source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2034

Last change on this file since 2034 was 2034, checked in by fielding@…, 7 years ago

Clarify when Content-Length SHOULD be sent. Addresses #420

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  • Property svn:mime-type set to text/xml
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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 "December">
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-type            "<xref target='Part2' x:rel='#media.type' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY content-codings        "<xref target='Part2' x:rel='#content.codings' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY CONNECT                "<xref target='Part2' x:rel='#CONNECT' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY content.negotiation    "<xref target='Part2' x:rel='#content.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY diff-mime              "<xref target='Part2' x:rel='#differences.between.http.and.mime' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY representation         "<xref target='Part2' x:rel='#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 selected-representation    "<xref target='Part2' x:rel='#selected.representation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
51  <!ENTITY status-codes           "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
52  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
53  <!ENTITY status-203             "<xref target='Part2' x:rel='#status.203' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
54  <!ENTITY status-3xx             "<xref target='Part2' x:rel='#status.3xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
55  <!ENTITY status-304             "<xref target='Part4' x:rel='#status.304' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
56  <!ENTITY status-4xx             "<xref target='Part2' x:rel='#status.4xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
57  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
58  <!ENTITY iana-header-registry   "<xref target='Part2' x:rel='#header.field.registry' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
59]>
60<?rfc toc="yes" ?>
61<?rfc symrefs="yes" ?>
62<?rfc sortrefs="yes" ?>
63<?rfc compact="yes"?>
64<?rfc subcompact="no" ?>
65<?rfc linkmailto="no" ?>
66<?rfc editing="no" ?>
67<?rfc comments="yes"?>
68<?rfc inline="yes"?>
69<?rfc rfcedstyle="yes"?>
70<?rfc-ext allow-markup-in-artwork="yes" ?>
71<?rfc-ext include-references-in-index="yes" ?>
72<rfc obsoletes="2145,2616" updates="2817,2818" category="std" x:maturity-level="proposed"
73     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
74     xmlns:x='http://purl.org/net/xml2rfc/ext'>
75<x:link rel="next" basename="p2-semantics"/>
76<x:feedback template="mailto:ietf-http-wg@w3.org?subject={docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
77<front>
78
79  <title abbrev="HTTP/1.1 Message Syntax and Routing">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
80
81  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
82    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
83    <address>
84      <postal>
85        <street>345 Park Ave</street>
86        <city>San Jose</city>
87        <region>CA</region>
88        <code>95110</code>
89        <country>USA</country>
90      </postal>
91      <email>fielding@gbiv.com</email>
92      <uri>http://roy.gbiv.com/</uri>
93    </address>
94  </author>
95
96  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
97    <organization abbrev="greenbytes">greenbytes GmbH</organization>
98    <address>
99      <postal>
100        <street>Hafenweg 16</street>
101        <city>Muenster</city><region>NW</region><code>48155</code>
102        <country>Germany</country>
103      </postal>
104      <email>julian.reschke@greenbytes.de</email>
105      <uri>http://greenbytes.de/tech/webdav/</uri>
106    </address>
107  </author>
108
109  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
110  <workgroup>HTTPbis Working Group</workgroup>
111
112<abstract>
113<t>
114   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
115   distributed, collaborative, hypertext information systems. HTTP has been in
116   use by the World Wide Web global information initiative since 1990.
117   This document provides an overview of HTTP architecture and its associated
118   terminology, defines the "http" and "https" Uniform Resource Identifier
119   (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements,
120   and describes general security concerns for implementations.
121</t>   
122</abstract>
123
124<note title="Editorial Note (To be removed by RFC Editor)">
125  <t>
126    Discussion of this draft takes place on the HTTPBIS working group
127    mailing list (ietf-http-wg@w3.org), which is archived at
128    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
129  </t>
130  <t>
131    The current issues list is at
132    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
133    documents (including fancy diffs) can be found at
134    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
135  </t>
136  <t>
137    The changes in this draft are summarized in <xref target="changes.since.21"/>.
138  </t>
139</note>
140</front>
141<middle>
142<section title="Introduction" anchor="introduction">
143<t>
144   The Hypertext Transfer Protocol (HTTP) is an application-level
145   request/response protocol that uses extensible semantics and MIME-like
146   message payloads for flexible interaction with network-based hypertext
147   information systems. This document is the first in a series of documents
148   that collectively form the HTTP/1.1 specification:
149   <list style="empty">
150    <t>RFC xxx1: Message Syntax and Routing</t>
151    <t><xref target="Part2" x:fmt="none">RFC xxx2</xref>: Semantics and Content</t>
152    <t><xref target="Part4" x:fmt="none">RFC xxx3</xref>: Conditional Requests</t>
153    <t><xref target="Part5" x:fmt="none">RFC xxx4</xref>: Range Requests</t>
154    <t><xref target="Part6" x:fmt="none">RFC xxx5</xref>: Caching</t>
155    <t><xref target="Part7" x:fmt="none">RFC xxx6</xref>: Authentication</t>
156   </list>
157</t>
158<t>
159   This HTTP/1.1 specification obsoletes and moves to historic status
160   <xref target="RFC2616" x:fmt="none">RFC 2616</xref>, its predecessor
161   <xref target="RFC2068" x:fmt="none">RFC 2068</xref>, and
162   <xref target="RFC2145" x:fmt="none">RFC 2145</xref> (on HTTP versioning).
163   This specification also updates the use of CONNECT to establish a tunnel,
164   previously defined in <xref target="RFC2817" x:fmt="none">RFC 2817</xref>,
165   and defines the "https" URI scheme that was described informally in
166   <xref target="RFC2818" x:fmt="none">RFC 2818</xref>.
167</t>
168<t>
169   HTTP is a generic interface protocol for information systems. It is
170   designed to hide the details of how a service is implemented by presenting
171   a uniform interface to clients that is independent of the types of
172   resources provided. Likewise, servers do not need to be aware of each
173   client's purpose: an HTTP request can be considered in isolation rather
174   than being associated with a specific type of client or a predetermined
175   sequence of application steps. The result is a protocol that can be used
176   effectively in many different contexts and for which implementations can
177   evolve independently over time.
178</t>
179<t>
180   HTTP is also designed for use as an intermediation protocol for translating
181   communication to and from non-HTTP information systems.
182   HTTP proxies and gateways can provide access to alternative information
183   services by translating their diverse protocols into a hypertext
184   format that can be viewed and manipulated by clients in the same way
185   as HTTP services.
186</t>
187<t>
188   One consequence of this flexibility is that the protocol cannot be
189   defined in terms of what occurs behind the interface. Instead, we
190   are limited to defining the syntax of communication, the intent
191   of received communication, and the expected behavior of recipients.
192   If the communication is considered in isolation, then successful
193   actions ought to be reflected in corresponding changes to the
194   observable interface provided by servers. However, since multiple
195   clients might act in parallel and perhaps at cross-purposes, we
196   cannot require that such changes be observable beyond the scope
197   of a single response.
198</t>
199<t>
200   This document describes the architectural elements that are used or
201   referred to in HTTP, defines the "http" and "https" URI schemes,
202   describes overall network operation and connection management,
203   and defines HTTP message framing and forwarding requirements.
204   Our goal is to define all of the mechanisms necessary for HTTP message
205   handling that are independent of message semantics, thereby defining the
206   complete set of requirements for message parsers and
207   message-forwarding intermediaries.
208</t>
209
210
211<section title="Requirement Notation" anchor="intro.requirements">
212<t>
213   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
214   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
215   document are to be interpreted as described in <xref target="RFC2119"/>.
216</t>
217<t>
218   Conformance criteria and considerations regarding error handling
219   are defined in <xref target="conformance"/>.
220</t>
221</section>
222
223<section title="Syntax Notation" anchor="notation">
224<iref primary="true" item="Grammar" subitem="ALPHA"/>
225<iref primary="true" item="Grammar" subitem="CR"/>
226<iref primary="true" item="Grammar" subitem="CRLF"/>
227<iref primary="true" item="Grammar" subitem="CTL"/>
228<iref primary="true" item="Grammar" subitem="DIGIT"/>
229<iref primary="true" item="Grammar" subitem="DQUOTE"/>
230<iref primary="true" item="Grammar" subitem="HEXDIG"/>
231<iref primary="true" item="Grammar" subitem="HTAB"/>
232<iref primary="true" item="Grammar" subitem="LF"/>
233<iref primary="true" item="Grammar" subitem="OCTET"/>
234<iref primary="true" item="Grammar" subitem="SP"/>
235<iref primary="true" item="Grammar" subitem="VCHAR"/>
236<t>
237   This specification uses the Augmented Backus-Naur Form (ABNF) notation
238   of <xref target="RFC5234"/> with the list rule extension defined in
239   <xref target="abnf.extension"/><xref target="collected.abnf"/> shows
240   the collected ABNF with the list rule expanded.
241</t>
242<t anchor="core.rules">
243  <x:anchor-alias value="ALPHA"/>
244  <x:anchor-alias value="CTL"/>
245  <x:anchor-alias value="CR"/>
246  <x:anchor-alias value="CRLF"/>
247  <x:anchor-alias value="DIGIT"/>
248  <x:anchor-alias value="DQUOTE"/>
249  <x:anchor-alias value="HEXDIG"/>
250  <x:anchor-alias value="HTAB"/>
251  <x:anchor-alias value="LF"/>
252  <x:anchor-alias value="OCTET"/>
253  <x:anchor-alias value="SP"/>
254  <x:anchor-alias value="VCHAR"/>
255   The following core rules are included by
256   reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
257   ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
258   DIGIT (decimal 0-9), DQUOTE (double quote),
259   HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed),
260   OCTET (any 8-bit sequence of data), SP (space), and
261   VCHAR (any visible <xref target="USASCII"/> character).
262</t>
263<t>
264   As a convention, ABNF rule names prefixed with "obs-" denote
265   "obsolete" grammar rules that appear for historical reasons.
266</t>
267</section>
268</section>
269
270<section title="Architecture" anchor="architecture">
271<t>
272   HTTP was created for the World Wide Web architecture
273   and has evolved over time to support the scalability needs of a worldwide
274   hypertext system. Much of that architecture is reflected in the terminology
275   and syntax productions used to define HTTP.
276</t>
277
278<section title="Client/Server Messaging" anchor="operation">
279<iref primary="true" item="client"/>
280<iref primary="true" item="server"/>
281<iref primary="true" item="connection"/>
282<t>
283   HTTP is a stateless request/response protocol that operates by exchanging
284   <x:dfn>messages</x:dfn> (<xref target="http.message"/>) across a reliable
285   transport or session-layer
286   "<x:dfn>connection</x:dfn>" (<xref target="connection.management"/>).
287   An HTTP "<x:dfn>client</x:dfn>" is a program that establishes a connection
288   to a server for the purpose of sending one or more HTTP requests.
289   An HTTP "<x:dfn>server</x:dfn>" is a program that accepts connections
290   in order to service HTTP requests by sending HTTP responses.
291</t>
292<iref primary="true" item="user agent"/>
293<iref primary="true" item="origin server"/>
294<iref primary="true" item="browser"/>
295<iref primary="true" item="spider"/>
296<iref primary="true" item="sender"/>
297<iref primary="true" item="recipient"/>
298<t>
299   The terms client and server refer only to the roles that
300   these programs perform for a particular connection.  The same program
301   might act as a client on some connections and a server on others.
302   We use the term "<x:dfn>user agent</x:dfn>" to refer to any of the various
303   client programs that initiate a request, including (but not limited to)
304   browsers, spiders (web-based robots), command-line tools, native
305   applications, and mobile apps.  The term "<x:dfn>origin server</x:dfn>" is
306   used to refer to the program that can originate authoritative responses to
307   a request. For general requirements, we use the terms
308   "<x:dfn>sender</x:dfn>" and "<x:dfn>recipient</x:dfn>" to refer to any
309   component that sends or receives, respectively, a given message.
310</t>
311<t>
312   HTTP relies upon the Uniform Resource Identifier (URI)
313   standard <xref target="RFC3986"/> to indicate the target resource
314   (<xref target="target-resource"/>) and relationships between resources.
315   Messages are passed in a format similar to that used by Internet mail
316   <xref target="RFC5322"/> and the Multipurpose Internet Mail Extensions
317   (MIME) <xref target="RFC2045"/> (see &diff-mime; for the differences
318   between HTTP and MIME messages).
319</t>
320<t>
321   Most HTTP communication consists of a retrieval request (GET) for
322   a representation of some resource identified by a URI.  In the
323   simplest case, this might be accomplished via a single bidirectional
324   connection (===) between the user agent (UA) and the origin server (O).
325</t>
326<figure><artwork type="drawing">
327         request   &gt;
328    <x:highlight>UA</x:highlight> ======================================= <x:highlight>O</x:highlight>
329                                &lt;   response
330</artwork></figure>
331<iref primary="true" item="message"/>
332<iref primary="true" item="request"/>
333<iref primary="true" item="response"/>
334<t>
335   A client sends an HTTP request to a server in the form of a <x:dfn>request</x:dfn>
336   message, beginning with a request-line that includes a method, URI, and
337   protocol version (<xref target="request.line"/>),
338   followed by header fields containing
339   request modifiers, client information, and representation metadata
340   (<xref target="header.fields"/>),
341   an empty line to indicate the end of the header section, and finally
342   a message body containing the payload body (if any,
343   <xref target="message.body"/>).
344</t>
345<t>
346   A server responds to a client's request by sending one or more HTTP
347   <x:dfn>response</x:dfn>
348   messages, each beginning with a status line that
349   includes the protocol version, a success or error code, and textual
350   reason phrase (<xref target="status.line"/>),
351   possibly followed by header fields containing server
352   information, resource metadata, and representation metadata
353   (<xref target="header.fields"/>),
354   an empty line to indicate the end of the header section, and finally
355   a message body containing the payload body (if any,
356   <xref target="message.body"/>).
357</t>
358<t>
359   A connection might be used for multiple request/response exchanges,
360   as defined in <xref target="persistent.connections"/>.
361</t>
362<t>
363   The following example illustrates a typical message exchange for a
364   GET request on the URI "http://www.example.com/hello.txt":
365</t>
366<figure><preamble>
367client request:
368</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
369GET /hello.txt HTTP/1.1
370User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
371Host: www.example.com
372Accept-Language: en, mi
373
374</artwork></figure>
375<figure><preamble>
376server response:
377</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
378HTTP/1.1 200 OK
379Date: Mon, 27 Jul 2009 12:28:53 GMT
380Server: Apache
381Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
382ETag: "34aa387-d-1568eb00"
383Accept-Ranges: bytes
384Content-Length: <x:length-of target="exbody"/>
385Vary: Accept-Encoding
386Content-Type: text/plain
387
388<x:span anchor="exbody">Hello World!
389</x:span></artwork></figure>
390</section>
391
392<section title="Implementation Diversity" anchor="implementation-diversity">
393<t>
394   When considering the design of HTTP, it is easy to fall into a trap of
395   thinking that all user agents are general-purpose browsers and all origin
396   servers are large public websites. That is not the case in practice.
397   Common HTTP user agents include household appliances, stereos, scales,
398   firmware update scripts, command-line programs, mobile apps,
399   and communication devices in a multitude of shapes and sizes.  Likewise,
400   common HTTP origin servers include home automation units, configurable
401   networking components, office machines, autonomous robots, news feeds,
402   traffic cameras, ad selectors, and video delivery platforms.
403</t>
404<t>
405   The term "user agent" does not imply that there is a human user directly
406   interacting with the software agent at the time of a request. In many
407   cases, a user agent is installed or configured to run in the background
408   and save its results for later inspection (or save only a subset of those
409   results that might be interesting or erroneous). Spiders, for example, are
410   typically given a start URI and configured to follow certain behavior while
411   crawling the Web as a hypertext graph.
412</t>
413<t>
414   The implementation diversity of HTTP means that we cannot assume the
415   user agent can make interactive suggestions to a user or provide adequate
416   warning for security or privacy options.  In the few cases where this
417   specification requires reporting of errors to the user, it is acceptable
418   for such reporting to only be observable in an error console or log file.
419   Likewise, requirements that an automated action be confirmed by the user
420   before proceeding can be met via advance configuration choices,
421   run-time options, or simply not proceeding with the unsafe action.
422</t>
423</section>
424
425<section title="Intermediaries" anchor="intermediaries">
426<iref primary="true" item="intermediary"/>
427<t>
428   HTTP enables the use of intermediaries to satisfy requests through
429   a chain of connections.  There are three common forms of HTTP
430   <x:dfn>intermediary</x:dfn>: proxy, gateway, and tunnel.  In some cases,
431   a single intermediary might act as an origin server, proxy, gateway,
432   or tunnel, switching behavior based on the nature of each request.
433</t>
434<figure><artwork type="drawing">
435         &gt;             &gt;             &gt;             &gt;
436    <x:highlight>UA</x:highlight> =========== <x:highlight>A</x:highlight> =========== <x:highlight>B</x:highlight> =========== <x:highlight>C</x:highlight> =========== <x:highlight>O</x:highlight>
437               &lt;             &lt;             &lt;             &lt;
438</artwork></figure>
439<t>
440   The figure above shows three intermediaries (A, B, and C) between the
441   user agent and origin server. A request or response message that
442   travels the whole chain will pass through four separate connections.
443   Some HTTP communication options
444   might apply only to the connection with the nearest, non-tunnel
445   neighbor, only to the end-points of the chain, or to all connections
446   along the chain. Although the diagram is linear, each participant might
447   be engaged in multiple, simultaneous communications. For example, B
448   might be receiving requests from many clients other than A, and/or
449   forwarding requests to servers other than C, at the same time that it
450   is handling A's request.
451</t>
452<t>
453<iref primary="true" item="upstream"/><iref primary="true" item="downstream"/>
454<iref primary="true" item="inbound"/><iref primary="true" item="outbound"/>
455   We use the terms "<x:dfn>upstream</x:dfn>" and "<x:dfn>downstream</x:dfn>"
456   to describe various requirements in relation to the directional flow of a
457   message: all messages flow from upstream to downstream.
458   Likewise, we use the terms inbound and outbound to refer to
459   directions in relation to the request path:
460   "<x:dfn>inbound</x:dfn>" means toward the origin server and
461   "<x:dfn>outbound</x:dfn>" means toward the user agent.
462</t>
463<t><iref primary="true" item="proxy"/>
464   A "<x:dfn>proxy</x:dfn>" is a message forwarding agent that is selected by the
465   client, usually via local configuration rules, to receive requests
466   for some type(s) of absolute URI and attempt to satisfy those
467   requests via translation through the HTTP interface.  Some translations
468   are minimal, such as for proxy requests for "http" URIs, whereas
469   other requests might require translation to and from entirely different
470   application-level protocols. Proxies are often used to group an
471   organization's HTTP requests through a common intermediary for the
472   sake of security, annotation services, or shared caching.
473</t>
474<t>
475<iref primary="true" item="transforming proxy"/>
476<iref primary="true" item="non-transforming proxy"/>
477   An HTTP-to-HTTP proxy is called a "<x:dfn>transforming proxy</x:dfn>" if it is designed
478   or configured to modify request or response messages in a semantically
479   meaningful way (i.e., modifications, beyond those required by normal
480   HTTP processing, that change the message in a way that would be
481   significant to the original sender or potentially significant to
482   downstream recipients).  For example, a transforming proxy might be
483   acting as a shared annotation server (modifying responses to include
484   references to a local annotation database), a malware filter, a
485   format transcoder, or an intranet-to-Internet privacy filter.  Such
486   transformations are presumed to be desired by the client (or client
487   organization) that selected the proxy and are beyond the scope of
488   this specification.  However, when a proxy is not intended to transform
489   a given message, we use the term "<x:dfn>non-transforming proxy</x:dfn>" to target
490   requirements that preserve HTTP message semantics. See &status-203; and
491   &header-warning; for status and warning codes related to transformations.
492</t>
493<t><iref primary="true" item="gateway"/><iref primary="true" item="reverse proxy"/>
494<iref primary="true" item="accelerator"/>
495   A "<x:dfn>gateway</x:dfn>" (a.k.a., "<x:dfn>reverse proxy</x:dfn>")
496   is a receiving agent that acts
497   as a layer above some other server(s) and translates the received
498   requests to the underlying server's protocol.  Gateways are often
499   used to encapsulate legacy or untrusted information services, to
500   improve server performance through "<x:dfn>accelerator</x:dfn>" caching, and to
501   enable partitioning or load-balancing of HTTP services across
502   multiple machines.
503</t>
504<t>
505   A gateway behaves as an origin server on its outbound connection and
506   as a user agent on its inbound connection.
507   All HTTP requirements applicable to an origin server
508   also apply to the outbound communication of a gateway.
509   A gateway communicates with inbound servers using any protocol that
510   it desires, including private extensions to HTTP that are outside
511   the scope of this specification.  However, an HTTP-to-HTTP gateway
512   that wishes to interoperate with third-party HTTP servers &MUST;
513   conform to HTTP user agent requirements on the gateway's inbound
514   connection and &MUST; implement the <x:ref>Connection</x:ref>
515   (<xref target="header.connection"/>) and <x:ref>Via</x:ref>
516   (<xref target="header.via"/>) header fields for both connections.
517</t>
518<t><iref primary="true" item="tunnel"/>
519   A "<x:dfn>tunnel</x:dfn>" acts as a blind relay between two connections
520   without changing the messages. Once active, a tunnel is not
521   considered a party to the HTTP communication, though the tunnel might
522   have been initiated by an HTTP request. A tunnel ceases to exist when
523   both ends of the relayed connection are closed. Tunnels are used to
524   extend a virtual connection through an intermediary, such as when
525   Transport Layer Security (TLS, <xref target="RFC5246"/>) is used to
526   establish confidential communication through a shared firewall proxy.
527</t>
528<t><iref primary="true" item="interception proxy"/>
529<iref primary="true" item="transparent proxy"/>
530<iref primary="true" item="captive portal"/>
531   The above categories for intermediary only consider those acting as
532   participants in the HTTP communication.  There are also intermediaries
533   that can act on lower layers of the network protocol stack, filtering or
534   redirecting HTTP traffic without the knowledge or permission of message
535   senders. Network intermediaries often introduce security flaws or
536   interoperability problems by violating HTTP semantics.  For example, an
537   "<x:dfn>interception proxy</x:dfn>" <xref target="RFC3040"/> (also commonly
538   known as a "<x:dfn>transparent proxy</x:dfn>" <xref target="RFC1919"/> or
539   "<x:dfn>captive portal</x:dfn>")
540   differs from an HTTP proxy because it is not selected by the client.
541   Instead, an interception proxy filters or redirects outgoing TCP port 80
542   packets (and occasionally other common port traffic).
543   Interception proxies are commonly found on public network access points,
544   as a means of enforcing account subscription prior to allowing use of
545   non-local Internet services, and within corporate firewalls to enforce
546   network usage policies.
547   They are indistinguishable from a man-in-the-middle attack.
548</t>
549<t>
550   HTTP is defined as a stateless protocol, meaning that each request message
551   can be understood in isolation.  Many implementations depend on HTTP's
552   stateless design in order to reuse proxied connections or dynamically
553   load-balance requests across multiple servers.  Hence, servers &MUST-NOT;
554   assume that two requests on the same connection are from the same user
555   agent unless the connection is secured and specific to that agent.
556   Some non-standard HTTP extensions (e.g., <xref target="RFC4559"/>) have
557   been known to violate this requirement, resulting in security and
558   interoperability problems.
559</t>
560</section>
561
562<section title="Caches" anchor="caches">
563<iref primary="true" item="cache"/>
564<t>
565   A "<x:dfn>cache</x:dfn>" is a local store of previous response messages and the
566   subsystem that controls its message storage, retrieval, and deletion.
567   A cache stores cacheable responses in order to reduce the response
568   time and network bandwidth consumption on future, equivalent
569   requests. Any client or server &MAY; employ a cache, though a cache
570   cannot be used by a server while it is acting as a tunnel.
571</t>
572<t>
573   The effect of a cache is that the request/response chain is shortened
574   if one of the participants along the chain has a cached response
575   applicable to that request. The following illustrates the resulting
576   chain if B has a cached copy of an earlier response from O (via C)
577   for a request which has not been cached by UA or A.
