source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2392

Last change on this file since 2392 was 2392, checked in by fielding@…, 7 years ago

clarify that recipients MUST parse for empty list elements; addresses #475

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 237.0 KB
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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "September">
16  <!ENTITY ID-YEAR "2013">
17  <!ENTITY mdash "&#8212;">
18  <!ENTITY Note "<x:h xmlns:x='http://purl.org/net/xml2rfc/ext'>Note:</x:h>">
19  <!ENTITY caching-overview       "<xref target='Part6' x:rel='#caching.overview' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY cache-incomplete       "<xref target='Part6' x:rel='#response.cacheability' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY payload                "<xref target='Part2' x:rel='#payload' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY media-type            "<xref target='Part2' x:rel='#media.type' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY content-codings        "<xref target='Part2' x:rel='#content.codings' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY CONNECT                "<xref target='Part2' x:rel='#CONNECT' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY content.negotiation    "<xref target='Part2' x:rel='#content.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY diff-mime              "<xref target='Part2' x:rel='#differences.between.http.and.mime' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY representation         "<xref target='Part2' x:rel='#representations' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY HEAD                   "<xref target='Part2' x:rel='#HEAD' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-allow           "<xref target='Part2' x:rel='#header.allow' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-cache-control   "<xref target='Part6' x:rel='#header.cache-control' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY header-content-encoding    "<xref target='Part2' x:rel='#header.content-encoding' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY header-content-location    "<xref target='Part2' x:rel='#header.content-location' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY header-content-range   "<xref target='Part5' x:rel='#header.content-range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY header-content-type    "<xref target='Part2' x:rel='#header.content-type' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY header-date            "<xref target='Part2' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY header-etag            "<xref target='Part4' x:rel='#header.etag' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY header-expires         "<xref target='Part6' x:rel='#header.expires' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY header-last-modified   "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY header-mime-version    "<xref target='Part2' x:rel='#mime-version' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40  <!ENTITY header-pragma          "<xref target='Part6' x:rel='#header.pragma' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41  <!ENTITY header-proxy-authenticate  "<xref target='Part7' x:rel='#header.proxy-authenticate' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
42  <!ENTITY header-proxy-authorization "<xref target='Part7' x:rel='#header.proxy-authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
43  <!ENTITY header-server          "<xref target='Part2' x:rel='#header.server' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
44  <!ENTITY header-warning         "<xref target='Part6' x:rel='#header.warning' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
45  <!ENTITY idempotent-methods     "<xref target='Part2' x:rel='#idempotent.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
46  <!ENTITY safe-methods           "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
47  <!ENTITY methods                "<xref target='Part2' x:rel='#methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
48  <!ENTITY OPTIONS                "<xref target='Part2' x:rel='#OPTIONS' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
49  <!ENTITY qvalue                 "<xref target='Part2' x:rel='#quality.values' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
50  <!ENTITY resource               "<xref target='Part2' x:rel='#resources' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
51  <!ENTITY status-codes           "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
52  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
53  <!ENTITY status-203             "<xref target='Part2' x:rel='#status.203' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
54  <!ENTITY status-3xx             "<xref target='Part2' x:rel='#status.3xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
55  <!ENTITY status-304             "<xref target='Part4' x:rel='#status.304' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
56  <!ENTITY status-4xx             "<xref target='Part2' x:rel='#status.4xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
57  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
58  <!ENTITY iana-header-registry   "<xref target='Part2' x:rel='#header.field.registry' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
59]>
60<?rfc toc="yes" ?>
61<?rfc symrefs="yes" ?>
62<?rfc sortrefs="yes" ?>
63<?rfc compact="yes"?>
64<?rfc subcompact="no" ?>
65<?rfc linkmailto="no" ?>
66<?rfc editing="no" ?>
67<?rfc comments="yes"?>
68<?rfc inline="yes"?>
69<?rfc rfcedstyle="yes"?>
70<?rfc-ext allow-markup-in-artwork="yes" ?>
71<?rfc-ext include-references-in-index="yes" ?>
72<rfc obsoletes="2145,2616" updates="2817,2818" category="std" x:maturity-level="proposed"
73     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
74     xmlns:x='http://purl.org/net/xml2rfc/ext'>
75<x:link rel="next" basename="p2-semantics"/>
76<x:feedback template="mailto:ietf-http-wg@w3.org?subject={docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
77<front>
78
79  <title abbrev="HTTP/1.1 Message Syntax and Routing">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
80
81  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
82    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
83    <address>
84      <postal>
85        <street>345 Park Ave</street>
86        <city>San Jose</city>
87        <region>CA</region>
88        <code>95110</code>
89        <country>USA</country>
90      </postal>
91      <email>fielding@gbiv.com</email>
92      <uri>http://roy.gbiv.com/</uri>
93    </address>
94  </author>
95
96  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
97    <organization abbrev="greenbytes">greenbytes GmbH</organization>
98    <address>
99      <postal>
100        <street>Hafenweg 16</street>
101        <city>Muenster</city><region>NW</region><code>48155</code>
102        <country>Germany</country>
103      </postal>
104      <email>julian.reschke@greenbytes.de</email>
105      <uri>http://greenbytes.de/tech/webdav/</uri>
106    </address>
107  </author>
108
109  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
110  <workgroup>HTTPbis Working Group</workgroup>
111
112<abstract>
113<t>
114   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
115   distributed, collaborative, hypertext information systems. HTTP has been in
116   use by the World Wide Web global information initiative since 1990.
117   This document provides an overview of HTTP architecture and its associated
118   terminology, defines the "http" and "https" Uniform Resource Identifier
119   (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements,
120   and describes general security concerns for implementations.
121</t>   
122</abstract>
123
124<note title="Editorial Note (To be removed by RFC Editor)">
125  <t>
126    Discussion of this draft takes place on the HTTPBIS working group
127    mailing list (ietf-http-wg@w3.org), which is archived at
128    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
129  </t>
130  <t>
131    The current issues list is at
132    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
133    documents (including fancy diffs) can be found at
134    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
135  </t>
136  <t>
137    The changes in this draft are summarized in <xref target="changes.since.23"/>.
138  </t>
139</note>
140</front>
141<middle>
142<section title="Introduction" anchor="introduction">
143<t>
144   The Hypertext Transfer Protocol (HTTP) is an application-level
145   request/response protocol that uses extensible semantics and self-descriptive
146   message payloads for flexible interaction with network-based hypertext
147   information systems. This document is the first in a series of documents
148   that collectively form the HTTP/1.1 specification:
149   <list style="empty">
150    <t>RFC xxx1: Message Syntax and Routing</t>
151    <t><xref target="Part2" x:fmt="none">RFC xxx2</xref>: Semantics and Content</t>
152    <t><xref target="Part4" x:fmt="none">RFC xxx3</xref>: Conditional Requests</t>
153    <t><xref target="Part5" x:fmt="none">RFC xxx4</xref>: Range Requests</t>
154    <t><xref target="Part6" x:fmt="none">RFC xxx5</xref>: Caching</t>
155    <t><xref target="Part7" x:fmt="none">RFC xxx6</xref>: Authentication</t>
156   </list>
157</t>
158<t>
159   This HTTP/1.1 specification obsoletes and moves to historic status
160   <xref target="RFC2616" x:fmt="none">RFC 2616</xref>, its predecessor
161   <xref target="RFC2068" x:fmt="none">RFC 2068</xref>, and
162   <xref target="RFC2145" x:fmt="none">RFC 2145</xref> (on HTTP versioning).
163   This specification also updates the use of CONNECT to establish a tunnel,
164   previously defined in <xref target="RFC2817" x:fmt="none">RFC 2817</xref>,
165   and defines the "https" URI scheme that was described informally in
166   <xref target="RFC2818" x:fmt="none">RFC 2818</xref>.
167</t>
168<t>
169   HTTP is a generic interface protocol for information systems. It is
170   designed to hide the details of how a service is implemented by presenting
171   a uniform interface to clients that is independent of the types of
172   resources provided. Likewise, servers do not need to be aware of each
173   client's purpose: an HTTP request can be considered in isolation rather
174   than being associated with a specific type of client or a predetermined
175   sequence of application steps. The result is a protocol that can be used
176   effectively in many different contexts and for which implementations can
177   evolve independently over time.
178</t>
179<t>
180   HTTP is also designed for use as an intermediation protocol for translating
181   communication to and from non-HTTP information systems.
182   HTTP proxies and gateways can provide access to alternative information
183   services by translating their diverse protocols into a hypertext
184   format that can be viewed and manipulated by clients in the same way
185   as HTTP services.
186</t>
187<t>
188   One consequence of this flexibility is that the protocol cannot be
189   defined in terms of what occurs behind the interface. Instead, we
190   are limited to defining the syntax of communication, the intent
191   of received communication, and the expected behavior of recipients.
192   If the communication is considered in isolation, then successful
193   actions ought to be reflected in corresponding changes to the
194   observable interface provided by servers. However, since multiple
195   clients might act in parallel and perhaps at cross-purposes, we
196   cannot require that such changes be observable beyond the scope
197   of a single response.
198</t>
199<t>
200   This document describes the architectural elements that are used or
201   referred to in HTTP, defines the "http" and "https" URI schemes,
202   describes overall network operation and connection management,
203   and defines HTTP message framing and forwarding requirements.
204   Our goal is to define all of the mechanisms necessary for HTTP message
205   handling that are independent of message semantics, thereby defining the
206   complete set of requirements for message parsers and
207   message-forwarding intermediaries.
