source: draft-ietf-httpbis/latest/p1-messaging.xml @ 1930

Last change on this file since 1930 was 1930, checked in by julian.reschke@…, 7 years ago

Prune logs of pre-WGLC changes

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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 "October">
16  <!ENTITY ID-YEAR "2012">
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-types            "<xref target='Part2' x:rel='#media.types' 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='#representation' 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 methods                "<xref target='Part2' x:rel='#methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
47  <!ENTITY OPTIONS                "<xref target='Part2' x:rel='#OPTIONS' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
48  <!ENTITY qvalue                 "<xref target='Part2' x:rel='#quality.values' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
49  <!ENTITY resource               "<xref target='Part2' x:rel='#resource' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
50  <!ENTITY status-codes           "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
51  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
52  <!ENTITY status-203             "<xref target='Part2' x:rel='#status.203' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
53  <!ENTITY status-3xx             "<xref target='Part2' x:rel='#status.3xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
54  <!ENTITY status-304             "<xref target='Part4' x:rel='#status.304' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
55  <!ENTITY status-4xx             "<xref target='Part2' x:rel='#status.4xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
56  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
57  <!ENTITY iana-header-registry   "<xref target='Part2' x:rel='#header.field.registry' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
58]>
59<?rfc toc="yes" ?>
60<?rfc symrefs="yes" ?>
61<?rfc sortrefs="yes" ?>
62<?rfc compact="yes"?>
63<?rfc subcompact="no" ?>
64<?rfc linkmailto="no" ?>
65<?rfc editing="no" ?>
66<?rfc comments="yes"?>
67<?rfc inline="yes"?>
68<?rfc rfcedstyle="yes"?>
69<?rfc-ext allow-markup-in-artwork="yes" ?>
70<?rfc-ext include-references-in-index="yes" ?>
71<rfc obsoletes="2145,2616" updates="2817" category="std" x:maturity-level="proposed"
72     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
73     xmlns:x='http://purl.org/net/xml2rfc/ext'>
74<x:link rel="next" basename="p2-semantics"/>
75<x:feedback template="mailto:ietf-http-wg@w3.org?subject={docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
76<front>
77
78  <title abbrev="HTTP/1.1 Message Syntax and Routing">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
79
80  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
81    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
82    <address>
83      <postal>
84        <street>345 Park Ave</street>
85        <city>San Jose</city>
86        <region>CA</region>
87        <code>95110</code>
88        <country>USA</country>
89      </postal>
90      <email>fielding@gbiv.com</email>
91      <uri>http://roy.gbiv.com/</uri>
92    </address>
93  </author>
94
95  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
96    <organization abbrev="greenbytes">greenbytes GmbH</organization>
97    <address>
98      <postal>
99        <street>Hafenweg 16</street>
100        <city>Muenster</city><region>NW</region><code>48155</code>
101        <country>Germany</country>
102      </postal>
103      <email>julian.reschke@greenbytes.de</email>
104      <uri>http://greenbytes.de/tech/webdav/</uri>
105    </address>
106  </author>
107
108  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
109  <workgroup>HTTPbis Working Group</workgroup>
110
111<abstract>
112<t>
113   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
114   distributed, collaborative, hypertext information systems. HTTP has been in
115   use by the World Wide Web global information initiative since 1990.
116   This document provides an overview of HTTP architecture and its associated
117   terminology, defines the "http" and "https" Uniform Resource Identifier
118   (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements,
119   and describes general security concerns for implementations.
120</t>   
121</abstract>
122
123<note title="Editorial Note (To be removed by RFC Editor)">
124  <t>
125    Discussion of this draft takes place on the HTTPBIS working group
126    mailing list (ietf-http-wg@w3.org), which is archived at
127    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
128  </t>
129  <t>
130    The current issues list is at
131    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
132    documents (including fancy diffs) can be found at
133    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
134  </t>
135  <t>
136    The changes in this draft are summarized in <xref target="changes.since.21"/>.
137  </t>
138</note>
139</front>
140<middle>
141<section title="Introduction" anchor="introduction">
142<t>
143   The Hypertext Transfer Protocol (HTTP) is an application-level
144   request/response protocol that uses extensible semantics and MIME-like
145   message payloads for flexible interaction with network-based hypertext
146   information systems. This document is the first in a series of documents
147   that collectively form the HTTP/1.1 specification:
148   <list style="empty">
149    <t>RFC xxx1: Message Syntax and Routing</t>
150    <t><xref target="Part2" x:fmt="none">RFC xxx2</xref>: Semantics and Content</t>
151    <t><xref target="Part4" x:fmt="none">RFC xxx3</xref>: Conditional Requests</t>
152    <t><xref target="Part5" x:fmt="none">RFC xxx4</xref>: Range Requests</t>
153    <t><xref target="Part6" x:fmt="none">RFC xxx5</xref>: Caching</t>
154    <t><xref target="Part7" x:fmt="none">RFC xxx6</xref>: Authentication</t>
155   </list>
156</t>
157<t>
158   This HTTP/1.1 specification obsoletes and moves to historic status
159   <xref target="RFC2616" x:fmt="none">RFC 2616</xref>, its predecessor
160   <xref target="RFC2068" x:fmt="none">RFC 2068</xref>,
161   <xref target="RFC2145" x:fmt="none">RFC 2145</xref> (on HTTP versioning),
162   and <xref target="RFC2817" x:fmt="none">RFC 2817</xref> (on using CONNECT
163   for TLS upgrades).
164</t>
165<t>
166   HTTP is a generic interface protocol for information systems. It is
167   designed to hide the details of how a service is implemented by presenting
168   a uniform interface to clients that is independent of the types of
169   resources provided. Likewise, servers do not need to be aware of each
170   client's purpose: an HTTP request can be considered in isolation rather
171   than being associated with a specific type of client or a predetermined
172   sequence of application steps. The result is a protocol that can be used
173   effectively in many different contexts and for which implementations can
174   evolve independently over time.
175</t>
176<t>
177   HTTP is also designed for use as an intermediation protocol for translating
178   communication to and from non-HTTP information systems.
179   HTTP proxies and gateways can provide access to alternative information
180   services by translating their diverse protocols into a hypertext
181   format that can be viewed and manipulated by clients in the same way
182   as HTTP services.
183</t>
184<t>
185   One consequence of HTTP flexibility is that the protocol cannot be
186   defined in terms of what occurs behind the interface. Instead, we
187   are limited to defining the syntax of communication, the intent
188   of received communication, and the expected behavior of recipients.
189   If the communication is considered in isolation, then successful
190   actions ought to be reflected in corresponding changes to the
191   observable interface provided by servers. However, since multiple
192   clients might act in parallel and perhaps at cross-purposes, we
193   cannot require that such changes be observable beyond the scope
194   of a single response.
195</t>
196<t>
197   This document describes the architectural elements that are used or
198   referred to in HTTP, defines the "http" and "https" URI schemes,
199   describes overall network operation and connection management,
200   and defines HTTP message framing and forwarding requirements.
201   Our goal is to define all of the mechanisms necessary for HTTP message
202   handling that are independent of message semantics, thereby defining the
203   complete set of requirements for message parsers and
204   message-forwarding intermediaries.
205</t>
206
207
208<section title="Requirement Notation" anchor="intro.requirements">
209<t>
210   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
211   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
212   document are to be interpreted as described in <xref target="RFC2119"/>.
213</t>
214<t>
215   Conformance criteria and considerations regarding error handling
216   are defined in <xref target="conformance"/>.
217</t>
218</section>
219
220<section title="Syntax Notation" anchor="notation">
221<iref primary="true" item="Grammar" subitem="ALPHA"/>
222<iref primary="true" item="Grammar" subitem="CR"/>
223<iref primary="true" item="Grammar" subitem="CRLF"/>
224<iref primary="true" item="Grammar" subitem="CTL"/>
225<iref primary="true" item="Grammar" subitem="DIGIT"/>
226<iref primary="true" item="Grammar" subitem="DQUOTE"/>
227<iref primary="true" item="Grammar" subitem="HEXDIG"/>
228<iref primary="true" item="Grammar" subitem="HTAB"/>
229<iref primary="true" item="Grammar" subitem="LF"/>
230<iref primary="true" item="Grammar" subitem="OCTET"/>
231<iref primary="true" item="Grammar" subitem="SP"/>
232<iref primary="true" item="Grammar" subitem="VCHAR"/>
233<t>
234   This specification uses the Augmented Backus-Naur Form (ABNF) notation
235   of <xref target="RFC5234"/> with the list rule extension defined in
236   <xref target="abnf.extension"/><xref target="collected.abnf"/> shows
237   the collected ABNF with the list rule expanded.
238</t>
239<t anchor="core.rules">
240  <x:anchor-alias value="ALPHA"/>
241  <x:anchor-alias value="CTL"/>
242  <x:anchor-alias value="CR"/>
243  <x:anchor-alias value="CRLF"/>
244  <x:anchor-alias value="DIGIT"/>
245  <x:anchor-alias value="DQUOTE"/>
246  <x:anchor-alias value="HEXDIG"/>
247  <x:anchor-alias value="HTAB"/>
248  <x:anchor-alias value="LF"/>
249  <x:anchor-alias value="OCTET"/>
250  <x:anchor-alias value="SP"/>
251  <x:anchor-alias value="VCHAR"/>
252   The following core rules are included by
253   reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
254   ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
255   DIGIT (decimal 0-9), DQUOTE (double quote),
256   HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed),
257   OCTET (any 8-bit sequence of data), SP (space), and
258   VCHAR (any visible <xref target="USASCII"/> character).
259</t>
260<t>
261   As a convention, ABNF rule names prefixed with "obs-" denote
262   "obsolete" grammar rules that appear for historical reasons.
263</t>
264</section>
265</section>
266
267<section title="Architecture" anchor="architecture">
268<t>
269   HTTP was created for the World Wide Web architecture
270   and has evolved over time to support the scalability needs of a worldwide
271   hypertext system. Much of that architecture is reflected in the terminology
272   and syntax productions used to define HTTP.
273</t>
274
275<section title="Client/Server Messaging" anchor="operation">
276<iref primary="true" item="client"/>
277<iref primary="true" item="server"/>
278<iref primary="true" item="connection"/>
279<t>
280   HTTP is a stateless request/response protocol that operates by exchanging
281   <x:dfn>messages</x:dfn> (<xref target="http.message"/>) across a reliable
282   transport or session-layer
283   "<x:dfn>connection</x:dfn>" (<xref target="connection.management"/>).
284   An HTTP "<x:dfn>client</x:dfn>" is a program that establishes a connection
285   to a server for the purpose of sending one or more HTTP requests.
286   An HTTP "<x:dfn>server</x:dfn>" is a program that accepts connections
287   in order to service HTTP requests by sending HTTP responses.
288</t>
289<iref primary="true" item="user agent"/>
290<iref primary="true" item="origin server"/>
291<iref primary="true" item="browser"/>
292<iref primary="true" item="spider"/>
293<iref primary="true" item="sender"/>
294<iref primary="true" item="recipient"/>
295<t>
296   The terms client and server refer only to the roles that
297   these programs perform for a particular connection.  The same program
298   might act as a client on some connections and a server on others.  We use
299   the term "<x:dfn>user agent</x:dfn>" to refer to the program that initiates a request,
300   such as a WWW browser, editor, or spider (web-traversing robot), and
301   the term "<x:dfn>origin server</x:dfn>" to refer to the program that can originate
302   authoritative responses to a request.  For general requirements, we use
303   the term "<x:dfn>sender</x:dfn>" to refer to whichever component sent a given message
304   and the term "<x:dfn>recipient</x:dfn>" to refer to any component that receives the
305   message.
306</t>
307<t>
308   HTTP relies upon the Uniform Resource Identifier (URI)
309   standard <xref target="RFC3986"/> to indicate the target resource
310   (<xref target="target-resource"/>) and relationships between resources.
311   Messages are passed in a format similar to that used by Internet mail
312   <xref target="RFC5322"/> and the Multipurpose Internet Mail Extensions
313   (MIME) <xref target="RFC2045"/> (see &diff-mime; for the differences
314   between HTTP and MIME messages).
315</t>
316<t>
317   Most HTTP communication consists of a retrieval request (GET) for
318   a representation of some resource identified by a URI.  In the
319   simplest case, this might be accomplished via a single bidirectional
320   connection (===) between the user agent (UA) and the origin server (O).
321</t>
322<figure><artwork type="drawing">
323         request   &gt;
324    <x:highlight>UA</x:highlight> ======================================= <x:highlight>O</x:highlight>
325                                &lt;   response
326</artwork></figure>
327<iref primary="true" item="message"/>
328<iref primary="true" item="request"/>
329<iref primary="true" item="response"/>
330<t>
331   A client sends an HTTP request to a server in the form of a <x:dfn>request</x:dfn>
332   message, beginning with a request-line that includes a method, URI, and
333   protocol version (<xref target="request.line"/>),
334   followed by header fields containing
335   request modifiers, client information, and representation metadata
336   (<xref target="header.fields"/>),
337   an empty line to indicate the end of the header section, and finally
338   a message body containing the payload body (if any,
339   <xref target="message.body"/>).
340</t>
341<t>
342   A server responds to a client's request by sending one or more HTTP
343   <x:dfn>response</x:dfn>
344   messages, each beginning with a status line that
345   includes the protocol version, a success or error code, and textual
346   reason phrase (<xref target="status.line"/>),
347   possibly followed by header fields containing server
348   information, resource metadata, and representation metadata
349   (<xref target="header.fields"/>),
350   an empty line to indicate the end of the header section, and finally
351   a message body containing the payload body (if any,
352   <xref target="message.body"/>).
353</t>
354<t>
355   A connection might be used for multiple request/response exchanges,
356   as defined in <xref target="persistent.connections"/>.
357</t>
358<t>
359   The following example illustrates a typical message exchange for a
360   GET request on the URI "http://www.example.com/hello.txt":
361</t>
362<figure><preamble>
363client request:
364</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
365GET /hello.txt HTTP/1.1
366User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
367Host: www.example.com
368Accept-Language: en, mi
369
370</artwork></figure>
371<figure><preamble>
372server response:
373</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
374HTTP/1.1 200 OK
375Date: Mon, 27 Jul 2009 12:28:53 GMT
376Server: Apache
377Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
378ETag: "34aa387-d-1568eb00"
379Accept-Ranges: bytes
380Content-Length: <x:length-of target="exbody"/>
381Vary: Accept-Encoding
382Content-Type: text/plain
383
384<x:span anchor="exbody">Hello World!
385</x:span></artwork></figure>
386</section>
387
388<section title="Implementation Diversity" anchor="implementation-diversity">
389<t>
390   When considering the design of HTTP, it is easy to fall into a trap of
391   thinking that all user agents are general-purpose browsers and all origin
392   servers are large public websites. That is not the case in practice.
393   Common HTTP user agents include household appliances, stereos, scales,
394   firmware update scripts, command-line programs, mobile apps,
395   and communication devices in a multitude of shapes and sizes.  Likewise,
396   common HTTP origin servers include home automation units, configurable
397   networking components, office machines, autonomous robots, news feeds,
398   traffic cameras, ad selectors, and video delivery platforms.
399</t>
400<t>
401   The term "user agent" does not imply that there is a human user directly
402   interacting with the software agent at the time of a request. In many
403   cases, a user agent is installed or configured to run in the background
404   and save its results for later inspection (or save only a subset of those
405   results that might be interesting or erroneous). Spiders, for example, are
406   typically given a start URI and configured to follow certain behavior while
407   crawling the Web as a hypertext graph.
408</t>
409<t>
410   The implementation diversity of HTTP means that we cannot assume the
411   user agent can make interactive suggestions to a user or provide adequate
412   warning for security or privacy options.  In the few cases where this
413   specification requires reporting of errors to the user, it is acceptable
414   for such reporting to only be observable in an error console or log file.
415   Likewise, requirements that an automated action be confirmed by the user
416   before proceeding can me met via advance configuration choices,
417   run-time options, or simply not proceeding with the unsafe action.
418</t>
419</section>
420
421<section title="Intermediaries" anchor="intermediaries">
422<iref primary="true" item="intermediary"/>
423<t>
424   HTTP enables the use of intermediaries to satisfy requests through
425   a chain of connections.  There are three common forms of HTTP
426   <x:dfn>intermediary</x:dfn>: proxy, gateway, and tunnel.  In some cases,
427   a single intermediary might act as an origin server, proxy, gateway,
428   or tunnel, switching behavior based on the nature of each request.
429</t>
430<figure><artwork type="drawing">
431         &gt;             &gt;             &gt;             &gt;
432    <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>
433               &lt;             &lt;             &lt;             &lt;
434</artwork></figure>
435<t>
436   The figure above shows three intermediaries (A, B, and C) between the
437   user agent and origin server. A request or response message that
438   travels the whole chain will pass through four separate connections.
439   Some HTTP communication options
440   might apply only to the connection with the nearest, non-tunnel
441   neighbor, only to the end-points of the chain, or to all connections
442   along the chain. Although the diagram is linear, each participant might
443   be engaged in multiple, simultaneous communications. For example, B
444   might be receiving requests from many clients other than A, and/or
445   forwarding requests to servers other than C, at the same time that it
446   is handling A's request.
447</t>
448<t>
449<iref primary="true" item="upstream"/><iref primary="true" item="downstream"/>
450<iref primary="true" item="inbound"/><iref primary="true" item="outbound"/>
451   We use the terms "<x:dfn>upstream</x:dfn>" and "<x:dfn>downstream</x:dfn>"
452   to describe various requirements in relation to the directional flow of a
453   message: all messages flow from upstream to downstream.
454   Likewise, we use the terms inbound and outbound to refer to
455   directions in relation to the request path:
456   "<x:dfn>inbound</x:dfn>" means toward the origin server and
457   "<x:dfn>outbound</x:dfn>" means toward the user agent.
458</t>
459<t><iref primary="true" item="proxy"/>
460   A "<x:dfn>proxy</x:dfn>" is a message forwarding agent that is selected by the
461   client, usually via local configuration rules, to receive requests
462   for some type(s) of absolute URI and attempt to satisfy those
463   requests via translation through the HTTP interface.  Some translations
464   are minimal, such as for proxy requests for "http" URIs, whereas
465   other requests might require translation to and from entirely different
466   application-level protocols. Proxies are often used to group an
467   organization's HTTP requests through a common intermediary for the
468   sake of security, annotation services, or shared caching.
469</t>
470<t>
471<iref primary="true" item="transforming proxy"/>
472<iref primary="true" item="non-transforming proxy"/>
473   An HTTP-to-HTTP proxy is called a "<x:dfn>transforming proxy</x:dfn>" if it is designed
474   or configured to modify request or response messages in a semantically
475   meaningful way (i.e., modifications, beyond those required by normal
476   HTTP processing, that change the message in a way that would be
477   significant to the original sender or potentially significant to
478   downstream recipients).  For example, a transforming proxy might be
479   acting as a shared annotation server (modifying responses to include
480   references to a local annotation database), a malware filter, a
481   format transcoder, or an intranet-to-Internet privacy filter.  Such
482   transformations are presumed to be desired by the client (or client
483   organization) that selected the proxy and are beyond the scope of
484   this specification.  However, when a proxy is not intended to transform
485   a given message, we use the term "<x:dfn>non-transforming proxy</x:dfn>" to target
486   requirements that preserve HTTP message semantics. See &status-203; and
487   &header-warning; for status and warning codes related to transformations.
488</t>
489<t><iref primary="true" item="gateway"/><iref primary="true" item="reverse proxy"/>
490<iref primary="true" item="accelerator"/>
491   A "<x:dfn>gateway</x:dfn>" (a.k.a., "<x:dfn>reverse proxy</x:dfn>")
492   is a receiving agent that acts
493   as a layer above some other server(s) and translates the received
494   requests to the underlying server's protocol.  Gateways are often
495   used to encapsulate legacy or untrusted information services, to
496   improve server performance through "<x:dfn>accelerator</x:dfn>" caching, and to
497   enable partitioning or load-balancing of HTTP services across
498   multiple machines.
499</t>
500<t>
501   A gateway behaves as an origin server on its outbound connection and
502   as a user agent on its inbound connection.
503   All HTTP requirements applicable to an origin server
504   also apply to the outbound communication of a gateway.
505   A gateway communicates with inbound servers using any protocol that
506   it desires, including private extensions to HTTP that are outside
507   the scope of this specification.  However, an HTTP-to-HTTP gateway
508   that wishes to interoperate with third-party HTTP servers &MUST;
509   conform to HTTP user agent requirements on the gateway's inbound
510   connection and &MUST; implement the <x:ref>Connection</x:ref>
511   (<xref target="header.connection"/>) and <x:ref>Via</x:ref>
512   (<xref target="header.via"/>) header fields for both connections.
513</t>
514<t><iref primary="true" item="tunnel"/>
515   A "<x:dfn>tunnel</x:dfn>" acts as a blind relay between two connections
516   without changing the messages. Once active, a tunnel is not
517   considered a party to the HTTP communication, though the tunnel might
518   have been initiated by an HTTP request. A tunnel ceases to exist when
519   both ends of the relayed connection are closed. Tunnels are used to
520   extend a virtual connection through an intermediary, such as when
521   Transport Layer Security (TLS, <xref target="RFC5246"/>) is used to
522   establish confidential communication through a shared firewall proxy.
523</t>
524<t><iref primary="true" item="interception proxy"/>
525<iref primary="true" item="transparent proxy"/>
526<iref primary="true" item="captive portal"/>
527   The above categories for intermediary only consider those acting as
528   participants in the HTTP communication.  There are also intermediaries
529   that can act on lower layers of the network protocol stack, filtering or
530   redirecting HTTP traffic without the knowledge or permission of message
531   senders. Network intermediaries often introduce security flaws or
532   interoperability problems by violating HTTP semantics.  For example, an
533   "<x:dfn>interception proxy</x:dfn>" <xref target="RFC3040"/> (also commonly
534   known as a "<x:dfn>transparent proxy</x:dfn>" <xref target="RFC1919"/> or
535   "<x:dfn>captive portal</x:dfn>")
536   differs from an HTTP proxy because it is not selected by the client.
537   Instead, an interception proxy filters or redirects outgoing TCP port 80
538   packets (and occasionally other common port traffic).
539   Interception proxies are commonly found on public network access points,
540   as a means of enforcing account subscription prior to allowing use of
541   non-local Internet services, and within corporate firewalls to enforce
542   network usage policies.
543   They are indistinguishable from a man-in-the-middle attack.
544</t>
545<t>
546   HTTP is defined as a stateless protocol, meaning that each request message
547   can be understood in isolation.  Many implementations depend on HTTP's
548   stateless design in order to reuse proxied connections or dynamically
549   load balance requests across multiple servers.  Hence, servers &MUST-NOT;
550   assume that two requests on the same connection are from the same user
551   agent unless the connection is secured and specific to that agent.
552   Some non-standard HTTP extensions (e.g., <xref target="RFC4559"/>) have
553   been known to violate this requirement, resulting in security and
554   interoperability problems.
555</t>
556</section>
557
558<section title="Caches" anchor="caches">
559<iref primary="true" item="cache"/>
560<t>
561   A "<x:dfn>cache</x:dfn>" is a local store of previous response messages and the
562   subsystem that controls its message storage, retrieval, and deletion.
563   A cache stores cacheable responses in order to reduce the response
564   time and network bandwidth consumption on future, equivalent
565   requests. Any client or server &MAY; employ a cache, though a cache
566   cannot be used by a server while it is acting as a tunnel.
567</t>
568<t>
569   The effect of a cache is that the request/response chain is shortened
570   if one of the participants along the chain has a cached response
571   applicable to that request. The following illustrates the resulting
572   chain if B has a cached copy of an earlier response from O (via C)
573   for a request which has not been cached by UA or A.
