source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2030

Last change on this file since 2030 was 2030, checked in by fielding@…, 7 years ago

(editorial) Deal with first half of mnot p1 feedback; addresses #408

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 222.6 KB
Line 
1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "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 status-codes           "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
51  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
52  <!ENTITY status-203             "<xref target='Part2' x:rel='#status.203' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
53  <!ENTITY status-3xx             "<xref target='Part2' x:rel='#status.3xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
54  <!ENTITY status-304             "<xref target='Part4' x:rel='#status.304' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
55  <!ENTITY status-4xx             "<xref target='Part2' x:rel='#status.4xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
56  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
57  <!ENTITY iana-header-registry   "<xref target='Part2' x:rel='#header.field.registry' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
58]>
59<?rfc toc="yes" ?>
60<?rfc symrefs="yes" ?>
61<?rfc sortrefs="yes" ?>
62<?rfc compact="yes"?>
63<?rfc subcompact="no" ?>
64<?rfc linkmailto="no" ?>
65<?rfc editing="no" ?>
66<?rfc comments="yes"?>
67<?rfc inline="yes"?>
68<?rfc rfcedstyle="yes"?>
69<?rfc-ext allow-markup-in-artwork="yes" ?>
70<?rfc-ext include-references-in-index="yes" ?>
71<rfc obsoletes="2145,2616" updates="2817,2818" category="std" x:maturity-level="proposed"
72     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
73     xmlns:x='http://purl.org/net/xml2rfc/ext'>
74<x:link rel="next" basename="p2-semantics"/>
75<x:feedback template="mailto:ietf-http-wg@w3.org?subject={docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
76<front>
77
78  <title abbrev="HTTP/1.1 Message Syntax and Routing">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
79
80  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
81    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
82    <address>
83      <postal>
84        <street>345 Park Ave</street>
85        <city>San Jose</city>
86        <region>CA</region>
87        <code>95110</code>
88        <country>USA</country>
89      </postal>
90      <email>fielding@gbiv.com</email>
91      <uri>http://roy.gbiv.com/</uri>
92    </address>
93  </author>
94
95  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
96    <organization abbrev="greenbytes">greenbytes GmbH</organization>
97    <address>
98      <postal>
99        <street>Hafenweg 16</street>
100        <city>Muenster</city><region>NW</region><code>48155</code>
101        <country>Germany</country>
102      </postal>
103      <email>julian.reschke@greenbytes.de</email>
104      <uri>http://greenbytes.de/tech/webdav/</uri>
105    </address>
106  </author>
107
108  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
109  <workgroup>HTTPbis Working Group</workgroup>
110
111<abstract>
112<t>
113   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
114   distributed, collaborative, hypertext information systems. HTTP has been in
115   use by the World Wide Web global information initiative since 1990.
116   This document provides an overview of HTTP architecture and its associated
117   terminology, defines the "http" and "https" Uniform Resource Identifier
118   (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements,
119   and describes general security concerns for implementations.
120</t>   
121</abstract>
122
123<note title="Editorial Note (To be removed by RFC Editor)">
124  <t>
125    Discussion of this draft takes place on the HTTPBIS working group
126    mailing list (ietf-http-wg@w3.org), which is archived at
127    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
128  </t>
129  <t>
130    The current issues list is at
131    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
132    documents (including fancy diffs) can be found at
133    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
134  </t>
135  <t>
136    The changes in this draft are summarized in <xref target="changes.since.21"/>.
137  </t>
138</note>
139</front>
140<middle>
141<section title="Introduction" anchor="introduction">
142<t>
143   The Hypertext Transfer Protocol (HTTP) is an application-level
144   request/response protocol that uses extensible semantics and MIME-like
145   message payloads for flexible interaction with network-based hypertext
146   information systems. This document is the first in a series of documents
147   that collectively form the HTTP/1.1 specification:
148   <list style="empty">
149    <t>RFC xxx1: Message Syntax and Routing</t>
150    <t><xref target="Part2" x:fmt="none">RFC xxx2</xref>: Semantics and Content</t>
151    <t><xref target="Part4" x:fmt="none">RFC xxx3</xref>: Conditional Requests</t>
152    <t><xref target="Part5" x:fmt="none">RFC xxx4</xref>: Range Requests</t>
153    <t><xref target="Part6" x:fmt="none">RFC xxx5</xref>: Caching</t>
154    <t><xref target="Part7" x:fmt="none">RFC xxx6</xref>: Authentication</t>
155   </list>
156</t>
157<t>
158   This HTTP/1.1 specification obsoletes and moves to historic status
159   <xref target="RFC2616" x:fmt="none">RFC 2616</xref>, its predecessor
160   <xref target="RFC2068" x:fmt="none">RFC 2068</xref>, and
161   <xref target="RFC2145" x:fmt="none">RFC 2145</xref> (on HTTP versioning).
162   This specification also updates the use of CONNECT to establish a tunnel,
163   previously defined in <xref target="RFC2817" x:fmt="none">RFC 2817</xref>,
164   and defines the "https" URI scheme that was described informally in
165   <xref target="RFC2818" x:fmt="none">RFC 2818</xref>.
166</t>
167<t>
168   HTTP is a generic interface protocol for information systems. It is
169   designed to hide the details of how a service is implemented by presenting
170   a uniform interface to clients that is independent of the types of
171   resources provided. Likewise, servers do not need to be aware of each
172   client's purpose: an HTTP request can be considered in isolation rather
173   than being associated with a specific type of client or a predetermined
174   sequence of application steps. The result is a protocol that can be used
175   effectively in many different contexts and for which implementations can
176   evolve independently over time.
177</t>
178<t>
179   HTTP is also designed for use as an intermediation protocol for translating
180   communication to and from non-HTTP information systems.
181   HTTP proxies and gateways can provide access to alternative information
182   services by translating their diverse protocols into a hypertext
183   format that can be viewed and manipulated by clients in the same way
184   as HTTP services.
185</t>
186<t>
187   One consequence of this flexibility is that the protocol cannot be
188   defined in terms of what occurs behind the interface. Instead, we
189   are limited to defining the syntax of communication, the intent
190   of received communication, and the expected behavior of recipients.
191   If the communication is considered in isolation, then successful
192   actions ought to be reflected in corresponding changes to the
193   observable interface provided by servers. However, since multiple
194   clients might act in parallel and perhaps at cross-purposes, we
195   cannot require that such changes be observable beyond the scope
196   of a single response.
197</t>
198<t>
199   This document describes the architectural elements that are used or
200   referred to in HTTP, defines the "http" and "https" URI schemes,
201   describes overall network operation and connection management,
202   and defines HTTP message framing and forwarding requirements.
203   Our goal is to define all of the mechanisms necessary for HTTP message
204   handling that are independent of message semantics, thereby defining the
205   complete set of requirements for message parsers and
206   message-forwarding intermediaries.
207</t>
208
209
210<section title="Requirement Notation" anchor="intro.requirements">
211<t>
212   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
213   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
214   document are to be interpreted as described in <xref target="RFC2119"/>.
215</t>
216<t>
217   Conformance criteria and considerations regarding error handling
218   are defined in <xref target="conformance"/>.
219</t>
220</section>
221
222<section title="Syntax Notation" anchor="notation">
223<iref primary="true" item="Grammar" subitem="ALPHA"/>
224<iref primary="true" item="Grammar" subitem="CR"/>
225<iref primary="true" item="Grammar" subitem="CRLF"/>
226<iref primary="true" item="Grammar" subitem="CTL"/>
227<iref primary="true" item="Grammar" subitem="DIGIT"/>
228<iref primary="true" item="Grammar" subitem="DQUOTE"/>
229<iref primary="true" item="Grammar" subitem="HEXDIG"/>
230<iref primary="true" item="Grammar" subitem="HTAB"/>
231<iref primary="true" item="Grammar" subitem="LF"/>
232<iref primary="true" item="Grammar" subitem="OCTET"/>
233<iref primary="true" item="Grammar" subitem="SP"/>
234<iref primary="true" item="Grammar" subitem="VCHAR"/>
235<t>
236   This specification uses the Augmented Backus-Naur Form (ABNF) notation
237   of <xref target="RFC5234"/> with the list rule extension defined in
238   <xref target="abnf.extension"/><xref target="collected.abnf"/> shows
239   the collected ABNF with the list rule expanded.
240</t>
241<t anchor="core.rules">
242  <x:anchor-alias value="ALPHA"/>
243  <x:anchor-alias value="CTL"/>
244  <x:anchor-alias value="CR"/>
245  <x:anchor-alias value="CRLF"/>
246  <x:anchor-alias value="DIGIT"/>
247  <x:anchor-alias value="DQUOTE"/>
248  <x:anchor-alias value="HEXDIG"/>
249  <x:anchor-alias value="HTAB"/>
250  <x:anchor-alias value="LF"/>
251  <x:anchor-alias value="OCTET"/>
252  <x:anchor-alias value="SP"/>
253  <x:anchor-alias value="VCHAR"/>
254   The following core rules are included by
255   reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
256   ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
257   DIGIT (decimal 0-9), DQUOTE (double quote),
258   HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed),
259   OCTET (any 8-bit sequence of data), SP (space), and
260   VCHAR (any visible <xref target="USASCII"/> character).
261</t>
262<t>
263   As a convention, ABNF rule names prefixed with "obs-" denote
264   "obsolete" grammar rules that appear for historical reasons.
265</t>
266</section>
267</section>
268
269<section title="Architecture" anchor="architecture">
270<t>
271   HTTP was created for the World Wide Web architecture
272   and has evolved over time to support the scalability needs of a worldwide
273   hypertext system. Much of that architecture is reflected in the terminology
274   and syntax productions used to define HTTP.
275</t>
276
277<section title="Client/Server Messaging" anchor="operation">
278<iref primary="true" item="client"/>
279<iref primary="true" item="server"/>
280<iref primary="true" item="connection"/>
281<t>
282   HTTP is a stateless request/response protocol that operates by exchanging
283   <x:dfn>messages</x:dfn> (<xref target="http.message"/>) across a reliable
284   transport or session-layer
285   "<x:dfn>connection</x:dfn>" (<xref target="connection.management"/>).
286   An HTTP "<x:dfn>client</x:dfn>" is a program that establishes a connection
287   to a server for the purpose of sending one or more HTTP requests.
288   An HTTP "<x:dfn>server</x:dfn>" is a program that accepts connections
289   in order to service HTTP requests by sending HTTP responses.
290</t>
291<iref primary="true" item="user agent"/>
292<iref primary="true" item="origin server"/>
293<iref primary="true" item="browser"/>
294<iref primary="true" item="spider"/>
295<iref primary="true" item="sender"/>
296<iref primary="true" item="recipient"/>
297<t>
298   The terms client and server refer only to the roles that
299   these programs perform for a particular connection.  The same program
300   might act as a client on some connections and a server on others.
301   We use the term "<x:dfn>user agent</x:dfn>" to refer to any of the various
302   client programs that initiate a request, including (but not limited to)
303   browsers, spiders (web-based robots), command-line tools, native
304   applications, and mobile apps.  The term "<x:dfn>origin server</x:dfn>" is
305   used to refer to the program that can originate authoritative responses to
306   a request. For general requirements, we use the terms
307   "<x:dfn>sender</x:dfn>" and "<x:dfn>recipient</x:dfn>" to refer to any
308   component that sends or receives, respectively, a given message.
309</t>
310<t>
311   HTTP relies upon the Uniform Resource Identifier (URI)
312   standard <xref target="RFC3986"/> to indicate the target resource
313   (<xref target="target-resource"/>) and relationships between resources.
314   Messages are passed in a format similar to that used by Internet mail
315   <xref target="RFC5322"/> and the Multipurpose Internet Mail Extensions
316   (MIME) <xref target="RFC2045"/> (see &diff-mime; for the differences
317   between HTTP and MIME messages).
318</t>
319<t>
320   Most HTTP communication consists of a retrieval request (GET) for
321   a representation of some resource identified by a URI.  In the
322   simplest case, this might be accomplished via a single bidirectional
323   connection (===) between the user agent (UA) and the origin server (O).
324</t>
325<figure><artwork type="drawing">
326         request   &gt;
327    <x:highlight>UA</x:highlight> ======================================= <x:highlight>O</x:highlight>
328                                &lt;   response
329</artwork></figure>
330<iref primary="true" item="message"/>
331<iref primary="true" item="request"/>
332<iref primary="true" item="response"/>
333<t>
334   A client sends an HTTP request to a server in the form of a <x:dfn>request</x:dfn>
335   message, beginning with a request-line that includes a method, URI, and
336   protocol version (<xref target="request.line"/>),
337   followed by header fields containing
338   request modifiers, client information, and representation metadata
339   (<xref target="header.fields"/>),
340   an empty line to indicate the end of the header section, and finally
341   a message body containing the payload body (if any,
342   <xref target="message.body"/>).
343</t>
344<t>
345   A server responds to a client's request by sending one or more HTTP
346   <x:dfn>response</x:dfn>
347   messages, each beginning with a status line that
348   includes the protocol version, a success or error code, and textual
349   reason phrase (<xref target="status.line"/>),
350   possibly followed by header fields containing server
351   information, resource metadata, and representation metadata
352   (<xref target="header.fields"/>),
353   an empty line to indicate the end of the header section, and finally
354   a message body containing the payload body (if any,
355   <xref target="message.body"/>).
356</t>
357<t>
358   A connection might be used for multiple request/response exchanges,
359   as defined in <xref target="persistent.connections"/>.
360</t>
361<t>
362   The following example illustrates a typical message exchange for a
363   GET request on the URI "http://www.example.com/hello.txt":
364</t>
365<figure><preamble>
366client request:
367</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
368GET /hello.txt HTTP/1.1
369User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
370Host: www.example.com
371Accept-Language: en, mi
372
373</artwork></figure>
374<figure><preamble>
375server response:
376</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
377HTTP/1.1 200 OK
378Date: Mon, 27 Jul 2009 12:28:53 GMT
379Server: Apache
380Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
381ETag: "34aa387-d-1568eb00"
382Accept-Ranges: bytes
383Content-Length: <x:length-of target="exbody"/>
384Vary: Accept-Encoding
385Content-Type: text/plain
386
387<x:span anchor="exbody">Hello World!
388</x:span></artwork></figure>
389</section>
390
391<section title="Implementation Diversity" anchor="implementation-diversity">
392<t>
393   When considering the design of HTTP, it is easy to fall into a trap of
394   thinking that all user agents are general-purpose browsers and all origin
395   servers are large public websites. That is not the case in practice.
396   Common HTTP user agents include household appliances, stereos, scales,
397   firmware update scripts, command-line programs, mobile apps,
398   and communication devices in a multitude of shapes and sizes.  Likewise,
399   common HTTP origin servers include home automation units, configurable
400   networking components, office machines, autonomous robots, news feeds,
401   traffic cameras, ad selectors, and video delivery platforms.
402</t>
403<t>
404   The term "user agent" does not imply that there is a human user directly
405   interacting with the software agent at the time of a request. In many
406   cases, a user agent is installed or configured to run in the background
407   and save its results for later inspection (or save only a subset of those
408   results that might be interesting or erroneous). Spiders, for example, are
409   typically given a start URI and configured to follow certain behavior while
410   crawling the Web as a hypertext graph.
411</t>
412<t>
413   The implementation diversity of HTTP means that we cannot assume the
414   user agent can make interactive suggestions to a user or provide adequate
415   warning for security or privacy options.  In the few cases where this
416   specification requires reporting of errors to the user, it is acceptable
417   for such reporting to only be observable in an error console or log file.
418   Likewise, requirements that an automated action be confirmed by the user
419   before proceeding can be met via advance configuration choices,
420   run-time options, or simply not proceeding with the unsafe action.
421</t>
422</section>
423
424<section title="Intermediaries" anchor="intermediaries">
425<iref primary="true" item="intermediary"/>
426<t>
427   HTTP enables the use of intermediaries to satisfy requests through
428   a chain of connections.  There are three common forms of HTTP
429   <x:dfn>intermediary</x:dfn>: proxy, gateway, and tunnel.  In some cases,
430   a single intermediary might act as an origin server, proxy, gateway,
431   or tunnel, switching behavior based on the nature of each request.
432</t>
433<figure><artwork type="drawing">
434         &gt;             &gt;             &gt;             &gt;
435    <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>
436               &lt;             &lt;             &lt;             &lt;
437</artwork></figure>
438<t>
439   The figure above shows three intermediaries (A, B, and C) between the
440   user agent and origin server. A request or response message that
441   travels the whole chain will pass through four separate connections.
442   Some HTTP communication options
443   might apply only to the connection with the nearest, non-tunnel
444   neighbor, only to the end-points of the chain, or to all connections
445   along the chain. Although the diagram is linear, each participant might
446   be engaged in multiple, simultaneous communications. For example, B
447   might be receiving requests from many clients other than A, and/or
448   forwarding requests to servers other than C, at the same time that it
449   is handling A's request.
450</t>
451<t>
452<iref primary="true" item="upstream"/><iref primary="true" item="downstream"/>
453<iref primary="true" item="inbound"/><iref primary="true" item="outbound"/>
454   We use the terms "<x:dfn>upstream</x:dfn>" and "<x:dfn>downstream</x:dfn>"
455   to describe various requirements in relation to the directional flow of a
456   message: all messages flow from upstream to downstream.
457   Likewise, we use the terms inbound and outbound to refer to
458   directions in relation to the request path:
459   "<x:dfn>inbound</x:dfn>" means toward the origin server and
460   "<x:dfn>outbound</x:dfn>" means toward the user agent.
461</t>
462<t><iref primary="true" item="proxy"/>
463   A "<x:dfn>proxy</x:dfn>" is a message forwarding agent that is selected by the
464   client, usually via local configuration rules, to receive requests
465   for some type(s) of absolute URI and attempt to satisfy those
466   requests via translation through the HTTP interface.  Some translations
467   are minimal, such as for proxy requests for "http" URIs, whereas
468   other requests might require translation to and from entirely different
469   application-level protocols. Proxies are often used to group an
470   organization's HTTP requests through a common intermediary for the
471   sake of security, annotation services, or shared caching.
472</t>
473<t>
474<iref primary="true" item="transforming proxy"/>
475<iref primary="true" item="non-transforming proxy"/>
476   An HTTP-to-HTTP proxy is called a "<x:dfn>transforming proxy</x:dfn>" if it is designed
477   or configured to modify request or response messages in a semantically
478   meaningful way (i.e., modifications, beyond those required by normal
479   HTTP processing, that change the message in a way that would be
480   significant to the original sender or potentially significant to
481   downstream recipients).  For example, a transforming proxy might be
482   acting as a shared annotation server (modifying responses to include
483   references to a local annotation database), a malware filter, a
484   format transcoder, or an intranet-to-Internet privacy filter.  Such
485   transformations are presumed to be desired by the client (or client
486   organization) that selected the proxy and are beyond the scope of
487   this specification.  However, when a proxy is not intended to transform
488   a given message, we use the term "<x:dfn>non-transforming proxy</x:dfn>" to target
489   requirements that preserve HTTP message semantics. See &status-203; and
490   &header-warning; for status and warning codes related to transformations.
491</t>
492<t><iref primary="true" item="gateway"/><iref primary="true" item="reverse proxy"/>
493<iref primary="true" item="accelerator"/>
494   A "<x:dfn>gateway</x:dfn>" (a.k.a., "<x:dfn>reverse proxy</x:dfn>")
495   is a receiving agent that acts
496   as a layer above some other server(s) and translates the received
497   requests to the underlying server's protocol.  Gateways are often
498   used to encapsulate legacy or untrusted information services, to
499   improve server performance through "<x:dfn>accelerator</x:dfn>" caching, and to
500   enable partitioning or load-balancing of HTTP services across
501   multiple machines.
502</t>
503<t>
504   A gateway behaves as an origin server on its outbound connection and
505   as a user agent on its inbound connection.
506   All HTTP requirements applicable to an origin server
507   also apply to the outbound communication of a gateway.
508   A gateway communicates with inbound servers using any protocol that
509   it desires, including private extensions to HTTP that are outside
510   the scope of this specification.  However, an HTTP-to-HTTP gateway
511   that wishes to interoperate with third-party HTTP servers &MUST;
512   conform to HTTP user agent requirements on the gateway's inbound
513   connection and &MUST; implement the <x:ref>Connection</x:ref>
514   (<xref target="header.connection"/>) and <x:ref>Via</x:ref>
515   (<xref target="header.via"/>) header fields for both connections.
516</t>
517<t><iref primary="true" item="tunnel"/>
518   A "<x:dfn>tunnel</x:dfn>" acts as a blind relay between two connections
519   without changing the messages. Once active, a tunnel is not
520   considered a party to the HTTP communication, though the tunnel might
521   have been initiated by an HTTP request. A tunnel ceases to exist when
522   both ends of the relayed connection are closed. Tunnels are used to
523   extend a virtual connection through an intermediary, such as when
524   Transport Layer Security (TLS, <xref target="RFC5246"/>) is used to
525   establish confidential communication through a shared firewall proxy.
526</t>
527<t><iref primary="true" item="interception proxy"/>
528<iref primary="true" item="transparent proxy"/>
529<iref primary="true" item="captive portal"/>
530   The above categories for intermediary only consider those acting as
531   participants in the HTTP communication.  There are also intermediaries
532   that can act on lower layers of the network protocol stack, filtering or
533   redirecting HTTP traffic without the knowledge or permission of message
534   senders. Network intermediaries often introduce security flaws or
535   interoperability problems by violating HTTP semantics.  For example, an
536   "<x:dfn>interception proxy</x:dfn>" <xref target="RFC3040"/> (also commonly
537   known as a "<x:dfn>transparent proxy</x:dfn>" <xref target="RFC1919"/> or
538   "<x:dfn>captive portal</x:dfn>")
539   differs from an HTTP proxy because it is not selected by the client.
540   Instead, an interception proxy filters or redirects outgoing TCP port 80
541   packets (and occasionally other common port traffic).
542   Interception proxies are commonly found on public network access points,
543   as a means of enforcing account subscription prior to allowing use of
544   non-local Internet services, and within corporate firewalls to enforce
545   network usage policies.
546   They are indistinguishable from a man-in-the-middle attack.
547</t>
548<t>
549   HTTP is defined as a stateless protocol, meaning that each request message
550   can be understood in isolation.  Many implementations depend on HTTP's
551   stateless design in order to reuse proxied connections or dynamically
552   load-balance requests across multiple servers.  Hence, servers &MUST-NOT;
553   assume that two requests on the same connection are from the same user
554   agent unless the connection is secured and specific to that agent.
555   Some non-standard HTTP extensions (e.g., <xref target="RFC4559"/>) have
556   been known to violate this requirement, resulting in security and
557   interoperability problems.
558</t>
559</section>
560
561<section title="Caches" anchor="caches">
562<iref primary="true" item="cache"/>
563<t>
564   A "<x:dfn>cache</x:dfn>" is a local store of previous response messages and the
565   subsystem that controls its message storage, retrieval, and deletion.
566   A cache stores cacheable responses in order to reduce the response
567   time and network bandwidth consumption on future, equivalent
568   requests. Any client or server &MAY; employ a cache, though a cache
569   cannot be used by a server while it is acting as a tunnel.
570</t>
571<t>
572   The effect of a cache is that the request/response chain is shortened
573   if one of the participants along the chain has a cached response
574   applicable to that request. The following illustrates the resulting
575   chain if B has a cached copy of an earlier response from O (via C)
576   for a request which has not been cached by UA or A.
577</t>
578<figure><artwork type="drawing">
579            &gt;             &gt;
580       <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>
581                  &lt;             &lt;
582</artwork></figure>
583<t><iref primary="true" item="cacheable"/>
584   A response is "<x:dfn>cacheable</x:dfn>" if a cache is allowed to store a copy of
585   the response message for use in answering subsequent requests.