208</t>
209
210
211<section title="Requirement Notation" anchor="intro.requirements">
212<t>
213   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
214   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
215   document are to be interpreted as described in <xref target="RFC2119"/>.
216</t>
217<t>
218   Conformance criteria and considerations regarding error handling
219   are defined in <xref target="conformance"/>.
220</t>
221</section>
222
223<section title="Syntax Notation" anchor="notation">
224<iref primary="true" item="Grammar" subitem="ALPHA"/>
225<iref primary="true" item="Grammar" subitem="CR"/>
226<iref primary="true" item="Grammar" subitem="CRLF"/>
227<iref primary="true" item="Grammar" subitem="CTL"/>
228<iref primary="true" item="Grammar" subitem="DIGIT"/>
229<iref primary="true" item="Grammar" subitem="DQUOTE"/>
230<iref primary="true" item="Grammar" subitem="HEXDIG"/>
231<iref primary="true" item="Grammar" subitem="HTAB"/>
232<iref primary="true" item="Grammar" subitem="LF"/>
233<iref primary="true" item="Grammar" subitem="OCTET"/>
234<iref primary="true" item="Grammar" subitem="SP"/>
235<iref primary="true" item="Grammar" subitem="VCHAR"/>
236<t>
237   This specification uses the Augmented Backus-Naur Form (ABNF) notation
238   of <xref target="RFC5234"/> with the list rule extension defined in
239   <xref target="abnf.extension"/>.  <xref target="collected.abnf"/> shows
240   the collected ABNF with the list rule expanded.
241</t>
242<t anchor="core.rules">
243  <x:anchor-alias value="ALPHA"/>
244  <x:anchor-alias value="CTL"/>
245  <x:anchor-alias value="CR"/>
246  <x:anchor-alias value="CRLF"/>
247  <x:anchor-alias value="DIGIT"/>
248  <x:anchor-alias value="DQUOTE"/>
249  <x:anchor-alias value="HEXDIG"/>
250  <x:anchor-alias value="HTAB"/>
251  <x:anchor-alias value="LF"/>
252  <x:anchor-alias value="OCTET"/>
253  <x:anchor-alias value="SP"/>
254  <x:anchor-alias value="VCHAR"/>
255   The following core rules are included by
256   reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
257   ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
258   DIGIT (decimal 0-9), DQUOTE (double quote),
259   HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed),
260   OCTET (any 8-bit sequence of data), SP (space), and
261   VCHAR (any visible <xref target="USASCII"/> character).
262</t>
263<t>
264   As a convention, ABNF rule names prefixed with "obs-" denote
265   "obsolete" grammar rules that appear for historical reasons.
266</t>
267</section>
268</section>
269
270<section title="Architecture" anchor="architecture">
271<t>
272   HTTP was created for the World Wide Web architecture
273   and has evolved over time to support the scalability needs of a worldwide
274   hypertext system. Much of that architecture is reflected in the terminology
275   and syntax productions used to define HTTP.
276</t>
277
278<section title="Client/Server Messaging" anchor="operation">
279<iref primary="true" item="client"/>
280<iref primary="true" item="server"/>
281<iref primary="true" item="connection"/>
282<t>
283   HTTP is a stateless request/response protocol that operates by exchanging
284   <x:dfn>messages</x:dfn> (<xref target="http.message"/>) across a reliable
285   transport or session-layer
286   "<x:dfn>connection</x:dfn>" (<xref target="connection.management"/>).
287   An HTTP "<x:dfn>client</x:dfn>" is a program that establishes a connection
288   to a server for the purpose of sending one or more HTTP requests.
289   An HTTP "<x:dfn>server</x:dfn>" is a program that accepts connections
290   in order to service HTTP requests by sending HTTP responses.
291</t>
292<iref primary="true" item="user agent"/>
293<iref primary="true" item="origin server"/>
294<iref primary="true" item="browser"/>
295<iref primary="true" item="spider"/>
296<iref primary="true" item="sender"/>
297<iref primary="true" item="recipient"/>
298<t>
299   The terms client and server refer only to the roles that
300   these programs perform for a particular connection.  The same program
301   might act as a client on some connections and a server on others.
302   We use the term "<x:dfn>user agent</x:dfn>" to refer to any of the various
303   client programs that initiate a request, including (but not limited to)
304   browsers, spiders (web-based robots), command-line tools, native
305   applications, and mobile apps.  The term "<x:dfn>origin server</x:dfn>" is
306   used to refer to the program that can originate authoritative responses to
307   a request. For general requirements, we use the terms
308   "<x:dfn>sender</x:dfn>" and "<x:dfn>recipient</x:dfn>" to refer to any
309   component that sends or receives, respectively, a given message.
310</t>
311<t>
312   HTTP relies upon the Uniform Resource Identifier (URI)
313   standard <xref target="RFC3986"/> to indicate the target resource
314   (<xref target="target-resource"/>) and relationships between resources.
315   Messages are passed in a format similar to that used by Internet mail
316   <xref target="RFC5322"/> and the Multipurpose Internet Mail Extensions
317   (MIME) <xref target="RFC2045"/> (see &diff-mime; for the differences
318   between HTTP and MIME messages).
319</t>
320<t>
321   Most HTTP communication consists of a retrieval request (GET) for
322   a representation of some resource identified by a URI.  In the
323   simplest case, this might be accomplished via a single bidirectional
324   connection (===) between the user agent (UA) and the origin server (O).
325</t>
326<figure><artwork type="drawing">
327         request   &gt;
328    <x:highlight>UA</x:highlight> ======================================= <x:highlight>O</x:highlight>
329                                &lt;   response
330</artwork></figure>
331<iref primary="true" item="message"/>
332<iref primary="true" item="request"/>
333<iref primary="true" item="response"/>
334<t>
335   A client sends an HTTP request to a server in the form of a <x:dfn>request</x:dfn>
336   message, beginning with a request-line that includes a method, URI, and
337   protocol version (<xref target="request.line"/>),
338   followed by header fields containing
339   request modifiers, client information, and representation metadata
340   (<xref target="header.fields"/>),
341   an empty line to indicate the end of the header section, and finally
342   a message body containing the payload body (if any,
343   <xref target="message.body"/>).
344</t>
345<t>
346   A server responds to a client's request by sending one or more HTTP
347   <x:dfn>response</x:dfn>
348   messages, each beginning with a status line that
349   includes the protocol version, a success or error code, and textual
350   reason phrase (<xref target="status.line"/>),
351   possibly followed by header fields containing server
352   information, resource metadata, and representation metadata
353   (<xref target="header.fields"/>),
354   an empty line to indicate the end of the header section, and finally
355   a message body containing the payload body (if any,
356   <xref target="message.body"/>).
357</t>
358<t>
359   A connection might be used for multiple request/response exchanges,
360   as defined in <xref target="persistent.connections"/>.
361</t>
362<t>
363   The following example illustrates a typical message exchange for a
364   GET request on the URI "http://www.example.com/hello.txt":
365</t>
366<figure><preamble>
367client request:
368</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
369GET /hello.txt HTTP/1.1
370User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
371Host: www.example.com
372Accept-Language: en, mi
373
374</artwork></figure>
375<figure><preamble>
376server response:
377</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
378HTTP/1.1 200 OK
379Date: Mon, 27 Jul 2009 12:28:53 GMT
380Server: Apache
381Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
382ETag: "34aa387-d-1568eb00"
383Accept-Ranges: bytes
384Content-Length: <x:length-of target="exbody"/>
385Vary: Accept-Encoding
386Content-Type: text/plain
387
388<x:span anchor="exbody">Hello World! My payload includes a trailing CRLF.
389</x:span></artwork>
390</figure>
391</section>
392
393<section title="Implementation Diversity" anchor="implementation-diversity">
394<t>
395   When considering the design of HTTP, it is easy to fall into a trap of
396   thinking that all user agents are general-purpose browsers and all origin
397   servers are large public websites. That is not the case in practice.
398   Common HTTP user agents include household appliances, stereos, scales,
399   firmware update scripts, command-line programs, mobile apps,
400   and communication devices in a multitude of shapes and sizes.  Likewise,
401   common HTTP origin servers include home automation units, configurable
402   networking components, office machines, autonomous robots, news feeds,
403   traffic cameras, ad selectors, and video delivery platforms.
404</t>
405<t>
406   The term "user agent" does not imply that there is a human user directly
407   interacting with the software agent at the time of a request. In many
408   cases, a user agent is installed or configured to run in the background
409   and save its results for later inspection (or save only a subset of those
410   results that might be interesting or erroneous). Spiders, for example, are
411   typically given a start URI and configured to follow certain behavior while
412   crawling the Web as a hypertext graph.
413</t>
414<t>
415   The implementation diversity of HTTP means that we cannot assume the
416   user agent can make interactive suggestions to a user or provide adequate
417   warning for security or privacy options.  In the few cases where this
418   specification requires reporting of errors to the user, it is acceptable
419   for such reporting to only be observable in an error console or log file.
420   Likewise, requirements that an automated action be confirmed by the user
421   before proceeding might be met via advance configuration choices,
422   run-time options, or simple avoidance of the unsafe action; confirmation
423   does not imply any specific user interface or interruption of normal
424   processing if the user has already made that choice.
425</t>
426</section>
427
428<section title="Intermediaries" anchor="intermediaries">
429<iref primary="true" item="intermediary"/>
430<t>
431   HTTP enables the use of intermediaries to satisfy requests through
432   a chain of connections.  There are three common forms of HTTP
433   <x:dfn>intermediary</x:dfn>: proxy, gateway, and tunnel.  In some cases,
434   a single intermediary might act as an origin server, proxy, gateway,
435   or tunnel, switching behavior based on the nature of each request.
