source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2261

Last change on this file since 2261 was 2261, checked in by fielding@…, 7 years ago

(editorial) replace error status code with error response to clarify that we do want clients to stop sending if a premature error is signaled by the server; noted by Zhong Yu in <http://www.w3.org/mid/CACuKZqFMCrbQQx7tb2eAhx68Sz3dAfiVLaoytGxq9=iHxmMmnw@mail.gmail.com>

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 227.2 KB
Line 
1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "May">
16  <!ENTITY ID-YEAR "2013">
17  <!ENTITY mdash "&#8212;">
18  <!ENTITY Note "<x:h xmlns:x='http://purl.org/net/xml2rfc/ext'>Note:</x:h>">
19  <!ENTITY caching-overview       "<xref target='Part6' x:rel='#caching.overview' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY cache-incomplete       "<xref target='Part6' x:rel='#response.cacheability' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY payload                "<xref target='Part2' x:rel='#payload' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY media-type            "<xref target='Part2' x:rel='#media.type' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY content-codings        "<xref target='Part2' x:rel='#content.codings' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY CONNECT                "<xref target='Part2' x:rel='#CONNECT' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY content.negotiation    "<xref target='Part2' x:rel='#content.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY diff-mime              "<xref target='Part2' x:rel='#differences.between.http.and.mime' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY representation         "<xref target='Part2' x:rel='#representations' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY HEAD                   "<xref target='Part2' x:rel='#HEAD' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-allow           "<xref target='Part2' x:rel='#header.allow' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-cache-control   "<xref target='Part6' x:rel='#header.cache-control' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY header-content-encoding    "<xref target='Part2' x:rel='#header.content-encoding' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY header-content-location    "<xref target='Part2' x:rel='#header.content-location' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY header-content-range   "<xref target='Part5' x:rel='#header.content-range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY header-content-type    "<xref target='Part2' x:rel='#header.content-type' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY header-date            "<xref target='Part2' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY header-etag            "<xref target='Part4' x:rel='#header.etag' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY header-expires         "<xref target='Part6' x:rel='#header.expires' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY header-last-modified   "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY header-mime-version    "<xref target='Part2' x:rel='#mime-version' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40  <!ENTITY header-pragma          "<xref target='Part6' x:rel='#header.pragma' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41  <!ENTITY header-proxy-authenticate  "<xref target='Part7' x:rel='#header.proxy-authenticate' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
42  <!ENTITY header-proxy-authorization "<xref target='Part7' x:rel='#header.proxy-authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
43  <!ENTITY header-server          "<xref target='Part2' x:rel='#header.server' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
44  <!ENTITY header-warning         "<xref target='Part6' x:rel='#header.warning' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
45  <!ENTITY idempotent-methods     "<xref target='Part2' x:rel='#idempotent.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
46  <!ENTITY safe-methods           "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
47  <!ENTITY methods                "<xref target='Part2' x:rel='#methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
48  <!ENTITY OPTIONS                "<xref target='Part2' x:rel='#OPTIONS' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
49  <!ENTITY qvalue                 "<xref target='Part2' x:rel='#quality.values' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
50  <!ENTITY resource               "<xref target='Part2' x:rel='#resources' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
51  <!ENTITY status-codes           "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
52  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
53  <!ENTITY status-203             "<xref target='Part2' x:rel='#status.203' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
54  <!ENTITY status-3xx             "<xref target='Part2' x:rel='#status.3xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
55  <!ENTITY status-304             "<xref target='Part4' x:rel='#status.304' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
56  <!ENTITY status-4xx             "<xref target='Part2' x:rel='#status.4xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
57  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
58  <!ENTITY iana-header-registry   "<xref target='Part2' x:rel='#header.field.registry' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
59]>
60<?rfc toc="yes" ?>
61<?rfc symrefs="yes" ?>
62<?rfc sortrefs="yes" ?>
63<?rfc compact="yes"?>
64<?rfc subcompact="no" ?>
65<?rfc linkmailto="no" ?>
66<?rfc editing="no" ?>
67<?rfc comments="yes"?>
68<?rfc inline="yes"?>
69<?rfc rfcedstyle="yes"?>
70<?rfc-ext allow-markup-in-artwork="yes" ?>
71<?rfc-ext include-references-in-index="yes" ?>
72<rfc obsoletes="2145,2616" updates="2817,2818" category="std" x:maturity-level="proposed"
73     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
74     xmlns:x='http://purl.org/net/xml2rfc/ext'>
75<x:link rel="next" basename="p2-semantics"/>
76<x:feedback template="mailto:ietf-http-wg@w3.org?subject={docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
77<front>
78
79  <title abbrev="HTTP/1.1 Message Syntax and Routing">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
80
81  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
82    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
83    <address>
84      <postal>
85        <street>345 Park Ave</street>
86        <city>San Jose</city>
87        <region>CA</region>
88        <code>95110</code>
89        <country>USA</country>
90      </postal>
91      <email>fielding@gbiv.com</email>
92      <uri>http://roy.gbiv.com/</uri>
93    </address>
94  </author>
95
96  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
97    <organization abbrev="greenbytes">greenbytes GmbH</organization>
98    <address>
99      <postal>
100        <street>Hafenweg 16</street>
101        <city>Muenster</city><region>NW</region><code>48155</code>
102        <country>Germany</country>
103      </postal>
104      <email>julian.reschke@greenbytes.de</email>
105      <uri>http://greenbytes.de/tech/webdav/</uri>
106    </address>
107  </author>
108
109  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
110  <workgroup>HTTPbis Working Group</workgroup>
111
112<abstract>
113<t>
114   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
115   distributed, collaborative, hypertext information systems. HTTP has been in
116   use by the World Wide Web global information initiative since 1990.
117   This document provides an overview of HTTP architecture and its associated
118   terminology, defines the "http" and "https" Uniform Resource Identifier
119   (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements,
120   and describes general security concerns for implementations.
121</t>   
122</abstract>
123
124<note title="Editorial Note (To be removed by RFC Editor)">
125  <t>
126    Discussion of this draft takes place on the HTTPBIS working group
127    mailing list (ietf-http-wg@w3.org), which is archived at
128    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
129  </t>
130  <t>
131    The current issues list is at
132    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
133    documents (including fancy diffs) can be found at
134    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
135  </t>
136  <t>
137    The changes in this draft are summarized in <xref target="changes.since.22"/>.
138  </t>
139</note>
140</front>
141<middle>
142<section title="Introduction" anchor="introduction">
143<t>
144   The Hypertext Transfer Protocol (HTTP) is an application-level
145   request/response protocol that uses extensible semantics and self-descriptive
146   message payloads for flexible interaction with network-based hypertext
147   information systems. This document is the first in a series of documents
148   that collectively form the HTTP/1.1 specification:
149   <list style="empty">
150    <t>RFC xxx1: Message Syntax and Routing</t>
151    <t><xref target="Part2" x:fmt="none">RFC xxx2</xref>: Semantics and Content</t>
152    <t><xref target="Part4" x:fmt="none">RFC xxx3</xref>: Conditional Requests</t>
153    <t><xref target="Part5" x:fmt="none">RFC xxx4</xref>: Range Requests</t>
154    <t><xref target="Part6" x:fmt="none">RFC xxx5</xref>: Caching</t>
155    <t><xref target="Part7" x:fmt="none">RFC xxx6</xref>: Authentication</t>
156   </list>
157</t>
158<t>
159   This HTTP/1.1 specification obsoletes and moves to historic status
160   <xref target="RFC2616" x:fmt="none">RFC 2616</xref>, its predecessor
161   <xref target="RFC2068" x:fmt="none">RFC 2068</xref>, and
162   <xref target="RFC2145" x:fmt="none">RFC 2145</xref> (on HTTP versioning).
163   This specification also updates the use of CONNECT to establish a tunnel,
164   previously defined in <xref target="RFC2817" x:fmt="none">RFC 2817</xref>,
165   and defines the "https" URI scheme that was described informally in
166   <xref target="RFC2818" x:fmt="none">RFC 2818</xref>.
167</t>
168<t>
169   HTTP is a generic interface protocol for information systems. It is
170   designed to hide the details of how a service is implemented by presenting
171   a uniform interface to clients that is independent of the types of
172   resources provided. Likewise, servers do not need to be aware of each
173   client's purpose: an HTTP request can be considered in isolation rather
174   than being associated with a specific type of client or a predetermined
175   sequence of application steps. The result is a protocol that can be used
176   effectively in many different contexts and for which implementations can
177   evolve independently over time.
178</t>
179<t>
180   HTTP is also designed for use as an intermediation protocol for translating
181   communication to and from non-HTTP information systems.
182   HTTP proxies and gateways can provide access to alternative information
183   services by translating their diverse protocols into a hypertext
184   format that can be viewed and manipulated by clients in the same way
185   as HTTP services.
186</t>
187<t>
188   One consequence of this flexibility is that the protocol cannot be
189   defined in terms of what occurs behind the interface. Instead, we
190   are limited to defining the syntax of communication, the intent
191   of received communication, and the expected behavior of recipients.
192   If the communication is considered in isolation, then successful
193   actions ought to be reflected in corresponding changes to the
194   observable interface provided by servers. However, since multiple
195   clients might act in parallel and perhaps at cross-purposes, we
196   cannot require that such changes be observable beyond the scope
197   of a single response.
198</t>
199<t>
200   This document describes the architectural elements that are used or
201   referred to in HTTP, defines the "http" and "https" URI schemes,
202   describes overall network operation and connection management,
203   and defines HTTP message framing and forwarding requirements.
204   Our goal is to define all of the mechanisms necessary for HTTP message
205   handling that are independent of message semantics, thereby defining the
206   complete set of requirements for message parsers and
207   message-forwarding intermediaries.
208</t>
209
210
211<section title="Requirement Notation" anchor="intro.requirements">
212<t>
213   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
214   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
215   document are to be interpreted as described in <xref target="RFC2119"/>.
216</t>
217<t>
218   Conformance criteria and considerations regarding error handling
219   are defined in <xref target="conformance"/>.
220</t>
221</section>
222
223<section title="Syntax Notation" anchor="notation">
224<iref primary="true" item="Grammar" subitem="ALPHA"/>
225<iref primary="true" item="Grammar" subitem="CR"/>
226<iref primary="true" item="Grammar" subitem="CRLF"/>
227<iref primary="true" item="Grammar" subitem="CTL"/>
228<iref primary="true" item="Grammar" subitem="DIGIT"/>
229<iref primary="true" item="Grammar" subitem="DQUOTE"/>
230<iref primary="true" item="Grammar" subitem="HEXDIG"/>
231<iref primary="true" item="Grammar" subitem="HTAB"/>
232<iref primary="true" item="Grammar" subitem="LF"/>
233<iref primary="true" item="Grammar" subitem="OCTET"/>
234<iref primary="true" item="Grammar" subitem="SP"/>
235<iref primary="true" item="Grammar" subitem="VCHAR"/>
236<t>
237   This specification uses the Augmented Backus-Naur Form (ABNF) notation
238   of <xref target="RFC5234"/> with the list rule extension defined in
239   <xref target="abnf.extension"/>.  <xref target="collected.abnf"/> shows
240   the collected ABNF with the list rule expanded.
241</t>
242<t anchor="core.rules">
243  <x:anchor-alias value="ALPHA"/>
244  <x:anchor-alias value="CTL"/>
245  <x:anchor-alias value="CR"/>
246  <x:anchor-alias value="CRLF"/>
247  <x:anchor-alias value="DIGIT"/>
248  <x:anchor-alias value="DQUOTE"/>
249  <x:anchor-alias value="HEXDIG"/>
250  <x:anchor-alias value="HTAB"/>
251  <x:anchor-alias value="LF"/>
252  <x:anchor-alias value="OCTET"/>
253  <x:anchor-alias value="SP"/>
254  <x:anchor-alias value="VCHAR"/>
255   The following core rules are included by
256   reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
257   ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
258   DIGIT (decimal 0-9), DQUOTE (double quote),
259   HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed),
260   OCTET (any 8-bit sequence of data), SP (space), and
261   VCHAR (any visible <xref target="USASCII"/> character).
262</t>
263<t>
264   As a convention, ABNF rule names prefixed with "obs-" denote
265   "obsolete" grammar rules that appear for historical reasons.
266</t>
267</section>
268</section>
269
270<section title="Architecture" anchor="architecture">
271<t>
272   HTTP was created for the World Wide Web architecture
273   and has evolved over time to support the scalability needs of a worldwide
274   hypertext system. Much of that architecture is reflected in the terminology
275   and syntax productions used to define HTTP.
276</t>
277
278<section title="Client/Server Messaging" anchor="operation">
279<iref primary="true" item="client"/>
280<iref primary="true" item="server"/>
281<iref primary="true" item="connection"/>
282<t>
283   HTTP is a stateless request/response protocol that operates by exchanging
284   <x:dfn>messages</x:dfn> (<xref target="http.message"/>) across a reliable
285   transport or session-layer
286   "<x:dfn>connection</x:dfn>" (<xref target="connection.management"/>).
287   An HTTP "<x:dfn>client</x:dfn>" is a program that establishes a connection
288   to a server for the purpose of sending one or more HTTP requests.
289   An HTTP "<x:dfn>server</x:dfn>" is a program that accepts connections
290   in order to service HTTP requests by sending HTTP responses.
291</t>
292<iref primary="true" item="user agent"/>
293<iref primary="true" item="origin server"/>
294<iref primary="true" item="browser"/>
295<iref primary="true" item="spider"/>
296<iref primary="true" item="sender"/>
297<iref primary="true" item="recipient"/>
298<t>
299   The terms client and server refer only to the roles that
300   these programs perform for a particular connection.  The same program
301   might act as a client on some connections and a server on others.
302   We use the term "<x:dfn>user agent</x:dfn>" to refer to any of the various
303   client programs that initiate a request, including (but not limited to)
304   browsers, spiders (web-based robots), command-line tools, native
305   applications, and mobile apps.  The term "<x:dfn>origin server</x:dfn>" is
306   used to refer to the program that can originate authoritative responses to
307   a request. For general requirements, we use the terms
308   "<x:dfn>sender</x:dfn>" and "<x:dfn>recipient</x:dfn>" to refer to any
309   component that sends or receives, respectively, a given message.
310</t>
311<t>
312   HTTP relies upon the Uniform Resource Identifier (URI)
313   standard <xref target="RFC3986"/> to indicate the target resource
314   (<xref target="target-resource"/>) and relationships between resources.
315   Messages are passed in a format similar to that used by Internet mail
316   <xref target="RFC5322"/> and the Multipurpose Internet Mail Extensions
317   (MIME) <xref target="RFC2045"/> (see &diff-mime; for the differences
318   between HTTP and MIME messages).
319</t>
320<t>
321   Most HTTP communication consists of a retrieval request (GET) for
322   a representation of some resource identified by a URI.  In the
323   simplest case, this might be accomplished via a single bidirectional
324   connection (===) between the user agent (UA) and the origin server (O).
325</t>
326<figure><artwork type="drawing">
327         request   &gt;
328    <x:highlight>UA</x:highlight> ======================================= <x:highlight>O</x:highlight>
329                                &lt;   response
330</artwork></figure>
331<iref primary="true" item="message"/>
332<iref primary="true" item="request"/>
333<iref primary="true" item="response"/>
334<t>
335   A client sends an HTTP request to a server in the form of a <x:dfn>request</x:dfn>
336   message, beginning with a request-line that includes a method, URI, and
337   protocol version (<xref target="request.line"/>),
338   followed by header fields containing
339   request modifiers, client information, and representation metadata
340   (<xref target="header.fields"/>),
341   an empty line to indicate the end of the header section, and finally
342   a message body containing the payload body (if any,
343   <xref target="message.body"/>).
344</t>
345<t>
346   A server responds to a client's request by sending one or more HTTP
347   <x:dfn>response</x:dfn>
348   messages, each beginning with a status line that
349   includes the protocol version, a success or error code, and textual
350   reason phrase (<xref target="status.line"/>),
351   possibly followed by header fields containing server
352   information, resource metadata, and representation metadata
353   (<xref target="header.fields"/>),
354   an empty line to indicate the end of the header section, and finally
355   a message body containing the payload body (if any,
356   <xref target="message.body"/>).
357</t>
358<t>
359   A connection might be used for multiple request/response exchanges,
360   as defined in <xref target="persistent.connections"/>.
361</t>
362<t>
363   The following example illustrates a typical message exchange for a
364   GET request on the URI "http://www.example.com/hello.txt":
365</t>
366<figure><preamble>
367client request:
368</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
369GET /hello.txt HTTP/1.1
370User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
371Host: www.example.com
372Accept-Language: en, mi
373
374</artwork></figure>
375<figure><preamble>
376server response:
377</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
378HTTP/1.1 200 OK
379Date: Mon, 27 Jul 2009 12:28:53 GMT
380Server: Apache
381Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
382ETag: "34aa387-d-1568eb00"
383Accept-Ranges: bytes
384Content-Length: <x:length-of target="exbody"/>
385Vary: Accept-Encoding
386Content-Type: text/plain
387
388<x:span anchor="exbody">Hello World! My payload includes a trailing CRLF.
389</x:span></artwork>
390</figure>
391</section>
392
393<section title="Implementation Diversity" anchor="implementation-diversity">
394<t>
395   When considering the design of HTTP, it is easy to fall into a trap of
396   thinking that all user agents are general-purpose browsers and all origin
397   servers are large public websites. That is not the case in practice.
398   Common HTTP user agents include household appliances, stereos, scales,
399   firmware update scripts, command-line programs, mobile apps,
400   and communication devices in a multitude of shapes and sizes.  Likewise,
401   common HTTP origin servers include home automation units, configurable
402   networking components, office machines, autonomous robots, news feeds,
403   traffic cameras, ad selectors, and video delivery platforms.
404</t>
405<t>
406   The term "user agent" does not imply that there is a human user directly
407   interacting with the software agent at the time of a request. In many
408   cases, a user agent is installed or configured to run in the background
409   and save its results for later inspection (or save only a subset of those
410   results that might be interesting or erroneous). Spiders, for example, are
411   typically given a start URI and configured to follow certain behavior while
412   crawling the Web as a hypertext graph.
413</t>
414<t>
415   The implementation diversity of HTTP means that we cannot assume the
416   user agent can make interactive suggestions to a user or provide adequate
417   warning for security or privacy options.  In the few cases where this
418   specification requires reporting of errors to the user, it is acceptable
419   for such reporting to only be observable in an error console or log file.
420   Likewise, requirements that an automated action be confirmed by the user
421   before proceeding can be met via advance configuration choices,
422   run-time options, or simply not proceeding with the unsafe action.
423</t>
424</section>
425
426<section title="Intermediaries" anchor="intermediaries">
427<iref primary="true" item="intermediary"/>
428<t>
429   HTTP enables the use of intermediaries to satisfy requests through
430   a chain of connections.  There are three common forms of HTTP
431   <x:dfn>intermediary</x:dfn>: proxy, gateway, and tunnel.  In some cases,
432   a single intermediary might act as an origin server, proxy, gateway,
433   or tunnel, switching behavior based on the nature of each request.
434</t>
435<figure><artwork type="drawing">
436         &gt;             &gt;             &gt;             &gt;
437    <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>
438               &lt;             &lt;             &lt;             &lt;
439</artwork></figure>
440<t>
441   The figure above shows three intermediaries (A, B, and C) between the
442   user agent and origin server. A request or response message that
443   travels the whole chain will pass through four separate connections.
444   Some HTTP communication options
445   might apply only to the connection with the nearest, non-tunnel
446   neighbor, only to the end-points of the chain, or to all connections
447   along the chain. Although the diagram is linear, each participant might
448   be engaged in multiple, simultaneous communications. For example, B
449   might be receiving requests from many clients other than A, and/or
450   forwarding requests to servers other than C, at the same time that it
451   is handling A's request.
452</t>
453<t>
454<iref primary="true" item="upstream"/><iref primary="true" item="downstream"/>
455<iref primary="true" item="inbound"/><iref primary="true" item="outbound"/>
456   We use the terms "<x:dfn>upstream</x:dfn>" and "<x:dfn>downstream</x:dfn>"
457   to describe various requirements in relation to the directional flow of a
458   message: all messages flow from upstream to downstream.
459   Likewise, we use the terms inbound and outbound to refer to
460   directions in relation to the request path:
461   "<x:dfn>inbound</x:dfn>" means toward the origin server and
462   "<x:dfn>outbound</x:dfn>" means toward the user agent.
463</t>
464<t><iref primary="true" item="proxy"/>
465   A "<x:dfn>proxy</x:dfn>" is a message forwarding agent that is selected by the
466   client, usually via local configuration rules, to receive requests
467   for some type(s) of absolute URI and attempt to satisfy those
468   requests via translation through the HTTP interface.  Some translations
469   are minimal, such as for proxy requests for "http" URIs, whereas
470   other requests might require translation to and from entirely different
471   application-level protocols. Proxies are often used to group an
472   organization's HTTP requests through a common intermediary for the
473   sake of security, annotation services, or shared caching.
474</t>
475<t>
476<iref primary="true" item="transforming proxy"/>
477<iref primary="true" item="non-transforming proxy"/>
478   An HTTP-to-HTTP proxy is called a "<x:dfn>transforming proxy</x:dfn>" if it is designed
479   or configured to modify request or response messages in a semantically
480   meaningful way (i.e., modifications, beyond those required by normal
481   HTTP processing, that change the message in a way that would be
482   significant to the original sender or potentially significant to
483   downstream recipients).  For example, a transforming proxy might be
484   acting as a shared annotation server (modifying responses to include
485   references to a local annotation database), a malware filter, a
486   format transcoder, or an intranet-to-Internet privacy filter.  Such
487   transformations are presumed to be desired by the client (or client
488   organization) that selected the proxy and are beyond the scope of
489   this specification.  However, when a proxy is not intended to transform
490   a given message, we use the term "<x:dfn>non-transforming proxy</x:dfn>" to target
491   requirements that preserve HTTP message semantics. See &status-203; and
492   &header-warning; for status and warning codes related to transformations.
493</t>
494<t><iref primary="true" item="gateway"/><iref primary="true" item="reverse proxy"/>
495<iref primary="true" item="accelerator"/>
496   A "<x:dfn>gateway</x:dfn>" (a.k.a., "<x:dfn>reverse proxy</x:dfn>")
497   is a receiving agent that acts
498   as a layer above some other server(s) and translates the received
499   requests to the underlying server's protocol.  Gateways are often
500   used to encapsulate legacy or untrusted information services, to
501   improve server performance through "<x:dfn>accelerator</x:dfn>" caching, and to
502   enable partitioning or load-balancing of HTTP services across
503   multiple machines.
504</t>
505<t>
506   A gateway behaves as an origin server on its outbound connection and
507   as a user agent on its inbound connection.
508   All HTTP requirements applicable to an origin server
509   also apply to the outbound communication of a gateway.
510   A gateway communicates with inbound servers using any protocol that
511   it desires, including private extensions to HTTP that are outside
512   the scope of this specification.  However, an HTTP-to-HTTP gateway
513   that wishes to interoperate with third-party HTTP servers &MUST;
514   conform to HTTP user agent requirements on the gateway's inbound
515   connection and &MUST; implement the <x:ref>Connection</x:ref>
516   (<xref target="header.connection"/>) and <x:ref>Via</x:ref>
517   (<xref target="header.via"/>) header fields for both connections.
518</t>
519<t><iref primary="true" item="tunnel"/>
520   A "<x:dfn>tunnel</x:dfn>" acts as a blind relay between two connections
521   without changing the messages. Once active, a tunnel is not
522   considered a party to the HTTP communication, though the tunnel might
523   have been initiated by an HTTP request. A tunnel ceases to exist when
524   both ends of the relayed connection are closed. Tunnels are used to
525   extend a virtual connection through an intermediary, such as when
526   Transport Layer Security (TLS, <xref target="RFC5246"/>) is used to
527   establish confidential communication through a shared firewall proxy.
528</t>
529<t><iref primary="true" item="interception proxy"/>
530<iref primary="true" item="transparent proxy"/>
531<iref primary="true" item="captive portal"/>
532   The above categories for intermediary only consider those acting as
533   participants in the HTTP communication.  There are also intermediaries
534   that can act on lower layers of the network protocol stack, filtering or
535   redirecting HTTP traffic without the knowledge or permission of message
536   senders. Network intermediaries often introduce security flaws or
537   interoperability problems by violating HTTP semantics.  For example, an
538   "<x:dfn>interception proxy</x:dfn>" <xref target="RFC3040"/> (also commonly
539   known as a "<x:dfn>transparent proxy</x:dfn>" <xref target="RFC1919"/> or
540   "<x:dfn>captive portal</x:dfn>")
541   differs from an HTTP proxy because it is not selected by the client.
542   Instead, an interception proxy filters or redirects outgoing TCP port 80
543   packets (and occasionally other common port traffic).