586   Even when a response is cacheable, there might be additional
587   constraints placed by the client or by the origin server on when
588   that cached response can be used for a particular request. HTTP
589   requirements for cache behavior and cacheable responses are
590   defined in &caching-overview;
591</t>
592<t>
593   There are a wide variety of architectures and configurations
594   of caches deployed across the World Wide Web and
595   inside large organizations. These include national hierarchies
596   of proxy caches to save transoceanic bandwidth, collaborative systems that
597   broadcast or multicast cache entries, archives of pre-fetched cache
598   entries for use in off-line or high-latency environments, and so on.
599</t>
600</section>
601
602<section title="Conformance and Error Handling" anchor="conformance">
603<t>
604   This specification targets conformance criteria according to the role of
605   a participant in HTTP communication.  Hence, HTTP requirements are placed
606   on senders, recipients, clients, servers, user agents, intermediaries,
607   origin servers, proxies, gateways, or caches, depending on what behavior
608   is being constrained by the requirement. Additional (social) requirements
609   are placed on implementations, resource owners, and protocol element
610   registrations when they apply beyond the scope of a single communication.
611</t>
612<t>
613   The verb "generate" is used instead of "send" where a requirement
614   differentiates between creating a protocol element and merely forwarding a
615   received element downstream.
616</t>
617<t>
618   An implementation is considered conformant if it complies with all of the
619   requirements associated with the roles it partakes in HTTP. Note that
620   SHOULD-level requirements are relevant here, unless one of the documented
621   exceptions is applicable.
622</t>
623<t>
624   Conformance applies to both the syntax and semantics of HTTP protocol
625   elements. A sender &MUST-NOT; generate protocol elements that convey a
626   meaning that is known by that sender to be false. A sender &MUST-NOT;
627   generate protocol elements that do not match the grammar defined by the
628   ABNF rules for those protocol elements that are applicable to the sender's
629   role. If a received protocol element is processed, the recipient &MUST; be
630   able to parse any value that would match the ABNF rules for that protocol
631   element, excluding only those rules not applicable to the recipient's role.
632</t>
633<t>
634   Unless noted otherwise, a recipient &MAY; attempt to recover a usable
635   protocol element from an invalid construct.  HTTP does not define
636   specific error handling mechanisms except when they have a direct impact
637   on security, since different applications of the protocol require
638   different error handling strategies.  For example, a Web browser might
639   wish to transparently recover from a response where the
640   <x:ref>Location</x:ref> header field doesn't parse according to the ABNF,
641   whereas a systems control client might consider any form of error recovery
642   to be dangerous.
643</t>
644</section>
645
646<section title="Protocol Versioning" anchor="http.version">
647  <x:anchor-alias value="HTTP-version"/>
648  <x:anchor-alias value="HTTP-name"/>
649<t>
650   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate
651   versions of the protocol. This specification defines version "1.1".
652   The protocol version as a whole indicates the sender's conformance
653   with the set of requirements laid out in that version's corresponding
654   specification of HTTP.
655</t>
656<t>
657   The version of an HTTP message is indicated by an HTTP-version field
658   in the first line of the message. HTTP-version is case-sensitive.
659</t>
660<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-version"/><iref primary="true" item="Grammar" subitem="HTTP-name"/>
661  <x:ref>HTTP-version</x:ref>  = <x:ref>HTTP-name</x:ref> "/" <x:ref>DIGIT</x:ref> "." <x:ref>DIGIT</x:ref>
662  <x:ref>HTTP-name</x:ref>     = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
663</artwork></figure>
664<t>
665   The HTTP version number consists of two decimal digits separated by a "."
666   (period or decimal point).  The first digit ("major version") indicates the
667   HTTP messaging syntax, whereas the second digit ("minor version") indicates
668   the highest minor version to which the sender is
669   conformant and able to understand for future communication.  The minor
670   version advertises the sender's communication capabilities even when the
671   sender is only using a backwards-compatible subset of the protocol,
672   thereby letting the recipient know that more advanced features can
673   be used in response (by servers) or in future requests (by clients).
674</t>
675<t>
676   When an HTTP/1.1 message is sent to an HTTP/1.0 recipient
677   <xref target="RFC1945"/> or a recipient whose version is unknown,
678   the HTTP/1.1 message is constructed such that it can be interpreted
679   as a valid HTTP/1.0 message if all of the newer features are ignored.
680   This specification places recipient-version requirements on some
681   new features so that a conformant sender will only use compatible
682   features until it has determined, through configuration or the
683   receipt of a message, that the recipient supports HTTP/1.1.
684</t>
685<t>
686   The interpretation of a header field does not change between minor
687   versions of the same major HTTP version, though the default
688   behavior of a recipient in the absence of such a field can change.
689   Unless specified otherwise, header fields defined in HTTP/1.1 are
690   defined for all versions of HTTP/1.x.  In particular, the <x:ref>Host</x:ref>
691   and <x:ref>Connection</x:ref> header fields ought to be implemented by all
692   HTTP/1.x implementations whether or not they advertise conformance with
693   HTTP/1.1.
694</t>
695<t>
696   New header fields can be defined such that, when they are
697   understood by a recipient, they might override or enhance the
698   interpretation of previously defined header fields.  When an
699   implementation receives an unrecognized header field, the recipient
700   &MUST; ignore that header field for local processing regardless of
701   the message's HTTP version.  An unrecognized header field received
702   by a proxy &MUST; be forwarded downstream unless the header field's
703   field-name is listed in the message's <x:ref>Connection</x:ref> header field
704   (see <xref target="header.connection"/>).
705   These requirements allow HTTP's functionality to be enhanced without
706   requiring prior update of deployed intermediaries.
707</t>
708<t>
709   Intermediaries that process HTTP messages (i.e., all intermediaries
710   other than those acting as tunnels) &MUST; send their own HTTP-version
711   in forwarded messages.  In other words, they &MUST-NOT; blindly
712   forward the first line of an HTTP message without ensuring that the
713   protocol version in that message matches a version to which that
714   intermediary is conformant for both the receiving and
715   sending of messages.  Forwarding an HTTP message without rewriting
716   the HTTP-version might result in communication errors when downstream
717   recipients use the message sender's version to determine what features
718   are safe to use for later communication with that sender.
719</t>
720<t>
721   An HTTP client &SHOULD; send a request version equal to the highest
722   version to which the client is conformant and
723   whose major version is no higher than the highest version supported
724   by the server, if this is known.  An HTTP client &MUST-NOT; send a
725   version to which it is not conformant.
726</t>
727<t>
728   An HTTP client &MAY; send a lower request version if it is known that
729   the server incorrectly implements the HTTP specification, but only
730   after the client has attempted at least one normal request and determined
731   from the response status or header fields (e.g., <x:ref>Server</x:ref>) that
732   the server improperly handles higher request versions.
733</t>
734<t>
735   An HTTP server &SHOULD; send a response version equal to the highest
736   version to which the server is conformant and
737   whose major version is less than or equal to the one received in the
738   request.  An HTTP server &MUST-NOT; send a version to which it is not
739   conformant.  A server &MAY; send a <x:ref>505 (HTTP Version Not
740   Supported)</x:ref> response if it cannot send a response using the
741   major version used in the client's request.
742</t>
743<t>
744   An HTTP server &MAY; send an HTTP/1.0 response to an HTTP/1.0 request
745   if it is known or suspected that the client incorrectly implements the
746   HTTP specification and is incapable of correctly processing later
747   version responses, such as when a client fails to parse the version
748   number correctly or when an intermediary is known to blindly forward
749   the HTTP-version even when it doesn't conform to the given minor
750   version of the protocol. Such protocol downgrades &SHOULD-NOT; be
751   performed unless triggered by specific client attributes, such as when
752   one or more of the request header fields (e.g., <x:ref>User-Agent</x:ref>)
753   uniquely match the values sent by a client known to be in error.
754</t>
755<t>
756   The intention of HTTP's versioning design is that the major number
757   will only be incremented if an incompatible message syntax is
758   introduced, and that the minor number will only be incremented when
759   changes made to the protocol have the effect of adding to the message
760   semantics or implying additional capabilities of the sender.  However,
761   the minor version was not incremented for the changes introduced between
762   <xref target="RFC2068"/> and <xref target="RFC2616"/>, and this revision
763   has specifically avoiding any such changes to the protocol.
764</t>
765</section>
766
767<section title="Uniform Resource Identifiers" anchor="uri">
768<iref primary="true" item="resource"/>
769<t>
770   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used
771   throughout HTTP as the means for identifying resources (&resource;).
772   URI references are used to target requests, indicate redirects, and define
773   relationships.
774</t>
775  <x:anchor-alias value="URI-reference"/>
776  <x:anchor-alias value="absolute-URI"/>
777  <x:anchor-alias value="relative-part"/>
778  <x:anchor-alias value="authority"/>
779  <x:anchor-alias value="path-abempty"/>
780  <x:anchor-alias value="path-absolute"/>
781  <x:anchor-alias value="port"/>
782  <x:anchor-alias value="query"/>
783  <x:anchor-alias value="uri-host"/>
784  <x:anchor-alias value="partial-URI"/>
785<t>
786   This specification adopts the definitions of "URI-reference",
787   "absolute-URI", "relative-part", "port", "host",
788   "path-abempty", "path-absolute", "query", and "authority" from the
789   URI generic syntax.
790   In addition, we define a partial-URI rule for protocol elements
791   that allow a relative URI but not a fragment.
792</t>
793<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>
794  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
795  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
796  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
797  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
798  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
799  <x:ref>path-absolute</x:ref> = &lt;path-absolute, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
800  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
801  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
802  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
803 
804  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
805</artwork></figure>
806<t>
807   Each protocol element in HTTP that allows a URI reference will indicate
808   in its ABNF production whether the element allows any form of reference
809   (URI-reference), only a URI in absolute form (absolute-URI), only the
810   path and optional query components, or some combination of the above.
811   Unless otherwise indicated, URI references are parsed
812   relative to the effective request URI
813   (<xref target="effective.request.uri"/>).
814</t>
815
816<section title="http URI scheme" anchor="http.uri">
817  <x:anchor-alias value="http-URI"/>
818  <iref item="http URI scheme" primary="true"/>
819  <iref item="URI scheme" subitem="http" primary="true"/>
820<t>
821   The "http" URI scheme is hereby defined for the purpose of minting
822   identifiers according to their association with the hierarchical
823   namespace governed by a potential HTTP origin server listening for
824   TCP connections on a given port.
825</t>
826<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"><!--terminal production--></iref>
827  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
828</artwork></figure>
829<t>
830   The HTTP origin server is identified by the generic syntax's
831   <x:ref>authority</x:ref> component, which includes a host identifier
832   and optional TCP port (<xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>).
833   The remainder of the URI, consisting of both the hierarchical path
834   component and optional query component, serves as an identifier for
835   a potential resource within that origin server's name space.
836</t>
837<t>
838   If the host identifier is provided as an IP address,
839   then the origin server is any listener on the indicated TCP port at
840   that IP address. If host is a registered name, then that name is
841   considered an indirect identifier and the recipient might use a name
842   resolution service, such as DNS, to find the address of a listener
843   for that host.
844   The host &MUST-NOT; be empty; if an "http" URI is received with an
845   empty host, then it &MUST; be rejected as invalid.
846   If the port subcomponent is empty or not given, then TCP port 80 is
847   assumed (the default reserved port for WWW services).
848</t>
849<t>
850   Regardless of the form of host identifier, access to that host is not
851   implied by the mere presence of its name or address. The host might or might
852   not exist and, even when it does exist, might or might not be running an
853   HTTP server or listening to the indicated port. The "http" URI scheme
854   makes use of the delegated nature of Internet names and addresses to
855   establish a naming authority (whatever entity has the ability to place
856   an HTTP server at that Internet name or address) and allows that
857   authority to determine which names are valid and how they might be used.
858</t>
859<t>
860   When an "http" URI is used within a context that calls for access to the
861   indicated resource, a client &MAY; attempt access by resolving
862   the host to an IP address, establishing a TCP connection to that address
863   on the indicated port, and sending an HTTP request message
864   (<xref target="http.message"/>) containing the URI's identifying data
865   (<xref target="message.routing"/>) to the server.
866   If the server responds to that request with a non-interim HTTP response
867   message, as described in &status-codes;, then that response
868   is considered an authoritative answer to the client's request.
869</t>
870<t>
871   Although HTTP is independent of the transport protocol, the "http"
872   scheme is specific to TCP-based services because the name delegation
873   process depends on TCP for establishing authority.
874   An HTTP service based on some other underlying connection protocol
875   would presumably be identified using a different URI scheme, just as
876   the "https" scheme (below) is used for resources that require an
877   end-to-end secured connection. Other protocols might also be used to
878   provide access to "http" identified resources &mdash; it is only the
879   authoritative interface used for mapping the namespace that is
880   specific to TCP.
881</t>
882<t>
883   The URI generic syntax for authority also includes a deprecated
884   userinfo subcomponent (<xref target="RFC3986" x:fmt="," x:sec="3.2.1"/>)
885   for including user authentication information in the URI.  Some
886   implementations make use of the userinfo component for internal
887   configuration of authentication information, such as within command
888   invocation options, configuration files, or bookmark lists, even
889   though such usage might expose a user identifier or password.
890   Senders &MUST; exclude the userinfo subcomponent (and its "@"
891   delimiter) when an "http" URI is transmitted within a message as a
892   request target or header field value.
893   Recipients of an "http" URI reference &SHOULD; parse for userinfo and
894   treat its presence as an error, since it is likely being used to obscure
895   the authority for the sake of phishing attacks.
896</t>
897</section>
898
899<section title="https URI scheme" anchor="https.uri">
900   <x:anchor-alias value="https-URI"/>
901   <iref item="https URI scheme"/>
902   <iref item="URI scheme" subitem="https"/>
903<t>
904   The "https" URI scheme is hereby defined for the purpose of minting
905   identifiers according to their association with the hierarchical
906   namespace governed by a potential HTTP origin server listening to a
907   given TCP port for TLS-secured connections <xref target="RFC5246"/>.
908</t>
909<t>
910   All of the requirements listed above for the "http" scheme are also
911   requirements for the "https" scheme, except that a default TCP port
912   of 443 is assumed if the port subcomponent is empty or not given,
913   and the TCP connection &MUST; be secured, end-to-end, through the
914   use of strong encryption prior to sending the first HTTP request.
915</t>
916<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="https-URI"><!--terminal production--></iref>
917  <x:ref>https-URI</x:ref> = "https:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
918</artwork></figure>
919<t>
920   Unlike the "http" scheme, responses to "https" identified requests
921   are never "public" and thus &MUST-NOT; be reused for shared caching.
922   They can, however, be reused in a private cache if the message is
923   cacheable by default in HTTP or specifically indicated as such by
924   the Cache-Control header field (&header-cache-control;).
925</t>
926<t>
927   Resources made available via the "https" scheme have no shared
928   identity with the "http" scheme even if their resource identifiers
929   indicate the same authority (the same host listening to the same
930   TCP port).  They are distinct name spaces and are considered to be
931   distinct origin servers.  However, an extension to HTTP that is
932   defined to apply to entire host domains, such as the Cookie protocol
933   <xref target="RFC6265"/>, can allow information
934   set by one service to impact communication with other services
935   within a matching group of host domains.
936</t>
937<t>
938   The process for authoritative access to an "https" identified
939   resource is defined in <xref target="RFC2818"/>.
940</t>
941</section>
942
943<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
944<t>
945   Since the "http" and "https" schemes conform to the URI generic syntax,
946   such URIs are normalized and compared according to the algorithm defined
947   in <xref target="RFC3986" x:fmt="," x:sec="6"/>, using the defaults
948   described above for each scheme.
949</t>
950<t>
951   If the port is equal to the default port for a scheme, the normal
952   form is to elide the port subcomponent. Likewise, an empty path
953   component is equivalent to an absolute path of "/", so the normal
954   form is to provide a path of "/" instead. The scheme and host
955   are case-insensitive and normally provided in lowercase; all
956   other components are compared in a case-sensitive manner.
957   Characters other than those in the "reserved" set are equivalent
958   to their percent-encoded octets (see <xref target="RFC3986"
959   x:fmt="," x:sec="2.1"/>): the normal form is to not encode them.
960</t>
961<t>
962   For example, the following three URIs are equivalent:
963</t>
964<figure><artwork type="example">
965   http://example.com:80/~smith/home.html
966   http://EXAMPLE.com/%7Esmith/home.html
967   http://EXAMPLE.com:/%7esmith/home.html
968</artwork></figure>
969</section>
970</section>
971</section>
972
973<section title="Message Format" anchor="http.message">
974<x:anchor-alias value="generic-message"/>
975<x:anchor-alias value="message.types"/>
976<x:anchor-alias value="HTTP-message"/>
977<x:anchor-alias value="start-line"/>
978<iref item="header section"/>
979<iref item="headers"/>
980<iref item="header field"/>
981<t>
982   All HTTP/1.1 messages consist of a start-line followed by a sequence of
983   octets in a format similar to the Internet Message Format
984   <xref target="RFC5322"/>: zero or more header fields (collectively
985   referred to as the "headers" or the "header section"), an empty line
986   indicating the end of the header section, and an optional message body.
987</t>
988<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"><!--terminal production--></iref>
989  <x:ref>HTTP-message</x:ref>   = <x:ref>start-line</x:ref>
990                   *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
991                   <x:ref>CRLF</x:ref>
992                   [ <x:ref>message-body</x:ref> ]
993</artwork></figure>
994<t>
995   The normal procedure for parsing an HTTP message is to read the
996   start-line into a structure, read each header field into a hash
997   table by field name until the empty line, and then use the parsed
998   data to determine if a message body is expected.  If a message body
999   has been indicated, then it is read as a stream until an amount
1000   of octets equal to the message body length is read or the connection
1001   is closed.
1002</t>
1003<t>
1004   Recipients &MUST; parse an HTTP message as a sequence of octets in an
1005   encoding that is a superset of US-ASCII <xref target="USASCII"/>.
1006   Parsing an HTTP message as a stream of Unicode characters, without regard
1007   for the specific encoding, creates security vulnerabilities due to the
1008   varying ways that string processing libraries handle invalid multibyte
1009   character sequences that contain the octet LF (%x0A).  String-based
1010   parsers can only be safely used within protocol elements after the element
1011   has been extracted from the message, such as within a header field-value
1012   after message parsing has delineated the individual fields.
1013</t>
1014<t>
1015   An HTTP message can be parsed as a stream for incremental processing or
1016   forwarding downstream.  However, recipients cannot rely on incremental
1017   delivery of partial messages, since some implementations will buffer or
1018   delay message forwarding for the sake of network efficiency, security
1019   checks, or payload transformations.
1020</t>
1021
1022<section title="Start Line" anchor="start.line">
1023  <x:anchor-alias value="Start-Line"/>
1024<t>
1025   An HTTP message can either be a request from client to server or a
1026   response from server to client.  Syntactically, the two types of message
1027   differ only in the start-line, which is either a request-line (for requests)
1028   or a status-line (for responses), and in the algorithm for determining
1029   the length of the message body (<xref target="message.body"/>).
1030</t>
1031<t>
1032   In theory, a client could receive requests and a server could receive
1033   responses, distinguishing them by their different start-line formats,
1034   but in practice servers are implemented to only expect a request
1035   (a response is interpreted as an unknown or invalid request method)
1036   and clients are implemented to only expect a response.
1037</t>
1038<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="start-line"/>
1039  <x:ref>start-line</x:ref>     = <x:ref>request-line</x:ref> / <x:ref>status-line</x:ref>
1040</artwork></figure>
1041<t>
1042   A sender &MUST-NOT; send whitespace between the start-line and
1043   the first header field. The presence of such whitespace in a request
1044   might be an attempt to trick a server into ignoring that field or
1045   processing the line after it as a new request, either of which might
1046   result in a security vulnerability if other implementations within
1047   the request chain interpret the same message differently.
1048   Likewise, the presence of such whitespace in a response might be
1049   ignored by some clients or cause others to cease parsing.
1050</t>
1051<t>
1052   A recipient that receives whitespace between the start-line and
1053   the first header field &MUST; either reject the message as invalid or
1054   consume each whitespace-preceded line without further processing of it
1055   (i.e., ignore the entire line, along with any subsequent lines preceded
1056   by whitespace, until a properly formed header field is received or the
1057   header block is terminated).
1058</t>
1059
1060<section title="Request Line" anchor="request.line">
1061  <x:anchor-alias value="Request"/>
1062  <x:anchor-alias value="request-line"/>
1063<t>
1064   A request-line begins with a method token, followed by a single
1065   space (SP), the request-target, another single space (SP), the
1066   protocol version, and ending with CRLF.
1067</t>
1068<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1069  <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>
1070</artwork></figure>
1071<iref primary="true" item="method"/>
1072<t anchor="method">
1073   The method token indicates the request method to be performed on the
1074   target resource. The request method is case-sensitive.
1075</t>
1076<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1077  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1078</artwork></figure>
1079<t>
1080   The methods defined by this specification can be found in
1081   &methods;, along with information regarding the HTTP method registry
1082   and considerations for defining new methods.
1083</t>
1084<iref item="request-target"/>
1085<t>
1086   The request-target identifies the target resource upon which to apply
1087   the request, as defined in <xref target="request-target"/>.
1088</t>
1089<t>
1090   No whitespace is allowed inside the method, request-target, and
1091   protocol version.  Hence, recipients typically parse the request-line
1092   into its component parts by splitting on whitespace
1093   (see <xref target="message.robustness"/>).
1094</t>
1095<t>
1096   Unfortunately, some user agents fail to properly encode hypertext
1097   references that have embedded whitespace, sending the characters directly
1098   instead of properly encoding or excluding the disallowed characters.
1099   Recipients of an invalid request-line &SHOULD; respond with either a
1100   <x:ref>400 (Bad Request)</x:ref> error or a <x:ref>301 (Moved Permanently)</x:ref>
1101   redirect with the request-target properly encoded.  Recipients &SHOULD-NOT;
1102   attempt to autocorrect and then process the request without a redirect,
1103   since the invalid request-line might be deliberately crafted to bypass
1104   security filters along the request chain.
1105</t>
1106<t>
1107   HTTP does not place a pre-defined limit on the length of a request-line.
1108   A server that receives a method longer than any that it implements
1109   &SHOULD; respond with either a <x:ref>405 (Method Not Allowed)</x:ref>, if it is an origin
1110   server, or a <x:ref>501 (Not Implemented)</x:ref> status code.
1111   A server &MUST; be prepared to receive URIs of unbounded length and
1112   respond with the <x:ref>414 (URI Too Long)</x:ref> status code if the received
1113   request-target would be longer than the server wishes to handle
1114   (see &status-414;).
1115</t>
1116<t>
1117   Various ad-hoc limitations on request-line length are found in practice.
1118   It is &RECOMMENDED; that all HTTP senders and recipients support, at a
1119   minimum, request-line lengths of 8000 octets.
1120</t>
1121</section>
1122
1123<section title="Status Line" anchor="status.line">
1124  <x:anchor-alias value="response"/>
1125  <x:anchor-alias value="status-line"/>
1126  <x:anchor-alias value="status-code"/>
1127  <x:anchor-alias value="reason-phrase"/>
1128<t>
1129   The first line of a response message is the status-line, consisting
1130   of the protocol version, a space (SP), the status code, another space,
1131   a possibly-empty textual phrase describing the status code, and
1132   ending with CRLF.
1133</t>
1134<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-line"/>
1135  <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>
1136</artwork></figure>
1137<t>
1138   The status-code element is a 3-digit integer code describing the
1139   result of the server's attempt to understand and satisfy the client's
1140   corresponding request. The rest of the response message is to be
1141   interpreted in light of the semantics defined for that status code.
1142   See &status-codes; for information about the semantics of status codes,
1143   including the classes of status code (indicated by the first digit),
1144   the status codes defined by this specification, considerations for the
1145   definition of new status codes, and the IANA registry.
1146</t>
1147<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-code"/>
1148  <x:ref>status-code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1149</artwork></figure>
1150<t>   
1151   The reason-phrase element exists for the sole purpose of providing a
1152   textual description associated with the numeric status code, mostly
1153   out of deference to earlier Internet application protocols that were more
1154   frequently used with interactive text clients. A client &SHOULD; ignore
1155   the reason-phrase content.