574</t>
575<figure><artwork type="drawing">
576            &gt;             &gt;
577       <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>
578                  &lt;             &lt;
579</artwork></figure>
580<t><iref primary="true" item="cacheable"/>
581   A response is "<x:dfn>cacheable</x:dfn>" if a cache is allowed to store a copy of
582   the response message for use in answering subsequent requests.
583   Even when a response is cacheable, there might be additional
584   constraints placed by the client or by the origin server on when
585   that cached response can be used for a particular request. HTTP
586   requirements for cache behavior and cacheable responses are
587   defined in &caching-overview;
588</t>
589<t>
590   There are a wide variety of architectures and configurations
591   of caches and proxies deployed across the World Wide Web and
592   inside large organizations. These systems include national hierarchies
593   of proxy caches to save transoceanic bandwidth, systems that
594   broadcast or multicast cache entries, organizations that distribute
595   subsets of cached data via optical media, and so on.
596</t>
597</section>
598
599<section title="Conformance and Error Handling" anchor="conformance">
600<t>
601   This specification targets conformance criteria according to the role of
602   a participant in HTTP communication.  Hence, HTTP requirements are placed
603   on senders, recipients, clients, servers, user agents, intermediaries,
604   origin servers, proxies, gateways, or caches, depending on what behavior
605   is being constrained by the requirement. Additional (social) requirements
606   are placed on implementations, resource owners, and protocol element
607   registrations when they apply beyond the scope of a single communication.
608</t>
609<t>
610   The verb "generate" is used instead of "send" where a requirement
611   differentiates between creating a protocol element and merely forwarding a
612   received element downstream.
613</t>
614<t>
615   An implementation is considered conformant if it complies with all of the
616   requirements associated with the roles it partakes in HTTP. Note that
617   SHOULD-level requirements are relevant here, unless one of the documented
618   exceptions is applicable.
619</t>
620<t>
621   Conformance applies to both the syntax and semantics of HTTP protocol
622   elements. A sender &MUST-NOT; generate protocol elements that convey a
623   meaning that is known by that sender to be false. A sender &MUST-NOT;
624   generate protocol elements that do not match the grammar defined by the
625   ABNF rules for those protocol elements that are applicable to the sender's
626   role. If a received protocol element is processed, the recipient &MUST; be
627   able to parse any value that would match the ABNF rules for that protocol
628   element, excluding only those rules not applicable to the recipient's role.
629</t>
630<t>
631   Unless noted otherwise, a recipient &MAY; attempt to recover a usable
632   protocol element from an invalid construct.  HTTP does not define
633   specific error handling mechanisms except when they have a direct impact
634   on security, since different applications of the protocol require
635   different error handling strategies.  For example, a Web browser might
636   wish to transparently recover from a response where the
637   <x:ref>Location</x:ref> header field doesn't parse according to the ABNF,
638   whereas a systems control client might consider any form of error recovery
639   to be dangerous.
640</t>
641</section>
642
643<section title="Protocol Versioning" anchor="http.version">
644  <x:anchor-alias value="HTTP-version"/>
645  <x:anchor-alias value="HTTP-name"/>
646<t>
647   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate
648   versions of the protocol. This specification defines version "1.1".
649   The protocol version as a whole indicates the sender's conformance
650   with the set of requirements laid out in that version's corresponding
651   specification of HTTP.
652</t>
653<t>
654   The version of an HTTP message is indicated by an HTTP-version field
655   in the first line of the message. HTTP-version is case-sensitive.
656</t>
657<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-version"/><iref primary="true" item="Grammar" subitem="HTTP-name"/>
658  <x:ref>HTTP-version</x:ref>  = <x:ref>HTTP-name</x:ref> "/" <x:ref>DIGIT</x:ref> "." <x:ref>DIGIT</x:ref>
659  <x:ref>HTTP-name</x:ref>     = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
660</artwork></figure>
661<t>
662   The HTTP version number consists of two decimal digits separated by a "."
663   (period or decimal point).  The first digit ("major version") indicates the
664   HTTP messaging syntax, whereas the second digit ("minor version") indicates
665   the highest minor version to which the sender is
666   conformant and able to understand for future communication.  The minor
667   version advertises the sender's communication capabilities even when the
668   sender is only using a backwards-compatible subset of the protocol,
669   thereby letting the recipient know that more advanced features can
670   be used in response (by servers) or in future requests (by clients).
671</t>
672<t>
673   When an HTTP/1.1 message is sent to an HTTP/1.0 recipient
674   <xref target="RFC1945"/> or a recipient whose version is unknown,
675   the HTTP/1.1 message is constructed such that it can be interpreted
676   as a valid HTTP/1.0 message if all of the newer features are ignored.
677   This specification places recipient-version requirements on some
678   new features so that a conformant sender will only use compatible
679   features until it has determined, through configuration or the
680   receipt of a message, that the recipient supports HTTP/1.1.
681</t>
682<t>
683   The interpretation of a header field does not change between minor
684   versions of the same major HTTP version, though the default
685   behavior of a recipient in the absence of such a field can change.
686   Unless specified otherwise, header fields defined in HTTP/1.1 are
687   defined for all versions of HTTP/1.x.  In particular, the <x:ref>Host</x:ref>
688   and <x:ref>Connection</x:ref> header fields ought to be implemented by all
689   HTTP/1.x implementations whether or not they advertise conformance with
690   HTTP/1.1.
691</t>
692<t>
693   New header fields can be defined such that, when they are
694   understood by a recipient, they might override or enhance the
695   interpretation of previously defined header fields.  When an
696   implementation receives an unrecognized header field, the recipient
697   &MUST; ignore that header field for local processing regardless of
698   the message's HTTP version.  An unrecognized header field received
699   by a proxy &MUST; be forwarded downstream unless the header field's
700   field-name is listed in the message's <x:ref>Connection</x:ref> header field
701   (see <xref target="header.connection"/>).
702   These requirements allow HTTP's functionality to be enhanced without
703   requiring prior update of deployed intermediaries.
704</t>
705<t>
706   Intermediaries that process HTTP messages (i.e., all intermediaries
707   other than those acting as tunnels) &MUST; send their own HTTP-version
708   in forwarded messages.  In other words, they &MUST-NOT; blindly
709   forward the first line of an HTTP message without ensuring that the
710   protocol version in that message matches a version to which that
711   intermediary is conformant for both the receiving and
712   sending of messages.  Forwarding an HTTP message without rewriting
713   the HTTP-version might result in communication errors when downstream
714   recipients use the message sender's version to determine what features
715   are safe to use for later communication with that sender.
716</t>
717<t>
718   An HTTP client &SHOULD; send a request version equal to the highest
719   version to which the client is conformant and
720   whose major version is no higher than the highest version supported
721   by the server, if this is known.  An HTTP client &MUST-NOT; send a
722   version to which it is not conformant.
723</t>
724<t>
725   An HTTP client &MAY; send a lower request version if it is known that
726   the server incorrectly implements the HTTP specification, but only
727   after the client has attempted at least one normal request and determined
728   from the response status or header fields (e.g., <x:ref>Server</x:ref>) that
729   the server improperly handles higher request versions.
730</t>
731<t>
732   An HTTP server &SHOULD; send a response version equal to the highest
733   version to which the server is conformant and
734   whose major version is less than or equal to the one received in the
735   request.  An HTTP server &MUST-NOT; send a version to which it is not
736   conformant.  A server &MAY; send a <x:ref>505 (HTTP Version Not
737   Supported)</x:ref> response if it cannot send a response using the
738   major version used in the client's request.
739</t>
740<t>
741   An HTTP server &MAY; send an HTTP/1.0 response to an HTTP/1.0 request
742   if it is known or suspected that the client incorrectly implements the
743   HTTP specification and is incapable of correctly processing later
744   version responses, such as when a client fails to parse the version
745   number correctly or when an intermediary is known to blindly forward
746   the HTTP-version even when it doesn't conform to the given minor
747   version of the protocol. Such protocol downgrades &SHOULD-NOT; be
748   performed unless triggered by specific client attributes, such as when
749   one or more of the request header fields (e.g., <x:ref>User-Agent</x:ref>)
750   uniquely match the values sent by a client known to be in error.
751</t>
752<t>
753   The intention of HTTP's versioning design is that the major number
754   will only be incremented if an incompatible message syntax is
755   introduced, and that the minor number will only be incremented when
756   changes made to the protocol have the effect of adding to the message
757   semantics or implying additional capabilities of the sender.  However,
758   the minor version was not incremented for the changes introduced between
759   <xref target="RFC2068"/> and <xref target="RFC2616"/>, and this revision
760   is specifically avoiding any such changes to the protocol.
761</t>
762</section>
763
764<section title="Uniform Resource Identifiers" anchor="uri">
765<iref primary="true" item="resource"/>
766<t>
767   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used
768   throughout HTTP as the means for identifying resources (&resource;).
769   URI references are used to target requests, indicate redirects, and define
770   relationships.
771</t>
772  <x:anchor-alias value="URI-reference"/>
773  <x:anchor-alias value="absolute-URI"/>
774  <x:anchor-alias value="relative-part"/>
775  <x:anchor-alias value="authority"/>
776  <x:anchor-alias value="path-abempty"/>
777  <x:anchor-alias value="path-absolute"/>
778  <x:anchor-alias value="port"/>
779  <x:anchor-alias value="query"/>
780  <x:anchor-alias value="uri-host"/>
781  <x:anchor-alias value="partial-URI"/>
782<t>
783   This specification adopts the definitions of "URI-reference",
784   "absolute-URI", "relative-part", "port", "host",
785   "path-abempty", "path-absolute", "query", and "authority" from the
786   URI generic syntax.
787   In addition, we define a partial-URI rule for protocol elements
788   that allow a relative URI but not a fragment.
789</t>
790<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>
791  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
792  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
793  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
794  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
795  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
796  <x:ref>path-absolute</x:ref> = &lt;path-absolute, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
797  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
798  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
799  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
800 
801  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
802</artwork></figure>
803<t>
804   Each protocol element in HTTP that allows a URI reference will indicate
805   in its ABNF production whether the element allows any form of reference
806   (URI-reference), only a URI in absolute form (absolute-URI), only the
807   path and optional query components, or some combination of the above.
808   Unless otherwise indicated, URI references are parsed
809   relative to the effective request URI
810   (<xref target="effective.request.uri"/>).
811</t>
812
813<section title="http URI scheme" anchor="http.uri">
814  <x:anchor-alias value="http-URI"/>
815  <iref item="http URI scheme" primary="true"/>
816  <iref item="URI scheme" subitem="http" primary="true"/>
817<t>
818   The "http" URI scheme is hereby defined for the purpose of minting
819   identifiers according to their association with the hierarchical
820   namespace governed by a potential HTTP origin server listening for
821   TCP connections on a given port.
822</t>
823<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"><!--terminal production--></iref>
824  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
825</artwork></figure>
826<t>
827   The HTTP origin server is identified by the generic syntax's
828   <x:ref>authority</x:ref> component, which includes a host identifier
829   and optional TCP port (<xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>).
830   The remainder of the URI, consisting of both the hierarchical path
831   component and optional query component, serves as an identifier for
832   a potential resource within that origin server's name space.
833</t>
834<t>
835   If the host identifier is provided as an IP literal or IPv4 address,
836   then the origin server is any listener on the indicated TCP port at
837   that IP address. If host is a registered name, then that name is
838   considered an indirect identifier and the recipient might use a name
839   resolution service, such as DNS, to find the address of a listener
840   for that host.
841   The host &MUST-NOT; be empty; if an "http" URI is received with an
842   empty host, then it &MUST; be rejected as invalid.
843   If the port subcomponent is empty or not given, then TCP port 80 is
844   assumed (the default reserved port for WWW services).
845</t>
846<t>
847   Regardless of the form of host identifier, access to that host is not
848   implied by the mere presence of its name or address. The host might or might
849   not exist and, even when it does exist, might or might not be running an
850   HTTP server or listening to the indicated port. The "http" URI scheme
851   makes use of the delegated nature of Internet names and addresses to
852   establish a naming authority (whatever entity has the ability to place
853   an HTTP server at that Internet name or address) and allows that
854   authority to determine which names are valid and how they might be used.
855</t>
856<t>
857   When an "http" URI is used within a context that calls for access to the
858   indicated resource, a client &MAY; attempt access by resolving
859   the host to an IP address, establishing a TCP connection to that address
860   on the indicated port, and sending an HTTP request message
861   (<xref target="http.message"/>) containing the URI's identifying data
862   (<xref target="message.routing"/>) to the server.
863   If the server responds to that request with a non-interim HTTP response
864   message, as described in &status-codes;, then that response
865   is considered an authoritative answer to the client's request.
866</t>
867<t>
868   Although HTTP is independent of the transport protocol, the "http"
869   scheme is specific to TCP-based services because the name delegation
870   process depends on TCP for establishing authority.
871   An HTTP service based on some other underlying connection protocol
872   would presumably be identified using a different URI scheme, just as
873   the "https" scheme (below) is used for resources that require an
874   end-to-end secured connection. Other protocols might also be used to
875   provide access to "http" identified resources &mdash; it is only the
876   authoritative interface used for mapping the namespace that is
877   specific to TCP.
878</t>
879<t>
880   The URI generic syntax for authority also includes a deprecated
881   userinfo subcomponent (<xref target="RFC3986" x:fmt="," x:sec="3.2.1"/>)
882   for including user authentication information in the URI.  Some
883   implementations make use of the userinfo component for internal
884   configuration of authentication information, such as within command
885   invocation options, configuration files, or bookmark lists, even
886   though such usage might expose a user identifier or password.
887   Senders &MUST-NOT; include a userinfo subcomponent (and its "@"
888   delimiter) when transmitting an "http" URI in a message.  Recipients
889   of HTTP messages that contain a URI reference &SHOULD; parse for the
890   existence of userinfo and treat its presence as an error, likely
891   indicating that the deprecated subcomponent is being used to obscure
892   the authority for the sake of phishing attacks.
893</t>
894</section>
895
896<section title="https URI scheme" anchor="https.uri">
897   <x:anchor-alias value="https-URI"/>
898   <iref item="https URI scheme"/>
899   <iref item="URI scheme" subitem="https"/>
900<t>
901   The "https" URI scheme is hereby defined for the purpose of minting
902   identifiers according to their association with the hierarchical
903   namespace governed by a potential HTTP origin server listening to a
904   given TCP port for TLS-secured connections <xref target="RFC5246"/>.
905</t>
906<t>
907   All of the requirements listed above for the "http" scheme are also
908   requirements for the "https" scheme, except that a default TCP port
909   of 443 is assumed if the port subcomponent is empty or not given,
910   and the TCP connection &MUST; be secured, end-to-end, through the
911   use of strong encryption prior to sending the first HTTP request.
912</t>
913<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="https-URI"><!--terminal production--></iref>
914  <x:ref>https-URI</x:ref> = "https:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
915</artwork></figure>
916<t>
917   Unlike the "http" scheme, responses to "https" identified requests
918   are never "public" and thus &MUST-NOT; be reused for shared caching.
919   They can, however, be reused in a private cache if the message is
920   cacheable by default in HTTP or specifically indicated as such by
921   the Cache-Control header field (&header-cache-control;).
922</t>
923<t>
924   Resources made available via the "https" scheme have no shared
925   identity with the "http" scheme even if their resource identifiers
926   indicate the same authority (the same host listening to the same
927   TCP port).  They are distinct name spaces and are considered to be
928   distinct origin servers.  However, an extension to HTTP that is
929   defined to apply to entire host domains, such as the Cookie protocol
930   <xref target="RFC6265"/>, can allow information
931   set by one service to impact communication with other services
932   within a matching group of host domains.
933</t>
934<t>
935   The process for authoritative access to an "https" identified
936   resource is defined in <xref target="RFC2818"/>.
937</t>
938</section>
939
940<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
941<t>
942   Since the "http" and "https" schemes conform to the URI generic syntax,
943   such URIs are normalized and compared according to the algorithm defined
944   in <xref target="RFC3986" x:fmt="," x:sec="6"/>, using the defaults
945   described above for each scheme.
946</t>
947<t>
948   If the port is equal to the default port for a scheme, the normal
949   form is to elide the port subcomponent. Likewise, an empty path
950   component is equivalent to an absolute path of "/", so the normal
951   form is to provide a path of "/" instead. The scheme and host
952   are case-insensitive and normally provided in lowercase; all
953   other components are compared in a case-sensitive manner.
954   Characters other than those in the "reserved" set are equivalent
955   to their percent-encoded octets (see <xref target="RFC3986"
956   x:fmt="," x:sec="2.1"/>): the normal form is to not encode them.
957</t>
958<t>
959   For example, the following three URIs are equivalent:
960</t>
961<figure><artwork type="example">
962   http://example.com:80/~smith/home.html
963   http://EXAMPLE.com/%7Esmith/home.html
964   http://EXAMPLE.com:/%7esmith/home.html
965</artwork></figure>
966</section>
967</section>
968</section>
969
970<section title="Message Format" anchor="http.message">
971<x:anchor-alias value="generic-message"/>
972<x:anchor-alias value="message.types"/>
973<x:anchor-alias value="HTTP-message"/>
974<x:anchor-alias value="start-line"/>
975<iref item="header section"/>
976<iref item="headers"/>
977<iref item="header field"/>
978<t>
979   All HTTP/1.1 messages consist of a start-line followed by a sequence of
980   octets in a format similar to the Internet Message Format
981   <xref target="RFC5322"/>: zero or more header fields (collectively
982   referred to as the "headers" or the "header section"), an empty line
983   indicating the end of the header section, and an optional message body.
984</t>
985<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"><!--terminal production--></iref>
986  <x:ref>HTTP-message</x:ref>   = <x:ref>start-line</x:ref>
987                   *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
988                   <x:ref>CRLF</x:ref>
989                   [ <x:ref>message-body</x:ref> ]
990</artwork></figure>
991<t>
992   The normal procedure for parsing an HTTP message is to read the
993   start-line into a structure, read each header field into a hash
994   table by field name until the empty line, and then use the parsed
995   data to determine if a message body is expected.  If a message body
996   has been indicated, then it is read as a stream until an amount
997   of octets equal to the message body length is read or the connection
998   is closed.
999</t>
1000<t>
1001   Recipients &MUST; parse an HTTP message as a sequence of octets in an
1002   encoding that is a superset of US-ASCII <xref target="USASCII"/>.
1003   Parsing an HTTP message as a stream of Unicode characters, without regard
1004   for the specific encoding, creates security vulnerabilities due to the
1005   varying ways that string processing libraries handle invalid multibyte
1006   character sequences that contain the octet LF (%x0A).  String-based
1007   parsers can only be safely used within protocol elements after the element
1008   has been extracted from the message, such as within a header field-value
1009   after message parsing has delineated the individual fields.
1010</t>
1011<t>
1012   An HTTP message can be parsed as a stream for incremental processing or
1013   forwarding downstream.  However, recipients cannot rely on incremental
1014   delivery of partial messages, since some implementations will buffer or
1015   delay message forwarding for the sake of network efficiency, security
1016   checks, or payload transformations.
1017</t>
1018
1019<section title="Start Line" anchor="start.line">
1020  <x:anchor-alias value="Start-Line"/>
1021<t>
1022   An HTTP message can either be a request from client to server or a
1023   response from server to client.  Syntactically, the two types of message
1024   differ only in the start-line, which is either a request-line (for requests)
1025   or a status-line (for responses), and in the algorithm for determining
1026   the length of the message body (<xref target="message.body"/>).
1027   In theory, a client could receive requests and a server could receive
1028   responses, distinguishing them by their different start-line formats,
1029   but in practice servers are implemented to only expect a request
1030   (a response is interpreted as an unknown or invalid request method)
1031   and clients are implemented to only expect a response.
1032</t>
1033<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="start-line"/>
1034  <x:ref>start-line</x:ref>     = <x:ref>request-line</x:ref> / <x:ref>status-line</x:ref>
1035</artwork></figure>
1036<t>
1037   A sender &MUST-NOT; send whitespace between the start-line and
1038   the first header field. The presence of such whitespace in a request
1039   might be an attempt to trick a server into ignoring that field or
1040   processing the line after it as a new request, either of which might
1041   result in a security vulnerability if other implementations within
1042   the request chain interpret the same message differently.
1043   Likewise, the presence of such whitespace in a response might be
1044   ignored by some clients or cause others to cease parsing.
1045</t>
1046
1047<section title="Request Line" anchor="request.line">
1048  <x:anchor-alias value="Request"/>
1049  <x:anchor-alias value="request-line"/>
1050<t>
1051   A request-line begins with a method token, followed by a single
1052   space (SP), the request-target, another single space (SP), the
1053   protocol version, and ending with CRLF.
1054</t>
1055<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1056  <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>
1057</artwork></figure>
1058<t>
1059   A server &MUST; be able to parse any received message that begins
1060   with a request-line and matches the ABNF rule for HTTP-message.
1061</t>
1062<iref primary="true" item="method"/>
1063<t anchor="method">
1064   The method token indicates the request method to be performed on the
1065   target resource. The request method is case-sensitive.
1066</t>
1067<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1068  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1069</artwork></figure>
1070<t>
1071   The methods defined by this specification can be found in
1072   &methods;, along with information regarding the HTTP method registry
1073   and considerations for defining new methods.
1074</t>
1075<iref item="request-target"/>
1076<t>
1077   The request-target identifies the target resource upon which to apply
1078   the request, as defined in <xref target="request-target"/>.
1079</t>
1080<t>
1081   No whitespace is allowed inside the method, request-target, and
1082   protocol version.  Hence, recipients typically parse the request-line
1083   into its component parts by splitting on the SP characters.
1084</t>
1085<t>
1086   Unfortunately, some user agents fail to properly encode hypertext
1087   references that have embedded whitespace, sending the characters
1088   directly instead of properly percent-encoding the disallowed characters.
1089   Recipients of an invalid request-line &SHOULD; respond with either a
1090   <x:ref>400 (Bad Request)</x:ref> error or a <x:ref>301 (Moved Permanently)</x:ref>
1091   redirect with the request-target properly encoded.  Recipients &SHOULD-NOT;
1092   attempt to autocorrect and then process the request without a redirect,
1093   since the invalid request-line might be deliberately crafted to bypass
1094   security filters along the request chain.
1095</t>
1096<t>
1097   HTTP does not place a pre-defined limit on the length of a request-line.
1098   A server that receives a method longer than any that it implements
1099   &SHOULD; respond with either a <x:ref>405 (Method Not Allowed)</x:ref>, if it is an origin
1100   server, or a <x:ref>501 (Not Implemented)</x:ref> status code.
1101   A server &MUST; be prepared to receive URIs of unbounded length and
1102   respond with the <x:ref>414 (URI Too Long)</x:ref> status code if the received
1103   request-target would be longer than the server wishes to handle
1104   (see &status-414;).
1105</t>
1106<t>
1107   Various ad-hoc limitations on request-line length are found in practice.
1108   It is &RECOMMENDED; that all HTTP senders and recipients support, at a
1109   minimum, request-line lengths of up to 8000 octets.
1110</t>
1111</section>
1112
1113<section title="Status Line" anchor="status.line">
1114  <x:anchor-alias value="response"/>
1115  <x:anchor-alias value="status-line"/>
1116  <x:anchor-alias value="status-code"/>
1117  <x:anchor-alias value="reason-phrase"/>
1118<t>
1119   The first line of a response message is the status-line, consisting
1120   of the protocol version, a space (SP), the status code, another space,
1121   a possibly-empty textual phrase describing the status code, and
1122   ending with CRLF.
1123</t>
1124<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-line"/>
1125  <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>
1126</artwork></figure>
1127<t>
1128   A client &MUST; be able to parse any received message that begins
1129   with a status-line and matches the ABNF rule for HTTP-message.