544   Interception proxies are commonly found on public network access points,
545   as a means of enforcing account subscription prior to allowing use of
546   non-local Internet services, and within corporate firewalls to enforce
547   network usage policies.
548   They are indistinguishable from a man-in-the-middle attack.
549</t>
550<t>
551   HTTP is defined as a stateless protocol, meaning that each request message
552   can be understood in isolation.  Many implementations depend on HTTP's
553   stateless design in order to reuse proxied connections or dynamically
554   load-balance requests across multiple servers.  Hence, servers &MUST-NOT;
555   assume that two requests on the same connection are from the same user
556   agent unless the connection is secured and specific to that agent.
557   Some non-standard HTTP extensions (e.g., <xref target="RFC4559"/>) have
558   been known to violate this requirement, resulting in security and
559   interoperability problems.
560</t>
561</section>
562
563<section title="Caches" anchor="caches">
564<iref primary="true" item="cache"/>
565<t>
566   A "<x:dfn>cache</x:dfn>" is a local store of previous response messages and the
567   subsystem that controls its message storage, retrieval, and deletion.
568   A cache stores cacheable responses in order to reduce the response
569   time and network bandwidth consumption on future, equivalent
570   requests. Any client or server &MAY; employ a cache, though a cache
571   cannot be used by a server while it is acting as a tunnel.
572</t>
573<t>
574   The effect of a cache is that the request/response chain is shortened
575   if one of the participants along the chain has a cached response
576   applicable to that request. The following illustrates the resulting
577   chain if B has a cached copy of an earlier response from O (via C)
578   for a request that has not been cached by UA or A.
579</t>
580<figure><artwork type="drawing">
581            &gt;             &gt;
582       <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>
583                  &lt;             &lt;
584</artwork></figure>
585<t><iref primary="true" item="cacheable"/>
586   A response is "<x:dfn>cacheable</x:dfn>" if a cache is allowed to store a copy of
587   the response message for use in answering subsequent requests.
588   Even when a response is cacheable, there might be additional
589   constraints placed by the client or by the origin server on when
590   that cached response can be used for a particular request. HTTP
591   requirements for cache behavior and cacheable responses are
592   defined in &caching-overview;. 
593</t>
594<t>
595   There are a wide variety of architectures and configurations
596   of caches deployed across the World Wide Web and
597   inside large organizations. These include national hierarchies
598   of proxy caches to save transoceanic bandwidth, collaborative systems that
599   broadcast or multicast cache entries, archives of pre-fetched cache
600   entries for use in off-line or high-latency environments, and so on.
601</t>
602</section>
603
604<section title="Conformance and Error Handling" anchor="conformance">
605<t>
606   This specification targets conformance criteria according to the role of
607   a participant in HTTP communication.  Hence, HTTP requirements are placed
608   on senders, recipients, clients, servers, user agents, intermediaries,
609   origin servers, proxies, gateways, or caches, depending on what behavior
610   is being constrained by the requirement. Additional (social) requirements
611   are placed on implementations, resource owners, and protocol element
612   registrations when they apply beyond the scope of a single communication.
613</t>
614<t>
615   The verb "generate" is used instead of "send" where a requirement
616   differentiates between creating a protocol element and merely forwarding a
617   received element downstream.
618</t>
619<t>
620   An implementation is considered conformant if it complies with all of the
621   requirements associated with the roles it partakes in HTTP.
622</t>
623<t>
624   Conformance applies to both the syntax and semantics of HTTP protocol
625   elements. A sender &MUST-NOT; generate protocol elements that convey a
626   meaning that is known by that sender to be false. A sender &MUST-NOT;
627   generate protocol elements that do not match the grammar defined by the
628   ABNF rules for those protocol elements that are applicable to the sender's
629   role. If a received protocol element is processed, the recipient &MUST; be
630   able to parse any value that would match the ABNF rules for that protocol
631   element, excluding only those rules not applicable to the recipient's role.
632</t>
633<t>
634   Unless noted otherwise, a recipient &MAY; attempt to recover a usable
635   protocol element from an invalid construct.  HTTP does not define
636   specific error handling mechanisms except when they have a direct impact
637   on security, since different applications of the protocol require
638   different error handling strategies.  For example, a Web browser might
639   wish to transparently recover from a response where the
640   <x:ref>Location</x:ref> header field doesn't parse according to the ABNF,
641   whereas a systems control client might consider any form of error recovery
642   to be dangerous.
643</t>
644</section>
645
646<section title="Protocol Versioning" anchor="http.version">
647  <x:anchor-alias value="HTTP-version"/>
648  <x:anchor-alias value="HTTP-name"/>
649<t>
650   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate
651   versions of the protocol. This specification defines version "1.1".
652   The protocol version as a whole indicates the sender's conformance
653   with the set of requirements laid out in that version's corresponding
654   specification of HTTP.
655</t>
656<t>
657   The version of an HTTP message is indicated by an HTTP-version field
658   in the first line of the message. HTTP-version is case-sensitive.
659</t>
660<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-version"/><iref primary="true" item="Grammar" subitem="HTTP-name"/>
661  <x:ref>HTTP-version</x:ref>  = <x:ref>HTTP-name</x:ref> "/" <x:ref>DIGIT</x:ref> "." <x:ref>DIGIT</x:ref>
662  <x:ref>HTTP-name</x:ref>     = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
663</artwork></figure>
664<t>
665   The HTTP version number consists of two decimal digits separated by a "."
666   (period or decimal point).  The first digit ("major version") indicates the
667   HTTP messaging syntax, whereas the second digit ("minor version") indicates
668   the highest minor version to which the sender is
669   conformant and able to understand for future communication.  The minor
670   version advertises the sender's communication capabilities even when the
671   sender is only using a backwards-compatible subset of the protocol,
672   thereby letting the recipient know that more advanced features can
673   be used in response (by servers) or in future requests (by clients).
674</t>
675<t>
676   When an HTTP/1.1 message is sent to an HTTP/1.0 recipient
677   <xref target="RFC1945"/> or a recipient whose version is unknown,
678   the HTTP/1.1 message is constructed such that it can be interpreted
679   as a valid HTTP/1.0 message if all of the newer features are ignored.
680   This specification places recipient-version requirements on some
681   new features so that a conformant sender will only use compatible
682   features until it has determined, through configuration or the
683   receipt of a message, that the recipient supports HTTP/1.1.
684</t>
685<t>
686   The interpretation of a header field does not change between minor
687   versions of the same major HTTP version, though the default
688   behavior of a recipient in the absence of such a field can change.
689   Unless specified otherwise, header fields defined in HTTP/1.1 are
690   defined for all versions of HTTP/1.x.  In particular, the <x:ref>Host</x:ref>
691   and <x:ref>Connection</x:ref> header fields ought to be implemented by all
692   HTTP/1.x implementations whether or not they advertise conformance with
693   HTTP/1.1.
694</t>
695<t>
696   New header fields can be defined such that, when they are
697   understood by a recipient, they might override or enhance the
698   interpretation of previously defined header fields.  When an
699   implementation receives an unrecognized header field, the recipient
700   &MUST; ignore that header field for local processing regardless of
701   the message's HTTP version.  An unrecognized header field received
702   by a proxy &MUST; be forwarded downstream unless the header field's
703   field-name is listed in the message's <x:ref>Connection</x:ref> header field
704   (see <xref target="header.connection"/>).
705   These requirements allow HTTP's functionality to be enhanced without
706   requiring prior update of deployed intermediaries.
707</t>
708<t>
709   Intermediaries that process HTTP messages (i.e., all intermediaries
710   other than those acting as tunnels) &MUST; send their own HTTP-version
711   in forwarded messages.  In other words, they &MUST-NOT; blindly
712   forward the first line of an HTTP message without ensuring that the
713   protocol version in that message matches a version to which that
714   intermediary is conformant for both the receiving and
715   sending of messages.  Forwarding an HTTP message without rewriting
716   the HTTP-version might result in communication errors when downstream
717   recipients use the message sender's version to determine what features
718   are safe to use for later communication with that sender.
719</t>
720<t>
721   An HTTP client &SHOULD; send a request version equal to the highest
722   version to which the client is conformant and
723   whose major version is no higher than the highest version supported
724   by the server, if this is known.  An HTTP client &MUST-NOT; send a
725   version to which it is not conformant.
726</t>
727<t>
728   An HTTP client &MAY; send a lower request version if it is known that
729   the server incorrectly implements the HTTP specification, but only
730   after the client has attempted at least one normal request and determined
731   from the response status or header fields (e.g., <x:ref>Server</x:ref>) that
732   the server improperly handles higher request versions.
733</t>
734<t>
735   An HTTP server &SHOULD; send a response version equal to the highest
736   version to which the server is conformant and
737   whose major version is less than or equal to the one received in the
738   request.  An HTTP server &MUST-NOT; send a version to which it is not
739   conformant.  A server &MAY; send a <x:ref>505 (HTTP Version Not
740   Supported)</x:ref> response if it cannot send a response using the
741   major version used in the client's request.
742</t>
743<t>
744   An HTTP server &MAY; send an HTTP/1.0 response to an HTTP/1.0 request
745   if it is known or suspected that the client incorrectly implements the
746   HTTP specification and is incapable of correctly processing later
747   version responses, such as when a client fails to parse the version
748   number correctly or when an intermediary is known to blindly forward
749   the HTTP-version even when it doesn't conform to the given minor
750   version of the protocol. Such protocol downgrades &SHOULD-NOT; be
751   performed unless triggered by specific client attributes, such as when
752   one or more of the request header fields (e.g., <x:ref>User-Agent</x:ref>)
753   uniquely match the values sent by a client known to be in error.
754</t>
755<t>
756   The intention of HTTP's versioning design is that the major number
757   will only be incremented if an incompatible message syntax is
758   introduced, and that the minor number will only be incremented when
759   changes made to the protocol have the effect of adding to the message
760   semantics or implying additional capabilities of the sender.  However,
761   the minor version was not incremented for the changes introduced between
762   <xref target="RFC2068"/> and <xref target="RFC2616"/>, and this revision
763   has specifically avoided any such changes to the protocol.
764</t>
765</section>
766
767<section title="Uniform Resource Identifiers" anchor="uri">
768<iref primary="true" item="resource"/>
769<t>
770   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used
771   throughout HTTP as the means for identifying resources (&resource;).
772   URI references are used to target requests, indicate redirects, and define
773   relationships.
774</t>
775  <x:anchor-alias value="URI-reference"/>
776  <x:anchor-alias value="absolute-URI"/>
777  <x:anchor-alias value="relative-part"/>
778  <x:anchor-alias value="authority"/>
779  <x:anchor-alias value="path-abempty"/>
780  <x:anchor-alias value="port"/>
781  <x:anchor-alias value="query"/>
782  <x:anchor-alias value="segment"/>
783  <x:anchor-alias value="uri-host"/>
784  <x:anchor-alias value="absolute-path"/>
785  <x:anchor-alias value="partial-URI"/>
786<t>
787   This specification adopts the definitions of "URI-reference",
788   "absolute-URI", "relative-part", "port", "host",
789   "path-abempty", "query", "segment", and "authority" from the
790   URI generic syntax.
791   In addition, we define an "absolute-path" rule (that differs from
792   RFC 3986's "path-absolute" in that it allows a leading "//")
793   and a "partial-URI" rule for protocol elements
794   that allow a relative URI but not a fragment.
795</t>
796<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>
797  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
798  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
799  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
800  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
801  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
802  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
803  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
804  <x:ref>segment</x:ref>       = &lt;segment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
805  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
806 
807  <x:ref>absolute-path</x:ref> = 1*( "/" segment )
808  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
809</artwork></figure>
810<t>
811   Each protocol element in HTTP that allows a URI reference will indicate
812   in its ABNF production whether the element allows any form of reference
813   (URI-reference), only a URI in absolute form (absolute-URI), only the
814   path and optional query components, or some combination of the above.
815   Unless otherwise indicated, URI references are parsed
816   relative to the effective request URI
817   (<xref target="effective.request.uri"/>).
818</t>
819
820<section title="http URI scheme" anchor="http.uri">
821  <x:anchor-alias value="http-URI"/>
822  <iref item="http URI scheme" primary="true"/>
823  <iref item="URI scheme" subitem="http" primary="true"/>
824<t>
825   The "http" URI scheme is hereby defined for the purpose of minting
826   identifiers according to their association with the hierarchical
827   namespace governed by a potential HTTP origin server listening for
828   TCP (<xref target="RFC0793"/>) connections on a given port.
829</t>
830<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"><!--terminal production--></iref>
831  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
832</artwork></figure>
833<t>
834   The HTTP origin server is identified by the generic syntax's
835   <x:ref>authority</x:ref> component, which includes a host identifier
836   and optional TCP port (<xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>).
837   The remainder of the URI, consisting of both the hierarchical path
838   component and optional query component, serves as an identifier for
839   a potential resource within that origin server's name space.
840</t>
841<t>
842   If the host identifier is provided as an IP address,
843   then the origin server is any listener on the indicated TCP port at
844   that IP address. If host is a registered name, then that name is
845   considered an indirect identifier and the recipient might use a name
846   resolution service, such as DNS, to find the address of a listener
847   for that host.
848   The host &MUST-NOT; be empty; if an "http" URI is received with an
849   empty host, then it &MUST; be rejected as invalid.
850   If the port subcomponent is empty or not given, then TCP port 80 is
851   assumed (the default reserved port for WWW services).
852</t>
853<t>
854   Regardless of the form of host identifier, access to that host is not
855   implied by the mere presence of its name or address. The host might or might
856   not exist and, even when it does exist, might or might not be running an
857   HTTP server or listening to the indicated port. The "http" URI scheme
858   makes use of the delegated nature of Internet names and addresses to
859   establish a naming authority (whatever entity has the ability to place
860   an HTTP server at that Internet name or address) and allows that
861   authority to determine which names are valid and how they might be used.
862</t>
863<t>
864   When an "http" URI is used within a context that calls for access to the
865   indicated resource, a client &MAY; attempt access by resolving
866   the host to an IP address, establishing a TCP connection to that address
867   on the indicated port, and sending an HTTP request message
868   (<xref target="http.message"/>) containing the URI's identifying data
869   (<xref target="message.routing"/>) to the server.
870   If the server responds to that request with a non-interim HTTP response
871   message, as described in &status-codes;, then that response
872   is considered an authoritative answer to the client's request.
873</t>
874<t>
875   Although HTTP is independent of the transport protocol, the "http"
876   scheme is specific to TCP-based services because the name delegation
877   process depends on TCP for establishing authority.
878   An HTTP service based on some other underlying connection protocol
879   would presumably be identified using a different URI scheme, just as
880   the "https" scheme (below) is used for resources that require an
881   end-to-end secured connection. Other protocols might also be used to
882   provide access to "http" identified resources &mdash; it is only the
883   authoritative interface used for mapping the namespace that is
884   specific to TCP.
885</t>
886<t>
887   The URI generic syntax for authority also includes a deprecated
888   userinfo subcomponent (<xref target="RFC3986" x:fmt="," x:sec="3.2.1"/>)
889   for including user authentication information in the URI.  Some
890   implementations make use of the userinfo component for internal
891   configuration of authentication information, such as within command
892   invocation options, configuration files, or bookmark lists, even
893   though such usage might expose a user identifier or password.
894   Senders &MUST; exclude the userinfo subcomponent (and its "@"
895   delimiter) when an "http" URI is transmitted within a message as a
896   request target or header field value.
897   Recipients of an "http" URI reference &SHOULD; parse for userinfo and
898   treat its presence as an error, since it is likely being used to obscure
899   the authority for the sake of phishing attacks.
900</t>
901</section>
902
903<section title="https URI scheme" anchor="https.uri">
904   <x:anchor-alias value="https-URI"/>
905   <iref item="https URI scheme"/>
906   <iref item="URI scheme" subitem="https"/>
907<t>
908   The "https" URI scheme is hereby defined for the purpose of minting
909   identifiers according to their association with the hierarchical
910   namespace governed by a potential HTTP origin server listening to a
911   given TCP port for TLS-secured connections
912   (<xref target="RFC0793"/>, <xref target="RFC5246"/>).
913</t>
914<t>
915   All of the requirements listed above for the "http" scheme are also
916   requirements for the "https" scheme, except that a default TCP port
917   of 443 is assumed if the port subcomponent is empty or not given,
918   and the TCP connection &MUST; be secured, end-to-end, through the
919   use of strong encryption prior to sending the first HTTP request.
920</t>
921<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="https-URI"><!--terminal production--></iref>
922  <x:ref>https-URI</x:ref> = "https:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
923</artwork></figure>
924<t>
925   Resources made available via the "https" scheme have no shared
926   identity with the "http" scheme even if their resource identifiers
927   indicate the same authority (the same host listening to the same
928   TCP port).  They are distinct name spaces and are considered to be
929   distinct origin servers.  However, an extension to HTTP that is
930   defined to apply to entire host domains, such as the Cookie protocol
931   <xref target="RFC6265"/>, can allow information
932   set by one service to impact communication with other services
933   within a matching group of host domains.
934</t>
935<t>
936   The process for authoritative access to an "https" identified
937   resource is defined in <xref target="RFC2818"/>.
938</t>
939</section>
940
941<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
942<t>
943   Since the "http" and "https" schemes conform to the URI generic syntax,
944   such URIs are normalized and compared according to the algorithm defined
945   in <xref target="RFC3986" x:fmt="," x:sec="6"/>, using the defaults
946   described above for each scheme.
947</t>
948<t>
949   If the port is equal to the default port for a scheme, the normal form is
950   to elide the port subcomponent. When not being used in absolute form as the
951   request target of an OPTIONS request, an empty path component is equivalent
952   to an absolute path of "/", so the normal form is to provide a path of "/"
953   instead. The scheme and host are case-insensitive and normally provided in
954   lowercase; all other components are compared in a case-sensitive manner.
955   Characters other than those in the "reserved" set are equivalent to their
956   percent-encoded octets (see <xref target="RFC3986" x:fmt=","
957   x:sec="2.1"/>): the normal form is to not encode them.
958</t>
959<t>
960   For example, the following three URIs are equivalent:
961</t>
962<figure><artwork type="example">
963   http://example.com:80/~smith/home.html
964   http://EXAMPLE.com/%7Esmith/home.html
965   http://EXAMPLE.com:/%7esmith/home.html
966</artwork></figure>
967</section>
968</section>
969</section>
970
971<section title="Message Format" anchor="http.message">
972<x:anchor-alias value="generic-message"/>
973<x:anchor-alias value="message.types"/>
974<x:anchor-alias value="HTTP-message"/>
975<x:anchor-alias value="start-line"/>
976<iref item="header section"/>
977<iref item="headers"/>
978<iref item="header field"/>
979<t>
980   All HTTP/1.1 messages consist of a start-line followed by a sequence of
981   octets in a format similar to the Internet Message Format
982   <xref target="RFC5322"/>: zero or more header fields (collectively
983   referred to as the "headers" or the "header section"), an empty line
984   indicating the end of the header section, and an optional message body.
985</t>
986<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"><!--terminal production--></iref>
987  <x:ref>HTTP-message</x:ref>   = <x:ref>start-line</x:ref>
988                   *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
989                   <x:ref>CRLF</x:ref>
990                   [ <x:ref>message-body</x:ref> ]
991</artwork></figure>
992<t>
993   The normal procedure for parsing an HTTP message is to read the
994   start-line into a structure, read each header field into a hash
995   table by field name until the empty line, and then use the parsed
996   data to determine if a message body is expected.  If a message body
997   has been indicated, then it is read as a stream until an amount
998   of octets equal to the message body length is read or the connection
999   is closed.
1000</t>
1001<t>
1002   Recipients &MUST; parse an HTTP message as a sequence of octets in an
1003   encoding that is a superset of US-ASCII <xref target="USASCII"/>.
1004   Parsing an HTTP message as a stream of Unicode characters, without regard
1005   for the specific encoding, creates security vulnerabilities due to the
1006   varying ways that string processing libraries handle invalid multibyte
1007   character sequences that contain the octet LF (%x0A).  String-based
1008   parsers can only be safely used within protocol elements after the element
1009   has been extracted from the message, such as within a header field-value
1010   after message parsing has delineated the individual fields.
1011</t>
1012<t>
1013   An HTTP message can be parsed as a stream for incremental processing or
1014   forwarding downstream.  However, recipients cannot rely on incremental
1015   delivery of partial messages, since some implementations will buffer or
1016   delay message forwarding for the sake of network efficiency, security
1017   checks, or payload transformations.
1018</t>
1019<t>
1020   A sender &MUST-NOT; send whitespace between the start-line and
1021   the first header field.
1022   A recipient that receives whitespace between the start-line and
1023   the first header field &MUST; either reject the message as invalid or
1024   consume each whitespace-preceded line without further processing of it
1025   (i.e., ignore the entire line, along with any subsequent lines preceded
1026   by whitespace, until a properly formed header field is received or the
1027   header block is terminated).
1028</t>
1029<t>
1030   The presence of such whitespace in a request
1031   might be an attempt to trick a server into ignoring that field or
1032   processing the line after it as a new request, either of which might
1033   result in a security vulnerability if other implementations within
1034   the request chain interpret the same message differently.
1035   Likewise, the presence of such whitespace in a response might be
1036   ignored by some clients or cause others to cease parsing.
1037</t>
1038
1039<section title="Start Line" anchor="start.line">
1040  <x:anchor-alias value="Start-Line"/>
1041<t>
1042   An HTTP message can either be a request from client to server or a
1043   response from server to client.  Syntactically, the two types of message
1044   differ only in the start-line, which is either a request-line (for requests)
1045   or a status-line (for responses), and in the algorithm for determining
1046   the length of the message body (<xref target="message.body"/>).
1047</t>
1048<t>
1049   In theory, a client could receive requests and a server could receive
1050   responses, distinguishing them by their different start-line formats,
1051   but in practice servers are implemented to only expect a request
1052   (a response is interpreted as an unknown or invalid request method)
1053   and clients are implemented to only expect a response.
1054</t>
1055<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="start-line"/>
1056  <x:ref>start-line</x:ref>     = <x:ref>request-line</x:ref> / <x:ref>status-line</x:ref>
1057</artwork></figure>
1058
1059<section title="Request Line" anchor="request.line">
1060  <x:anchor-alias value="Request"/>
1061  <x:anchor-alias value="request-line"/>
1062<t>
1063   A request-line begins with a method token, followed by a single
1064   space (SP), the request-target, another single space (SP), the
1065   protocol version, and ending with CRLF.
1066</t>
1067<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1068  <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>
1069</artwork></figure>
1070<iref primary="true" item="method"/>
1071<t anchor="method">
1072   The method token indicates the request method to be performed on the
1073   target resource. The request method is case-sensitive.
1074</t>
1075<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1076  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1077</artwork></figure>
1078<t>
1079   The methods defined by this specification can be found in
1080   &methods;, along with information regarding the HTTP method registry
1081   and considerations for defining new methods.
1082</t>
1083<iref item="request-target"/>
1084<t>
1085   The request-target identifies the target resource upon which to apply
1086   the request, as defined in <xref target="request-target"/>.
1087</t>
1088<t>
1089   Recipients typically parse the request-line into its component parts by
1090   splitting on whitespace (see <xref target="message.robustness"/>), since
1091   no whitespace is allowed in the three components.
1092   Unfortunately, some user agents fail to properly encode or exclude
1093   whitespace found in hypertext references, resulting in those disallowed
1094   characters being sent in a request-target.
1095</t>
1096<t>
1097   Recipients of an invalid request-line &SHOULD; respond with either a
1098   <x:ref>400 (Bad Request)</x:ref> error or a <x:ref>301 (Moved Permanently)</x:ref>
1099   redirect with the request-target properly encoded.  Recipients &SHOULD-NOT;
1100   attempt to autocorrect and then process the request without a redirect,
1101   since the invalid request-line might be deliberately crafted to bypass
1102   security filters along the request chain.
1103</t>
1104<t>
1105   HTTP does not place a pre-defined limit on the length of a request-line.
1106   A server that receives a method longer than any that it implements
1107   &SHOULD; respond with a <x:ref>501 (Not Implemented)</x:ref> status code.
1108   A server &MUST; be prepared to receive URIs of unbounded length and
1109   respond with the <x:ref>414 (URI Too Long)</x:ref> status code if the received
1110   request-target would be longer than the server wishes to handle
1111   (see &status-414;).
1112</t>
1113<t>
1114   Various ad-hoc limitations on request-line length are found in practice.
1115   It is &RECOMMENDED; that all HTTP senders and recipients support, at a
1116   minimum, request-line lengths of 8000 octets.
1117</t>
1118</section>
1119
1120<section title="Status Line" anchor="status.line">
1121  <x:anchor-alias value="response"/>
1122  <x:anchor-alias value="status-line"/>
1123  <x:anchor-alias value="status-code"/>
1124  <x:anchor-alias value="reason-phrase"/>
1125<t>
1126   The first line of a response message is the status-line, consisting
1127   of the protocol version, a space (SP), the status code, another space,
1128   a possibly-empty textual phrase describing the status code, and
1129   ending with CRLF.
