source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2221

Last change on this file since 2221 was 2221, 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: 226.7 KB
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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "April">
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!
389</x:span></artwork>
390<postamble>
391(Note that the content length includes the trailing CR/LF sequence of the body text)
392</postamble></figure>
393</section>
394
395<section title="Implementation Diversity" anchor="implementation-diversity">
396<t>
397   When considering the design of HTTP, it is easy to fall into a trap of
398   thinking that all user agents are general-purpose browsers and all origin
399   servers are large public websites. That is not the case in practice.
400   Common HTTP user agents include household appliances, stereos, scales,
401   firmware update scripts, command-line programs, mobile apps,
402   and communication devices in a multitude of shapes and sizes.  Likewise,
403   common HTTP origin servers include home automation units, configurable
404   networking components, office machines, autonomous robots, news feeds,
405   traffic cameras, ad selectors, and video delivery platforms.
406</t>
407<t>
408   The term "user agent" does not imply that there is a human user directly
409   interacting with the software agent at the time of a request. In many
410   cases, a user agent is installed or configured to run in the background
411   and save its results for later inspection (or save only a subset of those
412   results that might be interesting or erroneous). Spiders, for example, are
413   typically given a start URI and configured to follow certain behavior while
414   crawling the Web as a hypertext graph.
415</t>
416<t>
417   The implementation diversity of HTTP means that we cannot assume the
418   user agent can make interactive suggestions to a user or provide adequate
419   warning for security or privacy options.  In the few cases where this
420   specification requires reporting of errors to the user, it is acceptable
421   for such reporting to only be observable in an error console or log file.
422   Likewise, requirements that an automated action be confirmed by the user
423   before proceeding can be met via advance configuration choices,
424   run-time options, or simply not proceeding with the unsafe action.
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.
454</t>
455<t>
456<iref primary="true" item="upstream"/><iref primary="true" item="downstream"/>
457<iref primary="true" item="inbound"/><iref primary="true" item="outbound"/>
458   We use the terms "<x:dfn>upstream</x:dfn>" and "<x:dfn>downstream</x:dfn>"
459   to describe various requirements in relation to the directional flow of a
460   message: all messages flow from upstream to downstream.
461   Likewise, we use the terms inbound and outbound to refer to
462   directions in relation to the request path:
463   "<x:dfn>inbound</x:dfn>" means toward the origin server and
464   "<x:dfn>outbound</x:dfn>" means toward the user agent.
465</t>
466<t><iref primary="true" item="proxy"/>
467   A "<x:dfn>proxy</x:dfn>" is a message forwarding agent that is selected by the
468   client, usually via local configuration rules, to receive requests
469   for some type(s) of absolute URI and attempt to satisfy those
470   requests via translation through the HTTP interface.  Some translations
471   are minimal, such as for proxy requests for "http" URIs, whereas
472   other requests might require translation to and from entirely different
473   application-level protocols. Proxies are often used to group an
474   organization's HTTP requests through a common intermediary for the
475   sake of security, annotation services, or shared caching.
476</t>
477<t>
478<iref primary="true" item="transforming proxy"/>
479<iref primary="true" item="non-transforming proxy"/>
480   An HTTP-to-HTTP proxy is called a "<x:dfn>transforming proxy</x:dfn>" if it is designed
481   or configured to modify request or response messages in a semantically
482   meaningful way (i.e., modifications, beyond those required by normal
483   HTTP processing, that change the message in a way that would be
484   significant to the original sender or potentially significant to
485   downstream recipients).  For example, a transforming proxy might be
486   acting as a shared annotation server (modifying responses to include
487   references to a local annotation database), a malware filter, a
488   format transcoder, or an intranet-to-Internet privacy filter.  Such
489   transformations are presumed to be desired by the client (or client
490   organization) that selected the proxy and are beyond the scope of
491   this specification.  However, when a proxy is not intended to transform
492   a given message, we use the term "<x:dfn>non-transforming proxy</x:dfn>" to target
493   requirements that preserve HTTP message semantics. See &status-203; and
494   &header-warning; for status and warning codes related to transformations.
495</t>
496<t><iref primary="true" item="gateway"/><iref primary="true" item="reverse proxy"/>
497<iref primary="true" item="accelerator"/>
498   A "<x:dfn>gateway</x:dfn>" (a.k.a., "<x:dfn>reverse proxy</x:dfn>")
499   is a receiving agent that acts
500   as a layer above some other server(s) and translates the received
501   requests to the underlying server's protocol.  Gateways are often
502   used to encapsulate legacy or untrusted information services, to
503   improve server performance through "<x:dfn>accelerator</x:dfn>" caching, and to
504   enable partitioning or load-balancing of HTTP services across
505   multiple machines.
506</t>
507<t>
508   A gateway behaves as an origin server on its outbound connection and
509   as a user agent on its inbound connection.
510   All HTTP requirements applicable to an origin server
511   also apply to the outbound communication of a gateway.
512   A gateway communicates with inbound servers using any protocol that
513   it desires, including private extensions to HTTP that are outside
514   the scope of this specification.  However, an HTTP-to-HTTP gateway
515   that wishes to interoperate with third-party HTTP servers &MUST;
516   conform to HTTP user agent requirements on the gateway's inbound
517   connection and &MUST; implement the <x:ref>Connection</x:ref>
518   (<xref target="header.connection"/>) and <x:ref>Via</x:ref>
519   (<xref target="header.via"/>) header fields for both 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. Note that
624   SHOULD-level requirements are relevant here, unless one of the documented
625   exceptions is applicable.
626</t>
627<t>
628   Conformance applies to both the syntax and semantics of HTTP protocol
629   elements. A sender &MUST-NOT; generate protocol elements that convey a
630   meaning that is known by that sender to be false. A sender &MUST-NOT;
631   generate protocol elements that do not match the grammar defined by the
632   ABNF rules for those protocol elements that are applicable to the sender's
633   role. If a received protocol element is processed, the recipient &MUST; be
634   able to parse any value that would match the ABNF rules for that protocol
635   element, excluding only those rules not applicable to the recipient's role.
636</t>
637<t>
638   Unless noted otherwise, a recipient &MAY; attempt to recover a usable
639   protocol element from an invalid construct.  HTTP does not define
640   specific error handling mechanisms except when they have a direct impact
641   on security, since different applications of the protocol require
642   different error handling strategies.  For example, a Web browser might
643   wish to transparently recover from a response where the
644   <x:ref>Location</x:ref> header field doesn't parse according to the ABNF,
645   whereas a systems control client might consider any form of error recovery
646   to be dangerous.
647</t>
648</section>
649
650<section title="Protocol Versioning" anchor="http.version">
651  <x:anchor-alias value="HTTP-version"/>
652  <x:anchor-alias value="HTTP-name"/>
653<t>
654   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate
655   versions of the protocol. This specification defines version "1.1".
656   The protocol version as a whole indicates the sender's conformance
657   with the set of requirements laid out in that version's corresponding
658   specification of HTTP.
659</t>
660<t>
661   The version of an HTTP message is indicated by an HTTP-version field
662   in the first line of the message. HTTP-version is case-sensitive.
663</t>
664<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-version"/><iref primary="true" item="Grammar" subitem="HTTP-name"/>
665  <x:ref>HTTP-version</x:ref>  = <x:ref>HTTP-name</x:ref> "/" <x:ref>DIGIT</x:ref> "." <x:ref>DIGIT</x:ref>
666  <x:ref>HTTP-name</x:ref>     = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
667</artwork></figure>
668<t>
669   The HTTP version number consists of two decimal digits separated by a "."
670   (period or decimal point).  The first digit ("major version") indicates the
671   HTTP messaging syntax, whereas the second digit ("minor version") indicates
672   the highest minor version to which the sender is
673   conformant and able to understand for future communication.  The minor
674   version advertises the sender's communication capabilities even when the
675   sender is only using a backwards-compatible subset of the protocol,
676   thereby letting the recipient know that more advanced features can
677   be used in response (by servers) or in future requests (by clients).
678</t>
679<t>
680   When an HTTP/1.1 message is sent to an HTTP/1.0 recipient
681   <xref target="RFC1945"/> or a recipient whose version is unknown,
682   the HTTP/1.1 message is constructed such that it can be interpreted
683   as a valid HTTP/1.0 message if all of the newer features are ignored.
684   This specification places recipient-version requirements on some
685   new features so that a conformant sender will only use compatible
686   features until it has determined, through configuration or the
687   receipt of a message, that the recipient supports HTTP/1.1.
688</t>
689<t>
690   The interpretation of a header field does not change between minor
691   versions of the same major HTTP version, though the default
692   behavior of a recipient in the absence of such a field can change.
693   Unless specified otherwise, header fields defined in HTTP/1.1 are
694   defined for all versions of HTTP/1.x.  In particular, the <x:ref>Host</x:ref>
695   and <x:ref>Connection</x:ref> header fields ought to be implemented by all
696   HTTP/1.x implementations whether or not they advertise conformance with
697   HTTP/1.1.
698</t>
699<t>
700   New header fields can be defined such that, when they are
701   understood by a recipient, they might override or enhance the
702   interpretation of previously defined header fields.  When an
703   implementation receives an unrecognized header field, the recipient
704   &MUST; ignore that header field for local processing regardless of
705   the message's HTTP version.  An unrecognized header field received
706   by a proxy &MUST; be forwarded downstream unless the header field's
707   field-name is listed in the message's <x:ref>Connection</x:ref> header field
708   (see <xref target="header.connection"/>).
709   These requirements allow HTTP's functionality to be enhanced without
710   requiring prior update of deployed intermediaries.
711</t>
712<t>
713   Intermediaries that process HTTP messages (i.e., all intermediaries
714   other than those acting as tunnels) &MUST; send their own HTTP-version
715   in forwarded messages.  In other words, they &MUST-NOT; blindly
716   forward the first line of an HTTP message without ensuring that the
717   protocol version in that message matches a version to which that
718   intermediary is conformant for both the receiving and
719   sending of messages.  Forwarding an HTTP message without rewriting
720   the HTTP-version might result in communication errors when downstream
721   recipients use the message sender's version to determine what features
722   are safe to use for later communication with that sender.
723</t>
724<t>
725   An HTTP client &SHOULD; send a request version equal to the highest
726   version to which the client is conformant and
727   whose major version is no higher than the highest version supported
728   by the server, if this is known.  An HTTP client &MUST-NOT; send a
729   version to which it is not conformant.
730</t>
731<t>
732   An HTTP client &MAY; send a lower request version if it is known that
733   the server incorrectly implements the HTTP specification, but only
734   after the client has attempted at least one normal request and determined
735   from the response status or header fields (e.g., <x:ref>Server</x:ref>) that
736   the server improperly handles higher request versions.
737</t>
738<t>
739   An HTTP server &SHOULD; send a response version equal to the highest
740   version to which the server is conformant and
741   whose major version is less than or equal to the one received in the
742   request.  An HTTP server &MUST-NOT; send a version to which it is not
743   conformant.  A server &MAY; send a <x:ref>505 (HTTP Version Not
744   Supported)</x:ref> response if it cannot send a response using the
745   major version used in the client's request.
746</t>
747<t>
748   An HTTP server &MAY; send an HTTP/1.0 response to an HTTP/1.0 request
749   if it is known or suspected that the client incorrectly implements the
750   HTTP specification and is incapable of correctly processing later
751   version responses, such as when a client fails to parse the version
752   number correctly or when an intermediary is known to blindly forward
753   the HTTP-version even when it doesn't conform to the given minor
754   version of the protocol. Such protocol downgrades &SHOULD-NOT; be
755   performed unless triggered by specific client attributes, such as when
756   one or more of the request header fields (e.g., <x:ref>User-Agent</x:ref>)
757   uniquely match the values sent by a client known to be in error.
758</t>
759<t>
760   The intention of HTTP's versioning design is that the major number
761   will only be incremented if an incompatible message syntax is
762   introduced, and that the minor number will only be incremented when
763   changes made to the protocol have the effect of adding to the message
764   semantics or implying additional capabilities of the sender.  However,
765   the minor version was not incremented for the changes introduced between
766   <xref target="RFC2068"/> and <xref target="RFC2616"/>, and this revision
767   has specifically avoided any such changes to the protocol.
768</t>
769</section>
770
771<section title="Uniform Resource Identifiers" anchor="uri">
772<iref primary="true" item="resource"/>
773<t>
774   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used
775   throughout HTTP as the means for identifying resources (&resource;).
776   URI references are used to target requests, indicate redirects, and define
777   relationships.
778</t>
779  <x:anchor-alias value="URI-reference"/>
780  <x:anchor-alias value="absolute-URI"/>
781  <x:anchor-alias value="relative-part"/>
782  <x:anchor-alias value="authority"/>
783  <x:anchor-alias value="path-abempty"/>
784  <x:anchor-alias value="port"/>
785  <x:anchor-alias value="query"/>
786  <x:anchor-alias value="segment"/>
787  <x:anchor-alias value="uri-host"/>
788  <x:anchor-alias value="absolute-path"/>
789  <x:anchor-alias value="partial-URI"/>
790<t>
791   This specification adopts the definitions of "URI-reference",
792   "absolute-URI", "relative-part", "port", "host",
793   "path-abempty", "query", "segment", and "authority" from the
794   URI generic syntax.
795   In addition, we define an "absolute-path" rule (that differs from
796   RFC 3986's "path-absolute" in that it allows a leading "//")
797   and a "partial-URI" rule for protocol elements
798   that allow a relative URI but not a fragment.
799</t>
800<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>
801  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
802  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
803  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
804  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
805  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
806  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
807  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
808  <x:ref>segment</x:ref>       = &lt;segment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
809  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
810 
811  <x:ref>absolute-path</x:ref> = 1*( "/" segment )
812  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
813</artwork></figure>
814<t>
815   Each protocol element in HTTP that allows a URI reference will indicate
816   in its ABNF production whether the element allows any form of reference
817   (URI-reference), only a URI in absolute form (absolute-URI), only the
818   path and optional query components, or some combination of the above.
819   Unless otherwise indicated, URI references are parsed
820   relative to the effective request URI
821   (<xref target="effective.request.uri"/>).
822</t>
823
824<section title="http URI scheme" anchor="http.uri">
825  <x:anchor-alias value="http-URI"/>
826  <iref item="http URI scheme" primary="true"/>
827  <iref item="URI scheme" subitem="http" primary="true"/>
828<t>
829   The "http" URI scheme is hereby defined for the purpose of minting
830   identifiers according to their association with the hierarchical
831   namespace governed by a potential HTTP origin server listening for
832   TCP (<xref target="RFC0793"/>) connections on a given port.
833</t>
834<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"><!--terminal production--></iref>
835  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
836</artwork></figure>
837<t>
838   The HTTP origin server is identified by the generic syntax's
839   <x:ref>authority</x:ref> component, which includes a host identifier
840   and optional TCP port (<xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>).
841   The remainder of the URI, consisting of both the hierarchical path
842   component and optional query component, serves as an identifier for
843   a potential resource within that origin server's name space.
844</t>
845<t>
846   If the host identifier is provided as an IP address,
847   then the origin server is any listener on the indicated TCP port at
848   that IP address. If host is a registered name, then that name is
849   considered an indirect identifier and the recipient might use a name
850   resolution service, such as DNS, to find the address of a listener
851   for that host.
852   The host &MUST-NOT; be empty; if an "http" URI is received with an
853   empty host, then it &MUST; be rejected as invalid.
854   If the port subcomponent is empty or not given, then TCP port 80 is
855   assumed (the default reserved port for WWW services).
856</t>
857<t>
858   Regardless of the form of host identifier, access to that host is not
859   implied by the mere presence of its name or address. The host might or might
860   not exist and, even when it does exist, might or might not be running an
861   HTTP server or listening to the indicated port. The "http" URI scheme
862   makes use of the delegated nature of Internet names and addresses to
863   establish a naming authority (whatever entity has the ability to place
864   an HTTP server at that Internet name or address) and allows that
865   authority to determine which names are valid and how they might be used.
866</t>
867<t>
868   When an "http" URI is used within a context that calls for access to the
869   indicated resource, a client &MAY; attempt access by resolving
870   the host to an IP address, establishing a TCP connection to that address
871   on the indicated port, and sending an HTTP request message
872   (<xref target="http.message"/>) containing the URI's identifying data
873   (<xref target="message.routing"/>) to the server.
874   If the server responds to that request with a non-interim HTTP response
875   message, as described in &status-codes;, then that response
876   is considered an authoritative answer to the client's request.
877</t>
878<t>
879   Although HTTP is independent of the transport protocol, the "http"
880   scheme is specific to TCP-based services because the name delegation
881   process depends on TCP for establishing authority.
882   An HTTP service based on some other underlying connection protocol
883   would presumably be identified using a different URI scheme, just as
884   the "https" scheme (below) is used for resources that require an
885   end-to-end secured connection. Other protocols might also be used to
886   provide access to "http" identified resources &mdash; it is only the
887   authoritative interface used for mapping the namespace that is
888   specific to TCP.
889</t>
890<t>
891   The URI generic syntax for authority also includes a deprecated
892   userinfo subcomponent (<xref target="RFC3986" x:fmt="," x:sec="3.2.1"/>)
893   for including user authentication information in the URI.  Some
894   implementations make use of the userinfo component for internal
895   configuration of authentication information, such as within command
896   invocation options, configuration files, or bookmark lists, even
897   though such usage might expose a user identifier or password.
898   Senders &MUST; exclude the userinfo subcomponent (and its "@"
899   delimiter) when an "http" URI is transmitted within a message as a
900   request target or header field value.
901   Recipients of an "http" URI reference &SHOULD; parse for userinfo and
902   treat its presence as an error, since it is likely being used to obscure
903   the authority for the sake of phishing attacks.
904</t>
905</section>
906
907<section title="https URI scheme" anchor="https.uri">
908   <x:anchor-alias value="https-URI"/>
909   <iref item="https URI scheme"/>
910   <iref item="URI scheme" subitem="https"/>
911<t>
912   The "https" URI scheme is hereby defined for the purpose of minting
913   identifiers according to their association with the hierarchical
914   namespace governed by a potential HTTP origin server listening to a
915   given TCP port for TLS-secured connections
916   (<xref target="RFC0793"/>, <xref target="RFC5246"/>).
917</t>
918<t>
919   All of the requirements listed above for the "http" scheme are also
920   requirements for the "https" scheme, except that a default TCP port
921   of 443 is assumed if the port subcomponent is empty or not given,
922   and the TCP connection &MUST; be secured, end-to-end, through the
923   use of strong encryption prior to sending the first HTTP request.
924</t>
925<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="https-URI"><!--terminal production--></iref>
926  <x:ref>https-URI</x:ref> = "https:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
927</artwork></figure>
928<t>
929   Resources made available via the "https" scheme have no shared
930   identity with the "http" scheme even if their resource identifiers
931   indicate the same authority (the same host listening to the same
932   TCP port).  They are distinct name spaces and are considered to be
933   distinct origin servers.  However, an extension to HTTP that is
934   defined to apply to entire host domains, such as the Cookie protocol
935   <xref target="RFC6265"/>, can allow information
936   set by one service to impact communication with other services
937   within a matching group of host domains.
938</t>
939<t>
940   The process for authoritative access to an "https" identified
941   resource is defined in <xref target="RFC2818"/>.
942</t>
943</section>
944
945<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
946<t>
947   Since the "http" and "https" schemes conform to the URI generic syntax,
948   such URIs are normalized and compared according to the algorithm defined
949   in <xref target="RFC3986" x:fmt="," x:sec="6"/>, using the defaults
950   described above for each scheme.
951</t>
952<t>
953   If the port is equal to the default port for a scheme, the normal form is
954   to elide the port subcomponent. When not being used in absolute form as the
955   request target of an OPTIONS request, an empty path component is equivalent
956   to an absolute path of "/", so the normal form is to provide a path of "/"
957   instead. The scheme and host are case-insensitive and normally provided in
958   lowercase; all other components are compared in a case-sensitive manner.
959   Characters other than those in the "reserved" set are equivalent to their
960   percent-encoded octets (see <xref target="RFC3986" x:fmt=","
961   x:sec="2.1"/>): the normal form is to not encode them.
962</t>
963<t>
964   For example, the following three URIs are equivalent:
965</t>
966<figure><artwork type="example">
967   http://example.com:80/~smith/home.html
968   http://EXAMPLE.com/%7Esmith/home.html
969   http://EXAMPLE.com:/%7esmith/home.html
970</artwork></figure>
971</section>
972</section>
973</section>
974
975<section title="Message Format" anchor="http.message">
976<x:anchor-alias value="generic-message"/>
977<x:anchor-alias value="message.types"/>
978<x:anchor-alias value="HTTP-message"/>
979<x:anchor-alias value="start-line"/>
980<iref item="header section"/>
981<iref item="headers"/>
982<iref item="header field"/>
983<t>
984   All HTTP/1.1 messages consist of a start-line followed by a sequence of
985   octets in a format similar to the Internet Message Format
986   <xref target="RFC5322"/>: zero or more header fields (collectively
987   referred to as the "headers" or the "header section"), an empty line
988   indicating the end of the header section, and an optional message body.
989</t>
990<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"><!--terminal production--></iref>
991  <x:ref>HTTP-message</x:ref>   = <x:ref>start-line</x:ref>
992                   *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
993                   <x:ref>CRLF</x:ref>
994                   [ <x:ref>message-body</x:ref> ]
995</artwork></figure>
996<t>
997   The normal procedure for parsing an HTTP message is to read the
998   start-line into a structure, read each header field into a hash
999   table by field name until the empty line, and then use the parsed
1000   data to determine if a message body is expected.  If a message body
1001   has been indicated, then it is read as a stream until an amount
1002   of octets equal to the message body length is read or the connection
1003   is closed.
1004</t>
1005<t>
1006   Recipients &MUST; parse an HTTP message as a sequence of octets in an
1007   encoding that is a superset of US-ASCII <xref target="USASCII"/>.