1156</t>
1157<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
1158  <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> )
1159</artwork></figure>
1160</section>
1161</section>
1162
1163<section title="Header Fields" anchor="header.fields">
1164  <x:anchor-alias value="header-field"/>
1165  <x:anchor-alias value="field-content"/>
1166  <x:anchor-alias value="field-name"/>
1167  <x:anchor-alias value="field-value"/>
1168  <x:anchor-alias value="obs-fold"/>
1169<t>
1170   Each HTTP header field consists of a case-insensitive field name
1171   followed by a colon (":"), optional whitespace, and the field value.
1172</t>
1173<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"/>
1174  <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>
1175  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1176  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>obs-fold</x:ref> )
1177  <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> )
1178  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref> ( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1179                 ; obsolete line folding
1180                 ; see <xref target="field.parsing"/>
1181</artwork></figure>
1182<t>
1183   The field-name token labels the corresponding field-value as having the
1184   semantics defined by that header field.  For example, the <x:ref>Date</x:ref>
1185   header field is defined in &header-date; as containing the origination
1186   timestamp for the message in which it appears.
1187</t>
1188
1189<section title="Field Extensibility" anchor="field.extensibility">
1190<t>
1191   HTTP header fields are fully extensible: there is no limit on the
1192   introduction of new field names, each presumably defining new semantics,
1193   nor on the number of header fields used in a given message.  Existing
1194   fields are defined in each part of this specification and in many other
1195   specifications outside the core standard.
1196   New header fields can be introduced without changing the protocol version
1197   if their defined semantics allow them to be safely ignored by recipients
1198   that do not recognize them.
1199</t>
1200<t>
1201   New HTTP header fields &SHOULD; be registered with IANA in the
1202   Message Header Field Registry, as described in &iana-header-registry;.
1203   Unrecognized header fields &MUST; be forwarded by a proxy unless the
1204   field-name is listed in the <x:ref>Connection</x:ref> header field
1205   (<xref target="header.connection"/>) or the proxy is specifically
1206   configured to block or otherwise transform such fields.
1207   Unrecognized header fields &SHOULD; be ignored by other recipients.
1208</t>
1209</section>
1210
1211<section title="Field Order" anchor="field.order">
1212<t>
1213   The order in which header fields with differing field names are
1214   received is not significant. However, it is "good practice" to send
1215   header fields that contain control data first, such as <x:ref>Host</x:ref>
1216   on requests and <x:ref>Date</x:ref> on responses, so that implementations
1217   can decide when not to handle a message as early as possible.  A server
1218   &MUST; wait until the entire header section is received before interpreting
1219   a request message, since later header fields might include conditionals,
1220   authentication credentials, or deliberately misleading duplicate
1221   header fields that would impact request processing.
1222</t>
1223<t>
1224   Multiple header fields with the same field name &MUST-NOT; be
1225   sent in a message unless the entire field value for that
1226   header field is defined as a comma-separated list [i.e., #(values)].
1227</t>
1228<t>
1229   Multiple header fields with the same field name can be combined into
1230   one "field-name: field-value" pair, without changing the semantics of the
1231   message, by appending each subsequent field value to the combined
1232   field value in order, separated by a comma. The order in which
1233   header fields with the same field name are received is therefore
1234   significant to the interpretation of the combined field value;
1235   a proxy &MUST-NOT; change the order of these field values when
1236   forwarding a message.
1237</t>
1238<x:note>
1239  <t>
1240   &Note; In practice, the "Set-Cookie" header field (<xref target="RFC6265"/>)
1241   often appears multiple times in a response message and does not use the
1242   list syntax, violating the above requirements on multiple header fields
1243   with the same name. Since it cannot be combined into a single field-value,
1244   recipients ought to handle "Set-Cookie" as a special case while processing
1245   header fields. (See Appendix A.2.3 of <xref target="Kri2001"/> for details.)
1246  </t>
1247</x:note>
1248</section>
1249
1250<section title="Whitespace" anchor="whitespace">
1251<t anchor="rule.LWS">
1252   This specification uses three rules to denote the use of linear
1253   whitespace: OWS (optional whitespace), RWS (required whitespace), and
1254   BWS ("bad" whitespace).
1255</t>
1256<t anchor="rule.OWS">
1257   The OWS rule is used where zero or more linear whitespace octets might
1258   appear. OWS &SHOULD; either not be generated or be generated as a single
1259   SP. Multiple OWS octets that occur within field-content &SHOULD; either
1260   be replaced with a single SP or transformed to all SP octets (each
1261   octet other than SP replaced with SP) before interpreting the field value
1262   or forwarding the message downstream.
1263</t>
1264<t anchor="rule.RWS">
1265   RWS is used when at least one linear whitespace octet is required to
1266   separate field tokens. RWS &SHOULD; be generated as a single SP.
1267   Multiple RWS octets that occur within field-content &SHOULD; either
1268   be replaced with a single SP or transformed to all SP octets before
1269   interpreting the field value or forwarding the message downstream.
1270</t>
1271<t anchor="rule.BWS">
1272   BWS is used where the grammar allows optional whitespace, for historical
1273   reasons, but senders &SHOULD-NOT; generate it in messages;
1274   recipients &MUST; accept such bad optional whitespace and remove it before
1275   interpreting the field value or forwarding the message downstream.
1276</t>
1277<t anchor="rule.whitespace">
1278  <x:anchor-alias value="BWS"/>
1279  <x:anchor-alias value="OWS"/>
1280  <x:anchor-alias value="RWS"/>
1281</t>
1282<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"/>
1283  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1284                 ; optional whitespace
1285  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1286                 ; required whitespace
1287  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1288                 ; "bad" whitespace
1289</artwork></figure>
1290</section>
1291
1292<section title="Field Parsing" anchor="field.parsing">
1293<t>
1294   No whitespace is allowed between the header field-name and colon.
1295   In the past, differences in the handling of such whitespace have led to
1296   security vulnerabilities in request routing and response handling.
1297   Any received request message that contains whitespace between a header
1298   field-name and colon &MUST; be rejected with a response code of 400
1299   (Bad Request).  A proxy &MUST; remove any such whitespace from a response
1300   message before forwarding the message downstream.
1301</t>
1302<t>
1303   A field value is preceded by optional whitespace (OWS); a single SP is
1304   preferred. The field value does not include any leading or trailing white
1305   space: OWS occurring before the first non-whitespace octet of the
1306   field value or after the last non-whitespace octet of the field value
1307   is ignored and &SHOULD; be removed before further processing (as this does
1308   not change the meaning of the header field).
1309</t>
1310<t>
1311   Historically, HTTP header field values could be extended over multiple
1312   lines by preceding each extra line with at least one space or horizontal
1313   tab (obs-fold). This specification deprecates such line
1314   folding except within the message/http media type
1315   (<xref target="internet.media.type.message.http"/>).
1316   HTTP senders &MUST-NOT; generate messages that include line folding
1317   (i.e., that contain any field-value that matches the obs-fold rule) unless
1318   the message is intended for packaging within the message/http media type.
1319   HTTP recipients &SHOULD; accept line folding and replace any embedded
1320   obs-fold whitespace with either a single SP or a matching number of SP
1321   octets (to avoid buffer copying) prior to interpreting the field value or
1322   forwarding the message downstream.
1323</t>
1324<t>
1325   Historically, HTTP has allowed field content with text in the ISO-8859-1
1326   <xref target="ISO-8859-1"/> charset, supporting other charsets only
1327   through use of <xref target="RFC2047"/> encoding.
1328   In practice, most HTTP header field values use only a subset of the
1329   US-ASCII charset <xref target="USASCII"/>. Newly defined
1330   header fields &SHOULD; limit their field values to US-ASCII octets.
1331   Recipients &SHOULD; treat other octets in field content (obs-text) as
1332   opaque data.
1333</t>
1334</section>
1335
1336<section title="Field Limits" anchor="field.limits">
1337<t>
1338   HTTP does not place a pre-defined limit on the length of each header field
1339   or on the length of the header block as a whole.  Various ad-hoc
1340   limitations on individual header field length are found in practice,
1341   often depending on the specific field semantics.
1342</t>
1343<t>
1344   A server &MUST; be prepared to receive request header fields of unbounded
1345   length and respond with an appropriate <x:ref>4xx (Client Error)</x:ref>
1346   status code if the received header field(s) are larger than the server
1347   wishes to process.
1348</t>
1349<t>
1350   A client &MUST; be prepared to receive response header fields of unbounded
1351   length. A client &MAY; discard or truncate received header fields that are
1352   larger than the client wishes to process if the field semantics are such
1353   that the dropped value(s) can be safely ignored without changing the
1354   response semantics.
1355</t>
1356</section>
1357
1358<section title="Field value components" anchor="field.components">
1359<t anchor="rule.token.separators">
1360  <x:anchor-alias value="tchar"/>
1361  <x:anchor-alias value="token"/>
1362  <x:anchor-alias value="special"/>
1363  <x:anchor-alias value="word"/>
1364   Many HTTP header field values consist of words (token or quoted-string)
1365   separated by whitespace or special characters. These special characters
1366   &MUST; be in a quoted string to be used within a parameter value (as defined
1367   in <xref target="transfer.codings"/>).
1368</t>
1369<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>
1370  <x:ref>word</x:ref>           = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1371
1372  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1373<!--
1374  IMPORTANT: when editing "tchar" make sure that "special" is updated accordingly!!!
1375 -->
1376  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1377                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1378                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1379                 ; any <x:ref>VCHAR</x:ref>, except <x:ref>special</x:ref>
1380
1381  <x:ref>special</x:ref>        = "(" / ")" / "&lt;" / ">" / "@" / ","
1382                 / ";" / ":" / "\" / DQUOTE / "/" / "["
1383                 / "]" / "?" / "=" / "{" / "}"
1384</artwork></figure>
1385<t anchor="rule.quoted-string">
1386  <x:anchor-alias value="quoted-string"/>
1387  <x:anchor-alias value="qdtext"/>
1388  <x:anchor-alias value="obs-text"/>
1389   A string of text is parsed as a single word if it is quoted using
1390   double-quote marks.
1391</t>
1392<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"/>
1393  <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>
1394  <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>
1395  <x:ref>obs-text</x:ref>       = %x80-FF
1396</artwork></figure>
1397<t anchor="rule.quoted-pair">
1398  <x:anchor-alias value="quoted-pair"/>
1399   The backslash octet ("\") can be used as a single-octet
1400   quoting mechanism within quoted-string constructs:
1401</t>
1402<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1403  <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> )
1404</artwork></figure>
1405<t>
1406   Recipients that process the value of a quoted-string &MUST; handle a
1407   quoted-pair as if it were replaced by the octet following the backslash.
1408</t>
1409<t>
1410   Senders &SHOULD-NOT; generate a quoted-pair in a quoted-string except where
1411   necessary to quote DQUOTE and backslash octets occurring within that string.
1412</t>
1413<t anchor="rule.comment">
1414  <x:anchor-alias value="comment"/>
1415  <x:anchor-alias value="ctext"/>
1416   Comments can be included in some HTTP header fields by surrounding
1417   the comment text with parentheses. Comments are only allowed in
1418   fields containing "comment" as part of their field value definition.
1419</t>
1420<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1421  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-cpair</x:ref> / <x:ref>comment</x:ref> ) ")"
1422  <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>
1423</artwork></figure>
1424<t anchor="rule.quoted-cpair">
1425  <x:anchor-alias value="quoted-cpair"/>
1426   The backslash octet ("\") can be used as a single-octet
1427   quoting mechanism within comment constructs:
1428</t>
1429<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-cpair"/>
1430  <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> )
1431</artwork></figure>
1432<t>
1433   Senders &SHOULD-NOT; escape octets in comments that do not require escaping
1434   (i.e., other than the backslash octet "\" and the parentheses "(" and ")").
1435</t>
1436</section>
1437
1438</section>
1439
1440<section title="Message Body" anchor="message.body">
1441  <x:anchor-alias value="message-body"/>
1442<t>
1443   The message body (if any) of an HTTP message is used to carry the
1444   payload body of that request or response.  The message body is
1445   identical to the payload body unless a transfer coding has been
1446   applied, as described in <xref target="header.transfer-encoding"/>.
1447</t>
1448<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1449  <x:ref>message-body</x:ref> = *OCTET
1450</artwork></figure>
1451<t>
1452   The rules for when a message body is allowed in a message differ for
1453   requests and responses.
1454</t>
1455<t>
1456   The presence of a message body in a request is signaled by a
1457   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1458   field. Request message framing is independent of method semantics,
1459   even if the method does not define any use for a message body.
1460</t>
1461<t>
1462   The presence of a message body in a response depends on both
1463   the request method to which it is responding and the response
1464   status code (<xref target="status.line"/>).
1465   Responses to the HEAD request method never include a message body
1466   because the associated response header fields (e.g.,
1467   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1468   if present, indicate only what their values would have been if the request
1469   method had been GET (&HEAD;).
1470   <x:ref>2xx (Successful)</x:ref> responses to CONNECT switch to tunnel
1471   mode instead of having a message body (&CONNECT;).
1472   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1473   <x:ref>304 (Not Modified)</x:ref> responses &MUST-NOT; include a message body.
1474   All other responses do include a message body, although the body
1475   &MAY; be of zero length.
1476</t>
1477
1478<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1479  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1480  <x:anchor-alias value="Transfer-Encoding"/>
1481<t>
1482   When one or more transfer codings are applied to a payload body in order
1483   to form the message body, a Transfer-Encoding header field &MUST; be sent
1484   in the message and &MUST; contain the list of corresponding
1485   transfer-coding names in the same order that they were applied.
1486   Transfer codings are defined in <xref target="transfer.codings"/>.
1487</t>
1488<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1489  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1490</artwork></figure>
1491<t>
1492   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1493   MIME, which was designed to enable safe transport of binary data over a
1494   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1495   However, safe transport has a different focus for an 8bit-clean transfer
1496   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1497   accurately delimit a dynamically generated payload and to distinguish
1498   payload encodings that are only applied for transport efficiency or
1499   security from those that are characteristics of the target resource.
1500</t>
1501<t>
1502   The "chunked" transfer-coding (<xref target="chunked.encoding"/>)
1503   &MUST; be implemented by all HTTP/1.1 recipients because it plays a
1504   crucial role in delimiting messages when the payload body size is not
1505   known in advance.
1506   When the "chunked" transfer-coding is used, it &MUST; be the last
1507   transfer-coding applied to form the message body and &MUST-NOT;
1508   be applied more than once in a message body.
1509   If any transfer-coding is applied to a request payload body,
1510   the final transfer-coding applied &MUST; be "chunked".
1511   If any transfer-coding is applied to a response payload body, then either
1512   the final transfer-coding applied &MUST; be "chunked" or
1513   the message &MUST; be terminated by closing the connection.
1514</t>
1515<figure><preamble>
1516   For example,
1517</preamble><artwork type="example">
1518  Transfer-Encoding: gzip, chunked
1519</artwork><postamble>
1520   indicates that the payload body has been compressed using the gzip
1521   coding and then chunked using the chunked coding while forming the
1522   message body.
1523</postamble></figure>
1524<t>
1525   If more than one Transfer-Encoding header field is present in a message,
1526   the multiple field-values &MUST; be combined into one field-value,
1527   according to the algorithm defined in <xref target="header.fields"/>,
1528   before determining the message body length.
1529</t>
1530<t>
1531   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1532   Transfer-Encoding is a property of the message, not of the payload, and thus
1533   &MAY; be added or removed by any implementation along the request/response
1534   chain. Additional information about the encoding parameters &MAY; be
1535   provided by other header fields not defined by this specification.
1536</t>
1537<t>
1538   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1539   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1540   neither of which includes a message body,
1541   to indicate that the origin server would have applied a transfer coding
1542   to the message body if the request had been an unconditional GET.
1543   This indication is not required, however, because any recipient on
1544   the response chain (including the origin server) can remove transfer
1545   codings when they are not needed.
1546</t>
1547<t>
1548   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1549   implementations advertising only HTTP/1.0 support will not understand
1550   how to process a transfer-encoded payload.
1551   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1552   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1553   might be in the form of specific user configuration or by remembering the
1554   version of a prior received response.
1555   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1556   the corresponding request indicates HTTP/1.1 (or later).
1557</t>
1558<t>
1559   A server that receives a request message with a transfer-coding it does
1560   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref> and then
1561   close the connection.
1562</t>
1563</section>
1564
1565<section title="Content-Length" anchor="header.content-length">
1566  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1567  <x:anchor-alias value="Content-Length"/>
1568<t>
1569   When a message is allowed to contain a message body, does not have a
1570   <x:ref>Transfer-Encoding</x:ref> header field, and has a payload body
1571   length that is known to the sender before the message header section has
1572   been sent, the sender &SHOULD; send a Content-Length header field to
1573   indicate the length of the payload body as a decimal number of octets.
1574</t>
1575<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1576  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1577</artwork></figure>
1578<t>
1579   An example is
1580</t>
1581<figure><artwork type="example">
1582  Content-Length: 3495
1583</artwork></figure>
1584<t>
1585   A sender &MUST-NOT; send a Content-Length header field in any message that
1586   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1587</t>
1588<t>
1589   A server &MAY; send a Content-Length header field in a response to a HEAD
1590   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1591   response unless its field-value equals the decimal number of octets that
1592   would have been sent in the payload body of a response if the same
1593   request had used the GET method.
1594</t>
1595<t>
1596   A server &MAY; send a Content-Length header field in a
1597   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1598   (&status-304;); 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 <x:ref>200 (OK)</x:ref>
1601   response to the same request.
1602</t>
1603<t>
1604   A server &MUST-NOT; send a Content-Length header field in any response
1605   with a status code of
1606   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1607   A server &SHOULD-NOT; send a Content-Length header field in any
1608   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1609</t>
1610<t>
1611   Any Content-Length field value greater than or equal to zero is valid.
1612   Since there is no predefined limit to the length of an HTTP payload,
1613   recipients &SHOULD; anticipate potentially large decimal numerals and
1614   prevent parsing errors due to integer conversion overflows
1615   (<xref target="attack.protocol.element.size.overflows"/>).
1616</t>
1617<t>
1618   If a message is received that has multiple Content-Length header fields
1619   with field-values consisting of the same decimal value, or a single
1620   Content-Length header field with a field value containing a list of
1621   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1622   duplicate Content-Length header fields have been generated or combined by an
1623   upstream message processor, then the recipient &MUST; either reject the
1624   message as invalid or replace the duplicated field-values with a single
1625   valid Content-Length field containing that decimal value prior to
1626   determining the message body length.
1627</t>
1628<x:note>
1629  <t>
1630   &Note; HTTP's use of Content-Length for message framing differs
1631   significantly from the same field's use in MIME, where it is an optional
1632   field used only within the "message/external-body" media-type.
1633  </t>
1634</x:note>
1635</section>
1636
1637<section title="Message Body Length" anchor="message.body.length">
1638<t>
1639   The length of a message body is determined by one of the following
1640   (in order of precedence):
1641</t>
1642<t>
1643  <list style="numbers">
1644    <x:lt><t>
1645     Any response to a HEAD request and any response with a
1646     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1647     <x:ref>304 (Not Modified)</x:ref> status code is always
1648     terminated by the first empty line after the header fields, regardless of
1649     the header fields present in the message, and thus cannot contain a
1650     message body.
1651    </t></x:lt>
1652    <x:lt><t>
1653     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1654     connection will become a tunnel immediately after the empty line that
1655     concludes the header fields.  A client &MUST; ignore any
1656     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1657     fields received in such a message.
1658    </t></x:lt>
1659    <x:lt><t>
1660     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1661     and the "chunked" transfer-coding (<xref target="chunked.encoding"/>)
1662     is the final encoding, the message body length is determined by reading
1663     and decoding the chunked data until the transfer-coding indicates the
1664     data is complete.
1665    </t>
1666    <t>
1667     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1668     response and the "chunked" transfer-coding is not the final encoding, the
1669     message body length is determined by reading the connection until it is
1670     closed by the server.
1671     If a Transfer-Encoding header field is present in a request and the
1672     "chunked" transfer-coding is not the final encoding, the message body
1673     length cannot be determined reliably; the server &MUST; respond with
1674     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1675    </t>
1676    <t>
1677     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1678     and a <x:ref>Content-Length</x:ref> header field, the
1679     Transfer-Encoding overrides the Content-Length.
1680     Such a message might indicate an attempt to perform request or response
1681     smuggling (bypass of security-related checks on message routing or content)
1682     and thus ought to be handled as an error.  The provided Content-Length &MUST;
1683     be removed, prior to forwarding the message downstream, or replaced with
1684     the real message body length after the transfer-coding is decoded.
1685    </t></x:lt>
1686    <x:lt><t>
1687     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1688     either multiple <x:ref>Content-Length</x:ref> header fields having
1689     differing field-values or a single Content-Length header field having an
1690     invalid value, then the message framing is invalid and &MUST; be treated
1691     as an error to prevent request or response smuggling.
1692     If this is a request message, the server &MUST; respond with
1693     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1694     If this is a response message received by a proxy, the proxy
1695     &MUST; discard the received response, send a <x:ref>502 (Bad Gateway)</x:ref>
1696     status code as its downstream response, and then close the connection.
1697     If this is a response message received by a user agent, it &MUST; be
1698     treated as an error by discarding the message and closing the connection.
1699    </t></x:lt>
1700    <x:lt><t>
1701     If a valid <x:ref>Content-Length</x:ref> header field is present without
1702     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1703     message body length in octets.  If the actual number of octets sent in
1704     the message is less than the indicated Content-Length, the recipient
1705     &MUST; consider the message to be incomplete and treat the connection
1706     as no longer usable.
1707     If the actual number of octets sent in the message is more than the indicated
1708     Content-Length, the recipient &MUST; only process the message body up to the
1709     field value's number of octets; the remainder of the message &MUST; either
1710     be discarded or treated as the next message in a pipeline.  For the sake of
1711     robustness, a user agent &MAY; attempt to detect and correct such an error
1712     in message framing if it is parsing the response to the last request on
1713     a connection and the connection has been closed by the server.
1714    </t></x:lt>
1715    <x:lt><t>
1716     If this is a request message and none of the above are true, then the
1717     message body length is zero (no message body is present).
1718    </t></x:lt>
1719    <x:lt><t>
1720     Otherwise, this is a response message without a declared message body
1721     length, so the message body length is determined by the number of octets
1722     received prior to the server closing the connection.
1723    </t></x:lt>
1724  </list>
1725</t>
1726<t>
1727   Since there is no way to distinguish a successfully completed,
1728   close-delimited message from a partially-received message interrupted
1729   by network failure, a server &SHOULD; use encoding or
1730   length-delimited messages whenever possible.  The close-delimiting
1731   feature exists primarily for backwards compatibility with HTTP/1.0.
1732</t>
1733<t>
1734   A server &MAY; reject a request that contains a message body but
1735   not a <x:ref>Content-Length</x:ref> by responding with
1736   <x:ref>411 (Length Required)</x:ref>.
1737</t>
1738<t>
1739   Unless a transfer-coding other than "chunked" has been applied,
1740   a client that sends a request containing a message body &SHOULD;
1741   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1742   length is known in advance, rather than the "chunked" encoding, since some
1743   existing services respond to "chunked" with a <x:ref>411 (Length Required)</x:ref>
1744   status code even though they understand the chunked encoding.  This
1745   is typically because such services are implemented via a gateway that
1746   requires a content-length in advance of being called and the server
1747   is unable or unwilling to buffer the entire request before processing.
1748</t>
1749<t>
1750   A client that sends a request containing a message body &MUST; include a
1751   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1752   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1753   the form of specific user configuration or by remembering the version of a
1754   prior received response.
1755</t>
1756</section>
1757</section>
1758
1759<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1760<t>
1761   Request messages that are prematurely terminated, possibly due to a
1762   canceled connection or a server-imposed time-out exception, &MUST;
1763   result in closure of the connection; sending an error response
1764   prior to closing the connection is &OPTIONAL;.
1765</t>
1766<t>
1767   Response messages that are prematurely terminated, usually by closure
1768   of the connection prior to receiving the expected number of octets or by
1769   failure to decode a transfer-encoded message body, &MUST; be recorded
1770   as incomplete.  A response that terminates in the middle of the header
1771   block (before the empty line is received) cannot be assumed to convey the
1772   full semantics of the response and &MUST; be treated as an error.