1130</t>
1131<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-line"/>
1132  <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>
1133</artwork></figure>
1134<t>
1135   The status-code element is a 3-digit integer code describing the
1136   result of the server's attempt to understand and satisfy the client's
1137   corresponding request. The rest of the response message is to be
1138   interpreted in light of the semantics defined for that status code.
1139   See &status-codes; for information about the semantics of status codes,
1140   including the classes of status code (indicated by the first digit),
1141   the status codes defined by this specification, considerations for the
1142   definition of new status codes, and the IANA registry.
1143</t>
1144<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-code"/>
1145  <x:ref>status-code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1146</artwork></figure>
1147<t>  
1148   The reason-phrase element exists for the sole purpose of providing a
1149   textual description associated with the numeric status code, mostly
1150   out of deference to earlier Internet application protocols that were more
1151   frequently used with interactive text clients. A client &SHOULD; ignore
1152   the reason-phrase content.
1153</t>
1154<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
1155  <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> )
1156</artwork></figure>
1157</section>
1158</section>
1159
1160<section title="Header Fields" anchor="header.fields">
1161  <x:anchor-alias value="header-field"/>
1162  <x:anchor-alias value="field-content"/>
1163  <x:anchor-alias value="field-name"/>
1164  <x:anchor-alias value="field-value"/>
1165  <x:anchor-alias value="obs-fold"/>
1166<t>
1167   Each HTTP header field consists of a case-insensitive field name
1168   followed by a colon (":"), optional whitespace, and the field value.
1169</t>
1170<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"/>
1171  <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>
1172  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1173  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>obs-fold</x:ref> )
1174  <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> )
1175  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref> ( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1176                 ; obsolete line folding
1177                 ; see <xref target="field.parsing"/>
1178</artwork></figure>
1179<t>
1180   The field-name token labels the corresponding field-value as having the
1181   semantics defined by that header field.  For example, the <x:ref>Date</x:ref>
1182   header field is defined in &header-date; as containing the origination
1183   timestamp for the message in which it appears.
1184</t>
1185
1186<section title="Field Extensibility" anchor="field.extensibility">
1187<t>
1188   HTTP header fields are fully extensible: there is no limit on the
1189   introduction of new field names, each presumably defining new semantics,
1190   nor on the number of header fields used in a given message.  Existing
1191   fields are defined in each part of this specification and in many other
1192   specifications outside the core standard.
1193   New header fields can be introduced without changing the protocol version
1194   if their defined semantics allow them to be safely ignored by recipients
1195   that do not recognize them.
1196</t>
1197<t>
1198   New HTTP header fields ought to be registered with IANA in the
1199   Message Header Field Registry, as described in &iana-header-registry;.
1200   A proxy &MUST; forward unrecognized header fields unless the
1201   field-name is listed in the <x:ref>Connection</x:ref> header field
1202   (<xref target="header.connection"/>) or the proxy is specifically
1203   configured to block, or otherwise transform, such fields.
1204   Other recipients &SHOULD; ignore unrecognized header fields.
1205</t>
1206</section>
1207
1208<section title="Field Order" anchor="field.order">
1209<t>
1210   The order in which header fields with differing field names are
1211   received is not significant. However, it is "good practice" to send
1212   header fields that contain control data first, such as <x:ref>Host</x:ref>
1213   on requests and <x:ref>Date</x:ref> on responses, so that implementations
1214   can decide when not to handle a message as early as possible.  A server
1215   &MUST; wait until the entire header section is received before interpreting
1216   a request message, since later header fields might include conditionals,
1217   authentication credentials, or deliberately misleading duplicate
1218   header fields that would impact request processing.
1219</t>
1220<t>
1221   A sender &MUST-NOT; generate multiple header fields with the same field
1222   name in a message unless either the entire field value for that
1223   header field is defined as a comma-separated list [i.e., #(values)]
1224   or the header field is a well-known exception (as noted below).
1225</t>
1226<t>
1227   Multiple header fields with the same field name can be combined into
1228   one "field-name: field-value" pair, without changing the semantics of the
1229   message, by appending each subsequent field value to the combined
1230   field value in order, separated by a comma. The order in which
1231   header fields with the same field name are received is therefore
1232   significant to the interpretation of the combined field value;
1233   a proxy &MUST-NOT; change the order of these field values when
1234   forwarding a message.
1235</t>
1236<x:note>
1237  <t>
1238   &Note; In practice, the "Set-Cookie" header field (<xref target="RFC6265"/>)
1239   often appears multiple times in a response message and does not use the
1240   list syntax, violating the above requirements on multiple header fields
1241   with the same name. Since it cannot be combined into a single field-value,
1242   recipients ought to handle "Set-Cookie" as a special case while processing
1243   header fields. (See Appendix A.2.3 of <xref target="Kri2001"/> for details.)
1244  </t>
1245</x:note>
1246</section>
1247
1248<section title="Whitespace" anchor="whitespace">
1249<t anchor="rule.LWS">
1250   This specification uses three rules to denote the use of linear
1251   whitespace: OWS (optional whitespace), RWS (required whitespace), and
1252   BWS ("bad" whitespace).
1253</t>
1254<t anchor="rule.OWS">
1255   The OWS rule is used where zero or more linear whitespace octets might
1256   appear. For protocol elements where optional whitespace is preferred to
1257   improve readability, a sender &SHOULD; generate the optional whitespace
1258   as a single SP; otherwise, a sender &SHOULD-NOT; generate optional
1259   whitespace except as needed to white-out invalid or unwanted protocol
1260   elements during in-place message filtering.
1261</t>
1262<t anchor="rule.RWS">
1263   The RWS rule is used when at least one linear whitespace octet is required
1264   to separate field tokens. A sender &SHOULD; generate RWS as a single SP.
1265</t>
1266<t anchor="rule.BWS">
1267   The BWS rule is used where the grammar allows optional whitespace only for
1268   historical reasons. A sender &MUST-NOT; generate BWS in messages.
1269   A recipient &MUST; parse for such bad whitespace and remove it before
1270   interpreting the protocol element.
1271</t>
1272<t anchor="rule.whitespace">
1273  <x:anchor-alias value="BWS"/>
1274  <x:anchor-alias value="OWS"/>
1275  <x:anchor-alias value="RWS"/>
1276</t>
1277<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"/>
1278  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1279                 ; optional whitespace
1280  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1281                 ; required whitespace
1282  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1283                 ; "bad" whitespace
1284</artwork></figure>
1285</section>
1286
1287<section title="Field Parsing" anchor="field.parsing">
1288<t>
1289   No whitespace is allowed between the header field-name and colon.
1290   In the past, differences in the handling of such whitespace have led to
1291   security vulnerabilities in request routing and response handling.
1292   A server &MUST; reject any received request message that contains
1293   whitespace between a header field-name and colon with a response code of
1294   <x:ref>400 (Bad Request)</x:ref>. A proxy &MUST; remove any such whitespace
1295   from a response message before forwarding the message downstream.
1296</t>
1297<t>
1298   A field value is preceded by optional whitespace (OWS); a single SP is
1299   preferred. The field value does not include any leading or trailing white
1300   space: OWS occurring before the first non-whitespace octet of the
1301   field value or after the last non-whitespace octet of the field value
1302   is ignored and &SHOULD; be removed before further processing (as this does
1303   not change the meaning of the header field).
1304</t>
1305<t>
1306   A recipient of field-content containing multiple sequential octets of
1307   optional (OWS) or required (RWS) whitespace &SHOULD; either replace the
1308   sequence with a single SP or transform any non-SP octets in the sequence to
1309   SP octets before interpreting the field value or forwarding the message
1310   downstream.
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.
1322</t>
1323<t>
1324   A server that receives an <x:ref>obs-fold</x:ref> in a request message that
1325   is not within a message/http container &MUST; either reject the message by
1326   sending a <x:ref>400 (Bad Request)</x:ref>, preferably with a
1327   representation explaining that obsolete line folding is unacceptable, or
1328   replace each received <x:ref>obs-fold</x:ref> with one or more
1329   <x:ref>SP</x:ref> octets prior to interpreting the field value or
1330   forwarding the message downstream.
1331</t>
1332<t>
1333   A proxy or gateway that receives an <x:ref>obs-fold</x:ref> in a response
1334   message that is not within a message/http container &MUST; either discard
1335   the message and replace it with a <x:ref>502 (Bad Gateway)</x:ref>
1336   response, preferably with a representation explaining that unacceptable
1337   line folding was received, or replace each received <x:ref>obs-fold</x:ref>
1338   with one or more <x:ref>SP</x:ref> octets prior to interpreting the field
1339   value or forwarding the message downstream.
1340</t>
1341<t>
1342   A user agent that receives an <x:ref>obs-fold</x:ref> in a response message
1343   that is not within a message/http container &MUST; replace each received
1344   <x:ref>obs-fold</x:ref> with one or more <x:ref>SP</x:ref> octets prior to
1345   interpreting the field value.
1346</t>
1347<t>
1348   Historically, HTTP has allowed field content with text in the ISO-8859-1
1349   <xref target="ISO-8859-1"/> charset, supporting other charsets only
1350   through use of <xref target="RFC2047"/> encoding.
1351   In practice, most HTTP header field values use only a subset of the
1352   US-ASCII charset <xref target="USASCII"/>. Newly defined
1353   header fields &SHOULD; limit their field values to US-ASCII octets.
1354   Recipients &SHOULD; treat other octets in field content (obs-text) as
1355   opaque data.
1356</t>
1357</section>
1358
1359<section title="Field Limits" anchor="field.limits">
1360<t>
1361   HTTP does not place a pre-defined limit on the length of each header field
1362   or on the length of the header block as a whole.  Various ad-hoc
1363   limitations on individual header field length are found in practice,
1364   often depending on the specific field semantics.
1365</t>
1366<t>
1367   A server &MUST; be prepared to receive request header fields of unbounded
1368   length and respond with an appropriate <x:ref>4xx (Client Error)</x:ref>
1369   status code if the received header field(s) are larger than the server
1370   wishes to process.
1371</t>
1372<t>
1373   A client &MUST; be prepared to receive response header fields of unbounded
1374   length. A client &MAY; discard or truncate received header fields that are
1375   larger than the client wishes to process if the field semantics are such
1376   that the dropped value(s) can be safely ignored without changing the
1377   response semantics.
1378</t>
1379</section>
1380
1381<section title="Field value components" anchor="field.components">
1382<t anchor="rule.token.separators">
1383  <x:anchor-alias value="tchar"/>
1384  <x:anchor-alias value="token"/>
1385  <x:anchor-alias value="special"/>
1386  <x:anchor-alias value="word"/>
1387   Many HTTP header field values consist of words (token or quoted-string)
1388   separated by whitespace or special characters. These special characters
1389   &MUST; be in a quoted string to be used within a parameter value (as defined
1390   in <xref target="transfer.codings"/>).
1391</t>
1392<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>
1393  <x:ref>word</x:ref>           = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1394
1395  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1396<!--
1397  IMPORTANT: when editing "tchar" make sure that "special" is updated accordingly!!!
1398 -->
1399  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1400                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1401                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1402                 ; any <x:ref>VCHAR</x:ref>, except <x:ref>special</x:ref>
1403
1404  <x:ref>special</x:ref>        = "(" / ")" / "&lt;" / ">" / "@" / ","
1405                 / ";" / ":" / "\" / DQUOTE / "/" / "["
1406                 / "]" / "?" / "=" / "{" / "}"
1407</artwork></figure>
1408<t anchor="rule.quoted-string">
1409  <x:anchor-alias value="quoted-string"/>
1410  <x:anchor-alias value="qdtext"/>
1411  <x:anchor-alias value="obs-text"/>
1412   A string of text is parsed as a single word if it is quoted using
1413   double-quote marks.
1414</t>
1415<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"/>
1416  <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>
1417  <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>
1418  <x:ref>obs-text</x:ref>       = %x80-FF
1419</artwork></figure>
1420<t anchor="rule.quoted-pair">
1421  <x:anchor-alias value="quoted-pair"/>
1422   The backslash octet ("\") can be used as a single-octet
1423   quoting mechanism within quoted-string constructs:
1424</t>
1425<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1426  <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> )
1427</artwork></figure>
1428<t>
1429   Recipients that process the value of a quoted-string &MUST; handle a
1430   quoted-pair as if it were replaced by the octet following the backslash.
1431</t>
1432<t>
1433   Senders &SHOULD-NOT; generate a quoted-pair in a quoted-string except where
1434   necessary to quote DQUOTE and backslash octets occurring within that string.
1435</t>
1436<t anchor="rule.comment">
1437  <x:anchor-alias value="comment"/>
1438  <x:anchor-alias value="ctext"/>
1439   Comments can be included in some HTTP header fields by surrounding
1440   the comment text with parentheses. Comments are only allowed in
1441   fields containing "comment" as part of their field value definition.
1442</t>
1443<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1444  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-cpair</x:ref> / <x:ref>comment</x:ref> ) ")"
1445  <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>
1446</artwork></figure>
1447<t anchor="rule.quoted-cpair">
1448  <x:anchor-alias value="quoted-cpair"/>
1449   The backslash octet ("\") can be used as a single-octet
1450   quoting mechanism within comment constructs:
1451</t>
1452<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-cpair"/>
1453  <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> )
1454</artwork></figure>
1455<t>
1456   Senders &SHOULD-NOT; escape octets in comments that do not require escaping
1457   (i.e., other than the backslash octet "\" and the parentheses "(" and ")").
1458</t>
1459</section>
1460
1461</section>
1462
1463<section title="Message Body" anchor="message.body">
1464  <x:anchor-alias value="message-body"/>
1465<t>
1466   The message body (if any) of an HTTP message is used to carry the
1467   payload body of that request or response.  The message body is
1468   identical to the payload body unless a transfer coding has been
1469   applied, as described in <xref target="header.transfer-encoding"/>.
1470</t>
1471<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1472  <x:ref>message-body</x:ref> = *OCTET
1473</artwork></figure>
1474<t>
1475   The rules for when a message body is allowed in a message differ for
1476   requests and responses.
1477</t>
1478<t>
1479   The presence of a message body in a request is signaled by a
1480   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1481   field. Request message framing is independent of method semantics,
1482   even if the method does not define any use for a message body.
1483</t>
1484<t>
1485   The presence of a message body in a response depends on both
1486   the request method to which it is responding and the response
1487   status code (<xref target="status.line"/>).
1488   Responses to the HEAD request method never include a message body
1489   because the associated response header fields (e.g.,
1490   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1491   if present, indicate only what their values would have been if the request
1492   method had been GET (&HEAD;).
1493   <x:ref>2xx (Successful)</x:ref> responses to CONNECT switch to tunnel
1494   mode instead of having a message body (&CONNECT;).
1495   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1496   <x:ref>304 (Not Modified)</x:ref> responses do not include a message body.
1497   All other responses do include a message body, although the body
1498   might be of zero length.
1499</t>
1500
1501<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1502  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1503  <iref item="chunked (Coding Format)"/>
1504  <x:anchor-alias value="Transfer-Encoding"/>
1505<t>
1506   The Transfer-Encoding header field lists the transfer coding names
1507   corresponding to the sequence of transfer codings that have been
1508   (or will be) applied to the payload body in order to form the message body.
1509   Transfer codings are defined in <xref target="transfer.codings"/>.
1510</t>
1511<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1512  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1513</artwork></figure>
1514<t>
1515   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1516   MIME, which was designed to enable safe transport of binary data over a
1517   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1518   However, safe transport has a different focus for an 8bit-clean transfer
1519   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1520   accurately delimit a dynamically generated payload and to distinguish
1521   payload encodings that are only applied for transport efficiency or
1522   security from those that are characteristics of the selected resource.
1523</t>
1524<t>
1525   All HTTP/1.1 recipients &MUST; implement the chunked transfer coding
1526   (<xref target="chunked.encoding"/>) because it plays a crucial role in
1527   framing messages when the payload body size is not known in advance.
1528   If chunked is applied to a payload body, the sender &MUST-NOT; apply
1529   chunked more than once (i.e., chunking an already chunked message is not
1530   allowed).
1531   If any transfer coding is applied to a request payload body, the
1532   sender &MUST; apply chunked as the final transfer coding to ensure that
1533   the message is properly framed.
1534   If any transfer coding is applied to a response payload body, the
1535   sender &MUST; either apply chunked as the final transfer coding or
1536   terminate the message by closing the connection.
1537</t>
1538<figure><preamble>
1539   For example,
1540</preamble><artwork type="example">
1541  Transfer-Encoding: gzip, chunked
1542</artwork><postamble>
1543   indicates that the payload body has been compressed using the gzip
1544   coding and then chunked using the chunked coding while forming the
1545   message body.
1546</postamble></figure>
1547<t>
1548   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1549   Transfer-Encoding is a property of the message, not of the representation, and
1550   any recipient along the request/response chain &MAY; decode the received
1551   transfer coding(s) or apply additional transfer coding(s) to the message
1552   body, assuming that corresponding changes are made to the Transfer-Encoding
1553   field-value. Additional information about the encoding parameters &MAY; be
1554   provided by other header fields not defined by this specification.
1555</t>
1556<t>
1557   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1558   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1559   neither of which includes a message body,
1560   to indicate that the origin server would have applied a transfer coding
1561   to the message body if the request had been an unconditional GET.
1562   This indication is not required, however, because any recipient on
1563   the response chain (including the origin server) can remove transfer
1564   codings when they are not needed.
1565</t>
1566<t>
1567   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1568   implementations advertising only HTTP/1.0 support will not understand
1569   how to process a transfer-encoded payload.
1570   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1571   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1572   might be in the form of specific user configuration or by remembering the
1573   version of a prior received response.
1574   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1575   the corresponding request indicates HTTP/1.1 (or later).
1576</t>
1577<t>
1578   A server that receives a request message with a transfer coding it does
1579   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref>.
1580</t>
1581</section>
1582
1583<section title="Content-Length" anchor="header.content-length">
1584  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1585  <x:anchor-alias value="Content-Length"/>
1586<t>
1587   When a message does not have a <x:ref>Transfer-Encoding</x:ref> header
1588   field, a Content-Length header field can provide the anticipated size,
1589   as a decimal number of octets, for a potential payload body.
1590   For messages that do include a payload body, the Content-Length field-value
1591   provides the framing information necessary for determining where the body
1592   (and message) ends.  For messages that do not include a payload body, the
1593   Content-Length indicates the size of the selected representation
1594   (&representation;).
1595</t>
1596<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1597  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1598</artwork></figure>
1599<t>
1600   An example is
1601</t>
1602<figure><artwork type="example">
1603  Content-Length: 3495
1604</artwork></figure>
1605<t>
1606   A sender &MUST-NOT; send a Content-Length header field in any message that
1607   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1608</t>
1609<t>
1610   A user agent &SHOULD; send a Content-Length in a request message when no
1611   <x:ref>Transfer-Encoding</x:ref> is sent and the request method defines
1612   a meaning for an enclosed payload body. For example, a Content-Length
1613   header field is normally sent in a POST request even when the value is
1614   0 (indicating an empty payload body).  A user agent &SHOULD-NOT; send a
1615   Content-Length header field when the request message does not contain a
1616   payload body and the method semantics do not anticipate such a body.
1617</t>
1618<t>
1619   A server &MAY; send a Content-Length header field in a response to a HEAD
1620   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1621   response unless its field-value equals the decimal number of octets that
1622   would have been sent in the payload body of a response if the same
1623   request had used the GET method.
1624</t>
1625<t>
1626   A server &MAY; send a Content-Length header field in a
1627   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1628   (&status-304;); a server &MUST-NOT; send Content-Length in such a
1629   response unless its field-value equals the decimal number of octets that
1630   would have been sent in the payload body of a <x:ref>200 (OK)</x:ref>
1631   response to the same request.
1632</t>
1633<t>
1634   A server &MUST-NOT; send a Content-Length header field in any response
1635   with a status code of
1636   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1637   A server &SHOULD-NOT; send a Content-Length header field in any
1638   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1639</t>
1640<t>
1641   Aside from the cases defined above, in the absence of Transfer-Encoding,
1642   an origin server &SHOULD; send a Content-Length header field when the
1643   payload body size is known prior to sending the complete header block.
1644   This will allow downstream recipients to measure transfer progress,
1645   know when a received message is complete, and potentially reuse the
1646   connection for additional requests.
1647</t>
1648<t>
1649   Any Content-Length field value greater than or equal to zero is valid.
1650   Since there is no predefined limit to the length of a payload,
1651   recipients &SHOULD; anticipate potentially large decimal numerals and
1652   prevent parsing errors due to integer conversion overflows
1653   (<xref target="attack.protocol.element.size.overflows"/>).
1654</t>
1655<t>
1656   If a message is received that has multiple Content-Length header fields
1657   with field-values consisting of the same decimal value, or a single
1658   Content-Length header field with a field value containing a list of
1659   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1660   duplicate Content-Length header fields have been generated or combined by an
1661   upstream message processor, then the recipient &MUST; either reject the
1662   message as invalid or replace the duplicated field-values with a single
1663   valid Content-Length field containing that decimal value prior to
1664   determining the message body length.
1665</t>
1666<x:note>
1667  <t>
1668   &Note; HTTP's use of Content-Length for message framing differs
1669   significantly from the same field's use in MIME, where it is an optional
1670   field used only within the "message/external-body" media-type.
1671  </t>
1672</x:note>
1673</section>
1674
1675<section title="Message Body Length" anchor="message.body.length">
1676  <iref item="chunked (Coding Format)"/>
1677<t>
1678   The length of a message body is determined by one of the following
1679   (in order of precedence):
1680</t>
1681<t>
1682  <list style="numbers">
1683    <x:lt><t>
1684     Any response to a HEAD request and any response with a
1685     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1686     <x:ref>304 (Not Modified)</x:ref> status code is always
1687     terminated by the first empty line after the header fields, regardless of
1688     the header fields present in the message, and thus cannot contain a
1689     message body.
1690    </t></x:lt>
1691    <x:lt><t>
1692     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1693     connection will become a tunnel immediately after the empty line that
1694     concludes the header fields.  A client &MUST; ignore any
1695     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1696     fields received in such a message.
1697    </t></x:lt>
1698    <x:lt><t>
1699     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1700     and the chunked transfer coding (<xref target="chunked.encoding"/>)
1701     is the final encoding, the message body length is determined by reading
1702     and decoding the chunked data until the transfer coding indicates the
1703     data is complete.
1704    </t>
1705    <t>
1706     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1707     response and the chunked transfer coding is not the final encoding, the
1708     message body length is determined by reading the connection until it is
1709     closed by the server.
1710     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a request and the
1711     chunked transfer coding is not the final encoding, the message body
1712     length cannot be determined reliably; the server &MUST; respond with
1713     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1714    </t>
1715    <t>
1716     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1717     and a <x:ref>Content-Length</x:ref> header field, the Transfer-Encoding
1718     overrides the Content-Length. Such a message might indicate an attempt
1719     to perform request or response smuggling (bypass of security-related
1720     checks on message routing or content) and thus ought to be handled as
1721     an error.  A sender &MUST; remove the received Content-Length field
1722     prior to forwarding such a message downstream.
1723    </t></x:lt>
1724    <x:lt><t>
1725     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1726     either multiple <x:ref>Content-Length</x:ref> header fields having
1727     differing field-values or a single Content-Length header field having an
1728     invalid value, then the message framing is invalid and &MUST; be treated
1729     as an error to prevent request or response smuggling.
1730     If this is a request message, the server &MUST; respond with
1731     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1732     If this is a response message received by a proxy, the proxy
1733     &MUST; discard the received response, send a <x:ref>502 (Bad Gateway)</x:ref>
1734     status code as its downstream response, and then close the connection.
1735     If this is a response message received by a user agent, it &MUST; be
1736     treated as an error by discarding the message and closing the connection.
1737    </t></x:lt>
1738    <x:lt><t>
1739     If a valid <x:ref>Content-Length</x:ref> header field is present without
1740     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1741     expected message body length in octets.
1742     If the sender closes the connection or the recipient times out before the
1743     indicated number of octets are received, the recipient &MUST; consider
1744     the message to be incomplete and close the connection.
1745    </t></x:lt>
1746    <x:lt><t>
1747     If this is a request message and none of the above are true, then the
1748     message body length is zero (no message body is present).
1749    </t></x:lt>
1750    <x:lt><t>
1751     Otherwise, this is a response message without a declared message body
1752     length, so the message body length is determined by the number of octets
1753     received prior to the server closing the connection.
1754    </t></x:lt>
1755  </list>
1756</t>
1757<t>
1758   Since there is no way to distinguish a successfully completed,
1759   close-delimited message from a partially-received message interrupted
1760   by network failure, a server &SHOULD; use encoding or
1761   length-delimited messages whenever possible.  The close-delimiting
1762   feature exists primarily for backwards compatibility with HTTP/1.0.
