source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2177

Last change on this file since 2177 was 2177, checked in by julian.reschke@…, 7 years ago

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

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  • Property svn:mime-type set to text/xml
File size: 224.8 KB
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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "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"/>
385Vary: Accept-Encoding
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="path-absolute"/>
782  <x:anchor-alias value="port"/>
783  <x:anchor-alias value="query"/>
784  <x:anchor-alias value="uri-host"/>
785  <x:anchor-alias value="partial-URI"/>
786<t>
787   This specification adopts the definitions of "URI-reference",
788   "absolute-URI", "relative-part", "port", "host",
789   "path-abempty", "path-absolute", "query", and "authority" from the
790   URI generic syntax.
791   In addition, we define a partial-URI rule for protocol elements
792   that allow a relative URI but not a fragment.
793</t>
794<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="path-absolute"/><iref primary="true" item="Grammar" subitem="port"/><iref primary="true" item="Grammar" subitem="query"/><iref primary="true" item="Grammar" subitem="uri-host"/><iref primary="true" item="Grammar" subitem="partial-URI"><!--exported production--></iref>
795  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
796  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
797  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
798  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
799  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
800  <x:ref>path-absolute</x:ref> = &lt;path-absolute, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
801  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
802  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
803  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
804 
805  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
806</artwork></figure>
807<t>
808   Each protocol element in HTTP that allows a URI reference will indicate
809   in its ABNF production whether the element allows any form of reference
810   (URI-reference), only a URI in absolute form (absolute-URI), only the
811   path and optional query components, or some combination of the above.
812   Unless otherwise indicated, URI references are parsed
813   relative to the effective request URI
814   (<xref target="effective.request.uri"/>).
815</t>
816
817<section title="http URI scheme" anchor="http.uri">
818  <x:anchor-alias value="http-URI"/>
819  <iref item="http URI scheme" primary="true"/>
820  <iref item="URI scheme" subitem="http" primary="true"/>
821<t>
822   The "http" URI scheme is hereby defined for the purpose of minting
823   identifiers according to their association with the hierarchical
824   namespace governed by a potential HTTP origin server listening for
825   TCP connections on a given port.
826</t>
827<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"><!--terminal production--></iref>
828  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
829</artwork></figure>
830<t>
831   The HTTP origin server is identified by the generic syntax's
832   <x:ref>authority</x:ref> component, which includes a host identifier
833   and optional TCP port (<xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>).
834   The remainder of the URI, consisting of both the hierarchical path
835   component and optional query component, serves as an identifier for
836   a potential resource within that origin server's name space.
837</t>
838<t>
839   If the host identifier is provided as an IP address,
840   then the origin server is any listener on the indicated TCP port at
841   that IP address. If host is a registered name, then that name is
842   considered an indirect identifier and the recipient might use a name
843   resolution service, such as DNS, to find the address of a listener
844   for that host.
845   The host &MUST-NOT; be empty; if an "http" URI is received with an
846   empty host, then it &MUST; be rejected as invalid.
847   If the port subcomponent is empty or not given, then TCP port 80 is
848   assumed (the default reserved port for WWW services).
849</t>
850<t>
851   Regardless of the form of host identifier, access to that host is not
852   implied by the mere presence of its name or address. The host might or might
853   not exist and, even when it does exist, might or might not be running an
854   HTTP server or listening to the indicated port. The "http" URI scheme
855   makes use of the delegated nature of Internet names and addresses to
856   establish a naming authority (whatever entity has the ability to place
857   an HTTP server at that Internet name or address) and allows that
858   authority to determine which names are valid and how they might be used.
859</t>
860<t>
861   When an "http" URI is used within a context that calls for access to the
862   indicated resource, a client &MAY; attempt access by resolving
863   the host to an IP address, establishing a TCP connection to that address
864   on the indicated port, and sending an HTTP request message
865   (<xref target="http.message"/>) containing the URI's identifying data
866   (<xref target="message.routing"/>) to the server.
867   If the server responds to that request with a non-interim HTTP response
868   message, as described in &status-codes;, then that response
869   is considered an authoritative answer to the client's request.
870</t>
871<t>
872   Although HTTP is independent of the transport protocol, the "http"
873   scheme is specific to TCP-based services because the name delegation
874   process depends on TCP for establishing authority.
875   An HTTP service based on some other underlying connection protocol
876   would presumably be identified using a different URI scheme, just as
877   the "https" scheme (below) is used for resources that require an
878   end-to-end secured connection. Other protocols might also be used to
879   provide access to "http" identified resources &mdash; it is only the
880   authoritative interface used for mapping the namespace that is
881   specific to TCP.
882</t>
883<t>
884   The URI generic syntax for authority also includes a deprecated
885   userinfo subcomponent (<xref target="RFC3986" x:fmt="," x:sec="3.2.1"/>)
886   for including user authentication information in the URI.  Some
887   implementations make use of the userinfo component for internal
888   configuration of authentication information, such as within command
889   invocation options, configuration files, or bookmark lists, even
890   though such usage might expose a user identifier or password.
891   Senders &MUST; exclude the userinfo subcomponent (and its "@"
892   delimiter) when an "http" URI is transmitted within a message as a
893   request target or header field value.
894   Recipients of an "http" URI reference &SHOULD; parse for userinfo and
895   treat its presence as an error, since it is likely being used to obscure
896   the authority for the sake of phishing attacks.
897</t>
898</section>
899
900<section title="https URI scheme" anchor="https.uri">
901   <x:anchor-alias value="https-URI"/>
902   <iref item="https URI scheme"/>
903   <iref item="URI scheme" subitem="https"/>
904<t>
905   The "https" URI scheme is hereby defined for the purpose of minting
906   identifiers according to their association with the hierarchical
907   namespace governed by a potential HTTP origin server listening to a
908   given TCP port for TLS-secured connections <xref target="RFC5246"/>.
909</t>
910<t>
911   All of the requirements listed above for the "http" scheme are also
912   requirements for the "https" scheme, except that a default TCP port
913   of 443 is assumed if the port subcomponent is empty or not given,
914   and the TCP connection &MUST; be secured, end-to-end, through the
915   use of strong encryption prior to sending the first HTTP request.
916</t>
917<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="https-URI"><!--terminal production--></iref>
918  <x:ref>https-URI</x:ref> = "https:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
919</artwork></figure>
920<t>
921   Resources made available via the "https" scheme have no shared
922   identity with the "http" scheme even if their resource identifiers
923   indicate the same authority (the same host listening to the same
924   TCP port).  They are distinct name spaces and are considered to be
925   distinct origin servers.  However, an extension to HTTP that is
926   defined to apply to entire host domains, such as the Cookie protocol
927   <xref target="RFC6265"/>, can allow information
928   set by one service to impact communication with other services
929   within a matching group of host domains.
930</t>
931<t>
932   The process for authoritative access to an "https" identified
933   resource is defined in <xref target="RFC2818"/>.
934</t>
935</section>
936
937<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
938<t>
939   Since the "http" and "https" schemes conform to the URI generic syntax,
940   such URIs are normalized and compared according to the algorithm defined
941   in <xref target="RFC3986" x:fmt="," x:sec="6"/>, using the defaults
942   described above for each scheme.
943</t>
944<t>
945   If the port is equal to the default port for a scheme, the normal form is
946   to elide the port subcomponent. When not being used in absolute form as the
947   request target of an OPTIONS request, an empty path component is equivalent
948   to an absolute path of "/", so the normal form is to provide a path of "/"
949   instead. The scheme and host are case-insensitive and normally provided in
950   lowercase; all other components are compared in a case-sensitive manner.
951   Characters other than those in the "reserved" set are equivalent to their
952   percent-encoded octets (see <xref target="RFC3986" x:fmt=","
953   x:sec="2.1"/>): the normal form is to not encode them.
954</t>
955<t>
956   For example, the following three URIs are equivalent:
957</t>
958<figure><artwork type="example">
959   http://example.com:80/~smith/home.html
960   http://EXAMPLE.com/%7Esmith/home.html
961   http://EXAMPLE.com:/%7esmith/home.html
962</artwork></figure>
963</section>
964</section>
965</section>
966
967<section title="Message Format" anchor="http.message">
968<x:anchor-alias value="generic-message"/>
969<x:anchor-alias value="message.types"/>
970<x:anchor-alias value="HTTP-message"/>
971<x:anchor-alias value="start-line"/>
972<iref item="header section"/>
973<iref item="headers"/>
974<iref item="header field"/>
975<t>
976   All HTTP/1.1 messages consist of a start-line followed by a sequence of
977   octets in a format similar to the Internet Message Format
978   <xref target="RFC5322"/>: zero or more header fields (collectively
979   referred to as the "headers" or the "header section"), an empty line
980   indicating the end of the header section, and an optional message body.
981</t>
982<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"><!--terminal production--></iref>
983  <x:ref>HTTP-message</x:ref>   = <x:ref>start-line</x:ref>
984                   *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
985                   <x:ref>CRLF</x:ref>
986                   [ <x:ref>message-body</x:ref> ]
987</artwork></figure>
988<t>
989   The normal procedure for parsing an HTTP message is to read the
990   start-line into a structure, read each header field into a hash
991   table by field name until the empty line, and then use the parsed
992   data to determine if a message body is expected.  If a message body
993   has been indicated, then it is read as a stream until an amount
994   of octets equal to the message body length is read or the connection
995   is closed.
996</t>
997<t>
998   Recipients &MUST; parse an HTTP message as a sequence of octets in an
999   encoding that is a superset of US-ASCII <xref target="USASCII"/>.
1000   Parsing an HTTP message as a stream of Unicode characters, without regard
1001   for the specific encoding, creates security vulnerabilities due to the
1002   varying ways that string processing libraries handle invalid multibyte
1003   character sequences that contain the octet LF (%x0A).  String-based
1004   parsers can only be safely used within protocol elements after the element
1005   has been extracted from the message, such as within a header field-value
1006   after message parsing has delineated the individual fields.
1007</t>
1008<t>
1009   An HTTP message can be parsed as a stream for incremental processing or
1010   forwarding downstream.  However, recipients cannot rely on incremental
1011   delivery of partial messages, since some implementations will buffer or
1012   delay message forwarding for the sake of network efficiency, security
1013   checks, or payload transformations.
1014</t>
1015
1016<section title="Start Line" anchor="start.line">
1017  <x:anchor-alias value="Start-Line"/>
1018<t>
1019   An HTTP message can either be a request from client to server or a
1020   response from server to client.  Syntactically, the two types of message
1021   differ only in the start-line, which is either a request-line (for requests)
1022   or a status-line (for responses), and in the algorithm for determining
1023   the length of the message body (<xref target="message.body"/>).
1024</t>
1025<t>
1026   In theory, a client could receive requests and a server could receive
1027   responses, distinguishing them by their different start-line formats,
1028   but in practice servers are implemented to only expect a request
1029   (a response is interpreted as an unknown or invalid request method)
1030   and clients are implemented to only expect a response.
1031</t>
1032<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="start-line"/>
1033  <x:ref>start-line</x:ref>     = <x:ref>request-line</x:ref> / <x:ref>status-line</x:ref>
1034</artwork></figure>
1035<t>
1036   A sender &MUST-NOT; send whitespace between the start-line and
1037   the first header field. The presence of such whitespace in a request
1038   might be an attempt to trick a server into ignoring that field or
1039   processing the line after it as a new request, either of which might
1040   result in a security vulnerability if other implementations within
1041   the request chain interpret the same message differently.
1042   Likewise, the presence of such whitespace in a response might be
1043   ignored by some clients or cause others to cease parsing.
1044</t>
1045<t>
1046   A recipient that receives whitespace between the start-line and
1047   the first header field &MUST; either reject the message as invalid or
1048   consume each whitespace-preceded line without further processing of it
1049   (i.e., ignore the entire line, along with any subsequent lines preceded
1050   by whitespace, until a properly formed header field is received or the
1051   header block is terminated).
1052</t>
1053
1054<section title="Request Line" anchor="request.line">
1055  <x:anchor-alias value="Request"/>
1056  <x:anchor-alias value="request-line"/>
1057<t>
1058   A request-line begins with a method token, followed by a single
1059   space (SP), the request-target, another single space (SP), the
1060   protocol version, and ending with CRLF.
1061</t>
1062<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1063  <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>
1064</artwork></figure>
1065<iref primary="true" item="method"/>
1066<t anchor="method">
1067   The method token indicates the request method to be performed on the
1068   target resource. The request method is case-sensitive.
1069</t>
1070<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1071  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1072</artwork></figure>
1073<t>
1074   The methods defined by this specification can be found in
1075   &methods;, along with information regarding the HTTP method registry
1076   and considerations for defining new methods.
1077</t>
1078<iref item="request-target"/>
1079<t>
1080   The request-target identifies the target resource upon which to apply
1081   the request, as defined in <xref target="request-target"/>.
1082</t>
1083<t>
1084   No whitespace is allowed inside the method, request-target, and
1085   protocol version.  Hence, recipients typically parse the request-line
1086   into its component parts by splitting on whitespace
1087   (see <xref target="message.robustness"/>).
1088</t>
1089<t>
1090   Unfortunately, some user agents fail to properly encode hypertext
1091   references that have embedded whitespace, sending the characters directly
1092   instead of properly encoding or excluding the disallowed characters.
1093   Recipients of an invalid request-line &SHOULD; respond with either a
1094   <x:ref>400 (Bad Request)</x:ref> error or a <x:ref>301 (Moved Permanently)</x:ref>
1095   redirect with the request-target properly encoded.  Recipients &SHOULD-NOT;
1096   attempt to autocorrect and then process the request without a redirect,
1097   since the invalid request-line might be deliberately crafted to bypass
1098   security filters along the request chain.
1099</t>
1100<t>
1101   HTTP does not place a pre-defined limit on the length of a request-line.
1102   A server that receives a method longer than any that it implements
1103   &SHOULD; respond with a <x:ref>501 (Not Implemented)</x:ref> status code.
1104   A server &MUST; be prepared to receive URIs of unbounded length and
1105   respond with the <x:ref>414 (URI Too Long)</x:ref> status code if the received
1106   request-target would be longer than the server wishes to handle
1107   (see &status-414;).
1108</t>
1109<t>
1110   Various ad-hoc limitations on request-line length are found in practice.
1111   It is &RECOMMENDED; that all HTTP senders and recipients support, at a
1112   minimum, request-line lengths of 8000 octets.
1113</t>
1114</section>
1115
1116<section title="Status Line" anchor="status.line">
1117  <x:anchor-alias value="response"/>
1118  <x:anchor-alias value="status-line"/>
1119  <x:anchor-alias value="status-code"/>
1120  <x:anchor-alias value="reason-phrase"/>
1121<t>
1122   The first line of a response message is the status-line, consisting
1123   of the protocol version, a space (SP), the status code, another space,
1124   a possibly-empty textual phrase describing the status code, and
1125   ending with CRLF.
1126</t>
1127<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-line"/>
1128  <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>
1129</artwork></figure>
1130<t>
1131   The status-code element is a 3-digit integer code describing the
1132   result of the server's attempt to understand and satisfy the client's
1133   corresponding request. The rest of the response message is to be
1134   interpreted in light of the semantics defined for that status code.
1135   See &status-codes; for information about the semantics of status codes,
1136   including the classes of status code (indicated by the first digit),
1137   the status codes defined by this specification, considerations for the
1138   definition of new status codes, and the IANA registry.
1139</t>
1140<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-code"/>
1141  <x:ref>status-code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1142</artwork></figure>
1143<t>   
1144   The reason-phrase element exists for the sole purpose of providing a
1145   textual description associated with the numeric status code, mostly
1146   out of deference to earlier Internet application protocols that were more
1147   frequently used with interactive text clients. A client &SHOULD; ignore
1148   the reason-phrase content.
1149</t>
1150<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
1151  <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> )
1152</artwork></figure>
1153</section>
1154</section>
1155
1156<section title="Header Fields" anchor="header.fields">
1157  <x:anchor-alias value="header-field"/>
1158  <x:anchor-alias value="field-content"/>
1159  <x:anchor-alias value="field-name"/>
1160  <x:anchor-alias value="field-value"/>
1161  <x:anchor-alias value="obs-fold"/>
1162<t>
1163   Each HTTP header field consists of a case-insensitive field name
1164   followed by a colon (":"), optional whitespace, and the field value.
1165</t>
1166<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"/>
1167  <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>
1168  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1169  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>obs-fold</x:ref> )
1170  <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> )
1171  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref> ( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1172                 ; obsolete line folding
1173                 ; see <xref target="field.parsing"/>
1174</artwork></figure>
1175<t>
1176   The field-name token labels the corresponding field-value as having the
1177   semantics defined by that header field.  For example, the <x:ref>Date</x:ref>
1178   header field is defined in &header-date; as containing the origination
1179   timestamp for the message in which it appears.
1180</t>
1181
1182<section title="Field Extensibility" anchor="field.extensibility">
1183<t>
1184   HTTP header fields are fully extensible: there is no limit on the
1185   introduction of new field names, each presumably defining new semantics,
1186   nor on the number of header fields used in a given message.  Existing
1187   fields are defined in each part of this specification and in many other
1188   specifications outside the core standard.
1189   New header fields can be introduced without changing the protocol version
1190   if their defined semantics allow them to be safely ignored by recipients
1191   that do not recognize them.
1192</t>
1193<t>
1194   New HTTP header fields ought to be be registered with IANA in the
1195   Message Header Field Registry, as described in &iana-header-registry;.
1196   A proxy &MUST; forward unrecognized header fields unless the
1197   field-name is listed in the <x:ref>Connection</x:ref> header field
1198   (<xref target="header.connection"/>) or the proxy is specifically
1199   configured to block, or otherwise transform, such fields.
1200   Other recipients &SHOULD; ignore unrecognized header fields.
1201</t>
1202</section>
1203
1204<section title="Field Order" anchor="field.order">
1205<t>
1206   The order in which header fields with differing field names are
1207   received is not significant. However, it is "good practice" to send
1208   header fields that contain control data first, such as <x:ref>Host</x:ref>
1209   on requests and <x:ref>Date</x:ref> on responses, so that implementations
1210   can decide when not to handle a message as early as possible.  A server
1211   &MUST; wait until the entire header section is received before interpreting
1212   a request message, since later header fields might include conditionals,
1213   authentication credentials, or deliberately misleading duplicate
1214   header fields that would impact request processing.
1215</t>
1216<t>
1217   A sender &MUST-NOT; generate multiple header fields with the same field
1218   name in a message unless either the entire field value for that
1219   header field is defined as a comma-separated list [i.e., #(values)]
1220   or the header field is a well-known exception (as noted below).
1221</t>
1222<t>
1223   Multiple header fields with the same field name can be combined into
1224   one "field-name: field-value" pair, without changing the semantics of the
1225   message, by appending each subsequent field value to the combined
1226   field value in order, separated by a comma. The order in which
1227   header fields with the same field name are received is therefore
1228   significant to the interpretation of the combined field value;
1229   a proxy &MUST-NOT; change the order of these field values when
1230   forwarding a message.
1231</t>
1232<x:note>
1233  <t>
1234   &Note; In practice, the "Set-Cookie" header field (<xref target="RFC6265"/>)
1235   often appears multiple times in a response message and does not use the
1236   list syntax, violating the above requirements on multiple header fields
1237   with the same name. Since it cannot be combined into a single field-value,
1238   recipients ought to handle "Set-Cookie" as a special case while processing
1239   header fields. (See Appendix A.2.3 of <xref target="Kri2001"/> for details.)
1240  </t>
1241</x:note>
1242</section>
1243
1244<section title="Whitespace" anchor="whitespace">
1245<t anchor="rule.LWS">
1246   This specification uses three rules to denote the use of linear
1247   whitespace: OWS (optional whitespace), RWS (required whitespace), and
1248   BWS ("bad" whitespace).
1249</t>
1250<t anchor="rule.OWS">
1251   The OWS rule is used where zero or more linear whitespace octets might
1252   appear. OWS &SHOULD; either not be generated or be generated as a single
1253   SP. Multiple OWS octets that occur within field-content &SHOULD; either
1254   be replaced with a single SP or transformed to all SP octets (each
1255   octet other than SP replaced with SP) before interpreting the field value
1256   or forwarding the message downstream.