1008   Parsing an HTTP message as a stream of Unicode characters, without regard
1009   for the specific encoding, creates security vulnerabilities due to the
1010   varying ways that string processing libraries handle invalid multibyte
1011   character sequences that contain the octet LF (%x0A).  String-based
1012   parsers can only be safely used within protocol elements after the element
1013   has been extracted from the message, such as within a header field-value
1014   after message parsing has delineated the individual fields.
1015</t>
1016<t>
1017   An HTTP message can be parsed as a stream for incremental processing or
1018   forwarding downstream.  However, recipients cannot rely on incremental
1019   delivery of partial messages, since some implementations will buffer or
1020   delay message forwarding for the sake of network efficiency, security
1021   checks, or payload transformations.
1022</t>
1023
1024<section title="Start Line" anchor="start.line">
1025  <x:anchor-alias value="Start-Line"/>
1026<t>
1027   An HTTP message can either be a request from client to server or a
1028   response from server to client.  Syntactically, the two types of message
1029   differ only in the start-line, which is either a request-line (for requests)
1030   or a status-line (for responses), and in the algorithm for determining
1031   the length of the message body (<xref target="message.body"/>).
1032</t>
1033<t>
1034   In theory, a client could receive requests and a server could receive
1035   responses, distinguishing them by their different start-line formats,
1036   but in practice servers are implemented to only expect a request
1037   (a response is interpreted as an unknown or invalid request method)
1038   and clients are implemented to only expect a response.
1039</t>
1040<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="start-line"/>
1041  <x:ref>start-line</x:ref>     = <x:ref>request-line</x:ref> / <x:ref>status-line</x:ref>
1042</artwork></figure>
1043<t>
1044   A sender &MUST-NOT; send whitespace between the start-line and
1045   the first header field. The presence of such whitespace in a request
1046   might be an attempt to trick a server into ignoring that field or
1047   processing the line after it as a new request, either of which might
1048   result in a security vulnerability if other implementations within
1049   the request chain interpret the same message differently.
1050   Likewise, the presence of such whitespace in a response might be
1051   ignored by some clients or cause others to cease parsing.
1052</t>
1053<t>
1054   A recipient that receives whitespace between the start-line and
1055   the first header field &MUST; either reject the message as invalid or
1056   consume each whitespace-preceded line without further processing of it
1057   (i.e., ignore the entire line, along with any subsequent lines preceded
1058   by whitespace, until a properly formed header field is received or the
1059   header block is terminated).
1060</t>
1061
1062<section title="Request Line" anchor="request.line">
1063  <x:anchor-alias value="Request"/>
1064  <x:anchor-alias value="request-line"/>
1065<t>
1066   A request-line begins with a method token, followed by a single
1067   space (SP), the request-target, another single space (SP), the
1068   protocol version, and ending with CRLF.
1069</t>
1070<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1071  <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>
1072</artwork></figure>
1073<iref primary="true" item="method"/>
1074<t anchor="method">
1075   The method token indicates the request method to be performed on the
1076   target resource. The request method is case-sensitive.
1077</t>
1078<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1079  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1080</artwork></figure>
1081<t>
1082   The methods defined by this specification can be found in
1083   &methods;, along with information regarding the HTTP method registry
1084   and considerations for defining new methods.
1085</t>
1086<iref item="request-target"/>
1087<t>
1088   The request-target identifies the target resource upon which to apply
1089   the request, as defined in <xref target="request-target"/>.
1090</t>
1091<t>
1092   No whitespace is allowed inside the method, request-target, and
1093   protocol version.  Hence, recipients typically parse the request-line
1094   into its component parts by splitting on whitespace
1095   (see <xref target="message.robustness"/>).
1096</t>
1097<t>
1098   Unfortunately, some user agents fail to properly encode hypertext
1099   references that have embedded whitespace, sending the characters directly
1100   instead of properly encoding or excluding the disallowed characters.
1101   Recipients of an invalid request-line &SHOULD; respond with either a
1102   <x:ref>400 (Bad Request)</x:ref> error or a <x:ref>301 (Moved Permanently)</x:ref>
1103   redirect with the request-target properly encoded.  Recipients &SHOULD-NOT;
1104   attempt to autocorrect and then process the request without a redirect,
1105   since the invalid request-line might be deliberately crafted to bypass
1106   security filters along the request chain.
1107</t>
1108<t>
1109   HTTP does not place a pre-defined limit on the length of a request-line.
1110   A server that receives a method longer than any that it implements
1111   &SHOULD; respond with a <x:ref>501 (Not Implemented)</x:ref> status code.
1112   A server &MUST; be prepared to receive URIs of unbounded length and
1113   respond with the <x:ref>414 (URI Too Long)</x:ref> status code if the received
1114   request-target would be longer than the server wishes to handle
1115   (see &status-414;).
1116</t>
1117<t>
1118   Various ad-hoc limitations on request-line length are found in practice.
1119   It is &RECOMMENDED; that all HTTP senders and recipients support, at a
1120   minimum, request-line lengths of 8000 octets.
1121</t>
1122</section>
1123
1124<section title="Status Line" anchor="status.line">
1125  <x:anchor-alias value="response"/>
1126  <x:anchor-alias value="status-line"/>
1127  <x:anchor-alias value="status-code"/>
1128  <x:anchor-alias value="reason-phrase"/>
1129<t>
1130   The first line of a response message is the status-line, consisting
1131   of the protocol version, a space (SP), the status code, another space,
1132   a possibly-empty textual phrase describing the status code, and
1133   ending with CRLF.
1134</t>
1135<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-line"/>
1136  <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>
1137</artwork></figure>
1138<t>
1139   The status-code element is a 3-digit integer code describing the
1140   result of the server's attempt to understand and satisfy the client's
1141   corresponding request. The rest of the response message is to be
1142   interpreted in light of the semantics defined for that status code.
1143   See &status-codes; for information about the semantics of status codes,
1144   including the classes of status code (indicated by the first digit),
1145   the status codes defined by this specification, considerations for the
1146   definition of new status codes, and the IANA registry.
1147</t>
1148<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-code"/>
1149  <x:ref>status-code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1150</artwork></figure>
1151<t>   
1152   The reason-phrase element exists for the sole purpose of providing a
1153   textual description associated with the numeric status code, mostly
1154   out of deference to earlier Internet application protocols that were more
1155   frequently used with interactive text clients. A client &SHOULD; ignore
1156   the reason-phrase content.
1157</t>
1158<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
1159  <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> )
1160</artwork></figure>
1161</section>
1162</section>
1163
1164<section title="Header Fields" anchor="header.fields">
1165  <x:anchor-alias value="header-field"/>
1166  <x:anchor-alias value="field-content"/>
1167  <x:anchor-alias value="field-name"/>
1168  <x:anchor-alias value="field-value"/>
1169  <x:anchor-alias value="obs-fold"/>
1170<t>
1171   Each HTTP header field consists of a case-insensitive field name
1172   followed by a colon (":"), optional whitespace, and the field value.
1173</t>
1174<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"/>
1175  <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>BWS</x:ref>
1176  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1177  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>obs-fold</x:ref> )
1178  <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> )
1179  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref> ( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1180                 ; obsolete line folding
1181                 ; see <xref target="field.parsing"/>
1182</artwork></figure>
1183<t>
1184   The field-name token labels the corresponding field-value as having the
1185   semantics defined by that header field.  For example, the <x:ref>Date</x:ref>
1186   header field is defined in &header-date; as containing the origination
1187   timestamp for the message in which it appears.
1188</t>
1189
1190<section title="Field Extensibility" anchor="field.extensibility">
1191<t>
1192   HTTP header fields are fully extensible: there is no limit on the
1193   introduction of new field names, each presumably defining new semantics,
1194   nor on the number of header fields used in a given message.  Existing
1195   fields are defined in each part of this specification and in many other
1196   specifications outside the core standard.
1197   New header fields can be introduced without changing the protocol version
1198   if their defined semantics allow them to be safely ignored by recipients
1199   that do not recognize them.
1200</t>
1201<t>
1202   New HTTP header fields ought to be registered with IANA in the
1203   Message Header Field Registry, as described in &iana-header-registry;.
1204   A proxy &MUST; forward unrecognized header fields unless the
1205   field-name is listed in the <x:ref>Connection</x:ref> header field
1206   (<xref target="header.connection"/>) or the proxy is specifically
1207   configured to block, or otherwise transform, such fields.
1208   Other recipients &SHOULD; ignore unrecognized header fields.
1209</t>
1210</section>
1211
1212<section title="Field Order" anchor="field.order">
1213<t>
1214   The order in which header fields with differing field names are
1215   received is not significant. However, it is "good practice" to send
1216   header fields that contain control data first, such as <x:ref>Host</x:ref>
1217   on requests and <x:ref>Date</x:ref> on responses, so that implementations
1218   can decide when not to handle a message as early as possible.  A server
1219   &MUST; wait until the entire header section is received before interpreting
1220   a request message, since later header fields might include conditionals,
1221   authentication credentials, or deliberately misleading duplicate
1222   header fields that would impact request processing.
1223</t>
1224<t>
1225   A sender &MUST-NOT; generate multiple header fields with the same field
1226   name in a message unless either the entire field value for that
1227   header field is defined as a comma-separated list [i.e., #(values)]
1228   or the header field is a well-known exception (as noted below).
1229</t>
1230<t>
1231   Multiple header fields with the same field name can be combined into
1232   one "field-name: field-value" pair, without changing the semantics of the
1233   message, by appending each subsequent field value to the combined
1234   field value in order, separated by a comma. The order in which
1235   header fields with the same field name are received is therefore
1236   significant to the interpretation of the combined field value;
1237   a proxy &MUST-NOT; change the order of these field values when
1238   forwarding a message.
1239</t>
1240<x:note>
1241  <t>
1242   &Note; In practice, the "Set-Cookie" header field (<xref target="RFC6265"/>)
1243   often appears multiple times in a response message and does not use the
1244   list syntax, violating the above requirements on multiple header fields
1245   with the same name. Since it cannot be combined into a single field-value,
1246   recipients ought to handle "Set-Cookie" as a special case while processing
1247   header fields. (See Appendix A.2.3 of <xref target="Kri2001"/> for details.)
1248  </t>
1249</x:note>
1250</section>
1251
1252<section title="Whitespace" anchor="whitespace">
1253<t anchor="rule.LWS">
1254   This specification uses three rules to denote the use of linear
1255   whitespace: OWS (optional whitespace), RWS (required whitespace), and
1256   BWS ("bad" whitespace).
1257</t>
1258<t anchor="rule.OWS">
1259   The OWS rule is used where zero or more linear whitespace octets might
1260   appear. OWS &SHOULD; either not be generated or be generated as a single
1261   SP. Multiple OWS octets that occur within field-content &SHOULD; either
1262   be replaced with a single SP or transformed to all SP octets (each
1263   octet other than SP replaced with SP) before interpreting the field value
1264   or forwarding the message downstream.
1265</t>
1266<t anchor="rule.RWS">
1267   RWS is used when at least one linear whitespace octet is required to
1268   separate field tokens. RWS &SHOULD; be generated as a single SP.
1269   Multiple RWS octets that occur within field-content &SHOULD; either
1270   be replaced with a single SP or transformed to all SP octets before
1271   interpreting the field value or forwarding the message downstream.
1272</t>
1273<t anchor="rule.BWS">
1274   BWS is used where the grammar allows optional whitespace, for historical
1275   reasons, but senders &SHOULD-NOT; generate it in messages;
1276   recipients &MUST; accept such bad optional whitespace and remove it before
1277   interpreting the field value or forwarding the message downstream.
1278</t>
1279<t anchor="rule.whitespace">
1280  <x:anchor-alias value="BWS"/>
1281  <x:anchor-alias value="OWS"/>
1282  <x:anchor-alias value="RWS"/>
1283</t>
1284<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"/>
1285  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1286                 ; optional whitespace
1287  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1288                 ; required whitespace
1289  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1290                 ; "bad" whitespace
1291</artwork></figure>
1292</section>
1293
1294<section title="Field Parsing" anchor="field.parsing">
1295<t>
1296   No whitespace is allowed between the header field-name and colon.
1297   In the past, differences in the handling of such whitespace have led to
1298   security vulnerabilities in request routing and response handling.
1299   A server &MUST; reject any received request message that contains
1300   whitespace between a header field-name and colon with a response code of
1301   <x:ref>400 (Bad Request)</x:ref>. A proxy &MUST; remove any such whitespace
1302   from a response message before forwarding the message downstream.
1303</t>
1304<t>
1305   A field value is preceded by optional whitespace (OWS); a single SP is
1306   preferred. The field value does not include any leading or trailing white
1307   space: OWS occurring before the first non-whitespace octet of the
1308   field value or after the last non-whitespace octet of the field value
1309   is ignored and &SHOULD; be removed before further processing (as this does
1310   not change the meaning of the header field).
1311</t>
1312<t>
1313   Historically, HTTP header field values could be extended over multiple
1314   lines by preceding each extra line with at least one space or horizontal
1315   tab (obs-fold). This specification deprecates such line folding except
1316   within the message/http media type
1317   (<xref target="internet.media.type.message.http"/>).
1318   Senders &MUST-NOT; generate messages that include line folding
1319   (i.e., that contain any field-value that contains a match to the
1320   <x:ref>obs-fold</x:ref> rule) unless the message is intended for packaging
1321   within the message/http media type. When an <x:ref>obs-fold</x:ref> is
1322   received in a message, recipients &MUST; do one of:
1323   <list style="symbols">
1324      <t>accept the message and replace any embedded <x:ref>obs-fold</x:ref>
1325         whitespace with either a single <x:ref>SP</x:ref> or a matching
1326         number of <x:ref>SP</x:ref> octets (to avoid buffer copying) prior to
1327         interpreting the field value or forwarding the message
1328         downstream;</t>
1329
1330      <t>if it is a request, reject the message by sending a
1331         <x:ref>400 (Bad Request)</x:ref> response with a representation
1332         explaining that obsolete line folding is unacceptable; or,</t>
1333         
1334      <t>if it is a response, discard the message and generate a
1335         <x:ref>502 (Bad Gateway)</x:ref> response with a representation
1336         explaining that unacceptable line folding was received.</t>
1337   </list>
1338   Recipients that choose not to implement <x:ref>obs-fold</x:ref> processing
1339   (as described above) &MUST-NOT; accept messages containing header fields
1340   with leading whitespace, as this can expose them to attacks that exploit
1341   this difference in processing.
1342</t>
1343<t>
1344   Historically, HTTP has allowed field content with text in the ISO-8859-1
1345   <xref target="ISO-8859-1"/> charset, supporting other charsets only
1346   through use of <xref target="RFC2047"/> encoding.
1347   In practice, most HTTP header field values use only a subset of the
1348   US-ASCII charset <xref target="USASCII"/>. Newly defined
1349   header fields &SHOULD; limit their field values to US-ASCII octets.
1350   Recipients &SHOULD; treat other octets in field content (obs-text) as
1351   opaque data.
1352</t>
1353</section>
1354
1355<section title="Field Limits" anchor="field.limits">
1356<t>
1357   HTTP does not place a pre-defined limit on the length of each header field
1358   or on the length of the header block as a whole.  Various ad-hoc
1359   limitations on individual header field length are found in practice,
1360   often depending on the specific field semantics.
1361</t>
1362<t>
1363   A server &MUST; be prepared to receive request header fields of unbounded
1364   length and respond with an appropriate <x:ref>4xx (Client Error)</x:ref>
1365   status code if the received header field(s) are larger than the server
1366   wishes to process.
1367</t>
1368<t>
1369   A client &MUST; be prepared to receive response header fields of unbounded
1370   length. A client &MAY; discard or truncate received header fields that are
1371   larger than the client wishes to process if the field semantics are such
1372   that the dropped value(s) can be safely ignored without changing the
1373   response semantics.
1374</t>
1375</section>
1376
1377<section title="Field value components" anchor="field.components">
1378<t anchor="rule.token.separators">
1379  <x:anchor-alias value="tchar"/>
1380  <x:anchor-alias value="token"/>
1381  <x:anchor-alias value="special"/>
1382  <x:anchor-alias value="word"/>
1383   Many HTTP header field values consist of words (token or quoted-string)
1384   separated by whitespace or special characters. These special characters
1385   &MUST; be in a quoted string to be used within a parameter value (as defined
1386   in <xref target="transfer.codings"/>).
1387</t>
1388<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>
1389  <x:ref>word</x:ref>           = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1390
1391  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1392<!--
1393  IMPORTANT: when editing "tchar" make sure that "special" is updated accordingly!!!
1394 -->
1395  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1396                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1397                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1398                 ; any <x:ref>VCHAR</x:ref>, except <x:ref>special</x:ref>
1399
1400  <x:ref>special</x:ref>        = "(" / ")" / "&lt;" / ">" / "@" / ","
1401                 / ";" / ":" / "\" / DQUOTE / "/" / "["
1402                 / "]" / "?" / "=" / "{" / "}"
1403</artwork></figure>
1404<t anchor="rule.quoted-string">
1405  <x:anchor-alias value="quoted-string"/>
1406  <x:anchor-alias value="qdtext"/>
1407  <x:anchor-alias value="obs-text"/>
1408   A string of text is parsed as a single word if it is quoted using
1409   double-quote marks.
1410</t>
1411<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"/>
1412  <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>
1413  <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>
1414  <x:ref>obs-text</x:ref>       = %x80-FF
1415</artwork></figure>
1416<t anchor="rule.quoted-pair">
1417  <x:anchor-alias value="quoted-pair"/>
1418   The backslash octet ("\") can be used as a single-octet
1419   quoting mechanism within quoted-string constructs:
1420</t>
1421<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1422  <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> )
1423</artwork></figure>
1424<t>
1425   Recipients that process the value of a quoted-string &MUST; handle a
1426   quoted-pair as if it were replaced by the octet following the backslash.
1427</t>
1428<t>
1429   Senders &SHOULD-NOT; generate a quoted-pair in a quoted-string except where
1430   necessary to quote DQUOTE and backslash octets occurring within that string.
1431</t>
1432<t anchor="rule.comment">
1433  <x:anchor-alias value="comment"/>
1434  <x:anchor-alias value="ctext"/>
1435   Comments can be included in some HTTP header fields by surrounding
1436   the comment text with parentheses. Comments are only allowed in
1437   fields containing "comment" as part of their field value definition.
1438</t>
1439<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1440  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-cpair</x:ref> / <x:ref>comment</x:ref> ) ")"
1441  <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>
1442</artwork></figure>
1443<t anchor="rule.quoted-cpair">
1444  <x:anchor-alias value="quoted-cpair"/>
1445   The backslash octet ("\") can be used as a single-octet
1446   quoting mechanism within comment constructs:
1447</t>
1448<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-cpair"/>
1449  <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> )
1450</artwork></figure>
1451<t>
1452   Senders &SHOULD-NOT; escape octets in comments that do not require escaping
1453   (i.e., other than the backslash octet "\" and the parentheses "(" and ")").
1454</t>
1455</section>
1456
1457</section>
1458
1459<section title="Message Body" anchor="message.body">
1460  <x:anchor-alias value="message-body"/>
1461<t>
1462   The message body (if any) of an HTTP message is used to carry the
1463   payload body of that request or response.  The message body is
1464   identical to the payload body unless a transfer coding has been
1465   applied, as described in <xref target="header.transfer-encoding"/>.
1466</t>
1467<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1468  <x:ref>message-body</x:ref> = *OCTET
1469</artwork></figure>
1470<t>
1471   The rules for when a message body is allowed in a message differ for
1472   requests and responses.
1473</t>
1474<t>
1475   The presence of a message body in a request is signaled by a
1476   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1477   field. Request message framing is independent of method semantics,
1478   even if the method does not define any use for a message body.
1479</t>
1480<t>
1481   The presence of a message body in a response depends on both
1482   the request method to which it is responding and the response
1483   status code (<xref target="status.line"/>).
1484   Responses to the HEAD request method never include a message body
1485   because the associated response header fields (e.g.,
1486   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1487   if present, indicate only what their values would have been if the request
1488   method had been GET (&HEAD;).
1489   <x:ref>2xx (Successful)</x:ref> responses to CONNECT switch to tunnel
1490   mode instead of having a message body (&CONNECT;).
1491   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1492   <x:ref>304 (Not Modified)</x:ref> responses do not include a message body.
1493   All other responses do include a message body, although the body
1494   might be of zero length.
1495</t>
1496
1497<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1498  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1499  <iref item="chunked (Coding Format)"/>
1500  <x:anchor-alias value="Transfer-Encoding"/>
1501<t>
1502   The Transfer-Encoding header field lists the transfer coding names
1503   corresponding to the sequence of transfer codings that have been
1504   (or will be) applied to the payload body in order to form the message body.
1505   Transfer codings are defined in <xref target="transfer.codings"/>.
1506</t>
1507<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1508  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1509</artwork></figure>
1510<t>
1511   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1512   MIME, which was designed to enable safe transport of binary data over a
1513   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1514   However, safe transport has a different focus for an 8bit-clean transfer
1515   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1516   accurately delimit a dynamically generated payload and to distinguish
1517   payload encodings that are only applied for transport efficiency or
1518   security from those that are characteristics of the selected resource.
1519</t>
1520<t>
1521   All HTTP/1.1 recipients &MUST; implement the chunked transfer coding
1522   (<xref target="chunked.encoding"/>) because it plays a crucial role in
1523   framing messages when the payload body size is not known in advance.
1524   If chunked is applied to a payload body, the sender &MUST-NOT; apply
1525   chunked more than once (i.e., chunking an already chunked message is not
1526   allowed).
1527   If any transfer coding is applied to a request payload body, the
1528   sender &MUST; apply chunked as the final transfer coding to ensure that
1529   the message is properly framed.
1530   If any transfer coding is applied to a response payload body, the
1531   sender &MUST; either apply chunked as the final transfer coding or
1532   terminate the message by closing the connection.
1533</t>
1534<figure><preamble>
1535   For example,
1536</preamble><artwork type="example">
1537  Transfer-Encoding: gzip, chunked
1538</artwork><postamble>
1539   indicates that the payload body has been compressed using the gzip
1540   coding and then chunked using the chunked coding while forming the
1541   message body.