1773</t>
1774<t>
1775   A message body that uses the chunked transfer encoding is
1776   incomplete if the zero-sized chunk that terminates the encoding has not
1777   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1778   incomplete if the size of the message body received (in octets) is less than
1779   the value given by Content-Length.  A response that has neither chunked
1780   transfer encoding nor Content-Length is terminated by closure of the
1781   connection, and thus is considered complete regardless of the number of
1782   message body octets received, provided that the header block was received
1783   intact.
1784</t>
1785<t>
1786   A user agent &MUST-NOT; render an incomplete response message body as if
1787   it were complete (i.e., some indication needs to be given to the user that an
1788   error occurred).  Cache requirements for incomplete responses are defined
1789   in &cache-incomplete;.
1790</t>
1791<t>
1792   A server &MUST; read the entire request message body or close
1793   the connection after sending its response, since otherwise the
1794   remaining data on a persistent connection would be misinterpreted
1795   as the next request.  Likewise,
1796   a client &MUST; read the entire response message body if it intends
1797   to reuse the same connection for a subsequent request.  Pipelining
1798   multiple requests on a connection is described in <xref target="pipelining"/>.
1799</t>
1800</section>
1801
1802<section title="Message Parsing Robustness" anchor="message.robustness">
1803<t>
1804   Older HTTP/1.0 client implementations might send an extra CRLF
1805   after a POST request as a lame workaround for some early server
1806   applications that failed to read message body content that was
1807   not terminated by a line-ending. An HTTP/1.1 client &MUST-NOT;
1808   preface or follow a request with an extra CRLF.  If terminating
1809   the request message body with a line-ending is desired, then the
1810   client &MUST; include the terminating CRLF octets as part of the
1811   message body length.
1812</t>
1813<t>
1814   In the interest of robustness, servers &SHOULD; ignore at least one
1815   empty line received where a request-line is expected. In other words, if
1816   the server is reading the protocol stream at the beginning of a
1817   message and receives a CRLF first, it &SHOULD; ignore the CRLF.
1818</t>
1819<t>
1820   Although the line terminator for the start-line and header
1821   fields is the sequence CRLF, recipients &MAY; recognize a
1822   single LF as a line terminator and ignore any preceding CR.
1823</t>
1824<t>
1825   Although the request-line and status-line grammar rules require that each
1826   of the component elements be separated by a single SP octet, recipients
1827   &MAY; instead parse on whitespace-delimited word boundaries and, aside
1828   from the CRLF terminator, treat any form of whitespace as the SP separator
1829   while ignoring preceding or trailing whitespace;
1830   such whitespace includes one or more of the following octets:
1831   SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
1832</t>
1833<t>
1834   When a server listening only for HTTP request messages, or processing
1835   what appears from the start-line to be an HTTP request message,
1836   receives a sequence of octets that does not match the HTTP-message
1837   grammar aside from the robustness exceptions listed above, the
1838   server &SHOULD; respond with a <x:ref>400 (Bad Request)</x:ref> response. 
1839</t>
1840</section>
1841</section>
1842
1843<section title="Transfer Codings" anchor="transfer.codings">
1844  <x:anchor-alias value="transfer-coding"/>
1845  <x:anchor-alias value="transfer-extension"/>
1846<t>
1847   Transfer-coding values are used to indicate an encoding
1848   transformation that has been, can be, or might need to be applied to a
1849   payload body in order to ensure "safe transport" through the network.
1850   This differs from a content coding in that the transfer-coding is a
1851   property of the message rather than a property of the representation
1852   that is being transferred.
1853</t>
1854<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1855  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1856                     / "compress" ; <xref target="compress.coding"/>
1857                     / "deflate" ; <xref target="deflate.coding"/>
1858                     / "gzip" ; <xref target="gzip.coding"/>
1859                     / <x:ref>transfer-extension</x:ref>
1860  <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> )
1861</artwork></figure>
1862<t anchor="rule.parameter">
1863  <x:anchor-alias value="attribute"/>
1864  <x:anchor-alias value="transfer-parameter"/>
1865  <x:anchor-alias value="value"/>
1866   Parameters are in the form of attribute/value pairs.
1867</t>
1868<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"/>
1869  <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>
1870  <x:ref>attribute</x:ref>          = <x:ref>token</x:ref>
1871  <x:ref>value</x:ref>              = <x:ref>word</x:ref>
1872</artwork></figure>
1873<t>
1874   All transfer-coding values are case-insensitive and &SHOULD; be registered
1875   within the HTTP Transfer Coding registry, as defined in
1876   <xref target="transfer.coding.registry"/>.
1877   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1878   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1879   header fields.
1880</t>
1881
1882<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1883  <iref item="chunked (Coding Format)"/>
1884  <x:anchor-alias value="chunk"/>
1885  <x:anchor-alias value="chunked-body"/>
1886  <x:anchor-alias value="chunk-data"/>
1887  <x:anchor-alias value="chunk-ext"/>
1888  <x:anchor-alias value="chunk-ext-name"/>
1889  <x:anchor-alias value="chunk-ext-val"/>
1890  <x:anchor-alias value="chunk-size"/>
1891  <x:anchor-alias value="last-chunk"/>
1892  <x:anchor-alias value="trailer-part"/>
1893  <x:anchor-alias value="quoted-str-nf"/>
1894  <x:anchor-alias value="qdtext-nf"/>
1895<t>
1896   The chunked encoding modifies the body of a message in order to
1897   transfer it as a series of chunks, each with its own size indicator,
1898   followed by an &OPTIONAL; trailer containing header fields. This
1899   allows dynamically generated content to be transferred along with the
1900   information necessary for the recipient to verify that it has
1901   received the full message.
1902</t>
1903<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"/>
1904  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1905                   <x:ref>last-chunk</x:ref>
1906                   <x:ref>trailer-part</x:ref>
1907                   <x:ref>CRLF</x:ref>
1908 
1909  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1910                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1911  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1912  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1913 
1914  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
1915  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
1916  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-str-nf</x:ref>
1917  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1918  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1919 
1920  <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>
1921                 ; like <x:ref>quoted-string</x:ref>, but disallowing line folding
1922  <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>
1923</artwork></figure>
1924<t>
1925   Chunk extensions within the chucked encoding are deprecated.
1926   Senders &SHOULD-NOT; send chunk-ext.
1927   Definition of new chunk extensions is discouraged.
1928</t>
1929<t>
1930   The chunk-size field is a string of hex digits indicating the size of
1931   the chunk-data in octets. The chunked encoding is ended by any chunk whose size is
1932   zero, followed by the trailer, which is terminated by an empty line.
1933</t>
1934
1935<section title="Trailer" anchor="header.trailer">
1936  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
1937  <x:anchor-alias value="Trailer"/>
1938<t>
1939   A trailer allows the sender to include additional fields at the end of a
1940   chunked message in order to supply metadata that might be dynamically
1941   generated while the message body is sent, such as a message integrity
1942   check, digital signature, or post-processing status.
1943   The trailer &MUST-NOT; contain fields that need to be known before a
1944   recipient processes the body, such as <x:ref>Transfer-Encoding</x:ref>,
1945   <x:ref>Content-Length</x:ref>, and <x:ref>Trailer</x:ref>.
1946</t>
1947<t>
1948   When a message includes a message body encoded with the chunked
1949   transfer-coding and the sender desires to send metadata in the form of
1950   trailer fields at the end of the message, the sender &SHOULD; send a
1951   <x:ref>Trailer</x:ref> header field before the message body to indicate
1952   which fields will be present in the trailers. This allows the recipient
1953   to prepare for receipt of that metadata before it starts processing the body,
1954   which is useful if the message is being streamed and the recipient wishes
1955   to confirm an integrity check on the fly.
1956</t>
1957<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
1958  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
1959</artwork></figure>
1960<t>
1961   If no <x:ref>Trailer</x:ref> header field is present, the sender of a
1962   chunked message body &SHOULD; send an empty trailer.
1963</t>
1964<t>
1965   A server &MUST; send an empty trailer with the chunked transfer-coding
1966   unless at least one of the following is true:
1967  <list style="numbers">
1968    <t>the request included a <x:ref>TE</x:ref> header field that indicates
1969    "trailers" is acceptable in the transfer-coding of the response, as
1970    described in <xref target="header.te"/>; or,</t>
1971     
1972    <t>the trailer fields consist entirely of optional metadata and the
1973    recipient could use the message (in a manner acceptable to the server where
1974    the field originated) without receiving that metadata. In other words,
1975    the server that generated the header field is willing to accept the
1976    possibility that the trailer fields might be silently discarded along
1977    the path to the client.</t>
1978  </list>
1979</t>
1980<t>
1981   The above requirement prevents the need for an infinite buffer when a
1982   message is being received by an HTTP/1.1 (or later) proxy and forwarded to
1983   an HTTP/1.0 recipient.
1984</t>
1985</section>
1986
1987<section title="Decoding chunked" anchor="decoding.chunked">
1988<t>
1989   A process for decoding the "chunked" transfer-coding
1990   can be represented in pseudo-code as:
1991</t>
1992<figure><artwork type="code">
1993  length := 0
1994  read chunk-size, chunk-ext (if any) and CRLF
1995  while (chunk-size &gt; 0) {
1996     read chunk-data and CRLF
1997     append chunk-data to decoded-body
1998     length := length + chunk-size
1999     read chunk-size and CRLF
2000  }
2001  read header-field
2002  while (header-field not empty) {
2003     append header-field to existing header fields
2004     read header-field
2005  }
2006  Content-Length := length
2007  Remove "chunked" from Transfer-Encoding
2008  Remove Trailer from existing header fields
2009</artwork></figure>
2010<t>
2011   All recipients &MUST; be able to receive and decode the
2012   "chunked" transfer-coding and &MUST; ignore chunk-ext extensions
2013   they do not understand.
2014</t>
2015</section>
2016</section>
2017
2018<section title="Compression Codings" anchor="compression.codings">
2019<t>
2020   The codings defined below can be used to compress the payload of a
2021   message.
2022</t>
2023
2024<section title="Compress Coding" anchor="compress.coding">
2025<iref item="compress (Coding Format)"/>
2026<t>
2027   The "compress" format is produced by the common UNIX file compression
2028   program "compress". This format is an adaptive Lempel-Ziv-Welch
2029   coding (LZW). Recipients &SHOULD; consider "x-compress" to be
2030   equivalent to "compress".
2031</t>
2032</section>
2033
2034<section title="Deflate Coding" anchor="deflate.coding">
2035<iref item="deflate (Coding Format)"/>
2036<t>
2037   The "deflate" format is defined as the "deflate" compression mechanism
2038   (described in <xref target="RFC1951"/>) used inside the "zlib"
2039   data format (<xref target="RFC1950"/>).
2040</t>
2041<x:note>
2042  <t>
2043    &Note; Some incorrect implementations send the "deflate"
2044    compressed data without the zlib wrapper.
2045   </t>
2046</x:note>
2047</section>
2048
2049<section title="Gzip Coding" anchor="gzip.coding">
2050<iref item="gzip (Coding Format)"/>
2051<t>
2052   The "gzip" format is produced by the file compression program
2053   "gzip" (GNU zip), as described in <xref target="RFC1952"/>. This format is a
2054   Lempel-Ziv coding (LZ77) with a 32 bit CRC.
2055   Recipients &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2056</t>
2057</section>
2058
2059</section>
2060
2061<section title="TE" anchor="header.te">
2062  <iref primary="true" item="TE header field" x:for-anchor=""/>
2063  <x:anchor-alias value="TE"/>
2064  <x:anchor-alias value="t-codings"/>
2065  <x:anchor-alias value="t-ranking"/>
2066  <x:anchor-alias value="rank"/>
2067<t>
2068   The "TE" header field in a request indicates what transfer-codings,
2069   besides "chunked", the client is willing to accept in response, and
2070   whether or not the client is willing to accept trailer fields in a
2071   chunked transfer-coding.
2072</t>
2073<t>
2074   The TE field-value consists of a comma-separated list of transfer-coding
2075   names, each allowing for optional parameters (as described in
2076   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2077   Clients &MUST-NOT; send the chunked transfer-coding name in TE;
2078   chunked is always acceptable for HTTP/1.1 recipients.
2079</t>
2080<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"/>
2081  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2082  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2083  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2084  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2085             / ( "1" [ "." 0*3("0") ] )
2086</artwork></figure>
2087<t>
2088   Three examples of TE use are below.
2089</t>
2090<figure><artwork type="example">
2091  TE: deflate
2092  TE:
2093  TE: trailers, deflate;q=0.5
2094</artwork></figure>
2095<t>
2096   The presence of the keyword "trailers" indicates that the client is
2097   willing to accept trailer fields in a chunked transfer-coding,
2098   as defined in <xref target="chunked.encoding"/>, on behalf of itself and
2099   any downstream clients. For chained requests, this implies that either:
2100   (a) all downstream clients are willing to accept trailer fields in the
2101   forwarded response; or,
2102   (b) the client will attempt to buffer the response on behalf of downstream
2103   recipients.
2104   Note that HTTP/1.1 does not define any means to limit the size of a
2105   chunked response such that a client can be assured of buffering the
2106   entire response.
2107</t>
2108<t>
2109   When multiple transfer-codings are acceptable, the client &MAY; rank the
2110   codings by preference using a case-insensitive "q" parameter (similar to
2111   the qvalues used in content negotiation fields, &qvalue;). The rank value
2112   is a real number in the range 0 through 1, where 0.001 is the least
2113   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2114</t>
2115<t>
2116   If the TE field-value is empty or if no TE field is present, the only
2117   acceptable transfer-coding is "chunked". A message with no transfer-coding
2118   is always acceptable.
2119</t>
2120<t>
2121   Since the TE header field only applies to the immediate connection,
2122   a sender of TE &MUST; also send a "TE" connection option within the
2123   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2124   in order to prevent the TE field from being forwarded by intermediaries
2125   that do not support its semantics.
2126</t>
2127</section>
2128</section>
2129
2130<section title="Message Routing" anchor="message.routing">
2131<t>
2132   HTTP request message routing is determined by each client based on the
2133   target resource, the client's proxy configuration, and
2134   establishment or reuse of an inbound connection.  The corresponding
2135   response routing follows the same connection chain back to the client.
2136</t>
2137
2138<section title="Identifying a Target Resource" anchor="target-resource">
2139  <iref primary="true" item="target resource"/>
2140  <iref primary="true" item="target URI"/>
2141  <x:anchor-alias value="target resource"/>
2142  <x:anchor-alias value="target URI"/>
2143<t>
2144   HTTP is used in a wide variety of applications, ranging from
2145   general-purpose computers to home appliances.  In some cases,
2146   communication options are hard-coded in a client's configuration.
2147   However, most HTTP clients rely on the same resource identification
2148   mechanism and configuration techniques as general-purpose Web browsers.
2149</t>
2150<t>
2151   HTTP communication is initiated by a user agent for some purpose.
2152   The purpose is a combination of request semantics, which are defined in
2153   <xref target="Part2"/>, and a target resource upon which to apply those
2154   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2155   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2156   would resolve to its absolute form in order to obtain the
2157   "<x:dfn>target URI</x:dfn>".  The target URI
2158   excludes the reference's fragment identifier component, if any,
2159   since fragment identifiers are reserved for client-side processing
2160   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2161</t>
2162</section>
2163
2164<section title="Connecting Inbound" anchor="connecting.inbound">
2165<t>
2166   Once the target URI is determined, a client needs to decide whether
2167   a network request is necessary to accomplish the desired semantics and,
2168   if so, where that request is to be directed.
2169</t>
2170<t>
2171   If the client has a response cache and the request semantics can be
2172   satisfied by a cache (<xref target="Part6"/>), then the request is
2173   usually directed to the cache first.
2174</t>
2175<t>
2176   If the request is not satisfied by a cache, then a typical client will
2177   check its configuration to determine whether a proxy is to be used to
2178   satisfy the request.  Proxy configuration is implementation-dependent,
2179   but is often based on URI prefix matching, selective authority matching,
2180   or both, and the proxy itself is usually identified by an "http" or
2181   "https" URI.  If a proxy is applicable, the client connects inbound by
2182   establishing (or reusing) a connection to that proxy.
2183</t>
2184<t>
2185   If no proxy is applicable, a typical client will invoke a handler routine,
2186   usually specific to the target URI's scheme, to connect directly
2187   to an authority for the target resource.  How that is accomplished is
2188   dependent on the target URI scheme and defined by its associated
2189   specification, similar to how this specification defines origin server
2190   access for resolution of the "http" (<xref target="http.uri"/>) and
2191   "https" (<xref target="https.uri"/>) schemes.
2192</t>
2193<t>
2194   HTTP requirements regarding connection management are defined in
2195   <xref target="connection.management"/>.
2196</t>
2197</section>
2198
2199<section title="Request Target" anchor="request-target">
2200<t>
2201   Once an inbound connection is obtained,
2202   the client sends an HTTP request message (<xref target="http.message"/>)
2203   with a request-target derived from the target URI.
2204   There are four distinct formats for the request-target, depending on both
2205   the method being requested and whether the request is to a proxy.
2206</t>   
2207<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"/>
2208  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2209                 / <x:ref>absolute-form</x:ref>
2210                 / <x:ref>authority-form</x:ref>
2211                 / <x:ref>asterisk-form</x:ref>
2212
2213  <x:ref>origin-form</x:ref>    = <x:ref>path-absolute</x:ref> [ "?" <x:ref>query</x:ref> ]
2214  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2215  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2216  <x:ref>asterisk-form</x:ref>  = "*"
2217</artwork></figure>
2218<t anchor="origin-form"><iref item="origin-form (of request-target)"/>
2219   The most common form of request-target is the origin-form.
2220   When making a request directly to an origin server, other than a CONNECT
2221   or server-wide OPTIONS request (as detailed below),
2222   a client &MUST; send only the absolute path and query components of
2223   the target URI as the request-target.
2224   If the target URI's path component is empty, then the client &MUST; send
2225   "/" as the path within the origin-form of request-target.
2226   A <x:ref>Host</x:ref> header field is also sent, as defined in
2227   <xref target="header.host"/>, containing the target URI's
2228   authority component (excluding any userinfo).
2229</t>
2230<t>
2231   For example, a client wishing to retrieve a representation of the resource
2232   identified as
2233</t>
2234<figure><artwork x:indent-with="  " type="example">
2235http://www.example.org/where?q=now
2236</artwork></figure>
2237<t>
2238   directly from the origin server would open (or reuse) a TCP connection
2239   to port 80 of the host "www.example.org" and send the lines:
2240</t>
2241<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2242GET /where?q=now HTTP/1.1
2243Host: www.example.org
2244</artwork></figure>
2245<t>
2246   followed by the remainder of the request message.
2247</t>
2248<t anchor="absolute-form"><iref item="absolute-form (of request-target)"/>
2249   When making a request to a proxy, other than a CONNECT or server-wide
2250   OPTIONS request (as detailed below), a client &MUST; send the target URI
2251   in absolute-form as the request-target.
2252   The proxy is requested to either service that request from a valid cache,
2253   if possible, or make the same request on the client's behalf to either
2254   the next inbound proxy server or directly to the origin server indicated
2255   by the request-target.  Requirements on such "forwarding" of messages are
2256   defined in <xref target="message.forwarding"/>.
2257</t>
2258<t>
2259   An example absolute-form of request-line would be:
2260</t>
2261<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2262GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
2263</artwork></figure>
2264<t>
2265   To allow for transition to the absolute-form for all requests in some
2266   future version of HTTP, HTTP/1.1 servers &MUST; accept the absolute-form
2267   in requests, even though HTTP/1.1 clients will only send them in requests
2268   to proxies.
2269</t>
2270<t anchor="authority-form"><iref item="authority-form (of request-target)"/>
2271   The authority-form of request-target is only used for CONNECT requests
2272   (&CONNECT;).  When making a CONNECT request to establish a tunnel through
2273   one or more proxies, a client &MUST; send only the target URI's
2274   authority component (excluding any userinfo) as the request-target.
2275   For example,
2276</t>
2277<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2278CONNECT www.example.com:80 HTTP/1.1
2279</artwork></figure>
2280<t anchor="asterisk-form"><iref item="asterisk-form (of request-target)"/>
2281   The asterisk-form of request-target is only used for a server-wide
2282   OPTIONS request (&OPTIONS;).  When a client wishes to request OPTIONS
2283   for the server as a whole, as opposed to a specific named resource of
2284   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2285   For example,
2286</t>
2287<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2288OPTIONS * HTTP/1.1
2289</artwork></figure>
2290<t>
2291   If a proxy receives an OPTIONS request with an absolute-form of
2292   request-target in which the URI has an empty path and no query component,
2293   then the last proxy on the request chain &MUST; send a request-target
2294   of "*" when it forwards the request to the indicated origin server.
2295</t>
2296<figure><preamble>   
2297   For example, the request
2298</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2299OPTIONS http://www.example.org:8001 HTTP/1.1
2300</artwork></figure>
2301<figure><preamble>   
2302  would be forwarded by the final proxy as
2303</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2304OPTIONS * HTTP/1.1
2305Host: www.example.org:8001
2306</artwork>
2307<postamble>
2308   after connecting to port 8001 of host "www.example.org".
2309</postamble>
2310</figure>
2311</section>
2312
2313<section title="Host" anchor="header.host">
2314  <iref primary="true" item="Host header field" x:for-anchor=""/>
2315  <x:anchor-alias value="Host"/>
2316<t>
2317   The "Host" header field in a request provides the host and port
2318   information from the target URI, enabling the origin
2319   server to distinguish among resources while servicing requests
2320   for multiple host names on a single IP address.  Since the Host
2321   field-value is critical information for handling a request, it
2322   &SHOULD; be sent as the first header field following the request-line.
2323</t>
2324<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2325  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2326</artwork></figure>
2327<t>
2328   A client &MUST; send a Host header field in all HTTP/1.1 request
2329   messages.  If the target URI includes an authority component, then
2330   the Host field-value &MUST; be identical to that authority component
2331   after excluding any userinfo (<xref target="http.uri"/>).
2332   If the authority component is missing or undefined for the target URI,
2333   then the Host header field &MUST; be sent with an empty field-value.
2334</t>
2335<t>
2336   For example, a GET request to the origin server for
2337   &lt;http://www.example.org/pub/WWW/&gt; would begin with:
2338</t>
2339<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2340GET /pub/WWW/ HTTP/1.1
2341Host: www.example.org
2342</artwork></figure>
2343<t>
2344   The Host header field &MUST; be sent in an HTTP/1.1 request even
2345   if the request-target is in the absolute-form, since this
2346   allows the Host information to be forwarded through ancient HTTP/1.0
2347   proxies that might not have implemented Host.
2348</t>
2349<t>
2350   When a proxy receives a request with an absolute-form of
2351   request-target, the proxy &MUST; ignore the received
2352   Host header field (if any) and instead replace it with the host
2353   information of the request-target.  If the proxy forwards the request,
2354   it &MUST; generate a new Host field-value based on the received
2355   request-target rather than forward the received Host field-value.
2356</t>
2357<t>
2358   Since the Host header field acts as an application-level routing
2359   mechanism, it is a frequent target for malware seeking to poison
2360   a shared cache or redirect a request to an unintended server.
2361   An interception proxy is particularly vulnerable if it relies on
2362   the Host field-value for redirecting requests to internal
2363   servers, or for use as a cache key in a shared cache, without
2364   first verifying that the intercepted connection is targeting a
2365   valid IP address for that host.
2366</t>
2367<t>
2368   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2369   to any HTTP/1.1 request message that lacks a Host header field and
2370   to any request message that contains more than one Host header field
2371   or a Host header field with an invalid field-value.
2372</t>
2373</section>
2374
2375<section title="Effective Request URI" anchor="effective.request.uri">
2376  <iref primary="true" item="effective request URI"/>
2377<t>
2378   A server that receives an HTTP request message &MUST; reconstruct
2379   the user agent's original target URI, based on the pieces of information
2380   learned from the request-target, <x:ref>Host</x:ref> header field, and
2381   connection context, in order to identify the intended target resource and
2382   properly service the request. The URI derived from this reconstruction
2383   process is referred to as the "<x:dfn>effective request URI</x:dfn>".
