding
1880   transformation that has been, can be, or might need to be applied to a
1881   payload body in order to ensure "safe transport" through the network.
1882   This differs from a content coding in that the transfer coding is a
1883   property of the message rather than a property of the representation
1884   that is being transferred.
1885</t>
1886<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1887  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1888                     / "compress" ; <xref target="compress.coding"/>
1889                     / "deflate" ; <xref target="deflate.coding"/>
1890                     / "gzip" ; <xref target="gzip.coding"/>
1891                     / <x:ref>transfer-extension</x:ref>
1892  <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> )
1893</artwork></figure>
1894<t anchor="rule.parameter">
1895  <x:anchor-alias value="attribute"/>
1896  <x:anchor-alias value="transfer-parameter"/>
1897  <x:anchor-alias value="value"/>
1898   Parameters are in the form of attribute/value pairs.
1899</t>
1900<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"/>
1901  <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>
1902  <x:ref>attribute</x:ref>          = <x:ref>token</x:ref>
1903  <x:ref>value</x:ref>              = <x:ref>word</x:ref>
1904</artwork></figure>
1905<t>
1906   All transfer-coding names are case-insensitive and ought to be registered
1907   within the HTTP Transfer Coding registry, as defined in
1908   <xref target="transfer.coding.registry"/>.
1909   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1910   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1911   header fields.
1912</t>
1913
1914<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1915  <iref primary="true" item="chunked (Coding Format)"/>
1916  <x:anchor-alias value="chunk"/>
1917  <x:anchor-alias value="chunked-body"/>
1918  <x:anchor-alias value="chunk-data"/>
1919  <x:anchor-alias value="chunk-ext"/>
1920  <x:anchor-alias value="chunk-ext-name"/>
1921  <x:anchor-alias value="chunk-ext-val"/>
1922  <x:anchor-alias value="chunk-size"/>
1923  <x:anchor-alias value="last-chunk"/>
1924  <x:anchor-alias value="trailer-part"/>
1925  <x:anchor-alias value="quoted-str-nf"/>
1926  <x:anchor-alias value="qdtext-nf"/>
1927<t>
1928   The chunked transfer coding modifies the body of a message in order to
1929   transfer it as a series of chunks, each with its own size indicator,
1930   followed by an &OPTIONAL; trailer containing header fields. This
1931   allows dynamically generated content to be transferred along with the
1932   information necessary for the recipient to verify that it has
1933   received the full message.
1934</t>
1935<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"/>
1936  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1937                   <x:ref>last-chunk</x:ref>
1938                   <x:ref>trailer-part</x:ref>
1939                   <x:ref>CRLF</x:ref>
1940 
1941  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1942                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1943  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1944  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1945 
1946  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
1947  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
1948  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-str-nf</x:ref>
1949  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1950  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1951 
1952  <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>
1953                 ; like <x:ref>quoted-string</x:ref>, but disallowing line folding
1954  <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>
1955</artwork></figure>
1956<t>
1957   Chunk extensions within the chunked transfer coding are deprecated.
1958   Senders &SHOULD-NOT; send chunk-ext.
1959   Definition of new chunk extensions is discouraged.
1960</t>
1961<t>
1962   The chunk-size field is a string of hex digits indicating the size of
1963   the chunk-data in octets. The chunked transfer coding is complete when a
1964   chunk with a chunk-size of zero is received, possibly followed by a
1965   trailer, and finally terminated by an empty line.
1966</t>
1967
1968<section title="Trailer" anchor="header.trailer">
1969  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
1970  <x:anchor-alias value="Trailer"/>
1971<t>
1972   A trailer allows the sender to include additional fields at the end of a
1973   chunked message in order to supply metadata that might be dynamically
1974   generated while the message body is sent, such as a message integrity
1975   check, digital signature, or post-processing status.
1976   The trailer &MUST-NOT; contain fields that need to be known before a
1977   recipient processes the body, such as <x:ref>Transfer-Encoding</x:ref>,
1978   <x:ref>Content-Length</x:ref>, and <x:ref>Trailer</x:ref>.
1979</t>
1980<t>
1981   When a message includes a message body encoded with the chunked
1982   transfer coding and the sender desires to send metadata in the form of
1983   trailer fields at the end of the message, the sender &SHOULD; send a
1984   <x:ref>Trailer</x:ref> header field before the message body to indicate
1985   which fields will be present in the trailers. This allows the recipient
1986   to prepare for receipt of that metadata before it starts processing the body,
1987   which is useful if the message is being streamed and the recipient wishes
1988   to confirm an integrity check on the fly.
1989</t>
1990<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
1991  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
1992</artwork></figure>
1993<t>
1994   If no <x:ref>Trailer</x:ref> header field is present, the sender of a
1995   chunked message body &SHOULD; send an empty trailer.
1996</t>
1997<t>
1998   A server &MUST; send an empty trailer with the chunked transfer coding
1999   unless at least one of the following is true:
2000  <list style="numbers">
2001    <t>the request included a <x:ref>TE</x:ref> header field that indicates
2002    "trailers" is acceptable in the transfer coding of the response, as
2003    described in <xref target="header.te"/>; or,</t>
2004     
2005    <t>the trailer fields consist entirely of optional metadata and the
2006    recipient could use the message (in a manner acceptable to the server where
2007    the field originated) without receiving that metadata. In other words,
2008    the server that generated the header field is willing to accept the
2009    possibility that the trailer fields might be silently discarded along
2010    the path to the client.</t>
2011  </list>
2012</t>
2013<t>
2014   The above requirement prevents the need for an infinite buffer when a
2015   message is being received by an HTTP/1.1 (or later) proxy and forwarded to
2016   an HTTP/1.0 recipient.
2017</t>
2018</section>
2019
2020<section title="Decoding chunked" anchor="decoding.chunked">
2021<t>
2022   A process for decoding the chunked transfer coding
2023   can be represented in pseudo-code as:
2024</t>
2025<figure><artwork type="code">
2026  length := 0
2027  read chunk-size, chunk-ext (if any), and CRLF
2028  while (chunk-size &gt; 0) {
2029     read chunk-data and CRLF
2030     append chunk-data to decoded-body
2031     length := length + chunk-size
2032     read chunk-size, chunk-ext (if any), and CRLF
2033  }
2034  read header-field
2035  while (header-field not empty) {
2036     append header-field to existing header fields
2037     read header-field
2038  }
2039  Content-Length := length
2040  Remove "chunked" from Transfer-Encoding
2041  Remove Trailer from existing header fields
2042</artwork></figure>
2043<t>
2044   All recipients &MUST; be able to receive and decode the
2045   chunked transfer coding and &MUST; ignore chunk-ext extensions
2046   they do not understand.
2047</t>
2048</section>
2049</section>
2050
2051<section title="Compression Codings" anchor="compression.codings">
2052<t>
2053   The codings defined below can be used to compress the payload of a
2054   message.
2055</t>
2056
2057<section title="Compress Coding" anchor="compress.coding">
2058<iref item="compress (Coding Format)"/>
2059<t>
2060   The "compress" format is produced by the common UNIX file compression
2061   program "compress". This format is an adaptive Lempel-Ziv-Welch
2062   coding (LZW). Recipients &SHOULD; consider "x-compress" to be
2063   equivalent to "compress".
2064</t>
2065</section>
2066
2067<section title="Deflate Coding" anchor="deflate.coding">
2068<iref item="deflate (Coding Format)"/>
2069<t>
2070   The "deflate" format is defined as the "deflate" compression mechanism
2071   (described in <xref target="RFC1951"/>) used inside the "zlib"
2072   data format (<xref target="RFC1950"/>).
2073</t>
2074<x:note>
2075  <t>
2076    &Note; Some incorrect implementations send the "deflate"
2077    compressed data without the zlib wrapper.
2078   </t>
2079</x:note>
2080</section>
2081
2082<section title="Gzip Coding" anchor="gzip.coding">
2083<iref item="gzip (Coding Format)"/>
2084<t>
2085   The "gzip" format is produced by the file compression program
2086   "gzip" (GNU zip), as described in <xref target="RFC1952"/>. This format is a
2087   Lempel-Ziv coding (LZ77) with a 32 bit CRC.
2088   Recipients &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2089</t>
2090</section>
2091
2092</section>
2093
2094<section title="TE" anchor="header.te">
2095  <iref primary="true" item="TE header field" x:for-anchor=""/>
2096  <x:anchor-alias value="TE"/>
2097  <x:anchor-alias value="t-codings"/>
2098  <x:anchor-alias value="t-ranking"/>
2099  <x:anchor-alias value="rank"/>
2100<t>
2101   The "TE" header field in a request indicates what transfer codings,
2102   besides chunked, the client is willing to accept in response, and
2103   whether or not the client is willing to accept trailer fields in a
2104   chunked transfer coding.
2105</t>
2106<t>
2107   The TE field-value consists of a comma-separated list of transfer coding
2108   names, each allowing for optional parameters (as described in
2109   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2110   Clients &MUST-NOT; send the chunked transfer coding name in TE;
2111   chunked is always acceptable for HTTP/1.1 recipients.
2112</t>
2113<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"/>
2114  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2115  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2116  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2117  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2118             / ( "1" [ "." 0*3("0") ] )
2119</artwork></figure>
2120<t>
2121   Three examples of TE use are below.
2122</t>
2123<figure><artwork type="example">
2124  TE: deflate
2125  TE:
2126  TE: trailers, deflate;q=0.5
2127</artwork></figure>
2128<t>
2129   The presence of the keyword "trailers" indicates that the client is willing
2130   to accept trailer fields in a chunked transfer coding, as defined in
2131   <xref target="chunked.encoding"/>, on behalf of itself and any downstream
2132   clients. For requests from an intermediary, this implies that either:
2133   (a) all downstream clients are willing to accept trailer fields in the
2134   forwarded response; or,
2135   (b) the intermediary will attempt to buffer the response on behalf of
2136   downstream recipients.
2137   Note that HTTP/1.1 does not define any means to limit the size of a
2138   chunked response such that an intermediary can be assured of buffering the
2139   entire response.
2140</t>
2141<t>
2142   When multiple transfer codings are acceptable, the client &MAY; rank the
2143   codings by preference using a case-insensitive "q" parameter (similar to
2144   the qvalues used in content negotiation fields, &qvalue;). The rank value
2145   is a real number in the range 0 through 1, where 0.001 is the least
2146   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2147</t>
2148<t>
2149   If the TE field-value is empty or if no TE field is present, the only
2150   acceptable transfer coding is chunked. A message with no transfer coding
2151   is always acceptable.
2152</t>
2153<t>
2154   Since the TE header field only applies to the immediate connection,
2155   a sender of TE &MUST; also send a "TE" connection option within the
2156   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2157   in order to prevent the TE field from being forwarded by intermediaries
2158   that do not support its semantics.
2159</t>
2160</section>
2161</section>
2162
2163<section title="Message Routing" anchor="message.routing">
2164<t>
2165   HTTP request message routing is determined by each client based on the
2166   target resource, the client's proxy configuration, and
2167   establishment or reuse of an inbound connection.  The corresponding
2168   response routing follows the same connection chain back to the client.
2169</t>
2170
2171<section title="Identifying a Target Resource" anchor="target-resource">
2172  <iref primary="true" item="target resource"/>
2173  <iref primary="true" item="target URI"/>
2174  <x:anchor-alias value="target resource"/>
2175  <x:anchor-alias value="target URI"/>
2176<t>
2177   HTTP is used in a wide variety of applications, ranging from
2178   general-purpose computers to home appliances.  In some cases,
2179   communication options are hard-coded in a client's configuration.
2180   However, most HTTP clients rely on the same resource identification
2181   mechanism and configuration techniques as general-purpose Web browsers.
2182</t>
2183<t>
2184   HTTP communication is initiated by a user agent for some purpose.
2185   The purpose is a combination of request semantics, which are defined in
2186   <xref target="Part2"/>, and a target resource upon which to apply those
2187   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2188   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2189   would resolve to its absolute form in order to obtain the
2190   "<x:dfn>target URI</x:dfn>".  The target URI
2191   excludes the reference's fragment identifier component, if any,
2192   since fragment identifiers are reserved for client-side processing
2193   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2194</t>
2195</section>
2196
2197<section title="Connecting Inbound" anchor="connecting.inbound">
2198<t>
2199   Once the target URI is determined, a client needs to decide whether
2200   a network request is necessary to accomplish the desired semantics and,
2201   if so, where that request is to be directed.
2202</t>
2203<t>
2204   If the client has a cache <xref target="Part6"/> and the request can be
2205   satisfied by it, then the request is
2206   usually directed there first.
2207</t>
2208<t>
2209   If the request is not satisfied by a cache, then a typical client will
2210   check its configuration to determine whether a proxy is to be used to
2211   satisfy the request.  Proxy configuration is implementation-dependent,
2212   but is often based on URI prefix matching, selective authority matching,
2213   or both, and the proxy itself is usually identified by an "http" or
2214   "https" URI.  If a proxy is applicable, the client connects inbound by
2215   establishing (or reusing) a connection to that proxy.
2216</t>
2217<t>
2218   If no proxy is applicable, a typical client will invoke a handler routine,
2219   usually specific to the target URI's scheme, to connect directly
2220   to an authority for the target resource.  How that is accomplished is
2221   dependent on the target URI scheme and defined by its associated
2222   specification, similar to how this specification defines origin server
2223   access for resolution of the "http" (<xref target="http.uri"/>) and
2224   "https" (<xref target="https.uri"/>) schemes.
2225</t>
2226<t>
2227   HTTP requirements regarding connection management are defined in
2228   <xref target="connection.management"/>.
2229</t>
2230</section>
2231
2232<section title="Request Target" anchor="request-target">
2233<t>
2234   Once an inbound connection is obtained,
2235   the client sends an HTTP request message (<xref target="http.message"/>)
2236   with a request-target derived from the target URI.
2237   There are four distinct formats for the request-target, depending on both
2238   the method being requested and whether the request is to a proxy.
2239</t>   
2240<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"/>
2241  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2242                 / <x:ref>absolute-form</x:ref>
2243                 / <x:ref>authority-form</x:ref>
2244                 / <x:ref>asterisk-form</x:ref>
2245
2246  <x:ref>origin-form</x:ref>    = <x:ref>absolute-path</x:ref> [ "?" <x:ref>query</x:ref> ]
2247  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2248  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2249  <x:ref>asterisk-form</x:ref>  = "*"
2250</artwork></figure>
2251<t anchor="origin-form"><iref item="origin-form (of request-target)"/>
2252  <x:h>origin-form</x:h>
2253</t>
2254<t>
2255   The most common form of request-target is the <x:dfn>origin-form</x:dfn>.
2256   When making a request directly to an origin server, other than a CONNECT
2257   or server-wide OPTIONS request (as detailed below),
2258   a client &MUST; send only the absolute path and query components of
2259   the target URI as the request-target.
2260   If the target URI's path component is empty, then the client &MUST; send
2261   "/" as the path within the origin-form of request-target.
2262   A <x:ref>Host</x:ref> header field is also sent, as defined in
2263   <xref target="header.host"/>, containing the target URI's
2264   authority component (excluding any userinfo).