578</t>
579<figure><artwork type="drawing">
580            &gt;             &gt;
581       <x:highlight>UA</x:highlight> =========== <x:highlight>A</x:highlight> =========== <x:highlight>B</x:highlight> - - - - - - <x:highlight>C</x:highlight> - - - - - - <x:highlight>O</x:highlight>
582                  &lt;             &lt;
583</artwork></figure>
584<t><iref primary="true" item="cacheable"/>
585   A response is "<x:dfn>cacheable</x:dfn>" if a cache is allowed to store a copy of
586   the response message for use in answering subsequent requests.
587   Even when a response is cacheable, there might be additional
588   constraints placed by the client or by the origin server on when
589   that cached response can be used for a particular request. HTTP
590   requirements for cache behavior and cacheable responses are
591   defined in &caching-overview;
592</t>
593<t>
594   There are a wide variety of architectures and configurations
595   of caches deployed across the World Wide Web and
596   inside large organizations. These include national hierarchies
597   of proxy caches to save transoceanic bandwidth, collaborative systems that
598   broadcast or multicast cache entries, archives of pre-fetched cache
599   entries for use in off-line or high-latency environments, and so on.
600</t>
601</section>
602
603<section title="Conformance and Error Handling" anchor="conformance">
604<t>
605   This specification targets conformance criteria according to the role of
606   a participant in HTTP communication.  Hence, HTTP requirements are placed
607   on senders, recipients, clients, servers, user agents, intermediaries,
608   origin servers, proxies, gateways, or caches, depending on what behavior
609   is being constrained by the requirement. Additional (social) requirements
610   are placed on implementations, resource owners, and protocol element
611   registrations when they apply beyond the scope of a single communication.
612</t>
613<t>
614   The verb "generate" is used instead of "send" where a requirement
615   differentiates between creating a protocol element and merely forwarding a
616   received element downstream.
617</t>
618<t>
619   An implementation is considered conformant if it complies with all of the
620   requirements associated with the roles it partakes in HTTP. Note that
621   SHOULD-level requirements are relevant here, unless one of the documented
622   exceptions is applicable.
623</t>
624<t>
625   Conformance applies to both the syntax and semantics of HTTP protocol
626   elements. A sender &MUST-NOT; generate protocol elements that convey a
627   meaning that is known by that sender to be false. A sender &MUST-NOT;
628   generate protocol elements that do not match the grammar defined by the
629   ABNF rules for those protocol elements that are applicable to the sender's
630   role. If a received protocol element is processed, the recipient &MUST; be
631   able to parse any value that would match the ABNF rules for that protocol
632   element, excluding only those rules not applicable to the recipient's role.
633</t>
634<t>
635   Unless noted otherwise, a recipient &MAY; attempt to recover a usable
636   protocol element from an invalid construct.  HTTP does not define
637   specific error handling mechanisms except when they have a direct impact
638   on security, since different applications of the protocol require
639   different error handling strategies.  For example, a Web browser might
640   wish to transparently recover from a response where the
641   <x:ref>Location</x:ref> header field doesn't parse according to the ABNF,
642   whereas a systems control client might consider any form of error recovery
643   to be dangerous.
644</t>
645</section>
646
647<section title="Protocol Versioning" anchor="http.version">
648  <x:anchor-alias value="HTTP-version"/>
649  <x:anchor-alias value="HTTP-name"/>
650<t>
651   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate
652   versions of the protocol. This specification defines version "1.1".
653   The protocol version as a whole indicates the sender's conformance
654   with the set of requirements laid out in that version's corresponding
655   specification of HTTP.
656</t>
657<t>
658   The version of an HTTP message is indicated by an HTTP-version field
659   in the first line of the message. HTTP-version is case-sensitive.
660</t>
661<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-version"/><iref primary="true" item="Grammar" subitem="HTTP-name"/>
662  <x:ref>HTTP-version</x:ref>  = <x:ref>HTTP-name</x:ref> "/" <x:ref>DIGIT</x:ref> "." <x:ref>DIGIT</x:ref>
663  <x:ref>HTTP-name</x:ref>     = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
664</artwork></figure>
665<t>
666   The HTTP version number consists of two decimal digits separated by a "."
667   (period or decimal point).  The first digit ("major version") indicates the
668   HTTP messaging syntax, whereas the second digit ("minor version") indicates
669   the highest minor version to which the sender is
670   conformant and able to understand for future communication.  The minor
671   version advertises the sender's communication capabilities even when the
672   sender is only using a backwards-compatible subset of the protocol,
673   thereby letting the recipient know that more advanced features can
674   be used in response (by servers) or in future requests (by clients).
675</t>
676<t>
677   When an HTTP/1.1 message is sent to an HTTP/1.0 recipient
678   <xref target="RFC1945"/> or a recipient whose version is unknown,
679   the HTTP/1.1 message is constructed such that it can be interpreted
680   as a valid HTTP/1.0 message if all of the newer features are ignored.
681   This specification places recipient-version requirements on some
682   new features so that a conformant sender will only use compatible
683   features until it has determined, through configuration or the
684   receipt of a message, that the recipient supports HTTP/1.1.
685</t>
686<t>
687   The interpretation of a header field does not change between minor
688   versions of the same major HTTP version, though the default
689   behavior of a recipient in the absence of such a field can change.
690   Unless specified otherwise, header fields defined in HTTP/1.1 are
691   defined for all versions of HTTP/1.x.  In particular, the <x:ref>Host</x:ref>
692   and <x:ref>Connection</x:ref> header fields ought to be implemented by all
693   HTTP/1.x implementations whether or not they advertise conformance with
694   HTTP/1.1.
695</t>
696<t>
697   New header fields can be defined such that, when they are
698   understood by a recipient, they might override or enhance the
699   interpretation of previously defined header fields.  When an
700   implementation receives an unrecognized header field, the recipient
701   &MUST; ignore that header field for local processing regardless of
702   the message's HTTP version.  An unrecognized header field received
703   by a proxy &MUST; be forwarded downstream unless the header field's
704   field-name is listed in the message's <x:ref>Connection</x:ref> header field
705   (see <xref target="header.connection"/>).
706   These requirements allow HTTP's functionality to be enhanced without
707   requiring prior update of deployed intermediaries.
708</t>
709<t>
710   Intermediaries that process HTTP messages (i.e., all intermediaries
711   other than those acting as tunnels) &MUST; send their own HTTP-version
712   in forwarded messages.  In other words, they &MUST-NOT; blindly
713   forward the first line of an HTTP message without ensuring that the
714   protocol version in that message matches a version to which that
715   intermediary is conformant for both the receiving and
716   sending of messages.  Forwarding an HTTP message without rewriting
717   the HTTP-version might result in communication errors when downstream
718   recipients use the message sender's version to determine what features
719   are safe to use for later communication with that sender.
720</t>
721<t>
722   An HTTP client &SHOULD; send a request version equal to the highest
723   version to which the client is conformant and
724   whose major version is no higher than the highest version supported
725   by the server, if this is known.  An HTTP client &MUST-NOT; send a
726   version to which it is not conformant.
727</t>
728<t>
729   An HTTP client &MAY; send a lower request version if it is known that
730   the server incorrectly implements the HTTP specification, but only
731   after the client has attempted at least one normal request and determined
732   from the response status or header fields (e.g., <x:ref>Server</x:ref>) that
733   the server improperly handles higher request versions.
734</t>
735<t>
736   An HTTP server &SHOULD; send a response version equal to the highest
737   version to which the server is conformant and
738   whose major version is less than or equal to the one received in the
739   request.  An HTTP server &MUST-NOT; send a version to which it is not
740   conformant.  A server &MAY; send a <x:ref>505 (HTTP Version Not
741   Supported)</x:ref> response if it cannot send a response using the
742   major version used in the client's request.
743</t>
744<t>
745   An HTTP server &MAY; send an HTTP/1.0 response to an HTTP/1.0 request
746   if it is known or suspected that the client incorrectly implements the
747   HTTP specification and is incapable of correctly processing later
748   version responses, such as when a client fails to parse the version
749   number correctly or when an intermediary is known to blindly forward
750   the HTTP-version even when it doesn't conform to the given minor
751   version of the protocol. Such protocol downgrades &SHOULD-NOT; be
752   performed unless triggered by specific client attributes, such as when
753   one or more of the request header fields (e.g., <x:ref>User-Agent</x:ref>)
754   uniquely match the values sent by a client known to be in error.
755</t>
756<t>
757   The intention of HTTP's versioning design is that the major number
758   will only be incremented if an incompatible message syntax is
759   introduced, and that the minor number will only be incremented when
760   changes made to the protocol have the effect of adding to the message
761   semantics or implying additional capabilities of the sender.  However,
762   the minor version was not incremented for the changes introduced between
763   <xref target="RFC2068"/> and <xref target="RFC2616"/>, and this revision
764   has specifically avoiding any such changes to the protocol.
765</t>
766</section>
767
768<section title="Uniform Resource Identifiers" anchor="uri">
769<iref primary="true" item="resource"/>
770<t>
771   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used
772   throughout HTTP as the means for identifying resources (&resource;).
773   URI references are used to target requests, indicate redirects, and define
774   relationships.
775</t>
776  <x:anchor-alias value="URI-reference"/>
777  <x:anchor-alias value="absolute-URI"/>
778  <x:anchor-alias value="relative-part"/>
779  <x:anchor-alias value="authority"/>
780  <x:anchor-alias value="path-abempty"/>
781  <x:anchor-alias value="path-absolute"/>
782  <x:anchor-alias value="port"/>
783  <x:anchor-alias value="query"/>
784  <x:anchor-alias value="uri-host"/>
785  <x:anchor-alias value="partial-URI"/>
786<t>
787   This specification adopts the definitions of "URI-reference",
788   "absolute-URI", "relative-part", "port", "host",
789   "path-abempty", "path-absolute", "query", and "authority" from the
790   URI generic syntax.
791   In addition, we define a partial-URI rule for protocol elements
792   that allow a relative URI but not a fragment.
793</t>
794<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="URI-reference"><!--exported production--></iref><iref primary="true" item="Grammar" subitem="absolute-URI"/><iref primary="true" item="Grammar" subitem="authority"/><iref primary="true" item="Grammar" subitem="path-absolute"/><iref primary="true" item="Grammar" subitem="port"/><iref primary="true" item="Grammar" subitem="query"/><iref primary="true" item="Grammar" subitem="uri-host"/><iref primary="true" item="Grammar" subitem="partial-URI"><!--exported production--></iref>
795  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
796  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
797  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
798  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
799  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
800  <x:ref>path-absolute</x:ref> = &lt;path-absolute, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
801  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
802  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
803  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
804 
805  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
806</artwork></figure>
807<t>
808   Each protocol element in HTTP that allows a URI reference will indicate
809   in its ABNF production whether the element allows any form of reference
810   (URI-reference), only a URI in absolute form (absolute-URI), only the
811   path and optional query components, or some combination of the above.
812   Unless otherwise indicated, URI references are parsed
813   relative to the effective request URI
814   (<xref target="effective.request.uri"/>).
815</t>
816
817<section title="http URI scheme" anchor="http.uri">
818  <x:anchor-alias value="http-URI"/>
819  <iref item="http URI scheme" primary="true"/>
820  <iref item="URI scheme" subitem="http" primary="true"/>
821<t>
822   The "http" URI scheme is hereby defined for the purpose of minting
823   identifiers according to their association with the hierarchical
824   namespace governed by a potential HTTP origin server listening for
825   TCP connections on a given port.
826</t>
827<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"><!--terminal production--></iref>
828  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
829</artwork></figure>
830<t>
831   The HTTP origin server is identified by the generic syntax's
832   <x:ref>authority</x:ref> component, which includes a host identifier
833   and optional TCP port (<xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>).
834   The remainder of the URI, consisting of both the hierarchical path
835   component and optional query component, serves as an identifier for
836   a potential resource within that origin server's name space.
837</t>
838<t>
839   If the host identifier is provided as an IP address,
840   then the origin server is any listener on the indicated TCP port at
841   that IP address. If host is a registered name, then that name is
842   considered an indirect identifier and the recipient might use a name
843   resolution service, such as DNS, to find the address of a listener
844   for that host.
845   The host &MUST-NOT; be empty; if an "http" URI is received with an
846   empty host, then it &MUST; be rejected as invalid.
847   If the port subcomponent is empty or not given, then TCP port 80 is
848   assumed (the default reserved port for WWW services).
849</t>
850<t>
851   Regardless of the form of host identifier, access to that host is not
852   implied by the mere presence of its name or address. The host might or might
853   not exist and, even when it does exist, might or might not be running an
854   HTTP server or listening to the indicated port. The "http" URI scheme
855   makes use of the delegated nature of Internet names and addresses to
856   establish a naming authority (whatever entity has the ability to place
857   an HTTP server at that Internet name or address) and allows that
858   authority to determine which names are valid and how they might be used.
859</t>
860<t>
861   When an "http" URI is used within a context that calls for access to the
862   indicated resource, a client &MAY; attempt access by resolving
863   the host to an IP address, establishing a TCP connection to that address
864   on the indicated port, and sending an HTTP request message
865   (<xref target="http.message"/>) containing the URI's identifying data
866   (<xref target="message.routing"/>) to the server.
867   If the server responds to that request with a non-interim HTTP response
868   message, as described in &status-codes;, then that response
869   is considered an authoritative answer to the client's request.
870</t>
871<t>
872   Although HTTP is independent of the transport protocol, the "http"
873   scheme is specific to TCP-based services because the name delegation
874   process depends on TCP for establishing authority.
875   An HTTP service based on some other underlying connection protocol
876   would presumably be identified using a different URI scheme, just as
877   the "https" scheme (below) is used for resources that require an
878   end-to-end secured connection. Other protocols might also be used to
879   provide access to "http" identified resources &mdash; it is only the
880   authoritative interface used for mapping the namespace that is
881   specific to TCP.
882</t>
883<t>
884   The URI generic syntax for authority also includes a deprecated
885   userinfo subcomponent (<xref target="RFC3986" x:fmt="," x:sec="3.2.1"/>)
886   for including user authentication information in the URI.  Some
887   implementations make use of the userinfo component for internal
888   configuration of authentication information, such as within command
889   invocation options, configuration files, or bookmark lists, even
890   though such usage might expose a user identifier or password.
891   Senders &MUST; exclude the userinfo subcomponent (and its "@"
892   delimiter) when an "http" URI is transmitted within a message as a
893   request target or header field value.
894   Recipients of an "http" URI reference &SHOULD; parse for userinfo and
895   treat its presence as an error, since it is likely being used to obscure
896   the authority for the sake of phishing attacks.
897</t>
898</section>
899
900<section title="https URI scheme" anchor="https.uri">
901   <x:anchor-alias value="https-URI"/>
902   <iref item="https URI scheme"/>
903   <iref item="URI scheme" subitem="https"/>
904<t>
905   The "https" URI scheme is hereby defined for the purpose of minting
906   identifiers according to their association with the hierarchical
907   namespace governed by a potential HTTP origin server listening to a
908   given TCP port for TLS-secured connections <xref target="RFC5246"/>.
909</t>
910<t>
911   All of the requirements listed above for the "http" scheme are also
912   requirements for the "https" scheme, except that a default TCP port
913   of 443 is assumed if the port subcomponent is empty or not given,
914   and the TCP connection &MUST; be secured, end-to-end, through the
915   use of strong encryption prior to sending the first HTTP request.
916</t>
917<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="https-URI"><!--terminal production--></iref>
918  <x:ref>https-URI</x:ref> = "https:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
919</artwork></figure>
920<t>
921   Unlike the "http" scheme, responses to "https" identified requests
922   are never "public" and thus &MUST-NOT; be reused for shared caching.
923   They can, however, be reused in a private cache if the message is
924   cacheable by default in HTTP or specifically indicated as such by
925   the Cache-Control header field (&header-cache-control;).
926</t>
927<t>
928   Resources made available via the "https" scheme have no shared
929   identity with the "http" scheme even if their resource identifiers
930   indicate the same authority (the same host listening to the same
931   TCP port).  They are distinct name spaces and are considered to be
932   distinct origin servers.  However, an extension to HTTP that is
933   defined to apply to entire host domains, such as the Cookie protocol
934   <xref target="RFC6265"/>, can allow information
935   set by one service to impact communication with other services
936   within a matching group of host domains.
937</t>
938<t>
939   The process for authoritative access to an "https" identified
940   resource is defined in <xref target="RFC2818"/>.
941</t>
942</section>
943
944<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
945<t>
946   Since the "http" and "https" schemes conform to the URI generic syntax,
947   such URIs are normalized and compared according to the algorithm defined
948   in <xref target="RFC3986" x:fmt="," x:sec="6"/>, using the defaults
949   described above for each scheme.
950</t>
951<t>
952   If the port is equal to the default port for a scheme, the normal
953   form is to elide the port subcomponent. Likewise, an empty path
954   component is equivalent to an absolute path of "/", so the normal
955   form is to provide a path of "/" instead. The scheme and host
956   are case-insensitive and normally provided in lowercase; all
957   other components are compared in a case-sensitive manner.
958   Characters other than those in the "reserved" set are equivalent
959   to their percent-encoded octets (see <xref target="RFC3986"
960   x:fmt="," x:sec="2.1"/>): the normal form is to not encode them.
961</t>
962<t>
963   For example, the following three URIs are equivalent:
964</t>
965<figure><artwork type="example">
966   http://example.com:80/~smith/home.html
967   http://EXAMPLE.com/%7Esmith/home.html
968   http://EXAMPLE.com:/%7esmith/home.html
969</artwork></figure>
970</section>
971</section>
972</section>
973
974<section title="Message Format" anchor="http.message">
975<x:anchor-alias value="generic-message"/>
976<x:anchor-alias value="message.types"/>
977<x:anchor-alias value="HTTP-message"/>
978<x:anchor-alias value="start-line"/>
979<iref item="header section"/>
980<iref item="headers"/>
981<iref item="header field"/>
982<t>
983   All HTTP/1.1 messages consist of a start-line followed by a sequence of
984   octets in a format similar to the Internet Message Format
985   <xref target="RFC5322"/>: zero or more header fields (collectively
986   referred to as the "headers" or the "header section"), an empty line
987   indicating the end of the header section, and an optional message body.
988</t>
989<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"><!--terminal production--></iref>
990  <x:ref>HTTP-message</x:ref>   = <x:ref>start-line</x:ref>
991                   *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
992                   <x:ref>CRLF</x:ref>
993                   [ <x:ref>message-body</x:ref> ]
994</artwork></figure>
995<t>
996   The normal procedure for parsing an HTTP message is to read the
997   start-line into a structure, read each header field into a hash
998   table by field name until the empty line, and then use the parsed
999   data to determine if a message body is expected.  If a message body
1000   has been indicated, then it is read as a stream until an amount
1001   of octets equal to the message body length is read or the connection
1002   is closed.
1003</t>
1004<t>
1005   Recipients &MUST; parse an HTTP message as a sequence of octets in an
1006   encoding that is a superset of US-ASCII <xref target="USASCII"/>.
1007   Parsing an HTTP message as a stream of Unicode characters, without regard
1008   for the specific encoding, creates security vulnerabilities due to the
1009   varying ways that string processing libraries handle invalid multibyte
1010   character sequences that contain the octet LF (%x0A).  String-based
1011   parsers can only be safely used within protocol elements after the element
1012   has been extracted from the message, such as within a header field-value
1013   after message parsing has delineated the individual fields.
1014</t>
1015<t>
1016   An HTTP message can be parsed as a stream for incremental processing or
1017   forwarding downstream.  However, recipients cannot rely on incremental
1018   delivery of partial messages, since some implementations will buffer or
1019   delay message forwarding for the sake of network efficiency, security
1020   checks, or payload transformations.
1021</t>
1022
1023<section title="Start Line" anchor="start.line">
1024  <x:anchor-alias value="Start-Line"/>
1025<t>
1026   An HTTP message can either be a request from client to server or a
1027   response from server to client.  Syntactically, the two types of message
1028   differ only in the start-line, which is either a request-line (for requests)
1029   or a status-line (for responses), and in the algorithm for determining
1030   the length of the message body (<xref target="message.body"/>).
1031</t>
1032<t>
1033   In theory, a client could receive requests and a server could receive
1034   responses, distinguishing them by their different start-line formats,
1035   but in practice servers are implemented to only expect a request
1036   (a response is interpreted as an unknown or invalid request method)
1037   and clients are implemented to only expect a response.
1038</t>
1039<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="start-line"/>
1040  <x:ref>start-line</x:ref>     = <x:ref>request-line</x:ref> / <x:ref>status-line</x:ref>
1041</artwork></figure>
1042<t>
1043   A sender &MUST-NOT; send whitespace between the start-line and
1044   the first header field. The presence of such whitespace in a request
1045   might be an attempt to trick a server into ignoring that field or
1046   processing the line after it as a new request, either of which might
1047   result in a security vulnerability if other implementations within
1048   the request chain interpret the same message differently.
1049   Likewise, the presence of such whitespace in a response might be
1050   ignored by some clients or cause others to cease parsing.
1051</t>
1052<t>
1053   A recipient that receives whitespace between the start-line and
1054   the first header field &MUST; either reject the message as invalid or
1055   consume each whitespace-preceded line without further processing of it
1056   (i.e., ignore the entire line, along with any subsequent lines preceded
1057   by whitespace, until a properly formed header field is received or the
1058   header block is terminated).
1059</t>
1060
1061<section title="Request Line" anchor="request.line">
1062  <x:anchor-alias value="Request"/>
1063  <x:anchor-alias value="request-line"/>
1064<t>
1065   A request-line begins with a method token, followed by a single
1066   space (SP), the request-target, another single space (SP), the
1067   protocol version, and ending with CRLF.
1068</t>
1069<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1070  <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>
1071</artwork></figure>
1072<iref primary="true" item="method"/>
1073<t anchor="method">
1074   The method token indicates the request method to be performed on the
1075   target resource. The request method is case-sensitive.
1076</t>
1077<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1078  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1079</artwork></figure>
1080<t>
1081   The methods defined by this specification can be found in
1082   &methods;, along with information regarding the HTTP method registry
1083   and considerations for defining new methods.
1084</t>
1085<iref item="request-target"/>
1086<t>
1087   The request-target identifies the target resource upon which to apply
1088   the request, as defined in <xref target="request-target"/>.
1089</t>
1090<t>
1091   No whitespace is allowed inside the method, request-target, and
1092   protocol version.  Hence, recipients typically parse the request-line
1093   into its component parts by splitting on whitespace
1094   (see <xref target="message.robustness"/>).
1095</t>
1096<t>
1097   Unfortunately, some user agents fail to properly encode hypertext
1098   references that have embedded whitespace, sending the characters directly
1099   instead of properly encoding or excluding the disallowed characters.
1100   Recipients of an invalid request-line &SHOULD; respond with either a
1101   <x:ref>400 (Bad Request)</x:ref> error or a <x:ref>301 (Moved Permanently)</x:ref>
1102   redirect with the request-target properly encoded.  Recipients &SHOULD-NOT;
1103   attempt to autocorrect and then process the request without a redirect,
1104   since the invalid request-line might be deliberately crafted to bypass
1105   security filters along the request chain.
1106</t>
1107<t>
1108   HTTP does not place a pre-defined limit on the length of a request-line.
1109   A server that receives a method longer than any that it implements
1110   &SHOULD; respond with either a <x:ref>405 (Method Not Allowed)</x:ref>, if it is an origin
1111   server, or a <x:ref>501 (Not Implemented)</x:ref> status code.
1112   A server &MUST; be prepared to receive URIs of unbounded length and
1113   respond with the <x:ref>414 (URI Too Long)</x:ref> status code if the received
1114   request-target would be longer than the server wishes to handle
1115   (see &status-414;).
1116</t>
1117<t>
1118   Various ad-hoc limitations on request-line length are found in practice.
1119   It is &RECOMMENDED; that all HTTP senders and recipients support, at a
1120   minimum, request-line lengths of 8000 octets.
1121</t>
1122</section>
1123
1124<section title="Status Line" anchor="status.line">
1125  <x:anchor-alias value="response"/>
1126  <x:anchor-alias value="status-line"/>
1127  <x:anchor-alias value="status-code"/>
1128  <x:anchor-alias value="reason-phrase"/>
1129<t>
1130   The first line of a response message is the status-line, consisting
1131   of the protocol version, a space (SP), the status code, another space,
1132   a possibly-empty textual phrase describing the status code, and
1133   ending with CRLF.