1130</t>
1131<t>
1132   The status-code element is a 3-digit integer code describing the
1133   result of the server's attempt to understand and satisfy the client's
1134   corresponding request. The rest of the response message is to be
1135   interpreted in light of the semantics defined for that status code.
1136   See &status-codes; for information about the semantics of status codes,
1137   including the classes of status code (indicated by the first digit),
1138   the status codes defined by this specification, considerations for the
1139   definition of new status codes, and the IANA registry.
1140</t>
1141<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-code"/>
1142  <x:ref>status-code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1143</artwork></figure>
1144<t>   
1145   The reason-phrase element exists for the sole purpose of providing a
1146   textual description associated with the numeric status code, mostly
1147   out of deference to earlier Internet application protocols that were more
1148   frequently used with interactive text clients. A client &SHOULD; ignore
1149   the reason-phrase content.
1150</t>
1151<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
1152  <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> )
1153</artwork></figure>
1154</section>
1155</section>
1156
1157<section title="Header Fields" anchor="header.fields">
1158  <x:anchor-alias value="header-field"/>
1159  <x:anchor-alias value="field-content"/>
1160  <x:anchor-alias value="field-name"/>
1161  <x:anchor-alias value="field-value"/>
1162  <x:anchor-alias value="obs-fold"/>
1163<t>
1164   Each HTTP header field consists of a case-insensitive field name
1165   followed by a colon (":"), optional whitespace, and the field value.
1166</t>
1167<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"/>
1168  <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>
1169  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1170  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>obs-fold</x:ref> )
1171  <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> )
1172  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref> ( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1173                 ; obsolete line folding
1174                 ; see <xref target="field.parsing"/>
1175</artwork></figure>
1176<t>
1177   The field-name token labels the corresponding field-value as having the
1178   semantics defined by that header field.  For example, the <x:ref>Date</x:ref>
1179   header field is defined in &header-date; as containing the origination
1180   timestamp for the message in which it appears.
1181</t>
1182<t>
1183   HTTP header fields are fully extensible: there is no limit on the
1184   introduction of new field names, each presumably defining new semantics,
1185   or on the number of header fields used in a given message.  Existing
1186   fields are defined in each part of this specification and in many other
1187   specifications outside the standards process.
1188   New header fields can be introduced without changing the protocol version
1189   if their defined semantics allow them to be safely ignored by recipients
1190   that do not recognize them.
1191</t>
1192<t>
1193   New HTTP header fields &SHOULD; be registered with IANA in the
1194   Message Header Field Registry, as described in &iana-header-registry;.
1195   Unrecognized header fields &MUST; be forwarded by a proxy unless the
1196   field-name is listed in the <x:ref>Connection</x:ref> header field
1197   (<xref target="header.connection"/>) or the proxy is specifically
1198   configured to block or otherwise transform such fields.
1199   Unrecognized header fields &SHOULD; be ignored by other recipients.
1200</t>
1201<t>
1202   The order in which header fields with differing field names are
1203   received is not significant. However, it is "good practice" to send
1204   header fields that contain control data first, such as <x:ref>Host</x:ref>
1205   on requests and <x:ref>Date</x:ref> on responses, so that implementations
1206   can decide when not to handle a message as early as possible.  A server
1207   &MUST; wait until the entire header section is received before interpreting
1208   a request message, since later header fields might include conditionals,
1209   authentication credentials, or deliberately misleading duplicate
1210   header fields that would impact request processing.
1211</t>
1212<t>
1213   Multiple header fields with the same field name &MUST-NOT; be
1214   sent in a message unless the entire field value for that
1215   header field is defined as a comma-separated list [i.e., #(values)].
1216   Multiple header fields with the same field name can be combined into
1217   one "field-name: field-value" pair, without changing the semantics of the
1218   message, by appending each subsequent field value to the combined
1219   field value in order, separated by a comma. The order in which
1220   header fields with the same field name are received is therefore
1221   significant to the interpretation of the combined field value;
1222   a proxy &MUST-NOT; change the order of these field values when
1223   forwarding a message.
1224</t>
1225<x:note>
1226  <t>
1227   &Note; The "Set-Cookie" header field as implemented in
1228   practice can occur multiple times, but does not use the list syntax, and
1229   thus cannot be combined into a single line (<xref target="RFC6265"/>). (See Appendix A.2.3 of <xref target="Kri2001"/>
1230   for details.) Also note that the Set-Cookie2 header field specified in
1231   <xref target="RFC2965"/> does not share this problem.
1232  </t>
1233</x:note>
1234
1235<section title="Whitespace" anchor="whitespace">
1236<t anchor="rule.LWS">
1237   This specification uses three rules to denote the use of linear
1238   whitespace: OWS (optional whitespace), RWS (required whitespace), and
1239   BWS ("bad" whitespace).
1240</t>
1241<t anchor="rule.OWS">
1242   The OWS rule is used where zero or more linear whitespace octets might
1243   appear. OWS &SHOULD; either not be produced or be produced as a single
1244   SP. Multiple OWS octets that occur within field-content &SHOULD; either
1245   be replaced with a single SP or transformed to all SP octets (each
1246   octet other than SP replaced with SP) before interpreting the field value
1247   or forwarding the message downstream.
1248</t>
1249<t anchor="rule.RWS">
1250   RWS is used when at least one linear whitespace octet is required to
1251   separate field tokens. RWS &SHOULD; be produced as a single SP.
1252   Multiple RWS octets that occur within field-content &SHOULD; either
1253   be replaced with a single SP or transformed to all SP octets before
1254   interpreting the field value or forwarding the message downstream.
1255</t>
1256<t anchor="rule.BWS">
1257   BWS is used where the grammar allows optional whitespace, for historical
1258   reasons, but senders &SHOULD-NOT; produce it in messages;
1259   recipients &MUST; accept such bad optional whitespace and remove it before
1260   interpreting the field value or forwarding the message downstream.
1261</t>
1262<t anchor="rule.whitespace">
1263  <x:anchor-alias value="BWS"/>
1264  <x:anchor-alias value="OWS"/>
1265  <x:anchor-alias value="RWS"/>
1266</t>
1267<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"/>
1268  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1269                 ; "optional" whitespace
1270  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1271                 ; "required" whitespace
1272  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1273                 ; "bad" whitespace
1274</artwork></figure>
1275</section>
1276
1277<section title="Field Parsing" anchor="field.parsing">
1278<t>
1279   No whitespace is allowed between the header field-name and colon.
1280   In the past, differences in the handling of such whitespace have led to
1281   security vulnerabilities in request routing and response handling.
1282   Any received request message that contains whitespace between a header
1283   field-name and colon &MUST; be rejected with a response code of 400
1284   (Bad Request).  A proxy &MUST; remove any such whitespace from a response
1285   message before forwarding the message downstream.
1286</t>
1287<t>
1288   A field value &MAY; be preceded by optional whitespace (OWS); a single SP is
1289   preferred. The field value does not include any leading or trailing white
1290   space: OWS occurring before the first non-whitespace octet of the
1291   field value or after the last non-whitespace octet of the field value
1292   is ignored and &SHOULD; be removed before further processing (as this does
1293   not change the meaning of the header field).
1294</t>
1295<t>
1296   Historically, HTTP header field values could be extended over multiple
1297   lines by preceding each extra line with at least one space or horizontal
1298   tab (obs-fold). This specification deprecates such line
1299   folding except within the message/http media type
1300   (<xref target="internet.media.type.message.http"/>).
1301   HTTP senders &MUST-NOT; produce messages that include line folding
1302   (i.e., that contain any field-value that matches the obs-fold rule) unless
1303   the message is intended for packaging within the message/http media type.
1304   HTTP recipients &SHOULD; accept line folding and replace any embedded
1305   obs-fold whitespace with either a single SP or a matching number of SP
1306   octets (to avoid buffer copying) prior to interpreting the field value or
1307   forwarding the message downstream.
1308</t>
1309<t>
1310   Historically, HTTP has allowed field content with text in the ISO-8859-1
1311   <xref target="ISO-8859-1"/> character encoding and supported other
1312   character sets only through use of <xref target="RFC2047"/> encoding.
1313   In practice, most HTTP header field values use only a subset of the
1314   US-ASCII character encoding <xref target="USASCII"/>. Newly defined
1315   header fields &SHOULD; limit their field values to US-ASCII octets.
1316   Recipients &SHOULD; treat other (obs-text) octets in field content as
1317   opaque data.
1318</t>
1319</section>
1320
1321<section title="Field Length" anchor="field.length">
1322<t>
1323   HTTP does not place a pre-defined limit on the length of header fields,
1324   either in isolation or as a set. A server &MUST; be prepared to receive
1325   request header fields of unbounded length and respond with a <x:ref>4xx
1326   (Client Error)</x:ref> status code if the received header field(s) would be
1327   longer than the server wishes to handle.
1328</t>
1329<t>
1330   A client that receives response header fields that are longer than it wishes
1331   to handle can only treat it as a server error.
1332</t>
1333<t>
1334   Various ad-hoc limitations on header field length are found in practice. It
1335   is &RECOMMENDED; that all HTTP senders and recipients support messages whose
1336   combined header fields have 4000 or more octets.
1337</t>
1338</section>
1339
1340<section title="Field value components" anchor="field.components">
1341<t anchor="rule.token.separators">
1342  <x:anchor-alias value="tchar"/>
1343  <x:anchor-alias value="token"/>
1344  <x:anchor-alias value="special"/>
1345  <x:anchor-alias value="word"/>
1346   Many HTTP header field values consist of words (token or quoted-string)
1347   separated by whitespace or special characters. These special characters
1348   &MUST; be in a quoted string to be used within a parameter value (as defined
1349   in <xref target="transfer.codings"/>).
1350</t>
1351<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>
1352  <x:ref>word</x:ref>           = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1353
1354  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1355<!--
1356  IMPORTANT: when editing "tchar" make sure that "special" is updated accordingly!!!
1357 -->
1358  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1359                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1360                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1361                 ; any <x:ref>VCHAR</x:ref>, except <x:ref>special</x:ref>
1362
1363  <x:ref>special</x:ref>        = "(" / ")" / "&lt;" / ">" / "@" / ","
1364                 / ";" / ":" / "\" / DQUOTE / "/" / "["
1365                 / "]" / "?" / "=" / "{" / "}"
1366</artwork></figure>
1367<t anchor="rule.quoted-string">
1368  <x:anchor-alias value="quoted-string"/>
1369  <x:anchor-alias value="qdtext"/>
1370  <x:anchor-alias value="obs-text"/>
1371   A string of text is parsed as a single word if it is quoted using
1372   double-quote marks.
1373</t>
1374<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"/>
1375  <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>
1376  <x:ref>qdtext</x:ref>         = <x:ref>OWS</x:ref> / %x21 / %x23-5B / %x5D-7E / <x:ref>obs-text</x:ref>
1377  <x:ref>obs-text</x:ref>       = %x80-FF
1378</artwork></figure>
1379<t anchor="rule.quoted-pair">
1380  <x:anchor-alias value="quoted-pair"/>
1381   The backslash octet ("\") can be used as a single-octet
1382   quoting mechanism within quoted-string constructs:
1383</t>
1384<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1385  <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> )
1386</artwork></figure>
1387<t>
1388   Recipients that process the value of the quoted-string &MUST; handle a
1389   quoted-pair as if it were replaced by the octet following the backslash.
1390</t>
1391<t>
1392   Senders &SHOULD-NOT; escape octets in quoted-strings that do not require
1393   escaping (i.e., other than DQUOTE and the backslash octet).
1394</t>
1395<t anchor="rule.comment">
1396  <x:anchor-alias value="comment"/>
1397  <x:anchor-alias value="ctext"/>
1398   Comments can be included in some HTTP header fields by surrounding
1399   the comment text with parentheses. Comments are only allowed in
1400   fields containing "comment" as part of their field value definition.
1401</t>
1402<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1403  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-cpair</x:ref> / <x:ref>comment</x:ref> ) ")"
1404  <x:ref>ctext</x:ref>          = <x:ref>OWS</x:ref> / %x21-27 / %x2A-5B / %x5D-7E / <x:ref>obs-text</x:ref>
1405</artwork></figure>
1406<t anchor="rule.quoted-cpair">
1407  <x:anchor-alias value="quoted-cpair"/>
1408   The backslash octet ("\") can be used as a single-octet
1409   quoting mechanism within comment constructs:
1410</t>
1411<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-cpair"/>
1412  <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> )
1413</artwork></figure>
1414<t>
1415   Senders &SHOULD-NOT; escape octets in comments that do not require escaping
1416   (i.e., other than the backslash octet "\" and the parentheses "(" and ")").
1417</t>
1418</section>
1419
1420</section>
1421
1422<section title="Message Body" anchor="message.body">
1423  <x:anchor-alias value="message-body"/>
1424<t>
1425   The message body (if any) of an HTTP message is used to carry the
1426   payload body of that request or response.  The message body is
1427   identical to the payload body unless a transfer coding has been
1428   applied, as described in <xref target="header.transfer-encoding"/>.
1429</t>
1430<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1431  <x:ref>message-body</x:ref> = *OCTET
1432</artwork></figure>
1433<t>
1434   The rules for when a message body is allowed in a message differ for
1435   requests and responses.
1436</t>
1437<t>
1438   The presence of a message body in a request is signaled by a
1439   a <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1440   field. Request message framing is independent of method semantics,
1441   even if the method does not define any use for a message body.
1442</t>
1443<t>
1444   The presence of a message body in a response depends on both
1445   the request method to which it is responding and the response
1446   status code (<xref target="status.line"/>).
1447   Responses to the HEAD request method never include a message body
1448   because the associated response header fields (e.g.,
1449   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1450   if present, indicate only what their values would have been if the request
1451   method had been GET (&HEAD;).
1452   <x:ref>2xx (Successful)</x:ref> responses to CONNECT switch to tunnel
1453   mode instead of having a message body (&CONNECT;).
1454   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1455   <x:ref>304 (Not Modified)</x:ref> responses &MUST-NOT; include a message body.
1456   All other responses do include a message body, although the body
1457   &MAY; be of zero length.
1458</t>
1459
1460<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1461  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1462  <x:anchor-alias value="Transfer-Encoding"/>
1463<t>
1464   When one or more transfer codings are applied to a payload body in order
1465   to form the message body, a Transfer-Encoding header field &MUST; be sent
1466   in the message and &MUST; contain the list of corresponding
1467   transfer-coding names in the same order that they were applied.
1468   Transfer codings are defined in <xref target="transfer.codings"/>.
1469</t>
1470<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1471  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1472</artwork></figure>
1473<t>
1474   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1475   MIME, which was designed to enable safe transport of binary data over a
1476   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1477   However, safe transport has a different focus for an 8bit-clean transfer
1478   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1479   accurately delimit a dynamically generated payload and to distinguish
1480   payload encodings that are only applied for transport efficiency or
1481   security from those that are characteristics of the target resource.
1482</t>
1483<t>
1484   The "chunked" transfer-coding (<xref target="chunked.encoding"/>)
1485   &MUST; be implemented by all HTTP/1.1 recipients because it plays a
1486   crucial role in delimiting messages when the payload body size is not
1487   known in advance.
1488   When the "chunked" transfer-coding is used, it &MUST; be the last
1489   transfer-coding applied to form the message body and &MUST-NOT;
1490   be applied more than once in a message body.
1491   If any transfer-coding is applied to a request payload body,
1492   the final transfer-coding applied &MUST; be "chunked".
1493   If any transfer-coding is applied to a response payload body, then either
1494   the final transfer-coding applied &MUST; be "chunked" or
1495   the message &MUST; be terminated by closing the connection.
1496</t>
1497<figure><preamble>
1498   For example,
1499</preamble><artwork type="example">
1500  Transfer-Encoding: gzip, chunked
1501</artwork><postamble>
1502   indicates that the payload body has been compressed using the gzip
1503   coding and then chunked using the chunked coding while forming the
1504   message body.
1505</postamble></figure>
1506<t>
1507   If more than one Transfer-Encoding header field is present in a message,
1508   the multiple field-values &MUST; be combined into one field-value,
1509   according to the algorithm defined in <xref target="header.fields"/>,
1510   before determining the message body length.
1511</t>
1512<t>
1513   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1514   Transfer-Encoding is a property of the message, not of the payload, and thus
1515   &MAY; be added or removed by any implementation along the request/response
1516   chain. Additional information about the encoding parameters &MAY; be
1517   provided by other header fields not defined by this specification.
1518</t>
1519<t>
1520   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1521   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1522   neither of which includes a message body,
1523   to indicate that the origin server would have applied a transfer coding
1524   to the message body if the request had been an unconditional GET.
1525   This indication is not required, however, because any recipient on
1526   the response chain (including the origin server) can remove transfer
1527   codings when they are not needed.
1528</t>
1529<t>
1530   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1531   implementations advertising only HTTP/1.0 support will not understand
1532   how to process a transfer-encoded payload.
1533   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1534   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1535   might be in the form of specific user configuration or by remembering the
1536   version of a prior received response.
1537   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1538   the corresponding request indicates HTTP/1.1 (or later).
1539</t>
1540<t>
1541   A server that receives a request message with a transfer-coding it does
1542   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref> and then
1543   close the connection.
1544</t>
1545</section>
1546
1547<section title="Content-Length" anchor="header.content-length">
1548  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1549  <x:anchor-alias value="Content-Length"/>
1550<t>
1551   When a message is allowed to contain a message body, does not have a
1552   <x:ref>Transfer-Encoding</x:ref> header field, and has a payload body
1553   length that is known to the sender before the message header section has
1554   been sent, the sender &SHOULD; send a Content-Length header field to
1555   indicate the length of the payload body as a decimal number of octets.
1556</t>
1557<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1558  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1559</artwork></figure>
1560<t>
1561   An example is
1562</t>
1563<figure><artwork type="example">
1564  Content-Length: 3495
1565</artwork></figure>
1566<t>
1567   A sender &MUST-NOT; send a Content-Length header field in any message that
1568   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1569</t>
1570<t>
1571   A server &MAY; send a Content-Length header field in a response to a HEAD
1572   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1573   response unless its field-value equals the decimal number of octets that
1574   would have been sent in the payload body of a response if the same
1575   request had used the GET method.
1576</t>
1577<t>
1578   A server &MAY; send a Content-Length header field in a
1579   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1580   (&status-304;); a server &MUST-NOT; send Content-Length in such a
1581   response unless its field-value equals the decimal number of octets that
1582   would have been sent in the payload body of a <x:ref>200 (OK)</x:ref>
1583   response to the same request.
1584</t>
1585<t>
1586   A server &MUST-NOT; send a Content-Length header field in any response
1587   with a status code of
1588   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1589   A server &SHOULD-NOT; send a Content-Length header field in any
1590   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1591</t>
1592<t>
1593   Any Content-Length field value greater than or equal to zero is valid.
1594   Since there is no predefined limit to the length of an HTTP payload,
1595   recipients &SHOULD; anticipate potentially large decimal numerals and
1596   prevent parsing errors due to integer conversion overflows
1597   (<xref target="attack.protocol.element.size.overflows"/>).
1598</t>
1599<t>
1600   If a message is received that has multiple Content-Length header fields
1601   with field-values consisting of the same decimal value, or a single
1602   Content-Length header field with a field value containing a list of
1603   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1604   duplicate Content-Length header fields have been generated or combined by an
1605   upstream message processor, then the recipient &MUST; either reject the
1606   message as invalid or replace the duplicated field-values with a single
1607   valid Content-Length field containing that decimal value prior to
1608   determining the message body length.
1609</t>
1610<x:note>
1611  <t>
1612   &Note; HTTP's use of Content-Length for message framing differs
1613   significantly from the same field's use in MIME, where it is an optional
1614   field used only within the "message/external-body" media-type.
1615  </t>
1616</x:note>
1617</section>
1618
1619<section title="Message Body Length" anchor="message.body.length">
1620<t>
1621   The length of a message body is determined by one of the following
1622   (in order of precedence):
1623</t>
1624<t>
1625  <list style="numbers">
1626    <x:lt><t>
1627     Any response to a HEAD request and any response with a
1628     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1629     <x:ref>304 (Not Modified)</x:ref> status code is always
1630     terminated by the first empty line after the header fields, regardless of
1631     the header fields present in the message, and thus cannot contain a
1632     message body.
1633    </t></x:lt>
1634    <x:lt><t>
1635     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1636     connection will become a tunnel immediately after the empty line that
1637     concludes the header fields.  A client &MUST; ignore any
1638     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1639     fields received in such a message.
1640    </t></x:lt>
1641    <x:lt><t>
1642     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1643     and the "chunked" transfer-coding (<xref target="chunked.encoding"/>)
1644     is the final encoding, the message body length is determined by reading
1645     and decoding the chunked data until the transfer-coding indicates the
1646     data is complete.
1647    </t>
1648    <t>
1649     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1650     response and the "chunked" transfer-coding is not the final encoding, the
1651     message body length is determined by reading the connection until it is
1652     closed by the server.
1653     If a Transfer-Encoding header field is present in a request and the
1654     "chunked" transfer-coding is not the final encoding, the message body
1655     length cannot be determined reliably; the server &MUST; respond with
1656     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1657    </t>
1658    <t>
1659     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1660     and a <x:ref>Content-Length</x:ref> header field, the
1661     Transfer-Encoding overrides the Content-Length.
1662     Such a message might indicate an attempt to perform request or response
1663     smuggling (bypass of security-related checks on message routing or content)
1664     and thus ought to be handled as an error.  The provided Content-Length &MUST;
1665     be removed, prior to forwarding the message downstream, or replaced with
1666     the real message body length after the transfer-coding is decoded.
1667    </t></x:lt>
1668    <x:lt><t>
1669     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1670     either multiple <x:ref>Content-Length</x:ref> header fields having
1671     differing field-values or a single Content-Length header field having an
1672     invalid value, then the message framing is invalid and &MUST; be treated
1673     as an error to prevent request or response smuggling.
1674     If this is a request message, the server &MUST; respond with
1675     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1676     If this is a response message received by a proxy, the proxy
1677     &MUST; discard the received response, send a <x:ref>502 (Bad Gateway)</x:ref>
1678     status code as its downstream response, and then close the connection.
1679     If this is a response message received by a user-agent, it &MUST; be
1680     treated as an error by discarding the message and closing the connection.
1681    </t></x:lt>
1682    <x:lt><t>
1683     If a valid <x:ref>Content-Length</x:ref> header field is present without
1684     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1685     message body length in octets.  If the actual number of octets sent in
1686     the message is less than the indicated Content-Length, the recipient
1687     &MUST; consider the message to be incomplete and treat the connection
1688     as no longer usable.
1689     If the actual number of octets sent in the message is more than the indicated
1690     Content-Length, the recipient &MUST; only process the message body up to the
1691     field value's number of octets; the remainder of the message &MUST; either
1692     be discarded or treated as the next message in a pipeline.  For the sake of
1693     robustness, a user-agent &MAY; attempt to detect and correct such an error
1694     in message framing if it is parsing the response to the last request on
1695     a connection and the connection has been closed by the server.
1696    </t></x:lt>
1697    <x:lt><t>
1698     If this is a request message and none of the above are true, then the
1699     message body length is zero (no message body is present).
1700    </t></x:lt>
1701    <x:lt><t>
1702     Otherwise, this is a response message without a declared message body
1703     length, so the message body length is determined by the number of octets
1704     received prior to the server closing the connection.
1705    </t></x:lt>
1706  </list>
1707</t>
1708<t>
1709   Since there is no way to distinguish a successfully completed,
1710   close-delimited message from a partially-received message interrupted
1711   by network failure, a server &SHOULD; use encoding or
1712   length-delimited messages whenever possible.  The close-delimiting
1713   feature exists primarily for backwards compatibility with HTTP/1.0.