2265</t>
2266<t>
2267   For example, a client wishing to retrieve a representation of the resource
2268   identified as
2269</t>
2270<figure><artwork x:indent-with="  " type="example">
2271http://www.example.org/where?q=now
2272</artwork></figure>
2273<t>
2274   directly from the origin server would open (or reuse) a TCP connection
2275   to port 80 of the host "www.example.org" and send the lines:
2276</t>
2277<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2278GET /where?q=now HTTP/1.1
2279Host: www.example.org
2280</artwork></figure>
2281<t>
2282   followed by the remainder of the request message.
2283</t>
2284<t anchor="absolute-form"><iref item="absolute-form (of request-target)"/>
2285  <x:h>absolute-form</x:h>
2286</t>
2287<t>
2288   When making a request to a proxy, other than a CONNECT or server-wide
2289   OPTIONS request (as detailed below), a client &MUST; send the target URI
2290   in <x:dfn>absolute-form</x:dfn> as the request-target.
2291   The proxy is requested to either service that request from a valid cache,
2292   if possible, or make the same request on the client's behalf to either
2293   the next inbound proxy server or directly to the origin server indicated
2294   by the request-target.  Requirements on such "forwarding" of messages are
2295   defined in <xref target="message.forwarding"/>.
2296</t>
2297<t>
2298   An example absolute-form of request-line would be:
2299</t>
2300<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2301GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
2302</artwork></figure>
2303<t>
2304   To allow for transition to the absolute-form for all requests in some
2305   future version of HTTP, HTTP/1.1 servers &MUST; accept the absolute-form
2306   in requests, even though HTTP/1.1 clients will only send them in requests
2307   to proxies.
2308</t>
2309<t anchor="authority-form"><iref item="authority-form (of request-target)"/>
2310  <x:h>authority-form</x:h>
2311</t>
2312<t>
2313   The <x:dfn>authority-form</x:dfn> of request-target is only used for CONNECT requests
2314   (&CONNECT;).  When making a CONNECT request to establish a tunnel through
2315   one or more proxies, a client &MUST; send only the target URI's
2316   authority component (excluding any userinfo) as the request-target.
2317   For example,
2318</t>
2319<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2320CONNECT www.example.com:80 HTTP/1.1
2321</artwork></figure>
2322<t anchor="asterisk-form"><iref item="asterisk-form (of request-target)"/>
2323  <x:h>asterisk-form</x:h>
2324</t>
2325<t>
2326   The <x:dfn>asterisk-form</x:dfn> of request-target is only used for a server-wide
2327   OPTIONS request (&OPTIONS;).  When a client wishes to request OPTIONS
2328   for the server as a whole, as opposed to a specific named resource of
2329   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2330   For example,
2331</t>
2332<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2333OPTIONS * HTTP/1.1
2334</artwork></figure>
2335<t>
2336   If a proxy receives an OPTIONS request with an absolute-form of
2337   request-target in which the URI has an empty path and no query component,
2338   then the last proxy on the request chain &MUST; send a request-target
2339   of "*" when it forwards the request to the indicated origin server.
2340</t>
2341<figure><preamble>  
2342   For example, the request
2343</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2344OPTIONS http://www.example.org:8001 HTTP/1.1
2345</artwork></figure>
2346<figure><preamble>  
2347  would be forwarded by the final proxy as
2348</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2349OPTIONS * HTTP/1.1
2350Host: www.example.org:8001
2351</artwork>
2352<postamble>
2353   after connecting to port 8001 of host "www.example.org".
2354</postamble>
2355</figure>
2356</section>
2357
2358<section title="Host" anchor="header.host">
2359  <iref primary="true" item="Host header field" x:for-anchor=""/>
2360  <x:anchor-alias value="Host"/>
2361<t>
2362   The "Host" header field in a request provides the host and port
2363   information from the target URI, enabling the origin
2364   server to distinguish among resources while servicing requests
2365   for multiple host names on a single IP address.  Since the Host
2366   field-value is critical information for handling a request, it
2367   &SHOULD; be sent as the first header field following the request-line.
2368</t>
2369<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2370  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2371</artwork></figure>
2372<t>
2373   A client &MUST; send a Host header field in all HTTP/1.1 request
2374   messages.  If the target URI includes an authority component, then
2375   the Host field-value &MUST; be identical to that authority component
2376   after excluding any userinfo (<xref target="http.uri"/>).
2377   If the authority component is missing or undefined for the target URI,
2378   then the Host header field &MUST; be sent with an empty field-value.
2379</t>
2380<t>
2381   For example, a GET request to the origin server for
2382   &lt;http://www.example.org/pub/WWW/&gt; would begin with:
2383</t>
2384<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2385GET /pub/WWW/ HTTP/1.1
2386Host: www.example.org
2387</artwork></figure>
2388<t>
2389   The Host header field &MUST; be sent in an HTTP/1.1 request even
2390   if the request-target is in the absolute-form, since this
2391   allows the Host information to be forwarded through ancient HTTP/1.0
2392   proxies that might not have implemented Host.
2393</t>
2394<t>
2395   When a proxy receives a request with an absolute-form of
2396   request-target, the proxy &MUST; ignore the received
2397   Host header field (if any) and instead replace it with the host
2398   information of the request-target.  If the proxy forwards the request,
2399   it &MUST; generate a new Host field-value based on the received
2400   request-target rather than forward the received Host field-value.
2401</t>
2402<t>
2403   Since the Host header field acts as an application-level routing
2404   mechanism, it is a frequent target for malware seeking to poison
2405   a shared cache or redirect a request to an unintended server.
2406   An interception proxy is particularly vulnerable if it relies on
2407   the Host field-value for redirecting requests to internal
2408   servers, or for use as a cache key in a shared cache, without
2409   first verifying that the intercepted connection is targeting a
2410   valid IP address for that host.
2411</t>
2412<t>
2413   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2414   to any HTTP/1.1 request message that lacks a Host header field and
2415   to any request message that contains more than one Host header field
2416   or a Host header field with an invalid field-value.
2417</t>
2418</section>
2419
2420<section title="Effective Request URI" anchor="effective.request.uri">
2421  <iref primary="true" item="effective request URI"/>
2422  <x:anchor-alias value="effective request URI"/>
2423<t>
2424   A server that receives an HTTP request message &MUST; reconstruct
2425   the user agent's original target URI, based on the pieces of information
2426   learned from the request-target, <x:ref>Host</x:ref> header field, and
2427   connection context, in order to identify the intended target resource and
2428   properly service the request. The URI derived from this reconstruction
2429   process is referred to as the "<x:dfn>effective request URI</x:dfn>".
2430</t>
2431<t>
2432   For a user agent, the effective request URI is the target URI.
2433</t>
2434<t>
2435   If the request-target is in absolute-form, then the effective request URI
2436   is the same as the request-target.  Otherwise, the effective request URI
2437   is constructed as follows.
2438</t>
2439<t>
2440   If the request is received over a TLS-secured TCP connection,
2441   then the effective request URI's scheme is "https"; otherwise, the
2442   scheme is "http".
2443</t>
2444<t>
2445   If the request-target is in authority-form, then the effective
2446   request URI's authority component is the same as the request-target.
2447   Otherwise, if a <x:ref>Host</x:ref> header field is supplied with a
2448   non-empty field-value, then the authority component is the same as the
2449   Host field-value. Otherwise, the authority component is the concatenation of
2450   the default host name configured for the server, a colon (":"), and the
2451   connection's incoming TCP port number in decimal form.
2452</t>
2453<t>
2454   If the request-target is in authority-form or asterisk-form, then the
2455   effective request URI's combined path and query component is empty.
2456   Otherwise, the combined path and query component is the same as the
2457   request-target.
2458</t>
2459<t>
2460   The components of the effective request URI, once determined as above,
2461   can be combined into absolute-URI form by concatenating the scheme,
2462   "://", authority, and combined path and query component.
2463</t>
2464<figure>
2465<preamble>
2466   Example 1: the following message received over an insecure TCP connection
2467</preamble>
2468<artwork type="example" x:indent-with="  ">
2469GET /pub/WWW/TheProject.html HTTP/1.1
2470Host: www.example.org:8080
2471</artwork>
2472</figure>
2473<figure>
2474<preamble>
2475  has an effective request URI of
2476</preamble>
2477<artwork type="example" x:indent-with="  ">
2478http://www.example.org:8080/pub/WWW/TheProject.html
2479</artwork>
2480</figure>
2481<figure>
2482<preamble>
2483   Example 2: the following message received over a TLS-secured TCP connection
2484</preamble>
2485<artwork type="example" x:indent-with="  ">
2486OPTIONS * HTTP/1.1
2487Host: www.example.org
2488</artwork>
2489</figure>
2490<figure>
2491<preamble>
2492  has an effective request URI of
2493</preamble>
2494<artwork type="example" x:indent-with="  ">
2495https://www.example.org
2496</artwork>
2497</figure>
2498<t>
2499   An origin server that does not allow resources to differ by requested
2500   host &MAY; ignore the <x:ref>Host</x:ref> field-value and instead replace it
2501   with a configured server name when constructing the effective request URI.
2502</t>
2503<t>
2504   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2505   field &MAY; attempt to use heuristics (e.g., examination of the URI path for
2506   something unique to a particular host) in order to guess the
2507   effective request URI's authority component.
2508</t>
2509</section>
2510
2511<section title="Associating a Response to a Request" anchor="associating.response.to.request">
2512<t>
2513   HTTP does not include a request identifier for associating a given
2514   request message with its corresponding one or more response messages.
2515   Hence, it relies on the order of response arrival to correspond exactly
2516   to the order in which requests are made on the same connection.
2517   More than one response message per request only occurs when one or more
2518   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a
2519   final response to the same request.
2520</t>
2521<t>
2522   A client that has more than one outstanding request on a connection &MUST;
2523   maintain a list of outstanding requests in the order sent and &MUST;
2524   associate each received response message on that connection to the highest
2525   ordered request that has not yet received a final (non-<x:ref>1xx</x:ref>)
2526   response.
2527</t>
2528</section>
2529
2530<section title="Message Forwarding" anchor="message.forwarding">
2531<t>
2532   As described in <xref target="intermediaries"/>, intermediaries can serve
2533   a variety of roles in the processing of HTTP requests and responses.
2534   Some intermediaries are used to improve performance or availability.
2535   Others are used for access control or to filter content.
2536   Since an HTTP stream has characteristics similar to a pipe-and-filter
2537   architecture, there are no inherent limits to the extent an intermediary
2538   can enhance (or interfere) with either direction of the stream.
2539</t>
2540<t>
2541   Intermediaries that forward a message &MUST; implement the
2542   <x:ref>Connection</x:ref> header field, as specified in
2543   <xref target="header.connection"/>, to exclude fields that are only
2544   intended for the incoming connection.
2545</t>
2546<t>
2547   In order to avoid request loops, a proxy that forwards requests to other
2548   proxies &MUST; be able to recognize and exclude all of its own server
2549   names, including any aliases, local variations, or literal IP addresses.
2550</t>
2551
2552<section title="Via" anchor="header.via">
2553  <iref primary="true" item="Via header field" x:for-anchor=""/>
2554  <x:anchor-alias value="pseudonym"/>
2555  <x:anchor-alias value="received-by"/>
2556  <x:anchor-alias value="received-protocol"/>
2557  <x:anchor-alias value="Via"/>
2558<t>
2559   The "Via" header field &MUST; be sent by a proxy or gateway in forwarded
2560   messages to indicate the intermediate protocols and recipients between the
2561   user agent and the server on requests, and between the origin server and
2562   the client on responses. It is analogous to the "Received" field
2563   used by email systems (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2564   Via is used in HTTP for tracking message forwards,
2565   avoiding request loops, and identifying the protocol capabilities of
2566   all senders along the request/response chain.
2567</t>
2568<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"/>
2569  <x:ref>Via</x:ref>               = 1#( <x:ref>received-protocol</x:ref> <x:ref>RWS</x:ref> <x:ref>received-by</x:ref>
2570                          [ <x:ref>RWS</x:ref> <x:ref>comment</x:ref> ] )
2571  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2572                      ; see <xref target="header.upgrade"/>
2573  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2574  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2575</artwork></figure>
2576<t>
2577   The received-protocol indicates the protocol version of the message
2578   received by the server or client along each segment of the
2579   request/response chain. The received-protocol version is appended to
2580   the Via field value when the message is forwarded so that information
2581   about the protocol capabilities of upstream applications remains
2582   visible to all recipients.
2583</t>
2584<t>
2585   The protocol-name is excluded if and only if it would be "HTTP". The
2586   received-by field is normally the host and optional port number of a
2587   recipient server or client that subsequently forwarded the message.
2588   However, if the real host is considered to be sensitive information,
2589   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2590   be assumed to be the default port of the received-protocol.
2591</t>
2592<t>
2593   Multiple Via field values represent each proxy or gateway that has
2594   forwarded the message. Each recipient &MUST; append its information
2595   such that the end result is ordered according to the sequence of
2596   forwarding applications.
2597</t>
2598<t>
2599   Comments &MAY; be used in the Via header field to identify the software
2600   of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2601   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2602   are optional and &MAY; be removed by any recipient prior to forwarding the
2603   message.
2604</t>
2605<t>
2606   For example, a request message could be sent from an HTTP/1.0 user
2607   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2608   forward the request to a public proxy at p.example.net, which completes
2609   the request by forwarding it to the origin server at www.example.com.
2610   The request received by www.example.com would then have the following
2611   Via header field:
2612</t>
2613<figure><artwork type="example">
2614  Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
2615</artwork></figure>
2616<t>
2617   A proxy or gateway used as a portal through a network firewall
2618   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2619   region unless it is explicitly enabled to do so. If not enabled, the
2620   received-by host of any host behind the firewall &SHOULD; be replaced
2621   by an appropriate pseudonym for that host.
2622</t>
2623<t>
2624   A proxy or gateway &MAY; combine an ordered subsequence of Via header
2625   field entries into a single such entry if the entries have identical
2626   received-protocol values. For example,
2627</t>
2628<figure><artwork type="example">
2629  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2630</artwork></figure>
2631<t>
2632  could be collapsed to
2633</t>
2634<figure><artwork type="example">
2635  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2636</artwork></figure>
2637<t>
2638   Senders &SHOULD-NOT; combine multiple entries unless they are all
2639   under the same organizational control and the hosts have already been
2640   replaced by pseudonyms. Senders &MUST-NOT; combine entries that
2641   have different received-protocol values.
2642</t>
2643</section>
2644
2645<section title="Transformations" anchor="message.transformations">
2646<t>
2647   Some intermediaries include features for transforming messages and their
2648   payloads.  A transforming proxy might, for example, convert between image
2649   formats in order to save cache space or to reduce the amount of traffic on
2650   a slow link. However, operational problems might occur when these
2651   transformations are applied to payloads intended for critical applications,
2652   such as medical imaging or scientific data analysis, particularly when
2653   integrity checks or digital signatures are used to ensure that the payload
2654   received is identical to the original.
2655</t>
2656<t>
2657   If a proxy receives a request-target with a host name that is not a
2658   fully qualified domain name, it &MAY; add its own domain to the host name
2659   it received when forwarding the request.  A proxy &MUST-NOT; change the
2660   host name if it is a fully qualified domain name.
2661</t>
2662<t>
2663   A proxy &MUST-NOT; modify the "absolute-path" and "query" parts of the
2664   received request-target when forwarding it to the next inbound server,
2665   except as noted above to replace an empty path with "/" or "*".