1134</t>
1135<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-line"/>
1136  <x:ref>status-line</x:ref> = <x:ref>HTTP-version</x:ref> <x:ref>SP</x:ref> <x:ref>status-code</x:ref> <x:ref>SP</x:ref> <x:ref>reason-phrase</x:ref> <x:ref>CRLF</x:ref>
1137</artwork></figure>
1138<t>
1139   The status-code element is a 3-digit integer code describing the
1140   result of the server's attempt to understand and satisfy the client's
1141   corresponding request. The rest of the response message is to be
1142   interpreted in light of the semantics defined for that status code.
1143   See &status-codes; for information about the semantics of status codes,
1144   including the classes of status code (indicated by the first digit),
1145   the status codes defined by this specification, considerations for the
1146   definition of new status codes, and the IANA registry.
1147</t>
1148<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-code"/>
1149  <x:ref>status-code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1150</artwork></figure>
1151<t>   
1152   The reason-phrase element exists for the sole purpose of providing a
1153   textual description associated with the numeric status code, mostly
1154   out of deference to earlier Internet application protocols that were more
1155   frequently used with interactive text clients. A client &SHOULD; ignore
1156   the reason-phrase content.
1157</t>
1158<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
1159  <x:ref>reason-phrase</x:ref>  = *( <x:ref>HTAB</x:ref> / <x:ref>SP</x:ref> / <x:ref>VCHAR</x:ref> / <x:ref>obs-text</x:ref> )
1160</artwork></figure>
1161</section>
1162</section>
1163
1164<section title="Header Fields" anchor="header.fields">
1165  <x:anchor-alias value="header-field"/>
1166  <x:anchor-alias value="field-content"/>
1167  <x:anchor-alias value="field-name"/>
1168  <x:anchor-alias value="field-value"/>
1169  <x:anchor-alias value="obs-fold"/>
1170<t>
1171   Each HTTP header field consists of a case-insensitive field name
1172   followed by a colon (":"), optional whitespace, and the field value.
1173</t>
1174<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="header-field"/><iref primary="true" item="Grammar" subitem="field-name"/><iref primary="true" item="Grammar" subitem="field-value"/><iref primary="true" item="Grammar" subitem="field-content"/><iref primary="true" item="Grammar" subitem="obs-fold"/>
1175  <x:ref>header-field</x:ref>   = <x:ref>field-name</x:ref> ":" <x:ref>OWS</x:ref> <x:ref>field-value</x:ref> <x:ref>BWS</x:ref>
1176  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1177  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>obs-fold</x:ref> )
1178  <x:ref>field-content</x:ref>  = *( <x:ref>HTAB</x:ref> / <x:ref>SP</x:ref> / <x:ref>VCHAR</x:ref> / <x:ref>obs-text</x:ref> )
1179  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref> ( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1180                 ; obsolete line folding
1181                 ; see <xref target="field.parsing"/>
1182</artwork></figure>
1183<t>
1184   The field-name token labels the corresponding field-value as having the
1185   semantics defined by that header field.  For example, the <x:ref>Date</x:ref>
1186   header field is defined in &header-date; as containing the origination
1187   timestamp for the message in which it appears.
1188</t>
1189
1190<section title="Field Extensibility" anchor="field.extensibility">
1191<t>
1192   HTTP header fields are fully extensible: there is no limit on the
1193   introduction of new field names, each presumably defining new semantics,
1194   nor on the number of header fields used in a given message.  Existing
1195   fields are defined in each part of this specification and in many other
1196   specifications outside the core standard.
1197   New header fields can be introduced without changing the protocol version
1198   if their defined semantics allow them to be safely ignored by recipients
1199   that do not recognize them.
1200</t>
1201<t>
1202   New HTTP header fields &SHOULD; be registered with IANA in the
1203   Message Header Field Registry, as described in &iana-header-registry;.
1204   Unrecognized header fields &MUST; be forwarded by a proxy unless the
1205   field-name is listed in the <x:ref>Connection</x:ref> header field
1206   (<xref target="header.connection"/>) or the proxy is specifically
1207   configured to block or otherwise transform such fields.
1208   Unrecognized header fields &SHOULD; be ignored by other recipients.
1209</t>
1210</section>
1211
1212<section title="Field Order" anchor="field.order">
1213<t>
1214   The order in which header fields with differing field names are
1215   received is not significant. However, it is "good practice" to send
1216   header fields that contain control data first, such as <x:ref>Host</x:ref>
1217   on requests and <x:ref>Date</x:ref> on responses, so that implementations
1218   can decide when not to handle a message as early as possible.  A server
1219   &MUST; wait until the entire header section is received before interpreting
1220   a request message, since later header fields might include conditionals,
1221   authentication credentials, or deliberately misleading duplicate
1222   header fields that would impact request processing.
1223</t>
1224<t>
1225   Multiple header fields with the same field name &MUST-NOT; be
1226   sent in a message unless the entire field value for that
1227   header field is defined as a comma-separated list [i.e., #(values)].
1228</t>
1229<t>
1230   Multiple header fields with the same field name can be combined into
1231   one "field-name: field-value" pair, without changing the semantics of the
1232   message, by appending each subsequent field value to the combined
1233   field value in order, separated by a comma. The order in which
1234   header fields with the same field name are received is therefore
1235   significant to the interpretation of the combined field value;
1236   a proxy &MUST-NOT; change the order of these field values when
1237   forwarding a message.
1238</t>
1239<x:note>
1240  <t>
1241   &Note; In practice, the "Set-Cookie" header field (<xref target="RFC6265"/>)
1242   often appears multiple times in a response message and does not use the
1243   list syntax, violating the above requirements on multiple header fields
1244   with the same name. Since it cannot be combined into a single field-value,
1245   recipients ought to handle "Set-Cookie" as a special case while processing
1246   header fields. (See Appendix A.2.3 of <xref target="Kri2001"/> for details.)
1247  </t>
1248</x:note>
1249</section>
1250
1251<section title="Whitespace" anchor="whitespace">
1252<t anchor="rule.LWS">
1253   This specification uses three rules to denote the use of linear
1254   whitespace: OWS (optional whitespace), RWS (required whitespace), and
1255   BWS ("bad" whitespace).
1256</t>
1257<t anchor="rule.OWS">
1258   The OWS rule is used where zero or more linear whitespace octets might
1259   appear. OWS &SHOULD; either not be generated or be generated as a single
1260   SP. Multiple OWS octets that occur within field-content &SHOULD; either
1261   be replaced with a single SP or transformed to all SP octets (each
1262   octet other than SP replaced with SP) before interpreting the field value
1263   or forwarding the message downstream.
1264</t>
1265<t anchor="rule.RWS">
1266   RWS is used when at least one linear whitespace octet is required to
1267   separate field tokens. RWS &SHOULD; be generated as a single SP.
1268   Multiple RWS octets that occur within field-content &SHOULD; either
1269   be replaced with a single SP or transformed to all SP octets before
1270   interpreting the field value or forwarding the message downstream.
1271</t>
1272<t anchor="rule.BWS">
1273   BWS is used where the grammar allows optional whitespace, for historical
1274   reasons, but senders &SHOULD-NOT; generate it in messages;
1275   recipients &MUST; accept such bad optional whitespace and remove it before
1276   interpreting the field value or forwarding the message downstream.
1277</t>
1278<t anchor="rule.whitespace">
1279  <x:anchor-alias value="BWS"/>
1280  <x:anchor-alias value="OWS"/>
1281  <x:anchor-alias value="RWS"/>
1282</t>
1283<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"/>
1284  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1285                 ; optional whitespace
1286  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1287                 ; required whitespace
1288  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1289                 ; "bad" whitespace
1290</artwork></figure>
1291</section>
1292
1293<section title="Field Parsing" anchor="field.parsing">
1294<t>
1295   No whitespace is allowed between the header field-name and colon.
1296   In the past, differences in the handling of such whitespace have led to
1297   security vulnerabilities in request routing and response handling.
1298   Any received request message that contains whitespace between a header
1299   field-name and colon &MUST; be rejected with a response code of 400
1300   (Bad Request).  A proxy &MUST; remove any such whitespace from a response
1301   message before forwarding the message downstream.
1302</t>
1303<t>
1304   A field value is preceded by optional whitespace (OWS); a single SP is
1305   preferred. The field value does not include any leading or trailing white
1306   space: OWS occurring before the first non-whitespace octet of the
1307   field value or after the last non-whitespace octet of the field value
1308   is ignored and &SHOULD; be removed before further processing (as this does
1309   not change the meaning of the header field).
1310</t>
1311<t>
1312   Historically, HTTP header field values could be extended over multiple
1313   lines by preceding each extra line with at least one space or horizontal
1314   tab (obs-fold). This specification deprecates such line
1315   folding except within the message/http media type
1316   (<xref target="internet.media.type.message.http"/>).
1317   HTTP senders &MUST-NOT; generate messages that include line folding
1318   (i.e., that contain any field-value that matches the obs-fold rule) unless
1319   the message is intended for packaging within the message/http media type.
1320   HTTP recipients &SHOULD; accept line folding and replace any embedded
1321   obs-fold whitespace with either a single SP or a matching number of SP
1322   octets (to avoid buffer copying) prior to interpreting the field value or
1323   forwarding the message downstream.
1324</t>
1325<t>
1326   Historically, HTTP has allowed field content with text in the ISO-8859-1
1327   <xref target="ISO-8859-1"/> charset, supporting other charsets only
1328   through use of <xref target="RFC2047"/> encoding.
1329   In practice, most HTTP header field values use only a subset of the
1330   US-ASCII charset <xref target="USASCII"/>. Newly defined
1331   header fields &SHOULD; limit their field values to US-ASCII octets.
1332   Recipients &SHOULD; treat other octets in field content (obs-text) as
1333   opaque data.
1334</t>
1335</section>
1336
1337<section title="Field Limits" anchor="field.limits">
1338<t>
1339   HTTP does not place a pre-defined limit on the length of each header field
1340   or on the length of the header block as a whole.  Various ad-hoc
1341   limitations on individual header field length are found in practice,
1342   often depending on the specific field semantics.
1343</t>
1344<t>
1345   A server &MUST; be prepared to receive request header fields of unbounded
1346   length and respond with an appropriate <x:ref>4xx (Client Error)</x:ref>
1347   status code if the received header field(s) are larger than the server
1348   wishes to process.
1349</t>
1350<t>
1351   A client &MUST; be prepared to receive response header fields of unbounded
1352   length. A client &MAY; discard or truncate received header fields that are
1353   larger than the client wishes to process if the field semantics are such
1354   that the dropped value(s) can be safely ignored without changing the
1355   response semantics.
1356</t>
1357</section>
1358
1359<section title="Field value components" anchor="field.components">
1360<t anchor="rule.token.separators">
1361  <x:anchor-alias value="tchar"/>
1362  <x:anchor-alias value="token"/>
1363  <x:anchor-alias value="special"/>
1364  <x:anchor-alias value="word"/>
1365   Many HTTP header field values consist of words (token or quoted-string)
1366   separated by whitespace or special characters. These special characters
1367   &MUST; be in a quoted string to be used within a parameter value (as defined
1368   in <xref target="transfer.codings"/>).
1369</t>
1370<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>
1371  <x:ref>word</x:ref>           = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1372
1373  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1374<!--
1375  IMPORTANT: when editing "tchar" make sure that "special" is updated accordingly!!!
1376 -->
1377  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1378                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1379                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1380                 ; any <x:ref>VCHAR</x:ref>, except <x:ref>special</x:ref>
1381
1382  <x:ref>special</x:ref>        = "(" / ")" / "&lt;" / ">" / "@" / ","
1383                 / ";" / ":" / "\" / DQUOTE / "/" / "["
1384                 / "]" / "?" / "=" / "{" / "}"
1385</artwork></figure>
1386<t anchor="rule.quoted-string">
1387  <x:anchor-alias value="quoted-string"/>
1388  <x:anchor-alias value="qdtext"/>
1389  <x:anchor-alias value="obs-text"/>
1390   A string of text is parsed as a single word if it is quoted using
1391   double-quote marks.
1392</t>
1393<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"/>
1394  <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>
1395  <x:ref>qdtext</x:ref>         = <x:ref>HTAB</x:ref> / <x:ref>SP</x:ref> /%x21 / %x23-5B / %x5D-7E / <x:ref>obs-text</x:ref>
1396  <x:ref>obs-text</x:ref>       = %x80-FF
1397</artwork></figure>
1398<t anchor="rule.quoted-pair">
1399  <x:anchor-alias value="quoted-pair"/>
1400   The backslash octet ("\") can be used as a single-octet
1401   quoting mechanism within quoted-string constructs:
1402</t>
1403<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1404  <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> )
1405</artwork></figure>
1406<t>
1407   Recipients that process the value of a quoted-string &MUST; handle a
1408   quoted-pair as if it were replaced by the octet following the backslash.
1409</t>
1410<t>
1411   Senders &SHOULD-NOT; generate a quoted-pair in a quoted-string except where
1412   necessary to quote DQUOTE and backslash octets occurring within that string.
1413</t>
1414<t anchor="rule.comment">
1415  <x:anchor-alias value="comment"/>
1416  <x:anchor-alias value="ctext"/>
1417   Comments can be included in some HTTP header fields by surrounding
1418   the comment text with parentheses. Comments are only allowed in
1419   fields containing "comment" as part of their field value definition.
1420</t>
1421<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1422  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-cpair</x:ref> / <x:ref>comment</x:ref> ) ")"
1423  <x:ref>ctext</x:ref>          = <x:ref>HTAB</x:ref> / <x:ref>SP</x:ref> / %x21-27 / %x2A-5B / %x5D-7E / <x:ref>obs-text</x:ref>
1424</artwork></figure>
1425<t anchor="rule.quoted-cpair">
1426  <x:anchor-alias value="quoted-cpair"/>
1427   The backslash octet ("\") can be used as a single-octet
1428   quoting mechanism within comment constructs:
1429</t>
1430<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-cpair"/>
1431  <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> )
1432</artwork></figure>
1433<t>
1434   Senders &SHOULD-NOT; escape octets in comments that do not require escaping
1435   (i.e., other than the backslash octet "\" and the parentheses "(" and ")").
1436</t>
1437</section>
1438
1439</section>
1440
1441<section title="Message Body" anchor="message.body">
1442  <x:anchor-alias value="message-body"/>
1443<t>
1444   The message body (if any) of an HTTP message is used to carry the
1445   payload body of that request or response.  The message body is
1446   identical to the payload body unless a transfer coding has been
1447   applied, as described in <xref target="header.transfer-encoding"/>.
1448</t>
1449<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1450  <x:ref>message-body</x:ref> = *OCTET
1451</artwork></figure>
1452<t>
1453   The rules for when a message body is allowed in a message differ for
1454   requests and responses.
1455</t>
1456<t>
1457   The presence of a message body in a request is signaled by a
1458   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1459   field. Request message framing is independent of method semantics,
1460   even if the method does not define any use for a message body.
1461</t>
1462<t>
1463   The presence of a message body in a response depends on both
1464   the request method to which it is responding and the response
1465   status code (<xref target="status.line"/>).
1466   Responses to the HEAD request method never include a message body
1467   because the associated response header fields (e.g.,
1468   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1469   if present, indicate only what their values would have been if the request
1470   method had been GET (&HEAD;).
1471   <x:ref>2xx (Successful)</x:ref> responses to CONNECT switch to tunnel
1472   mode instead of having a message body (&CONNECT;).
1473   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1474   <x:ref>304 (Not Modified)</x:ref> responses &MUST-NOT; include a message body.
1475   All other responses do include a message body, although the body
1476   &MAY; be of zero length.
1477</t>
1478
1479<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1480  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1481  <iref item="chunked (Coding Format)"/>
1482  <x:anchor-alias value="Transfer-Encoding"/>
1483<t>
1484   The Transfer-Encoding header field lists the transfer coding names
1485   corresponding to the sequence of transfer codings that have been
1486   (or will be) applied to the payload body in order to form the message body.
1487   Transfer codings are defined in <xref target="transfer.codings"/>.
1488</t>
1489<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1490  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1491</artwork></figure>
1492<t>
1493   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1494   MIME, which was designed to enable safe transport of binary data over a
1495   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1496   However, safe transport has a different focus for an 8bit-clean transfer
1497   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1498   accurately delimit a dynamically generated payload and to distinguish
1499   payload encodings that are only applied for transport efficiency or
1500   security from those that are characteristics of the selected resource.
1501</t>
1502<t>
1503   All HTTP/1.1 recipients &MUST; implement the chunked transfer coding
1504   (<xref target="chunked.encoding"/>) because it plays a crucial role in
1505   framing messages when the payload body size is not known in advance.
1506   If chunked is applied to a payload body, the sender &MUST-NOT; apply
1507   chunked more than once (i.e., chunking an already chunked message is not
1508   allowed).
1509   If any transfer coding is applied to a request payload body, the
1510   sender &MUST; apply chunked as the final transfer coding to ensure that
1511   the message is properly framed.
1512   If any transfer coding is applied to a response payload body, the
1513   sender &MUST; either apply chunked as the final transfer coding or
1514   terminate the message by closing the connection.
1515</t>
1516<figure><preamble>
1517   For example,
1518</preamble><artwork type="example">
1519  Transfer-Encoding: gzip, chunked
1520</artwork><postamble>
1521   indicates that the payload body has been compressed using the gzip
1522   coding and then chunked using the chunked coding while forming the
1523   message body.
1524</postamble></figure>
1525<t>
1526   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1527   Transfer-Encoding is a property of the message, not of the payload, and
1528   any recipient along the request/response chain &MAY; decode the received
1529   transfer coding(s) or apply additional transfer coding(s) to the message
1530   body, assuming that corresponding changes are made to the Transfer-Encoding
1531   field-value. Additional information about the encoding parameters &MAY; be
1532   provided by other header fields not defined by this specification.
1533</t>
1534<t>
1535   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1536   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1537   neither of which includes a message body,
1538   to indicate that the origin server would have applied a transfer coding
1539   to the message body if the request had been an unconditional GET.
1540   This indication is not required, however, because any recipient on
1541   the response chain (including the origin server) can remove transfer
1542   codings when they are not needed.
1543</t>
1544<t>
1545   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1546   implementations advertising only HTTP/1.0 support will not understand
1547   how to process a transfer-encoded payload.
1548   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1549   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1550   might be in the form of specific user configuration or by remembering the
1551   version of a prior received response.
1552   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1553   the corresponding request indicates HTTP/1.1 (or later).
1554</t>
1555<t>
1556   A server that receives a request message with a transfer coding it does
1557   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref>.
1558</t>
1559</section>
1560
1561<section title="Content-Length" anchor="header.content-length">
1562  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1563  <x:anchor-alias value="Content-Length"/>
1564<t>
1565   When a message does not have a <x:ref>Transfer-Encoding</x:ref> header
1566   field, a Content-Length header field can provide the anticipated size,
1567   as a decimal number of octets, for a potential payload body.
1568   For messages that do include a payload body, the Content-Length field-value
1569   provides the framing information necessary for determining where the body
1570   (and message) ends.  For messages that do not include a payload body, the
1571   Content-Length indicates the size of the selected representation
1572   (&selected-representation;).
1573</t>
1574<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1575  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1576</artwork></figure>
1577<t>
1578   An example is
1579</t>
1580<figure><artwork type="example">
1581  Content-Length: 3495
1582</artwork></figure>
1583<t>
1584   A sender &MUST-NOT; send a Content-Length header field in any message that
1585   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1586</t>
1587<t>
1588   A user agent &SHOULD; send a Content-Length in a request message when no
1589   <x:ref>Transfer-Encoding</x:ref> is sent and the request method defines
1590   a meaning for an enclosed payload body. For example, a Content-Length
1591   header field is normally sent in a POST request even when the value is
1592   0 (indicating an empty payload body).  A user agent &SHOULD-NOT; send a
1593   Content-Length header field when the request message does not contain a
1594   payload body and the method semantics do not anticipate such a body.
1595</t>
1596<t>
1597   A server &MAY; send a Content-Length header field in a response to a HEAD
1598   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1599   response unless its field-value equals the decimal number of octets that
1600   would have been sent in the payload body of a response if the same
1601   request had used the GET method.
1602</t>
1603<t>
1604   A server &MAY; send a Content-Length header field in a
1605   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1606   (&status-304;); a server &MUST-NOT; send Content-Length in such a
1607   response unless its field-value equals the decimal number of octets that
1608   would have been sent in the payload body of a <x:ref>200 (OK)</x:ref>
1609   response to the same request.
1610</t>
1611<t>
1612   A server &MUST-NOT; send a Content-Length header field in any response
1613   with a status code of
1614   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1615   A server &SHOULD-NOT; send a Content-Length header field in any
1616   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1617</t>
1618<t>
1619   Aside from the cases defined above, in the absence of Transfer-Encoding,
1620   an origin server &SHOULD; send a Content-Length header field when the
1621   payload body size is known prior to sending the complete header block.
1622   This will allow downstream recipients to measure transfer progress,
1623   know when a received message is complete, and potentially reuse the
1624   connection for additional requests.
1625</t>
1626<t>
1627   Any Content-Length field value greater than or equal to zero is valid.
1628   Since there is no predefined limit to the length of an HTTP payload,
1629   recipients &SHOULD; anticipate potentially large decimal numerals and
1630   prevent parsing errors due to integer conversion overflows
1631   (<xref target="attack.protocol.element.size.overflows"/>).
1632</t>
1633<t>
1634   If a message is received that has multiple Content-Length header fields
1635   with field-values consisting of the same decimal value, or a single
1636   Content-Length header field with a field value containing a list of
1637   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1638   duplicate Content-Length header fields have been generated or combined by an
1639   upstream message processor, then the recipient &MUST; either reject the
1640   message as invalid or replace the duplicated field-values with a single
1641   valid Content-Length field containing that decimal value prior to
1642   determining the message body length.
1643</t>
1644<x:note>
1645  <t>
1646   &Note; HTTP's use of Content-Length for message framing differs
1647   significantly from the same field's use in MIME, where it is an optional
1648   field used only within the "message/external-body" media-type.
1649  </t>
1650</x:note>
1651</section>
1652
1653<section title="Message Body Length" anchor="message.body.length">
1654  <iref item="chunked (Coding Format)"/>
1655<t>
1656   The length of a message body is determined by one of the following
1657   (in order of precedence):
1658</t>
1659<t>
1660  <list style="numbers">
1661    <x:lt><t>
1662     Any response to a HEAD request and any response with a
1663     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1664     <x:ref>304 (Not Modified)</x:ref> status code is always
1665     terminated by the first empty line after the header fields, regardless of
1666     the header fields present in the message, and thus cannot contain a
1667     message body.
1668    </t></x:lt>
1669    <x:lt><t>
1670     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1671     connection will become a tunnel immediately after the empty line that
1672     concludes the header fields.  A client &MUST; ignore any
1673     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1674     fields received in such a message.
1675    </t></x:lt>
1676    <x:lt><t>
1677     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1678     and the chunked transfer coding (<xref target="chunked.encoding"/>)
1679     is the final encoding, the message body length is determined by reading
1680     and decoding the chunked data until the transfer coding indicates the
1681     data is complete.
1682    </t>
1683    <t>
1684     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1685     response and the chunked transfer coding is not the final encoding, the
1686     message body length is determined by reading the connection until it is
1687     closed by the server.
1688     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a request and the
1689     chunked transfer coding is not the final encoding, the message body
1690     length cannot be determined reliably; the server &MUST; respond with
1691     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1692    </t>
1693    <t>
1694     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1695     and a <x:ref>Content-Length</x:ref> header field, the Transfer-Encoding
1696     overrides the Content-Length. Such a message might indicate an attempt
1697     to perform request or response smuggling (bypass of security-related
1698     checks on message routing or content) and thus ought to be handled as
1699     an error.  A sender &MUST; remove the received Content-Length field
1700     prior to forwarding such a message downstream.
1701    </t></x:lt>
1702    <x:lt><t>
1703     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1704     either multiple <x:ref>Content-Length</x:ref> header fields having
1705     differing field-values or a single Content-Length header field having an
1706     invalid value, then the message framing is invalid and &MUST; be treated
1707     as an error to prevent request or response smuggling.
1708     If this is a request message, the server &MUST; respond with
1709     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1710     If this is a response message received by a proxy, the proxy
1711     &MUST; discard the received response, send a <x:ref>502 (Bad Gateway)</x:ref>
1712     status code as its downstream response, and then close the connection.
1713     If this is a response message received by a user agent, it &MUST; be
1714     treated as an error by discarding the message and closing the connection.
1715    </t></x:lt>
1716    <x:lt><t>
1717     If a valid <x:ref>Content-Length</x:ref> header field is present without
1718     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1719     message body length in octets.  If the actual number of octets sent in
1720     the message is less than the indicated Content-Length, the recipient
1721     &MUST; consider the message to be incomplete and treat the connection
1722     as no longer usable.