1714</t>
1715<t>
1716   A server &MAY; reject a request that contains a message body but
1717   not a <x:ref>Content-Length</x:ref> by responding with
1718   <x:ref>411 (Length Required)</x:ref>.
1719</t>
1720<t>
1721   Unless a transfer-coding other than "chunked" has been applied,
1722   a client that sends a request containing a message body &SHOULD;
1723   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1724   length is known in advance, rather than the "chunked" encoding, since some
1725   existing services respond to "chunked" with a <x:ref>411 (Length Required)</x:ref>
1726   status code even though they understand the chunked encoding.  This
1727   is typically because such services are implemented via a gateway that
1728   requires a content-length in advance of being called and the server
1729   is unable or unwilling to buffer the entire request before processing.
1730</t>
1731<t>
1732   A client that sends a request containing a message body &MUST; include a
1733   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1734   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1735   the form of specific user configuration or by remembering the version of a
1736   prior received response.
1737</t>
1738</section>
1739</section>
1740
1741<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1742<t>
1743   Request messages that are prematurely terminated, possibly due to a
1744   canceled connection or a server-imposed time-out exception, &MUST;
1745   result in closure of the connection; sending an error response
1746   prior to closing the connection is &OPTIONAL;.
1747</t>
1748<t>
1749   Response messages that are prematurely terminated, usually by closure
1750   of the connection prior to receiving the expected number of octets or by
1751   failure to decode a transfer-encoded message body, &MUST; be recorded
1752   as incomplete.  A response that terminates in the middle of the header
1753   block (before the empty line is received) cannot be assumed to convey the
1754   full semantics of the response and &MUST; be treated as an error.
1755</t>
1756<t>
1757   A message body that uses the chunked transfer encoding is
1758   incomplete if the zero-sized chunk that terminates the encoding has not
1759   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1760   incomplete if the size of the message body received (in octets) is less than
1761   the value given by Content-Length.  A response that has neither chunked
1762   transfer encoding nor Content-Length is terminated by closure of the
1763   connection, and thus is considered complete regardless of the number of
1764   message body octets received, provided that the header block was received
1765   intact.
1766</t>
1767<t>
1768   A user agent &MUST-NOT; render an incomplete response message body as if
1769   it were complete (i.e., some indication needs to be given to the user that an
1770   error occurred).  Cache requirements for incomplete responses are defined
1771   in &cache-incomplete;.
1772</t>
1773<t>
1774   A server &MUST; read the entire request message body or close
1775   the connection after sending its response, since otherwise the
1776   remaining data on a persistent connection would be misinterpreted
1777   as the next request.  Likewise,
1778   a client &MUST; read the entire response message body if it intends
1779   to reuse the same connection for a subsequent request.  Pipelining
1780   multiple requests on a connection is described in <xref target="pipelining"/>.
1781</t>
1782</section>
1783
1784<section title="Message Parsing Robustness" anchor="message.robustness">
1785<t>
1786   Older HTTP/1.0 client implementations might send an extra CRLF
1787   after a POST request as a lame workaround for some early server
1788   applications that failed to read message body content that was
1789   not terminated by a line-ending. An HTTP/1.1 client &MUST-NOT;
1790   preface or follow a request with an extra CRLF.  If terminating
1791   the request message body with a line-ending is desired, then the
1792   client &MUST; include the terminating CRLF octets as part of the
1793   message body length.
1794</t>
1795<t>
1796   In the interest of robustness, servers &SHOULD; ignore at least one
1797   empty line received where a request-line is expected. In other words, if
1798   the server is reading the protocol stream at the beginning of a
1799   message and receives a CRLF first, it &SHOULD; ignore the CRLF.
1800   Likewise, although the line terminator for the start-line and header
1801   fields is the sequence CRLF, we recommend that recipients recognize a
1802   single LF as a line terminator and ignore any CR.
1803</t>
1804<t>
1805   When a server listening only for HTTP request messages, or processing
1806   what appears from the start-line to be an HTTP request message,
1807   receives a sequence of octets that does not match the HTTP-message
1808   grammar aside from the robustness exceptions listed above, the
1809   server &MUST; respond with an HTTP/1.1 <x:ref>400 (Bad Request)</x:ref> response. 
1810</t>
1811</section>
1812</section>
1813
1814<section title="Transfer Codings" anchor="transfer.codings">
1815  <x:anchor-alias value="transfer-coding"/>
1816  <x:anchor-alias value="transfer-extension"/>
1817<t>
1818   Transfer-coding values are used to indicate an encoding
1819   transformation that has been, can be, or might need to be applied to a
1820   payload body in order to ensure "safe transport" through the network.
1821   This differs from a content coding in that the transfer-coding is a
1822   property of the message rather than a property of the representation
1823   that is being transferred.
1824</t>
1825<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1826  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1827                     / "compress" ; <xref target="compress.coding"/>
1828                     / "deflate" ; <xref target="deflate.coding"/>
1829                     / "gzip" ; <xref target="gzip.coding"/>
1830                     / <x:ref>transfer-extension</x:ref>
1831  <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> )
1832</artwork></figure>
1833<t anchor="rule.parameter">
1834  <x:anchor-alias value="attribute"/>
1835  <x:anchor-alias value="transfer-parameter"/>
1836  <x:anchor-alias value="value"/>
1837   Parameters are in the form of attribute/value pairs.
1838</t>
1839<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"/>
1840  <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>
1841  <x:ref>attribute</x:ref>          = <x:ref>token</x:ref>
1842  <x:ref>value</x:ref>              = <x:ref>word</x:ref>
1843</artwork></figure>
1844<t>
1845   All transfer-coding values are case-insensitive and &SHOULD; be registered
1846   within the HTTP Transfer Coding registry, as defined in
1847   <xref target="transfer.coding.registry"/>.
1848   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1849   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1850   header fields.
1851</t>
1852
1853<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1854  <iref item="chunked (Coding Format)"/>
1855  <x:anchor-alias value="chunk"/>
1856  <x:anchor-alias value="chunked-body"/>
1857  <x:anchor-alias value="chunk-data"/>
1858  <x:anchor-alias value="chunk-ext"/>
1859  <x:anchor-alias value="chunk-ext-name"/>
1860  <x:anchor-alias value="chunk-ext-val"/>
1861  <x:anchor-alias value="chunk-size"/>
1862  <x:anchor-alias value="last-chunk"/>
1863  <x:anchor-alias value="trailer-part"/>
1864  <x:anchor-alias value="quoted-str-nf"/>
1865  <x:anchor-alias value="qdtext-nf"/>
1866<t>
1867   The chunked encoding modifies the body of a message in order to
1868   transfer it as a series of chunks, each with its own size indicator,
1869   followed by an &OPTIONAL; trailer containing header fields. This
1870   allows dynamically produced content to be transferred along with the
1871   information necessary for the recipient to verify that it has
1872   received the full message.
1873</t>
1874<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"/>
1875  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1876                   <x:ref>last-chunk</x:ref>
1877                   <x:ref>trailer-part</x:ref>
1878                   <x:ref>CRLF</x:ref>
1879 
1880  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1881                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1882  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1883  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1884 
1885  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
1886  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
1887  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-str-nf</x:ref>
1888  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1889  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1890 
1891  <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>
1892                 ; like <x:ref>quoted-string</x:ref>, but disallowing line folding
1893  <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>
1894</artwork></figure>
1895<t>
1896   Chunk extensions within the chucked encoding are deprecated.
1897   Senders &SHOULD-NOT; send chunk-ext.
1898   Definition of new chunk extensions is discouraged.
1899</t>
1900<t>
1901   The chunk-size field is a string of hex digits indicating the size of
1902   the chunk-data in octets. The chunked encoding is ended by any chunk whose size is
1903   zero, followed by the trailer, which is terminated by an empty line.
1904</t>
1905
1906<section title="Trailer" anchor="header.trailer">
1907  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
1908  <x:anchor-alias value="Trailer"/>
1909<t>
1910   A trailer allows the sender to include additional fields at the end of a
1911   chunked message in order to supply metadata that might be dynamically
1912   generated while the message body is sent, such as a message integrity
1913   check, digital signature, or post-processing status.
1914   The trailer &MUST-NOT; contain fields that need to be known before a
1915   recipient processes the body, such as <x:ref>Transfer-Encoding</x:ref>,
1916   <x:ref>Content-Length</x:ref>, and <x:ref>Trailer</x:ref>.
1917</t>
1918<t>
1919   When a message includes a message body encoded with the chunked
1920   transfer-coding and the sender desires to send metadata in the form of
1921   trailer fields at the end of the message, the sender &SHOULD; send a
1922   <x:ref>Trailer</x:ref> header field before the message body to indicate
1923   which fields will be present in the trailers. This allows the recipient
1924   to prepare for receipt of that metadata before it starts processing the body,
1925   which is useful if the message is being streamed and the recipient wishes
1926   to confirm an integrity check on the fly.
1927</t>
1928<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
1929  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
1930</artwork></figure>
1931<t>
1932   If no <x:ref>Trailer</x:ref> header field is present, the sender of a
1933   chunked message body &SHOULD; send an empty trailer.
1934</t>
1935<t>
1936   A server &MUST; send an empty trailer with the chunked transfer-coding
1937   unless at least one of the following is true:
1938  <list style="numbers">
1939    <t>the request included a <x:ref>TE</x:ref> header field that indicates
1940    "trailers" is acceptable in the transfer-coding of the response, as
1941    described in <xref target="header.te"/>; or,</t>
1942     
1943    <t>the trailer fields consist entirely of optional metadata and the
1944    recipient could use the message (in a manner acceptable to the server where
1945    the field originated) without receiving that metadata. In other words,
1946    the server that generated the header field is willing to accept the
1947    possibility that the trailer fields might be silently discarded along
1948    the path to the client.</t>
1949  </list>
1950</t>
1951<t>
1952   The above requirement prevents the need for an infinite buffer when a
1953   message is being received by an HTTP/1.1 (or later) proxy and forwarded to
1954   an HTTP/1.0 recipient.
1955</t>
1956</section>
1957
1958<section title="Decoding chunked" anchor="decoding.chunked">
1959<t>
1960   A process for decoding the "chunked" transfer-coding
1961   can be represented in pseudo-code as:
1962</t>
1963<figure><artwork type="code">
1964  length := 0
1965  read chunk-size, chunk-ext (if any) and CRLF
1966  while (chunk-size &gt; 0) {
1967     read chunk-data and CRLF
1968     append chunk-data to decoded-body
1969     length := length + chunk-size
1970     read chunk-size and CRLF
1971  }
1972  read header-field
1973  while (header-field not empty) {
1974     append header-field to existing header fields
1975     read header-field
1976  }
1977  Content-Length := length
1978  Remove "chunked" from Transfer-Encoding
1979  Remove Trailer from existing header fields
1980</artwork></figure>
1981<t>
1982   All recipients &MUST; be able to receive and decode the
1983   "chunked" transfer-coding and &MUST; ignore chunk-ext extensions
1984   they do not understand.
1985</t>
1986</section>
1987</section>
1988
1989<section title="Compression Codings" anchor="compression.codings">
1990<t>
1991   The codings defined below can be used to compress the payload of a
1992   message.
1993</t>
1994
1995<section title="Compress Coding" anchor="compress.coding">
1996<iref item="compress (Coding Format)"/>
1997<t>
1998   The "compress" format is produced by the common UNIX file compression
1999   program "compress". This format is an adaptive Lempel-Ziv-Welch
2000   coding (LZW). Recipients &SHOULD; consider "x-compress" to be
2001   equivalent to "compress".
2002</t>
2003</section>
2004
2005<section title="Deflate Coding" anchor="deflate.coding">
2006<iref item="deflate (Coding Format)"/>
2007<t>
2008   The "deflate" format is defined as the "deflate" compression mechanism
2009   (described in <xref target="RFC1951"/>) used inside the "zlib"
2010   data format (<xref target="RFC1950"/>).
2011</t>
2012<x:note>
2013  <t>
2014    &Note; Some incorrect implementations send the "deflate"
2015    compressed data without the zlib wrapper.
2016   </t>
2017</x:note>
2018</section>
2019
2020<section title="Gzip Coding" anchor="gzip.coding">
2021<iref item="gzip (Coding Format)"/>
2022<t>
2023   The "gzip" format is produced by the file compression program
2024   "gzip" (GNU zip), as described in <xref target="RFC1952"/>. This format is a
2025   Lempel-Ziv coding (LZ77) with a 32 bit CRC.
2026   Recipients &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2027</t>
2028</section>
2029
2030</section>
2031
2032<section title="TE" anchor="header.te">
2033  <iref primary="true" item="TE header field" x:for-anchor=""/>
2034  <x:anchor-alias value="TE"/>
2035  <x:anchor-alias value="t-codings"/>
2036  <x:anchor-alias value="t-ranking"/>
2037  <x:anchor-alias value="rank"/>
2038<t>
2039   The "TE" header field in a request indicates what transfer-codings,
2040   besides "chunked", the client is willing to accept in response, and
2041   whether or not the client is willing to accept trailer fields in a
2042   chunked transfer-coding.
2043</t>
2044<t>
2045   The TE field-value consists of a comma-separated list of transfer-coding
2046   names, each allowing for optional parameters (as described in
2047   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2048   Clients &MUST-NOT; send the chunked transfer-coding name in TE;
2049   chunked is always acceptable for HTTP/1.1 recipients.
2050</t>
2051<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"/>
2052  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2053  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2054  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2055  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2056             / ( "1" [ "." 0*3("0") ] )
2057</artwork></figure>
2058<t>
2059   Three examples of TE use are below.
2060</t>
2061<figure><artwork type="example">
2062  TE: deflate
2063  TE:
2064  TE: trailers, deflate;q=0.5
2065</artwork></figure>
2066<t>
2067   The presence of the keyword "trailers" indicates that the client is
2068   willing to accept trailer fields in a chunked transfer-coding,
2069   as defined in <xref target="chunked.encoding"/>, on behalf of itself and
2070   any downstream clients. For chained requests, this implies that either:
2071   (a) all downstream clients are willing to accept trailer fields in the
2072   forwarded response; or,
2073   (b) the client will attempt to buffer the response on behalf of downstream
2074   recipients.
2075   Note that HTTP/1.1 does not define any means to limit the size of a
2076   chunked response such that a client can be assured of buffering the
2077   entire response.
2078</t>
2079<t>
2080   When multiple transfer-codings are acceptable, the client &MAY; rank the
2081   codings by preference using a case-insensitive "q" parameter (similar to
2082   the qvalues used in content negotiation fields, &qvalue;). The rank value
2083   is a real number in the range 0 through 1, where 0.001 is the least
2084   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2085</t>
2086<t>
2087   If the TE field-value is empty or if no TE field is present, the only
2088   acceptable transfer-coding is "chunked". A message with no transfer-coding
2089   is always acceptable.
2090</t>
2091<t>
2092   Since the TE header field only applies to the immediate connection,
2093   a sender of TE &MUST; also send a "TE" connection option within the
2094   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2095   in order to prevent the TE field from being forwarded by intermediaries
2096   that do not support its semantics.
2097</t>
2098</section>
2099</section>
2100
2101<section title="Message Routing" anchor="message.routing">
2102<t>
2103   HTTP request message routing is determined by each client based on the
2104   target resource, the client's proxy configuration, and
2105   establishment or reuse of an inbound connection.  The corresponding
2106   response routing follows the same connection chain back to the client.
2107</t>
2108
2109<section title="Identifying a Target Resource" anchor="target-resource">
2110  <iref primary="true" item="target resource"/>
2111  <iref primary="true" item="target URI"/>
2112  <x:anchor-alias value="target resource"/>
2113  <x:anchor-alias value="target URI"/>
2114<t>
2115   HTTP is used in a wide variety of applications, ranging from
2116   general-purpose computers to home appliances.  In some cases,
2117   communication options are hard-coded in a client's configuration.
2118   However, most HTTP clients rely on the same resource identification
2119   mechanism and configuration techniques as general-purpose Web browsers.
2120</t>
2121<t>
2122   HTTP communication is initiated by a user agent for some purpose.
2123   The purpose is a combination of request semantics, which are defined in
2124   <xref target="Part2"/>, and a target resource upon which to apply those
2125   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2126   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2127   would resolve to its absolute form in order to obtain the
2128   "<x:dfn>target URI</x:dfn>".  The target URI
2129   excludes the reference's fragment identifier component, if any,
2130   since fragment identifiers are reserved for client-side processing
2131   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2132</t>
2133</section>
2134
2135<section title="Connecting Inbound" anchor="connecting.inbound">
2136<t>
2137   Once the target URI is determined, a client needs to decide whether
2138   a network request is necessary to accomplish the desired semantics and,
2139   if so, where that request is to be directed.
2140</t>
2141<t>
2142   If the client has a response cache and the request semantics can be
2143   satisfied by a cache (<xref target="Part6"/>), then the request is
2144   usually directed to the cache first.
2145</t>
2146<t>
2147   If the request is not satisfied by a cache, then a typical client will
2148   check its configuration to determine whether a proxy is to be used to
2149   satisfy the request.  Proxy configuration is implementation-dependent,
2150   but is often based on URI prefix matching, selective authority matching,
2151   or both, and the proxy itself is usually identified by an "http" or
2152   "https" URI.  If a proxy is applicable, the client connects inbound by
2153   establishing (or reusing) a connection to that proxy.
2154</t>
2155<t>
2156   If no proxy is applicable, a typical client will invoke a handler routine,
2157   usually specific to the target URI's scheme, to connect directly
2158   to an authority for the target resource.  How that is accomplished is
2159   dependent on the target URI scheme and defined by its associated
2160   specification, similar to how this specification defines origin server
2161   access for resolution of the "http" (<xref target="http.uri"/>) and
2162   "https" (<xref target="https.uri"/>) schemes.
2163</t>
2164<t>
2165   HTTP requirements regarding connection management are defined in
2166   <xref target="connection.management"/>.
2167</t>
2168</section>
2169
2170<section title="Request Target" anchor="request-target">
2171<t>
2172   Once an inbound connection is obtained,
2173   the client sends an HTTP request message (<xref target="http.message"/>)
2174   with a request-target derived from the target URI.
2175   There are four distinct formats for the request-target, depending on both
2176   the method being requested and whether the request is to a proxy.
2177</t>   
2178<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"/>
2179  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2180                 / <x:ref>absolute-form</x:ref>
2181                 / <x:ref>authority-form</x:ref>
2182                 / <x:ref>asterisk-form</x:ref>
2183
2184  <x:ref>origin-form</x:ref>    = <x:ref>path-absolute</x:ref> [ "?" <x:ref>query</x:ref> ]
2185  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2186  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2187  <x:ref>asterisk-form</x:ref>  = "*"
2188</artwork></figure>
2189<t anchor="origin-form"><iref item="origin-form (of request-target)"/>
2190   The most common form of request-target is the origin-form.
2191   When making a request directly to an origin server, other than a CONNECT
2192   or server-wide OPTIONS request (as detailed below),
2193   a client &MUST; send only the absolute path and query components of
2194   the target URI as the request-target.
2195   If the target URI's path component is empty, then the client &MUST; send
2196   "/" as the path within the origin-form of request-target.
2197   A <x:ref>Host</x:ref> header field is also sent, as defined in
2198   <xref target="header.host"/>, containing the target URI's
2199   authority component (excluding any userinfo).
2200</t>
2201<t>
2202   For example, a client wishing to retrieve a representation of the resource
2203   identified as
2204</t>
2205<figure><artwork x:indent-with="  " type="example">
2206http://www.example.org/where?q=now
2207</artwork></figure>
2208<t>
2209   directly from the origin server would open (or reuse) a TCP connection
2210   to port 80 of the host "www.example.org" and send the lines:
2211</t>
2212<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2213GET /where?q=now HTTP/1.1
2214Host: www.example.org
2215</artwork></figure>
2216<t>
2217   followed by the remainder of the request message.
2218</t>
2219<t anchor="absolute-form"><iref item="absolute-form (of request-target)"/>
2220   When making a request to a proxy, other than a CONNECT or server-wide
2221   OPTIONS request (as detailed below), a client &MUST; send the target URI
2222   in absolute-form as the request-target.
2223   The proxy is requested to either service that request from a valid cache,
2224   if possible, or make the same request on the client's behalf to either
2225   the next inbound proxy server or directly to the origin server indicated
2226   by the request-target.  Requirements on such "forwarding" of messages are
2227   defined in <xref target="message.forwarding"/>.
2228</t>
2229<t>
2230   An example absolute-form of request-line would be:
2231</t>
2232<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2233GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
2234</artwork></figure>
2235<t>
2236   To allow for transition to the absolute-form for all requests in some
2237   future version of HTTP, HTTP/1.1 servers &MUST; accept the absolute-form
2238   in requests, even though HTTP/1.1 clients will only send them in requests
2239   to proxies.
2240</t>
2241<t anchor="authority-form"><iref item="authority-form (of request-target)"/>
2242   The authority-form of request-target is only used for CONNECT requests
2243   (&CONNECT;).  When making a CONNECT request to establish a tunnel through
2244   one or more proxies, a client &MUST; send only the target URI's
2245   authority component (excluding any userinfo) as the request-target.
2246   For example,
2247</t>
2248<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2249CONNECT www.example.com:80 HTTP/1.1
2250</artwork></figure>
2251<t anchor="asterisk-form"><iref item="asterisk-form (of request-target)"/>
2252   The asterisk-form of request-target is only used for a server-wide
2253   OPTIONS request (&OPTIONS;).  When a client wishes to request OPTIONS
2254   for the server as a whole, as opposed to a specific named resource of
2255   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2256   For example,
2257</t>
2258<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2259OPTIONS * HTTP/1.1
2260</artwork></figure>
2261<t>
2262   If a proxy receives an OPTIONS request with an absolute-form of
2263   request-target in which the URI has an empty path and no query component,
2264   then the last proxy on the request chain &MUST; send a request-target
2265   of "*" when it forwards the request to the indicated origin server.
2266</t>
2267<figure><preamble>   
2268   For example, the request
2269</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2270OPTIONS http://www.example.org:8001 HTTP/1.1
2271</artwork></figure>
2272<figure><preamble>   
2273  would be forwarded by the final proxy as
2274</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2275OPTIONS * HTTP/1.1
2276Host: www.example.org:8001
2277</artwork>
2278<postamble>
2279   after connecting to port 8001 of host "www.example.org".
2280</postamble>
2281</figure>
2282</section>
2283
2284<section title="Host" anchor="header.host">
2285  <iref primary="true" item="Host header field" x:for-anchor=""/>
2286  <x:anchor-alias value="Host"/>
2287<t>
2288   The "Host" header field in a request provides the host and port
2289   information from the target URI, enabling the origin
2290   server to distinguish among resources while servicing requests
2291   for multiple host names on a single IP address.  Since the Host
2292   field-value is critical information for handling a request, it
2293   &SHOULD; be sent as the first header field following the request-line.
2294</t>
2295<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2296  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2297</artwork></figure>
2298<t>
2299   A client &MUST; send a Host header field in all HTTP/1.1 request
2300   messages.  If the target URI includes an authority component, then
2301   the Host field-value &MUST; be identical to that authority component
2302   after excluding any userinfo (<xref target="http.uri"/>).
2303   If the authority component is missing or undefined for the target URI,
2304   then the Host header field &MUST; be sent with an empty field-value.
2305</t>
2306<t>
2307   For example, a GET request to the origin server for
2308   &lt;http://www.example.org/pub/WWW/&gt; would begin with:
2309</t>
2310<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2311GET /pub/WWW/ HTTP/1.1
2312Host: www.example.org
2313</artwork></figure>
2314<t>
2315   The Host header field &MUST; be sent in an HTTP/1.1 request even
2316   if the request-target is in the absolute-form, since this
2317   allows the Host information to be forwarded through ancient HTTP/1.0
2318   proxies that might not have implemented Host.
