source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2181

Last change on this file since 2181 was 2181, checked in by julian.reschke@…, 8 years ago

allow origin-form to use a path starting with "" (see #431)

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 225.4 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 "February">
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.21"/>.
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"/>
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386Content-Type: text/plain
387
388<x:span anchor="exbody">Hello World!
389</x:span></artwork></figure>
390</section>
391
392<section title="Implementation Diversity" anchor="implementation-diversity">
393<t>
394   When considering the design of HTTP, it is easy to fall into a trap of
395   thinking that all user agents are general-purpose browsers and all origin
396   servers are large public websites. That is not the case in practice.
397   Common HTTP user agents include household appliances, stereos, scales,
398   firmware update scripts, command-line programs, mobile apps,
399   and communication devices in a multitude of shapes and sizes.  Likewise,
400   common HTTP origin servers include home automation units, configurable
401   networking components, office machines, autonomous robots, news feeds,
402   traffic cameras, ad selectors, and video delivery platforms.
403</t>
404<t>
405   The term "user agent" does not imply that there is a human user directly
406   interacting with the software agent at the time of a request. In many
407   cases, a user agent is installed or configured to run in the background
408   and save its results for later inspection (or save only a subset of those
409   results that might be interesting or erroneous). Spiders, for example, are
410   typically given a start URI and configured to follow certain behavior while
411   crawling the Web as a hypertext graph.
412</t>
413<t>
414   The implementation diversity of HTTP means that we cannot assume the
415   user agent can make interactive suggestions to a user or provide adequate
416   warning for security or privacy options.  In the few cases where this
417   specification requires reporting of errors to the user, it is acceptable
418   for such reporting to only be observable in an error console or log file.
419   Likewise, requirements that an automated action be confirmed by the user
420   before proceeding can be met via advance configuration choices,
421   run-time options, or simply not proceeding with the unsafe action.
422</t>
423</section>
424
425<section title="Intermediaries" anchor="intermediaries">
426<iref primary="true" item="intermediary"/>
427<t>
428   HTTP enables the use of intermediaries to satisfy requests through
429   a chain of connections.  There are three common forms of HTTP
430   <x:dfn>intermediary</x:dfn>: proxy, gateway, and tunnel.  In some cases,
431   a single intermediary might act as an origin server, proxy, gateway,
432   or tunnel, switching behavior based on the nature of each request.
433</t>
434<figure><artwork type="drawing">
435         &gt;             &gt;             &gt;             &gt;
436    <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>
437               &lt;             &lt;             &lt;             &lt;
438</artwork></figure>
439<t>
440   The figure above shows three intermediaries (A, B, and C) between the
441   user agent and origin server. A request or response message that
442   travels the whole chain will pass through four separate connections.
443   Some HTTP communication options
444   might apply only to the connection with the nearest, non-tunnel
445   neighbor, only to the end-points of the chain, or to all connections
446   along the chain. Although the diagram is linear, each participant might
447   be engaged in multiple, simultaneous communications. For example, B
448   might be receiving requests from many clients other than A, and/or
449   forwarding requests to servers other than C, at the same time that it
450   is handling A's request.
451</t>
452<t>
453<iref primary="true" item="upstream"/><iref primary="true" item="downstream"/>
454<iref primary="true" item="inbound"/><iref primary="true" item="outbound"/>
455   We use the terms "<x:dfn>upstream</x:dfn>" and "<x:dfn>downstream</x:dfn>"
456   to describe various requirements in relation to the directional flow of a
457   message: all messages flow from upstream to downstream.
458   Likewise, we use the terms inbound and outbound to refer to
459   directions in relation to the request path:
460   "<x:dfn>inbound</x:dfn>" means toward the origin server and
461   "<x:dfn>outbound</x:dfn>" means toward the user agent.
462</t>
463<t><iref primary="true" item="proxy"/>
464   A "<x:dfn>proxy</x:dfn>" is a message forwarding agent that is selected by the
465   client, usually via local configuration rules, to receive requests
466   for some type(s) of absolute URI and attempt to satisfy those
467   requests via translation through the HTTP interface.  Some translations
468   are minimal, such as for proxy requests for "http" URIs, whereas
469   other requests might require translation to and from entirely different
470   application-level protocols. Proxies are often used to group an
471   organization's HTTP requests through a common intermediary for the
472   sake of security, annotation services, or shared caching.
473</t>
474<t>
475<iref primary="true" item="transforming proxy"/>
476<iref primary="true" item="non-transforming proxy"/>
477   An HTTP-to-HTTP proxy is called a "<x:dfn>transforming proxy</x:dfn>" if it is designed
478   or configured to modify request or response messages in a semantically
479   meaningful way (i.e., modifications, beyond those required by normal
480   HTTP processing, that change the message in a way that would be
481   significant to the original sender or potentially significant to
482   downstream recipients).  For example, a transforming proxy might be
483   acting as a shared annotation server (modifying responses to include
484   references to a local annotation database), a malware filter, a
485   format transcoder, or an intranet-to-Internet privacy filter.  Such
486   transformations are presumed to be desired by the client (or client
487   organization) that selected the proxy and are beyond the scope of
488   this specification.  However, when a proxy is not intended to transform
489   a given message, we use the term "<x:dfn>non-transforming proxy</x:dfn>" to target
490   requirements that preserve HTTP message semantics. See &status-203; and
491   &header-warning; for status and warning codes related to transformations.
492</t>
493<t><iref primary="true" item="gateway"/><iref primary="true" item="reverse proxy"/>
494<iref primary="true" item="accelerator"/>
495   A "<x:dfn>gateway</x:dfn>" (a.k.a., "<x:dfn>reverse proxy</x:dfn>")
496   is a receiving agent that acts
497   as a layer above some other server(s) and translates the received
498   requests to the underlying server's protocol.  Gateways are often
499   used to encapsulate legacy or untrusted information services, to
500   improve server performance through "<x:dfn>accelerator</x:dfn>" caching, and to
501   enable partitioning or load-balancing of HTTP services across
502   multiple machines.
503</t>
504<t>
505   A gateway behaves as an origin server on its outbound connection and
506   as a user agent on its inbound connection.
507   All HTTP requirements applicable to an origin server
508   also apply to the outbound communication of a gateway.
509   A gateway communicates with inbound servers using any protocol that
510   it desires, including private extensions to HTTP that are outside
511   the scope of this specification.  However, an HTTP-to-HTTP gateway
512   that wishes to interoperate with third-party HTTP servers &MUST;
513   conform to HTTP user agent requirements on the gateway's inbound
514   connection and &MUST; implement the <x:ref>Connection</x:ref>
515   (<xref target="header.connection"/>) and <x:ref>Via</x:ref>
516   (<xref target="header.via"/>) header fields for both connections.
517</t>
518<t><iref primary="true" item="tunnel"/>
519   A "<x:dfn>tunnel</x:dfn>" acts as a blind relay between two connections
520   without changing the messages. Once active, a tunnel is not
521   considered a party to the HTTP communication, though the tunnel might
522   have been initiated by an HTTP request. A tunnel ceases to exist when
523   both ends of the relayed connection are closed. Tunnels are used to
524   extend a virtual connection through an intermediary, such as when
525   Transport Layer Security (TLS, <xref target="RFC5246"/>) is used to
526   establish confidential communication through a shared firewall proxy.
527</t>
528<t><iref primary="true" item="interception proxy"/>
529<iref primary="true" item="transparent proxy"/>
530<iref primary="true" item="captive portal"/>
531   The above categories for intermediary only consider those acting as
532   participants in the HTTP communication.  There are also intermediaries
533   that can act on lower layers of the network protocol stack, filtering or
534   redirecting HTTP traffic without the knowledge or permission of message
535   senders. Network intermediaries often introduce security flaws or
536   interoperability problems by violating HTTP semantics.  For example, an
537   "<x:dfn>interception proxy</x:dfn>" <xref target="RFC3040"/> (also commonly
538   known as a "<x:dfn>transparent proxy</x:dfn>" <xref target="RFC1919"/> or
539   "<x:dfn>captive portal</x:dfn>")
540   differs from an HTTP proxy because it is not selected by the client.
541   Instead, an interception proxy filters or redirects outgoing TCP port 80
542   packets (and occasionally other common port traffic).
543   Interception proxies are commonly found on public network access points,
544   as a means of enforcing account subscription prior to allowing use of
545   non-local Internet services, and within corporate firewalls to enforce
546   network usage policies.
547   They are indistinguishable from a man-in-the-middle attack.
548</t>
549<t>
550   HTTP is defined as a stateless protocol, meaning that each request message
551   can be understood in isolation.  Many implementations depend on HTTP's
552   stateless design in order to reuse proxied connections or dynamically
553   load-balance requests across multiple servers.  Hence, servers &MUST-NOT;
554   assume that two requests on the same connection are from the same user
555   agent unless the connection is secured and specific to that agent.
556   Some non-standard HTTP extensions (e.g., <xref target="RFC4559"/>) have
557   been known to violate this requirement, resulting in security and
558   interoperability problems.
559</t>
560</section>
561
562<section title="Caches" anchor="caches">
563<iref primary="true" item="cache"/>
564<t>
565   A "<x:dfn>cache</x:dfn>" is a local store of previous response messages and the
566   subsystem that controls its message storage, retrieval, and deletion.
567   A cache stores cacheable responses in order to reduce the response
568   time and network bandwidth consumption on future, equivalent
569   requests. Any client or server &MAY; employ a cache, though a cache
570   cannot be used by a server while it is acting as a tunnel.
571</t>
572<t>
573   The effect of a cache is that the request/response chain is shortened
574   if one of the participants along the chain has a cached response
575   applicable to that request. The following illustrates the resulting
576   chain if B has a cached copy of an earlier response from O (via C)
577   for a request that has not been cached by UA or A.
578</t>
579<figure><artwork type="drawing">
580            &gt;             &gt;
581       <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>
582                  &lt;             &lt;
583</artwork></figure>
584<t><iref primary="true" item="cacheable"/>
585   A response is "<x:dfn>cacheable</x:dfn>" if a cache is allowed to store a copy of
586   the response message for use in answering subsequent requests.
587   Even when a response is cacheable, there might be additional
588   constraints placed by the client or by the origin server on when
589   that cached response can be used for a particular request. HTTP
590   requirements for cache behavior and cacheable responses are
591   defined in &caching-overview;. 
592</t>
593<t>
594   There are a wide variety of architectures and configurations
595   of caches deployed across the World Wide Web and
596   inside large organizations. These include national hierarchies
597   of proxy caches to save transoceanic bandwidth, collaborative systems that
598   broadcast or multicast cache entries, archives of pre-fetched cache
599   entries for use in off-line or high-latency environments, and so on.
600</t>
601</section>
602
603<section title="Conformance and Error Handling" anchor="conformance">
604<t>
605   This specification targets conformance criteria according to the role of
606   a participant in HTTP communication.  Hence, HTTP requirements are placed
607   on senders, recipients, clients, servers, user agents, intermediaries,
608   origin servers, proxies, gateways, or caches, depending on what behavior
609   is being constrained by the requirement. Additional (social) requirements
610   are placed on implementations, resource owners, and protocol element
611   registrations when they apply beyond the scope of a single communication.
612</t>
613<t>
614   The verb "generate" is used instead of "send" where a requirement
615   differentiates between creating a protocol element and merely forwarding a
616   received element downstream.
617</t>
618<t>
619   An implementation is considered conformant if it complies with all of the
620   requirements associated with the roles it partakes in HTTP. Note that
621   SHOULD-level requirements are relevant here, unless one of the documented
622   exceptions is applicable.
623</t>
624<t>
625   Conformance applies to both the syntax and semantics of HTTP protocol
626   elements. A sender &MUST-NOT; generate protocol elements that convey a
627   meaning that is known by that sender to be false. A sender &MUST-NOT;
628   generate protocol elements that do not match the grammar defined by the
629   ABNF rules for those protocol elements that are applicable to the sender's
630   role. If a received protocol element is processed, the recipient &MUST; be
631   able to parse any value that would match the ABNF rules for that protocol
632   element, excluding only those rules not applicable to the recipient's role.
633</t>
634<t>
635   Unless noted otherwise, a recipient &MAY; attempt to recover a usable
636   protocol element from an invalid construct.  HTTP does not define
637   specific error handling mechanisms except when they have a direct impact
638   on security, since different applications of the protocol require
639   different error handling strategies.  For example, a Web browser might
640   wish to transparently recover from a response where the
641   <x:ref>Location</x:ref> header field doesn't parse according to the ABNF,
642   whereas a systems control client might consider any form of error recovery
643   to be dangerous.
644</t>
645</section>
646
647<section title="Protocol Versioning" anchor="http.version">
648  <x:anchor-alias value="HTTP-version"/>
649  <x:anchor-alias value="HTTP-name"/>
650<t>
651   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate
652   versions of the protocol. This specification defines version "1.1".
653   The protocol version as a whole indicates the sender's conformance
654   with the set of requirements laid out in that version's corresponding
655   specification of HTTP.
656</t>
657<t>
658   The version of an HTTP message is indicated by an HTTP-version field
659   in the first line of the message. HTTP-version is case-sensitive.
660</t>
661<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-version"/><iref primary="true" item="Grammar" subitem="HTTP-name"/>
662  <x:ref>HTTP-version</x:ref>  = <x:ref>HTTP-name</x:ref> "/" <x:ref>DIGIT</x:ref> "." <x:ref>DIGIT</x:ref>
663  <x:ref>HTTP-name</x:ref>     = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
664</artwork></figure>
665<t>
666   The HTTP version number consists of two decimal digits separated by a "."
667   (period or decimal point).  The first digit ("major version") indicates the
668   HTTP messaging syntax, whereas the second digit ("minor version") indicates
669   the highest minor version to which the sender is
670   conformant and able to understand for future communication.  The minor
671   version advertises the sender's communication capabilities even when the
672   sender is only using a backwards-compatible subset of the protocol,
673   thereby letting the recipient know that more advanced features can
674   be used in response (by servers) or in future requests (by clients).
675</t>
676<t>
677   When an HTTP/1.1 message is sent to an HTTP/1.0 recipient
678   <xref target="RFC1945"/> or a recipient whose version is unknown,
679   the HTTP/1.1 message is constructed such that it can be interpreted
680   as a valid HTTP/1.0 message if all of the newer features are ignored.
681   This specification places recipient-version requirements on some
682   new features so that a conformant sender will only use compatible
683   features until it has determined, through configuration or the
684   receipt of a message, that the recipient supports HTTP/1.1.
685</t>
686<t>
687   The interpretation of a header field does not change between minor
688   versions of the same major HTTP version, though the default
689   behavior of a recipient in the absence of such a field can change.
690   Unless specified otherwise, header fields defined in HTTP/1.1 are
691   defined for all versions of HTTP/1.x.  In particular, the <x:ref>Host</x:ref>
692   and <x:ref>Connection</x:ref> header fields ought to be implemented by all
693   HTTP/1.x implementations whether or not they advertise conformance with
694   HTTP/1.1.
695</t>
696<t>
697   New header fields can be defined such that, when they are
698   understood by a recipient, they might override or enhance the
699   interpretation of previously defined header fields.  When an
700   implementation receives an unrecognized header field, the recipient
701   &MUST; ignore that header field for local processing regardless of
702   the message's HTTP version.  An unrecognized header field received
703   by a proxy &MUST; be forwarded downstream unless the header field's
704   field-name is listed in the message's <x:ref>Connection</x:ref> header field
705   (see <xref target="header.connection"/>).
706   These requirements allow HTTP's functionality to be enhanced without
707   requiring prior update of deployed intermediaries.
708</t>
709<t>
710   Intermediaries that process HTTP messages (i.e., all intermediaries
711   other than those acting as tunnels) &MUST; send their own HTTP-version
712   in forwarded messages.  In other words, they &MUST-NOT; blindly
713   forward the first line of an HTTP message without ensuring that the
714   protocol version in that message matches a version to which that
715   intermediary is conformant for both the receiving and
716   sending of messages.  Forwarding an HTTP message without rewriting
717   the HTTP-version might result in communication errors when downstream
718   recipients use the message sender's version to determine what features
719   are safe to use for later communication with that sender.
720</t>
721<t>
722   An HTTP client &SHOULD; send a request version equal to the highest
723   version to which the client is conformant and
724   whose major version is no higher than the highest version supported
725   by the server, if this is known.  An HTTP client &MUST-NOT; send a
726   version to which it is not conformant.
727</t>
728<t>
729   An HTTP client &MAY; send a lower request version if it is known that
730   the server incorrectly implements the HTTP specification, but only
731   after the client has attempted at least one normal request and determined
732   from the response status or header fields (e.g., <x:ref>Server</x:ref>) that
733   the server improperly handles higher request versions.
734</t>
735<t>
736   An HTTP server &SHOULD; send a response version equal to the highest
737   version to which the server is conformant and
738   whose major version is less than or equal to the one received in the
739   request.  An HTTP server &MUST-NOT; send a version to which it is not
740   conformant.  A server &MAY; send a <x:ref>505 (HTTP Version Not
741   Supported)</x:ref> response if it cannot send a response using the
742   major version used in the client's request.
743</t>
744<t>
745   An HTTP server &MAY; send an HTTP/1.0 response to an HTTP/1.0 request
746   if it is known or suspected that the client incorrectly implements the
747   HTTP specification and is incapable of correctly processing later
748   version responses, such as when a client fails to parse the version
749   number correctly or when an intermediary is known to blindly forward
750   the HTTP-version even when it doesn't conform to the given minor
751   version of the protocol. Such protocol downgrades &SHOULD-NOT; be
752   performed unless triggered by specific client attributes, such as when
753   one or more of the request header fields (e.g., <x:ref>User-Agent</x:ref>)
754   uniquely match the values sent by a client known to be in error.
755</t>
756<t>
757   The intention of HTTP's versioning design is that the major number
758   will only be incremented if an incompatible message syntax is
759   introduced, and that the minor number will only be incremented when
760   changes made to the protocol have the effect of adding to the message
761   semantics or implying additional capabilities of the sender.  However,
762   the minor version was not incremented for the changes introduced between
763   <xref target="RFC2068"/> and <xref target="RFC2616"/>, and this revision
764   has specifically avoiding any such changes to the protocol.
765</t>
766</section>
767
768<section title="Uniform Resource Identifiers" anchor="uri">
769<iref primary="true" item="resource"/>
770<t>
771   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used
772   throughout HTTP as the means for identifying resources (&resource;).
773   URI references are used to target requests, indicate redirects, and define
774   relationships.
775</t>
776  <x:anchor-alias value="URI-reference"/>
777  <x:anchor-alias value="absolute-URI"/>
778  <x:anchor-alias value="relative-part"/>
779  <x:anchor-alias value="authority"/>
780  <x:anchor-alias value="path-abempty"/>
781  <x:anchor-alias value="port"/>
782  <x:anchor-alias value="query"/>
783  <x:anchor-alias value="segment"/>
784  <x:anchor-alias value="uri-host"/>
785  <x:anchor-alias value="absolute-path"/>
786  <x:anchor-alias value="partial-URI"/>
787<t>
788   This specification adopts the definitions of "URI-reference",
789   "absolute-URI", "relative-part", "port", "host",
790   "path-abempty", "query", "segment", and "authority" from the
791   URI generic syntax.
792   In addition, we define an "absolute-path" rule (that differs from
793   RFC 3986's "path-absolute" in that it allows a leading "//")
794   and a "partial-URI" rule for protocol elements
795   that allow a relative URI but not a fragment.
796</t>
797<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>
798  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
799  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
800  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
801  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
802  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
803  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
804  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
805  <x:ref>segment</x:ref>       = &lt;segment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
806  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
807 
808  <x:ref>absolute-path</x:ref> = 1*( "/" segment )
809  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
810</artwork></figure>
811<t>
812   Each protocol element in HTTP that allows a URI reference will indicate
813   in its ABNF production whether the element allows any form of reference
814   (URI-reference), only a URI in absolute form (absolute-URI), only the
815   path and optional query components, or some combination of the above.
816   Unless otherwise indicated, URI references are parsed
817   relative to the effective request URI
818   (<xref target="effective.request.uri"/>).
819</t>
820
821<section title="http URI scheme" anchor="http.uri">
822  <x:anchor-alias value="http-URI"/>
823  <iref item="http URI scheme" primary="true"/>
824  <iref item="URI scheme" subitem="http" primary="true"/>
825<t>
826   The "http" URI scheme is hereby defined for the purpose of minting
827   identifiers according to their association with the hierarchical
828   namespace governed by a potential HTTP origin server listening for
829   TCP connections on a given port.
830</t>
831<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"><!--terminal production--></iref>
832  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
833</artwork></figure>
834<t>
835   The HTTP origin server is identified by the generic syntax's
836   <x:ref>authority</x:ref> component, which includes a host identifier
837   and optional TCP port (<xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>).
838   The remainder of the URI, consisting of both the hierarchical path
839   component and optional query component, serves as an identifier for
840   a potential resource within that origin server's name space.
841</t>
842<t>
843   If the host identifier is provided as an IP address,
844   then the origin server is any listener on the indicated TCP port at
845   that IP address. If host is a registered name, then that name is
846   considered an indirect identifier and the recipient might use a name
847   resolution service, such as DNS, to find the address of a listener
848   for that host.