2666</t>
2667<t>
2668   A proxy &MUST-NOT; modify header fields that provide information about the
2669   end points of the communication chain, the resource state, or the selected
2670   representation. A proxy &MAY; change the message body through application
2671   or removal of a transfer coding (<xref target="transfer.codings"/>).
2672</t>
2673<t>
2674   A non-transforming proxy &MUST-NOT; modify the message payload (&payload;).
2675   A transforming proxy &MUST-NOT; modify the payload of a message that
2676   contains the no-transform cache-control directive.
2677</t>
2678<t>
2679   A transforming proxy &MAY; transform the payload of a message
2680   that does not contain the no-transform cache-control directive;
2681   if the payload is transformed, the transforming proxy &MUST; add a
2682   Warning 214 (Transformation applied) header field if one does not
2683   already appear in the message (see &header-warning;).
2684</t>
2685</section>
2686</section>
2687</section>
2688
2689<section title="Connection Management" anchor="connection.management">
2690<t>
2691   HTTP messaging is independent of the underlying transport or
2692   session-layer connection protocol(s).  HTTP only presumes a reliable
2693   transport with in-order delivery of requests and the corresponding
2694   in-order delivery of responses.  The mapping of HTTP request and
2695   response structures onto the data units of an underlying transport
2696   protocol is outside the scope of this specification.
2697</t>
2698<t>
2699   As described in <xref target="connecting.inbound"/>, the specific
2700   connection protocols to be used for an HTTP interaction are determined by
2701   client configuration and the <x:ref>target URI</x:ref>.
2702   For example, the "http" URI scheme
2703   (<xref target="http.uri"/>) indicates a default connection of TCP
2704   over IP, with a default TCP port of 80, but the client might be
2705   configured to use a proxy via some other connection, port, or protocol.
2706</t>
2707<t>
2708   HTTP implementations are expected to engage in connection management,
2709   which includes maintaining the state of current connections,
2710   establishing a new connection or reusing an existing connection,
2711   processing messages received on a connection, detecting connection
2712   failures, and closing each connection.
2713   Most clients maintain multiple connections in parallel, including
2714   more than one connection per server endpoint.
2715   Most servers are designed to maintain thousands of concurrent connections,
2716   while controlling request queues to enable fair use and detect
2717   denial of service attacks.
2718</t>
2719
2720<section title="Connection" anchor="header.connection">
2721  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2722  <iref primary="true" item="close" x:for-anchor=""/>
2723  <x:anchor-alias value="Connection"/>
2724  <x:anchor-alias value="connection-option"/>
2725  <x:anchor-alias value="close"/>
2726<t>
2727   The "Connection" header field allows the sender to indicate desired
2728   control options for the current connection.  In order to avoid confusing
2729   downstream recipients, a proxy or gateway &MUST; remove or replace any
2730   received connection options before forwarding the message.
2731</t>
2732<t>
2733   When a header field aside from Connection is used to supply control
2734   information for or about the current connection, the sender &MUST; list
2735   the corresponding field-name within the "Connection" header field.
2736   A proxy or gateway &MUST; parse a received Connection
2737   header field before a message is forwarded and, for each
2738   connection-option in this field, remove any header field(s) from
2739   the message with the same name as the connection-option, and then
2740   remove the Connection header field itself (or replace it with the
2741   intermediary's own connection options for the forwarded message).
2742</t>
2743<t>
2744   Hence, the Connection header field provides a declarative way of
2745   distinguishing header fields that are only intended for the
2746   immediate recipient ("hop-by-hop") from those fields that are
2747   intended for all recipients on the chain ("end-to-end"), enabling the
2748   message to be self-descriptive and allowing future connection-specific
2749   extensions to be deployed without fear that they will be blindly
2750   forwarded by older intermediaries.
2751</t>
2752<t>
2753   The Connection header field's value has the following grammar:
2754</t>
2755<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2756  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2757  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2758</artwork></figure>
2759<t>
2760   Connection options are case-insensitive.
2761</t>
2762<t>
2763   A sender &MUST-NOT; send a connection option corresponding to a header
2764   field that is intended for all recipients of the payload.
2765   For example, <x:ref>Cache-Control</x:ref> is never appropriate as a
2766   connection option (&header-cache-control;).
2767</t>
2768<t>
2769   The connection options do not have to correspond to a header field
2770   present in the message, since a connection-specific header field
2771   might not be needed if there are no parameters associated with that
2772   connection option.  Recipients that trigger certain connection
2773   behavior based on the presence of connection options &MUST; do so
2774   based on the presence of the connection-option rather than only the
2775   presence of the optional header field.  In other words, if the
2776   connection option is received as a header field but not indicated
2777   within the Connection field-value, then the recipient &MUST; ignore
2778   the connection-specific header field because it has likely been
2779   forwarded by an intermediary that is only partially conformant.
2780</t>
2781<t>
2782   When defining new connection options, specifications ought to
2783   carefully consider existing deployed header fields and ensure
2784   that the new connection option does not share the same name as
2785   an unrelated header field that might already be deployed.
2786   Defining a new connection option essentially reserves that potential
2787   field-name for carrying additional information related to the
2788   connection option, since it would be unwise for senders to use
2789   that field-name for anything else.
2790</t>
2791<t>
2792   The "<x:dfn>close</x:dfn>" connection option is defined for a
2793   sender to signal that this connection will be closed after completion of
2794   the response. For example,
2795</t>
2796<figure><artwork type="example">
2797  Connection: close
2798</artwork></figure>
2799<t>
2800   in either the request or the response header fields indicates that
2801   the connection &MUST; be closed after the current request/response
2802   is complete (<xref target="persistent.tear-down"/>).
2803</t>
2804<t>
2805   A client that does not support <x:ref>persistent connections</x:ref> &MUST;
2806   send the "close" connection option in every request message.
2807</t>
2808<t>
2809   A server that does not support <x:ref>persistent connections</x:ref> &MUST;
2810   send the "close" connection option in every response message that
2811   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2812</t>
2813</section>
2814
2815<section title="Establishment" anchor="persistent.establishment">
2816<t>
2817   It is beyond the scope of this specification to describe how connections
2818   are established via various transport or session-layer protocols.
2819   Each connection applies to only one transport link.
2820</t>
2821</section>
2822
2823<section title="Persistence" anchor="persistent.connections">
2824   <x:anchor-alias value="persistent connections"/>
2825<t>
2826   HTTP/1.1 defaults to the use of "<x:dfn>persistent connections</x:dfn>",
2827   allowing multiple requests and responses to be carried over a single
2828   connection. The "<x:ref>close</x:ref>" connection-option is used to signal
2829   that a connection will not persist after the current request/response.
2830   HTTP implementations &SHOULD; support persistent connections.
2831</t>
2832<t>
2833   A recipient determines whether a connection is persistent or not based on
2834   the most recently received message's protocol version and
2835   <x:ref>Connection</x:ref> header field (if any):
2836   <list style="symbols">
2837     <t>If the <x:ref>close</x:ref> connection option is present, the
2838        connection will not persist after the current response; else,</t>
2839     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2840        persist after the current response; else,</t>
2841     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2842        connection option is present, the recipient is not a proxy, and
2843        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2844        the connection will persist after the current response; otherwise,</t>
2845     <t>The connection will close after the current response.</t>
2846   </list>
2847</t>
2848<t>
2849   A server &MAY; assume that an HTTP/1.1 client intends to maintain a
2850   persistent connection until a <x:ref>close</x:ref> connection option
2851   is received in a request.
2852</t>
2853<t>
2854   A client &MAY; reuse a persistent connection until it sends or receives
2855   a <x:ref>close</x:ref> connection option or receives an HTTP/1.0 response
2856   without a "keep-alive" connection option.
2857</t>
2858<t>
2859   In order to remain persistent, all messages on a connection &MUST;
2860   have a self-defined message length (i.e., one not defined by closure
2861   of the connection), as described in <xref target="message.body"/>.
2862   A server &MUST; read the entire request message body or close
2863   the connection after sending its response, since otherwise the
2864   remaining data on a persistent connection would be misinterpreted
2865   as the next request.  Likewise,
2866   a client &MUST; read the entire response message body if it intends
2867   to reuse the same connection for a subsequent request.
2868</t>
2869<t>
2870   A proxy server &MUST-NOT; maintain a persistent connection with an
2871   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
2872   information and discussion of the problems with the Keep-Alive header field
2873   implemented by many HTTP/1.0 clients).
2874</t>
2875<t>
2876   Clients and servers &SHOULD-NOT; assume that a persistent connection is
2877   maintained for HTTP versions less than 1.1 unless it is explicitly
2878   signaled.
2879   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
2880   for more information on backward compatibility with HTTP/1.0 clients.
2881</t>
2882
2883<section title="Retrying Requests" anchor="persistent.retrying.requests">
2884<t>
2885   Connections can be closed at any time, with or without intention.
2886   Implementations ought to anticipate the need to recover
2887   from asynchronous close events.
2888</t>
2889<t>
2890   When an inbound connection is closed prematurely, a client &MAY; open a new
2891   connection and automatically retransmit an aborted sequence of requests if
2892   all of those requests have idempotent methods (&idempotent-methods;).
2893   A proxy &MUST-NOT; automatically retry non-idempotent requests.
2894</t>
2895<t>
2896   A user agent &MUST-NOT; automatically retry a request with a non-idempotent
2897   method unless it has some means to know that the request semantics are
2898   actually idempotent, regardless of the method, or some means to detect that
2899   the original request was never applied. For example, a user agent that
2900   knows (through design or configuration) that a POST request to a given
2901   resource is safe can repeat that request automatically.
2902   Likewise, a user agent designed specifically to operate on a version
2903   control repository might be able to recover from partial failure conditions
2904   by checking the target resource revision(s) after a failed connection,
2905   reverting or fixing any changes that were partially applied, and then
2906   automatically retrying the requests that failed.
2907</t>
2908<t>
2909   An automatic retry &SHOULD-NOT; be repeated if it fails.
2910</t>
2911</section>
2912
2913<section title="Pipelining" anchor="pipelining">
2914   <x:anchor-alias value="pipeline"/>
2915<t>
2916   A client that supports persistent connections &MAY; "<x:dfn>pipeline</x:dfn>"
2917   its requests (i.e., send multiple requests without waiting for each
2918   response). A server &MAY; process a sequence of pipelined requests in
2919   parallel if they all have safe methods (&safe-methods;), but &MUST; send
2920   the corresponding responses in the same order that the requests were
2921   received.
2922</t>
2923<t>
2924   A client that pipelines requests &MUST; be prepared to retry those
2925   requests if the connection closes before it receives all of the
2926   corresponding responses. A client that assumes a persistent connection and
2927   pipelines immediately after connection establishment &MUST-NOT; pipeline
2928   on a retry connection until it knows the connection is persistent.
2929</t>
2930<t>
2931   Idempotent methods (&idempotent-methods;) are significant to pipelining
2932   because they can be automatically retried after a connection failure.
2933   A user agent &SHOULD-NOT; pipeline requests after a non-idempotent method
2934   until the final response status code for that method has been received,
2935   unless the user agent has a means to detect and recover from partial
2936   failure conditions involving the pipelined sequence.
2937</t>
2938<t>
2939   An intermediary that receives pipelined requests &MAY; pipeline those
2940   requests when forwarding them inbound, since it can rely on the outbound
2941   user agent(s) to determine what requests can be safely pipelined. If the
2942   inbound connection fails before receiving a response, the pipelining
2943   intermediary &MAY; attempt to retry a sequence of requests that have yet
2944   to receive a response if the requests all have idempotent methods;
2945   otherwise, the pipelining intermediary &SHOULD; forward any received
2946   responses and then close the corresponding outbound connection(s) so that
2947   the outbound user agent(s) can recover accordingly.
2948</t>
2949</section>
2950</section>
2951   
2952<section title="Concurrency" anchor="persistent.concurrency">
2953<t>
2954   Clients &SHOULD; limit the number of simultaneous
2955   connections that they maintain to a given server.
2956</t>
2957<t>
2958   Previous revisions of HTTP gave a specific number of connections as a
2959   ceiling, but this was found to be impractical for many applications. As a
2960   result, this specification does not mandate a particular maximum number of
2961   connections, but instead encourages clients to be conservative when opening
2962   multiple connections.
2963</t>
2964<t>
2965   Multiple connections are typically used to avoid the "head-of-line
2966   blocking" problem, wherein a request that takes significant server-side
2967   processing and/or has a large payload blocks subsequent requests on the
2968   same connection. However, each connection consumes server resources.
2969   Furthermore, using multiple connections can cause undesirable side effects
2970   in congested networks.
2971</t>
2972<t>
2973   Note that servers might reject traffic that they deem abusive, including an
2974   excessive number of connections from a client.
2975</t>
2976</section>
2977
2978<section title="Failures and Time-outs" anchor="persistent.failures">
2979<t>
2980   Servers will usually have some time-out value beyond which they will
2981   no longer maintain an inactive connection. Proxy servers might make
2982   this a higher value since it is likely that the client will be making
2983   more connections through the same server. The use of persistent
2984   connections places no requirements on the length (or existence) of
2985   this time-out for either the client or the server.
2986</t>
2987<t>
2988   When a client or server wishes to time-out it &SHOULD; issue a graceful
2989   close on the transport connection. Clients and servers &SHOULD; both
2990   constantly watch for the other side of the transport close, and
2991   respond to it as appropriate. If a client or server does not detect
2992   the other side's close promptly it could cause unnecessary resource
2993   drain on the network.
2994</t>
2995<t>
2996   A client, server, or proxy &MAY; close the transport connection at any
2997   time. For example, a client might have started to send a new request
2998   at the same time that the server has decided to close the "idle"
2999   connection. From the server's point of view, the connection is being
3000   closed while it was idle, but from the client's point of view, a
3001   request is in progress.
3002</t>
3003<t>
3004   Servers &SHOULD; maintain persistent connections and allow the underlying
3005   transport's flow control mechanisms to resolve temporary overloads, rather
3006   than terminate connections with the expectation that clients will retry.
3007   The latter technique can exacerbate network congestion.
3008</t>
3009<t>
3010   A client sending a message body &SHOULD; monitor
3011   the network connection for an error response while it is transmitting
3012   the request. If the client sees an error response, it &SHOULD;
3013   immediately cease transmitting the body and close the connection.
3014</t>
3015</section>
3016   
3017<section title="Tear-down" anchor="persistent.tear-down">
3018  <iref primary="false" item="Connection header field" x:for-anchor=""/>
3019  <iref primary="false" item="close" x:for-anchor=""/>
3020<t>
3021   The <x:ref>Connection</x:ref> header field
3022   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
3023   connection option that a sender &SHOULD; send when it wishes to close
3024   the connection after the current request/response pair.
3025</t>
3026<t>
3027   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
3028   send further requests on that connection (after the one containing
3029   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
3030   final response message corresponding to this request.
3031</t>
3032<t>
3033   A server that receives a <x:ref>close</x:ref> connection option &MUST;
3034   initiate a close of the connection (see below) after it sends the
3035   final response to the request that contained <x:ref>close</x:ref>.
3036   The server &SHOULD; send a <x:ref>close</x:ref> connection option
3037   in its final response on that connection. The server &MUST-NOT; process
3038   any further requests received on that connection.
3039</t>
3040<t>
3041   A server that sends a <x:ref>close</x:ref> connection option &MUST;
3042   initiate a close of the connection (see below) after it sends the
3043   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
3044   any further requests received on that connection.