1723     If the actual number of octets sent in the message is more than the indicated
1724     Content-Length, the recipient &MUST; only process the message body up to the
1725     field value's number of octets; the remainder of the message &MUST; either
1726     be discarded or treated as the next message in a pipeline.  For the sake of
1727     robustness, a user agent &MAY; attempt to detect and correct such an error
1728     in message framing if it is parsing the response to the last request on
1729     a connection and the connection has been closed by the server.
1730    </t></x:lt>
1731    <x:lt><t>
1732     If this is a request message and none of the above are true, then the
1733     message body length is zero (no message body is present).
1734    </t></x:lt>
1735    <x:lt><t>
1736     Otherwise, this is a response message without a declared message body
1737     length, so the message body length is determined by the number of octets
1738     received prior to the server closing the connection.
1739    </t></x:lt>
1740  </list>
1741</t>
1742<t>
1743   Since there is no way to distinguish a successfully completed,
1744   close-delimited message from a partially-received message interrupted
1745   by network failure, a server &SHOULD; use encoding or
1746   length-delimited messages whenever possible.  The close-delimiting
1747   feature exists primarily for backwards compatibility with HTTP/1.0.
1748</t>
1749<t>
1750   A server &MAY; reject a request that contains a message body but
1751   not a <x:ref>Content-Length</x:ref> by responding with
1752   <x:ref>411 (Length Required)</x:ref>.
1753</t>
1754<t>
1755   Unless a transfer coding other than chunked has been applied,
1756   a client that sends a request containing a message body &SHOULD;
1757   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1758   length is known in advance, rather than the chunked transfer coding, since some
1759   existing services respond to chunked with a <x:ref>411 (Length Required)</x:ref>
1760   status code even though they understand the chunked transfer coding.  This
1761   is typically because such services are implemented via a gateway that
1762   requires a content-length in advance of being called and the server
1763   is unable or unwilling to buffer the entire request before processing.
1764</t>
1765<t>
1766   A client that sends a request containing a message body &MUST; include a
1767   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1768   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1769   the form of specific user configuration or by remembering the version of a
1770   prior received response.
1771</t>
1772</section>
1773</section>
1774
1775<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1776<t>
1777   A server that receives an incomplete request message, usually due to a
1778   canceled request or a triggered time-out exception, &MAY; send an error
1779   response prior to closing the connection.
1780</t>
1781<t>
1782   A client that receives an incomplete response message, which can occur
1783   when a connection is closed prematurely or when decoding a supposedly
1784   chunked transfer coding fails, &MUST; record the message as incomplete.
1785   Cache requirements for incomplete responses are defined in
1786   &cache-incomplete;.
1787</t>
1788<t>
1789   If a response terminates in the middle of the header block (before the
1790   empty line is received) and the status code might rely on header fields to
1791   convey the full meaning of the response, then the client cannot assume
1792   that meaning has been conveyed; the client might need to repeat the
1793   request in order to determine what action to take next.
1794</t>
1795<t>
1796   A message body that uses the chunked transfer coding is
1797   incomplete if the zero-sized chunk that terminates the encoding has not
1798   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1799   incomplete if the size of the message body received (in octets) is less than
1800   the value given by Content-Length.  A response that has neither chunked
1801   transfer coding nor Content-Length is terminated by closure of the
1802   connection, and thus is considered complete regardless of the number of
1803   message body octets received, provided that the header block was received
1804   intact.
1805</t>
1806</section>
1807
1808<section title="Message Parsing Robustness" anchor="message.robustness">
1809<t>
1810   Older HTTP/1.0 user agent implementations might send an extra CRLF
1811   after a POST request as a lame workaround for some early server
1812   applications that failed to read message body content that was
1813   not terminated by a line-ending. An HTTP/1.1 user agent &MUST-NOT;
1814   preface or follow a request with an extra CRLF.  If terminating
1815   the request message body with a line-ending is desired, then the
1816   user agent &MUST; include the terminating CRLF octets as part of the
1817   message body length.
1818</t>
1819<t>
1820   In the interest of robustness, servers &SHOULD; ignore at least one
1821   empty line received where a request-line is expected. In other words, if
1822   a server is reading the protocol stream at the beginning of a
1823   message and receives a CRLF first, the server &SHOULD; ignore the CRLF.
1824</t>
1825<t>
1826   Although the line terminator for the start-line and header
1827   fields is the sequence CRLF, recipients &MAY; recognize a
1828   single LF as a line terminator and ignore any preceding CR.
1829</t>
1830<t>
1831   Although the request-line and status-line grammar rules require that each
1832   of the component elements be separated by a single SP octet, recipients
1833   &MAY; instead parse on whitespace-delimited word boundaries and, aside
1834   from the CRLF terminator, treat any form of whitespace as the SP separator
1835   while ignoring preceding or trailing whitespace;
1836   such whitespace includes one or more of the following octets:
1837   SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
1838</t>
1839<t>
1840   When a server listening only for HTTP request messages, or processing
1841   what appears from the start-line to be an HTTP request message,
1842   receives a sequence of octets that does not match the HTTP-message
1843   grammar aside from the robustness exceptions listed above, the
1844   server &SHOULD; respond with a <x:ref>400 (Bad Request)</x:ref> response. 
1845</t>
1846</section>
1847</section>
1848
1849<section title="Transfer Codings" anchor="transfer.codings">
1850  <x:anchor-alias value="transfer-coding"/>
1851  <x:anchor-alias value="transfer-extension"/>
1852<t>
1853   Transfer coding names are used to indicate an encoding
1854   transformation that has been, can be, or might need to be applied to a
1855   payload body in order to ensure "safe transport" through the network.
1856   This differs from a content coding in that the transfer coding is a
1857   property of the message rather than a property of the representation
1858   that is being transferred.
1859</t>
1860<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1861  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1862                     / "compress" ; <xref target="compress.coding"/>
1863                     / "deflate" ; <xref target="deflate.coding"/>
1864                     / "gzip" ; <xref target="gzip.coding"/>
1865                     / <x:ref>transfer-extension</x:ref>
1866  <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> )
1867</artwork></figure>
1868<t anchor="rule.parameter">
1869  <x:anchor-alias value="attribute"/>
1870  <x:anchor-alias value="transfer-parameter"/>
1871  <x:anchor-alias value="value"/>
1872   Parameters are in the form of attribute/value pairs.
1873</t>
1874<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"/>
1875  <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>
1876  <x:ref>attribute</x:ref>          = <x:ref>token</x:ref>
1877  <x:ref>value</x:ref>              = <x:ref>word</x:ref>
1878</artwork></figure>
1879<t>
1880   All transfer-coding names are case-insensitive and &SHOULD; be registered
1881   within the HTTP Transfer Coding registry, as defined in
1882   <xref target="transfer.coding.registry"/>.
1883   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1884   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1885   header fields.
1886</t>
1887
1888<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1889  <iref primary="true" item="chunked (Coding Format)"/>
1890  <x:anchor-alias value="chunk"/>
1891  <x:anchor-alias value="chunked-body"/>
1892  <x:anchor-alias value="chunk-data"/>
1893  <x:anchor-alias value="chunk-ext"/>
1894  <x:anchor-alias value="chunk-ext-name"/>
1895  <x:anchor-alias value="chunk-ext-val"/>
1896  <x:anchor-alias value="chunk-size"/>
1897  <x:anchor-alias value="last-chunk"/>
1898  <x:anchor-alias value="trailer-part"/>
1899  <x:anchor-alias value="quoted-str-nf"/>
1900  <x:anchor-alias value="qdtext-nf"/>
1901<t>
1902   The chunked transfer coding modifies the body of a message in order to
1903   transfer it as a series of chunks, each with its own size indicator,
1904   followed by an &OPTIONAL; trailer containing header fields. This
1905   allows dynamically generated content to be transferred along with the
1906   information necessary for the recipient to verify that it has
1907   received the full message.
1908</t>
1909<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"/>
1910  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1911                   <x:ref>last-chunk</x:ref>
1912                   <x:ref>trailer-part</x:ref>
1913                   <x:ref>CRLF</x:ref>
1914 
1915  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1916                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1917  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1918  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1919 
1920  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
1921  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
1922  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-str-nf</x:ref>
1923  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1924  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1925 
1926  <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>
1927                 ; like <x:ref>quoted-string</x:ref>, but disallowing line folding
1928  <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>
1929</artwork></figure>
1930<t>
1931   Chunk extensions within the chunked transfer coding are deprecated.
1932   Senders &SHOULD-NOT; send chunk-ext.
1933   Definition of new chunk extensions is discouraged.
1934</t>
1935<t>
1936   The chunk-size field is a string of hex digits indicating the size of
1937   the chunk-data in octets. The chunked transfer coding is complete when a
1938   chunk with a chunk-size of zero is received, possibly followed by a
1939   trailer, and finally terminated by an empty line.
1940</t>
1941
1942<section title="Trailer" anchor="header.trailer">
1943  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
1944  <x:anchor-alias value="Trailer"/>
1945<t>
1946   A trailer allows the sender to include additional fields at the end of a
1947   chunked message in order to supply metadata that might be dynamically
1948   generated while the message body is sent, such as a message integrity
1949   check, digital signature, or post-processing status.
1950   The trailer &MUST-NOT; contain fields that need to be known before a
1951   recipient processes the body, such as <x:ref>Transfer-Encoding</x:ref>,
1952   <x:ref>Content-Length</x:ref>, and <x:ref>Trailer</x:ref>.
1953</t>
1954<t>
1955   When a message includes a message body encoded with the chunked
1956   transfer coding and the sender desires to send metadata in the form of
1957   trailer fields at the end of the message, the sender &SHOULD; send a
1958   <x:ref>Trailer</x:ref> header field before the message body to indicate
1959   which fields will be present in the trailers. This allows the recipient
1960   to prepare for receipt of that metadata before it starts processing the body,
1961   which is useful if the message is being streamed and the recipient wishes
1962   to confirm an integrity check on the fly.
1963</t>
1964<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
1965  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
1966</artwork></figure>
1967<t>
1968   If no <x:ref>Trailer</x:ref> header field is present, the sender of a
1969   chunked message body &SHOULD; send an empty trailer.
1970</t>
1971<t>
1972   A server &MUST; send an empty trailer with the chunked transfer coding
1973   unless at least one of the following is true:
1974  <list style="numbers">
1975    <t>the request included a <x:ref>TE</x:ref> header field that indicates
1976    "trailers" is acceptable in the transfer coding of the response, as
1977    described in <xref target="header.te"/>; or,</t>
1978     
1979    <t>the trailer fields consist entirely of optional metadata and the
1980    recipient could use the message (in a manner acceptable to the server where
1981    the field originated) without receiving that metadata. In other words,
1982    the server that generated the header field is willing to accept the
1983    possibility that the trailer fields might be silently discarded along
1984    the path to the client.</t>
1985  </list>
1986</t>
1987<t>
1988   The above requirement prevents the need for an infinite buffer when a
1989   message is being received by an HTTP/1.1 (or later) proxy and forwarded to
1990   an HTTP/1.0 recipient.
1991</t>
1992</section>
1993
1994<section title="Decoding chunked" anchor="decoding.chunked">
1995<t>
1996   A process for decoding the chunked transfer coding
1997   can be represented in pseudo-code as:
1998</t>
1999<figure><artwork type="code">
2000  length := 0
2001  read chunk-size, chunk-ext (if any) and CRLF
2002  while (chunk-size &gt; 0) {
2003     read chunk-data and CRLF
2004     append chunk-data to decoded-body
2005     length := length + chunk-size
2006     read chunk-size and CRLF
2007  }
2008  read header-field
2009  while (header-field not empty) {
2010     append header-field to existing header fields
2011     read header-field
2012  }
2013  Content-Length := length
2014  Remove "chunked" from Transfer-Encoding
2015  Remove Trailer from existing header fields
2016</artwork></figure>
2017<t>
2018   All recipients &MUST; be able to receive and decode the
2019   chunked transfer coding and &MUST; ignore chunk-ext extensions
2020   they do not understand.
2021</t>
2022</section>
2023</section>
2024
2025<section title="Compression Codings" anchor="compression.codings">
2026<t>
2027   The codings defined below can be used to compress the payload of a
2028   message.
2029</t>
2030
2031<section title="Compress Coding" anchor="compress.coding">
2032<iref item="compress (Coding Format)"/>
2033<t>
2034   The "compress" format is produced by the common UNIX file compression
2035   program "compress". This format is an adaptive Lempel-Ziv-Welch
2036   coding (LZW). Recipients &SHOULD; consider "x-compress" to be
2037   equivalent to "compress".
2038</t>
2039</section>
2040
2041<section title="Deflate Coding" anchor="deflate.coding">
2042<iref item="deflate (Coding Format)"/>
2043<t>
2044   The "deflate" format is defined as the "deflate" compression mechanism
2045   (described in <xref target="RFC1951"/>) used inside the "zlib"
2046   data format (<xref target="RFC1950"/>).
2047</t>
2048<x:note>
2049  <t>
2050    &Note; Some incorrect implementations send the "deflate"
2051    compressed data without the zlib wrapper.
2052   </t>
2053</x:note>
2054</section>
2055
2056<section title="Gzip Coding" anchor="gzip.coding">
2057<iref item="gzip (Coding Format)"/>
2058<t>
2059   The "gzip" format is produced by the file compression program
2060   "gzip" (GNU zip), as described in <xref target="RFC1952"/>. This format is a
2061   Lempel-Ziv coding (LZ77) with a 32 bit CRC.
2062   Recipients &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2063</t>
2064</section>
2065
2066</section>
2067
2068<section title="TE" anchor="header.te">
2069  <iref primary="true" item="TE header field" x:for-anchor=""/>
2070  <x:anchor-alias value="TE"/>
2071  <x:anchor-alias value="t-codings"/>
2072  <x:anchor-alias value="t-ranking"/>
2073  <x:anchor-alias value="rank"/>
2074<t>
2075   The "TE" header field in a request indicates what transfer codings,
2076   besides chunked, the client is willing to accept in response, and
2077   whether or not the client is willing to accept trailer fields in a
2078   chunked transfer coding.
2079</t>
2080<t>
2081   The TE field-value consists of a comma-separated list of transfer coding
2082   names, each allowing for optional parameters (as described in
2083   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2084   Clients &MUST-NOT; send the chunked transfer coding name in TE;
2085   chunked is always acceptable for HTTP/1.1 recipients.
2086</t>
2087<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"/>
2088  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2089  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2090  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2091  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2092             / ( "1" [ "." 0*3("0") ] )
2093</artwork></figure>
2094<t>
2095   Three examples of TE use are below.
2096</t>
2097<figure><artwork type="example">
2098  TE: deflate
2099  TE:
2100  TE: trailers, deflate;q=0.5
2101</artwork></figure>
2102<t>
2103   The presence of the keyword "trailers" indicates that the client is
2104   willing to accept trailer fields in a chunked transfer coding,
2105   as defined in <xref target="chunked.encoding"/>, on behalf of itself and
2106   any downstream clients. For chained requests, this implies that either:
2107   (a) all downstream clients are willing to accept trailer fields in the
2108   forwarded response; or,
2109   (b) the client will attempt to buffer the response on behalf of downstream
2110   recipients.
2111   Note that HTTP/1.1 does not define any means to limit the size of a
2112   chunked response such that a client can be assured of buffering the
2113   entire response.
2114</t>
2115<t>
2116   When multiple transfer codings are acceptable, the client &MAY; rank the
2117   codings by preference using a case-insensitive "q" parameter (similar to
2118   the qvalues used in content negotiation fields, &qvalue;). The rank value
2119   is a real number in the range 0 through 1, where 0.001 is the least
2120   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2121</t>
2122<t>
2123   If the TE field-value is empty or if no TE field is present, the only
2124   acceptable transfer coding is chunked. A message with no transfer coding
2125   is always acceptable.
2126</t>
2127<t>
2128   Since the TE header field only applies to the immediate connection,
2129   a sender of TE &MUST; also send a "TE" connection option within the
2130   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2131   in order to prevent the TE field from being forwarded by intermediaries
2132   that do not support its semantics.
2133</t>
2134</section>
2135</section>
2136
2137<section title="Message Routing" anchor="message.routing">
2138<t>
2139   HTTP request message routing is determined by each client based on the
2140   target resource, the client's proxy configuration, and
2141   establishment or reuse of an inbound connection.  The corresponding
2142   response routing follows the same connection chain back to the client.
2143</t>
2144
2145<section title="Identifying a Target Resource" anchor="target-resource">
2146  <iref primary="true" item="target resource"/>
2147  <iref primary="true" item="target URI"/>
2148  <x:anchor-alias value="target resource"/>
2149  <x:anchor-alias value="target URI"/>
2150<t>
2151   HTTP is used in a wide variety of applications, ranging from
2152   general-purpose computers to home appliances.  In some cases,
2153   communication options are hard-coded in a client's configuration.
2154   However, most HTTP clients rely on the same resource identification
2155   mechanism and configuration techniques as general-purpose Web browsers.
2156</t>
2157<t>
2158   HTTP communication is initiated by a user agent for some purpose.
2159   The purpose is a combination of request semantics, which are defined in
2160   <xref target="Part2"/>, and a target resource upon which to apply those
2161   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2162   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2163   would resolve to its absolute form in order to obtain the
2164   "<x:dfn>target URI</x:dfn>".  The target URI
2165   excludes the reference's fragment identifier component, if any,
2166   since fragment identifiers are reserved for client-side processing
2167   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2168</t>
2169</section>
2170
2171<section title="Connecting Inbound" anchor="connecting.inbound">
2172<t>
2173   Once the target URI is determined, a client needs to decide whether
2174   a network request is necessary to accomplish the desired semantics and,
2175   if so, where that request is to be directed.
2176</t>
2177<t>
2178   If the client has a response cache and the request semantics can be
2179   satisfied by a cache (<xref target="Part6"/>), then the request is
2180   usually directed to the cache first.
2181</t>
2182<t>
2183   If the request is not satisfied by a cache, then a typical client will
2184   check its configuration to determine whether a proxy is to be used to
2185   satisfy the request.  Proxy configuration is implementation-dependent,
2186   but is often based on URI prefix matching, selective authority matching,
2187   or both, and the proxy itself is usually identified by an "http" or
2188   "https" URI.  If a proxy is applicable, the client connects inbound by
2189   establishing (or reusing) a connection to that proxy.
2190</t>
2191<t>
2192   If no proxy is applicable, a typical client will invoke a handler routine,
2193   usually specific to the target URI's scheme, to connect directly
2194   to an authority for the target resource.  How that is accomplished is
2195   dependent on the target URI scheme and defined by its associated
2196   specification, similar to how this specification defines origin server
2197   access for resolution of the "http" (<xref target="http.uri"/>) and
2198   "https" (<xref target="https.uri"/>) schemes.
2199</t>
2200<t>
2201   HTTP requirements regarding connection management are defined in
2202   <xref target="connection.management"/>.
2203</t>
2204</section>
2205
2206<section title="Request Target" anchor="request-target">
2207<t>
2208   Once an inbound connection is obtained,
2209   the client sends an HTTP request message (<xref target="http.message"/>)
2210   with a request-target derived from the target URI.
2211   There are four distinct formats for the request-target, depending on both
2212   the method being requested and whether the request is to a proxy.
2213</t>   
2214<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"/>
2215  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2216                 / <x:ref>absolute-form</x:ref>
2217                 / <x:ref>authority-form</x:ref>
2218                 / <x:ref>asterisk-form</x:ref>
2219
2220  <x:ref>origin-form</x:ref>    = <x:ref>path-absolute</x:ref> [ "?" <x:ref>query</x:ref> ]
2221  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2222  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2223  <x:ref>asterisk-form</x:ref>  = "*"
2224</artwork></figure>
2225<t anchor="origin-form"><iref item="origin-form (of request-target)"/>
2226   The most common form of request-target is the origin-form.
2227   When making a request directly to an origin server, other than a CONNECT
2228   or server-wide OPTIONS request (as detailed below),
2229   a client &MUST; send only the absolute path and query components of
2230   the target URI as the request-target.
2231   If the target URI's path component is empty, then the client &MUST; send
2232   "/" as the path within the origin-form of request-target.
2233   A <x:ref>Host</x:ref> header field is also sent, as defined in
2234   <xref target="header.host"/>, containing the target URI's
2235   authority component (excluding any userinfo).
2236</t>
2237<t>
2238   For example, a client wishing to retrieve a representation of the resource
2239   identified as
2240</t>
2241<figure><artwork x:indent-with="  " type="example">
2242http://www.example.org/where?q=now
2243</artwork></figure>
2244<t>
2245   directly from the origin server would open (or reuse) a TCP connection
2246   to port 80 of the host "www.example.org" and send the lines:
2247</t>
2248<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2249GET /where?q=now HTTP/1.1
2250Host: www.example.org
2251</artwork></figure>
2252<t>
2253   followed by the remainder of the request message.
2254</t>
2255<t anchor="absolute-form"><iref item="absolute-form (of request-target)"/>
2256   When making a request to a proxy, other than a CONNECT or server-wide
2257   OPTIONS request (as detailed below), a client &MUST; send the target URI
2258   in absolute-form as the request-target.
2259   The proxy is requested to either service that request from a valid cache,
2260   if possible, or make the same request on the client's behalf to either
2261   the next inbound proxy server or directly to the origin server indicated
2262   by the request-target.  Requirements on such "forwarding" of messages are
2263   defined in <xref target="message.forwarding"/>.
2264</t>
2265<t>
2266   An example absolute-form of request-line would be:
2267</t>
2268<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2269GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
2270</artwork></figure>
2271<t>
2272   To allow for transition to the absolute-form for all requests in some
2273   future version of HTTP, HTTP/1.1 servers &MUST; accept the absolute-form
2274   in requests, even though HTTP/1.1 clients will only send them in requests
2275   to proxies.
2276</t>
2277<t anchor="authority-form"><iref item="authority-form (of request-target)"/>
2278   The authority-form of request-target is only used for CONNECT requests
2279   (&CONNECT;).  When making a CONNECT request to establish a tunnel through
2280   one or more proxies, a client &MUST; send only the target URI's
2281   authority component (excluding any userinfo) as the request-target.
2282   For example,
2283</t>
2284<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2285CONNECT www.example.com:80 HTTP/1.1
2286</artwork></figure>
2287<t anchor="asterisk-form"><iref item="asterisk-form (of request-target)"/>
2288   The asterisk-form of request-target is only used for a server-wide
2289   OPTIONS request (&OPTIONS;).  When a client wishes to request OPTIONS
2290   for the server as a whole, as opposed to a specific named resource of
2291   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2292   For example,
2293</t>
2294<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2295OPTIONS * HTTP/1.1
2296</artwork></figure>
2297<t>
2298   If a proxy receives an OPTIONS request with an absolute-form of
2299   request-target in which the URI has an empty path and no query component,
2300   then the last proxy on the request chain &MUST; send a request-target
2301   of "*" when it forwards the request to the indicated origin server.
2302</t>
2303<figure><preamble>   
2304   For example, the request
2305</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2306OPTIONS http://www.example.org:8001 HTTP/1.1
2307</artwork></figure>
2308<figure><preamble>   
2309  would be forwarded by the final proxy as
2310</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2311OPTIONS * HTTP/1.1
2312Host: www.example.org:8001
2313</artwork>
2314<postamble>
2315   after connecting to port 8001 of host "www.example.org".
2316</postamble>
2317</figure>
2318</section>
2319
2320<section title="Host" anchor="header.host">
2321  <iref primary="true" item="Host header field" x:for-anchor=""/>
2322  <x:anchor-alias value="Host"/>
2323<t>
2324   The "Host" header field in a request provides the host and port
2325   information from the target URI, enabling the origin
2326   server to distinguish among resources while servicing requests
2327   for multiple host names on a single IP address.  Since the Host
2328   field-value is critical information for handling a request, it
2329   &SHOULD; be sent as the first header field following the request-line.
2330</t>
2331<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2332  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2333</artwork></figure>
2334<t>
2335   A client &MUST; send a Host header field in all HTTP/1.1 request
2336   messages.  If the target URI includes an authority component, then
2337   the Host field-value &MUST; be identical to that authority component
2338   after excluding any userinfo (<xref target="http.uri"/>).
2339   If the authority component is missing or undefined for the target URI,
2340   then the Host header field &MUST; be sent with an empty field-value.