1542</postamble></figure>
1543<t>
1544   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1545   Transfer-Encoding is a property of the message, not of the representation, and
1546   any recipient along the request/response chain &MAY; decode the received
1547   transfer coding(s) or apply additional transfer coding(s) to the message
1548   body, assuming that corresponding changes are made to the Transfer-Encoding
1549   field-value. Additional information about the encoding parameters &MAY; be
1550   provided by other header fields not defined by this specification.
1551</t>
1552<t>
1553   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1554   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1555   neither of which includes a message body,
1556   to indicate that the origin server would have applied a transfer coding
1557   to the message body if the request had been an unconditional GET.
1558   This indication is not required, however, because any recipient on
1559   the response chain (including the origin server) can remove transfer
1560   codings when they are not needed.
1561</t>
1562<t>
1563   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1564   implementations advertising only HTTP/1.0 support will not understand
1565   how to process a transfer-encoded payload.
1566   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1567   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1568   might be in the form of specific user configuration or by remembering the
1569   version of a prior received response.
1570   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1571   the corresponding request indicates HTTP/1.1 (or later).
1572</t>
1573<t>
1574   A server that receives a request message with a transfer coding it does
1575   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref>.
1576</t>
1577</section>
1578
1579<section title="Content-Length" anchor="header.content-length">
1580  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1581  <x:anchor-alias value="Content-Length"/>
1582<t>
1583   When a message does not have a <x:ref>Transfer-Encoding</x:ref> header
1584   field, a Content-Length header field can provide the anticipated size,
1585   as a decimal number of octets, for a potential payload body.
1586   For messages that do include a payload body, the Content-Length field-value
1587   provides the framing information necessary for determining where the body
1588   (and message) ends.  For messages that do not include a payload body, the
1589   Content-Length indicates the size of the selected representation
1590   (&representation;).
1591</t>
1592<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1593  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1594</artwork></figure>
1595<t>
1596   An example is
1597</t>
1598<figure><artwork type="example">
1599  Content-Length: 3495
1600</artwork></figure>
1601<t>
1602   A sender &MUST-NOT; send a Content-Length header field in any message that
1603   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1604</t>
1605<t>
1606   A user agent &SHOULD; send a Content-Length in a request message when no
1607   <x:ref>Transfer-Encoding</x:ref> is sent and the request method defines
1608   a meaning for an enclosed payload body. For example, a Content-Length
1609   header field is normally sent in a POST request even when the value is
1610   0 (indicating an empty payload body).  A user agent &SHOULD-NOT; send a
1611   Content-Length header field when the request message does not contain a
1612   payload body and the method semantics do not anticipate such a body.
1613</t>
1614<t>
1615   A server &MAY; send a Content-Length header field in a response to a HEAD
1616   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1617   response unless its field-value equals the decimal number of octets that
1618   would have been sent in the payload body of a response if the same
1619   request had used the GET method.
1620</t>
1621<t>
1622   A server &MAY; send a Content-Length header field in a
1623   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1624   (&status-304;); a server &MUST-NOT; send Content-Length in such a
1625   response unless its field-value equals the decimal number of octets that
1626   would have been sent in the payload body of a <x:ref>200 (OK)</x:ref>
1627   response to the same request.
1628</t>
1629<t>
1630   A server &MUST-NOT; send a Content-Length header field in any response
1631   with a status code of
1632   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1633   A server &SHOULD-NOT; send a Content-Length header field in any
1634   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1635</t>
1636<t>
1637   Aside from the cases defined above, in the absence of Transfer-Encoding,
1638   an origin server &SHOULD; send a Content-Length header field when the
1639   payload body size is known prior to sending the complete header block.
1640   This will allow downstream recipients to measure transfer progress,
1641   know when a received message is complete, and potentially reuse the
1642   connection for additional requests.
1643</t>
1644<t>
1645   Any Content-Length field value greater than or equal to zero is valid.
1646   Since there is no predefined limit to the length of a payload,
1647   recipients &SHOULD; anticipate potentially large decimal numerals and
1648   prevent parsing errors due to integer conversion overflows
1649   (<xref target="attack.protocol.element.size.overflows"/>).
1650</t>
1651<t>
1652   If a message is received that has multiple Content-Length header fields
1653   with field-values consisting of the same decimal value, or a single
1654   Content-Length header field with a field value containing a list of
1655   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1656   duplicate Content-Length header fields have been generated or combined by an
1657   upstream message processor, then the recipient &MUST; either reject the
1658   message as invalid or replace the duplicated field-values with a single
1659   valid Content-Length field containing that decimal value prior to
1660   determining the message body length.
1661</t>
1662<x:note>
1663  <t>
1664   &Note; HTTP's use of Content-Length for message framing differs
1665   significantly from the same field's use in MIME, where it is an optional
1666   field used only within the "message/external-body" media-type.
1667  </t>
1668</x:note>
1669</section>
1670
1671<section title="Message Body Length" anchor="message.body.length">
1672  <iref item="chunked (Coding Format)"/>
1673<t>
1674   The length of a message body is determined by one of the following
1675   (in order of precedence):
1676</t>
1677<t>
1678  <list style="numbers">
1679    <x:lt><t>
1680     Any response to a HEAD request and any response with a
1681     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1682     <x:ref>304 (Not Modified)</x:ref> status code is always
1683     terminated by the first empty line after the header fields, regardless of
1684     the header fields present in the message, and thus cannot contain a
1685     message body.
1686    </t></x:lt>
1687    <x:lt><t>
1688     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1689     connection will become a tunnel immediately after the empty line that
1690     concludes the header fields.  A client &MUST; ignore any
1691     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1692     fields received in such a message.
1693    </t></x:lt>
1694    <x:lt><t>
1695     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1696     and the chunked transfer coding (<xref target="chunked.encoding"/>)
1697     is the final encoding, the message body length is determined by reading
1698     and decoding the chunked data until the transfer coding indicates the
1699     data is complete.
1700    </t>
1701    <t>
1702     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1703     response and the chunked transfer coding is not the final encoding, the
1704     message body length is determined by reading the connection until it is
1705     closed by the server.
1706     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a request and the
1707     chunked transfer coding is not the final encoding, the message body
1708     length cannot be determined reliably; the server &MUST; respond with
1709     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1710    </t>
1711    <t>
1712     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1713     and a <x:ref>Content-Length</x:ref> header field, the Transfer-Encoding
1714     overrides the Content-Length. Such a message might indicate an attempt
1715     to perform request or response smuggling (bypass of security-related
1716     checks on message routing or content) and thus ought to be handled as
1717     an error.  A sender &MUST; remove the received Content-Length field
1718     prior to forwarding such a message downstream.
1719    </t></x:lt>
1720    <x:lt><t>
1721     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1722     either multiple <x:ref>Content-Length</x:ref> header fields having
1723     differing field-values or a single Content-Length header field having an
1724     invalid value, then the message framing is invalid and &MUST; be treated
1725     as an error to prevent request or response smuggling.
1726     If this is a request message, the server &MUST; respond with
1727     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1728     If this is a response message received by a proxy, the proxy
1729     &MUST; discard the received response, send a <x:ref>502 (Bad Gateway)</x:ref>
1730     status code as its downstream response, and then close the connection.
1731     If this is a response message received by a user agent, it &MUST; be
1732     treated as an error by discarding the message and closing the connection.
1733    </t></x:lt>
1734    <x:lt><t>
1735     If a valid <x:ref>Content-Length</x:ref> header field is present without
1736     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1737     expected message body length in octets.
1738     If the sender closes the connection or the recipient times out before the
1739     indicated number of octets are received, the recipient &MUST; consider
1740     the message to be incomplete and close the connection.
1741    </t></x:lt>
1742    <x:lt><t>
1743     If this is a request message and none of the above are true, then the
1744     message body length is zero (no message body is present).
1745    </t></x:lt>
1746    <x:lt><t>
1747     Otherwise, this is a response message without a declared message body
1748     length, so the message body length is determined by the number of octets
1749     received prior to the server closing the connection.
1750    </t></x:lt>
1751  </list>
1752</t>
1753<t>
1754   Since there is no way to distinguish a successfully completed,
1755   close-delimited message from a partially-received message interrupted
1756   by network failure, a server &SHOULD; use encoding or
1757   length-delimited messages whenever possible.  The close-delimiting
1758   feature exists primarily for backwards compatibility with HTTP/1.0.
1759</t>
1760<t>
1761   A server &MAY; reject a request that contains a message body but
1762   not a <x:ref>Content-Length</x:ref> by responding with
1763   <x:ref>411 (Length Required)</x:ref>.
1764</t>
1765<t>
1766   Unless a transfer coding other than chunked has been applied,
1767   a client that sends a request containing a message body &SHOULD;
1768   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1769   length is known in advance, rather than the chunked transfer coding, since some
1770   existing services respond to chunked with a <x:ref>411 (Length Required)</x:ref>
1771   status code even though they understand the chunked transfer coding.  This
1772   is typically because such services are implemented via a gateway that
1773   requires a content-length in advance of being called and the server
1774   is unable or unwilling to buffer the entire request before processing.
1775</t>
1776<t>
1777   A user agent that sends a request containing a message body &MUST; send a
1778   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1779   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1780   the form of specific user configuration or by remembering the version of a
1781   prior received response.
1782</t>
1783<t>
1784   If the final response to the last request on a connection has been
1785   completely received and there remains additional data to read, a user agent
1786   &MAY; discard the remaining data or attempt to determine if that data
1787   belongs as part of the prior response body, which might be the case if the
1788   prior message's Content-Length value is incorrect. A client &MUST-NOT;
1789   process, cache, or forward such extra data as a separate response, since
1790   such behavior would be vulnerable to cache poisoning.
1791</t>
1792</section>
1793</section>
1794
1795<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1796<t>
1797   A server that receives an incomplete request message, usually due to a
1798   canceled request or a triggered time-out exception, &MAY; send an error
1799   response prior to closing the connection.
1800</t>
1801<t>
1802   A client that receives an incomplete response message, which can occur
1803   when a connection is closed prematurely or when decoding a supposedly
1804   chunked transfer coding fails, &MUST; record the message as incomplete.
1805   Cache requirements for incomplete responses are defined in
1806   &cache-incomplete;.
1807</t>
1808<t>
1809   If a response terminates in the middle of the header block (before the
1810   empty line is received) and the status code might rely on header fields to
1811   convey the full meaning of the response, then the client cannot assume
1812   that meaning has been conveyed; the client might need to repeat the
1813   request in order to determine what action to take next.
1814</t>
1815<t>
1816   A message body that uses the chunked transfer coding is
1817   incomplete if the zero-sized chunk that terminates the encoding has not
1818   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1819   incomplete if the size of the message body received (in octets) is less than
1820   the value given by Content-Length.  A response that has neither chunked
1821   transfer coding nor Content-Length is terminated by closure of the
1822   connection, and thus is considered complete regardless of the number of
1823   message body octets received, provided that the header block was received
1824   intact.
1825</t>
1826</section>
1827
1828<section title="Message Parsing Robustness" anchor="message.robustness">
1829<t>
1830   Older HTTP/1.0 user agent implementations might send an extra CRLF
1831   after a POST request as a lame workaround for some early server
1832   applications that failed to read message body content that was
1833   not terminated by a line-ending. An HTTP/1.1 user agent &MUST-NOT;
1834   preface or follow a request with an extra CRLF.  If terminating
1835   the request message body with a line-ending is desired, then the
1836   user agent &MUST; count the terminating CRLF octets as part of the
1837   message body length.
1838</t>
1839<t>
1840   In the interest of robustness, servers &SHOULD; ignore at least one
1841   empty line received where a request-line is expected. In other words, if
1842   a server is reading the protocol stream at the beginning of a
1843   message and receives a CRLF first, the server &SHOULD; ignore the CRLF.
1844</t>
1845<t>
1846   Although the line terminator for the start-line and header
1847   fields is the sequence CRLF, recipients &MAY; recognize a
1848   single LF as a line terminator and ignore any preceding CR.
1849</t>
1850<t>
1851   Although the request-line and status-line grammar rules require that each
1852   of the component elements be separated by a single SP octet, recipients
1853   &MAY; instead parse on whitespace-delimited word boundaries and, aside
1854   from the CRLF terminator, treat any form of whitespace as the SP separator
1855   while ignoring preceding or trailing whitespace;
1856   such whitespace includes one or more of the following octets:
1857   SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
1858</t>
1859<t>
1860   When a server listening only for HTTP request messages, or processing
1861   what appears from the start-line to be an HTTP request message,
1862   receives a sequence of octets that does not match the HTTP-message
1863   grammar aside from the robustness exceptions listed above, the
1864   server &SHOULD; respond with a <x:ref>400 (Bad Request)</x:ref> response. 
1865</t>
1866</section>
1867</section>
1868
1869<section title="Transfer Codings" anchor="transfer.codings">
1870  <x:anchor-alias value="transfer-coding"/>
1871  <x:anchor-alias value="transfer-extension"/>
1872<t>
1873   Transfer coding names are used to indicate an encoding
1874   transformation that has been, can be, or might need to be applied to a
1875   payload body in order to ensure "safe transport" through the network.
1876   This differs from a content coding in that the transfer coding is a
1877   property of the message rather than a property of the representation
1878   that is being transferred.
1879</t>
1880<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1881  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1882                     / "compress" ; <xref target="compress.coding"/>
1883                     / "deflate" ; <xref target="deflate.coding"/>
1884                     / "gzip" ; <xref target="gzip.coding"/>
1885                     / <x:ref>transfer-extension</x:ref>
1886  <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> )
1887</artwork></figure>
1888<t anchor="rule.parameter">
1889  <x:anchor-alias value="attribute"/>
1890  <x:anchor-alias value="transfer-parameter"/>
1891  <x:anchor-alias value="value"/>
1892   Parameters are in the form of attribute/value pairs.
1893</t>
1894<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"/>
1895  <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>
1896  <x:ref>attribute</x:ref>          = <x:ref>token</x:ref>
1897  <x:ref>value</x:ref>              = <x:ref>word</x:ref>
1898</artwork></figure>
1899<t>
1900   All transfer-coding names are case-insensitive and ought to be registered
1901   within the HTTP Transfer Coding registry, as defined in
1902   <xref target="transfer.coding.registry"/>.
1903   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1904   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1905   header fields.
1906</t>
1907
1908<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1909  <iref primary="true" item="chunked (Coding Format)"/>
1910  <x:anchor-alias value="chunk"/>
1911  <x:anchor-alias value="chunked-body"/>
1912  <x:anchor-alias value="chunk-data"/>
1913  <x:anchor-alias value="chunk-ext"/>
1914  <x:anchor-alias value="chunk-ext-name"/>
1915  <x:anchor-alias value="chunk-ext-val"/>
1916  <x:anchor-alias value="chunk-size"/>
1917  <x:anchor-alias value="last-chunk"/>
1918  <x:anchor-alias value="trailer-part"/>
1919  <x:anchor-alias value="quoted-str-nf"/>
1920  <x:anchor-alias value="qdtext-nf"/>
1921<t>
1922   The chunked transfer coding modifies the body of a message in order to
1923   transfer it as a series of chunks, each with its own size indicator,
1924   followed by an &OPTIONAL; trailer containing header fields. This
1925   allows dynamically generated content to be transferred along with the
1926   information necessary for the recipient to verify that it has
1927   received the full message.
1928</t>
1929<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"/>
1930  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1931                   <x:ref>last-chunk</x:ref>
1932                   <x:ref>trailer-part</x:ref>
1933                   <x:ref>CRLF</x:ref>
1934 
1935  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1936                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1937  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1938  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1939 
1940  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
1941  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
1942  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-str-nf</x:ref>
1943  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1944  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1945 
1946  <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>
1947                 ; like <x:ref>quoted-string</x:ref>, but disallowing line folding
1948  <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>
1949</artwork></figure>
1950<t>
1951   Chunk extensions within the chunked transfer coding are deprecated.
1952   Senders &SHOULD-NOT; send chunk-ext.
1953   Definition of new chunk extensions is discouraged.
1954</t>
1955<t>
1956   The chunk-size field is a string of hex digits indicating the size of
1957   the chunk-data in octets. The chunked transfer coding is complete when a
1958   chunk with a chunk-size of zero is received, possibly followed by a
1959   trailer, and finally terminated by an empty line.
1960</t>
1961
1962<section title="Trailer" anchor="header.trailer">
1963  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
1964  <x:anchor-alias value="Trailer"/>
1965<t>
1966   A trailer allows the sender to include additional fields at the end of a
1967   chunked message in order to supply metadata that might be dynamically
1968   generated while the message body is sent, such as a message integrity
1969   check, digital signature, or post-processing status.
1970   The trailer &MUST-NOT; contain fields that need to be known before a
1971   recipient processes the body, such as <x:ref>Transfer-Encoding</x:ref>,
1972   <x:ref>Content-Length</x:ref>, and <x:ref>Trailer</x:ref>.
1973</t>
1974<t>
1975   When a message includes a message body encoded with the chunked
1976   transfer coding and the sender desires to send metadata in the form of
1977   trailer fields at the end of the message, the sender &SHOULD; send a
1978   <x:ref>Trailer</x:ref> header field before the message body to indicate
1979   which fields will be present in the trailers. This allows the recipient
1980   to prepare for receipt of that metadata before it starts processing the body,
1981   which is useful if the message is being streamed and the recipient wishes
1982   to confirm an integrity check on the fly.
1983</t>
1984<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
1985  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
1986</artwork></figure>
1987<t>
1988   If no <x:ref>Trailer</x:ref> header field is present, the sender of a
1989   chunked message body &SHOULD; send an empty trailer.
1990</t>
1991<t>
1992   A server &MUST; send an empty trailer with the chunked transfer coding
1993   unless at least one of the following is true:
1994  <list style="numbers">
1995    <t>the request included a <x:ref>TE</x:ref> header field that indicates
1996    "trailers" is acceptable in the transfer coding of the response, as
1997    described in <xref target="header.te"/>; or,</t>
1998     
1999    <t>the trailer fields consist entirely of optional metadata and the
2000    recipient could use the message (in a manner acceptable to the server where
2001    the field originated) without receiving that metadata. In other words,
2002    the server that generated the header field is willing to accept the
2003    possibility that the trailer fields might be silently discarded along
2004    the path to the client.</t>
2005  </list>
2006</t>
2007<t>
2008   The above requirement prevents the need for an infinite buffer when a
2009   message is being received by an HTTP/1.1 (or later) proxy and forwarded to
2010   an HTTP/1.0 recipient.
2011</t>
2012</section>
2013
2014<section title="Decoding chunked" anchor="decoding.chunked">
2015<t>
2016   A process for decoding the chunked transfer coding
2017   can be represented in pseudo-code as:
2018</t>
2019<figure><artwork type="code">
2020  length := 0
2021  read chunk-size, chunk-ext (if any) and CRLF
2022  while (chunk-size &gt; 0) {
2023     read chunk-data and CRLF
2024     append chunk-data to decoded-body
2025     length := length + chunk-size
2026     read chunk-size and CRLF
2027  }
2028  read header-field
2029  while (header-field not empty) {
2030     append header-field to existing header fields
2031     read header-field
2032  }
2033  Content-Length := length
2034  Remove "chunked" from Transfer-Encoding
2035  Remove Trailer from existing header fields
2036</artwork></figure>
2037<t>
2038   All recipients &MUST; be able to receive and decode the
2039   chunked transfer coding and &MUST; ignore chunk-ext extensions
2040   they do not understand.
2041</t>
2042</section>
2043</section>
2044
2045<section title="Compression Codings" anchor="compression.codings">
2046<t>
2047   The codings defined below can be used to compress the payload of a
2048   message.
2049</t>
2050
2051<section title="Compress Coding" anchor="compress.coding">
2052<iref item="compress (Coding Format)"/>
2053<t>
2054   The "compress" format is produced by the common UNIX file compression
2055   program "compress". This format is an adaptive Lempel-Ziv-Welch
2056   coding (LZW). Recipients &SHOULD; consider "x-compress" to be
2057   equivalent to "compress".
2058</t>
2059</section>
2060
2061<section title="Deflate Coding" anchor="deflate.coding">
2062<iref item="deflate (Coding Format)"/>
2063<t>
2064   The "deflate" format is defined as the "deflate" compression mechanism
2065   (described in <xref target="RFC1951"/>) used inside the "zlib"
2066   data format (<xref target="RFC1950"/>).
2067</t>
2068<x:note>
2069  <t>
2070    &Note; Some incorrect implementations send the "deflate"
2071    compressed data without the zlib wrapper.
2072   </t>
2073</x:note>
2074</section>
2075
2076<section title="Gzip Coding" anchor="gzip.coding">
2077<iref item="gzip (Coding Format)"/>
2078<t>
2079   The "gzip" format is produced by the file compression program
2080   "gzip" (GNU zip), as described in <xref target="RFC1952"/>. This format is a
2081   Lempel-Ziv coding (LZ77) with a 32 bit CRC.
2082   Recipients &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2083</t>
2084</section>
2085
2086</section>
2087
2088<section title="TE" anchor="header.te">
2089  <iref primary="true" item="TE header field" x:for-anchor=""/>
2090  <x:anchor-alias value="TE"/>
2091  <x:anchor-alias value="t-codings"/>
2092  <x:anchor-alias value="t-ranking"/>
2093  <x:anchor-alias value="rank"/>
2094<t>
2095   The "TE" header field in a request indicates what transfer codings,
2096   besides chunked, the client is willing to accept in response, and
2097   whether or not the client is willing to accept trailer fields in a
2098   chunked transfer coding.
2099</t>
2100<t>
2101   The TE field-value consists of a comma-separated list of transfer coding
2102   names, each allowing for optional parameters (as described in
2103   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2104   Clients &MUST-NOT; send the chunked transfer coding name in TE;
2105   chunked is always acceptable for HTTP/1.1 recipients.