849   The host &MUST-NOT; be empty; if an "http" URI is received with an
850   empty host, then it &MUST; be rejected as invalid.
851   If the port subcomponent is empty or not given, then TCP port 80 is
852   assumed (the default reserved port for WWW services).
853</t>
854<t>
855   Regardless of the form of host identifier, access to that host is not
856   implied by the mere presence of its name or address. The host might or might
857   not exist and, even when it does exist, might or might not be running an
858   HTTP server or listening to the indicated port. The "http" URI scheme
859   makes use of the delegated nature of Internet names and addresses to
860   establish a naming authority (whatever entity has the ability to place
861   an HTTP server at that Internet name or address) and allows that
862   authority to determine which names are valid and how they might be used.
863</t>
864<t>
865   When an "http" URI is used within a context that calls for access to the
866   indicated resource, a client &MAY; attempt access by resolving
867   the host to an IP address, establishing a TCP connection to that address
868   on the indicated port, and sending an HTTP request message
869   (<xref target="http.message"/>) containing the URI's identifying data
870   (<xref target="message.routing"/>) to the server.
871   If the server responds to that request with a non-interim HTTP response
872   message, as described in &status-codes;, then that response
873   is considered an authoritative answer to the client's request.
874</t>
875<t>
876   Although HTTP is independent of the transport protocol, the "http"
877   scheme is specific to TCP-based services because the name delegation
878   process depends on TCP for establishing authority.
879   An HTTP service based on some other underlying connection protocol
880   would presumably be identified using a different URI scheme, just as
881   the "https" scheme (below) is used for resources that require an
882   end-to-end secured connection. Other protocols might also be used to
883   provide access to "http" identified resources &mdash; it is only the
884   authoritative interface used for mapping the namespace that is
885   specific to TCP.
886</t>
887<t>
888   The URI generic syntax for authority also includes a deprecated
889   userinfo subcomponent (<xref target="RFC3986" x:fmt="," x:sec="3.2.1"/>)
890   for including user authentication information in the URI.  Some
891   implementations make use of the userinfo component for internal
892   configuration of authentication information, such as within command
893   invocation options, configuration files, or bookmark lists, even
894   though such usage might expose a user identifier or password.
895   Senders &MUST; exclude the userinfo subcomponent (and its "@"
896   delimiter) when an "http" URI is transmitted within a message as a
897   request target or header field value.
898   Recipients of an "http" URI reference &SHOULD; parse for userinfo and
899   treat its presence as an error, since it is likely being used to obscure
900   the authority for the sake of phishing attacks.
901</t>
902</section>
903
904<section title="https URI scheme" anchor="https.uri">
905   <x:anchor-alias value="https-URI"/>
906   <iref item="https URI scheme"/>
907   <iref item="URI scheme" subitem="https"/>
908<t>
909   The "https" URI scheme is hereby defined for the purpose of minting
910   identifiers according to their association with the hierarchical
911   namespace governed by a potential HTTP origin server listening to a
912   given TCP port for TLS-secured connections <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
1020<section title="Start Line" anchor="start.line">
1021  <x:anchor-alias value="Start-Line"/>
1022<t>
1023   An HTTP message can either be a request from client to server or a
1024   response from server to client.  Syntactically, the two types of message
1025   differ only in the start-line, which is either a request-line (for requests)
1026   or a status-line (for responses), and in the algorithm for determining
1027   the length of the message body (<xref target="message.body"/>).
1028</t>
1029<t>
1030   In theory, a client could receive requests and a server could receive
1031   responses, distinguishing them by their different start-line formats,
1032   but in practice servers are implemented to only expect a request
1033   (a response is interpreted as an unknown or invalid request method)
1034   and clients are implemented to only expect a response.
1035</t>
1036<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="start-line"/>
1037  <x:ref>start-line</x:ref>     = <x:ref>request-line</x:ref> / <x:ref>status-line</x:ref>
1038</artwork></figure>
1039<t>
1040   A sender &MUST-NOT; send whitespace between the start-line and
1041   the first header field. The presence of such whitespace in a request
1042   might be an attempt to trick a server into ignoring that field or
1043   processing the line after it as a new request, either of which might
1044   result in a security vulnerability if other implementations within
1045   the request chain interpret the same message differently.
1046   Likewise, the presence of such whitespace in a response might be
1047   ignored by some clients or cause others to cease parsing.
1048</t>
1049<t>
1050   A recipient that receives whitespace between the start-line and
1051   the first header field &MUST; either reject the message as invalid or
1052   consume each whitespace-preceded line without further processing of it
1053   (i.e., ignore the entire line, along with any subsequent lines preceded
1054   by whitespace, until a properly formed header field is received or the
1055   header block is terminated).
1056</t>
1057
1058<section title="Request Line" anchor="request.line">
1059  <x:anchor-alias value="Request"/>
1060  <x:anchor-alias value="request-line"/>
1061<t>
1062   A request-line begins with a method token, followed by a single
1063   space (SP), the request-target, another single space (SP), the
1064   protocol version, and ending with CRLF.
1065</t>
1066<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1067  <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>
1068</artwork></figure>
1069<iref primary="true" item="method"/>
1070<t anchor="method">
1071   The method token indicates the request method to be performed on the
1072   target resource. The request method is case-sensitive.
1073</t>
1074<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1075  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1076</artwork></figure>
1077<t>
1078   The methods defined by this specification can be found in
1079   &methods;, along with information regarding the HTTP method registry
1080   and considerations for defining new methods.
1081</t>
1082<iref item="request-target"/>
1083<t>
1084   The request-target identifies the target resource upon which to apply
1085   the request, as defined in <xref target="request-target"/>.
1086</t>
1087<t>
1088   No whitespace is allowed inside the method, request-target, and
1089   protocol version.  Hence, recipients typically parse the request-line
1090   into its component parts by splitting on whitespace
1091   (see <xref target="message.robustness"/>).
1092</t>
1093<t>
1094   Unfortunately, some user agents fail to properly encode hypertext
1095   references that have embedded whitespace, sending the characters directly
1096   instead of properly encoding or excluding the disallowed characters.
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 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. OWS &SHOULD; either not be generated or be generated as a single
1257   SP. Multiple OWS octets that occur within field-content &SHOULD; either
1258   be replaced with a single SP or transformed to all SP octets (each
1259   octet other than SP replaced with SP) before interpreting the field value
1260   or forwarding the message downstream.
1261</t>
1262<t anchor="rule.RWS">
1263   RWS is used when at least one linear whitespace octet is required to
1264   separate field tokens. RWS &SHOULD; be generated as a single SP.
1265   Multiple RWS octets that occur within field-content &SHOULD; either
1266   be replaced with a single SP or transformed to all SP octets before
1267   interpreting the field value or forwarding the message downstream.
1268</t>
1269<t anchor="rule.BWS">
1270   BWS is used where the grammar allows optional whitespace, for historical
1271   reasons, but senders &SHOULD-NOT; generate it in messages;
1272   recipients &MUST; accept such bad optional whitespace and remove it before
1273   interpreting the field value or forwarding the message downstream.
1274</t>
1275<t anchor="rule.whitespace">
1276  <x:anchor-alias value="BWS"/>
1277  <x:anchor-alias value="OWS"/>
1278  <x:anchor-alias value="RWS"/>
1279</t>
1280<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="OWS"/><iref primary="true" item="Grammar" subitem="RWS"/><iref primary="true" item="Grammar" subitem="BWS"/>
1281  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1282                 ; optional whitespace
1283  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1284                 ; required whitespace
1285  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1286                 ; "bad" whitespace
1287</artwork></figure>
1288</section>
1289
1290<section title="Field Parsing" anchor="field.parsing">
1291<t>
1292   No whitespace is allowed between the header field-name and colon.
1293   In the past, differences in the handling of such whitespace have led to
1294   security vulnerabilities in request routing and response handling.
1295   A server &MUST; reject any received request message that contains
1296   whitespace between a header field-name and colon with a response code of
1297   <x:ref>400 (Bad Request)</x:ref>. A proxy &MUST; remove any such whitespace
1298   from a response message before forwarding the message downstream.
1299</t>
1300<t>
1301   A field value is preceded by optional whitespace (OWS); a single SP is
1302   preferred. The field value does not include any leading or trailing white
1303   space: OWS occurring before the first non-whitespace octet of the
1304   field value or after the last non-whitespace octet of the field value
1305   is ignored and &SHOULD; be removed before further processing (as this does
1306   not change the meaning of the header field).
1307</t>
1308<t>
1309   Historically, HTTP header field values could be extended over multiple
1310   lines by preceding each extra line with at least one space or horizontal
1311   tab (obs-fold). This specification deprecates such line folding except
1312   within the message/http media type
1313   (<xref target="internet.media.type.message.http"/>).
1314   Senders &MUST-NOT; generate messages that include line folding
1315   (i.e., that contain any field-value that contains a match to the
1316   <x:ref>obs-fold</x:ref> rule) unless the message is intended for packaging
1317   within the message/http media type. When an <x:ref>obs-fold</x:ref> is
1318   received in a message, recipients &MUST; do one of:
1319   <list style="symbols">
1320      <t>accept the message and replace any embedded <x:ref>obs-fold</x:ref>
1321         whitespace with either a single <x:ref>SP</x:ref> or a matching
1322         number of <x:ref>SP</x:ref> octets (to avoid buffer copying) prior to
1323         interpreting the field value or forwarding the message
1324         downstream;</t>
1325
1326      <t>if it is a request, reject the message by sending a
1327         <x:ref>400 (Bad Request)</x:ref> response with a representation
1328         explaining that obsolete line folding is unacceptable; or,</t>
1329         
1330      <t>if it is a response, discard the message and generate a
1331         <x:ref>502 (Bad Gateway)</x:ref> response with a representation
1332         explaining that unacceptable line folding was received.</t>
1333   </list>
1334   Recipients that choose not to implement <x:ref>obs-fold</x:ref> processing
1335   (as described above) &MUST-NOT; accept messages containing header fields
1336   with leading whitespace, as this can expose them to attacks that exploit
1337   this difference in processing.
1338</t>
1339<t>
1340   Historically, HTTP has allowed field content with text in the ISO-8859-1
1341   <xref target="ISO-8859-1"/> charset, supporting other charsets only
1342   through use of <xref target="RFC2047"/> encoding.
1343   In practice, most HTTP header field values use only a subset of the
1344   US-ASCII charset <xref target="USASCII"/>. Newly defined
1345   header fields &SHOULD; limit their field values to US-ASCII octets.
1346   Recipients &SHOULD; treat other octets in field content (obs-text) as
1347   opaque data.
1348</t>
1349</section>
1350
1351<section title="Field Limits" anchor="field.limits">
1352<t>
1353   HTTP does not place a pre-defined limit on the length of each header field
1354   or on the length of the header block as a whole.  Various ad-hoc
1355   limitations on individual header field length are found in practice,
1356   often depending on the specific field semantics.
1357</t>
1358<t>
1359   A server &MUST; be prepared to receive request header fields of unbounded
1360   length and respond with an appropriate <x:ref>4xx (Client Error)</x:ref>
1361   status code if the received header field(s) are larger than the server
1362   wishes to process.
1363</t>
1364<t>
1365   A client &MUST; be prepared to receive response header fields of unbounded
1366   length. A client &MAY; discard or truncate received header fields that are
1367   larger than the client wishes to process if the field semantics are such
1368   that the dropped value(s) can be safely ignored without changing the
1369   response semantics.
1370</t>
1371</section>
1372
1373<section title="Field value components" anchor="field.components">
1374<t anchor="rule.token.separators">
1375  <x:anchor-alias value="tchar"/>
1376  <x:anchor-alias value="token"/>
1377  <x:anchor-alias value="special"/>
1378  <x:anchor-alias value="word"/>
1379   Many HTTP header field values consist of words (token or quoted-string)
1380   separated by whitespace or special characters. These special characters
1381   &MUST; be in a quoted string to be used within a parameter value (as defined
1382   in <xref target="transfer.codings"/>).
1383</t>
1384<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>
1385  <x:ref>word</x:ref>           = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1386
1387  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1388<!--
1389  IMPORTANT: when editing "tchar" make sure that "special" is updated accordingly!!!
1390 -->
1391  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1392                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1393                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1394                 ; any <x:ref>VCHAR</x:ref>, except <x:ref>special</x:ref>
1395
1396  <x:ref>special</x:ref>        = "(" / ")" / "&lt;" / ">" / "@" / ","
1397                 / ";" / ":" / "\" / DQUOTE / "/" / "["
1398                 / "]" / "?" / "=" / "{" / "}"
1399</artwork></figure>
1400<t anchor="rule.quoted-string">
1401  <x:anchor-alias value="quoted-string"/>
1402  <x:anchor-alias value="qdtext"/>
1403  <x:anchor-alias value="obs-text"/>
1404   A string of text is parsed as a single word if it is quoted using
1405   double-quote marks.
1406</t>
1407<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"/>
1408  <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>
1409  <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>
1410  <x:ref>obs-text</x:ref>       = %x80-FF
1411</artwork></figure>
1412<t anchor="rule.quoted-pair">
1413  <x:anchor-alias value="quoted-pair"/>
1414   The backslash octet ("\") can be used as a single-octet
1415   quoting mechanism within quoted-string constructs:
1416</t>
1417<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1418  <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> )
1419</artwork></figure>
1420<t>
1421   Recipients that process the value of a quoted-string &MUST; handle a
1422   quoted-pair as if it were replaced by the octet following the backslash.
1423</t>
1424<t>
1425   Senders &SHOULD-NOT; generate a quoted-pair in a quoted-string except where
1426   necessary to quote DQUOTE and backslash octets occurring within that string.
1427</t>
1428<t anchor="rule.comment">
1429  <x:anchor-alias value="comment"/>
1430  <x:anchor-alias value="ctext"/>
1431   Comments can be included in some HTTP header fields by surrounding
1432   the comment text with parentheses. Comments are only allowed in
1433   fields containing "comment" as part of their field value definition.
1434</t>
1435<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1436  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-cpair</x:ref> / <x:ref>comment</x:ref> ) ")"
1437  <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>
1438</artwork></figure>
1439<t anchor="rule.quoted-cpair">
1440  <x:anchor-alias value="quoted-cpair"/>
1441   The backslash octet ("\") can be used as a single-octet
1442   quoting mechanism within comment constructs:
1443</t>
1444<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-cpair"/>
1445  <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> )
1446</artwork></figure>
1447<t>
1448   Senders &SHOULD-NOT; escape octets in comments that do not require escaping
1449   (i.e., other than the backslash octet "\" and the parentheses "(" and ")").
1450</t>
1451</section>
1452
1453</section>
1454
1455<section title="Message Body" anchor="message.body">
1456  <x:anchor-alias value="message-body"/>
1457<t>
1458   The message body (if any) of an HTTP message is used to carry the
1459   payload body of that request or response.  The message body is
1460   identical to the payload body unless a transfer coding has been
1461   applied, as described in <xref target="header.transfer-encoding"/>.
1462</t>
1463<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1464  <x:ref>message-body</x:ref> = *OCTET
1465</artwork></figure>
1466<t>
1467   The rules for when a message body is allowed in a message differ for
1468   requests and responses.
1469</t>
1470<t>
1471   The presence of a message body in a request is signaled by a
1472   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1473   field. Request message framing is independent of method semantics,
1474   even if the method does not define any use for a message body.
1475</t>
1476<t>
1477   The presence of a message body in a response depends on both
1478   the request method to which it is responding and the response
1479   status code (<xref target="status.line"/>).
1480   Responses to the HEAD request method never include a message body
1481   because the associated response header fields (e.g.,
1482   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1483   if present, indicate only what their values would have been if the request
1484   method had been GET (&HEAD;).
1485   <x:ref>2xx (Successful)</x:ref> responses to CONNECT switch to tunnel
1486   mode instead of having a message body (&CONNECT;).
1487   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1488   <x:ref>304 (Not Modified)</x:ref> responses do not include a message body.
1489   All other responses do include a message body, although the body
1490   might be of zero length.
1491</t>
1492
1493<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1494  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1495  <iref item="chunked (Coding Format)"/>
1496  <x:anchor-alias value="Transfer-Encoding"/>
1497<t>
1498   The Transfer-Encoding header field lists the transfer coding names
1499   corresponding to the sequence of transfer codings that have been
1500   (or will be) applied to the payload body in order to form the message body.
1501   Transfer codings are defined in <xref target="transfer.codings"/>.
1502</t>
1503<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1504  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1505</artwork></figure>
1506<t>
1507   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1508   MIME, which was designed to enable safe transport of binary data over a
1509   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1510   However, safe transport has a different focus for an 8bit-clean transfer
1511   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1512   accurately delimit a dynamically generated payload and to distinguish
1513   payload encodings that are only applied for transport efficiency or
1514   security from those that are characteristics of the selected resource.
1515</t>
1516<t>
1517   All HTTP/1.1 recipients &MUST; implement the chunked transfer coding
1518   (<xref target="chunked.encoding"/>) because it plays a crucial role in
1519   framing messages when the payload body size is not known in advance.
1520   If chunked is applied to a payload body, the sender &MUST-NOT; apply
1521   chunked more than once (i.e., chunking an already chunked message is not
1522   allowed).
1523   If any transfer coding is applied to a request payload body, the
1524   sender &MUST; apply chunked as the final transfer coding to ensure that
1525   the message is properly framed.
1526   If any transfer coding is applied to a response payload body, the
1527   sender &MUST; either apply chunked as the final transfer coding or
1528   terminate the message by closing the connection.
1529</t>
1530<figure><preamble>
1531   For example,
1532</preamble><artwork type="example">
1533  Transfer-Encoding: gzip, chunked
1534</artwork><postamble>
1535   indicates that the payload body has been compressed using the gzip
1536   coding and then chunked using the chunked coding while forming the
1537   message body.
1538</postamble></figure>
1539<t>
1540   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1541   Transfer-Encoding is a property of the message, not of the representation, and
1542   any recipient along the request/response chain &MAY; decode the received
1543   transfer coding(s) or apply additional transfer coding(s) to the message
1544   body, assuming that corresponding changes are made to the Transfer-Encoding
1545   field-value. Additional information about the encoding parameters &MAY; be
1546   provided by other header fields not defined by this specification.
1547</t>
1548<t>
1549   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1550   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1551   neither of which includes a message body,
1552   to indicate that the origin server would have applied a transfer coding
1553   to the message body if the request had been an unconditional GET.
1554   This indication is not required, however, because any recipient on
1555   the response chain (including the origin server) can remove transfer
1556   codings when they are not needed.
1557</t>
1558<t>
1559   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1560   implementations advertising only HTTP/1.0 support will not understand
1561   how to process a transfer-encoded payload.
1562   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1563   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1564   might be in the form of specific user configuration or by remembering the
1565   version of a prior received response.
1566   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1567   the corresponding request indicates HTTP/1.1 (or later).
1568</t>
1569<t>
1570   A server that receives a request message with a transfer coding it does
1571   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref>.
1572</t>
1573</section>
1574
1575<section title="Content-Length" anchor="header.content-length">
1576  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1577  <x:anchor-alias value="Content-Length"/>
1578<t>
1579   When a message does not have a <x:ref>Transfer-Encoding</x:ref> header
1580   field, a Content-Length header field can provide the anticipated size,
1581   as a decimal number of octets, for a potential payload body.
1582   For messages that do include a payload body, the Content-Length field-value
1583   provides the framing information necessary for determining where the body
1584   (and message) ends.  For messages that do not include a payload body, the
1585   Content-Length indicates the size of the selected representation
1586   (&representation;).
1587</t>
1588<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1589  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1590</artwork></figure>
1591<t>
1592   An example is
1593</t>
1594<figure><artwork type="example">
1595  Content-Length: 3495
1596</artwork></figure>
1597<t>
1598   A sender &MUST-NOT; send a Content-Length header field in any message that
1599   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1600</t>
1601<t>
1602   A user agent &SHOULD; send a Content-Length in a request message when no
1603   <x:ref>Transfer-Encoding</x:ref> is sent and the request method defines
1604   a meaning for an enclosed payload body. For example, a Content-Length
1605   header field is normally sent in a POST request even when the value is
1606   0 (indicating an empty payload body).  A user agent &SHOULD-NOT; send a
1607   Content-Length header field when the request message does not contain a
1608   payload body and the method semantics do not anticipate such a body.
1609</t>
1610<t>
1611   A server &MAY; send a Content-Length header field in a response to a HEAD
1612   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1613   response unless its field-value equals the decimal number of octets that
1614   would have been sent in the payload body of a response if the same
1615   request had used the GET method.
1616</t>
1617<t>
1618   A server &MAY; send a Content-Length header field in a
1619   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1620   (&status-304;); 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 <x:ref>200 (OK)</x:ref>
1623   response to the same request.
1624</t>
1625<t>
1626   A server &MUST-NOT; send a Content-Length header field in any response
1627   with a status code of
1628   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1629   A server &SHOULD-NOT; send a Content-Length header field in any
1630   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1631</t>
1632<t>
1633   Aside from the cases defined above, in the absence of Transfer-Encoding,
1634   an origin server &SHOULD; send a Content-Length header field when the
1635   payload body size is known prior to sending the complete header block.
1636   This will allow downstream recipients to measure transfer progress,
1637   know when a received message is complete, and potentially reuse the
1638   connection for additional requests.