2341</t>
2342<t>
2343   For example, a GET request to the origin server for
2344   &lt;http://www.example.org/pub/WWW/&gt; would begin with:
2345</t>
2346<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2347GET /pub/WWW/ HTTP/1.1
2348Host: www.example.org
2349</artwork></figure>
2350<t>
2351   The Host header field &MUST; be sent in an HTTP/1.1 request even
2352   if the request-target is in the absolute-form, since this
2353   allows the Host information to be forwarded through ancient HTTP/1.0
2354   proxies that might not have implemented Host.
2355</t>
2356<t>
2357   When a proxy receives a request with an absolute-form of
2358   request-target, the proxy &MUST; ignore the received
2359   Host header field (if any) and instead replace it with the host
2360   information of the request-target.  If the proxy forwards the request,
2361   it &MUST; generate a new Host field-value based on the received
2362   request-target rather than forward the received Host field-value.
2363</t>
2364<t>
2365   Since the Host header field acts as an application-level routing
2366   mechanism, it is a frequent target for malware seeking to poison
2367   a shared cache or redirect a request to an unintended server.
2368   An interception proxy is particularly vulnerable if it relies on
2369   the Host field-value for redirecting requests to internal
2370   servers, or for use as a cache key in a shared cache, without
2371   first verifying that the intercepted connection is targeting a
2372   valid IP address for that host.
2373</t>
2374<t>
2375   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2376   to any HTTP/1.1 request message that lacks a Host header field and
2377   to any request message that contains more than one Host header field
2378   or a Host header field with an invalid field-value.
2379</t>
2380</section>
2381
2382<section title="Effective Request URI" anchor="effective.request.uri">
2383  <iref primary="true" item="effective request URI"/>
2384<t>
2385   A server that receives an HTTP request message &MUST; reconstruct
2386   the user agent's original target URI, based on the pieces of information
2387   learned from the request-target, <x:ref>Host</x:ref> header field, and
2388   connection context, in order to identify the intended target resource and
2389   properly service the request. The URI derived from this reconstruction
2390   process is referred to as the "<x:dfn>effective request URI</x:dfn>".
2391</t>
2392<t>
2393   For a user agent, the effective request URI is the target URI.
2394</t>
2395<t>
2396   If the request-target is in absolute-form, then the effective request URI
2397   is the same as the request-target.  Otherwise, the effective request URI
2398   is constructed as follows.
2399</t>
2400<t>
2401   If the request is received over a TLS-secured TCP connection,
2402   then the effective request URI's scheme is "https"; otherwise, the
2403   scheme is "http".
2404</t>
2405<t>
2406   If the request-target is in authority-form, then the effective
2407   request URI's authority component is the same as the request-target.
2408   Otherwise, if a <x:ref>Host</x:ref> header field is supplied with a
2409   non-empty field-value, then the authority component is the same as the
2410   Host field-value. Otherwise, the authority component is the concatenation of
2411   the default host name configured for the server, a colon (":"), and the
2412   connection's incoming TCP port number in decimal form.
2413</t>
2414<t>
2415   If the request-target is in authority-form or asterisk-form, then the
2416   effective request URI's combined path and query component is empty.
2417   Otherwise, the combined path and query component is the same as the
2418   request-target.
2419</t>
2420<t>
2421   The components of the effective request URI, once determined as above,
2422   can be combined into absolute-URI form by concatenating the scheme,
2423   "://", authority, and combined path and query component.
2424</t>
2425<figure>
2426<preamble>
2427   Example 1: the following message received over an insecure TCP connection
2428</preamble> 
2429<artwork type="example" x:indent-with="  ">
2430GET /pub/WWW/TheProject.html HTTP/1.1
2431Host: www.example.org:8080
2432</artwork>
2433</figure>
2434<figure>
2435<preamble>
2436  has an effective request URI of
2437</preamble>
2438<artwork type="example" x:indent-with="  ">
2439http://www.example.org:8080/pub/WWW/TheProject.html
2440</artwork>
2441</figure>
2442<figure>
2443<preamble>
2444   Example 2: the following message received over a TLS-secured TCP connection
2445</preamble> 
2446<artwork type="example" x:indent-with="  ">
2447OPTIONS * HTTP/1.1
2448Host: www.example.org
2449</artwork>
2450</figure>
2451<figure>
2452<preamble>
2453  has an effective request URI of
2454</preamble>
2455<artwork type="example" x:indent-with="  ">
2456https://www.example.org
2457</artwork>
2458</figure>
2459<t>
2460   An origin server that does not allow resources to differ by requested
2461   host &MAY; ignore the <x:ref>Host</x:ref> field-value and instead replace it
2462   with a configured server name when constructing the effective request URI.
2463</t>
2464<t>
2465   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2466   field &MAY; attempt to use heuristics (e.g., examination of the URI path for
2467   something unique to a particular host) in order to guess the
2468   effective request URI's authority component.
2469</t>
2470</section>
2471
2472<section title="Associating a Response to a Request" anchor="associating.response.to.request">
2473<t>
2474   HTTP does not include a request identifier for associating a given
2475   request message with its corresponding one or more response messages.
2476   Hence, it relies on the order of response arrival to correspond exactly
2477   to the order in which requests are made on the same connection.
2478   More than one response message per request only occurs when one or more
2479   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a
2480   final response to the same request.
2481</t>
2482<t>
2483   A client that has more than one outstanding request on a connection &MUST;
2484   maintain a list of outstanding requests in the order sent and &MUST;
2485   associate each received response message on that connection to the highest
2486   ordered request that has not yet received a final (non-<x:ref>1xx</x:ref>)
2487   response.
2488</t>
2489</section>
2490
2491<section title="Message Forwarding" anchor="message.forwarding">
2492<t>
2493   As described in <xref target="intermediaries"/>, intermediaries can serve
2494   a variety of roles in the processing of HTTP requests and responses.
2495   Some intermediaries are used to improve performance or availability.
2496   Others are used for access control or to filter content.
2497   Since an HTTP stream has characteristics similar to a pipe-and-filter
2498   architecture, there are no inherent limits to the extent an intermediary
2499   can enhance (or interfere) with either direction of the stream.
2500</t>
2501<t>
2502   Intermediaries that forward a message &MUST; implement the
2503   <x:ref>Connection</x:ref> header field, as specified in
2504   <xref target="header.connection"/>, to exclude fields that are only
2505   intended for the incoming connection.
2506</t>
2507<t>
2508   In order to avoid request loops, a proxy that forwards requests to other
2509   proxies &MUST; be able to recognize and exclude all of its own server
2510   names, including any aliases, local variations, or literal IP addresses.
2511</t>
2512
2513<section title="Via" anchor="header.via">
2514  <iref primary="true" item="Via header field" x:for-anchor=""/>
2515  <x:anchor-alias value="pseudonym"/>
2516  <x:anchor-alias value="received-by"/>
2517  <x:anchor-alias value="received-protocol"/>
2518  <x:anchor-alias value="Via"/>
2519<t>
2520   The "Via" header field &MUST; be sent by a proxy or gateway in forwarded
2521   messages to indicate the intermediate protocols and recipients between the
2522   user agent and the server on requests, and between the origin server and
2523   the client on responses. It is analogous to the "Received" field
2524   used by email systems (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2525   Via is used in HTTP for tracking message forwards,
2526   avoiding request loops, and identifying the protocol capabilities of
2527   all senders along the request/response chain.
2528</t>
2529<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"/>
2530  <x:ref>Via</x:ref>               = 1#( <x:ref>received-protocol</x:ref> <x:ref>RWS</x:ref> <x:ref>received-by</x:ref>
2531                          [ <x:ref>RWS</x:ref> <x:ref>comment</x:ref> ] )
2532  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2533                      ; see <xref target="header.upgrade"/>
2534  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2535  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2536</artwork></figure>
2537<t>
2538   The received-protocol indicates the protocol version of the message
2539   received by the server or client along each segment of the
2540   request/response chain. The received-protocol version is appended to
2541   the Via field value when the message is forwarded so that information
2542   about the protocol capabilities of upstream applications remains
2543   visible to all recipients.
2544</t>
2545<t>
2546   The protocol-name is excluded if and only if it would be "HTTP". The
2547   received-by field is normally the host and optional port number of a
2548   recipient server or client that subsequently forwarded the message.
2549   However, if the real host is considered to be sensitive information,
2550   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2551   be assumed to be the default port of the received-protocol.
2552</t>
2553<t>
2554   Multiple Via field values represent each proxy or gateway that has
2555   forwarded the message. Each recipient &MUST; append its information
2556   such that the end result is ordered according to the sequence of
2557   forwarding applications.
2558</t>
2559<t>
2560   Comments &MAY; be used in the Via header field to identify the software
2561   of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2562   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2563   are optional and &MAY; be removed by any recipient prior to forwarding the
2564   message.
2565</t>
2566<t>
2567   For example, a request message could be sent from an HTTP/1.0 user
2568   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2569   forward the request to a public proxy at p.example.net, which completes
2570   the request by forwarding it to the origin server at www.example.com.
2571   The request received by www.example.com would then have the following
2572   Via header field:
2573</t>
2574<figure><artwork type="example">
2575  Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
2576</artwork></figure>
2577<t>
2578   A proxy or gateway used as a portal through a network firewall
2579   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2580   region unless it is explicitly enabled to do so. If not enabled, the
2581   received-by host of any host behind the firewall &SHOULD; be replaced
2582   by an appropriate pseudonym for that host.
2583</t>
2584<t>
2585   A proxy or gateway &MAY; combine an ordered subsequence of Via header
2586   field entries into a single such entry if the entries have identical
2587   received-protocol values. For example,
2588</t>
2589<figure><artwork type="example">
2590  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2591</artwork></figure>
2592<t>
2593  could be collapsed to
2594</t>
2595<figure><artwork type="example">
2596  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2597</artwork></figure>
2598<t>
2599   Senders &SHOULD-NOT; combine multiple entries unless they are all
2600   under the same organizational control and the hosts have already been
2601   replaced by pseudonyms. Senders &MUST-NOT; combine entries which
2602   have different received-protocol values.
2603</t>
2604</section>
2605
2606<section title="Transformation" anchor="message.transformation">
2607<t>
2608   If a proxy receives a request-target with a host name that is not a
2609   fully qualified domain name, it &MAY; add its own domain to the host name
2610   it received when forwarding the request.  A proxy &MUST-NOT; change the
2611   host name if it is a fully qualified domain name.
2612</t>
2613<t>
2614   A non-transforming proxy &MUST-NOT; modify the "path-absolute" and "query"
2615   parts of the received request-target when forwarding it to the next inbound
2616   server, except as noted above to replace an empty path with "/" or "*".
2617</t>
2618<t>
2619   A non-transforming proxy &MUST; preserve the message payload (&payload;),
2620   though it &MAY; change the message body through application or removal
2621   of a transfer coding (<xref target="transfer.codings"/>).
2622</t>
2623<t>
2624   A non-transforming proxy &SHOULD-NOT; modify header fields that provide
2625   information about the end points of the communication chain, the resource
2626   state, or the selected representation.
2627</t>
2628<t>
2629   A non-transforming proxy &MUST-NOT; modify any of the following fields in a
2630   request or response, and it &MUST-NOT; add any of these fields if not
2631   already present:
2632  <list style="symbols">
2633    <t><x:ref>Allow</x:ref> (&header-allow;)</t>
2634    <t><x:ref>Content-Location</x:ref> (&header-content-location;)</t>
2635    <t>Content-MD5 (<xref target="RFC2616" x:fmt="of" x:sec="14.15"/>)</t>
2636    <t><x:ref>ETag</x:ref> (&header-etag;)</t>
2637    <t><x:ref>Last-Modified</x:ref> (&header-last-modified;)</t>
2638    <t><x:ref>Server</x:ref> (&header-server;)</t>
2639  </list>
2640</t>
2641<t>
2642   A non-transforming proxy &MUST-NOT; modify an <x:ref>Expires</x:ref>
2643   header field (&header-expires;) if already present in a response, but
2644   it &MAY; add an <x:ref>Expires</x:ref> header field with a field-value
2645   identical to that of the <x:ref>Date</x:ref> header field.
2646</t>
2647<t>
2648   A proxy &MUST-NOT; modify or add any of the following fields in a
2649   message that contains the no-transform cache-control directive:
2650  <list style="symbols">
2651    <t><x:ref>Content-Encoding</x:ref> (&header-content-encoding;)</t>
2652    <t><x:ref>Content-Range</x:ref> (&header-content-range;)</t>
2653    <t><x:ref>Content-Type</x:ref> (&header-content-type;)</t>
2654  </list>
2655</t>
2656<t>
2657   A transforming proxy &MAY; modify or add these fields to a message
2658   that does not include no-transform, but if it does so, it &MUST; add a
2659   Warning 214 (Transformation applied) if one does not already appear
2660   in the message (see &header-warning;).
2661</t>
2662<x:note>
2663  <t>
2664    <x:h>Warning:</x:h> Unnecessary modification of header fields might
2665    cause authentication failures if stronger authentication
2666    mechanisms are introduced in later versions of HTTP. Such
2667    authentication mechanisms &MAY; rely on the values of header fields
2668    not listed here.
2669  </t>
2670</x:note>
2671</section>
2672</section>
2673</section>
2674
2675<section title="Connection Management" anchor="connection.management">
2676<t>
2677   HTTP messaging is independent of the underlying transport or
2678   session-layer connection protocol(s).  HTTP only presumes a reliable
2679   transport with in-order delivery of requests and the corresponding
2680   in-order delivery of responses.  The mapping of HTTP request and
2681   response structures onto the data units of an underlying transport
2682   protocol is outside the scope of this specification.
2683</t>
2684<t>
2685   As described in <xref target="connecting.inbound"/>, the specific
2686   connection protocols to be used for an HTTP interaction are determined by
2687   client configuration and the <x:ref>target URI</x:ref>.
2688   For example, the "http" URI scheme
2689   (<xref target="http.uri"/>) indicates a default connection of TCP
2690   over IP, with a default TCP port of 80, but the client might be
2691   configured to use a proxy via some other connection, port, or protocol.
2692</t>
2693<t>
2694   HTTP implementations are expected to engage in connection management,
2695   which includes maintaining the state of current connections,
2696   establishing a new connection or reusing an existing connection,
2697   processing messages received on a connection, detecting connection
2698   failures, and closing each connection.
2699   Most clients maintain multiple connections in parallel, including
2700   more than one connection per server endpoint.
2701   Most servers are designed to maintain thousands of concurrent connections,
2702   while controlling request queues to enable fair use and detect
2703   denial of service attacks.
2704</t>
2705
2706<section title="Connection" anchor="header.connection">
2707  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2708  <iref primary="true" item="close" x:for-anchor=""/>
2709  <x:anchor-alias value="Connection"/>
2710  <x:anchor-alias value="connection-option"/>
2711  <x:anchor-alias value="close"/>
2712<t>
2713   The "Connection" header field allows the sender to indicate desired
2714   control options for the current connection.  In order to avoid confusing
2715   downstream recipients, a proxy or gateway &MUST; remove or replace any
2716   received connection options before forwarding the message.
2717</t>
2718<t>
2719   When a header field is used to supply control information for or about
2720   the current connection, the sender &SHOULD; list the corresponding
2721   field-name within the "Connection" header field.
2722   A proxy or gateway &MUST; parse a received Connection
2723   header field before a message is forwarded and, for each
2724   connection-option in this field, remove any header field(s) from
2725   the message with the same name as the connection-option, and then
2726   remove the Connection header field itself (or replace it with the
2727   intermediary's own connection options for the forwarded message).
2728</t>
2729<t>
2730   Hence, the Connection header field provides a declarative way of
2731   distinguishing header fields that are only intended for the
2732   immediate recipient ("hop-by-hop") from those fields that are
2733   intended for all recipients on the chain ("end-to-end"), enabling the
2734   message to be self-descriptive and allowing future connection-specific
2735   extensions to be deployed without fear that they will be blindly
2736   forwarded by older intermediaries.
2737</t>
2738<t>
2739   The Connection header field's value has the following grammar:
2740</t>
2741<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2742  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2743  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2744</artwork></figure>
2745<t>
2746   Connection options are case-insensitive.
2747</t>
2748<t>
2749   A sender &MUST-NOT; include field-names in the Connection header
2750   field-value for fields that are defined as expressing constraints
2751   for all recipients in the request or response chain, such as the
2752   Cache-Control header field (&header-cache-control;).
2753</t>
2754<t>
2755   The connection options do not have to correspond to a header field
2756   present in the message, since a connection-specific header field
2757   might not be needed if there are no parameters associated with that
2758   connection option.  Recipients that trigger certain connection
2759   behavior based on the presence of connection options &MUST; do so
2760   based on the presence of the connection-option rather than only the
2761   presence of the optional header field.  In other words, if the
2762   connection option is received as a header field but not indicated
2763   within the Connection field-value, then the recipient &MUST; ignore
2764   the connection-specific header field because it has likely been
2765   forwarded by an intermediary that is only partially conformant.
2766</t>
2767<t>
2768   When defining new connection options, specifications ought to
2769   carefully consider existing deployed header fields and ensure
2770   that the new connection option does not share the same name as
2771   an unrelated header field that might already be deployed.
2772   Defining a new connection option essentially reserves that potential
2773   field-name for carrying additional information related to the
2774   connection option, since it would be unwise for senders to use
2775   that field-name for anything else.
2776</t>
2777<t>
2778   The "<x:dfn>close</x:dfn>" connection option is defined for a
2779   sender to signal that this connection will be closed after completion of
2780   the response. For example,
2781</t>
2782<figure><artwork type="example">
2783  Connection: close
2784</artwork></figure>
2785<t>
2786   in either the request or the response header fields indicates that
2787   the connection &SHOULD; be closed after the current request/response
2788   is complete (<xref target="persistent.tear-down"/>).
2789</t>
2790<t>
2791   A client that does not support <x:ref>persistent connections</x:ref> &MUST;
2792   send the "close" connection option in every request message.
2793</t>
2794<t>
2795   A server that does not support <x:ref>persistent connections</x:ref> &MUST;
2796   send the "close" connection option in every response message that
2797   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2798</t>
2799</section>
2800
2801<section title="Establishment" anchor="persistent.establishment">
2802<t>
2803   It is beyond the scope of this specification to describe how connections
2804   are established via various transport or session-layer protocols.
2805   Each connection applies to only one transport link.
2806</t>
2807</section>
2808
2809<section title="Persistence" anchor="persistent.connections">
2810   <x:anchor-alias value="persistent connections"/>
2811<t>
2812   HTTP/1.1 defaults to the use of "<x:dfn>persistent connections</x:dfn>",
2813   which allow multiple requests and responses to be carried over a single
2814   connection. The "<x:ref>close</x:ref>" connection-option is used to signal
2815   that a connection will not persist after the current request/response.
2816   HTTP implementations &SHOULD; support persistent connections.
2817</t>
2818<t>
2819   A recipient determines whether a connection is persistent or not based on
2820   the most recently received message's protocol version and
2821   <x:ref>Connection</x:ref> header field (if any):
2822   <list style="symbols">
2823     <t>If the <x:ref>close</x:ref> connection option is present, the
2824        connection will not persist after the current response; else,</t>
2825     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2826        persist after the current response; else,</t>
2827     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2828        connection option is present, the recipient is not a proxy, and
2829        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2830        the connection will persist after the current response; otherwise,</t>
2831     <t>The connection will close after the current response.</t>
2832   </list>
2833</t>
2834<t>
2835   A server &MAY; assume that an HTTP/1.1 client intends to maintain a
2836   persistent connection until a <x:ref>close</x:ref> connection option
2837   is received in a request.
2838</t>
2839<t>
2840   A client &MAY; reuse a persistent connection until it sends or receives
2841   a <x:ref>close</x:ref> connection option or receives an HTTP/1.0 response
2842   without a "keep-alive" connection option.
2843</t>
2844<t>
2845   In order to remain persistent, all messages on a connection &MUST;
2846   have a self-defined message length (i.e., one not defined by closure
2847   of the connection), as described in <xref target="message.body"/>.
2848   A server &MUST; read the entire request message body or close
2849   the connection after sending its response, since otherwise the
2850   remaining data on a persistent connection would be misinterpreted
2851   as the next request.  Likewise,
2852   a client &MUST; read the entire response message body if it intends
2853   to reuse the same connection for a subsequent request.
2854</t>
2855<t>
2856   A proxy server &MUST-NOT; maintain a persistent connection with an
2857   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
2858   information and discussion of the problems with the Keep-Alive header field
2859   implemented by many HTTP/1.0 clients).
2860</t>
2861<t>
2862   Clients and servers &SHOULD-NOT; assume that a persistent connection is
2863   maintained for HTTP versions less than 1.1 unless it is explicitly
2864   signaled.
2865   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
2866   for more information on backward compatibility with HTTP/1.0 clients.
2867</t>
2868
2869<section title="Pipelining" anchor="pipelining">
2870<t>
2871   A client that supports persistent connections &MAY; "pipeline" its
2872   requests (i.e., send multiple requests without waiting for each
2873   response). A server &MUST; send its responses to those requests in the
2874   same order that the requests were received.
2875</t>
2876<t>
2877   Clients which assume persistent connections and pipeline immediately
2878   after connection establishment &SHOULD; be prepared to retry their
2879   connection if the first pipelined attempt fails. If a client does
2880   such a retry, it &MUST-NOT; pipeline before it knows the connection is
2881   persistent. Clients &MUST; also be prepared to resend their requests if
2882   the server closes the connection before sending all of the
2883   corresponding responses.
2884</t>
2885<t>
2886   Clients &SHOULD-NOT; pipeline requests using non-idempotent request methods
2887   or non-idempotent sequences of request methods (see &idempotent-methods;).
2888   Otherwise, a premature termination of the transport connection could lead
2889   to indeterminate results. A client wishing to send a non-idempotent
2890   request &SHOULD; wait to send that request until it has received the
2891   response status line for the previous request.
2892</t>
2893</section>
2894
2895<section title="Retrying Requests" anchor="persistent.retrying.requests">
2896<t>
2897   Connections can be closed at any time, with or without intention.
2898   Implementations ought to anticipate the need to recover
2899   from asynchronous close events.
2900   A client &MAY; open a new connection and retransmit an aborted sequence
2901   of requests without user interaction so long as the request sequence is
2902   idempotent (see &idempotent-methods;).
2903   A client &MUST-NOT; automatically retry non-idempotent request sequences,
2904   although user agents &MAY; offer a human operator the choice of retrying
2905   the request(s). Confirmation by
2906   user agent software with semantic understanding of the application
2907   &MAY; substitute for user confirmation. An automatic retry &SHOULD-NOT; 
2908   be repeated if a second sequence of requests fails.
2909</t>
2910</section>
2911</section>
2912   
2913<section title="Concurrency" anchor="persistent.concurrency">
2914<t>
2915   Clients &SHOULD; limit the number of simultaneous
2916   connections that they maintain to a given server.
2917</t>
2918<t>
2919   Previous revisions of HTTP gave a specific number of connections as a
2920   ceiling, but this was found to be impractical for many applications. As a
2921   result, this specification does not mandate a particular maximum number of
2922   connections, but instead encourages clients to be conservative when opening
2923   multiple connections.
2924</t>
2925<t>
2926   Multiple connections are typically used to avoid the "head-of-line
2927   blocking" problem, wherein a request that takes significant server-side
2928   processing and/or has a large payload blocks subsequent requests on the
2929   same connection. However, each connection consumes server resources.
2930   Furthermore, using multiple connections can cause undesirable side effects
2931   in congested networks.
2932</t>
2933<t>
2934   Note that servers might reject traffic that they deem abusive, including an
2935   excessive number of connections from a client.
2936</t>
2937</section>
2938
2939<section title="Failures and Time-outs" anchor="persistent.failures">
2940<t>
2941   Servers will usually have some time-out value beyond which they will
2942   no longer maintain an inactive connection. Proxy servers might make
2943   this a higher value since it is likely that the client will be making
2944   more connections through the same server. The use of persistent
2945   connections places no requirements on the length (or existence) of
2946   this time-out for either the client or the server.
2947</t>
2948<t>
2949   When a client or server wishes to time-out it &SHOULD; issue a graceful
2950   close on the transport connection. Clients and servers &SHOULD; both
2951   constantly watch for the other side of the transport close, and
2952   respond to it as appropriate. If a client or server does not detect
2953   the other side's close promptly it could cause unnecessary resource
2954   drain on the network.