3045</t>
3046<t>
3047   A client that receives a <x:ref>close</x:ref> connection option &MUST;
3048   cease sending requests on that connection and close the connection
3049   after reading the response message containing the close; if additional
3050   pipelined requests had been sent on the connection, the client &SHOULD-NOT;
3051   assume that they will be processed by the server.
3052</t>
3053<t>
3054   If a server performs an immediate close of a TCP connection, there is a
3055   significant risk that the client will not be able to read the last HTTP
3056   response.  If the server receives additional data from the client on a
3057   fully-closed connection, such as another request that was sent by the
3058   client before receiving the server's response, the server's TCP stack will
3059   send a reset packet to the client; unfortunately, the reset packet might
3060   erase the client's unacknowledged input buffers before they can be read
3061   and interpreted by the client's HTTP parser.
3062</t>
3063<t>
3064   To avoid the TCP reset problem, servers typically close a connection in
3065   stages. First, the server performs a half-close by closing only the write
3066   side of the read/write connection. The server then continues to read from
3067   the connection until it receives a corresponding close by the client, or
3068   until the server is reasonably certain that its own TCP stack has received
3069   the client's acknowledgement of the packet(s) containing the server's last
3070   response. Finally, the server fully closes the connection.
3071</t>
3072<t>
3073   It is unknown whether the reset problem is exclusive to TCP or might also
3074   be found in other transport connection protocols.
3075</t>
3076</section>
3077
3078<section title="Upgrade" anchor="header.upgrade">
3079  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3080  <x:anchor-alias value="Upgrade"/>
3081  <x:anchor-alias value="protocol"/>
3082  <x:anchor-alias value="protocol-name"/>
3083  <x:anchor-alias value="protocol-version"/>
3084<t>
3085   The "Upgrade" header field is intended to provide a simple mechanism
3086   for transitioning from HTTP/1.1 to some other protocol on the same
3087   connection.  A client &MAY; send a list of protocols in the Upgrade
3088   header field of a request to invite the server to switch to one or
3089   more of those protocols, in order of descending preference, before sending
3090   the final response. A server &MAY; ignore a received Upgrade header field
3091   if it wishes to continue using the current protocol on that connection.
3092</t>
3093<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3094  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3095
3096  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3097  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3098  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3099</artwork></figure>
3100<t>
3101   A server that sends a <x:ref>101 (Switching Protocols)</x:ref> response
3102   &MUST; send an Upgrade header field to indicate the new protocol(s) to
3103   which the connection is being switched; if multiple protocol layers are
3104   being switched, the new protocols &MUST; be listed in layer-ascending
3105   order. A server &MUST-NOT; switch to a protocol that was not indicated by
3106   the client in the corresponding request's Upgrade header field.
3107   A server &MAY; choose to ignore the order of preference indicated by the
3108   client and select the new protocol(s) based on other factors, such as the
3109   nature of the request or the current load on the server.
3110</t>
3111<t>
3112   A server that sends a <x:ref>426 (Upgrade Required)</x:ref> response
3113   &MUST; send an Upgrade header field to indicate the acceptable protocols,
3114   in order of descending preference.
3115</t>
3116<t>
3117   A server &MAY; send an Upgrade header field in any other response to
3118   advertise that it implements support for upgrading to the listed protocols,
3119   in order of descending preference, when appropriate for a future request.
3120</t>
3121<figure><preamble>
3122   The following is a hypothetical example sent by a client:
3123</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
3124GET /hello.txt HTTP/1.1
3125Host: www.example.com
3126Connection: upgrade
3127Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3128
3129</artwork></figure>
3130<t>
3131   Upgrade cannot be used to insist on a protocol change; its acceptance and
3132   use by the server is optional. The capabilities and nature of the
3133   application-level communication after the protocol change is entirely
3134   dependent upon the new protocol(s) chosen, although the first action
3135   after changing the protocol &MUST; be a response to the initial HTTP
3136   request that contained the Upgrade header field.
3137</t>
3138<t>
3139   For example, if the Upgrade header field is received in a GET request
3140   and the server decides to switch protocols, it first responds
3141   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3142   then immediately follows that with the new protocol's equivalent of a
3143   response to a GET on the target resource.  This allows a connection to be
3144   upgraded to protocols with the same semantics as HTTP without the
3145   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3146   protocols unless the received message semantics can be honored by the new
3147   protocol; an OPTIONS request can be honored by any protocol.
3148</t>
3149<figure><preamble>
3150   The following is an example response to the above hypothetical request:
3151</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
3152HTTP/1.1 101 Switching Protocols
3153Connection: upgrade
3154Upgrade: HTTP/2.0
3155
3156[... data stream switches to HTTP/2.0 with an appropriate response
3157(as defined by new protocol) to the "GET /hello.txt" request ...]
3158</artwork></figure>
3159<t>
3160   When Upgrade is sent, the sender &MUST; also send a
3161   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3162   that contains an "upgrade" connection option, in order to prevent Upgrade
3163   from being accidentally forwarded by intermediaries that might not implement
3164   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3165   is received in an HTTP/1.0 request.
3166</t>
3167<t>
3168   The Upgrade header field only applies to switching protocols on top of the
3169   existing connection; it cannot be used to switch the underlying connection
3170   (transport) protocol, nor to switch the existing communication to a
3171   different connection. For those purposes, it is more appropriate to use a
3172   <x:ref>3xx (Redirection)</x:ref> response (&status-3xx;).
3173</t>
3174<t>
3175   This specification only defines the protocol name "HTTP" for use by
3176   the family of Hypertext Transfer Protocols, as defined by the HTTP
3177   version rules of <xref target="http.version"/> and future updates to this
3178   specification. Additional tokens ought to be registered with IANA using the
3179   registration procedure defined in <xref target="upgrade.token.registry"/>.
3180</t>
3181</section>
3182</section>
3183
3184<section title="IANA Considerations" anchor="IANA.considerations">
3185
3186<section title="Header Field Registration" anchor="header.field.registration">
3187<t>
3188   HTTP header fields are registered within the Message Header Field Registry
3189   maintained at
3190   <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
3191</t>
3192<t>
3193   This document defines the following HTTP header fields, so their
3194   associated registry entries shall be updated according to the permanent
3195   registrations below (see <xref target="BCP90"/>):
3196</t>
3197<?BEGININC p1-messaging.iana-headers ?>
3198<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3199<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3200   <ttcol>Header Field Name</ttcol>
3201   <ttcol>Protocol</ttcol>
3202   <ttcol>Status</ttcol>
3203   <ttcol>Reference</ttcol>
3204
3205   <c>Connection</c>
3206   <c>http</c>
3207   <c>standard</c>
3208   <c>
3209      <xref target="header.connection"/>
3210   </c>
3211   <c>Content-Length</c>
3212   <c>http</c>
3213   <c>standard</c>
3214   <c>
3215      <xref target="header.content-length"/>
3216   </c>
3217   <c>Host</c>
3218   <c>http</c>
3219   <c>standard</c>
3220   <c>
3221      <xref target="header.host"/>
3222   </c>
3223   <c>TE</c>
3224   <c>http</c>
3225   <c>standard</c>
3226   <c>
3227      <xref target="header.te"/>
3228   </c>
3229   <c>Trailer</c>
3230   <c>http</c>
3231   <c>standard</c>
3232   <c>
3233      <xref target="header.trailer"/>
3234   </c>
3235   <c>Transfer-Encoding</c>
3236   <c>http</c>
3237   <c>standard</c>
3238   <c>
3239      <xref target="header.transfer-encoding"/>
3240   </c>
3241   <c>Upgrade</c>
3242   <c>http</c>
3243   <c>standard</c>
3244   <c>
3245      <xref target="header.upgrade"/>
3246   </c>
3247   <c>Via</c>
3248   <c>http</c>
3249   <c>standard</c>
3250   <c>
3251      <xref target="header.via"/>
3252   </c>
3253</texttable>
3254<!--(END)-->
3255<?ENDINC p1-messaging.iana-headers ?>
3256<t>
3257   Furthermore, the header field-name "Close" shall be registered as
3258   "reserved", since using that name as an HTTP header field might
3259   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3260   header field (<xref target="header.connection"/>).
3261</t>
3262<texttable align="left" suppress-title="true">
3263   <ttcol>Header Field Name</ttcol>
3264   <ttcol>Protocol</ttcol>
3265   <ttcol>Status</ttcol>
3266   <ttcol>Reference</ttcol>
3267
3268   <c>Close</c>
3269   <c>http</c>
3270   <c>reserved</c>
3271   <c>
3272      <xref target="header.field.registration"/>
3273   </c>
3274</texttable>
3275<t>
3276   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3277</t>
3278</section>
3279
3280<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3281<t>
3282   IANA maintains the registry of URI Schemes <xref target="BCP115"/> at
3283   <eref target="http://www.iana.org/assignments/uri-schemes.html"/>.
3284</t>
3285<t>
3286   This document defines the following URI schemes, so their
3287   associated registry entries shall be updated according to the permanent
3288   registrations below:
3289</t>
3290<texttable align="left" suppress-title="true">
3291   <ttcol>URI Scheme</ttcol>
3292   <ttcol>Description</ttcol>
3293   <ttcol>Reference</ttcol>
3294
3295   <c>http</c>
3296   <c>Hypertext Transfer Protocol</c>
3297   <c><xref target="http.uri"/></c>
3298
3299   <c>https</c>
3300   <c>Hypertext Transfer Protocol Secure</c>
3301   <c><xref target="https.uri"/></c>
3302</texttable>
3303</section>
3304
3305<section title="Internet Media Type Registration" anchor="internet.media.type.http">
3306<t>
3307   This document serves as the specification for the Internet media types
3308   "message/http" and "application/http". The following is to be registered with
3309   IANA (see <xref target="BCP13"/>).
3310</t>
3311<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
3312<iref item="Media Type" subitem="message/http" primary="true"/>
3313<iref item="message/http Media Type" primary="true"/>
3314<t>
3315   The message/http type can be used to enclose a single HTTP request or
3316   response message, provided that it obeys the MIME restrictions for all
3317   "message" types regarding line length and encodings.
3318</t>
3319<t>
3320  <list style="hanging" x:indent="12em">
3321    <t hangText="Type name:">
3322      message
3323    </t>
3324    <t hangText="Subtype name:">
3325      http
3326    </t>
3327    <t hangText="Required parameters:">
3328      none
3329    </t>
3330    <t hangText="Optional parameters:">
3331      version, msgtype
3332      <list style="hanging">
3333        <t hangText="version:">
3334          The HTTP-version number of the enclosed message
3335          (e.g., "1.1"). If not present, the version can be
3336          determined from the first line of the body.
3337        </t>
3338        <t hangText="msgtype:">
3339          The message type &mdash; "request" or "response". If not
3340          present, the type can be determined from the first
3341          line of the body.
3342        </t>
3343      </list>
3344    </t>
3345    <t hangText="Encoding considerations:">
3346      only "7bit", "8bit", or "binary" are permitted
3347    </t>
3348    <t hangText="Security considerations:">
3349      none
3350    </t>
3351    <t hangText="Interoperability considerations:">
3352      none
3353    </t>
3354    <t hangText="Published specification:">
3355      This specification (see <xref target="internet.media.type.message.http"/>).
3356    </t>
3357    <t hangText="Applications that use this media type:">
3358    </t>
3359    <t hangText="Additional information:">
3360      <list style="hanging">
3361        <t hangText="Magic number(s):">none</t>
3362        <t hangText="File extension(s):">none</t>
3363        <t hangText="Macintosh file type code(s):">none</t>
3364      </list>
3365    </t>
3366    <t hangText="Person and email address to contact for further information:">
3367      See Authors Section.
3368    </t>
3369    <t hangText="Intended usage:">
3370      COMMON
3371    </t>
3372    <t hangText="Restrictions on usage:">
3373      none
3374    </t>
3375    <t hangText="Author:">
3376      See Authors Section.
3377    </t>
3378    <t hangText="Change controller:">
3379      IESG
3380    </t>
3381  </list>
3382</t>
3383</section>
3384<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
3385<iref item="Media Type" subitem="application/http" primary="true"/>
3386<iref item="application/http Media Type" primary="true"/>
3387<t>
3388   The application/http type can be used to enclose a pipeline of one or more
3389   HTTP request or response messages (not intermixed).
3390</t>
3391<t>
3392  <list style="hanging" x:indent="12em">
3393    <t hangText="Type name:">
3394      application
3395    </t>
3396    <t hangText="Subtype name:">
3397      http
3398    </t>
3399    <t hangText="Required parameters:">
3400      none
3401    </t>
3402    <t hangText="Optional parameters:">
3403      version, msgtype
3404      <list style="hanging">
3405        <t hangText="version:">
3406          The HTTP-version number of the enclosed messages
3407          (e.g., "1.1"). If not present, the version can be
3408          determined from the first line of the body.
3409        </t>
3410        <t hangText="msgtype:">
3411          The message type &mdash; "request" or "response". If not
3412          present, the type can be determined from the first
3413          line of the body.
3414        </t>
3415      </list>
3416    </t>
3417    <t hangText="Encoding considerations:">
3418      HTTP messages enclosed by this type
3419      are in "binary" format; use of an appropriate
3420      Content-Transfer-Encoding is required when
3421      transmitted via E-mail.
3422    </t>
3423    <t hangText="Security considerations:">
3424      none
3425    </t>
3426    <t hangText="Interoperability considerations:">
3427      none
3428    </t>
3429    <t hangText="Published specification:">
3430      This specification (see <xref target="internet.media.type.application.http"/>).
3431    </t>
3432    <t hangText="Applications that use this media type:">
3433    </t>
3434    <t hangText="Additional information:">
3435      <list style="hanging">
3436        <t hangText="Magic number(s):">none</t>
3437        <t hangText="File extension(s):">none</t>
3438        <t hangText="Macintosh file type code(s):">none</t>
3439      </list>
3440    </t>
3441    <t hangText="Person and email address to contact for further information:">
3442      See Authors Section.
3443    </t>
3444    <t hangText="Intended usage:">
3445      COMMON
3446    </t>
3447    <t hangText="Restrictions on usage:">
3448      none
3449    </t>
3450    <t hangText="Author:">
3451      See Authors Section.
3452    </t>
3453    <t hangText="Change controller:">
3454      IESG
3455    </t>
3456  </list>
3457</t>
3458</section>
3459</section>
3460
3461<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3462<t>
3463   The HTTP Transfer Coding Registry defines the name space for transfer
3464   coding names. It is maintained at <eref target="http://www.iana.org/assignments/http-parameters"/>.
3465</t>
3466
3467<section title="Procedure" anchor="transfer.coding.registry.procedure">
3468<t>
3469   Registrations &MUST; include the following fields:
3470   <list style="symbols">
3471     <t>Name</t>
3472     <t>Description</t>
3473     <t>Pointer to specification text</t>
3474   </list>
3475</t>
3476<t>
3477   Names of transfer codings &MUST-NOT; overlap with names of content codings
3478   (&content-codings;) unless the encoding transformation is identical, as
3479   is the case for the compression codings defined in
3480   <xref target="compression.codings"/>.
3481</t>
3482<t>
3483   Values to be added to this name space require IETF Review (see
3484   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3485   conform to the purpose of transfer coding defined in this specification.