1257</t>
1258<t anchor="rule.RWS">
1259   RWS is used when at least one linear whitespace octet is required to
1260   separate field tokens. RWS &SHOULD; be generated as a single SP.
1261   Multiple RWS octets that occur within field-content &SHOULD; either
1262   be replaced with a single SP or transformed to all SP octets before
1263   interpreting the field value or forwarding the message downstream.
1264</t>
1265<t anchor="rule.BWS">
1266   BWS is used where the grammar allows optional whitespace, for historical
1267   reasons, but senders &SHOULD-NOT; generate it in messages;
1268   recipients &MUST; accept such bad optional whitespace and remove it before
1269   interpreting the field value or forwarding the message downstream.
1270</t>
1271<t anchor="rule.whitespace">
1272  <x:anchor-alias value="BWS"/>
1273  <x:anchor-alias value="OWS"/>
1274  <x:anchor-alias value="RWS"/>
1275</t>
1276<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"/>
1277  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1278                 ; optional whitespace
1279  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1280                 ; required whitespace
1281  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1282                 ; "bad" whitespace
1283</artwork></figure>
1284</section>
1285
1286<section title="Field Parsing" anchor="field.parsing">
1287<t>
1288   No whitespace is allowed between the header field-name and colon.
1289   In the past, differences in the handling of such whitespace have led to
1290   security vulnerabilities in request routing and response handling.
1291   A server &MUST; reject any received request message that contains
1292   whitespace between a header field-name and colon with a response code of
1293   <x:ref>400 (Bad Request)</x:ref>. A proxy &MUST; remove any such whitespace
1294   from a response message before forwarding the message downstream.
1295</t>
1296<t>
1297   A field value is preceded by optional whitespace (OWS); a single SP is
1298   preferred. The field value does not include any leading or trailing white
1299   space: OWS occurring before the first non-whitespace octet of the
1300   field value or after the last non-whitespace octet of the field value
1301   is ignored and &SHOULD; be removed before further processing (as this does
1302   not change the meaning of the header field).
1303</t>
1304<t>
1305   Historically, HTTP header field values could be extended over multiple
1306   lines by preceding each extra line with at least one space or horizontal
1307   tab (obs-fold). This specification deprecates such line folding except
1308   within the message/http media type
1309   (<xref target="internet.media.type.message.http"/>).
1310   Senders &MUST-NOT; generate messages that include line folding
1311   (i.e., that contain any field-value that contains a match to the
1312   <x:ref>obs-fold</x:ref> rule) unless the message is intended for packaging
1313   within the message/http media type. When an <x:ref>obs-fold</x:ref> is
1314   received in a message, recipients &MUST; do one of:
1315   <list style="symbols">
1316      <t>accept the message and replace any embedded <x:ref>obs-fold</x:ref>
1317         whitespace with either a single <x:ref>SP</x:ref> or a matching
1318         number of <x:ref>SP</x:ref> octets (to avoid buffer copying) prior to
1319         interpreting the field value or forwarding the message
1320         downstream;</t>
1321
1322      <t>if it is a request, reject the message by sending a
1323         <x:ref>400 (Bad Request)</x:ref> response with a representation
1324         explaining that obsolete line folding is unacceptable; or,</t>
1325         
1326      <t>if it is a response, discard the message and generate a
1327         <x:ref>502 (Bad Gateway)</x:ref> response with a representation
1328         explaining that unacceptable line folding was received.</t>
1329   </list>
1330   Recipients that choose not to implement <x:ref>obs-fold</x:ref> processing
1331   (as described above) &MUST-NOT; accept messages containing header fields
1332   with leading whitespace, as this can expose them to attacks that exploit
1333   this difference in processing.
1334</t>
1335<t>
1336   Historically, HTTP has allowed field content with text in the ISO-8859-1
1337   <xref target="ISO-8859-1"/> charset, supporting other charsets only
1338   through use of <xref target="RFC2047"/> encoding.
1339   In practice, most HTTP header field values use only a subset of the
1340   US-ASCII charset <xref target="USASCII"/>. Newly defined
1341   header fields &SHOULD; limit their field values to US-ASCII octets.
1342   Recipients &SHOULD; treat other octets in field content (obs-text) as
1343   opaque data.
1344</t>
1345</section>
1346
1347<section title="Field Limits" anchor="field.limits">
1348<t>
1349   HTTP does not place a pre-defined limit on the length of each header field
1350   or on the length of the header block as a whole.  Various ad-hoc
1351   limitations on individual header field length are found in practice,
1352   often depending on the specific field semantics.
1353</t>
1354<t>
1355   A server &MUST; be prepared to receive request header fields of unbounded
1356   length and respond with an appropriate <x:ref>4xx (Client Error)</x:ref>
1357   status code if the received header field(s) are larger than the server
1358   wishes to process.
1359</t>
1360<t>
1361   A client &MUST; be prepared to receive response header fields of unbounded
1362   length. A client &MAY; discard or truncate received header fields that are
1363   larger than the client wishes to process if the field semantics are such
1364   that the dropped value(s) can be safely ignored without changing the
1365   response semantics.
1366</t>
1367</section>
1368
1369<section title="Field value components" anchor="field.components">
1370<t anchor="rule.token.separators">
1371  <x:anchor-alias value="tchar"/>
1372  <x:anchor-alias value="token"/>
1373  <x:anchor-alias value="special"/>
1374  <x:anchor-alias value="word"/>
1375   Many HTTP header field values consist of words (token or quoted-string)
1376   separated by whitespace or special characters. These special characters
1377   &MUST; be in a quoted string to be used within a parameter value (as defined
1378   in <xref target="transfer.codings"/>).
1379</t>
1380<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>
1381  <x:ref>word</x:ref>           = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1382
1383  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1384<!--
1385  IMPORTANT: when editing "tchar" make sure that "special" is updated accordingly!!!
1386 -->
1387  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1388                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1389                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1390                 ; any <x:ref>VCHAR</x:ref>, except <x:ref>special</x:ref>
1391
1392  <x:ref>special</x:ref>        = "(" / ")" / "&lt;" / ">" / "@" / ","
1393                 / ";" / ":" / "\" / DQUOTE / "/" / "["
1394                 / "]" / "?" / "=" / "{" / "}"
1395</artwork></figure>
1396<t anchor="rule.quoted-string">
1397  <x:anchor-alias value="quoted-string"/>
1398  <x:anchor-alias value="qdtext"/>
1399  <x:anchor-alias value="obs-text"/>
1400   A string of text is parsed as a single word if it is quoted using
1401   double-quote marks.
1402</t>
1403<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"/>
1404  <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>
1405  <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>
1406  <x:ref>obs-text</x:ref>       = %x80-FF
1407</artwork></figure>
1408<t anchor="rule.quoted-pair">
1409  <x:anchor-alias value="quoted-pair"/>
1410   The backslash octet ("\") can be used as a single-octet
1411   quoting mechanism within quoted-string constructs:
1412</t>
1413<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1414  <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> )
1415</artwork></figure>
1416<t>
1417   Recipients that process the value of a quoted-string &MUST; handle a
1418   quoted-pair as if it were replaced by the octet following the backslash.
1419</t>
1420<t>
1421   Senders &SHOULD-NOT; generate a quoted-pair in a quoted-string except where
1422   necessary to quote DQUOTE and backslash octets occurring within that string.
1423</t>
1424<t anchor="rule.comment">
1425  <x:anchor-alias value="comment"/>
1426  <x:anchor-alias value="ctext"/>
1427   Comments can be included in some HTTP header fields by surrounding
1428   the comment text with parentheses. Comments are only allowed in
1429   fields containing "comment" as part of their field value definition.
1430</t>
1431<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1432  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-cpair</x:ref> / <x:ref>comment</x:ref> ) ")"
1433  <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>
1434</artwork></figure>
1435<t anchor="rule.quoted-cpair">
1436  <x:anchor-alias value="quoted-cpair"/>
1437   The backslash octet ("\") can be used as a single-octet
1438   quoting mechanism within comment constructs:
1439</t>
1440<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-cpair"/>
1441  <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> )
1442</artwork></figure>
1443<t>
1444   Senders &SHOULD-NOT; escape octets in comments that do not require escaping
1445   (i.e., other than the backslash octet "\" and the parentheses "(" and ")").
1446</t>
1447</section>
1448
1449</section>
1450
1451<section title="Message Body" anchor="message.body">
1452  <x:anchor-alias value="message-body"/>
1453<t>
1454   The message body (if any) of an HTTP message is used to carry the
1455   payload body of that request or response.  The message body is
1456   identical to the payload body unless a transfer coding has been
1457   applied, as described in <xref target="header.transfer-encoding"/>.
1458</t>
1459<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1460  <x:ref>message-body</x:ref> = *OCTET
1461</artwork></figure>
1462<t>
1463   The rules for when a message body is allowed in a message differ for
1464   requests and responses.
1465</t>
1466<t>
1467   The presence of a message body in a request is signaled by a
1468   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1469   field. Request message framing is independent of method semantics,
1470   even if the method does not define any use for a message body.
1471</t>
1472<t>
1473   The presence of a message body in a response depends on both
1474   the request method to which it is responding and the response
1475   status code (<xref target="status.line"/>).
1476   Responses to the HEAD request method never include a message body
1477   because the associated response header fields (e.g.,
1478   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1479   if present, indicate only what their values would have been if the request
1480   method had been GET (&HEAD;).
1481   <x:ref>2xx (Successful)</x:ref> responses to CONNECT switch to tunnel
1482   mode instead of having a message body (&CONNECT;).
1483   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1484   <x:ref>304 (Not Modified)</x:ref> responses do not include a message body.
1485   All other responses do include a message body, although the body
1486   might be of zero length.
1487</t>
1488
1489<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1490  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1491  <iref item="chunked (Coding Format)"/>
1492  <x:anchor-alias value="Transfer-Encoding"/>
1493<t>
1494   The Transfer-Encoding header field lists the transfer coding names
1495   corresponding to the sequence of transfer codings that have been
1496   (or will be) applied to the payload body in order to form the message body.
1497   Transfer codings are defined in <xref target="transfer.codings"/>.
1498</t>
1499<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1500  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1501</artwork></figure>
1502<t>
1503   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1504   MIME, which was designed to enable safe transport of binary data over a
1505   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1506   However, safe transport has a different focus for an 8bit-clean transfer
1507   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1508   accurately delimit a dynamically generated payload and to distinguish
1509   payload encodings that are only applied for transport efficiency or
1510   security from those that are characteristics of the selected resource.
1511</t>
1512<t>
1513   All HTTP/1.1 recipients &MUST; implement the chunked transfer coding
1514   (<xref target="chunked.encoding"/>) because it plays a crucial role in
1515   framing messages when the payload body size is not known in advance.
1516   If chunked is applied to a payload body, the sender &MUST-NOT; apply
1517   chunked more than once (i.e., chunking an already chunked message is not
1518   allowed).
1519   If any transfer coding is applied to a request payload body, the
1520   sender &MUST; apply chunked as the final transfer coding to ensure that
1521   the message is properly framed.
1522   If any transfer coding is applied to a response payload body, the
1523   sender &MUST; either apply chunked as the final transfer coding or
1524   terminate the message by closing the connection.
1525</t>
1526<figure><preamble>
1527   For example,
1528</preamble><artwork type="example">
1529  Transfer-Encoding: gzip, chunked
1530</artwork><postamble>
1531   indicates that the payload body has been compressed using the gzip
1532   coding and then chunked using the chunked coding while forming the
1533   message body.
1534</postamble></figure>
1535<t>
1536   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1537   Transfer-Encoding is a property of the message, not of the representation, and
1538   any recipient along the request/response chain &MAY; decode the received
1539   transfer coding(s) or apply additional transfer coding(s) to the message
1540   body, assuming that corresponding changes are made to the Transfer-Encoding
1541   field-value. Additional information about the encoding parameters &MAY; be
1542   provided by other header fields not defined by this specification.
1543</t>
1544<t>
1545   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1546   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1547   neither of which includes a message body,
1548   to indicate that the origin server would have applied a transfer coding
1549   to the message body if the request had been an unconditional GET.
1550   This indication is not required, however, because any recipient on
1551   the response chain (including the origin server) can remove transfer
1552   codings when they are not needed.
1553</t>
1554<t>
1555   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1556   implementations advertising only HTTP/1.0 support will not understand
1557   how to process a transfer-encoded payload.
1558   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1559   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1560   might be in the form of specific user configuration or by remembering the
1561   version of a prior received response.
1562   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1563   the corresponding request indicates HTTP/1.1 (or later).
1564</t>
1565<t>
1566   A server that receives a request message with a transfer coding it does
1567   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref>.
1568</t>
1569</section>
1570
1571<section title="Content-Length" anchor="header.content-length">
1572  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1573  <x:anchor-alias value="Content-Length"/>
1574<t>
1575   When a message does not have a <x:ref>Transfer-Encoding</x:ref> header
1576   field, a Content-Length header field can provide the anticipated size,
1577   as a decimal number of octets, for a potential payload body.
1578   For messages that do include a payload body, the Content-Length field-value
1579   provides the framing information necessary for determining where the body
1580   (and message) ends.  For messages that do not include a payload body, the
1581   Content-Length indicates the size of the selected representation
1582   (&representation;).
1583</t>
1584<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1585  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1586</artwork></figure>
1587<t>
1588   An example is
1589</t>
1590<figure><artwork type="example">
1591  Content-Length: 3495
1592</artwork></figure>
1593<t>
1594   A sender &MUST-NOT; send a Content-Length header field in any message that
1595   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1596</t>
1597<t>
1598   A user agent &SHOULD; send a Content-Length in a request message when no
1599   <x:ref>Transfer-Encoding</x:ref> is sent and the request method defines
1600   a meaning for an enclosed payload body. For example, a Content-Length
1601   header field is normally sent in a POST request even when the value is
1602   0 (indicating an empty payload body).  A user agent &SHOULD-NOT; send a
1603   Content-Length header field when the request message does not contain a
1604   payload body and the method semantics do not anticipate such a body.
1605</t>
1606<t>
1607   A server &MAY; send a Content-Length header field in a response to a HEAD
1608   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1609   response unless its field-value equals the decimal number of octets that
1610   would have been sent in the payload body of a response if the same
1611   request had used the GET method.
1612</t>
1613<t>
1614   A server &MAY; send a Content-Length header field in a
1615   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1616   (&status-304;); a server &MUST-NOT; send Content-Length in such a
1617   response unless its field-value equals the decimal number of octets that
1618   would have been sent in the payload body of a <x:ref>200 (OK)</x:ref>
1619   response to the same request.
1620</t>
1621<t>
1622   A server &MUST-NOT; send a Content-Length header field in any response
1623   with a status code of
1624   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1625   A server &SHOULD-NOT; send a Content-Length header field in any
1626   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1627</t>
1628<t>
1629   Aside from the cases defined above, in the absence of Transfer-Encoding,
1630   an origin server &SHOULD; send a Content-Length header field when the
1631   payload body size is known prior to sending the complete header block.
1632   This will allow downstream recipients to measure transfer progress,
1633   know when a received message is complete, and potentially reuse the
1634   connection for additional requests.
1635</t>
1636<t>
1637   Any Content-Length field value greater than or equal to zero is valid.
1638   Since there is no predefined limit to the length of a payload,
1639   recipients &SHOULD; anticipate potentially large decimal numerals and
1640   prevent parsing errors due to integer conversion overflows
1641   (<xref target="attack.protocol.element.size.overflows"/>).
1642</t>
1643<t>
1644   If a message is received that has multiple Content-Length header fields
1645   with field-values consisting of the same decimal value, or a single
1646   Content-Length header field with a field value containing a list of
1647   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1648   duplicate Content-Length header fields have been generated or combined by an
1649   upstream message processor, then the recipient &MUST; either reject the
1650   message as invalid or replace the duplicated field-values with a single
1651   valid Content-Length field containing that decimal value prior to
1652   determining the message body length.
1653</t>
1654<x:note>
1655  <t>
1656   &Note; HTTP's use of Content-Length for message framing differs
1657   significantly from the same field's use in MIME, where it is an optional
1658   field used only within the "message/external-body" media-type.
1659  </t>
1660</x:note>
1661</section>
1662
1663<section title="Message Body Length" anchor="message.body.length">
1664  <iref item="chunked (Coding Format)"/>
1665<t>
1666   The length of a message body is determined by one of the following
1667   (in order of precedence):
1668</t>
1669<t>
1670  <list style="numbers">
1671    <x:lt><t>
1672     Any response to a HEAD request and any response with a
1673     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1674     <x:ref>304 (Not Modified)</x:ref> status code is always
1675     terminated by the first empty line after the header fields, regardless of
1676     the header fields present in the message, and thus cannot contain a
1677     message body.
1678    </t></x:lt>
1679    <x:lt><t>
1680     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1681     connection will become a tunnel immediately after the empty line that
1682     concludes the header fields.  A client &MUST; ignore any
1683     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1684     fields received in such a message.
1685    </t></x:lt>
1686    <x:lt><t>
1687     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1688     and the chunked transfer coding (<xref target="chunked.encoding"/>)
1689     is the final encoding, the message body length is determined by reading
1690     and decoding the chunked data until the transfer coding indicates the
1691     data is complete.
1692    </t>
1693    <t>
1694     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1695     response and the chunked transfer coding is not the final encoding, the
1696     message body length is determined by reading the connection until it is
1697     closed by the server.
1698     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a request and the
1699     chunked transfer coding is not the final encoding, the message body
1700     length cannot be determined reliably; the server &MUST; respond with
1701     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1702    </t>
1703    <t>
1704     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1705     and a <x:ref>Content-Length</x:ref> header field, the Transfer-Encoding
1706     overrides the Content-Length. Such a message might indicate an attempt
1707     to perform request or response smuggling (bypass of security-related
1708     checks on message routing or content) and thus ought to be handled as
1709     an error.  A sender &MUST; remove the received Content-Length field
1710     prior to forwarding such a message downstream.
1711    </t></x:lt>
1712    <x:lt><t>
1713     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1714     either multiple <x:ref>Content-Length</x:ref> header fields having
1715     differing field-values or a single Content-Length header field having an
1716     invalid value, then the message framing is invalid and &MUST; be treated
1717     as an error to prevent request or response smuggling.
1718     If this is a request message, the server &MUST; respond with
1719     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1720     If this is a response message received by a proxy, the proxy
1721     &MUST; discard the received response, send a <x:ref>502 (Bad Gateway)</x:ref>
1722     status code as its downstream response, and then close the connection.
1723     If this is a response message received by a user agent, it &MUST; be
1724     treated as an error by discarding the message and closing the connection.
1725    </t></x:lt>
1726    <x:lt><t>
1727     If a valid <x:ref>Content-Length</x:ref> header field is present without
1728     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1729     expected message body length in octets.
1730     If the sender closes the connection or the recipient times out before the
1731     indicated number of octets are received, the recipient &MUST; consider
1732     the message to be incomplete and close the connection.
1733    </t></x:lt>
1734    <x:lt><t>
1735     If this is a request message and none of the above are true, then the
1736     message body length is zero (no message body is present).
1737    </t></x:lt>
1738    <x:lt><t>
1739     Otherwise, this is a response message without a declared message body
1740     length, so the message body length is determined by the number of octets
1741     received prior to the server closing the connection.
1742    </t></x:lt>
1743  </list>
1744</t>
1745<t>
1746   Since there is no way to distinguish a successfully completed,
1747   close-delimited message from a partially-received message interrupted
1748   by network failure, a server &SHOULD; use encoding or
1749   length-delimited messages whenever possible.  The close-delimiting
1750   feature exists primarily for backwards compatibility with HTTP/1.0.
1751</t>
1752<t>
1753   A server &MAY; reject a request that contains a message body but
1754   not a <x:ref>Content-Length</x:ref> by responding with
1755   <x:ref>411 (Length Required)</x:ref>.
1756</t>
1757<t>
1758   Unless a transfer coding other than chunked has been applied,
1759   a client that sends a request containing a message body &SHOULD;
1760   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1761   length is known in advance, rather than the chunked transfer coding, since some
1762   existing services respond to chunked with a <x:ref>411 (Length Required)</x:ref>
1763   status code even though they understand the chunked transfer coding.  This
1764   is typically because such services are implemented via a gateway that
1765   requires a content-length in advance of being called and the server
1766   is unable or unwilling to buffer the entire request before processing.