2955</t>
2956<t>
2957   A client, server, or proxy &MAY; close the transport connection at any
2958   time. For example, a client might have started to send a new request
2959   at the same time that the server has decided to close the "idle"
2960   connection. From the server's point of view, the connection is being
2961   closed while it was idle, but from the client's point of view, a
2962   request is in progress.
2963</t>
2964<t>
2965   Servers &SHOULD; maintain persistent connections and allow the underlying
2966   transport's flow control mechanisms to resolve temporary overloads, rather
2967   than terminate connections with the expectation that clients will retry.
2968   The latter technique can exacerbate network congestion.
2969</t>
2970<t>
2971   A client sending a message body &SHOULD; monitor
2972   the network connection for an error status code while it is transmitting
2973   the request. If the client sees an error status code, it &SHOULD;
2974   immediately cease transmitting the body and close the connection.
2975</t>
2976</section>
2977   
2978<section title="Tear-down" anchor="persistent.tear-down">
2979  <iref primary="false" item="Connection header field" x:for-anchor=""/>
2980  <iref primary="false" item="close" x:for-anchor=""/>
2981<t>
2982   The <x:ref>Connection</x:ref> header field
2983   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
2984   connection option that a sender &SHOULD; send when it wishes to close
2985   the connection after the current request/response pair.
2986</t>
2987<t>
2988   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
2989   send further requests on that connection (after the one containing
2990   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
2991   final response message corresponding to this request.
2992</t>
2993<t>
2994   A server that receives a <x:ref>close</x:ref> connection option &MUST;
2995   initiate a lingering close (see below) of the connection after it sends the
2996   final response to the request that contained <x:ref>close</x:ref>.
2997   The server &SHOULD; include a <x:ref>close</x:ref> connection option
2998   in its final response on that connection. The server &MUST-NOT; process
2999   any further requests received on that connection.
3000</t>
3001<t>
3002   A server that sends a <x:ref>close</x:ref> connection option &MUST;
3003   initiate a lingering close of the connection after it sends the
3004   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
3005   any further requests received on that connection.
3006</t>
3007<t>
3008   A client that receives a <x:ref>close</x:ref> connection option &MUST;
3009   cease sending requests on that connection and close the connection
3010   after reading the response message containing the close; if additional
3011   pipelined requests had been sent on the connection, the client &SHOULD;
3012   assume that they will not be processed by the server.
3013</t>
3014<t>
3015   If a server performs an immediate close of a TCP connection, there is a
3016   significant risk that the client will not be able to read the last HTTP
3017   response.  If the server receives additional data from the client on a
3018   fully-closed connection, such as another request that was sent by the
3019   client before receiving the server's response, the server's TCP stack will
3020   send a reset packet to the client; unfortunately, the reset packet might
3021   erase the client's unacknowledged input buffers before they can be read
3022   and interpreted by the client's HTTP parser.
3023</t>
3024<t>
3025   To avoid the TCP reset problem, a server can perform a lingering close on a
3026   connection by closing only the write side of the read/write connection
3027   (a half-close) and continuing to read from the connection until the
3028   connection is closed by the client or the server is reasonably certain
3029   that its own TCP stack has received the client's acknowledgement of the
3030   packet(s) containing the server's last response. It is then safe for the
3031   server to fully close the connection.
3032</t>
3033<t>
3034   It is unknown whether the reset problem is exclusive to TCP or might also
3035   be found in other transport connection protocols.
3036</t>
3037</section>
3038
3039<section title="Upgrade" anchor="header.upgrade">
3040  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3041  <x:anchor-alias value="Upgrade"/>
3042  <x:anchor-alias value="protocol"/>
3043  <x:anchor-alias value="protocol-name"/>
3044  <x:anchor-alias value="protocol-version"/>
3045<t>
3046   The "Upgrade" header field is intended to provide a simple mechanism
3047   for transitioning from HTTP/1.1 to some other protocol on the same
3048   connection.  A client &MAY; send a list of protocols in the Upgrade
3049   header field of a request to invite the server to switch to one or
3050   more of those protocols before sending the final response.
3051   A server &MUST; send an Upgrade header field in <x:ref>101 (Switching
3052   Protocols)</x:ref> responses to indicate which protocol(s) are being
3053   switched to, and &MUST; send it in <x:ref>426 (Upgrade Required)</x:ref>
3054   responses to indicate acceptable protocols.
3055   A server &MAY; send an Upgrade header field in any other response to
3056   indicate that they might be willing to upgrade to one of the
3057   specified protocols for a future request.
3058</t>
3059<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3060  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3061
3062  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3063  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3064  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3065</artwork></figure>
3066<t>
3067   For example,
3068</t>
3069<figure><artwork type="example">
3070  Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3071</artwork></figure>
3072<t>
3073   Upgrade eases the difficult transition between incompatible protocols by
3074   allowing the client to initiate a request in the more commonly
3075   supported protocol while indicating to the server that it would like
3076   to use a "better" protocol if available (where "better" is determined
3077   by the server, possibly according to the nature of the request method
3078   or target resource).
3079</t>
3080<t>
3081   Upgrade cannot be used to insist on a protocol change; its acceptance and
3082   use by the server is optional. The capabilities and nature of the
3083   application-level communication after the protocol change is entirely
3084   dependent upon the new protocol chosen, although the first action
3085   after changing the protocol &MUST; be a response to the initial HTTP
3086   request that contained the Upgrade header field.
3087</t>
3088<t>
3089   For example, if the Upgrade header field is received in a GET request
3090   and the server decides to switch protocols, then it &MUST; first respond
3091   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3092   then immediately follow that with the new protocol's equivalent of a
3093   response to a GET on the target resource.  This allows a connection to be
3094   upgraded to protocols with the same semantics as HTTP without the
3095   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3096   protocols unless the received message semantics can be honored by the new
3097   protocol; an OPTIONS request can be honored by any protocol.
3098</t>
3099<t>
3100   When Upgrade is sent, a sender &MUST; also send a
3101   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3102   that contains the "upgrade" connection option, in order to prevent Upgrade
3103   from being accidentally forwarded by intermediaries that might not implement
3104   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3105   is received in an HTTP/1.0 request.
3106</t>
3107<t>
3108   The Upgrade header field only applies to switching application-level
3109   protocols on the existing connection; it cannot be used
3110   to switch to a protocol on a different connection. For that purpose, it is
3111   more appropriate to use a <x:ref>3xx (Redirection)</x:ref> response
3112   (&status-3xx;).
3113</t>
3114<t>
3115   This specification only defines the protocol name "HTTP" for use by
3116   the family of Hypertext Transfer Protocols, as defined by the HTTP
3117   version rules of <xref target="http.version"/> and future updates to this
3118   specification. Additional tokens can be registered with IANA using the
3119   registration procedure defined in <xref target="upgrade.token.registry"/>.
3120</t>
3121</section>
3122</section>
3123
3124<section title="IANA Considerations" anchor="IANA.considerations">
3125
3126<section title="Header Field Registration" anchor="header.field.registration">
3127<t>
3128   HTTP header fields are registered within the Message Header Field Registry
3129   <xref target="RFC3864"/> maintained by IANA at
3130   <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
3131</t>
3132<t>
3133   This document defines the following HTTP header fields, so their
3134   associated registry entries shall be updated according to the permanent
3135   registrations below:
3136</t>
3137<?BEGININC p1-messaging.iana-headers ?>
3138<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3139<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3140   <ttcol>Header Field Name</ttcol>
3141   <ttcol>Protocol</ttcol>
3142   <ttcol>Status</ttcol>
3143   <ttcol>Reference</ttcol>
3144
3145   <c>Connection</c>
3146   <c>http</c>
3147   <c>standard</c>
3148   <c>
3149      <xref target="header.connection"/>
3150   </c>
3151   <c>Content-Length</c>
3152   <c>http</c>
3153   <c>standard</c>
3154   <c>
3155      <xref target="header.content-length"/>
3156   </c>
3157   <c>Host</c>
3158   <c>http</c>
3159   <c>standard</c>
3160   <c>
3161      <xref target="header.host"/>
3162   </c>
3163   <c>TE</c>
3164   <c>http</c>
3165   <c>standard</c>
3166   <c>
3167      <xref target="header.te"/>
3168   </c>
3169   <c>Trailer</c>
3170   <c>http</c>
3171   <c>standard</c>
3172   <c>
3173      <xref target="header.trailer"/>
3174   </c>
3175   <c>Transfer-Encoding</c>
3176   <c>http</c>
3177   <c>standard</c>
3178   <c>
3179      <xref target="header.transfer-encoding"/>
3180   </c>
3181   <c>Upgrade</c>
3182   <c>http</c>
3183   <c>standard</c>
3184   <c>
3185      <xref target="header.upgrade"/>
3186   </c>
3187   <c>Via</c>
3188   <c>http</c>
3189   <c>standard</c>
3190   <c>
3191      <xref target="header.via"/>
3192   </c>
3193</texttable>
3194<!--(END)-->
3195<?ENDINC p1-messaging.iana-headers ?>
3196<t>
3197   Furthermore, the header field-name "Close" shall be registered as
3198   "reserved", since using that name as an HTTP header field might
3199   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3200   header field (<xref target="header.connection"/>).
3201</t>
3202<texttable align="left" suppress-title="true">
3203   <ttcol>Header Field Name</ttcol>
3204   <ttcol>Protocol</ttcol>
3205   <ttcol>Status</ttcol>
3206   <ttcol>Reference</ttcol>
3207
3208   <c>Close</c>
3209   <c>http</c>
3210   <c>reserved</c>
3211   <c>
3212      <xref target="header.field.registration"/>
3213   </c>
3214</texttable>
3215<t>
3216   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3217</t>
3218</section>
3219
3220<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3221<t>
3222   IANA maintains the registry of URI Schemes <xref target="RFC4395"/> at
3223   <eref target="http://www.iana.org/assignments/uri-schemes.html"/>.
3224</t>
3225<t>
3226   This document defines the following URI schemes, so their
3227   associated registry entries shall be updated according to the permanent
3228   registrations below:
3229</t>
3230<texttable align="left" suppress-title="true">
3231   <ttcol>URI Scheme</ttcol>
3232   <ttcol>Description</ttcol>
3233   <ttcol>Reference</ttcol>
3234
3235   <c>http</c>
3236   <c>Hypertext Transfer Protocol</c>
3237   <c><xref target="http.uri"/></c>
3238
3239   <c>https</c>
3240   <c>Hypertext Transfer Protocol Secure</c>
3241   <c><xref target="https.uri"/></c>
3242</texttable>
3243</section>
3244
3245<section title="Internet Media Type Registrations" anchor="internet.media.type.http">
3246<t>
3247   This document serves as the specification for the Internet media types
3248   "message/http" and "application/http". The following is to be registered with
3249   IANA (see <xref target="RFC4288"/>).
3250</t>
3251<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
3252<iref item="Media Type" subitem="message/http" primary="true"/>
3253<iref item="message/http Media Type" primary="true"/>
3254<t>
3255   The message/http type can be used to enclose a single HTTP request or
3256   response message, provided that it obeys the MIME restrictions for all
3257   "message" types regarding line length and encodings.
3258</t>
3259<t>
3260  <list style="hanging" x:indent="12em">
3261    <t hangText="Type name:">
3262      message
3263    </t>
3264    <t hangText="Subtype name:">
3265      http
3266    </t>
3267    <t hangText="Required parameters:">
3268      none
3269    </t>
3270    <t hangText="Optional parameters:">
3271      version, msgtype
3272      <list style="hanging">
3273        <t hangText="version:">
3274          The HTTP-version number of the enclosed message
3275          (e.g., "1.1"). If not present, the version can be
3276          determined from the first line of the body.
3277        </t>
3278        <t hangText="msgtype:">
3279          The message type &mdash; "request" or "response". If not
3280          present, the type can be determined from the first
3281          line of the body.
3282        </t>
3283      </list>
3284    </t>
3285    <t hangText="Encoding considerations:">
3286      only "7bit", "8bit", or "binary" are permitted
3287    </t>
3288    <t hangText="Security considerations:">
3289      none
3290    </t>
3291    <t hangText="Interoperability considerations:">
3292      none
3293    </t>
3294    <t hangText="Published specification:">
3295      This specification (see <xref target="internet.media.type.message.http"/>).
3296    </t>
3297    <t hangText="Applications that use this media type:">
3298    </t>
3299    <t hangText="Additional information:">
3300      <list style="hanging">
3301        <t hangText="Magic number(s):">none</t>
3302        <t hangText="File extension(s):">none</t>
3303        <t hangText="Macintosh file type code(s):">none</t>
3304      </list>
3305    </t>
3306    <t hangText="Person and email address to contact for further information:">
3307      See Authors Section.
3308    </t>
3309    <t hangText="Intended usage:">
3310      COMMON
3311    </t>
3312    <t hangText="Restrictions on usage:">
3313      none
3314    </t>
3315    <t hangText="Author/Change controller:">
3316      IESG
3317    </t>
3318  </list>
3319</t>
3320</section>
3321<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
3322<iref item="Media Type" subitem="application/http" primary="true"/>
3323<iref item="application/http Media Type" primary="true"/>
3324<t>
3325   The application/http type can be used to enclose a pipeline of one or more
3326   HTTP request or response messages (not intermixed).
3327</t>
3328<t>
3329  <list style="hanging" x:indent="12em">
3330    <t hangText="Type name:">
3331      application
3332    </t>
3333    <t hangText="Subtype name:">
3334      http
3335    </t>
3336    <t hangText="Required parameters:">
3337      none
3338    </t>
3339    <t hangText="Optional parameters:">
3340      version, msgtype
3341      <list style="hanging">
3342        <t hangText="version:">
3343          The HTTP-version number of the enclosed messages
3344          (e.g., "1.1"). If not present, the version can be
3345          determined from the first line of the body.
3346        </t>
3347        <t hangText="msgtype:">
3348          The message type &mdash; "request" or "response". If not
3349          present, the type can be determined from the first
3350          line of the body.
3351        </t>
3352      </list>
3353    </t>
3354    <t hangText="Encoding considerations:">
3355      HTTP messages enclosed by this type
3356      are in "binary" format; use of an appropriate
3357      Content-Transfer-Encoding is required when
3358      transmitted via E-mail.
3359    </t>
3360    <t hangText="Security considerations:">
3361      none
3362    </t>
3363    <t hangText="Interoperability considerations:">
3364      none
3365    </t>
3366    <t hangText="Published specification:">
3367      This specification (see <xref target="internet.media.type.application.http"/>).
3368    </t>
3369    <t hangText="Applications that use this media type:">
3370    </t>
3371    <t hangText="Additional information:">
3372      <list style="hanging">
3373        <t hangText="Magic number(s):">none</t>
3374        <t hangText="File extension(s):">none</t>
3375        <t hangText="Macintosh file type code(s):">none</t>
3376      </list>
3377    </t>
3378    <t hangText="Person and email address to contact for further information:">
3379      See Authors Section.
3380    </t>
3381    <t hangText="Intended usage:">
3382      COMMON
3383    </t>
3384    <t hangText="Restrictions on usage:">
3385      none
3386    </t>
3387    <t hangText="Author/Change controller:">
3388      IESG
3389    </t>
3390  </list>
3391</t>
3392</section>
3393</section>
3394
3395<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3396<t>
3397   The HTTP Transfer Coding Registry defines the name space for transfer
3398   coding names.
3399</t>
3400<t>
3401   Registrations &MUST; include the following fields:
3402   <list style="symbols">
3403     <t>Name</t>
3404     <t>Description</t>
3405     <t>Pointer to specification text</t>
3406   </list>
3407</t>
3408<t>
3409   Names of transfer codings &MUST-NOT; overlap with names of content codings
3410   (&content-codings;) unless the encoding transformation is identical, as
3411   is the case for the compression codings defined in
3412   <xref target="compression.codings"/>.
3413</t>
3414<t>
3415   Values to be added to this name space require IETF Review (see
3416   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3417   conform to the purpose of transfer coding defined in this section.
3418   Use of program names for the identification of encoding formats
3419   is not desirable and is discouraged for future encodings.
3420</t>
3421<t>
3422   The registry itself is maintained at
3423   <eref target="http://www.iana.org/assignments/http-parameters"/>.
3424</t>
3425</section>
3426
3427<section title="Transfer Coding Registrations" anchor="transfer.coding.registration">
3428<t>
3429   The HTTP Transfer Coding Registry shall be updated with the registrations
3430   below:
3431</t>
3432<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3433   <ttcol>Name</ttcol>
3434   <ttcol>Description</ttcol>
3435   <ttcol>Reference</ttcol>
3436   <c>chunked</c>
3437   <c>Transfer in a series of chunks</c>
3438   <c>
3439      <xref target="chunked.encoding"/>
3440   </c>
3441   <c>compress</c>
3442   <c>UNIX "compress" program method</c>
3443   <c>
3444      <xref target="compress.coding"/>
3445   </c>
3446   <c>deflate</c>
3447   <c>"deflate" compression mechanism (<xref target="RFC1951"/>) used inside
3448   the "zlib" data format (<xref target="RFC1950"/>)
3449   </c>
3450   <c>
3451      <xref target="deflate.coding"/>
3452   </c>
3453   <c>gzip</c>
3454   <c>Same as GNU zip <xref target="RFC1952"/></c>
3455   <c>
3456      <xref target="gzip.coding"/>
3457   </c>
3458</texttable>
3459</section>
3460
3461<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3462<t>
3463   The HTTP Upgrade Token Registry defines the name space for protocol-name
3464   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3465   field. Each registered protocol name is associated with contact information
3466   and an optional set of specifications that details how the connection
3467   will be processed after it has been upgraded.
3468</t>
3469<t>
3470   Registrations happen on a "First Come First Served" basis (see
3471   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3472   following rules:
3473  <list style="numbers">
3474    <t>A protocol-name token, once registered, stays registered forever.</t>
3475    <t>The registration &MUST; name a responsible party for the
3476       registration.</t>
3477    <t>The registration &MUST; name a point of contact.</t>
3478    <t>The registration &MAY; name a set of specifications associated with
3479       that token. Such specifications need not be publicly available.</t>
3480    <t>The registration &SHOULD; name a set of expected "protocol-version"
3481       tokens associated with that token at the time of registration.</t>
3482    <t>The responsible party &MAY; change the registration at any time.
3483       The IANA will keep a record of all such changes, and make them
3484       available upon request.</t>
3485    <t>The IESG &MAY; reassign responsibility for a protocol token.
3486       This will normally only be used in the case when a
3487       responsible party cannot be contacted.</t>
3488  </list>
3489</t>
3490<t>
3491   This registration procedure for HTTP Upgrade Tokens replaces that
3492   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3493</t>
3494</section>
3495
3496<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3497<t>
3498   The HTTP Upgrade Token Registry shall be updated with the registration
3499   below:
3500</t>
3501<texttable align="left" suppress-title="true">
3502   <ttcol>Value</ttcol>
3503   <ttcol>Description</ttcol>
3504   <ttcol>Expected Version Tokens</ttcol>
3505   <ttcol>Reference</ttcol>
3506
3507   <c>HTTP</c>
3508   <c>Hypertext Transfer Protocol</c>
3509   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3510   <c><xref target="http.version"/></c>
3511</texttable>
3512<t>
3513   The responsible party is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3514</t>
3515</section>
3516
3517</section>
3518
3519<section title="Security Considerations" anchor="security.considerations">
3520<t>
3521   This section is meant to inform application developers, information
3522   providers, and users of the security limitations in HTTP/1.1 as
3523   described by this document. The discussion does not include
3524   definitive solutions to the problems revealed, though it does make
3525   some suggestions for reducing security risks.
3526</t>
3527
3528<section title="Personal Information" anchor="personal.information">
3529<t>
3530   HTTP clients are often privy to large amounts of personal information,
3531   including both information provided by the user to interact with resources
3532   (e.g., the user's name, location, mail address, passwords, encryption
3533   keys, etc.) and information about the user's browsing activity over
3534   time (e.g., history, bookmarks, etc.). HTTP implementations need to
3535   prevent unintentional leakage of this information.
3536</t>
3537</section>
3538
3539<section title="Abuse of Server Log Information" anchor="abuse.of.server.log.information">
3540<t>
3541   A server is in the position to save personal data about a user's
3542   requests which might identify their reading patterns or subjects of
3543   interest.  In particular, log information gathered at an intermediary
3544   often contains a history of user agent interaction, across a multitude
3545   of sites, that can be traced to individual users.
3546</t>
3547<t>
3548   HTTP log information is confidential in nature; its handling is often
3549   constrained by laws and regulations.  Log information needs to be securely
3550   stored and appropriate guidelines followed for its analysis.
3551   Anonymization of personal information within individual entries helps,
3552   but is generally not sufficient to prevent real log traces from being
3553   re-identified based on correlation with other access characteristics.
3554   As such, access traces that are keyed to a specific client should not
3555   be published even if the key is pseudonymous.
3556</t>
3557<t>
3558   To minimize the risk of theft or accidental publication, log information
3559   should be purged of personally identifiable information, including
3560   user identifiers, IP addresses, and user-provided query parameters,
3561   as soon as that information is no longer necessary to support operational
3562   needs for security, auditing, or fraud control.
3563</t>
3564</section>
3565
3566<section title="Attacks Based On File and Path Names" anchor="attack.pathname">
3567<t>
3568   Origin servers &SHOULD; be careful to restrict
3569   the documents returned by HTTP requests to be only those that were
3570   intended by the server administrators. If an HTTP server translates
3571   HTTP URIs directly into file system calls, the server &MUST; take
3572   special care not to serve files that were not intended to be
3573   delivered to HTTP clients. For example, UNIX, Microsoft Windows, and
3574   other operating systems use ".." as a path component to indicate a
3575   directory level above the current one. On such a system, an HTTP
3576   server &MUST; disallow any such construct in the request-target if it
3577   would otherwise allow access to a resource outside those intended to
3578   be accessible via the HTTP server. Similarly, files intended for
3579   reference only internally to the server (such as access control
3580   files, configuration files, and script code) &MUST; be protected from
3581   inappropriate retrieval, since they might contain sensitive
3582   information.
3583</t>
3584</section>
3585
3586<section title="DNS-related Attacks" anchor="dns.related.attacks">
3587<t>
3588   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3589   generally prone to security attacks based on the deliberate misassociation
3590   of IP addresses and DNS names not protected by DNSSEC. Clients need to be
3591   cautious in assuming the validity of an IP number/DNS name association unless
3592   the response is protected by DNSSEC (<xref target="RFC4033"/>).
3593</t>
3594</section>
3595
3596<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3597<t>
3598   By their very nature, HTTP intermediaries are men-in-the-middle, and
3599   represent an opportunity for man-in-the-middle attacks. Compromise of
3600   the systems on which the intermediaries run can result in serious security
3601   and privacy problems. Intermediaries have access to security-related
3602   information, personal information about individual users and
3603   organizations, and proprietary information belonging to users and
3604   content providers. A compromised intermediary, or an intermediary
3605   implemented or configured without regard to security and privacy
3606   considerations, might be used in the commission of a wide range of
3607   potential attacks.
3608</t>
3609<t>
3610   Intermediaries that contain a shared cache are especially vulnerable
3611   to cache poisoning attacks.
3612</t>
3613<t>
3614   Implementers need to consider the privacy and security
3615   implications of their design and coding decisions, and of the
3616   configuration options they provide to operators (especially the
3617   default configuration).
3618</t>
3619<t>
3620   Users need to be aware that intermediaries are no more trustworthy than
3621   the people who run them; HTTP itself cannot solve this problem.
3622</t>
3623</section>
3624
3625<section title="Buffer Overflows" anchor="attack.protocol.element.size.overflows">
3626<t>
3627   Because HTTP uses mostly textual, character-delimited fields, attackers can
3628   overflow buffers in implementations, and/or perform a Denial of Service
3629   against implementations that accept fields with unlimited lengths.
3630</t>
3631<t>
3632   To promote interoperability, this specification makes specific
3633   recommendations for minimum size limits on request-line
3634   (<xref target="request.line"/>)
3635   and blocks of header fields (<xref target="header.fields"/>). These are
3636   minimum recommendations, chosen to be supportable even by implementations
3637   with limited resources; it is expected that most implementations will
3638   choose substantially higher limits.
3639</t>
3640<t>
3641   This specification also provides a way for servers to reject messages that
3642   have request-targets that are too long (&status-414;) or request entities
3643   that are too large (&status-4xx;).
3644</t>
3645<t>
3646   Recipients &SHOULD; carefully limit the extent to which they read other
3647   fields, including (but not limited to) request methods, response status
3648   phrases, header field-names, and body chunks, so as to avoid denial of
3649   service attacks without impeding interoperability.