1763</t>
1764<t>
1765   A server &MAY; reject a request that contains a message body but
1766   not a <x:ref>Content-Length</x:ref> by responding with
1767   <x:ref>411 (Length Required)</x:ref>.
1768</t>
1769<t>
1770   Unless a transfer coding other than chunked has been applied,
1771   a client that sends a request containing a message body &SHOULD;
1772   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1773   length is known in advance, rather than the chunked transfer coding, since some
1774   existing services respond to chunked with a <x:ref>411 (Length Required)</x:ref>
1775   status code even though they understand the chunked transfer coding.  This
1776   is typically because such services are implemented via a gateway that
1777   requires a content-length in advance of being called and the server
1778   is unable or unwilling to buffer the entire request before processing.
1779</t>
1780<t>
1781   A user agent that sends a request containing a message body &MUST; send a
1782   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1783   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1784   the form of specific user configuration or by remembering the version of a
1785   prior received response.
1786</t>
1787<t>
1788   If the final response to the last request on a connection has been
1789   completely received and there remains additional data to read, a user agent
1790   &MAY; discard the remaining data or attempt to determine if that data
1791   belongs as part of the prior response body, which might be the case if the
1792   prior message's Content-Length value is incorrect. A client &MUST-NOT;
1793   process, cache, or forward such extra data as a separate response, since
1794   such behavior would be vulnerable to cache poisoning.
1795</t>
1796</section>
1797</section>
1798
1799<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1800<t>
1801   A server that receives an incomplete request message, usually due to a
1802   canceled request or a triggered time-out exception, &MAY; send an error
1803   response prior to closing the connection.
1804</t>
1805<t>
1806   A client that receives an incomplete response message, which can occur
1807   when a connection is closed prematurely or when decoding a supposedly
1808   chunked transfer coding fails, &MUST; record the message as incomplete.
1809   Cache requirements for incomplete responses are defined in
1810   &cache-incomplete;.
1811</t>
1812<t>
1813   If a response terminates in the middle of the header block (before the
1814   empty line is received) and the status code might rely on header fields to
1815   convey the full meaning of the response, then the client cannot assume
1816   that meaning has been conveyed; the client might need to repeat the
1817   request in order to determine what action to take next.
1818</t>
1819<t>
1820   A message body that uses the chunked transfer coding is
1821   incomplete if the zero-sized chunk that terminates the encoding has not
1822   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1823   incomplete if the size of the message body received (in octets) is less than
1824   the value given by Content-Length.  A response that has neither chunked
1825   transfer coding nor Content-Length is terminated by closure of the
1826   connection, and thus is considered complete regardless of the number of
1827   message body octets received, provided that the header block was received
1828   intact.
1829</t>
1830</section>
1831
1832<section title="Message Parsing Robustness" anchor="message.robustness">
1833<t>
1834   Older HTTP/1.0 user agent implementations might send an extra CRLF
1835   after a POST request as a lame workaround for some early server
1836   applications that failed to read message body content that was
1837   not terminated by a line-ending. An HTTP/1.1 user agent &MUST-NOT;
1838   preface or follow a request with an extra CRLF.  If terminating
1839   the request message body with a line-ending is desired, then the
1840   user agent &MUST; count the terminating CRLF octets as part of the
1841   message body length.
1842</t>
1843<t>
1844   In the interest of robustness, servers &SHOULD; ignore at least one
1845   empty line received where a request-line is expected. In other words, if
1846   a server is reading the protocol stream at the beginning of a
1847   message and receives a CRLF first, the server &SHOULD; ignore the CRLF.
1848</t>
1849<t>
1850   Although the line terminator for the start-line and header
1851   fields is the sequence CRLF, recipients &MAY; recognize a
1852   single LF as a line terminator and ignore any preceding CR.
1853</t>
1854<t>
1855   Although the request-line and status-line grammar rules require that each
1856   of the component elements be separated by a single SP octet, recipients
1857   &MAY; instead parse on whitespace-delimited word boundaries and, aside
1858   from the CRLF terminator, treat any form of whitespace as the SP separator
1859   while ignoring preceding or trailing whitespace;
1860   such whitespace includes one or more of the following octets:
1861   SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
1862</t>
1863<t>
1864   When a server listening only for HTTP request messages, or processing
1865   what appears from the start-line to be an HTTP request message,
1866   receives a sequence of octets that does not match the HTTP-message
1867   grammar aside from the robustness exceptions listed above, the
1868   server &SHOULD; respond with a <x:ref>400 (Bad Request)</x:ref> response. 
1869</t>
1870</section>
1871</section>
1872
1873<section title="Transfer Codings" anchor="transfer.codings">
1874  <x:anchor-alias value="transfer-coding"/>
1875  <x:anchor-alias value="transfer-extension"/>
1876<t>
1877   Transfer coding names are used to indicate an encoding
1878   transformation that has been, can be, or might need to be applied to a
1879   payload body in order to ensure "safe transport" through the network.
1880   This differs from a content coding in that the transfer coding is a
1881   property of the message rather than a property of the representation
1882   that is being transferred.
1883</t>
1884<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1885  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1886                     / "compress" ; <xref target="compress.coding"/>
1887                     / "deflate" ; <xref target="deflate.coding"/>
1888                     / "gzip" ; <xref target="gzip.coding"/>
1889                     / <x:ref>transfer-extension</x:ref>
1890  <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> )
1891</artwork></figure>
1892<t anchor="rule.parameter">
1893  <x:anchor-alias value="attribute"/>
1894  <x:anchor-alias value="transfer-parameter"/>
1895  <x:anchor-alias value="value"/>
1896   Parameters are in the form of attribute/value pairs.
1897</t>
1898<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"/>
1899  <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>
1900  <x:ref>attribute</x:ref>          = <x:ref>token</x:ref>
1901  <x:ref>value</x:ref>              = <x:ref>word</x:ref>
1902</artwork></figure>
1903<t>
1904   All transfer-coding names are case-insensitive and ought to be registered
1905   within the HTTP Transfer Coding registry, as defined in
1906   <xref target="transfer.coding.registry"/>.
1907   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1908   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1909   header fields.
1910</t>
1911
1912<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1913  <iref primary="true" item="chunked (Coding Format)"/>
1914  <x:anchor-alias value="chunk"/>
1915  <x:anchor-alias value="chunked-body"/>
1916  <x:anchor-alias value="chunk-data"/>
1917  <x:anchor-alias value="chunk-ext"/>
1918  <x:anchor-alias value="chunk-ext-name"/>
1919  <x:anchor-alias value="chunk-ext-val"/>
1920  <x:anchor-alias value="chunk-size"/>
1921  <x:anchor-alias value="last-chunk"/>
1922  <x:anchor-alias value="trailer-part"/>
1923  <x:anchor-alias value="quoted-str-nf"/>
1924  <x:anchor-alias value="qdtext-nf"/>
1925<t>
1926   The chunked transfer coding modifies the body of a message in order to
1927   transfer it as a series of chunks, each with its own size indicator,
1928   followed by an &OPTIONAL; trailer containing header fields. This
1929   allows dynamically generated content to be transferred along with the
1930   information necessary for the recipient to verify that it has
1931   received the full message.
1932</t>
1933<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"/>
1934  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1935                   <x:ref>last-chunk</x:ref>
1936                   <x:ref>trailer-part</x:ref>
1937                   <x:ref>CRLF</x:ref>
1938 
1939  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1940                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1941  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1942  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1943 
1944  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
1945  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
1946  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-str-nf</x:ref>
1947  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1948  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1949 
1950  <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>
1951                 ; like <x:ref>quoted-string</x:ref>, but disallowing line folding
1952  <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>
1953</artwork></figure>
1954<t>
1955   Chunk extensions within the chunked transfer coding are deprecated.
1956   Senders &SHOULD-NOT; send chunk-ext.
1957   Definition of new chunk extensions is discouraged.
1958</t>
1959<t>
1960   The chunk-size field is a string of hex digits indicating the size of
1961   the chunk-data in octets. The chunked transfer coding is complete when a
1962   chunk with a chunk-size of zero is received, possibly followed by a
1963   trailer, and finally terminated by an empty line.
1964</t>
1965
1966<section title="Trailer" anchor="header.trailer">
1967  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
1968  <x:anchor-alias value="Trailer"/>
1969<t>
1970   A trailer allows the sender to include additional fields at the end of a
1971   chunked message in order to supply metadata that might be dynamically
1972   generated while the message body is sent, such as a message integrity
1973   check, digital signature, or post-processing status.
1974   The trailer &MUST-NOT; contain fields that need to be known before a
1975   recipient processes the body, such as <x:ref>Transfer-Encoding</x:ref>,
1976   <x:ref>Content-Length</x:ref>, and <x:ref>Trailer</x:ref>.
1977</t>
1978<t>
1979   When a message includes a message body encoded with the chunked
1980   transfer coding and the sender desires to send metadata in the form of
1981   trailer fields at the end of the message, the sender &SHOULD; send a
1982   <x:ref>Trailer</x:ref> header field before the message body to indicate
1983   which fields will be present in the trailers. This allows the recipient
1984   to prepare for receipt of that metadata before it starts processing the body,
1985   which is useful if the message is being streamed and the recipient wishes
1986   to confirm an integrity check on the fly.
1987</t>
1988<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
1989  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
1990</artwork></figure>
1991<t>
1992   If no <x:ref>Trailer</x:ref> header field is present, the sender of a
1993   chunked message body &SHOULD; send an empty trailer.
1994</t>
1995<t>
1996   A server &MUST; send an empty trailer with the chunked transfer coding
1997   unless at least one of the following is true:
1998  <list style="numbers">
1999    <t>the request included a <x:ref>TE</x:ref> header field that indicates
2000    "trailers" is acceptable in the transfer coding of the response, as
2001    described in <xref target="header.te"/>; or,</t>
2002     
2003    <t>the trailer fields consist entirely of optional metadata and the
2004    recipient could use the message (in a manner acceptable to the server where
2005    the field originated) without receiving that metadata. In other words,
2006    the server that generated the header field is willing to accept the
2007    possibility that the trailer fields might be silently discarded along
2008    the path to the client.</t>
2009  </list>
2010</t>
2011<t>
2012   The above requirement prevents the need for an infinite buffer when a
2013   message is being received by an HTTP/1.1 (or later) proxy and forwarded to
2014   an HTTP/1.0 recipient.
2015</t>
2016</section>
2017
2018<section title="Decoding chunked" anchor="decoding.chunked">
2019<t>
2020   A process for decoding the chunked transfer coding
2021   can be represented in pseudo-code as:
2022</t>
2023<figure><artwork type="code">
2024  length := 0
2025  read chunk-size, chunk-ext (if any), and CRLF
2026  while (chunk-size &gt; 0) {
2027     read chunk-data and CRLF
2028     append chunk-data to decoded-body
2029     length := length + chunk-size
2030     read chunk-size, chunk-ext (if any), and CRLF
2031  }
2032  read header-field
2033  while (header-field not empty) {
2034     append header-field to existing header fields
2035     read header-field
2036  }
2037  Content-Length := length
2038  Remove "chunked" from Transfer-Encoding
2039  Remove Trailer from existing header fields
2040</artwork></figure>
2041<t>
2042   All recipients &MUST; be able to receive and decode the
2043   chunked transfer coding and &MUST; ignore chunk-ext extensions
2044   they do not understand.
2045</t>
2046</section>
2047</section>
2048
2049<section title="Compression Codings" anchor="compression.codings">
2050<t>
2051   The codings defined below can be used to compress the payload of a
2052   message.
2053</t>
2054
2055<section title="Compress Coding" anchor="compress.coding">
2056<iref item="compress (Coding Format)"/>
2057<t>
2058   The "compress" format is produced by the common UNIX file compression
2059   program "compress". This format is an adaptive Lempel-Ziv-Welch
2060   coding (LZW). Recipients &SHOULD; consider "x-compress" to be
2061   equivalent to "compress".
2062</t>
2063</section>
2064
2065<section title="Deflate Coding" anchor="deflate.coding">
2066<iref item="deflate (Coding Format)"/>
2067<t>
2068   The "deflate" format is defined as the "deflate" compression mechanism
2069   (described in <xref target="RFC1951"/>) used inside the "zlib"
2070   data format (<xref target="RFC1950"/>).
2071</t>
2072<x:note>
2073  <t>
2074    &Note; Some incorrect implementations send the "deflate"
2075    compressed data without the zlib wrapper.
2076   </t>
2077</x:note>
2078</section>
2079
2080<section title="Gzip Coding" anchor="gzip.coding">
2081<iref item="gzip (Coding Format)"/>
2082<t>
2083   The "gzip" format is produced by the file compression program
2084   "gzip" (GNU zip), as described in <xref target="RFC1952"/>. This format is a
2085   Lempel-Ziv coding (LZ77) with a 32 bit CRC.
2086   Recipients &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2087</t>
2088</section>
2089
2090</section>
2091
2092<section title="TE" anchor="header.te">
2093  <iref primary="true" item="TE header field" x:for-anchor=""/>
2094  <x:anchor-alias value="TE"/>
2095  <x:anchor-alias value="t-codings"/>
2096  <x:anchor-alias value="t-ranking"/>
2097  <x:anchor-alias value="rank"/>
2098<t>
2099   The "TE" header field in a request indicates what transfer codings,
2100   besides chunked, the client is willing to accept in response, and
2101   whether or not the client is willing to accept trailer fields in a
2102   chunked transfer coding.
2103</t>
2104<t>
2105   The TE field-value consists of a comma-separated list of transfer coding
2106   names, each allowing for optional parameters (as described in
2107   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2108   Clients &MUST-NOT; send the chunked transfer coding name in TE;
2109   chunked is always acceptable for HTTP/1.1 recipients.
2110</t>
2111<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"/>
2112  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2113  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2114  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2115  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2116             / ( "1" [ "." 0*3("0") ] )
2117</artwork></figure>
2118<t>
2119   Three examples of TE use are below.
2120</t>
2121<figure><artwork type="example">
2122  TE: deflate
2123  TE:
2124  TE: trailers, deflate;q=0.5
2125</artwork></figure>
2126<t>
2127   The presence of the keyword "trailers" indicates that the client is
2128   willing to accept trailer fields in a chunked transfer coding,
2129   as defined in <xref target="chunked.encoding"/>, on behalf of itself and
2130   any downstream clients. For chained requests, this implies that either:
2131   (a) all downstream clients are willing to accept trailer fields in the
2132   forwarded response; or,
2133   (b) the client will attempt to buffer the response on behalf of downstream
2134   recipients.
2135   Note that HTTP/1.1 does not define any means to limit the size of a
2136   chunked response such that a client can be assured of buffering the
2137   entire response.
2138</t>
2139<t>
2140   When multiple transfer codings are acceptable, the client &MAY; rank the
2141   codings by preference using a case-insensitive "q" parameter (similar to
2142   the qvalues used in content negotiation fields, &qvalue;). The rank value
2143   is a real number in the range 0 through 1, where 0.001 is the least
2144   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2145</t>
2146<t>
2147   If the TE field-value is empty or if no TE field is present, the only
2148   acceptable transfer coding is chunked. A message with no transfer coding
2149   is always acceptable.
2150</t>
2151<t>
2152   Since the TE header field only applies to the immediate connection,
2153   a sender of TE &MUST; also send a "TE" connection option within the
2154   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2155   in order to prevent the TE field from being forwarded by intermediaries
2156   that do not support its semantics.
2157</t>
2158</section>
2159</section>
2160
2161<section title="Message Routing" anchor="message.routing">
2162<t>
2163   HTTP request message routing is determined by each client based on the
2164   target resource, the client's proxy configuration, and
2165   establishment or reuse of an inbound connection.  The corresponding
2166   response routing follows the same connection chain back to the client.
2167</t>
2168
2169<section title="Identifying a Target Resource" anchor="target-resource">
2170  <iref primary="true" item="target resource"/>
2171  <iref primary="true" item="target URI"/>
2172  <x:anchor-alias value="target resource"/>
2173  <x:anchor-alias value="target URI"/>
2174<t>
2175   HTTP is used in a wide variety of applications, ranging from
2176   general-purpose computers to home appliances.  In some cases,
2177   communication options are hard-coded in a client's configuration.
2178   However, most HTTP clients rely on the same resource identification
2179   mechanism and configuration techniques as general-purpose Web browsers.
2180</t>
2181<t>
2182   HTTP communication is initiated by a user agent for some purpose.
2183   The purpose is a combination of request semantics, which are defined in
2184   <xref target="Part2"/>, and a target resource upon which to apply those
2185   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2186   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2187   would resolve to its absolute form in order to obtain the
2188   "<x:dfn>target URI</x:dfn>".  The target URI
2189   excludes the reference's fragment identifier component, if any,
2190   since fragment identifiers are reserved for client-side processing
2191   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2192</t>
2193</section>
2194
2195<section title="Connecting Inbound" anchor="connecting.inbound">
2196<t>
2197   Once the target URI is determined, a client needs to decide whether
2198   a network request is necessary to accomplish the desired semantics and,
2199   if so, where that request is to be directed.
2200</t>
2201<t>
2202   If the client has a response cache and the request semantics can be
2203   satisfied by a cache (<xref target="Part6"/>), then the request is
2204   usually directed to the cache first.
2205</t>
2206<t>
2207   If the request is not satisfied by a cache, then a typical client will
2208   check its configuration to determine whether a proxy is to be used to
2209   satisfy the request.  Proxy configuration is implementation-dependent,
2210   but is often based on URI prefix matching, selective authority matching,
2211   or both, and the proxy itself is usually identified by an "http" or
2212   "https" URI.  If a proxy is applicable, the client connects inbound by
2213   establishing (or reusing) a connection to that proxy.
2214</t>
2215<t>
2216   If no proxy is applicable, a typical client will invoke a handler routine,
2217   usually specific to the target URI's scheme, to connect directly
2218   to an authority for the target resource.  How that is accomplished is
2219   dependent on the target URI scheme and defined by its associated
2220   specification, similar to how this specification defines origin server
2221   access for resolution of the "http" (<xref target="http.uri"/>) and
2222   "https" (<xref target="https.uri"/>) schemes.
2223</t>
2224<t>
2225   HTTP requirements regarding connection management are defined in
2226   <xref target="connection.management"/>.
2227</t>
2228</section>
2229
2230<section title="Request Target" anchor="request-target">
2231<t>
2232   Once an inbound connection is obtained,
2233   the client sends an HTTP request message (<xref target="http.message"/>)
2234   with a request-target derived from the target URI.
2235   There are four distinct formats for the request-target, depending on both
2236   the method being requested and whether the request is to a proxy.
2237</t>   
2238<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"/>
2239  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2240                 / <x:ref>absolute-form</x:ref>
2241                 / <x:ref>authority-form</x:ref>
2242                 / <x:ref>asterisk-form</x:ref>
2243
2244  <x:ref>origin-form</x:ref>    = <x:ref>absolute-path</x:ref> [ "?" <x:ref>query</x:ref> ]
2245  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2246  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2247  <x:ref>asterisk-form</x:ref>  = "*"
2248</artwork></figure>
2249<t anchor="origin-form"><iref item="origin-form (of request-target)"/>
2250  <x:h>origin-form</x:h>
2251</t>
2252<t>
2253   The most common form of request-target is the <x:dfn>origin-form</x:dfn>.
2254   When making a request directly to an origin server, other than a CONNECT
2255   or server-wide OPTIONS request (as detailed below),
2256   a client &MUST; send only the absolute path and query components of
2257   the target URI as the request-target.
2258   If the target URI's path component is empty, then the client &MUST; send
2259   "/" as the path within the origin-form of request-target.
2260   A <x:ref>Host</x:ref> header field is also sent, as defined in
2261   <xref target="header.host"/>, containing the target URI's
2262   authority component (excluding any userinfo).
2263</t>
2264<t>
2265   For example, a client wishing to retrieve a representation of the resource
2266   identified as
2267</t>
2268<figure><artwork x:indent-with="  " type="example">
2269http://www.example.org/where?q=now
2270</artwork></figure>
2271<t>
2272   directly from the origin server would open (or reuse) a TCP connection
2273   to port 80 of the host "www.example.org" and send the lines:
2274</t>
2275<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2276GET /where?q=now HTTP/1.1
2277Host: www.example.org
2278</artwork></figure>
2279<t>
2280   followed by the remainder of the request message.
2281</t>
2282<t anchor="absolute-form"><iref item="absolute-form (of request-target)"/>
2283  <x:h>absolute-form</x:h>
2284</t>
2285<t>
2286   When making a request to a proxy, other than a CONNECT or server-wide
2287   OPTIONS request (as detailed below), a client &MUST; send the target URI
2288   in <x:dfn>absolute-form</x:dfn> as the request-target.
2289   The proxy is requested to either service that request from a valid cache,
2290   if possible, or make the same request on the client's behalf to either
2291   the next inbound proxy server or directly to the origin server indicated
2292   by the request-target.  Requirements on such "forwarding" of messages are
2293   defined in <xref target="message.forwarding"/>.
2294</t>
2295<t>
2296   An example absolute-form of request-line would be:
2297</t>
2298<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2299GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
2300</artwork></figure>
2301<t>
2302   To allow for transition to the absolute-form for all requests in some
2303   future version of HTTP, HTTP/1.1 servers &MUST; accept the absolute-form
2304   in requests, even though HTTP/1.1 clients will only send them in requests
2305   to proxies.
2306</t>
2307<t anchor="authority-form"><iref item="authority-form (of request-target)"/>
2308  <x:h>authority-form</x:h>
2309</t>
2310<t>
2311   The <x:dfn>authority-form</x:dfn> of request-target is only used for CONNECT requests
2312   (&CONNECT;).  When making a CONNECT request to establish a tunnel through
2313   one or more proxies, a client &MUST; send only the target URI's
2314   authority component (excluding any userinfo) as the request-target.
2315   For example,
2316</t>
2317<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2318CONNECT www.example.com:80 HTTP/1.1
2319</artwork></figure>
2320<t anchor="asterisk-form"><iref item="asterisk-form (of request-target)"/>
2321  <x:h>asterisk-form</x:h>
2322</t>
2323<t>
2324   The <x:dfn>asterisk-form</x:dfn> of request-target is only used for a server-wide
2325   OPTIONS request (&OPTIONS;).  When a client wishes to request OPTIONS
2326   for the server as a whole, as opposed to a specific named resource of
2327   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2328   For example,
2329</t>
2330<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2331OPTIONS * HTTP/1.1
2332</artwork></figure>
2333<t>
2334   If a proxy receives an OPTIONS request with an absolute-form of
2335   request-target in which the URI has an empty path and no query component,
2336   then the last proxy on the request chain &MUST; send a request-target
2337   of "*" when it forwards the request to the indicated origin server.
2338</t>
2339<figure><preamble>  
2340   For example, the request
2341</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2342OPTIONS http://www.example.org:8001 HTTP/1.1
2343</artwork></figure>
2344<figure><preamble>  
2345  would be forwarded by the final proxy as
2346</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2347OPTIONS * HTTP/1.1
2348Host: www.example.org:8001
2349</artwork>
2350<postamble>
2351   after connecting to port 8001 of host "www.example.org".
2352</postamble>
2353</figure>
2354</section>
2355
2356<section title="Host" anchor="header.host">
2357  <iref primary="true" item="Host header field" x:for-anchor=""/>
2358  <x:anchor-alias value="Host"/>
2359<t>
2360   The "Host" header field in a request provides the host and port
2361   information from the target URI, enabling the origin
2362   server to distinguish among resources while servicing requests
2363   for multiple host names on a single IP address.  Since the Host
2364   field-value is critical information for handling a request, it
2365   &SHOULD; be sent as the first header field following the request-line.
2366</t>
2367<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2368  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2369</artwork></figure>
2370<t>
2371   A client &MUST; send a Host header field in all HTTP/1.1 request
2372   messages.  If the target URI includes an authority component, then
2373   the Host field-value &MUST; be identical to that authority component
2374   after excluding any userinfo (<xref target="http.uri"/>).
2375   If the authority component is missing or undefined for the target URI,
2376   then the Host header field &MUST; be sent with an empty field-value.
2377</t>
2378<t>
2379   For example, a GET request to the origin server for
2380   &lt;http://www.example.org/pub/WWW/&gt; would begin with:
2381</t>
2382<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2383GET /pub/WWW/ HTTP/1.1
2384Host: www.example.org
2385</artwork></figure>
2386<t>
2387   The Host header field &MUST; be sent in an HTTP/1.1 request even
2388   if the request-target is in the absolute-form, since this
2389   allows the Host information to be forwarded through ancient HTTP/1.0
2390   proxies that might not have implemented Host.
2391</t>
2392<t>
2393   When a proxy receives a request with an absolute-form of
2394   request-target, the proxy &MUST; ignore the received
2395   Host header field (if any) and instead replace it with the host
2396   information of the request-target.  If the proxy forwards the request,
2397   it &MUST; generate a new Host field-value based on the received
2398   request-target rather than forward the received Host field-value.