2106</t>
2107<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"/>
2108  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2109  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2110  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2111  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2112             / ( "1" [ "." 0*3("0") ] )
2113</artwork></figure>
2114<t>
2115   Three examples of TE use are below.
2116</t>
2117<figure><artwork type="example">
2118  TE: deflate
2119  TE:
2120  TE: trailers, deflate;q=0.5
2121</artwork></figure>
2122<t>
2123   The presence of the keyword "trailers" indicates that the client is
2124   willing to accept trailer fields in a chunked transfer coding,
2125   as defined in <xref target="chunked.encoding"/>, on behalf of itself and
2126   any downstream clients. For chained requests, this implies that either:
2127   (a) all downstream clients are willing to accept trailer fields in the
2128   forwarded response; or,
2129   (b) the client will attempt to buffer the response on behalf of downstream
2130   recipients.
2131   Note that HTTP/1.1 does not define any means to limit the size of a
2132   chunked response such that a client can be assured of buffering the
2133   entire response.
2134</t>
2135<t>
2136   When multiple transfer codings are acceptable, the client &MAY; rank the
2137   codings by preference using a case-insensitive "q" parameter (similar to
2138   the qvalues used in content negotiation fields, &qvalue;). The rank value
2139   is a real number in the range 0 through 1, where 0.001 is the least
2140   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2141</t>
2142<t>
2143   If the TE field-value is empty or if no TE field is present, the only
2144   acceptable transfer coding is chunked. A message with no transfer coding
2145   is always acceptable.
2146</t>
2147<t>
2148   Since the TE header field only applies to the immediate connection,
2149   a sender of TE &MUST; also send a "TE" connection option within the
2150   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2151   in order to prevent the TE field from being forwarded by intermediaries
2152   that do not support its semantics.
2153</t>
2154</section>
2155</section>
2156
2157<section title="Message Routing" anchor="message.routing">
2158<t>
2159   HTTP request message routing is determined by each client based on the
2160   target resource, the client's proxy configuration, and
2161   establishment or reuse of an inbound connection.  The corresponding
2162   response routing follows the same connection chain back to the client.
2163</t>
2164
2165<section title="Identifying a Target Resource" anchor="target-resource">
2166  <iref primary="true" item="target resource"/>
2167  <iref primary="true" item="target URI"/>
2168  <x:anchor-alias value="target resource"/>
2169  <x:anchor-alias value="target URI"/>
2170<t>
2171   HTTP is used in a wide variety of applications, ranging from
2172   general-purpose computers to home appliances.  In some cases,
2173   communication options are hard-coded in a client's configuration.
2174   However, most HTTP clients rely on the same resource identification
2175   mechanism and configuration techniques as general-purpose Web browsers.
2176</t>
2177<t>
2178   HTTP communication is initiated by a user agent for some purpose.
2179   The purpose is a combination of request semantics, which are defined in
2180   <xref target="Part2"/>, and a target resource upon which to apply those
2181   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2182   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2183   would resolve to its absolute form in order to obtain the
2184   "<x:dfn>target URI</x:dfn>".  The target URI
2185   excludes the reference's fragment identifier component, if any,
2186   since fragment identifiers are reserved for client-side processing
2187   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2188</t>
2189</section>
2190
2191<section title="Connecting Inbound" anchor="connecting.inbound">
2192<t>
2193   Once the target URI is determined, a client needs to decide whether
2194   a network request is necessary to accomplish the desired semantics and,
2195   if so, where that request is to be directed.
2196</t>
2197<t>
2198   If the client has a response cache and the request semantics can be
2199   satisfied by a cache (<xref target="Part6"/>), then the request is
2200   usually directed to the cache first.
2201</t>
2202<t>
2203   If the request is not satisfied by a cache, then a typical client will
2204   check its configuration to determine whether a proxy is to be used to
2205   satisfy the request.  Proxy configuration is implementation-dependent,
2206   but is often based on URI prefix matching, selective authority matching,
2207   or both, and the proxy itself is usually identified by an "http" or
2208   "https" URI.  If a proxy is applicable, the client connects inbound by
2209   establishing (or reusing) a connection to that proxy.
2210</t>
2211<t>
2212   If no proxy is applicable, a typical client will invoke a handler routine,
2213   usually specific to the target URI's scheme, to connect directly
2214   to an authority for the target resource.  How that is accomplished is
2215   dependent on the target URI scheme and defined by its associated
2216   specification, similar to how this specification defines origin server
2217   access for resolution of the "http" (<xref target="http.uri"/>) and
2218   "https" (<xref target="https.uri"/>) schemes.
2219</t>
2220<t>
2221   HTTP requirements regarding connection management are defined in
2222   <xref target="connection.management"/>.
2223</t>
2224</section>
2225
2226<section title="Request Target" anchor="request-target">
2227<t>
2228   Once an inbound connection is obtained,
2229   the client sends an HTTP request message (<xref target="http.message"/>)
2230   with a request-target derived from the target URI.
2231   There are four distinct formats for the request-target, depending on both
2232   the method being requested and whether the request is to a proxy.
2233</t>   
2234<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"/>
2235  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2236                 / <x:ref>absolute-form</x:ref>
2237                 / <x:ref>authority-form</x:ref>
2238                 / <x:ref>asterisk-form</x:ref>
2239
2240  <x:ref>origin-form</x:ref>    = <x:ref>absolute-path</x:ref> [ "?" <x:ref>query</x:ref> ]
2241  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2242  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2243  <x:ref>asterisk-form</x:ref>  = "*"
2244</artwork></figure>
2245<t anchor="origin-form"><iref item="origin-form (of request-target)"/>
2246  <x:h>origin-form</x:h>
2247</t>
2248<t>
2249   The most common form of request-target is the <x:dfn>origin-form</x:dfn>.
2250   When making a request directly to an origin server, other than a CONNECT
2251   or server-wide OPTIONS request (as detailed below),
2252   a client &MUST; send only the absolute path and query components of
2253   the target URI as the request-target.
2254   If the target URI's path component is empty, then the client &MUST; send
2255   "/" as the path within the origin-form of request-target.
2256   A <x:ref>Host</x:ref> header field is also sent, as defined in
2257   <xref target="header.host"/>, containing the target URI's
2258   authority component (excluding any userinfo).
2259</t>
2260<t>
2261   For example, a client wishing to retrieve a representation of the resource
2262   identified as
2263</t>
2264<figure><artwork x:indent-with="  " type="example">
2265http://www.example.org/where?q=now
2266</artwork></figure>
2267<t>
2268   directly from the origin server would open (or reuse) a TCP connection
2269   to port 80 of the host "www.example.org" and send the lines:
2270</t>
2271<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2272GET /where?q=now HTTP/1.1
2273Host: www.example.org
2274</artwork></figure>
2275<t>
2276   followed by the remainder of the request message.
2277</t>
2278<t anchor="absolute-form"><iref item="absolute-form (of request-target)"/>
2279  <x:h>absolute-form</x:h>
2280</t>
2281<t>
2282   When making a request to a proxy, other than a CONNECT or server-wide
2283   OPTIONS request (as detailed below), a client &MUST; send the target URI
2284   in <x:dfn>absolute-form</x:dfn> as the request-target.
2285   The proxy is requested to either service that request from a valid cache,
2286   if possible, or make the same request on the client's behalf to either
2287   the next inbound proxy server or directly to the origin server indicated
2288   by the request-target.  Requirements on such "forwarding" of messages are
2289   defined in <xref target="message.forwarding"/>.
2290</t>
2291<t>
2292   An example absolute-form of request-line would be:
2293</t>
2294<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2295GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
2296</artwork></figure>
2297<t>
2298   To allow for transition to the absolute-form for all requests in some
2299   future version of HTTP, HTTP/1.1 servers &MUST; accept the absolute-form
2300   in requests, even though HTTP/1.1 clients will only send them in requests
2301   to proxies.
2302</t>
2303<t anchor="authority-form"><iref item="authority-form (of request-target)"/>
2304  <x:h>authority-form</x:h>
2305</t>
2306<t>
2307   The <x:dfn>authority-form</x:dfn> of request-target is only used for CONNECT requests
2308   (&CONNECT;).  When making a CONNECT request to establish a tunnel through
2309   one or more proxies, a client &MUST; send only the target URI's
2310   authority component (excluding any userinfo) as the request-target.
2311   For example,
2312</t>
2313<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2314CONNECT www.example.com:80 HTTP/1.1
2315</artwork></figure>
2316<t anchor="asterisk-form"><iref item="asterisk-form (of request-target)"/>
2317  <x:h>asterisk-form</x:h>
2318</t>
2319<t>
2320   The <x:dfn>asterisk-form</x:dfn> of request-target is only used for a server-wide
2321   OPTIONS request (&OPTIONS;).  When a client wishes to request OPTIONS
2322   for the server as a whole, as opposed to a specific named resource of
2323   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2324   For example,
2325</t>
2326<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2327OPTIONS * HTTP/1.1
2328</artwork></figure>
2329<t>
2330   If a proxy receives an OPTIONS request with an absolute-form of
2331   request-target in which the URI has an empty path and no query component,
2332   then the last proxy on the request chain &MUST; send a request-target
2333   of "*" when it forwards the request to the indicated origin server.
2334</t>
2335<figure><preamble>   
2336   For example, the request
2337</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2338OPTIONS http://www.example.org:8001 HTTP/1.1
2339</artwork></figure>
2340<figure><preamble>   
2341  would be forwarded by the final proxy as
2342</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2343OPTIONS * HTTP/1.1
2344Host: www.example.org:8001
2345</artwork>
2346<postamble>
2347   after connecting to port 8001 of host "www.example.org".
2348</postamble>
2349</figure>
2350</section>
2351
2352<section title="Host" anchor="header.host">
2353  <iref primary="true" item="Host header field" x:for-anchor=""/>
2354  <x:anchor-alias value="Host"/>
2355<t>
2356   The "Host" header field in a request provides the host and port
2357   information from the target URI, enabling the origin
2358   server to distinguish among resources while servicing requests
2359   for multiple host names on a single IP address.  Since the Host
2360   field-value is critical information for handling a request, it
2361   &SHOULD; be sent as the first header field following the request-line.
2362</t>
2363<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2364  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2365</artwork></figure>
2366<t>
2367   A client &MUST; send a Host header field in all HTTP/1.1 request
2368   messages.  If the target URI includes an authority component, then
2369   the Host field-value &MUST; be identical to that authority component
2370   after excluding any userinfo (<xref target="http.uri"/>).
2371   If the authority component is missing or undefined for the target URI,
2372   then the Host header field &MUST; be sent with an empty field-value.
2373</t>
2374<t>
2375   For example, a GET request to the origin server for
2376   &lt;http://www.example.org/pub/WWW/&gt; would begin with:
2377</t>
2378<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2379GET /pub/WWW/ HTTP/1.1
2380Host: www.example.org
2381</artwork></figure>
2382<t>
2383   The Host header field &MUST; be sent in an HTTP/1.1 request even
2384   if the request-target is in the absolute-form, since this
2385   allows the Host information to be forwarded through ancient HTTP/1.0
2386   proxies that might not have implemented Host.
2387</t>
2388<t>
2389   When a proxy receives a request with an absolute-form of
2390   request-target, the proxy &MUST; ignore the received
2391   Host header field (if any) and instead replace it with the host
2392   information of the request-target.  If the proxy forwards the request,
2393   it &MUST; generate a new Host field-value based on the received
2394   request-target rather than forward the received Host field-value.
2395</t>
2396<t>
2397   Since the Host header field acts as an application-level routing
2398   mechanism, it is a frequent target for malware seeking to poison
2399   a shared cache or redirect a request to an unintended server.
2400   An interception proxy is particularly vulnerable if it relies on
2401   the Host field-value for redirecting requests to internal
2402   servers, or for use as a cache key in a shared cache, without
2403   first verifying that the intercepted connection is targeting a
2404   valid IP address for that host.
2405</t>
2406<t>
2407   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2408   to any HTTP/1.1 request message that lacks a Host header field and
2409   to any request message that contains more than one Host header field
2410   or a Host header field with an invalid field-value.
2411</t>
2412</section>
2413
2414<section title="Effective Request URI" anchor="effective.request.uri">
2415  <iref primary="true" item="effective request URI"/>
2416  <x:anchor-alias value="effective request URI"/>
2417<t>
2418   A server that receives an HTTP request message &MUST; reconstruct
2419   the user agent's original target URI, based on the pieces of information
2420   learned from the request-target, <x:ref>Host</x:ref> header field, and
2421   connection context, in order to identify the intended target resource and
2422   properly service the request. The URI derived from this reconstruction
2423   process is referred to as the "<x:dfn>effective request URI</x:dfn>".
2424</t>
2425<t>
2426   For a user agent, the effective request URI is the target URI.
2427</t>
2428<t>
2429   If the request-target is in absolute-form, then the effective request URI
2430   is the same as the request-target.  Otherwise, the effective request URI
2431   is constructed as follows.
2432</t>
2433<t>
2434   If the request is received over a TLS-secured TCP connection,
2435   then the effective request URI's scheme is "https"; otherwise, the
2436   scheme is "http".
2437</t>
2438<t>
2439   If the request-target is in authority-form, then the effective
2440   request URI's authority component is the same as the request-target.
2441   Otherwise, if a <x:ref>Host</x:ref> header field is supplied with a
2442   non-empty field-value, then the authority component is the same as the
2443   Host field-value. Otherwise, the authority component is the concatenation of
2444   the default host name configured for the server, a colon (":"), and the
2445   connection's incoming TCP port number in decimal form.
2446</t>
2447<t>
2448   If the request-target is in authority-form or asterisk-form, then the
2449   effective request URI's combined path and query component is empty.
2450   Otherwise, the combined path and query component is the same as the
2451   request-target.
2452</t>
2453<t>
2454   The components of the effective request URI, once determined as above,
2455   can be combined into absolute-URI form by concatenating the scheme,
2456   "://", authority, and combined path and query component.
2457</t>
2458<figure>
2459<preamble>
2460   Example 1: the following message received over an insecure TCP connection
2461</preamble> 
2462<artwork type="example" x:indent-with="  ">
2463GET /pub/WWW/TheProject.html HTTP/1.1
2464Host: www.example.org:8080
2465</artwork>
2466</figure>
2467<figure>
2468<preamble>
2469  has an effective request URI of
2470</preamble>
2471<artwork type="example" x:indent-with="  ">
2472http://www.example.org:8080/pub/WWW/TheProject.html
2473</artwork>
2474</figure>
2475<figure>
2476<preamble>
2477   Example 2: the following message received over a TLS-secured TCP connection
2478</preamble> 
2479<artwork type="example" x:indent-with="  ">
2480OPTIONS * HTTP/1.1
2481Host: www.example.org
2482</artwork>
2483</figure>
2484<figure>
2485<preamble>
2486  has an effective request URI of
2487</preamble>
2488<artwork type="example" x:indent-with="  ">
2489https://www.example.org
2490</artwork>
2491</figure>
2492<t>
2493   An origin server that does not allow resources to differ by requested
2494   host &MAY; ignore the <x:ref>Host</x:ref> field-value and instead replace it
2495   with a configured server name when constructing the effective request URI.
2496</t>
2497<t>
2498   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2499   field &MAY; attempt to use heuristics (e.g., examination of the URI path for
2500   something unique to a particular host) in order to guess the
2501   effective request URI's authority component.
2502</t>
2503</section>
2504
2505<section title="Associating a Response to a Request" anchor="associating.response.to.request">
2506<t>
2507   HTTP does not include a request identifier for associating a given
2508   request message with its corresponding one or more response messages.
2509   Hence, it relies on the order of response arrival to correspond exactly
2510   to the order in which requests are made on the same connection.
2511   More than one response message per request only occurs when one or more
2512   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a
2513   final response to the same request.
2514</t>
2515<t>
2516   A client that has more than one outstanding request on a connection &MUST;
2517   maintain a list of outstanding requests in the order sent and &MUST;
2518   associate each received response message on that connection to the highest
2519   ordered request that has not yet received a final (non-<x:ref>1xx</x:ref>)
2520   response.
2521</t>
2522</section>
2523
2524<section title="Message Forwarding" anchor="message.forwarding">
2525<t>
2526   As described in <xref target="intermediaries"/>, intermediaries can serve
2527   a variety of roles in the processing of HTTP requests and responses.
2528   Some intermediaries are used to improve performance or availability.
2529   Others are used for access control or to filter content.
2530   Since an HTTP stream has characteristics similar to a pipe-and-filter
2531   architecture, there are no inherent limits to the extent an intermediary
2532   can enhance (or interfere) with either direction of the stream.
2533</t>
2534<t>
2535   Intermediaries that forward a message &MUST; implement the
2536   <x:ref>Connection</x:ref> header field, as specified in
2537   <xref target="header.connection"/>, to exclude fields that are only
2538   intended for the incoming connection.
2539</t>
2540<t>
2541   In order to avoid request loops, a proxy that forwards requests to other
2542   proxies &MUST; be able to recognize and exclude all of its own server
2543   names, including any aliases, local variations, or literal IP addresses.
2544</t>
2545
2546<section title="Via" anchor="header.via">
2547  <iref primary="true" item="Via header field" x:for-anchor=""/>
2548  <x:anchor-alias value="pseudonym"/>
2549  <x:anchor-alias value="received-by"/>
2550  <x:anchor-alias value="received-protocol"/>
2551  <x:anchor-alias value="Via"/>
2552<t>
2553   The "Via" header field &MUST; be sent by a proxy or gateway in forwarded
2554   messages to indicate the intermediate protocols and recipients between the
2555   user agent and the server on requests, and between the origin server and
2556   the client on responses. It is analogous to the "Received" field
2557   used by email systems (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2558   Via is used in HTTP for tracking message forwards,
2559   avoiding request loops, and identifying the protocol capabilities of
2560   all senders along the request/response chain.
2561</t>
2562<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"/>
2563  <x:ref>Via</x:ref>               = 1#( <x:ref>received-protocol</x:ref> <x:ref>RWS</x:ref> <x:ref>received-by</x:ref>
2564                          [ <x:ref>RWS</x:ref> <x:ref>comment</x:ref> ] )
2565  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2566                      ; see <xref target="header.upgrade"/>
2567  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2568  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2569</artwork></figure>
2570<t>
2571   The received-protocol indicates the protocol version of the message
2572   received by the server or client along each segment of the
2573   request/response chain. The received-protocol version is appended to
2574   the Via field value when the message is forwarded so that information
2575   about the protocol capabilities of upstream applications remains
2576   visible to all recipients.
2577</t>
2578<t>
2579   The protocol-name is excluded if and only if it would be "HTTP". The
2580   received-by field is normally the host and optional port number of a
2581   recipient server or client that subsequently forwarded the message.
2582   However, if the real host is considered to be sensitive information,
2583   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2584   be assumed to be the default port of the received-protocol.
2585</t>
2586<t>
2587   Multiple Via field values represent each proxy or gateway that has
2588   forwarded the message. Each recipient &MUST; append its information
2589   such that the end result is ordered according to the sequence of
2590   forwarding applications.
2591</t>
2592<t>
2593   Comments &MAY; be used in the Via header field to identify the software
2594   of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2595   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2596   are optional and &MAY; be removed by any recipient prior to forwarding the
2597   message.
2598</t>
2599<t>
2600   For example, a request message could be sent from an HTTP/1.0 user
2601   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2602   forward the request to a public proxy at p.example.net, which completes
2603   the request by forwarding it to the origin server at www.example.com.
2604   The request received by www.example.com would then have the following
2605   Via header field:
2606</t>
2607<figure><artwork type="example">
2608  Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
2609</artwork></figure>
2610<t>
2611   A proxy or gateway used as a portal through a network firewall
2612   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2613   region unless it is explicitly enabled to do so. If not enabled, the
2614   received-by host of any host behind the firewall &SHOULD; be replaced
2615   by an appropriate pseudonym for that host.
2616</t>
2617<t>
2618   A proxy or gateway &MAY; combine an ordered subsequence of Via header
2619   field entries into a single such entry if the entries have identical
2620   received-protocol values. For example,
2621</t>
2622<figure><artwork type="example">
2623  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2624</artwork></figure>
2625<t>
2626  could be collapsed to
2627</t>
2628<figure><artwork type="example">
2629  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2630</artwork></figure>
2631<t>
2632   Senders &SHOULD-NOT; combine multiple entries unless they are all
2633   under the same organizational control and the hosts have already been
2634   replaced by pseudonyms. Senders &MUST-NOT; combine entries that
2635   have different received-protocol values.
2636</t>
2637</section>
2638
2639<section title="Transformations" anchor="message.transformations">
2640<t>
2641   Some intermediaries include features for transforming messages and their
2642   payloads.  A transforming proxy might, for example, convert between image
2643   formats in order to save cache space or to reduce the amount of traffic on
2644   a slow link. However, operational problems might occur when these
2645   transformations are applied to payloads intended for critical applications,
2646   such as medical imaging or scientific data analysis, particularly when
2647   integrity checks or digital signatures are used to ensure that the payload
2648   received is identical to the original.
2649</t>
2650<t>
2651   If a proxy receives a request-target with a host name that is not a
2652   fully qualified domain name, it &MAY; add its own domain to the host name
2653   it received when forwarding the request.  A proxy &MUST-NOT; change the
2654   host name if it is a fully qualified domain name.
2655</t>
2656<t>
2657   A proxy &MUST-NOT; modify the "absolute-path" and "query" parts of the
2658   received request-target when forwarding it to the next inbound server,
2659   except as noted above to replace an empty path with "/" or "*".
2660</t>
2661<t>
2662   A proxy &MUST-NOT; modify header fields that provide information about the
2663   end points of the communication chain, the resource state, or the selected
2664   representation. A proxy &MAY; change the message body through application
2665   or removal of a transfer coding (<xref target="transfer.codings"/>).
2666</t>
2667<t>
2668   A non-transforming proxy &MUST; preserve the message payload (&payload;).
2669   A transforming proxy &MUST; preserve the payload of a message that
2670   contains the no-transform cache-control directive.
2671</t>
2672<t>
2673   A transforming proxy &MAY; transform the payload of a message
2674   that does not contain the no-transform cache-control directive;
2675   if the payload is transformed, the transforming proxy &MUST; add a
2676   Warning 214 (Transformation applied) header field if one does not
2677   already appear in the message (see &header-warning;).