1639</t>
1640<t>
1641   Any Content-Length field value greater than or equal to zero is valid.
1642   Since there is no predefined limit to the length of a payload,
1643   recipients &SHOULD; anticipate potentially large decimal numerals and
1644   prevent parsing errors due to integer conversion overflows
1645   (<xref target="attack.protocol.element.size.overflows"/>).
1646</t>
1647<t>
1648   If a message is received that has multiple Content-Length header fields
1649   with field-values consisting of the same decimal value, or a single
1650   Content-Length header field with a field value containing a list of
1651   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1652   duplicate Content-Length header fields have been generated or combined by an
1653   upstream message processor, then the recipient &MUST; either reject the
1654   message as invalid or replace the duplicated field-values with a single
1655   valid Content-Length field containing that decimal value prior to
1656   determining the message body length.
1657</t>
1658<x:note>
1659  <t>
1660   &Note; HTTP's use of Content-Length for message framing differs
1661   significantly from the same field's use in MIME, where it is an optional
1662   field used only within the "message/external-body" media-type.
1663  </t>
1664</x:note>
1665</section>
1666
1667<section title="Message Body Length" anchor="message.body.length">
1668  <iref item="chunked (Coding Format)"/>
1669<t>
1670   The length of a message body is determined by one of the following
1671   (in order of precedence):
1672</t>
1673<t>
1674  <list style="numbers">
1675    <x:lt><t>
1676     Any response to a HEAD request and any response with a
1677     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1678     <x:ref>304 (Not Modified)</x:ref> status code is always
1679     terminated by the first empty line after the header fields, regardless of
1680     the header fields present in the message, and thus cannot contain a
1681     message body.
1682    </t></x:lt>
1683    <x:lt><t>
1684     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1685     connection will become a tunnel immediately after the empty line that
1686     concludes the header fields.  A client &MUST; ignore any
1687     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1688     fields received in such a message.
1689    </t></x:lt>
1690    <x:lt><t>
1691     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1692     and the chunked transfer coding (<xref target="chunked.encoding"/>)
1693     is the final encoding, the message body length is determined by reading
1694     and decoding the chunked data until the transfer coding indicates the
1695     data is complete.
1696    </t>
1697    <t>
1698     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1699     response and the chunked transfer coding is not the final encoding, the
1700     message body length is determined by reading the connection until it is
1701     closed by the server.
1702     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a request and the
1703     chunked transfer coding is not the final encoding, the message body
1704     length cannot be determined reliably; the server &MUST; respond with
1705     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1706    </t>
1707    <t>
1708     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1709     and a <x:ref>Content-Length</x:ref> header field, the Transfer-Encoding
1710     overrides the Content-Length. Such a message might indicate an attempt
1711     to perform request or response smuggling (bypass of security-related
1712     checks on message routing or content) and thus ought to be handled as
1713     an error.  A sender &MUST; remove the received Content-Length field
1714     prior to forwarding such a message downstream.
1715    </t></x:lt>
1716    <x:lt><t>
1717     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1718     either multiple <x:ref>Content-Length</x:ref> header fields having
1719     differing field-values or a single Content-Length header field having an
1720     invalid value, then the message framing is invalid and &MUST; be treated
1721     as an error to prevent request or response smuggling.
1722     If this is a request message, the server &MUST; respond with
1723     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1724     If this is a response message received by a proxy, the proxy
1725     &MUST; discard the received response, send a <x:ref>502 (Bad Gateway)</x:ref>
1726     status code as its downstream response, and then close the connection.
1727     If this is a response message received by a user agent, it &MUST; be
1728     treated as an error by discarding the message and closing the connection.
1729    </t></x:lt>
1730    <x:lt><t>
1731     If a valid <x:ref>Content-Length</x:ref> header field is present without
1732     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1733     expected message body length in octets.
1734     If the sender closes the connection or the recipient times out before the
1735     indicated number of octets are received, the recipient &MUST; consider
1736     the message to be incomplete and close the connection.
1737    </t></x:lt>
1738    <x:lt><t>
1739     If this is a request message and none of the above are true, then the
1740     message body length is zero (no message body is present).
1741    </t></x:lt>
1742    <x:lt><t>
1743     Otherwise, this is a response message without a declared message body
1744     length, so the message body length is determined by the number of octets
1745     received prior to the server closing the connection.
1746    </t></x:lt>
1747  </list>
1748</t>
1749<t>
1750   Since there is no way to distinguish a successfully completed,
1751   close-delimited message from a partially-received message interrupted
1752   by network failure, a server &SHOULD; use encoding or
1753   length-delimited messages whenever possible.  The close-delimiting
1754   feature exists primarily for backwards compatibility with HTTP/1.0.
1755</t>
1756<t>
1757   A server &MAY; reject a request that contains a message body but
1758   not a <x:ref>Content-Length</x:ref> by responding with
1759   <x:ref>411 (Length Required)</x:ref>.
1760</t>
1761<t>
1762   Unless a transfer coding other than chunked has been applied,
1763   a client that sends a request containing a message body &SHOULD;
1764   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1765   length is known in advance, rather than the chunked transfer coding, since some
1766   existing services respond to chunked with a <x:ref>411 (Length Required)</x:ref>
1767   status code even though they understand the chunked transfer coding.  This
1768   is typically because such services are implemented via a gateway that
1769   requires a content-length in advance of being called and the server
1770   is unable or unwilling to buffer the entire request before processing.
1771</t>
1772<t>
1773   A user agent that sends a request containing a message body &MUST; send a
1774   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1775   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1776   the form of specific user configuration or by remembering the version of a
1777   prior received response.
1778</t>
1779<t>
1780   If the final response to the last request on a connection has been
1781   completely received and there remains additional data to read, a user agent
1782   &MAY; discard the remaining data or attempt to determine if that data
1783   belongs as part of the prior response body, which might be the case if the
1784   prior message's Content-Length value is incorrect. A client &MUST-NOT;
1785   process, cache, or forward such extra data as a separate response, since
1786   such behavior would be vulnerable to cache poisoning.
1787</t>
1788</section>
1789</section>
1790
1791<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1792<t>
1793   A server that receives an incomplete request message, usually due to a
1794   canceled request or a triggered time-out exception, &MAY; send an error
1795   response prior to closing the connection.
1796</t>
1797<t>
1798   A client that receives an incomplete response message, which can occur
1799   when a connection is closed prematurely or when decoding a supposedly
1800   chunked transfer coding fails, &MUST; record the message as incomplete.
1801   Cache requirements for incomplete responses are defined in
1802   &cache-incomplete;.
1803</t>
1804<t>
1805   If a response terminates in the middle of the header block (before the
1806   empty line is received) and the status code might rely on header fields to
1807   convey the full meaning of the response, then the client cannot assume
1808   that meaning has been conveyed; the client might need to repeat the
1809   request in order to determine what action to take next.
1810</t>
1811<t>
1812   A message body that uses the chunked transfer coding is
1813   incomplete if the zero-sized chunk that terminates the encoding has not
1814   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1815   incomplete if the size of the message body received (in octets) is less than
1816   the value given by Content-Length.  A response that has neither chunked
1817   transfer coding nor Content-Length is terminated by closure of the
1818   connection, and thus is considered complete regardless of the number of
1819   message body octets received, provided that the header block was received
1820   intact.
1821</t>
1822</section>
1823
1824<section title="Message Parsing Robustness" anchor="message.robustness">
1825<t>
1826   Older HTTP/1.0 user agent implementations might send an extra CRLF
1827   after a POST request as a lame workaround for some early server
1828   applications that failed to read message body content that was
1829   not terminated by a line-ending. An HTTP/1.1 user agent &MUST-NOT;
1830   preface or follow a request with an extra CRLF.  If terminating
1831   the request message body with a line-ending is desired, then the
1832   user agent &MUST; count the terminating CRLF octets as part of the
1833   message body length.
1834</t>
1835<t>
1836   In the interest of robustness, servers &SHOULD; ignore at least one
1837   empty line received where a request-line is expected. In other words, if
1838   a server is reading the protocol stream at the beginning of a
1839   message and receives a CRLF first, the server &SHOULD; ignore the CRLF.
1840</t>
1841<t>
1842   Although the line terminator for the start-line and header
1843   fields is the sequence CRLF, recipients &MAY; recognize a
1844   single LF as a line terminator and ignore any preceding CR.
1845</t>
1846<t>
1847   Although the request-line and status-line grammar rules require that each
1848   of the component elements be separated by a single SP octet, recipients
1849   &MAY; instead parse on whitespace-delimited word boundaries and, aside
1850   from the CRLF terminator, treat any form of whitespace as the SP separator
1851   while ignoring preceding or trailing whitespace;
1852   such whitespace includes one or more of the following octets:
1853   SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
1854</t>
1855<t>
1856   When a server listening only for HTTP request messages, or processing
1857   what appears from the start-line to be an HTTP request message,
1858   receives a sequence of octets that does not match the HTTP-message
1859   grammar aside from the robustness exceptions listed above, the
1860   server &SHOULD; respond with a <x:ref>400 (Bad Request)</x:ref> response. 
1861</t>
1862</section>
1863</section>
1864
1865<section title="Transfer Codings" anchor="transfer.codings">
1866  <x:anchor-alias value="transfer-coding"/>
1867  <x:anchor-alias value="transfer-extension"/>
1868<t>
1869   Transfer coding names are used to indicate an encoding
1870   transformation that has been, can be, or might need to be applied to a
1871   payload body in order to ensure "safe transport" through the network.
1872   This differs from a content coding in that the transfer coding is a
1873   property of the message rather than a property of the representation
1874   that is being transferred.
1875</t>
1876<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1877  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1878                     / "compress" ; <xref target="compress.coding"/>
1879                     / "deflate" ; <xref target="deflate.coding"/>
1880                     / "gzip" ; <xref target="gzip.coding"/>
1881                     / <x:ref>transfer-extension</x:ref>
1882  <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> )
1883</artwork></figure>
1884<t anchor="rule.parameter">
1885  <x:anchor-alias value="attribute"/>
1886  <x:anchor-alias value="transfer-parameter"/>
1887  <x:anchor-alias value="value"/>
1888   Parameters are in the form of attribute/value pairs.
1889</t>
1890<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"/>
1891  <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>
1892  <x:ref>attribute</x:ref>          = <x:ref>token</x:ref>
1893  <x:ref>value</x:ref>              = <x:ref>word</x:ref>
1894</artwork></figure>
1895<t>
1896   All transfer-coding names are case-insensitive and ought to be registered
1897   within the HTTP Transfer Coding registry, as defined in
1898   <xref target="transfer.coding.registry"/>.
1899   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1900   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1901   header fields.
1902</t>
1903
1904<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1905  <iref primary="true" item="chunked (Coding Format)"/>
1906  <x:anchor-alias value="chunk"/>
1907  <x:anchor-alias value="chunked-body"/>
1908  <x:anchor-alias value="chunk-data"/>
1909  <x:anchor-alias value="chunk-ext"/>
1910  <x:anchor-alias value="chunk-ext-name"/>
1911  <x:anchor-alias value="chunk-ext-val"/>
1912  <x:anchor-alias value="chunk-size"/>
1913  <x:anchor-alias value="last-chunk"/>
1914  <x:anchor-alias value="trailer-part"/>
1915  <x:anchor-alias value="quoted-str-nf"/>
1916  <x:anchor-alias value="qdtext-nf"/>
1917<t>
1918   The chunked transfer coding modifies the body of a message in order to
1919   transfer it as a series of chunks, each with its own size indicator,
1920   followed by an &OPTIONAL; trailer containing header fields. This
1921   allows dynamically generated content to be transferred along with the
1922   information necessary for the recipient to verify that it has
1923   received the full message.
1924</t>
1925<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"/>
1926  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1927                   <x:ref>last-chunk</x:ref>
1928                   <x:ref>trailer-part</x:ref>
1929                   <x:ref>CRLF</x:ref>
1930 
1931  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1932                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1933  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1934  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1935 
1936  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
1937  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
1938  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-str-nf</x:ref>
1939  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1940  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1941 
1942  <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>
1943                 ; like <x:ref>quoted-string</x:ref>, but disallowing line folding
1944  <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>
1945</artwork></figure>
1946<t>
1947   Chunk extensions within the chunked transfer coding are deprecated.
1948   Senders &SHOULD-NOT; send chunk-ext.
1949   Definition of new chunk extensions is discouraged.
1950</t>
1951<t>
1952   The chunk-size field is a string of hex digits indicating the size of
1953   the chunk-data in octets. The chunked transfer coding is complete when a
1954   chunk with a chunk-size of zero is received, possibly followed by a
1955   trailer, and finally terminated by an empty line.
1956</t>
1957
1958<section title="Trailer" anchor="header.trailer">
1959  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
1960  <x:anchor-alias value="Trailer"/>
1961<t>
1962   A trailer allows the sender to include additional fields at the end of a
1963   chunked message in order to supply metadata that might be dynamically
1964   generated while the message body is sent, such as a message integrity
1965   check, digital signature, or post-processing status.
1966   The trailer &MUST-NOT; contain fields that need to be known before a
1967   recipient processes the body, such as <x:ref>Transfer-Encoding</x:ref>,
1968   <x:ref>Content-Length</x:ref>, and <x:ref>Trailer</x:ref>.
1969</t>
1970<t>
1971   When a message includes a message body encoded with the chunked
1972   transfer coding and the sender desires to send metadata in the form of
1973   trailer fields at the end of the message, the sender &SHOULD; send a
1974   <x:ref>Trailer</x:ref> header field before the message body to indicate
1975   which fields will be present in the trailers. This allows the recipient
1976   to prepare for receipt of that metadata before it starts processing the body,
1977   which is useful if the message is being streamed and the recipient wishes
1978   to confirm an integrity check on the fly.
1979</t>
1980<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
1981  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
1982</artwork></figure>
1983<t>
1984   If no <x:ref>Trailer</x:ref> header field is present, the sender of a
1985   chunked message body &SHOULD; send an empty trailer.
1986</t>
1987<t>
1988   A server &MUST; send an empty trailer with the chunked transfer coding
1989   unless at least one of the following is true:
1990  <list style="numbers">
1991    <t>the request included a <x:ref>TE</x:ref> header field that indicates
1992    "trailers" is acceptable in the transfer coding of the response, as
1993    described in <xref target="header.te"/>; or,</t>
1994     
1995    <t>the trailer fields consist entirely of optional metadata and the
1996    recipient could use the message (in a manner acceptable to the server where
1997    the field originated) without receiving that metadata. In other words,
1998    the server that generated the header field is willing to accept the
1999    possibility that the trailer fields might be silently discarded along
2000    the path to the client.</t>
2001  </list>
2002</t>
2003<t>
2004   The above requirement prevents the need for an infinite buffer when a
2005   message is being received by an HTTP/1.1 (or later) proxy and forwarded to
2006   an HTTP/1.0 recipient.
2007</t>
2008</section>
2009
2010<section title="Decoding chunked" anchor="decoding.chunked">
2011<t>
2012   A process for decoding the chunked transfer coding
2013   can be represented in pseudo-code as:
2014</t>
2015<figure><artwork type="code">
2016  length := 0
2017  read chunk-size, chunk-ext (if any) and CRLF
2018  while (chunk-size &gt; 0) {
2019     read chunk-data and CRLF
2020     append chunk-data to decoded-body
2021     length := length + chunk-size
2022     read chunk-size and CRLF
2023  }
2024  read header-field
2025  while (header-field not empty) {
2026     append header-field to existing header fields
2027     read header-field
2028  }
2029  Content-Length := length
2030  Remove "chunked" from Transfer-Encoding
2031  Remove Trailer from existing header fields
2032</artwork></figure>
2033<t>
2034   All recipients &MUST; be able to receive and decode the
2035   chunked transfer coding and &MUST; ignore chunk-ext extensions
2036   they do not understand.
2037</t>
2038</section>
2039</section>
2040
2041<section title="Compression Codings" anchor="compression.codings">
2042<t>
2043   The codings defined below can be used to compress the payload of a
2044   message.
2045</t>
2046
2047<section title="Compress Coding" anchor="compress.coding">
2048<iref item="compress (Coding Format)"/>
2049<t>
2050   The "compress" format is produced by the common UNIX file compression
2051   program "compress". This format is an adaptive Lempel-Ziv-Welch
2052   coding (LZW). Recipients &SHOULD; consider "x-compress" to be
2053   equivalent to "compress".
2054</t>
2055</section>
2056
2057<section title="Deflate Coding" anchor="deflate.coding">
2058<iref item="deflate (Coding Format)"/>
2059<t>
2060   The "deflate" format is defined as the "deflate" compression mechanism
2061   (described in <xref target="RFC1951"/>) used inside the "zlib"
2062   data format (<xref target="RFC1950"/>).
2063</t>
2064<x:note>
2065  <t>
2066    &Note; Some incorrect implementations send the "deflate"
2067    compressed data without the zlib wrapper.
2068   </t>
2069</x:note>
2070</section>
2071
2072<section title="Gzip Coding" anchor="gzip.coding">
2073<iref item="gzip (Coding Format)"/>
2074<t>
2075   The "gzip" format is produced by the file compression program
2076   "gzip" (GNU zip), as described in <xref target="RFC1952"/>. This format is a
2077   Lempel-Ziv coding (LZ77) with a 32 bit CRC.
2078   Recipients &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2079</t>
2080</section>
2081
2082</section>
2083
2084<section title="TE" anchor="header.te">
2085  <iref primary="true" item="TE header field" x:for-anchor=""/>
2086  <x:anchor-alias value="TE"/>
2087  <x:anchor-alias value="t-codings"/>
2088  <x:anchor-alias value="t-ranking"/>
2089  <x:anchor-alias value="rank"/>
2090<t>
2091   The "TE" header field in a request indicates what transfer codings,
2092   besides chunked, the client is willing to accept in response, and
2093   whether or not the client is willing to accept trailer fields in a
2094   chunked transfer coding.
2095</t>
2096<t>
2097   The TE field-value consists of a comma-separated list of transfer coding
2098   names, each allowing for optional parameters (as described in
2099   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2100   Clients &MUST-NOT; send the chunked transfer coding name in TE;
2101   chunked is always acceptable for HTTP/1.1 recipients.
2102</t>
2103<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"/>
2104  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2105  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2106  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2107  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2108             / ( "1" [ "." 0*3("0") ] )
2109</artwork></figure>
2110<t>
2111   Three examples of TE use are below.
2112</t>
2113<figure><artwork type="example">
2114  TE: deflate
2115  TE:
2116  TE: trailers, deflate;q=0.5
2117</artwork></figure>
2118<t>
2119   The presence of the keyword "trailers" indicates that the client is
2120   willing to accept trailer fields in a chunked transfer coding,
2121   as defined in <xref target="chunked.encoding"/>, on behalf of itself and
2122   any downstream clients. For chained requests, this implies that either:
2123   (a) all downstream clients are willing to accept trailer fields in the
2124   forwarded response; or,
2125   (b) the client will attempt to buffer the response on behalf of downstream
2126   recipients.
2127   Note that HTTP/1.1 does not define any means to limit the size of a
2128   chunked response such that a client can be assured of buffering the
2129   entire response.
2130</t>
2131<t>
2132   When multiple transfer codings are acceptable, the client &MAY; rank the
2133   codings by preference using a case-insensitive "q" parameter (similar to
2134   the qvalues used in content negotiation fields, &qvalue;). The rank value
2135   is a real number in the range 0 through 1, where 0.001 is the least
2136   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2137</t>
2138<t>
2139   If the TE field-value is empty or if no TE field is present, the only
2140   acceptable transfer coding is chunked. A message with no transfer coding
2141   is always acceptable.
2142</t>
2143<t>
2144   Since the TE header field only applies to the immediate connection,
2145   a sender of TE &MUST; also send a "TE" connection option within the
2146   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2147   in order to prevent the TE field from being forwarded by intermediaries
2148   that do not support its semantics.
2149</t>
2150</section>
2151</section>
2152
2153<section title="Message Routing" anchor="message.routing">
2154<t>
2155   HTTP request message routing is determined by each client based on the
2156   target resource, the client's proxy configuration, and
2157   establishment or reuse of an inbound connection.  The corresponding
2158   response routing follows the same connection chain back to the client.
2159</t>
2160
2161<section title="Identifying a Target Resource" anchor="target-resource">
2162  <iref primary="true" item="target resource"/>
2163  <iref primary="true" item="target URI"/>
2164  <x:anchor-alias value="target resource"/>
2165  <x:anchor-alias value="target URI"/>
2166<t>
2167   HTTP is used in a wide variety of applications, ranging from
2168   general-purpose computers to home appliances.  In some cases,
2169   communication options are hard-coded in a client's configuration.
2170   However, most HTTP clients rely on the same resource identification
2171   mechanism and configuration techniques as general-purpose Web browsers.
2172</t>
2173<t>
2174   HTTP communication is initiated by a user agent for some purpose.
2175   The purpose is a combination of request semantics, which are defined in
2176   <xref target="Part2"/>, and a target resource upon which to apply those
2177   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2178   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2179   would resolve to its absolute form in order to obtain the
2180   "<x:dfn>target URI</x:dfn>".  The target URI
2181   excludes the reference's fragment identifier component, if any,
2182   since fragment identifiers are reserved for client-side processing
2183   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2184</t>
2185</section>
2186
2187<section title="Connecting Inbound" anchor="connecting.inbound">
2188<t>
2189   Once the target URI is determined, a client needs to decide whether
2190   a network request is necessary to accomplish the desired semantics and,
2191   if so, where that request is to be directed.