2399</t>
2400<t>
2401   Since the Host header field acts as an application-level routing
2402   mechanism, it is a frequent target for malware seeking to poison
2403   a shared cache or redirect a request to an unintended server.
2404   An interception proxy is particularly vulnerable if it relies on
2405   the Host field-value for redirecting requests to internal
2406   servers, or for use as a cache key in a shared cache, without
2407   first verifying that the intercepted connection is targeting a
2408   valid IP address for that host.
2409</t>
2410<t>
2411   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2412   to any HTTP/1.1 request message that lacks a Host header field and
2413   to any request message that contains more than one Host header field
2414   or a Host header field with an invalid field-value.
2415</t>
2416</section>
2417
2418<section title="Effective Request URI" anchor="effective.request.uri">
2419  <iref primary="true" item="effective request URI"/>
2420  <x:anchor-alias value="effective request URI"/>
2421<t>
2422   A server that receives an HTTP request message &MUST; reconstruct
2423   the user agent's original target URI, based on the pieces of information
2424   learned from the request-target, <x:ref>Host</x:ref> header field, and
2425   connection context, in order to identify the intended target resource and
2426   properly service the request. The URI derived from this reconstruction
2427   process is referred to as the "<x:dfn>effective request URI</x:dfn>".
2428</t>
2429<t>
2430   For a user agent, the effective request URI is the target URI.
2431</t>
2432<t>
2433   If the request-target is in absolute-form, then the effective request URI
2434   is the same as the request-target.  Otherwise, the effective request URI
2435   is constructed as follows.
2436</t>
2437<t>
2438   If the request is received over a TLS-secured TCP connection,
2439   then the effective request URI's scheme is "https"; otherwise, the
2440   scheme is "http".
2441</t>
2442<t>
2443   If the request-target is in authority-form, then the effective
2444   request URI's authority component is the same as the request-target.
2445   Otherwise, if a <x:ref>Host</x:ref> header field is supplied with a
2446   non-empty field-value, then the authority component is the same as the
2447   Host field-value. Otherwise, the authority component is the concatenation of
2448   the default host name configured for the server, a colon (":"), and the
2449   connection's incoming TCP port number in decimal form.
2450</t>
2451<t>
2452   If the request-target is in authority-form or asterisk-form, then the
2453   effective request URI's combined path and query component is empty.
2454   Otherwise, the combined path and query component is the same as the
2455   request-target.
2456</t>
2457<t>
2458   The components of the effective request URI, once determined as above,
2459   can be combined into absolute-URI form by concatenating the scheme,
2460   "://", authority, and combined path and query component.
2461</t>
2462<figure>
2463<preamble>
2464   Example 1: the following message received over an insecure TCP connection
2465</preamble>
2466<artwork type="example" x:indent-with="  ">
2467GET /pub/WWW/TheProject.html HTTP/1.1
2468Host: www.example.org:8080
2469</artwork>
2470</figure>
2471<figure>
2472<preamble>
2473  has an effective request URI of
2474</preamble>
2475<artwork type="example" x:indent-with="  ">
2476http://www.example.org:8080/pub/WWW/TheProject.html
2477</artwork>
2478</figure>
2479<figure>
2480<preamble>
2481   Example 2: the following message received over a TLS-secured TCP connection
2482</preamble>
2483<artwork type="example" x:indent-with="  ">
2484OPTIONS * HTTP/1.1
2485Host: www.example.org
2486</artwork>
2487</figure>
2488<figure>
2489<preamble>
2490  has an effective request URI of
2491</preamble>
2492<artwork type="example" x:indent-with="  ">
2493https://www.example.org
2494</artwork>
2495</figure>
2496<t>
2497   An origin server that does not allow resources to differ by requested
2498   host &MAY; ignore the <x:ref>Host</x:ref> field-value and instead replace it
2499   with a configured server name when constructing the effective request URI.
2500</t>
2501<t>
2502   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2503   field &MAY; attempt to use heuristics (e.g., examination of the URI path for
2504   something unique to a particular host) in order to guess the
2505   effective request URI's authority component.
2506</t>
2507</section>
2508
2509<section title="Associating a Response to a Request" anchor="associating.response.to.request">
2510<t>
2511   HTTP does not include a request identifier for associating a given
2512   request message with its corresponding one or more response messages.
2513   Hence, it relies on the order of response arrival to correspond exactly
2514   to the order in which requests are made on the same connection.
2515   More than one response message per request only occurs when one or more
2516   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a
2517   final response to the same request.
2518</t>
2519<t>
2520   A client that has more than one outstanding request on a connection &MUST;
2521   maintain a list of outstanding requests in the order sent and &MUST;
2522   associate each received response message on that connection to the highest
2523   ordered request that has not yet received a final (non-<x:ref>1xx</x:ref>)
2524   response.
2525</t>
2526</section>
2527
2528<section title="Message Forwarding" anchor="message.forwarding">
2529<t>
2530   As described in <xref target="intermediaries"/>, intermediaries can serve
2531   a variety of roles in the processing of HTTP requests and responses.
2532   Some intermediaries are used to improve performance or availability.
2533   Others are used for access control or to filter content.
2534   Since an HTTP stream has characteristics similar to a pipe-and-filter
2535   architecture, there are no inherent limits to the extent an intermediary
2536   can enhance (or interfere) with either direction of the stream.
2537</t>
2538<t>
2539   Intermediaries that forward a message &MUST; implement the
2540   <x:ref>Connection</x:ref> header field, as specified in
2541   <xref target="header.connection"/>, to exclude fields that are only
2542   intended for the incoming connection.
2543</t>
2544<t>
2545   In order to avoid request loops, a proxy that forwards requests to other
2546   proxies &MUST; be able to recognize and exclude all of its own server
2547   names, including any aliases, local variations, or literal IP addresses.
2548</t>
2549
2550<section title="Via" anchor="header.via">
2551  <iref primary="true" item="Via header field" x:for-anchor=""/>
2552  <x:anchor-alias value="pseudonym"/>
2553  <x:anchor-alias value="received-by"/>
2554  <x:anchor-alias value="received-protocol"/>
2555  <x:anchor-alias value="Via"/>
2556<t>
2557   The "Via" header field &MUST; be sent by a proxy or gateway in forwarded
2558   messages to indicate the intermediate protocols and recipients between the
2559   user agent and the server on requests, and between the origin server and
2560   the client on responses. It is analogous to the "Received" field
2561   used by email systems (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2562   Via is used in HTTP for tracking message forwards,
2563   avoiding request loops, and identifying the protocol capabilities of
2564   all senders along the request/response chain.
2565</t>
2566<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"/>
2567  <x:ref>Via</x:ref>               = 1#( <x:ref>received-protocol</x:ref> <x:ref>RWS</x:ref> <x:ref>received-by</x:ref>
2568                          [ <x:ref>RWS</x:ref> <x:ref>comment</x:ref> ] )
2569  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2570                      ; see <xref target="header.upgrade"/>
2571  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2572  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2573</artwork></figure>
2574<t>
2575   The received-protocol indicates the protocol version of the message
2576   received by the server or client along each segment of the
2577   request/response chain. The received-protocol version is appended to
2578   the Via field value when the message is forwarded so that information
2579   about the protocol capabilities of upstream applications remains
2580   visible to all recipients.
2581</t>
2582<t>
2583   The protocol-name is excluded if and only if it would be "HTTP". The
2584   received-by field is normally the host and optional port number of a
2585   recipient server or client that subsequently forwarded the message.
2586   However, if the real host is considered to be sensitive information,
2587   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2588   be assumed to be the default port of the received-protocol.
2589</t>
2590<t>
2591   Multiple Via field values represent each proxy or gateway that has
2592   forwarded the message. Each recipient &MUST; append its information
2593   such that the end result is ordered according to the sequence of
2594   forwarding applications.
2595</t>
2596<t>
2597   Comments &MAY; be used in the Via header field to identify the software
2598   of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2599   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2600   are optional and &MAY; be removed by any recipient prior to forwarding the
2601   message.
2602</t>
2603<t>
2604   For example, a request message could be sent from an HTTP/1.0 user
2605   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2606   forward the request to a public proxy at p.example.net, which completes
2607   the request by forwarding it to the origin server at www.example.com.
2608   The request received by www.example.com would then have the following
2609   Via header field:
2610</t>
2611<figure><artwork type="example">
2612  Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
2613</artwork></figure>
2614<t>
2615   A proxy or gateway used as a portal through a network firewall
2616   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2617   region unless it is explicitly enabled to do so. If not enabled, the
2618   received-by host of any host behind the firewall &SHOULD; be replaced
2619   by an appropriate pseudonym for that host.
2620</t>
2621<t>
2622   A proxy or gateway &MAY; combine an ordered subsequence of Via header
2623   field entries into a single such entry if the entries have identical
2624   received-protocol values. For example,
2625</t>
2626<figure><artwork type="example">
2627  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2628</artwork></figure>
2629<t>
2630  could be collapsed to
2631</t>
2632<figure><artwork type="example">
2633  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2634</artwork></figure>
2635<t>
2636   Senders &SHOULD-NOT; combine multiple entries unless they are all
2637   under the same organizational control and the hosts have already been
2638   replaced by pseudonyms. Senders &MUST-NOT; combine entries that
2639   have different received-protocol values.
2640</t>
2641</section>
2642
2643<section title="Transformations" anchor="message.transformations">
2644<t>
2645   Some intermediaries include features for transforming messages and their
2646   payloads.  A transforming proxy might, for example, convert between image
2647   formats in order to save cache space or to reduce the amount of traffic on
2648   a slow link. However, operational problems might occur when these
2649   transformations are applied to payloads intended for critical applications,
2650   such as medical imaging or scientific data analysis, particularly when
2651   integrity checks or digital signatures are used to ensure that the payload
2652   received is identical to the original.
2653</t>
2654<t>
2655   If a proxy receives a request-target with a host name that is not a
2656   fully qualified domain name, it &MAY; add its own domain to the host name
2657   it received when forwarding the request.  A proxy &MUST-NOT; change the
2658   host name if it is a fully qualified domain name.
2659</t>
2660<t>
2661   A proxy &MUST-NOT; modify the "absolute-path" and "query" parts of the
2662   received request-target when forwarding it to the next inbound server,
2663   except as noted above to replace an empty path with "/" or "*".
2664</t>
2665<t>
2666   A proxy &MUST-NOT; modify header fields that provide information about the
2667   end points of the communication chain, the resource state, or the selected
2668   representation. A proxy &MAY; change the message body through application
2669   or removal of a transfer coding (<xref target="transfer.codings"/>).
2670</t>
2671<t>
2672   A non-transforming proxy &MUST; preserve the message payload (&payload;).
2673   A transforming proxy &MUST; preserve the payload of a message that
2674   contains the no-transform cache-control directive.
2675</t>
2676<t>
2677   A transforming proxy &MAY; transform the payload of a message
2678   that does not contain the no-transform cache-control directive;
2679   if the payload is transformed, the transforming proxy &MUST; add a
2680   Warning 214 (Transformation applied) header field if one does not
2681   already appear in the message (see &header-warning;).
2682</t>
2683</section>
2684</section>
2685</section>
2686
2687<section title="Connection Management" anchor="connection.management">
2688<t>
2689   HTTP messaging is independent of the underlying transport or
2690   session-layer connection protocol(s).  HTTP only presumes a reliable
2691   transport with in-order delivery of requests and the corresponding
2692   in-order delivery of responses.  The mapping of HTTP request and
2693   response structures onto the data units of an underlying transport
2694   protocol is outside the scope of this specification.
2695</t>
2696<t>
2697   As described in <xref target="connecting.inbound"/>, the specific
2698   connection protocols to be used for an HTTP interaction are determined by
2699   client configuration and the <x:ref>target URI</x:ref>.
2700   For example, the "http" URI scheme
2701   (<xref target="http.uri"/>) indicates a default connection of TCP
2702   over IP, with a default TCP port of 80, but the client might be
2703   configured to use a proxy via some other connection, port, or protocol.
2704</t>
2705<t>
2706   HTTP implementations are expected to engage in connection management,
2707   which includes maintaining the state of current connections,
2708   establishing a new connection or reusing an existing connection,
2709   processing messages received on a connection, detecting connection
2710   failures, and closing each connection.
2711   Most clients maintain multiple connections in parallel, including
2712   more than one connection per server endpoint.
2713   Most servers are designed to maintain thousands of concurrent connections,
2714   while controlling request queues to enable fair use and detect
2715   denial of service attacks.
2716</t>
2717
2718<section title="Connection" anchor="header.connection">
2719  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2720  <iref primary="true" item="close" x:for-anchor=""/>
2721  <x:anchor-alias value="Connection"/>
2722  <x:anchor-alias value="connection-option"/>
2723  <x:anchor-alias value="close"/>
2724<t>
2725   The "Connection" header field allows the sender to indicate desired
2726   control options for the current connection.  In order to avoid confusing
2727   downstream recipients, a proxy or gateway &MUST; remove or replace any
2728   received connection options before forwarding the message.
2729</t>
2730<t>
2731   When a header field aside from Connection is used to supply control
2732   information for or about the current connection, the sender &MUST; list
2733   the corresponding field-name within the "Connection" header field.
2734   A proxy or gateway &MUST; parse a received Connection
2735   header field before a message is forwarded and, for each
2736   connection-option in this field, remove any header field(s) from
2737   the message with the same name as the connection-option, and then
2738   remove the Connection header field itself (or replace it with the
2739   intermediary's own connection options for the forwarded message).
2740</t>
2741<t>
2742   Hence, the Connection header field provides a declarative way of
2743   distinguishing header fields that are only intended for the
2744   immediate recipient ("hop-by-hop") from those fields that are
2745   intended for all recipients on the chain ("end-to-end"), enabling the
2746   message to be self-descriptive and allowing future connection-specific
2747   extensions to be deployed without fear that they will be blindly
2748   forwarded by older intermediaries.
2749</t>
2750<t>
2751   The Connection header field's value has the following grammar:
2752</t>
2753<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2754  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2755  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2756</artwork></figure>
2757<t>
2758   Connection options are case-insensitive.
2759</t>
2760<t>
2761   A sender &MUST-NOT; send a connection option corresponding to a header
2762   field that is intended for all recipients of the payload.
2763   For example, <x:ref>Cache-Control</x:ref> is never appropriate as a
2764   connection option (&header-cache-control;).
2765</t>
2766<t>
2767   The connection options do not have to correspond to a header field
2768   present in the message, since a connection-specific header field
2769   might not be needed if there are no parameters associated with that
2770   connection option.  Recipients that trigger certain connection
2771   behavior based on the presence of connection options &MUST; do so
2772   based on the presence of the connection-option rather than only the
2773   presence of the optional header field.  In other words, if the
2774   connection option is received as a header field but not indicated
2775   within the Connection field-value, then the recipient &MUST; ignore
2776   the connection-specific header field because it has likely been
2777   forwarded by an intermediary that is only partially conformant.
2778</t>
2779<t>
2780   When defining new connection options, specifications ought to
2781   carefully consider existing deployed header fields and ensure
2782   that the new connection option does not share the same name as
2783   an unrelated header field that might already be deployed.
2784   Defining a new connection option essentially reserves that potential
2785   field-name for carrying additional information related to the
2786   connection option, since it would be unwise for senders to use
2787   that field-name for anything else.
2788</t>
2789<t>
2790   The "<x:dfn>close</x:dfn>" connection option is defined for a
2791   sender to signal that this connection will be closed after completion of
2792   the response. For example,
2793</t>
2794<figure><artwork type="example">
2795  Connection: close
2796</artwork></figure>
2797<t>
2798   in either the request or the response header fields indicates that
2799   the connection &MUST; be closed after the current request/response
2800   is complete (<xref target="persistent.tear-down"/>).
2801</t>
2802<t>
2803   A client that does not support <x:ref>persistent connections</x:ref> &MUST;
2804   send the "close" connection option in every request message.
2805</t>
2806<t>
2807   A server that does not support <x:ref>persistent connections</x:ref> &MUST;
2808   send the "close" connection option in every response message that
2809   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2810</t>
2811</section>
2812
2813<section title="Establishment" anchor="persistent.establishment">
2814<t>
2815   It is beyond the scope of this specification to describe how connections
2816   are established via various transport or session-layer protocols.
2817   Each connection applies to only one transport link.
2818</t>
2819</section>
2820
2821<section title="Persistence" anchor="persistent.connections">
2822   <x:anchor-alias value="persistent connections"/>
2823<t>
2824   HTTP/1.1 defaults to the use of "<x:dfn>persistent connections</x:dfn>",
2825   allowing multiple requests and responses to be carried over a single
2826   connection. The "<x:ref>close</x:ref>" connection-option is used to signal
2827   that a connection will not persist after the current request/response.
2828   HTTP implementations &SHOULD; support persistent connections.
2829</t>
2830<t>
2831   A recipient determines whether a connection is persistent or not based on
2832   the most recently received message's protocol version and
2833   <x:ref>Connection</x:ref> header field (if any):
2834   <list style="symbols">
2835     <t>If the <x:ref>close</x:ref> connection option is present, the
2836        connection will not persist after the current response; else,</t>
2837     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2838        persist after the current response; else,</t>
2839     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2840        connection option is present, the recipient is not a proxy, and
2841        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2842        the connection will persist after the current response; otherwise,</t>
2843     <t>The connection will close after the current response.</t>
2844   </list>
2845</t>
2846<t>
2847   A server &MAY; assume that an HTTP/1.1 client intends to maintain a
2848   persistent connection until a <x:ref>close</x:ref> connection option
2849   is received in a request.
2850</t>
2851<t>
2852   A client &MAY; reuse a persistent connection until it sends or receives
2853   a <x:ref>close</x:ref> connection option or receives an HTTP/1.0 response
2854   without a "keep-alive" connection option.
2855</t>
2856<t>
2857   In order to remain persistent, all messages on a connection &MUST;
2858   have a self-defined message length (i.e., one not defined by closure
2859   of the connection), as described in <xref target="message.body"/>.
2860   A server &MUST; read the entire request message body or close
2861   the connection after sending its response, since otherwise the
2862   remaining data on a persistent connection would be misinterpreted
2863   as the next request.  Likewise,
2864   a client &MUST; read the entire response message body if it intends
2865   to reuse the same connection for a subsequent request.
2866</t>
2867<t>
2868   A proxy server &MUST-NOT; maintain a persistent connection with an
2869   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
2870   information and discussion of the problems with the Keep-Alive header field
2871   implemented by many HTTP/1.0 clients).
2872</t>
2873<t>
2874   Clients and servers &SHOULD-NOT; assume that a persistent connection is
2875   maintained for HTTP versions less than 1.1 unless it is explicitly
2876   signaled.
2877   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
2878   for more information on backward compatibility with HTTP/1.0 clients.
2879</t>
2880
2881<section title="Retrying Requests" anchor="persistent.retrying.requests">
2882<t>
2883   Connections can be closed at any time, with or without intention.
2884   Implementations ought to anticipate the need to recover
2885   from asynchronous close events.
2886</t>
2887<t>
2888   When an inbound connection is closed prematurely, a client &MAY; open a new
2889   connection and automatically retransmit an aborted sequence of requests if
2890   all of those requests have idempotent methods (&idempotent-methods;).
2891   A proxy &MUST-NOT; automatically retry non-idempotent requests.
2892</t>
2893<t>
2894   A user agent &MUST-NOT; automatically retry a request with a non-idempotent
2895   method unless it has some means to know that the request semantics are
2896   actually idempotent, regardless of the method, or some means to detect that
2897   the original request was never applied. For example, a user agent that
2898   knows (through design or configuration) that a POST request to a given
2899   resource is safe can repeat that request automatically.
2900   Likewise, a user agent designed specifically to operate on a version
2901   control repository might be able to recover from partial failure conditions
2902   by checking the target resource revision(s) after a failed connection,
2903   reverting or fixing any changes that were partially applied, and then
2904   automatically retrying the requests that failed.
2905</t>
2906<t>
2907   An automatic retry &SHOULD-NOT; be repeated if it fails.
2908</t>
2909</section>
2910
2911<section title="Pipelining" anchor="pipelining">
2912   <x:anchor-alias value="pipeline"/>
2913<t>
2914   A client that supports persistent connections &MAY; "<x:dfn>pipeline</x:dfn>"
2915   its requests (i.e., send multiple requests without waiting for each
2916   response). A server &MAY; process a sequence of pipelined requests in
2917   parallel if they all have safe methods (&safe-methods;), but &MUST; send
2918   the corresponding responses in the same order that the requests were
2919   received.
2920</t>
2921<t>
2922   A client that pipelines requests &MUST; be prepared to retry those
2923   requests if the connection closes before it receives all of the
2924   corresponding responses. A client that assumes a persistent connection and
2925   pipelines immediately after connection establishment &MUST-NOT; pipeline
2926   on a retry connection until it knows the connection is persistent.
2927</t>
2928<t>
2929   Idempotent methods (&idempotent-methods;) are significant to pipelining
2930   because they can be automatically retried after a connection failure.
2931   A user agent &SHOULD-NOT; pipeline requests after a non-idempotent method
2932   until the final response status code for that method has been received,
2933   unless the user agent has a means to detect and recover from partial
2934   failure conditions involving the pipelined sequence.
2935</t>
2936<t>
2937   An intermediary that receives pipelined requests &MAY; pipeline those
2938   requests when forwarding them inbound, since it can rely on the outbound
2939   user agent(s) to determine what requests can be safely pipelined. If the
2940   inbound connection fails before receiving a response, the pipelining
2941   intermediary &MAY; attempt to retry a sequence of requests that have yet
2942   to receive a response if the requests all have idempotent methods;
2943   otherwise, the pipelining intermediary &SHOULD; forward any received
2944   responses and then close the corresponding outbound connection(s) so that
2945   the outbound user agent(s) can recover accordingly.
2946</t>
2947</section>
2948</section>
2949   
2950<section title="Concurrency" anchor="persistent.concurrency">
2951<t>
2952   Clients &SHOULD; limit the number of simultaneous
2953   connections that they maintain to a given server.
2954</t>
2955<t>
2956   Previous revisions of HTTP gave a specific number of connections as a
2957   ceiling, but this was found to be impractical for many applications. As a
2958   result, this specification does not mandate a particular maximum number of
2959   connections, but instead encourages clients to be conservative when opening
2960   multiple connections.
2961</t>
2962<t>
2963   Multiple connections are typically used to avoid the "head-of-line
2964   blocking" problem, wherein a request that takes significant server-side
2965   processing and/or has a large payload blocks subsequent requests on the
2966   same connection. However, each connection consumes server resources.
2967   Furthermore, using multiple connections can cause undesirable side effects
2968   in congested networks.
2969</t>
2970<t>
2971   Note that servers might reject traffic that they deem abusive, including an
2972   excessive number of connections from a client.
2973</t>
2974</section>
2975
2976<section title="Failures and Time-outs" anchor="persistent.failures">
2977<t>
2978   Servers will usually have some time-out value beyond which they will
2979   no longer maintain an inactive connection. Proxy servers might make
2980   this a higher value since it is likely that the client will be making
2981   more connections through the same server. The use of persistent
2982   connections places no requirements on the length (or existence) of
2983   this time-out for either the client or the server.
2984</t>
2985<t>
2986   When a client or server wishes to time-out it &SHOULD; issue a graceful
2987   close on the transport connection. Clients and servers &SHOULD; both
2988   constantly watch for the other side of the transport close, and
2989   respond to it as appropriate. If a client or server does not detect
2990   the other side's close promptly it could cause unnecessary resource
2991   drain on the network.
2992</t>
2993<t>
2994   A client, server, or proxy &MAY; close the transport connection at any
2995   time. For example, a client might have started to send a new request
2996   at the same time that the server has decided to close the "idle"
2997   connection. From the server's point of view, the connection is being
2998   closed while it was idle, but from the client's point of view, a
2999   request is in progress.
3000</t>
3001<t>
3002   Servers &SHOULD; maintain persistent connections and allow the underlying
3003   transport's flow control mechanisms to resolve temporary overloads, rather
3004   than terminate connections with the expectation that clients will retry.
3005   The latter technique can exacerbate network congestion.
3006</t>
3007<t>
3008   A client sending a message body &SHOULD; monitor
3009   the network connection for an error response while it is transmitting
3010   the request. If the client sees an error response, it &SHOULD;
3011   immediately cease transmitting the body and close the connection.
3012</t>
3013</section>
3014   
3015<section title="Tear-down" anchor="persistent.tear-down">
3016  <iref primary="false" item="Connection header field" x:for-anchor=""/>
3017  <iref primary="false" item="close" x:for-anchor=""/>
3018<t>
3019   The <x:ref>Connection</x:ref> header field
3020   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
3021   connection option that a sender &SHOULD; send when it wishes to close
3022   the connection after the current request/response pair.
3023</t>
3024<t>
3025   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
3026   send further requests on that connection (after the one containing
3027   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
3028   final response message corresponding to this request.