2678</t>
2679</section>
2680</section>
2681</section>
2682
2683<section title="Connection Management" anchor="connection.management">
2684<t>
2685   HTTP messaging is independent of the underlying transport or
2686   session-layer connection protocol(s).  HTTP only presumes a reliable
2687   transport with in-order delivery of requests and the corresponding
2688   in-order delivery of responses.  The mapping of HTTP request and
2689   response structures onto the data units of an underlying transport
2690   protocol is outside the scope of this specification.
2691</t>
2692<t>
2693   As described in <xref target="connecting.inbound"/>, the specific
2694   connection protocols to be used for an HTTP interaction are determined by
2695   client configuration and the <x:ref>target URI</x:ref>.
2696   For example, the "http" URI scheme
2697   (<xref target="http.uri"/>) indicates a default connection of TCP
2698   over IP, with a default TCP port of 80, but the client might be
2699   configured to use a proxy via some other connection, port, or protocol.
2700</t>
2701<t>
2702   HTTP implementations are expected to engage in connection management,
2703   which includes maintaining the state of current connections,
2704   establishing a new connection or reusing an existing connection,
2705   processing messages received on a connection, detecting connection
2706   failures, and closing each connection.
2707   Most clients maintain multiple connections in parallel, including
2708   more than one connection per server endpoint.
2709   Most servers are designed to maintain thousands of concurrent connections,
2710   while controlling request queues to enable fair use and detect
2711   denial of service attacks.
2712</t>
2713
2714<section title="Connection" anchor="header.connection">
2715  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2716  <iref primary="true" item="close" x:for-anchor=""/>
2717  <x:anchor-alias value="Connection"/>
2718  <x:anchor-alias value="connection-option"/>
2719  <x:anchor-alias value="close"/>
2720<t>
2721   The "Connection" header field allows the sender to indicate desired
2722   control options for the current connection.  In order to avoid confusing
2723   downstream recipients, a proxy or gateway &MUST; remove or replace any
2724   received connection options before forwarding the message.
2725</t>
2726<t>
2727   When a header field aside from Connection is used to supply control
2728   information for or about the current connection, the sender &MUST; list
2729   the corresponding field-name within the "Connection" header field.
2730   A proxy or gateway &MUST; parse a received Connection
2731   header field before a message is forwarded and, for each
2732   connection-option in this field, remove any header field(s) from
2733   the message with the same name as the connection-option, and then
2734   remove the Connection header field itself (or replace it with the
2735   intermediary's own connection options for the forwarded message).
2736</t>
2737<t>
2738   Hence, the Connection header field provides a declarative way of
2739   distinguishing header fields that are only intended for the
2740   immediate recipient ("hop-by-hop") from those fields that are
2741   intended for all recipients on the chain ("end-to-end"), enabling the
2742   message to be self-descriptive and allowing future connection-specific
2743   extensions to be deployed without fear that they will be blindly
2744   forwarded by older intermediaries.
2745</t>
2746<t>
2747   The Connection header field's value has the following grammar:
2748</t>
2749<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2750  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2751  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2752</artwork></figure>
2753<t>
2754   Connection options are case-insensitive.
2755</t>
2756<t>
2757   A sender &MUST-NOT; send a connection option corresponding to a header
2758   field that is intended for all recipients of the payload.
2759   For example, <x:ref>Cache-Control</x:ref> is never appropriate as a
2760   connection option (&header-cache-control;).
2761</t>
2762<t>
2763   The connection options do not have to correspond to a header field
2764   present in the message, since a connection-specific header field
2765   might not be needed if there are no parameters associated with that
2766   connection option.  Recipients that trigger certain connection
2767   behavior based on the presence of connection options &MUST; do so
2768   based on the presence of the connection-option rather than only the
2769   presence of the optional header field.  In other words, if the
2770   connection option is received as a header field but not indicated
2771   within the Connection field-value, then the recipient &MUST; ignore
2772   the connection-specific header field because it has likely been
2773   forwarded by an intermediary that is only partially conformant.
2774</t>
2775<t>
2776   When defining new connection options, specifications ought to
2777   carefully consider existing deployed header fields and ensure
2778   that the new connection option does not share the same name as
2779   an unrelated header field that might already be deployed.
2780   Defining a new connection option essentially reserves that potential
2781   field-name for carrying additional information related to the
2782   connection option, since it would be unwise for senders to use
2783   that field-name for anything else.
2784</t>
2785<t>
2786   The "<x:dfn>close</x:dfn>" connection option is defined for a
2787   sender to signal that this connection will be closed after completion of
2788   the response. For example,
2789</t>
2790<figure><artwork type="example">
2791  Connection: close
2792</artwork></figure>
2793<t>
2794   in either the request or the response header fields indicates that
2795   the connection &MUST; be closed after the current request/response
2796   is complete (<xref target="persistent.tear-down"/>).
2797</t>
2798<t>
2799   A client that does not support <x:ref>persistent connections</x:ref> &MUST;
2800   send the "close" connection option in every request message.
2801</t>
2802<t>
2803   A server that does not support <x:ref>persistent connections</x:ref> &MUST;
2804   send the "close" connection option in every response message that
2805   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2806</t>
2807</section>
2808
2809<section title="Establishment" anchor="persistent.establishment">
2810<t>
2811   It is beyond the scope of this specification to describe how connections
2812   are established via various transport or session-layer protocols.
2813   Each connection applies to only one transport link.
2814</t>
2815</section>
2816
2817<section title="Persistence" anchor="persistent.connections">
2818   <x:anchor-alias value="persistent connections"/>
2819<t>
2820   HTTP/1.1 defaults to the use of "<x:dfn>persistent connections</x:dfn>",
2821   allowing multiple requests and responses to be carried over a single
2822   connection. The "<x:ref>close</x:ref>" connection-option is used to signal
2823   that a connection will not persist after the current request/response.
2824   HTTP implementations &SHOULD; support persistent connections.
2825</t>
2826<t>
2827   A recipient determines whether a connection is persistent or not based on
2828   the most recently received message's protocol version and
2829   <x:ref>Connection</x:ref> header field (if any):
2830   <list style="symbols">
2831     <t>If the <x:ref>close</x:ref> connection option is present, the
2832        connection will not persist after the current response; else,</t>
2833     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2834        persist after the current response; else,</t>
2835     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2836        connection option is present, the recipient is not a proxy, and
2837        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2838        the connection will persist after the current response; otherwise,</t>
2839     <t>The connection will close after the current response.</t>
2840   </list>
2841</t>
2842<t>
2843   A server &MAY; assume that an HTTP/1.1 client intends to maintain a
2844   persistent connection until a <x:ref>close</x:ref> connection option
2845   is received in a request.
2846</t>
2847<t>
2848   A client &MAY; reuse a persistent connection until it sends or receives
2849   a <x:ref>close</x:ref> connection option or receives an HTTP/1.0 response
2850   without a "keep-alive" connection option.
2851</t>
2852<t>
2853   In order to remain persistent, all messages on a connection &MUST;
2854   have a self-defined message length (i.e., one not defined by closure
2855   of the connection), as described in <xref target="message.body"/>.
2856   A server &MUST; read the entire request message body or close
2857   the connection after sending its response, since otherwise the
2858   remaining data on a persistent connection would be misinterpreted
2859   as the next request.  Likewise,
2860   a client &MUST; read the entire response message body if it intends
2861   to reuse the same connection for a subsequent request.
2862</t>
2863<t>
2864   A proxy server &MUST-NOT; maintain a persistent connection with an
2865   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
2866   information and discussion of the problems with the Keep-Alive header field
2867   implemented by many HTTP/1.0 clients).
2868</t>
2869<t>
2870   Clients and servers &SHOULD-NOT; assume that a persistent connection is
2871   maintained for HTTP versions less than 1.1 unless it is explicitly
2872   signaled.
2873   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
2874   for more information on backward compatibility with HTTP/1.0 clients.
2875</t>
2876
2877<section title="Retrying Requests" anchor="persistent.retrying.requests">
2878<t>
2879   Connections can be closed at any time, with or without intention.
2880   Implementations ought to anticipate the need to recover
2881   from asynchronous close events.
2882</t>
2883<t>
2884   When an inbound connection is closed prematurely, a client &MAY; open a new
2885   connection and automatically retransmit an aborted sequence of requests if
2886   all of those requests have idempotent methods (&idempotent-methods;).
2887   A proxy &MUST-NOT; automatically retry non-idempotent requests.
2888</t>
2889<t>
2890   A user agent &MUST-NOT; automatically retry a request with a non-idempotent
2891   method unless it has some means to know that the request semantics are
2892   actually idempotent, regardless of the method, or some means to detect that
2893   the original request was never applied. For example, a user agent that
2894   knows (through design or configuration) that a POST request to a given
2895   resource is safe can repeat that request automatically.
2896   Likewise, a user agent designed specifically to operate on a version
2897   control repository might be able to recover from partial failure conditions
2898   by checking the target resource revision(s) after a failed connection,
2899   reverting or fixing any changes that were partially applied, and then
2900   automatically retrying the requests that failed.
2901</t>
2902<t>
2903   An automatic retry &SHOULD-NOT; be repeated if it fails.
2904</t>
2905</section>
2906
2907<section title="Pipelining" anchor="pipelining">
2908   <x:anchor-alias value="pipeline"/>
2909<t>
2910   A client that supports persistent connections &MAY; "<x:dfn>pipeline</x:dfn>"
2911   its requests (i.e., send multiple requests without waiting for each
2912   response). A server &MAY; process a sequence of pipelined requests in
2913   parallel if they all have safe methods (&safe-methods;), but &MUST; send
2914   the corresponding responses in the same order that the requests were
2915   received.
2916</t>
2917<t>
2918   A client that pipelines requests &MUST; be prepared to retry those
2919   requests if the connection closes before it receives all of the
2920   corresponding responses. A client that assumes a persistent connection and
2921   pipelines immediately after connection establishment &MUST-NOT; pipeline
2922   on a retry connection until it knows the connection is persistent.
2923</t>
2924<t>
2925   Idempotent methods (&idempotent-methods;) are significant to pipelining
2926   because they can be automatically retried after a connection failure.
2927   A user agent &SHOULD-NOT; pipeline requests after a non-idempotent method
2928   until the final response status code for that method has been received,
2929   unless the user agent has a means to detect and recover from partial
2930   failure conditions involving the pipelined sequence.
2931</t>
2932<t>
2933   An intermediary that receives pipelined requests &MAY; pipeline those
2934   requests when forwarding them inbound, since it can rely on the outbound
2935   user agent(s) to determine what requests can be safely pipelined. If the
2936   inbound connection fails before receiving a response, the pipelining
2937   intermediary &MAY; attempt to retry a sequence of requests that have yet
2938   to receive a response if the requests all have idempotent methods;
2939   otherwise, the pipelining intermediary &SHOULD; forward any received
2940   responses and then close the corresponding outbound connection(s) so that
2941   the outbound user agent(s) can recover accordingly.
2942</t>
2943</section>
2944</section>
2945   
2946<section title="Concurrency" anchor="persistent.concurrency">
2947<t>
2948   Clients &SHOULD; limit the number of simultaneous
2949   connections that they maintain to a given server.
2950</t>
2951<t>
2952   Previous revisions of HTTP gave a specific number of connections as a
2953   ceiling, but this was found to be impractical for many applications. As a
2954   result, this specification does not mandate a particular maximum number of
2955   connections, but instead encourages clients to be conservative when opening
2956   multiple connections.
2957</t>
2958<t>
2959   Multiple connections are typically used to avoid the "head-of-line
2960   blocking" problem, wherein a request that takes significant server-side
2961   processing and/or has a large payload blocks subsequent requests on the
2962   same connection. However, each connection consumes server resources.
2963   Furthermore, using multiple connections can cause undesirable side effects
2964   in congested networks.
2965</t>
2966<t>
2967   Note that servers might reject traffic that they deem abusive, including an
2968   excessive number of connections from a client.
2969</t>
2970</section>
2971
2972<section title="Failures and Time-outs" anchor="persistent.failures">
2973<t>
2974   Servers will usually have some time-out value beyond which they will
2975   no longer maintain an inactive connection. Proxy servers might make
2976   this a higher value since it is likely that the client will be making
2977   more connections through the same server. The use of persistent
2978   connections places no requirements on the length (or existence) of
2979   this time-out for either the client or the server.
2980</t>
2981<t>
2982   When a client or server wishes to time-out it &SHOULD; issue a graceful
2983   close on the transport connection. Clients and servers &SHOULD; both
2984   constantly watch for the other side of the transport close, and
2985   respond to it as appropriate. If a client or server does not detect
2986   the other side's close promptly it could cause unnecessary resource
2987   drain on the network.
2988</t>
2989<t>
2990   A client, server, or proxy &MAY; close the transport connection at any
2991   time. For example, a client might have started to send a new request
2992   at the same time that the server has decided to close the "idle"
2993   connection. From the server's point of view, the connection is being
2994   closed while it was idle, but from the client's point of view, a
2995   request is in progress.
2996</t>
2997<t>
2998   Servers &SHOULD; maintain persistent connections and allow the underlying
2999   transport's flow control mechanisms to resolve temporary overloads, rather
3000   than terminate connections with the expectation that clients will retry.
3001   The latter technique can exacerbate network congestion.
3002</t>
3003<t>
3004   A client sending a message body &SHOULD; monitor
3005   the network connection for an error status code while it is transmitting
3006   the request. If the client sees an error status code, it &SHOULD;
3007   immediately cease transmitting the body and close the connection.
3008</t>
3009</section>
3010   
3011<section title="Tear-down" anchor="persistent.tear-down">
3012  <iref primary="false" item="Connection header field" x:for-anchor=""/>
3013  <iref primary="false" item="close" x:for-anchor=""/>
3014<t>
3015   The <x:ref>Connection</x:ref> header field
3016   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
3017   connection option that a sender &SHOULD; send when it wishes to close
3018   the connection after the current request/response pair.
3019</t>
3020<t>
3021   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
3022   send further requests on that connection (after the one containing
3023   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
3024   final response message corresponding to this request.
3025</t>
3026<t>
3027   A server that receives a <x:ref>close</x:ref> connection option &MUST;
3028   initiate a lingering close (see below) of the connection after it sends the
3029   final response to the request that contained <x:ref>close</x:ref>.
3030   The server &SHOULD; send a <x:ref>close</x:ref> connection option
3031   in its final response on that connection. The server &MUST-NOT; process
3032   any further requests received on that connection.
3033</t>
3034<t>
3035   A server that sends a <x:ref>close</x:ref> connection option &MUST;
3036   initiate a lingering close of the connection after it sends the
3037   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
3038   any further requests received on that connection.
3039</t>
3040<t>
3041   A client that receives a <x:ref>close</x:ref> connection option &MUST;
3042   cease sending requests on that connection and close the connection
3043   after reading the response message containing the close; if additional
3044   pipelined requests had been sent on the connection, the client &SHOULD;
3045   assume that they will not be processed by the server.
3046</t>
3047<t>
3048   If a server performs an immediate close of a TCP connection, there is a
3049   significant risk that the client will not be able to read the last HTTP
3050   response.  If the server receives additional data from the client on a
3051   fully-closed connection, such as another request that was sent by the
3052   client before receiving the server's response, the server's TCP stack will
3053   send a reset packet to the client; unfortunately, the reset packet might
3054   erase the client's unacknowledged input buffers before they can be read
3055   and interpreted by the client's HTTP parser.
3056</t>
3057<t>
3058   To avoid the TCP reset problem, a server can perform a lingering close on a
3059   connection by closing only the write side of the read/write connection
3060   (a half-close) and continuing to read from the connection until the
3061   connection is closed by the client or the server is reasonably certain
3062   that its own TCP stack has received the client's acknowledgement of the
3063   packet(s) containing the server's last response. It is then safe for the
3064   server to fully close the connection.
3065</t>
3066<t>
3067   It is unknown whether the reset problem is exclusive to TCP or might also
3068   be found in other transport connection protocols.
3069</t>
3070</section>
3071
3072<section title="Upgrade" anchor="header.upgrade">
3073  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3074  <x:anchor-alias value="Upgrade"/>
3075  <x:anchor-alias value="protocol"/>
3076  <x:anchor-alias value="protocol-name"/>
3077  <x:anchor-alias value="protocol-version"/>
3078<t>
3079   The "Upgrade" header field is intended to provide a simple mechanism
3080   for transitioning from HTTP/1.1 to some other protocol on the same
3081   connection.  A client &MAY; send a list of protocols in the Upgrade
3082   header field of a request to invite the server to switch to one or
3083   more of those protocols before sending the final response.
3084   A server &MUST; send an Upgrade header field in <x:ref>101 (Switching
3085   Protocols)</x:ref> responses to indicate which protocol(s) are being
3086   switched to, and &MUST; send it in <x:ref>426 (Upgrade Required)</x:ref>
3087   responses to indicate acceptable protocols.
3088   A server &MAY; send an Upgrade header field in any other response to
3089   indicate that they might be willing to upgrade to one of the
3090   specified protocols for a future request.
3091</t>
3092<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3093  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3094
3095  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3096  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3097  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3098</artwork></figure>
3099<t>
3100   For example,
3101</t>
3102<figure><artwork type="example">
3103  Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3104</artwork></figure>
3105<t>
3106   Upgrade eases the difficult transition between incompatible protocols by
3107   allowing the client to initiate a request in the more commonly
3108   supported protocol while indicating to the server that it would like
3109   to use a "better" protocol if available (where "better" is determined
3110   by the server, possibly according to the nature of the request method
3111   or target resource).
3112</t>
3113<t>
3114   Upgrade cannot be used to insist on a protocol change; its acceptance and
3115   use by the server is optional. The capabilities and nature of the
3116   application-level communication after the protocol change is entirely
3117   dependent upon the new protocol chosen, although the first action
3118   after changing the protocol &MUST; be a response to the initial HTTP
3119   request that contained the Upgrade header field.
3120</t>
3121<t>
3122   For example, if the Upgrade header field is received in a GET request
3123   and the server decides to switch protocols, then it first responds
3124   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3125   then immediately follows that with the new protocol's equivalent of a
3126   response to a GET on the target resource.  This allows a connection to be
3127   upgraded to protocols with the same semantics as HTTP without the
3128   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3129   protocols unless the received message semantics can be honored by the new
3130   protocol; an OPTIONS request can be honored by any protocol.
3131</t>
3132<t>
3133   When Upgrade is sent, a sender &MUST; also send a
3134   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3135   that contains the "upgrade" connection option, in order to prevent Upgrade
3136   from being accidentally forwarded by intermediaries that might not implement
3137   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3138   is received in an HTTP/1.0 request.
3139</t>
3140<t>
3141   The Upgrade header field only applies to switching application-level
3142   protocols on the existing connection; it cannot be used
3143   to switch to a protocol on a different connection. For that purpose, it is
3144   more appropriate to use a <x:ref>3xx (Redirection)</x:ref> response
3145   (&status-3xx;).
3146</t>
3147<t>
3148   This specification only defines the protocol name "HTTP" for use by
3149   the family of Hypertext Transfer Protocols, as defined by the HTTP
3150   version rules of <xref target="http.version"/> and future updates to this
3151   specification. Additional tokens ought to be registered with IANA using the
3152   registration procedure defined in <xref target="upgrade.token.registry"/>.
3153</t>
3154</section>
3155</section>
3156
3157<section title="IANA Considerations" anchor="IANA.considerations">
3158
3159<section title="Header Field Registration" anchor="header.field.registration">
3160<t>
3161   HTTP header fields are registered within the Message Header Field Registry
3162   <xref target="BCP90"/> maintained by IANA at
3163   <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
3164</t>
3165<t>
3166   This document defines the following HTTP header fields, so their
3167   associated registry entries shall be updated according to the permanent
3168   registrations below:
3169</t>
3170<?BEGININC p1-messaging.iana-headers ?>
3171<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3172<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3173   <ttcol>Header Field Name</ttcol>
3174   <ttcol>Protocol</ttcol>
3175   <ttcol>Status</ttcol>
3176   <ttcol>Reference</ttcol>
3177
3178   <c>Connection</c>
3179   <c>http</c>
3180   <c>standard</c>
3181   <c>
3182      <xref target="header.connection"/>
3183   </c>
3184   <c>Content-Length</c>
3185   <c>http</c>
3186   <c>standard</c>
3187   <c>
3188      <xref target="header.content-length"/>
3189   </c>
3190   <c>Host</c>
3191   <c>http</c>
3192   <c>standard</c>
3193   <c>
3194      <xref target="header.host"/>
3195   </c>
3196   <c>TE</c>
3197   <c>http</c>
3198   <c>standard</c>
3199   <c>
3200      <xref target="header.te"/>
3201   </c>
3202   <c>Trailer</c>
3203   <c>http</c>
3204   <c>standard</c>
3205   <c>
3206      <xref target="header.trailer"/>
3207   </c>
3208   <c>Transfer-Encoding</c>
3209   <c>http</c>
3210   <c>standard</c>
3211   <c>
3212      <xref target="header.transfer-encoding"/>
3213   </c>
3214   <c>Upgrade</c>
3215   <c>http</c>
3216   <c>standard</c>
3217   <c>
3218      <xref target="header.upgrade"/>
3219   </c>
3220   <c>Via</c>
3221   <c>http</c>
3222   <c>standard</c>
3223   <c>
3224      <xref target="header.via"/>
3225   </c>
3226</texttable>
3227<!--(END)-->
3228<?ENDINC p1-messaging.iana-headers ?>
3229<t>
3230   Furthermore, the header field-name "Close" shall be registered as
3231   "reserved", since using that name as an HTTP header field might
3232   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3233   header field (<xref target="header.connection"/>).
3234</t>
3235<texttable align="left" suppress-title="true">
3236   <ttcol>Header Field Name</ttcol>
3237   <ttcol>Protocol</ttcol>
3238   <ttcol>Status</ttcol>
3239   <ttcol>Reference</ttcol>
3240
3241   <c>Close</c>
3242   <c>http</c>
3243   <c>reserved</c>
3244   <c>
3245      <xref target="header.field.registration"/>
3246   </c>
3247</texttable>
3248<t>
3249   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3250</t>
3251</section>
3252
3253<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3254<t>
3255   IANA maintains the registry of URI Schemes <xref target="BCP115"/> at
3256   <eref target="http://www.iana.org/assignments/uri-schemes.html"/>.