2192</t>
2193<t>
2194   If the client has a response cache and the request semantics can be
2195   satisfied by a cache (<xref target="Part6"/>), then the request is
2196   usually directed to the cache first.
2197</t>
2198<t>
2199   If the request is not satisfied by a cache, then a typical client will
2200   check its configuration to determine whether a proxy is to be used to
2201   satisfy the request.  Proxy configuration is implementation-dependent,
2202   but is often based on URI prefix matching, selective authority matching,
2203   or both, and the proxy itself is usually identified by an "http" or
2204   "https" URI.  If a proxy is applicable, the client connects inbound by
2205   establishing (or reusing) a connection to that proxy.
2206</t>
2207<t>
2208   If no proxy is applicable, a typical client will invoke a handler routine,
2209   usually specific to the target URI's scheme, to connect directly
2210   to an authority for the target resource.  How that is accomplished is
2211   dependent on the target URI scheme and defined by its associated
2212   specification, similar to how this specification defines origin server
2213   access for resolution of the "http" (<xref target="http.uri"/>) and
2214   "https" (<xref target="https.uri"/>) schemes.
2215</t>
2216<t>
2217   HTTP requirements regarding connection management are defined in
2218   <xref target="connection.management"/>.
2219</t>
2220</section>
2221
2222<section title="Request Target" anchor="request-target">
2223<t>
2224   Once an inbound connection is obtained,
2225   the client sends an HTTP request message (<xref target="http.message"/>)
2226   with a request-target derived from the target URI.
2227   There are four distinct formats for the request-target, depending on both
2228   the method being requested and whether the request is to a proxy.
2229</t>   
2230<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"/>
2231  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2232                 / <x:ref>absolute-form</x:ref>
2233                 / <x:ref>authority-form</x:ref>
2234                 / <x:ref>asterisk-form</x:ref>
2235
2236  <x:ref>origin-form</x:ref>    = <x:ref>absolute-path</x:ref> [ "?" <x:ref>query</x:ref> ]
2237  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2238  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2239  <x:ref>asterisk-form</x:ref>  = "*"
2240</artwork></figure>
2241<t anchor="origin-form"><iref item="origin-form (of request-target)"/>
2242  <x:h>origin-form</x:h>
2243</t>
2244<t>
2245   The most common form of request-target is the <x:dfn>origin-form</x:dfn>.
2246   When making a request directly to an origin server, other than a CONNECT
2247   or server-wide OPTIONS request (as detailed below),
2248   a client &MUST; send only the absolute path and query components of
2249   the target URI as the request-target.
2250   If the target URI's path component is empty, then the client &MUST; send
2251   "/" as the path within the origin-form of request-target.
2252   A <x:ref>Host</x:ref> header field is also sent, as defined in
2253   <xref target="header.host"/>, containing the target URI's
2254   authority component (excluding any userinfo).
2255</t>
2256<t>
2257   For example, a client wishing to retrieve a representation of the resource
2258   identified as
2259</t>
2260<figure><artwork x:indent-with="  " type="example">
2261http://www.example.org/where?q=now
2262</artwork></figure>
2263<t>
2264   directly from the origin server would open (or reuse) a TCP connection
2265   to port 80 of the host "www.example.org" and send the lines:
2266</t>
2267<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2268GET /where?q=now HTTP/1.1
2269Host: www.example.org
2270</artwork></figure>
2271<t>
2272   followed by the remainder of the request message.
2273</t>
2274<t anchor="absolute-form"><iref item="absolute-form (of request-target)"/>
2275  <x:h>absolute-form</x:h>
2276</t>
2277<t>
2278   When making a request to a proxy, other than a CONNECT or server-wide
2279   OPTIONS request (as detailed below), a client &MUST; send the target URI
2280   in <x:dfn>absolute-form</x:dfn> as the request-target.
2281   The proxy is requested to either service that request from a valid cache,
2282   if possible, or make the same request on the client's behalf to either
2283   the next inbound proxy server or directly to the origin server indicated
2284   by the request-target.  Requirements on such "forwarding" of messages are
2285   defined in <xref target="message.forwarding"/>.
2286</t>
2287<t>
2288   An example absolute-form of request-line would be:
2289</t>
2290<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2291GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
2292</artwork></figure>
2293<t>
2294   To allow for transition to the absolute-form for all requests in some
2295   future version of HTTP, HTTP/1.1 servers &MUST; accept the absolute-form
2296   in requests, even though HTTP/1.1 clients will only send them in requests
2297   to proxies.
2298</t>
2299<t anchor="authority-form"><iref item="authority-form (of request-target)"/>
2300  <x:h>authority-form</x:h>
2301</t>
2302<t>
2303   The <x:dfn>authority-form</x:dfn> of request-target is only used for CONNECT requests
2304   (&CONNECT;).  When making a CONNECT request to establish a tunnel through
2305   one or more proxies, a client &MUST; send only the target URI's
2306   authority component (excluding any userinfo) as the request-target.
2307   For example,
2308</t>
2309<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2310CONNECT www.example.com:80 HTTP/1.1
2311</artwork></figure>
2312<t anchor="asterisk-form"><iref item="asterisk-form (of request-target)"/>
2313  <x:h>asterisk-form</x:h>
2314</t>
2315<t>
2316   The <x:dfn>asterisk-form</x:dfn> of request-target is only used for a server-wide
2317   OPTIONS request (&OPTIONS;).  When a client wishes to request OPTIONS
2318   for the server as a whole, as opposed to a specific named resource of
2319   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2320   For example,
2321</t>
2322<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2323OPTIONS * HTTP/1.1
2324</artwork></figure>
2325<t>
2326   If a proxy receives an OPTIONS request with an absolute-form of
2327   request-target in which the URI has an empty path and no query component,
2328   then the last proxy on the request chain &MUST; send a request-target
2329   of "*" when it forwards the request to the indicated origin server.
2330</t>
2331<figure><preamble>  
2332   For example, the request
2333</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2334OPTIONS http://www.example.org:8001 HTTP/1.1
2335</artwork></figure>
2336<figure><preamble>  
2337  would be forwarded by the final proxy as
2338</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2339OPTIONS * HTTP/1.1
2340Host: www.example.org:8001
2341</artwork>
2342<postamble>
2343   after connecting to port 8001 of host "www.example.org".
2344</postamble>
2345</figure>
2346</section>
2347
2348<section title="Host" anchor="header.host">
2349  <iref primary="true" item="Host header field" x:for-anchor=""/>
2350  <x:anchor-alias value="Host"/>
2351<t>
2352   The "Host" header field in a request provides the host and port
2353   information from the target URI, enabling the origin
2354   server to distinguish among resources while servicing requests
2355   for multiple host names on a single IP address.  Since the Host
2356   field-value is critical information for handling a request, it
2357   &SHOULD; be sent as the first header field following the request-line.
2358</t>
2359<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2360  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2361</artwork></figure>
2362<t>
2363   A client &MUST; send a Host header field in all HTTP/1.1 request
2364   messages.  If the target URI includes an authority component, then
2365   the Host field-value &MUST; be identical to that authority component
2366   after excluding any userinfo (<xref target="http.uri"/>).
2367   If the authority component is missing or undefined for the target URI,
2368   then the Host header field &MUST; be sent with an empty field-value.
2369</t>
2370<t>
2371   For example, a GET request to the origin server for
2372   &lt;http://www.example.org/pub/WWW/&gt; would begin with:
2373</t>
2374<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2375GET /pub/WWW/ HTTP/1.1
2376Host: www.example.org
2377</artwork></figure>
2378<t>
2379   The Host header field &MUST; be sent in an HTTP/1.1 request even
2380   if the request-target is in the absolute-form, since this
2381   allows the Host information to be forwarded through ancient HTTP/1.0
2382   proxies that might not have implemented Host.
2383</t>
2384<t>
2385   When a proxy receives a request with an absolute-form of
2386   request-target, the proxy &MUST; ignore the received
2387   Host header field (if any) and instead replace it with the host
2388   information of the request-target.  If the proxy forwards the request,
2389   it &MUST; generate a new Host field-value based on the received
2390   request-target rather than forward the received Host field-value.
2391</t>
2392<t>
2393   Since the Host header field acts as an application-level routing
2394   mechanism, it is a frequent target for malware seeking to poison
2395   a shared cache or redirect a request to an unintended server.
2396   An interception proxy is particularly vulnerable if it relies on
2397   the Host field-value for redirecting requests to internal
2398   servers, or for use as a cache key in a shared cache, without
2399   first verifying that the intercepted connection is targeting a
2400   valid IP address for that host.
2401</t>
2402<t>
2403   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2404   to any HTTP/1.1 request message that lacks a Host header field and
2405   to any request message that contains more than one Host header field
2406   or a Host header field with an invalid field-value.
2407</t>
2408</section>
2409
2410<section title="Effective Request URI" anchor="effective.request.uri">
2411  <iref primary="true" item="effective request URI"/>
2412  <x:anchor-alias value="effective request URI"/>
2413<t>
2414   A server that receives an HTTP request message &MUST; reconstruct
2415   the user agent's original target URI, based on the pieces of information
2416   learned from the request-target, <x:ref>Host</x:ref> header field, and
2417   connection context, in order to identify the intended target resource and
2418   properly service the request. The URI derived from this reconstruction
2419   process is referred to as the "<x:dfn>effective request URI</x:dfn>".
2420</t>
2421<t>
2422   For a user agent, the effective request URI is the target URI.
2423</t>
2424<t>
2425   If the request-target is in absolute-form, then the effective request URI
2426   is the same as the request-target.  Otherwise, the effective request URI
2427   is constructed as follows.
2428</t>
2429<t>
2430   If the request is received over a TLS-secured TCP connection,
2431   then the effective request URI's scheme is "https"; otherwise, the
2432   scheme is "http".
2433</t>
2434<t>
2435   If the request-target is in authority-form, then the effective
2436   request URI's authority component is the same as the request-target.
2437   Otherwise, if a <x:ref>Host</x:ref> header field is supplied with a
2438   non-empty field-value, then the authority component is the same as the
2439   Host field-value. Otherwise, the authority component is the concatenation of
2440   the default host name configured for the server, a colon (":"), and the
2441   connection's incoming TCP port number in decimal form.
2442</t>
2443<t>
2444   If the request-target is in authority-form or asterisk-form, then the
2445   effective request URI's combined path and query component is empty.
2446   Otherwise, the combined path and query component is the same as the
2447   request-target.
2448</t>
2449<t>
2450   The components of the effective request URI, once determined as above,
2451   can be combined into absolute-URI form by concatenating the scheme,
2452   "://", authority, and combined path and query component.
2453</t>
2454<figure>
2455<preamble>
2456   Example 1: the following message received over an insecure TCP connection
2457</preamble>
2458<artwork type="example" x:indent-with="  ">
2459GET /pub/WWW/TheProject.html HTTP/1.1
2460Host: www.example.org:8080
2461</artwork>
2462</figure>
2463<figure>
2464<preamble>
2465  has an effective request URI of
2466</preamble>
2467<artwork type="example" x:indent-with="  ">
2468http://www.example.org:8080/pub/WWW/TheProject.html
2469</artwork>
2470</figure>
2471<figure>
2472<preamble>
2473   Example 2: the following message received over a TLS-secured TCP connection
2474</preamble>
2475<artwork type="example" x:indent-with="  ">
2476OPTIONS * HTTP/1.1
2477Host: www.example.org
2478</artwork>
2479</figure>
2480<figure>
2481<preamble>
2482  has an effective request URI of
2483</preamble>
2484<artwork type="example" x:indent-with="  ">
2485https://www.example.org
2486</artwork>
2487</figure>
2488<t>
2489   An origin server that does not allow resources to differ by requested
2490   host &MAY; ignore the <x:ref>Host</x:ref> field-value and instead replace it
2491   with a configured server name when constructing the effective request URI.
2492</t>
2493<t>
2494   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2495   field &MAY; attempt to use heuristics (e.g., examination of the URI path for
2496   something unique to a particular host) in order to guess the
2497   effective request URI's authority component.
2498</t>
2499</section>
2500
2501<section title="Associating a Response to a Request" anchor="associating.response.to.request">
2502<t>
2503   HTTP does not include a request identifier for associating a given
2504   request message with its corresponding one or more response messages.
2505   Hence, it relies on the order of response arrival to correspond exactly
2506   to the order in which requests are made on the same connection.
2507   More than one response message per request only occurs when one or more
2508   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a
2509   final response to the same request.
2510</t>
2511<t>
2512   A client that has more than one outstanding request on a connection &MUST;
2513   maintain a list of outstanding requests in the order sent and &MUST;
2514   associate each received response message on that connection to the highest
2515   ordered request that has not yet received a final (non-<x:ref>1xx</x:ref>)
2516   response.
2517</t>
2518</section>
2519
2520<section title="Message Forwarding" anchor="message.forwarding">
2521<t>
2522   As described in <xref target="intermediaries"/>, intermediaries can serve
2523   a variety of roles in the processing of HTTP requests and responses.
2524   Some intermediaries are used to improve performance or availability.
2525   Others are used for access control or to filter content.
2526   Since an HTTP stream has characteristics similar to a pipe-and-filter
2527   architecture, there are no inherent limits to the extent an intermediary
2528   can enhance (or interfere) with either direction of the stream.
2529</t>
2530<t>
2531   Intermediaries that forward a message &MUST; implement the
2532   <x:ref>Connection</x:ref> header field, as specified in
2533   <xref target="header.connection"/>, to exclude fields that are only
2534   intended for the incoming connection.
2535</t>
2536<t>
2537   In order to avoid request loops, a proxy that forwards requests to other
2538   proxies &MUST; be able to recognize and exclude all of its own server
2539   names, including any aliases, local variations, or literal IP addresses.
2540</t>
2541
2542<section title="Via" anchor="header.via">
2543  <iref primary="true" item="Via header field" x:for-anchor=""/>
2544  <x:anchor-alias value="pseudonym"/>
2545  <x:anchor-alias value="received-by"/>
2546  <x:anchor-alias value="received-protocol"/>
2547  <x:anchor-alias value="Via"/>
2548<t>
2549   The "Via" header field &MUST; be sent by a proxy or gateway in forwarded
2550   messages to indicate the intermediate protocols and recipients between the
2551   user agent and the server on requests, and between the origin server and
2552   the client on responses. It is analogous to the "Received" field
2553   used by email systems (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2554   Via is used in HTTP for tracking message forwards,
2555   avoiding request loops, and identifying the protocol capabilities of
2556   all senders along the request/response chain.
2557</t>
2558<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"/>
2559  <x:ref>Via</x:ref>               = 1#( <x:ref>received-protocol</x:ref> <x:ref>RWS</x:ref> <x:ref>received-by</x:ref>
2560                          [ <x:ref>RWS</x:ref> <x:ref>comment</x:ref> ] )
2561  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2562                      ; see <xref target="header.upgrade"/>
2563  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2564  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2565</artwork></figure>
2566<t>
2567   The received-protocol indicates the protocol version of the message
2568   received by the server or client along each segment of the
2569   request/response chain. The received-protocol version is appended to
2570   the Via field value when the message is forwarded so that information
2571   about the protocol capabilities of upstream applications remains
2572   visible to all recipients.
2573</t>
2574<t>
2575   The protocol-name is excluded if and only if it would be "HTTP". The
2576   received-by field is normally the host and optional port number of a
2577   recipient server or client that subsequently forwarded the message.
2578   However, if the real host is considered to be sensitive information,
2579   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2580   be assumed to be the default port of the received-protocol.
2581</t>
2582<t>
2583   Multiple Via field values represent each proxy or gateway that has
2584   forwarded the message. Each recipient &MUST; append its information
2585   such that the end result is ordered according to the sequence of
2586   forwarding applications.
2587</t>
2588<t>
2589   Comments &MAY; be used in the Via header field to identify the software
2590   of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2591   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2592   are optional and &MAY; be removed by any recipient prior to forwarding the
2593   message.
2594</t>
2595<t>
2596   For example, a request message could be sent from an HTTP/1.0 user
2597   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2598   forward the request to a public proxy at p.example.net, which completes
2599   the request by forwarding it to the origin server at www.example.com.
2600   The request received by www.example.com would then have the following
2601   Via header field:
2602</t>
2603<figure><artwork type="example">
2604  Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
2605</artwork></figure>
2606<t>
2607   A proxy or gateway used as a portal through a network firewall
2608   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2609   region unless it is explicitly enabled to do so. If not enabled, the
2610   received-by host of any host behind the firewall &SHOULD; be replaced
2611   by an appropriate pseudonym for that host.
2612</t>
2613<t>
2614   A proxy or gateway &MAY; combine an ordered subsequence of Via header
2615   field entries into a single such entry if the entries have identical
2616   received-protocol values. For example,
2617</t>
2618<figure><artwork type="example">
2619  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2620</artwork></figure>
2621<t>
2622  could be collapsed to
2623</t>
2624<figure><artwork type="example">
2625  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2626</artwork></figure>
2627<t>
2628   Senders &SHOULD-NOT; combine multiple entries unless they are all
2629   under the same organizational control and the hosts have already been
2630   replaced by pseudonyms. Senders &MUST-NOT; combine entries that
2631   have different received-protocol values.
2632</t>
2633</section>
2634
2635<section title="Transformations" anchor="message.transformations">
2636<t>
2637   Some intermediaries include features for transforming messages and their
2638   payloads.  A transforming proxy might, for example, convert between image
2639   formats in order to save cache space or to reduce the amount of traffic on
2640   a slow link. However, operational problems might occur when these
2641   transformations are applied to payloads intended for critical applications,
2642   such as medical imaging or scientific data analysis, particularly when
2643   integrity checks or digital signatures are used to ensure that the payload
2644   received is identical to the original.
2645</t>
2646<t>
2647   If a proxy receives a request-target with a host name that is not a
2648   fully qualified domain name, it &MAY; add its own domain to the host name
2649   it received when forwarding the request.  A proxy &MUST-NOT; change the
2650   host name if it is a fully qualified domain name.
2651</t>
2652<t>
2653   A proxy &MUST-NOT; modify the "absolute-path" and "query" parts of the
2654   received request-target when forwarding it to the next inbound server,
2655   except as noted above to replace an empty path with "/" or "*".
2656</t>
2657<t>
2658   A proxy &MUST-NOT; modify header fields that provide information about the
2659   end points of the communication chain, the resource state, or the selected
2660   representation. A proxy &MAY; change the message body through application
2661   or removal of a transfer coding (<xref target="transfer.codings"/>).
2662</t>
2663<t>
2664   A non-transforming proxy &MUST; preserve the message payload (&payload;).
2665   A transforming proxy &MUST; preserve the payload of a message that
2666   contains the no-transform cache-control directive.
2667</t>
2668<t>
2669   A transforming proxy &MAY; transform the payload of a message
2670   that does not contain the no-transform cache-control directive;
2671   if the payload is transformed, the transforming proxy &MUST; add a
2672   Warning 214 (Transformation applied) header field if one does not
2673   already appear in the message (see &header-warning;).
2674</t>
2675</section>
2676</section>
2677</section>
2678
2679<section title="Connection Management" anchor="connection.management">
2680<t>
2681   HTTP messaging is independent of the underlying transport or
2682   session-layer connection protocol(s).  HTTP only presumes a reliable
2683   transport with in-order delivery of requests and the corresponding
2684   in-order delivery of responses.  The mapping of HTTP request and
2685   response structures onto the data units of an underlying transport
2686   protocol is outside the scope of this specification.
2687</t>
2688<t>
2689   As described in <xref target="connecting.inbound"/>, the specific
2690   connection protocols to be used for an HTTP interaction are determined by
2691   client configuration and the <x:ref>target URI</x:ref>.
2692   For example, the "http" URI scheme
2693   (<xref target="http.uri"/>) indicates a default connection of TCP
2694   over IP, with a default TCP port of 80, but the client might be
2695   configured to use a proxy via some other connection, port, or protocol.
2696</t>
2697<t>
2698   HTTP implementations are expected to engage in connection management,
2699   which includes maintaining the state of current connections,
2700   establishing a new connection or reusing an existing connection,
2701   processing messages received on a connection, detecting connection
2702   failures, and closing each connection.
2703   Most clients maintain multiple connections in parallel, including
2704   more than one connection per server endpoint.
2705   Most servers are designed to maintain thousands of concurrent connections,
2706   while controlling request queues to enable fair use and detect
2707   denial of service attacks.
2708</t>
2709
2710<section title="Connection" anchor="header.connection">
2711  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2712  <iref primary="true" item="close" x:for-anchor=""/>
2713  <x:anchor-alias value="Connection"/>
2714  <x:anchor-alias value="connection-option"/>
2715  <x:anchor-alias value="close"/>
2716<t>
2717   The "Connection" header field allows the sender to indicate desired
2718   control options for the current connection.  In order to avoid confusing
2719   downstream recipients, a proxy or gateway &MUST; remove or replace any
2720   received connection options before forwarding the message.