3486</t>
3487<t>
3488   Use of program names for the identification of encoding formats
3489   is not desirable and is discouraged for future encodings.
3490</t>
3491</section>
3492
3493<section title="Registration" anchor="transfer.coding.registration">
3494<t>
3495   The HTTP Transfer Coding Registry shall be updated with the registrations
3496   below:
3497</t>
3498<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3499   <ttcol>Name</ttcol>
3500   <ttcol>Description</ttcol>
3501   <ttcol>Reference</ttcol>
3502   <c>chunked</c>
3503   <c>Transfer in a series of chunks</c>
3504   <c>
3505      <xref target="chunked.encoding"/>
3506   </c>
3507   <c>compress</c>
3508   <c>UNIX "compress" program method</c>
3509   <c>
3510      <xref target="compress.coding"/>
3511   </c>
3512   <c>deflate</c>
3513   <c>"deflate" compression mechanism (<xref target="RFC1951"/>) used inside
3514   the "zlib" data format (<xref target="RFC1950"/>)
3515   </c>
3516   <c>
3517      <xref target="deflate.coding"/>
3518   </c>
3519   <c>gzip</c>
3520   <c>Same as GNU zip <xref target="RFC1952"/></c>
3521   <c>
3522      <xref target="gzip.coding"/>
3523   </c>
3524</texttable>
3525</section>
3526</section>
3527
3528<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3529<t>
3530   The HTTP Upgrade Token Registry defines the name space for protocol-name
3531   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3532   field. The registry is maintained at <eref target="http://www.iana.org/assignments/http-upgrade-tokens"/>.
3533</t>
3534
3535<section title="Procedure" anchor="upgrade.token.registry.procedure">  
3536<t>
3537   Each registered protocol name is associated with contact information
3538   and an optional set of specifications that details how the connection
3539   will be processed after it has been upgraded.
3540</t>
3541<t>
3542   Registrations happen on a "First Come First Served" basis (see
3543   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3544   following rules:
3545  <list style="numbers">
3546    <t>A protocol-name token, once registered, stays registered forever.</t>
3547    <t>The registration &MUST; name a responsible party for the
3548       registration.</t>
3549    <t>The registration &MUST; name a point of contact.</t>
3550    <t>The registration &MAY; name a set of specifications associated with
3551       that token. Such specifications need not be publicly available.</t>
3552    <t>The registration &SHOULD; name a set of expected "protocol-version"
3553       tokens associated with that token at the time of registration.</t>
3554    <t>The responsible party &MAY; change the registration at any time.
3555       The IANA will keep a record of all such changes, and make them
3556       available upon request.</t>
3557    <t>The IESG &MAY; reassign responsibility for a protocol token.
3558       This will normally only be used in the case when a
3559       responsible party cannot be contacted.</t>
3560  </list>
3561</t>
3562<t>
3563   This registration procedure for HTTP Upgrade Tokens replaces that
3564   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3565</t>
3566</section>
3567
3568<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3569<t>
3570   The HTTP Upgrade Token Registry shall be updated with the registration
3571   below:
3572</t>
3573<texttable align="left" suppress-title="true">
3574   <ttcol>Value</ttcol>
3575   <ttcol>Description</ttcol>
3576   <ttcol>Expected Version Tokens</ttcol>
3577   <ttcol>Reference</ttcol>
3578
3579   <c>HTTP</c>
3580   <c>Hypertext Transfer Protocol</c>
3581   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3582   <c><xref target="http.version"/></c>
3583</texttable>
3584<t>
3585   The responsible party is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3586</t>
3587</section>
3588</section>
3589
3590</section>
3591
3592<section title="Security Considerations" anchor="security.considerations">
3593<t>
3594   This section is meant to inform developers, information providers, and
3595   users of known security concerns relevant to HTTP/1.1 message syntax,
3596   parsing, and routing.
3597</t>
3598
3599<section title="DNS-related Attacks" anchor="dns.related.attacks">
3600<t>
3601   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3602   generally prone to security attacks based on the deliberate misassociation
3603   of IP addresses and DNS names not protected by DNSSEC. Clients need to be
3604   cautious in assuming the validity of an IP number/DNS name association unless
3605   the response is protected by DNSSEC (<xref target="RFC4033"/>).
3606</t>
3607</section>
3608
3609<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3610<t>
3611   By their very nature, HTTP intermediaries are men-in-the-middle, and
3612   represent an opportunity for man-in-the-middle attacks. Compromise of
3613   the systems on which the intermediaries run can result in serious security
3614   and privacy problems. Intermediaries have access to security-related
3615   information, personal information about individual users and
3616   organizations, and proprietary information belonging to users and
3617   content providers. A compromised intermediary, or an intermediary
3618   implemented or configured without regard to security and privacy
3619   considerations, might be used in the commission of a wide range of
3620   potential attacks.
3621</t>
3622<t>
3623   Intermediaries that contain a shared cache are especially vulnerable
3624   to cache poisoning attacks.
3625</t>
3626<t>
3627   Implementers need to consider the privacy and security
3628   implications of their design and coding decisions, and of the
3629   configuration options they provide to operators (especially the
3630   default configuration).
3631</t>
3632<t>
3633   Users need to be aware that intermediaries are no more trustworthy than
3634   the people who run them; HTTP itself cannot solve this problem.
3635</t>
3636</section>
3637
3638<section title="Buffer Overflows" anchor="attack.protocol.element.size.overflows">
3639<t>
3640   Because HTTP uses mostly textual, character-delimited fields, attackers can
3641   overflow buffers in implementations, and/or perform a Denial of Service
3642   against implementations that accept fields with unlimited lengths.
3643</t>
3644<t>
3645   To promote interoperability, this specification makes specific
3646   recommendations for minimum size limits on request-line
3647   (<xref target="request.line"/>)
3648   and blocks of header fields (<xref target="header.fields"/>). These are
3649   minimum recommendations, chosen to be supportable even by implementations
3650   with limited resources; it is expected that most implementations will
3651   choose substantially higher limits.
3652</t>
3653<t>
3654   This specification also provides a way for servers to reject messages that
3655   have request-targets that are too long (&status-414;) or request entities
3656   that are too large (&status-4xx;). Additional status codes related to
3657   capacity limits have been defined by extensions to HTTP
3658   <xref target="RFC6585"/>.
3659</t>
3660<t>
3661   Recipients &SHOULD; carefully limit the extent to which they read other
3662   fields, including (but not limited to) request methods, response status
3663   phrases, header field-names, and body chunks, so as to avoid denial of
3664   service attacks without impeding interoperability.
3665</t>
3666</section>
3667
3668<section title="Message Integrity" anchor="message.integrity">
3669<t>
3670   HTTP does not define a specific mechanism for ensuring message integrity,
3671   instead relying on the error-detection ability of underlying transport
3672   protocols and the use of length or chunk-delimited framing to detect
3673   completeness. Additional integrity mechanisms, such as hash functions or
3674   digital signatures applied to the content, can be selectively added to
3675   messages via extensible metadata header fields. Historically, the lack of
3676   a single integrity mechanism has been justified by the informal nature of
3677   most HTTP communication.  However, the prevalence of HTTP as an information
3678   access mechanism has resulted in its increasing use within environments
3679   where verification of message integrity is crucial.
3680</t>
3681<t>
3682   User agents are encouraged to implement configurable means for detecting
3683   and reporting failures of message integrity such that those means can be
3684   enabled within environments for which integrity is necessary. For example,
3685   a browser being used to view medical history or drug interaction
3686   information needs to indicate to the user when such information is detected
3687   by the protocol to be incomplete, expired, or corrupted during transfer.
3688   Such mechanisms might be selectively enabled via user agent extensions or
3689   the presence of message integrity metadata in a response.
3690   At a minimum, user agents ought to provide some indication that allows a
3691   user to distinguish between a complete and incomplete response message
3692   (<xref target="incomplete.messages"/>) when such verification is desired.
3693</t>
3694</section>
3695
3696<section title="Server Log Information" anchor="abuse.of.server.log.information">
3697<t>
3698   A server is in the position to save personal data about a user's requests
3699   over time, which might identify their reading patterns or subjects of
3700   interest.  In particular, log information gathered at an intermediary
3701   often contains a history of user agent interaction, across a multitude
3702   of sites, that can be traced to individual users.
3703</t>
3704<t>
3705   HTTP log information is confidential in nature; its handling is often
3706   constrained by laws and regulations.  Log information needs to be securely
3707   stored and appropriate guidelines followed for its analysis.
3708   Anonymization of personal information within individual entries helps,
3709   but is generally not sufficient to prevent real log traces from being
3710   re-identified based on correlation with other access characteristics.
3711   As such, access traces that are keyed to a specific client should not
3712   be published even if the key is pseudonymous.
3713</t>
3714<t>
3715   To minimize the risk of theft or accidental publication, log information
3716   should be purged of personally identifiable information, including
3717   user identifiers, IP addresses, and user-provided query parameters,
3718   as soon as that information is no longer necessary to support operational
3719   needs for security, auditing, or fraud control.
3720</t>
3721</section>
3722</section>
3723
3724<section title="Acknowledgments" anchor="acks">
3725<t>
3726   This edition of HTTP/1.1 builds on the many contributions that went into
3727   <xref target="RFC1945" format="none">RFC 1945</xref>,
3728   <xref target="RFC2068" format="none">RFC 2068</xref>,
3729   <xref target="RFC2145" format="none">RFC 2145</xref>, and
3730   <xref target="RFC2616" format="none">RFC 2616</xref>, including
3731   substantial contributions made by the previous authors, editors, and
3732   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
3733   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
3734   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
3735</t>
3736<t>
3737   Since 1999, the following contributors have helped improve the HTTP
3738   specification by reporting bugs, asking smart questions, drafting or
3739   reviewing text, and evaluating open issues:
3740</t>
3741<?BEGININC acks ?>
3742<t>Adam Barth,
3743Adam Roach,
3744Addison Phillips,
3745Adrian Chadd,
3746Adrien W. de Croy,
3747Alan Ford,
3748Alan Ruttenberg,
3749Albert Lunde,
3750Alek Storm,
3751Alex Rousskov,
3752Alexandre Morgaut,
3753Alexey Melnikov,
3754Alisha Smith,
3755Amichai Rothman,
3756Amit Klein,
3757Amos Jeffries,
3758Andreas Maier,
3759Andreas Petersson,
3760Anil Sharma,
3761Anne van Kesteren,
3762Anthony Bryan,
3763Asbjorn Ulsberg,
3764Ashok Kumar,
3765Balachander Krishnamurthy,
3766Barry Leiba,
3767Ben Laurie,
3768Benjamin Carlyle,
3769Benjamin Niven-Jenkins,
3770Bil Corry,
3771Bill Burke,
3772Bjoern Hoehrmann,
3773Bob Scheifler,
3774Boris Zbarsky,
3775Brett Slatkin,
3776Brian Kell,
3777Brian McBarron,
3778Brian Pane,
3779Brian Raymor,
3780Brian Smith,
3781Bryce Nesbitt,
3782Cameron Heavon-Jones,
3783Carl Kugler,
3784Carsten Bormann,
3785Charles Fry,
3786Chris Newman,
3787Cyrus Daboo,
3788Dale Robert Anderson,
3789Dan Wing,
3790Dan Winship,
3791Daniel Stenberg,
3792Darrel Miller,
3793Dave Cridland,
3794Dave Crocker,
3795Dave Kristol,
3796Dave Thaler,
3797David Booth,
3798David Singer,
3799David W. Morris,
3800Diwakar Shetty,
3801Dmitry Kurochkin,
3802Drummond Reed,
3803Duane Wessels,
3804Edward Lee,
3805Eitan Adler,
3806Eliot Lear,
3807Eran Hammer-Lahav,
3808Eric D. Williams,
3809Eric J. Bowman,
3810Eric Lawrence,
3811Eric Rescorla,
3812Erik Aronesty,
3813Evan Prodromou,
3814Felix Geisendoerfer,
3815Florian Weimer,
3816Frank Ellermann,
3817Fred Bohle,
3818Frederic Kayser,
3819Gabriel Montenegro,
3820Geoffrey Sneddon,
3821Gervase Markham,
3822Grahame Grieve,
3823Greg Wilkins,
3824Grzegorz Calkowski,
3825Harald Tveit Alvestrand,
3826Harry Halpin,
3827Helge Hess,
3828Henrik Nordstrom,
3829Henry S. Thompson,
3830Henry Story,
3831Herbert van de Sompel,
3832Herve Ruellan,
3833Howard Melman,
3834Hugo Haas,
3835Ian Fette,
3836Ian Hickson,
3837Ido Safruti,
3838Ilari Liusvaara,
3839Ilya Grigorik,
3840Ingo Struck,
3841J. Ross Nicoll,
3842James Cloos,
3843James H. Manger,
3844James Lacey,
3845James M. Snell,
3846Jamie Lokier,
3847Jan Algermissen,
3848Jeff Hodges (who came up with the term 'effective Request-URI'),
3849Jeff Pinner,
3850Jeff Walden,
3851Jim Luther,
3852Jitu Padhye,
3853Joe D. Williams,
3854Joe Gregorio,
3855Joe Orton,
3856John C. Klensin,
3857John C. Mallery,
3858John Cowan,
3859John Kemp,
3860John Panzer,
3861John Schneider,
3862John Stracke,
3863John Sullivan,
3864Jonas Sicking,
3865Jonathan A. Rees,
3866Jonathan Billington,
3867Jonathan Moore,
3868Jonathan Silvera,
3869Jordi Ros,
3870Joris Dobbelsteen,
3871Josh Cohen,
3872Julien Pierre,
3873Jungshik Shin,
3874Justin Chapweske,
3875Justin Erenkrantz,
3876Justin James,
3877Kalvinder Singh,
3878Karl Dubost,
3879Keith Hoffman,
3880Keith Moore,
3881Ken Murchison,
3882Koen Holtman,
3883Konstantin Voronkov,
3884Kris Zyp,
3885Lisa Dusseault,
3886Maciej Stachowiak,
3887Manu Sporny,
3888Marc Schneider,
3889Marc Slemko,
3890Mark Baker,
3891Mark Pauley,
3892Mark Watson,
3893Markus Isomaki,
3894Markus Lanthaler,
3895Martin J. Duerst,
3896Martin Musatov,
3897Martin Nilsson,
3898Martin Thomson,
3899Matt Lynch,
3900Matthew Cox,
3901Max Clark,
3902Michael Burrows,
3903Michael Hausenblas,
3904Mike Amundsen,
3905Mike Belshe,
3906Mike Kelly,
3907Mike Schinkel,
3908Miles Sabin,
3909Murray S. Kucherawy,
3910Mykyta Yevstifeyev,
3911Nathan Rixham,
3912Nicholas Shanks,
3913Nico Williams,
3914Nicolas Alvarez,
3915Nicolas Mailhot,
3916Noah Slater,
3917Osama Mazahir,
3918Pablo Castro,
3919Pat Hayes,
3920Patrick R. McManus,
3921Paul E. Jones,
3922Paul Hoffman,
3923Paul Marquess,
3924Peter Lepeska,
3925Peter Occil,
3926Peter Saint-Andre,
3927Peter Watkins,
3928Phil Archer,
3929Philippe Mougin,
3930Phillip Hallam-Baker,
3931Piotr Dobrogost,
3932Poul-Henning Kamp,
3933Preethi Natarajan,
3934Rajeev Bector,
3935Ray Polk,
3936Reto Bachmann-Gmuer,
3937Richard Cyganiak,
3938Robby Simpson,
3939Robert Brewer,
3940Robert Collins,
3941Robert Mattson,
3942Robert O'Callahan,
3943Robert Olofsson,
3944Robert Sayre,
3945Robert Siemer,
3946Robert de Wilde,
3947Roberto Javier Godoy,
3948Roberto Peon,
3949Roland Zink,
3950Ronny Widjaja,
3951S. Mike Dierken,
3952Salvatore Loreto,
3953Sam Johnston,
3954Sam Ruby,
3955Scott Lawrence (who maintained the original issues list),
3956Sean B. Palmer,
3957Shane McCarron,
3958Stefan Eissing,
3959Stefan Tilkov,
3960Stefanos Harhalakis,
3961Stephane Bortzmeyer,
3962Stephen Farrell,
3963Stephen Ludin,
3964Stuart Williams,
3965Subbu Allamaraju,
3966Sylvain Hellegouarch,
3967Tapan Divekar,
3968Tatsuya Hayashi,
3969Ted Hardie,
3970Thomas Broyer,
3971Thomas Fossati,
3972Thomas Maslen,
3973Thomas Nordin,
3974Thomas Roessler,
3975Tim Bray,
3976Tim Morgan,
3977Tim Olsen,
3978Tom Zhou,
3979Travis Snoozy,
3980Tyler Close,
3981Vincent Murphy,
3982Wenbo Zhu,
3983Werner Baumann,
3984Wilbur Streett,
3985Wilfredo Sanchez Vega,
3986William A. Rowe Jr.,
3987William Chan,
3988Willy Tarreau,
3989Xiaoshu Wang,
3990Yaron Goland,
3991Yngve Nysaeter Pettersen,
3992Yoav Nir,
3993Yogesh Bang,
3994Yutaka Oiwa,
3995Yves Lafon (long-time member of the editor team),
3996Zed A. Shaw, and
3997Zhong Yu.