1767</t>
1768<t>
1769   A user agent that sends a request containing a message body &MUST; send a
1770   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1771   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1772   the form of specific user configuration or by remembering the version of a
1773   prior received response.
1774</t>
1775<t>
1776   If the final response to the last request on a connection has been
1777   completely received and there remains additional data to read, a user agent
1778   &MAY; discard the remaining data or attempt to determine if that data
1779   belongs as part of the prior response body, which might be the case if the
1780   prior message's Content-Length value is incorrect. A client &MUST-NOT;
1781   process, cache, or forward such extra data as a separate response, since
1782   such behavior would be vulnerable to cache poisoning.
1783</t>
1784</section>
1785</section>
1786
1787<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1788<t>
1789   A server that receives an incomplete request message, usually due to a
1790   canceled request or a triggered time-out exception, &MAY; send an error
1791   response prior to closing the connection.
1792</t>
1793<t>
1794   A client that receives an incomplete response message, which can occur
1795   when a connection is closed prematurely or when decoding a supposedly
1796   chunked transfer coding fails, &MUST; record the message as incomplete.
1797   Cache requirements for incomplete responses are defined in
1798   &cache-incomplete;.
1799</t>
1800<t>
1801   If a response terminates in the middle of the header block (before the
1802   empty line is received) and the status code might rely on header fields to
1803   convey the full meaning of the response, then the client cannot assume
1804   that meaning has been conveyed; the client might need to repeat the
1805   request in order to determine what action to take next.
1806</t>
1807<t>
1808   A message body that uses the chunked transfer coding is
1809   incomplete if the zero-sized chunk that terminates the encoding has not
1810   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1811   incomplete if the size of the message body received (in octets) is less than
1812   the value given by Content-Length.  A response that has neither chunked
1813   transfer coding nor Content-Length is terminated by closure of the
1814   connection, and thus is considered complete regardless of the number of
1815   message body octets received, provided that the header block was received
1816   intact.
1817</t>
1818</section>
1819
1820<section title="Message Parsing Robustness" anchor="message.robustness">
1821<t>
1822   Older HTTP/1.0 user agent implementations might send an extra CRLF
1823   after a POST request as a lame workaround for some early server
1824   applications that failed to read message body content that was
1825   not terminated by a line-ending. An HTTP/1.1 user agent &MUST-NOT;
1826   preface or follow a request with an extra CRLF.  If terminating
1827   the request message body with a line-ending is desired, then the
1828   user agent &MUST; count the terminating CRLF octets as part of the
1829   message body length.
1830</t>
1831<t>
1832   In the interest of robustness, servers &SHOULD; ignore at least one
1833   empty line received where a request-line is expected. In other words, if
1834   a server is reading the protocol stream at the beginning of a
1835   message and receives a CRLF first, the server &SHOULD; ignore the CRLF.
1836</t>
1837<t>
1838   Although the line terminator for the start-line and header
1839   fields is the sequence CRLF, recipients &MAY; recognize a
1840   single LF as a line terminator and ignore any preceding CR.
1841</t>
1842<t>
1843   Although the request-line and status-line grammar rules require that each
1844   of the component elements be separated by a single SP octet, recipients
1845   &MAY; instead parse on whitespace-delimited word boundaries and, aside
1846   from the CRLF terminator, treat any form of whitespace as the SP separator
1847   while ignoring preceding or trailing whitespace;
1848   such whitespace includes one or more of the following octets:
1849   SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
1850</t>
1851<t>
1852   When a server listening only for HTTP request messages, or processing
1853   what appears from the start-line to be an HTTP request message,
1854   receives a sequence of octets that does not match the HTTP-message
1855   grammar aside from the robustness exceptions listed above, the
1856   server &SHOULD; respond with a <x:ref>400 (Bad Request)</x:ref> response. 
1857</t>
1858</section>
1859</section>
1860
1861<section title="Transfer Codings" anchor="transfer.codings">
1862  <x:anchor-alias value="transfer-coding"/>
1863  <x:anchor-alias value="transfer-extension"/>
1864<t>
1865   Transfer coding names are used to indicate an encoding
1866   transformation that has been, can be, or might need to be applied to a
1867   payload body in order to ensure "safe transport" through the network.
1868   This differs from a content coding in that the transfer coding is a
1869   property of the message rather than a property of the representation
1870   that is being transferred.
1871</t>
1872<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1873  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1874                     / "compress" ; <xref target="compress.coding"/>
1875                     / "deflate" ; <xref target="deflate.coding"/>
1876                     / "gzip" ; <xref target="gzip.coding"/>
1877                     / <x:ref>transfer-extension</x:ref>
1878  <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> )
1879</artwork></figure>
1880<t anchor="rule.parameter">
1881  <x:anchor-alias value="attribute"/>
1882  <x:anchor-alias value="transfer-parameter"/>
1883  <x:anchor-alias value="value"/>
1884   Parameters are in the form of attribute/value pairs.
1885</t>
1886<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"/>
1887  <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>
1888  <x:ref>attribute</x:ref>          = <x:ref>token</x:ref>
1889  <x:ref>value</x:ref>              = <x:ref>word</x:ref>
1890</artwork></figure>
1891<t>
1892   All transfer-coding names are case-insensitive and ought to be registered
1893   within the HTTP Transfer Coding registry, as defined in
1894   <xref target="transfer.coding.registry"/>.
1895   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1896   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1897   header fields.
1898</t>
1899
1900<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1901  <iref primary="true" item="chunked (Coding Format)"/>
1902  <x:anchor-alias value="chunk"/>
1903  <x:anchor-alias value="chunked-body"/>
1904  <x:anchor-alias value="chunk-data"/>
1905  <x:anchor-alias value="chunk-ext"/>
1906  <x:anchor-alias value="chunk-ext-name"/>
1907  <x:anchor-alias value="chunk-ext-val"/>
1908  <x:anchor-alias value="chunk-size"/>
1909  <x:anchor-alias value="last-chunk"/>
1910  <x:anchor-alias value="trailer-part"/>
1911  <x:anchor-alias value="quoted-str-nf"/>
1912  <x:anchor-alias value="qdtext-nf"/>
1913<t>
1914   The chunked transfer coding modifies the body of a message in order to
1915   transfer it as a series of chunks, each with its own size indicator,
1916   followed by an &OPTIONAL; trailer containing header fields. This
1917   allows dynamically generated content to be transferred along with the
1918   information necessary for the recipient to verify that it has
1919   received the full message.
1920</t>
1921<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"/>
1922  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1923                   <x:ref>last-chunk</x:ref>
1924                   <x:ref>trailer-part</x:ref>
1925                   <x:ref>CRLF</x:ref>
1926 
1927  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1928                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1929  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1930  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1931 
1932  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
1933  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
1934  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-str-nf</x:ref>
1935  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1936  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1937 
1938  <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>
1939                 ; like <x:ref>quoted-string</x:ref>, but disallowing line folding
1940  <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>
1941</artwork></figure>
1942<t>
1943   Chunk extensions within the chunked transfer coding are deprecated.
1944   Senders &SHOULD-NOT; send chunk-ext.
1945   Definition of new chunk extensions is discouraged.
1946</t>
1947<t>
1948   The chunk-size field is a string of hex digits indicating the size of
1949   the chunk-data in octets. The chunked transfer coding is complete when a
1950   chunk with a chunk-size of zero is received, possibly followed by a
1951   trailer, and finally terminated by an empty line.
1952</t>
1953
1954<section title="Trailer" anchor="header.trailer">
1955  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
1956  <x:anchor-alias value="Trailer"/>
1957<t>
1958   A trailer allows the sender to include additional fields at the end of a
1959   chunked message in order to supply metadata that might be dynamically
1960   generated while the message body is sent, such as a message integrity
1961   check, digital signature, or post-processing status.
1962   The trailer &MUST-NOT; contain fields that need to be known before a
1963   recipient processes the body, such as <x:ref>Transfer-Encoding</x:ref>,
1964   <x:ref>Content-Length</x:ref>, and <x:ref>Trailer</x:ref>.
1965</t>
1966<t>
1967   When a message includes a message body encoded with the chunked
1968   transfer coding and the sender desires to send metadata in the form of
1969   trailer fields at the end of the message, the sender &SHOULD; send a
1970   <x:ref>Trailer</x:ref> header field before the message body to indicate
1971   which fields will be present in the trailers. This allows the recipient
1972   to prepare for receipt of that metadata before it starts processing the body,
1973   which is useful if the message is being streamed and the recipient wishes
1974   to confirm an integrity check on the fly.
1975</t>
1976<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
1977  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
1978</artwork></figure>
1979<t>
1980   If no <x:ref>Trailer</x:ref> header field is present, the sender of a
1981   chunked message body &SHOULD; send an empty trailer.
1982</t>
1983<t>
1984   A server &MUST; send an empty trailer with the chunked transfer coding
1985   unless at least one of the following is true:
1986  <list style="numbers">
1987    <t>the request included a <x:ref>TE</x:ref> header field that indicates
1988    "trailers" is acceptable in the transfer coding of the response, as
1989    described in <xref target="header.te"/>; or,</t>
1990     
1991    <t>the trailer fields consist entirely of optional metadata and the
1992    recipient could use the message (in a manner acceptable to the server where
1993    the field originated) without receiving that metadata. In other words,
1994    the server that generated the header field is willing to accept the
1995    possibility that the trailer fields might be silently discarded along
1996    the path to the client.</t>
1997  </list>
1998</t>
1999<t>
2000   The above requirement prevents the need for an infinite buffer when a
2001   message is being received by an HTTP/1.1 (or later) proxy and forwarded to
2002   an HTTP/1.0 recipient.
2003</t>
2004</section>
2005
2006<section title="Decoding chunked" anchor="decoding.chunked">
2007<t>
2008   A process for decoding the chunked transfer coding
2009   can be represented in pseudo-code as:
2010</t>
2011<figure><artwork type="code">
2012  length := 0
2013  read chunk-size, chunk-ext (if any) and CRLF
2014  while (chunk-size &gt; 0) {
2015     read chunk-data and CRLF
2016     append chunk-data to decoded-body
2017     length := length + chunk-size
2018     read chunk-size and CRLF
2019  }
2020  read header-field
2021  while (header-field not empty) {
2022     append header-field to existing header fields
2023     read header-field
2024  }
2025  Content-Length := length
2026  Remove "chunked" from Transfer-Encoding
2027  Remove Trailer from existing header fields
2028</artwork></figure>
2029<t>
2030   All recipients &MUST; be able to receive and decode the
2031   chunked transfer coding and &MUST; ignore chunk-ext extensions
2032   they do not understand.
2033</t>
2034</section>
2035</section>
2036
2037<section title="Compression Codings" anchor="compression.codings">
2038<t>
2039   The codings defined below can be used to compress the payload of a
2040   message.
2041</t>
2042
2043<section title="Compress Coding" anchor="compress.coding">
2044<iref item="compress (Coding Format)"/>
2045<t>
2046   The "compress" format is produced by the common UNIX file compression
2047   program "compress". This format is an adaptive Lempel-Ziv-Welch
2048   coding (LZW). Recipients &SHOULD; consider "x-compress" to be
2049   equivalent to "compress".
2050</t>
2051</section>
2052
2053<section title="Deflate Coding" anchor="deflate.coding">
2054<iref item="deflate (Coding Format)"/>
2055<t>
2056   The "deflate" format is defined as the "deflate" compression mechanism
2057   (described in <xref target="RFC1951"/>) used inside the "zlib"
2058   data format (<xref target="RFC1950"/>).
2059</t>
2060<x:note>
2061  <t>
2062    &Note; Some incorrect implementations send the "deflate"
2063    compressed data without the zlib wrapper.
2064   </t>
2065</x:note>
2066</section>
2067
2068<section title="Gzip Coding" anchor="gzip.coding">
2069<iref item="gzip (Coding Format)"/>
2070<t>
2071   The "gzip" format is produced by the file compression program
2072   "gzip" (GNU zip), as described in <xref target="RFC1952"/>. This format is a
2073   Lempel-Ziv coding (LZ77) with a 32 bit CRC.
2074   Recipients &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2075</t>
2076</section>
2077
2078</section>
2079
2080<section title="TE" anchor="header.te">
2081  <iref primary="true" item="TE header field" x:for-anchor=""/>
2082  <x:anchor-alias value="TE"/>
2083  <x:anchor-alias value="t-codings"/>
2084  <x:anchor-alias value="t-ranking"/>
2085  <x:anchor-alias value="rank"/>
2086<t>
2087   The "TE" header field in a request indicates what transfer codings,
2088   besides chunked, the client is willing to accept in response, and
2089   whether or not the client is willing to accept trailer fields in a
2090   chunked transfer coding.
2091</t>
2092<t>
2093   The TE field-value consists of a comma-separated list of transfer coding
2094   names, each allowing for optional parameters (as described in
2095   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2096   Clients &MUST-NOT; send the chunked transfer coding name in TE;
2097   chunked is always acceptable for HTTP/1.1 recipients.
2098</t>
2099<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"/>
2100  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2101  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2102  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2103  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2104             / ( "1" [ "." 0*3("0") ] )
2105</artwork></figure>
2106<t>
2107   Three examples of TE use are below.
2108</t>
2109<figure><artwork type="example">
2110  TE: deflate
2111  TE:
2112  TE: trailers, deflate;q=0.5
2113</artwork></figure>
2114<t>
2115   The presence of the keyword "trailers" indicates that the client is
2116   willing to accept trailer fields in a chunked transfer coding,
2117   as defined in <xref target="chunked.encoding"/>, on behalf of itself and
2118   any downstream clients. For chained requests, this implies that either:
2119   (a) all downstream clients are willing to accept trailer fields in the
2120   forwarded response; or,
2121   (b) the client will attempt to buffer the response on behalf of downstream
2122   recipients.
2123   Note that HTTP/1.1 does not define any means to limit the size of a
2124   chunked response such that a client can be assured of buffering the
2125   entire response.
2126</t>
2127<t>
2128   When multiple transfer codings are acceptable, the client &MAY; rank the
2129   codings by preference using a case-insensitive "q" parameter (similar to
2130   the qvalues used in content negotiation fields, &qvalue;). The rank value
2131   is a real number in the range 0 through 1, where 0.001 is the least
2132   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2133</t>
2134<t>
2135   If the TE field-value is empty or if no TE field is present, the only
2136   acceptable transfer coding is chunked. A message with no transfer coding
2137   is always acceptable.
2138</t>
2139<t>
2140   Since the TE header field only applies to the immediate connection,
2141   a sender of TE &MUST; also send a "TE" connection option within the
2142   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2143   in order to prevent the TE field from being forwarded by intermediaries
2144   that do not support its semantics.
2145</t>
2146</section>
2147</section>
2148
2149<section title="Message Routing" anchor="message.routing">
2150<t>
2151   HTTP request message routing is determined by each client based on the
2152   target resource, the client's proxy configuration, and
2153   establishment or reuse of an inbound connection.  The corresponding
2154   response routing follows the same connection chain back to the client.
2155</t>
2156
2157<section title="Identifying a Target Resource" anchor="target-resource">
2158  <iref primary="true" item="target resource"/>
2159  <iref primary="true" item="target URI"/>
2160  <x:anchor-alias value="target resource"/>
2161  <x:anchor-alias value="target URI"/>
2162<t>
2163   HTTP is used in a wide variety of applications, ranging from
2164   general-purpose computers to home appliances.  In some cases,
2165   communication options are hard-coded in a client's configuration.
2166   However, most HTTP clients rely on the same resource identification
2167   mechanism and configuration techniques as general-purpose Web browsers.
2168</t>
2169<t>
2170   HTTP communication is initiated by a user agent for some purpose.
2171   The purpose is a combination of request semantics, which are defined in
2172   <xref target="Part2"/>, and a target resource upon which to apply those
2173   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2174   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2175   would resolve to its absolute form in order to obtain the
2176   "<x:dfn>target URI</x:dfn>".  The target URI
2177   excludes the reference's fragment identifier component, if any,
2178   since fragment identifiers are reserved for client-side processing
2179   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2180</t>
2181</section>
2182
2183<section title="Connecting Inbound" anchor="connecting.inbound">
2184<t>
2185   Once the target URI is determined, a client needs to decide whether
2186   a network request is necessary to accomplish the desired semantics and,
2187   if so, where that request is to be directed.
2188</t>
2189<t>
2190   If the client has a response cache and the request semantics can be
2191   satisfied by a cache (<xref target="Part6"/>), then the request is
2192   usually directed to the cache first.
2193</t>
2194<t>
2195   If the request is not satisfied by a cache, then a typical client will
2196   check its configuration to determine whether a proxy is to be used to
2197   satisfy the request.  Proxy configuration is implementation-dependent,
2198   but is often based on URI prefix matching, selective authority matching,
2199   or both, and the proxy itself is usually identified by an "http" or
2200   "https" URI.  If a proxy is applicable, the client connects inbound by
2201   establishing (or reusing) a connection to that proxy.
2202</t>
2203<t>
2204   If no proxy is applicable, a typical client will invoke a handler routine,
2205   usually specific to the target URI's scheme, to connect directly
2206   to an authority for the target resource.  How that is accomplished is
2207   dependent on the target URI scheme and defined by its associated
2208   specification, similar to how this specification defines origin server
2209   access for resolution of the "http" (<xref target="http.uri"/>) and
2210   "https" (<xref target="https.uri"/>) schemes.
2211</t>
2212<t>
2213   HTTP requirements regarding connection management are defined in
2214   <xref target="connection.management"/>.
2215</t>
2216</section>
2217
2218<section title="Request Target" anchor="request-target">
2219<t>
2220   Once an inbound connection is obtained,
2221   the client sends an HTTP request message (<xref target="http.message"/>)
2222   with a request-target derived from the target URI.
2223   There are four distinct formats for the request-target, depending on both
2224   the method being requested and whether the request is to a proxy.
2225</t>   
2226<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"/>
2227  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2228                 / <x:ref>absolute-form</x:ref>
2229                 / <x:ref>authority-form</x:ref>
2230                 / <x:ref>asterisk-form</x:ref>
2231
2232  <x:ref>origin-form</x:ref>    = <x:ref>path-absolute</x:ref> [ "?" <x:ref>query</x:ref> ]
2233  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2234  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2235  <x:ref>asterisk-form</x:ref>  = "*"
2236</artwork></figure>
2237<t anchor="origin-form"><iref item="origin-form (of request-target)"/>
2238   The most common form of request-target is the origin-form.
2239   When making a request directly to an origin server, other than a CONNECT
2240   or server-wide OPTIONS request (as detailed below),
2241   a client &MUST; send only the absolute path and query components of
2242   the target URI as the request-target.
2243   If the target URI's path component is empty, then the client &MUST; send
2244   "/" as the path within the origin-form of request-target.
2245   A <x:ref>Host</x:ref> header field is also sent, as defined in
2246   <xref target="header.host"/>, containing the target URI's
2247   authority component (excluding any userinfo).
2248</t>
2249<t>
2250   For example, a client wishing to retrieve a representation of the resource
2251   identified as
2252</t>
2253<figure><artwork x:indent-with="  " type="example">
2254http://www.example.org/where?q=now
2255</artwork></figure>
2256<t>
2257   directly from the origin server would open (or reuse) a TCP connection
2258   to port 80 of the host "www.example.org" and send the lines:
2259</t>
2260<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2261GET /where?q=now HTTP/1.1
2262Host: www.example.org
2263</artwork></figure>
2264<t>
2265   followed by the remainder of the request message.
2266</t>
2267<t anchor="absolute-form"><iref item="absolute-form (of request-target)"/>
2268   When making a request to a proxy, other than a CONNECT or server-wide
2269   OPTIONS request (as detailed below), a client &MUST; send the target URI
2270   in absolute-form as the request-target.
2271   The proxy is requested to either service that request from a valid cache,
2272   if possible, or make the same request on the client's behalf to either
2273   the next inbound proxy server or directly to the origin server indicated
2274   by the request-target.  Requirements on such "forwarding" of messages are
2275   defined in <xref target="message.forwarding"/>.
2276</t>
2277<t>
2278   An example absolute-form of request-line would be:
2279</t>
2280<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2281GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
2282</artwork></figure>
2283<t>
2284   To allow for transition to the absolute-form for all requests in some
2285   future version of HTTP, HTTP/1.1 servers &MUST; accept the absolute-form
2286   in requests, even though HTTP/1.1 clients will only send them in requests
2287   to proxies.