2721</t>
2722<t>
2723   When a header field aside from Connection is used to supply control
2724   information for or about the current connection, the sender &MUST; list
2725   the corresponding field-name within the "Connection" header field.
2726   A proxy or gateway &MUST; parse a received Connection
2727   header field before a message is forwarded and, for each
2728   connection-option in this field, remove any header field(s) from
2729   the message with the same name as the connection-option, and then
2730   remove the Connection header field itself (or replace it with the
2731   intermediary's own connection options for the forwarded message).
2732</t>
2733<t>
2734   Hence, the Connection header field provides a declarative way of
2735   distinguishing header fields that are only intended for the
2736   immediate recipient ("hop-by-hop") from those fields that are
2737   intended for all recipients on the chain ("end-to-end"), enabling the
2738   message to be self-descriptive and allowing future connection-specific
2739   extensions to be deployed without fear that they will be blindly
2740   forwarded by older intermediaries.
2741</t>
2742<t>
2743   The Connection header field's value has the following grammar:
2744</t>
2745<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2746  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2747  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2748</artwork></figure>
2749<t>
2750   Connection options are case-insensitive.
2751</t>
2752<t>
2753   A sender &MUST-NOT; send a connection option corresponding to a header
2754   field that is intended for all recipients of the payload.
2755   For example, <x:ref>Cache-Control</x:ref> is never appropriate as a
2756   connection option (&header-cache-control;).
2757</t>
2758<t>
2759   The connection options do not have to correspond to a header field
2760   present in the message, since a connection-specific header field
2761   might not be needed if there are no parameters associated with that
2762   connection option.  Recipients that trigger certain connection
2763   behavior based on the presence of connection options &MUST; do so
2764   based on the presence of the connection-option rather than only the
2765   presence of the optional header field.  In other words, if the
2766   connection option is received as a header field but not indicated
2767   within the Connection field-value, then the recipient &MUST; ignore
2768   the connection-specific header field because it has likely been
2769   forwarded by an intermediary that is only partially conformant.
2770</t>
2771<t>
2772   When defining new connection options, specifications ought to
2773   carefully consider existing deployed header fields and ensure
2774   that the new connection option does not share the same name as
2775   an unrelated header field that might already be deployed.
2776   Defining a new connection option essentially reserves that potential
2777   field-name for carrying additional information related to the
2778   connection option, since it would be unwise for senders to use
2779   that field-name for anything else.
2780</t>
2781<t>
2782   The "<x:dfn>close</x:dfn>" connection option is defined for a
2783   sender to signal that this connection will be closed after completion of
2784   the response. For example,
2785</t>
2786<figure><artwork type="example">
2787  Connection: close
2788</artwork></figure>
2789<t>
2790   in either the request or the response header fields indicates that
2791   the connection &MUST; be closed after the current request/response
2792   is complete (<xref target="persistent.tear-down"/>).
2793</t>
2794<t>
2795   A client that does not support <x:ref>persistent connections</x:ref> &MUST;
2796   send the "close" connection option in every request message.
2797</t>
2798<t>
2799   A server that does not support <x:ref>persistent connections</x:ref> &MUST;
2800   send the "close" connection option in every response message that
2801   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2802</t>
2803</section>
2804
2805<section title="Establishment" anchor="persistent.establishment">
2806<t>
2807   It is beyond the scope of this specification to describe how connections
2808   are established via various transport or session-layer protocols.
2809   Each connection applies to only one transport link.
2810</t>
2811</section>
2812
2813<section title="Persistence" anchor="persistent.connections">
2814   <x:anchor-alias value="persistent connections"/>
2815<t>
2816   HTTP/1.1 defaults to the use of "<x:dfn>persistent connections</x:dfn>",
2817   allowing multiple requests and responses to be carried over a single
2818   connection. The "<x:ref>close</x:ref>" connection-option is used to signal
2819   that a connection will not persist after the current request/response.
2820   HTTP implementations &SHOULD; support persistent connections.
2821</t>
2822<t>
2823   A recipient determines whether a connection is persistent or not based on
2824   the most recently received message's protocol version and
2825   <x:ref>Connection</x:ref> header field (if any):
2826   <list style="symbols">
2827     <t>If the <x:ref>close</x:ref> connection option is present, the
2828        connection will not persist after the current response; else,</t>
2829     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2830        persist after the current response; else,</t>
2831     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2832        connection option is present, the recipient is not a proxy, and
2833        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2834        the connection will persist after the current response; otherwise,</t>
2835     <t>The connection will close after the current response.</t>
2836   </list>
2837</t>
2838<t>
2839   A server &MAY; assume that an HTTP/1.1 client intends to maintain a
2840   persistent connection until a <x:ref>close</x:ref> connection option
2841   is received in a request.
2842</t>
2843<t>
2844   A client &MAY; reuse a persistent connection until it sends or receives
2845   a <x:ref>close</x:ref> connection option or receives an HTTP/1.0 response
2846   without a "keep-alive" connection option.
2847</t>
2848<t>
2849   In order to remain persistent, all messages on a connection &MUST;
2850   have a self-defined message length (i.e., one not defined by closure
2851   of the connection), as described in <xref target="message.body"/>.
2852   A server &MUST; read the entire request message body or close
2853   the connection after sending its response, since otherwise the
2854   remaining data on a persistent connection would be misinterpreted
2855   as the next request.  Likewise,
2856   a client &MUST; read the entire response message body if it intends
2857   to reuse the same connection for a subsequent request.
2858</t>
2859<t>
2860   A proxy server &MUST-NOT; maintain a persistent connection with an
2861   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
2862   information and discussion of the problems with the Keep-Alive header field
2863   implemented by many HTTP/1.0 clients).
2864</t>
2865<t>
2866   Clients and servers &SHOULD-NOT; assume that a persistent connection is
2867   maintained for HTTP versions less than 1.1 unless it is explicitly
2868   signaled.
2869   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
2870   for more information on backward compatibility with HTTP/1.0 clients.
2871</t>
2872
2873<section title="Retrying Requests" anchor="persistent.retrying.requests">
2874<t>
2875   Connections can be closed at any time, with or without intention.
2876   Implementations ought to anticipate the need to recover
2877   from asynchronous close events.
2878</t>
2879<t>
2880   When an inbound connection is closed prematurely, a client &MAY; open a new
2881   connection and automatically retransmit an aborted sequence of requests if
2882   all of those requests have idempotent methods (&idempotent-methods;).
2883   A proxy &MUST-NOT; automatically retry non-idempotent requests.
2884</t>
2885<t>
2886   A user agent &MUST-NOT; automatically retry a request with a non-idempotent
2887   method unless it has some means to know that the request semantics are
2888   actually idempotent, regardless of the method, or some means to detect that
2889   the original request was never applied. For example, a user agent that
2890   knows (through design or configuration) that a POST request to a given
2891   resource is safe can repeat that request automatically.
2892   Likewise, a user agent designed specifically to operate on a version
2893   control repository might be able to recover from partial failure conditions
2894   by checking the target resource revision(s) after a failed connection,
2895   reverting or fixing any changes that were partially applied, and then
2896   automatically retrying the requests that failed.
2897</t>
2898<t>
2899   An automatic retry &SHOULD-NOT; be repeated if it fails.
2900</t>
2901</section>
2902
2903<section title="Pipelining" anchor="pipelining">
2904   <x:anchor-alias value="pipeline"/>
2905<t>
2906   A client that supports persistent connections &MAY; "<x:dfn>pipeline</x:dfn>"
2907   its requests (i.e., send multiple requests without waiting for each
2908   response). A server &MAY; process a sequence of pipelined requests in
2909   parallel if they all have safe methods (&safe-methods;), but &MUST; send
2910   the corresponding responses in the same order that the requests were
2911   received.
2912</t>
2913<t>
2914   A client that pipelines requests &MUST; be prepared to retry those
2915   requests if the connection closes before it receives all of the
2916   corresponding responses. A client that assumes a persistent connection and
2917   pipelines immediately after connection establishment &MUST-NOT; pipeline
2918   on a retry connection until it knows the connection is persistent.
2919</t>
2920<t>
2921   Idempotent methods (&idempotent-methods;) are significant to pipelining
2922   because they can be automatically retried after a connection failure.
2923   A user agent &SHOULD-NOT; pipeline requests after a non-idempotent method
2924   until the final response status code for that method has been received,
2925   unless the user agent has a means to detect and recover from partial
2926   failure conditions involving the pipelined sequence.
2927</t>
2928<t>
2929   An intermediary that receives pipelined requests &MAY; pipeline those
2930   requests when forwarding them inbound, since it can rely on the outbound
2931   user agent(s) to determine what requests can be safely pipelined. If the
2932   inbound connection fails before receiving a response, the pipelining
2933   intermediary &MAY; attempt to retry a sequence of requests that have yet
2934   to receive a response if the requests all have idempotent methods;
2935   otherwise, the pipelining intermediary &SHOULD; forward any received
2936   responses and then close the corresponding outbound connection(s) so that
2937   the outbound user agent(s) can recover accordingly.
2938</t>
2939</section>
2940</section>
2941   
2942<section title="Concurrency" anchor="persistent.concurrency">
2943<t>
2944   Clients &SHOULD; limit the number of simultaneous
2945   connections that they maintain to a given server.
2946</t>
2947<t>
2948   Previous revisions of HTTP gave a specific number of connections as a
2949   ceiling, but this was found to be impractical for many applications. As a
2950   result, this specification does not mandate a particular maximum number of
2951   connections, but instead encourages clients to be conservative when opening
2952   multiple connections.
2953</t>
2954<t>
2955   Multiple connections are typically used to avoid the "head-of-line
2956   blocking" problem, wherein a request that takes significant server-side
2957   processing and/or has a large payload blocks subsequent requests on the
2958   same connection. However, each connection consumes server resources.
2959   Furthermore, using multiple connections can cause undesirable side effects
2960   in congested networks.
2961</t>
2962<t>
2963   Note that servers might reject traffic that they deem abusive, including an
2964   excessive number of connections from a client.
2965</t>
2966</section>
2967
2968<section title="Failures and Time-outs" anchor="persistent.failures">
2969<t>
2970   Servers will usually have some time-out value beyond which they will
2971   no longer maintain an inactive connection. Proxy servers might make
2972   this a higher value since it is likely that the client will be making
2973   more connections through the same server. The use of persistent
2974   connections places no requirements on the length (or existence) of
2975   this time-out for either the client or the server.
2976</t>
2977<t>
2978   When a client or server wishes to time-out it &SHOULD; issue a graceful
2979   close on the transport connection. Clients and servers &SHOULD; both
2980   constantly watch for the other side of the transport close, and
2981   respond to it as appropriate. If a client or server does not detect
2982   the other side's close promptly it could cause unnecessary resource
2983   drain on the network.
2984</t>
2985<t>
2986   A client, server, or proxy &MAY; close the transport connection at any
2987   time. For example, a client might have started to send a new request
2988   at the same time that the server has decided to close the "idle"
2989   connection. From the server's point of view, the connection is being
2990   closed while it was idle, but from the client's point of view, a
2991   request is in progress.
2992</t>
2993<t>
2994   Servers &SHOULD; maintain persistent connections and allow the underlying
2995   transport's flow control mechanisms to resolve temporary overloads, rather
2996   than terminate connections with the expectation that clients will retry.
2997   The latter technique can exacerbate network congestion.
2998</t>
2999<t>
3000   A client sending a message body &SHOULD; monitor
3001   the network connection for an error status code while it is transmitting
3002   the request. If the client sees an error status code, it &SHOULD;
3003   immediately cease transmitting the body and close the connection.
3004</t>
3005</section>
3006   
3007<section title="Tear-down" anchor="persistent.tear-down">
3008  <iref primary="false" item="Connection header field" x:for-anchor=""/>
3009  <iref primary="false" item="close" x:for-anchor=""/>
3010<t>
3011   The <x:ref>Connection</x:ref> header field
3012   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
3013   connection option that a sender &SHOULD; send when it wishes to close
3014   the connection after the current request/response pair.
3015</t>
3016<t>
3017   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
3018   send further requests on that connection (after the one containing
3019   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
3020   final response message corresponding to this request.
3021</t>
3022<t>
3023   A server that receives a <x:ref>close</x:ref> connection option &MUST;
3024   initiate a lingering close (see below) of the connection after it sends the
3025   final response to the request that contained <x:ref>close</x:ref>.
3026   The server &SHOULD; send a <x:ref>close</x:ref> connection option
3027   in its final response on that connection. The server &MUST-NOT; process
3028   any further requests received on that connection.
3029</t>
3030<t>
3031   A server that sends a <x:ref>close</x:ref> connection option &MUST;
3032   initiate a lingering close of the connection after it sends the
3033   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
3034   any further requests received on that connection.
3035</t>
3036<t>
3037   A client that receives a <x:ref>close</x:ref> connection option &MUST;
3038   cease sending requests on that connection and close the connection
3039   after reading the response message containing the close; if additional
3040   pipelined requests had been sent on the connection, the client &SHOULD;
3041   assume that they will not be processed by the server.
3042</t>
3043<t>
3044   If a server performs an immediate close of a TCP connection, there is a
3045   significant risk that the client will not be able to read the last HTTP
3046   response.  If the server receives additional data from the client on a
3047   fully-closed connection, such as another request that was sent by the
3048   client before receiving the server's response, the server's TCP stack will
3049   send a reset packet to the client; unfortunately, the reset packet might
3050   erase the client's unacknowledged input buffers before they can be read
3051   and interpreted by the client's HTTP parser.
3052</t>
3053<t>
3054   To avoid the TCP reset problem, a server can perform a lingering close on a
3055   connection by closing only the write side of the read/write connection
3056   (a half-close) and continuing to read from the connection until the
3057   connection is closed by the client or the server is reasonably certain
3058   that its own TCP stack has received the client's acknowledgement of the
3059   packet(s) containing the server's last response. It is then safe for the
3060   server to fully close the connection.
3061</t>
3062<t>
3063   It is unknown whether the reset problem is exclusive to TCP or might also
3064   be found in other transport connection protocols.
3065</t>
3066</section>
3067
3068<section title="Upgrade" anchor="header.upgrade">
3069  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3070  <x:anchor-alias value="Upgrade"/>
3071  <x:anchor-alias value="protocol"/>
3072  <x:anchor-alias value="protocol-name"/>
3073  <x:anchor-alias value="protocol-version"/>
3074<t>
3075   The "Upgrade" header field is intended to provide a simple mechanism
3076   for transitioning from HTTP/1.1 to some other protocol on the same
3077   connection.  A client &MAY; send a list of protocols in the Upgrade
3078   header field of a request to invite the server to switch to one or
3079   more of those protocols before sending the final response.
3080   A server &MUST; send an Upgrade header field in <x:ref>101 (Switching
3081   Protocols)</x:ref> responses to indicate which protocol(s) are being
3082   switched to, and &MUST; send it in <x:ref>426 (Upgrade Required)</x:ref>
3083   responses to indicate acceptable protocols.
3084   A server &MAY; send an Upgrade header field in any other response to
3085   indicate that they might be willing to upgrade to one of the
3086   specified protocols for a future request.
3087</t>
3088<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3089  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3090
3091  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3092  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3093  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3094</artwork></figure>
3095<t>
3096   For example,
3097</t>
3098<figure><artwork type="example">
3099  Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3100</artwork></figure>
3101<t>
3102   Upgrade eases the difficult transition between incompatible protocols by
3103   allowing the client to initiate a request in the more commonly
3104   supported protocol while indicating to the server that it would like
3105   to use a "better" protocol if available (where "better" is determined
3106   by the server, possibly according to the nature of the request method
3107   or target resource).
3108</t>
3109<t>
3110   Upgrade cannot be used to insist on a protocol change; its acceptance and
3111   use by the server is optional. The capabilities and nature of the
3112   application-level communication after the protocol change is entirely
3113   dependent upon the new protocol chosen, although the first action
3114   after changing the protocol &MUST; be a response to the initial HTTP
3115   request that contained the Upgrade header field.
3116</t>
3117<t>
3118   For example, if the Upgrade header field is received in a GET request
3119   and the server decides to switch protocols, then it first responds
3120   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3121   then immediately follows that with the new protocol's equivalent of a
3122   response to a GET on the target resource.  This allows a connection to be
3123   upgraded to protocols with the same semantics as HTTP without the
3124   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3125   protocols unless the received message semantics can be honored by the new
3126   protocol; an OPTIONS request can be honored by any protocol.
3127</t>
3128<t>
3129   When Upgrade is sent, a sender &MUST; also send a
3130   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3131   that contains the "upgrade" connection option, in order to prevent Upgrade
3132   from being accidentally forwarded by intermediaries that might not implement
3133   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3134   is received in an HTTP/1.0 request.
3135</t>
3136<t>
3137   The Upgrade header field only applies to switching application-level
3138   protocols on the existing connection; it cannot be used
3139   to switch to a protocol on a different connection. For that purpose, it is
3140   more appropriate to use a <x:ref>3xx (Redirection)</x:ref> response
3141   (&status-3xx;).
3142</t>
3143<t>
3144   This specification only defines the protocol name "HTTP" for use by
3145   the family of Hypertext Transfer Protocols, as defined by the HTTP
3146   version rules of <xref target="http.version"/> and future updates to this
3147   specification. Additional tokens ought to be registered with IANA using the
3148   registration procedure defined in <xref target="upgrade.token.registry"/>.
3149</t>
3150</section>
3151</section>
3152
3153<section title="IANA Considerations" anchor="IANA.considerations">
3154
3155<section title="Header Field Registration" anchor="header.field.registration">
3156<t>
3157   HTTP header fields are registered within the Message Header Field Registry
3158   <xref target="BCP90"/> maintained by IANA at
3159   <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
3160</t>
3161<t>
3162   This document defines the following HTTP header fields, so their
3163   associated registry entries shall be updated according to the permanent
3164   registrations below:
3165</t>
3166<?BEGININC p1-messaging.iana-headers ?>
3167<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3168<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3169   <ttcol>Header Field Name</ttcol>
3170   <ttcol>Protocol</ttcol>
3171   <ttcol>Status</ttcol>
3172   <ttcol>Reference</ttcol>
3173
3174   <c>Connection</c>
3175   <c>http</c>
3176   <c>standard</c>
3177   <c>
3178      <xref target="header.connection"/>
3179   </c>
3180   <c>Content-Length</c>
3181   <c>http</c>
3182   <c>standard</c>
3183   <c>
3184      <xref target="header.content-length"/>
3185   </c>
3186   <c>Host</c>
3187   <c>http</c>
3188   <c>standard</c>
3189   <c>
3190      <xref target="header.host"/>
3191   </c>
3192   <c>TE</c>
3193   <c>http</c>
3194   <c>standard</c>
3195   <c>
3196      <xref target="header.te"/>
3197   </c>
3198   <c>Trailer</c>
3199   <c>http</c>
3200   <c>standard</c>
3201   <c>
3202      <xref target="header.trailer"/>
3203   </c>
3204   <c>Transfer-Encoding</c>
3205   <c>http</c>
3206   <c>standard</c>
3207   <c>
3208      <xref target="header.transfer-encoding"/>
3209   </c>
3210   <c>Upgrade</c>
3211   <c>http</c>
3212   <c>standard</c>
3213   <c>
3214      <xref target="header.upgrade"/>
3215   </c>
3216   <c>Via</c>
3217   <c>http</c>
3218   <c>standard</c>
3219   <c>
3220      <xref target="header.via"/>
3221   </c>
3222</texttable>
3223<!--(END)-->
3224<?ENDINC p1-messaging.iana-headers ?>
3225<t>
3226   Furthermore, the header field-name "Close" shall be registered as
3227   "reserved", since using that name as an HTTP header field might
3228   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3229   header field (<xref target="header.connection"/>).
3230</t>
3231<texttable align="left" suppress-title="true">
3232   <ttcol>Header Field Name</ttcol>
3233   <ttcol>Protocol</ttcol>
3234   <ttcol>Status</ttcol>
3235   <ttcol>Reference</ttcol>
3236
3237   <c>Close</c>
3238   <c>http</c>
3239   <c>reserved</c>
3240   <c>
3241      <xref target="header.field.registration"/>
3242   </c>
3243</texttable>
3244<t>
3245   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3246</t>
3247</section>
3248
3249<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3250<t>
3251   IANA maintains the registry of URI Schemes <xref target="BCP115"/> at
3252   <eref target="http://www.iana.org/assignments/uri-schemes.html"/>.
3253</t>
3254<t>
3255   This document defines the following URI schemes, so their
3256   associated registry entries shall be updated according to the permanent
3257   registrations below:
3258</t>
3259<texttable align="left" suppress-title="true">
3260   <ttcol>URI Scheme</ttcol>
3261   <ttcol>Description</ttcol>
3262   <ttcol>Reference</ttcol>
3263
3264   <c>http</c>
3265   <c>Hypertext Transfer Protocol</c>
3266   <c><xref target="http.uri"/></c>
3267
3268   <c>https</c>
3269   <c>Hypertext Transfer Protocol Secure</c>
3270   <c><xref target="https.uri"/></c>
3271</texttable>
3272</section>
3273
3274<section title="Internet Media Type Registration" anchor="internet.media.type.http">
3275<t>
3276   This document serves as the specification for the Internet media types
3277   "message/http" and "application/http". The following is to be registered with
3278   IANA (see <xref target="BCP13"/>).
3279</t>
3280<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
3281<iref item="Media Type" subitem="message/http" primary="true"/>
3282<iref item="message/http Media Type" primary="true"/>
3283<t>
3284   The message/http type can be used to enclose a single HTTP request or
3285   response message, provided that it obeys the MIME restrictions for all
3286   "message" types regarding line length and encodings.