2319</t>
2320<t>
2321   When a proxy receives a request with an absolute-form of
2322   request-target, the proxy &MUST; ignore the received
2323   Host header field (if any) and instead replace it with the host
2324   information of the request-target.  If the proxy forwards the request,
2325   it &MUST; generate a new Host field-value based on the received
2326   request-target rather than forward the received Host field-value.
2327</t>
2328<t>
2329   Since the Host header field acts as an application-level routing
2330   mechanism, it is a frequent target for malware seeking to poison
2331   a shared cache or redirect a request to an unintended server.
2332   An interception proxy is particularly vulnerable if it relies on
2333   the Host field-value for redirecting requests to internal
2334   servers, or for use as a cache key in a shared cache, without
2335   first verifying that the intercepted connection is targeting a
2336   valid IP address for that host.
2337</t>
2338<t>
2339   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2340   to any HTTP/1.1 request message that lacks a Host header field and
2341   to any request message that contains more than one Host header field
2342   or a Host header field with an invalid field-value.
2343</t>
2344</section>
2345
2346<section title="Effective Request URI" anchor="effective.request.uri">
2347  <iref primary="true" item="effective request URI"/>
2348<t>
2349   A server that receives an HTTP request message &MUST; reconstruct
2350   the user agent's original target URI, based on the pieces of information
2351   learned from the request-target, <x:ref>Host</x:ref> header field, and
2352   connection context, in order to identify the intended target resource and
2353   properly service the request. The URI derived from this reconstruction
2354   process is referred to as the "<x:dfn>effective request URI</x:dfn>".
2355</t>
2356<t>
2357   For a user agent, the effective request URI is the target URI.
2358</t>
2359<t>
2360   If the request-target is in absolute-form, then the effective request URI
2361   is the same as the request-target.  Otherwise, the effective request URI
2362   is constructed as follows.
2363</t>
2364<t>
2365   If the request is received over a TLS-secured TCP connection,
2366   then the effective request URI's scheme is "https"; otherwise, the
2367   scheme is "http".
2368</t>
2369<t>
2370   If the request-target is in authority-form, then the effective
2371   request URI's authority component is the same as the request-target.
2372   Otherwise, if a <x:ref>Host</x:ref> header field is supplied with a
2373   non-empty field-value, then the authority component is the same as the
2374   Host field-value. Otherwise, the authority component is the concatenation of
2375   the default host name configured for the server, a colon (":"), and the
2376   connection's incoming TCP port number in decimal form.
2377</t>
2378<t>
2379   If the request-target is in authority-form or asterisk-form, then the
2380   effective request URI's combined path and query component is empty.
2381   Otherwise, the combined path and query component is the same as the
2382   request-target.
2383</t>
2384<t>
2385   The components of the effective request URI, once determined as above,
2386   can be combined into absolute-URI form by concatenating the scheme,
2387   "://", authority, and combined path and query component.
2388</t>
2389<figure>
2390<preamble>
2391   Example 1: the following message received over an insecure TCP connection
2392</preamble> 
2393<artwork type="example" x:indent-with="  ">
2394GET /pub/WWW/TheProject.html HTTP/1.1
2395Host: www.example.org:8080
2396</artwork>
2397</figure>
2398<figure>
2399<preamble>
2400  has an effective request URI of
2401</preamble>
2402<artwork type="example" x:indent-with="  ">
2403http://www.example.org:8080/pub/WWW/TheProject.html
2404</artwork>
2405</figure>
2406<figure>
2407<preamble>
2408   Example 2: the following message received over a TLS-secured TCP connection
2409</preamble> 
2410<artwork type="example" x:indent-with="  ">
2411OPTIONS * HTTP/1.1
2412Host: www.example.org
2413</artwork>
2414</figure>
2415<figure>
2416<preamble>
2417  has an effective request URI of
2418</preamble>
2419<artwork type="example" x:indent-with="  ">
2420https://www.example.org
2421</artwork>
2422</figure>
2423<t>
2424   An origin server that does not allow resources to differ by requested
2425   host &MAY; ignore the <x:ref>Host</x:ref> field-value and instead replace it
2426   with a configured server name when constructing the effective request URI.
2427</t>
2428<t>
2429   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2430   field &MAY; attempt to use heuristics (e.g., examination of the URI path for
2431   something unique to a particular host) in order to guess the
2432   effective request URI's authority component.
2433</t>
2434</section>
2435
2436<section title="Message Forwarding" anchor="message.forwarding">
2437<t>
2438   As described in <xref target="intermediaries"/>, intermediaries can serve
2439   a variety of roles in the processing of HTTP requests and responses.
2440   Some intermediaries are used to improve performance or availability.
2441   Others are used for access control or to filter content.
2442   Since an HTTP stream has characteristics similar to a pipe-and-filter
2443   architecture, there are no inherent limits to the extent an intermediary
2444   can enhance (or interfere) with either direction of the stream.
2445</t>
2446<t>
2447   Intermediaries that forward a message &MUST; implement the
2448   <x:ref>Connection</x:ref> header field, as specified in
2449   <xref target="header.connection"/>, to exclude fields that are only
2450   intended for the incoming connection.
2451</t>
2452<t>
2453   In order to avoid request loops, a proxy that forwards requests to other
2454   proxies &MUST; be able to recognize and exclude all of its own server
2455   names, including any aliases, local variations, or literal IP addresses.
2456</t>
2457</section>
2458
2459<section title="Via" anchor="header.via">
2460  <iref primary="true" item="Via header field" x:for-anchor=""/>
2461  <x:anchor-alias value="pseudonym"/>
2462  <x:anchor-alias value="received-by"/>
2463  <x:anchor-alias value="received-protocol"/>
2464  <x:anchor-alias value="Via"/>
2465<t>
2466   The "Via" header field &MUST; be sent by a proxy or gateway
2467   in forwarded messages to
2468   indicate the intermediate protocols and recipients between the user
2469   agent and the server on requests, and between the origin server and
2470   the client on responses. It is analogous to the "Received" field
2471   used by email systems (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2472   Via is used in HTTP for tracking message forwards,
2473   avoiding request loops, and identifying the protocol capabilities of
2474   all senders along the request/response chain.
2475</t>
2476<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"/>
2477  <x:ref>Via</x:ref>               = 1#( <x:ref>received-protocol</x:ref> <x:ref>RWS</x:ref> <x:ref>received-by</x:ref>
2478                          [ <x:ref>RWS</x:ref> <x:ref>comment</x:ref> ] )
2479  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2480  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2481  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2482</artwork></figure>
2483<t>
2484   The received-protocol indicates the protocol version of the message
2485   received by the server or client along each segment of the
2486   request/response chain. The received-protocol version is appended to
2487   the Via field value when the message is forwarded so that information
2488   about the protocol capabilities of upstream applications remains
2489   visible to all recipients.
2490</t>
2491<t>
2492   The protocol-name is excluded if and only if it would be "HTTP". The
2493   received-by field is normally the host and optional port number of a
2494   recipient server or client that subsequently forwarded the message.
2495   However, if the real host is considered to be sensitive information,
2496   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2497   be assumed to be the default port of the received-protocol.
2498</t>
2499<t>
2500   Multiple Via field values represent each proxy or gateway that has
2501   forwarded the message. Each recipient &MUST; append its information
2502   such that the end result is ordered according to the sequence of
2503   forwarding applications.
2504</t>
2505<t>
2506   Comments &MAY; be used in the Via header field to identify the software
2507   of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2508   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2509   are optional and &MAY; be removed by any recipient prior to forwarding the
2510   message.
2511</t>
2512<t>
2513   For example, a request message could be sent from an HTTP/1.0 user
2514   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2515   forward the request to a public proxy at p.example.net, which completes
2516   the request by forwarding it to the origin server at www.example.com.
2517   The request received by www.example.com would then have the following
2518   Via header field:
2519</t>
2520<figure><artwork type="example">
2521  Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
2522</artwork></figure>
2523<t>
2524   A proxy or gateway used as a portal through a network firewall
2525   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2526   region unless it is explicitly enabled to do so. If not enabled, the
2527   received-by host of any host behind the firewall &SHOULD; be replaced
2528   by an appropriate pseudonym for that host.
2529</t>
2530<t>
2531   A proxy or gateway &MAY; combine an ordered subsequence of Via header
2532   field entries into a single such entry if the entries have identical
2533   received-protocol values. For example,
2534</t>
2535<figure><artwork type="example">
2536  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2537</artwork></figure>
2538<t>
2539  could be collapsed to
2540</t>
2541<figure><artwork type="example">
2542  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2543</artwork></figure>
2544<t>
2545   Senders &SHOULD-NOT; combine multiple entries unless they are all
2546   under the same organizational control and the hosts have already been
2547   replaced by pseudonyms. Senders &MUST-NOT; combine entries which
2548   have different received-protocol values.
2549</t>
2550</section>
2551
2552<section title="Message Transforming" anchor="message.transforming">
2553<t>
2554   If a proxy receives a request-target with a host name that is not a
2555   fully qualified domain name, it &MAY; add its own domain to the host name
2556   it received when forwarding the request.  A proxy &MUST-NOT; change the
2557   host name if it is a fully qualified domain name.
2558</t>
2559<t>
2560   A non-transforming proxy &MUST-NOT; modify the "path-absolute" and "query"
2561   parts of the received request-target when forwarding it to the next inbound
2562   server, except as noted above to replace an empty path with "/" or "*".
2563</t>
2564<t>
2565   A non-transforming proxy &MUST; preserve the message payload (&payload;),
2566   though it &MAY; change the message body through application or removal
2567   of a transfer-coding (<xref target="transfer.codings"/>).
2568</t>
2569<t>
2570   A non-transforming proxy &SHOULD-NOT; modify header fields that provide
2571   information about the end points of the communication chain, the resource
2572   state, or the selected representation.
2573</t>
2574<t>
2575   A non-transforming proxy &MUST-NOT; modify any of the following fields in a
2576   request or response, and it &MUST-NOT; add any of these fields if not
2577   already present:
2578  <list style="symbols">
2579    <t><x:ref>Allow</x:ref> (&header-allow;)</t>
2580    <t><x:ref>Content-Location</x:ref> (&header-content-location;)</t>
2581    <t>Content-MD5 (<xref target="RFC2616" x:fmt="of" x:sec="14.15"/>)</t>
2582    <t><x:ref>ETag</x:ref> (&header-etag;)</t>
2583    <t><x:ref>Last-Modified</x:ref> (&header-last-modified;)</t>
2584    <t><x:ref>Server</x:ref> (&header-server;)</t>
2585  </list>
2586</t>
2587<t>
2588   A non-transforming proxy &MUST-NOT; modify an <x:ref>Expires</x:ref>
2589   header field (&header-expires;) if already present in a response, but
2590   it &MAY; add an <x:ref>Expires</x:ref> header field with a field-value
2591   identical to that of the <x:ref>Date</x:ref> header field.
2592</t>
2593<t>
2594   A proxy &MUST-NOT; modify or add any of the following fields in a
2595   message that contains the no-transform cache-control directive:
2596  <list style="symbols">
2597    <t><x:ref>Content-Encoding</x:ref> (&header-content-encoding;)</t>
2598    <t><x:ref>Content-Range</x:ref> (&header-content-range;)</t>
2599    <t><x:ref>Content-Type</x:ref> (&header-content-type;)</t>
2600  </list>
2601</t>
2602<t>
2603   A transforming proxy &MAY; modify or add these fields to a message
2604   that does not include no-transform, but if it does so, it &MUST; add a
2605   Warning 214 (Transformation applied) if one does not already appear
2606   in the message (see &header-warning;).
2607</t>
2608<x:note>
2609  <t>
2610    <x:h>Warning:</x:h> Unnecessary modification of header fields might
2611    cause authentication failures if stronger authentication
2612    mechanisms are introduced in later versions of HTTP. Such
2613    authentication mechanisms &MAY; rely on the values of header fields
2614    not listed here.
2615  </t>
2616</x:note>
2617</section>
2618
2619<section title="Associating a Response to a Request" anchor="associating.response.to.request">
2620<t>
2621   HTTP does not include a request identifier for associating a given
2622   request message with its corresponding one or more response messages.
2623   Hence, it relies on the order of response arrival to correspond exactly
2624   to the order in which requests are made on the same connection.
2625   More than one response message per request only occurs when one or more
2626   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a final response
2627   to the same request.
2628</t>
2629<t>
2630   A client that uses persistent connections and sends more than one request
2631   per connection &MUST; maintain a list of outstanding requests in the
2632   order sent on that connection and &MUST; associate each received response
2633   message to the highest ordered request that has not yet received a final
2634   (non-<x:ref>1xx</x:ref>) response.
2635</t>
2636</section>
2637</section>
2638
2639<section title="Connection Management" anchor="connection.management">
2640<t>
2641   HTTP messaging is independent of the underlying transport or
2642   session-layer connection protocol(s).  HTTP only presumes a reliable
2643   transport with in-order delivery of requests and the corresponding
2644   in-order delivery of responses.  The mapping of HTTP request and
2645   response structures onto the data units of an underlying transport
2646   protocol is outside the scope of this specification.
2647</t>
2648<t>
2649   As described in <xref target="connecting.inbound"/>, the specific
2650   connection protocols to be used for an HTTP interaction are determined by
2651   client configuration and the <x:ref>target URI</x:ref>.
2652   For example, the "http" URI scheme
2653   (<xref target="http.uri"/>) indicates a default connection of TCP
2654   over IP, with a default TCP port of 80, but the client might be
2655   configured to use a proxy via some other connection, port, or protocol.
2656</t>
2657<t>
2658   HTTP implementations are expected to engage in connection management,
2659   which includes maintaining the state of current connections,
2660   establishing a new connection or reusing an existing connection,
2661   processing messages received on a connection, detecting connection
2662   failures, and closing each connection.
2663   Most clients maintain multiple connections in parallel, including
2664   more than one connection per server endpoint.
2665   Most servers are designed to maintain thousands of concurrent connections,
2666   while controlling request queues to enable fair use and detect
2667   denial of service attacks.
2668</t>
2669
2670<section title="Connection" anchor="header.connection">
2671  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2672  <iref primary="true" item="close" x:for-anchor=""/>
2673  <x:anchor-alias value="Connection"/>
2674  <x:anchor-alias value="connection-option"/>
2675  <x:anchor-alias value="close"/>
2676<t>
2677   The "Connection" header field allows the sender to indicate desired
2678   control options for the current connection.  In order to avoid confusing
2679   downstream recipients, a proxy or gateway &MUST; remove or replace any
2680   received connection options before forwarding the message.
2681</t>
2682<t>
2683   When a header field is used to supply control information for or about
2684   the current connection, the sender &SHOULD; list the corresponding
2685   field-name within the "Connection" header field.
2686   A proxy or gateway &MUST; parse a received Connection
2687   header field before a message is forwarded and, for each
2688   connection-option in this field, remove any header field(s) from
2689   the message with the same name as the connection-option, and then
2690   remove the Connection header field itself (or replace it with the
2691   intermediary's own connection options for the forwarded message).
2692</t>
2693<t>
2694   Hence, the Connection header field provides a declarative way of
2695   distinguishing header fields that are only intended for the
2696   immediate recipient ("hop-by-hop") from those fields that are
2697   intended for all recipients on the chain ("end-to-end"), enabling the
2698   message to be self-descriptive and allowing future connection-specific
2699   extensions to be deployed without fear that they will be blindly
2700   forwarded by older intermediaries.
2701</t>
2702<t>
2703   The Connection header field's value has the following grammar:
2704</t>
2705<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2706  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2707  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2708</artwork></figure>
2709<t>
2710   Connection options are case-insensitive.
2711</t>
2712<t>
2713   A sender &MUST-NOT; include field-names in the Connection header
2714   field-value for fields that are defined as expressing constraints
2715   for all recipients in the request or response chain, such as the
2716   Cache-Control header field (&header-cache-control;).
2717</t>
2718<t>
2719   The connection options do not have to correspond to a header field
2720   present in the message, since a connection-specific header field
2721   might not be needed if there are no parameters associated with that
2722   connection option.  Recipients that trigger certain connection
2723   behavior based on the presence of connection options &MUST; do so
2724   based on the presence of the connection-option rather than only the
2725   presence of the optional header field.  In other words, if the
2726   connection option is received as a header field but not indicated
2727   within the Connection field-value, then the recipient &MUST; ignore
2728   the connection-specific header field because it has likely been
2729   forwarded by an intermediary that is only partially conformant.
2730</t>
2731<t>
2732   When defining new connection options, specifications ought to
2733   carefully consider existing deployed header fields and ensure
2734   that the new connection option does not share the same name as
2735   an unrelated header field that might already be deployed.
2736   Defining a new connection option essentially reserves that potential
2737   field-name for carrying additional information related to the
2738   connection option, since it would be unwise for senders to use
2739   that field-name for anything else.
2740</t>
2741<t>
2742   The "<x:dfn>close</x:dfn>" connection option is defined for a
2743   sender to signal that this connection will be closed after completion of
2744   the response. For example,
2745</t>
2746<figure><artwork type="example">
2747  Connection: close
2748</artwork></figure>
2749<t>
2750   in either the request or the response header fields indicates that
2751   the connection &SHOULD; be closed after the current request/response
2752   is complete (<xref target="persistent.tear-down"/>).
2753</t>
2754<t>
2755   A client that does not support persistent connections &MUST;
2756   send the "close" connection option in every request message.
2757</t>
2758<t>
2759   A server that does not support persistent connections &MUST;
2760   send the "close" connection option in every response message that
2761   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2762</t>
2763</section>
2764
2765<section title="Persistent Connections" anchor="persistent.connections">
2766  <x:anchor-alias value="persistent connections"/>
2767<t>
2768   HTTP was originally designed to use a separate connection for each
2769   request/response pair. As the Web evolved and embedded requests became
2770   common for inline images, the connection establishment overhead was
2771   a significant drain on performance and a concern for Internet congestion.
2772   Message framing (via <x:ref>Content-Length</x:ref>) and optional
2773   long-lived connections (via Keep-Alive) were added to HTTP/1.0 in order
2774   to improve performance for some requests. However, these extensions were
2775   insufficient for dynamically generated responses and difficult to use
2776   with intermediaries.
2777</t>
2778<t>
2779   HTTP/1.1 defaults to the use of "<x:ref>persistent connections</x:ref>",
2780   which allow multiple requests and responses to be carried over a single
2781   connection. The "<x:ref>close</x:ref>" connection-option is used to
2782   signal that a connection will close after the current request/response.
2783   Persistent connections have a number of advantages:
2784  <list style="symbols">
2785      <t>
2786        By opening and closing fewer connections, CPU time is saved
2787        in routers and hosts (clients, servers, proxies, gateways,
2788        tunnels, or caches), and memory used for protocol control
2789        blocks can be saved in hosts.
2790      </t>
2791      <t>
2792        Most requests and responses can be pipelined on a connection.
2793        Pipelining allows a client to make multiple requests without
2794        waiting for each response, allowing a single connection to
2795        be used much more efficiently and with less overall latency.
2796      </t>
2797      <t>
2798        For TCP connections, network congestion is reduced by eliminating the
2799        packets associated with the three way handshake and graceful close
2800        procedures, and by allowing sufficient time to determine the
2801        congestion state of the network.
2802      </t>
2803      <t>
2804        Latency on subsequent requests is reduced since there is no time
2805        spent in the connection opening handshake.
2806      </t>
2807      <t>
2808        HTTP can evolve more gracefully, since most errors can be reported
2809        without the penalty of closing the connection. Clients using
2810        future versions of HTTP might optimistically try a new feature,
2811        but if communicating with an older server, retry with old
2812        semantics after an error is reported.
2813      </t>
2814    </list>
2815</t>
2816<t>
2817   HTTP implementations &SHOULD; implement persistent connections.
2818</t>
2819   
2820<section title="Establishment" anchor="persistent.establishment">
2821<t>
2822   It is beyond the scope of this specification to describe how connections
2823   are established via various transport or session-layer protocols.
2824   Each connection applies to only one transport link.
2825</t>
2826<t>
2827   A recipient determines whether a connection is persistent or not based on
2828   the most recently received message's protocol version and
2829   <x:ref>Connection</x:ref> header field (if any):
2830   <list style="symbols">
2831     <t>If the <x:ref>close</x:ref> connection option is present, the
2832        connection will not persist after the current response; else,</t>
2833     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2834        persist after the current response; else,</t>
2835     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2836        connection option is present, the recipient is not a proxy, and
2837        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2838        the connection will persist after the current response; otherwise,</t>
2839     <t>The connection will close after the current response.</t>
2840   </list>
2841</t>
2842<t>
2843   A proxy server &MUST-NOT; maintain a persistent connection with an
2844   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
2845   information and discussion of the problems with the Keep-Alive header field
2846   implemented by many HTTP/1.0 clients).
2847</t>
2848</section>
2849
2850<section title="Reuse" anchor="persistent.reuse">
2851<t>
2852   In order to remain persistent, all messages on a connection &MUST;
2853   have a self-defined message length (i.e., one not defined by closure
2854   of the connection), as described in <xref target="message.body"/>.
2855</t>
2856<t>
2857   A server &MAY; assume that an HTTP/1.1 client intends to maintain a
2858   persistent connection until a <x:ref>close</x:ref> connection option
2859   is received in a request.
2860</t>
2861<t>
2862   A client &MAY; reuse a persistent connection until it sends or receives
2863   a <x:ref>close</x:ref> connection option or receives an HTTP/1.0 response
2864   without a "keep-alive" connection option.
2865</t>
2866<t>
2867   Clients and servers &SHOULD-NOT; assume that a persistent connection is
2868   maintained for HTTP versions less than 1.1 unless it is explicitly
2869   signaled.
2870   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
2871   for more information on backward compatibility with HTTP/1.0 clients.
2872</t>
2873
2874<section title="Pipelining" anchor="pipelining">
2875<t>
2876   A client that supports persistent connections &MAY; "pipeline" its
2877   requests (i.e., send multiple requests without waiting for each
2878   response). A server &MUST; send its responses to those requests in the
2879   same order that the requests were received.
2880</t>
2881<t>
2882   Clients which assume persistent connections and pipeline immediately
2883   after connection establishment &SHOULD; be prepared to retry their
2884   connection if the first pipelined attempt fails. If a client does
2885   such a retry, it &MUST-NOT; pipeline before it knows the connection is
2886   persistent. Clients &MUST; also be prepared to resend their requests if
2887   the server closes the connection before sending all of the
2888   corresponding responses.
2889</t>
2890<t>
2891   Clients &SHOULD-NOT; pipeline requests using non-idempotent request methods
2892   or non-idempotent sequences of request methods (see &idempotent-methods;).
2893   Otherwise, a premature termination of the transport connection could lead
2894   to indeterminate results. A client wishing to send a non-idempotent
2895   request &SHOULD; wait to send that request until it has received the
2896   response status line for the previous request.
2897</t>
2898</section>
2899
2900<section title="Retrying Requests" anchor="persistent.retrying.requests">
2901<t>
2902   Senders can close the transport connection at any time. Therefore,
2903   clients, servers, and proxies &MUST; be able to recover
2904   from asynchronous close events. Client software &MAY; reopen the
2905   transport connection and retransmit the aborted sequence of requests
2906   without user interaction so long as the request sequence is
2907   idempotent (see &idempotent-methods;). Non-idempotent request methods or sequences
2908   &MUST-NOT; be automatically retried, although user agents &MAY; offer a
2909   human operator the choice of retrying the request(s). Confirmation by
2910   user-agent software with semantic understanding of the application
2911   &MAY; substitute for user confirmation. The automatic retry &SHOULD-NOT; 
2912   be repeated if the second sequence of requests fails.
2913</t>
2914</section>
2915</section>
2916   
2917<section title="Concurrency" anchor="persistent.concurrency">
2918<t>
2919   Clients &SHOULD; limit the number of simultaneous
2920   connections that they maintain to a given server.