2384</t>
2385<t>
2386   For a user agent, the effective request URI is the target URI.
2387</t>
2388<t>
2389   If the request-target is in absolute-form, then the effective request URI
2390   is the same as the request-target.  Otherwise, the effective request URI
2391   is constructed as follows.
2392</t>
2393<t>
2394   If the request is received over a TLS-secured TCP connection,
2395   then the effective request URI's scheme is "https"; otherwise, the
2396   scheme is "http".
2397</t>
2398<t>
2399   If the request-target is in authority-form, then the effective
2400   request URI's authority component is the same as the request-target.
2401   Otherwise, if a <x:ref>Host</x:ref> header field is supplied with a
2402   non-empty field-value, then the authority component is the same as the
2403   Host field-value. Otherwise, the authority component is the concatenation of
2404   the default host name configured for the server, a colon (":"), and the
2405   connection's incoming TCP port number in decimal form.
2406</t>
2407<t>
2408   If the request-target is in authority-form or asterisk-form, then the
2409   effective request URI's combined path and query component is empty.
2410   Otherwise, the combined path and query component is the same as the
2411   request-target.
2412</t>
2413<t>
2414   The components of the effective request URI, once determined as above,
2415   can be combined into absolute-URI form by concatenating the scheme,
2416   "://", authority, and combined path and query component.
2417</t>
2418<figure>
2419<preamble>
2420   Example 1: the following message received over an insecure TCP connection
2421</preamble> 
2422<artwork type="example" x:indent-with="  ">
2423GET /pub/WWW/TheProject.html HTTP/1.1
2424Host: www.example.org:8080
2425</artwork>
2426</figure>
2427<figure>
2428<preamble>
2429  has an effective request URI of
2430</preamble>
2431<artwork type="example" x:indent-with="  ">
2432http://www.example.org:8080/pub/WWW/TheProject.html
2433</artwork>
2434</figure>
2435<figure>
2436<preamble>
2437   Example 2: the following message received over a TLS-secured TCP connection
2438</preamble> 
2439<artwork type="example" x:indent-with="  ">
2440OPTIONS * HTTP/1.1
2441Host: www.example.org
2442</artwork>
2443</figure>
2444<figure>
2445<preamble>
2446  has an effective request URI of
2447</preamble>
2448<artwork type="example" x:indent-with="  ">
2449https://www.example.org
2450</artwork>
2451</figure>
2452<t>
2453   An origin server that does not allow resources to differ by requested
2454   host &MAY; ignore the <x:ref>Host</x:ref> field-value and instead replace it
2455   with a configured server name when constructing the effective request URI.
2456</t>
2457<t>
2458   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2459   field &MAY; attempt to use heuristics (e.g., examination of the URI path for
2460   something unique to a particular host) in order to guess the
2461   effective request URI's authority component.
2462</t>
2463</section>
2464
2465<section title="Message Forwarding" anchor="message.forwarding">
2466<t>
2467   As described in <xref target="intermediaries"/>, intermediaries can serve
2468   a variety of roles in the processing of HTTP requests and responses.
2469   Some intermediaries are used to improve performance or availability.
2470   Others are used for access control or to filter content.
2471   Since an HTTP stream has characteristics similar to a pipe-and-filter
2472   architecture, there are no inherent limits to the extent an intermediary
2473   can enhance (or interfere) with either direction of the stream.
2474</t>
2475<t>
2476   Intermediaries that forward a message &MUST; implement the
2477   <x:ref>Connection</x:ref> header field, as specified in
2478   <xref target="header.connection"/>, to exclude fields that are only
2479   intended for the incoming connection.
2480</t>
2481<t>
2482   In order to avoid request loops, a proxy that forwards requests to other
2483   proxies &MUST; be able to recognize and exclude all of its own server
2484   names, including any aliases, local variations, or literal IP addresses.
2485</t>
2486</section>
2487
2488<section title="Via" anchor="header.via">
2489  <iref primary="true" item="Via header field" x:for-anchor=""/>
2490  <x:anchor-alias value="pseudonym"/>
2491  <x:anchor-alias value="received-by"/>
2492  <x:anchor-alias value="received-protocol"/>
2493  <x:anchor-alias value="Via"/>
2494<t>
2495   The "Via" header field &MUST; be sent by a proxy or gateway in forwarded
2496   messages to indicate the intermediate protocols and recipients between the
2497   user agent and the server on requests, and between the origin server and
2498   the client on responses. It is analogous to the "Received" field
2499   used by email systems (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2500   Via is used in HTTP for tracking message forwards,
2501   avoiding request loops, and identifying the protocol capabilities of
2502   all senders along the request/response chain.
2503</t>
2504<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"/>
2505  <x:ref>Via</x:ref>               = 1#( <x:ref>received-protocol</x:ref> <x:ref>RWS</x:ref> <x:ref>received-by</x:ref>
2506                          [ <x:ref>RWS</x:ref> <x:ref>comment</x:ref> ] )
2507  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2508                      ; see <xref target="header.upgrade"/>
2509  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2510  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2511</artwork></figure>
2512<t>
2513   The received-protocol indicates the protocol version of the message
2514   received by the server or client along each segment of the
2515   request/response chain. The received-protocol version is appended to
2516   the Via field value when the message is forwarded so that information
2517   about the protocol capabilities of upstream applications remains
2518   visible to all recipients.
2519</t>
2520<t>
2521   The protocol-name is excluded if and only if it would be "HTTP". The
2522   received-by field is normally the host and optional port number of a
2523   recipient server or client that subsequently forwarded the message.
2524   However, if the real host is considered to be sensitive information,
2525   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2526   be assumed to be the default port of the received-protocol.
2527</t>
2528<t>
2529   Multiple Via field values represent each proxy or gateway that has
2530   forwarded the message. Each recipient &MUST; append its information
2531   such that the end result is ordered according to the sequence of
2532   forwarding applications.
2533</t>
2534<t>
2535   Comments &MAY; be used in the Via header field to identify the software
2536   of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2537   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2538   are optional and &MAY; be removed by any recipient prior to forwarding the
2539   message.
2540</t>
2541<t>
2542   For example, a request message could be sent from an HTTP/1.0 user
2543   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2544   forward the request to a public proxy at p.example.net, which completes
2545   the request by forwarding it to the origin server at www.example.com.
2546   The request received by www.example.com would then have the following
2547   Via header field:
2548</t>
2549<figure><artwork type="example">
2550  Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
2551</artwork></figure>
2552<t>
2553   A proxy or gateway used as a portal through a network firewall
2554   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2555   region unless it is explicitly enabled to do so. If not enabled, the
2556   received-by host of any host behind the firewall &SHOULD; be replaced
2557   by an appropriate pseudonym for that host.
2558</t>
2559<t>
2560   A proxy or gateway &MAY; combine an ordered subsequence of Via header
2561   field entries into a single such entry if the entries have identical
2562   received-protocol values. For example,
2563</t>
2564<figure><artwork type="example">
2565  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2566</artwork></figure>
2567<t>
2568  could be collapsed to
2569</t>
2570<figure><artwork type="example">
2571  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2572</artwork></figure>
2573<t>
2574   Senders &SHOULD-NOT; combine multiple entries unless they are all
2575   under the same organizational control and the hosts have already been
2576   replaced by pseudonyms. Senders &MUST-NOT; combine entries which
2577   have different received-protocol values.
2578</t>
2579</section>
2580
2581<section title="Message Transforming" anchor="message.transforming">
2582<t>
2583   If a proxy receives a request-target with a host name that is not a
2584   fully qualified domain name, it &MAY; add its own domain to the host name
2585   it received when forwarding the request.  A proxy &MUST-NOT; change the
2586   host name if it is a fully qualified domain name.
2587</t>
2588<t>
2589   A non-transforming proxy &MUST-NOT; modify the "path-absolute" and "query"
2590   parts of the received request-target when forwarding it to the next inbound
2591   server, except as noted above to replace an empty path with "/" or "*".
2592</t>
2593<t>
2594   A non-transforming proxy &MUST; preserve the message payload (&payload;),
2595   though it &MAY; change the message body through application or removal
2596   of a transfer-coding (<xref target="transfer.codings"/>).
2597</t>
2598<t>
2599   A non-transforming proxy &SHOULD-NOT; modify header fields that provide
2600   information about the end points of the communication chain, the resource
2601   state, or the selected representation.
2602</t>
2603<t>
2604   A non-transforming proxy &MUST-NOT; modify any of the following fields in a
2605   request or response, and it &MUST-NOT; add any of these fields if not
2606   already present:
2607  <list style="symbols">
2608    <t><x:ref>Allow</x:ref> (&header-allow;)</t>
2609    <t><x:ref>Content-Location</x:ref> (&header-content-location;)</t>
2610    <t>Content-MD5 (<xref target="RFC2616" x:fmt="of" x:sec="14.15"/>)</t>
2611    <t><x:ref>ETag</x:ref> (&header-etag;)</t>
2612    <t><x:ref>Last-Modified</x:ref> (&header-last-modified;)</t>
2613    <t><x:ref>Server</x:ref> (&header-server;)</t>
2614  </list>
2615</t>
2616<t>
2617   A non-transforming proxy &MUST-NOT; modify an <x:ref>Expires</x:ref>
2618   header field (&header-expires;) if already present in a response, but
2619   it &MAY; add an <x:ref>Expires</x:ref> header field with a field-value
2620   identical to that of the <x:ref>Date</x:ref> header field.
2621</t>
2622<t>
2623   A proxy &MUST-NOT; modify or add any of the following fields in a
2624   message that contains the no-transform cache-control directive:
2625  <list style="symbols">
2626    <t><x:ref>Content-Encoding</x:ref> (&header-content-encoding;)</t>
2627    <t><x:ref>Content-Range</x:ref> (&header-content-range;)</t>
2628    <t><x:ref>Content-Type</x:ref> (&header-content-type;)</t>
2629  </list>
2630</t>
2631<t>
2632   A transforming proxy &MAY; modify or add these fields to a message
2633   that does not include no-transform, but if it does so, it &MUST; add a
2634   Warning 214 (Transformation applied) if one does not already appear
2635   in the message (see &header-warning;).
2636</t>
2637<x:note>
2638  <t>
2639    <x:h>Warning:</x:h> Unnecessary modification of header fields might
2640    cause authentication failures if stronger authentication
2641    mechanisms are introduced in later versions of HTTP. Such
2642    authentication mechanisms &MAY; rely on the values of header fields
2643    not listed here.
2644  </t>
2645</x:note>
2646</section>
2647
2648<section title="Associating a Response to a Request" anchor="associating.response.to.request">
2649<t>
2650   HTTP does not include a request identifier for associating a given
2651   request message with its corresponding one or more response messages.
2652   Hence, it relies on the order of response arrival to correspond exactly
2653   to the order in which requests are made on the same connection.
2654   More than one response message per request only occurs when one or more
2655   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a final response
2656   to the same request.
2657</t>
2658<t>
2659   A client that uses persistent connections and sends more than one request
2660   per connection &MUST; maintain a list of outstanding requests in the
2661   order sent on that connection and &MUST; associate each received response
2662   message to the highest ordered request that has not yet received a final
2663   (non-<x:ref>1xx</x:ref>) response.
2664</t>
2665</section>
2666</section>
2667
2668<section title="Connection Management" anchor="connection.management">
2669<t>
2670   HTTP messaging is independent of the underlying transport or
2671   session-layer connection protocol(s).  HTTP only presumes a reliable
2672   transport with in-order delivery of requests and the corresponding
2673   in-order delivery of responses.  The mapping of HTTP request and
2674   response structures onto the data units of an underlying transport
2675   protocol is outside the scope of this specification.
2676</t>
2677<t>
2678   As described in <xref target="connecting.inbound"/>, the specific
2679   connection protocols to be used for an HTTP interaction are determined by
2680   client configuration and the <x:ref>target URI</x:ref>.
2681   For example, the "http" URI scheme
2682   (<xref target="http.uri"/>) indicates a default connection of TCP
2683   over IP, with a default TCP port of 80, but the client might be
2684   configured to use a proxy via some other connection, port, or protocol.
2685</t>
2686<t>
2687   HTTP implementations are expected to engage in connection management,
2688   which includes maintaining the state of current connections,
2689   establishing a new connection or reusing an existing connection,
2690   processing messages received on a connection, detecting connection
2691   failures, and closing each connection.
2692   Most clients maintain multiple connections in parallel, including
2693   more than one connection per server endpoint.
2694   Most servers are designed to maintain thousands of concurrent connections,
2695   while controlling request queues to enable fair use and detect
2696   denial of service attacks.
2697</t>
2698
2699<section title="Connection" anchor="header.connection">
2700  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2701  <iref primary="true" item="close" x:for-anchor=""/>
2702  <x:anchor-alias value="Connection"/>
2703  <x:anchor-alias value="connection-option"/>
2704  <x:anchor-alias value="close"/>
2705<t>
2706   The "Connection" header field allows the sender to indicate desired
2707   control options for the current connection.  In order to avoid confusing
2708   downstream recipients, a proxy or gateway &MUST; remove or replace any
2709   received connection options before forwarding the message.
2710</t>
2711<t>
2712   When a header field is used to supply control information for or about
2713   the current connection, the sender &SHOULD; list the corresponding
2714   field-name within the "Connection" header field.
2715   A proxy or gateway &MUST; parse a received Connection
2716   header field before a message is forwarded and, for each
2717   connection-option in this field, remove any header field(s) from
2718   the message with the same name as the connection-option, and then
2719   remove the Connection header field itself (or replace it with the
2720   intermediary's own connection options for the forwarded message).
2721</t>
2722<t>
2723   Hence, the Connection header field provides a declarative way of
2724   distinguishing header fields that are only intended for the
2725   immediate recipient ("hop-by-hop") from those fields that are
2726   intended for all recipients on the chain ("end-to-end"), enabling the
2727   message to be self-descriptive and allowing future connection-specific
2728   extensions to be deployed without fear that they will be blindly
2729   forwarded by older intermediaries.
2730</t>
2731<t>
2732   The Connection header field's value has the following grammar:
2733</t>
2734<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2735  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2736  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2737</artwork></figure>
2738<t>
2739   Connection options are case-insensitive.
2740</t>
2741<t>
2742   A sender &MUST-NOT; include field-names in the Connection header
2743   field-value for fields that are defined as expressing constraints
2744   for all recipients in the request or response chain, such as the
2745   Cache-Control header field (&header-cache-control;).
2746</t>
2747<t>
2748   The connection options do not have to correspond to a header field
2749   present in the message, since a connection-specific header field
2750   might not be needed if there are no parameters associated with that
2751   connection option.  Recipients that trigger certain connection
2752   behavior based on the presence of connection options &MUST; do so
2753   based on the presence of the connection-option rather than only the
2754   presence of the optional header field.  In other words, if the
2755   connection option is received as a header field but not indicated
2756   within the Connection field-value, then the recipient &MUST; ignore
2757   the connection-specific header field because it has likely been
2758   forwarded by an intermediary that is only partially conformant.
2759</t>
2760<t>
2761   When defining new connection options, specifications ought to
2762   carefully consider existing deployed header fields and ensure
2763   that the new connection option does not share the same name as
2764   an unrelated header field that might already be deployed.
2765   Defining a new connection option essentially reserves that potential
2766   field-name for carrying additional information related to the
2767   connection option, since it would be unwise for senders to use
2768   that field-name for anything else.
2769</t>
2770<t>
2771   The "<x:dfn>close</x:dfn>" connection option is defined for a
2772   sender to signal that this connection will be closed after completion of
2773   the response. For example,
2774</t>
2775<figure><artwork type="example">
2776  Connection: close
2777</artwork></figure>
2778<t>
2779   in either the request or the response header fields indicates that
2780   the connection &SHOULD; be closed after the current request/response
2781   is complete (<xref target="persistent.tear-down"/>).
2782</t>
2783<t>
2784   A client that does not support persistent connections &MUST;
2785   send the "close" connection option in every request message.
2786</t>
2787<t>
2788   A server that does not support persistent connections &MUST;
2789   send the "close" connection option in every response message that
2790   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2791</t>
2792</section>
2793
2794<section title="Persistent Connections" anchor="persistent.connections">
2795  <x:anchor-alias value="persistent connections"/>
2796<t>
2797   HTTP was originally designed to use a separate connection for each
2798   request/response pair. As the Web evolved and embedded requests became
2799   common for inline images, the connection establishment overhead was
2800   a significant drain on performance and a concern for Internet congestion.
2801   Message framing (via <x:ref>Content-Length</x:ref>) and optional
2802   long-lived connections (via Keep-Alive) were added to HTTP/1.0 in order
2803   to improve performance for some requests. However, these extensions were
2804   insufficient for dynamically generated responses and difficult to use
2805   with intermediaries.
2806</t>
2807<t>
2808   HTTP/1.1 defaults to the use of "<x:ref>persistent connections</x:ref>",
2809   which allow multiple requests and responses to be carried over a single
2810   connection. The "<x:ref>close</x:ref>" connection-option is used to
2811   signal that a connection will close after the current request/response.
2812   Persistent connections have a number of advantages:
2813  <list style="symbols">
2814      <t>
2815        By opening and closing fewer connections, CPU time is saved
2816        in routers and hosts (clients, servers, proxies, gateways,
2817        tunnels, or caches), and memory used for protocol control
2818        blocks can be saved in hosts.
2819      </t>
2820      <t>
2821        Most requests and responses can be pipelined on a connection.
2822        Pipelining allows a client to make multiple requests without
2823        waiting for each response, allowing a single connection to
2824        be used much more efficiently and with less overall latency.
2825      </t>
2826      <t>
2827        For TCP connections, network congestion is reduced by eliminating the
2828        packets associated with the three way handshake and graceful close
2829        procedures, and by allowing sufficient time to determine the
2830        congestion state of the network.
2831      </t>
2832      <t>
2833        Latency on subsequent requests is reduced since there is no time
2834        spent in the connection opening handshake.
2835      </t>
2836      <t>
2837        HTTP can evolve more gracefully, since most errors can be reported
2838        without the penalty of closing the connection. Clients using
2839        future versions of HTTP might optimistically try a new feature,
2840        but if communicating with an older server, retry with old
2841        semantics after an error is reported.
2842      </t>
2843    </list>
2844</t>
2845<t>
2846   HTTP implementations &SHOULD; implement persistent connections.
2847</t>
2848   
2849<section title="Establishment" anchor="persistent.establishment">
2850<t>
2851   It is beyond the scope of this specification to describe how connections
2852   are established via various transport or session-layer protocols.
2853   Each connection applies to only one transport link.
2854</t>
2855<t>
2856   A recipient determines whether a connection is persistent or not based on
2857   the most recently received message's protocol version and
2858   <x:ref>Connection</x:ref> header field (if any):
2859   <list style="symbols">
2860     <t>If the <x:ref>close</x:ref> connection option is present, the
2861        connection will not persist after the current response; else,</t>
2862     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2863        persist after the current response; else,</t>
2864     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2865        connection option is present, the recipient is not a proxy, and
2866        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2867        the connection will persist after the current response; otherwise,</t>
2868     <t>The connection will close after the current response.</t>
2869   </list>
2870</t>
2871<t>
2872   A proxy server &MUST-NOT; maintain a persistent connection with an
2873   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
2874   information and discussion of the problems with the Keep-Alive header field
2875   implemented by many HTTP/1.0 clients).
2876</t>
2877</section>
2878
2879<section title="Reuse" anchor="persistent.reuse">
2880<t>
2881   In order to remain persistent, all messages on a connection &MUST;
2882   have a self-defined message length (i.e., one not defined by closure
2883   of the connection), as described in <xref target="message.body"/>.
2884</t>
2885<t>
2886   A server &MAY; assume that an HTTP/1.1 client intends to maintain a
2887   persistent connection until a <x:ref>close</x:ref> connection option
2888   is received in a request.
2889</t>
2890<t>
2891   A client &MAY; reuse a persistent connection until it sends or receives
2892   a <x:ref>close</x:ref> connection option or receives an HTTP/1.0 response
2893   without a "keep-alive" connection option.
2894</t>
2895<t>
2896   Clients and servers &SHOULD-NOT; assume that a persistent connection is
2897   maintained for HTTP versions less than 1.1 unless it is explicitly
2898   signaled.
2899   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
2900   for more information on backward compatibility with HTTP/1.0 clients.
2901</t>
2902
2903<section title="Pipelining" anchor="pipelining">
2904<t>
2905   A client that supports persistent connections &MAY; "pipeline" its
2906   requests (i.e., send multiple requests without waiting for each
2907   response). A server &MUST; send its responses to those requests in the
2908   same order that the requests were received.
2909</t>
2910<t>
2911   Clients which assume persistent connections and pipeline immediately
2912   after connection establishment &SHOULD; be prepared to retry their
2913   connection if the first pipelined attempt fails. If a client does
2914   such a retry, it &MUST-NOT; pipeline before it knows the connection is
2915   persistent. Clients &MUST; also be prepared to resend their requests if
2916   the server closes the connection before sending all of the
2917   corresponding responses.
2918</t>
2919<t>
2920   Clients &SHOULD-NOT; pipeline requests using non-idempotent request methods
2921   or non-idempotent sequences of request methods (see &idempotent-methods;).
2922   Otherwise, a premature termination of the transport connection could lead
2923   to indeterminate results. A client wishing to send a non-idempotent
2924   request &SHOULD; wait to send that request until it has received the
2925   response status line for the previous request.
2926</t>
2927</section>
2928
2929<section title="Retrying Requests" anchor="persistent.retrying.requests">
2930<t>
2931   Connections can be closed at any time, with or without intention.
2932   Implementations ought to anticipate the need to recover
2933   from asynchronous close events.
2934   A client &MAY; open a new connection and retransmit an aborted sequence
2935   of requests without user interaction so long as the request sequence is
2936   idempotent (see &idempotent-methods;).
2937   A client &MUST-NOT; automatically retry non-idempotent request sequences,
2938   although user agents &MAY; offer a human operator the choice of retrying
2939   the request(s). Confirmation by
2940   user agent software with semantic understanding of the application
2941   &MAY; substitute for user confirmation. An automatic retry &SHOULD-NOT; 
2942   be repeated if a second sequence of requests fails.
2943</t>
2944</section>
2945</section>
2946   
2947<section title="Concurrency" anchor="persistent.concurrency">
2948<t>
2949   Clients &SHOULD; limit the number of simultaneous
2950   connections that they maintain to a given server.
2951</t>
2952<t>
2953   Previous revisions of HTTP gave a specific number of connections as a
2954   ceiling, but this was found to be impractical for many applications. As a
2955   result, this specification does not mandate a particular maximum number of
2956   connections, but instead encourages clients to be conservative when opening
2957   multiple connections.
2958</t>
2959<t>
2960   Multiple connections are typically used to avoid the "head-of-line
2961   blocking" problem, wherein a request that takes significant server-side
2962   processing and/or has a large payload blocks subsequent requests on the
2963   same connection. However, each connection consumes server resources.
2964   Furthermore, using multiple connections can cause undesirable side effects
2965   in congested networks.
2966</t>
2967<t>
2968   Note that servers might reject traffic that they deem abusive, including an
2969   excessive number of connections from a client.
2970</t>
2971</section>
2972
2973<section title="Failures and Time-outs" anchor="persistent.failures">
2974<t>
2975   Servers will usually have some time-out value beyond which they will
2976   no longer maintain an inactive connection. Proxy servers might make
2977   this a higher value since it is likely that the client will be making
2978   more connections through the same server. The use of persistent
2979   connections places no requirements on the length (or existence) of
2980   this time-out for either the client or the server.
2981</t>
2982<t>
2983   When a client or server wishes to time-out it &SHOULD; issue a graceful
2984   close on the transport connection. Clients and servers &SHOULD; both
2985   constantly watch for the other side of the transport close, and
2986   respond to it as appropriate. If a client or server does not detect
2987   the other side's close promptly it could cause unnecessary resource
2988   drain on the network.