3650</t>
3651</section>
3652
3653<section title="Message Integrity" anchor="message.integrity">
3654<t>
3655   HTTP does not define a specific mechanism for ensuring message integrity,
3656   instead relying on the error-detection ability of underlying transport
3657   protocols and the use of length or chunk-delimited framing to detect
3658   completeness. Additional integrity mechanisms, such as hash functions or
3659   digital signatures applied to the content, can be selectively added to
3660   messages via extensible metadata header fields. Historically, the lack of
3661   a single integrity mechanism has been justified by the informal nature of
3662   most HTTP communication.  However, the prevalence of HTTP as an information
3663   access mechanism has resulted in its increasing use within environments
3664   where verification of message integrity is crucial.
3665</t>
3666<t>
3667   User agents are encouraged to implement configurable means for detecting
3668   and reporting failures of message integrity such that those means can be
3669   enabled within environments for which integrity is necessary. For example,
3670   a browser being used to view medical history or drug interaction
3671   information needs to indicate to the user when such information is detected
3672   by the protocol to be incomplete, expired, or corrupted during transfer.
3673   Such mechanisms might be selectively enabled via user agent extensions or
3674   the presence of message integrity metadata in a response.
3675   At a minimum, user agents ought to provide some indication that allows a
3676   user to distinguish between a complete and incomplete response message
3677   (<xref target="incomplete.messages"/>) when such verification is desired.
3678</t>
3679</section>
3680</section>
3681
3682<section title="Acknowledgments" anchor="acks">
3683<t>
3684   This edition of HTTP/1.1 builds on the many contributions that went into
3685   <xref target="RFC1945" format="none">RFC 1945</xref>,
3686   <xref target="RFC2068" format="none">RFC 2068</xref>,
3687   <xref target="RFC2145" format="none">RFC 2145</xref>, and
3688   <xref target="RFC2616" format="none">RFC 2616</xref>, including
3689   substantial contributions made by the previous authors, editors, and
3690   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
3691   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
3692   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
3693</t>
3694<t>
3695   Since 1999, the following contributors have helped improve the HTTP
3696   specification by reporting bugs, asking smart questions, drafting or
3697   reviewing text, and evaluating open issues:
3698</t>
3699<?BEGININC acks ?>
3700<t>Adam Barth,
3701Adam Roach,
3702Addison Phillips,
3703Adrian Chadd,
3704Adrien W. de Croy,
3705Alan Ford,
3706Alan Ruttenberg,
3707Albert Lunde,
3708Alek Storm,
3709Alex Rousskov,
3710Alexandre Morgaut,
3711Alexey Melnikov,
3712Alisha Smith,
3713Amichai Rothman,
3714Amit Klein,
3715Amos Jeffries,
3716Andreas Maier,
3717Andreas Petersson,
3718Anil Sharma,
3719Anne van Kesteren,
3720Anthony Bryan,
3721Asbjorn Ulsberg,
3722Ashok Kumar,
3723Balachander Krishnamurthy,
3724Barry Leiba,
3725Ben Laurie,
3726Benjamin Niven-Jenkins,
3727Bil Corry,
3728Bill Burke,
3729Bjoern Hoehrmann,
3730Bob Scheifler,
3731Boris Zbarsky,
3732Brett Slatkin,
3733Brian Kell,
3734Brian McBarron,
3735Brian Pane,
3736Brian Smith,
3737Bryce Nesbitt,
3738Cameron Heavon-Jones,
3739Carl Kugler,
3740Carsten Bormann,
3741Charles Fry,
3742Chris Newman,
3743Cyrus Daboo,
3744Dale Robert Anderson,
3745Dan Wing,
3746Dan Winship,
3747Daniel Stenberg,
3748Dave Cridland,
3749Dave Crocker,
3750Dave Kristol,
3751David Booth,
3752David Singer,
3753David W. Morris,
3754Diwakar Shetty,
3755Dmitry Kurochkin,
3756Drummond Reed,
3757Duane Wessels,
3758Edward Lee,
3759Eliot Lear,
3760Eran Hammer-Lahav,
3761Eric D. Williams,
3762Eric J. Bowman,
3763Eric Lawrence,
3764Eric Rescorla,
3765Erik Aronesty,
3766Evan Prodromou,
3767Florian Weimer,
3768Frank Ellermann,
3769Fred Bohle,
3770Gabriel Montenegro,
3771Geoffrey Sneddon,
3772Gervase Markham,
3773Grahame Grieve,
3774Greg Wilkins,
3775Harald Tveit Alvestrand,
3776Harry Halpin,
3777Helge Hess,
3778Henrik Nordstrom,
3779Henry S. Thompson,
3780Henry Story,
3781Herbert van de Sompel,
3782Howard Melman,
3783Hugo Haas,
3784Ian Fette,
3785Ian Hickson,
3786Ido Safruti,
3787Ilya Grigorik,
3788Ingo Struck,
3789J. Ross Nicoll,
3790James H. Manger,
3791James Lacey,
3792James M. Snell,
3793Jamie Lokier,
3794Jan Algermissen,
3795Jeff Hodges (who came up with the term 'effective Request-URI'),
3796Jeff Walden,
3797Jim Luther,
3798Joe D. Williams,
3799Joe Gregorio,
3800Joe Orton,
3801John C. Klensin,
3802John C. Mallery,
3803John Cowan,
3804John Kemp,
3805John Panzer,
3806John Schneider,
3807John Stracke,
3808John Sullivan,
3809Jonas Sicking,
3810Jonathan Billington,
3811Jonathan Moore,
3812Jonathan Rees,
3813Jonathan Silvera,
3814Jordi Ros,
3815Joris Dobbelsteen,
3816Josh Cohen,
3817Julien Pierre,
3818Jungshik Shin,
3819Justin Chapweske,
3820Justin Erenkrantz,
3821Justin James,
3822Kalvinder Singh,
3823Karl Dubost,
3824Keith Hoffman,
3825Keith Moore,
3826Ken Murchison,
3827Koen Holtman,
3828Konstantin Voronkov,
3829Kris Zyp,
3830Lisa Dusseault,
3831Maciej Stachowiak,
3832Marc Schneider,
3833Marc Slemko,
3834Mark Baker,
3835Mark Pauley,
3836Mark Watson,
3837Markus Isomaki,
3838Markus Lanthaler,
3839Martin J. Duerst,
3840Martin Musatov,
3841Martin Nilsson,
3842Martin Thomson,
3843Matt Lynch,
3844Matthew Cox,
3845Max Clark,
3846Michael Burrows,
3847Michael Hausenblas,
3848Mike Amundsen,
3849Mike Belshe,
3850Mike Kelly,
3851Mike Schinkel,
3852Miles Sabin,
3853Murray S. Kucherawy,
3854Mykyta Yevstifeyev,
3855Nathan Rixham,
3856Nicholas Shanks,
3857Nico Williams,
3858Nicolas Alvarez,
3859Nicolas Mailhot,
3860Noah Slater,
3861Pablo Castro,
3862Pat Hayes,
3863Patrick R. McManus,
3864Paul E. Jones,
3865Paul Hoffman,
3866Paul Marquess,
3867Peter Lepeska,
3868Peter Saint-Andre,
3869Peter Watkins,
3870Phil Archer,
3871Philippe Mougin,
3872Phillip Hallam-Baker,
3873Poul-Henning Kamp,
3874Preethi Natarajan,
3875Rajeev Bector,
3876Ray Polk,
3877Reto Bachmann-Gmuer,
3878Richard Cyganiak,
3879Robert Brewer,
3880Robert Collins,
3881Robert O'Callahan,
3882Robert Olofsson,
3883Robert Sayre,
3884Robert Siemer,
3885Robert de Wilde,
3886Roberto Javier Godoy,
3887Roberto Peon,
3888Roland Zink,
3889Ronny Widjaja,
3890S. Mike Dierken,
3891Salvatore Loreto,
3892Sam Johnston,
3893Sam Ruby,
3894Scott Lawrence (who maintained the original issues list),
3895Sean B. Palmer,
3896Shane McCarron,
3897Stefan Eissing,
3898Stefan Tilkov,
3899Stefanos Harhalakis,
3900Stephane Bortzmeyer,
3901Stephen Farrell,
3902Stephen Ludin,
3903Stuart Williams,
3904Subbu Allamaraju,
3905Sylvain Hellegouarch,
3906Tapan Divekar,
3907Tatsuya Hayashi,
3908Ted Hardie,
3909Thomas Broyer,
3910Thomas Fossati,
3911Thomas Nordin,
3912Thomas Roessler,
3913Tim Bray,
3914Tim Morgan,
3915Tim Olsen,
3916Tom Zhou,
3917Travis Snoozy,
3918Tyler Close,
3919Vincent Murphy,
3920Wenbo Zhu,
3921Werner Baumann,
3922Wilbur Streett,
3923Wilfredo Sanchez Vega,
3924William A. Rowe Jr.,
3925William Chan,
3926Willy Tarreau,
3927Xiaoshu Wang,
3928Yaron Goland,
3929Yngve Nysaeter Pettersen,
3930Yoav Nir,
3931Yogesh Bang,
3932Yutaka Oiwa,
3933Yves Lafon (long-time member of the editor team),
3934Zed A. Shaw, and
3935Zhong Yu.
3936</t>
3937<?ENDINC acks ?>
3938<t>
3939   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
3940   acknowledgements from prior revisions.
3941</t>
3942</section>
3943
3944</middle>
3945<back>
3946
3947<references title="Normative References">
3948
3949<reference anchor="Part2">
3950  <front>
3951    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
3952    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3953      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
3954      <address><email>fielding@gbiv.com</email></address>
3955    </author>
3956    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3957      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3958      <address><email>julian.reschke@greenbytes.de</email></address>
3959    </author>
3960    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3961  </front>
3962  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
3963  <x:source href="p2-semantics.xml" basename="p2-semantics">
3964    <x:defines>1xx (Informational)</x:defines>
3965    <x:defines>1xx</x:defines>
3966    <x:defines>100 (Continue)</x:defines>
3967    <x:defines>101 (Switching Protocols)</x:defines>
3968    <x:defines>2xx (Successful)</x:defines>
3969    <x:defines>2xx</x:defines>
3970    <x:defines>200 (OK)</x:defines>
3971    <x:defines>204 (No Content)</x:defines>
3972    <x:defines>3xx (Redirection)</x:defines>
3973    <x:defines>3xx</x:defines>
3974    <x:defines>301 (Moved Permanently)</x:defines>
3975    <x:defines>4xx (Client Error)</x:defines>
3976    <x:defines>4xx</x:defines>
3977    <x:defines>400 (Bad Request)</x:defines>
3978    <x:defines>405 (Method Not Allowed)</x:defines>
3979    <x:defines>411 (Length Required)</x:defines>
3980    <x:defines>414 (URI Too Long)</x:defines>
3981    <x:defines>417 (Expectation Failed)</x:defines>
3982    <x:defines>426 (Upgrade Required)</x:defines>
3983    <x:defines>501 (Not Implemented)</x:defines>
3984    <x:defines>502 (Bad Gateway)</x:defines>
3985    <x:defines>505 (HTTP Version Not Supported)</x:defines>
3986    <x:defines>Allow</x:defines>
3987    <x:defines>Content-Encoding</x:defines>
3988    <x:defines>Content-Location</x:defines>
3989    <x:defines>Content-Type</x:defines>
3990    <x:defines>Date</x:defines>
3991    <x:defines>Expect</x:defines>
3992    <x:defines>Location</x:defines>
3993    <x:defines>Server</x:defines>
3994    <x:defines>User-Agent</x:defines>
3995  </x:source>
3996</reference>
3997
3998<reference anchor="Part4">
3999  <front>
4000    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
4001    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
4002      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4003      <address><email>fielding@gbiv.com</email></address>
4004    </author>
4005    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
4006      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4007      <address><email>julian.reschke@greenbytes.de</email></address>
4008    </author>
4009    <date month="&ID-MONTH;" year="&ID-YEAR;" />
4010  </front>
4011  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
4012  <x:source basename="p4-conditional" href="p4-conditional.xml">
4013    <x:defines>304 (Not Modified)</x:defines>
4014    <x:defines>ETag</x:defines>
4015    <x:defines>Last-Modified</x:defines>
4016  </x:source>
4017</reference>
4018
4019<reference anchor="Part5">
4020  <front>
4021    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
4022    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4023      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4024      <address><email>fielding@gbiv.com</email></address>
4025    </author>
4026    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
4027      <organization abbrev="W3C">World Wide Web Consortium</organization>
4028      <address><email>ylafon@w3.org</email></address>
4029    </author>
4030    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4031      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4032      <address><email>julian.reschke@greenbytes.de</email></address>
4033    </author>
4034    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4035  </front>
4036  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4037  <x:source href="p5-range.xml" basename="p5-range">
4038    <x:defines>Content-Range</x:defines>
4039  </x:source>
4040</reference>
4041
4042<reference anchor="Part6">
4043  <front>
4044    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4045    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4046      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4047      <address><email>fielding@gbiv.com</email></address>
4048    </author>
4049    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4050      <organization>Akamai</organization>
4051      <address><email>mnot@mnot.net</email></address>
4052    </author>
4053    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4054      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4055      <address><email>julian.reschke@greenbytes.de</email></address>
4056    </author>
4057    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4058  </front>
4059  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4060  <x:source href="p6-cache.xml" basename="p6-cache">
4061    <x:defines>Expires</x:defines>
4062  </x:source>
4063</reference>
4064
4065<reference anchor="Part7">
4066  <front>
4067    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4068    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4069      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4070      <address><email>fielding@gbiv.com</email></address>
4071    </author>
4072    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4073      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4074      <address><email>julian.reschke@greenbytes.de</email></address>
4075    </author>
4076    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4077  </front>
4078  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4079  <x:source href="p7-auth.xml" basename="p7-auth">
4080    <x:defines>Proxy-Authenticate</x:defines>
4081    <x:defines>Proxy-Authorization</x:defines>
4082  </x:source>
4083</reference>
4084
4085<reference anchor="RFC5234">
4086  <front>
4087    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4088    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4089      <organization>Brandenburg InternetWorking</organization>
4090      <address>
4091        <email>dcrocker@bbiw.net</email>
4092      </address> 
4093    </author>
4094    <author initials="P." surname="Overell" fullname="Paul Overell">
4095      <organization>THUS plc.</organization>
4096      <address>
4097        <email>paul.overell@thus.net</email>
4098      </address>
4099    </author>
4100    <date month="January" year="2008"/>
4101  </front>
4102  <seriesInfo name="STD" value="68"/>
4103  <seriesInfo name="RFC" value="5234"/>
4104</reference>
4105
4106<reference anchor="RFC2119">
4107  <front>
4108    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4109    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4110      <organization>Harvard University</organization>
4111      <address><email>sob@harvard.edu</email></address>
4112    </author>
4113    <date month="March" year="1997"/>
4114  </front>
4115  <seriesInfo name="BCP" value="14"/>
4116  <seriesInfo name="RFC" value="2119"/>
4117</reference>
4118
4119<reference anchor="RFC3986">
4120 <front>
4121  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4122  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4123    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4124    <address>
4125       <email>timbl@w3.org</email>
4126       <uri>http://www.w3.org/People/Berners-Lee/</uri>
4127    </address>
4128  </author>
4129  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4130    <organization abbrev="Day Software">Day Software</organization>
4131    <address>
4132      <email>fielding@gbiv.com</email>
4133      <uri>http://roy.gbiv.com/</uri>
4134    </address>
4135  </author>
4136  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4137    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4138    <address>
4139      <email>LMM@acm.org</email>
4140      <uri>http://larry.masinter.net/</uri>
4141    </address>
4142  </author>
4143  <date month='January' year='2005'></date>
4144 </front>
4145 <seriesInfo name="STD" value="66"/>
4146 <seriesInfo name="RFC" value="3986"/>
4147</reference>
4148
4149<reference anchor="USASCII">
4150  <front>
4151    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4152    <author>
4153      <organization>American National Standards Institute</organization>
4154    </author>
4155    <date year="1986"/>
4156  </front>
4157  <seriesInfo name="ANSI" value="X3.4"/>
4158</reference>
4159
4160<reference anchor="RFC1950">
4161  <front>
4162    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4163    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4164      <organization>Aladdin Enterprises</organization>
4165      <address><email>ghost@aladdin.com</email></address>
4166    </author>
4167    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4168    <date month="May" year="1996"/>
4169  </front>
4170  <seriesInfo name="RFC" value="1950"/>
4171  <!--<annotation>
4172    RFC 1950 is an Informational RFC, thus it might be less stable than
4173    this specification. On the other hand, this downward reference was
4174    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4175    therefore it is unlikely to cause problems in practice. See also
4176    <xref target="BCP97"/>.
4177  </annotation>-->
4178</reference>
4179
4180<reference anchor="RFC1951">
4181  <front>
4182    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4183    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4184      <organization>Aladdin Enterprises</organization>
4185      <address><email>ghost@aladdin.com</email></address>
4186    </author>
4187    <date month="May" year="1996"/>
4188  </front>
4189  <seriesInfo name="RFC" value="1951"/>
4190  <!--<annotation>
4191    RFC 1951 is an Informational RFC, thus it might be less stable than
4192    this specification. On the other hand, this downward reference was
4193    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4194    therefore it is unlikely to cause problems in practice. See also
4195    <xref target="BCP97"/>.
4196  </annotation>-->
4197</reference>
4198
4199<reference anchor="RFC1952">
4200  <front>
4201    <title>GZIP file format specification version 4.3</title>
4202    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4203      <organization>Aladdin Enterprises</organization>
4204      <address><email>ghost@aladdin.com</email></address>
4205    </author>
4206    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4207      <address><email>gzip@prep.ai.mit.edu</email></address>
4208    </author>
4209    <author initials="M." surname="Adler" fullname="Mark Adler">
4210      <address><email>madler@alumni.caltech.edu</email></address>
4211    </author>
4212    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4213      <address><email>ghost@aladdin.com</email></address>
4214    </author>
4215    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4216      <address><email>randeg@alumni.rpi.edu</email></address>
4217    </author>
4218    <date month="May" year="1996"/>
4219  </front>
4220  <seriesInfo name="RFC" value="1952"/>
4221  <!--<annotation>
4222    RFC 1952 is an Informational RFC, thus it might be less stable than
4223    this specification. On the other hand, this downward reference was
4224    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4225    therefore it is unlikely to cause problems in practice. See also
4226    <xref target="BCP97"/>.