436</t>
437<figure><artwork type="drawing">
438         &gt;             &gt;             &gt;             &gt;
439    <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>
440               &lt;             &lt;             &lt;             &lt;
441</artwork></figure>
442<t>
443   The figure above shows three intermediaries (A, B, and C) between the
444   user agent and origin server. A request or response message that
445   travels the whole chain will pass through four separate connections.
446   Some HTTP communication options
447   might apply only to the connection with the nearest, non-tunnel
448   neighbor, only to the end-points of the chain, or to all connections
449   along the chain. Although the diagram is linear, each participant might
450   be engaged in multiple, simultaneous communications. For example, B
451   might be receiving requests from many clients other than A, and/or
452   forwarding requests to servers other than C, at the same time that it
453   is handling A's request. Likewise, later requests might be sent through a
454   different path of connections, often based on dynamic configuration for
455   load balancing.   
456</t>
457<t>
458<iref primary="true" item="upstream"/><iref primary="true" item="downstream"/>
459<iref primary="true" item="inbound"/><iref primary="true" item="outbound"/>
460   We use the terms "<x:dfn>upstream</x:dfn>" and "<x:dfn>downstream</x:dfn>"
461   to describe various requirements in relation to the directional flow of a
462   message: all messages flow from upstream to downstream.
463   Likewise, we use the terms inbound and outbound to refer to
464   directions in relation to the request path:
465   "<x:dfn>inbound</x:dfn>" means toward the origin server and
466   "<x:dfn>outbound</x:dfn>" means toward the user agent.
467</t>
468<t><iref primary="true" item="proxy"/>
469   A "<x:dfn>proxy</x:dfn>" is a message forwarding agent that is selected by the
470   client, usually via local configuration rules, to receive requests
471   for some type(s) of absolute URI and attempt to satisfy those
472   requests via translation through the HTTP interface.  Some translations
473   are minimal, such as for proxy requests for "http" URIs, whereas
474   other requests might require translation to and from entirely different
475   application-level protocols. Proxies are often used to group an
476   organization's HTTP requests through a common intermediary for the
477   sake of security, annotation services, or shared caching.
478</t>
479<t>
480<iref primary="true" item="transforming proxy"/>
481<iref primary="true" item="non-transforming proxy"/>
482   An HTTP-to-HTTP proxy is called a "<x:dfn>transforming proxy</x:dfn>" if it is designed
483   or configured to modify request or response messages in a semantically
484   meaningful way (i.e., modifications, beyond those required by normal
485   HTTP processing, that change the message in a way that would be
486   significant to the original sender or potentially significant to
487   downstream recipients).  For example, a transforming proxy might be
488   acting as a shared annotation server (modifying responses to include
489   references to a local annotation database), a malware filter, a
490   format transcoder, or an intranet-to-Internet privacy filter.  Such
491   transformations are presumed to be desired by the client (or client
492   organization) that selected the proxy and are beyond the scope of
493   this specification.  However, when a proxy is not intended to transform
494   a given message, we use the term "<x:dfn>non-transforming proxy</x:dfn>" to target
495   requirements that preserve HTTP message semantics. See &status-203; and
496   &header-warning; for status and warning codes related to transformations.
497</t>
498<t><iref primary="true" item="gateway"/><iref primary="true" item="reverse proxy"/>
499<iref primary="true" item="accelerator"/>
500   A "<x:dfn>gateway</x:dfn>" (a.k.a., "<x:dfn>reverse proxy</x:dfn>") is an
501   intermediary that acts as an origin server for the outbound connection, but
502   translates received requests and forwards them inbound to another server or
503   servers. Gateways are often used to encapsulate legacy or untrusted
504   information services, to improve server performance through
505   "<x:dfn>accelerator</x:dfn>" caching, and to enable partitioning or load
506   balancing of HTTP services across multiple machines.
507</t>
508<t>
509   All HTTP requirements applicable to an origin server
510   also apply to the outbound communication of a gateway.
511   A gateway communicates with inbound servers using any protocol that
512   it desires, including private extensions to HTTP that are outside
513   the scope of this specification.  However, an HTTP-to-HTTP gateway
514   that wishes to interoperate with third-party HTTP servers ought to conform
515   to user agent requirements on the gateway's inbound connection.
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 that 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.
620</t>
621<t>
622   Conformance includes both the syntax and semantics of HTTP protocol
623   elements. A sender &MUST-NOT; generate protocol elements that convey a
624   meaning that is known by that sender to be false. A sender &MUST-NOT;
625   generate protocol elements that do not match the grammar defined by the
626   corresponding ABNF rules. Within a given message, a sender &MUST-NOT;
627   generate protocol elements or syntax alternatives that are only allowed to
628   be generated by participants in other roles (i.e., a role that the sender
629   does not have for that message).
630</t>
631<t>
632   When a received protocol element is parsed, the recipient &MUST; be able to
633   parse any value of reasonable length that is applicable to the recipient's
634   role and matches the grammar defined by the corresponding ABNF rules.
635   Note, however, that some received protocol elements might not be parsed.
636   For example, an intermediary forwarding a message might parse a
637   header-field into generic field-name and field-value components, but then
638   forward the header field without further parsing inside the field-value.
639</t>
640<t>
641   HTTP does not have specific length limitations for many of its protocol
642   elements because the lengths that might be appropriate will vary widely,
643   depending on the deployment context and purpose of the implementation.
644   Hence, interoperability between senders and recipients depends on shared
645   expectations regarding what is a reasonable length for each protocol
646   element. Furthermore, what is commonly understood to be a reasonable length
647   for some protocol elements has changed over the course of the past two
648   decades of HTTP use, and is expected to continue changing in the future.
649</t>
650<t>
651   At a minimum, a recipient &MUST; be able to parse and process protocol
652   element lengths that are at least as long as the values that it generates
653   for those same protocol elements in other messages. For example, an origin
654   server that publishes very long URI references to its own resources needs
655   to be able to parse and process those same references when received as a
656   request target.
657</t>
658<t>
659   A recipient &MUST; interpret a received protocol element according to the
660   semantics defined for it by this specification, including extensions to
661   this specification, unless the recipient has determined (through experience
662   or configuration) that the sender incorrectly implements what is implied by
663   those semantics.
664   For example, an origin server might disregard the contents of a received
665   <x:ref>Accept-Encoding</x:ref> header field if inspection of the
666   <x:ref>User-Agent</x:ref> header field indicates a specific implementation
667   version that is known to fail on receipt of certain content codings.
668</t>
669<t>
670   Unless noted otherwise, a recipient &MAY; attempt to recover a usable
671   protocol element from an invalid construct.  HTTP does not define
672   specific error handling mechanisms except when they have a direct impact
673   on security, since different applications of the protocol require
674   different error handling strategies.  For example, a Web browser might
675   wish to transparently recover from a response where the
676   <x:ref>Location</x:ref> header field doesn't parse according to the ABNF,
677   whereas a systems control client might consider any form of error recovery
678   to be dangerous.
679</t>
680</section>
681
682<section title="Protocol Versioning" anchor="http.version">
683  <x:anchor-alias value="HTTP-version"/>
684  <x:anchor-alias value="HTTP-name"/>
685<t>
686   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate
687   versions of the protocol. This specification defines version "1.1".
688   The protocol version as a whole indicates the sender's conformance
689   with the set of requirements laid out in that version's corresponding
690   specification of HTTP.
691</t>
692<t>
693   The version of an HTTP message is indicated by an HTTP-version field
694   in the first line of the message. HTTP-version is case-sensitive.
695</t>
696<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-version"/><iref primary="true" item="Grammar" subitem="HTTP-name"/>
697  <x:ref>HTTP-version</x:ref>  = <x:ref>HTTP-name</x:ref> "/" <x:ref>DIGIT</x:ref> "." <x:ref>DIGIT</x:ref>
698  <x:ref>HTTP-name</x:ref>     = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
699</artwork></figure>
700<t>
701   The HTTP version number consists of two decimal digits separated by a "."
702   (period or decimal point).  The first digit ("major version") indicates the
703   HTTP messaging syntax, whereas the second digit ("minor version") indicates
704   the highest minor version within that major version to which the sender is
705   conformant and able to understand for future communication.  The minor
706   version advertises the sender's communication capabilities even when the
707   sender is only using a backwards-compatible subset of the protocol,
708   thereby letting the recipient know that more advanced features can
709   be used in response (by servers) or in future requests (by clients).
710</t>
711<t>
712   When an HTTP/1.1 message is sent to an HTTP/1.0 recipient
713   <xref target="RFC1945"/> or a recipient whose version is unknown,
714   the HTTP/1.1 message is constructed such that it can be interpreted
715   as a valid HTTP/1.0 message if all of the newer features are ignored.
716   This specification places recipient-version requirements on some
717   new features so that a conformant sender will only use compatible
718   features until it has determined, through configuration or the
719   receipt of a message, that the recipient supports HTTP/1.1.
720</t>
721<t>
722   The interpretation of a header field does not change between minor
723   versions of the same major HTTP version, though the default
724   behavior of a recipient in the absence of such a field can change.