2989</t>
2990<t>
2991   A client, server, or proxy &MAY; close the transport connection at any
2992   time. For example, a client might have started to send a new request
2993   at the same time that the server has decided to close the "idle"
2994   connection. From the server's point of view, the connection is being
2995   closed while it was idle, but from the client's point of view, a
2996   request is in progress.
2997</t>
2998<t>
2999   Servers &SHOULD; maintain persistent connections and allow the underlying
3000   transport's flow control mechanisms to resolve temporary overloads, rather
3001   than terminate connections with the expectation that clients will retry.
3002   The latter technique can exacerbate network congestion.
3003</t>
3004<t>
3005   A client sending a message body &SHOULD; monitor
3006   the network connection for an error status code while it is transmitting
3007   the request. If the client sees an error status code, it &SHOULD;
3008   immediately cease transmitting the body and close the connection.
3009</t>
3010</section>
3011   
3012<section title="Tear-down" anchor="persistent.tear-down">
3013  <iref primary="false" item="Connection header field" x:for-anchor=""/>
3014  <iref primary="false" item="close" x:for-anchor=""/>
3015<t>
3016   The <x:ref>Connection</x:ref> header field
3017   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
3018   connection option that a sender &SHOULD; send when it wishes to close
3019   the connection after the current request/response pair.
3020</t>
3021<t>
3022   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
3023   send further requests on that connection (after the one containing
3024   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
3025   final response message corresponding to this request.
3026</t>
3027<t>
3028   A server that receives a <x:ref>close</x:ref> connection option &MUST;
3029   initiate a lingering close (see below) of the connection after it sends the
3030   final response to the request that contained <x:ref>close</x:ref>.
3031   The server &SHOULD; include a <x:ref>close</x:ref> connection option
3032   in its final response on that connection. The server &MUST-NOT; process
3033   any further requests received on that connection.
3034</t>
3035<t>
3036   A server that sends a <x:ref>close</x:ref> connection option &MUST;
3037   initiate a lingering close of the connection after it sends the
3038   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
3039   any further requests received on that connection.
3040</t>
3041<t>
3042   A client that receives a <x:ref>close</x:ref> connection option &MUST;
3043   cease sending requests on that connection and close the connection
3044   after reading the response message containing the close; if additional
3045   pipelined requests had been sent on the connection, the client &SHOULD;
3046   assume that they will not be processed by the server.
3047</t>
3048<t>
3049   If a server performs an immediate close of a TCP connection, there is a
3050   significant risk that the client will not be able to read the last HTTP
3051   response.  If the server receives additional data from the client on a
3052   fully-closed connection, such as another request that was sent by the
3053   client before receiving the server's response, the server's TCP stack will
3054   send a reset packet to the client; unfortunately, the reset packet might
3055   erase the client's unacknowledged input buffers before they can be read
3056   and interpreted by the client's HTTP parser.
3057</t>
3058<t>
3059   To avoid the TCP reset problem, a server can perform a lingering close on a
3060   connection by closing only the write side of the read/write connection
3061   (a half-close) and continuing to read from the connection until the
3062   connection is closed by the client or the server is reasonably certain
3063   that its own TCP stack has received the client's acknowledgement of the
3064   packet(s) containing the server's last response. It is then safe for the
3065   server to fully close the connection.
3066</t>
3067<t>
3068   It is unknown whether the reset problem is exclusive to TCP or might also
3069   be found in other transport connection protocols.
3070</t>
3071</section>
3072</section>
3073
3074<section title="Upgrade" anchor="header.upgrade">
3075  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3076  <x:anchor-alias value="Upgrade"/>
3077  <x:anchor-alias value="protocol"/>
3078  <x:anchor-alias value="protocol-name"/>
3079  <x:anchor-alias value="protocol-version"/>
3080<t>
3081   The "Upgrade" header field is intended to provide a simple mechanism
3082   for transitioning from HTTP/1.1 to some other protocol on the same
3083   connection.  A client &MAY; send a list of protocols in the Upgrade
3084   header field of a request to invite the server to switch to one or
3085   more of those protocols before sending the final response.
3086   A server &MUST; send an Upgrade header field in <x:ref>101 (Switching
3087   Protocols)</x:ref> responses to indicate which protocol(s) are being
3088   switched to, and &MUST; send it in <x:ref>426 (Upgrade Required)</x:ref>
3089   responses to indicate acceptable protocols.
3090   A server &MAY; send an Upgrade header field in any other response to
3091   indicate that they might be willing to upgrade to one of the
3092   specified protocols for a future request.
3093</t>
3094<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3095  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3096
3097  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3098  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3099  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3100</artwork></figure>
3101<t>
3102   For example,
3103</t>
3104<figure><artwork type="example">
3105  Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3106</artwork></figure>
3107<t>
3108   Upgrade eases the difficult transition between incompatible protocols by
3109   allowing the client to initiate a request in the more commonly
3110   supported protocol while indicating to the server that it would like
3111   to use a "better" protocol if available (where "better" is determined
3112   by the server, possibly according to the nature of the request method
3113   or target resource).
3114</t>
3115<t>
3116   Upgrade cannot be used to insist on a protocol change; its acceptance and
3117   use by the server is optional. The capabilities and nature of the
3118   application-level communication after the protocol change is entirely
3119   dependent upon the new protocol chosen, although the first action
3120   after changing the protocol &MUST; be a response to the initial HTTP
3121   request that contained the Upgrade header field.
3122</t>
3123<t>
3124   For example, if the Upgrade header field is received in a GET request
3125   and the server decides to switch protocols, then it &MUST; first respond
3126   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3127   then immediately follow that with the new protocol's equivalent of a
3128   response to a GET on the target resource.  This allows a connection to be
3129   upgraded to protocols with the same semantics as HTTP without the
3130   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3131   protocols unless the received message semantics can be honored by the new
3132   protocol; an OPTIONS request can be honored by any protocol.
3133</t>
3134<t>
3135   When Upgrade is sent, a sender &MUST; also send a
3136   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3137   that contains the "upgrade" connection option, in order to prevent Upgrade
3138   from being accidentally forwarded by intermediaries that might not implement
3139   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3140   is received in an HTTP/1.0 request.
3141</t>
3142<t>
3143   The Upgrade header field only applies to switching application-level
3144   protocols on the existing connection; it cannot be used
3145   to switch to a protocol on a different connection. For that purpose, it is
3146   more appropriate to use a <x:ref>3xx (Redirection)</x:ref> response
3147   (&status-3xx;).
3148</t>
3149<t>
3150   This specification only defines the protocol name "HTTP" for use by
3151   the family of Hypertext Transfer Protocols, as defined by the HTTP
3152   version rules of <xref target="http.version"/> and future updates to this
3153   specification. Additional tokens can be registered with IANA using the
3154   registration procedure defined in <xref target="upgrade.token.registry"/>.
3155</t>
3156</section>
3157</section>
3158
3159<section title="IANA Considerations" anchor="IANA.considerations">
3160
3161<section title="Header Field Registration" anchor="header.field.registration">
3162<t>
3163   HTTP header fields are registered within the Message Header Field Registry
3164   <xref target="RFC3864"/> maintained by IANA at
3165   <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
3166</t>
3167<t>
3168   This document defines the following HTTP header fields, so their
3169   associated registry entries shall be updated according to the permanent
3170   registrations below:
3171</t>
3172<?BEGININC p1-messaging.iana-headers ?>
3173<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3174<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3175   <ttcol>Header Field Name</ttcol>
3176   <ttcol>Protocol</ttcol>
3177   <ttcol>Status</ttcol>
3178   <ttcol>Reference</ttcol>
3179
3180   <c>Connection</c>
3181   <c>http</c>
3182   <c>standard</c>
3183   <c>
3184      <xref target="header.connection"/>
3185   </c>
3186   <c>Content-Length</c>
3187   <c>http</c>
3188   <c>standard</c>
3189   <c>
3190      <xref target="header.content-length"/>
3191   </c>
3192   <c>Host</c>
3193   <c>http</c>
3194   <c>standard</c>
3195   <c>
3196      <xref target="header.host"/>
3197   </c>
3198   <c>TE</c>
3199   <c>http</c>
3200   <c>standard</c>
3201   <c>
3202      <xref target="header.te"/>
3203   </c>
3204   <c>Trailer</c>
3205   <c>http</c>
3206   <c>standard</c>
3207   <c>
3208      <xref target="header.trailer"/>
3209   </c>
3210   <c>Transfer-Encoding</c>
3211   <c>http</c>
3212   <c>standard</c>
3213   <c>
3214      <xref target="header.transfer-encoding"/>
3215   </c>
3216   <c>Upgrade</c>
3217   <c>http</c>
3218   <c>standard</c>
3219   <c>
3220      <xref target="header.upgrade"/>
3221   </c>
3222   <c>Via</c>
3223   <c>http</c>
3224   <c>standard</c>
3225   <c>
3226      <xref target="header.via"/>
3227   </c>
3228</texttable>
3229<!--(END)-->
3230<?ENDINC p1-messaging.iana-headers ?>
3231<t>
3232   Furthermore, the header field-name "Close" shall be registered as
3233   "reserved", since using that name as an HTTP header field might
3234   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3235   header field (<xref target="header.connection"/>).
3236</t>
3237<texttable align="left" suppress-title="true">
3238   <ttcol>Header Field Name</ttcol>
3239   <ttcol>Protocol</ttcol>
3240   <ttcol>Status</ttcol>
3241   <ttcol>Reference</ttcol>
3242
3243   <c>Close</c>
3244   <c>http</c>
3245   <c>reserved</c>
3246   <c>
3247      <xref target="header.field.registration"/>
3248   </c>
3249</texttable>
3250<t>
3251   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3252</t>
3253</section>
3254
3255<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3256<t>
3257   IANA maintains the registry of URI Schemes <xref target="RFC4395"/> at
3258   <eref target="http://www.iana.org/assignments/uri-schemes.html"/>.
3259</t>
3260<t>
3261   This document defines the following URI schemes, so their
3262   associated registry entries shall be updated according to the permanent
3263   registrations below:
3264</t>
3265<texttable align="left" suppress-title="true">
3266   <ttcol>URI Scheme</ttcol>
3267   <ttcol>Description</ttcol>
3268   <ttcol>Reference</ttcol>
3269
3270   <c>http</c>
3271   <c>Hypertext Transfer Protocol</c>
3272   <c><xref target="http.uri"/></c>
3273
3274   <c>https</c>
3275   <c>Hypertext Transfer Protocol Secure</c>
3276   <c><xref target="https.uri"/></c>
3277</texttable>
3278</section>
3279
3280<section title="Internet Media Type Registrations" anchor="internet.media.type.http">
3281<t>
3282   This document serves as the specification for the Internet media types
3283   "message/http" and "application/http". The following is to be registered with
3284   IANA (see <xref target="RFC4288"/>).
3285</t>
3286<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
3287<iref item="Media Type" subitem="message/http" primary="true"/>
3288<iref item="message/http Media Type" primary="true"/>
3289<t>
3290   The message/http type can be used to enclose a single HTTP request or
3291   response message, provided that it obeys the MIME restrictions for all
3292   "message" types regarding line length and encodings.
3293</t>
3294<t>
3295  <list style="hanging" x:indent="12em">
3296    <t hangText="Type name:">
3297      message
3298    </t>
3299    <t hangText="Subtype name:">
3300      http
3301    </t>
3302    <t hangText="Required parameters:">
3303      none
3304    </t>
3305    <t hangText="Optional parameters:">
3306      version, msgtype
3307      <list style="hanging">
3308        <t hangText="version:">
3309          The HTTP-version number of the enclosed message
3310          (e.g., "1.1"). If not present, the version can be
3311          determined from the first line of the body.
3312        </t>
3313        <t hangText="msgtype:">
3314          The message type &mdash; "request" or "response". If not
3315          present, the type can be determined from the first
3316          line of the body.
3317        </t>
3318      </list>
3319    </t>
3320    <t hangText="Encoding considerations:">
3321      only "7bit", "8bit", or "binary" are permitted
3322    </t>
3323    <t hangText="Security considerations:">
3324      none
3325    </t>
3326    <t hangText="Interoperability considerations:">
3327      none
3328    </t>
3329    <t hangText="Published specification:">
3330      This specification (see <xref target="internet.media.type.message.http"/>).
3331    </t>
3332    <t hangText="Applications that use this media type:">
3333    </t>
3334    <t hangText="Additional information:">
3335      <list style="hanging">
3336        <t hangText="Magic number(s):">none</t>
3337        <t hangText="File extension(s):">none</t>
3338        <t hangText="Macintosh file type code(s):">none</t>
3339      </list>
3340    </t>
3341    <t hangText="Person and email address to contact for further information:">
3342      See Authors Section.
3343    </t>
3344    <t hangText="Intended usage:">
3345      COMMON
3346    </t>
3347    <t hangText="Restrictions on usage:">
3348      none
3349    </t>
3350    <t hangText="Author/Change controller:">
3351      IESG
3352    </t>
3353  </list>
3354</t>
3355</section>
3356<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
3357<iref item="Media Type" subitem="application/http" primary="true"/>
3358<iref item="application/http Media Type" primary="true"/>
3359<t>
3360   The application/http type can be used to enclose a pipeline of one or more
3361   HTTP request or response messages (not intermixed).
3362</t>
3363<t>
3364  <list style="hanging" x:indent="12em">
3365    <t hangText="Type name:">
3366      application
3367    </t>
3368    <t hangText="Subtype name:">
3369      http
3370    </t>
3371    <t hangText="Required parameters:">
3372      none
3373    </t>
3374    <t hangText="Optional parameters:">
3375      version, msgtype
3376      <list style="hanging">
3377        <t hangText="version:">
3378          The HTTP-version number of the enclosed messages
3379          (e.g., "1.1"). If not present, the version can be
3380          determined from the first line of the body.
3381        </t>
3382        <t hangText="msgtype:">
3383          The message type &mdash; "request" or "response". If not
3384          present, the type can be determined from the first
3385          line of the body.
3386        </t>
3387      </list>
3388    </t>
3389    <t hangText="Encoding considerations:">
3390      HTTP messages enclosed by this type
3391      are in "binary" format; use of an appropriate
3392      Content-Transfer-Encoding is required when
3393      transmitted via E-mail.
3394    </t>
3395    <t hangText="Security considerations:">
3396      none
3397    </t>
3398    <t hangText="Interoperability considerations:">
3399      none
3400    </t>
3401    <t hangText="Published specification:">
3402      This specification (see <xref target="internet.media.type.application.http"/>).
3403    </t>
3404    <t hangText="Applications that use this media type:">
3405    </t>
3406    <t hangText="Additional information:">
3407      <list style="hanging">
3408        <t hangText="Magic number(s):">none</t>
3409        <t hangText="File extension(s):">none</t>
3410        <t hangText="Macintosh file type code(s):">none</t>
3411      </list>
3412    </t>
3413    <t hangText="Person and email address to contact for further information:">
3414      See Authors Section.
3415    </t>
3416    <t hangText="Intended usage:">
3417      COMMON
3418    </t>
3419    <t hangText="Restrictions on usage:">
3420      none
3421    </t>
3422    <t hangText="Author/Change controller:">
3423      IESG
3424    </t>
3425  </list>
3426</t>
3427</section>
3428</section>
3429
3430<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3431<t>
3432   The HTTP Transfer Coding Registry defines the name space for transfer
3433   coding names.
3434</t>
3435<t>
3436   Registrations &MUST; include the following fields:
3437   <list style="symbols">
3438     <t>Name</t>
3439     <t>Description</t>
3440     <t>Pointer to specification text</t>
3441   </list>
3442</t>
3443<t>
3444   Names of transfer codings &MUST-NOT; overlap with names of content codings
3445   (&content-codings;) unless the encoding transformation is identical, as
3446   is the case for the compression codings defined in
3447   <xref target="compression.codings"/>.
3448</t>
3449<t>
3450   Values to be added to this name space require IETF Review (see
3451   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3452   conform to the purpose of transfer coding defined in this section.
3453   Use of program names for the identification of encoding formats
3454   is not desirable and is discouraged for future encodings.
3455</t>
3456<t>
3457   The registry itself is maintained at
3458   <eref target="http://www.iana.org/assignments/http-parameters"/>.
3459</t>
3460</section>
3461
3462<section title="Transfer Coding Registrations" anchor="transfer.coding.registration">
3463<t>
3464   The HTTP Transfer Coding Registry shall be updated with the registrations
3465   below:
3466</t>
3467<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3468   <ttcol>Name</ttcol>
3469   <ttcol>Description</ttcol>
3470   <ttcol>Reference</ttcol>
3471   <c>chunked</c>
3472   <c>Transfer in a series of chunks</c>
3473   <c>
3474      <xref target="chunked.encoding"/>
3475   </c>
3476   <c>compress</c>
3477   <c>UNIX "compress" program method</c>
3478   <c>
3479      <xref target="compress.coding"/>
3480   </c>
3481   <c>deflate</c>
3482   <c>"deflate" compression mechanism (<xref target="RFC1951"/>) used inside
3483   the "zlib" data format (<xref target="RFC1950"/>)
3484   </c>
3485   <c>
3486      <xref target="deflate.coding"/>
3487   </c>
3488   <c>gzip</c>
3489   <c>Same as GNU zip <xref target="RFC1952"/></c>
3490   <c>
3491      <xref target="gzip.coding"/>
3492   </c>
3493</texttable>
3494</section>
3495
3496<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3497<t>
3498   The HTTP Upgrade Token Registry defines the name space for protocol-name
3499   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3500   field. Each registered protocol name is associated with contact information
3501   and an optional set of specifications that details how the connection
3502   will be processed after it has been upgraded.
3503</t>
3504<t>
3505   Registrations happen on a "First Come First Served" basis (see
3506   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3507   following rules:
3508  <list style="numbers">
3509    <t>A protocol-name token, once registered, stays registered forever.</t>
3510    <t>The registration &MUST; name a responsible party for the
3511       registration.</t>
3512    <t>The registration &MUST; name a point of contact.</t>
3513    <t>The registration &MAY; name a set of specifications associated with
3514       that token. Such specifications need not be publicly available.</t>
3515    <t>The registration &SHOULD; name a set of expected "protocol-version"
3516       tokens associated with that token at the time of registration.</t>
3517    <t>The responsible party &MAY; change the registration at any time.
3518       The IANA will keep a record of all such changes, and make them
3519       available upon request.</t>
3520    <t>The IESG &MAY; reassign responsibility for a protocol token.
3521       This will normally only be used in the case when a
3522       responsible party cannot be contacted.</t>
3523  </list>
3524</t>
3525<t>
3526   This registration procedure for HTTP Upgrade Tokens replaces that
3527   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3528</t>
3529</section>
3530
3531<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3532<t>
3533   The HTTP Upgrade Token Registry shall be updated with the registration
3534   below:
3535</t>
3536<texttable align="left" suppress-title="true">
3537   <ttcol>Value</ttcol>
3538   <ttcol>Description</ttcol>
3539   <ttcol>Expected Version Tokens</ttcol>
3540   <ttcol>Reference</ttcol>
3541
3542   <c>HTTP</c>
3543   <c>Hypertext Transfer Protocol</c>
3544   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3545   <c><xref target="http.version"/></c>
3546</texttable>
3547<t>
3548   The responsible party is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3549</t>
3550</section>
3551
3552</section>
3553
3554<section title="Security Considerations" anchor="security.considerations">
3555<t>
3556   This section is meant to inform application developers, information
3557   providers, and users of the security limitations in HTTP/1.1 as
3558   described by this document. The discussion does not include
3559   definitive solutions to the problems revealed, though it does make
3560   some suggestions for reducing security risks.
3561</t>
3562
3563<section title="Personal Information" anchor="personal.information">
3564<t>
3565   HTTP clients are often privy to large amounts of personal information,
3566   including both information provided by the user to interact with resources
3567   (e.g., the user's name, location, mail address, passwords, encryption
3568   keys, etc.) and information about the user's browsing activity over
3569   time (e.g., history, bookmarks, etc.). HTTP implementations need to
3570   prevent unintentional leakage of this information.
3571</t>
3572</section>
3573
3574<section title="Abuse of Server Log Information" anchor="abuse.of.server.log.information">
3575<t>
3576   A server is in the position to save personal data about a user's
3577   requests which might identify their reading patterns or subjects of
3578   interest.  In particular, log information gathered at an intermediary
3579   often contains a history of user agent interaction, across a multitude
3580   of sites, that can be traced to individual users.
3581</t>
3582<t>
3583   HTTP log information is confidential in nature; its handling is often
3584   constrained by laws and regulations.  Log information needs to be securely
3585   stored and appropriate guidelines followed for its analysis.
3586   Anonymization of personal information within individual entries helps,
3587   but is generally not sufficient to prevent real log traces from being
3588   re-identified based on correlation with other access characteristics.
3589   As such, access traces that are keyed to a specific client should not
3590   be published even if the key is pseudonymous.
3591</t>
3592<t>
3593   To minimize the risk of theft or accidental publication, log information
3594   should be purged of personally identifiable information, including
3595   user identifiers, IP addresses, and user-provided query parameters,
3596   as soon as that information is no longer necessary to support operational
3597   needs for security, auditing, or fraud control.
3598</t>
3599</section>
3600
3601<section title="Attacks Based On File and Path Names" anchor="attack.pathname">
3602<t>
3603   Origin servers &SHOULD; be careful to restrict
3604   the documents returned by HTTP requests to be only those that were
3605   intended by the server administrators. If an HTTP server translates
3606   HTTP URIs directly into file system calls, the server &MUST; take
3607   special care not to serve files that were not intended to be
3608   delivered to HTTP clients. For example, UNIX, Microsoft Windows, and
3609   other operating systems use ".." as a path component to indicate a
3610   directory level above the current one. On such a system, an HTTP
3611   server &MUST; disallow any such construct in the request-target if it
3612   would otherwise allow access to a resource outside those intended to
3613   be accessible via the HTTP server. Similarly, files intended for
3614   reference only internally to the server (such as access control
3615   files, configuration files, and script code) &MUST; be protected from
3616   inappropriate retrieval, since they might contain sensitive
3617   information.
3618</t>
3619</section>
3620
3621<section title="DNS-related Attacks" anchor="dns.related.attacks">
3622<t>
3623   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3624   generally prone to security attacks based on the deliberate misassociation
3625   of IP addresses and DNS names not protected by DNSSEC. Clients need to be
3626   cautious in assuming the validity of an IP number/DNS name association unless
3627   the response is protected by DNSSEC (<xref target="RFC4033"/>).
3628</t>
3629</section>
3630
3631<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3632<t>
3633   By their very nature, HTTP intermediaries are men-in-the-middle, and
3634   represent an opportunity for man-in-the-middle attacks. Compromise of
3635   the systems on which the intermediaries run can result in serious security
3636   and privacy problems. Intermediaries have access to security-related
3637   information, personal information about individual users and
3638   organizations, and proprietary information belonging to users and
3639   content providers. A compromised intermediary, or an intermediary
3640   implemented or configured without regard to security and privacy
3641   considerations, might be used in the commission of a wide range of
3642   potential attacks.
3643</t>
3644<t>
3645   Intermediaries that contain a shared cache are especially vulnerable
3646   to cache poisoning attacks.
3647</t>
3648<t>
3649   Implementers need to consider the privacy and security
3650   implications of their design and coding decisions, and of the
3651   configuration options they provide to operators (especially the
3652   default configuration).
3653</t>
3654<t>
3655   Users need to be aware that intermediaries are no more trustworthy than
3656   the people who run them; HTTP itself cannot solve this problem.
3657</t>
3658</section>
3659
3660<section title="Protocol Element Size Overflows" anchor="attack.protocol.element.size.overflows">
3661<t>
3662   Because HTTP uses mostly textual, character-delimited fields, attackers can
3663   overflow buffers in implementations, and/or perform a Denial of Service
3664   against implementations that accept fields with unlimited lengths.