2288</t>
2289<t anchor="authority-form"><iref item="authority-form (of request-target)"/>
2290   The authority-form of request-target is only used for CONNECT requests
2291   (&CONNECT;).  When making a CONNECT request to establish a tunnel through
2292   one or more proxies, a client &MUST; send only the target URI's
2293   authority component (excluding any userinfo) as the request-target.
2294   For example,
2295</t>
2296<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2297CONNECT www.example.com:80 HTTP/1.1
2298</artwork></figure>
2299<t anchor="asterisk-form"><iref item="asterisk-form (of request-target)"/>
2300   The asterisk-form of request-target is only used for a server-wide
2301   OPTIONS request (&OPTIONS;).  When a client wishes to request OPTIONS
2302   for the server as a whole, as opposed to a specific named resource of
2303   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2304   For example,
2305</t>
2306<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2307OPTIONS * HTTP/1.1
2308</artwork></figure>
2309<t>
2310   If a proxy receives an OPTIONS request with an absolute-form of
2311   request-target in which the URI has an empty path and no query component,
2312   then the last proxy on the request chain &MUST; send a request-target
2313   of "*" when it forwards the request to the indicated origin server.
2314</t>
2315<figure><preamble>   
2316   For example, the request
2317</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2318OPTIONS http://www.example.org:8001 HTTP/1.1
2319</artwork></figure>
2320<figure><preamble>   
2321  would be forwarded by the final proxy as
2322</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2323OPTIONS * HTTP/1.1
2324Host: www.example.org:8001
2325</artwork>
2326<postamble>
2327   after connecting to port 8001 of host "www.example.org".
2328</postamble>
2329</figure>
2330</section>
2331
2332<section title="Host" anchor="header.host">
2333  <iref primary="true" item="Host header field" x:for-anchor=""/>
2334  <x:anchor-alias value="Host"/>
2335<t>
2336   The "Host" header field in a request provides the host and port
2337   information from the target URI, enabling the origin
2338   server to distinguish among resources while servicing requests
2339   for multiple host names on a single IP address.  Since the Host
2340   field-value is critical information for handling a request, it
2341   &SHOULD; be sent as the first header field following the request-line.
2342</t>
2343<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2344  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2345</artwork></figure>
2346<t>
2347   A client &MUST; send a Host header field in all HTTP/1.1 request
2348   messages.  If the target URI includes an authority component, then
2349   the Host field-value &MUST; be identical to that authority component
2350   after excluding any userinfo (<xref target="http.uri"/>).
2351   If the authority component is missing or undefined for the target URI,
2352   then the Host header field &MUST; be sent with an empty field-value.
2353</t>
2354<t>
2355   For example, a GET request to the origin server for
2356   &lt;http://www.example.org/pub/WWW/&gt; would begin with:
2357</t>
2358<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2359GET /pub/WWW/ HTTP/1.1
2360Host: www.example.org
2361</artwork></figure>
2362<t>
2363   The Host header field &MUST; be sent in an HTTP/1.1 request even
2364   if the request-target is in the absolute-form, since this
2365   allows the Host information to be forwarded through ancient HTTP/1.0
2366   proxies that might not have implemented Host.
2367</t>
2368<t>
2369   When a proxy receives a request with an absolute-form of
2370   request-target, the proxy &MUST; ignore the received
2371   Host header field (if any) and instead replace it with the host
2372   information of the request-target.  If the proxy forwards the request,
2373   it &MUST; generate a new Host field-value based on the received
2374   request-target rather than forward the received Host field-value.
2375</t>
2376<t>
2377   Since the Host header field acts as an application-level routing
2378   mechanism, it is a frequent target for malware seeking to poison
2379   a shared cache or redirect a request to an unintended server.
2380   An interception proxy is particularly vulnerable if it relies on
2381   the Host field-value for redirecting requests to internal
2382   servers, or for use as a cache key in a shared cache, without
2383   first verifying that the intercepted connection is targeting a
2384   valid IP address for that host.
2385</t>
2386<t>
2387   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2388   to any HTTP/1.1 request message that lacks a Host header field and
2389   to any request message that contains more than one Host header field
2390   or a Host header field with an invalid field-value.
2391</t>
2392</section>
2393
2394<section title="Effective Request URI" anchor="effective.request.uri">
2395  <iref primary="true" item="effective request URI"/>
2396  <x:anchor-alias value="effective request URI"/>
2397<t>
2398   A server that receives an HTTP request message &MUST; reconstruct
2399   the user agent's original target URI, based on the pieces of information
2400   learned from the request-target, <x:ref>Host</x:ref> header field, and
2401   connection context, in order to identify the intended target resource and
2402   properly service the request. The URI derived from this reconstruction
2403   process is referred to as the "<x:dfn>effective request URI</x:dfn>".
2404</t>
2405<t>
2406   For a user agent, the effective request URI is the target URI.
2407</t>
2408<t>
2409   If the request-target is in absolute-form, then the effective request URI
2410   is the same as the request-target.  Otherwise, the effective request URI
2411   is constructed as follows.
2412</t>
2413<t>
2414   If the request is received over a TLS-secured TCP connection,
2415   then the effective request URI's scheme is "https"; otherwise, the
2416   scheme is "http".
2417</t>
2418<t>
2419   If the request-target is in authority-form, then the effective
2420   request URI's authority component is the same as the request-target.
2421   Otherwise, if a <x:ref>Host</x:ref> header field is supplied with a
2422   non-empty field-value, then the authority component is the same as the
2423   Host field-value. Otherwise, the authority component is the concatenation of
2424   the default host name configured for the server, a colon (":"), and the
2425   connection's incoming TCP port number in decimal form.
2426</t>
2427<t>
2428   If the request-target is in authority-form or asterisk-form, then the
2429   effective request URI's combined path and query component is empty.
2430   Otherwise, the combined path and query component is the same as the
2431   request-target.
2432</t>
2433<t>
2434   The components of the effective request URI, once determined as above,
2435   can be combined into absolute-URI form by concatenating the scheme,
2436   "://", authority, and combined path and query component.
2437</t>
2438<figure>
2439<preamble>
2440   Example 1: the following message received over an insecure TCP connection
2441</preamble> 
2442<artwork type="example" x:indent-with="  ">
2443GET /pub/WWW/TheProject.html HTTP/1.1
2444Host: www.example.org:8080
2445</artwork>
2446</figure>
2447<figure>
2448<preamble>
2449  has an effective request URI of
2450</preamble>
2451<artwork type="example" x:indent-with="  ">
2452http://www.example.org:8080/pub/WWW/TheProject.html
2453</artwork>
2454</figure>
2455<figure>
2456<preamble>
2457   Example 2: the following message received over a TLS-secured TCP connection
2458</preamble> 
2459<artwork type="example" x:indent-with="  ">
2460OPTIONS * HTTP/1.1
2461Host: www.example.org
2462</artwork>
2463</figure>
2464<figure>
2465<preamble>
2466  has an effective request URI of
2467</preamble>
2468<artwork type="example" x:indent-with="  ">
2469https://www.example.org
2470</artwork>
2471</figure>
2472<t>
2473   An origin server that does not allow resources to differ by requested
2474   host &MAY; ignore the <x:ref>Host</x:ref> field-value and instead replace it
2475   with a configured server name when constructing the effective request URI.
2476</t>
2477<t>
2478   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2479   field &MAY; attempt to use heuristics (e.g., examination of the URI path for
2480   something unique to a particular host) in order to guess the
2481   effective request URI's authority component.
2482</t>
2483</section>
2484
2485<section title="Associating a Response to a Request" anchor="associating.response.to.request">
2486<t>
2487   HTTP does not include a request identifier for associating a given
2488   request message with its corresponding one or more response messages.
2489   Hence, it relies on the order of response arrival to correspond exactly
2490   to the order in which requests are made on the same connection.
2491   More than one response message per request only occurs when one or more
2492   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a
2493   final response to the same request.
2494</t>
2495<t>
2496   A client that has more than one outstanding request on a connection &MUST;
2497   maintain a list of outstanding requests in the order sent and &MUST;
2498   associate each received response message on that connection to the highest
2499   ordered request that has not yet received a final (non-<x:ref>1xx</x:ref>)
2500   response.
2501</t>
2502</section>
2503
2504<section title="Message Forwarding" anchor="message.forwarding">
2505<t>
2506   As described in <xref target="intermediaries"/>, intermediaries can serve
2507   a variety of roles in the processing of HTTP requests and responses.
2508   Some intermediaries are used to improve performance or availability.
2509   Others are used for access control or to filter content.
2510   Since an HTTP stream has characteristics similar to a pipe-and-filter
2511   architecture, there are no inherent limits to the extent an intermediary
2512   can enhance (or interfere) with either direction of the stream.
2513</t>
2514<t>
2515   Intermediaries that forward a message &MUST; implement the
2516   <x:ref>Connection</x:ref> header field, as specified in
2517   <xref target="header.connection"/>, to exclude fields that are only
2518   intended for the incoming connection.
2519</t>
2520<t>
2521   In order to avoid request loops, a proxy that forwards requests to other
2522   proxies &MUST; be able to recognize and exclude all of its own server
2523   names, including any aliases, local variations, or literal IP addresses.
2524</t>
2525
2526<section title="Via" anchor="header.via">
2527  <iref primary="true" item="Via header field" x:for-anchor=""/>
2528  <x:anchor-alias value="pseudonym"/>
2529  <x:anchor-alias value="received-by"/>
2530  <x:anchor-alias value="received-protocol"/>
2531  <x:anchor-alias value="Via"/>
2532<t>
2533   The "Via" header field &MUST; be sent by a proxy or gateway in forwarded
2534   messages to indicate the intermediate protocols and recipients between the
2535   user agent and the server on requests, and between the origin server and
2536   the client on responses. It is analogous to the "Received" field
2537   used by email systems (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2538   Via is used in HTTP for tracking message forwards,
2539   avoiding request loops, and identifying the protocol capabilities of
2540   all senders along the request/response chain.
2541</t>
2542<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"/>
2543  <x:ref>Via</x:ref>               = 1#( <x:ref>received-protocol</x:ref> <x:ref>RWS</x:ref> <x:ref>received-by</x:ref>
2544                          [ <x:ref>RWS</x:ref> <x:ref>comment</x:ref> ] )
2545  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2546                      ; see <xref target="header.upgrade"/>
2547  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2548  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2549</artwork></figure>
2550<t>
2551   The received-protocol indicates the protocol version of the message
2552   received by the server or client along each segment of the
2553   request/response chain. The received-protocol version is appended to
2554   the Via field value when the message is forwarded so that information
2555   about the protocol capabilities of upstream applications remains
2556   visible to all recipients.
2557</t>
2558<t>
2559   The protocol-name is excluded if and only if it would be "HTTP". The
2560   received-by field is normally the host and optional port number of a
2561   recipient server or client that subsequently forwarded the message.
2562   However, if the real host is considered to be sensitive information,
2563   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2564   be assumed to be the default port of the received-protocol.
2565</t>
2566<t>
2567   Multiple Via field values represent each proxy or gateway that has
2568   forwarded the message. Each recipient &MUST; append its information
2569   such that the end result is ordered according to the sequence of
2570   forwarding applications.
2571</t>
2572<t>
2573   Comments &MAY; be used in the Via header field to identify the software
2574   of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2575   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2576   are optional and &MAY; be removed by any recipient prior to forwarding the
2577   message.
2578</t>
2579<t>
2580   For example, a request message could be sent from an HTTP/1.0 user
2581   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2582   forward the request to a public proxy at p.example.net, which completes
2583   the request by forwarding it to the origin server at www.example.com.
2584   The request received by www.example.com would then have the following
2585   Via header field:
2586</t>
2587<figure><artwork type="example">
2588  Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
2589</artwork></figure>
2590<t>
2591   A proxy or gateway used as a portal through a network firewall
2592   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2593   region unless it is explicitly enabled to do so. If not enabled, the
2594   received-by host of any host behind the firewall &SHOULD; be replaced
2595   by an appropriate pseudonym for that host.
2596</t>
2597<t>
2598   A proxy or gateway &MAY; combine an ordered subsequence of Via header
2599   field entries into a single such entry if the entries have identical
2600   received-protocol values. For example,
2601</t>
2602<figure><artwork type="example">
2603  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2604</artwork></figure>
2605<t>
2606  could be collapsed to
2607</t>
2608<figure><artwork type="example">
2609  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2610</artwork></figure>
2611<t>
2612   Senders &SHOULD-NOT; combine multiple entries unless they are all
2613   under the same organizational control and the hosts have already been
2614   replaced by pseudonyms. Senders &MUST-NOT; combine entries that
2615   have different received-protocol values.
2616</t>
2617</section>
2618
2619<section title="Transformations" anchor="message.transformations">
2620<t>
2621   Some intermediaries include features for transforming messages and their
2622   payloads.  A transforming proxy might, for example, convert between image
2623   formats in order to save cache space or to reduce the amount of traffic on
2624   a slow link. However, operational problems might occur when these
2625   transformations are applied to payloads intended for critical applications,
2626   such as medical imaging or scientific data analysis, particularly when
2627   integrity checks or digital signatures are used to ensure that the payload
2628   received is identical to the original.
2629</t>
2630<t>
2631   If a proxy receives a request-target with a host name that is not a
2632   fully qualified domain name, it &MAY; add its own domain to the host name
2633   it received when forwarding the request.  A proxy &MUST-NOT; change the
2634   host name if it is a fully qualified domain name.
2635</t>
2636<t>
2637   A proxy &MUST-NOT; modify the "path-absolute" and "query" parts of the
2638   received request-target when forwarding it to the next inbound server,
2639   except as noted above to replace an empty path with "/" or "*".
2640</t>
2641<t>
2642   A proxy &MUST-NOT; modify header fields that provide information about the
2643   end points of the communication chain, the resource state, or the selected
2644   representation. A proxy &MAY; change the message body through application
2645   or removal of a transfer coding (<xref target="transfer.codings"/>).
2646</t>
2647<t>
2648   A non-transforming proxy &MUST; preserve the message payload (&payload;).
2649   A transforming proxy &MUST; preserve the payload of a message that
2650   contains the no-transform cache-control directive.
2651</t>
2652<t>
2653   A transforming proxy &MAY; transform the payload of a message
2654   that does not contain the no-transform cache-control directive;
2655   if the payload is transformed, the transforming proxy &MUST; add a
2656   Warning 214 (Transformation applied) header field if one does not
2657   already appear in the message (see &header-warning;).
2658</t>
2659</section>
2660</section>
2661</section>
2662
2663<section title="Connection Management" anchor="connection.management">
2664<t>
2665   HTTP messaging is independent of the underlying transport or
2666   session-layer connection protocol(s).  HTTP only presumes a reliable
2667   transport with in-order delivery of requests and the corresponding
2668   in-order delivery of responses.  The mapping of HTTP request and
2669   response structures onto the data units of an underlying transport
2670   protocol is outside the scope of this specification.
2671</t>
2672<t>
2673   As described in <xref target="connecting.inbound"/>, the specific
2674   connection protocols to be used for an HTTP interaction are determined by
2675   client configuration and the <x:ref>target URI</x:ref>.
2676   For example, the "http" URI scheme
2677   (<xref target="http.uri"/>) indicates a default connection of TCP
2678   over IP, with a default TCP port of 80, but the client might be
2679   configured to use a proxy via some other connection, port, or protocol.
2680</t>
2681<t>
2682   HTTP implementations are expected to engage in connection management,
2683   which includes maintaining the state of current connections,
2684   establishing a new connection or reusing an existing connection,
2685   processing messages received on a connection, detecting connection
2686   failures, and closing each connection.
2687   Most clients maintain multiple connections in parallel, including
2688   more than one connection per server endpoint.
2689   Most servers are designed to maintain thousands of concurrent connections,
2690   while controlling request queues to enable fair use and detect
2691   denial of service attacks.
2692</t>
2693
2694<section title="Connection" anchor="header.connection">
2695  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2696  <iref primary="true" item="close" x:for-anchor=""/>
2697  <x:anchor-alias value="Connection"/>
2698  <x:anchor-alias value="connection-option"/>
2699  <x:anchor-alias value="close"/>
2700<t>
2701   The "Connection" header field allows the sender to indicate desired
2702   control options for the current connection.  In order to avoid confusing
2703   downstream recipients, a proxy or gateway &MUST; remove or replace any
2704   received connection options before forwarding the message.
2705</t>
2706<t>
2707   When a header field aside from Connection is used to supply control
2708   information for or about the current connection, the sender &MUST; list
2709   the corresponding field-name within the "Connection" header field.
2710   A proxy or gateway &MUST; parse a received Connection
2711   header field before a message is forwarded and, for each
2712   connection-option in this field, remove any header field(s) from
2713   the message with the same name as the connection-option, and then
2714   remove the Connection header field itself (or replace it with the
2715   intermediary's own connection options for the forwarded message).
2716</t>
2717<t>
2718   Hence, the Connection header field provides a declarative way of
2719   distinguishing header fields that are only intended for the
2720   immediate recipient ("hop-by-hop") from those fields that are
2721   intended for all recipients on the chain ("end-to-end"), enabling the
2722   message to be self-descriptive and allowing future connection-specific
2723   extensions to be deployed without fear that they will be blindly
2724   forwarded by older intermediaries.
2725</t>
2726<t>
2727   The Connection header field's value has the following grammar:
2728</t>
2729<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2730  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2731  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2732</artwork></figure>
2733<t>
2734   Connection options are case-insensitive.
2735</t>
2736<t>
2737   A sender &MUST-NOT; send a connection option corresponding to a header
2738   field that is intended for all recipients of the payload.
2739   For example, <x:ref>Cache-Control</x:ref> is never appropriate as a
2740   connection option (&header-cache-control;).
2741</t>
2742<t>
2743   The connection options do not have to correspond to a header field
2744   present in the message, since a connection-specific header field
2745   might not be needed if there are no parameters associated with that
2746   connection option.  Recipients that trigger certain connection
2747   behavior based on the presence of connection options &MUST; do so
2748   based on the presence of the connection-option rather than only the
2749   presence of the optional header field.  In other words, if the
2750   connection option is received as a header field but not indicated
2751   within the Connection field-value, then the recipient &MUST; ignore
2752   the connection-specific header field because it has likely been
2753   forwarded by an intermediary that is only partially conformant.
2754</t>
2755<t>
2756   When defining new connection options, specifications ought to
2757   carefully consider existing deployed header fields and ensure
2758   that the new connection option does not share the same name as
2759   an unrelated header field that might already be deployed.
2760   Defining a new connection option essentially reserves that potential
2761   field-name for carrying additional information related to the
2762   connection option, since it would be unwise for senders to use
2763   that field-name for anything else.
2764</t>
2765<t>
2766   The "<x:dfn>close</x:dfn>" connection option is defined for a
2767   sender to signal that this connection will be closed after completion of
2768   the response. For example,
2769</t>
2770<figure><artwork type="example">
2771  Connection: close
2772</artwork></figure>
2773<t>
2774   in either the request or the response header fields indicates that
2775   the connection &MUST; be closed after the current request/response
2776   is complete (<xref target="persistent.tear-down"/>).
2777</t>
2778<t>
2779   A client that does not support <x:ref>persistent connections</x:ref> &MUST;
2780   send the "close" connection option in every request message.
2781</t>
2782<t>
2783   A server that does not support <x:ref>persistent connections</x:ref> &MUST;
2784   send the "close" connection option in every response message that
2785   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2786</t>
2787</section>
2788
2789<section title="Establishment" anchor="persistent.establishment">
2790<t>
2791   It is beyond the scope of this specification to describe how connections
2792   are established via various transport or session-layer protocols.
2793   Each connection applies to only one transport link.
2794</t>
2795</section>
2796
2797<section title="Persistence" anchor="persistent.connections">
2798   <x:anchor-alias value="persistent connections"/>
2799<t>
2800   HTTP/1.1 defaults to the use of "<x:dfn>persistent connections</x:dfn>",
2801   allowing multiple requests and responses to be carried over a single
2802   connection. The "<x:ref>close</x:ref>" connection-option is used to signal
2803   that a connection will not persist after the current request/response.