3029</t>
3030<t>
3031   A server that receives a <x:ref>close</x:ref> connection option &MUST;
3032   initiate a close of the connection (see below) after it sends the
3033   final response to the request that contained <x:ref>close</x:ref>.
3034   The server &SHOULD; send a <x:ref>close</x:ref> connection option
3035   in its final response on that connection. The server &MUST-NOT; process
3036   any further requests received on that connection.
3037</t>
3038<t>
3039   A server that sends a <x:ref>close</x:ref> connection option &MUST;
3040   initiate a close of the connection (see below) after it sends the
3041   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
3042   any further requests received on that connection.
3043</t>
3044<t>
3045   A client that receives a <x:ref>close</x:ref> connection option &MUST;
3046   cease sending requests on that connection and close the connection
3047   after reading the response message containing the close; if additional
3048   pipelined requests had been sent on the connection, the client &SHOULD-NOT;
3049   assume that they will be processed by the server.
3050</t>
3051<t>
3052   If a server performs an immediate close of a TCP connection, there is a
3053   significant risk that the client will not be able to read the last HTTP
3054   response.  If the server receives additional data from the client on a
3055   fully-closed connection, such as another request that was sent by the
3056   client before receiving the server's response, the server's TCP stack will
3057   send a reset packet to the client; unfortunately, the reset packet might
3058   erase the client's unacknowledged input buffers before they can be read
3059   and interpreted by the client's HTTP parser.
3060</t>
3061<t>
3062   To avoid the TCP reset problem, servers typically close a connection in
3063   stages. First, the server performs a half-close by closing only the write
3064   side of the read/write connection. The server then continues to read from
3065   the connection until it receives a corresponding close by the client, or
3066   until the server is reasonably certain that its own TCP stack has received
3067   the client's acknowledgement of the packet(s) containing the server's last
3068   response. Finally, the server fully closes the connection.
3069</t>
3070<t>
3071   It is unknown whether the reset problem is exclusive to TCP or might also
3072   be found in other transport connection protocols.
3073</t>
3074</section>
3075
3076<section title="Upgrade" anchor="header.upgrade">
3077  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3078  <x:anchor-alias value="Upgrade"/>
3079  <x:anchor-alias value="protocol"/>
3080  <x:anchor-alias value="protocol-name"/>
3081  <x:anchor-alias value="protocol-version"/>
3082<t>
3083   The "Upgrade" header field is intended to provide a simple mechanism
3084   for transitioning from HTTP/1.1 to some other protocol on the same
3085   connection.  A client &MAY; send a list of protocols in the Upgrade
3086   header field of a request to invite the server to switch to one or
3087   more of those protocols before sending the final response.
3088   A server &MUST; send an Upgrade header field in <x:ref>101 (Switching
3089   Protocols)</x:ref> responses to indicate which protocol(s) are being
3090   switched to, and &MUST; send it in <x:ref>426 (Upgrade Required)</x:ref>
3091   responses to indicate acceptable protocols.
3092   A server &MAY; send an Upgrade header field in any other response to
3093   indicate that they might be willing to upgrade to one of the
3094   specified protocols for a future request.
3095</t>
3096<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3097  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3098
3099  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3100  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3101  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3102</artwork></figure>
3103<t>
3104   For example,
3105</t>
3106<figure><artwork type="example">
3107  Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3108</artwork></figure>
3109<t>
3110   Upgrade eases the difficult transition between incompatible protocols by
3111   allowing the client to initiate a request in the more commonly
3112   supported protocol while indicating to the server that it would like
3113   to use a "better" protocol if available (where "better" is determined
3114   by the server, possibly according to the nature of the request method
3115   or target resource).
3116</t>
3117<t>
3118   Upgrade cannot be used to insist on a protocol change; its acceptance and
3119   use by the server is optional. The capabilities and nature of the
3120   application-level communication after the protocol change is entirely
3121   dependent upon the new protocol chosen, although the first action
3122   after changing the protocol &MUST; be a response to the initial HTTP
3123   request that contained the Upgrade header field.
3124</t>
3125<t>
3126   For example, if the Upgrade header field is received in a GET request
3127   and the server decides to switch protocols, then it first responds
3128   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3129   then immediately follows that with the new protocol's equivalent of a
3130   response to a GET on the target resource.  This allows a connection to be
3131   upgraded to protocols with the same semantics as HTTP without the
3132   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3133   protocols unless the received message semantics can be honored by the new
3134   protocol; an OPTIONS request can be honored by any protocol.
3135</t>
3136<t>
3137   When Upgrade is sent, a sender &MUST; also send a
3138   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3139   that contains the "upgrade" connection option, in order to prevent Upgrade
3140   from being accidentally forwarded by intermediaries that might not implement
3141   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3142   is received in an HTTP/1.0 request.
3143</t>
3144<t>
3145   The Upgrade header field only applies to switching application-level
3146   protocols on the existing connection; it cannot be used
3147   to switch to a protocol on a different connection. For that purpose, it is
3148   more appropriate to use a <x:ref>3xx (Redirection)</x:ref> response
3149   (&status-3xx;).
3150</t>
3151<t>
3152   This specification only defines the protocol name "HTTP" for use by
3153   the family of Hypertext Transfer Protocols, as defined by the HTTP
3154   version rules of <xref target="http.version"/> and future updates to this
3155   specification. Additional tokens ought to be registered with IANA using the
3156   registration procedure defined in <xref target="upgrade.token.registry"/>.
3157</t>
3158</section>
3159</section>
3160
3161<section title="IANA Considerations" anchor="IANA.considerations">
3162
3163<section title="Header Field Registration" anchor="header.field.registration">
3164<t>
3165   HTTP header fields are registered within the Message Header Field Registry
3166   maintained at
3167   <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
3168</t>
3169<t>
3170   This document defines the following HTTP header fields, so their
3171   associated registry entries shall be updated according to the permanent
3172   registrations below (see <xref target="BCP90"/>):
3173</t>
3174<?BEGININC p1-messaging.iana-headers ?>
3175<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3176<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3177   <ttcol>Header Field Name</ttcol>
3178   <ttcol>Protocol</ttcol>
3179   <ttcol>Status</ttcol>
3180   <ttcol>Reference</ttcol>
3181
3182   <c>Connection</c>
3183   <c>http</c>
3184   <c>standard</c>
3185   <c>
3186      <xref target="header.connection"/>
3187   </c>
3188   <c>Content-Length</c>
3189   <c>http</c>
3190   <c>standard</c>
3191   <c>
3192      <xref target="header.content-length"/>
3193   </c>
3194   <c>Host</c>
3195   <c>http</c>
3196   <c>standard</c>
3197   <c>
3198      <xref target="header.host"/>
3199   </c>
3200   <c>TE</c>
3201   <c>http</c>
3202   <c>standard</c>
3203   <c>
3204      <xref target="header.te"/>
3205   </c>
3206   <c>Trailer</c>
3207   <c>http</c>
3208   <c>standard</c>
3209   <c>
3210      <xref target="header.trailer"/>
3211   </c>
3212   <c>Transfer-Encoding</c>
3213   <c>http</c>
3214   <c>standard</c>
3215   <c>
3216      <xref target="header.transfer-encoding"/>
3217   </c>
3218   <c>Upgrade</c>
3219   <c>http</c>
3220   <c>standard</c>
3221   <c>
3222      <xref target="header.upgrade"/>
3223   </c>
3224   <c>Via</c>
3225   <c>http</c>
3226   <c>standard</c>
3227   <c>
3228      <xref target="header.via"/>
3229   </c>
3230</texttable>
3231<!--(END)-->
3232<?ENDINC p1-messaging.iana-headers ?>
3233<t>
3234   Furthermore, the header field-name "Close" shall be registered as
3235   "reserved", since using that name as an HTTP header field might
3236   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3237   header field (<xref target="header.connection"/>).
3238</t>
3239<texttable align="left" suppress-title="true">
3240   <ttcol>Header Field Name</ttcol>
3241   <ttcol>Protocol</ttcol>
3242   <ttcol>Status</ttcol>
3243   <ttcol>Reference</ttcol>
3244
3245   <c>Close</c>
3246   <c>http</c>
3247   <c>reserved</c>
3248   <c>
3249      <xref target="header.field.registration"/>
3250   </c>
3251</texttable>
3252<t>
3253   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3254</t>
3255</section>
3256
3257<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3258<t>
3259   IANA maintains the registry of URI Schemes <xref target="BCP115"/> at
3260   <eref target="http://www.iana.org/assignments/uri-schemes.html"/>.
3261</t>
3262<t>
3263   This document defines the following URI schemes, so their
3264   associated registry entries shall be updated according to the permanent
3265   registrations below:
3266</t>
3267<texttable align="left" suppress-title="true">
3268   <ttcol>URI Scheme</ttcol>
3269   <ttcol>Description</ttcol>
3270   <ttcol>Reference</ttcol>
3271
3272   <c>http</c>
3273   <c>Hypertext Transfer Protocol</c>
3274   <c><xref target="http.uri"/></c>
3275
3276   <c>https</c>
3277   <c>Hypertext Transfer Protocol Secure</c>
3278   <c><xref target="https.uri"/></c>
3279</texttable>
3280</section>
3281
3282<section title="Internet Media Type Registration" anchor="internet.media.type.http">
3283<t>
3284   This document serves as the specification for the Internet media types
3285   "message/http" and "application/http". The following is to be registered with
3286   IANA (see <xref target="BCP13"/>).
3287</t>
3288<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
3289<iref item="Media Type" subitem="message/http" primary="true"/>
3290<iref item="message/http Media Type" primary="true"/>
3291<t>
3292   The message/http type can be used to enclose a single HTTP request or
3293   response message, provided that it obeys the MIME restrictions for all
3294   "message" types regarding line length and encodings.
3295</t>
3296<t>
3297  <list style="hanging" x:indent="12em">
3298    <t hangText="Type name:">
3299      message
3300    </t>
3301    <t hangText="Subtype name:">
3302      http
3303    </t>
3304    <t hangText="Required parameters:">
3305      none
3306    </t>
3307    <t hangText="Optional parameters:">
3308      version, msgtype
3309      <list style="hanging">
3310        <t hangText="version:">
3311          The HTTP-version number of the enclosed message
3312          (e.g., "1.1"). If not present, the version can be
3313          determined from the first line of the body.
3314        </t>
3315        <t hangText="msgtype:">
3316          The message type &mdash; "request" or "response". If not
3317          present, the type can be determined from the first
3318          line of the body.
3319        </t>
3320      </list>
3321    </t>
3322    <t hangText="Encoding considerations:">
3323      only "7bit", "8bit", or "binary" are permitted
3324    </t>
3325    <t hangText="Security considerations:">
3326      none
3327    </t>
3328    <t hangText="Interoperability considerations:">
3329      none
3330    </t>
3331    <t hangText="Published specification:">
3332      This specification (see <xref target="internet.media.type.message.http"/>).
3333    </t>
3334    <t hangText="Applications that use this media type:">
3335    </t>
3336    <t hangText="Additional information:">
3337      <list style="hanging">
3338        <t hangText="Magic number(s):">none</t>
3339        <t hangText="File extension(s):">none</t>
3340        <t hangText="Macintosh file type code(s):">none</t>
3341      </list>
3342    </t>
3343    <t hangText="Person and email address to contact for further information:">
3344      See Authors Section.
3345    </t>
3346    <t hangText="Intended usage:">
3347      COMMON
3348    </t>
3349    <t hangText="Restrictions on usage:">
3350      none
3351    </t>
3352    <t hangText="Author:">
3353      See Authors Section.
3354    </t>
3355    <t hangText="Change controller:">
3356      IESG
3357    </t>
3358  </list>
3359</t>
3360</section>
3361<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
3362<iref item="Media Type" subitem="application/http" primary="true"/>
3363<iref item="application/http Media Type" primary="true"/>
3364<t>
3365   The application/http type can be used to enclose a pipeline of one or more
3366   HTTP request or response messages (not intermixed).
3367</t>
3368<t>
3369  <list style="hanging" x:indent="12em">
3370    <t hangText="Type name:">
3371      application
3372    </t>
3373    <t hangText="Subtype name:">
3374      http
3375    </t>
3376    <t hangText="Required parameters:">
3377      none
3378    </t>
3379    <t hangText="Optional parameters:">
3380      version, msgtype
3381      <list style="hanging">
3382        <t hangText="version:">
3383          The HTTP-version number of the enclosed messages
3384          (e.g., "1.1"). If not present, the version can be
3385          determined from the first line of the body.
3386        </t>
3387        <t hangText="msgtype:">
3388          The message type &mdash; "request" or "response". If not
3389          present, the type can be determined from the first
3390          line of the body.
3391        </t>
3392      </list>
3393    </t>
3394    <t hangText="Encoding considerations:">
3395      HTTP messages enclosed by this type
3396      are in "binary" format; use of an appropriate
3397      Content-Transfer-Encoding is required when
3398      transmitted via E-mail.
3399    </t>
3400    <t hangText="Security considerations:">
3401      none
3402    </t>
3403    <t hangText="Interoperability considerations:">
3404      none
3405    </t>
3406    <t hangText="Published specification:">
3407      This specification (see <xref target="internet.media.type.application.http"/>).
3408    </t>
3409    <t hangText="Applications that use this media type:">
3410    </t>
3411    <t hangText="Additional information:">
3412      <list style="hanging">
3413        <t hangText="Magic number(s):">none</t>
3414        <t hangText="File extension(s):">none</t>
3415        <t hangText="Macintosh file type code(s):">none</t>
3416      </list>
3417    </t>
3418    <t hangText="Person and email address to contact for further information:">
3419      See Authors Section.
3420    </t>
3421    <t hangText="Intended usage:">
3422      COMMON
3423    </t>
3424    <t hangText="Restrictions on usage:">
3425      none
3426    </t>
3427    <t hangText="Author:">
3428      See Authors Section.
3429    </t>
3430    <t hangText="Change controller:">
3431      IESG
3432    </t>
3433  </list>
3434</t>
3435</section>
3436</section>
3437
3438<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3439<t>
3440   The HTTP Transfer Coding Registry defines the name space for transfer
3441   coding names. It is maintained at <eref target="http://www.iana.org/assignments/http-parameters"/>.
3442</t>
3443
3444<section title="Procedure" anchor="transfer.coding.registry.procedure">
3445<t>
3446   Registrations &MUST; include the following fields:
3447   <list style="symbols">
3448     <t>Name</t>
3449     <t>Description</t>
3450     <t>Pointer to specification text</t>
3451   </list>
3452</t>
3453<t>
3454   Names of transfer codings &MUST-NOT; overlap with names of content codings
3455   (&content-codings;) unless the encoding transformation is identical, as
3456   is the case for the compression codings defined in
3457   <xref target="compression.codings"/>.
3458</t>
3459<t>
3460   Values to be added to this name space require IETF Review (see
3461   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3462   conform to the purpose of transfer coding defined in this specification.
3463</t>
3464<t>
3465   Use of program names for the identification of encoding formats
3466   is not desirable and is discouraged for future encodings.
3467</t>
3468</section>
3469
3470<section title="Registration" anchor="transfer.coding.registration">
3471<t>
3472   The HTTP Transfer Coding Registry shall be updated with the registrations
3473   below:
3474</t>
3475<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3476   <ttcol>Name</ttcol>
3477   <ttcol>Description</ttcol>
3478   <ttcol>Reference</ttcol>
3479   <c>chunked</c>
3480   <c>Transfer in a series of chunks</c>
3481   <c>
3482      <xref target="chunked.encoding"/>
3483   </c>
3484   <c>compress</c>
3485   <c>UNIX "compress" program method</c>
3486   <c>
3487      <xref target="compress.coding"/>
3488   </c>
3489   <c>deflate</c>
3490   <c>"deflate" compression mechanism (<xref target="RFC1951"/>) used inside
3491   the "zlib" data format (<xref target="RFC1950"/>)
3492   </c>
3493   <c>
3494      <xref target="deflate.coding"/>
3495   </c>
3496   <c>gzip</c>
3497   <c>Same as GNU zip <xref target="RFC1952"/></c>
3498   <c>
3499      <xref target="gzip.coding"/>
3500   </c>
3501</texttable>
3502</section>
3503</section>
3504
3505<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3506<t>
3507   The HTTP Upgrade Token Registry defines the name space for protocol-name
3508   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3509   field. The registry is maintained at <eref target="http://www.iana.org/assignments/http-upgrade-tokens"/>.
3510</t>
3511
3512<section title="Procedure" anchor="upgrade.token.registry.procedure">  
3513<t>
3514   Each registered protocol name is associated with contact information
3515   and an optional set of specifications that details how the connection
3516   will be processed after it has been upgraded.
3517</t>
3518<t>
3519   Registrations happen on a "First Come First Served" basis (see
3520   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3521   following rules:
3522  <list style="numbers">
3523    <t>A protocol-name token, once registered, stays registered forever.</t>
3524    <t>The registration &MUST; name a responsible party for the
3525       registration.</t>
3526    <t>The registration &MUST; name a point of contact.</t>
3527    <t>The registration &MAY; name a set of specifications associated with
3528       that token. Such specifications need not be publicly available.</t>
3529    <t>The registration &SHOULD; name a set of expected "protocol-version"
3530       tokens associated with that token at the time of registration.</t>
3531    <t>The responsible party &MAY; change the registration at any time.
3532       The IANA will keep a record of all such changes, and make them
3533       available upon request.</t>
3534    <t>The IESG &MAY; reassign responsibility for a protocol token.
3535       This will normally only be used in the case when a
3536       responsible party cannot be contacted.</t>
3537  </list>
3538</t>
3539<t>
3540   This registration procedure for HTTP Upgrade Tokens replaces that
3541   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3542</t>
3543</section>
3544
3545<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3546<t>
3547   The HTTP Upgrade Token Registry shall be updated with the registration
3548   below:
3549</t>
3550<texttable align="left" suppress-title="true">
3551   <ttcol>Value</ttcol>
3552   <ttcol>Description</ttcol>
3553   <ttcol>Expected Version Tokens</ttcol>
3554   <ttcol>Reference</ttcol>
3555
3556   <c>HTTP</c>
3557   <c>Hypertext Transfer Protocol</c>
3558   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3559   <c><xref target="http.version"/></c>
3560</texttable>
3561<t>
3562   The responsible party is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3563</t>
3564</section>
3565</section>
3566
3567</section>
3568
3569<section title="Security Considerations" anchor="security.considerations">
3570<t>
3571   This section is meant to inform developers, information providers, and
3572   users of known security concerns relevant to HTTP/1.1 message syntax,
3573   parsing, and routing.
3574</t>
3575
3576<section title="DNS-related Attacks" anchor="dns.related.attacks">
3577<t>
3578   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3579   generally prone to security attacks based on the deliberate misassociation
3580   of IP addresses and DNS names not protected by DNSSEC. Clients need to be
3581   cautious in assuming the validity of an IP number/DNS name association unless
3582   the response is protected by DNSSEC (<xref target="RFC4033"/>).
3583</t>
3584</section>
3585
3586<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3587<t>
3588   By their very nature, HTTP intermediaries are men-in-the-middle, and
3589   represent an opportunity for man-in-the-middle attacks. Compromise of
3590   the systems on which the intermediaries run can result in serious security
3591   and privacy problems. Intermediaries have access to security-related
3592   information, personal information about individual users and
3593   organizations, and proprietary information belonging to users and
3594   content providers. A compromised intermediary, or an intermediary
3595   implemented or configured without regard to security and privacy
3596   considerations, might be used in the commission of a wide range of
3597   potential attacks.
3598</t>
3599<t>
3600   Intermediaries that contain a shared cache are especially vulnerable
3601   to cache poisoning attacks.
3602</t>
3603<t>
3604   Implementers need to consider the privacy and security
3605   implications of their design and coding decisions, and of the
3606   configuration options they provide to operators (especially the
3607   default configuration).
3608</t>
3609<t>
3610   Users need to be aware that intermediaries are no more trustworthy than
3611   the people who run them; HTTP itself cannot solve this problem.
3612</t>
3613</section>
3614
3615<section title="Buffer Overflows" anchor="attack.protocol.element.size.overflows">
3616<t>
3617   Because HTTP uses mostly textual, character-delimited fields, attackers can
3618   overflow buffers in implementations, and/or perform a Denial of Service
3619   against implementations that accept fields with unlimited lengths.
3620</t>
3621<t>
3622   To promote interoperability, this specification makes specific
3623   recommendations for minimum size limits on request-line
3624   (<xref target="request.line"/>)
3625   and blocks of header fields (<xref target="header.fields"/>). These are
3626   minimum recommendations, chosen to be supportable even by implementations
3627   with limited resources; it is expected that most implementations will
3628   choose substantially higher limits.
3629</t>
3630<t>
3631   This specification also provides a way for servers to reject messages that
3632   have request-targets that are too long (&status-414;) or request entities
3633   that are too large (&status-4xx;).
3634</t>
3635<t>
3636   Recipients &SHOULD; carefully limit the extent to which they read other
3637   fields, including (but not limited to) request methods, response status
3638   phrases, header field-names, and body chunks, so as to avoid denial of
3639   service attacks without impeding interoperability.
3640</t>
3641</section>
3642
3643<section title="Message Integrity" anchor="message.integrity">
3644<t>
3645   HTTP does not define a specific mechanism for ensuring message integrity,
3646   instead relying on the error-detection ability of underlying transport
3647   protocols and the use of length or chunk-delimited framing to detect
3648   completeness. Additional integrity mechanisms, such as hash functions or
3649   digital signatures applied to the content, can be selectively added to
3650   messages via extensible metadata header fields. Historically, the lack of
3651   a single integrity mechanism has been justified by the informal nature of
3652   most HTTP communication.  However, the prevalence of HTTP as an information
3653   access mechanism has resulted in its increasing use within environments
3654   where verification of message integrity is crucial.
3655</t>
3656<t>
3657   User agents are encouraged to implement configurable means for detecting
3658   and reporting failures of message integrity such that those means can be
3659   enabled within environments for which integrity is necessary. For example,
3660   a browser being used to view medical history or drug interaction
3661   information needs to indicate to the user when such information is detected
3662   by the protocol to be incomplete, expired, or corrupted during transfer.
3663   Such mechanisms might be selectively enabled via user agent extensions or
3664   the presence of message integrity metadata in a response.
3665   At a minimum, user agents ought to provide some indication that allows a
3666   user to distinguish between a complete and incomplete response message
3667   (<xref target="incomplete.messages"/>) when such verification is desired.
3668</t>
3669</section>
3670
3671<section title="Server Log Information" anchor="abuse.of.server.log.information">
3672<t>
3673   A server is in the position to save personal data about a user's requests
3674   over time, which might identify their reading patterns or subjects of
3675   interest.  In particular, log information gathered at an intermediary
3676   often contains a history of user agent interaction, across a multitude
3677   of sites, that can be traced to individual users.
3678</t>
3679<t>
3680   HTTP log information is confidential in nature; its handling is often
3681   constrained by laws and regulations.  Log information needs to be securely
3682   stored and appropriate guidelines followed for its analysis.
3683   Anonymization of personal information within individual entries helps,
3684   but is generally not sufficient to prevent real log traces from being
3685   re-identified based on correlation with other access characteristics.
3686   As such, access traces that are keyed to a specific client should not
3687   be published even if the key is pseudonymous.
3688</t>
3689<t>
3690   To minimize the risk of theft or accidental publication, log information
3691   should be purged of personally identifiable information, including
3692   user identifiers, IP addresses, and user-provided query parameters,
3693   as soon as that information is no longer necessary to support operational
3694   needs for security, auditing, or fraud control.