3665</t>
3666<t>
3667   To promote interoperability, this specification makes specific
3668   recommendations for minimum size limits on request-line
3669   (<xref target="request.line"/>)
3670   and blocks of header fields (<xref target="header.fields"/>). These are
3671   minimum recommendations, chosen to be supportable even by implementations
3672   with limited resources; it is expected that most implementations will
3673   choose substantially higher limits.
3674</t>
3675<t>
3676   This specification also provides a way for servers to reject messages that
3677   have request-targets that are too long (&status-414;) or request entities
3678   that are too large (&status-4xx;).
3679</t>
3680<t>
3681   Recipients &SHOULD; carefully limit the extent to which they read other
3682   fields, including (but not limited to) request methods, response status
3683   phrases, header field-names, and body chunks, so as to avoid denial of
3684   service attacks without impeding interoperability.
3685</t>
3686</section>
3687</section>
3688
3689<section title="Acknowledgments" anchor="acks">
3690<t>
3691   This edition of HTTP/1.1 builds on the many contributions that went into
3692   <xref target="RFC1945" format="none">RFC 1945</xref>,
3693   <xref target="RFC2068" format="none">RFC 2068</xref>,
3694   <xref target="RFC2145" format="none">RFC 2145</xref>, and
3695   <xref target="RFC2616" format="none">RFC 2616</xref>, including
3696   substantial contributions made by the previous authors, editors, and
3697   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
3698   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
3699   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
3700</t>
3701<t>
3702   Since 1999, the following contributors have helped improve the HTTP
3703   specification by reporting bugs, asking smart questions, drafting or
3704   reviewing text, and evaluating open issues:
3705</t>
3706<?BEGININC acks ?>
3707<t>Adam Barth,
3708Adam Roach,
3709Addison Phillips,
3710Adrian Chadd,
3711Adrien W. de Croy,
3712Alan Ford,
3713Alan Ruttenberg,
3714Albert Lunde,
3715Alek Storm,
3716Alex Rousskov,
3717Alexandre Morgaut,
3718Alexey Melnikov,
3719Alisha Smith,
3720Amichai Rothman,
3721Amit Klein,
3722Amos Jeffries,
3723Andreas Maier,
3724Andreas Petersson,
3725Anil Sharma,
3726Anne van Kesteren,
3727Anthony Bryan,
3728Asbjorn Ulsberg,
3729Ashok Kumar,
3730Balachander Krishnamurthy,
3731Barry Leiba,
3732Ben Laurie,
3733Benjamin Niven-Jenkins,
3734Bil Corry,
3735Bill Burke,
3736Bjoern Hoehrmann,
3737Bob Scheifler,
3738Boris Zbarsky,
3739Brett Slatkin,
3740Brian Kell,
3741Brian McBarron,
3742Brian Pane,
3743Brian Smith,
3744Bryce Nesbitt,
3745Cameron Heavon-Jones,
3746Carl Kugler,
3747Carsten Bormann,
3748Charles Fry,
3749Chris Newman,
3750Cyrus Daboo,
3751Dale Robert Anderson,
3752Dan Wing,
3753Dan Winship,
3754Daniel Stenberg,
3755Dave Cridland,
3756Dave Crocker,
3757Dave Kristol,
3758David Booth,
3759David Singer,
3760David W. Morris,
3761Diwakar Shetty,
3762Dmitry Kurochkin,
3763Drummond Reed,
3764Duane Wessels,
3765Edward Lee,
3766Eliot Lear,
3767Eran Hammer-Lahav,
3768Eric D. Williams,
3769Eric J. Bowman,
3770Eric Lawrence,
3771Eric Rescorla,
3772Erik Aronesty,
3773Evan Prodromou,
3774Florian Weimer,
3775Frank Ellermann,
3776Fred Bohle,
3777Gabriel Montenegro,
3778Geoffrey Sneddon,
3779Gervase Markham,
3780Grahame Grieve,
3781Greg Wilkins,
3782Harald Tveit Alvestrand,
3783Harry Halpin,
3784Helge Hess,
3785Henrik Nordstrom,
3786Henry S. Thompson,
3787Henry Story,
3788Herbert van de Sompel,
3789Howard Melman,
3790Hugo Haas,
3791Ian Fette,
3792Ian Hickson,
3793Ido Safruti,
3794Ilya Grigorik,
3795Ingo Struck,
3796J. Ross Nicoll,
3797James H. Manger,
3798James Lacey,
3799James M. Snell,
3800Jamie Lokier,
3801Jan Algermissen,
3802Jeff Hodges (who came up with the term 'effective Request-URI'),
3803Jeff Walden,
3804Jim Luther,
3805Joe D. Williams,
3806Joe Gregorio,
3807Joe Orton,
3808John C. Klensin,
3809John C. Mallery,
3810John Cowan,
3811John Kemp,
3812John Panzer,
3813John Schneider,
3814John Stracke,
3815John Sullivan,
3816Jonas Sicking,
3817Jonathan Billington,
3818Jonathan Moore,
3819Jonathan Rees,
3820Jonathan Silvera,
3821Jordi Ros,
3822Joris Dobbelsteen,
3823Josh Cohen,
3824Julien Pierre,
3825Jungshik Shin,
3826Justin Chapweske,
3827Justin Erenkrantz,
3828Justin James,
3829Kalvinder Singh,
3830Karl Dubost,
3831Keith Hoffman,
3832Keith Moore,
3833Ken Murchison,
3834Koen Holtman,
3835Konstantin Voronkov,
3836Kris Zyp,
3837Lisa Dusseault,
3838Maciej Stachowiak,
3839Marc Schneider,
3840Marc Slemko,
3841Mark Baker,
3842Mark Pauley,
3843Mark Watson,
3844Markus Isomaki,
3845Markus Lanthaler,
3846Martin J. Duerst,
3847Martin Musatov,
3848Martin Nilsson,
3849Martin Thomson,
3850Matt Lynch,
3851Matthew Cox,
3852Max Clark,
3853Michael Burrows,
3854Michael Hausenblas,
3855Mike Amundsen,
3856Mike Belshe,
3857Mike Kelly,
3858Mike Schinkel,
3859Miles Sabin,
3860Murray S. Kucherawy,
3861Mykyta Yevstifeyev,
3862Nathan Rixham,
3863Nicholas Shanks,
3864Nico Williams,
3865Nicolas Alvarez,
3866Nicolas Mailhot,
3867Noah Slater,
3868Pablo Castro,
3869Pat Hayes,
3870Patrick R. McManus,
3871Paul E. Jones,
3872Paul Hoffman,
3873Paul Marquess,
3874Peter Lepeska,
3875Peter Saint-Andre,
3876Peter Watkins,
3877Phil Archer,
3878Philippe Mougin,
3879Phillip Hallam-Baker,
3880Poul-Henning Kamp,
3881Preethi Natarajan,
3882Rajeev Bector,
3883Ray Polk,
3884Reto Bachmann-Gmuer,
3885Richard Cyganiak,
3886Robert Brewer,
3887Robert Collins,
3888Robert O'Callahan,
3889Robert Olofsson,
3890Robert Sayre,
3891Robert Siemer,
3892Robert de Wilde,
3893Roberto Javier Godoy,
3894Roberto Peon,
3895Roland Zink,
3896Ronny Widjaja,
3897S. Mike Dierken,
3898Salvatore Loreto,
3899Sam Johnston,
3900Sam Ruby,
3901Scott Lawrence (who maintained the original issues list),
3902Sean B. Palmer,
3903Shane McCarron,
3904Stefan Eissing,
3905Stefan Tilkov,
3906Stefanos Harhalakis,
3907Stephane Bortzmeyer,
3908Stephen Farrell,
3909Stephen Ludin,
3910Stuart Williams,
3911Subbu Allamaraju,
3912Sylvain Hellegouarch,
3913Tapan Divekar,
3914Tatsuya Hayashi,
3915Ted Hardie,
3916Thomas Broyer,
3917Thomas Fossati,
3918Thomas Nordin,
3919Thomas Roessler,
3920Tim Bray,
3921Tim Morgan,
3922Tim Olsen,
3923Tom Zhou,
3924Travis Snoozy,
3925Tyler Close,
3926Vincent Murphy,
3927Wenbo Zhu,
3928Werner Baumann,
3929Wilbur Streett,
3930Wilfredo Sanchez Vega,
3931William A. Rowe Jr.,
3932William Chan,
3933Willy Tarreau,
3934Xiaoshu Wang,
3935Yaron Goland,
3936Yngve Nysaeter Pettersen,
3937Yoav Nir,
3938Yogesh Bang,
3939Yutaka Oiwa,
3940Yves Lafon (long-time member of the editor team),
3941Zed A. Shaw, and
3942Zhong Yu.
3943</t>
3944<?ENDINC acks ?>
3945<t>
3946   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
3947   acknowledgements from prior revisions.
3948</t>
3949</section>
3950
3951</middle>
3952<back>
3953
3954<references title="Normative References">
3955
3956<reference anchor="Part2">
3957  <front>
3958    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
3959    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3960      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
3961      <address><email>fielding@gbiv.com</email></address>
3962    </author>
3963    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3964      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3965      <address><email>julian.reschke@greenbytes.de</email></address>
3966    </author>
3967    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3968  </front>
3969  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
3970  <x:source href="p2-semantics.xml" basename="p2-semantics">
3971    <x:defines>1xx (Informational)</x:defines>
3972    <x:defines>1xx</x:defines>
3973    <x:defines>100 (Continue)</x:defines>
3974    <x:defines>101 (Switching Protocols)</x:defines>
3975    <x:defines>2xx (Successful)</x:defines>
3976    <x:defines>2xx</x:defines>
3977    <x:defines>200 (OK)</x:defines>
3978    <x:defines>204 (No Content)</x:defines>
3979    <x:defines>3xx (Redirection)</x:defines>
3980    <x:defines>3xx</x:defines>
3981    <x:defines>301 (Moved Permanently)</x:defines>
3982    <x:defines>4xx (Client Error)</x:defines>
3983    <x:defines>4xx</x:defines>
3984    <x:defines>400 (Bad Request)</x:defines>
3985    <x:defines>405 (Method Not Allowed)</x:defines>
3986    <x:defines>411 (Length Required)</x:defines>
3987    <x:defines>414 (URI Too Long)</x:defines>
3988    <x:defines>417 (Expectation Failed)</x:defines>
3989    <x:defines>426 (Upgrade Required)</x:defines>
3990    <x:defines>501 (Not Implemented)</x:defines>
3991    <x:defines>502 (Bad Gateway)</x:defines>
3992    <x:defines>505 (HTTP Version Not Supported)</x:defines>
3993    <x:defines>Allow</x:defines>
3994    <x:defines>Content-Encoding</x:defines>
3995    <x:defines>Content-Location</x:defines>
3996    <x:defines>Content-Type</x:defines>
3997    <x:defines>Date</x:defines>
3998    <x:defines>Expect</x:defines>
3999    <x:defines>Location</x:defines>
4000    <x:defines>Server</x:defines>
4001    <x:defines>User-Agent</x:defines>
4002  </x:source>
4003</reference>
4004
4005<reference anchor="Part4">
4006  <front>
4007    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
4008    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
4009      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4010      <address><email>fielding@gbiv.com</email></address>
4011    </author>
4012    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
4013      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4014      <address><email>julian.reschke@greenbytes.de</email></address>
4015    </author>
4016    <date month="&ID-MONTH;" year="&ID-YEAR;" />
4017  </front>
4018  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
4019  <x:source basename="p4-conditional" href="p4-conditional.xml">
4020    <x:defines>304 (Not Modified)</x:defines>
4021    <x:defines>ETag</x:defines>
4022    <x:defines>Last-Modified</x:defines>
4023  </x:source>
4024</reference>
4025
4026<reference anchor="Part5">
4027  <front>
4028    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
4029    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4030      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4031      <address><email>fielding@gbiv.com</email></address>
4032    </author>
4033    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
4034      <organization abbrev="W3C">World Wide Web Consortium</organization>
4035      <address><email>ylafon@w3.org</email></address>
4036    </author>
4037    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4038      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4039      <address><email>julian.reschke@greenbytes.de</email></address>
4040    </author>
4041    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4042  </front>
4043  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4044  <x:source href="p5-range.xml" basename="p5-range">
4045    <x:defines>Content-Range</x:defines>
4046  </x:source>
4047</reference>
4048
4049<reference anchor="Part6">
4050  <front>
4051    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4052    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4053      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4054      <address><email>fielding@gbiv.com</email></address>
4055    </author>
4056    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4057      <organization>Akamai</organization>
4058      <address><email>mnot@mnot.net</email></address>
4059    </author>
4060    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4061      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4062      <address><email>julian.reschke@greenbytes.de</email></address>
4063    </author>
4064    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4065  </front>
4066  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4067  <x:source href="p6-cache.xml" basename="p6-cache">
4068    <x:defines>Expires</x:defines>
4069  </x:source>
4070</reference>
4071
4072<reference anchor="Part7">
4073  <front>
4074    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4075    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4076      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4077      <address><email>fielding@gbiv.com</email></address>
4078    </author>
4079    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4080      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4081      <address><email>julian.reschke@greenbytes.de</email></address>
4082    </author>
4083    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4084  </front>
4085  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4086  <x:source href="p7-auth.xml" basename="p7-auth">
4087    <x:defines>Proxy-Authenticate</x:defines>
4088    <x:defines>Proxy-Authorization</x:defines>
4089  </x:source>
4090</reference>
4091
4092<reference anchor="RFC5234">
4093  <front>
4094    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4095    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4096      <organization>Brandenburg InternetWorking</organization>
4097      <address>
4098        <email>dcrocker@bbiw.net</email>
4099      </address> 
4100    </author>
4101    <author initials="P." surname="Overell" fullname="Paul Overell">
4102      <organization>THUS plc.</organization>
4103      <address>
4104        <email>paul.overell@thus.net</email>
4105      </address>
4106    </author>
4107    <date month="January" year="2008"/>
4108  </front>
4109  <seriesInfo name="STD" value="68"/>
4110  <seriesInfo name="RFC" value="5234"/>
4111</reference>
4112
4113<reference anchor="RFC2119">
4114  <front>
4115    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4116    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4117      <organization>Harvard University</organization>
4118      <address><email>sob@harvard.edu</email></address>
4119    </author>
4120    <date month="March" year="1997"/>
4121  </front>
4122  <seriesInfo name="BCP" value="14"/>
4123  <seriesInfo name="RFC" value="2119"/>
4124</reference>
4125
4126<reference anchor="RFC3986">
4127 <front>
4128  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4129  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4130    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4131    <address>
4132       <email>timbl@w3.org</email>
4133       <uri>http://www.w3.org/People/Berners-Lee/</uri>
4134    </address>
4135  </author>
4136  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4137    <organization abbrev="Day Software">Day Software</organization>
4138    <address>
4139      <email>fielding@gbiv.com</email>
4140      <uri>http://roy.gbiv.com/</uri>
4141    </address>
4142  </author>
4143  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4144    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4145    <address>
4146      <email>LMM@acm.org</email>
4147      <uri>http://larry.masinter.net/</uri>
4148    </address>
4149  </author>
4150  <date month='January' year='2005'></date>
4151 </front>
4152 <seriesInfo name="STD" value="66"/>
4153 <seriesInfo name="RFC" value="3986"/>
4154</reference>
4155
4156<reference anchor="USASCII">
4157  <front>
4158    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4159    <author>
4160      <organization>American National Standards Institute</organization>
4161    </author>
4162    <date year="1986"/>
4163  </front>
4164  <seriesInfo name="ANSI" value="X3.4"/>
4165</reference>
4166
4167<reference anchor="RFC1950">
4168  <front>
4169    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4170    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4171      <organization>Aladdin Enterprises</organization>
4172      <address><email>ghost@aladdin.com</email></address>
4173    </author>
4174    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4175    <date month="May" year="1996"/>
4176  </front>
4177  <seriesInfo name="RFC" value="1950"/>
4178  <!--<annotation>
4179    RFC 1950 is an Informational RFC, thus it might be less stable than
4180    this specification. On the other hand, this downward reference was
4181    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4182    therefore it is unlikely to cause problems in practice. See also
4183    <xref target="BCP97"/>.
4184  </annotation>-->
4185</reference>
4186
4187<reference anchor="RFC1951">
4188  <front>
4189    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4190    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4191      <organization>Aladdin Enterprises</organization>
4192      <address><email>ghost@aladdin.com</email></address>
4193    </author>
4194    <date month="May" year="1996"/>
4195  </front>
4196  <seriesInfo name="RFC" value="1951"/>
4197  <!--<annotation>
4198    RFC 1951 is an Informational RFC, thus it might be less stable than
4199    this specification. On the other hand, this downward reference was
4200    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4201    therefore it is unlikely to cause problems in practice. See also
4202    <xref target="BCP97"/>.
4203  </annotation>-->
4204</reference>
4205
4206<reference anchor="RFC1952">
4207  <front>
4208    <title>GZIP file format specification version 4.3</title>
4209    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4210      <organization>Aladdin Enterprises</organization>
4211      <address><email>ghost@aladdin.com</email></address>
4212    </author>
4213    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4214      <address><email>gzip@prep.ai.mit.edu</email></address>
4215    </author>
4216    <author initials="M." surname="Adler" fullname="Mark Adler">
4217      <address><email>madler@alumni.caltech.edu</email></address>
4218    </author>
4219    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4220      <address><email>ghost@aladdin.com</email></address>
4221    </author>
4222    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4223      <address><email>randeg@alumni.rpi.edu</email></address>
4224    </author>
4225    <date month="May" year="1996"/>
4226  </front>
4227  <seriesInfo name="RFC" value="1952"/>
4228  <!--<annotation>
4229    RFC 1952 is an Informational RFC, thus it might be less stable than
4230    this specification. On the other hand, this downward reference was
4231    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4232    therefore it is unlikely to cause problems in practice. See also
4233    <xref target="BCP97"/>.
4234  </annotation>-->
4235</reference>
4236
4237</references>
4238
4239<references title="Informative References">
4240
4241<reference anchor="ISO-8859-1">
4242  <front>
4243    <title>
4244     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4245    </title>
4246    <author>
4247      <organization>International Organization for Standardization</organization>
4248    </author>
4249    <date year="1998"/>
4250  </front>
4251  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4252</reference>
4253
4254<reference anchor='RFC1919'>
4255  <front>
4256    <title>Classical versus Transparent IP Proxies</title>
4257    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4258      <address><email>mchatel@pax.eunet.ch</email></address>
4259    </author>
4260    <date year='1996' month='March' />
4261  </front>
4262  <seriesInfo name='RFC' value='1919' />
4263</reference>
4264
4265<reference anchor="RFC1945">
4266  <front>
4267    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4268    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4269      <organization>MIT, Laboratory for Computer Science</organization>
4270      <address><email>timbl@w3.org</email></address>
4271    </author>
4272    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4273      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4274      <address><email>fielding@ics.uci.edu</email></address>
4275    </author>
4276    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4277      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4278      <address><email>frystyk@w3.org</email></address>
4279    </author>
4280    <date month="May" year="1996"/>
4281  </front>
4282  <seriesInfo name="RFC" value="1945"/>
4283</reference>
4284
4285<reference anchor="RFC2045">
4286  <front>
4287    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4288    <author initials="N." surname="Freed" fullname="Ned Freed">
4289      <organization>Innosoft International, Inc.</organization>
4290      <address><email>ned@innosoft.com</email></address>
4291    </author>
4292    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4293      <organization>First Virtual Holdings</organization>
4294      <address><email>nsb@nsb.fv.com</email></address>
4295    </author>
4296    <date month="November" year="1996"/>
4297  </front>
4298  <seriesInfo name="RFC" value="2045"/>
4299</reference>
4300
4301<reference anchor="RFC2047">
4302  <front>
4303    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4304    <author initials="K." surname="Moore" fullname="Keith Moore">
4305      <organization>University of Tennessee</organization>
4306      <address><email>moore@cs.utk.edu</email></address>
4307    </author>
4308    <date month="November" year="1996"/>
4309  </front>
4310  <seriesInfo name="RFC" value="2047"/>
4311</reference>
4312
4313<reference anchor="RFC2068">
4314  <front>
4315    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4316    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
4317      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4318      <address><email>fielding@ics.uci.edu</email></address>
4319    </author>
4320    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4321      <organization>MIT Laboratory for Computer Science</organization>
4322      <address><email>jg@w3.org</email></address>
4323    </author>
4324    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4325      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4326      <address><email>mogul@wrl.dec.com</email></address>
4327    </author>
4328    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4329      <organization>MIT Laboratory for Computer Science</organization>
4330      <address><email>frystyk@w3.org</email></address>
4331    </author>
4332    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4333      <organization>MIT Laboratory for Computer Science</organization>
4334      <address><email>timbl@w3.org</email></address>
4335    </author>
4336    <date month="January" year="1997"/>
4337  </front>
4338  <seriesInfo name="RFC" value="2068"/>
4339</reference>
4340
4341<reference anchor="RFC2145">
4342  <front>
4343    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4344    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4345      <organization>Western Research Laboratory</organization>
4346      <address><email>mogul@wrl.dec.com</email></address>
4347    </author>
4348    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4349      <organization>Department of Information and Computer Science</organization>
4350      <address><email>fielding@ics.uci.edu</email></address>
4351    </author>
4352    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4353      <organization>MIT Laboratory for Computer Science</organization>
4354      <address><email>jg@w3.org</email></address>
4355    </author>
4356    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4357      <organization>W3 Consortium</organization>
4358      <address><email>frystyk@w3.org</email></address>
4359    </author>
4360    <date month="May" year="1997"/>
4361  </front>
4362  <seriesInfo name="RFC" value="2145"/>
4363</reference>
4364
4365<reference anchor="RFC2616">
4366  <front>
4367    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4368    <author initials="R." surname="Fielding" fullname="R. Fielding">
4369      <organization>University of California, Irvine</organization>
4370      <address><email>fielding@ics.uci.edu</email></address>
4371    </author>
4372    <author initials="J." surname="Gettys" fullname="J. Gettys">
4373      <organization>W3C</organization>
4374      <address><email>jg@w3.org</email></address>
4375    </author>
4376    <author initials="J." surname="Mogul" fullname="J. Mogul">
4377      <organization>Compaq Computer Corporation</organization>
4378      <address><email>mogul@wrl.dec.com</email></address>
4379    </author>
4380    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4381      <organization>MIT Laboratory for Computer Science</organization>
4382      <address><email>frystyk@w3.org</email></address>
4383    </author>
4384    <author initials="L." surname="Masinter" fullname="L. Masinter">
4385      <organization>Xerox Corporation</organization>
4386      <address><email>masinter@parc.xerox.com</email></address>
4387    </author>
4388    <author initials="P." surname="Leach" fullname="P. Leach">
4389      <organization>Microsoft Corporation</organization>
4390      <address><email>paulle@microsoft.com</email></address>
4391    </author>
4392    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4393      <organization>W3C</organization>
4394      <address><email>timbl@w3.org</email></address>
4395    </author>
4396    <date month="June" year="1999"/>
4397  </front>
4398  <seriesInfo name="RFC" value="2616"/>
4399</reference>
4400
4401<reference anchor='RFC2817'>
4402  <front>
4403    <title>Upgrading to TLS Within HTTP/1.1</title>
4404    <author initials='R.' surname='Khare' fullname='R. Khare'>
4405      <organization>4K Associates / UC Irvine</organization>
4406      <address><email>rohit@4K-associates.com</email></address>
4407    </author>
4408    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4409      <organization>Agranat Systems, Inc.</organization>
4410      <address><email>lawrence@agranat.com</email></address>
4411    </author>
4412    <date year='2000' month='May' />
4413  </front>
4414  <seriesInfo name='RFC' value='2817' />
4415</reference>
4416
4417<reference anchor='RFC2818'>
4418  <front>
4419    <title>HTTP Over TLS</title>
4420    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4421      <organization>RTFM, Inc.</organization>
4422      <address><email>ekr@rtfm.com</email></address>
4423    </author>
4424    <date year='2000' month='May' />
4425  </front>
4426  <seriesInfo name='RFC' value='2818' />
4427</reference>
4428
4429<reference anchor='RFC3040'>
4430  <front>
4431    <title>Internet Web Replication and Caching Taxonomy</title>
4432    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4433      <organization>Equinix, Inc.</organization>
4434    </author>
4435    <author initials='I.' surname='Melve' fullname='I. Melve'>
4436      <organization>UNINETT</organization>
4437    </author>
4438    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4439      <organization>CacheFlow Inc.</organization>
4440    </author>
4441    <date year='2001' month='January' />
4442  </front>
4443  <seriesInfo name='RFC' value='3040' />
4444</reference>
4445
4446<reference anchor='RFC3864'>
4447  <front>
4448    <title>Registration Procedures for Message Header Fields</title>
4449    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4450      <organization>Nine by Nine</organization>
4451      <address><email>GK-IETF@ninebynine.org</email></address>
4452    </author>
4453    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4454      <organization>BEA Systems</organization>
4455      <address><email>mnot@pobox.com</email></address>
4456    </author>
4457    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4458      <organization>HP Labs</organization>
4459      <address><email>JeffMogul@acm.org</email></address>
4460    </author>
4461    <date year='2004' month='September' />
4462  </front>
4463  <seriesInfo name='BCP' value='90' />
4464  <seriesInfo name='RFC' value='3864' />
4465</reference>
4466
4467<reference anchor='RFC4033'>
4468  <front>
4469    <title>DNS Security Introduction and Requirements</title>
4470    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4471    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4472    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4473    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4474    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4475    <date year='2005' month='March' />
4476  </front>
4477  <seriesInfo name='RFC' value='4033' />
4478</reference>
4479
4480<reference anchor="RFC4288">
4481  <front>
4482    <title>Media Type Specifications and Registration Procedures</title>
4483    <author initials="N." surname="Freed" fullname="N. Freed">
4484      <organization>Sun Microsystems</organization>
4485      <address>
4486        <email>ned.freed@mrochek.com</email>
4487      </address>
4488    </author>
4489    <author initials="J." surname="Klensin" fullname="J. Klensin">
4490      <address>
4491        <email>klensin+ietf@jck.com</email>
4492      </address>
4493    </author>
4494    <date year="2005" month="December"/>
4495  </front>
4496  <seriesInfo name="BCP" value="13"/>
4497  <seriesInfo name="RFC" value="4288"/>
4498</reference>
4499
4500<reference anchor='RFC4395'>
4501  <front>
4502    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4503    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4504      <organization>AT&amp;T Laboratories</organization>
4505      <address>
4506        <email>tony+urireg@maillennium.att.com</email>
4507      </address>
4508    </author>
4509    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4510      <organization>Qualcomm, Inc.</organization>
4511      <address>
4512        <email>hardie@qualcomm.com</email>
4513      </address>
4514    </author>
4515    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4516      <organization>Adobe Systems</organization>
4517      <address>
4518        <email>LMM@acm.org</email>
4519      </address>
4520    </author>
4521    <date year='2006' month='February' />
4522  </front>
4523  <seriesInfo name='BCP' value='115' />
4524  <seriesInfo name='RFC' value='4395' />
4525</reference>
4526
4527<reference anchor='RFC4559'>
4528  <front>
4529    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4530    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4531    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4532    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4533    <date year='2006' month='June' />
4534  </front>
4535  <seriesInfo name='RFC' value='4559' />
4536</reference>
4537
4538<reference anchor='RFC5226'>
4539  <front>
4540    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4541    <author initials='T.' surname='Narten' fullname='T. Narten'>
4542      <organization>IBM</organization>
4543      <address><email>narten@us.ibm.com</email></address>
4544    </author>
4545    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4546      <organization>Google</organization>
4547      <address><email>Harald@Alvestrand.no</email></address>
4548    </author>
4549    <date year='2008' month='May' />
4550  </front>
4551  <seriesInfo name='BCP' value='26' />
4552  <seriesInfo name='RFC' value='5226' />
4553</reference>
4554
4555<reference anchor='RFC5246'>
4556   <front>
4557      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4558      <author initials='T.' surname='Dierks' fullname='T. Dierks'>
4559         <organization />
4560      </author>
4561      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4562         <organization>RTFM, Inc.</organization>
4563      </author>
4564      <date year='2008' month='August' />
4565   </front>
4566   <seriesInfo name='RFC' value='5246' />
4567</reference>
4568
4569<reference anchor="RFC5322">
4570  <front>
4571    <title>Internet Message Format</title>
4572    <author initials="P." surname="Resnick" fullname="P. Resnick">
4573      <organization>Qualcomm Incorporated</organization>
4574    </author>
4575    <date year="2008" month="October"/>
4576  </front> 
4577  <seriesInfo name="RFC" value="5322"/>
4578</reference>
4579
4580<reference anchor="RFC6265">
4581  <front>
4582    <title>HTTP State Management Mechanism</title>
4583    <author initials="A." surname="Barth" fullname="Adam Barth">
4584      <organization abbrev="U.C. Berkeley">
4585        University of California, Berkeley
4586      </organization>
4587      <address><email>abarth@eecs.berkeley.edu</email></address>
4588    </author>
4589    <date year="2011" month="April" />
4590  </front>
4591  <seriesInfo name="RFC" value="6265"/>
4592</reference>
4593
4594<!--<reference anchor='BCP97'>
4595  <front>
4596    <title>Handling Normative References to Standards-Track Documents</title>
4597    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
4598      <address>
4599        <email>klensin+ietf@jck.com</email>
4600      </address>
4601    </author>
4602    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
4603      <organization>MIT</organization>
4604      <address>
4605        <email>hartmans-ietf@mit.edu</email>
4606      </address>
4607    </author>
4608    <date year='2007' month='June' />
4609  </front>
4610  <seriesInfo name='BCP' value='97' />
4611  <seriesInfo name='RFC' value='4897' />
4612</reference>-->
4613
4614<reference anchor="Kri2001" target="http://arxiv.org/abs/cs.SE/0105018">
4615  <front>
4616    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
4617    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
4618    <date year="2001" month="November"/>
4619  </front>
4620  <seriesInfo name="ACM Transactions on Internet Technology" value="Vol. 1, #2"/>
4621</reference>
4622
4623</references>
4624
4625
4626<section title="HTTP Version History" anchor="compatibility">
4627<t>
4628   HTTP has been in use by the World-Wide Web global information initiative
4629   since 1990. The first version of HTTP, later referred to as HTTP/0.9,
4630   was a simple protocol for hypertext data transfer across the Internet
4631   with only a single request method (GET) and no metadata.