2804   HTTP implementations &SHOULD; support persistent connections.
2805</t>
2806<t>
2807   A recipient determines whether a connection is persistent or not based on
2808   the most recently received message's protocol version and
2809   <x:ref>Connection</x:ref> header field (if any):
2810   <list style="symbols">
2811     <t>If the <x:ref>close</x:ref> connection option is present, the
2812        connection will not persist after the current response; else,</t>
2813     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2814        persist after the current response; else,</t>
2815     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2816        connection option is present, the recipient is not a proxy, and
2817        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2818        the connection will persist after the current response; otherwise,</t>
2819     <t>The connection will close after the current response.</t>
2820   </list>
2821</t>
2822<t>
2823   A server &MAY; assume that an HTTP/1.1 client intends to maintain a
2824   persistent connection until a <x:ref>close</x:ref> connection option
2825   is received in a request.
2826</t>
2827<t>
2828   A client &MAY; reuse a persistent connection until it sends or receives
2829   a <x:ref>close</x:ref> connection option or receives an HTTP/1.0 response
2830   without a "keep-alive" connection option.
2831</t>
2832<t>
2833   In order to remain persistent, all messages on a connection &MUST;
2834   have a self-defined message length (i.e., one not defined by closure
2835   of the connection), as described in <xref target="message.body"/>.
2836   A server &MUST; read the entire request message body or close
2837   the connection after sending its response, since otherwise the
2838   remaining data on a persistent connection would be misinterpreted
2839   as the next request.  Likewise,
2840   a client &MUST; read the entire response message body if it intends
2841   to reuse the same connection for a subsequent request.
2842</t>
2843<t>
2844   A proxy server &MUST-NOT; maintain a persistent connection with an
2845   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
2846   information and discussion of the problems with the Keep-Alive header field
2847   implemented by many HTTP/1.0 clients).
2848</t>
2849<t>
2850   Clients and servers &SHOULD-NOT; assume that a persistent connection is
2851   maintained for HTTP versions less than 1.1 unless it is explicitly
2852   signaled.
2853   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
2854   for more information on backward compatibility with HTTP/1.0 clients.
2855</t>
2856
2857<section title="Retrying Requests" anchor="persistent.retrying.requests">
2858<t>
2859   Connections can be closed at any time, with or without intention.
2860   Implementations ought to anticipate the need to recover
2861   from asynchronous close events.
2862</t>
2863<t>
2864   When an inbound connection is closed prematurely, a client &MAY; open a new
2865   connection and automatically retransmit an aborted sequence of requests if
2866   all of those requests have idempotent methods (&idempotent-methods;).
2867   A proxy &MUST-NOT; automatically retry non-idempotent requests.
2868</t>
2869<t>
2870   A user agent &MUST-NOT; automatically retry a request with a non-idempotent
2871   method unless it has some means to know that the request semantics are
2872   actually idempotent, regardless of the method, or some means to detect that
2873   the original request was never applied. For example, a user agent that
2874   knows (through design or configuration) that a POST request to a given
2875   resource is safe can repeat that request automatically.
2876   Likewise, a user agent designed specifically to operate on a version
2877   control repository might be able to recover from partial failure conditions
2878   by checking the target resource revision(s) after a failed connection,
2879   reverting or fixing any changes that were partially applied, and then
2880   automatically retrying the requests that failed.
2881</t>
2882<t>
2883   An automatic retry &SHOULD-NOT; be repeated if it fails.
2884</t>
2885</section>
2886
2887<section title="Pipelining" anchor="pipelining">
2888   <x:anchor-alias value="pipeline"/>
2889<t>
2890   A client that supports persistent connections &MAY; "<x:dfn>pipeline</x:dfn>"
2891   its requests (i.e., send multiple requests without waiting for each
2892   response). A server &MAY; process a sequence of pipelined requests in
2893   parallel if they all have safe methods (&safe-methods;), but &MUST; send
2894   the corresponding responses in the same order that the requests were
2895   received.
2896</t>
2897<t>
2898   A client that pipelines requests &MUST; be prepared to retry those
2899   requests if the connection closes before it receives all of the
2900   corresponding responses. A client that assumes a persistent connection and
2901   pipelines immediately after connection establishment &MUST-NOT; pipeline
2902   on a retry connection until it knows the connection is persistent.
2903</t>
2904<t>
2905   Idempotent methods (&idempotent-methods;) are significant to pipelining
2906   because they can be automatically retried after a connection failure.
2907   A user agent &SHOULD-NOT; pipeline requests after a non-idempotent method
2908   until the final response status code for that method has been received,
2909   unless the user agent has a means to detect and recover from partial
2910   failure conditions involving the pipelined sequence.
2911</t>
2912<t>
2913   An intermediary that receives pipelined requests &MAY; pipeline those
2914   requests when forwarding them inbound, since it can rely on the outbound
2915   user agent(s) to determine what requests can be safely pipelined. If the
2916   inbound connection fails before receiving a response, the pipelining
2917   intermediary &MAY; attempt to retry a sequence of requests that have yet
2918   to receive a response if the requests all have idempotent methods;
2919   otherwise, the pipelining intermediary &SHOULD; forward any received
2920   responses and then close the corresponding outbound connection(s) so that
2921   the outbound user agent(s) can recover accordingly.
2922</t>
2923</section>
2924</section>
2925   
2926<section title="Concurrency" anchor="persistent.concurrency">
2927<t>
2928   Clients &SHOULD; limit the number of simultaneous
2929   connections that they maintain to a given server.
2930</t>
2931<t>
2932   Previous revisions of HTTP gave a specific number of connections as a
2933   ceiling, but this was found to be impractical for many applications. As a
2934   result, this specification does not mandate a particular maximum number of
2935   connections, but instead encourages clients to be conservative when opening
2936   multiple connections.
2937</t>
2938<t>
2939   Multiple connections are typically used to avoid the "head-of-line
2940   blocking" problem, wherein a request that takes significant server-side
2941   processing and/or has a large payload blocks subsequent requests on the
2942   same connection. However, each connection consumes server resources.
2943   Furthermore, using multiple connections can cause undesirable side effects
2944   in congested networks.
2945</t>
2946<t>
2947   Note that servers might reject traffic that they deem abusive, including an
2948   excessive number of connections from a client.
2949</t>
2950</section>
2951
2952<section title="Failures and Time-outs" anchor="persistent.failures">
2953<t>
2954   Servers will usually have some time-out value beyond which they will
2955   no longer maintain an inactive connection. Proxy servers might make
2956   this a higher value since it is likely that the client will be making
2957   more connections through the same server. The use of persistent
2958   connections places no requirements on the length (or existence) of
2959   this time-out for either the client or the server.
2960</t>
2961<t>
2962   When a client or server wishes to time-out it &SHOULD; issue a graceful
2963   close on the transport connection. Clients and servers &SHOULD; both
2964   constantly watch for the other side of the transport close, and
2965   respond to it as appropriate. If a client or server does not detect
2966   the other side's close promptly it could cause unnecessary resource
2967   drain on the network.
2968</t>
2969<t>
2970   A client, server, or proxy &MAY; close the transport connection at any
2971   time. For example, a client might have started to send a new request
2972   at the same time that the server has decided to close the "idle"
2973   connection. From the server's point of view, the connection is being
2974   closed while it was idle, but from the client's point of view, a
2975   request is in progress.
2976</t>
2977<t>
2978   Servers &SHOULD; maintain persistent connections and allow the underlying
2979   transport's flow control mechanisms to resolve temporary overloads, rather
2980   than terminate connections with the expectation that clients will retry.
2981   The latter technique can exacerbate network congestion.
2982</t>
2983<t>
2984   A client sending a message body &SHOULD; monitor
2985   the network connection for an error status code while it is transmitting
2986   the request. If the client sees an error status code, it &SHOULD;
2987   immediately cease transmitting the body and close the connection.
2988</t>
2989</section>
2990   
2991<section title="Tear-down" anchor="persistent.tear-down">
2992  <iref primary="false" item="Connection header field" x:for-anchor=""/>
2993  <iref primary="false" item="close" x:for-anchor=""/>
2994<t>
2995   The <x:ref>Connection</x:ref> header field
2996   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
2997   connection option that a sender &SHOULD; send when it wishes to close
2998   the connection after the current request/response pair.
2999</t>
3000<t>
3001   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
3002   send further requests on that connection (after the one containing
3003   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
3004   final response message corresponding to this request.
3005</t>
3006<t>
3007   A server that receives a <x:ref>close</x:ref> connection option &MUST;
3008   initiate a lingering close (see below) of the connection after it sends the
3009   final response to the request that contained <x:ref>close</x:ref>.
3010   The server &SHOULD; send a <x:ref>close</x:ref> connection option
3011   in its final response on that connection. The server &MUST-NOT; process
3012   any further requests received on that connection.
3013</t>
3014<t>
3015   A server that sends a <x:ref>close</x:ref> connection option &MUST;
3016   initiate a lingering close of the connection after it sends the
3017   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
3018   any further requests received on that connection.
3019</t>
3020<t>
3021   A client that receives a <x:ref>close</x:ref> connection option &MUST;
3022   cease sending requests on that connection and close the connection
3023   after reading the response message containing the close; if additional
3024   pipelined requests had been sent on the connection, the client &SHOULD;
3025   assume that they will not be processed by the server.
3026</t>
3027<t>
3028   If a server performs an immediate close of a TCP connection, there is a
3029   significant risk that the client will not be able to read the last HTTP
3030   response.  If the server receives additional data from the client on a
3031   fully-closed connection, such as another request that was sent by the
3032   client before receiving the server's response, the server's TCP stack will
3033   send a reset packet to the client; unfortunately, the reset packet might
3034   erase the client's unacknowledged input buffers before they can be read
3035   and interpreted by the client's HTTP parser.
3036</t>
3037<t>
3038   To avoid the TCP reset problem, a server can perform a lingering close on a
3039   connection by closing only the write side of the read/write connection
3040   (a half-close) and continuing to read from the connection until the
3041   connection is closed by the client or the server is reasonably certain
3042   that its own TCP stack has received the client's acknowledgement of the
3043   packet(s) containing the server's last response. It is then safe for the
3044   server to fully close the connection.
3045</t>
3046<t>
3047   It is unknown whether the reset problem is exclusive to TCP or might also
3048   be found in other transport connection protocols.
3049</t>
3050</section>
3051
3052<section title="Upgrade" anchor="header.upgrade">
3053  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3054  <x:anchor-alias value="Upgrade"/>
3055  <x:anchor-alias value="protocol"/>
3056  <x:anchor-alias value="protocol-name"/>
3057  <x:anchor-alias value="protocol-version"/>
3058<t>
3059   The "Upgrade" header field is intended to provide a simple mechanism
3060   for transitioning from HTTP/1.1 to some other protocol on the same
3061   connection.  A client &MAY; send a list of protocols in the Upgrade
3062   header field of a request to invite the server to switch to one or
3063   more of those protocols before sending the final response.
3064   A server &MUST; send an Upgrade header field in <x:ref>101 (Switching
3065   Protocols)</x:ref> responses to indicate which protocol(s) are being
3066   switched to, and &MUST; send it in <x:ref>426 (Upgrade Required)</x:ref>
3067   responses to indicate acceptable protocols.
3068   A server &MAY; send an Upgrade header field in any other response to
3069   indicate that they might be willing to upgrade to one of the
3070   specified protocols for a future request.
3071</t>
3072<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3073  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3074
3075  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3076  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3077  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3078</artwork></figure>
3079<t>
3080   For example,
3081</t>
3082<figure><artwork type="example">
3083  Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3084</artwork></figure>
3085<t>
3086   Upgrade eases the difficult transition between incompatible protocols by
3087   allowing the client to initiate a request in the more commonly
3088   supported protocol while indicating to the server that it would like
3089   to use a "better" protocol if available (where "better" is determined
3090   by the server, possibly according to the nature of the request method
3091   or target resource).
3092</t>
3093<t>
3094   Upgrade cannot be used to insist on a protocol change; its acceptance and
3095   use by the server is optional. The capabilities and nature of the
3096   application-level communication after the protocol change is entirely
3097   dependent upon the new protocol chosen, although the first action
3098   after changing the protocol &MUST; be a response to the initial HTTP
3099   request that contained the Upgrade header field.
3100</t>
3101<t>
3102   For example, if the Upgrade header field is received in a GET request
3103   and the server decides to switch protocols, then it first responds
3104   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3105   then immediately follows that with the new protocol's equivalent of a
3106   response to a GET on the target resource.  This allows a connection to be
3107   upgraded to protocols with the same semantics as HTTP without the
3108   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3109   protocols unless the received message semantics can be honored by the new
3110   protocol; an OPTIONS request can be honored by any protocol.
3111</t>
3112<t>
3113   When Upgrade is sent, a sender &MUST; also send a
3114   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3115   that contains the "upgrade" connection option, in order to prevent Upgrade
3116   from being accidentally forwarded by intermediaries that might not implement
3117   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3118   is received in an HTTP/1.0 request.
3119</t>
3120<t>
3121   The Upgrade header field only applies to switching application-level
3122   protocols on the existing connection; it cannot be used
3123   to switch to a protocol on a different connection. For that purpose, it is
3124   more appropriate to use a <x:ref>3xx (Redirection)</x:ref> response
3125   (&status-3xx;).
3126</t>
3127<t>
3128   This specification only defines the protocol name "HTTP" for use by
3129   the family of Hypertext Transfer Protocols, as defined by the HTTP
3130   version rules of <xref target="http.version"/> and future updates to this
3131   specification. Additional tokens ought to be registered with IANA using the
3132   registration procedure defined in <xref target="upgrade.token.registry"/>.
3133</t>
3134</section>
3135</section>
3136
3137<section title="IANA Considerations" anchor="IANA.considerations">
3138
3139<section title="Header Field Registration" anchor="header.field.registration">
3140<t>
3141   HTTP header fields are registered within the Message Header Field Registry
3142   <xref target="BCP90"/> maintained by IANA at
3143   <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
3144</t>
3145<t>
3146   This document defines the following HTTP header fields, so their
3147   associated registry entries shall be updated according to the permanent
3148   registrations below:
3149</t>
3150<?BEGININC p1-messaging.iana-headers ?>
3151<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3152<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3153   <ttcol>Header Field Name</ttcol>
3154   <ttcol>Protocol</ttcol>
3155   <ttcol>Status</ttcol>
3156   <ttcol>Reference</ttcol>
3157
3158   <c>Connection</c>
3159   <c>http</c>
3160   <c>standard</c>
3161   <c>
3162      <xref target="header.connection"/>
3163   </c>
3164   <c>Content-Length</c>
3165   <c>http</c>
3166   <c>standard</c>
3167   <c>
3168      <xref target="header.content-length"/>
3169   </c>
3170   <c>Host</c>
3171   <c>http</c>
3172   <c>standard</c>
3173   <c>
3174      <xref target="header.host"/>
3175   </c>
3176   <c>TE</c>
3177   <c>http</c>
3178   <c>standard</c>
3179   <c>
3180      <xref target="header.te"/>
3181   </c>
3182   <c>Trailer</c>
3183   <c>http</c>
3184   <c>standard</c>
3185   <c>
3186      <xref target="header.trailer"/>
3187   </c>
3188   <c>Transfer-Encoding</c>
3189   <c>http</c>
3190   <c>standard</c>
3191   <c>
3192      <xref target="header.transfer-encoding"/>
3193   </c>
3194   <c>Upgrade</c>
3195   <c>http</c>
3196   <c>standard</c>
3197   <c>
3198      <xref target="header.upgrade"/>
3199   </c>
3200   <c>Via</c>
3201   <c>http</c>
3202   <c>standard</c>
3203   <c>
3204      <xref target="header.via"/>
3205   </c>
3206</texttable>
3207<!--(END)-->
3208<?ENDINC p1-messaging.iana-headers ?>
3209<t>
3210   Furthermore, the header field-name "Close" shall be registered as
3211   "reserved", since using that name as an HTTP header field might
3212   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3213   header field (<xref target="header.connection"/>).
3214</t>
3215<texttable align="left" suppress-title="true">
3216   <ttcol>Header Field Name</ttcol>
3217   <ttcol>Protocol</ttcol>
3218   <ttcol>Status</ttcol>
3219   <ttcol>Reference</ttcol>
3220
3221   <c>Close</c>
3222   <c>http</c>
3223   <c>reserved</c>
3224   <c>
3225      <xref target="header.field.registration"/>
3226   </c>
3227</texttable>
3228<t>
3229   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3230</t>
3231</section>
3232
3233<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3234<t>
3235   IANA maintains the registry of URI Schemes <xref target="BCP115"/> at
3236   <eref target="http://www.iana.org/assignments/uri-schemes.html"/>.
3237</t>
3238<t>
3239   This document defines the following URI schemes, so their
3240   associated registry entries shall be updated according to the permanent
3241   registrations below:
3242</t>
3243<texttable align="left" suppress-title="true">
3244   <ttcol>URI Scheme</ttcol>
3245   <ttcol>Description</ttcol>
3246   <ttcol>Reference</ttcol>
3247
3248   <c>http</c>
3249   <c>Hypertext Transfer Protocol</c>
3250   <c><xref target="http.uri"/></c>
3251
3252   <c>https</c>
3253   <c>Hypertext Transfer Protocol Secure</c>
3254   <c><xref target="https.uri"/></c>
3255</texttable>
3256</section>
3257
3258<section title="Internet Media Type Registration" anchor="internet.media.type.http">
3259<t>
3260   This document serves as the specification for the Internet media types
3261   "message/http" and "application/http". The following is to be registered with
3262   IANA (see <xref target="BCP13"/>).
3263</t>
3264<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
3265<iref item="Media Type" subitem="message/http" primary="true"/>
3266<iref item="message/http Media Type" primary="true"/>
3267<t>
3268   The message/http type can be used to enclose a single HTTP request or
3269   response message, provided that it obeys the MIME restrictions for all
3270   "message" types regarding line length and encodings.
3271</t>
3272<t>
3273  <list style="hanging" x:indent="12em">
3274    <t hangText="Type name:">
3275      message
3276    </t>
3277    <t hangText="Subtype name:">
3278      http
3279    </t>
3280    <t hangText="Required parameters:">
3281      none
3282    </t>
3283    <t hangText="Optional parameters:">
3284      version, msgtype
3285      <list style="hanging">
3286        <t hangText="version:">
3287          The HTTP-version number of the enclosed message
3288          (e.g., "1.1"). If not present, the version can be
3289          determined from the first line of the body.
3290        </t>
3291        <t hangText="msgtype:">
3292          The message type &mdash; "request" or "response". If not
3293          present, the type can be determined from the first
3294          line of the body.
3295        </t>
3296      </list>
3297    </t>
3298    <t hangText="Encoding considerations:">
3299      only "7bit", "8bit", or "binary" are permitted
3300    </t>
3301    <t hangText="Security considerations:">
3302      none
3303    </t>
3304    <t hangText="Interoperability considerations:">
3305      none
3306    </t>
3307    <t hangText="Published specification:">
3308      This specification (see <xref target="internet.media.type.message.http"/>).
3309    </t>
3310    <t hangText="Applications that use this media type:">
3311    </t>
3312    <t hangText="Additional information:">
3313      <list style="hanging">
3314        <t hangText="Magic number(s):">none</t>
3315        <t hangText="File extension(s):">none</t>
3316        <t hangText="Macintosh file type code(s):">none</t>
3317      </list>
3318    </t>
3319    <t hangText="Person and email address to contact for further information:">
3320      See Authors Section.
3321    </t>
3322    <t hangText="Intended usage:">
3323      COMMON
3324    </t>
3325    <t hangText="Restrictions on usage:">
3326      none
3327    </t>
3328    <t hangText="Author/Change controller:">
3329      IESG
3330    </t>
3331  </list>
3332</t>
3333</section>
3334<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
3335<iref item="Media Type" subitem="application/http" primary="true"/>
3336<iref item="application/http Media Type" primary="true"/>
3337<t>
3338   The application/http type can be used to enclose a pipeline of one or more
3339   HTTP request or response messages (not intermixed).
3340</t>
3341<t>
3342  <list style="hanging" x:indent="12em">
3343    <t hangText="Type name:">
3344      application
3345    </t>
3346    <t hangText="Subtype name:">
3347      http
3348    </t>
3349    <t hangText="Required parameters:">
3350      none
3351    </t>
3352    <t hangText="Optional parameters:">
3353      version, msgtype
3354      <list style="hanging">
3355        <t hangText="version:">
3356          The HTTP-version number of the enclosed messages
3357          (e.g., "1.1"). If not present, the version can be
3358          determined from the first line of the body.