2921</t>
2922<t>
2923   Previous revisions of HTTP gave a specific number of connections as a
2924   ceiling, but this was found to be impractical for many applications. As a
2925   result, this specification does not mandate a particular maximum number of
2926   connections, but instead encourages clients to be conservative when opening
2927   multiple connections.
2928</t>
2929<t>
2930   Multiple connections are typically used to avoid the "head-of-line
2931   blocking" problem, wherein a request that takes significant server-side
2932   processing and/or has a large payload blocks subsequent requests on the
2933   same connection. However, each connection consumes server resources.
2934   Furthermore, using multiple connections can cause undesirable side effects
2935   in congested networks.
2936</t>
2937<t>
2938   Note that servers might reject traffic that they deem abusive, including an
2939   excessive number of connections from a client.
2940</t>
2941</section>
2942
2943<section title="Failures and Time-outs" anchor="persistent.failures">
2944<t>
2945   Servers will usually have some time-out value beyond which they will
2946   no longer maintain an inactive connection. Proxy servers might make
2947   this a higher value since it is likely that the client will be making
2948   more connections through the same server. The use of persistent
2949   connections places no requirements on the length (or existence) of
2950   this time-out for either the client or the server.
2951</t>
2952<t>
2953   When a client or server wishes to time-out it &SHOULD; issue a graceful
2954   close on the transport connection. Clients and servers &SHOULD; both
2955   constantly watch for the other side of the transport close, and
2956   respond to it as appropriate. If a client or server does not detect
2957   the other side's close promptly it could cause unnecessary resource
2958   drain on the network.
2959</t>
2960<t>
2961   A client, server, or proxy &MAY; close the transport connection at any
2962   time. For example, a client might have started to send a new request
2963   at the same time that the server has decided to close the "idle"
2964   connection. From the server's point of view, the connection is being
2965   closed while it was idle, but from the client's point of view, a
2966   request is in progress.
2967</t>
2968<t>
2969   Servers &SHOULD; maintain persistent connections and allow the underlying
2970   transport's flow control mechanisms to resolve temporary overloads, rather
2971   than terminate connections with the expectation that clients will retry.
2972   The latter technique can exacerbate network congestion.
2973</t>
2974<t>
2975   A client sending a message body &SHOULD; monitor
2976   the network connection for an error status code while it is transmitting
2977   the request. If the client sees an error status code, it &SHOULD;
2978   immediately cease transmitting the body and close the connection.
2979</t>
2980</section>
2981   
2982<section title="Tear-down" anchor="persistent.tear-down">
2983  <iref primary="false" item="Connection header field" x:for-anchor=""/>
2984  <iref primary="false" item="close" x:for-anchor=""/>
2985<t>
2986   The <x:ref>Connection</x:ref> header field
2987   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
2988   connection option that a sender &SHOULD; send when it wishes to close
2989   the connection after the current request/response pair.
2990</t>
2991<t>
2992   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
2993   send further requests on that connection (after the one containing
2994   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
2995   final response message corresponding to this request.
2996</t>
2997<t>
2998   A server that receives a <x:ref>close</x:ref> connection option &MUST;
2999   initiate a lingering close of the connection after it sends the final
3000   response to the request that contained <x:ref>close</x:ref>.
3001   The server &SHOULD; include a <x:ref>close</x:ref> connection option
3002   in its final response on that connection. The server &MUST-NOT; process
3003   any further requests received on that connection.
3004</t>
3005<t>
3006   A server that sends a <x:ref>close</x:ref> connection option &MUST;
3007   initiate a lingering close of the connection after it sends the
3008   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
3009   any further requests received on that connection.
3010</t>
3011<t>
3012   A client that receives a <x:ref>close</x:ref> connection option &MUST;
3013   cease sending requests on that connection and close the connection
3014   after reading the response message containing the close; if additional
3015   pipelined requests had been sent on the connection, the client &SHOULD;
3016   assume that they will not be processed by the server.
3017</t>
3018<t>
3019   If a server performs an immediate close of a TCP connection, there is a
3020   significant risk that the client will not be able to read the last HTTP
3021   response.  If the server receives additional data from the client on a
3022   fully-closed connection, such as another request that was sent by the
3023   client before receiving the server's response, the server's TCP stack will
3024   send a reset packet to the client; unfortunately, the reset packet might
3025   erase the client's unacknowledged input buffers before they can be read
3026   and interpreted by the client's HTTP parser.
3027</t>
3028<t>
3029   To avoid the TCP reset problem, a server can perform a lingering close on a
3030   connection by closing only the write side of the read/write connection
3031   (a half-close) and continuing to read from the connection until the
3032   connection is closed by the client or the server is reasonably certain
3033   that its own TCP stack has received the client's acknowledgement of the
3034   packet(s) containing the server's last response. It is then safe for the
3035   server to fully close the connection.
3036</t>
3037<t>
3038   It is unknown whether the reset problem is exclusive to TCP or might also
3039   be found in other transport connection protocols.
3040</t>
3041</section>
3042</section>
3043
3044<section title="Upgrade" anchor="header.upgrade">
3045  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3046  <x:anchor-alias value="Upgrade"/>
3047  <x:anchor-alias value="protocol"/>
3048  <x:anchor-alias value="protocol-name"/>
3049  <x:anchor-alias value="protocol-version"/>
3050<t>
3051   The "Upgrade" header field is intended to provide a simple mechanism
3052   for transitioning from HTTP/1.1 to some other protocol on the same
3053   connection.  A client &MAY; send a list of protocols in the Upgrade
3054   header field of a request to invite the server to switch to one or
3055   more of those protocols before sending the final response.
3056   A server &MUST; send an Upgrade header field in <x:ref>101 (Switching
3057   Protocols)</x:ref> responses to indicate which protocol(s) are being
3058   switched to, and &MUST; send it in <x:ref>426 (Upgrade Required)</x:ref>
3059   responses to indicate acceptable protocols.
3060   A server &MAY; send an Upgrade header field in any other response to
3061   indicate that they might be willing to upgrade to one of the
3062   specified protocols for a future request.
3063</t>
3064<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3065  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3066
3067  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3068  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3069  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3070</artwork></figure>
3071<t>
3072   For example,
3073</t>
3074<figure><artwork type="example">
3075  Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3076</artwork></figure>
3077<t>
3078   Upgrade eases the difficult transition between incompatible protocols by
3079   allowing the client to initiate a request in the more commonly
3080   supported protocol while indicating to the server that it would like
3081   to use a "better" protocol if available (where "better" is determined
3082   by the server, possibly according to the nature of the request method
3083   or target resource).
3084</t>
3085<t>
3086   Upgrade cannot be used to insist on a protocol change; its acceptance and
3087   use by the server is optional. The capabilities and nature of the
3088   application-level communication after the protocol change is entirely
3089   dependent upon the new protocol chosen, although the first action
3090   after changing the protocol &MUST; be a response to the initial HTTP
3091   request that contained the Upgrade header field.
3092</t>
3093<t>
3094   For example, if the Upgrade header field is received in a GET request
3095   and the server decides to switch protocols, then it &MUST; first respond
3096   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3097   then immediately follow that with the new protocol's equivalent of a
3098   response to a GET on the target resource.  This allows a connection to be
3099   upgraded to protocols with the same semantics as HTTP without the
3100   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3101   protocols unless the received message semantics can be honored by the new
3102   protocol; an OPTIONS request can be honored by any protocol.
3103</t>
3104<t>
3105   When Upgrade is sent, a sender &MUST; also send a
3106   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3107   that contains the "upgrade" connection option, in order to prevent Upgrade
3108   from being accidentally forwarded by intermediaries that might not implement
3109   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3110   is received in an HTTP/1.0 request.
3111</t>
3112<t>
3113   The Upgrade header field only applies to switching application-level
3114   protocols on the existing connection; it cannot be used
3115   to switch to a protocol on a different connection. For that purpose, it is
3116   more appropriate to use a <x:ref>3xx (Redirection)</x:ref> response
3117   (&status-3xx;).
3118</t>
3119<t>
3120   This specification only defines the protocol name "HTTP" for use by
3121   the family of Hypertext Transfer Protocols, as defined by the HTTP
3122   version rules of <xref target="http.version"/> and future updates to this
3123   specification. Additional tokens can be registered with IANA using the
3124   registration procedure defined in <xref target="upgrade.token.registry"/>.
3125</t>
3126</section>
3127</section>
3128
3129<section title="IANA Considerations" anchor="IANA.considerations">
3130
3131<section title="Header Field Registration" anchor="header.field.registration">
3132<t>
3133   HTTP header fields are registered within the Message Header Field Registry
3134   <xref target="RFC3864"/> maintained by IANA at
3135   <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
3136</t>
3137<t>
3138   This document defines the following HTTP header fields, so their
3139   associated registry entries shall be updated according to the permanent
3140   registrations below:
3141</t>
3142<?BEGININC p1-messaging.iana-headers ?>
3143<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3144<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3145   <ttcol>Header Field Name</ttcol>
3146   <ttcol>Protocol</ttcol>
3147   <ttcol>Status</ttcol>
3148   <ttcol>Reference</ttcol>
3149
3150   <c>Connection</c>
3151   <c>http</c>
3152   <c>standard</c>
3153   <c>
3154      <xref target="header.connection"/>
3155   </c>
3156   <c>Content-Length</c>
3157   <c>http</c>
3158   <c>standard</c>
3159   <c>
3160      <xref target="header.content-length"/>
3161   </c>
3162   <c>Host</c>
3163   <c>http</c>
3164   <c>standard</c>
3165   <c>
3166      <xref target="header.host"/>
3167   </c>
3168   <c>TE</c>
3169   <c>http</c>
3170   <c>standard</c>
3171   <c>
3172      <xref target="header.te"/>
3173   </c>
3174   <c>Trailer</c>
3175   <c>http</c>
3176   <c>standard</c>
3177   <c>
3178      <xref target="header.trailer"/>
3179   </c>
3180   <c>Transfer-Encoding</c>
3181   <c>http</c>
3182   <c>standard</c>
3183   <c>
3184      <xref target="header.transfer-encoding"/>
3185   </c>
3186   <c>Upgrade</c>
3187   <c>http</c>
3188   <c>standard</c>
3189   <c>
3190      <xref target="header.upgrade"/>
3191   </c>
3192   <c>Via</c>
3193   <c>http</c>
3194   <c>standard</c>
3195   <c>
3196      <xref target="header.via"/>
3197   </c>
3198</texttable>
3199<!--(END)-->
3200<?ENDINC p1-messaging.iana-headers ?>
3201<t>
3202   Furthermore, the header field-name "Close" shall be registered as
3203   "reserved", since using that name as an HTTP header field might
3204   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3205   header field (<xref target="header.connection"/>).
3206</t>
3207<texttable align="left" suppress-title="true">
3208   <ttcol>Header Field Name</ttcol>
3209   <ttcol>Protocol</ttcol>
3210   <ttcol>Status</ttcol>
3211   <ttcol>Reference</ttcol>
3212
3213   <c>Close</c>
3214   <c>http</c>
3215   <c>reserved</c>
3216   <c>
3217      <xref target="header.field.registration"/>
3218   </c>
3219</texttable>
3220<t>
3221   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3222</t>
3223</section>
3224
3225<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3226<t>
3227   IANA maintains the registry of URI Schemes <xref target="RFC4395"/> at
3228   <eref target="http://www.iana.org/assignments/uri-schemes.html"/>.
3229</t>
3230<t>
3231   This document defines the following URI schemes, so their
3232   associated registry entries shall be updated according to the permanent
3233   registrations below:
3234</t>
3235<texttable align="left" suppress-title="true">
3236   <ttcol>URI Scheme</ttcol>
3237   <ttcol>Description</ttcol>
3238   <ttcol>Reference</ttcol>
3239
3240   <c>http</c>
3241   <c>Hypertext Transfer Protocol</c>
3242   <c><xref target="http.uri"/></c>
3243
3244   <c>https</c>
3245   <c>Hypertext Transfer Protocol Secure</c>
3246   <c><xref target="https.uri"/></c>
3247</texttable>
3248</section>
3249
3250<section title="Internet Media Type Registrations" anchor="internet.media.type.http">
3251<t>
3252   This document serves as the specification for the Internet media types
3253   "message/http" and "application/http". The following is to be registered with
3254   IANA (see <xref target="RFC4288"/>).
3255</t>
3256<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
3257<iref item="Media Type" subitem="message/http" primary="true"/>
3258<iref item="message/http Media Type" primary="true"/>
3259<t>
3260   The message/http type can be used to enclose a single HTTP request or
3261   response message, provided that it obeys the MIME restrictions for all
3262   "message" types regarding line length and encodings.
3263</t>
3264<t>
3265  <list style="hanging" x:indent="12em">
3266    <t hangText="Type name:">
3267      message
3268    </t>
3269    <t hangText="Subtype name:">
3270      http
3271    </t>
3272    <t hangText="Required parameters:">
3273      none
3274    </t>
3275    <t hangText="Optional parameters:">
3276      version, msgtype
3277      <list style="hanging">
3278        <t hangText="version:">
3279          The HTTP-version number of the enclosed message
3280          (e.g., "1.1"). If not present, the version can be
3281          determined from the first line of the body.
3282        </t>
3283        <t hangText="msgtype:">
3284          The message type &mdash; "request" or "response". If not
3285          present, the type can be determined from the first
3286          line of the body.
3287        </t>
3288      </list>
3289    </t>
3290    <t hangText="Encoding considerations:">
3291      only "7bit", "8bit", or "binary" are permitted
3292    </t>
3293    <t hangText="Security considerations:">
3294      none
3295    </t>
3296    <t hangText="Interoperability considerations:">
3297      none
3298    </t>
3299    <t hangText="Published specification:">
3300      This specification (see <xref target="internet.media.type.message.http"/>).
3301    </t>
3302    <t hangText="Applications that use this media type:">
3303    </t>
3304    <t hangText="Additional information:">
3305      <list style="hanging">
3306        <t hangText="Magic number(s):">none</t>
3307        <t hangText="File extension(s):">none</t>
3308        <t hangText="Macintosh file type code(s):">none</t>
3309      </list>
3310    </t>
3311    <t hangText="Person and email address to contact for further information:">
3312      See Authors Section.
3313    </t>
3314    <t hangText="Intended usage:">
3315      COMMON
3316    </t>
3317    <t hangText="Restrictions on usage:">
3318      none
3319    </t>
3320    <t hangText="Author/Change controller:">
3321      IESG
3322    </t>
3323  </list>
3324</t>
3325</section>
3326<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
3327<iref item="Media Type" subitem="application/http" primary="true"/>
3328<iref item="application/http Media Type" primary="true"/>
3329<t>
3330   The application/http type can be used to enclose a pipeline of one or more
3331   HTTP request or response messages (not intermixed).
3332</t>
3333<t>
3334  <list style="hanging" x:indent="12em">
3335    <t hangText="Type name:">
3336      application
3337    </t>
3338    <t hangText="Subtype name:">
3339      http
3340    </t>
3341    <t hangText="Required parameters:">
3342      none
3343    </t>
3344    <t hangText="Optional parameters:">
3345      version, msgtype
3346      <list style="hanging">
3347        <t hangText="version:">
3348          The HTTP-version number of the enclosed messages
3349          (e.g., "1.1"). If not present, the version can be
3350          determined from the first line of the body.
3351        </t>
3352        <t hangText="msgtype:">
3353          The message type &mdash; "request" or "response". If not
3354          present, the type can be determined from the first
3355          line of the body.
3356        </t>
3357      </list>
3358    </t>
3359    <t hangText="Encoding considerations:">
3360      HTTP messages enclosed by this type
3361      are in "binary" format; use of an appropriate
3362      Content-Transfer-Encoding is required when
3363      transmitted via E-mail.
3364    </t>
3365    <t hangText="Security considerations:">
3366      none
3367    </t>
3368    <t hangText="Interoperability considerations:">
3369      none
3370    </t>
3371    <t hangText="Published specification:">
3372      This specification (see <xref target="internet.media.type.application.http"/>).
3373    </t>
3374    <t hangText="Applications that use this media type:">
3375    </t>
3376    <t hangText="Additional information:">
3377      <list style="hanging">
3378        <t hangText="Magic number(s):">none</t>
3379        <t hangText="File extension(s):">none</t>
3380        <t hangText="Macintosh file type code(s):">none</t>
3381      </list>
3382    </t>
3383    <t hangText="Person and email address to contact for further information:">
3384      See Authors Section.
3385    </t>
3386    <t hangText="Intended usage:">
3387      COMMON
3388    </t>
3389    <t hangText="Restrictions on usage:">
3390      none
3391    </t>
3392    <t hangText="Author/Change controller:">
3393      IESG
3394    </t>
3395  </list>
3396</t>
3397</section>
3398</section>
3399
3400<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3401<t>
3402   The HTTP Transfer Coding Registry defines the name space for transfer
3403   coding names.
3404</t>
3405<t>
3406   Registrations &MUST; include the following fields:
3407   <list style="symbols">
3408     <t>Name</t>
3409     <t>Description</t>
3410     <t>Pointer to specification text</t>
3411   </list>
3412</t>
3413<t>
3414   Names of transfer codings &MUST-NOT; overlap with names of content codings
3415   (&content-codings;) unless the encoding transformation is identical, as
3416   is the case for the compression codings defined in
3417   <xref target="compression.codings"/>.
3418</t>
3419<t>
3420   Values to be added to this name space require IETF Review (see
3421   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3422   conform to the purpose of transfer coding defined in this section.
3423   Use of program names for the identification of encoding formats
3424   is not desirable and is discouraged for future encodings.
3425</t>
3426<t>
3427   The registry itself is maintained at
3428   <eref target="http://www.iana.org/assignments/http-parameters"/>.
3429</t>
3430</section>
3431
3432<section title="Transfer Coding Registrations" anchor="transfer.coding.registration">
3433<t>
3434   The HTTP Transfer Coding Registry shall be updated with the registrations
3435   below:
3436</t>
3437<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3438   <ttcol>Name</ttcol>
3439   <ttcol>Description</ttcol>
3440   <ttcol>Reference</ttcol>
3441   <c>chunked</c>
3442   <c>Transfer in a series of chunks</c>
3443   <c>
3444      <xref target="chunked.encoding"/>
3445   </c>
3446   <c>compress</c>
3447   <c>UNIX "compress" program method</c>
3448   <c>
3449      <xref target="compress.coding"/>
3450   </c>
3451   <c>deflate</c>
3452   <c>"deflate" compression mechanism (<xref target="RFC1951"/>) used inside
3453   the "zlib" data format (<xref target="RFC1950"/>)
3454   </c>
3455   <c>
3456      <xref target="deflate.coding"/>
3457   </c>
3458   <c>gzip</c>
3459   <c>Same as GNU zip <xref target="RFC1952"/></c>
3460   <c>
3461      <xref target="gzip.coding"/>
3462   </c>
3463</texttable>
3464</section>
3465
3466<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3467<t>
3468   The HTTP Upgrade Token Registry defines the name space for protocol-name
3469   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3470   field. Each registered protocol name is associated with contact information
3471   and an optional set of specifications that details how the connection
3472   will be processed after it has been upgraded.
3473</t>
3474<t>
3475   Registrations happen on a "First Come First Served" basis (see
3476   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3477   following rules:
3478  <list style="numbers">
3479    <t>A protocol-name token, once registered, stays registered forever.</t>
3480    <t>The registration &MUST; name a responsible party for the
3481       registration.</t>
3482    <t>The registration &MUST; name a point of contact.</t>
3483    <t>The registration &MAY; name a set of specifications associated with
3484       that token. Such specifications need not be publicly available.</t>
3485    <t>The registration &SHOULD; name a set of expected "protocol-version"
3486       tokens associated with that token at the time of registration.</t>
3487    <t>The responsible party &MAY; change the registration at any time.
3488       The IANA will keep a record of all such changes, and make them
3489       available upon request.</t>
3490    <t>The IESG &MAY; reassign responsibility for a protocol token.
3491       This will normally only be used in the case when a
3492       responsible party cannot be contacted.</t>
3493  </list>
3494</t>
3495<t>
3496   This registration procedure for HTTP Upgrade Tokens replaces that
3497   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3498</t>
3499</section>
3500
3501<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3502<t>
3503   The HTTP Upgrade Token Registry shall be updated with the registration
3504   below:
3505</t>
3506<texttable align="left" suppress-title="true">
3507   <ttcol>Value</ttcol>
3508   <ttcol>Description</ttcol>
3509   <ttcol>Expected Version Tokens</ttcol>
3510   <ttcol>Reference</ttcol>
3511
3512   <c>HTTP</c>
3513   <c>Hypertext Transfer Protocol</c>
3514   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3515   <c><xref target="http.version"/></c>
3516</texttable>
3517<t>
3518   The responsible party is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3519</t>
3520</section>
3521
3522</section>
3523
3524<section title="Security Considerations" anchor="security.considerations">
3525<t>
3526   This section is meant to inform application developers, information
3527   providers, and users of the security limitations in HTTP/1.1 as
3528   described by this document. The discussion does not include
3529   definitive solutions to the problems revealed, though it does make
3530   some suggestions for reducing security risks.
3531</t>
3532
3533<section title="Personal Information" anchor="personal.information">
3534<t>
3535   HTTP clients are often privy to large amounts of personal information,
3536   including both information provided by the user to interact with resources
3537   (e.g., the user's name, location, mail address, passwords, encryption
3538   keys, etc.) and information about the user's browsing activity over
3539   time (e.g., history, bookmarks, etc.). HTTP implementations need to
3540   prevent unintentional leakage of this information.
3541</t>
3542</section>
3543
3544<section title="Abuse of Server Log Information" anchor="abuse.of.server.log.information">
3545<t>
3546   A server is in the position to save personal data about a user's
3547   requests which might identify their reading patterns or subjects of
3548   interest.  In particular, log information gathered at an intermediary
3549   often contains a history of user agent interaction, across a multitude
3550   of sites, that can be traced to individual users.
3551</t>
3552<t>
3553   HTTP log information is confidential in nature; its handling is often
3554   constrained by laws and regulations.  Log information needs to be securely
3555   stored and appropriate guidelines followed for its analysis.
3556   Anonymization of personal information within individual entries helps,
3557   but is generally not sufficient to prevent real log traces from being
3558   re-identified based on correlation with other access characteristics.
3559   As such, access traces that are keyed to a specific client should not
3560   be published even if the key is pseudonymous.
3561</t>
3562<t>
3563   To minimize the risk of theft or accidental publication, log information
3564   should be purged of personally identifiable information, including
3565   user identifiers, IP addresses, and user-provided query parameters,
3566   as soon as that information is no longer necessary to support operational
3567   needs for security, auditing, or fraud control.
3568</t>
3569</section>
3570
3571<section title="Attacks Based On File and Path Names" anchor="attack.pathname">
3572<t>
3573   Origin servers &SHOULD; be careful to restrict
3574   the documents returned by HTTP requests to be only those that were
3575   intended by the server administrators. If an HTTP server translates
3576   HTTP URIs directly into file system calls, the server &MUST; take
3577   special care not to serve files that were not intended to be
3578   delivered to HTTP clients. For example, UNIX, Microsoft Windows, and
3579   other operating systems use ".." as a path component to indicate a
3580   directory level above the current one. On such a system, an HTTP
3581   server &MUST; disallow any such construct in the request-target if it
3582   would otherwise allow access to a resource outside those intended to
3583   be accessible via the HTTP server. Similarly, files intended for
3584   reference only internally to the server (such as access control
3585   files, configuration files, and script code) &MUST; be protected from
3586   inappropriate retrieval, since they might contain sensitive
3587   information.