3287</t>
3288<t>
3289  <list style="hanging" x:indent="12em">
3290    <t hangText="Type name:">
3291      message
3292    </t>
3293    <t hangText="Subtype name:">
3294      http
3295    </t>
3296    <t hangText="Required parameters:">
3297      none
3298    </t>
3299    <t hangText="Optional parameters:">
3300      version, msgtype
3301      <list style="hanging">
3302        <t hangText="version:">
3303          The HTTP-version number of the enclosed message
3304          (e.g., "1.1"). If not present, the version can be
3305          determined from the first line of the body.
3306        </t>
3307        <t hangText="msgtype:">
3308          The message type &mdash; "request" or "response". If not
3309          present, the type can be determined from the first
3310          line of the body.
3311        </t>
3312      </list>
3313    </t>
3314    <t hangText="Encoding considerations:">
3315      only "7bit", "8bit", or "binary" are permitted
3316    </t>
3317    <t hangText="Security considerations:">
3318      none
3319    </t>
3320    <t hangText="Interoperability considerations:">
3321      none
3322    </t>
3323    <t hangText="Published specification:">
3324      This specification (see <xref target="internet.media.type.message.http"/>).
3325    </t>
3326    <t hangText="Applications that use this media type:">
3327    </t>
3328    <t hangText="Additional information:">
3329      <list style="hanging">
3330        <t hangText="Magic number(s):">none</t>
3331        <t hangText="File extension(s):">none</t>
3332        <t hangText="Macintosh file type code(s):">none</t>
3333      </list>
3334    </t>
3335    <t hangText="Person and email address to contact for further information:">
3336      See Authors Section.
3337    </t>
3338    <t hangText="Intended usage:">
3339      COMMON
3340    </t>
3341    <t hangText="Restrictions on usage:">
3342      none
3343    </t>
3344    <t hangText="Author/Change controller:">
3345      IESG
3346    </t>
3347  </list>
3348</t>
3349</section>
3350<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
3351<iref item="Media Type" subitem="application/http" primary="true"/>
3352<iref item="application/http Media Type" primary="true"/>
3353<t>
3354   The application/http type can be used to enclose a pipeline of one or more
3355   HTTP request or response messages (not intermixed).
3356</t>
3357<t>
3358  <list style="hanging" x:indent="12em">
3359    <t hangText="Type name:">
3360      application
3361    </t>
3362    <t hangText="Subtype name:">
3363      http
3364    </t>
3365    <t hangText="Required parameters:">
3366      none
3367    </t>
3368    <t hangText="Optional parameters:">
3369      version, msgtype
3370      <list style="hanging">
3371        <t hangText="version:">
3372          The HTTP-version number of the enclosed messages
3373          (e.g., "1.1"). If not present, the version can be
3374          determined from the first line of the body.
3375        </t>
3376        <t hangText="msgtype:">
3377          The message type &mdash; "request" or "response". If not
3378          present, the type can be determined from the first
3379          line of the body.
3380        </t>
3381      </list>
3382    </t>
3383    <t hangText="Encoding considerations:">
3384      HTTP messages enclosed by this type
3385      are in "binary" format; use of an appropriate
3386      Content-Transfer-Encoding is required when
3387      transmitted via E-mail.
3388    </t>
3389    <t hangText="Security considerations:">
3390      none
3391    </t>
3392    <t hangText="Interoperability considerations:">
3393      none
3394    </t>
3395    <t hangText="Published specification:">
3396      This specification (see <xref target="internet.media.type.application.http"/>).
3397    </t>
3398    <t hangText="Applications that use this media type:">
3399    </t>
3400    <t hangText="Additional information:">
3401      <list style="hanging">
3402        <t hangText="Magic number(s):">none</t>
3403        <t hangText="File extension(s):">none</t>
3404        <t hangText="Macintosh file type code(s):">none</t>
3405      </list>
3406    </t>
3407    <t hangText="Person and email address to contact for further information:">
3408      See Authors Section.
3409    </t>
3410    <t hangText="Intended usage:">
3411      COMMON
3412    </t>
3413    <t hangText="Restrictions on usage:">
3414      none
3415    </t>
3416    <t hangText="Author/Change controller:">
3417      IESG
3418    </t>
3419  </list>
3420</t>
3421</section>
3422</section>
3423
3424<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3425<t>
3426   The HTTP Transfer Coding Registry defines the name space for transfer
3427   coding names.
3428</t>
3429<t>
3430   Registrations &MUST; include the following fields:
3431   <list style="symbols">
3432     <t>Name</t>
3433     <t>Description</t>
3434     <t>Pointer to specification text</t>
3435   </list>
3436</t>
3437<t>
3438   Names of transfer codings &MUST-NOT; overlap with names of content codings
3439   (&content-codings;) unless the encoding transformation is identical, as
3440   is the case for the compression codings defined in
3441   <xref target="compression.codings"/>.
3442</t>
3443<t>
3444   Values to be added to this name space require IETF Review (see
3445   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3446   conform to the purpose of transfer coding defined in this section.
3447   Use of program names for the identification of encoding formats
3448   is not desirable and is discouraged for future encodings.
3449</t>
3450<t>
3451   The registry itself is maintained at
3452   <eref target="http://www.iana.org/assignments/http-parameters"/>.
3453</t>
3454</section>
3455
3456<section title="Transfer Coding Registration" anchor="transfer.coding.registration">
3457<t>
3458   The HTTP Transfer Coding Registry shall be updated with the registrations
3459   below:
3460</t>
3461<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3462   <ttcol>Name</ttcol>
3463   <ttcol>Description</ttcol>
3464   <ttcol>Reference</ttcol>
3465   <c>chunked</c>
3466   <c>Transfer in a series of chunks</c>
3467   <c>
3468      <xref target="chunked.encoding"/>
3469   </c>
3470   <c>compress</c>
3471   <c>UNIX "compress" program method</c>
3472   <c>
3473      <xref target="compress.coding"/>
3474   </c>
3475   <c>deflate</c>
3476   <c>"deflate" compression mechanism (<xref target="RFC1951"/>) used inside
3477   the "zlib" data format (<xref target="RFC1950"/>)
3478   </c>
3479   <c>
3480      <xref target="deflate.coding"/>
3481   </c>
3482   <c>gzip</c>
3483   <c>Same as GNU zip <xref target="RFC1952"/></c>
3484   <c>
3485      <xref target="gzip.coding"/>
3486   </c>
3487</texttable>
3488</section>
3489
3490<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3491<t>
3492   The HTTP Upgrade Token Registry defines the name space for protocol-name
3493   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3494   field. Each registered protocol name is associated with contact information
3495   and an optional set of specifications that details how the connection
3496   will be processed after it has been upgraded.
3497</t>
3498<t>
3499   Registrations happen on a "First Come First Served" basis (see
3500   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3501   following rules:
3502  <list style="numbers">
3503    <t>A protocol-name token, once registered, stays registered forever.</t>
3504    <t>The registration &MUST; name a responsible party for the
3505       registration.</t>
3506    <t>The registration &MUST; name a point of contact.</t>
3507    <t>The registration &MAY; name a set of specifications associated with
3508       that token. Such specifications need not be publicly available.</t>
3509    <t>The registration &SHOULD; name a set of expected "protocol-version"
3510       tokens associated with that token at the time of registration.</t>
3511    <t>The responsible party &MAY; change the registration at any time.
3512       The IANA will keep a record of all such changes, and make them
3513       available upon request.</t>
3514    <t>The IESG &MAY; reassign responsibility for a protocol token.
3515       This will normally only be used in the case when a
3516       responsible party cannot be contacted.</t>
3517  </list>
3518</t>
3519<t>
3520   This registration procedure for HTTP Upgrade Tokens replaces that
3521   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3522</t>
3523</section>
3524
3525<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3526<t>
3527   The HTTP Upgrade Token Registry shall be updated with the registration
3528   below:
3529</t>
3530<texttable align="left" suppress-title="true">
3531   <ttcol>Value</ttcol>
3532   <ttcol>Description</ttcol>
3533   <ttcol>Expected Version Tokens</ttcol>
3534   <ttcol>Reference</ttcol>
3535
3536   <c>HTTP</c>
3537   <c>Hypertext Transfer Protocol</c>
3538   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3539   <c><xref target="http.version"/></c>
3540</texttable>
3541<t>
3542   The responsible party is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3543</t>
3544</section>
3545
3546</section>
3547
3548<section title="Security Considerations" anchor="security.considerations">
3549<t>
3550   This section is meant to inform developers, information providers, and
3551   users of known security concerns relevant to HTTP/1.1 message syntax,
3552   parsing, and routing.
3553</t>
3554
3555<section title="DNS-related Attacks" anchor="dns.related.attacks">
3556<t>
3557   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3558   generally prone to security attacks based on the deliberate misassociation
3559   of IP addresses and DNS names not protected by DNSSEC. Clients need to be
3560   cautious in assuming the validity of an IP number/DNS name association unless
3561   the response is protected by DNSSEC (<xref target="RFC4033"/>).
3562</t>
3563</section>
3564
3565<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3566<t>
3567   By their very nature, HTTP intermediaries are men-in-the-middle, and
3568   represent an opportunity for man-in-the-middle attacks. Compromise of
3569   the systems on which the intermediaries run can result in serious security
3570   and privacy problems. Intermediaries have access to security-related
3571   information, personal information about individual users and
3572   organizations, and proprietary information belonging to users and
3573   content providers. A compromised intermediary, or an intermediary
3574   implemented or configured without regard to security and privacy
3575   considerations, might be used in the commission of a wide range of
3576   potential attacks.
3577</t>
3578<t>
3579   Intermediaries that contain a shared cache are especially vulnerable
3580   to cache poisoning attacks.
3581</t>
3582<t>
3583   Implementers need to consider the privacy and security
3584   implications of their design and coding decisions, and of the
3585   configuration options they provide to operators (especially the
3586   default configuration).
3587</t>
3588<t>
3589   Users need to be aware that intermediaries are no more trustworthy than
3590   the people who run them; HTTP itself cannot solve this problem.
3591</t>
3592</section>
3593
3594<section title="Buffer Overflows" anchor="attack.protocol.element.size.overflows">
3595<t>
3596   Because HTTP uses mostly textual, character-delimited fields, attackers can
3597   overflow buffers in implementations, and/or perform a Denial of Service
3598   against implementations that accept fields with unlimited lengths.
3599</t>
3600<t>
3601   To promote interoperability, this specification makes specific
3602   recommendations for minimum size limits on request-line
3603   (<xref target="request.line"/>)
3604   and blocks of header fields (<xref target="header.fields"/>). These are
3605   minimum recommendations, chosen to be supportable even by implementations
3606   with limited resources; it is expected that most implementations will
3607   choose substantially higher limits.
3608</t>
3609<t>
3610   This specification also provides a way for servers to reject messages that
3611   have request-targets that are too long (&status-414;) or request entities
3612   that are too large (&status-4xx;).
3613</t>
3614<t>
3615   Recipients &SHOULD; carefully limit the extent to which they read other
3616   fields, including (but not limited to) request methods, response status
3617   phrases, header field-names, and body chunks, so as to avoid denial of
3618   service attacks without impeding interoperability.
3619</t>
3620</section>
3621
3622<section title="Message Integrity" anchor="message.integrity">
3623<t>
3624   HTTP does not define a specific mechanism for ensuring message integrity,
3625   instead relying on the error-detection ability of underlying transport
3626   protocols and the use of length or chunk-delimited framing to detect
3627   completeness. Additional integrity mechanisms, such as hash functions or
3628   digital signatures applied to the content, can be selectively added to
3629   messages via extensible metadata header fields. Historically, the lack of
3630   a single integrity mechanism has been justified by the informal nature of
3631   most HTTP communication.  However, the prevalence of HTTP as an information
3632   access mechanism has resulted in its increasing use within environments
3633   where verification of message integrity is crucial.
3634</t>
3635<t>
3636   User agents are encouraged to implement configurable means for detecting
3637   and reporting failures of message integrity such that those means can be
3638   enabled within environments for which integrity is necessary. For example,
3639   a browser being used to view medical history or drug interaction
3640   information needs to indicate to the user when such information is detected
3641   by the protocol to be incomplete, expired, or corrupted during transfer.
3642   Such mechanisms might be selectively enabled via user agent extensions or
3643   the presence of message integrity metadata in a response.
3644   At a minimum, user agents ought to provide some indication that allows a
3645   user to distinguish between a complete and incomplete response message
3646   (<xref target="incomplete.messages"/>) when such verification is desired.
3647</t>
3648</section>
3649
3650<section title="Server Log Information" anchor="abuse.of.server.log.information">
3651<t>
3652   A server is in the position to save personal data about a user's requests
3653   over time, which might identify their reading patterns or subjects of
3654   interest.  In particular, log information gathered at an intermediary
3655   often contains a history of user agent interaction, across a multitude
3656   of sites, that can be traced to individual users.
3657</t>
3658<t>
3659   HTTP log information is confidential in nature; its handling is often
3660   constrained by laws and regulations.  Log information needs to be securely
3661   stored and appropriate guidelines followed for its analysis.
3662   Anonymization of personal information within individual entries helps,
3663   but is generally not sufficient to prevent real log traces from being
3664   re-identified based on correlation with other access characteristics.
3665   As such, access traces that are keyed to a specific client should not
3666   be published even if the key is pseudonymous.
3667</t>
3668<t>
3669   To minimize the risk of theft or accidental publication, log information
3670   should be purged of personally identifiable information, including
3671   user identifiers, IP addresses, and user-provided query parameters,
3672   as soon as that information is no longer necessary to support operational
3673   needs for security, auditing, or fraud control.
3674</t>
3675</section>
3676</section>
3677
3678<section title="Acknowledgments" anchor="acks">
3679<t>
3680   This edition of HTTP/1.1 builds on the many contributions that went into
3681   <xref target="RFC1945" format="none">RFC 1945</xref>,
3682   <xref target="RFC2068" format="none">RFC 2068</xref>,
3683   <xref target="RFC2145" format="none">RFC 2145</xref>, and
3684   <xref target="RFC2616" format="none">RFC 2616</xref>, including
3685   substantial contributions made by the previous authors, editors, and
3686   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
3687   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
3688   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
3689</t>
3690<t>
3691   Since 1999, the following contributors have helped improve the HTTP
3692   specification by reporting bugs, asking smart questions, drafting or
3693   reviewing text, and evaluating open issues:
3694</t>
3695<?BEGININC acks ?>
3696<t>Adam Barth,
3697Adam Roach,
3698Addison Phillips,
3699Adrian Chadd,
3700Adrien W. de Croy,
3701Alan Ford,
3702Alan Ruttenberg,
3703Albert Lunde,
3704Alek Storm,
3705Alex Rousskov,
3706Alexandre Morgaut,
3707Alexey Melnikov,
3708Alisha Smith,
3709Amichai Rothman,
3710Amit Klein,
3711Amos Jeffries,
3712Andreas Maier,
3713Andreas Petersson,
3714Anil Sharma,
3715Anne van Kesteren,
3716Anthony Bryan,
3717Asbjorn Ulsberg,
3718Ashok Kumar,
3719Balachander Krishnamurthy,
3720Barry Leiba,
3721Ben Laurie,
3722Benjamin Niven-Jenkins,
3723Bil Corry,
3724Bill Burke,
3725Bjoern Hoehrmann,
3726Bob Scheifler,
3727Boris Zbarsky,
3728Brett Slatkin,
3729Brian Kell,
3730Brian McBarron,
3731Brian Pane,
3732Brian Smith,
3733Bryce Nesbitt,
3734Cameron Heavon-Jones,
3735Carl Kugler,
3736Carsten Bormann,
3737Charles Fry,
3738Chris Newman,
3739Chris Weber,
3740Cyrus Daboo,
3741Dale Robert Anderson,
3742Dan Wing,
3743Dan Winship,
3744Daniel Stenberg,
3745Darrel Miller,
3746Dave Cridland,
3747Dave Crocker,
3748Dave Kristol,
3749David Booth,
3750David Singer,
3751David W. Morris,
3752Diwakar Shetty,
3753Dmitry Kurochkin,
3754Drummond Reed,
3755Duane Wessels,
3756Duncan Cragg,
3757Edward Lee,
3758Eliot Lear,
3759Eran Hammer-Lahav,
3760Eric D. Williams,
3761Eric J. Bowman,
3762Eric Lawrence,
3763Eric Rescorla,
3764Erik Aronesty,
3765Evan Prodromou,
3766Florian Weimer,
3767Frank Ellermann,
3768Fred Bohle,
3769Gabriel Montenegro,
3770Geoffrey Sneddon,
3771Gervase Markham,
3772Grahame Grieve,
3773Greg Wilkins,
3774Harald Tveit Alvestrand,
3775Harry Halpin,
3776Helge Hess,
3777Henrik Nordstrom,
3778Henry S. Thompson,
3779Henry Story,
3780Herbert van de Sompel,
3781Howard Melman,
3782Hugo Haas,
3783Ian Fette,
3784Ian Hickson,
3785Ido Safruti,
3786Ilya Grigorik,
3787Ingo Struck,
3788J. Ross Nicoll,
3789James H. Manger,
3790James Lacey,
3791James M. Snell,
3792Jamie Lokier,
3793Jan Algermissen,
3794Jeff Hodges (who came up with the term 'effective Request-URI'),
3795Jeff Walden,
3796Jeroen de Borst,
3797Jim Luther,
3798Joe D. Williams,
3799Joe Gregorio,
3800Joe Orton,
3801John C. Klensin,
3802John C. Mallery,
3803John Cowan,
3804John Kemp,
3805John Panzer,
3806John Schneider,
3807John Stracke,
3808John Sullivan,
3809Jonas Sicking,
3810Jonathan A. Rees,
3811Jonathan Billington,
3812Jonathan Moore,
3813Jonathan Rees,
3814Jonathan Silvera,
3815Jordi Ros,
3816Joris Dobbelsteen,
3817Josh Cohen,
3818Julien Pierre,
3819Jungshik Shin,
3820Justin Chapweske,
3821Justin Erenkrantz,
3822Justin James,
3823Kalvinder Singh,
3824Karl Dubost,
3825Keith Hoffman,
3826Keith Moore,
3827Ken Murchison,
3828Koen Holtman,
3829Konstantin Voronkov,
3830Kris Zyp,
3831Lisa Dusseault,
3832Maciej Stachowiak,
3833Marc Schneider,
3834Marc Slemko,
3835Mark Baker,
3836Mark Pauley,
3837Mark Watson,
3838Markus Isomaki,
3839Markus Lanthaler,
3840Martin J. Duerst,
3841Martin Musatov,
3842Martin Nilsson,
3843Martin Thomson,
3844Matt Lynch,
3845Matthew Cox,
3846Max Clark,
3847Michael Burrows,
3848Michael Hausenblas,
3849Mike Amundsen,
3850Mike Belshe,
3851Mike Kelly,
3852Mike Schinkel,
3853Miles Sabin,
3854Murray S. Kucherawy,
3855Mykyta Yevstifeyev,
3856Nathan Rixham,
3857Nicholas Shanks,
3858Nico Williams,
3859Nicolas Alvarez,
3860Nicolas Mailhot,
3861Noah Slater,
3862Pablo Castro,
3863Pat Hayes,
3864Patrick R. McManus,
3865Patrik Faltstrom,
3866Paul E. Jones,
3867Paul Hoffman,
3868Paul Marquess,
3869Peter Lepeska,
3870Peter Saint-Andre,
3871Peter Watkins,
3872Phil Archer,
3873Philippe Mougin,
3874Phillip Hallam-Baker,
3875Poul-Henning Kamp,
3876Preethi Natarajan,
3877Rajeev Bector,
3878Ray Polk,
3879Reto Bachmann-Gmuer,
3880Richard Cyganiak,
3881Robert Brewer,
3882Robert Collins,
3883Robert O'Callahan,
3884Robert Olofsson,
3885Robert Sayre,
3886Robert Siemer,
3887Robert de Wilde,
3888Roberto Javier Godoy,
3889Roberto Peon,
3890Roland Zink,
3891Ronny Widjaja,
3892S. Mike Dierken,
3893Salvatore Loreto,
3894Sam Johnston,
3895Sam Ruby,
3896Scott Lawrence (who maintained the original issues list),
3897Sean B. Palmer,
3898Shane McCarron,
3899Stefan Eissing,
3900Stefan Tilkov,
3901Stefanos Harhalakis,
3902Stephane Bortzmeyer,
3903Stephen Farrell,
3904Stephen Ludin,
3905Stuart Williams,
3906Subbu Allamaraju,
3907Subramanian Moonesamy,
3908Sylvain Hellegouarch,
3909Tapan Divekar,
3910Tatsuya Hayashi,
3911Ted Hardie,
3912Thomas Broyer,
3913Thomas Fossati,
3914Thomas Nordin,
3915Thomas Roessler,
3916Tim Bray,
3917Tim Morgan,
3918Tim Olsen,
3919Tobias Oberstein,
3920Tom Zhou,
3921Travis Snoozy,
3922Tyler Close,
3923Vincent Murphy,
3924Wenbo Zhu,
3925Werner Baumann,
3926Wilbur Streett,
3927Wilfredo Sanchez Vega,
3928William A. Rowe Jr.,
3929William Chan,
3930Willy Tarreau,
3931Xiaoshu Wang,
3932Yaron Goland,
3933Yngve Nysaeter Pettersen,
3934Yoav Nir,
3935Yogesh Bang,
3936Yutaka Oiwa,
3937Yves Lafon (long-time member of the editor team),
3938Zed A. Shaw, and
3939Zhong Yu.