3359        </t>
3360        <t hangText="msgtype:">
3361          The message type &mdash; "request" or "response". If not
3362          present, the type can be determined from the first
3363          line of the body.
3364        </t>
3365      </list>
3366    </t>
3367    <t hangText="Encoding considerations:">
3368      HTTP messages enclosed by this type
3369      are in "binary" format; use of an appropriate
3370      Content-Transfer-Encoding is required when
3371      transmitted via E-mail.
3372    </t>
3373    <t hangText="Security considerations:">
3374      none
3375    </t>
3376    <t hangText="Interoperability considerations:">
3377      none
3378    </t>
3379    <t hangText="Published specification:">
3380      This specification (see <xref target="internet.media.type.application.http"/>).
3381    </t>
3382    <t hangText="Applications that use this media type:">
3383    </t>
3384    <t hangText="Additional information:">
3385      <list style="hanging">
3386        <t hangText="Magic number(s):">none</t>
3387        <t hangText="File extension(s):">none</t>
3388        <t hangText="Macintosh file type code(s):">none</t>
3389      </list>
3390    </t>
3391    <t hangText="Person and email address to contact for further information:">
3392      See Authors Section.
3393    </t>
3394    <t hangText="Intended usage:">
3395      COMMON
3396    </t>
3397    <t hangText="Restrictions on usage:">
3398      none
3399    </t>
3400    <t hangText="Author/Change controller:">
3401      IESG
3402    </t>
3403  </list>
3404</t>
3405</section>
3406</section>
3407
3408<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3409<t>
3410   The HTTP Transfer Coding Registry defines the name space for transfer
3411   coding names.
3412</t>
3413<t>
3414   Registrations &MUST; include the following fields:
3415   <list style="symbols">
3416     <t>Name</t>
3417     <t>Description</t>
3418     <t>Pointer to specification text</t>
3419   </list>
3420</t>
3421<t>
3422   Names of transfer codings &MUST-NOT; overlap with names of content codings
3423   (&content-codings;) unless the encoding transformation is identical, as
3424   is the case for the compression codings defined in
3425   <xref target="compression.codings"/>.
3426</t>
3427<t>
3428   Values to be added to this name space require IETF Review (see
3429   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3430   conform to the purpose of transfer coding defined in this section.
3431   Use of program names for the identification of encoding formats
3432   is not desirable and is discouraged for future encodings.
3433</t>
3434<t>
3435   The registry itself is maintained at
3436   <eref target="http://www.iana.org/assignments/http-parameters"/>.
3437</t>
3438</section>
3439
3440<section title="Transfer Coding Registration" anchor="transfer.coding.registration">
3441<t>
3442   The HTTP Transfer Coding Registry shall be updated with the registrations
3443   below:
3444</t>
3445<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3446   <ttcol>Name</ttcol>
3447   <ttcol>Description</ttcol>
3448   <ttcol>Reference</ttcol>
3449   <c>chunked</c>
3450   <c>Transfer in a series of chunks</c>
3451   <c>
3452      <xref target="chunked.encoding"/>
3453   </c>
3454   <c>compress</c>
3455   <c>UNIX "compress" program method</c>
3456   <c>
3457      <xref target="compress.coding"/>
3458   </c>
3459   <c>deflate</c>
3460   <c>"deflate" compression mechanism (<xref target="RFC1951"/>) used inside
3461   the "zlib" data format (<xref target="RFC1950"/>)
3462   </c>
3463   <c>
3464      <xref target="deflate.coding"/>
3465   </c>
3466   <c>gzip</c>
3467   <c>Same as GNU zip <xref target="RFC1952"/></c>
3468   <c>
3469      <xref target="gzip.coding"/>
3470   </c>
3471</texttable>
3472</section>
3473
3474<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3475<t>
3476   The HTTP Upgrade Token Registry defines the name space for protocol-name
3477   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3478   field. Each registered protocol name is associated with contact information
3479   and an optional set of specifications that details how the connection
3480   will be processed after it has been upgraded.
3481</t>
3482<t>
3483   Registrations happen on a "First Come First Served" basis (see
3484   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3485   following rules:
3486  <list style="numbers">
3487    <t>A protocol-name token, once registered, stays registered forever.</t>
3488    <t>The registration &MUST; name a responsible party for the
3489       registration.</t>
3490    <t>The registration &MUST; name a point of contact.</t>
3491    <t>The registration &MAY; name a set of specifications associated with
3492       that token. Such specifications need not be publicly available.</t>
3493    <t>The registration &SHOULD; name a set of expected "protocol-version"
3494       tokens associated with that token at the time of registration.</t>
3495    <t>The responsible party &MAY; change the registration at any time.
3496       The IANA will keep a record of all such changes, and make them
3497       available upon request.</t>
3498    <t>The IESG &MAY; reassign responsibility for a protocol token.
3499       This will normally only be used in the case when a
3500       responsible party cannot be contacted.</t>
3501  </list>
3502</t>
3503<t>
3504   This registration procedure for HTTP Upgrade Tokens replaces that
3505   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3506</t>
3507</section>
3508
3509<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3510<t>
3511   The HTTP Upgrade Token Registry shall be updated with the registration
3512   below:
3513</t>
3514<texttable align="left" suppress-title="true">
3515   <ttcol>Value</ttcol>
3516   <ttcol>Description</ttcol>
3517   <ttcol>Expected Version Tokens</ttcol>
3518   <ttcol>Reference</ttcol>
3519
3520   <c>HTTP</c>
3521   <c>Hypertext Transfer Protocol</c>
3522   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3523   <c><xref target="http.version"/></c>
3524</texttable>
3525<t>
3526   The responsible party is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3527</t>
3528</section>
3529
3530</section>
3531
3532<section title="Security Considerations" anchor="security.considerations">
3533<t>
3534   This section is meant to inform developers, information providers, and
3535   users of known security concerns relevant to HTTP/1.1 message syntax,
3536   parsing, and routing.
3537</t>
3538
3539<section title="DNS-related Attacks" anchor="dns.related.attacks">
3540<t>
3541   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3542   generally prone to security attacks based on the deliberate misassociation
3543   of IP addresses and DNS names not protected by DNSSEC. Clients need to be
3544   cautious in assuming the validity of an IP number/DNS name association unless
3545   the response is protected by DNSSEC (<xref target="RFC4033"/>).
3546</t>
3547</section>
3548
3549<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3550<t>
3551   By their very nature, HTTP intermediaries are men-in-the-middle, and
3552   represent an opportunity for man-in-the-middle attacks. Compromise of
3553   the systems on which the intermediaries run can result in serious security
3554   and privacy problems. Intermediaries have access to security-related
3555   information, personal information about individual users and
3556   organizations, and proprietary information belonging to users and
3557   content providers. A compromised intermediary, or an intermediary
3558   implemented or configured without regard to security and privacy
3559   considerations, might be used in the commission of a wide range of
3560   potential attacks.
3561</t>
3562<t>
3563   Intermediaries that contain a shared cache are especially vulnerable
3564   to cache poisoning attacks.
3565</t>
3566<t>
3567   Implementers need to consider the privacy and security
3568   implications of their design and coding decisions, and of the
3569   configuration options they provide to operators (especially the
3570   default configuration).
3571</t>
3572<t>
3573   Users need to be aware that intermediaries are no more trustworthy than
3574   the people who run them; HTTP itself cannot solve this problem.
3575</t>
3576</section>
3577
3578<section title="Buffer Overflows" anchor="attack.protocol.element.size.overflows">
3579<t>
3580   Because HTTP uses mostly textual, character-delimited fields, attackers can
3581   overflow buffers in implementations, and/or perform a Denial of Service
3582   against implementations that accept fields with unlimited lengths.
3583</t>
3584<t>
3585   To promote interoperability, this specification makes specific
3586   recommendations for minimum size limits on request-line
3587   (<xref target="request.line"/>)
3588   and blocks of header fields (<xref target="header.fields"/>). These are
3589   minimum recommendations, chosen to be supportable even by implementations
3590   with limited resources; it is expected that most implementations will
3591   choose substantially higher limits.
3592</t>
3593<t>
3594   This specification also provides a way for servers to reject messages that
3595   have request-targets that are too long (&status-414;) or request entities
3596   that are too large (&status-4xx;).
3597</t>
3598<t>
3599   Recipients &SHOULD; carefully limit the extent to which they read other
3600   fields, including (but not limited to) request methods, response status
3601   phrases, header field-names, and body chunks, so as to avoid denial of
3602   service attacks without impeding interoperability.
3603</t>
3604</section>
3605
3606<section title="Message Integrity" anchor="message.integrity">
3607<t>
3608   HTTP does not define a specific mechanism for ensuring message integrity,
3609   instead relying on the error-detection ability of underlying transport
3610   protocols and the use of length or chunk-delimited framing to detect
3611   completeness. Additional integrity mechanisms, such as hash functions or
3612   digital signatures applied to the content, can be selectively added to
3613   messages via extensible metadata header fields. Historically, the lack of
3614   a single integrity mechanism has been justified by the informal nature of
3615   most HTTP communication.  However, the prevalence of HTTP as an information
3616   access mechanism has resulted in its increasing use within environments
3617   where verification of message integrity is crucial.
3618</t>
3619<t>
3620   User agents are encouraged to implement configurable means for detecting
3621   and reporting failures of message integrity such that those means can be
3622   enabled within environments for which integrity is necessary. For example,
3623   a browser being used to view medical history or drug interaction
3624   information needs to indicate to the user when such information is detected
3625   by the protocol to be incomplete, expired, or corrupted during transfer.
3626   Such mechanisms might be selectively enabled via user agent extensions or
3627   the presence of message integrity metadata in a response.
3628   At a minimum, user agents ought to provide some indication that allows a
3629   user to distinguish between a complete and incomplete response message
3630   (<xref target="incomplete.messages"/>) when such verification is desired.
3631</t>
3632</section>
3633
3634<section title="Server Log Information" anchor="abuse.of.server.log.information">
3635<t>
3636   A server is in the position to save personal data about a user's requests
3637   over time, which might identify their reading patterns or subjects of
3638   interest.  In particular, log information gathered at an intermediary
3639   often contains a history of user agent interaction, across a multitude
3640   of sites, that can be traced to individual users.
3641</t>
3642<t>
3643   HTTP log information is confidential in nature; its handling is often
3644   constrained by laws and regulations.  Log information needs to be securely
3645   stored and appropriate guidelines followed for its analysis.
3646   Anonymization of personal information within individual entries helps,
3647   but is generally not sufficient to prevent real log traces from being
3648   re-identified based on correlation with other access characteristics.
3649   As such, access traces that are keyed to a specific client should not
3650   be published even if the key is pseudonymous.
3651</t>
3652<t>
3653   To minimize the risk of theft or accidental publication, log information
3654   should be purged of personally identifiable information, including
3655   user identifiers, IP addresses, and user-provided query parameters,
3656   as soon as that information is no longer necessary to support operational
3657   needs for security, auditing, or fraud control.
3658</t>
3659</section>
3660</section>
3661
3662<section title="Acknowledgments" anchor="acks">
3663<t>
3664   This edition of HTTP/1.1 builds on the many contributions that went into
3665   <xref target="RFC1945" format="none">RFC 1945</xref>,
3666   <xref target="RFC2068" format="none">RFC 2068</xref>,
3667   <xref target="RFC2145" format="none">RFC 2145</xref>, and
3668   <xref target="RFC2616" format="none">RFC 2616</xref>, including
3669   substantial contributions made by the previous authors, editors, and
3670   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
3671   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
3672   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
3673</t>
3674<t>
3675   Since 1999, the following contributors have helped improve the HTTP
3676   specification by reporting bugs, asking smart questions, drafting or
3677   reviewing text, and evaluating open issues:
3678</t>
3679<?BEGININC acks ?>
3680<t>Adam Barth,
3681Adam Roach,
3682Addison Phillips,
3683Adrian Chadd,
3684Adrien W. de Croy,
3685Alan Ford,
3686Alan Ruttenberg,
3687Albert Lunde,
3688Alek Storm,
3689Alex Rousskov,
3690Alexandre Morgaut,
3691Alexey Melnikov,
3692Alisha Smith,
3693Amichai Rothman,
3694Amit Klein,
3695Amos Jeffries,
3696Andreas Maier,
3697Andreas Petersson,
3698Anil Sharma,
3699Anne van Kesteren,
3700Anthony Bryan,
3701Asbjorn Ulsberg,
3702Ashok Kumar,
3703Balachander Krishnamurthy,
3704Barry Leiba,
3705Ben Laurie,
3706Benjamin Niven-Jenkins,
3707Bil Corry,
3708Bill Burke,
3709Bjoern Hoehrmann,
3710Bob Scheifler,
3711Boris Zbarsky,
3712Brett Slatkin,
3713Brian Kell,
3714Brian McBarron,
3715Brian Pane,
3716Brian Smith,
3717Bryce Nesbitt,
3718Cameron Heavon-Jones,
3719Carl Kugler,
3720Carsten Bormann,
3721Charles Fry,
3722Chris Newman,
3723Chris Weber,
3724Cyrus Daboo,
3725Dale Robert Anderson,
3726Dan Wing,
3727Dan Winship,
3728Daniel Stenberg,
3729Darrel Miller,
3730Dave Cridland,
3731Dave Crocker,
3732Dave Kristol,
3733David Booth,
3734David Singer,
3735David W. Morris,
3736Diwakar Shetty,
3737Dmitry Kurochkin,
3738Drummond Reed,
3739Duane Wessels,
3740Duncan Cragg,
3741Edward Lee,
3742Eliot Lear,
3743Eran Hammer-Lahav,
3744Eric D. Williams,
3745Eric J. Bowman,
3746Eric Lawrence,
3747Eric Rescorla,
3748Erik Aronesty,
3749Evan Prodromou,
3750Florian Weimer,
3751Frank Ellermann,
3752Fred Bohle,
3753Gabriel Montenegro,
3754Geoffrey Sneddon,
3755Gervase Markham,
3756Grahame Grieve,
3757Greg Wilkins,
3758Harald Tveit Alvestrand,
3759Harry Halpin,
3760Helge Hess,
3761Henrik Nordstrom,
3762Henry S. Thompson,
3763Henry Story,
3764Herbert van de Sompel,
3765Howard Melman,
3766Hugo Haas,
3767Ian Fette,
3768Ian Hickson,
3769Ido Safruti,
3770Ilya Grigorik,
3771Ingo Struck,
3772J. Ross Nicoll,
3773James H. Manger,
3774James Lacey,
3775James M. Snell,
3776Jamie Lokier,
3777Jan Algermissen,
3778Jeff Hodges (who came up with the term 'effective Request-URI'),
3779Jeff Walden,
3780Jeroen de Borst,
3781Jim Luther,
3782Joe D. Williams,
3783Joe Gregorio,
3784Joe Orton,
3785John C. Klensin,
3786John C. Mallery,
3787John Cowan,
3788John Kemp,
3789John Panzer,
3790John Schneider,
3791John Stracke,
3792John Sullivan,
3793Jonas Sicking,
3794Jonathan A. Rees,
3795Jonathan Billington,
3796Jonathan Moore,
3797Jonathan Rees,
3798Jonathan Silvera,
3799Jordi Ros,
3800Joris Dobbelsteen,
3801Josh Cohen,
3802Julien Pierre,
3803Jungshik Shin,
3804Justin Chapweske,
3805Justin Erenkrantz,
3806Justin James,
3807Kalvinder Singh,
3808Karl Dubost,
3809Keith Hoffman,
3810Keith Moore,
3811Ken Murchison,
3812Koen Holtman,
3813Konstantin Voronkov,
3814Kris Zyp,
3815Lisa Dusseault,
3816Maciej Stachowiak,
3817Marc Schneider,
3818Marc Slemko,
3819Mark Baker,
3820Mark Pauley,
3821Mark Watson,
3822Markus Isomaki,
3823Markus Lanthaler,
3824Martin J. Duerst,
3825Martin Musatov,
3826Martin Nilsson,
3827Martin Thomson,
3828Matt Lynch,
3829Matthew Cox,
3830Max Clark,
3831Michael Burrows,
3832Michael Hausenblas,
3833Mike Amundsen,
3834Mike Belshe,
3835Mike Kelly,
3836Mike Schinkel,
3837Miles Sabin,
3838Murray S. Kucherawy,
3839Mykyta Yevstifeyev,
3840Nathan Rixham,
3841Nicholas Shanks,
3842Nico Williams,
3843Nicolas Alvarez,
3844Nicolas Mailhot,
3845Noah Slater,
3846Pablo Castro,
3847Pat Hayes,
3848Patrick R. McManus,
3849Patrik Faltstrom,
3850Paul E. Jones,
3851Paul Hoffman,
3852Paul Marquess,
3853Peter Lepeska,
3854Peter Saint-Andre,
3855Peter Watkins,
3856Phil Archer,
3857Philippe Mougin,
3858Phillip Hallam-Baker,
3859Poul-Henning Kamp,
3860Preethi Natarajan,
3861Rajeev Bector,
3862Ray Polk,
3863Reto Bachmann-Gmuer,
3864Richard Cyganiak,
3865Robert Brewer,
3866Robert Collins,
3867Robert O'Callahan,
3868Robert Olofsson,
3869Robert Sayre,
3870Robert Siemer,
3871Robert de Wilde,
3872Roberto Javier Godoy,
3873Roberto Peon,
3874Roland Zink,
3875Ronny Widjaja,
3876S. Mike Dierken,
3877Salvatore Loreto,
3878Sam Johnston,
3879Sam Ruby,
3880Scott Lawrence (who maintained the original issues list),
3881Sean B. Palmer,
3882Shane McCarron,
3883Stefan Eissing,
3884Stefan Tilkov,
3885Stefanos Harhalakis,
3886Stephane Bortzmeyer,
3887Stephen Farrell,
3888Stephen Ludin,
3889Stuart Williams,
3890Subbu Allamaraju,
3891Subramanian Moonesamy,
3892Sylvain Hellegouarch,
3893Tapan Divekar,
3894Tatsuya Hayashi,
3895Ted Hardie,
3896Thomas Broyer,
3897Thomas Fossati,
3898Thomas Nordin,
3899Thomas Roessler,
3900Tim Bray,
3901Tim Morgan,
3902Tim Olsen,
3903Tobias Oberstein,
3904Tom Zhou,
3905Travis Snoozy,
3906Tyler Close,
3907Vincent Murphy,
3908Wenbo Zhu,
3909Werner Baumann,
3910Wilbur Streett,
3911Wilfredo Sanchez Vega,
3912William A. Rowe Jr.,
3913William Chan,
3914Willy Tarreau,
3915Xiaoshu Wang,
3916Yaron Goland,
3917Yngve Nysaeter Pettersen,
3918Yoav Nir,
3919Yogesh Bang,
3920Yutaka Oiwa,
3921Yves Lafon (long-time member of the editor team),
3922Zed A. Shaw, and
3923Zhong Yu.
3924</t>
3925<?ENDINC acks ?>
3926<t>
3927   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
3928   acknowledgements from prior revisions.