3588</t>
3589</section>
3590
3591<section title="DNS-related Attacks" anchor="dns.related.attacks">
3592<t>
3593   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3594   generally prone to security attacks based on the deliberate misassociation
3595   of IP addresses and DNS names not protected by DNSSec. Clients need to be
3596   cautious in assuming the validity of an IP number/DNS name association unless
3597   the response is protected by DNSSec (<xref target="RFC4033"/>).
3598</t>
3599</section>
3600
3601<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3602<t>
3603   By their very nature, HTTP intermediaries are men-in-the-middle, and
3604   represent an opportunity for man-in-the-middle attacks. Compromise of
3605   the systems on which the intermediaries run can result in serious security
3606   and privacy problems. Intermediaries have access to security-related
3607   information, personal information about individual users and
3608   organizations, and proprietary information belonging to users and
3609   content providers. A compromised intermediary, or an intermediary
3610   implemented or configured without regard to security and privacy
3611   considerations, might be used in the commission of a wide range of
3612   potential attacks.
3613</t>
3614<t>
3615   Intermediaries that contain a shared cache are especially vulnerable
3616   to cache poisoning attacks.
3617</t>
3618<t>
3619   Implementers need to consider the privacy and security
3620   implications of their design and coding decisions, and of the
3621   configuration options they provide to operators (especially the
3622   default configuration).
3623</t>
3624<t>
3625   Users need to be aware that intermediaries are no more trustworthy than
3626   the people who run them; HTTP itself cannot solve this problem.
3627</t>
3628</section>
3629
3630<section title="Protocol Element Size Overflows" anchor="attack.protocol.element.size.overflows">
3631<t>
3632   Because HTTP uses mostly textual, character-delimited fields, attackers can
3633   overflow buffers in implementations, and/or perform a Denial of Service
3634   against implementations that accept fields with unlimited lengths.
3635</t>
3636<t>
3637   To promote interoperability, this specification makes specific
3638   recommendations for minimum size limits on request-line
3639   (<xref target="request.line"/>)
3640   and blocks of header fields (<xref target="header.fields"/>). These are
3641   minimum recommendations, chosen to be supportable even by implementations
3642   with limited resources; it is expected that most implementations will
3643   choose substantially higher limits.
3644</t>
3645<t>
3646   This specification also provides a way for servers to reject messages that
3647   have request-targets that are too long (&status-414;) or request entities
3648   that are too large (&status-4xx;).
3649</t>
3650<t>
3651   Recipients &SHOULD; carefully limit the extent to which they read other
3652   fields, including (but not limited to) request methods, response status
3653   phrases, header field-names, and body chunks, so as to avoid denial of
3654   service attacks without impeding interoperability.
3655</t>
3656</section>
3657</section>
3658
3659<section title="Acknowledgments" anchor="acks">
3660<t>
3661   This edition of HTTP builds on the many contributions that went into
3662   <xref target="RFC1945" format="none">RFC 1945</xref>,
3663   <xref target="RFC2068" format="none">RFC 2068</xref>,
3664   <xref target="RFC2145" format="none">RFC 2145</xref>, and
3665   <xref target="RFC2616" format="none">RFC 2616</xref>, including
3666   substantial contributions made by the previous authors, editors, and
3667   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
3668   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
3669   Paul J. Leach, and Mark Nottingham.
3670   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
3671   acknowledgements from prior revisions.
3672</t>
3673<t>
3674   Since 1999, the following contributors have helped improve the HTTP
3675   specification by reporting bugs, asking smart questions, drafting or
3676   reviewing text, and evaluating open issues:
3677</t>
3678<?BEGININC acks ?>
3679<t>Adam Barth,
3680Adam Roach,
3681Addison Phillips,
3682Adrian Chadd,
3683Adrien W. de Croy,
3684Alan Ford,
3685Alan Ruttenberg,
3686Albert Lunde,
3687Alek Storm,
3688Alex Rousskov,
3689Alexandre Morgaut,
3690Alexey Melnikov,
3691Alisha Smith,
3692Amichai Rothman,
3693Amit Klein,
3694Amos Jeffries,
3695Andreas Maier,
3696Andreas Petersson,
3697Anil Sharma,
3698Anne van Kesteren,
3699Anthony Bryan,
3700Asbjorn Ulsberg,
3701Balachander Krishnamurthy,
3702Barry Leiba,
3703Ben Laurie,
3704Benjamin Niven-Jenkins,
3705Bil Corry,
3706Bill Burke,
3707Bjoern Hoehrmann,
3708Bob Scheifler,
3709Boris Zbarsky,
3710Brett Slatkin,
3711Brian Kell,
3712Brian McBarron,
3713Brian Pane,
3714Brian Smith,
3715Bryce Nesbitt,
3716Cameron Heavon-Jones,
3717Carl Kugler,
3718Carsten Bormann,
3719Charles Fry,
3720Chris Newman,
3721Cyrus Daboo,
3722Dale Robert Anderson,
3723Dan Wing,
3724Dan Winship,
3725Daniel Stenberg,
3726Dave Cridland,
3727Dave Crocker,
3728Dave Kristol,
3729David Booth,
3730David Singer,
3731David W. Morris,
3732Diwakar Shetty,
3733Dmitry Kurochkin,
3734Drummond Reed,
3735Duane Wessels,
3736Edward Lee,
3737Eliot Lear,
3738Eran Hammer-Lahav,
3739Eric D. Williams,
3740Eric J. Bowman,
3741Eric Lawrence,
3742Eric Rescorla,
3743Erik Aronesty,
3744Evan Prodromou,
3745Florian Weimer,
3746Frank Ellermann,
3747Fred Bohle,
3748Gabriel Montenegro,
3749Geoffrey Sneddon,
3750Gervase Markham,
3751Grahame Grieve,
3752Greg Wilkins,
3753Harald Tveit Alvestrand,
3754Harry Halpin,
3755Helge Hess,
3756Henrik Nordstrom,
3757Henry S. Thompson,
3758Henry Story,
3759Herbert van de Sompel,
3760Howard Melman,
3761Hugo Haas,
3762Ian Fette,
3763Ian Hickson,
3764Ido Safruti,
3765Ingo Struck,
3766J. Ross Nicoll,
3767James H. Manger,
3768James Lacey,
3769James M. Snell,
3770Jamie Lokier,
3771Jan Algermissen,
3772Jeff Hodges (who came up with the term 'effective Request-URI'),
3773Jeff Walden,
3774Jim Luther,
3775Joe D. Williams,
3776Joe Gregorio,
3777Joe Orton,
3778John C. Klensin,
3779John C. Mallery,
3780John Cowan,
3781John Kemp,
3782John Panzer,
3783John Schneider,
3784John Stracke,
3785John Sullivan,
3786Jonas Sicking,
3787Jonathan Billington,
3788Jonathan Moore,
3789Jonathan Rees,
3790Jonathan Silvera,
3791Jordi Ros,
3792Joris Dobbelsteen,
3793Josh Cohen,
3794Julien Pierre,
3795Jungshik Shin,
3796Justin Chapweske,
3797Justin Erenkrantz,
3798Justin James,
3799Kalvinder Singh,
3800Karl Dubost,
3801Keith Hoffman,
3802Keith Moore,
3803Koen Holtman,
3804Konstantin Voronkov,
3805Kris Zyp,
3806Lisa Dusseault,
3807Maciej Stachowiak,
3808Marc Schneider,
3809Marc Slemko,
3810Mark Baker,
3811Mark Pauley,
3812Mark Watson,
3813Markus Isomaki,
3814Markus Lanthaler,
3815Martin J. Duerst,
3816Martin Musatov,
3817Martin Nilsson,
3818Martin Thomson,
3819Matt Lynch,
3820Matthew Cox,
3821Max Clark,
3822Michael Burrows,
3823Michael Hausenblas,
3824Mike Amundsen,
3825Mike Belshe,
3826Mike Kelly,
3827Mike Schinkel,
3828Miles Sabin,
3829Murray S. Kucherawy,
3830Mykyta Yevstifeyev,
3831Nathan Rixham,
3832Nicholas Shanks,
3833Nico Williams,
3834Nicolas Alvarez,
3835Nicolas Mailhot,
3836Noah Slater,
3837Pablo Castro,
3838Pat Hayes,
3839Patrick R. McManus,
3840Paul E. Jones,
3841Paul Hoffman,
3842Paul Marquess,
3843Peter Lepeska,
3844Peter Saint-Andre,
3845Peter Watkins,
3846Phil Archer,
3847Philippe Mougin,
3848Phillip Hallam-Baker,
3849Poul-Henning Kamp,
3850Preethi Natarajan,
3851Rajeev Bector,
3852Ray Polk,
3853Reto Bachmann-Gmuer,
3854Richard Cyganiak,
3855Robert Brewer,
3856Robert Collins,
3857Robert O'Callahan,
3858Robert Olofsson,
3859Robert Sayre,
3860Robert Siemer,
3861Robert de Wilde,
3862Roberto Javier Godoy,
3863Roberto Peon,
3864Ronny Widjaja,
3865S. Mike Dierken,
3866Salvatore Loreto,
3867Sam Johnston,
3868Sam Ruby,
3869Scott Lawrence (who maintained the original issues list),
3870Sean B. Palmer,
3871Shane McCarron,
3872Stefan Eissing,
3873Stefan Tilkov,
3874Stefanos Harhalakis,
3875Stephane Bortzmeyer,
3876Stephen Farrell,
3877Stephen Ludin,
3878Stuart Williams,
3879Subbu Allamaraju,
3880Sylvain Hellegouarch,
3881Tapan Divekar,
3882Tatsuya Hayashi,
3883Ted Hardie,
3884Thomas Broyer,
3885Thomas Nordin,
3886Thomas Roessler,
3887Tim Bray,
3888Tim Morgan,
3889Tim Olsen,
3890Tom Zhou,
3891Travis Snoozy,
3892Tyler Close,
3893Vincent Murphy,
3894Wenbo Zhu,
3895Werner Baumann,
3896Wilbur Streett,
3897Wilfredo Sanchez Vega,
3898William A. Rowe Jr.,
3899William Chan,
3900Willy Tarreau,
3901Xiaoshu Wang,
3902Yaron Goland,
3903Yngve Nysaeter Pettersen,
3904Yoav Nir,
3905Yogesh Bang,
3906Yutaka Oiwa,
3907Yves Lafon (long-time member of the editor team),
3908Zed A. Shaw, and
3909Zhong Yu.
3910</t>
3911<?ENDINC acks ?>
3912</section>
3913
3914</middle>
3915<back>
3916
3917<references title="Normative References">
3918
3919<reference anchor="Part2">
3920  <front>
3921    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
3922    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3923      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
3924      <address><email>fielding@gbiv.com</email></address>
3925    </author>
3926    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3927      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3928      <address><email>julian.reschke@greenbytes.de</email></address>
3929    </author>
3930    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3931  </front>
3932  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
3933  <x:source href="p2-semantics.xml" basename="p2-semantics">
3934    <x:defines>1xx (Informational)</x:defines>
3935    <x:defines>1xx</x:defines>
3936    <x:defines>100 (Continue)</x:defines>
3937    <x:defines>101 (Switching Protocols)</x:defines>
3938    <x:defines>2xx (Successful)</x:defines>
3939    <x:defines>2xx</x:defines>
3940    <x:defines>200 (OK)</x:defines>
3941    <x:defines>204 (No Content)</x:defines>
3942    <x:defines>3xx (Redirection)</x:defines>
3943    <x:defines>3xx</x:defines>
3944    <x:defines>301 (Moved Permanently)</x:defines>
3945    <x:defines>4xx (Client Error)</x:defines>
3946    <x:defines>4xx</x:defines>
3947    <x:defines>400 (Bad Request)</x:defines>
3948    <x:defines>405 (Method Not Allowed)</x:defines>
3949    <x:defines>411 (Length Required)</x:defines>
3950    <x:defines>414 (URI Too Long)</x:defines>
3951    <x:defines>417 (Expectation Failed)</x:defines>
3952    <x:defines>426 (Upgrade Required)</x:defines>
3953    <x:defines>501 (Not Implemented)</x:defines>
3954    <x:defines>502 (Bad Gateway)</x:defines>
3955    <x:defines>505 (HTTP Version Not Supported)</x:defines>
3956    <x:defines>Allow</x:defines>
3957    <x:defines>Content-Encoding</x:defines>
3958    <x:defines>Content-Location</x:defines>
3959    <x:defines>Content-Type</x:defines>
3960    <x:defines>Date</x:defines>
3961    <x:defines>Expect</x:defines>
3962    <x:defines>Location</x:defines>
3963    <x:defines>Server</x:defines>
3964    <x:defines>User-Agent</x:defines>
3965  </x:source>
3966</reference>
3967
3968<reference anchor="Part4">
3969  <front>
3970    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
3971    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
3972      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
3973      <address><email>fielding@gbiv.com</email></address>
3974    </author>
3975    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
3976      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3977      <address><email>julian.reschke@greenbytes.de</email></address>
3978    </author>
3979    <date month="&ID-MONTH;" year="&ID-YEAR;" />
3980  </front>
3981  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
3982  <x:source basename="p4-conditional" href="p4-conditional.xml">
3983    <x:defines>304 (Not Modified)</x:defines>
3984    <x:defines>ETag</x:defines>
3985    <x:defines>Last-Modified</x:defines>
3986  </x:source>
3987</reference>
3988
3989<reference anchor="Part5">
3990  <front>
3991    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
3992    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3993      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
3994      <address><email>fielding@gbiv.com</email></address>
3995    </author>
3996    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3997      <organization abbrev="W3C">World Wide Web Consortium</organization>
3998      <address><email>ylafon@w3.org</email></address>
3999    </author>
4000    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4001      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4002      <address><email>julian.reschke@greenbytes.de</email></address>
4003    </author>
4004    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4005  </front>
4006  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4007  <x:source href="p5-range.xml" basename="p5-range">
4008    <x:defines>Content-Range</x:defines>
4009  </x:source>
4010</reference>
4011
4012<reference anchor="Part6">
4013  <front>
4014    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4015    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4016      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4017      <address><email>fielding@gbiv.com</email></address>
4018    </author>
4019    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4020      <organization>Akamai</organization>
4021      <address><email>mnot@mnot.net</email></address>
4022    </author>
4023    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4024      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4025      <address><email>julian.reschke@greenbytes.de</email></address>
4026    </author>
4027    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4028  </front>
4029  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4030  <x:source href="p6-cache.xml" basename="p6-cache">
4031    <x:defines>Expires</x:defines>
4032  </x:source>
4033</reference>
4034
4035<reference anchor="Part7">
4036  <front>
4037    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4038    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4039      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4040      <address><email>fielding@gbiv.com</email></address>
4041    </author>
4042    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4043      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4044      <address><email>julian.reschke@greenbytes.de</email></address>
4045    </author>
4046    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4047  </front>
4048  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4049  <x:source href="p7-auth.xml" basename="p7-auth">
4050    <x:defines>Proxy-Authenticate</x:defines>
4051    <x:defines>Proxy-Authorization</x:defines>
4052  </x:source>
4053</reference>
4054
4055<reference anchor="RFC5234">
4056  <front>
4057    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4058    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4059      <organization>Brandenburg InternetWorking</organization>
4060      <address>
4061        <email>dcrocker@bbiw.net</email>
4062      </address> 
4063    </author>
4064    <author initials="P." surname="Overell" fullname="Paul Overell">
4065      <organization>THUS plc.</organization>
4066      <address>
4067        <email>paul.overell@thus.net</email>
4068      </address>
4069    </author>
4070    <date month="January" year="2008"/>
4071  </front>
4072  <seriesInfo name="STD" value="68"/>
4073  <seriesInfo name="RFC" value="5234"/>
4074</reference>
4075
4076<reference anchor="RFC2119">
4077  <front>
4078    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4079    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4080      <organization>Harvard University</organization>
4081      <address><email>sob@harvard.edu</email></address>
4082    </author>
4083    <date month="March" year="1997"/>
4084  </front>
4085  <seriesInfo name="BCP" value="14"/>
4086  <seriesInfo name="RFC" value="2119"/>
4087</reference>
4088
4089<reference anchor="RFC3986">
4090 <front>
4091  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4092  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4093    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4094    <address>
4095       <email>timbl@w3.org</email>
4096       <uri>http://www.w3.org/People/Berners-Lee/</uri>
4097    </address>
4098  </author>
4099  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4100    <organization abbrev="Day Software">Day Software</organization>
4101    <address>
4102      <email>fielding@gbiv.com</email>
4103      <uri>http://roy.gbiv.com/</uri>
4104    </address>
4105  </author>
4106  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4107    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4108    <address>
4109      <email>LMM@acm.org</email>
4110      <uri>http://larry.masinter.net/</uri>
4111    </address>
4112  </author>
4113  <date month='January' year='2005'></date>
4114 </front>
4115 <seriesInfo name="STD" value="66"/>
4116 <seriesInfo name="RFC" value="3986"/>
4117</reference>
4118
4119<reference anchor="USASCII">
4120  <front>
4121    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4122    <author>
4123      <organization>American National Standards Institute</organization>
4124    </author>
4125    <date year="1986"/>
4126  </front>
4127  <seriesInfo name="ANSI" value="X3.4"/>
4128</reference>
4129
4130<reference anchor="RFC1950">
4131  <front>
4132    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4133    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4134      <organization>Aladdin Enterprises</organization>
4135      <address><email>ghost@aladdin.com</email></address>
4136    </author>
4137    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4138    <date month="May" year="1996"/>
4139  </front>
4140  <seriesInfo name="RFC" value="1950"/>
4141  <!--<annotation>
4142    RFC 1950 is an Informational RFC, thus it might be less stable than
4143    this specification. On the other hand, this downward reference was
4144    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4145    therefore it is unlikely to cause problems in practice. See also
4146    <xref target="BCP97"/>.
4147  </annotation>-->
4148</reference>
4149
4150<reference anchor="RFC1951">
4151  <front>
4152    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4153    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4154      <organization>Aladdin Enterprises</organization>
4155      <address><email>ghost@aladdin.com</email></address>
4156    </author>
4157    <date month="May" year="1996"/>
4158  </front>
4159  <seriesInfo name="RFC" value="1951"/>
4160  <!--<annotation>
4161    RFC 1951 is an Informational RFC, thus it might be less stable than
4162    this specification. On the other hand, this downward reference was
4163    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4164    therefore it is unlikely to cause problems in practice. See also
4165    <xref target="BCP97"/>.
4166  </annotation>-->
4167</reference>
4168
4169<reference anchor="RFC1952">
4170  <front>
4171    <title>GZIP file format specification version 4.3</title>
4172    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4173      <organization>Aladdin Enterprises</organization>
4174      <address><email>ghost@aladdin.com</email></address>
4175    </author>
4176    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4177      <address><email>gzip@prep.ai.mit.edu</email></address>
4178    </author>
4179    <author initials="M." surname="Adler" fullname="Mark Adler">
4180      <address><email>madler@alumni.caltech.edu</email></address>
4181    </author>
4182    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4183      <address><email>ghost@aladdin.com</email></address>
4184    </author>
4185    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4186      <address><email>randeg@alumni.rpi.edu</email></address>
4187    </author>
4188    <date month="May" year="1996"/>
4189  </front>
4190  <seriesInfo name="RFC" value="1952"/>
4191  <!--<annotation>
4192    RFC 1952 is an Informational RFC, thus it might be less stable than
4193    this specification. On the other hand, this downward reference was
4194    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4195    therefore it is unlikely to cause problems in practice. See also
4196    <xref target="BCP97"/>.