3940</t>
3941<?ENDINC acks ?>
3942<t>
3943   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
3944   acknowledgements from prior revisions.
3945</t>
3946</section>
3947
3948</middle>
3949<back>
3950
3951<references title="Normative References">
3952
3953<reference anchor="Part2">
3954  <front>
3955    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
3956    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3957      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
3958      <address><email>fielding@gbiv.com</email></address>
3959    </author>
3960    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3961      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3962      <address><email>julian.reschke@greenbytes.de</email></address>
3963    </author>
3964    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3965  </front>
3966  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
3967  <x:source href="p2-semantics.xml" basename="p2-semantics">
3968    <x:defines>1xx (Informational)</x:defines>
3969    <x:defines>1xx</x:defines>
3970    <x:defines>100 (Continue)</x:defines>
3971    <x:defines>101 (Switching Protocols)</x:defines>
3972    <x:defines>2xx (Successful)</x:defines>
3973    <x:defines>2xx</x:defines>
3974    <x:defines>200 (OK)</x:defines>
3975    <x:defines>204 (No Content)</x:defines>
3976    <x:defines>3xx (Redirection)</x:defines>
3977    <x:defines>3xx</x:defines>
3978    <x:defines>301 (Moved Permanently)</x:defines>
3979    <x:defines>4xx (Client Error)</x:defines>
3980    <x:defines>4xx</x:defines>
3981    <x:defines>400 (Bad Request)</x:defines>
3982    <x:defines>411 (Length Required)</x:defines>
3983    <x:defines>414 (URI Too Long)</x:defines>
3984    <x:defines>417 (Expectation Failed)</x:defines>
3985    <x:defines>426 (Upgrade Required)</x:defines>
3986    <x:defines>501 (Not Implemented)</x:defines>
3987    <x:defines>502 (Bad Gateway)</x:defines>
3988    <x:defines>505 (HTTP Version Not Supported)</x:defines>
3989    <x:defines>Allow</x:defines>
3990    <x:defines>Content-Encoding</x:defines>
3991    <x:defines>Content-Location</x:defines>
3992    <x:defines>Content-Type</x:defines>
3993    <x:defines>Date</x:defines>
3994    <x:defines>Expect</x:defines>
3995    <x:defines>Location</x:defines>
3996    <x:defines>Server</x:defines>
3997    <x:defines>User-Agent</x:defines>
3998  </x:source>
3999</reference>
4000
4001<reference anchor="Part4">
4002  <front>
4003    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
4004    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
4005      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4006      <address><email>fielding@gbiv.com</email></address>
4007    </author>
4008    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
4009      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4010      <address><email>julian.reschke@greenbytes.de</email></address>
4011    </author>
4012    <date month="&ID-MONTH;" year="&ID-YEAR;" />
4013  </front>
4014  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
4015  <x:source basename="p4-conditional" href="p4-conditional.xml">
4016    <x:defines>304 (Not Modified)</x:defines>
4017    <x:defines>ETag</x:defines>
4018    <x:defines>Last-Modified</x:defines>
4019  </x:source>
4020</reference>
4021
4022<reference anchor="Part5">
4023  <front>
4024    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
4025    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4026      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4027      <address><email>fielding@gbiv.com</email></address>
4028    </author>
4029    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
4030      <organization abbrev="W3C">World Wide Web Consortium</organization>
4031      <address><email>ylafon@w3.org</email></address>
4032    </author>
4033    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4034      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4035      <address><email>julian.reschke@greenbytes.de</email></address>
4036    </author>
4037    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4038  </front>
4039  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4040  <x:source href="p5-range.xml" basename="p5-range">
4041    <x:defines>Content-Range</x:defines>
4042  </x:source>
4043</reference>
4044
4045<reference anchor="Part6">
4046  <front>
4047    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4048    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4049      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4050      <address><email>fielding@gbiv.com</email></address>
4051    </author>
4052    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4053      <organization>Akamai</organization>
4054      <address><email>mnot@mnot.net</email></address>
4055    </author>
4056    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4057      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4058      <address><email>julian.reschke@greenbytes.de</email></address>
4059    </author>
4060    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4061  </front>
4062  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4063  <x:source href="p6-cache.xml" basename="p6-cache">
4064    <x:defines>Cache-Control</x:defines>
4065    <x:defines>Expires</x:defines>
4066  </x:source>
4067</reference>
4068
4069<reference anchor="Part7">
4070  <front>
4071    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4072    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4073      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4074      <address><email>fielding@gbiv.com</email></address>
4075    </author>
4076    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4077      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4078      <address><email>julian.reschke@greenbytes.de</email></address>
4079    </author>
4080    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4081  </front>
4082  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4083  <x:source href="p7-auth.xml" basename="p7-auth">
4084    <x:defines>Proxy-Authenticate</x:defines>
4085    <x:defines>Proxy-Authorization</x:defines>
4086  </x:source>
4087</reference>
4088
4089<reference anchor="RFC5234">
4090  <front>
4091    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4092    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4093      <organization>Brandenburg InternetWorking</organization>
4094      <address>
4095        <email>dcrocker@bbiw.net</email>
4096      </address> 
4097    </author>
4098    <author initials="P." surname="Overell" fullname="Paul Overell">
4099      <organization>THUS plc.</organization>
4100      <address>
4101        <email>paul.overell@thus.net</email>
4102      </address>
4103    </author>
4104    <date month="January" year="2008"/>
4105  </front>
4106  <seriesInfo name="STD" value="68"/>
4107  <seriesInfo name="RFC" value="5234"/>
4108</reference>
4109
4110<reference anchor="RFC2119">
4111  <front>
4112    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4113    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4114      <organization>Harvard University</organization>
4115      <address><email>sob@harvard.edu</email></address>
4116    </author>
4117    <date month="March" year="1997"/>
4118  </front>
4119  <seriesInfo name="BCP" value="14"/>
4120  <seriesInfo name="RFC" value="2119"/>
4121</reference>
4122
4123<reference anchor="RFC3986">
4124 <front>
4125  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4126  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4127    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4128    <address>
4129       <email>timbl@w3.org</email>
4130       <uri>http://www.w3.org/People/Berners-Lee/</uri>
4131    </address>
4132  </author>
4133  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4134    <organization abbrev="Day Software">Day Software</organization>
4135    <address>
4136      <email>fielding@gbiv.com</email>
4137      <uri>http://roy.gbiv.com/</uri>
4138    </address>
4139  </author>
4140  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4141    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4142    <address>
4143      <email>LMM@acm.org</email>
4144      <uri>http://larry.masinter.net/</uri>
4145    </address>
4146  </author>
4147  <date month='January' year='2005'></date>
4148 </front>
4149 <seriesInfo name="STD" value="66"/>
4150 <seriesInfo name="RFC" value="3986"/>
4151</reference>
4152
4153<reference anchor="USASCII">
4154  <front>
4155    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4156    <author>
4157      <organization>American National Standards Institute</organization>
4158    </author>
4159    <date year="1986"/>
4160  </front>
4161  <seriesInfo name="ANSI" value="X3.4"/>
4162</reference>
4163
4164<reference anchor="RFC1950">
4165  <front>
4166    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4167    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4168      <organization>Aladdin Enterprises</organization>
4169      <address><email>ghost@aladdin.com</email></address>
4170    </author>
4171    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4172    <date month="May" year="1996"/>
4173  </front>
4174  <seriesInfo name="RFC" value="1950"/>
4175  <!--<annotation>
4176    RFC 1950 is an Informational RFC, thus it might be less stable than
4177    this specification. On the other hand, this downward reference was
4178    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4179    therefore it is unlikely to cause problems in practice. See also
4180    <xref target="BCP97"/>.
4181  </annotation>-->
4182</reference>
4183
4184<reference anchor="RFC1951">
4185  <front>
4186    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4187    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4188      <organization>Aladdin Enterprises</organization>
4189      <address><email>ghost@aladdin.com</email></address>
4190    </author>
4191    <date month="May" year="1996"/>
4192  </front>
4193  <seriesInfo name="RFC" value="1951"/>
4194  <!--<annotation>
4195    RFC 1951 is an Informational RFC, thus it might be less stable than
4196    this specification. On the other hand, this downward reference was
4197    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4198    therefore it is unlikely to cause problems in practice. See also
4199    <xref target="BCP97"/>.
4200  </annotation>-->
4201</reference>
4202
4203<reference anchor="RFC1952">
4204  <front>
4205    <title>GZIP file format specification version 4.3</title>
4206    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4207      <organization>Aladdin Enterprises</organization>
4208      <address><email>ghost@aladdin.com</email></address>
4209    </author>
4210    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4211      <address><email>gzip@prep.ai.mit.edu</email></address>
4212    </author>
4213    <author initials="M." surname="Adler" fullname="Mark Adler">
4214      <address><email>madler@alumni.caltech.edu</email></address>
4215    </author>
4216    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4217      <address><email>ghost@aladdin.com</email></address>
4218    </author>
4219    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4220      <address><email>randeg@alumni.rpi.edu</email></address>
4221    </author>
4222    <date month="May" year="1996"/>
4223  </front>
4224  <seriesInfo name="RFC" value="1952"/>
4225  <!--<annotation>
4226    RFC 1952 is an Informational RFC, thus it might be less stable than
4227    this specification. On the other hand, this downward reference was
4228    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4229    therefore it is unlikely to cause problems in practice. See also
4230    <xref target="BCP97"/>.
4231  </annotation>-->
4232</reference>
4233
4234</references>
4235
4236<references title="Informative References">
4237
4238<reference anchor="ISO-8859-1">
4239  <front>
4240    <title>
4241     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4242    </title>
4243    <author>
4244      <organization>International Organization for Standardization</organization>
4245    </author>
4246    <date year="1998"/>
4247  </front>
4248  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4249</reference>
4250
4251<reference anchor='RFC1919'>
4252  <front>
4253    <title>Classical versus Transparent IP Proxies</title>
4254    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4255      <address><email>mchatel@pax.eunet.ch</email></address>
4256    </author>
4257    <date year='1996' month='March' />
4258  </front>
4259  <seriesInfo name='RFC' value='1919' />
4260</reference>
4261
4262<reference anchor="RFC1945">
4263  <front>
4264    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4265    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4266      <organization>MIT, Laboratory for Computer Science</organization>
4267      <address><email>timbl@w3.org</email></address>
4268    </author>
4269    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4270      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4271      <address><email>fielding@ics.uci.edu</email></address>
4272    </author>
4273    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4274      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4275      <address><email>frystyk@w3.org</email></address>
4276    </author>
4277    <date month="May" year="1996"/>
4278  </front>
4279  <seriesInfo name="RFC" value="1945"/>
4280</reference>
4281
4282<reference anchor="RFC2045">
4283  <front>
4284    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4285    <author initials="N." surname="Freed" fullname="Ned Freed">
4286      <organization>Innosoft International, Inc.</organization>
4287      <address><email>ned@innosoft.com</email></address>
4288    </author>
4289    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4290      <organization>First Virtual Holdings</organization>
4291      <address><email>nsb@nsb.fv.com</email></address>
4292    </author>
4293    <date month="November" year="1996"/>
4294  </front>
4295  <seriesInfo name="RFC" value="2045"/>
4296</reference>
4297
4298<reference anchor="RFC2047">
4299  <front>
4300    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4301    <author initials="K." surname="Moore" fullname="Keith Moore">
4302      <organization>University of Tennessee</organization>
4303      <address><email>moore@cs.utk.edu</email></address>
4304    </author>
4305    <date month="November" year="1996"/>
4306  </front>
4307  <seriesInfo name="RFC" value="2047"/>
4308</reference>
4309
4310<reference anchor="RFC2068">
4311  <front>
4312    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4313    <author initials="R." 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="J." surname="Gettys" fullname="Jim Gettys">
4318      <organization>MIT Laboratory for Computer Science</organization>
4319      <address><email>jg@w3.org</email></address>
4320    </author>
4321    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4322      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4323      <address><email>mogul@wrl.dec.com</email></address>
4324    </author>
4325    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4326      <organization>MIT Laboratory for Computer Science</organization>
4327      <address><email>frystyk@w3.org</email></address>
4328    </author>
4329    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4330      <organization>MIT Laboratory for Computer Science</organization>
4331      <address><email>timbl@w3.org</email></address>
4332    </author>
4333    <date month="January" year="1997"/>
4334  </front>
4335  <seriesInfo name="RFC" value="2068"/>
4336</reference>
4337
4338<reference anchor="RFC2145">
4339  <front>
4340    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4341    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4342      <organization>Western Research Laboratory</organization>
4343      <address><email>mogul@wrl.dec.com</email></address>
4344    </author>
4345    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4346      <organization>Department of Information and Computer Science</organization>
4347      <address><email>fielding@ics.uci.edu</email></address>
4348    </author>
4349    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4350      <organization>MIT Laboratory for Computer Science</organization>
4351      <address><email>jg@w3.org</email></address>
4352    </author>
4353    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4354      <organization>W3 Consortium</organization>
4355      <address><email>frystyk@w3.org</email></address>
4356    </author>
4357    <date month="May" year="1997"/>
4358  </front>
4359  <seriesInfo name="RFC" value="2145"/>
4360</reference>
4361
4362<reference anchor="RFC2616">
4363  <front>
4364    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4365    <author initials="R." surname="Fielding" fullname="R. Fielding">
4366      <organization>University of California, Irvine</organization>
4367      <address><email>fielding@ics.uci.edu</email></address>
4368    </author>
4369    <author initials="J." surname="Gettys" fullname="J. Gettys">
4370      <organization>W3C</organization>
4371      <address><email>jg@w3.org</email></address>
4372    </author>
4373    <author initials="J." surname="Mogul" fullname="J. Mogul">
4374      <organization>Compaq Computer Corporation</organization>
4375      <address><email>mogul@wrl.dec.com</email></address>
4376    </author>
4377    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4378      <organization>MIT Laboratory for Computer Science</organization>
4379      <address><email>frystyk@w3.org</email></address>
4380    </author>
4381    <author initials="L." surname="Masinter" fullname="L. Masinter">
4382      <organization>Xerox Corporation</organization>
4383      <address><email>masinter@parc.xerox.com</email></address>
4384    </author>
4385    <author initials="P." surname="Leach" fullname="P. Leach">
4386      <organization>Microsoft Corporation</organization>
4387      <address><email>paulle@microsoft.com</email></address>
4388    </author>
4389    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4390      <organization>W3C</organization>
4391      <address><email>timbl@w3.org</email></address>
4392    </author>
4393    <date month="June" year="1999"/>
4394  </front>
4395  <seriesInfo name="RFC" value="2616"/>
4396</reference>
4397
4398<reference anchor='RFC2817'>
4399  <front>
4400    <title>Upgrading to TLS Within HTTP/1.1</title>
4401    <author initials='R.' surname='Khare' fullname='R. Khare'>
4402      <organization>4K Associates / UC Irvine</organization>
4403      <address><email>rohit@4K-associates.com</email></address>
4404    </author>
4405    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4406      <organization>Agranat Systems, Inc.</organization>
4407      <address><email>lawrence@agranat.com</email></address>
4408    </author>
4409    <date year='2000' month='May' />
4410  </front>
4411  <seriesInfo name='RFC' value='2817' />
4412</reference>
4413
4414<reference anchor='RFC2818'>
4415  <front>
4416    <title>HTTP Over TLS</title>
4417    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4418      <organization>RTFM, Inc.</organization>
4419      <address><email>ekr@rtfm.com</email></address>
4420    </author>
4421    <date year='2000' month='May' />
4422  </front>
4423  <seriesInfo name='RFC' value='2818' />
4424</reference>
4425
4426<reference anchor='RFC3040'>
4427  <front>
4428    <title>Internet Web Replication and Caching Taxonomy</title>
4429    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4430      <organization>Equinix, Inc.</organization>
4431    </author>
4432    <author initials='I.' surname='Melve' fullname='I. Melve'>
4433      <organization>UNINETT</organization>
4434    </author>
4435    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4436      <organization>CacheFlow Inc.</organization>
4437    </author>
4438    <date year='2001' month='January' />
4439  </front>
4440  <seriesInfo name='RFC' value='3040' />
4441</reference>
4442
4443<reference anchor='BCP90'>
4444  <front>
4445    <title>Registration Procedures for Message Header Fields</title>
4446    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4447      <organization>Nine by Nine</organization>
4448      <address><email>GK-IETF@ninebynine.org</email></address>
4449    </author>
4450    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4451      <organization>BEA Systems</organization>
4452      <address><email>mnot@pobox.com</email></address>
4453    </author>
4454    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4455      <organization>HP Labs</organization>
4456      <address><email>JeffMogul@acm.org</email></address>
4457    </author>
4458    <date year='2004' month='September' />
4459  </front>
4460  <seriesInfo name='BCP' value='90' />
4461  <seriesInfo name='RFC' value='3864' />
4462</reference>
4463
4464<reference anchor='RFC4033'>
4465  <front>
4466    <title>DNS Security Introduction and Requirements</title>
4467    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4468    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4469    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4470    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4471    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4472    <date year='2005' month='March' />
4473  </front>
4474  <seriesInfo name='RFC' value='4033' />
4475</reference>
4476
4477<reference anchor="BCP13">
4478  <front>
4479    <title>Media Type Specifications and Registration Procedures</title>
4480    <author initials="N." surname="Freed" fullname="Ned Freed">
4481      <organization>Oracle</organization>
4482      <address>
4483        <email>ned+ietf@mrochek.com</email>
4484      </address>
4485    </author>
4486    <author initials="J." surname="Klensin" fullname="John C. Klensin">
4487      <address>
4488        <email>john+ietf@jck.com</email>
4489      </address>
4490    </author>
4491    <author initials="T." surname="Hansen" fullname="Tony Hansen">
4492      <organization>AT&amp;T Laboratories</organization>
4493      <address>
4494        <email>tony+mtsuffix@maillennium.att.com</email>
4495      </address>
4496    </author>
4497    <date year="2013" month="January"/>
4498  </front>
4499  <seriesInfo name="BCP" value="13"/>
4500  <seriesInfo name="RFC" value="6838"/>
4501</reference>
4502
4503<reference anchor='BCP115'>
4504  <front>
4505    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4506    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4507      <organization>AT&amp;T Laboratories</organization>
4508      <address>
4509        <email>tony+urireg@maillennium.att.com</email>
4510      </address>
4511    </author>
4512    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4513      <organization>Qualcomm, Inc.</organization>
4514      <address>
4515        <email>hardie@qualcomm.com</email>
4516      </address>
4517    </author>
4518    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4519      <organization>Adobe Systems</organization>
4520      <address>
4521        <email>LMM@acm.org</email>
4522      </address>
4523    </author>
4524    <date year='2006' month='February' />
4525  </front>
4526  <seriesInfo name='BCP' value='115' />
4527  <seriesInfo name='RFC' value='4395' />
4528</reference>
4529
4530<reference anchor='RFC4559'>
4531  <front>
4532    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4533    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4534    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4535    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4536    <date year='2006' month='June' />
4537  </front>
4538  <seriesInfo name='RFC' value='4559' />
4539</reference>
4540
4541<reference anchor='RFC5226'>
4542  <front>
4543    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4544    <author initials='T.' surname='Narten' fullname='T. Narten'>
4545      <organization>IBM</organization>
4546      <address><email>narten@us.ibm.com</email></address>
4547    </author>
4548    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4549      <organization>Google</organization>
4550      <address><email>Harald@Alvestrand.no</email></address>
4551    </author>
4552    <date year='2008' month='May' />
4553  </front>
4554  <seriesInfo name='BCP' value='26' />
4555  <seriesInfo name='RFC' value='5226' />
4556</reference>
4557
4558<reference anchor='RFC5246'>
4559   <front>
4560      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4561      <author initials='T.' surname='Dierks' fullname='T. Dierks'>
4562         <organization />
4563      </author>
4564      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4565         <organization>RTFM, Inc.</organization>
4566      </author>
4567      <date year='2008' month='August' />
4568   </front>
4569   <seriesInfo name='RFC' value='5246' />
4570</reference>
4571
4572<reference anchor="RFC5322">
4573  <front>
4574    <title>Internet Message Format</title>
4575    <author initials="P." surname="Resnick" fullname="P. Resnick">
4576      <organization>Qualcomm Incorporated</organization>
4577    </author>
4578    <date year="2008" month="October"/>
4579  </front>
4580  <seriesInfo name="RFC" value="5322"/>
4581</reference>
4582
4583<reference anchor="RFC6265">
4584  <front>
4585    <title>HTTP State Management Mechanism</title>
4586    <author initials="A." surname="Barth" fullname="Adam Barth">
4587      <organization abbrev="U.C. Berkeley">
4588        University of California, Berkeley
4589      </organization>
4590      <address><email>abarth@eecs.berkeley.edu</email></address>
4591    </author>
4592    <date year="2011" month="April" />
4593  </front>
4594  <seriesInfo name="RFC" value="6265"/>
4595</reference>
4596
4597<!--<reference anchor='BCP97'>
4598  <front>
4599    <title>Handling Normative References to Standards-Track Documents</title>
4600    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
4601      <address>
4602        <email>klensin+ietf@jck.com</email>
4603      </address>
4604    </author>
4605    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
4606      <organization>MIT</organization>
4607      <address>
4608        <email>hartmans-ietf@mit.edu</email>
4609      </address>
4610    </author>
4611    <date year='2007' month='June' />
4612  </front>
4613  <seriesInfo name='BCP' value='97' />
4614  <seriesInfo name='RFC' value='4897' />
4615</reference>-->
4616
4617<reference anchor="Kri2001" target="http://arxiv.org/abs/cs.SE/0105018">
4618  <front>
4619    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
4620    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
4621    <date year="2001" month="November"/>
4622  </front>
4623  <seriesInfo name="ACM Transactions on Internet Technology" value="Vol. 1, #2"/>
4624</reference>
4625
4626</references>
4627
4628
4629<section title="HTTP Version History" anchor="compatibility">
4630<t>
4631   HTTP has been in use by the World-Wide Web global information initiative
4632   since 1990. The first version of HTTP, later referred to as HTTP/0.9,
4633   was a simple protocol for hypertext data transfer across the Internet
4634   with only a single request method (GET) and no metadata.