3929</t>
3930</section>
3931
3932</middle>
3933<back>
3934
3935<references title="Normative References">
3936
3937<reference anchor="Part2">
3938  <front>
3939    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
3940    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3941      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
3942      <address><email>fielding@gbiv.com</email></address>
3943    </author>
3944    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3945      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3946      <address><email>julian.reschke@greenbytes.de</email></address>
3947    </author>
3948    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3949  </front>
3950  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
3951  <x:source href="p2-semantics.xml" basename="p2-semantics">
3952    <x:defines>1xx (Informational)</x:defines>
3953    <x:defines>1xx</x:defines>
3954    <x:defines>100 (Continue)</x:defines>
3955    <x:defines>101 (Switching Protocols)</x:defines>
3956    <x:defines>2xx (Successful)</x:defines>
3957    <x:defines>2xx</x:defines>
3958    <x:defines>200 (OK)</x:defines>
3959    <x:defines>204 (No Content)</x:defines>
3960    <x:defines>3xx (Redirection)</x:defines>
3961    <x:defines>3xx</x:defines>
3962    <x:defines>301 (Moved Permanently)</x:defines>
3963    <x:defines>4xx (Client Error)</x:defines>
3964    <x:defines>4xx</x:defines>
3965    <x:defines>400 (Bad Request)</x:defines>
3966    <x:defines>411 (Length Required)</x:defines>
3967    <x:defines>414 (URI Too Long)</x:defines>
3968    <x:defines>417 (Expectation Failed)</x:defines>
3969    <x:defines>426 (Upgrade Required)</x:defines>
3970    <x:defines>501 (Not Implemented)</x:defines>
3971    <x:defines>502 (Bad Gateway)</x:defines>
3972    <x:defines>505 (HTTP Version Not Supported)</x:defines>
3973    <x:defines>Allow</x:defines>
3974    <x:defines>Content-Encoding</x:defines>
3975    <x:defines>Content-Location</x:defines>
3976    <x:defines>Content-Type</x:defines>
3977    <x:defines>Date</x:defines>
3978    <x:defines>Expect</x:defines>
3979    <x:defines>Location</x:defines>
3980    <x:defines>Server</x:defines>
3981    <x:defines>User-Agent</x:defines>
3982  </x:source>
3983</reference>
3984
3985<reference anchor="Part4">
3986  <front>
3987    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
3988    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
3989      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
3990      <address><email>fielding@gbiv.com</email></address>
3991    </author>
3992    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
3993      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3994      <address><email>julian.reschke@greenbytes.de</email></address>
3995    </author>
3996    <date month="&ID-MONTH;" year="&ID-YEAR;" />
3997  </front>
3998  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
3999  <x:source basename="p4-conditional" href="p4-conditional.xml">
4000    <x:defines>304 (Not Modified)</x:defines>
4001    <x:defines>ETag</x:defines>
4002    <x:defines>Last-Modified</x:defines>
4003  </x:source>
4004</reference>
4005
4006<reference anchor="Part5">
4007  <front>
4008    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
4009    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4010      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4011      <address><email>fielding@gbiv.com</email></address>
4012    </author>
4013    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
4014      <organization abbrev="W3C">World Wide Web Consortium</organization>
4015      <address><email>ylafon@w3.org</email></address>
4016    </author>
4017    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4018      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4019      <address><email>julian.reschke@greenbytes.de</email></address>
4020    </author>
4021    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4022  </front>
4023  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4024  <x:source href="p5-range.xml" basename="p5-range">
4025    <x:defines>Content-Range</x:defines>
4026  </x:source>
4027</reference>
4028
4029<reference anchor="Part6">
4030  <front>
4031    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4032    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4033      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4034      <address><email>fielding@gbiv.com</email></address>
4035    </author>
4036    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4037      <organization>Akamai</organization>
4038      <address><email>mnot@mnot.net</email></address>
4039    </author>
4040    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4041      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4042      <address><email>julian.reschke@greenbytes.de</email></address>
4043    </author>
4044    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4045  </front>
4046  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4047  <x:source href="p6-cache.xml" basename="p6-cache">
4048    <x:defines>Cache-Control</x:defines>
4049    <x:defines>Expires</x:defines>
4050  </x:source>
4051</reference>
4052
4053<reference anchor="Part7">
4054  <front>
4055    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4056    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4057      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4058      <address><email>fielding@gbiv.com</email></address>
4059    </author>
4060    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4061      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4062      <address><email>julian.reschke@greenbytes.de</email></address>
4063    </author>
4064    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4065  </front>
4066  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4067  <x:source href="p7-auth.xml" basename="p7-auth">
4068    <x:defines>Proxy-Authenticate</x:defines>
4069    <x:defines>Proxy-Authorization</x:defines>
4070  </x:source>
4071</reference>
4072
4073<reference anchor="RFC5234">
4074  <front>
4075    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4076    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4077      <organization>Brandenburg InternetWorking</organization>
4078      <address>
4079        <email>dcrocker@bbiw.net</email>
4080      </address> 
4081    </author>
4082    <author initials="P." surname="Overell" fullname="Paul Overell">
4083      <organization>THUS plc.</organization>
4084      <address>
4085        <email>paul.overell@thus.net</email>
4086      </address>
4087    </author>
4088    <date month="January" year="2008"/>
4089  </front>
4090  <seriesInfo name="STD" value="68"/>
4091  <seriesInfo name="RFC" value="5234"/>
4092</reference>
4093
4094<reference anchor="RFC2119">
4095  <front>
4096    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4097    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4098      <organization>Harvard University</organization>
4099      <address><email>sob@harvard.edu</email></address>
4100    </author>
4101    <date month="March" year="1997"/>
4102  </front>
4103  <seriesInfo name="BCP" value="14"/>
4104  <seriesInfo name="RFC" value="2119"/>
4105</reference>
4106
4107<reference anchor="RFC3986">
4108 <front>
4109  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4110  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4111    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4112    <address>
4113       <email>timbl@w3.org</email>
4114       <uri>http://www.w3.org/People/Berners-Lee/</uri>
4115    </address>
4116  </author>
4117  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4118    <organization abbrev="Day Software">Day Software</organization>
4119    <address>
4120      <email>fielding@gbiv.com</email>
4121      <uri>http://roy.gbiv.com/</uri>
4122    </address>
4123  </author>
4124  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4125    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4126    <address>
4127      <email>LMM@acm.org</email>
4128      <uri>http://larry.masinter.net/</uri>
4129    </address>
4130  </author>
4131  <date month='January' year='2005'></date>
4132 </front>
4133 <seriesInfo name="STD" value="66"/>
4134 <seriesInfo name="RFC" value="3986"/>
4135</reference>
4136
4137<reference anchor="USASCII">
4138  <front>
4139    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4140    <author>
4141      <organization>American National Standards Institute</organization>
4142    </author>
4143    <date year="1986"/>
4144  </front>
4145  <seriesInfo name="ANSI" value="X3.4"/>
4146</reference>
4147
4148<reference anchor="RFC1950">
4149  <front>
4150    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4151    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4152      <organization>Aladdin Enterprises</organization>
4153      <address><email>ghost@aladdin.com</email></address>
4154    </author>
4155    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4156    <date month="May" year="1996"/>
4157  </front>
4158  <seriesInfo name="RFC" value="1950"/>
4159  <!--<annotation>
4160    RFC 1950 is an Informational RFC, thus it might be less stable than
4161    this specification. On the other hand, this downward reference was
4162    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4163    therefore it is unlikely to cause problems in practice. See also
4164    <xref target="BCP97"/>.
4165  </annotation>-->
4166</reference>
4167
4168<reference anchor="RFC1951">
4169  <front>
4170    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4171    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4172      <organization>Aladdin Enterprises</organization>
4173      <address><email>ghost@aladdin.com</email></address>
4174    </author>
4175    <date month="May" year="1996"/>
4176  </front>
4177  <seriesInfo name="RFC" value="1951"/>
4178  <!--<annotation>
4179    RFC 1951 is an Informational RFC, thus it might be less stable than
4180    this specification. On the other hand, this downward reference was
4181    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4182    therefore it is unlikely to cause problems in practice. See also
4183    <xref target="BCP97"/>.
4184  </annotation>-->
4185</reference>
4186
4187<reference anchor="RFC1952">
4188  <front>
4189    <title>GZIP file format specification version 4.3</title>
4190    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4191      <organization>Aladdin Enterprises</organization>
4192      <address><email>ghost@aladdin.com</email></address>
4193    </author>
4194    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4195      <address><email>gzip@prep.ai.mit.edu</email></address>
4196    </author>
4197    <author initials="M." surname="Adler" fullname="Mark Adler">
4198      <address><email>madler@alumni.caltech.edu</email></address>
4199    </author>
4200    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4201      <address><email>ghost@aladdin.com</email></address>
4202    </author>
4203    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4204      <address><email>randeg@alumni.rpi.edu</email></address>
4205    </author>
4206    <date month="May" year="1996"/>
4207  </front>
4208  <seriesInfo name="RFC" value="1952"/>
4209  <!--<annotation>
4210    RFC 1952 is an Informational RFC, thus it might be less stable than
4211    this specification. On the other hand, this downward reference was
4212    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4213    therefore it is unlikely to cause problems in practice. See also
4214    <xref target="BCP97"/>.
4215  </annotation>-->
4216</reference>
4217
4218</references>
4219
4220<references title="Informative References">
4221
4222<reference anchor="ISO-8859-1">
4223  <front>
4224    <title>
4225     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4226    </title>
4227    <author>
4228      <organization>International Organization for Standardization</organization>
4229    </author>
4230    <date year="1998"/>
4231  </front>
4232  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4233</reference>
4234
4235<reference anchor='RFC1919'>
4236  <front>
4237    <title>Classical versus Transparent IP Proxies</title>
4238    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4239      <address><email>mchatel@pax.eunet.ch</email></address>
4240    </author>
4241    <date year='1996' month='March' />
4242  </front>
4243  <seriesInfo name='RFC' value='1919' />
4244</reference>
4245
4246<reference anchor="RFC1945">
4247  <front>
4248    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4249    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4250      <organization>MIT, Laboratory for Computer Science</organization>
4251      <address><email>timbl@w3.org</email></address>
4252    </author>
4253    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4254      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4255      <address><email>fielding@ics.uci.edu</email></address>
4256    </author>
4257    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4258      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4259      <address><email>frystyk@w3.org</email></address>
4260    </author>
4261    <date month="May" year="1996"/>
4262  </front>
4263  <seriesInfo name="RFC" value="1945"/>
4264</reference>
4265
4266<reference anchor="RFC2045">
4267  <front>
4268    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4269    <author initials="N." surname="Freed" fullname="Ned Freed">
4270      <organization>Innosoft International, Inc.</organization>
4271      <address><email>ned@innosoft.com</email></address>
4272    </author>
4273    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4274      <organization>First Virtual Holdings</organization>
4275      <address><email>nsb@nsb.fv.com</email></address>
4276    </author>
4277    <date month="November" year="1996"/>
4278  </front>
4279  <seriesInfo name="RFC" value="2045"/>
4280</reference>
4281
4282<reference anchor="RFC2047">
4283  <front>
4284    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4285    <author initials="K." surname="Moore" fullname="Keith Moore">
4286      <organization>University of Tennessee</organization>
4287      <address><email>moore@cs.utk.edu</email></address>
4288    </author>
4289    <date month="November" year="1996"/>
4290  </front>
4291  <seriesInfo name="RFC" value="2047"/>
4292</reference>
4293
4294<reference anchor="RFC2068">
4295  <front>
4296    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4297    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
4298      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4299      <address><email>fielding@ics.uci.edu</email></address>
4300    </author>
4301    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4302      <organization>MIT Laboratory for Computer Science</organization>
4303      <address><email>jg@w3.org</email></address>
4304    </author>
4305    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4306      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4307      <address><email>mogul@wrl.dec.com</email></address>
4308    </author>
4309    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4310      <organization>MIT Laboratory for Computer Science</organization>
4311      <address><email>frystyk@w3.org</email></address>
4312    </author>
4313    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4314      <organization>MIT Laboratory for Computer Science</organization>
4315      <address><email>timbl@w3.org</email></address>
4316    </author>
4317    <date month="January" year="1997"/>
4318  </front>
4319  <seriesInfo name="RFC" value="2068"/>
4320</reference>
4321
4322<reference anchor="RFC2145">
4323  <front>
4324    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4325    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4326      <organization>Western Research Laboratory</organization>
4327      <address><email>mogul@wrl.dec.com</email></address>
4328    </author>
4329    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4330      <organization>Department of Information and Computer Science</organization>
4331      <address><email>fielding@ics.uci.edu</email></address>
4332    </author>
4333    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4334      <organization>MIT Laboratory for Computer Science</organization>
4335      <address><email>jg@w3.org</email></address>
4336    </author>
4337    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4338      <organization>W3 Consortium</organization>
4339      <address><email>frystyk@w3.org</email></address>
4340    </author>
4341    <date month="May" year="1997"/>
4342  </front>
4343  <seriesInfo name="RFC" value="2145"/>
4344</reference>
4345
4346<reference anchor="RFC2616">
4347  <front>
4348    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4349    <author initials="R." surname="Fielding" fullname="R. Fielding">
4350      <organization>University of California, Irvine</organization>
4351      <address><email>fielding@ics.uci.edu</email></address>
4352    </author>
4353    <author initials="J." surname="Gettys" fullname="J. Gettys">
4354      <organization>W3C</organization>
4355      <address><email>jg@w3.org</email></address>
4356    </author>
4357    <author initials="J." surname="Mogul" fullname="J. Mogul">
4358      <organization>Compaq Computer Corporation</organization>
4359      <address><email>mogul@wrl.dec.com</email></address>
4360    </author>
4361    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4362      <organization>MIT Laboratory for Computer Science</organization>
4363      <address><email>frystyk@w3.org</email></address>
4364    </author>
4365    <author initials="L." surname="Masinter" fullname="L. Masinter">
4366      <organization>Xerox Corporation</organization>
4367      <address><email>masinter@parc.xerox.com</email></address>
4368    </author>
4369    <author initials="P." surname="Leach" fullname="P. Leach">
4370      <organization>Microsoft Corporation</organization>
4371      <address><email>paulle@microsoft.com</email></address>
4372    </author>
4373    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4374      <organization>W3C</organization>
4375      <address><email>timbl@w3.org</email></address>
4376    </author>
4377    <date month="June" year="1999"/>
4378  </front>
4379  <seriesInfo name="RFC" value="2616"/>
4380</reference>
4381
4382<reference anchor='RFC2817'>
4383  <front>
4384    <title>Upgrading to TLS Within HTTP/1.1</title>
4385    <author initials='R.' surname='Khare' fullname='R. Khare'>
4386      <organization>4K Associates / UC Irvine</organization>
4387      <address><email>rohit@4K-associates.com</email></address>
4388    </author>
4389    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4390      <organization>Agranat Systems, Inc.</organization>
4391      <address><email>lawrence@agranat.com</email></address>
4392    </author>
4393    <date year='2000' month='May' />
4394  </front>
4395  <seriesInfo name='RFC' value='2817' />
4396</reference>
4397
4398<reference anchor='RFC2818'>
4399  <front>
4400    <title>HTTP Over TLS</title>
4401    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4402      <organization>RTFM, Inc.</organization>
4403      <address><email>ekr@rtfm.com</email></address>
4404    </author>
4405    <date year='2000' month='May' />
4406  </front>
4407  <seriesInfo name='RFC' value='2818' />
4408</reference>
4409
4410<reference anchor='RFC3040'>
4411  <front>
4412    <title>Internet Web Replication and Caching Taxonomy</title>
4413    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4414      <organization>Equinix, Inc.</organization>
4415    </author>
4416    <author initials='I.' surname='Melve' fullname='I. Melve'>
4417      <organization>UNINETT</organization>
4418    </author>
4419    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4420      <organization>CacheFlow Inc.</organization>
4421    </author>
4422    <date year='2001' month='January' />
4423  </front>
4424  <seriesInfo name='RFC' value='3040' />
4425</reference>
4426
4427<reference anchor='BCP90'>
4428  <front>
4429    <title>Registration Procedures for Message Header Fields</title>
4430    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4431      <organization>Nine by Nine</organization>
4432      <address><email>GK-IETF@ninebynine.org</email></address>
4433    </author>
4434    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4435      <organization>BEA Systems</organization>
4436      <address><email>mnot@pobox.com</email></address>
4437    </author>
4438    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4439      <organization>HP Labs</organization>
4440      <address><email>JeffMogul@acm.org</email></address>
4441    </author>
4442    <date year='2004' month='September' />
4443  </front>
4444  <seriesInfo name='BCP' value='90' />
4445  <seriesInfo name='RFC' value='3864' />
4446</reference>
4447
4448<reference anchor='RFC4033'>
4449  <front>
4450    <title>DNS Security Introduction and Requirements</title>
4451    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4452    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4453    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4454    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4455    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4456    <date year='2005' month='March' />
4457  </front>
4458  <seriesInfo name='RFC' value='4033' />
4459</reference>
4460
4461<reference anchor="BCP13">
4462  <front>
4463    <title>Media Type Specifications and Registration Procedures</title>
4464    <author initials="N." surname="Freed" fullname="Ned Freed">
4465      <organization>Oracle</organization>
4466      <address>
4467        <email>ned+ietf@mrochek.com</email>
4468      </address>
4469    </author>
4470    <author initials="J." surname="Klensin" fullname="John C. Klensin">
4471      <address>
4472        <email>john+ietf@jck.com</email>
4473      </address>
4474    </author>
4475    <author initials="T." surname="Hansen" fullname="Tony Hansen">
4476      <organization>AT&amp;T Laboratories</organization>
4477      <address>
4478        <email>tony+mtsuffix@maillennium.att.com</email>
4479      </address>
4480    </author>
4481    <date year="2013" month="January"/>
4482  </front>
4483  <seriesInfo name="BCP" value="13"/>
4484  <seriesInfo name="RFC" value="6838"/>
4485</reference>
4486
4487<reference anchor='BCP115'>
4488  <front>
4489    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4490    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4491      <organization>AT&amp;T Laboratories</organization>
4492      <address>
4493        <email>tony+urireg@maillennium.att.com</email>
4494      </address>
4495    </author>
4496    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4497      <organization>Qualcomm, Inc.</organization>
4498      <address>
4499        <email>hardie@qualcomm.com</email>
4500      </address>
4501    </author>
4502    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4503      <organization>Adobe Systems</organization>
4504      <address>
4505        <email>LMM@acm.org</email>
4506      </address>
4507    </author>
4508    <date year='2006' month='February' />
4509  </front>
4510  <seriesInfo name='BCP' value='115' />
4511  <seriesInfo name='RFC' value='4395' />
4512</reference>
4513
4514<reference anchor='RFC4559'>
4515  <front>
4516    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4517    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4518    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4519    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4520    <date year='2006' month='June' />
4521  </front>
4522  <seriesInfo name='RFC' value='4559' />
4523</reference>
4524
4525<reference anchor='RFC5226'>
4526  <front>
4527    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4528    <author initials='T.' surname='Narten' fullname='T. Narten'>
4529      <organization>IBM</organization>
4530      <address><email>narten@us.ibm.com</email></address>
4531    </author>
4532    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4533      <organization>Google</organization>
4534      <address><email>Harald@Alvestrand.no</email></address>
4535    </author>
4536    <date year='2008' month='May' />
4537  </front>
4538  <seriesInfo name='BCP' value='26' />
4539  <seriesInfo name='RFC' value='5226' />
4540</reference>
4541
4542<reference anchor='RFC5246'>
4543   <front>
4544      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4545      <author initials='T.' surname='Dierks' fullname='T. Dierks'>
4546         <organization />
4547      </author>
4548      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4549         <organization>RTFM, Inc.</organization>
4550      </author>
4551      <date year='2008' month='August' />
4552   </front>
4553   <seriesInfo name='RFC' value='5246' />
4554</reference>
4555
4556<reference anchor="RFC5322">
4557  <front>
4558    <title>Internet Message Format</title>
4559    <author initials="P." surname="Resnick" fullname="P. Resnick">
4560      <organization>Qualcomm Incorporated</organization>
4561    </author>
4562    <date year="2008" month="October"/>
4563  </front> 
4564  <seriesInfo name="RFC" value="5322"/>
4565</reference>
4566
4567<reference anchor="RFC6265">
4568  <front>
4569    <title>HTTP State Management Mechanism</title>
4570    <author initials="A." surname="Barth" fullname="Adam Barth">
4571      <organization abbrev="U.C. Berkeley">
4572        University of California, Berkeley
4573      </organization>
4574      <address><email>abarth@eecs.berkeley.edu</email></address>
4575    </author>
4576    <date year="2011" month="April" />
4577  </front>
4578  <seriesInfo name="RFC" value="6265"/>
4579</reference>
4580
4581<!--<reference anchor='BCP97'>
4582  <front>
4583    <title>Handling Normative References to Standards-Track Documents</title>
4584    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
4585      <address>
4586        <email>klensin+ietf@jck.com</email>
4587      </address>
4588    </author>
4589    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
4590      <organization>MIT</organization>
4591      <address>
4592        <email>hartmans-ietf@mit.edu</email>
4593      </address>
4594    </author>
4595    <date year='2007' month='June' />
4596  </front>
4597  <seriesInfo name='BCP' value='97' />
4598  <seriesInfo name='RFC' value='4897' />
4599</reference>-->
4600
4601<reference anchor="Kri2001" target="http://arxiv.org/abs/cs.SE/0105018">
4602  <front>
4603    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
4604    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
4605    <date year="2001" month="November"/>
4606  </front>
4607  <seriesInfo name="ACM Transactions on Internet Technology" value="Vol. 1, #2"/>
4608</reference>
4609
4610</references>
4611
4612
4613<section title="HTTP Version History" anchor="compatibility">
4614<t>
4615   HTTP has been in use by the World-Wide Web global information initiative
4616   since 1990. The first version of HTTP, later referred to as HTTP/0.9,
4617   was a simple protocol for hypertext data transfer across the Internet
4618   with only a single request method (GET) and no metadata.