3998</t>
3999<?ENDINC acks ?>
4000<t>
4001   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
4002   acknowledgements from prior revisions.
4003</t>
4004</section>
4005
4006</middle>
4007<back>
4008
4009<references title="Normative References">
4010
4011<reference anchor="Part2">
4012  <front>
4013    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
4014    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4015      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4016      <address><email>fielding@gbiv.com</email></address>
4017    </author>
4018    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4019      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4020      <address><email>julian.reschke@greenbytes.de</email></address>
4021    </author>
4022    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4023  </front>
4024  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
4025  <x:source href="p2-semantics.xml" basename="p2-semantics">
4026    <x:defines>1xx (Informational)</x:defines>
4027    <x:defines>1xx</x:defines>
4028    <x:defines>100 (Continue)</x:defines>
4029    <x:defines>101 (Switching Protocols)</x:defines>
4030    <x:defines>2xx (Successful)</x:defines>
4031    <x:defines>2xx</x:defines>
4032    <x:defines>200 (OK)</x:defines>
4033    <x:defines>204 (No Content)</x:defines>
4034    <x:defines>3xx (Redirection)</x:defines>
4035    <x:defines>3xx</x:defines>
4036    <x:defines>301 (Moved Permanently)</x:defines>
4037    <x:defines>4xx (Client Error)</x:defines>
4038    <x:defines>4xx</x:defines>
4039    <x:defines>400 (Bad Request)</x:defines>
4040    <x:defines>411 (Length Required)</x:defines>
4041    <x:defines>414 (URI Too Long)</x:defines>
4042    <x:defines>417 (Expectation Failed)</x:defines>
4043    <x:defines>426 (Upgrade Required)</x:defines>
4044    <x:defines>501 (Not Implemented)</x:defines>
4045    <x:defines>502 (Bad Gateway)</x:defines>
4046    <x:defines>505 (HTTP Version Not Supported)</x:defines>
4047    <x:defines>Allow</x:defines>
4048    <x:defines>Content-Encoding</x:defines>
4049    <x:defines>Content-Location</x:defines>
4050    <x:defines>Content-Type</x:defines>
4051    <x:defines>Date</x:defines>
4052    <x:defines>Expect</x:defines>
4053    <x:defines>Location</x:defines>
4054    <x:defines>Server</x:defines>
4055    <x:defines>User-Agent</x:defines>
4056  </x:source>
4057</reference>
4058
4059<reference anchor="Part4">
4060  <front>
4061    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
4062    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
4063      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4064      <address><email>fielding@gbiv.com</email></address>
4065    </author>
4066    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
4067      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4068      <address><email>julian.reschke@greenbytes.de</email></address>
4069    </author>
4070    <date month="&ID-MONTH;" year="&ID-YEAR;" />
4071  </front>
4072  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
4073  <x:source basename="p4-conditional" href="p4-conditional.xml">
4074    <x:defines>304 (Not Modified)</x:defines>
4075    <x:defines>ETag</x:defines>
4076    <x:defines>Last-Modified</x:defines>
4077  </x:source>
4078</reference>
4079
4080<reference anchor="Part5">
4081  <front>
4082    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
4083    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4084      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4085      <address><email>fielding@gbiv.com</email></address>
4086    </author>
4087    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
4088      <organization abbrev="W3C">World Wide Web Consortium</organization>
4089      <address><email>ylafon@w3.org</email></address>
4090    </author>
4091    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4092      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4093      <address><email>julian.reschke@greenbytes.de</email></address>
4094    </author>
4095    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4096  </front>
4097  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4098  <x:source href="p5-range.xml" basename="p5-range">
4099    <x:defines>Content-Range</x:defines>
4100  </x:source>
4101</reference>
4102
4103<reference anchor="Part6">
4104  <front>
4105    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4106    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4107      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4108      <address><email>fielding@gbiv.com</email></address>
4109    </author>
4110    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4111      <organization>Akamai</organization>
4112      <address><email>mnot@mnot.net</email></address>
4113    </author>
4114    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4115      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4116      <address><email>julian.reschke@greenbytes.de</email></address>
4117    </author>
4118    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4119  </front>
4120  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4121  <x:source href="p6-cache.xml" basename="p6-cache">
4122    <x:defines>Cache-Control</x:defines>
4123    <x:defines>Expires</x:defines>
4124  </x:source>
4125</reference>
4126
4127<reference anchor="Part7">
4128  <front>
4129    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4130    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4131      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4132      <address><email>fielding@gbiv.com</email></address>
4133    </author>
4134    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4135      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4136      <address><email>julian.reschke@greenbytes.de</email></address>
4137    </author>
4138    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4139  </front>
4140  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4141  <x:source href="p7-auth.xml" basename="p7-auth">
4142    <x:defines>Proxy-Authenticate</x:defines>
4143    <x:defines>Proxy-Authorization</x:defines>
4144  </x:source>
4145</reference>
4146
4147<reference anchor="RFC5234">
4148  <front>
4149    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4150    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4151      <organization>Brandenburg InternetWorking</organization>
4152      <address>
4153        <email>dcrocker@bbiw.net</email>
4154      </address> 
4155    </author>
4156    <author initials="P." surname="Overell" fullname="Paul Overell">
4157      <organization>THUS plc.</organization>
4158      <address>
4159        <email>paul.overell@thus.net</email>
4160      </address>
4161    </author>
4162    <date month="January" year="2008"/>
4163  </front>
4164  <seriesInfo name="STD" value="68"/>
4165  <seriesInfo name="RFC" value="5234"/>
4166</reference>
4167
4168<reference anchor="RFC2119">
4169  <front>
4170    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4171    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4172      <organization>Harvard University</organization>
4173      <address><email>sob@harvard.edu</email></address>
4174    </author>
4175    <date month="March" year="1997"/>
4176  </front>
4177  <seriesInfo name="BCP" value="14"/>
4178  <seriesInfo name="RFC" value="2119"/>
4179</reference>
4180
4181<reference anchor="RFC3986">
4182 <front>
4183  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4184  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4185    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4186    <address>
4187       <email>timbl@w3.org</email>
4188       <uri>http://www.w3.org/People/Berners-Lee/</uri>
4189    </address>
4190  </author>
4191  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4192    <organization abbrev="Day Software">Day Software</organization>
4193    <address>
4194      <email>fielding@gbiv.com</email>
4195      <uri>http://roy.gbiv.com/</uri>
4196    </address>
4197  </author>
4198  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4199    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4200    <address>
4201      <email>LMM@acm.org</email>
4202      <uri>http://larry.masinter.net/</uri>
4203    </address>
4204  </author>
4205  <date month='January' year='2005'></date>
4206 </front>
4207 <seriesInfo name="STD" value="66"/>
4208 <seriesInfo name="RFC" value="3986"/>
4209</reference>
4210
4211<reference anchor="RFC0793">
4212  <front>
4213    <title>Transmission Control Protocol</title>
4214    <author initials='J.' surname='Postel' fullname='Jon Postel'>
4215      <organization>University of Southern California (USC)/Information Sciences Institute</organization>
4216    </author>
4217    <date year='1981' month='September' />
4218  </front>
4219  <seriesInfo name='STD' value='7' />
4220  <seriesInfo name='RFC' value='793' />
4221</reference>
4222
4223<reference anchor="USASCII">
4224  <front>
4225    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4226    <author>
4227      <organization>American National Standards Institute</organization>
4228    </author>
4229    <date year="1986"/>
4230  </front>
4231  <seriesInfo name="ANSI" value="X3.4"/>
4232</reference>
4233
4234<reference anchor="RFC1950">
4235  <front>
4236    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4237    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4238      <organization>Aladdin Enterprises</organization>
4239      <address><email>ghost@aladdin.com</email></address>
4240    </author>
4241    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4242    <date month="May" year="1996"/>
4243  </front>
4244  <seriesInfo name="RFC" value="1950"/>
4245  <!--<annotation>
4246    RFC 1950 is an Informational RFC, thus it might be less stable than
4247    this specification. On the other hand, this downward reference was
4248    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4249    therefore it is unlikely to cause problems in practice. See also
4250    <xref target="BCP97"/>.
4251  </annotation>-->
4252</reference>
4253
4254<reference anchor="RFC1951">
4255  <front>
4256    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4257    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4258      <organization>Aladdin Enterprises</organization>
4259      <address><email>ghost@aladdin.com</email></address>
4260    </author>
4261    <date month="May" year="1996"/>
4262  </front>
4263  <seriesInfo name="RFC" value="1951"/>
4264  <!--<annotation>
4265    RFC 1951 is an Informational RFC, thus it might be less stable than
4266    this specification. On the other hand, this downward reference was
4267    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4268    therefore it is unlikely to cause problems in practice. See also
4269    <xref target="BCP97"/>.
4270  </annotation>-->
4271</reference>
4272
4273<reference anchor="RFC1952">
4274  <front>
4275    <title>GZIP file format specification version 4.3</title>
4276    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4277      <organization>Aladdin Enterprises</organization>
4278      <address><email>ghost@aladdin.com</email></address>
4279    </author>
4280    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4281      <address><email>gzip@prep.ai.mit.edu</email></address>
4282    </author>
4283    <author initials="M." surname="Adler" fullname="Mark Adler">
4284      <address><email>madler@alumni.caltech.edu</email></address>
4285    </author>
4286    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4287      <address><email>ghost@aladdin.com</email></address>
4288    </author>
4289    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4290      <address><email>randeg@alumni.rpi.edu</email></address>
4291    </author>
4292    <date month="May" year="1996"/>
4293  </front>
4294  <seriesInfo name="RFC" value="1952"/>
4295  <!--<annotation>
4296    RFC 1952 is an Informational RFC, thus it might be less stable than
4297    this specification. On the other hand, this downward reference was
4298    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4299    therefore it is unlikely to cause problems in practice. See also
4300    <xref target="BCP97"/>.