725   Unless specified otherwise, header fields defined in HTTP/1.1 are
726   defined for all versions of HTTP/1.x.  In particular, the <x:ref>Host</x:ref>
727   and <x:ref>Connection</x:ref> header fields ought to be implemented by all
728   HTTP/1.x implementations whether or not they advertise conformance with
729   HTTP/1.1.
730</t>
731<t>
732   New header fields can be introduced without changing the protocol version
733   if their defined semantics allow them to be safely ignored by recipients
734   that do not recognize them. Header field extensibility is discussed in
735   <xref target="field.extensibility"/>.
736</t>
737<t>
738   Intermediaries that process HTTP messages (i.e., all intermediaries
739   other than those acting as tunnels) &MUST; send their own HTTP-version
740   in forwarded messages.  In other words, they &MUST-NOT; blindly
741   forward the first line of an HTTP message without ensuring that the
742   protocol version in that message matches a version to which that
743   intermediary is conformant for both the receiving and
744   sending of messages.  Forwarding an HTTP message without rewriting
745   the HTTP-version might result in communication errors when downstream
746   recipients use the message sender's version to determine what features
747   are safe to use for later communication with that sender.
748</t>
749<t>
750   A client &SHOULD; send a request version equal to the highest
751   version to which the client is conformant and
752   whose major version is no higher than the highest version supported
753   by the server, if this is known.  A client &MUST-NOT; send a
754   version to which it is not conformant.
755</t>
756<t>
757   A client &MAY; send a lower request version if it is known that
758   the server incorrectly implements the HTTP specification, but only
759   after the client has attempted at least one normal request and determined
760   from the response status or header fields (e.g., <x:ref>Server</x:ref>) that
761   the server improperly handles higher request versions.
762</t>
763<t>
764   A server &SHOULD; send a response version equal to the highest
765   version to which the server is conformant and
766   whose major version is less than or equal to the one received in the
767   request.  A server &MUST-NOT; send a version to which it is not
768   conformant.  A server &MAY; send a <x:ref>505 (HTTP Version Not
769   Supported)</x:ref> response if it cannot send a response using the
770   major version used in the client's request.
771</t>
772<t>
773   A server &MAY; send an HTTP/1.0 response to a request
774   if it is known or suspected that the client incorrectly implements the
775   HTTP specification and is incapable of correctly processing later
776   version responses, such as when a client fails to parse the version
777   number correctly or when an intermediary is known to blindly forward
778   the HTTP-version even when it doesn't conform to the given minor
779   version of the protocol. Such protocol downgrades &SHOULD-NOT; be
780   performed unless triggered by specific client attributes, such as when
781   one or more of the request header fields (e.g., <x:ref>User-Agent</x:ref>)
782   uniquely match the values sent by a client known to be in error.
783</t>
784<t>
785   The intention of HTTP's versioning design is that the major number
786   will only be incremented if an incompatible message syntax is
787   introduced, and that the minor number will only be incremented when
788   changes made to the protocol have the effect of adding to the message
789   semantics or implying additional capabilities of the sender.  However,
790   the minor version was not incremented for the changes introduced between
791   <xref target="RFC2068"/> and <xref target="RFC2616"/>, and this revision
792   has specifically avoided any such changes to the protocol.
793</t>
794<t>
795   When an HTTP message is received with a major version number that the
796   recipient implements, but a higher minor version number than what the
797   recipient implements, the recipient &SHOULD; process the message as if it
798   were in the highest minor version within that major version to which the
799   recipient is conformant. A recipient can assume that a message with a
800   higher minor version, when sent to a recipient that has not yet indicated
801   support for that higher version, is sufficiently backwards-compatible to be
802   safely processed by any implementation of the same major version.
803</t>
804</section>
805
806<section title="Uniform Resource Identifiers" anchor="uri">
807<iref primary="true" item="resource"/>
808<t>
809   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used
810   throughout HTTP as the means for identifying resources (&resource;).
811   URI references are used to target requests, indicate redirects, and define
812   relationships.
813</t>
814  <x:anchor-alias value="URI-reference"/>
815  <x:anchor-alias value="absolute-URI"/>
816  <x:anchor-alias value="relative-part"/>
817  <x:anchor-alias value="authority"/>
818  <x:anchor-alias value="uri-host"/>
819  <x:anchor-alias value="port"/>
820  <x:anchor-alias value="path-abempty"/>
821  <x:anchor-alias value="segment"/>
822  <x:anchor-alias value="query"/>
823  <x:anchor-alias value="fragment"/>
824  <x:anchor-alias value="absolute-path"/>
825  <x:anchor-alias value="partial-URI"/>
826<t>
827   This specification adopts the definitions of "URI-reference",
828   "absolute-URI", "relative-part", "authority", "port", "host",
829   "path-abempty", "segment", "query", and "fragment" from the
830   URI generic syntax.
831   In addition, we define an "absolute-path" rule (that differs from
832   RFC 3986's "path-absolute" in that it allows a leading "//")
833   and a "partial-URI" rule for protocol elements
834   that allow a relative URI but not a fragment.
835</t>
836<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="URI-reference"><!--exported production--></iref><iref primary="true" item="Grammar" subitem="absolute-URI"/><iref primary="true" item="Grammar" subitem="authority"/><iref primary="true" item="Grammar" subitem="absolute-path"/><iref primary="true" item="Grammar" subitem="port"/><iref primary="true" item="Grammar" subitem="query"/><iref primary="true" item="Grammar" subitem="fragment"/><iref primary="true" item="Grammar" subitem="segment"/><iref primary="true" item="Grammar" subitem="uri-host"/><iref primary="true" item="Grammar" subitem="partial-URI"><!--exported production--></iref>
837  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
838  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
839  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
840  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
841  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
842  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
843  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
844  <x:ref>segment</x:ref>       = &lt;segment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
845  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
846  <x:ref>fragment</x:ref>      = &lt;fragment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.5"/>&gt;
847 
848  <x:ref>absolute-path</x:ref> = 1*( "/" segment )
849  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
850</artwork></figure>
851<t>
852   Each protocol element in HTTP that allows a URI reference will indicate
853   in its ABNF production whether the element allows any form of reference
854   (URI-reference), only a URI in absolute form (absolute-URI), only the
855   path and optional query components, or some combination of the above.
856   Unless otherwise indicated, URI references are parsed
857   relative to the effective request URI
858   (<xref target="effective.request.uri"/>).
859</t>
860
861<section title="http URI scheme" anchor="http.uri">
862  <x:anchor-alias value="http-URI"/>
863  <iref item="http URI scheme" primary="true"/>
864  <iref item="URI scheme" subitem="http" primary="true"/>
865<t>
866   The "http" URI scheme is hereby defined for the purpose of minting
867   identifiers according to their association with the hierarchical
868   namespace governed by a potential HTTP origin server listening for
869   TCP (<xref target="RFC0793"/>) connections on a given port.
870</t>
871<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"><!--terminal production--></iref>
872  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
873             [ "#" <x:ref>fragment</x:ref> ]
874</artwork></figure>
875<t>
876   The HTTP origin server is identified by the generic syntax's
877   <x:ref>authority</x:ref> component, which includes a host identifier
878   and optional TCP port (<xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>).
879   The remainder of the URI, consisting of both the hierarchical path
880   component and optional query component, serves as an identifier for
881   a potential resource within that origin server's name space.
882</t>
883<t>
884   A sender &MUST-NOT; generate an "http" URI with an empty host identifier.
885   A recipient that processes such a URI reference &MUST; reject it as invalid.
886</t>
887<t>
888   If the host identifier is provided as an IP address,
889   then the origin server is any listener on the indicated TCP port at
890   that IP address. If host is a registered name, then that name is
891   considered an indirect identifier and the recipient might use a name
892   resolution service, such as DNS, to find the address of a listener
893   for that host.
894   If the port subcomponent is empty or not given, then TCP port 80 is
895   assumed (the default reserved port for WWW services).
896</t>
897<t>
898   Regardless of the form of host identifier, access to that host is not
899   implied by the mere presence of its name or address. The host might or might
900   not exist and, even when it does exist, might or might not be running an
901   HTTP server or listening to the indicated port. The "http" URI scheme
902   makes use of the delegated nature of Internet names and addresses to
903   establish a naming authority (whatever entity has the ability to place
904   an HTTP server at that Internet name or address) and allows that
905   authority to determine which names are valid and how they might be used.
906</t>
907<t>
908   When an "http" URI is used within a context that calls for access to the
909   indicated resource, a client &MAY; attempt access by resolving
910   the host to an IP address, establishing a TCP connection to that address
911   on the indicated port, and sending an HTTP request message
912   (<xref target="http.message"/>) containing the URI's identifying data
913   (<xref target="message.routing"/>) to the server.
914   If the server responds to that request with a non-interim HTTP response
915   message, as described in &status-codes;, then that response
916   is considered an authoritative answer to the client's request.
917</t>
918<t>
919   Although HTTP is independent of the transport protocol, the "http"
920   scheme is specific to TCP-based services because the name delegation
921   process depends on TCP for establishing authority.
922   An HTTP service based on some other underlying connection protocol
923   would presumably be identified using a different URI scheme, just as
924   the "https" scheme (below) is used for resources that require an
925   end-to-end secured connection. Other protocols might also be used to
926   provide access to "http" identified resources &mdash; it is only the
927   authoritative interface that is specific to TCP.
928</t>
929<t>
930   The URI generic syntax for authority also includes a deprecated
931   userinfo subcomponent (<xref target="RFC3986" x:fmt="," x:sec="3.2.1"/>)
932   for including user authentication information in the URI.  Some
933   implementations make use of the userinfo component for internal
934   configuration of authentication information, such as within command
935   invocation options, configuration files, or bookmark lists, even
936   though such usage might expose a user identifier or password.