4227  </annotation>-->
4228</reference>
4229
4230</references>
4231
4232<references title="Informative References">
4233
4234<reference anchor="ISO-8859-1">
4235  <front>
4236    <title>
4237     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4238    </title>
4239    <author>
4240      <organization>International Organization for Standardization</organization>
4241    </author>
4242    <date year="1998"/>
4243  </front>
4244  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4245</reference>
4246
4247<reference anchor='RFC1919'>
4248  <front>
4249    <title>Classical versus Transparent IP Proxies</title>
4250    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4251      <address><email>mchatel@pax.eunet.ch</email></address>
4252    </author>
4253    <date year='1996' month='March' />
4254  </front>
4255  <seriesInfo name='RFC' value='1919' />
4256</reference>
4257
4258<reference anchor="RFC1945">
4259  <front>
4260    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4261    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4262      <organization>MIT, Laboratory for Computer Science</organization>
4263      <address><email>timbl@w3.org</email></address>
4264    </author>
4265    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4266      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4267      <address><email>fielding@ics.uci.edu</email></address>
4268    </author>
4269    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4270      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4271      <address><email>frystyk@w3.org</email></address>
4272    </author>
4273    <date month="May" year="1996"/>
4274  </front>
4275  <seriesInfo name="RFC" value="1945"/>
4276</reference>
4277
4278<reference anchor="RFC2045">
4279  <front>
4280    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4281    <author initials="N." surname="Freed" fullname="Ned Freed">
4282      <organization>Innosoft International, Inc.</organization>
4283      <address><email>ned@innosoft.com</email></address>
4284    </author>
4285    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4286      <organization>First Virtual Holdings</organization>
4287      <address><email>nsb@nsb.fv.com</email></address>
4288    </author>
4289    <date month="November" year="1996"/>
4290  </front>
4291  <seriesInfo name="RFC" value="2045"/>
4292</reference>
4293
4294<reference anchor="RFC2047">
4295  <front>
4296    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4297    <author initials="K." surname="Moore" fullname="Keith Moore">
4298      <organization>University of Tennessee</organization>
4299      <address><email>moore@cs.utk.edu</email></address>
4300    </author>
4301    <date month="November" year="1996"/>
4302  </front>
4303  <seriesInfo name="RFC" value="2047"/>
4304</reference>
4305
4306<reference anchor="RFC2068">
4307  <front>
4308    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4309    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
4310      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4311      <address><email>fielding@ics.uci.edu</email></address>
4312    </author>
4313    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4314      <organization>MIT Laboratory for Computer Science</organization>
4315      <address><email>jg@w3.org</email></address>
4316    </author>
4317    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4318      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4319      <address><email>mogul@wrl.dec.com</email></address>
4320    </author>
4321    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4322      <organization>MIT Laboratory for Computer Science</organization>
4323      <address><email>frystyk@w3.org</email></address>
4324    </author>
4325    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4326      <organization>MIT Laboratory for Computer Science</organization>
4327      <address><email>timbl@w3.org</email></address>
4328    </author>
4329    <date month="January" year="1997"/>
4330  </front>
4331  <seriesInfo name="RFC" value="2068"/>
4332</reference>
4333
4334<reference anchor="RFC2145">
4335  <front>
4336    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4337    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4338      <organization>Western Research Laboratory</organization>
4339      <address><email>mogul@wrl.dec.com</email></address>
4340    </author>
4341    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4342      <organization>Department of Information and Computer Science</organization>
4343      <address><email>fielding@ics.uci.edu</email></address>
4344    </author>
4345    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4346      <organization>MIT Laboratory for Computer Science</organization>
4347      <address><email>jg@w3.org</email></address>
4348    </author>
4349    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4350      <organization>W3 Consortium</organization>
4351      <address><email>frystyk@w3.org</email></address>
4352    </author>
4353    <date month="May" year="1997"/>
4354  </front>
4355  <seriesInfo name="RFC" value="2145"/>
4356</reference>
4357
4358<reference anchor="RFC2616">
4359  <front>
4360    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4361    <author initials="R." surname="Fielding" fullname="R. Fielding">
4362      <organization>University of California, Irvine</organization>
4363      <address><email>fielding@ics.uci.edu</email></address>
4364    </author>
4365    <author initials="J." surname="Gettys" fullname="J. Gettys">
4366      <organization>W3C</organization>
4367      <address><email>jg@w3.org</email></address>
4368    </author>
4369    <author initials="J." surname="Mogul" fullname="J. Mogul">
4370      <organization>Compaq Computer Corporation</organization>
4371      <address><email>mogul@wrl.dec.com</email></address>
4372    </author>
4373    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4374      <organization>MIT Laboratory for Computer Science</organization>
4375      <address><email>frystyk@w3.org</email></address>
4376    </author>
4377    <author initials="L." surname="Masinter" fullname="L. Masinter">
4378      <organization>Xerox Corporation</organization>
4379      <address><email>masinter@parc.xerox.com</email></address>
4380    </author>
4381    <author initials="P." surname="Leach" fullname="P. Leach">
4382      <organization>Microsoft Corporation</organization>
4383      <address><email>paulle@microsoft.com</email></address>
4384    </author>
4385    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4386      <organization>W3C</organization>
4387      <address><email>timbl@w3.org</email></address>
4388    </author>
4389    <date month="June" year="1999"/>
4390  </front>
4391  <seriesInfo name="RFC" value="2616"/>
4392</reference>
4393
4394<reference anchor='RFC2817'>
4395  <front>
4396    <title>Upgrading to TLS Within HTTP/1.1</title>
4397    <author initials='R.' surname='Khare' fullname='R. Khare'>
4398      <organization>4K Associates / UC Irvine</organization>
4399      <address><email>rohit@4K-associates.com</email></address>
4400    </author>
4401    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4402      <organization>Agranat Systems, Inc.</organization>
4403      <address><email>lawrence@agranat.com</email></address>
4404    </author>
4405    <date year='2000' month='May' />
4406  </front>
4407  <seriesInfo name='RFC' value='2817' />
4408</reference>
4409
4410<reference anchor='RFC2818'>
4411  <front>
4412    <title>HTTP Over TLS</title>
4413    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4414      <organization>RTFM, Inc.</organization>
4415      <address><email>ekr@rtfm.com</email></address>
4416    </author>
4417    <date year='2000' month='May' />
4418  </front>
4419  <seriesInfo name='RFC' value='2818' />
4420</reference>
4421
4422<reference anchor='RFC3040'>
4423  <front>
4424    <title>Internet Web Replication and Caching Taxonomy</title>
4425    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4426      <organization>Equinix, Inc.</organization>
4427    </author>
4428    <author initials='I.' surname='Melve' fullname='I. Melve'>
4429      <organization>UNINETT</organization>
4430    </author>
4431    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4432      <organization>CacheFlow Inc.</organization>
4433    </author>
4434    <date year='2001' month='January' />
4435  </front>
4436  <seriesInfo name='RFC' value='3040' />
4437</reference>
4438
4439<reference anchor='RFC3864'>
4440  <front>
4441    <title>Registration Procedures for Message Header Fields</title>
4442    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4443      <organization>Nine by Nine</organization>
4444      <address><email>GK-IETF@ninebynine.org</email></address>
4445    </author>
4446    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4447      <organization>BEA Systems</organization>
4448      <address><email>mnot@pobox.com</email></address>
4449    </author>
4450    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4451      <organization>HP Labs</organization>
4452      <address><email>JeffMogul@acm.org</email></address>
4453    </author>
4454    <date year='2004' month='September' />
4455  </front>
4456  <seriesInfo name='BCP' value='90' />
4457  <seriesInfo name='RFC' value='3864' />
4458</reference>
4459
4460<reference anchor='RFC4033'>
4461  <front>
4462    <title>DNS Security Introduction and Requirements</title>
4463    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4464    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4465    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4466    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4467    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4468    <date year='2005' month='March' />
4469  </front>
4470  <seriesInfo name='RFC' value='4033' />
4471</reference>
4472
4473<reference anchor="RFC4288">
4474  <front>
4475    <title>Media Type Specifications and Registration Procedures</title>
4476    <author initials="N." surname="Freed" fullname="N. Freed">
4477      <organization>Sun Microsystems</organization>
4478      <address>
4479        <email>ned.freed@mrochek.com</email>
4480      </address>
4481    </author>
4482    <author initials="J." surname="Klensin" fullname="J. Klensin">
4483      <address>
4484        <email>klensin+ietf@jck.com</email>
4485      </address>
4486    </author>
4487    <date year="2005" month="December"/>
4488  </front>
4489  <seriesInfo name="BCP" value="13"/>
4490  <seriesInfo name="RFC" value="4288"/>
4491</reference>
4492
4493<reference anchor='RFC4395'>
4494  <front>
4495    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4496    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4497      <organization>AT&amp;T Laboratories</organization>
4498      <address>
4499        <email>tony+urireg@maillennium.att.com</email>
4500      </address>
4501    </author>
4502    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4503      <organization>Qualcomm, Inc.</organization>
4504      <address>
4505        <email>hardie@qualcomm.com</email>
4506      </address>
4507    </author>
4508    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4509      <organization>Adobe Systems</organization>
4510      <address>
4511        <email>LMM@acm.org</email>
4512      </address>
4513    </author>
4514    <date year='2006' month='February' />
4515  </front>
4516  <seriesInfo name='BCP' value='115' />
4517  <seriesInfo name='RFC' value='4395' />
4518</reference>
4519
4520<reference anchor='RFC4559'>
4521  <front>
4522    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4523    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4524    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4525    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4526    <date year='2006' month='June' />
4527  </front>
4528  <seriesInfo name='RFC' value='4559' />
4529</reference>
4530
4531<reference anchor='RFC5226'>
4532  <front>
4533    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4534    <author initials='T.' surname='Narten' fullname='T. Narten'>
4535      <organization>IBM</organization>
4536      <address><email>narten@us.ibm.com</email></address>
4537    </author>
4538    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4539      <organization>Google</organization>
4540      <address><email>Harald@Alvestrand.no</email></address>
4541    </author>
4542    <date year='2008' month='May' />
4543  </front>
4544  <seriesInfo name='BCP' value='26' />
4545  <seriesInfo name='RFC' value='5226' />
4546</reference>
4547
4548<reference anchor='RFC5246'>
4549   <front>
4550      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4551      <author initials='T.' surname='Dierks' fullname='T. Dierks'>
4552         <organization />
4553      </author>
4554      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4555         <organization>RTFM, Inc.</organization>
4556      </author>
4557      <date year='2008' month='August' />
4558   </front>
4559   <seriesInfo name='RFC' value='5246' />
4560</reference>
4561
4562<reference anchor="RFC5322">
4563  <front>
4564    <title>Internet Message Format</title>
4565    <author initials="P." surname="Resnick" fullname="P. Resnick">
4566      <organization>Qualcomm Incorporated</organization>
4567    </author>
4568    <date year="2008" month="October"/>
4569  </front> 
4570  <seriesInfo name="RFC" value="5322"/>
4571</reference>
4572
4573<reference anchor="RFC6265">
4574  <front>
4575    <title>HTTP State Management Mechanism</title>
4576    <author initials="A." surname="Barth" fullname="Adam Barth">
4577      <organization abbrev="U.C. Berkeley">
4578        University of California, Berkeley
4579      </organization>
4580      <address><email>abarth@eecs.berkeley.edu</email></address>
4581    </author>
4582    <date year="2011" month="April" />
4583  </front>
4584  <seriesInfo name="RFC" value="6265"/>
4585</reference>
4586
4587<!--<reference anchor='BCP97'>
4588  <front>
4589    <title>Handling Normative References to Standards-Track Documents</title>
4590    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
4591      <address>
4592        <email>klensin+ietf@jck.com</email>
4593      </address>
4594    </author>
4595    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
4596      <organization>MIT</organization>
4597      <address>
4598        <email>hartmans-ietf@mit.edu</email>
4599      </address>
4600    </author>
4601    <date year='2007' month='June' />
4602  </front>
4603  <seriesInfo name='BCP' value='97' />
4604  <seriesInfo name='RFC' value='4897' />
4605</reference>-->
4606
4607<reference anchor="Kri2001" target="http://arxiv.org/abs/cs.SE/0105018">
4608  <front>
4609    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
4610    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
4611    <date year="2001" month="November"/>
4612  </front>
4613  <seriesInfo name="ACM Transactions on Internet Technology" value="Vol. 1, #2"/>
4614</reference>
4615
4616</references>
4617
4618
4619<section title="HTTP Version History" anchor="compatibility">
4620<t>
4621   HTTP has been in use by the World-Wide Web global information initiative
4622   since 1990. The first version of HTTP, later referred to as HTTP/0.9,
4623   was a simple protocol for hypertext data transfer across the Internet
4624   with only a single request method (GET) and no metadata.
4625   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
4626   methods and MIME-like messaging that could include metadata about the data
4627   transferred and modifiers on the request/response semantics. However,
4628   HTTP/1.0 did not sufficiently take into consideration the effects of
4629   hierarchical proxies, caching, the need for persistent connections, or
4630   name-based virtual hosts. The proliferation of incompletely-implemented
4631   applications calling themselves "HTTP/1.0" further necessitated a
4632   protocol version change in order for two communicating applications
4633   to determine each other's true capabilities.
4634</t>
4635<t>
4636   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
4637   requirements that enable reliable implementations, adding only
4638   those new features that will either be safely ignored by an HTTP/1.0
4639   recipient or only sent when communicating with a party advertising
4640   conformance with HTTP/1.1.
4641</t>
4642<t>
4643   It is beyond the scope of a protocol specification to mandate
4644   conformance with previous versions. HTTP/1.1 was deliberately
4645   designed, however, to make supporting previous versions easy.
4646   We would expect a general-purpose HTTP/1.1 server to understand
4647   any valid request in the format of HTTP/1.0 and respond appropriately
4648   with an HTTP/1.1 message that only uses features understood (or
4649   safely ignored) by HTTP/1.0 clients.  Likewise, we would expect
4650   an HTTP/1.1 client to understand any valid HTTP/1.0 response.
4651</t>
4652<t>
4653   Since HTTP/0.9 did not support header fields in a request,
4654   there is no mechanism for it to support name-based virtual
4655   hosts (selection of resource by inspection of the <x:ref>Host</x:ref> header
4656   field).  Any server that implements name-based virtual hosts
4657   ought to disable support for HTTP/0.9.  Most requests that
4658   appear to be HTTP/0.9 are, in fact, badly constructed HTTP/1.x
4659   requests wherein a buggy client failed to properly encode
4660   linear whitespace found in a URI reference and placed in
4661   the request-target.
4662</t>
4663
4664<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
4665<t>
4666   This section summarizes major differences between versions HTTP/1.0
4667   and HTTP/1.1.
4668</t>
4669
4670<section title="Multi-homed Web Servers" anchor="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">
4671<t>
4672   The requirements that clients and servers support the <x:ref>Host</x:ref>
4673   header field (<xref target="header.host"/>), report an error if it is
4674   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
4675   are among the most important changes defined by HTTP/1.1.
4676</t>
4677<t>
4678   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4679   addresses and servers; there was no other established mechanism for
4680   distinguishing the intended server of a request than the IP address
4681   to which that request was directed. The <x:ref>Host</x:ref> header field was
4682   introduced during the development of HTTP/1.1 and, though it was
4683   quickly implemented by most HTTP/1.0 browsers, additional requirements
4684   were placed on all HTTP/1.1 requests in order to ensure complete
4685   adoption.  At the time of this writing, most HTTP-based services
4686   are dependent upon the Host header field for targeting requests.
4687</t>
4688</section>
4689
4690<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4691<t>
4692   In HTTP/1.0, each connection is established by the client prior to the
4693   request and closed by the server after sending the response. However, some
4694   implementations implement the explicitly negotiated ("Keep-Alive") version
4695   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
4696   target="RFC2068"/>.
4697</t>
4698<t>
4699   Some clients and servers might wish to be compatible with these previous
4700   approaches to persistent connections, by explicitly negotiating for them
4701   with a "Connection: keep-alive" request header field. However, some
4702   experimental implementations of HTTP/1.0 persistent connections are faulty;
4703   for example, if an HTTP/1.0 proxy server doesn't understand
4704   <x:ref>Connection</x:ref>, it will erroneously forward that header field
4705   to the next inbound server, which would result in a hung connection.
4706</t>
4707<t>
4708   One attempted solution was the introduction of a Proxy-Connection header
4709   field, targeted specifically at proxies. In practice, this was also
4710   unworkable, because proxies are often deployed in multiple layers, bringing
4711   about the same problem discussed above.
4712</t>
4713<t>
4714   As a result, clients are encouraged not to send the Proxy-Connection header
4715   field in any requests.
4716</t>
4717<t>
4718   Clients are also encouraged to consider the use of Connection: keep-alive
4719   in requests carefully; while they can enable persistent connections with
4720   HTTP/1.0 servers, clients using them need will need to monitor the
4721   connection for "hung" requests (which indicate that the client ought stop
4722   sending the header field), and this mechanism ought not be used by clients
4723   at all when a proxy is being used.
4724</t>
4725</section>
4726
4727<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
4728<t>
4729   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
4730   (<xref target="header.transfer-encoding"/>). Proxies/gateways &MUST; remove
4731   any transfer coding prior to forwarding a message via a MIME-compliant
4732   protocol.
4733</t>
4734</section>
4735
4736</section>
4737
4738<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4739<t>
4740  HTTP's approach to error handling has been explained.
4741  (<xref target="conformance"/>)
4742</t>
4743<t>
4744  The expectation to support HTTP/0.9 requests has been removed.
4745</t>
4746<t>
4747  The term "Effective Request URI" has been introduced.
4748  (<xref target="effective.request.uri" />)
4749</t>
4750<t>
4751  HTTP messages can be (and often are) buffered by implementations; despite
4752  it sometimes being available as a stream, HTTP is fundamentally a
4753  message-oriented protocol.
4754  (<xref target="http.message" />)
4755</t>
4756<t>
4757  Minimum supported sizes for various protocol elements have been
4758  suggested, to improve interoperability.
4759</t>
4760<t>
4761  Header fields that span multiple lines ("line folding") are deprecated.
4762  (<xref target="field.parsing" />)
4763</t>
4764<t>
4765  The HTTP-version ABNF production has been clarified to be case-sensitive.
4766  Additionally, version numbers has been restricted to single digits, due
4767  to the fact that implementations are known to handle multi-digit version
4768  numbers incorrectly.
4769  (<xref target="http.version"/>)
4770</t>
4771<t>
4772  The HTTPS URI scheme is now defined by this specification; previously,
4773  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
4774  (<xref target="https.uri"/>)
4775</t>
4776<t>
4777  The HTTPS URI scheme implies end-to-end security.
4778  (<xref target="https.uri"/>)
4779</t>
4780<t>
4781  Userinfo (i.e., username and password) are now disallowed in HTTP and
4782  HTTPS URIs, because of security issues related to their transmission on the
4783  wire.
4784  (<xref target="http.uri" />)
4785</t>
4786<t>
4787  Invalid whitespace around field-names is now required to be rejected,
4788  because accepting it represents a security vulnerability.
4789  (<xref target="header.fields"/>)
4790</t>
4791<t>
4792  The ABNF productions defining header fields now only list the field value.
4793  (<xref target="header.fields"/>)
4794</t>
4795<t>
4796  Rules about implicit linear whitespace between certain grammar productions
4797  have been removed; now whitespace is only allowed where specifically
4798  defined in the ABNF.
4799  (<xref target="whitespace"/>)
4800</t>
4801<t> 
4802  The NUL octet is no longer allowed in comment and quoted-string text, and
4803  handling of backslash-escaping in them has been clarified.
4804  (<xref target="field.components"/>)
4805</t> 
4806<t>
4807  The quoted-pair rule no longer allows escaping control characters other than
4808  HTAB.
4809  (<xref target="field.components"/>)
4810</t>
4811<t>
4812  Non-ASCII content in header fields and the reason phrase has been obsoleted
4813  and made opaque (the TEXT rule was removed).
4814  (<xref target="field.components"/>)
4815</t>
4816<t>
4817  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
4818  handled as errors by recipients.
4819  (<xref target="header.content-length"/>)
4820</t>
4821<t>
4822  The "identity" transfer coding token has been removed.
4823  (Sections <xref format="counter" target="message.body"/> and
4824  <xref format="counter" target="transfer.codings"/>)
4825</t>
4826<t>
4827  The algorithm for determining the message body length has been clarified
4828  to indicate all of the special cases (e.g., driven by methods or status
4829  codes) that affect it, and that new protocol elements cannot define such
4830  special cases.
4831  (<xref target="message.body.length"/>)
4832</t>
4833<t>
4834  "multipart/byteranges" is no longer a way of determining message body length
4835  detection.
4836  (<xref target="message.body.length"/>)
4837</t>
4838<t>
4839  CONNECT is a new, special case in determining message body length.
4840  (<xref target="message.body.length"/>)
4841</t>
4842<t>
4843  Chunk length does not include the count of the octets in the
4844  chunk header and trailer.
4845  (<xref target="chunked.encoding"/>)
4846</t>
4847<t>
4848  Use of chunk extensions is deprecated, and line folding in them is
4849  disallowed.
4850  (<xref target="chunked.encoding"/>)
4851</t>
4852<t>
4853  The path-absolute + query components of RFC3986 have been used to define the
4854  request-target, instead of abs_path from RFC 1808.
4855  (<xref target="request-target"/>)
4856</t>
4857<t>
4858  The asterisk form of the request-target is only allowed in the OPTIONS
4859  method.
4860  (<xref target="request-target"/>)
4861</t>
4862<t>
4863  Exactly when "close" connection options have to be sent has been clarified.
4864  (<xref target="header.connection"/>)
4865</t>
4866<t> 
4867  "hop-by-hop" header fields are required to appear in the Connection header
4868  field; just because they're defined as hop-by-hop in this specification
4869  doesn't exempt them.
4870  (<xref target="header.connection"/>)
4871</t>
4872<t>
4873  The limit of two connections per server has been removed.
4874  (<xref target="persistent.connections"/>)
4875</t>
4876<t>
4877  An idempotent sequence of requests is no longer required to be retried.
4878  (<xref target="persistent.connections"/>)
4879</t>
4880<t>
4881  The requirement to retry requests under certain circumstances when the
4882  server prematurely closes the connection has been removed.
4883  (<xref target="persistent.connections"/>)
4884</t>
4885<t>
4886  Some extraneous requirements about when servers are allowed to close
4887  connections prematurely have been removed.
4888  (<xref target="persistent.connections"/>)
4889</t>
4890<t>
4891  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
4892  responses other than 101 (this was incorporated from <xref
4893  target="RFC2817"/>).
4894  (<xref target="header.upgrade"/>)
4895</t>
4896<t>
4897  Registration of Transfer Codings now requires IETF Review
4898  (<xref target="transfer.coding.registry"/>)
4899</t>
4900<t>
4901  The meaning of the "deflate" content coding has been clarified.
4902  (<xref target="deflate.coding" />)
4903</t>
4904<t>
4905  This specification now defines the Upgrade Token Registry, previously
4906  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
4907  (<xref target="upgrade.token.registry"/>)
4908</t>
4909<t>
4910  Empty list elements in list productions (e.g., a list header containing
4911  ", ,") have been deprecated.
4912  (<xref target="abnf.extension"/>)
4913</t>
4914<t>
4915  Issues with the Keep-Alive and Proxy-Connection headers in requests
4916  are pointed out, with use of the latter being discouraged altogether.
4917  (<xref target="compatibility.with.http.1.0.persistent.connections" />)
4918</t>
4919</section>
4920</section>
4921
4922<section title="ABNF list extension: #rule" anchor="abnf.extension">
4923<t>
4924  A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
4925  improve readability in the definitions of some header field values.
4926</t>
4927<t>
4928  A construct "#" is defined, similar to "*", for defining comma-delimited
4929  lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
4930  at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
4931  comma (",") and optional whitespace (OWS).   
4932</t>
4933<figure><preamble>
4934  Thus,
4935</preamble><artwork type="example">
4936  1#element =&gt; element *( OWS "," OWS element )
4937</artwork></figure>
4938<figure><preamble>
4939  and:
4940</preamble><artwork type="example">
4941  #element =&gt; [ 1#element ]
4942</artwork></figure>
4943<figure><preamble>
4944  and for n &gt;= 1 and m &gt; 1:
4945</preamble><artwork type="example">
4946  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
4947</artwork></figure>
4948<t>
4949  For compatibility with legacy list rules, recipients &SHOULD; accept empty
4950  list elements. In other words, consumers would follow the list productions:
4951</t>
4952<figure><artwork type="example">
4953  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
4954 
4955  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
4956</artwork></figure>
4957<t>
4958  Note that empty elements do not contribute to the count of elements present,
4959  though.
4960</t>
4961<t>
4962  For example, given these ABNF productions:
4963</t>
4964<figure><artwork type="example">
4965  example-list      = 1#example-list-elmt
4966  example-list-elmt = token ; see <xref target="field.components"/> 
4967</artwork></figure>
4968<t>
4969  Then these are valid values for example-list (not including the double
4970  quotes, which are present for delimitation only):
4971</t>
4972<figure><artwork type="example">
4973  "foo,bar"
4974  "foo ,bar,"
4975  "foo , ,bar,charlie   "
4976</artwork></figure>
4977<t>
4978  But these values would be invalid, as at least one non-empty element is
4979  required:
4980</t>
4981<figure><artwork type="example">
4982  ""
4983  ","
4984  ",   ,"
4985</artwork></figure>
4986<t>
4987  <xref target="collected.abnf"/> shows the collected ABNF, with the list rules
4988  expanded as explained above.
4989</t>
4990</section>
4991
4992<?BEGININC p1-messaging.abnf-appendix ?>
4993<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
4994<figure>
4995<artwork type="abnf" name="p1-messaging.parsed-abnf">
4996<x:ref>BWS</x:ref> = OWS
4997
4998<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
4999 connection-option ] )
5000<x:ref>Content-Length</x:ref> = 1*DIGIT
5001
5002<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
5003 ]
5004<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
5005<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
5006<x:ref>Host</x:ref> = uri-host [ ":" port ]
5007
5008<x:ref>OWS</x:ref> = *( SP / HTAB )
5009
5010<x:ref>RWS</x:ref> = 1*( SP / HTAB )
5011
5012<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
5013<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
5014<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
5015 transfer-coding ] )
5016
5017<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
5018<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
5019
5020<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
5021 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
5022 comment ] ) ] )
5023
5024<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
5025<x:ref>absolute-form</x:ref> = absolute-URI
5026<x:ref>asterisk-form</x:ref> = "*"
5027<x:ref>attribute</x:ref> = token
5028<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
5029<x:ref>authority-form</x:ref> = authority
5030
5031<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
5032<x:ref>chunk-data</x:ref> = 1*OCTET
5033<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
5034<x:ref>chunk-ext-name</x:ref> = token
5035<x:ref>chunk-ext-val</x:ref> = token / quoted-str-nf
5036<x:ref>chunk-size</x:ref> = 1*HEXDIG
5037<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
5038<x:ref>comment</x:ref> = "(" *( ctext / quoted-cpair / comment ) ")"
5039<x:ref>connection-option</x:ref> = token
5040<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
5041 / %x2A-5B ; '*'-'['
5042 / %x5D-7E ; ']'-'~'
5043 / obs-text
5044
5045<x:ref>field-content</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5046<x:ref>field-name</x:ref> = token
5047<x:ref>field-value</x:ref> = *( field-content / obs-fold )
5048
5049<x:ref>header-field</x:ref> = field-name ":" OWS field-value BWS
5050<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ]
5051<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ]
5052
5053<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
5054
5055<x:ref>message-body</x:ref> = *OCTET
5056<x:ref>method</x:ref> = token
5057
5058<x:ref>obs-fold</x:ref> = CRLF ( SP / HTAB )
5059<x:ref>obs-text</x:ref> = %x80-FF
5060<x:ref>origin-form</x:ref> = path-absolute [ "?" query ]
5061
5062<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
5063<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
5064<x:ref>path-absolute</x:ref> = &lt;path-absolute, defined in [RFC3986], Section 3.3&gt;
5065<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
5066<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
5067<x:ref>protocol-name</x:ref> = token
5068<x:ref>protocol-version</x:ref> = token
5069<x:ref>pseudonym</x:ref> = token
5070
5071<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5072 / %x5D-7E ; ']'-'~'
5073 / obs-text
5074<x:ref>qdtext-nf</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5075 / %x5D-7E ; ']'-'~'
5076 / obs-text
5077<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5078<x:ref>quoted-cpair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5079<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5080<x:ref>quoted-str-nf</x:ref> = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
5081<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5082
5083<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5084<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5085<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5086<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5087<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5088<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5089<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5090 asterisk-form
5091
5092<x:ref>special</x:ref> = "(" / ")" / "&lt;" / "&gt;" / "@" / "," / ";" / ":" / "\" /
5093 DQUOTE / "/" / "[" / "]" / "?" / "=" / "{" / "}"
5094<x:ref>start-line</x:ref> = request-line / status-line
5095<x:ref>status-code</x:ref> = 3DIGIT
5096<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5097
5098<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5099<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5100<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5101 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5102<x:ref>token</x:ref> = 1*tchar
5103<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5104<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5105 transfer-extension
5106<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5107<x:ref>transfer-parameter</x:ref> = attribute BWS "=" BWS value
5108
5109<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5110
5111<x:ref>value</x:ref> = word
5112
5113<x:ref>word</x:ref> = token / quoted-string
5114</artwork>
5115</figure>
5116</section>
5117<?ENDINC p1-messaging.abnf-appendix ?>
5118
5119<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5120
5121<section title="Since RFC 2616">
5122<t>
5123  Changes up to the first Working Group Last Call draft are summarized
5124  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-21#appendix-D"/>.
5125</t>
5126</section>
5127
5128<section title="Since draft-ietf-httpbis-p1-messaging-21" anchor="changes.since.21">
5129<t>
5130  Closed issues:
5131  <list style="symbols">
5132    <t>
5133      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/128"/>:
5134      "Cite HTTPS URI scheme definition" (the spec now includes the HTTPs
5135      scheme definition and thus updates RFC 2818)
5136    </t>
5137    <t>
5138      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/389"/>:
5139      "mention of 'proxies' in section about caches"
5140    </t>
5141  </list>
5142</t>
5143</section>
5144
5145</section>
5146
5147</back>
5148</rfc>
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