4301  </annotation>-->
4302</reference>
4303
4304</references>
4305
4306<references title="Informative References">
4307
4308<reference anchor="ISO-8859-1">
4309  <front>
4310    <title>
4311     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4312    </title>
4313    <author>
4314      <organization>International Organization for Standardization</organization>
4315    </author>
4316    <date year="1998"/>
4317  </front>
4318  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4319</reference>
4320
4321<reference anchor='RFC1919'>
4322  <front>
4323    <title>Classical versus Transparent IP Proxies</title>
4324    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4325      <address><email>mchatel@pax.eunet.ch</email></address>
4326    </author>
4327    <date year='1996' month='March' />
4328  </front>
4329  <seriesInfo name='RFC' value='1919' />
4330</reference>
4331
4332<reference anchor="RFC1945">
4333  <front>
4334    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4335    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4336      <organization>MIT, Laboratory for Computer Science</organization>
4337      <address><email>timbl@w3.org</email></address>
4338    </author>
4339    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4340      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4341      <address><email>fielding@ics.uci.edu</email></address>
4342    </author>
4343    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4344      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4345      <address><email>frystyk@w3.org</email></address>
4346    </author>
4347    <date month="May" year="1996"/>
4348  </front>
4349  <seriesInfo name="RFC" value="1945"/>
4350</reference>
4351
4352<reference anchor="RFC2045">
4353  <front>
4354    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4355    <author initials="N." surname="Freed" fullname="Ned Freed">
4356      <organization>Innosoft International, Inc.</organization>
4357      <address><email>ned@innosoft.com</email></address>
4358    </author>
4359    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4360      <organization>First Virtual Holdings</organization>
4361      <address><email>nsb@nsb.fv.com</email></address>
4362    </author>
4363    <date month="November" year="1996"/>
4364  </front>
4365  <seriesInfo name="RFC" value="2045"/>
4366</reference>
4367
4368<reference anchor="RFC2047">
4369  <front>
4370    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4371    <author initials="K." surname="Moore" fullname="Keith Moore">
4372      <organization>University of Tennessee</organization>
4373      <address><email>moore@cs.utk.edu</email></address>
4374    </author>
4375    <date month="November" year="1996"/>
4376  </front>
4377  <seriesInfo name="RFC" value="2047"/>
4378</reference>
4379
4380<reference anchor="RFC2068">
4381  <front>
4382    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4383    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
4384      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4385      <address><email>fielding@ics.uci.edu</email></address>
4386    </author>
4387    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4388      <organization>MIT Laboratory for Computer Science</organization>
4389      <address><email>jg@w3.org</email></address>
4390    </author>
4391    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4392      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4393      <address><email>mogul@wrl.dec.com</email></address>
4394    </author>
4395    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4396      <organization>MIT Laboratory for Computer Science</organization>
4397      <address><email>frystyk@w3.org</email></address>
4398    </author>
4399    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4400      <organization>MIT Laboratory for Computer Science</organization>
4401      <address><email>timbl@w3.org</email></address>
4402    </author>
4403    <date month="January" year="1997"/>
4404  </front>
4405  <seriesInfo name="RFC" value="2068"/>
4406</reference>
4407
4408<reference anchor="RFC2145">
4409  <front>
4410    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4411    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4412      <organization>Western Research Laboratory</organization>
4413      <address><email>mogul@wrl.dec.com</email></address>
4414    </author>
4415    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4416      <organization>Department of Information and Computer Science</organization>
4417      <address><email>fielding@ics.uci.edu</email></address>
4418    </author>
4419    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4420      <organization>MIT Laboratory for Computer Science</organization>
4421      <address><email>jg@w3.org</email></address>
4422    </author>
4423    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4424      <organization>W3 Consortium</organization>
4425      <address><email>frystyk@w3.org</email></address>
4426    </author>
4427    <date month="May" year="1997"/>
4428  </front>
4429  <seriesInfo name="RFC" value="2145"/>
4430</reference>
4431
4432<reference anchor="RFC2616">
4433  <front>
4434    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4435    <author initials="R." surname="Fielding" fullname="R. Fielding">
4436      <organization>University of California, Irvine</organization>
4437      <address><email>fielding@ics.uci.edu</email></address>
4438    </author>
4439    <author initials="J." surname="Gettys" fullname="J. Gettys">
4440      <organization>W3C</organization>
4441      <address><email>jg@w3.org</email></address>
4442    </author>
4443    <author initials="J." surname="Mogul" fullname="J. Mogul">
4444      <organization>Compaq Computer Corporation</organization>
4445      <address><email>mogul@wrl.dec.com</email></address>
4446    </author>
4447    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4448      <organization>MIT Laboratory for Computer Science</organization>
4449      <address><email>frystyk@w3.org</email></address>
4450    </author>
4451    <author initials="L." surname="Masinter" fullname="L. Masinter">
4452      <organization>Xerox Corporation</organization>
4453      <address><email>masinter@parc.xerox.com</email></address>
4454    </author>
4455    <author initials="P." surname="Leach" fullname="P. Leach">
4456      <organization>Microsoft Corporation</organization>
4457      <address><email>paulle@microsoft.com</email></address>
4458    </author>
4459    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4460      <organization>W3C</organization>
4461      <address><email>timbl@w3.org</email></address>
4462    </author>
4463    <date month="June" year="1999"/>
4464  </front>
4465  <seriesInfo name="RFC" value="2616"/>
4466</reference>
4467
4468<reference anchor='RFC2817'>
4469  <front>
4470    <title>Upgrading to TLS Within HTTP/1.1</title>
4471    <author initials='R.' surname='Khare' fullname='R. Khare'>
4472      <organization>4K Associates / UC Irvine</organization>
4473      <address><email>rohit@4K-associates.com</email></address>
4474    </author>
4475    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4476      <organization>Agranat Systems, Inc.</organization>
4477      <address><email>lawrence@agranat.com</email></address>
4478    </author>
4479    <date year='2000' month='May' />
4480  </front>
4481  <seriesInfo name='RFC' value='2817' />
4482</reference>
4483
4484<reference anchor='RFC2818'>
4485  <front>
4486    <title>HTTP Over TLS</title>
4487    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4488      <organization>RTFM, Inc.</organization>
4489      <address><email>ekr@rtfm.com</email></address>
4490    </author>
4491    <date year='2000' month='May' />
4492  </front>
4493  <seriesInfo name='RFC' value='2818' />
4494</reference>
4495
4496<reference anchor='RFC3040'>
4497  <front>
4498    <title>Internet Web Replication and Caching Taxonomy</title>
4499    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4500      <organization>Equinix, Inc.</organization>
4501    </author>
4502    <author initials='I.' surname='Melve' fullname='I. Melve'>
4503      <organization>UNINETT</organization>
4504    </author>
4505    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4506      <organization>CacheFlow Inc.</organization>
4507    </author>
4508    <date year='2001' month='January' />
4509  </front>
4510  <seriesInfo name='RFC' value='3040' />
4511</reference>
4512
4513<reference anchor='BCP90'>
4514  <front>
4515    <title>Registration Procedures for Message Header Fields</title>
4516    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4517      <organization>Nine by Nine</organization>
4518      <address><email>GK-IETF@ninebynine.org</email></address>
4519    </author>
4520    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4521      <organization>BEA Systems</organization>
4522      <address><email>mnot@pobox.com</email></address>
4523    </author>
4524    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4525      <organization>HP Labs</organization>
4526      <address><email>JeffMogul@acm.org</email></address>
4527    </author>
4528    <date year='2004' month='September' />
4529  </front>
4530  <seriesInfo name='BCP' value='90' />
4531  <seriesInfo name='RFC' value='3864' />
4532</reference>
4533
4534<reference anchor='RFC4033'>
4535  <front>
4536    <title>DNS Security Introduction and Requirements</title>
4537    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4538    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4539    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4540    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4541    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4542    <date year='2005' month='March' />
4543  </front>
4544  <seriesInfo name='RFC' value='4033' />
4545</reference>
4546
4547<reference anchor="BCP13">
4548  <front>
4549    <title>Media Type Specifications and Registration Procedures</title>
4550    <author initials="N." surname="Freed" fullname="Ned Freed">
4551      <organization>Oracle</organization>
4552      <address>
4553        <email>ned+ietf@mrochek.com</email>
4554      </address>
4555    </author>
4556    <author initials="J." surname="Klensin" fullname="John C. Klensin">
4557      <address>
4558        <email>john+ietf@jck.com</email>
4559      </address>
4560    </author>
4561    <author initials="T." surname="Hansen" fullname="Tony Hansen">
4562      <organization>AT&amp;T Laboratories</organization>
4563      <address>
4564        <email>tony+mtsuffix@maillennium.att.com</email>
4565      </address>
4566    </author>
4567    <date year="2013" month="January"/>
4568  </front>
4569  <seriesInfo name="BCP" value="13"/>
4570  <seriesInfo name="RFC" value="6838"/>
4571</reference>
4572
4573<reference anchor='BCP115'>
4574  <front>
4575    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4576    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4577      <organization>AT&amp;T Laboratories</organization>
4578      <address>
4579        <email>tony+urireg@maillennium.att.com</email>
4580      </address>
4581    </author>
4582    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4583      <organization>Qualcomm, Inc.</organization>
4584      <address>
4585        <email>hardie@qualcomm.com</email>
4586      </address>
4587    </author>
4588    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4589      <organization>Adobe Systems</organization>
4590      <address>
4591        <email>LMM@acm.org</email>
4592      </address>
4593    </author>
4594    <date year='2006' month='February' />
4595  </front>
4596  <seriesInfo name='BCP' value='115' />
4597  <seriesInfo name='RFC' value='4395' />
4598</reference>
4599
4600<reference anchor='RFC4559'>
4601  <front>
4602    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4603    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4604    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4605    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4606    <date year='2006' month='June' />
4607  </front>
4608  <seriesInfo name='RFC' value='4559' />
4609</reference>
4610
4611<reference anchor='RFC5226'>
4612  <front>
4613    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4614    <author initials='T.' surname='Narten' fullname='T. Narten'>
4615      <organization>IBM</organization>
4616      <address><email>narten@us.ibm.com</email></address>
4617    </author>
4618    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4619      <organization>Google</organization>
4620      <address><email>Harald@Alvestrand.no</email></address>
4621    </author>
4622    <date year='2008' month='May' />
4623  </front>
4624  <seriesInfo name='BCP' value='26' />
4625  <seriesInfo name='RFC' value='5226' />
4626</reference>
4627
4628<reference anchor='RFC5246'>
4629   <front>
4630      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4631      <author initials='T.' surname='Dierks' fullname='T. Dierks'>
4632         <organization />
4633      </author>
4634      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4635         <organization>RTFM, Inc.</organization>
4636      </author>
4637      <date year='2008' month='August' />
4638   </front>
4639   <seriesInfo name='RFC' value='5246' />
4640</reference>
4641
4642<reference anchor="RFC5322">
4643  <front>
4644    <title>Internet Message Format</title>
4645    <author initials="P." surname="Resnick" fullname="P. Resnick">
4646      <organization>Qualcomm Incorporated</organization>
4647    </author>
4648    <date year="2008" month="October"/>
4649  </front>
4650  <seriesInfo name="RFC" value="5322"/>
4651</reference>
4652
4653<reference anchor="RFC6265">
4654  <front>
4655    <title>HTTP State Management Mechanism</title>
4656    <author initials="A." surname="Barth" fullname="Adam Barth">
4657      <organization abbrev="U.C. Berkeley">
4658        University of California, Berkeley
4659      </organization>
4660      <address><email>abarth@eecs.berkeley.edu</email></address>
4661    </author>
4662    <date year="2011" month="April" />
4663  </front>
4664  <seriesInfo name="RFC" value="6265"/>
4665</reference>
4666
4667<reference anchor='RFC6585'>
4668  <front>
4669    <title>Additional HTTP Status Codes</title>
4670    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4671      <organization>Rackspace</organization>
4672    </author>
4673    <author initials='R.' surname='Fielding' fullname='R. Fielding'>
4674      <organization>Adobe</organization>
4675    </author>
4676    <date year='2012' month='April' />
4677   </front>
4678   <seriesInfo name='RFC' value='6585' />
4679</reference>
4680
4681<!--<reference anchor='BCP97'>
4682  <front>
4683    <title>Handling Normative References to Standards-Track Documents</title>
4684    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
4685      <address>
4686        <email>klensin+ietf@jck.com</email>
4687      </address>
4688    </author>
4689    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
4690      <organization>MIT</organization>
4691      <address>
4692        <email>hartmans-ietf@mit.edu</email>
4693      </address>
4694    </author>
4695    <date year='2007' month='June' />
4696  </front>
4697  <seriesInfo name='BCP' value='97' />
4698  <seriesInfo name='RFC' value='4897' />
4699</reference>-->
4700
4701<reference anchor="Kri2001" target="http://arxiv.org/abs/cs.SE/0105018">
4702  <front>
4703    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
4704    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
4705    <date year="2001" month="November"/>
4706  </front>
4707  <seriesInfo name="ACM Transactions on Internet Technology" value="Vol. 1, #2"/>
4708</reference>
4709
4710</references>
4711
4712
4713<section title="HTTP Version History" anchor="compatibility">
4714<t>
4715   HTTP has been in use by the World-Wide Web global information initiative
4716   since 1990. The first version of HTTP, later referred to as HTTP/0.9,
4717   was a simple protocol for hypertext data transfer across the Internet
4718   with only a single request method (GET) and no metadata.
4719   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
4720   methods and MIME-like messaging that could include metadata about the data
4721   transferred and modifiers on the request/response semantics. However,
4722   HTTP/1.0 did not sufficiently take into consideration the effects of
4723   hierarchical proxies, caching, the need for persistent connections, or
4724   name-based virtual hosts. The proliferation of incompletely-implemented
4725   applications calling themselves "HTTP/1.0" further necessitated a
4726   protocol version change in order for two communicating applications
4727   to determine each other's true capabilities.
4728</t>
4729<t>
4730   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
4731   requirements that enable reliable implementations, adding only
4732   those new features that will either be safely ignored by an HTTP/1.0
4733   recipient or only sent when communicating with a party advertising
4734   conformance with HTTP/1.1.
4735</t>
4736<t>
4737   It is beyond the scope of a protocol specification to mandate
4738   conformance with previous versions. HTTP/1.1 was deliberately
4739   designed, however, to make supporting previous versions easy.
4740   We would expect a general-purpose HTTP/1.1 server to understand
4741   any valid request in the format of HTTP/1.0 and respond appropriately
4742   with an HTTP/1.1 message that only uses features understood (or
4743   safely ignored) by HTTP/1.0 clients.  Likewise, we would expect
4744   an HTTP/1.1 client to understand any valid HTTP/1.0 response.
4745</t>
4746<t>
4747   Since HTTP/0.9 did not support header fields in a request,
4748   there is no mechanism for it to support name-based virtual
4749   hosts (selection of resource by inspection of the <x:ref>Host</x:ref> header
4750   field).  Any server that implements name-based virtual hosts
4751   ought to disable support for HTTP/0.9.  Most requests that
4752   appear to be HTTP/0.9 are, in fact, badly constructed HTTP/1.x
4753   requests wherein a buggy client failed to properly encode
4754   linear whitespace found in a URI reference and placed in
4755   the request-target.
4756</t>
4757
4758<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
4759<t>
4760   This section summarizes major differences between versions HTTP/1.0
4761   and HTTP/1.1.
4762</t>
4763
4764<section title="Multi-homed Web Servers" anchor="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">
4765<t>
4766   The requirements that clients and servers support the <x:ref>Host</x:ref>
4767   header field (<xref target="header.host"/>), report an error if it is
4768   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
4769   are among the most important changes defined by HTTP/1.1.
4770</t>
4771<t>
4772   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4773   addresses and servers; there was no other established mechanism for
4774   distinguishing the intended server of a request than the IP address
4775   to which that request was directed. The <x:ref>Host</x:ref> header field was
4776   introduced during the development of HTTP/1.1 and, though it was
4777   quickly implemented by most HTTP/1.0 browsers, additional requirements
4778   were placed on all HTTP/1.1 requests in order to ensure complete
4779   adoption.  At the time of this writing, most HTTP-based services
4780   are dependent upon the Host header field for targeting requests.
4781</t>
4782</section>
4783
4784<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4785<t>
4786   In HTTP/1.0, each connection is established by the client prior to the
4787   request and closed by the server after sending the response. However, some
4788   implementations implement the explicitly negotiated ("Keep-Alive") version
4789   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
4790   target="RFC2068"/>.
4791</t>
4792<t>
4793   Some clients and servers might wish to be compatible with these previous
4794   approaches to persistent connections, by explicitly negotiating for them
4795   with a "Connection: keep-alive" request header field. However, some
4796   experimental implementations of HTTP/1.0 persistent connections are faulty;
4797   for example, if an HTTP/1.0 proxy server doesn't understand
4798   <x:ref>Connection</x:ref>, it will erroneously forward that header field
4799   to the next inbound server, which would result in a hung connection.
4800</t>
4801<t>
4802   One attempted solution was the introduction of a Proxy-Connection header
4803   field, targeted specifically at proxies. In practice, this was also
4804   unworkable, because proxies are often deployed in multiple layers, bringing
4805   about the same problem discussed above.
4806</t>
4807<t>
4808   As a result, clients are encouraged not to send the Proxy-Connection header
4809   field in any requests.
4810</t>
4811<t>
4812   Clients are also encouraged to consider the use of Connection: keep-alive
4813   in requests carefully; while they can enable persistent connections with
4814   HTTP/1.0 servers, clients using them will need to monitor the
4815   connection for "hung" requests (which indicate that the client ought stop
4816   sending the header field), and this mechanism ought not be used by clients
4817   at all when a proxy is being used.
4818</t>
4819</section>
4820
4821<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
4822<t>
4823   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
4824   (<xref target="header.transfer-encoding"/>).
4825   Transfer codings need to be decoded prior to forwarding an HTTP message
4826   over a MIME-compliant protocol.
4827</t>
4828</section>
4829
4830</section>
4831
4832<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4833<t>
4834  HTTP's approach to error handling has been explained.
4835  (<xref target="conformance"/>)
4836</t>
4837<t>
4838  The expectation to support HTTP/0.9 requests has been removed.
4839</t>
4840<t>
4841  The term "Effective Request URI" has been introduced.
4842  (<xref target="effective.request.uri" />)
4843</t>
4844<t>
4845  HTTP messages can be (and often are) buffered by implementations; despite
4846  it sometimes being available as a stream, HTTP is fundamentally a
4847  message-oriented protocol.
4848  (<xref target="http.message" />)
4849</t>
4850<t>
4851  Minimum supported sizes for various protocol elements have been
4852  suggested, to improve interoperability.