937   Senders &MUST-NOT; generate the userinfo subcomponent (and its "@"
938   delimiter) when an "http" URI reference is generated within a message as a
939   request target or header field value.
940   Recipients of an "http" URI reference &SHOULD; parse for userinfo and
941   treat its presence as an error, since it is likely being used to obscure
942   the authority for the sake of phishing attacks.
943</t>
944</section>
945
946<section title="https URI scheme" anchor="https.uri">
947   <x:anchor-alias value="https-URI"/>
948   <iref item="https URI scheme"/>
949   <iref item="URI scheme" subitem="https"/>
950<t>
951   The "https" URI scheme is hereby defined for the purpose of minting
952   identifiers according to their association with the hierarchical
953   namespace governed by a potential HTTP origin server listening to a
954   given TCP port for TLS-secured connections
955   (<xref target="RFC0793"/>, <xref target="RFC5246"/>).
956</t>
957<t>
958   All of the requirements listed above for the "http" scheme are also
959   requirements for the "https" scheme, except that a default TCP port
960   of 443 is assumed if the port subcomponent is empty or not given,
961   and the user agent &MUST; ensure that its connection to the origin
962   server is secured through the use of strong encryption, end-to-end,
963   prior to sending the first HTTP request.
964</t>
965<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="https-URI"><!--terminal production--></iref>
966  <x:ref>https-URI</x:ref> = "https:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
967              [ "#" <x:ref>fragment</x:ref> ]
968</artwork></figure>
969<t>
970   Note that the "https" URI scheme depends on both TLS and TCP for
971   establishing authority.
972   Resources made available via the "https" scheme have no shared
973   identity with the "http" scheme even if their resource identifiers
974   indicate the same authority (the same host listening to the same
975   TCP port).  They are distinct name spaces and are considered to be
976   distinct origin servers.  However, an extension to HTTP that is
977   defined to apply to entire host domains, such as the Cookie protocol
978   <xref target="RFC6265"/>, can allow information
979   set by one service to impact communication with other services
980   within a matching group of host domains.
981</t>
982<t>
983   The process for authoritative access to an "https" identified
984   resource is defined in <xref target="RFC2818"/>.
985</t>
986</section>
987
988<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
989<t>
990   Since the "http" and "https" schemes conform to the URI generic syntax,
991   such URIs are normalized and compared according to the algorithm defined
992   in <xref target="RFC3986" x:fmt="," x:sec="6"/>, using the defaults
993   described above for each scheme.
994</t>
995<t>
996   If the port is equal to the default port for a scheme, the normal form is
997   to omit the port subcomponent. When not being used in absolute form as the
998   request target of an OPTIONS request, an empty path component is equivalent
999   to an absolute path of "/", so the normal form is to provide a path of "/"
1000   instead. The scheme and host are case-insensitive and normally provided in
1001   lowercase; all other components are compared in a case-sensitive manner.
1002   Characters other than those in the "reserved" set are equivalent to their
1003   percent-encoded octets (see <xref target="RFC3986" x:fmt=","
1004   x:sec="2.1"/>): the normal form is to not encode them.
1005</t>
1006<t>
1007   For example, the following three URIs are equivalent:
1008</t>
1009<figure><artwork type="example">
1010   http://example.com:80/~smith/home.html
1011   http://EXAMPLE.com/%7Esmith/home.html
1012   http://EXAMPLE.com:/%7esmith/home.html
1013</artwork></figure>
1014</section>
1015</section>
1016</section>
1017
1018<section title="Message Format" anchor="http.message">
1019<x:anchor-alias value="generic-message"/>
1020<x:anchor-alias value="message.types"/>
1021<x:anchor-alias value="HTTP-message"/>
1022<x:anchor-alias value="start-line"/>
1023<iref item="header section"/>
1024<iref item="headers"/>
1025<iref item="header field"/>
1026<t>
1027   All HTTP/1.1 messages consist of a start-line followed by a sequence of
1028   octets in a format similar to the Internet Message Format
1029   <xref target="RFC5322"/>: zero or more header fields (collectively
1030   referred to as the "headers" or the "header section"), an empty line
1031   indicating the end of the header section, and an optional message body.
1032</t>
1033<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"><!--terminal production--></iref>
1034  <x:ref>HTTP-message</x:ref>   = <x:ref>start-line</x:ref>
1035                   *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1036                   <x:ref>CRLF</x:ref>
1037                   [ <x:ref>message-body</x:ref> ]
1038</artwork></figure>
1039<t>
1040   The normal procedure for parsing an HTTP message is to read the
1041   start-line into a structure, read each header field into a hash
1042   table by field name until the empty line, and then use the parsed
1043   data to determine if a message body is expected.  If a message body
1044   has been indicated, then it is read as a stream until an amount
1045   of octets equal to the message body length is read or the connection
1046   is closed.
1047</t>
1048<t>
1049   Recipients &MUST; parse an HTTP message as a sequence of octets in an
1050   encoding that is a superset of US-ASCII <xref target="USASCII"/>.
1051   Parsing an HTTP message as a stream of Unicode characters, without regard
1052   for the specific encoding, creates security vulnerabilities due to the
1053   varying ways that string processing libraries handle invalid multibyte
1054   character sequences that contain the octet LF (%x0A).  String-based
1055   parsers can only be safely used within protocol elements after the element
1056   has been extracted from the message, such as within a header field-value
1057   after message parsing has delineated the individual fields.
1058</t>
1059<t>
1060   An HTTP message can be parsed as a stream for incremental processing or
1061   forwarding downstream.  However, recipients cannot rely on incremental
1062   delivery of partial messages, since some implementations will buffer or
1063   delay message forwarding for the sake of network efficiency, security
1064   checks, or payload transformations.
1065</t>
1066<t>
1067   A sender &MUST-NOT; send whitespace between the start-line and
1068   the first header field.
1069   A recipient that receives whitespace between the start-line and
1070   the first header field &MUST; either reject the message as invalid or
1071   consume each whitespace-preceded line without further processing of it
1072   (i.e., ignore the entire line, along with any subsequent lines preceded
1073   by whitespace, until a properly formed header field is received or the
1074   header block is terminated).
1075</t>
1076<t>
1077   The presence of such whitespace in a request
1078   might be an attempt to trick a server into ignoring that field or
1079   processing the line after it as a new request, either of which might
1080   result in a security vulnerability if other implementations within
1081   the request chain interpret the same message differently.
1082   Likewise, the presence of such whitespace in a response might be
1083   ignored by some clients or cause others to cease parsing.
1084</t>
1085
1086<section title="Start Line" anchor="start.line">
1087  <x:anchor-alias value="Start-Line"/>
1088<t>
1089   An HTTP message can either be a request from client to server or a
1090   response from server to client.  Syntactically, the two types of message
1091   differ only in the start-line, which is either a request-line (for requests)
1092   or a status-line (for responses), and in the algorithm for determining
1093   the length of the message body (<xref target="message.body"/>).
1094</t>
1095<t>
1096   In theory, a client could receive requests and a server could receive
1097   responses, distinguishing them by their different start-line formats,
1098   but in practice servers are implemented to only expect a request
1099   (a response is interpreted as an unknown or invalid request method)
1100   and clients are implemented to only expect a response.
1101</t>
1102<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="start-line"/>
1103  <x:ref>start-line</x:ref>     = <x:ref>request-line</x:ref> / <x:ref>status-line</x:ref>
1104</artwork></figure>
1105
1106<section title="Request Line" anchor="request.line">
1107  <x:anchor-alias value="Request"/>
1108  <x:anchor-alias value="request-line"/>
1109<t>
1110   A request-line begins with a method token, followed by a single
1111   space (SP), the request-target, another single space (SP), the
1112   protocol version, and ending with CRLF.
1113</t>
1114<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1115  <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>
1116</artwork></figure>
1117<iref primary="true" item="method"/>
1118<t anchor="method">
1119   The method token indicates the request method to be performed on the
1120   target resource. The request method is case-sensitive.
1121</t>
1122<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1123  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1124</artwork></figure>
1125<t>
1126   The methods defined by this specification can be found in
1127   &methods;, along with information regarding the HTTP method registry
1128   and considerations for defining new methods.
1129</t>
1130<iref item="request-target"/>
1131<t>
1132   The request-target identifies the target resource upon which to apply
1133   the request, as defined in <xref target="request-target"/>.
1134</t>
1135<t>
1136   Recipients typically parse the request-line into its component parts by
1137   splitting on whitespace (see <xref target="message.robustness"/>), since
1138   no whitespace is allowed in the three components.
1139   Unfortunately, some user agents fail to properly encode or exclude
1140   whitespace found in hypertext references, resulting in those disallowed
1141   characters being sent in a request-target.
1142</t>
1143<t>
1144   Recipients of an invalid request-line &SHOULD; respond with either a
1145   <x:ref>400 (Bad Request)</x:ref> error or a <x:ref>301 (Moved Permanently)</x:ref>
1146   redirect with the request-target properly encoded.  Recipients &SHOULD-NOT;
1147   attempt to autocorrect and then process the request without a redirect,
1148   since the invalid request-line might be deliberately crafted to bypass
1149   security filters along the request chain.
1150</t>
1151<t>
1152   HTTP does not place a pre-defined limit on the length of a request-line.
1153   A server that receives a method longer than any that it implements
1154   &SHOULD; respond with a <x:ref>501 (Not Implemented)</x:ref> status code.
1155   A server ought to be prepared to receive URIs of unbounded length, as
1156   described in <xref target="conformance"/>, and &MUST; respond with a
1157   <x:ref>414 (URI Too Long)</x:ref> status code if the received
1158   request-target is longer than the server wishes to parse (see &status-414;).