4619   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
4620   methods and MIME-like messaging that could include metadata about the data
4621   transferred and modifiers on the request/response semantics. However,
4622   HTTP/1.0 did not sufficiently take into consideration the effects of
4623   hierarchical proxies, caching, the need for persistent connections, or
4624   name-based virtual hosts. The proliferation of incompletely-implemented
4625   applications calling themselves "HTTP/1.0" further necessitated a
4626   protocol version change in order for two communicating applications
4627   to determine each other's true capabilities.
4628</t>
4629<t>
4630   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
4631   requirements that enable reliable implementations, adding only
4632   those new features that will either be safely ignored by an HTTP/1.0
4633   recipient or only sent when communicating with a party advertising
4634   conformance with HTTP/1.1.
4635</t>
4636<t>
4637   It is beyond the scope of a protocol specification to mandate
4638   conformance with previous versions. HTTP/1.1 was deliberately
4639   designed, however, to make supporting previous versions easy.
4640   We would expect a general-purpose HTTP/1.1 server to understand
4641   any valid request in the format of HTTP/1.0 and respond appropriately
4642   with an HTTP/1.1 message that only uses features understood (or
4643   safely ignored) by HTTP/1.0 clients.  Likewise, we would expect
4644   an HTTP/1.1 client to understand any valid HTTP/1.0 response.
4645</t>
4646<t>
4647   Since HTTP/0.9 did not support header fields in a request,
4648   there is no mechanism for it to support name-based virtual
4649   hosts (selection of resource by inspection of the <x:ref>Host</x:ref> header
4650   field).  Any server that implements name-based virtual hosts
4651   ought to disable support for HTTP/0.9.  Most requests that
4652   appear to be HTTP/0.9 are, in fact, badly constructed HTTP/1.x
4653   requests wherein a buggy client failed to properly encode
4654   linear whitespace found in a URI reference and placed in
4655   the request-target.
4656</t>
4657
4658<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
4659<t>
4660   This section summarizes major differences between versions HTTP/1.0
4661   and HTTP/1.1.
4662</t>
4663
4664<section title="Multi-homed Web Servers" anchor="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">
4665<t>
4666   The requirements that clients and servers support the <x:ref>Host</x:ref>
4667   header field (<xref target="header.host"/>), report an error if it is
4668   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
4669   are among the most important changes defined by HTTP/1.1.
4670</t>
4671<t>
4672   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4673   addresses and servers; there was no other established mechanism for
4674   distinguishing the intended server of a request than the IP address
4675   to which that request was directed. The <x:ref>Host</x:ref> header field was
4676   introduced during the development of HTTP/1.1 and, though it was
4677   quickly implemented by most HTTP/1.0 browsers, additional requirements
4678   were placed on all HTTP/1.1 requests in order to ensure complete
4679   adoption.  At the time of this writing, most HTTP-based services
4680   are dependent upon the Host header field for targeting requests.
4681</t>
4682</section>
4683
4684<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4685<t>
4686   In HTTP/1.0, each connection is established by the client prior to the
4687   request and closed by the server after sending the response. However, some
4688   implementations implement the explicitly negotiated ("Keep-Alive") version
4689   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
4690   target="RFC2068"/>.
4691</t>
4692<t>
4693   Some clients and servers might wish to be compatible with these previous
4694   approaches to persistent connections, by explicitly negotiating for them
4695   with a "Connection: keep-alive" request header field. However, some
4696   experimental implementations of HTTP/1.0 persistent connections are faulty;
4697   for example, if an HTTP/1.0 proxy server doesn't understand
4698   <x:ref>Connection</x:ref>, it will erroneously forward that header field
4699   to the next inbound server, which would result in a hung connection.
4700</t>
4701<t>
4702   One attempted solution was the introduction of a Proxy-Connection header
4703   field, targeted specifically at proxies. In practice, this was also
4704   unworkable, because proxies are often deployed in multiple layers, bringing
4705   about the same problem discussed above.
4706</t>
4707<t>
4708   As a result, clients are encouraged not to send the Proxy-Connection header
4709   field in any requests.
4710</t>
4711<t>
4712   Clients are also encouraged to consider the use of Connection: keep-alive
4713   in requests carefully; while they can enable persistent connections with
4714   HTTP/1.0 servers, clients using them need will need to monitor the
4715   connection for "hung" requests (which indicate that the client ought stop
4716   sending the header field), and this mechanism ought not be used by clients
4717   at all when a proxy is being used.
4718</t>
4719</section>
4720
4721<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
4722<t>
4723   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
4724   (<xref target="header.transfer-encoding"/>).
4725   Transfer codings need to be decoded prior to forwarding an HTTP message
4726   over a MIME-compliant protocol.
4727</t>
4728</section>
4729
4730</section>
4731
4732<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4733<t>
4734  HTTP's approach to error handling has been explained.
4735  (<xref target="conformance"/>)
4736</t>
4737<t>
4738  The expectation to support HTTP/0.9 requests has been removed.
4739</t>
4740<t>
4741  The term "Effective Request URI" has been introduced.
4742  (<xref target="effective.request.uri" />)
4743</t>
4744<t>
4745  HTTP messages can be (and often are) buffered by implementations; despite
4746  it sometimes being available as a stream, HTTP is fundamentally a
4747  message-oriented protocol.
4748  (<xref target="http.message" />)
4749</t>
4750<t>
4751  Minimum supported sizes for various protocol elements have been
4752  suggested, to improve interoperability.
4753</t>
4754<t>
4755  Header fields that span multiple lines ("line folding") are deprecated.
4756  (<xref target="field.parsing" />)
4757</t>
4758<t>
4759  The HTTP-version ABNF production has been clarified to be case-sensitive.
4760  Additionally, version numbers has been restricted to single digits, due
4761  to the fact that implementations are known to handle multi-digit version
4762  numbers incorrectly.
4763  (<xref target="http.version"/>)
4764</t>
4765<t>
4766  The HTTPS URI scheme is now defined by this specification; previously,
4767  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
4768  (<xref target="https.uri"/>)
4769</t>
4770<t>
4771  The HTTPS URI scheme implies end-to-end security.
4772  (<xref target="https.uri"/>)
4773</t>
4774<t>
4775  Userinfo (i.e., username and password) are now disallowed in HTTP and
4776  HTTPS URIs, because of security issues related to their transmission on the
4777  wire.
4778  (<xref target="http.uri" />)
4779</t>
4780<t>
4781  Invalid whitespace around field-names is now required to be rejected,
4782  because accepting it represents a security vulnerability.
4783  (<xref target="header.fields"/>)
4784</t>
4785<t>
4786  The ABNF productions defining header fields now only list the field value.
4787  (<xref target="header.fields"/>)
4788</t>
4789<t>
4790  Rules about implicit linear whitespace between certain grammar productions
4791  have been removed; now whitespace is only allowed where specifically
4792  defined in the ABNF.
4793  (<xref target="whitespace"/>)
4794</t>
4795<t> 
4796  The NUL octet is no longer allowed in comment and quoted-string text, and
4797  handling of backslash-escaping in them has been clarified.
4798  (<xref target="field.components"/>)
4799</t> 
4800<t>
4801  The quoted-pair rule no longer allows escaping control characters other than
4802  HTAB.
4803  (<xref target="field.components"/>)
4804</t>
4805<t>
4806  Non-ASCII content in header fields and the reason phrase has been obsoleted
4807  and made opaque (the TEXT rule was removed).
4808  (<xref target="field.components"/>)
4809</t>
4810<t>
4811  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
4812  handled as errors by recipients.
4813  (<xref target="header.content-length"/>)
4814</t>
4815<t>
4816  The "identity" transfer coding token has been removed.
4817  (Sections <xref format="counter" target="message.body"/> and
4818  <xref format="counter" target="transfer.codings"/>)
4819</t>
4820<t>
4821  The algorithm for determining the message body length has been clarified
4822  to indicate all of the special cases (e.g., driven by methods or status
4823  codes) that affect it, and that new protocol elements cannot define such
4824  special cases.
4825  (<xref target="message.body.length"/>)
4826</t>
4827<t>
4828  "multipart/byteranges" is no longer a way of determining message body length
4829  detection.
4830  (<xref target="message.body.length"/>)
4831</t>
4832<t>
4833  CONNECT is a new, special case in determining message body length.
4834  (<xref target="message.body.length"/>)
4835</t>
4836<t>
4837  Chunk length does not include the count of the octets in the
4838  chunk header and trailer.
4839  (<xref target="chunked.encoding"/>)
4840</t>
4841<t>
4842  Use of chunk extensions is deprecated, and line folding in them is
4843  disallowed.
4844  (<xref target="chunked.encoding"/>)
4845</t>
4846<t>
4847  The path-absolute + query components of RFC3986 have been used to define the
4848  request-target, instead of abs_path from RFC 1808.
4849  (<xref target="request-target"/>)
4850</t>
4851<t>
4852  The asterisk form of the request-target is only allowed in the OPTIONS
4853  method.
4854  (<xref target="request-target"/>)
4855</t>
4856<t>
4857  Exactly when "close" connection options have to be sent has been clarified.
4858  (<xref target="header.connection"/>)
4859</t>
4860<t> 
4861  "hop-by-hop" header fields are required to appear in the Connection header
4862  field; just because they're defined as hop-by-hop in this specification
4863  doesn't exempt them.
4864  (<xref target="header.connection"/>)
4865</t>
4866<t>
4867  The limit of two connections per server has been removed.
4868  (<xref target="persistent.connections"/>)
4869</t>
4870<t>
4871  An idempotent sequence of requests is no longer required to be retried.
4872  (<xref target="persistent.connections"/>)
4873</t>
4874<t>
4875  The requirement to retry requests under certain circumstances when the
4876  server prematurely closes the connection has been removed.
4877  (<xref target="persistent.connections"/>)
4878</t>
4879<t>
4880  Some extraneous requirements about when servers are allowed to close
4881  connections prematurely have been removed.
4882  (<xref target="persistent.connections"/>)
4883</t>
4884<t>
4885  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
4886  responses other than 101 (this was incorporated from <xref
4887  target="RFC2817"/>).
4888  (<xref target="header.upgrade"/>)
4889</t>
4890<t>
4891  Registration of Transfer Codings now requires IETF Review
4892  (<xref target="transfer.coding.registry"/>)
4893</t>
4894<t>
4895  The meaning of the "deflate" content coding has been clarified.
4896  (<xref target="deflate.coding" />)
4897</t>
4898<t>
4899  This specification now defines the Upgrade Token Registry, previously
4900  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
4901  (<xref target="upgrade.token.registry"/>)
4902</t>
4903<t>
4904  Empty list elements in list productions (e.g., a list header containing
4905  ", ,") have been deprecated.
4906  (<xref target="abnf.extension"/>)
4907</t>
4908<t>
4909  Issues with the Keep-Alive and Proxy-Connection headers in requests
4910  are pointed out, with use of the latter being discouraged altogether.
4911  (<xref target="compatibility.with.http.1.0.persistent.connections" />)
4912</t>
4913</section>
4914</section>
4915
4916<section title="ABNF list extension: #rule" anchor="abnf.extension">
4917<t>
4918  A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
4919  improve readability in the definitions of some header field values.
4920</t>
4921<t>
4922  A construct "#" is defined, similar to "*", for defining comma-delimited
4923  lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
4924  at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
4925  comma (",") and optional whitespace (OWS).   
4926</t>
4927<figure><preamble>
4928  Thus,
4929</preamble><artwork type="example">
4930  1#element =&gt; element *( OWS "," OWS element )
4931</artwork></figure>
4932<figure><preamble>
4933  and:
4934</preamble><artwork type="example">
4935  #element =&gt; [ 1#element ]
4936</artwork></figure>
4937<figure><preamble>
4938  and for n &gt;= 1 and m &gt; 1:
4939</preamble><artwork type="example">
4940  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
4941</artwork></figure>
4942<t>
4943  For compatibility with legacy list rules, recipients &SHOULD; accept empty
4944  list elements. In other words, consumers would follow the list productions:
4945</t>
4946<figure><artwork type="example">
4947  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
4948 
4949  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
4950</artwork></figure>
4951<t>
4952  Note that empty elements do not contribute to the count of elements present,
4953  though.
4954</t>
4955<t>
4956  For example, given these ABNF productions:
4957</t>
4958<figure><artwork type="example">
4959  example-list      = 1#example-list-elmt
4960  example-list-elmt = token ; see <xref target="field.components"/> 
4961</artwork></figure>
4962<t>
4963  Then these are valid values for example-list (not including the double
4964  quotes, which are present for delimitation only):
4965</t>
4966<figure><artwork type="example">
4967  "foo,bar"
4968  "foo ,bar,"
4969  "foo , ,bar,charlie   "
4970</artwork></figure>
4971<t>
4972  But these values would be invalid, as at least one non-empty element is
4973  required:
4974</t>
4975<figure><artwork type="example">
4976  ""
4977  ","
4978  ",   ,"
4979</artwork></figure>
4980<t>
4981  <xref target="collected.abnf"/> shows the collected ABNF, with the list rules
4982  expanded as explained above.
4983</t>
4984</section>
4985
4986<?BEGININC p1-messaging.abnf-appendix ?>
4987<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
4988<figure>
4989<artwork type="abnf" name="p1-messaging.parsed-abnf">
4990<x:ref>BWS</x:ref> = OWS
4991
4992<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
4993 connection-option ] )
4994<x:ref>Content-Length</x:ref> = 1*DIGIT
4995
4996<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
4997 ]
4998<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
4999<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
5000<x:ref>Host</x:ref> = uri-host [ ":" port ]
5001
5002<x:ref>OWS</x:ref> = *( SP / HTAB )
5003
5004<x:ref>RWS</x:ref> = 1*( SP / HTAB )
5005
5006<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
5007<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
5008<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
5009 transfer-coding ] )
5010
5011<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
5012<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
5013
5014<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
5015 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
5016 comment ] ) ] )
5017
5018<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
5019<x:ref>absolute-form</x:ref> = absolute-URI
5020<x:ref>asterisk-form</x:ref> = "*"
5021<x:ref>attribute</x:ref> = token
5022<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
5023<x:ref>authority-form</x:ref> = authority
5024
5025<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
5026<x:ref>chunk-data</x:ref> = 1*OCTET
5027<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
5028<x:ref>chunk-ext-name</x:ref> = token
5029<x:ref>chunk-ext-val</x:ref> = token / quoted-str-nf
5030<x:ref>chunk-size</x:ref> = 1*HEXDIG
5031<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
5032<x:ref>comment</x:ref> = "(" *( ctext / quoted-cpair / comment ) ")"
5033<x:ref>connection-option</x:ref> = token
5034<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
5035 / %x2A-5B ; '*'-'['
5036 / %x5D-7E ; ']'-'~'
5037 / obs-text
5038
5039<x:ref>field-content</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5040<x:ref>field-name</x:ref> = token
5041<x:ref>field-value</x:ref> = *( field-content / obs-fold )
5042
5043<x:ref>header-field</x:ref> = field-name ":" OWS field-value BWS
5044<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ]
5045<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ]
5046
5047<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
5048
5049<x:ref>message-body</x:ref> = *OCTET
5050<x:ref>method</x:ref> = token
5051
5052<x:ref>obs-fold</x:ref> = CRLF ( SP / HTAB )
5053<x:ref>obs-text</x:ref> = %x80-FF
5054<x:ref>origin-form</x:ref> = path-absolute [ "?" query ]
5055
5056<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
5057<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
5058<x:ref>path-absolute</x:ref> = &lt;path-absolute, defined in [RFC3986], Section 3.3&gt;
5059<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
5060<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
5061<x:ref>protocol-name</x:ref> = token
5062<x:ref>protocol-version</x:ref> = token
5063<x:ref>pseudonym</x:ref> = token
5064
5065<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5066 / %x5D-7E ; ']'-'~'
5067 / obs-text
5068<x:ref>qdtext-nf</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5069 / %x5D-7E ; ']'-'~'
5070 / obs-text
5071<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5072<x:ref>quoted-cpair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5073<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5074<x:ref>quoted-str-nf</x:ref> = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
5075<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5076
5077<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5078<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5079<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5080<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5081<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5082<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5083<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5084 asterisk-form
5085
5086<x:ref>special</x:ref> = "(" / ")" / "&lt;" / "&gt;" / "@" / "," / ";" / ":" / "\" /
5087 DQUOTE / "/" / "[" / "]" / "?" / "=" / "{" / "}"
5088<x:ref>start-line</x:ref> = request-line / status-line
5089<x:ref>status-code</x:ref> = 3DIGIT
5090<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5091
5092<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5093<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5094<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5095 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5096<x:ref>token</x:ref> = 1*tchar
5097<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5098<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5099 transfer-extension
5100<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5101<x:ref>transfer-parameter</x:ref> = attribute BWS "=" BWS value
5102
5103<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5104
5105<x:ref>value</x:ref> = word
5106
5107<x:ref>word</x:ref> = token / quoted-string
5108</artwork>
5109</figure>
5110</section>
5111<?ENDINC p1-messaging.abnf-appendix ?>
5112
5113<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5114
5115<section title="Since RFC 2616">
5116<t>
5117  Changes up to the first Working Group Last Call draft are summarized
5118  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-21#appendix-D"/>.
5119</t>
5120</section>
5121
5122<section title="Since draft-ietf-httpbis-p1-messaging-21" anchor="changes.since.21">
5123<t>
5124  Closed issues:
5125  <list style="symbols">
5126    <t>
5127      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/128"/>:
5128      "Cite HTTPS URI scheme definition" (the spec now includes the HTTPs
5129      scheme definition and thus updates RFC 2818)
5130    </t>
5131    <t>
5132      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/389"/>:
5133      "mention of 'proxies' in section about caches"
5134    </t>
5135    <t>
5136      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/390"/>:
5137      "use of ABNF terms from RFC 3986"
5138    </t>
5139    <t>
5140      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/392"/>:
5141      "editorial improvements to message length definition"
5142    </t>
5143    <t>
5144      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/395"/>:
5145      "Connection header field MUST vs SHOULD"
5146    </t>
5147    <t>
5148      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/396"/>:
5149      "editorial improvements to persistent connections section"
5150    </t>
5151    <t>
5152      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/397"/>:
5153      "URI normalization vs empty path"
5154    </t>
5155    <t>
5156      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/408"/>:
5157      "p1 feedback"
5158    </t>
5159    <t>
5160      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/409"/>:
5161      "is parsing OBS-FOLD mandatory?"
5162    </t>
5163    <t>
5164      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/410"/>:
5165      "HTTPS and Shared Caching"
5166    </t>
5167    <t>
5168      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/411"/>:
5169      "Requirements for recipients of ws between start-line and first header field"
5170    </t>
5171    <t>
5172      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/412"/>:
5173      "SP and HT when being tolerant"
5174    </t>
5175    <t>
5176      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/414"/>:
5177      "Message Parsing Strictness"
5178    </t>
5179    <t>
5180      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/415"/>:
5181      "'Render'"
5182    </t>
5183    <t>
5184      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/418"/>:
5185      "No-Transform"
5186    </t>
5187    <t>
5188      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/419"/>:
5189      "p2 editorial feedback"
5190    </t>
5191    <t>
5192      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/420"/>:
5193      "Content-Length SHOULD be sent"
5194    </t>
5195  </list>
5196</t>
5197</section>
5198
5199<!--<section title="Since draft-ietf-httpbis-p1-messaging-22" anchor="changes.since.22">
5200<t>
5201  None yet.
5202</t>
5203</section>-->
5204</section>
5205
5206</back>
5207</rfc>
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