4853</t>
4854<t>
4855  Header fields that span multiple lines ("line folding") are deprecated.
4856  (<xref target="field.parsing" />)
4857</t>
4858<t>
4859  The HTTP-version ABNF production has been clarified to be case-sensitive.
4860  Additionally, version numbers has been restricted to single digits, due
4861  to the fact that implementations are known to handle multi-digit version
4862  numbers incorrectly.
4863  (<xref target="http.version"/>)
4864</t>
4865<t>
4866  The HTTPS URI scheme is now defined by this specification; previously,
4867  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
4868  (<xref target="https.uri"/>)
4869</t>
4870<t>
4871  The HTTPS URI scheme implies end-to-end security.
4872  (<xref target="https.uri"/>)
4873</t>
4874<t>
4875  Userinfo (i.e., username and password) are now disallowed in HTTP and
4876  HTTPS URIs, because of security issues related to their transmission on the
4877  wire.
4878  (<xref target="http.uri" />)
4879</t>
4880<t>
4881  Invalid whitespace around field-names is now required to be rejected,
4882  because accepting it represents a security vulnerability.
4883  (<xref target="header.fields"/>)
4884</t>
4885<t>
4886  The ABNF productions defining header fields now only list the field value.
4887  (<xref target="header.fields"/>)
4888</t>
4889<t>
4890  Rules about implicit linear whitespace between certain grammar productions
4891  have been removed; now whitespace is only allowed where specifically
4892  defined in the ABNF.
4893  (<xref target="whitespace"/>)
4894</t>
4895<t> 
4896  The NUL octet is no longer allowed in comment and quoted-string text, and
4897  handling of backslash-escaping in them has been clarified.
4898  (<xref target="field.components"/>)
4899</t> 
4900<t>
4901  The quoted-pair rule no longer allows escaping control characters other than
4902  HTAB.
4903  (<xref target="field.components"/>)
4904</t>
4905<t>
4906  Non-ASCII content in header fields and the reason phrase has been obsoleted
4907  and made opaque (the TEXT rule was removed).
4908  (<xref target="field.components"/>)
4909</t>
4910<t>
4911  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
4912  handled as errors by recipients.
4913  (<xref target="header.content-length"/>)
4914</t>
4915<t>
4916  The "identity" transfer coding token has been removed.
4917  (Sections <xref format="counter" target="message.body"/> and
4918  <xref format="counter" target="transfer.codings"/>)
4919</t>
4920<t>
4921  The algorithm for determining the message body length has been clarified
4922  to indicate all of the special cases (e.g., driven by methods or status
4923  codes) that affect it, and that new protocol elements cannot define such
4924  special cases.
4925  (<xref target="message.body.length"/>)
4926</t>
4927<t>
4928  "multipart/byteranges" is no longer a way of determining message body length
4929  detection.
4930  (<xref target="message.body.length"/>)
4931</t>
4932<t>
4933  CONNECT is a new, special case in determining message body length.
4934  (<xref target="message.body.length"/>)
4935</t>
4936<t>
4937  Chunk length does not include the count of the octets in the
4938  chunk header and trailer.
4939  (<xref target="chunked.encoding"/>)
4940</t>
4941<t>
4942  Use of chunk extensions is deprecated, and line folding in them is
4943  disallowed.
4944  (<xref target="chunked.encoding"/>)
4945</t>
4946<t>
4947  The segment + query components of RFC3986 have been used to define the
4948  request-target, instead of abs_path from RFC 1808.
4949  (<xref target="request-target"/>)
4950</t>
4951<t>
4952  The asterisk form of the request-target is only allowed in the OPTIONS
4953  method.
4954  (<xref target="request-target"/>)
4955</t>
4956<t>
4957  Exactly when "close" connection options have to be sent has been clarified.
4958  (<xref target="header.connection"/>)
4959</t>
4960<t> 
4961  "hop-by-hop" header fields are required to appear in the Connection header
4962  field; just because they're defined as hop-by-hop in this specification
4963  doesn't exempt them.
4964  (<xref target="header.connection"/>)
4965</t>
4966<t>
4967  The limit of two connections per server has been removed.
4968  (<xref target="persistent.connections"/>)
4969</t>
4970<t>
4971  An idempotent sequence of requests is no longer required to be retried.
4972  (<xref target="persistent.connections"/>)
4973</t>
4974<t>
4975  The requirement to retry requests under certain circumstances when the
4976  server prematurely closes the connection has been removed.
4977  (<xref target="persistent.connections"/>)
4978</t>
4979<t>
4980  Some extraneous requirements about when servers are allowed to close
4981  connections prematurely have been removed.
4982  (<xref target="persistent.connections"/>)
4983</t>
4984<t>
4985  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
4986  responses other than 101 (this was incorporated from <xref
4987  target="RFC2817"/>).
4988  (<xref target="header.upgrade"/>)
4989</t>
4990<t>
4991  Registration of Transfer Codings now requires IETF Review
4992  (<xref target="transfer.coding.registry"/>)
4993</t>
4994<t>
4995  The meaning of the "deflate" content coding has been clarified.
4996  (<xref target="deflate.coding" />)
4997</t>
4998<t>
4999  This specification now defines the Upgrade Token Registry, previously
5000  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
5001  (<xref target="upgrade.token.registry"/>)
5002</t>
5003<t>
5004  Issues with the Keep-Alive and Proxy-Connection header fields in requests
5005  are pointed out, with use of the latter being discouraged altogether.
5006  (<xref target="compatibility.with.http.1.0.persistent.connections" />)
5007</t>
5008<t>
5009  Empty list elements in list productions (e.g., a list header field containing
5010  ", ,") have been deprecated.
5011  (<xref target="abnf.extension"/>)
5012</t>
5013</section>
5014</section>
5015
5016<section title="ABNF list extension: #rule" anchor="abnf.extension">
5017<t>
5018  A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
5019  improve readability in the definitions of some header field values.
5020</t>
5021<t>
5022  A construct "#" is defined, similar to "*", for defining comma-delimited
5023  lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
5024  at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
5025  comma (",") and optional whitespace (OWS).   
5026</t>
5027<figure><preamble>
5028  Thus,
5029</preamble><artwork type="example">
5030  1#element =&gt; element *( OWS "," OWS element )
5031</artwork></figure>
5032<figure><preamble>
5033  and:
5034</preamble><artwork type="example">
5035  #element =&gt; [ 1#element ]
5036</artwork></figure>
5037<figure><preamble>
5038  and for n &gt;= 1 and m &gt; 1:
5039</preamble><artwork type="example">
5040  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
5041</artwork></figure>
5042<t>
5043  For compatibility with legacy list rules, recipients &SHOULD; accept empty
5044  list elements. In other words, consumers would follow the list productions:
5045</t>
5046<figure><artwork type="example">
5047  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
5048 
5049  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
5050</artwork></figure>
5051<t>
5052  Note that empty elements do not contribute to the count of elements present,
5053  though.
5054</t>
5055<t>
5056  For example, given these ABNF productions:
5057</t>
5058<figure><artwork type="example">
5059  example-list      = 1#example-list-elmt
5060  example-list-elmt = token ; see <xref target="field.components"/>
5061</artwork></figure>
5062<t>
5063  Then these are valid values for example-list (not including the double
5064  quotes, which are present for delimitation only):
5065</t>
5066<figure><artwork type="example">
5067  "foo,bar"
5068  "foo ,bar,"
5069  "foo , ,bar,charlie   "
5070</artwork></figure>
5071<t>
5072  But these values would be invalid, as at least one non-empty element is
5073  required:
5074</t>
5075<figure><artwork type="example">
5076  ""
5077  ","
5078  ",   ,"
5079</artwork></figure>
5080<t>
5081  <xref target="collected.abnf"/> shows the collected ABNF, with the list rules
5082  expanded as explained above.
5083</t>
5084</section>
5085
5086<?BEGININC p1-messaging.abnf-appendix ?>
5087<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
5088<figure>
5089<artwork type="abnf" name="p1-messaging.parsed-abnf">
5090<x:ref>BWS</x:ref> = OWS
5091
5092<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
5093 connection-option ] )
5094<x:ref>Content-Length</x:ref> = 1*DIGIT
5095
5096<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
5097 ]
5098<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
5099<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
5100<x:ref>Host</x:ref> = uri-host [ ":" port ]
5101
5102<x:ref>OWS</x:ref> = *( SP / HTAB )
5103
5104<x:ref>RWS</x:ref> = 1*( SP / HTAB )
5105
5106<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
5107<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
5108<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
5109 transfer-coding ] )
5110
5111<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
5112<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
5113
5114<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
5115 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
5116 comment ] ) ] )
5117
5118<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
5119<x:ref>absolute-form</x:ref> = absolute-URI
5120<x:ref>absolute-path</x:ref> = 1*( "/" segment )
5121<x:ref>asterisk-form</x:ref> = "*"
5122<x:ref>attribute</x:ref> = token
5123<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
5124<x:ref>authority-form</x:ref> = authority
5125
5126<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
5127<x:ref>chunk-data</x:ref> = 1*OCTET
5128<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
5129<x:ref>chunk-ext-name</x:ref> = token
5130<x:ref>chunk-ext-val</x:ref> = token / quoted-str-nf
5131<x:ref>chunk-size</x:ref> = 1*HEXDIG
5132<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
5133<x:ref>comment</x:ref> = "(" *( ctext / quoted-cpair / comment ) ")"
5134<x:ref>connection-option</x:ref> = token
5135<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
5136 / %x2A-5B ; '*'-'['
5137 / %x5D-7E ; ']'-'~'
5138 / obs-text
5139
5140<x:ref>field-content</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5141<x:ref>field-name</x:ref> = token
5142<x:ref>field-value</x:ref> = *( field-content / obs-fold )
5143
5144<x:ref>header-field</x:ref> = field-name ":" OWS field-value OWS
5145<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ]
5146<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ]
5147
5148<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
5149
5150<x:ref>message-body</x:ref> = *OCTET
5151<x:ref>method</x:ref> = token
5152
5153<x:ref>obs-fold</x:ref> = CRLF ( SP / HTAB )
5154<x:ref>obs-text</x:ref> = %x80-FF
5155<x:ref>origin-form</x:ref> = absolute-path [ "?" query ]
5156
5157<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
5158<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
5159<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
5160<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
5161<x:ref>protocol-name</x:ref> = token
5162<x:ref>protocol-version</x:ref> = token
5163<x:ref>pseudonym</x:ref> = token
5164
5165<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5166 / %x5D-7E ; ']'-'~'
5167 / obs-text
5168<x:ref>qdtext-nf</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5169 / %x5D-7E ; ']'-'~'
5170 / obs-text
5171<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5172<x:ref>quoted-cpair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5173<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5174<x:ref>quoted-str-nf</x:ref> = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
5175<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5176
5177<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5178<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5179<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5180<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5181<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5182<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5183<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5184 asterisk-form
5185
5186<x:ref>segment</x:ref> = &lt;segment, defined in [RFC3986], Section 3.3&gt;
5187<x:ref>special</x:ref> = "(" / ")" / "&lt;" / "&gt;" / "@" / "," / ";" / ":" / "\" /
5188 DQUOTE / "/" / "[" / "]" / "?" / "=" / "{" / "}"
5189<x:ref>start-line</x:ref> = request-line / status-line
5190<x:ref>status-code</x:ref> = 3DIGIT
5191<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5192
5193<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5194<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5195<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5196 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5197<x:ref>token</x:ref> = 1*tchar
5198<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5199<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5200 transfer-extension
5201<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5202<x:ref>transfer-parameter</x:ref> = attribute BWS "=" BWS value
5203
5204<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5205
5206<x:ref>value</x:ref> = word
5207
5208<x:ref>word</x:ref> = token / quoted-string
5209</artwork>
5210</figure>
5211</section>
5212<?ENDINC p1-messaging.abnf-appendix ?>
5213
5214<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5215
5216<section title="Since RFC 2616">
5217<t>
5218  Changes up to the first Working Group Last Call draft are summarized
5219  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-21#appendix-D"/>.
5220</t>
5221</section>
5222
5223<section title="Since draft-ietf-httpbis-p1-messaging-21" anchor="changes.since.21">
5224<t>
5225  Closed issues:
5226  <list style="symbols">
5227    <t>
5228      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/128"/>:
5229      "Cite HTTPS URI scheme definition" (the spec now includes the HTTPs
5230      scheme definition and thus updates RFC 2818)
5231    </t>
5232    <t>
5233      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/389"/>:
5234      "mention of 'proxies' in section about caches"
5235    </t>
5236    <t>
5237      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/390"/>:
5238      "use of ABNF terms from RFC 3986"
5239    </t>
5240    <t>
5241      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/391"/>:
5242      "transferring URIs with userinfo in payload"
5243    </t>
5244    <t>
5245      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/392"/>:
5246      "editorial improvements to message length definition"
5247    </t>
5248    <t>
5249      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/395"/>:
5250      "Connection header field MUST vs SHOULD"
5251    </t>
5252    <t>
5253      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/396"/>:
5254      "editorial improvements to persistent connections section"
5255    </t>
5256    <t>
5257      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/397"/>:
5258      "URI normalization vs empty path"
5259    </t>
5260    <t>
5261      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/408"/>:
5262      "p1 feedback"
5263    </t>
5264    <t>
5265      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/409"/>:
5266      "is parsing OBS-FOLD mandatory?"
5267    </t>
5268    <t>
5269      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/410"/>:
5270      "HTTPS and Shared Caching"
5271    </t>
5272    <t>
5273      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/411"/>:
5274      "Requirements for recipients of ws between start-line and first header field"
5275    </t>
5276    <t>
5277      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/412"/>:
5278      "SP and HT when being tolerant"
5279    </t>
5280    <t>
5281      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/414"/>:
5282      "Message Parsing Strictness"
5283    </t>
5284    <t>
5285      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/415"/>:
5286      "'Render'"
5287    </t>
5288    <t>
5289      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/418"/>:
5290      "No-Transform"
5291    </t>
5292    <t>
5293      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/419"/>:
5294      "p2 editorial feedback"
5295    </t>
5296    <t>
5297      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/420"/>:
5298      "Content-Length SHOULD be sent"
5299    </t>
5300    <t>
5301      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/431"/>:
5302      "origin-form does not allow path starting with "//""
5303    </t>
5304    <t>
5305      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/433"/>:
5306      "ambiguity in part 1 example"
5307    </t>
5308  </list>
5309</t>
5310</section>
5311
5312<section title="Since draft-ietf-httpbis-p1-messaging-22" anchor="changes.since.22">
5313<t>
5314  Closed issues:
5315  <list style="symbols">
5316    <t>
5317      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/435"/>:
5318      "Part1 should have a reference to TCP (RFC 793)"
5319    </t>
5320    <t>
5321      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/438"/>:
5322      "media type registration template issues"
5323    </t>
5324    <t>
5325      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/442"/>:
5326      "BWS" (vs conformance)
5327    </t>
5328    <t>
5329      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/444"/>:
5330      "obs-fold language"
5331    </t>
5332    <t>
5333      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/445"/>:
5334      "Ordering in Upgrade"
5335    </t>
5336    <t>
5337      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/446"/>:
5338      "p1 editorial feedback"
5339    </t>
5340    <t>
5341      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/476"/>:
5342      "SHOULD and conformance"
5343    </t>
5344    <t>
5345      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/477"/>:
5346      "Pipelining language"
5347    </t>
5348  </list>
5349</t>
5350</section>
5351</section>
5352
5353</back>
5354</rfc>
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