1159</t>
1160<t>
1161   Various ad-hoc limitations on request-line length are found in practice.
1162   It is &RECOMMENDED; that all HTTP senders and recipients support, at a
1163   minimum, request-line lengths of 8000 octets.
1164</t>
1165</section>
1166
1167<section title="Status Line" anchor="status.line">
1168  <x:anchor-alias value="response"/>
1169  <x:anchor-alias value="status-line"/>
1170  <x:anchor-alias value="status-code"/>
1171  <x:anchor-alias value="reason-phrase"/>
1172<t>
1173   The first line of a response message is the status-line, consisting
1174   of the protocol version, a space (SP), the status code, another space,
1175   a possibly-empty textual phrase describing the status code, and
1176   ending with CRLF.
1177</t>
1178<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-line"/>
1179  <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>
1180</artwork></figure>
1181<t>
1182   The status-code element is a 3-digit integer code describing the
1183   result of the server's attempt to understand and satisfy the client's
1184   corresponding request. The rest of the response message is to be
1185   interpreted in light of the semantics defined for that status code.
1186   See &status-codes; for information about the semantics of status codes,
1187   including the classes of status code (indicated by the first digit),
1188   the status codes defined by this specification, considerations for the
1189   definition of new status codes, and the IANA registry.
1190</t>
1191<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-code"/>
1192  <x:ref>status-code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1193</artwork></figure>
1194<t>  
1195   The reason-phrase element exists for the sole purpose of providing a
1196   textual description associated with the numeric status code, mostly
1197   out of deference to earlier Internet application protocols that were more
1198   frequently used with interactive text clients. A client &SHOULD; ignore
1199   the reason-phrase content.
1200</t>
1201<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
1202  <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> )
1203</artwork></figure>
1204</section>
1205</section>
1206
1207<section title="Header Fields" anchor="header.fields">
1208  <x:anchor-alias value="header-field"/>
1209  <x:anchor-alias value="field-content"/>
1210  <x:anchor-alias value="field-name"/>
1211  <x:anchor-alias value="field-value"/>
1212  <x:anchor-alias value="obs-fold"/>
1213<t>
1214   Each HTTP header field consists of a case-insensitive field name
1215   followed by a colon (":"), optional leading whitespace, the field value,
1216   and optional trailing whitespace.
1217</t>
1218<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"/>
1219  <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>OWS</x:ref>
1220  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1221  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>obs-fold</x:ref> )
1222  <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> )
1223  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref> ( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1224                 ; obsolete line folding
1225                 ; see <xref target="field.parsing"/>
1226</artwork></figure>
1227<t>
1228   The field-name token labels the corresponding field-value as having the
1229   semantics defined by that header field.  For example, the <x:ref>Date</x:ref>
1230   header field is defined in &header-date; as containing the origination
1231   timestamp for the message in which it appears.
1232</t>
1233
1234<section title="Field Extensibility" anchor="field.extensibility">
1235<t>
1236   Header fields are fully extensible: there is no limit on the
1237   introduction of new field names, each presumably defining new semantics,
1238   nor on the number of header fields used in a given message.  Existing
1239   fields are defined in each part of this specification and in many other
1240   specifications outside the core standard.
1241</t>
1242<t>
1243   New header fields can be defined such that, when they are understood by a
1244   recipient, they might override or enhance the interpretation of previously
1245   defined header fields, define preconditions on request evaluation, or
1246   refine the meaning of responses.
1247</t>
1248<t>
1249   A proxy &MUST; forward unrecognized header fields unless the
1250   field-name is listed in the <x:ref>Connection</x:ref> header field
1251   (<xref target="header.connection"/>) or the proxy is specifically
1252   configured to block, or otherwise transform, such fields.
1253   Other recipients &SHOULD; ignore unrecognized header fields.
1254   These requirements allow HTTP's functionality to be enhanced without
1255   requiring prior update of deployed intermediaries.
1256</t>
1257<t>
1258   All defined header fields ought to be registered with IANA in the
1259   Message Header Field Registry, as described in &iana-header-registry;.
1260</t>
1261</section>
1262
1263<section title="Field Order" anchor="field.order">
1264<t>
1265   The order in which header fields with differing field names are
1266   received is not significant. However, it is "good practice" to send
1267   header fields that contain control data first, such as <x:ref>Host</x:ref>
1268   on requests and <x:ref>Date</x:ref> on responses, so that implementations
1269   can decide when not to handle a message as early as possible.  A server
1270   &MUST; wait until the entire header section is received before interpreting
1271   a request message, since later header fields might include conditionals,
1272   authentication credentials, or deliberately misleading duplicate
1273   header fields that would impact request processing.
1274</t>
1275<t>
1276   A sender &MUST-NOT; generate multiple header fields with the same field
1277   name in a message unless either the entire field value for that
1278   header field is defined as a comma-separated list [i.e., #(values)]
1279   or the header field is a well-known exception (as noted below).
1280</t>
1281<t>
1282   A recipient &MAY; combine multiple header fields with the same field name
1283   into one "field-name: field-value" pair, without changing the semantics of
1284   the message, by appending each subsequent field value to the combined
1285   field value in order, separated by a comma. The order in which
1286   header fields with the same field name are received is therefore
1287   significant to the interpretation of the combined field value;
1288   a proxy &MUST-NOT; change the order of these field values when
1289   forwarding a message.
1290</t>
1291<x:note>
1292  <t>
1293   &Note; In practice, the "Set-Cookie" header field (<xref target="RFC6265"/>)
1294   often appears multiple times in a response message and does not use the
1295   list syntax, violating the above requirements on multiple header fields
1296   with the same name. Since it cannot be combined into a single field-value,
1297   recipients ought to handle "Set-Cookie" as a special case while processing
1298   header fields. (See Appendix A.2.3 of <xref target="Kri2001"/> for details.)
1299  </t>
1300</x:note>
1301</section>
1302
1303<section title="Whitespace" anchor="whitespace">
1304<t anchor="rule.LWS">
1305   This specification uses three rules to denote the use of linear
1306   whitespace: OWS (optional whitespace), RWS (required whitespace), and
1307   BWS ("bad" whitespace).
1308</t>
1309<t anchor="rule.OWS">
1310   The OWS rule is used where zero or more linear whitespace octets might
1311   appear. For protocol elements where optional whitespace is preferred to
1312   improve readability, a sender &SHOULD; generate the optional whitespace
1313   as a single SP; otherwise, a sender &SHOULD-NOT; generate optional
1314   whitespace except as needed to white-out invalid or unwanted protocol
1315   elements during in-place message filtering.
1316</t>
1317<t anchor="rule.RWS">
1318   The RWS rule is used when at least one linear whitespace octet is required
1319   to separate field tokens. A sender &SHOULD; generate RWS as a single SP.
1320</t>
1321<t anchor="rule.BWS">
1322   The BWS rule is used where the grammar allows optional whitespace only for
1323   historical reasons. A sender &MUST-NOT; generate BWS in messages.
1324   A recipient &MUST; parse for such bad whitespace and remove it before
1325   interpreting the protocol element.
1326</t>
1327<t anchor="rule.whitespace">
1328  <x:anchor-alias value="BWS"/>
1329  <x:anchor-alias value="OWS"/>
1330  <x:anchor-alias value="RWS"/>
1331</t>
1332<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"/>
1333  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1334                 ; optional whitespace
1335  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1336                 ; required whitespace
1337  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1338                 ; "bad" whitespace
1339</artwork></figure>
1340</section>
1341
1342<section title="Field Parsing" anchor="field.parsing">
1343<t>
1344   No whitespace is allowed between the header field-name and colon.
1345   In the past, differences in the handling of such whitespace have led to
1346   security vulnerabilities in request routing and response handling.
1347   A server &MUST; reject any received request message that contains
1348   whitespace between a header field-name and colon with a response code of
1349   <x:ref>400 (Bad Request)</x:ref>. A proxy &MUST; remove any such whitespace
1350   from a response message before forwarding the message downstream.
1351</t>
1352<t>
1353   A field value is preceded by optional whitespace (OWS); a single SP is
1354   preferred. The field value does not include any leading or trailing white
1355   space: OWS occurring before the first non-whitespace octet of the field
1356   value or after the last non-whitespace octet of the field value ought to be
1357   excluded by parsers when extracting the field value from a header field.
1358</t>
1359<t>
1360   A recipient of field-content containing multiple sequential octets of
1361   optional (OWS) or required (RWS) whitespace &SHOULD; either replace the
1362   sequence with a single SP or transform any non-SP octets in the sequence to
1363   SP octets before interpreting the field value or forwarding the message
1364   downstream.
1365</t>
1366<t>
1367   Historically, HTTP header field values could be extended over multiple
1368   lines by preceding each extra line with at least one space or horizontal
1369   tab (obs-fold). This specification deprecates such line folding except
1370   within the message/http media type
1371   (<xref target="internet.media.type.message.http"/>).
1372   Senders &MUST-NOT; generate messages that include line folding
1373   (i.e., that contain any field-value that contains a match to the
1374   <x:ref>obs-fold</x:ref> rule) unless the message is intended for packaging
1375   within the message/http media type.
1376</t>
1377<t>
1378   A server that receives an <x:ref>obs-fold</x:ref> in a request message that
1379   is not within a message/http container &MUST; either reject the message by
1380   sending a <x:ref>400 (Bad Request)</x:ref>, preferably with a
1381   representation explaining that obsolete line folding is unacceptable, or
1382   replace each received <x:ref>obs-fold</x:ref> with one or more
1383   <x:ref>SP</x:ref> octets prior to interpreting the field value or
1384   forwarding the message downstream.