3695</t>
3696</section>
3697</section>
3698
3699<section title="Acknowledgments" anchor="acks">
3700<t>
3701   This edition of HTTP/1.1 builds on the many contributions that went into
3702   <xref target="RFC1945" format="none">RFC 1945</xref>,
3703   <xref target="RFC2068" format="none">RFC 2068</xref>,
3704   <xref target="RFC2145" format="none">RFC 2145</xref>, and
3705   <xref target="RFC2616" format="none">RFC 2616</xref>, including
3706   substantial contributions made by the previous authors, editors, and
3707   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
3708   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
3709   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
3710</t>
3711<t>
3712   Since 1999, the following contributors have helped improve the HTTP
3713   specification by reporting bugs, asking smart questions, drafting or
3714   reviewing text, and evaluating open issues:
3715</t>
3716<?BEGININC acks ?>
3717<t>Adam Barth,
3718Adam Roach,
3719Addison Phillips,
3720Adrian Chadd,
3721Adrien W. de Croy,
3722Alan Ford,
3723Alan Ruttenberg,
3724Albert Lunde,
3725Alek Storm,
3726Alex Rousskov,
3727Alexandre Morgaut,
3728Alexey Melnikov,
3729Alisha Smith,
3730Amichai Rothman,
3731Amit Klein,
3732Amos Jeffries,
3733Andreas Maier,
3734Andreas Petersson,
3735Anil Sharma,
3736Anne van Kesteren,
3737Anthony Bryan,
3738Asbjorn Ulsberg,
3739Ashok Kumar,
3740Balachander Krishnamurthy,
3741Barry Leiba,
3742Ben Laurie,
3743Benjamin Carlyle,
3744Benjamin Niven-Jenkins,
3745Bil Corry,
3746Bill Burke,
3747Bjoern Hoehrmann,
3748Bob Scheifler,
3749Boris Zbarsky,
3750Brett Slatkin,
3751Brian Kell,
3752Brian McBarron,
3753Brian Pane,
3754Brian Raymor,
3755Brian Smith,
3756Bryce Nesbitt,
3757Cameron Heavon-Jones,
3758Carl Kugler,
3759Carsten Bormann,
3760Charles Fry,
3761Chris Newman,
3762Cyrus Daboo,
3763Dale Robert Anderson,
3764Dan Wing,
3765Dan Winship,
3766Daniel Stenberg,
3767Darrel Miller,
3768Dave Cridland,
3769Dave Crocker,
3770Dave Kristol,
3771Dave Thaler,
3772David Booth,
3773David Singer,
3774David W. Morris,
3775Diwakar Shetty,
3776Dmitry Kurochkin,
3777Drummond Reed,
3778Duane Wessels,
3779Edward Lee,
3780Eitan Adler,
3781Eliot Lear,
3782Eran Hammer-Lahav,
3783Eric D. Williams,
3784Eric J. Bowman,
3785Eric Lawrence,
3786Eric Rescorla,
3787Erik Aronesty,
3788Evan Prodromou,
3789Felix Geisendoerfer,
3790Florian Weimer,
3791Frank Ellermann,
3792Fred Bohle,
3793Frederic Kayser,
3794Gabriel Montenegro,
3795Geoffrey Sneddon,
3796Gervase Markham,
3797Grahame Grieve,
3798Greg Wilkins,
3799Grzegorz Calkowski,
3800Harald Tveit Alvestrand,
3801Harry Halpin,
3802Helge Hess,
3803Henrik Nordstrom,
3804Henry S. Thompson,
3805Henry Story,
3806Herbert van de Sompel,
3807Herve Ruellan,
3808Howard Melman,
3809Hugo Haas,
3810Ian Fette,
3811Ian Hickson,
3812Ido Safruti,
3813Ilari Liusvaara,
3814Ilya Grigorik,
3815Ingo Struck,
3816J. Ross Nicoll,
3817James Cloos,
3818James H. Manger,
3819James Lacey,
3820James M. Snell,
3821Jamie Lokier,
3822Jan Algermissen,
3823Jeff Hodges (who came up with the term 'effective Request-URI'),
3824Jeff Pinner,
3825Jeff Walden,
3826Jim Luther,
3827Jitu Padhye,
3828Joe D. Williams,
3829Joe Gregorio,
3830Joe Orton,
3831John C. Klensin,
3832John C. Mallery,
3833John Cowan,
3834John Kemp,
3835John Panzer,
3836John Schneider,
3837John Stracke,
3838John Sullivan,
3839Jonas Sicking,
3840Jonathan A. Rees,
3841Jonathan Billington,
3842Jonathan Moore,
3843Jonathan Silvera,
3844Jordi Ros,
3845Joris Dobbelsteen,
3846Josh Cohen,
3847Julien Pierre,
3848Jungshik Shin,
3849Justin Chapweske,
3850Justin Erenkrantz,
3851Justin James,
3852Kalvinder Singh,
3853Karl Dubost,
3854Keith Hoffman,
3855Keith Moore,
3856Ken Murchison,
3857Koen Holtman,
3858Konstantin Voronkov,
3859Kris Zyp,
3860Lisa Dusseault,
3861Maciej Stachowiak,
3862Manu Sporny,
3863Marc Schneider,
3864Marc Slemko,
3865Mark Baker,
3866Mark Pauley,
3867Mark Watson,
3868Markus Isomaki,
3869Markus Lanthaler,
3870Martin J. Duerst,
3871Martin Musatov,
3872Martin Nilsson,
3873Martin Thomson,
3874Matt Lynch,
3875Matthew Cox,
3876Max Clark,
3877Michael Burrows,
3878Michael Hausenblas,
3879Mike Amundsen,
3880Mike Belshe,
3881Mike Kelly,
3882Mike Schinkel,
3883Miles Sabin,
3884Murray S. Kucherawy,
3885Mykyta Yevstifeyev,
3886Nathan Rixham,
3887Nicholas Shanks,
3888Nico Williams,
3889Nicolas Alvarez,
3890Nicolas Mailhot,
3891Noah Slater,
3892Osama Mazahir,
3893Pablo Castro,
3894Pat Hayes,
3895Patrick R. McManus,
3896Paul E. Jones,
3897Paul Hoffman,
3898Paul Marquess,
3899Peter Lepeska,
3900Peter Saint-Andre,
3901Peter Watkins,
3902Phil Archer,
3903Philippe Mougin,
3904Phillip Hallam-Baker,
3905Piotr Dobrogost,
3906Poul-Henning Kamp,
3907Preethi Natarajan,
3908Rajeev Bector,
3909Ray Polk,
3910Reto Bachmann-Gmuer,
3911Richard Cyganiak,
3912Robby Simpson,
3913Robert Brewer,
3914Robert Collins,
3915Robert Mattson,
3916Robert O'Callahan,
3917Robert Olofsson,
3918Robert Sayre,
3919Robert Siemer,
3920Robert de Wilde,
3921Roberto Javier Godoy,
3922Roberto Peon,
3923Roland Zink,
3924Ronny Widjaja,
3925S. Mike Dierken,
3926Salvatore Loreto,
3927Sam Johnston,
3928Sam Ruby,
3929Scott Lawrence (who maintained the original issues list),
3930Sean B. Palmer,
3931Shane McCarron,
3932Stefan Eissing,
3933Stefan Tilkov,
3934Stefanos Harhalakis,
3935Stephane Bortzmeyer,
3936Stephen Farrell,
3937Stephen Ludin,
3938Stuart Williams,
3939Subbu Allamaraju,
3940Sylvain Hellegouarch,
3941Tapan Divekar,
3942Tatsuya Hayashi,
3943Ted Hardie,
3944Thomas Broyer,
3945Thomas Fossati,
3946Thomas Maslen,
3947Thomas Nordin,
3948Thomas Roessler,
3949Tim Bray,
3950Tim Morgan,
3951Tim Olsen,
3952Tom Zhou,
3953Travis Snoozy,
3954Tyler Close,
3955Vincent Murphy,
3956Wenbo Zhu,
3957Werner Baumann,
3958Wilbur Streett,
3959Wilfredo Sanchez Vega,
3960William A. Rowe Jr.,
3961William Chan,
3962Willy Tarreau,
3963Xiaoshu Wang,
3964Yaron Goland,
3965Yngve Nysaeter Pettersen,
3966Yoav Nir,
3967Yogesh Bang,
3968Yutaka Oiwa,
3969Yves Lafon (long-time member of the editor team),
3970Zed A. Shaw, and
3971Zhong Yu.
3972</t>
3973<?ENDINC acks ?>
3974<t>
3975   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
3976   acknowledgements from prior revisions.
3977</t>
3978</section>
3979
3980</middle>
3981<back>
3982
3983<references title="Normative References">
3984
3985<reference anchor="Part2">
3986  <front>
3987    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
3988    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3989      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
3990      <address><email>fielding@gbiv.com</email></address>
3991    </author>
3992    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3993      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3994      <address><email>julian.reschke@greenbytes.de</email></address>
3995    </author>
3996    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3997  </front>
3998  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
3999  <x:source href="p2-semantics.xml" basename="p2-semantics">
4000    <x:defines>1xx (Informational)</x:defines>
4001    <x:defines>1xx</x:defines>
4002    <x:defines>100 (Continue)</x:defines>
4003    <x:defines>101 (Switching Protocols)</x:defines>
4004    <x:defines>2xx (Successful)</x:defines>
4005    <x:defines>2xx</x:defines>
4006    <x:defines>200 (OK)</x:defines>
4007    <x:defines>204 (No Content)</x:defines>
4008    <x:defines>3xx (Redirection)</x:defines>
4009    <x:defines>3xx</x:defines>
4010    <x:defines>301 (Moved Permanently)</x:defines>
4011    <x:defines>4xx (Client Error)</x:defines>
4012    <x:defines>4xx</x:defines>
4013    <x:defines>400 (Bad Request)</x:defines>
4014    <x:defines>411 (Length Required)</x:defines>
4015    <x:defines>414 (URI Too Long)</x:defines>
4016    <x:defines>417 (Expectation Failed)</x:defines>
4017    <x:defines>426 (Upgrade Required)</x:defines>
4018    <x:defines>501 (Not Implemented)</x:defines>
4019    <x:defines>502 (Bad Gateway)</x:defines>
4020    <x:defines>505 (HTTP Version Not Supported)</x:defines>
4021    <x:defines>Allow</x:defines>
4022    <x:defines>Content-Encoding</x:defines>
4023    <x:defines>Content-Location</x:defines>
4024    <x:defines>Content-Type</x:defines>
4025    <x:defines>Date</x:defines>
4026    <x:defines>Expect</x:defines>
4027    <x:defines>Location</x:defines>
4028    <x:defines>Server</x:defines>
4029    <x:defines>User-Agent</x:defines>
4030  </x:source>
4031</reference>
4032
4033<reference anchor="Part4">
4034  <front>
4035    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
4036    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
4037      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4038      <address><email>fielding@gbiv.com</email></address>
4039    </author>
4040    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
4041      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4042      <address><email>julian.reschke@greenbytes.de</email></address>
4043    </author>
4044    <date month="&ID-MONTH;" year="&ID-YEAR;" />
4045  </front>
4046  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
4047  <x:source basename="p4-conditional" href="p4-conditional.xml">
4048    <x:defines>304 (Not Modified)</x:defines>
4049    <x:defines>ETag</x:defines>
4050    <x:defines>Last-Modified</x:defines>
4051  </x:source>
4052</reference>
4053
4054<reference anchor="Part5">
4055  <front>
4056    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
4057    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4058      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4059      <address><email>fielding@gbiv.com</email></address>
4060    </author>
4061    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
4062      <organization abbrev="W3C">World Wide Web Consortium</organization>
4063      <address><email>ylafon@w3.org</email></address>
4064    </author>
4065    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4066      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4067      <address><email>julian.reschke@greenbytes.de</email></address>
4068    </author>
4069    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4070  </front>
4071  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4072  <x:source href="p5-range.xml" basename="p5-range">
4073    <x:defines>Content-Range</x:defines>
4074  </x:source>
4075</reference>
4076
4077<reference anchor="Part6">
4078  <front>
4079    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4080    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4081      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4082      <address><email>fielding@gbiv.com</email></address>
4083    </author>
4084    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4085      <organization>Akamai</organization>
4086      <address><email>mnot@mnot.net</email></address>
4087    </author>
4088    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4089      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4090      <address><email>julian.reschke@greenbytes.de</email></address>
4091    </author>
4092    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4093  </front>
4094  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4095  <x:source href="p6-cache.xml" basename="p6-cache">
4096    <x:defines>Cache-Control</x:defines>
4097    <x:defines>Expires</x:defines>
4098  </x:source>
4099</reference>
4100
4101<reference anchor="Part7">
4102  <front>
4103    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4104    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4105      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4106      <address><email>fielding@gbiv.com</email></address>
4107    </author>
4108    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4109      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4110      <address><email>julian.reschke@greenbytes.de</email></address>
4111    </author>
4112    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4113  </front>
4114  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4115  <x:source href="p7-auth.xml" basename="p7-auth">
4116    <x:defines>Proxy-Authenticate</x:defines>
4117    <x:defines>Proxy-Authorization</x:defines>
4118  </x:source>
4119</reference>
4120
4121<reference anchor="RFC5234">
4122  <front>
4123    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4124    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4125      <organization>Brandenburg InternetWorking</organization>
4126      <address>
4127        <email>dcrocker@bbiw.net</email>
4128      </address> 
4129    </author>
4130    <author initials="P." surname="Overell" fullname="Paul Overell">
4131      <organization>THUS plc.</organization>
4132      <address>
4133        <email>paul.overell@thus.net</email>
4134      </address>
4135    </author>
4136    <date month="January" year="2008"/>
4137  </front>
4138  <seriesInfo name="STD" value="68"/>
4139  <seriesInfo name="RFC" value="5234"/>
4140</reference>
4141
4142<reference anchor="RFC2119">
4143  <front>
4144    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4145    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4146      <organization>Harvard University</organization>
4147      <address><email>sob@harvard.edu</email></address>
4148    </author>
4149    <date month="March" year="1997"/>
4150  </front>
4151  <seriesInfo name="BCP" value="14"/>
4152  <seriesInfo name="RFC" value="2119"/>
4153</reference>
4154
4155<reference anchor="RFC3986">
4156 <front>
4157  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4158  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4159    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4160    <address>
4161       <email>timbl@w3.org</email>
4162       <uri>http://www.w3.org/People/Berners-Lee/</uri>
4163    </address>
4164  </author>
4165  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4166    <organization abbrev="Day Software">Day Software</organization>
4167    <address>
4168      <email>fielding@gbiv.com</email>
4169      <uri>http://roy.gbiv.com/</uri>
4170    </address>
4171  </author>
4172  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4173    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4174    <address>
4175      <email>LMM@acm.org</email>
4176      <uri>http://larry.masinter.net/</uri>
4177    </address>
4178  </author>
4179  <date month='January' year='2005'></date>
4180 </front>
4181 <seriesInfo name="STD" value="66"/>
4182 <seriesInfo name="RFC" value="3986"/>
4183</reference>
4184
4185<reference anchor="RFC0793">
4186  <front>
4187    <title>Transmission Control Protocol</title>
4188    <author initials='J.' surname='Postel' fullname='Jon Postel'>
4189      <organization>University of Southern California (USC)/Information Sciences Institute</organization>
4190    </author>
4191    <date year='1981' month='September' />
4192  </front>
4193  <seriesInfo name='STD' value='7' />
4194  <seriesInfo name='RFC' value='793' />
4195</reference>
4196
4197<reference anchor="USASCII">
4198  <front>
4199    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4200    <author>
4201      <organization>American National Standards Institute</organization>
4202    </author>
4203    <date year="1986"/>
4204  </front>
4205  <seriesInfo name="ANSI" value="X3.4"/>
4206</reference>
4207
4208<reference anchor="RFC1950">
4209  <front>
4210    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4211    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4212      <organization>Aladdin Enterprises</organization>
4213      <address><email>ghost@aladdin.com</email></address>
4214    </author>
4215    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4216    <date month="May" year="1996"/>
4217  </front>
4218  <seriesInfo name="RFC" value="1950"/>
4219  <!--<annotation>
4220    RFC 1950 is an Informational RFC, thus it might be less stable than
4221    this specification. On the other hand, this downward reference was
4222    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4223    therefore it is unlikely to cause problems in practice. See also
4224    <xref target="BCP97"/>.
4225  </annotation>-->
4226</reference>
4227
4228<reference anchor="RFC1951">
4229  <front>
4230    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4231    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4232      <organization>Aladdin Enterprises</organization>
4233      <address><email>ghost@aladdin.com</email></address>
4234    </author>
4235    <date month="May" year="1996"/>
4236  </front>
4237  <seriesInfo name="RFC" value="1951"/>
4238  <!--<annotation>
4239    RFC 1951 is an Informational RFC, thus it might be less stable than
4240    this specification. On the other hand, this downward reference was
4241    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4242    therefore it is unlikely to cause problems in practice. See also
4243    <xref target="BCP97"/>.
4244  </annotation>-->
4245</reference>
4246
4247<reference anchor="RFC1952">
4248  <front>
4249    <title>GZIP file format specification version 4.3</title>
4250    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4251      <organization>Aladdin Enterprises</organization>
4252      <address><email>ghost@aladdin.com</email></address>
4253    </author>
4254    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4255      <address><email>gzip@prep.ai.mit.edu</email></address>
4256    </author>
4257    <author initials="M." surname="Adler" fullname="Mark Adler">
4258      <address><email>madler@alumni.caltech.edu</email></address>
4259    </author>
4260    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4261      <address><email>ghost@aladdin.com</email></address>
4262    </author>
4263    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4264      <address><email>randeg@alumni.rpi.edu</email></address>
4265    </author>
4266    <date month="May" year="1996"/>
4267  </front>
4268  <seriesInfo name="RFC" value="1952"/>
4269  <!--<annotation>
4270    RFC 1952 is an Informational RFC, thus it might be less stable than
4271    this specification. On the other hand, this downward reference was
4272    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4273    therefore it is unlikely to cause problems in practice. See also
4274    <xref target="BCP97"/>.
4275  </annotation>-->
4276</reference>
4277
4278</references>
4279
4280<references title="Informative References">
4281
4282<reference anchor="ISO-8859-1">
4283  <front>
4284    <title>
4285     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4286    </title>
4287    <author>
4288      <organization>International Organization for Standardization</organization>
4289    </author>
4290    <date year="1998"/>
4291  </front>
4292  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4293</reference>
4294
4295<reference anchor='RFC1919'>
4296  <front>
4297    <title>Classical versus Transparent IP Proxies</title>
4298    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4299      <address><email>mchatel@pax.eunet.ch</email></address>
4300    </author>
4301    <date year='1996' month='March' />
4302  </front>
4303  <seriesInfo name='RFC' value='1919' />
4304</reference>
4305
4306<reference anchor="RFC1945">
4307  <front>
4308    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4309    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4310      <organization>MIT, Laboratory for Computer Science</organization>
4311      <address><email>timbl@w3.org</email></address>
4312    </author>
4313    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4314      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4315      <address><email>fielding@ics.uci.edu</email></address>
4316    </author>
4317    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4318      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4319      <address><email>frystyk@w3.org</email></address>
4320    </author>
4321    <date month="May" year="1996"/>
4322  </front>
4323  <seriesInfo name="RFC" value="1945"/>
4324</reference>
4325
4326<reference anchor="RFC2045">
4327  <front>
4328    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4329    <author initials="N." surname="Freed" fullname="Ned Freed">
4330      <organization>Innosoft International, Inc.</organization>
4331      <address><email>ned@innosoft.com</email></address>
4332    </author>
4333    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4334      <organization>First Virtual Holdings</organization>
4335      <address><email>nsb@nsb.fv.com</email></address>
4336    </author>
4337    <date month="November" year="1996"/>
4338  </front>
4339  <seriesInfo name="RFC" value="2045"/>
4340</reference>
4341
4342<reference anchor="RFC2047">
4343  <front>
4344    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4345    <author initials="K." surname="Moore" fullname="Keith Moore">
4346      <organization>University of Tennessee</organization>
4347      <address><email>moore@cs.utk.edu</email></address>
4348    </author>
4349    <date month="November" year="1996"/>
4350  </front>
4351  <seriesInfo name="RFC" value="2047"/>
4352</reference>
4353
4354<reference anchor="RFC2068">
4355  <front>
4356    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4357    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
4358      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4359      <address><email>fielding@ics.uci.edu</email></address>
4360    </author>
4361    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4362      <organization>MIT Laboratory for Computer Science</organization>
4363      <address><email>jg@w3.org</email></address>
4364    </author>
4365    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4366      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4367      <address><email>mogul@wrl.dec.com</email></address>
4368    </author>
4369    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4370      <organization>MIT Laboratory for Computer Science</organization>
4371      <address><email>frystyk@w3.org</email></address>
4372    </author>
4373    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4374      <organization>MIT Laboratory for Computer Science</organization>
4375      <address><email>timbl@w3.org</email></address>
4376    </author>
4377    <date month="January" year="1997"/>
4378  </front>
4379  <seriesInfo name="RFC" value="2068"/>
4380</reference>
4381
4382<reference anchor="RFC2145">
4383  <front>
4384    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4385    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4386      <organization>Western Research Laboratory</organization>
4387      <address><email>mogul@wrl.dec.com</email></address>
4388    </author>
4389    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4390      <organization>Department of Information and Computer Science</organization>
4391      <address><email>fielding@ics.uci.edu</email></address>
4392    </author>
4393    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4394      <organization>MIT Laboratory for Computer Science</organization>
4395      <address><email>jg@w3.org</email></address>
4396    </author>
4397    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4398      <organization>W3 Consortium</organization>
4399      <address><email>frystyk@w3.org</email></address>
4400    </author>
4401    <date month="May" year="1997"/>
4402  </front>
4403  <seriesInfo name="RFC" value="2145"/>
4404</reference>
4405
4406<reference anchor="RFC2616">
4407  <front>
4408    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4409    <author initials="R." surname="Fielding" fullname="R. Fielding">
4410      <organization>University of California, Irvine</organization>
4411      <address><email>fielding@ics.uci.edu</email></address>
4412    </author>
4413    <author initials="J." surname="Gettys" fullname="J. Gettys">
4414      <organization>W3C</organization>
4415      <address><email>jg@w3.org</email></address>
4416    </author>
4417    <author initials="J." surname="Mogul" fullname="J. Mogul">
4418      <organization>Compaq Computer Corporation</organization>
4419      <address><email>mogul@wrl.dec.com</email></address>
4420    </author>
4421    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4422      <organization>MIT Laboratory for Computer Science</organization>
4423      <address><email>frystyk@w3.org</email></address>
4424    </author>
4425    <author initials="L." surname="Masinter" fullname="L. Masinter">
4426      <organization>Xerox Corporation</organization>
4427      <address><email>masinter@parc.xerox.com</email></address>
4428    </author>
4429    <author initials="P." surname="Leach" fullname="P. Leach">
4430      <organization>Microsoft Corporation</organization>
4431      <address><email>paulle@microsoft.com</email></address>
4432    </author>
4433    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4434      <organization>W3C</organization>
4435      <address><email>timbl@w3.org</email></address>
4436    </author>
4437    <date month="June" year="1999"/>
4438  </front>
4439  <seriesInfo name="RFC" value="2616"/>
4440</reference>
4441
4442<reference anchor='RFC2817'>
4443  <front>
4444    <title>Upgrading to TLS Within HTTP/1.1</title>
4445    <author initials='R.' surname='Khare' fullname='R. Khare'>
4446      <organization>4K Associates / UC Irvine</organization>
4447      <address><email>rohit@4K-associates.com</email></address>
4448    </author>
4449    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4450      <organization>Agranat Systems, Inc.</organization>
4451      <address><email>lawrence@agranat.com</email></address>
4452    </author>
4453    <date year='2000' month='May' />
4454  </front>
4455  <seriesInfo name='RFC' value='2817' />
4456</reference>
4457
4458<reference anchor='RFC2818'>
4459  <front>
4460    <title>HTTP Over TLS</title>
4461    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4462      <organization>RTFM, Inc.</organization>
4463      <address><email>ekr@rtfm.com</email></address>
4464    </author>
4465    <date year='2000' month='May' />
4466  </front>
4467  <seriesInfo name='RFC' value='2818' />
4468</reference>
4469
4470<reference anchor='RFC3040'>
4471  <front>
4472    <title>Internet Web Replication and Caching Taxonomy</title>
4473    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4474      <organization>Equinix, Inc.</organization>
4475    </author>
4476    <author initials='I.' surname='Melve' fullname='I. Melve'>
4477      <organization>UNINETT</organization>
4478    </author>
4479    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4480      <organization>CacheFlow Inc.</organization>
4481    </author>
4482    <date year='2001' month='January' />
4483  </front>
4484  <seriesInfo name='RFC' value='3040' />
4485</reference>
4486
4487<reference anchor='BCP90'>
4488  <front>
4489    <title>Registration Procedures for Message Header Fields</title>
4490    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4491      <organization>Nine by Nine</organization>
4492      <address><email>GK-IETF@ninebynine.org</email></address>
4493    </author>
4494    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4495      <organization>BEA Systems</organization>
4496      <address><email>mnot@pobox.com</email></address>
4497    </author>
4498    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4499      <organization>HP Labs</organization>
4500      <address><email>JeffMogul@acm.org</email></address>
4501    </author>
4502    <date year='2004' month='September' />
4503  </front>
4504  <seriesInfo name='BCP' value='90' />
4505  <seriesInfo name='RFC' value='3864' />
4506</reference>
4507
4508<reference anchor='RFC4033'>
4509  <front>
4510    <title>DNS Security Introduction and Requirements</title>
4511    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4512    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4513    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4514    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4515    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4516    <date year='2005' month='March' />
4517  </front>
4518  <seriesInfo name='RFC' value='4033' />
4519</reference>
4520
4521<reference anchor="BCP13">
4522  <front>
4523    <title>Media Type Specifications and Registration Procedures</title>
4524    <author initials="N." surname="Freed" fullname="Ned Freed">
4525      <organization>Oracle</organization>
4526      <address>
4527        <email>ned+ietf@mrochek.com</email>
4528      </address>
4529    </author>
4530    <author initials="J." surname="Klensin" fullname="John C. Klensin">
4531      <address>
4532        <email>john+ietf@jck.com</email>
4533      </address>
4534    </author>
4535    <author initials="T." surname="Hansen" fullname="Tony Hansen">
4536      <organization>AT&amp;T Laboratories</organization>
4537      <address>
4538        <email>tony+mtsuffix@maillennium.att.com</email>
4539      </address>
4540    </author>
4541    <date year="2013" month="January"/>
4542  </front>
4543  <seriesInfo name="BCP" value="13"/>
4544  <seriesInfo name="RFC" value="6838"/>
4545</reference>
4546
4547<reference anchor='BCP115'>
4548  <front>
4549    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4550    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4551      <organization>AT&amp;T Laboratories</organization>
4552      <address>
4553        <email>tony+urireg@maillennium.att.com</email>
4554      </address>
4555    </author>
4556    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4557      <organization>Qualcomm, Inc.</organization>
4558      <address>
4559        <email>hardie@qualcomm.com</email>
4560      </address>
4561    </author>
4562    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4563      <organization>Adobe Systems</organization>
4564      <address>
4565        <email>LMM@acm.org</email>
4566      </address>
4567    </author>
4568    <date year='2006' month='February' />
4569  </front>
4570  <seriesInfo name='BCP' value='115' />
4571  <seriesInfo name='RFC' value='4395' />
4572</reference>
4573
4574<reference anchor='RFC4559'>
4575  <front>
4576    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4577    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4578    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4579    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4580    <date year='2006' month='June' />
4581  </front>
4582  <seriesInfo name='RFC' value='4559' />
4583</reference>
4584
4585<reference anchor='RFC5226'>
4586  <front>
4587    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4588    <author initials='T.' surname='Narten' fullname='T. Narten'>
4589      <organization>IBM</organization>
4590      <address><email>narten@us.ibm.com</email></address>
4591    </author>
4592    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4593      <organization>Google</organization>
4594      <address><email>Harald@Alvestrand.no</email></address>
4595    </author>
4596    <date year='2008' month='May' />
4597  </front>
4598  <seriesInfo name='BCP' value='26' />
4599  <seriesInfo name='RFC' value='5226' />
4600</reference>
4601
4602<reference anchor='RFC5246'>
4603   <front>
4604      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4605      <author initials='T.' surname='Dierks' fullname='T. Dierks'>
4606         <organization />
4607      </author>
4608      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4609         <organization>RTFM, Inc.</organization>
4610      </author>
4611      <date year='2008' month='August' />
4612   </front>
4613   <seriesInfo name='RFC' value='5246' />
4614</reference>
4615
4616<reference anchor="RFC5322">
4617  <front>
4618    <title>Internet Message Format</title>
4619    <author initials="P." surname="Resnick" fullname="P. Resnick">
4620      <organization>Qualcomm Incorporated</organization>
4621    </author>
4622    <date year="2008" month="October"/>
4623  </front>
4624  <seriesInfo name="RFC" value="5322"/>
4625</reference>
4626
4627<reference anchor="RFC6265">
4628  <front>
4629    <title>HTTP State Management Mechanism</title>
4630    <author initials="A." surname="Barth" fullname="Adam Barth">
4631      <organization abbrev="U.C. Berkeley">
4632        University of California, Berkeley
4633      </organization>
4634      <address><email>abarth@eecs.berkeley.edu</email></address>
4635    </author>
4636    <date year="2011" month="April" />
4637  </front>
4638  <seriesInfo name="RFC" value="6265"/>
4639</reference>
4640
4641<!--<reference anchor='BCP97'>
4642  <front>
4643    <title>Handling Normative References to Standards-Track Documents</title>
4644    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
4645      <address>
4646        <email>klensin+ietf@jck.com</email>
4647      </address>
4648    </author>
4649    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
4650      <organization>MIT</organization>
4651      <address>
4652        <email>hartmans-ietf@mit.edu</email>
4653      </address>
4654    </author>
4655    <date year='2007' month='June' />
4656  </front>
4657  <seriesInfo name='BCP' value='97' />
4658  <seriesInfo name='RFC' value='4897' />
4659</reference>-->
4660
4661<reference anchor="Kri2001" target="http://arxiv.org/abs/cs.SE/0105018">
4662  <front>
4663    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
4664    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
4665    <date year="2001" month="November"/>
4666  </front>
4667  <seriesInfo name="ACM Transactions on Internet Technology" value="Vol. 1, #2"/>
4668</reference>
4669
4670</references>
4671
4672
4673<section title="HTTP Version History" anchor="compatibility">
4674<t>
4675   HTTP has been in use by the World-Wide Web global information initiative
4676   since 1990. The first version of HTTP, later referred to as HTTP/0.9,
4677   was a simple protocol for hypertext data transfer across the Internet
4678   with only a single request method (GET) and no metadata.