3257</t>
3258<t>
3259   This document defines the following URI schemes, so their
3260   associated registry entries shall be updated according to the permanent
3261   registrations below:
3262</t>
3263<texttable align="left" suppress-title="true">
3264   <ttcol>URI Scheme</ttcol>
3265   <ttcol>Description</ttcol>
3266   <ttcol>Reference</ttcol>
3267
3268   <c>http</c>
3269   <c>Hypertext Transfer Protocol</c>
3270   <c><xref target="http.uri"/></c>
3271
3272   <c>https</c>
3273   <c>Hypertext Transfer Protocol Secure</c>
3274   <c><xref target="https.uri"/></c>
3275</texttable>
3276</section>
3277
3278<section title="Internet Media Type Registration" anchor="internet.media.type.http">
3279<t>
3280   This document serves as the specification for the Internet media types
3281   "message/http" and "application/http". The following is to be registered with
3282   IANA (see <xref target="BCP13"/>).
3283</t>
3284<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
3285<iref item="Media Type" subitem="message/http" primary="true"/>
3286<iref item="message/http Media Type" primary="true"/>
3287<t>
3288   The message/http type can be used to enclose a single HTTP request or
3289   response message, provided that it obeys the MIME restrictions for all
3290   "message" types regarding line length and encodings.
3291</t>
3292<t>
3293  <list style="hanging" x:indent="12em">
3294    <t hangText="Type name:">
3295      message
3296    </t>
3297    <t hangText="Subtype name:">
3298      http
3299    </t>
3300    <t hangText="Required parameters:">
3301      none
3302    </t>
3303    <t hangText="Optional parameters:">
3304      version, msgtype
3305      <list style="hanging">
3306        <t hangText="version:">
3307          The HTTP-version number of the enclosed message
3308          (e.g., "1.1"). If not present, the version can be
3309          determined from the first line of the body.
3310        </t>
3311        <t hangText="msgtype:">
3312          The message type &mdash; "request" or "response". If not
3313          present, the type can be determined from the first
3314          line of the body.
3315        </t>
3316      </list>
3317    </t>
3318    <t hangText="Encoding considerations:">
3319      only "7bit", "8bit", or "binary" are permitted
3320    </t>
3321    <t hangText="Security considerations:">
3322      none
3323    </t>
3324    <t hangText="Interoperability considerations:">
3325      none
3326    </t>
3327    <t hangText="Published specification:">
3328      This specification (see <xref target="internet.media.type.message.http"/>).
3329    </t>
3330    <t hangText="Applications that use this media type:">
3331    </t>
3332    <t hangText="Additional information:">
3333      <list style="hanging">
3334        <t hangText="Magic number(s):">none</t>
3335        <t hangText="File extension(s):">none</t>
3336        <t hangText="Macintosh file type code(s):">none</t>
3337      </list>
3338    </t>
3339    <t hangText="Person and email address to contact for further information:">
3340      See Authors Section.
3341    </t>
3342    <t hangText="Intended usage:">
3343      COMMON
3344    </t>
3345    <t hangText="Restrictions on usage:">
3346      none
3347    </t>
3348    <t hangText="Author:">
3349      See Authors Section.
3350    </t>
3351    <t hangText="Change controller:">
3352      IESG
3353    </t>
3354  </list>
3355</t>
3356</section>
3357<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
3358<iref item="Media Type" subitem="application/http" primary="true"/>
3359<iref item="application/http Media Type" primary="true"/>
3360<t>
3361   The application/http type can be used to enclose a pipeline of one or more
3362   HTTP request or response messages (not intermixed).
3363</t>
3364<t>
3365  <list style="hanging" x:indent="12em">
3366    <t hangText="Type name:">
3367      application
3368    </t>
3369    <t hangText="Subtype name:">
3370      http
3371    </t>
3372    <t hangText="Required parameters:">
3373      none
3374    </t>
3375    <t hangText="Optional parameters:">
3376      version, msgtype
3377      <list style="hanging">
3378        <t hangText="version:">
3379          The HTTP-version number of the enclosed messages
3380          (e.g., "1.1"). If not present, the version can be
3381          determined from the first line of the body.
3382        </t>
3383        <t hangText="msgtype:">
3384          The message type &mdash; "request" or "response". If not
3385          present, the type can be determined from the first
3386          line of the body.
3387        </t>
3388      </list>
3389    </t>
3390    <t hangText="Encoding considerations:">
3391      HTTP messages enclosed by this type
3392      are in "binary" format; use of an appropriate
3393      Content-Transfer-Encoding is required when
3394      transmitted via E-mail.
3395    </t>
3396    <t hangText="Security considerations:">
3397      none
3398    </t>
3399    <t hangText="Interoperability considerations:">
3400      none
3401    </t>
3402    <t hangText="Published specification:">
3403      This specification (see <xref target="internet.media.type.application.http"/>).
3404    </t>
3405    <t hangText="Applications that use this media type:">
3406    </t>
3407    <t hangText="Additional information:">
3408      <list style="hanging">
3409        <t hangText="Magic number(s):">none</t>
3410        <t hangText="File extension(s):">none</t>
3411        <t hangText="Macintosh file type code(s):">none</t>
3412      </list>
3413    </t>
3414    <t hangText="Person and email address to contact for further information:">
3415      See Authors Section.
3416    </t>
3417    <t hangText="Intended usage:">
3418      COMMON
3419    </t>
3420    <t hangText="Restrictions on usage:">
3421      none
3422    </t>
3423    <t hangText="Author:">
3424      See Authors Section.
3425    </t>
3426    <t hangText="Change controller:">
3427      IESG
3428    </t>
3429  </list>
3430</t>
3431</section>
3432</section>
3433
3434<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3435<t>
3436   The HTTP Transfer Coding Registry defines the name space for transfer
3437   coding names.
3438</t>
3439<t>
3440   Registrations &MUST; include the following fields:
3441   <list style="symbols">
3442     <t>Name</t>
3443     <t>Description</t>
3444     <t>Pointer to specification text</t>
3445   </list>
3446</t>
3447<t>
3448   Names of transfer codings &MUST-NOT; overlap with names of content codings
3449   (&content-codings;) unless the encoding transformation is identical, as
3450   is the case for the compression codings defined in
3451   <xref target="compression.codings"/>.
3452</t>
3453<t>
3454   Values to be added to this name space require IETF Review (see
3455   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3456   conform to the purpose of transfer coding defined in this section.
3457   Use of program names for the identification of encoding formats
3458   is not desirable and is discouraged for future encodings.
3459</t>
3460<t>
3461   The registry itself is maintained at
3462   <eref target="http://www.iana.org/assignments/http-parameters"/>.
3463</t>
3464</section>
3465
3466<section title="Transfer Coding Registration" anchor="transfer.coding.registration">
3467<t>
3468   The HTTP Transfer Coding Registry shall be updated with the registrations
3469   below:
3470</t>
3471<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3472   <ttcol>Name</ttcol>
3473   <ttcol>Description</ttcol>
3474   <ttcol>Reference</ttcol>
3475   <c>chunked</c>
3476   <c>Transfer in a series of chunks</c>
3477   <c>
3478      <xref target="chunked.encoding"/>
3479   </c>
3480   <c>compress</c>
3481   <c>UNIX "compress" program method</c>
3482   <c>
3483      <xref target="compress.coding"/>
3484   </c>
3485   <c>deflate</c>
3486   <c>"deflate" compression mechanism (<xref target="RFC1951"/>) used inside
3487   the "zlib" data format (<xref target="RFC1950"/>)
3488   </c>
3489   <c>
3490      <xref target="deflate.coding"/>
3491   </c>
3492   <c>gzip</c>
3493   <c>Same as GNU zip <xref target="RFC1952"/></c>
3494   <c>
3495      <xref target="gzip.coding"/>
3496   </c>
3497</texttable>
3498</section>
3499
3500<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3501<t>
3502   The HTTP Upgrade Token Registry defines the name space for protocol-name
3503   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3504   field. Each registered protocol name is associated with contact information
3505   and an optional set of specifications that details how the connection
3506   will be processed after it has been upgraded.
3507</t>
3508<t>
3509   Registrations happen on a "First Come First Served" basis (see
3510   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3511   following rules:
3512  <list style="numbers">
3513    <t>A protocol-name token, once registered, stays registered forever.</t>
3514    <t>The registration &MUST; name a responsible party for the
3515       registration.</t>
3516    <t>The registration &MUST; name a point of contact.</t>
3517    <t>The registration &MAY; name a set of specifications associated with
3518       that token. Such specifications need not be publicly available.</t>
3519    <t>The registration &SHOULD; name a set of expected "protocol-version"
3520       tokens associated with that token at the time of registration.</t>
3521    <t>The responsible party &MAY; change the registration at any time.
3522       The IANA will keep a record of all such changes, and make them
3523       available upon request.</t>
3524    <t>The IESG &MAY; reassign responsibility for a protocol token.
3525       This will normally only be used in the case when a
3526       responsible party cannot be contacted.</t>
3527  </list>
3528</t>
3529<t>
3530   This registration procedure for HTTP Upgrade Tokens replaces that
3531   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3532</t>
3533</section>
3534
3535<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3536<t>
3537   The HTTP Upgrade Token Registry shall be updated with the registration
3538   below:
3539</t>
3540<texttable align="left" suppress-title="true">
3541   <ttcol>Value</ttcol>
3542   <ttcol>Description</ttcol>
3543   <ttcol>Expected Version Tokens</ttcol>
3544   <ttcol>Reference</ttcol>
3545
3546   <c>HTTP</c>
3547   <c>Hypertext Transfer Protocol</c>
3548   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3549   <c><xref target="http.version"/></c>
3550</texttable>
3551<t>
3552   The responsible party is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3553</t>
3554</section>
3555
3556</section>
3557
3558<section title="Security Considerations" anchor="security.considerations">
3559<t>
3560   This section is meant to inform developers, information providers, and
3561   users of known security concerns relevant to HTTP/1.1 message syntax,
3562   parsing, and routing.
3563</t>
3564
3565<section title="DNS-related Attacks" anchor="dns.related.attacks">
3566<t>
3567   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3568   generally prone to security attacks based on the deliberate misassociation
3569   of IP addresses and DNS names not protected by DNSSEC. Clients need to be
3570   cautious in assuming the validity of an IP number/DNS name association unless
3571   the response is protected by DNSSEC (<xref target="RFC4033"/>).
3572</t>
3573</section>
3574
3575<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3576<t>
3577   By their very nature, HTTP intermediaries are men-in-the-middle, and
3578   represent an opportunity for man-in-the-middle attacks. Compromise of
3579   the systems on which the intermediaries run can result in serious security
3580   and privacy problems. Intermediaries have access to security-related
3581   information, personal information about individual users and
3582   organizations, and proprietary information belonging to users and
3583   content providers. A compromised intermediary, or an intermediary
3584   implemented or configured without regard to security and privacy
3585   considerations, might be used in the commission of a wide range of
3586   potential attacks.
3587</t>
3588<t>
3589   Intermediaries that contain a shared cache are especially vulnerable
3590   to cache poisoning attacks.
3591</t>
3592<t>
3593   Implementers need to consider the privacy and security
3594   implications of their design and coding decisions, and of the
3595   configuration options they provide to operators (especially the
3596   default configuration).
3597</t>
3598<t>
3599   Users need to be aware that intermediaries are no more trustworthy than
3600   the people who run them; HTTP itself cannot solve this problem.
3601</t>
3602</section>
3603
3604<section title="Buffer Overflows" anchor="attack.protocol.element.size.overflows">
3605<t>
3606   Because HTTP uses mostly textual, character-delimited fields, attackers can
3607   overflow buffers in implementations, and/or perform a Denial of Service
3608   against implementations that accept fields with unlimited lengths.
3609</t>
3610<t>
3611   To promote interoperability, this specification makes specific
3612   recommendations for minimum size limits on request-line
3613   (<xref target="request.line"/>)
3614   and blocks of header fields (<xref target="header.fields"/>). These are
3615   minimum recommendations, chosen to be supportable even by implementations
3616   with limited resources; it is expected that most implementations will
3617   choose substantially higher limits.
3618</t>
3619<t>
3620   This specification also provides a way for servers to reject messages that
3621   have request-targets that are too long (&status-414;) or request entities
3622   that are too large (&status-4xx;).
3623</t>
3624<t>
3625   Recipients &SHOULD; carefully limit the extent to which they read other
3626   fields, including (but not limited to) request methods, response status
3627   phrases, header field-names, and body chunks, so as to avoid denial of
3628   service attacks without impeding interoperability.
3629</t>
3630</section>
3631
3632<section title="Message Integrity" anchor="message.integrity">
3633<t>
3634   HTTP does not define a specific mechanism for ensuring message integrity,
3635   instead relying on the error-detection ability of underlying transport
3636   protocols and the use of length or chunk-delimited framing to detect
3637   completeness. Additional integrity mechanisms, such as hash functions or
3638   digital signatures applied to the content, can be selectively added to
3639   messages via extensible metadata header fields. Historically, the lack of
3640   a single integrity mechanism has been justified by the informal nature of
3641   most HTTP communication.  However, the prevalence of HTTP as an information
3642   access mechanism has resulted in its increasing use within environments
3643   where verification of message integrity is crucial.
3644</t>
3645<t>
3646   User agents are encouraged to implement configurable means for detecting
3647   and reporting failures of message integrity such that those means can be
3648   enabled within environments for which integrity is necessary. For example,
3649   a browser being used to view medical history or drug interaction
3650   information needs to indicate to the user when such information is detected
3651   by the protocol to be incomplete, expired, or corrupted during transfer.
3652   Such mechanisms might be selectively enabled via user agent extensions or
3653   the presence of message integrity metadata in a response.
3654   At a minimum, user agents ought to provide some indication that allows a
3655   user to distinguish between a complete and incomplete response message
3656   (<xref target="incomplete.messages"/>) when such verification is desired.
3657</t>
3658</section>
3659
3660<section title="Server Log Information" anchor="abuse.of.server.log.information">
3661<t>
3662   A server is in the position to save personal data about a user's requests
3663   over time, which might identify their reading patterns or subjects of
3664   interest.  In particular, log information gathered at an intermediary
3665   often contains a history of user agent interaction, across a multitude
3666   of sites, that can be traced to individual users.
3667</t>
3668<t>
3669   HTTP log information is confidential in nature; its handling is often
3670   constrained by laws and regulations.  Log information needs to be securely
3671   stored and appropriate guidelines followed for its analysis.
3672   Anonymization of personal information within individual entries helps,
3673   but is generally not sufficient to prevent real log traces from being
3674   re-identified based on correlation with other access characteristics.
3675   As such, access traces that are keyed to a specific client should not
3676   be published even if the key is pseudonymous.
3677</t>
3678<t>
3679   To minimize the risk of theft or accidental publication, log information
3680   should be purged of personally identifiable information, including
3681   user identifiers, IP addresses, and user-provided query parameters,
3682   as soon as that information is no longer necessary to support operational
3683   needs for security, auditing, or fraud control.
3684</t>
3685</section>
3686</section>
3687
3688<section title="Acknowledgments" anchor="acks">
3689<t>
3690   This edition of HTTP/1.1 builds on the many contributions that went into
3691   <xref target="RFC1945" format="none">RFC 1945</xref>,
3692   <xref target="RFC2068" format="none">RFC 2068</xref>,
3693   <xref target="RFC2145" format="none">RFC 2145</xref>, and
3694   <xref target="RFC2616" format="none">RFC 2616</xref>, including
3695   substantial contributions made by the previous authors, editors, and
3696   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
3697   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
3698   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
3699</t>
3700<t>
3701   Since 1999, the following contributors have helped improve the HTTP
3702   specification by reporting bugs, asking smart questions, drafting or
3703   reviewing text, and evaluating open issues:
3704</t>
3705<?BEGININC acks ?>
3706<t>Adam Barth,
3707Adam Roach,
3708Addison Phillips,
3709Adrian Chadd,
3710Adrien W. de Croy,
3711Alan Ford,
3712Alan Ruttenberg,
3713Albert Lunde,
3714Alek Storm,
3715Alex Rousskov,
3716Alexandre Morgaut,
3717Alexey Melnikov,
3718Alisha Smith,
3719Amichai Rothman,
3720Amit Klein,
3721Amos Jeffries,
3722Andreas Maier,
3723Andreas Petersson,
3724Anil Sharma,
3725Anne van Kesteren,
3726Anthony Bryan,
3727Asbjorn Ulsberg,
3728Ashok Kumar,
3729Balachander Krishnamurthy,
3730Barry Leiba,
3731Ben Laurie,
3732Benjamin Niven-Jenkins,
3733Bil Corry,
3734Bill Burke,
3735Bjoern Hoehrmann,
3736Bob Scheifler,
3737Boris Zbarsky,
3738Brett Slatkin,
3739Brian Kell,
3740Brian McBarron,
3741Brian Pane,
3742Brian Raymor,
3743Brian Smith,
3744Bryce Nesbitt,
3745Cameron Heavon-Jones,
3746Carl Kugler,
3747Carsten Bormann,
3748Charles Fry,
3749Chris Newman,
3750Cyrus Daboo,
3751Dale Robert Anderson,
3752Dan Wing,
3753Dan Winship,
3754Daniel Stenberg,
3755Darrel Miller,
3756Dave Cridland,
3757Dave Crocker,
3758Dave Kristol,
3759David Booth,
3760David Singer,
3761David W. Morris,
3762Diwakar Shetty,
3763Dmitry Kurochkin,
3764Drummond Reed,
3765Duane Wessels,
3766Edward Lee,
3767Eitan Adler,
3768Eliot Lear,
3769Eran Hammer-Lahav,
3770Eric D. Williams,
3771Eric J. Bowman,
3772Eric Lawrence,
3773Eric Rescorla,
3774Erik Aronesty,
3775Evan Prodromou,
3776Florian Weimer,
3777Frank Ellermann,
3778Fred Bohle,
3779Frederic Kayser,
3780Gabriel Montenegro,
3781Geoffrey Sneddon,
3782Gervase Markham,
3783Grahame Grieve,
3784Greg Wilkins,
3785Harald Tveit Alvestrand,
3786Harry Halpin,
3787Helge Hess,
3788Henrik Nordstrom,
3789Henry S. Thompson,
3790Henry Story,
3791Herbert van de Sompel,
3792Herve Ruellan,
3793Howard Melman,
3794Hugo Haas,
3795Ian Fette,
3796Ian Hickson,
3797Ido Safruti,
3798Ilari Liusvaara,
3799Ilya Grigorik,
3800Ingo Struck,
3801J. Ross Nicoll,
3802James H. Manger,
3803James Lacey,
3804James M. Snell,
3805Jamie Lokier,
3806Jan Algermissen,
3807Jeff Hodges (who came up with the term 'effective Request-URI'),
3808Jeff Walden,
3809Jim Luther,
3810Jitu Padhye,
3811Joe D. Williams,
3812Joe Gregorio,
3813Joe Orton,
3814John C. Klensin,
3815John C. Mallery,
3816John Cowan,
3817John Kemp,
3818John Panzer,
3819John Schneider,
3820John Stracke,
3821John Sullivan,
3822Jonas Sicking,
3823Jonathan A. Rees,
3824Jonathan Billington,
3825Jonathan Moore,
3826Jonathan Silvera,
3827Jordi Ros,
3828Joris Dobbelsteen,
3829Josh Cohen,
3830Julien Pierre,
3831Jungshik Shin,
3832Justin Chapweske,
3833Justin Erenkrantz,
3834Justin James,
3835Kalvinder Singh,
3836Karl Dubost,
3837Keith Hoffman,
3838Keith Moore,
3839Ken Murchison,
3840Koen Holtman,
3841Konstantin Voronkov,
3842Kris Zyp,
3843Lisa Dusseault,
3844Maciej Stachowiak,
3845Marc Schneider,
3846Marc Slemko,
3847Mark Baker,
3848Mark Pauley,
3849Mark Watson,
3850Markus Isomaki,
3851Markus Lanthaler,
3852Martin J. Duerst,
3853Martin Musatov,
3854Martin Nilsson,
3855Martin Thomson,
3856Matt Lynch,
3857Matthew Cox,
3858Max Clark,
3859Michael Burrows,
3860Michael Hausenblas,
3861Mike Amundsen,
3862Mike Belshe,
3863Mike Kelly,
3864Mike Schinkel,
3865Miles Sabin,
3866Murray S. Kucherawy,
3867Mykyta Yevstifeyev,
3868Nathan Rixham,
3869Nicholas Shanks,
3870Nico Williams,
3871Nicolas Alvarez,
3872Nicolas Mailhot,
3873Noah Slater,
3874Osama Mazahir,
3875Pablo Castro,
3876Pat Hayes,
3877Patrick R. McManus,
3878Paul E. Jones,
3879Paul Hoffman,
3880Paul Marquess,
3881Peter Lepeska,
3882Peter Saint-Andre,
3883Peter Watkins,
3884Phil Archer,
3885Philippe Mougin,
3886Phillip Hallam-Baker,
3887Piotr Dobrogost,
3888Poul-Henning Kamp,
3889Preethi Natarajan,
3890Rajeev Bector,
3891Ray Polk,
3892Reto Bachmann-Gmuer,
3893Richard Cyganiak,
3894Robby Simpson,
3895Robert Brewer,
3896Robert Collins,
3897Robert Mattson,
3898Robert O'Callahan,
3899Robert Olofsson,
3900Robert Sayre,
3901Robert Siemer,
3902Robert de Wilde,
3903Roberto Javier Godoy,
3904Roberto Peon,
3905Roland Zink,
3906Ronny Widjaja,
3907S. Mike Dierken,
3908Salvatore Loreto,
3909Sam Johnston,
3910Sam Ruby,
3911Scott Lawrence (who maintained the original issues list),
3912Sean B. Palmer,
3913Shane McCarron,
3914Stefan Eissing,
3915Stefan Tilkov,
3916Stefanos Harhalakis,
3917Stephane Bortzmeyer,
3918Stephen Farrell,
3919Stephen Ludin,
3920Stuart Williams,
3921Subbu Allamaraju,
3922Sylvain Hellegouarch,
3923Tapan Divekar,
3924Tatsuya Hayashi,
3925Ted Hardie,
3926Thomas Broyer,
3927Thomas Fossati,
3928Thomas Maslen,
3929Thomas Nordin,
3930Thomas Roessler,
3931Tim Bray,
3932Tim Morgan,
3933Tim Olsen,
3934Tom Zhou,
3935Travis Snoozy,
3936Tyler Close,
3937Vincent Murphy,
3938Wenbo Zhu,
3939Werner Baumann,
3940Wilbur Streett,
3941Wilfredo Sanchez Vega,
3942William A. Rowe Jr.,
3943William Chan,
3944Willy Tarreau,
3945Xiaoshu Wang,
3946Yaron Goland,
3947Yngve Nysaeter Pettersen,
3948Yoav Nir,
3949Yogesh Bang,
3950Yutaka Oiwa,
3951Yves Lafon (long-time member of the editor team),
3952Zed A. Shaw, and
3953Zhong Yu.