1385</t>
1386<t>
1387   A proxy or gateway that receives an <x:ref>obs-fold</x:ref> in a response
1388   message that is not within a message/http container &MUST; either discard
1389   the message and replace it with a <x:ref>502 (Bad Gateway)</x:ref>
1390   response, preferably with a representation explaining that unacceptable
1391   line folding was received, or replace each received <x:ref>obs-fold</x:ref>
1392   with one or more <x:ref>SP</x:ref> octets prior to interpreting the field
1393   value or forwarding the message downstream.
1394</t>
1395<t>
1396   A user agent that receives an <x:ref>obs-fold</x:ref> in a response message
1397   that is not within a message/http container &MUST; replace each received
1398   <x:ref>obs-fold</x:ref> with one or more <x:ref>SP</x:ref> octets prior to
1399   interpreting the field value.
1400</t>
1401<t>
1402   Historically, HTTP has allowed field content with text in the ISO-8859-1
1403   <xref target="ISO-8859-1"/> charset, supporting other charsets only
1404   through use of <xref target="RFC2047"/> encoding.
1405   In practice, most HTTP header field values use only a subset of the
1406   US-ASCII charset <xref target="USASCII"/>. Newly defined
1407   header fields &SHOULD; limit their field values to US-ASCII octets.
1408   Recipients &SHOULD; treat other octets in field content (obs-text) as
1409   opaque data.
1410</t>
1411</section>
1412
1413<section title="Field Limits" anchor="field.limits">
1414<t>
1415   HTTP does not place a pre-defined limit on the length of each header field
1416   or on the length of the header block as a whole, as described in
1417   <xref target="conformance"/>. Various ad-hoc limitations on individual
1418   header field length are found in practice, often depending on the specific
1419   field semantics.
1420</t>
1421<t>
1422   A server ought to be prepared to receive request header fields of unbounded
1423   length and &MUST; respond with an appropriate
1424   <x:ref>4xx (Client Error)</x:ref> status code if the received header
1425   field(s) are larger than the server wishes to process.
1426</t>
1427<t>
1428   A client ought to be prepared to receive response header fields of
1429   unbounded length.
1430   A client &MAY; discard or truncate received header fields that are larger
1431   than the client wishes to process if the field semantics are such that the
1432   dropped value(s) can be safely ignored without changing the
1433   message framing or response semantics.
1434</t>
1435</section>
1436
1437<section title="Field value components" anchor="field.components">
1438<t anchor="rule.token.separators">
1439  <x:anchor-alias value="tchar"/>
1440  <x:anchor-alias value="token"/>
1441  <x:anchor-alias value="special"/>
1442  <x:anchor-alias value="word"/>
1443   Many HTTP header field values consist of words (token or quoted-string)
1444   separated by whitespace or special characters.
1445</t>
1446<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>
1447  <x:ref>word</x:ref>           = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1448
1449  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1450<!--
1451  IMPORTANT: when editing "tchar" make sure that "special" is updated accordingly!!!
1452 -->
1453  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1454                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1455                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1456                 ; any <x:ref>VCHAR</x:ref>, except <x:ref>special</x:ref>
1457
1458  <x:ref>special</x:ref>        = "(" / ")" / "&lt;" / ">" / "@" / ","
1459                 / ";" / ":" / "\" / DQUOTE / "/" / "["
1460                 / "]" / "?" / "=" / "{" / "}"
1461</artwork></figure>
1462<t anchor="rule.quoted-string">
1463  <x:anchor-alias value="quoted-string"/>
1464  <x:anchor-alias value="qdtext"/>
1465  <x:anchor-alias value="obs-text"/>
1466   A string of text is parsed as a single word if it is quoted using
1467   double-quote marks.
1468</t>
1469<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"/>
1470  <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>
1471  <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>
1472  <x:ref>obs-text</x:ref>       = %x80-FF
1473</artwork></figure>
1474<t anchor="rule.quoted-pair">
1475  <x:anchor-alias value="quoted-pair"/>
1476   The backslash octet ("\") can be used as a single-octet
1477   quoting mechanism within quoted-string constructs:
1478</t>
1479<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1480  <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> )
1481</artwork></figure>
1482<t>
1483   Recipients that process the value of a quoted-string &MUST; handle a
1484   quoted-pair as if it were replaced by the octet following the backslash.
1485</t>
1486<t>
1487   Senders &SHOULD-NOT; generate a quoted-pair in a quoted-string except where
1488   necessary to quote DQUOTE and backslash octets occurring within that string.
1489</t>
1490<t anchor="rule.comment">
1491  <x:anchor-alias value="comment"/>
1492  <x:anchor-alias value="ctext"/>
1493   Comments can be included in some HTTP header fields by surrounding
1494   the comment text with parentheses. Comments are only allowed in
1495   fields containing "comment" as part of their field value definition.
1496</t>
1497<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1498  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-cpair</x:ref> / <x:ref>comment</x:ref> ) ")"
1499  <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>
1500</artwork></figure>
1501<t anchor="rule.quoted-cpair">
1502  <x:anchor-alias value="quoted-cpair"/>
1503   The backslash octet ("\") can be used as a single-octet
1504   quoting mechanism within comment constructs:
1505</t>
1506<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-cpair"/>
1507  <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> )
1508</artwork></figure>
1509<t>
1510   Senders &SHOULD-NOT; escape octets in comments that do not require escaping
1511   (i.e., other than the backslash octet "\" and the parentheses "(" and ")").
1512</t>
1513</section>
1514
1515</section>
1516
1517<section title="Message Body" anchor="message.body">
1518  <x:anchor-alias value="message-body"/>
1519<t>
1520   The message body (if any) of an HTTP message is used to carry the
1521   payload body of that request or response.  The message body is
1522   identical to the payload body unless a transfer coding has been
1523   applied, as described in <xref target="header.transfer-encoding"/>.
1524</t>
1525<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1526  <x:ref>message-body</x:ref> = *OCTET
1527</artwork></figure>
1528<t>
1529   The rules for when a message body is allowed in a message differ for
1530   requests and responses.
1531</t>
1532<t>
1533   The presence of a message body in a request is signaled by a
1534   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1535   field. Request message framing is independent of method semantics,
1536   even if the method does not define any use for a message body.
1537</t>
1538<t>
1539   The presence of a message body in a response depends on both
1540   the request method to which it is responding and the response
1541   status code (<xref target="status.line"/>).
1542   Responses to the HEAD request method never include a message body
1543   because the associated response header fields (e.g.,
1544   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1545   if present, indicate only what their values would have been if the request
1546   method had been GET (&HEAD;).
1547   <x:ref>2xx (Successful)</x:ref> responses to CONNECT switch to tunnel
1548   mode instead of having a message body (&CONNECT;).
1549   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1550   <x:ref>304 (Not Modified)</x:ref> responses do not include a message body.
1551   All other responses do include a message body, although the body
1552   might be of zero length.
1553</t>
1554
1555<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1556  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1557  <iref item="chunked (Coding Format)"/>
1558  <x:anchor-alias value="Transfer-Encoding"/>
1559<t>
1560   The Transfer-Encoding header field lists the transfer coding names
1561   corresponding to the sequence of transfer codings that have been
1562   (or will be) applied to the payload body in order to form the message body.
1563   Transfer codings are defined in <xref target="transfer.codings"/>.
1564</t>
1565<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1566  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1567</artwork></figure>
1568<t>
1569   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1570   MIME, which was designed to enable safe transport of binary data over a
1571   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1572   However, safe transport has a different focus for an 8bit-clean transfer
1573   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1574   accurately delimit a dynamically generated payload and to distinguish
1575   payload encodings that are only applied for transport efficiency or
1576   security from those that are characteristics of the selected resource.
1577</t>
1578<t>
1579   All HTTP/1.1 recipients &MUST; be able to parse the chunked transfer coding
1580   (<xref target="chunked.encoding"/>) because it plays a crucial role in
1581   framing messages when the payload body size is not known in advance.
1582   A sender &MUST-NOT; apply chunked more than once to a message body
1583   (i.e., chunking an already chunked message is not allowed).
1584   If any transfer coding other than chunked is applied to a request payload
1585   body, the sender &MUST; apply chunked as the final transfer coding to
1586   ensure that the message is properly framed.
1587   If any transfer coding other than chunked is applied to a response payload
1588   body, the sender &MUST; either apply chunked as the final transfer coding
1589   or terminate the message by closing the connection.
1590</t>
1591<figure><preamble>
1592   For example,
1593</preamble><artwork type="example">
1594  Transfer-Encoding: gzip, chunked
1595</artwork><postamble>
1596   indicates that the payload body has been compressed using the gzip
1597   coding and then chunked using the chunked coding while forming the
1598   message body.
1599</postamble></figure>
1600<t>
1601   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1602   Transfer-Encoding is a property of the message, not of the representation, and
1603   any recipient along the request/response chain &MAY; decode the received
1604   transfer coding(s) or apply additional transfer coding(s) to the message
1605   body, assuming that corresponding changes are made to the Transfer-Encoding
1606   field-value. Additional information about the encoding parameters &MAY; be
1607   provided by other header fields not defined by this specification.
1608</t>
1609<t>
1610   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1611   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1612   neither of which includes a message body,
1613   to indicate that the origin server would have applied a transfer coding
1614   to the message body if the request had been an unconditional GET.