4632   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
4633   methods and MIME-like messaging that could include metadata about the data
4634   transferred and modifiers on the request/response semantics. However,
4635   HTTP/1.0 did not sufficiently take into consideration the effects of
4636   hierarchical proxies, caching, the need for persistent connections, or
4637   name-based virtual hosts. The proliferation of incompletely-implemented
4638   applications calling themselves "HTTP/1.0" further necessitated a
4639   protocol version change in order for two communicating applications
4640   to determine each other's true capabilities.
4641</t>
4642<t>
4643   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
4644   requirements that enable reliable implementations, adding only
4645   those new features that will either be safely ignored by an HTTP/1.0
4646   recipient or only sent when communicating with a party advertising
4647   conformance with HTTP/1.1.
4648</t>
4649<t>
4650   It is beyond the scope of a protocol specification to mandate
4651   conformance with previous versions. HTTP/1.1 was deliberately
4652   designed, however, to make supporting previous versions easy.
4653   We would expect a general-purpose HTTP/1.1 server to understand
4654   any valid request in the format of HTTP/1.0 and respond appropriately
4655   with an HTTP/1.1 message that only uses features understood (or
4656   safely ignored) by HTTP/1.0 clients.  Likewise, we would expect
4657   an HTTP/1.1 client to understand any valid HTTP/1.0 response.
4658</t>
4659<t>
4660   Since HTTP/0.9 did not support header fields in a request,
4661   there is no mechanism for it to support name-based virtual
4662   hosts (selection of resource by inspection of the <x:ref>Host</x:ref> header
4663   field).  Any server that implements name-based virtual hosts
4664   ought to disable support for HTTP/0.9.  Most requests that
4665   appear to be HTTP/0.9 are, in fact, badly constructed HTTP/1.x
4666   requests wherein a buggy client failed to properly encode
4667   linear whitespace found in a URI reference and placed in
4668   the request-target.
4669</t>
4670
4671<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
4672<t>
4673   This section summarizes major differences between versions HTTP/1.0
4674   and HTTP/1.1.
4675</t>
4676
4677<section title="Multi-homed Web Servers" anchor="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">
4678<t>
4679   The requirements that clients and servers support the <x:ref>Host</x:ref>
4680   header field (<xref target="header.host"/>), report an error if it is
4681   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
4682   are among the most important changes defined by HTTP/1.1.
4683</t>
4684<t>
4685   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4686   addresses and servers; there was no other established mechanism for
4687   distinguishing the intended server of a request than the IP address
4688   to which that request was directed. The <x:ref>Host</x:ref> header field was
4689   introduced during the development of HTTP/1.1 and, though it was
4690   quickly implemented by most HTTP/1.0 browsers, additional requirements
4691   were placed on all HTTP/1.1 requests in order to ensure complete
4692   adoption.  At the time of this writing, most HTTP-based services
4693   are dependent upon the Host header field for targeting requests.
4694</t>
4695</section>
4696
4697<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4698<t>
4699   In HTTP/1.0, each connection is established by the client prior to the
4700   request and closed by the server after sending the response. However, some
4701   implementations implement the explicitly negotiated ("Keep-Alive") version
4702   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
4703   target="RFC2068"/>.
4704</t>
4705<t>
4706   Some clients and servers might wish to be compatible with these previous
4707   approaches to persistent connections, by explicitly negotiating for them
4708   with a "Connection: keep-alive" request header field. However, some
4709   experimental implementations of HTTP/1.0 persistent connections are faulty;
4710   for example, if an HTTP/1.0 proxy server doesn't understand
4711   <x:ref>Connection</x:ref>, it will erroneously forward that header field
4712   to the next inbound server, which would result in a hung connection.
4713</t>
4714<t>
4715   One attempted solution was the introduction of a Proxy-Connection header
4716   field, targeted specifically at proxies. In practice, this was also
4717   unworkable, because proxies are often deployed in multiple layers, bringing
4718   about the same problem discussed above.
4719</t>
4720<t>
4721   As a result, clients are encouraged not to send the Proxy-Connection header
4722   field in any requests.
4723</t>
4724<t>
4725   Clients are also encouraged to consider the use of Connection: keep-alive
4726   in requests carefully; while they can enable persistent connections with
4727   HTTP/1.0 servers, clients using them need will need to monitor the
4728   connection for "hung" requests (which indicate that the client ought stop
4729   sending the header field), and this mechanism ought not be used by clients
4730   at all when a proxy is being used.
4731</t>
4732</section>
4733
4734<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
4735<t>
4736   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
4737   (<xref target="header.transfer-encoding"/>). Proxies/gateways &MUST; remove
4738   any transfer-coding prior to forwarding a message via a MIME-compliant
4739   protocol.
4740</t>
4741</section>
4742
4743</section>
4744
4745<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4746<t>
4747  HTTP's approach to error handling has been explained.
4748  (<xref target="conformance"/>)
4749</t>
4750<t>
4751  The expectation to support HTTP/0.9 requests has been removed.
4752</t>
4753<t>
4754  The term "Effective Request URI" has been introduced.
4755  (<xref target="effective.request.uri" />)
4756</t>
4757<t>
4758  HTTP messages can be (and often are) buffered by implementations; despite
4759  it sometimes being available as a stream, HTTP is fundamentally a
4760  message-oriented protocol.
4761  (<xref target="http.message" />)
4762</t>
4763<t>
4764  Minimum supported sizes for various protocol elements have been
4765  suggested, to improve interoperability.
4766</t>
4767<t>
4768  Header fields that span multiple lines ("line folding") are deprecated.
4769  (<xref target="field.parsing" />)
4770</t>
4771<t>
4772  The HTTP-version ABNF production has been clarified to be case-sensitive.
4773  Additionally, version numbers has been restricted to single digits, due
4774  to the fact that implementations are known to handle multi-digit version
4775  numbers incorrectly.
4776  (<xref target="http.version"/>)
4777</t>
4778<t>
4779  The HTTPS URI scheme is now defined by this specification; previously,
4780  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
4781  (<xref target="https.uri"/>)
4782</t>
4783<t>
4784  The HTTPS URI scheme implies end-to-end security.
4785  (<xref target="https.uri"/>)
4786</t>
4787<t>
4788  Userinfo (i.e., username and password) are now disallowed in HTTP and
4789  HTTPS URIs, because of security issues related to their transmission on the
4790  wire.
4791  (<xref target="http.uri" />)
4792</t>
4793<t>
4794  Invalid whitespace around field-names is now required to be rejected,
4795  because accepting it represents a security vulnerability.
4796  (<xref target="header.fields"/>)
4797</t>
4798<t>
4799  The ABNF productions defining header fields now only list the field value.
4800  (<xref target="header.fields"/>)
4801</t>
4802<t>
4803  Rules about implicit linear whitespace between certain grammar productions
4804  have been removed; now whitespace is only allowed where specifically
4805  defined in the ABNF.
4806  (<xref target="whitespace"/>)
4807</t>
4808<t> 
4809  The NUL octet is no longer allowed in comment and quoted-string text, and
4810  handling of backslash-escaping in them has been clarified.
4811  (<xref target="field.components"/>)
4812</t> 
4813<t>
4814  The quoted-pair rule no longer allows escaping control characters other than
4815  HTAB.
4816  (<xref target="field.components"/>)
4817</t>
4818<t>
4819  Non-ASCII content in header fields and the reason phrase has been obsoleted
4820  and made opaque (the TEXT rule was removed).
4821  (<xref target="field.components"/>)
4822</t>
4823<t>
4824  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
4825  handled as errors by recipients.
4826  (<xref target="header.content-length"/>)
4827</t>
4828<t>
4829  The "identity" transfer-coding value token has been removed.
4830  (Sections <xref format="counter" target="message.body"/> and
4831  <xref format="counter" target="transfer.codings"/>)
4832</t>
4833<t>
4834  The algorithm for determining the message body length has been clarified
4835  to indicate all of the special cases (e.g., driven by methods or status
4836  codes) that affect it, and that new protocol elements cannot define such
4837  special cases.
4838  (<xref target="message.body.length"/>)
4839</t>
4840<t>
4841  "multipart/byteranges" is no longer a way of determining message body length
4842  detection.
4843  (<xref target="message.body.length"/>)
4844</t>
4845<t>
4846  CONNECT is a new, special case in determining message body length.
4847  (<xref target="message.body.length"/>)
4848</t>
4849<t>
4850  Chunk length does not include the count of the octets in the
4851  chunk header and trailer.
4852  (<xref target="chunked.encoding"/>)
4853</t>
4854<t>
4855  Use of chunk extensions is deprecated, and line folding in them is
4856  disallowed.
4857  (<xref target="chunked.encoding"/>)
4858</t>
4859<t>
4860  The path-absolute + query components of RFC3986 have been used to define the
4861  request-target, instead of abs_path from RFC 1808.
4862  (<xref target="request-target"/>)
4863</t>
4864<t>
4865  The asterisk form of the request-target is only allowed in the OPTIONS
4866  method.
4867  (<xref target="request-target"/>)
4868</t>
4869<t>
4870  Exactly when "close" connection options have to be sent has been clarified.
4871  (<xref target="header.connection"/>)
4872</t>
4873<t> 
4874  "hop-by-hop" header fields are required to appear in the Connection header
4875  field; just because they're defined as hop-by-hop in this specification
4876  doesn't exempt them.
4877  (<xref target="header.connection"/>)
4878</t>
4879<t>
4880  The limit of two connections per server has been removed.
4881  (<xref target="persistent.connections"/>)
4882</t>
4883<t>
4884  An idempotent sequence of requests is no longer required to be retried.
4885  (<xref target="persistent.connections"/>)
4886</t>
4887<t>
4888  The requirement to retry requests under certain circumstances when the
4889  server prematurely closes the connection has been removed.
4890  (<xref target="persistent.reuse"/>)
4891</t>
4892<t>
4893  Some extraneous requirements about when servers are allowed to close
4894  connections prematurely have been removed.
4895  (<xref target="persistent.connections"/>)
4896</t>
4897<t>
4898  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
4899  responses other than 101 (this was incorporated from <xref
4900  target="RFC2817"/>).
4901  (<xref target="header.upgrade"/>)
4902</t>
4903<t>
4904  Registration of Transfer Codings now requires IETF Review
4905  (<xref target="transfer.coding.registry"/>)
4906</t>
4907<t>
4908  The meaning of the "deflate" content coding has been clarified.
4909  (<xref target="deflate.coding" />)
4910</t>
4911<t>
4912  This specification now defines the Upgrade Token Registry, previously
4913  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
4914  (<xref target="upgrade.token.registry"/>)
4915</t>
4916<t>
4917  Empty list elements in list productions (e.g., a list header containing
4918  ", ,") have been deprecated.
4919  (<xref target="abnf.extension"/>)
4920</t>
4921<t>
4922  Issues with the Keep-Alive and Proxy-Connection headers in requests
4923  are pointed out, with use of the latter being discouraged altogether.
4924  (<xref target="compatibility.with.http.1.0.persistent.connections" />)
4925</t>
4926</section>
4927</section>
4928
4929<section title="ABNF list extension: #rule" anchor="abnf.extension">
4930<t>
4931  A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
4932  improve readability in the definitions of some header field values.
4933</t>
4934<t>
4935  A construct "#" is defined, similar to "*", for defining comma-delimited
4936  lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
4937  at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
4938  comma (",") and optional whitespace (OWS).   
4939</t>
4940<figure><preamble>
4941  Thus,
4942</preamble><artwork type="example">
4943  1#element =&gt; element *( OWS "," OWS element )
4944</artwork></figure>
4945<figure><preamble>
4946  and:
4947</preamble><artwork type="example">
4948  #element =&gt; [ 1#element ]
4949</artwork></figure>
4950<figure><preamble>
4951  and for n &gt;= 1 and m &gt; 1:
4952</preamble><artwork type="example">
4953  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
4954</artwork></figure>
4955<t>
4956  For compatibility with legacy list rules, recipients &SHOULD; accept empty
4957  list elements. In other words, consumers would follow the list productions:
4958</t>
4959<figure><artwork type="example">
4960  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
4961 
4962  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
4963</artwork></figure>
4964<t>
4965  Note that empty elements do not contribute to the count of elements present,
4966  though.
4967</t>
4968<t>
4969  For example, given these ABNF productions:
4970</t>
4971<figure><artwork type="example">
4972  example-list      = 1#example-list-elmt
4973  example-list-elmt = token ; see <xref target="field.components"/> 
4974</artwork></figure>
4975<t>
4976  Then these are valid values for example-list (not including the double
4977  quotes, which are present for delimitation only):
4978</t>
4979<figure><artwork type="example">
4980  "foo,bar"
4981  "foo ,bar,"
4982  "foo , ,bar,charlie   "
4983</artwork></figure>
4984<t>
4985  But these values would be invalid, as at least one non-empty element is
4986  required:
4987</t>
4988<figure><artwork type="example">
4989  ""
4990  ","
4991  ",   ,"
4992</artwork></figure>
4993<t>
4994  <xref target="collected.abnf"/> shows the collected ABNF, with the list rules
4995  expanded as explained above.
4996</t>
4997</section>
4998
4999<?BEGININC p1-messaging.abnf-appendix ?>
5000<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
5001<figure>
5002<artwork type="abnf" name="p1-messaging.parsed-abnf">
5003<x:ref>BWS</x:ref> = OWS
5004
5005<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
5006 connection-option ] )
5007<x:ref>Content-Length</x:ref> = 1*DIGIT
5008
5009<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
5010 ]
5011<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
5012<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
5013<x:ref>Host</x:ref> = uri-host [ ":" port ]
5014
5015<x:ref>OWS</x:ref> = *( SP / HTAB )
5016
5017<x:ref>RWS</x:ref> = 1*( SP / HTAB )
5018
5019<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
5020<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
5021<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
5022 transfer-coding ] )
5023
5024<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
5025<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
5026
5027<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
5028 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
5029 comment ] ) ] )
5030
5031<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
5032<x:ref>absolute-form</x:ref> = absolute-URI
5033<x:ref>asterisk-form</x:ref> = "*"
5034<x:ref>attribute</x:ref> = token
5035<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
5036<x:ref>authority-form</x:ref> = authority
5037
5038<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
5039<x:ref>chunk-data</x:ref> = 1*OCTET
5040<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
5041<x:ref>chunk-ext-name</x:ref> = token
5042<x:ref>chunk-ext-val</x:ref> = token / quoted-str-nf
5043<x:ref>chunk-size</x:ref> = 1*HEXDIG
5044<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
5045<x:ref>comment</x:ref> = "(" *( ctext / quoted-cpair / comment ) ")"
5046<x:ref>connection-option</x:ref> = token
5047<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
5048 / %x2A-5B ; '*'-'['
5049 / %x5D-7E ; ']'-'~'
5050 / obs-text
5051
5052<x:ref>field-content</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5053<x:ref>field-name</x:ref> = token
5054<x:ref>field-value</x:ref> = *( field-content / obs-fold )
5055
5056<x:ref>header-field</x:ref> = field-name ":" OWS field-value BWS
5057<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ]
5058<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ]
5059
5060<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
5061
5062<x:ref>message-body</x:ref> = *OCTET
5063<x:ref>method</x:ref> = token
5064
5065<x:ref>obs-fold</x:ref> = CRLF ( SP / HTAB )
5066<x:ref>obs-text</x:ref> = %x80-FF
5067<x:ref>origin-form</x:ref> = path-absolute [ "?" query ]
5068
5069<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
5070<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
5071<x:ref>path-absolute</x:ref> = &lt;path-absolute, defined in [RFC3986], Section 3.3&gt;
5072<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
5073<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
5074<x:ref>protocol-name</x:ref> = token
5075<x:ref>protocol-version</x:ref> = token
5076<x:ref>pseudonym</x:ref> = token
5077
5078<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5079 / %x5D-7E ; ']'-'~'
5080 / obs-text
5081<x:ref>qdtext-nf</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5082 / %x5D-7E ; ']'-'~'
5083 / obs-text
5084<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5085<x:ref>quoted-cpair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5086<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5087<x:ref>quoted-str-nf</x:ref> = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
5088<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5089
5090<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5091<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5092<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5093<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5094<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5095<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5096<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5097 asterisk-form
5098
5099<x:ref>special</x:ref> = "(" / ")" / "&lt;" / "&gt;" / "@" / "," / ";" / ":" / "\" /
5100 DQUOTE / "/" / "[" / "]" / "?" / "=" / "{" / "}"
5101<x:ref>start-line</x:ref> = request-line / status-line
5102<x:ref>status-code</x:ref> = 3DIGIT
5103<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5104
5105<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5106<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5107<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5108 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5109<x:ref>token</x:ref> = 1*tchar
5110<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5111<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5112 transfer-extension
5113<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5114<x:ref>transfer-parameter</x:ref> = attribute BWS "=" BWS value
5115
5116<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5117
5118<x:ref>value</x:ref> = word
5119
5120<x:ref>word</x:ref> = token / quoted-string
5121</artwork>
5122</figure>
5123</section>
5124<?ENDINC p1-messaging.abnf-appendix ?>
5125
5126<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5127
5128<section title="Since RFC 2616">
5129<t>
5130  Changes up to the first Working Group Last Call draft are summarized
5131  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-21#appendix-D"/>.
5132</t>
5133</section>
5134
5135<section title="Since draft-ietf-httpbis-p1-messaging-21" anchor="changes.since.21">
5136<t>
5137  Closed issues:
5138  <list style="symbols">
5139    <t>
5140      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/128"/>:
5141      "Cite HTTPS URI scheme definition" (the spec now includes the HTTPs
5142      scheme definition and thus updates RFC 2818)
5143    </t>
5144    <t>
5145      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/389"/>:
5146      "mention of 'proxies' in section about caches"
5147    </t>
5148  </list>
5149</t>
5150</section>
5151
5152</section>
5153
5154</back>
5155</rfc>
Note: See TracBrowser for help on using the repository browser.