4635   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
4636   methods and MIME-like messaging that could include metadata about the data
4637   transferred and modifiers on the request/response semantics. However,
4638   HTTP/1.0 did not sufficiently take into consideration the effects of
4639   hierarchical proxies, caching, the need for persistent connections, or
4640   name-based virtual hosts. The proliferation of incompletely-implemented
4641   applications calling themselves "HTTP/1.0" further necessitated a
4642   protocol version change in order for two communicating applications
4643   to determine each other's true capabilities.
4644</t>
4645<t>
4646   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
4647   requirements that enable reliable implementations, adding only
4648   those new features that will either be safely ignored by an HTTP/1.0
4649   recipient or only sent when communicating with a party advertising
4650   conformance with HTTP/1.1.
4651</t>
4652<t>
4653   It is beyond the scope of a protocol specification to mandate
4654   conformance with previous versions. HTTP/1.1 was deliberately
4655   designed, however, to make supporting previous versions easy.
4656   We would expect a general-purpose HTTP/1.1 server to understand
4657   any valid request in the format of HTTP/1.0 and respond appropriately
4658   with an HTTP/1.1 message that only uses features understood (or
4659   safely ignored) by HTTP/1.0 clients.  Likewise, we would expect
4660   an HTTP/1.1 client to understand any valid HTTP/1.0 response.
4661</t>
4662<t>
4663   Since HTTP/0.9 did not support header fields in a request,
4664   there is no mechanism for it to support name-based virtual
4665   hosts (selection of resource by inspection of the <x:ref>Host</x:ref> header
4666   field).  Any server that implements name-based virtual hosts
4667   ought to disable support for HTTP/0.9.  Most requests that
4668   appear to be HTTP/0.9 are, in fact, badly constructed HTTP/1.x
4669   requests wherein a buggy client failed to properly encode
4670   linear whitespace found in a URI reference and placed in
4671   the request-target.
4672</t>
4673
4674<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
4675<t>
4676   This section summarizes major differences between versions HTTP/1.0
4677   and HTTP/1.1.
4678</t>
4679
4680<section title="Multi-homed Web Servers" anchor="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">
4681<t>
4682   The requirements that clients and servers support the <x:ref>Host</x:ref>
4683   header field (<xref target="header.host"/>), report an error if it is
4684   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
4685   are among the most important changes defined by HTTP/1.1.
4686</t>
4687<t>
4688   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4689   addresses and servers; there was no other established mechanism for
4690   distinguishing the intended server of a request than the IP address
4691   to which that request was directed. The <x:ref>Host</x:ref> header field was
4692   introduced during the development of HTTP/1.1 and, though it was
4693   quickly implemented by most HTTP/1.0 browsers, additional requirements
4694   were placed on all HTTP/1.1 requests in order to ensure complete
4695   adoption.  At the time of this writing, most HTTP-based services
4696   are dependent upon the Host header field for targeting requests.
4697</t>
4698</section>
4699
4700<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4701<t>
4702   In HTTP/1.0, each connection is established by the client prior to the
4703   request and closed by the server after sending the response. However, some
4704   implementations implement the explicitly negotiated ("Keep-Alive") version
4705   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
4706   target="RFC2068"/>.
4707</t>
4708<t>
4709   Some clients and servers might wish to be compatible with these previous
4710   approaches to persistent connections, by explicitly negotiating for them
4711   with a "Connection: keep-alive" request header field. However, some
4712   experimental implementations of HTTP/1.0 persistent connections are faulty;
4713   for example, if an HTTP/1.0 proxy server doesn't understand
4714   <x:ref>Connection</x:ref>, it will erroneously forward that header field
4715   to the next inbound server, which would result in a hung connection.
4716</t>
4717<t>
4718   One attempted solution was the introduction of a Proxy-Connection header
4719   field, targeted specifically at proxies. In practice, this was also
4720   unworkable, because proxies are often deployed in multiple layers, bringing
4721   about the same problem discussed above.
4722</t>
4723<t>
4724   As a result, clients are encouraged not to send the Proxy-Connection header
4725   field in any requests.
4726</t>
4727<t>
4728   Clients are also encouraged to consider the use of Connection: keep-alive
4729   in requests carefully; while they can enable persistent connections with
4730   HTTP/1.0 servers, clients using them need will need to monitor the
4731   connection for "hung" requests (which indicate that the client ought stop
4732   sending the header field), and this mechanism ought not be used by clients
4733   at all when a proxy is being used.
4734</t>
4735</section>
4736
4737<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
4738<t>
4739   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
4740   (<xref target="header.transfer-encoding"/>).
4741   Transfer codings need to be decoded prior to forwarding an HTTP message
4742   over a MIME-compliant protocol.
4743</t>
4744</section>
4745
4746</section>
4747
4748<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4749<t>
4750  HTTP's approach to error handling has been explained.
4751  (<xref target="conformance"/>)
4752</t>
4753<t>
4754  The expectation to support HTTP/0.9 requests has been removed.
4755</t>
4756<t>
4757  The term "Effective Request URI" has been introduced.
4758  (<xref target="effective.request.uri" />)
4759</t>
4760<t>
4761  HTTP messages can be (and often are) buffered by implementations; despite
4762  it sometimes being available as a stream, HTTP is fundamentally a
4763  message-oriented protocol.
4764  (<xref target="http.message" />)
4765</t>
4766<t>
4767  Minimum supported sizes for various protocol elements have been
4768  suggested, to improve interoperability.
4769</t>
4770<t>
4771  Header fields that span multiple lines ("line folding") are deprecated.
4772  (<xref target="field.parsing" />)
4773</t>
4774<t>
4775  The HTTP-version ABNF production has been clarified to be case-sensitive.
4776  Additionally, version numbers has been restricted to single digits, due
4777  to the fact that implementations are known to handle multi-digit version
4778  numbers incorrectly.
4779  (<xref target="http.version"/>)
4780</t>
4781<t>
4782  The HTTPS URI scheme is now defined by this specification; previously,
4783  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
4784  (<xref target="https.uri"/>)
4785</t>
4786<t>
4787  The HTTPS URI scheme implies end-to-end security.
4788  (<xref target="https.uri"/>)
4789</t>
4790<t>
4791  Userinfo (i.e., username and password) are now disallowed in HTTP and
4792  HTTPS URIs, because of security issues related to their transmission on the
4793  wire.
4794  (<xref target="http.uri" />)
4795</t>
4796<t>
4797  Invalid whitespace around field-names is now required to be rejected,
4798  because accepting it represents a security vulnerability.
4799  (<xref target="header.fields"/>)
4800</t>
4801<t>
4802  The ABNF productions defining header fields now only list the field value.
4803  (<xref target="header.fields"/>)
4804</t>
4805<t>
4806  Rules about implicit linear whitespace between certain grammar productions
4807  have been removed; now whitespace is only allowed where specifically
4808  defined in the ABNF.
4809  (<xref target="whitespace"/>)
4810</t>
4811<t> 
4812  The NUL octet is no longer allowed in comment and quoted-string text, and
4813  handling of backslash-escaping in them has been clarified.
4814  (<xref target="field.components"/>)
4815</t> 
4816<t>
4817  The quoted-pair rule no longer allows escaping control characters other than
4818  HTAB.
4819  (<xref target="field.components"/>)
4820</t>
4821<t>
4822  Non-ASCII content in header fields and the reason phrase has been obsoleted
4823  and made opaque (the TEXT rule was removed).
4824  (<xref target="field.components"/>)
4825</t>
4826<t>
4827  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
4828  handled as errors by recipients.
4829  (<xref target="header.content-length"/>)
4830</t>
4831<t>
4832  The "identity" transfer coding token has been removed.
4833  (Sections <xref format="counter" target="message.body"/> and
4834  <xref format="counter" target="transfer.codings"/>)
4835</t>
4836<t>
4837  The algorithm for determining the message body length has been clarified
4838  to indicate all of the special cases (e.g., driven by methods or status
4839  codes) that affect it, and that new protocol elements cannot define such
4840  special cases.
4841  (<xref target="message.body.length"/>)
4842</t>
4843<t>
4844  "multipart/byteranges" is no longer a way of determining message body length
4845  detection.
4846  (<xref target="message.body.length"/>)
4847</t>
4848<t>
4849  CONNECT is a new, special case in determining message body length.
4850  (<xref target="message.body.length"/>)
4851</t>
4852<t>
4853  Chunk length does not include the count of the octets in the
4854  chunk header and trailer.
4855  (<xref target="chunked.encoding"/>)
4856</t>
4857<t>
4858  Use of chunk extensions is deprecated, and line folding in them is
4859  disallowed.
4860  (<xref target="chunked.encoding"/>)
4861</t>
4862<t>
4863  The segment + query components of RFC3986 have been used to define the
4864  request-target, instead of abs_path from RFC 1808.
4865  (<xref target="request-target"/>)
4866</t>
4867<t>
4868  The asterisk form of the request-target is only allowed in the OPTIONS
4869  method.
4870  (<xref target="request-target"/>)
4871</t>
4872<t>
4873  Exactly when "close" connection options have to be sent has been clarified.
4874  (<xref target="header.connection"/>)
4875</t>
4876<t> 
4877  "hop-by-hop" header fields are required to appear in the Connection header
4878  field; just because they're defined as hop-by-hop in this specification
4879  doesn't exempt them.
4880  (<xref target="header.connection"/>)
4881</t>
4882<t>
4883  The limit of two connections per server has been removed.
4884  (<xref target="persistent.connections"/>)
4885</t>
4886<t>
4887  An idempotent sequence of requests is no longer required to be retried.
4888  (<xref target="persistent.connections"/>)
4889</t>
4890<t>
4891  The requirement to retry requests under certain circumstances when the
4892  server prematurely closes the connection has been removed.
4893  (<xref target="persistent.connections"/>)
4894</t>
4895<t>
4896  Some extraneous requirements about when servers are allowed to close
4897  connections prematurely have been removed.
4898  (<xref target="persistent.connections"/>)
4899</t>
4900<t>
4901  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
4902  responses other than 101 (this was incorporated from <xref
4903  target="RFC2817"/>).
4904  (<xref target="header.upgrade"/>)
4905</t>
4906<t>
4907  Registration of Transfer Codings now requires IETF Review
4908  (<xref target="transfer.coding.registry"/>)
4909</t>
4910<t>
4911  The meaning of the "deflate" content coding has been clarified.
4912  (<xref target="deflate.coding" />)
4913</t>
4914<t>
4915  This specification now defines the Upgrade Token Registry, previously
4916  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
4917  (<xref target="upgrade.token.registry"/>)
4918</t>
4919<t>
4920  Empty list elements in list productions (e.g., a list header containing
4921  ", ,") have been deprecated.
4922  (<xref target="abnf.extension"/>)
4923</t>
4924<t>
4925  Issues with the Keep-Alive and Proxy-Connection headers in requests
4926  are pointed out, with use of the latter being discouraged altogether.
4927  (<xref target="compatibility.with.http.1.0.persistent.connections" />)
4928</t>
4929</section>
4930</section>
4931
4932<section title="ABNF list extension: #rule" anchor="abnf.extension">
4933<t>
4934  A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
4935  improve readability in the definitions of some header field values.
4936</t>
4937<t>
4938  A construct "#" is defined, similar to "*", for defining comma-delimited
4939  lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
4940  at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
4941  comma (",") and optional whitespace (OWS).   
4942</t>
4943<figure><preamble>
4944  Thus,
4945</preamble><artwork type="example">
4946  1#element =&gt; element *( OWS "," OWS element )
4947</artwork></figure>
4948<figure><preamble>
4949  and:
4950</preamble><artwork type="example">
4951  #element =&gt; [ 1#element ]
4952</artwork></figure>
4953<figure><preamble>
4954  and for n &gt;= 1 and m &gt; 1:
4955</preamble><artwork type="example">
4956  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
4957</artwork></figure>
4958<t>
4959  For compatibility with legacy list rules, recipients &SHOULD; accept empty
4960  list elements. In other words, consumers would follow the list productions:
4961</t>
4962<figure><artwork type="example">
4963  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
4964 
4965  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
4966</artwork></figure>
4967<t>
4968  Note that empty elements do not contribute to the count of elements present,
4969  though.
4970</t>
4971<t>
4972  For example, given these ABNF productions:
4973</t>
4974<figure><artwork type="example">
4975  example-list      = 1#example-list-elmt
4976  example-list-elmt = token ; see <xref target="field.components"/>
4977</artwork></figure>
4978<t>
4979  Then these are valid values for example-list (not including the double
4980  quotes, which are present for delimitation only):
4981</t>
4982<figure><artwork type="example">
4983  "foo,bar"
4984  "foo ,bar,"
4985  "foo , ,bar,charlie   "
4986</artwork></figure>
4987<t>
4988  But these values would be invalid, as at least one non-empty element is
4989  required:
4990</t>
4991<figure><artwork type="example">
4992  ""
4993  ","
4994  ",   ,"
4995</artwork></figure>
4996<t>
4997  <xref target="collected.abnf"/> shows the collected ABNF, with the list rules
4998  expanded as explained above.
4999</t>
5000</section>
5001
5002<?BEGININC p1-messaging.abnf-appendix ?>
5003<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
5004<figure>
5005<artwork type="abnf" name="p1-messaging.parsed-abnf">
5006<x:ref>BWS</x:ref> = OWS
5007
5008<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
5009 connection-option ] )
5010<x:ref>Content-Length</x:ref> = 1*DIGIT
5011
5012<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
5013 ]
5014<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
5015<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
5016<x:ref>Host</x:ref> = uri-host [ ":" port ]
5017
5018<x:ref>OWS</x:ref> = *( SP / HTAB )
5019
5020<x:ref>RWS</x:ref> = 1*( SP / HTAB )
5021
5022<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
5023<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
5024<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
5025 transfer-coding ] )
5026
5027<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
5028<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
5029
5030<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
5031 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
5032 comment ] ) ] )
5033
5034<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
5035<x:ref>absolute-form</x:ref> = absolute-URI
5036<x:ref>absolute-path</x:ref> = 1*( "/" segment )
5037<x:ref>asterisk-form</x:ref> = "*"
5038<x:ref>attribute</x:ref> = token
5039<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
5040<x:ref>authority-form</x:ref> = authority
5041
5042<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
5043<x:ref>chunk-data</x:ref> = 1*OCTET
5044<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
5045<x:ref>chunk-ext-name</x:ref> = token
5046<x:ref>chunk-ext-val</x:ref> = token / quoted-str-nf
5047<x:ref>chunk-size</x:ref> = 1*HEXDIG
5048<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
5049<x:ref>comment</x:ref> = "(" *( ctext / quoted-cpair / comment ) ")"
5050<x:ref>connection-option</x:ref> = token
5051<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
5052 / %x2A-5B ; '*'-'['
5053 / %x5D-7E ; ']'-'~'
5054 / obs-text
5055
5056<x:ref>field-content</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5057<x:ref>field-name</x:ref> = token
5058<x:ref>field-value</x:ref> = *( field-content / obs-fold )
5059
5060<x:ref>header-field</x:ref> = field-name ":" OWS field-value BWS
5061<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ]
5062<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ]
5063
5064<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
5065
5066<x:ref>message-body</x:ref> = *OCTET
5067<x:ref>method</x:ref> = token
5068
5069<x:ref>obs-fold</x:ref> = CRLF ( SP / HTAB )
5070<x:ref>obs-text</x:ref> = %x80-FF
5071<x:ref>origin-form</x:ref> = absolute-path [ "?" query ]
5072
5073<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
5074<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
5075<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
5076<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
5077<x:ref>protocol-name</x:ref> = token
5078<x:ref>protocol-version</x:ref> = token
5079<x:ref>pseudonym</x:ref> = token
5080
5081<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5082 / %x5D-7E ; ']'-'~'
5083 / obs-text
5084<x:ref>qdtext-nf</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5085 / %x5D-7E ; ']'-'~'
5086 / obs-text
5087<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5088<x:ref>quoted-cpair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5089<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5090<x:ref>quoted-str-nf</x:ref> = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
5091<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5092
5093<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5094<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5095<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5096<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5097<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5098<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5099<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5100 asterisk-form
5101
5102<x:ref>segment</x:ref> = &lt;segment, defined in [RFC3986], Section 3.3&gt;
5103<x:ref>special</x:ref> = "(" / ")" / "&lt;" / "&gt;" / "@" / "," / ";" / ":" / "\" /
5104 DQUOTE / "/" / "[" / "]" / "?" / "=" / "{" / "}"
5105<x:ref>start-line</x:ref> = request-line / status-line
5106<x:ref>status-code</x:ref> = 3DIGIT
5107<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5108
5109<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5110<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5111<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5112 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5113<x:ref>token</x:ref> = 1*tchar
5114<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5115<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5116 transfer-extension
5117<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5118<x:ref>transfer-parameter</x:ref> = attribute BWS "=" BWS value
5119
5120<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5121
5122<x:ref>value</x:ref> = word
5123
5124<x:ref>word</x:ref> = token / quoted-string
5125</artwork>
5126</figure>
5127</section>
5128<?ENDINC p1-messaging.abnf-appendix ?>
5129
5130<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5131
5132<section title="Since RFC 2616">
5133<t>
5134  Changes up to the first Working Group Last Call draft are summarized
5135  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-21#appendix-D"/>.
5136</t>
5137</section>
5138
5139<section title="Since draft-ietf-httpbis-p1-messaging-21" anchor="changes.since.21">
5140<t>
5141  Closed issues:
5142  <list style="symbols">
5143    <t>
5144      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/128"/>:
5145      "Cite HTTPS URI scheme definition" (the spec now includes the HTTPs
5146      scheme definition and thus updates RFC 2818)
5147    </t>
5148    <t>
5149      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/389"/>:
5150      "mention of 'proxies' in section about caches"
5151    </t>
5152    <t>
5153      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/390"/>:
5154      "use of ABNF terms from RFC 3986"
5155    </t>
5156    <t>
5157      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/392"/>:
5158      "editorial improvements to message length definition"
5159    </t>
5160    <t>
5161      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/395"/>:
5162      "Connection header field MUST vs SHOULD"
5163    </t>
5164    <t>
5165      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/396"/>:
5166      "editorial improvements to persistent connections section"
5167    </t>
5168    <t>
5169      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/397"/>:
5170      "URI normalization vs empty path"
5171    </t>
5172    <t>
5173      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/408"/>:
5174      "p1 feedback"
5175    </t>
5176    <t>
5177      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/409"/>:
5178      "is parsing OBS-FOLD mandatory?"
5179    </t>
5180    <t>
5181      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/410"/>:
5182      "HTTPS and Shared Caching"
5183    </t>
5184    <t>
5185      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/411"/>:
5186      "Requirements for recipients of ws between start-line and first header field"
5187    </t>
5188    <t>
5189      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/412"/>:
5190      "SP and HT when being tolerant"
5191    </t>
5192    <t>
5193      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/414"/>:
5194      "Message Parsing Strictness"
5195    </t>
5196    <t>
5197      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/415"/>:
5198      "'Render'"
5199    </t>
5200    <t>
5201      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/418"/>:
5202      "No-Transform"
5203    </t>
5204    <t>
5205      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/419"/>:
5206      "p2 editorial feedback"
5207    </t>
5208    <t>
5209      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/420"/>:
5210      "Content-Length SHOULD be sent"
5211    </t>
5212    <t>
5213      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/431"/>:
5214      "origin-form does not allow path starting with "//""
5215    </t>
5216  </list>
5217</t>
5218</section>
5219
5220<!--<section title="Since draft-ietf-httpbis-p1-messaging-22" anchor="changes.since.22">
5221<t>
5222  None yet.
5223</t>
5224</section>-->
5225</section>
5226
5227</back>
5228</rfc>
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