3954</t>
3955<?ENDINC acks ?>
3956<t>
3957   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
3958   acknowledgements from prior revisions.
3959</t>
3960</section>
3961
3962</middle>
3963<back>
3964
3965<references title="Normative References">
3966
3967<reference anchor="Part2">
3968  <front>
3969    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
3970    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3971      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
3972      <address><email>fielding@gbiv.com</email></address>
3973    </author>
3974    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3975      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3976      <address><email>julian.reschke@greenbytes.de</email></address>
3977    </author>
3978    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3979  </front>
3980  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
3981  <x:source href="p2-semantics.xml" basename="p2-semantics">
3982    <x:defines>1xx (Informational)</x:defines>
3983    <x:defines>1xx</x:defines>
3984    <x:defines>100 (Continue)</x:defines>
3985    <x:defines>101 (Switching Protocols)</x:defines>
3986    <x:defines>2xx (Successful)</x:defines>
3987    <x:defines>2xx</x:defines>
3988    <x:defines>200 (OK)</x:defines>
3989    <x:defines>204 (No Content)</x:defines>
3990    <x:defines>3xx (Redirection)</x:defines>
3991    <x:defines>3xx</x:defines>
3992    <x:defines>301 (Moved Permanently)</x:defines>
3993    <x:defines>4xx (Client Error)</x:defines>
3994    <x:defines>4xx</x:defines>
3995    <x:defines>400 (Bad Request)</x:defines>
3996    <x:defines>411 (Length Required)</x:defines>
3997    <x:defines>414 (URI Too Long)</x:defines>
3998    <x:defines>417 (Expectation Failed)</x:defines>
3999    <x:defines>426 (Upgrade Required)</x:defines>
4000    <x:defines>501 (Not Implemented)</x:defines>
4001    <x:defines>502 (Bad Gateway)</x:defines>
4002    <x:defines>505 (HTTP Version Not Supported)</x:defines>
4003    <x:defines>Allow</x:defines>
4004    <x:defines>Content-Encoding</x:defines>
4005    <x:defines>Content-Location</x:defines>
4006    <x:defines>Content-Type</x:defines>
4007    <x:defines>Date</x:defines>
4008    <x:defines>Expect</x:defines>
4009    <x:defines>Location</x:defines>
4010    <x:defines>Server</x:defines>
4011    <x:defines>User-Agent</x:defines>
4012  </x:source>
4013</reference>
4014
4015<reference anchor="Part4">
4016  <front>
4017    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
4018    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
4019      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4020      <address><email>fielding@gbiv.com</email></address>
4021    </author>
4022    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
4023      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4024      <address><email>julian.reschke@greenbytes.de</email></address>
4025    </author>
4026    <date month="&ID-MONTH;" year="&ID-YEAR;" />
4027  </front>
4028  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
4029  <x:source basename="p4-conditional" href="p4-conditional.xml">
4030    <x:defines>304 (Not Modified)</x:defines>
4031    <x:defines>ETag</x:defines>
4032    <x:defines>Last-Modified</x:defines>
4033  </x:source>
4034</reference>
4035
4036<reference anchor="Part5">
4037  <front>
4038    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
4039    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4040      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4041      <address><email>fielding@gbiv.com</email></address>
4042    </author>
4043    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
4044      <organization abbrev="W3C">World Wide Web Consortium</organization>
4045      <address><email>ylafon@w3.org</email></address>
4046    </author>
4047    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4048      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4049      <address><email>julian.reschke@greenbytes.de</email></address>
4050    </author>
4051    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4052  </front>
4053  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4054  <x:source href="p5-range.xml" basename="p5-range">
4055    <x:defines>Content-Range</x:defines>
4056  </x:source>
4057</reference>
4058
4059<reference anchor="Part6">
4060  <front>
4061    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4062    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4063      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4064      <address><email>fielding@gbiv.com</email></address>
4065    </author>
4066    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4067      <organization>Akamai</organization>
4068      <address><email>mnot@mnot.net</email></address>
4069    </author>
4070    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4071      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4072      <address><email>julian.reschke@greenbytes.de</email></address>
4073    </author>
4074    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4075  </front>
4076  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4077  <x:source href="p6-cache.xml" basename="p6-cache">
4078    <x:defines>Cache-Control</x:defines>
4079    <x:defines>Expires</x:defines>
4080  </x:source>
4081</reference>
4082
4083<reference anchor="Part7">
4084  <front>
4085    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4086    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4087      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4088      <address><email>fielding@gbiv.com</email></address>
4089    </author>
4090    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4091      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4092      <address><email>julian.reschke@greenbytes.de</email></address>
4093    </author>
4094    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4095  </front>
4096  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4097  <x:source href="p7-auth.xml" basename="p7-auth">
4098    <x:defines>Proxy-Authenticate</x:defines>
4099    <x:defines>Proxy-Authorization</x:defines>
4100  </x:source>
4101</reference>
4102
4103<reference anchor="RFC5234">
4104  <front>
4105    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4106    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4107      <organization>Brandenburg InternetWorking</organization>
4108      <address>
4109        <email>dcrocker@bbiw.net</email>
4110      </address> 
4111    </author>
4112    <author initials="P." surname="Overell" fullname="Paul Overell">
4113      <organization>THUS plc.</organization>
4114      <address>
4115        <email>paul.overell@thus.net</email>
4116      </address>
4117    </author>
4118    <date month="January" year="2008"/>
4119  </front>
4120  <seriesInfo name="STD" value="68"/>
4121  <seriesInfo name="RFC" value="5234"/>
4122</reference>
4123
4124<reference anchor="RFC2119">
4125  <front>
4126    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4127    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4128      <organization>Harvard University</organization>
4129      <address><email>sob@harvard.edu</email></address>
4130    </author>
4131    <date month="March" year="1997"/>
4132  </front>
4133  <seriesInfo name="BCP" value="14"/>
4134  <seriesInfo name="RFC" value="2119"/>
4135</reference>
4136
4137<reference anchor="RFC3986">
4138 <front>
4139  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4140  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4141    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4142    <address>
4143       <email>timbl@w3.org</email>
4144       <uri>http://www.w3.org/People/Berners-Lee/</uri>
4145    </address>
4146  </author>
4147  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4148    <organization abbrev="Day Software">Day Software</organization>
4149    <address>
4150      <email>fielding@gbiv.com</email>
4151      <uri>http://roy.gbiv.com/</uri>
4152    </address>
4153  </author>
4154  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4155    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4156    <address>
4157      <email>LMM@acm.org</email>
4158      <uri>http://larry.masinter.net/</uri>
4159    </address>
4160  </author>
4161  <date month='January' year='2005'></date>
4162 </front>
4163 <seriesInfo name="STD" value="66"/>
4164 <seriesInfo name="RFC" value="3986"/>
4165</reference>
4166
4167<reference anchor="RFC0793">
4168  <front>
4169    <title>Transmission Control Protocol</title>
4170    <author initials='J.' surname='Postel' fullname='Jon Postel'>
4171      <organization>University of Southern California (USC)/Information Sciences Institute</organization>
4172    </author>
4173    <date year='1981' month='September' />
4174  </front>
4175  <seriesInfo name='STD' value='7' />
4176  <seriesInfo name='RFC' value='793' />
4177</reference>
4178
4179<reference anchor="USASCII">
4180  <front>
4181    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4182    <author>
4183      <organization>American National Standards Institute</organization>
4184    </author>
4185    <date year="1986"/>
4186  </front>
4187  <seriesInfo name="ANSI" value="X3.4"/>
4188</reference>
4189
4190<reference anchor="RFC1950">
4191  <front>
4192    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4193    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4194      <organization>Aladdin Enterprises</organization>
4195      <address><email>ghost@aladdin.com</email></address>
4196    </author>
4197    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4198    <date month="May" year="1996"/>
4199  </front>
4200  <seriesInfo name="RFC" value="1950"/>
4201  <!--<annotation>
4202    RFC 1950 is an Informational RFC, thus it might be less stable than
4203    this specification. On the other hand, this downward reference was
4204    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4205    therefore it is unlikely to cause problems in practice. See also
4206    <xref target="BCP97"/>.
4207  </annotation>-->
4208</reference>
4209
4210<reference anchor="RFC1951">
4211  <front>
4212    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4213    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4214      <organization>Aladdin Enterprises</organization>
4215      <address><email>ghost@aladdin.com</email></address>
4216    </author>
4217    <date month="May" year="1996"/>
4218  </front>
4219  <seriesInfo name="RFC" value="1951"/>
4220  <!--<annotation>
4221    RFC 1951 is an Informational RFC, thus it might be less stable than
4222    this specification. On the other hand, this downward reference was
4223    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4224    therefore it is unlikely to cause problems in practice. See also
4225    <xref target="BCP97"/>.
4226  </annotation>-->
4227</reference>
4228
4229<reference anchor="RFC1952">
4230  <front>
4231    <title>GZIP file format specification version 4.3</title>
4232    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4233      <organization>Aladdin Enterprises</organization>
4234      <address><email>ghost@aladdin.com</email></address>
4235    </author>
4236    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4237      <address><email>gzip@prep.ai.mit.edu</email></address>
4238    </author>
4239    <author initials="M." surname="Adler" fullname="Mark Adler">
4240      <address><email>madler@alumni.caltech.edu</email></address>
4241    </author>
4242    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4243      <address><email>ghost@aladdin.com</email></address>
4244    </author>
4245    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4246      <address><email>randeg@alumni.rpi.edu</email></address>
4247    </author>
4248    <date month="May" year="1996"/>
4249  </front>
4250  <seriesInfo name="RFC" value="1952"/>
4251  <!--<annotation>
4252    RFC 1952 is an Informational RFC, thus it might be less stable than
4253    this specification. On the other hand, this downward reference was
4254    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4255    therefore it is unlikely to cause problems in practice. See also
4256    <xref target="BCP97"/>.
4257  </annotation>-->
4258</reference>
4259
4260</references>
4261
4262<references title="Informative References">
4263
4264<reference anchor="ISO-8859-1">
4265  <front>
4266    <title>
4267     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4268    </title>
4269    <author>
4270      <organization>International Organization for Standardization</organization>
4271    </author>
4272    <date year="1998"/>
4273  </front>
4274  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4275</reference>
4276
4277<reference anchor='RFC1919'>
4278  <front>
4279    <title>Classical versus Transparent IP Proxies</title>
4280    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4281      <address><email>mchatel@pax.eunet.ch</email></address>
4282    </author>
4283    <date year='1996' month='March' />
4284  </front>
4285  <seriesInfo name='RFC' value='1919' />
4286</reference>
4287
4288<reference anchor="RFC1945">
4289  <front>
4290    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4291    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4292      <organization>MIT, Laboratory for Computer Science</organization>
4293      <address><email>timbl@w3.org</email></address>
4294    </author>
4295    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4296      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4297      <address><email>fielding@ics.uci.edu</email></address>
4298    </author>
4299    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4300      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4301      <address><email>frystyk@w3.org</email></address>
4302    </author>
4303    <date month="May" year="1996"/>
4304  </front>
4305  <seriesInfo name="RFC" value="1945"/>
4306</reference>
4307
4308<reference anchor="RFC2045">
4309  <front>
4310    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4311    <author initials="N." surname="Freed" fullname="Ned Freed">
4312      <organization>Innosoft International, Inc.</organization>
4313      <address><email>ned@innosoft.com</email></address>
4314    </author>
4315    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4316      <organization>First Virtual Holdings</organization>
4317      <address><email>nsb@nsb.fv.com</email></address>
4318    </author>
4319    <date month="November" year="1996"/>
4320  </front>
4321  <seriesInfo name="RFC" value="2045"/>
4322</reference>
4323
4324<reference anchor="RFC2047">
4325  <front>
4326    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4327    <author initials="K." surname="Moore" fullname="Keith Moore">
4328      <organization>University of Tennessee</organization>
4329      <address><email>moore@cs.utk.edu</email></address>
4330    </author>
4331    <date month="November" year="1996"/>
4332  </front>
4333  <seriesInfo name="RFC" value="2047"/>
4334</reference>
4335
4336<reference anchor="RFC2068">
4337  <front>
4338    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4339    <author initials="R." 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="J." surname="Gettys" fullname="Jim Gettys">
4344      <organization>MIT Laboratory for Computer Science</organization>
4345      <address><email>jg@w3.org</email></address>
4346    </author>
4347    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4348      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4349      <address><email>mogul@wrl.dec.com</email></address>
4350    </author>
4351    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4352      <organization>MIT Laboratory for Computer Science</organization>
4353      <address><email>frystyk@w3.org</email></address>
4354    </author>
4355    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4356      <organization>MIT Laboratory for Computer Science</organization>
4357      <address><email>timbl@w3.org</email></address>
4358    </author>
4359    <date month="January" year="1997"/>
4360  </front>
4361  <seriesInfo name="RFC" value="2068"/>
4362</reference>
4363
4364<reference anchor="RFC2145">
4365  <front>
4366    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4367    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4368      <organization>Western Research Laboratory</organization>
4369      <address><email>mogul@wrl.dec.com</email></address>
4370    </author>
4371    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4372      <organization>Department of Information and Computer Science</organization>
4373      <address><email>fielding@ics.uci.edu</email></address>
4374    </author>
4375    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4376      <organization>MIT Laboratory for Computer Science</organization>
4377      <address><email>jg@w3.org</email></address>
4378    </author>
4379    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4380      <organization>W3 Consortium</organization>
4381      <address><email>frystyk@w3.org</email></address>
4382    </author>
4383    <date month="May" year="1997"/>
4384  </front>
4385  <seriesInfo name="RFC" value="2145"/>
4386</reference>
4387
4388<reference anchor="RFC2616">
4389  <front>
4390    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4391    <author initials="R." surname="Fielding" fullname="R. Fielding">
4392      <organization>University of California, Irvine</organization>
4393      <address><email>fielding@ics.uci.edu</email></address>
4394    </author>
4395    <author initials="J." surname="Gettys" fullname="J. Gettys">
4396      <organization>W3C</organization>
4397      <address><email>jg@w3.org</email></address>
4398    </author>
4399    <author initials="J." surname="Mogul" fullname="J. Mogul">
4400      <organization>Compaq Computer Corporation</organization>
4401      <address><email>mogul@wrl.dec.com</email></address>
4402    </author>
4403    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4404      <organization>MIT Laboratory for Computer Science</organization>
4405      <address><email>frystyk@w3.org</email></address>
4406    </author>
4407    <author initials="L." surname="Masinter" fullname="L. Masinter">
4408      <organization>Xerox Corporation</organization>
4409      <address><email>masinter@parc.xerox.com</email></address>
4410    </author>
4411    <author initials="P." surname="Leach" fullname="P. Leach">
4412      <organization>Microsoft Corporation</organization>
4413      <address><email>paulle@microsoft.com</email></address>
4414    </author>
4415    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4416      <organization>W3C</organization>
4417      <address><email>timbl@w3.org</email></address>
4418    </author>
4419    <date month="June" year="1999"/>
4420  </front>
4421  <seriesInfo name="RFC" value="2616"/>
4422</reference>
4423
4424<reference anchor='RFC2817'>
4425  <front>
4426    <title>Upgrading to TLS Within HTTP/1.1</title>
4427    <author initials='R.' surname='Khare' fullname='R. Khare'>
4428      <organization>4K Associates / UC Irvine</organization>
4429      <address><email>rohit@4K-associates.com</email></address>
4430    </author>
4431    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4432      <organization>Agranat Systems, Inc.</organization>
4433      <address><email>lawrence@agranat.com</email></address>
4434    </author>
4435    <date year='2000' month='May' />
4436  </front>
4437  <seriesInfo name='RFC' value='2817' />
4438</reference>
4439
4440<reference anchor='RFC2818'>
4441  <front>
4442    <title>HTTP Over TLS</title>
4443    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4444      <organization>RTFM, Inc.</organization>
4445      <address><email>ekr@rtfm.com</email></address>
4446    </author>
4447    <date year='2000' month='May' />
4448  </front>
4449  <seriesInfo name='RFC' value='2818' />
4450</reference>
4451
4452<reference anchor='RFC3040'>
4453  <front>
4454    <title>Internet Web Replication and Caching Taxonomy</title>
4455    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4456      <organization>Equinix, Inc.</organization>
4457    </author>
4458    <author initials='I.' surname='Melve' fullname='I. Melve'>
4459      <organization>UNINETT</organization>
4460    </author>
4461    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4462      <organization>CacheFlow Inc.</organization>
4463    </author>
4464    <date year='2001' month='January' />
4465  </front>
4466  <seriesInfo name='RFC' value='3040' />
4467</reference>
4468
4469<reference anchor='BCP90'>
4470  <front>
4471    <title>Registration Procedures for Message Header Fields</title>
4472    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4473      <organization>Nine by Nine</organization>
4474      <address><email>GK-IETF@ninebynine.org</email></address>
4475    </author>
4476    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4477      <organization>BEA Systems</organization>
4478      <address><email>mnot@pobox.com</email></address>
4479    </author>
4480    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4481      <organization>HP Labs</organization>
4482      <address><email>JeffMogul@acm.org</email></address>
4483    </author>
4484    <date year='2004' month='September' />
4485  </front>
4486  <seriesInfo name='BCP' value='90' />
4487  <seriesInfo name='RFC' value='3864' />
4488</reference>
4489
4490<reference anchor='RFC4033'>
4491  <front>
4492    <title>DNS Security Introduction and Requirements</title>
4493    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4494    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4495    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4496    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4497    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4498    <date year='2005' month='March' />
4499  </front>
4500  <seriesInfo name='RFC' value='4033' />
4501</reference>
4502
4503<reference anchor="BCP13">
4504  <front>
4505    <title>Media Type Specifications and Registration Procedures</title>
4506    <author initials="N." surname="Freed" fullname="Ned Freed">
4507      <organization>Oracle</organization>
4508      <address>
4509        <email>ned+ietf@mrochek.com</email>
4510      </address>
4511    </author>
4512    <author initials="J." surname="Klensin" fullname="John C. Klensin">
4513      <address>
4514        <email>john+ietf@jck.com</email>
4515      </address>
4516    </author>
4517    <author initials="T." surname="Hansen" fullname="Tony Hansen">
4518      <organization>AT&amp;T Laboratories</organization>
4519      <address>
4520        <email>tony+mtsuffix@maillennium.att.com</email>
4521      </address>
4522    </author>
4523    <date year="2013" month="January"/>
4524  </front>
4525  <seriesInfo name="BCP" value="13"/>
4526  <seriesInfo name="RFC" value="6838"/>
4527</reference>
4528
4529<reference anchor='BCP115'>
4530  <front>
4531    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4532    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4533      <organization>AT&amp;T Laboratories</organization>
4534      <address>
4535        <email>tony+urireg@maillennium.att.com</email>
4536      </address>
4537    </author>
4538    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4539      <organization>Qualcomm, Inc.</organization>
4540      <address>
4541        <email>hardie@qualcomm.com</email>
4542      </address>
4543    </author>
4544    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4545      <organization>Adobe Systems</organization>
4546      <address>
4547        <email>LMM@acm.org</email>
4548      </address>
4549    </author>
4550    <date year='2006' month='February' />
4551  </front>
4552  <seriesInfo name='BCP' value='115' />
4553  <seriesInfo name='RFC' value='4395' />
4554</reference>
4555
4556<reference anchor='RFC4559'>
4557  <front>
4558    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4559    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4560    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4561    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4562    <date year='2006' month='June' />
4563  </front>
4564  <seriesInfo name='RFC' value='4559' />
4565</reference>
4566
4567<reference anchor='RFC5226'>
4568  <front>
4569    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4570    <author initials='T.' surname='Narten' fullname='T. Narten'>
4571      <organization>IBM</organization>
4572      <address><email>narten@us.ibm.com</email></address>
4573    </author>
4574    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4575      <organization>Google</organization>
4576      <address><email>Harald@Alvestrand.no</email></address>
4577    </author>
4578    <date year='2008' month='May' />
4579  </front>
4580  <seriesInfo name='BCP' value='26' />
4581  <seriesInfo name='RFC' value='5226' />
4582</reference>
4583
4584<reference anchor='RFC5246'>
4585   <front>
4586      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4587      <author initials='T.' surname='Dierks' fullname='T. Dierks'>
4588         <organization />
4589      </author>
4590      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4591         <organization>RTFM, Inc.</organization>
4592      </author>
4593      <date year='2008' month='August' />
4594   </front>
4595   <seriesInfo name='RFC' value='5246' />
4596</reference>
4597
4598<reference anchor="RFC5322">
4599  <front>
4600    <title>Internet Message Format</title>
4601    <author initials="P." surname="Resnick" fullname="P. Resnick">
4602      <organization>Qualcomm Incorporated</organization>
4603    </author>
4604    <date year="2008" month="October"/>
4605  </front> 
4606  <seriesInfo name="RFC" value="5322"/>
4607</reference>
4608
4609<reference anchor="RFC6265">
4610  <front>
4611    <title>HTTP State Management Mechanism</title>
4612    <author initials="A." surname="Barth" fullname="Adam Barth">
4613      <organization abbrev="U.C. Berkeley">
4614        University of California, Berkeley
4615      </organization>
4616      <address><email>abarth@eecs.berkeley.edu</email></address>
4617    </author>
4618    <date year="2011" month="April" />
4619  </front>
4620  <seriesInfo name="RFC" value="6265"/>
4621</reference>
4622
4623<!--<reference anchor='BCP97'>
4624  <front>
4625    <title>Handling Normative References to Standards-Track Documents</title>
4626    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
4627      <address>
4628        <email>klensin+ietf@jck.com</email>
4629      </address>
4630    </author>
4631    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
4632      <organization>MIT</organization>
4633      <address>
4634        <email>hartmans-ietf@mit.edu</email>
4635      </address>
4636    </author>
4637    <date year='2007' month='June' />
4638  </front>
4639  <seriesInfo name='BCP' value='97' />
4640  <seriesInfo name='RFC' value='4897' />
4641</reference>-->
4642
4643<reference anchor="Kri2001" target="http://arxiv.org/abs/cs.SE/0105018">
4644  <front>
4645    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
4646    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
4647    <date year="2001" month="November"/>
4648  </front>
4649  <seriesInfo name="ACM Transactions on Internet Technology" value="Vol. 1, #2"/>
4650</reference>
4651
4652</references>
4653
4654
4655<section title="HTTP Version History" anchor="compatibility">
4656<t>
4657   HTTP has been in use by the World-Wide Web global information initiative
4658   since 1990. The first version of HTTP, later referred to as HTTP/0.9,
4659   was a simple protocol for hypertext data transfer across the Internet
4660   with only a single request method (GET) and no metadata.