4197  </annotation>-->
4198</reference>
4199
4200</references>
4201
4202<references title="Informative References">
4203
4204<reference anchor="ISO-8859-1">
4205  <front>
4206    <title>
4207     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4208    </title>
4209    <author>
4210      <organization>International Organization for Standardization</organization>
4211    </author>
4212    <date year="1998"/>
4213  </front>
4214  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4215</reference>
4216
4217<reference anchor='RFC1919'>
4218  <front>
4219    <title>Classical versus Transparent IP Proxies</title>
4220    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4221      <address><email>mchatel@pax.eunet.ch</email></address>
4222    </author>
4223    <date year='1996' month='March' />
4224  </front>
4225  <seriesInfo name='RFC' value='1919' />
4226</reference>
4227
4228<reference anchor="RFC1945">
4229  <front>
4230    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4231    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4232      <organization>MIT, Laboratory for Computer Science</organization>
4233      <address><email>timbl@w3.org</email></address>
4234    </author>
4235    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4236      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4237      <address><email>fielding@ics.uci.edu</email></address>
4238    </author>
4239    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4240      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4241      <address><email>frystyk@w3.org</email></address>
4242    </author>
4243    <date month="May" year="1996"/>
4244  </front>
4245  <seriesInfo name="RFC" value="1945"/>
4246</reference>
4247
4248<reference anchor="RFC2045">
4249  <front>
4250    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4251    <author initials="N." surname="Freed" fullname="Ned Freed">
4252      <organization>Innosoft International, Inc.</organization>
4253      <address><email>ned@innosoft.com</email></address>
4254    </author>
4255    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4256      <organization>First Virtual Holdings</organization>
4257      <address><email>nsb@nsb.fv.com</email></address>
4258    </author>
4259    <date month="November" year="1996"/>
4260  </front>
4261  <seriesInfo name="RFC" value="2045"/>
4262</reference>
4263
4264<reference anchor="RFC2047">
4265  <front>
4266    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4267    <author initials="K." surname="Moore" fullname="Keith Moore">
4268      <organization>University of Tennessee</organization>
4269      <address><email>moore@cs.utk.edu</email></address>
4270    </author>
4271    <date month="November" year="1996"/>
4272  </front>
4273  <seriesInfo name="RFC" value="2047"/>
4274</reference>
4275
4276<reference anchor="RFC2068">
4277  <front>
4278    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4279    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
4280      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4281      <address><email>fielding@ics.uci.edu</email></address>
4282    </author>
4283    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4284      <organization>MIT Laboratory for Computer Science</organization>
4285      <address><email>jg@w3.org</email></address>
4286    </author>
4287    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4288      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4289      <address><email>mogul@wrl.dec.com</email></address>
4290    </author>
4291    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4292      <organization>MIT Laboratory for Computer Science</organization>
4293      <address><email>frystyk@w3.org</email></address>
4294    </author>
4295    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4296      <organization>MIT Laboratory for Computer Science</organization>
4297      <address><email>timbl@w3.org</email></address>
4298    </author>
4299    <date month="January" year="1997"/>
4300  </front>
4301  <seriesInfo name="RFC" value="2068"/>
4302</reference>
4303
4304<reference anchor="RFC2145">
4305  <front>
4306    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4307    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4308      <organization>Western Research Laboratory</organization>
4309      <address><email>mogul@wrl.dec.com</email></address>
4310    </author>
4311    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4312      <organization>Department of Information and Computer Science</organization>
4313      <address><email>fielding@ics.uci.edu</email></address>
4314    </author>
4315    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4316      <organization>MIT Laboratory for Computer Science</organization>
4317      <address><email>jg@w3.org</email></address>
4318    </author>
4319    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4320      <organization>W3 Consortium</organization>
4321      <address><email>frystyk@w3.org</email></address>
4322    </author>
4323    <date month="May" year="1997"/>
4324  </front>
4325  <seriesInfo name="RFC" value="2145"/>
4326</reference>
4327
4328<reference anchor="RFC2616">
4329  <front>
4330    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4331    <author initials="R." surname="Fielding" fullname="R. Fielding">
4332      <organization>University of California, Irvine</organization>
4333      <address><email>fielding@ics.uci.edu</email></address>
4334    </author>
4335    <author initials="J." surname="Gettys" fullname="J. Gettys">
4336      <organization>W3C</organization>
4337      <address><email>jg@w3.org</email></address>
4338    </author>
4339    <author initials="J." surname="Mogul" fullname="J. Mogul">
4340      <organization>Compaq Computer Corporation</organization>
4341      <address><email>mogul@wrl.dec.com</email></address>
4342    </author>
4343    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4344      <organization>MIT Laboratory for Computer Science</organization>
4345      <address><email>frystyk@w3.org</email></address>
4346    </author>
4347    <author initials="L." surname="Masinter" fullname="L. Masinter">
4348      <organization>Xerox Corporation</organization>
4349      <address><email>masinter@parc.xerox.com</email></address>
4350    </author>
4351    <author initials="P." surname="Leach" fullname="P. Leach">
4352      <organization>Microsoft Corporation</organization>
4353      <address><email>paulle@microsoft.com</email></address>
4354    </author>
4355    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4356      <organization>W3C</organization>
4357      <address><email>timbl@w3.org</email></address>
4358    </author>
4359    <date month="June" year="1999"/>
4360  </front>
4361  <seriesInfo name="RFC" value="2616"/>
4362</reference>
4363
4364<reference anchor='RFC2817'>
4365  <front>
4366    <title>Upgrading to TLS Within HTTP/1.1</title>
4367    <author initials='R.' surname='Khare' fullname='R. Khare'>
4368      <organization>4K Associates / UC Irvine</organization>
4369      <address><email>rohit@4K-associates.com</email></address>
4370    </author>
4371    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4372      <organization>Agranat Systems, Inc.</organization>
4373      <address><email>lawrence@agranat.com</email></address>
4374    </author>
4375    <date year='2000' month='May' />
4376  </front>
4377  <seriesInfo name='RFC' value='2817' />
4378</reference>
4379
4380<reference anchor='RFC2818'>
4381  <front>
4382    <title>HTTP Over TLS</title>
4383    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4384      <organization>RTFM, Inc.</organization>
4385      <address><email>ekr@rtfm.com</email></address>
4386    </author>
4387    <date year='2000' month='May' />
4388  </front>
4389  <seriesInfo name='RFC' value='2818' />
4390</reference>
4391
4392<reference anchor='RFC2965'>
4393  <front>
4394    <title>HTTP State Management Mechanism</title>
4395    <author initials='D. M.' surname='Kristol' fullname='David M. Kristol'>
4396      <organization>Bell Laboratories, Lucent Technologies</organization>
4397      <address><email>dmk@bell-labs.com</email></address>
4398    </author>
4399    <author initials='L.' surname='Montulli' fullname='Lou Montulli'>
4400      <organization>Epinions.com, Inc.</organization>
4401      <address><email>lou@montulli.org</email></address>
4402    </author>
4403    <date year='2000' month='October' />
4404  </front>
4405  <seriesInfo name='RFC' value='2965' />
4406</reference>
4407
4408<reference anchor='RFC3040'>
4409  <front>
4410    <title>Internet Web Replication and Caching Taxonomy</title>
4411    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4412      <organization>Equinix, Inc.</organization>
4413    </author>
4414    <author initials='I.' surname='Melve' fullname='I. Melve'>
4415      <organization>UNINETT</organization>
4416    </author>
4417    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4418      <organization>CacheFlow Inc.</organization>
4419    </author>
4420    <date year='2001' month='January' />
4421  </front>
4422  <seriesInfo name='RFC' value='3040' />
4423</reference>
4424
4425<reference anchor='RFC3864'>
4426  <front>
4427    <title>Registration Procedures for Message Header Fields</title>
4428    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4429      <organization>Nine by Nine</organization>
4430      <address><email>GK-IETF@ninebynine.org</email></address>
4431    </author>
4432    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4433      <organization>BEA Systems</organization>
4434      <address><email>mnot@pobox.com</email></address>
4435    </author>
4436    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4437      <organization>HP Labs</organization>
4438      <address><email>JeffMogul@acm.org</email></address>
4439    </author>
4440    <date year='2004' month='September' />
4441  </front>
4442  <seriesInfo name='BCP' value='90' />
4443  <seriesInfo name='RFC' value='3864' />
4444</reference>
4445
4446<reference anchor='RFC4033'>
4447  <front>
4448    <title>DNS Security Introduction and Requirements</title>
4449    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4450    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4451    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4452    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4453    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4454    <date year='2005' month='March' />
4455  </front>
4456  <seriesInfo name='RFC' value='4033' />
4457</reference>
4458
4459<reference anchor="RFC4288">
4460  <front>
4461    <title>Media Type Specifications and Registration Procedures</title>
4462    <author initials="N." surname="Freed" fullname="N. Freed">
4463      <organization>Sun Microsystems</organization>
4464      <address>
4465        <email>ned.freed@mrochek.com</email>
4466      </address>
4467    </author>
4468    <author initials="J." surname="Klensin" fullname="J. Klensin">
4469      <address>
4470        <email>klensin+ietf@jck.com</email>
4471      </address>
4472    </author>
4473    <date year="2005" month="December"/>
4474  </front>
4475  <seriesInfo name="BCP" value="13"/>
4476  <seriesInfo name="RFC" value="4288"/>
4477</reference>
4478
4479<reference anchor='RFC4395'>
4480  <front>
4481    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4482    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4483      <organization>AT&amp;T Laboratories</organization>
4484      <address>
4485        <email>tony+urireg@maillennium.att.com</email>
4486      </address>
4487    </author>
4488    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4489      <organization>Qualcomm, Inc.</organization>
4490      <address>
4491        <email>hardie@qualcomm.com</email>
4492      </address>
4493    </author>
4494    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4495      <organization>Adobe Systems</organization>
4496      <address>
4497        <email>LMM@acm.org</email>
4498      </address>
4499    </author>
4500    <date year='2006' month='February' />
4501  </front>
4502  <seriesInfo name='BCP' value='115' />
4503  <seriesInfo name='RFC' value='4395' />
4504</reference>
4505
4506<reference anchor='RFC4559'>
4507  <front>
4508    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4509    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4510    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4511    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4512    <date year='2006' month='June' />
4513  </front>
4514  <seriesInfo name='RFC' value='4559' />
4515</reference>
4516
4517<reference anchor='RFC5226'>
4518  <front>
4519    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4520    <author initials='T.' surname='Narten' fullname='T. Narten'>
4521      <organization>IBM</organization>
4522      <address><email>narten@us.ibm.com</email></address>
4523    </author>
4524    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4525      <organization>Google</organization>
4526      <address><email>Harald@Alvestrand.no</email></address>
4527    </author>
4528    <date year='2008' month='May' />
4529  </front>
4530  <seriesInfo name='BCP' value='26' />
4531  <seriesInfo name='RFC' value='5226' />
4532</reference>
4533
4534<reference anchor='RFC5246'>
4535   <front>
4536      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4537      <author initials='T.' surname='Dierks' fullname='T. Dierks'>
4538         <organization />
4539      </author>
4540      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4541         <organization>RTFM, Inc.</organization>
4542      </author>
4543      <date year='2008' month='August' />
4544   </front>
4545   <seriesInfo name='RFC' value='5246' />
4546</reference>
4547
4548<reference anchor="RFC5322">
4549  <front>
4550    <title>Internet Message Format</title>
4551    <author initials="P." surname="Resnick" fullname="P. Resnick">
4552      <organization>Qualcomm Incorporated</organization>
4553    </author>
4554    <date year="2008" month="October"/>
4555  </front> 
4556  <seriesInfo name="RFC" value="5322"/>
4557</reference>
4558
4559<reference anchor="RFC6265">
4560  <front>
4561    <title>HTTP State Management Mechanism</title>
4562    <author initials="A." surname="Barth" fullname="Adam Barth">
4563      <organization abbrev="U.C. Berkeley">
4564        University of California, Berkeley
4565      </organization>
4566      <address><email>abarth@eecs.berkeley.edu</email></address>
4567    </author>
4568    <date year="2011" month="April" />
4569  </front>
4570  <seriesInfo name="RFC" value="6265"/>
4571</reference>
4572
4573<!--<reference anchor='BCP97'>
4574  <front>
4575    <title>Handling Normative References to Standards-Track Documents</title>
4576    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
4577      <address>
4578        <email>klensin+ietf@jck.com</email>
4579      </address>
4580    </author>
4581    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
4582      <organization>MIT</organization>
4583      <address>
4584        <email>hartmans-ietf@mit.edu</email>
4585      </address>
4586    </author>
4587    <date year='2007' month='June' />
4588  </front>
4589  <seriesInfo name='BCP' value='97' />
4590  <seriesInfo name='RFC' value='4897' />
4591</reference>-->
4592
4593<reference anchor="Kri2001" target="http://arxiv.org/abs/cs.SE/0105018">
4594  <front>
4595    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
4596    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
4597    <date year="2001" month="November"/>
4598  </front>
4599  <seriesInfo name="ACM Transactions on Internet Technology" value="Vol. 1, #2"/>
4600</reference>
4601
4602</references>
4603
4604
4605<section title="HTTP Version History" anchor="compatibility">
4606<t>
4607   HTTP has been in use by the World-Wide Web global information initiative
4608   since 1990. The first version of HTTP, later referred to as HTTP/0.9,
4609   was a simple protocol for hypertext data transfer across the Internet
4610   with only a single request method (GET) and no metadata.
4611   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
4612   methods and MIME-like messaging that could include metadata about the data
4613   transferred and modifiers on the request/response semantics. However,
4614   HTTP/1.0 did not sufficiently take into consideration the effects of
4615   hierarchical proxies, caching, the need for persistent connections, or
4616   name-based virtual hosts. The proliferation of incompletely-implemented
4617   applications calling themselves "HTTP/1.0" further necessitated a
4618   protocol version change in order for two communicating applications
4619   to determine each other's true capabilities.
4620</t>
4621<t>
4622   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
4623   requirements that enable reliable implementations, adding only
4624   those new features that will either be safely ignored by an HTTP/1.0
4625   recipient or only sent when communicating with a party advertising
4626   conformance with HTTP/1.1.
4627</t>
4628<t>
4629   It is beyond the scope of a protocol specification to mandate
4630   conformance with previous versions. HTTP/1.1 was deliberately
4631   designed, however, to make supporting previous versions easy.
4632   We would expect a general-purpose HTTP/1.1 server to understand
4633   any valid request in the format of HTTP/1.0 and respond appropriately
4634   with an HTTP/1.1 message that only uses features understood (or
4635   safely ignored) by HTTP/1.0 clients.  Likewise, we would expect
4636   an HTTP/1.1 client to understand any valid HTTP/1.0 response.
4637</t>
4638<t>
4639   Since HTTP/0.9 did not support header fields in a request,
4640   there is no mechanism for it to support name-based virtual
4641   hosts (selection of resource by inspection of the <x:ref>Host</x:ref> header
4642   field).  Any server that implements name-based virtual hosts
4643   ought to disable support for HTTP/0.9.  Most requests that
4644   appear to be HTTP/0.9 are, in fact, badly constructed HTTP/1.x
4645   requests wherein a buggy client failed to properly encode
4646   linear whitespace found in a URI reference and placed in
4647   the request-target.
4648</t>
4649
4650<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
4651<t>
4652   This section summarizes major differences between versions HTTP/1.0
4653   and HTTP/1.1.
4654</t>
4655
4656<section title="Multi-homed Web Servers" anchor="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">
4657<t>
4658   The requirements that clients and servers support the <x:ref>Host</x:ref>
4659   header field (<xref target="header.host"/>), report an error if it is
4660   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
4661   are among the most important changes defined by HTTP/1.1.
4662</t>
4663<t>
4664   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4665   addresses and servers; there was no other established mechanism for
4666   distinguishing the intended server of a request than the IP address
4667   to which that request was directed. The <x:ref>Host</x:ref> header field was
4668   introduced during the development of HTTP/1.1 and, though it was
4669   quickly implemented by most HTTP/1.0 browsers, additional requirements
4670   were placed on all HTTP/1.1 requests in order to ensure complete
4671   adoption.  At the time of this writing, most HTTP-based services
4672   are dependent upon the Host header field for targeting requests.
4673</t>
4674</section>
4675
4676<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4677<t>
4678   In HTTP/1.0, each connection is established by the client prior to the
4679   request and closed by the server after sending the response. However, some
4680   implementations implement the explicitly negotiated ("Keep-Alive") version
4681   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
4682   target="RFC2068"/>.
4683</t>
4684<t>
4685   Some clients and servers might wish to be compatible with these previous
4686   approaches to persistent connections, by explicitly negotiating for them
4687   with a "Connection: keep-alive" request header field. However, some
4688   experimental implementations of HTTP/1.0 persistent connections are faulty;
4689   for example, if a HTTP/1.0 proxy server doesn't understand
4690   <x:ref>Connection</x:ref>, it will erroneously forward that header field
4691   to the next inbound server, which would result in a hung connection.
4692</t>
4693<t>
4694   One attempted solution was the introduction of a Proxy-Connection header
4695   field, targeted specifically at proxies. In practice, this was also
4696   unworkable, because proxies are often deployed in multiple layers, bringing
4697   about the same problem discussed above.
4698</t>
4699<t>
4700   As a result, clients are encouraged not to send the Proxy-Connection header
4701   field in any requests.
4702</t>
4703<t>
4704   Clients are also encouraged to consider the use of Connection: keep-alive
4705   in requests carefully; while they can enable persistent connections with
4706   HTTP/1.0 servers, clients using them need will need to monitor the
4707   connection for "hung" requests (which indicate that the client ought stop
4708   sending the header field), and this mechanism ought not be used by clients
4709   at all when a proxy is being used.
4710</t>
4711</section>
4712
4713<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
4714<t>
4715   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
4716   (<xref target="header.transfer-encoding"/>). Proxies/gateways &MUST; remove
4717   any transfer-coding prior to forwarding a message via a MIME-compliant
4718   protocol.
4719</t>
4720</section>
4721
4722</section>
4723
4724<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4725<t>
4726  Clarify that the string "HTTP" in the HTTP-version ABNF production is case
4727  sensitive. Restrict the version numbers to be single digits due to the fact
4728  that implementations are known to handle multi-digit version numbers
4729  incorrectly.
4730  (<xref target="http.version"/>)
4731</t>
4732<t>
4733  Require that invalid whitespace around field-names be rejected.
4734  Change ABNF productions for header fields to only define the field value.
4735  (<xref target="header.fields"/>)
4736</t>
4737<t>
4738  Rules about implicit linear whitespace between certain grammar productions
4739  have been removed; now whitespace is only allowed where specifically
4740  defined in the ABNF.
4741  (<xref target="whitespace"/>)
4742</t>
4743<t> 
4744  The NUL octet is no longer allowed in comment and quoted-string
4745  text. The quoted-pair rule no longer allows escaping control characters other than HTAB.
4746  Non-ASCII content in header fields and reason phrase has been obsoleted and
4747  made opaque (the TEXT rule was removed).
4748  (<xref target="field.components"/>)
4749</t>
4750<t>
4751  Require recipients to handle bogus "<x:ref>Content-Length</x:ref>" header
4752  fields as errors.
4753  (<xref target="message.body"/>)
4754</t>
4755<t>
4756  Remove reference to non-existent identity transfer-coding value tokens.
4757  (Sections <xref format="counter" target="message.body"/> and
4758  <xref format="counter" target="transfer.codings"/>)
4759</t>
4760<t>
4761  Clarification that the chunk length does not include the count of the octets
4762  in the chunk header and trailer. Furthermore disallowed line folding
4763  in chunk extensions, and deprecate their use.
4764  (<xref target="chunked.encoding"/>)
4765</t>
4766<t>
4767  Update use of abs_path production from RFC 1808 to the path-absolute + query
4768  components of RFC 3986. State that the asterisk form is allowed for the OPTIONS
4769  request method only.
4770  (<xref target="request-target"/>)
4771</t>
4772<t>
4773  Clarify exactly when "close" connection options have to be sent; drop
4774  notion of header fields being "hop-by-hop" without being listed in the
4775  Connection header field.
4776  (<xref target="header.connection"/>)
4777</t>
4778<t>
4779  Remove hard limit of two connections per server.
4780  Remove requirement to retry a sequence of requests as long it was idempotent.
4781  Remove requirements about when servers are allowed to close connections
4782  prematurely.
4783  (<xref target="persistent.connections"/>)
4784</t>
4785<t>
4786  Remove requirement to retry requests under certain circumstances when the
4787  server prematurely closes the connection.
4788  (<xref target="persistent.reuse"/>)
4789</t>
4790<t>
4791  Define the semantics of the <x:ref>Upgrade</x:ref> header field in responses
4792  other than 101 (this was incorporated from <xref target="RFC2817"/>).
4793  (<xref target="header.upgrade"/>)
4794</t>
4795<t>
4796  Registration of Transfer Codings now requires IETF Review
4797  (<xref target="transfer.coding.registry"/>)
4798</t>
4799<t>
4800  Take over the Upgrade Token Registry, previously defined in
4801  <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
4802  (<xref target="upgrade.token.registry"/>)
4803</t>
4804<t>
4805  Empty list elements in list productions have been deprecated.
4806  (<xref target="abnf.extension"/>)
4807</t>
4808</section>
4809</section>
4810
4811<section title="ABNF list extension: #rule" anchor="abnf.extension">
4812<t>
4813  A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
4814  improve readability in the definitions of some header field values.
4815</t>
4816<t>
4817  A construct "#" is defined, similar to "*", for defining comma-delimited
4818  lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
4819  at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
4820  comma (",") and optional whitespace (OWS).   
4821</t>
4822<figure><preamble>
4823  Thus,
4824</preamble><artwork type="example">
4825  1#element =&gt; element *( OWS "," OWS element )
4826</artwork></figure>
4827<figure><preamble>
4828  and:
4829</preamble><artwork type="example">
4830  #element =&gt; [ 1#element ]
4831</artwork></figure>
4832<figure><preamble>
4833  and for n &gt;= 1 and m &gt; 1:
4834</preamble><artwork type="example">
4835  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
4836</artwork></figure>
4837<t>
4838  For compatibility with legacy list rules, recipients &SHOULD; accept empty
4839  list elements. In other words, consumers would follow the list productions:
4840</t>
4841<figure><artwork type="example">
4842  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
4843 
4844  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
4845</artwork></figure>
4846<t>
4847  Note that empty elements do not contribute to the count of elements present,
4848  though.
4849</t>
4850<t>
4851  For example, given these ABNF productions:
4852</t>
4853<figure><artwork type="example">
4854  example-list      = 1#example-list-elmt
4855  example-list-elmt = token ; see <xref target="field.components"/> 
4856</artwork></figure>
4857<t>
4858  Then these are valid values for example-list (not including the double
4859  quotes, which are present for delimitation only):
4860</t>
4861<figure><artwork type="example">
4862  "foo,bar"
4863  "foo ,bar,"
4864  "foo , ,bar,charlie   "
4865</artwork></figure>
4866<t>
4867  But these values would be invalid, as at least one non-empty element is
4868  required:
4869</t>
4870<figure><artwork type="example">
4871  ""
4872  ","
4873  ",   ,"
4874</artwork></figure>
4875<t>
4876  <xref target="collected.abnf"/> shows the collected ABNF, with the list rules
4877  expanded as explained above.
4878</t>
4879</section>
4880
4881<?BEGININC p1-messaging.abnf-appendix ?>
4882<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
4883<figure>
4884<artwork type="abnf" name="p1-messaging.parsed-abnf">
4885<x:ref>BWS</x:ref> = OWS
4886
4887<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
4888 connection-option ] )
4889<x:ref>Content-Length</x:ref> = 1*DIGIT
4890
4891<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
4892 ]
4893<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
4894<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
4895<x:ref>Host</x:ref> = uri-host [ ":" port ]
4896
4897<x:ref>OWS</x:ref> = *( SP / HTAB )
4898
4899<x:ref>RWS</x:ref> = 1*( SP / HTAB )
4900
4901<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
4902<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
4903<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
4904 transfer-coding ] )
4905
4906<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
4907<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
4908
4909<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
4910 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
4911 comment ] ) ] )
4912
4913<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
4914<x:ref>absolute-form</x:ref> = absolute-URI
4915<x:ref>asterisk-form</x:ref> = "*"
4916<x:ref>attribute</x:ref> = token
4917<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
4918<x:ref>authority-form</x:ref> = authority
4919
4920<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
4921<x:ref>chunk-data</x:ref> = 1*OCTET
4922<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
4923<x:ref>chunk-ext-name</x:ref> = token
4924<x:ref>chunk-ext-val</x:ref> = token / quoted-str-nf
4925<x:ref>chunk-size</x:ref> = 1*HEXDIG
4926<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
4927<x:ref>comment</x:ref> = "(" *( ctext / quoted-cpair / comment ) ")"
4928<x:ref>connection-option</x:ref> = token
4929<x:ref>ctext</x:ref> = OWS / %x21-27 ; '!'-'''
4930 / %x2A-5B ; '*'-'['
4931 / %x5D-7E ; ']'-'~'
4932 / obs-text
4933
4934<x:ref>field-content</x:ref> = *( HTAB / SP / VCHAR / obs-text )
4935<x:ref>field-name</x:ref> = token
4936<x:ref>field-value</x:ref> = *( field-content / obs-fold )
4937
4938<x:ref>header-field</x:ref> = field-name ":" OWS field-value BWS
4939<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ]
4940<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ]
4941
4942<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
4943
4944<x:ref>message-body</x:ref> = *OCTET
4945<x:ref>method</x:ref> = token
4946
4947<x:ref>obs-fold</x:ref> = CRLF ( SP / HTAB )
4948<x:ref>obs-text</x:ref> = %x80-FF
4949<x:ref>origin-form</x:ref> = path-absolute [ "?" query ]
4950
4951<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
4952<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
4953<x:ref>path-absolute</x:ref> = &lt;path-absolute, defined in [RFC3986], Section 3.3&gt;
4954<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
4955<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
4956<x:ref>protocol-name</x:ref> = token
4957<x:ref>protocol-version</x:ref> = token
4958<x:ref>pseudonym</x:ref> = token
4959
4960<x:ref>qdtext</x:ref> = OWS / "!" / %x23-5B ; '#'-'['
4961 / %x5D-7E ; ']'-'~'
4962 / obs-text
4963<x:ref>qdtext-nf</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
4964 / %x5D-7E ; ']'-'~'
4965 / obs-text
4966<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
4967<x:ref>quoted-cpair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
4968<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
4969<x:ref>quoted-str-nf</x:ref> = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
4970<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
4971
4972<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
4973<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
4974<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
4975<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
4976<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
4977<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
4978<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
4979 asterisk-form
4980
4981<x:ref>special</x:ref> = "(" / ")" / "&lt;" / "&gt;" / "@" / "," / ";" / ":" / "\" /
4982 DQUOTE / "/" / "[" / "]" / "?" / "=" / "{" / "}"
4983<x:ref>start-line</x:ref> = request-line / status-line
4984<x:ref>status-code</x:ref> = 3DIGIT
4985<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
4986
4987<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
4988<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
4989<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
4990 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
4991<x:ref>token</x:ref> = 1*tchar
4992<x:ref>trailer-part</x:ref> = *( header-field CRLF )
4993<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
4994 transfer-extension
4995<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
4996<x:ref>transfer-parameter</x:ref> = attribute BWS "=" BWS value
4997
4998<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
4999
5000<x:ref>value</x:ref> = word
5001
5002<x:ref>word</x:ref> = token / quoted-string
5003</artwork>
5004</figure>
5005</section>
5006<?ENDINC p1-messaging.abnf-appendix ?>
5007
5008<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5009
5010<section title="Since RFC 2616">
5011<t>
5012  Changes up to the first Working Group Last Call draft are summarized
5013  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-21#appendix-D"/>.
5014</t>
5015</section>
5016
5017<section title="Since draft-ietf-httpbis-p1-messaging-21" anchor="changes.since.21">
5018<t>
5019  None yet.
5020</t>
5021</section>
5022
5023</section>
5024
5025</back>
5026</rfc>
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