4679   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
4680   methods and MIME-like messaging that could include metadata about the data
4681   transferred and modifiers on the request/response semantics. However,
4682   HTTP/1.0 did not sufficiently take into consideration the effects of
4683   hierarchical proxies, caching, the need for persistent connections, or
4684   name-based virtual hosts. The proliferation of incompletely-implemented
4685   applications calling themselves "HTTP/1.0" further necessitated a
4686   protocol version change in order for two communicating applications
4687   to determine each other's true capabilities.
4688</t>
4689<t>
4690   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
4691   requirements that enable reliable implementations, adding only
4692   those new features that will either be safely ignored by an HTTP/1.0
4693   recipient or only sent when communicating with a party advertising
4694   conformance with HTTP/1.1.
4695</t>
4696<t>
4697   It is beyond the scope of a protocol specification to mandate
4698   conformance with previous versions. HTTP/1.1 was deliberately
4699   designed, however, to make supporting previous versions easy.
4700   We would expect a general-purpose HTTP/1.1 server to understand
4701   any valid request in the format of HTTP/1.0 and respond appropriately
4702   with an HTTP/1.1 message that only uses features understood (or
4703   safely ignored) by HTTP/1.0 clients.  Likewise, we would expect
4704   an HTTP/1.1 client to understand any valid HTTP/1.0 response.
4705</t>
4706<t>
4707   Since HTTP/0.9 did not support header fields in a request,
4708   there is no mechanism for it to support name-based virtual
4709   hosts (selection of resource by inspection of the <x:ref>Host</x:ref> header
4710   field).  Any server that implements name-based virtual hosts
4711   ought to disable support for HTTP/0.9.  Most requests that
4712   appear to be HTTP/0.9 are, in fact, badly constructed HTTP/1.x
4713   requests wherein a buggy client failed to properly encode
4714   linear whitespace found in a URI reference and placed in
4715   the request-target.
4716</t>
4717
4718<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
4719<t>
4720   This section summarizes major differences between versions HTTP/1.0
4721   and HTTP/1.1.
4722</t>
4723
4724<section title="Multi-homed Web Servers" anchor="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">
4725<t>
4726   The requirements that clients and servers support the <x:ref>Host</x:ref>
4727   header field (<xref target="header.host"/>), report an error if it is
4728   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
4729   are among the most important changes defined by HTTP/1.1.
4730</t>
4731<t>
4732   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4733   addresses and servers; there was no other established mechanism for
4734   distinguishing the intended server of a request than the IP address
4735   to which that request was directed. The <x:ref>Host</x:ref> header field was
4736   introduced during the development of HTTP/1.1 and, though it was
4737   quickly implemented by most HTTP/1.0 browsers, additional requirements
4738   were placed on all HTTP/1.1 requests in order to ensure complete
4739   adoption.  At the time of this writing, most HTTP-based services
4740   are dependent upon the Host header field for targeting requests.
4741</t>
4742</section>
4743
4744<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4745<t>
4746   In HTTP/1.0, each connection is established by the client prior to the
4747   request and closed by the server after sending the response. However, some
4748   implementations implement the explicitly negotiated ("Keep-Alive") version
4749   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
4750   target="RFC2068"/>.
4751</t>
4752<t>
4753   Some clients and servers might wish to be compatible with these previous
4754   approaches to persistent connections, by explicitly negotiating for them
4755   with a "Connection: keep-alive" request header field. However, some
4756   experimental implementations of HTTP/1.0 persistent connections are faulty;
4757   for example, if an HTTP/1.0 proxy server doesn't understand
4758   <x:ref>Connection</x:ref>, it will erroneously forward that header field
4759   to the next inbound server, which would result in a hung connection.
4760</t>
4761<t>
4762   One attempted solution was the introduction of a Proxy-Connection header
4763   field, targeted specifically at proxies. In practice, this was also
4764   unworkable, because proxies are often deployed in multiple layers, bringing
4765   about the same problem discussed above.
4766</t>
4767<t>
4768   As a result, clients are encouraged not to send the Proxy-Connection header
4769   field in any requests.
4770</t>
4771<t>
4772   Clients are also encouraged to consider the use of Connection: keep-alive
4773   in requests carefully; while they can enable persistent connections with
4774   HTTP/1.0 servers, clients using them will need to monitor the
4775   connection for "hung" requests (which indicate that the client ought stop
4776   sending the header field), and this mechanism ought not be used by clients
4777   at all when a proxy is being used.
4778</t>
4779</section>
4780
4781<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
4782<t>
4783   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
4784   (<xref target="header.transfer-encoding"/>).
4785   Transfer codings need to be decoded prior to forwarding an HTTP message
4786   over a MIME-compliant protocol.
4787</t>
4788</section>
4789
4790</section>
4791
4792<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4793<t>
4794  HTTP's approach to error handling has been explained.
4795  (<xref target="conformance"/>)
4796</t>
4797<t>
4798  The expectation to support HTTP/0.9 requests has been removed.
4799</t>
4800<t>
4801  The term "Effective Request URI" has been introduced.
4802  (<xref target="effective.request.uri" />)
4803</t>
4804<t>
4805  HTTP messages can be (and often are) buffered by implementations; despite
4806  it sometimes being available as a stream, HTTP is fundamentally a
4807  message-oriented protocol.
4808  (<xref target="http.message" />)
4809</t>
4810<t>
4811  Minimum supported sizes for various protocol elements have been
4812  suggested, to improve interoperability.
4813</t>
4814<t>
4815  Header fields that span multiple lines ("line folding") are deprecated.
4816  (<xref target="field.parsing" />)
4817</t>
4818<t>
4819  The HTTP-version ABNF production has been clarified to be case-sensitive.
4820  Additionally, version numbers has been restricted to single digits, due
4821  to the fact that implementations are known to handle multi-digit version
4822  numbers incorrectly.
4823  (<xref target="http.version"/>)
4824</t>
4825<t>
4826  The HTTPS URI scheme is now defined by this specification; previously,
4827  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
4828  (<xref target="https.uri"/>)
4829</t>
4830<t>
4831  The HTTPS URI scheme implies end-to-end security.
4832  (<xref target="https.uri"/>)
4833</t>
4834<t>
4835  Userinfo (i.e., username and password) are now disallowed in HTTP and
4836  HTTPS URIs, because of security issues related to their transmission on the
4837  wire.
4838  (<xref target="http.uri" />)
4839</t>
4840<t>
4841  Invalid whitespace around field-names is now required to be rejected,
4842  because accepting it represents a security vulnerability.
4843  (<xref target="header.fields"/>)
4844</t>
4845<t>
4846  The ABNF productions defining header fields now only list the field value.
4847  (<xref target="header.fields"/>)
4848</t>
4849<t>
4850  Rules about implicit linear whitespace between certain grammar productions
4851  have been removed; now whitespace is only allowed where specifically
4852  defined in the ABNF.
4853  (<xref target="whitespace"/>)
4854</t>
4855<t> 
4856  The NUL octet is no longer allowed in comment and quoted-string text, and
4857  handling of backslash-escaping in them has been clarified.
4858  (<xref target="field.components"/>)
4859</t> 
4860<t>
4861  The quoted-pair rule no longer allows escaping control characters other than
4862  HTAB.
4863  (<xref target="field.components"/>)
4864</t>
4865<t>
4866  Non-ASCII content in header fields and the reason phrase has been obsoleted
4867  and made opaque (the TEXT rule was removed).
4868  (<xref target="field.components"/>)
4869</t>
4870<t>
4871  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
4872  handled as errors by recipients.
4873  (<xref target="header.content-length"/>)
4874</t>
4875<t>
4876  The "identity" transfer coding token has been removed.
4877  (Sections <xref format="counter" target="message.body"/> and
4878  <xref format="counter" target="transfer.codings"/>)
4879</t>
4880<t>
4881  The algorithm for determining the message body length has been clarified
4882  to indicate all of the special cases (e.g., driven by methods or status
4883  codes) that affect it, and that new protocol elements cannot define such
4884  special cases.
4885  (<xref target="message.body.length"/>)
4886</t>
4887<t>
4888  "multipart/byteranges" is no longer a way of determining message body length
4889  detection.
4890  (<xref target="message.body.length"/>)
4891</t>
4892<t>
4893  CONNECT is a new, special case in determining message body length.
4894  (<xref target="message.body.length"/>)
4895</t>
4896<t>
4897  Chunk length does not include the count of the octets in the
4898  chunk header and trailer.
4899  (<xref target="chunked.encoding"/>)
4900</t>
4901<t>
4902  Use of chunk extensions is deprecated, and line folding in them is
4903  disallowed.
4904  (<xref target="chunked.encoding"/>)
4905</t>
4906<t>
4907  The segment + query components of RFC3986 have been used to define the
4908  request-target, instead of abs_path from RFC 1808.
4909  (<xref target="request-target"/>)
4910</t>
4911<t>
4912  The asterisk form of the request-target is only allowed in the OPTIONS
4913  method.
4914  (<xref target="request-target"/>)
4915</t>
4916<t>
4917  Exactly when "close" connection options have to be sent has been clarified.
4918  (<xref target="header.connection"/>)
4919</t>
4920<t> 
4921  "hop-by-hop" header fields are required to appear in the Connection header
4922  field; just because they're defined as hop-by-hop in this specification
4923  doesn't exempt them.
4924  (<xref target="header.connection"/>)
4925</t>
4926<t>
4927  The limit of two connections per server has been removed.
4928  (<xref target="persistent.connections"/>)
4929</t>
4930<t>
4931  An idempotent sequence of requests is no longer required to be retried.
4932  (<xref target="persistent.connections"/>)
4933</t>
4934<t>
4935  The requirement to retry requests under certain circumstances when the
4936  server prematurely closes the connection has been removed.
4937  (<xref target="persistent.connections"/>)
4938</t>
4939<t>
4940  Some extraneous requirements about when servers are allowed to close
4941  connections prematurely have been removed.
4942  (<xref target="persistent.connections"/>)
4943</t>
4944<t>
4945  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
4946  responses other than 101 (this was incorporated from <xref
4947  target="RFC2817"/>).
4948  (<xref target="header.upgrade"/>)
4949</t>
4950<t>
4951  Registration of Transfer Codings now requires IETF Review
4952  (<xref target="transfer.coding.registry"/>)
4953</t>
4954<t>
4955  The meaning of the "deflate" content coding has been clarified.
4956  (<xref target="deflate.coding" />)
4957</t>
4958<t>
4959  This specification now defines the Upgrade Token Registry, previously
4960  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
4961  (<xref target="upgrade.token.registry"/>)
4962</t>
4963<t>
4964  Empty list elements in list productions (e.g., a list header containing
4965  ", ,") have been deprecated.
4966  (<xref target="abnf.extension"/>)
4967</t>
4968<t>
4969  Issues with the Keep-Alive and Proxy-Connection headers in requests
4970  are pointed out, with use of the latter being discouraged altogether.
4971  (<xref target="compatibility.with.http.1.0.persistent.connections" />)
4972</t>
4973</section>
4974</section>
4975
4976<section title="ABNF list extension: #rule" anchor="abnf.extension">
4977<t>
4978  A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
4979  improve readability in the definitions of some header field values.
4980</t>
4981<t>
4982  A construct "#" is defined, similar to "*", for defining comma-delimited
4983  lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
4984  at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
4985  comma (",") and optional whitespace (OWS).   
4986</t>
4987<figure><preamble>
4988  Thus,
4989</preamble><artwork type="example">
4990  1#element =&gt; element *( OWS "," OWS element )
4991</artwork></figure>
4992<figure><preamble>
4993  and:
4994</preamble><artwork type="example">
4995  #element =&gt; [ 1#element ]
4996</artwork></figure>
4997<figure><preamble>
4998  and for n &gt;= 1 and m &gt; 1:
4999</preamble><artwork type="example">
5000  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
5001</artwork></figure>
5002<t>
5003  For compatibility with legacy list rules, recipients &SHOULD; accept empty
5004  list elements. In other words, consumers would follow the list productions:
5005</t>
5006<figure><artwork type="example">
5007  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
5008 
5009  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
5010</artwork></figure>
5011<t>
5012  Note that empty elements do not contribute to the count of elements present,
5013  though.
5014</t>
5015<t>
5016  For example, given these ABNF productions:
5017</t>
5018<figure><artwork type="example">
5019  example-list      = 1#example-list-elmt
5020  example-list-elmt = token ; see <xref target="field.components"/>
5021</artwork></figure>
5022<t>
5023  Then these are valid values for example-list (not including the double
5024  quotes, which are present for delimitation only):
5025</t>
5026<figure><artwork type="example">
5027  "foo,bar"
5028  "foo ,bar,"
5029  "foo , ,bar,charlie   "
5030</artwork></figure>
5031<t>
5032  But these values would be invalid, as at least one non-empty element is
5033  required:
5034</t>
5035<figure><artwork type="example">
5036  ""
5037  ","
5038  ",   ,"
5039</artwork></figure>
5040<t>
5041  <xref target="collected.abnf"/> shows the collected ABNF, with the list rules
5042  expanded as explained above.
5043</t>
5044</section>
5045
5046<?BEGININC p1-messaging.abnf-appendix ?>
5047<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
5048<figure>
5049<artwork type="abnf" name="p1-messaging.parsed-abnf">
5050<x:ref>BWS</x:ref> = OWS
5051
5052<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
5053 connection-option ] )
5054<x:ref>Content-Length</x:ref> = 1*DIGIT
5055
5056<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
5057 ]
5058<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
5059<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
5060<x:ref>Host</x:ref> = uri-host [ ":" port ]
5061
5062<x:ref>OWS</x:ref> = *( SP / HTAB )
5063
5064<x:ref>RWS</x:ref> = 1*( SP / HTAB )
5065
5066<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
5067<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
5068<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
5069 transfer-coding ] )
5070
5071<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
5072<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
5073
5074<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
5075 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
5076 comment ] ) ] )
5077
5078<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
5079<x:ref>absolute-form</x:ref> = absolute-URI
5080<x:ref>absolute-path</x:ref> = 1*( "/" segment )
5081<x:ref>asterisk-form</x:ref> = "*"
5082<x:ref>attribute</x:ref> = token
5083<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
5084<x:ref>authority-form</x:ref> = authority
5085
5086<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
5087<x:ref>chunk-data</x:ref> = 1*OCTET
5088<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
5089<x:ref>chunk-ext-name</x:ref> = token
5090<x:ref>chunk-ext-val</x:ref> = token / quoted-str-nf
5091<x:ref>chunk-size</x:ref> = 1*HEXDIG
5092<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
5093<x:ref>comment</x:ref> = "(" *( ctext / quoted-cpair / comment ) ")"
5094<x:ref>connection-option</x:ref> = token
5095<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
5096 / %x2A-5B ; '*'-'['
5097 / %x5D-7E ; ']'-'~'
5098 / obs-text
5099
5100<x:ref>field-content</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5101<x:ref>field-name</x:ref> = token
5102<x:ref>field-value</x:ref> = *( field-content / obs-fold )
5103
5104<x:ref>header-field</x:ref> = field-name ":" OWS field-value BWS
5105<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ]
5106<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ]
5107
5108<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
5109
5110<x:ref>message-body</x:ref> = *OCTET
5111<x:ref>method</x:ref> = token
5112
5113<x:ref>obs-fold</x:ref> = CRLF ( SP / HTAB )
5114<x:ref>obs-text</x:ref> = %x80-FF
5115<x:ref>origin-form</x:ref> = absolute-path [ "?" query ]
5116
5117<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
5118<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
5119<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
5120<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
5121<x:ref>protocol-name</x:ref> = token
5122<x:ref>protocol-version</x:ref> = token
5123<x:ref>pseudonym</x:ref> = token
5124
5125<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5126 / %x5D-7E ; ']'-'~'
5127 / obs-text
5128<x:ref>qdtext-nf</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5129 / %x5D-7E ; ']'-'~'
5130 / obs-text
5131<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5132<x:ref>quoted-cpair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5133<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5134<x:ref>quoted-str-nf</x:ref> = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
5135<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5136
5137<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5138<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5139<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5140<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5141<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5142<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5143<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5144 asterisk-form
5145
5146<x:ref>segment</x:ref> = &lt;segment, defined in [RFC3986], Section 3.3&gt;
5147<x:ref>special</x:ref> = "(" / ")" / "&lt;" / "&gt;" / "@" / "," / ";" / ":" / "\" /
5148 DQUOTE / "/" / "[" / "]" / "?" / "=" / "{" / "}"
5149<x:ref>start-line</x:ref> = request-line / status-line
5150<x:ref>status-code</x:ref> = 3DIGIT
5151<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5152
5153<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5154<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5155<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5156 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5157<x:ref>token</x:ref> = 1*tchar
5158<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5159<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5160 transfer-extension
5161<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5162<x:ref>transfer-parameter</x:ref> = attribute BWS "=" BWS value
5163
5164<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5165
5166<x:ref>value</x:ref> = word
5167
5168<x:ref>word</x:ref> = token / quoted-string
5169</artwork>
5170</figure>
5171</section>
5172<?ENDINC p1-messaging.abnf-appendix ?>
5173
5174<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5175
5176<section title="Since RFC 2616">
5177<t>
5178  Changes up to the first Working Group Last Call draft are summarized
5179  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-21#appendix-D"/>.
5180</t>
5181</section>
5182
5183<section title="Since draft-ietf-httpbis-p1-messaging-21" anchor="changes.since.21">
5184<t>
5185  Closed issues:
5186  <list style="symbols">
5187    <t>
5188      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/128"/>:
5189      "Cite HTTPS URI scheme definition" (the spec now includes the HTTPs
5190      scheme definition and thus updates RFC 2818)
5191    </t>
5192    <t>
5193      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/389"/>:
5194      "mention of 'proxies' in section about caches"
5195    </t>
5196    <t>
5197      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/390"/>:
5198      "use of ABNF terms from RFC 3986"
5199    </t>
5200    <t>
5201      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/391"/>:
5202      "transferring URIs with userinfo in payload"
5203    </t>
5204    <t>
5205      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/392"/>:
5206      "editorial improvements to message length definition"
5207    </t>
5208    <t>
5209      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/395"/>:
5210      "Connection header field MUST vs SHOULD"
5211    </t>
5212    <t>
5213      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/396"/>:
5214      "editorial improvements to persistent connections section"
5215    </t>
5216    <t>
5217      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/397"/>:
5218      "URI normalization vs empty path"
5219    </t>
5220    <t>
5221      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/408"/>:
5222      "p1 feedback"
5223    </t>
5224    <t>
5225      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/409"/>:
5226      "is parsing OBS-FOLD mandatory?"
5227    </t>
5228    <t>
5229      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/410"/>:
5230      "HTTPS and Shared Caching"
5231    </t>
5232    <t>
5233      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/411"/>:
5234      "Requirements for recipients of ws between start-line and first header field"
5235    </t>
5236    <t>
5237      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/412"/>:
5238      "SP and HT when being tolerant"
5239    </t>
5240    <t>
5241      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/414"/>:
5242      "Message Parsing Strictness"
5243    </t>
5244    <t>
5245      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/415"/>:
5246      "'Render'"
5247    </t>
5248    <t>
5249      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/418"/>:
5250      "No-Transform"
5251    </t>
5252    <t>
5253      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/419"/>:
5254      "p2 editorial feedback"
5255    </t>
5256    <t>
5257      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/420"/>:
5258      "Content-Length SHOULD be sent"
5259    </t>
5260    <t>
5261      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/431"/>:
5262      "origin-form does not allow path starting with "//""
5263    </t>
5264    <t>
5265      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/433"/>:
5266      "ambiguity in part 1 example"
5267    </t>
5268  </list>
5269</t>
5270</section>
5271
5272<section title="Since draft-ietf-httpbis-p1-messaging-22" anchor="changes.since.22">
5273<t>
5274  Closed issues:
5275  <list style="symbols">
5276    <t>
5277      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/435"/>:
5278      "Part1 should have a reference to TCP (RFC 793)"
5279    </t>
5280    <t>
5281      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/438"/>:
5282      "media type registration template issues"
5283    </t>
5284    <t>
5285      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/442"/>:
5286      "BWS" (vs conformance)
5287    </t>
5288  </list>
5289</t>
5290</section>
5291</section>
5292
5293</back>
5294</rfc>
Note: See TracBrowser for help on using the repository browser.