4661   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
4662   methods and MIME-like messaging that could include metadata about the data
4663   transferred and modifiers on the request/response semantics. However,
4664   HTTP/1.0 did not sufficiently take into consideration the effects of
4665   hierarchical proxies, caching, the need for persistent connections, or
4666   name-based virtual hosts. The proliferation of incompletely-implemented
4667   applications calling themselves "HTTP/1.0" further necessitated a
4668   protocol version change in order for two communicating applications
4669   to determine each other's true capabilities.
4670</t>
4671<t>
4672   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
4673   requirements that enable reliable implementations, adding only
4674   those new features that will either be safely ignored by an HTTP/1.0
4675   recipient or only sent when communicating with a party advertising
4676   conformance with HTTP/1.1.
4677</t>
4678<t>
4679   It is beyond the scope of a protocol specification to mandate
4680   conformance with previous versions. HTTP/1.1 was deliberately
4681   designed, however, to make supporting previous versions easy.
4682   We would expect a general-purpose HTTP/1.1 server to understand
4683   any valid request in the format of HTTP/1.0 and respond appropriately
4684   with an HTTP/1.1 message that only uses features understood (or
4685   safely ignored) by HTTP/1.0 clients.  Likewise, we would expect
4686   an HTTP/1.1 client to understand any valid HTTP/1.0 response.
4687</t>
4688<t>
4689   Since HTTP/0.9 did not support header fields in a request,
4690   there is no mechanism for it to support name-based virtual
4691   hosts (selection of resource by inspection of the <x:ref>Host</x:ref> header
4692   field).  Any server that implements name-based virtual hosts
4693   ought to disable support for HTTP/0.9.  Most requests that
4694   appear to be HTTP/0.9 are, in fact, badly constructed HTTP/1.x
4695   requests wherein a buggy client failed to properly encode
4696   linear whitespace found in a URI reference and placed in
4697   the request-target.
4698</t>
4699
4700<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
4701<t>
4702   This section summarizes major differences between versions HTTP/1.0
4703   and HTTP/1.1.
4704</t>
4705
4706<section title="Multi-homed Web Servers" anchor="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">
4707<t>
4708   The requirements that clients and servers support the <x:ref>Host</x:ref>
4709   header field (<xref target="header.host"/>), report an error if it is
4710   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
4711   are among the most important changes defined by HTTP/1.1.
4712</t>
4713<t>
4714   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4715   addresses and servers; there was no other established mechanism for
4716   distinguishing the intended server of a request than the IP address
4717   to which that request was directed. The <x:ref>Host</x:ref> header field was
4718   introduced during the development of HTTP/1.1 and, though it was
4719   quickly implemented by most HTTP/1.0 browsers, additional requirements
4720   were placed on all HTTP/1.1 requests in order to ensure complete
4721   adoption.  At the time of this writing, most HTTP-based services
4722   are dependent upon the Host header field for targeting requests.
4723</t>
4724</section>
4725
4726<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4727<t>
4728   In HTTP/1.0, each connection is established by the client prior to the
4729   request and closed by the server after sending the response. However, some
4730   implementations implement the explicitly negotiated ("Keep-Alive") version
4731   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
4732   target="RFC2068"/>.
4733</t>
4734<t>
4735   Some clients and servers might wish to be compatible with these previous
4736   approaches to persistent connections, by explicitly negotiating for them
4737   with a "Connection: keep-alive" request header field. However, some
4738   experimental implementations of HTTP/1.0 persistent connections are faulty;
4739   for example, if an HTTP/1.0 proxy server doesn't understand
4740   <x:ref>Connection</x:ref>, it will erroneously forward that header field
4741   to the next inbound server, which would result in a hung connection.
4742</t>
4743<t>
4744   One attempted solution was the introduction of a Proxy-Connection header
4745   field, targeted specifically at proxies. In practice, this was also
4746   unworkable, because proxies are often deployed in multiple layers, bringing
4747   about the same problem discussed above.
4748</t>
4749<t>
4750   As a result, clients are encouraged not to send the Proxy-Connection header
4751   field in any requests.
4752</t>
4753<t>
4754   Clients are also encouraged to consider the use of Connection: keep-alive
4755   in requests carefully; while they can enable persistent connections with
4756   HTTP/1.0 servers, clients using them will need to monitor the
4757   connection for "hung" requests (which indicate that the client ought stop
4758   sending the header field), and this mechanism ought not be used by clients
4759   at all when a proxy is being used.
4760</t>
4761</section>
4762
4763<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
4764<t>
4765   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
4766   (<xref target="header.transfer-encoding"/>).
4767   Transfer codings need to be decoded prior to forwarding an HTTP message
4768   over a MIME-compliant protocol.
4769</t>
4770</section>
4771
4772</section>
4773
4774<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4775<t>
4776  HTTP's approach to error handling has been explained.
4777  (<xref target="conformance"/>)
4778</t>
4779<t>
4780  The expectation to support HTTP/0.9 requests has been removed.
4781</t>
4782<t>
4783  The term "Effective Request URI" has been introduced.
4784  (<xref target="effective.request.uri" />)
4785</t>
4786<t>
4787  HTTP messages can be (and often are) buffered by implementations; despite
4788  it sometimes being available as a stream, HTTP is fundamentally a
4789  message-oriented protocol.
4790  (<xref target="http.message" />)
4791</t>
4792<t>
4793  Minimum supported sizes for various protocol elements have been
4794  suggested, to improve interoperability.
4795</t>
4796<t>
4797  Header fields that span multiple lines ("line folding") are deprecated.
4798  (<xref target="field.parsing" />)
4799</t>
4800<t>
4801  The HTTP-version ABNF production has been clarified to be case-sensitive.
4802  Additionally, version numbers has been restricted to single digits, due
4803  to the fact that implementations are known to handle multi-digit version
4804  numbers incorrectly.
4805  (<xref target="http.version"/>)
4806</t>
4807<t>
4808  The HTTPS URI scheme is now defined by this specification; previously,
4809  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
4810  (<xref target="https.uri"/>)
4811</t>
4812<t>
4813  The HTTPS URI scheme implies end-to-end security.
4814  (<xref target="https.uri"/>)
4815</t>
4816<t>
4817  Userinfo (i.e., username and password) are now disallowed in HTTP and
4818  HTTPS URIs, because of security issues related to their transmission on the
4819  wire.
4820  (<xref target="http.uri" />)
4821</t>
4822<t>
4823  Invalid whitespace around field-names is now required to be rejected,
4824  because accepting it represents a security vulnerability.
4825  (<xref target="header.fields"/>)
4826</t>
4827<t>
4828  The ABNF productions defining header fields now only list the field value.
4829  (<xref target="header.fields"/>)
4830</t>
4831<t>
4832  Rules about implicit linear whitespace between certain grammar productions
4833  have been removed; now whitespace is only allowed where specifically
4834  defined in the ABNF.
4835  (<xref target="whitespace"/>)
4836</t>
4837<t> 
4838  The NUL octet is no longer allowed in comment and quoted-string text, and
4839  handling of backslash-escaping in them has been clarified.
4840  (<xref target="field.components"/>)
4841</t> 
4842<t>
4843  The quoted-pair rule no longer allows escaping control characters other than
4844  HTAB.
4845  (<xref target="field.components"/>)
4846</t>
4847<t>
4848  Non-ASCII content in header fields and the reason phrase has been obsoleted
4849  and made opaque (the TEXT rule was removed).
4850  (<xref target="field.components"/>)
4851</t>
4852<t>
4853  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
4854  handled as errors by recipients.
4855  (<xref target="header.content-length"/>)
4856</t>
4857<t>
4858  The "identity" transfer coding token has been removed.
4859  (Sections <xref format="counter" target="message.body"/> and
4860  <xref format="counter" target="transfer.codings"/>)
4861</t>
4862<t>
4863  The algorithm for determining the message body length has been clarified
4864  to indicate all of the special cases (e.g., driven by methods or status
4865  codes) that affect it, and that new protocol elements cannot define such
4866  special cases.
4867  (<xref target="message.body.length"/>)
4868</t>
4869<t>
4870  "multipart/byteranges" is no longer a way of determining message body length
4871  detection.
4872  (<xref target="message.body.length"/>)
4873</t>
4874<t>
4875  CONNECT is a new, special case in determining message body length.
4876  (<xref target="message.body.length"/>)
4877</t>
4878<t>
4879  Chunk length does not include the count of the octets in the
4880  chunk header and trailer.
4881  (<xref target="chunked.encoding"/>)
4882</t>
4883<t>
4884  Use of chunk extensions is deprecated, and line folding in them is
4885  disallowed.
4886  (<xref target="chunked.encoding"/>)
4887</t>
4888<t>
4889  The segment + query components of RFC3986 have been used to define the
4890  request-target, instead of abs_path from RFC 1808.
4891  (<xref target="request-target"/>)
4892</t>
4893<t>
4894  The asterisk form of the request-target is only allowed in the OPTIONS
4895  method.
4896  (<xref target="request-target"/>)
4897</t>
4898<t>
4899  Exactly when "close" connection options have to be sent has been clarified.
4900  (<xref target="header.connection"/>)
4901</t>
4902<t> 
4903  "hop-by-hop" header fields are required to appear in the Connection header
4904  field; just because they're defined as hop-by-hop in this specification
4905  doesn't exempt them.
4906  (<xref target="header.connection"/>)
4907</t>
4908<t>
4909  The limit of two connections per server has been removed.
4910  (<xref target="persistent.connections"/>)
4911</t>
4912<t>
4913  An idempotent sequence of requests is no longer required to be retried.
4914  (<xref target="persistent.connections"/>)
4915</t>
4916<t>
4917  The requirement to retry requests under certain circumstances when the
4918  server prematurely closes the connection has been removed.
4919  (<xref target="persistent.connections"/>)
4920</t>
4921<t>
4922  Some extraneous requirements about when servers are allowed to close
4923  connections prematurely have been removed.
4924  (<xref target="persistent.connections"/>)
4925</t>
4926<t>
4927  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
4928  responses other than 101 (this was incorporated from <xref
4929  target="RFC2817"/>).
4930  (<xref target="header.upgrade"/>)
4931</t>
4932<t>
4933  Registration of Transfer Codings now requires IETF Review
4934  (<xref target="transfer.coding.registry"/>)
4935</t>
4936<t>
4937  The meaning of the "deflate" content coding has been clarified.
4938  (<xref target="deflate.coding" />)
4939</t>
4940<t>
4941  This specification now defines the Upgrade Token Registry, previously
4942  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
4943  (<xref target="upgrade.token.registry"/>)
4944</t>
4945<t>
4946  Empty list elements in list productions (e.g., a list header containing
4947  ", ,") have been deprecated.
4948  (<xref target="abnf.extension"/>)
4949</t>
4950<t>
4951  Issues with the Keep-Alive and Proxy-Connection headers in requests
4952  are pointed out, with use of the latter being discouraged altogether.
4953  (<xref target="compatibility.with.http.1.0.persistent.connections" />)
4954</t>
4955</section>
4956</section>
4957
4958<section title="ABNF list extension: #rule" anchor="abnf.extension">
4959<t>
4960  A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
4961  improve readability in the definitions of some header field values.
4962</t>
4963<t>
4964  A construct "#" is defined, similar to "*", for defining comma-delimited
4965  lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
4966  at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
4967  comma (",") and optional whitespace (OWS).   
4968</t>
4969<figure><preamble>
4970  Thus,
4971</preamble><artwork type="example">
4972  1#element =&gt; element *( OWS "," OWS element )
4973</artwork></figure>
4974<figure><preamble>
4975  and:
4976</preamble><artwork type="example">
4977  #element =&gt; [ 1#element ]
4978</artwork></figure>
4979<figure><preamble>
4980  and for n &gt;= 1 and m &gt; 1:
4981</preamble><artwork type="example">
4982  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
4983</artwork></figure>
4984<t>
4985  For compatibility with legacy list rules, recipients &SHOULD; accept empty
4986  list elements. In other words, consumers would follow the list productions:
4987</t>
4988<figure><artwork type="example">
4989  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
4990 
4991  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
4992</artwork></figure>
4993<t>
4994  Note that empty elements do not contribute to the count of elements present,
4995  though.
4996</t>
4997<t>
4998  For example, given these ABNF productions:
4999</t>
5000<figure><artwork type="example">
5001  example-list      = 1#example-list-elmt
5002  example-list-elmt = token ; see <xref target="field.components"/> 
5003</artwork></figure>
5004<t>
5005  Then these are valid values for example-list (not including the double
5006  quotes, which are present for delimitation only):
5007</t>
5008<figure><artwork type="example">
5009  "foo,bar"
5010  "foo ,bar,"
5011  "foo , ,bar,charlie   "
5012</artwork></figure>
5013<t>
5014  But these values would be invalid, as at least one non-empty element is
5015  required:
5016</t>
5017<figure><artwork type="example">
5018  ""
5019  ","
5020  ",   ,"
5021</artwork></figure>
5022<t>
5023  <xref target="collected.abnf"/> shows the collected ABNF, with the list rules
5024  expanded as explained above.
5025</t>
5026</section>
5027
5028<?BEGININC p1-messaging.abnf-appendix ?>
5029<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
5030<figure>
5031<artwork type="abnf" name="p1-messaging.parsed-abnf">
5032<x:ref>BWS</x:ref> = OWS
5033
5034<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
5035 connection-option ] )
5036<x:ref>Content-Length</x:ref> = 1*DIGIT
5037
5038<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
5039 ]
5040<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
5041<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
5042<x:ref>Host</x:ref> = uri-host [ ":" port ]
5043
5044<x:ref>OWS</x:ref> = *( SP / HTAB )
5045
5046<x:ref>RWS</x:ref> = 1*( SP / HTAB )
5047
5048<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
5049<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
5050<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
5051 transfer-coding ] )
5052
5053<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
5054<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
5055
5056<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
5057 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
5058 comment ] ) ] )
5059
5060<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
5061<x:ref>absolute-form</x:ref> = absolute-URI
5062<x:ref>absolute-path</x:ref> = 1*( "/" segment )
5063<x:ref>asterisk-form</x:ref> = "*"
5064<x:ref>attribute</x:ref> = token
5065<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
5066<x:ref>authority-form</x:ref> = authority
5067
5068<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
5069<x:ref>chunk-data</x:ref> = 1*OCTET
5070<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
5071<x:ref>chunk-ext-name</x:ref> = token
5072<x:ref>chunk-ext-val</x:ref> = token / quoted-str-nf
5073<x:ref>chunk-size</x:ref> = 1*HEXDIG
5074<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
5075<x:ref>comment</x:ref> = "(" *( ctext / quoted-cpair / comment ) ")"
5076<x:ref>connection-option</x:ref> = token
5077<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
5078 / %x2A-5B ; '*'-'['
5079 / %x5D-7E ; ']'-'~'
5080 / obs-text
5081
5082<x:ref>field-content</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5083<x:ref>field-name</x:ref> = token
5084<x:ref>field-value</x:ref> = *( field-content / obs-fold )
5085
5086<x:ref>header-field</x:ref> = field-name ":" OWS field-value BWS
5087<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ]
5088<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ]
5089
5090<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
5091
5092<x:ref>message-body</x:ref> = *OCTET
5093<x:ref>method</x:ref> = token
5094
5095<x:ref>obs-fold</x:ref> = CRLF ( SP / HTAB )
5096<x:ref>obs-text</x:ref> = %x80-FF
5097<x:ref>origin-form</x:ref> = absolute-path [ "?" query ]
5098
5099<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
5100<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
5101<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
5102<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
5103<x:ref>protocol-name</x:ref> = token
5104<x:ref>protocol-version</x:ref> = token
5105<x:ref>pseudonym</x:ref> = token
5106
5107<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5108 / %x5D-7E ; ']'-'~'
5109 / obs-text
5110<x:ref>qdtext-nf</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5111 / %x5D-7E ; ']'-'~'
5112 / obs-text
5113<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5114<x:ref>quoted-cpair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5115<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5116<x:ref>quoted-str-nf</x:ref> = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
5117<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5118
5119<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5120<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5121<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5122<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5123<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5124<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5125<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5126 asterisk-form
5127
5128<x:ref>segment</x:ref> = &lt;segment, defined in [RFC3986], Section 3.3&gt;
5129<x:ref>special</x:ref> = "(" / ")" / "&lt;" / "&gt;" / "@" / "," / ";" / ":" / "\" /
5130 DQUOTE / "/" / "[" / "]" / "?" / "=" / "{" / "}"
5131<x:ref>start-line</x:ref> = request-line / status-line
5132<x:ref>status-code</x:ref> = 3DIGIT
5133<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5134
5135<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5136<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5137<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5138 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5139<x:ref>token</x:ref> = 1*tchar
5140<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5141<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5142 transfer-extension
5143<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5144<x:ref>transfer-parameter</x:ref> = attribute BWS "=" BWS value
5145
5146<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5147
5148<x:ref>value</x:ref> = word
5149
5150<x:ref>word</x:ref> = token / quoted-string
5151</artwork>
5152</figure>
5153</section>
5154<?ENDINC p1-messaging.abnf-appendix ?>
5155
5156<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5157
5158<section title="Since RFC 2616">
5159<t>
5160  Changes up to the first Working Group Last Call draft are summarized
5161  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-21#appendix-D"/>.
5162</t>
5163</section>
5164
5165<section title="Since draft-ietf-httpbis-p1-messaging-21" anchor="changes.since.21">
5166<t>
5167  Closed issues:
5168  <list style="symbols">
5169    <t>
5170      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/128"/>:
5171      "Cite HTTPS URI scheme definition" (the spec now includes the HTTPs
5172      scheme definition and thus updates RFC 2818)
5173    </t>
5174    <t>
5175      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/389"/>:
5176      "mention of 'proxies' in section about caches"
5177    </t>
5178    <t>
5179      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/390"/>:
5180      "use of ABNF terms from RFC 3986"
5181    </t>
5182    <t>
5183      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/391"/>:
5184      "transferring URIs with userinfo in payload"
5185    </t>
5186    <t>
5187      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/392"/>:
5188      "editorial improvements to message length definition"
5189    </t>
5190    <t>
5191      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/395"/>:
5192      "Connection header field MUST vs SHOULD"
5193    </t>
5194    <t>
5195      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/396"/>:
5196      "editorial improvements to persistent connections section"
5197    </t>
5198    <t>
5199      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/397"/>:
5200      "URI normalization vs empty path"
5201    </t>
5202    <t>
5203      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/408"/>:
5204      "p1 feedback"
5205    </t>
5206    <t>
5207      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/409"/>:
5208      "is parsing OBS-FOLD mandatory?"
5209    </t>
5210    <t>
5211      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/410"/>:
5212      "HTTPS and Shared Caching"
5213    </t>
5214    <t>
5215      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/411"/>:
5216      "Requirements for recipients of ws between start-line and first header field"
5217    </t>
5218    <t>
5219      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/412"/>:
5220      "SP and HT when being tolerant"
5221    </t>
5222    <t>
5223      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/414"/>:
5224      "Message Parsing Strictness"
5225    </t>
5226    <t>
5227      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/415"/>:
5228      "'Render'"
5229    </t>
5230    <t>
5231      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/418"/>:
5232      "No-Transform"
5233    </t>
5234    <t>
5235      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/419"/>:
5236      "p2 editorial feedback"
5237    </t>
5238    <t>
5239      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/420"/>:
5240      "Content-Length SHOULD be sent"
5241    </t>
5242    <t>
5243      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/431"/>:
5244      "origin-form does not allow path starting with "//""
5245    </t>
5246    <t>
5247      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/433"/>:
5248      "ambiguity in part 1 example"
5249    </t>
5250  </list>
5251</t>
5252</section>
5253
5254<section title="Since draft-ietf-httpbis-p1-messaging-22" anchor="changes.since.22">
5255<t>
5256  Closed issues:
5257  <list style="symbols">
5258    <t>
5259      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/435"/>:
5260      "Part1 should have a reference to TCP (RFC 793)"
5261    </t>
5262    <t>
5263      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/438"/>:
5264      "media type registration template issues"
5265    </t>
5266  </list>
5267</t>
5268</section>
5269</section>
5270
5271</back>
5272</rfc>
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