1615   This indication is not required, however, because any recipient on
1616   the response chain (including the origin server) can remove transfer
1617   codings when they are not needed.
1618</t>
1619<t>
1620   A server &MUST-NOT; send a Transfer-Encoding header field in any response
1621   with a status code of
1622   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1623   A server &MUST-NOT; send a Transfer-Encoding header field in any
1624   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1625</t>
1626<t>
1627   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1628   implementations advertising only HTTP/1.0 support will not understand
1629   how to process a transfer-encoded payload.
1630   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1631   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1632   might be in the form of specific user configuration or by remembering the
1633   version of a prior received response.
1634   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1635   the corresponding request indicates HTTP/1.1 (or later).
1636</t>
1637<t>
1638   A server that receives a request message with a transfer coding it does
1639   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref>.
1640</t>
1641</section>
1642
1643<section title="Content-Length" anchor="header.content-length">
1644  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1645  <x:anchor-alias value="Content-Length"/>
1646<t>
1647   When a message does not have a <x:ref>Transfer-Encoding</x:ref> header
1648   field, a Content-Length header field can provide the anticipated size,
1649   as a decimal number of octets, for a potential payload body.
1650   For messages that do include a payload body, the Content-Length field-value
1651   provides the framing information necessary for determining where the body
1652   (and message) ends.  For messages that do not include a payload body, the
1653   Content-Length indicates the size of the selected representation
1654   (&representation;).
1655</t>
1656<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1657  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1658</artwork></figure>
1659<t>
1660   An example is
1661</t>
1662<figure><artwork type="example">
1663  Content-Length: 3495
1664</artwork></figure>
1665<t>
1666   A sender &MUST-NOT; send a Content-Length header field in any message that
1667   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1668</t>
1669<t>
1670   A user agent &SHOULD; send a Content-Length in a request message when no
1671   <x:ref>Transfer-Encoding</x:ref> is sent and the request method defines
1672   a meaning for an enclosed payload body. For example, a Content-Length
1673   header field is normally sent in a POST request even when the value is
1674   0 (indicating an empty payload body).  A user agent &SHOULD-NOT; send a
1675   Content-Length header field when the request message does not contain a
1676   payload body and the method semantics do not anticipate such a body.
1677</t>
1678<t>
1679   A server &MAY; send a Content-Length header field in a response to a HEAD
1680   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1681   response unless its field-value equals the decimal number of octets that
1682   would have been sent in the payload body of a response if the same
1683   request had used the GET method.
1684</t>
1685<t>
1686   A server &MAY; send a Content-Length header field in a
1687   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1688   (&status-304;); a server &MUST-NOT; send Content-Length in such a
1689   response unless its field-value equals the decimal number of octets that
1690   would have been sent in the payload body of a <x:ref>200 (OK)</x:ref>
1691   response to the same request.
1692</t>
1693<t>
1694   A server &MUST-NOT; send a Content-Length header field in any response
1695   with a status code of
1696   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1697   A server &MUST-NOT; send a Content-Length header field in any
1698   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1699</t>
1700<t>
1701   Aside from the cases defined above, in the absence of Transfer-Encoding,
1702   an origin server &SHOULD; send a Content-Length header field when the
1703   payload body size is known prior to sending the complete header block.
1704   This will allow downstream recipients to measure transfer progress,
1705   know when a received message is complete, and potentially reuse the
1706   connection for additional requests.
1707</t>
1708<t>
1709   Any Content-Length field value greater than or equal to zero is valid.
1710   Since there is no predefined limit to the length of a payload,
1711   recipients &SHOULD; anticipate potentially large decimal numerals and
1712   prevent parsing errors due to integer conversion overflows
1713   (<xref target="attack.protocol.element.size.overflows"/>).
1714</t>
1715<t>
1716   If a message is received that has multiple Content-Length header fields
1717   with field-values consisting of the same decimal value, or a single
1718   Content-Length header field with a field value containing a list of
1719   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1720   duplicate Content-Length header fields have been generated or combined by an
1721   upstream message processor, then the recipient &MUST; either reject the
1722   message as invalid or replace the duplicated field-values with a single
1723   valid Content-Length field containing that decimal value prior to
1724   determining the message body length or forwarding the message.
1725</t>
1726<x:note>
1727  <t>
1728   &Note; HTTP's use of Content-Length for message framing differs
1729   significantly from the same field's use in MIME, where it is an optional
1730   field used only within the "message/external-body" media-type.
1731  </t>
1732</x:note>
1733</section>
1734
1735<section title="Message Body Length" anchor="message.body.length">
1736  <iref item="chunked (Coding Format)"/>
1737<t>
1738   The length of a message body is determined by one of the following
1739   (in order of precedence):
1740</t>
1741<t>
1742  <list style="numbers">
1743    <x:lt><t>
1744     Any response to a HEAD request and any response with a
1745     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1746     <x:ref>304 (Not Modified)</x:ref> status code is always
1747     terminated by the first empty line after the header fields, regardless of
1748     the header fields present in the message, and thus cannot contain a
1749     message body.
1750    </t></x:lt>
1751    <x:lt><t>
1752     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1753     connection will become a tunnel immediately after the empty line that
1754     concludes the header fields.  A client &MUST; ignore any
1755     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1756     fields received in such a message.
1757    </t></x:lt>
1758    <x:lt><t>
1759     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1760     and the chunked transfer coding (<xref target="chunked.encoding"/>)
1761     is the final encoding, the message body length is determined by reading
1762     and decoding the chunked data until the transfer coding indicates the
1763     data is complete.
1764    </t>
1765    <t>
1766     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1767     response and the chunked transfer coding is not the final encoding, the
1768     message body length is determined by reading the connection until it is
1769     closed by the server.
1770     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a request and the
1771     chunked transfer coding is not the final encoding, the message body
1772     length cannot be determined reliably; the server &MUST; respond with
1773     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1774    </t>
1775    <t>
1776     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1777     and a <x:ref>Content-Length</x:ref> header field, the Transfer-Encoding
1778     overrides the Content-Length. Such a message might indicate an attempt
1779     to perform request or response smuggling (bypass of security-related
1780     checks on message routing or content) and thus ought to be handled as
1781     an error.  A sender &MUST; remove the received Content-Length field
1782     prior to forwarding such a message downstream.
1783    </t></x:lt>
1784    <x:lt><t>
1785     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1786     either multiple <x:ref>Content-Length</x:ref> header fields having
1787     differing field-values or a single Content-Length header field having an
1788     invalid value, then the message framing is invalid and
1789     the recipient &MUST; treat it as an unrecoverable error to prevent
1790     request or response smuggling.
1791     If this is a request message, the server &MUST; respond with
1792     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1793     If this is a response message received by a proxy,
1794     the proxy &MUST; close the connection to the server, discard the received
1795     response, and send a <x:ref>502 (Bad Gateway)</x:ref> response to the
1796     client.
1797     If this is a response message received by a user agent,
1798     the user agent &MUST; close the connection to the server and discard the
1799     received response.
1800    </t></x:lt>
1801    <x:lt><t>
1802     If a valid <x:ref>Content-Length</x:ref> header field is present without
1803     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1804     expected message body length in octets.
1805     If the sender closes the connection or the recipient times out before the
1806     indicated number of octets are received, the recipient &MUST; consider
1807     the message to be incomplete and close the connection.
1808    </t></x:lt>
1809    <x:lt><t>
1810     If this is a request message and none of the above are true, then the
1811     message body length is zero (no message body is present).
1812    </t></x:lt>
1813    <x:lt><t>
1814     Otherwise, this is a response message without a declared message body
1815     length, so the message body length is determined by the number of octets
1816     received prior to the server closing the connection.
1817    </t></x:lt>
1818  </list>
1819</t>
1820<t>
1821   Since there is no way to distinguish a successfully completed,
1822   close-delimited message from a partially-received message interrupted
1823   by network failure, a server &SHOULD; use encoding or
1824   length-delimited messages whenever possible.  The close-delimiting
1825   feature exists primarily for backwards compatibility with HTTP/1.0.
1826</t>
1827<t>
1828   A server &MAY; reject a request that contains a message body but
1829   not a <x:ref>Content-Length</x:ref> by responding with
1830   <x:ref>411 (Length Required)</x:ref>.
1831</t>
1832<t>
1833   Unless a transfer coding other than chunked has been applied,
1834   a client that sends a request containing a message body &SHOULD;
1835   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1836   length is known in advance, rather than the chunked transfer coding, since some
1837   existing services respond to chunked with a <x:ref>411 (Length Required)</x:ref>
1838   status code even though they understand the chunked transfer coding.  This
1839   is typically because such services are implemented via a gateway that
1840   requires a content-length in advance of being called and the server
1841   is unable or unwilling to buffer the entire request before processing.
1842</t>
1843<t>
1844   A user agent that sends a request containing a message body &MUST; send a
1845   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1846   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1847   the form of specific user configuration or by remembering the version of a
1848   prior received response.
1849</t>
1850<t>
1851   If the final response to the last request on a connection has been
1852   completely received and there remains additional data to read, a user agent
1853   &MAY; discard the remaining data or attempt to determine if that data
1854   belongs as part of the prior response body, which might be the case if the
1855   prior message's Content-Length value is incorrect. A client &MUST-NOT;
1856   process, cache, or forward such extra data as a separate response, since
1857   such behavior would be vulnerable to cache poisoning.
1858</t>
1859</section>
1860</section>
1861
1862<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1863<t>
1864   A server that receives an incomplete request message, usually due to a
1865   canceled request or a triggered time-out exception, &MAY; send an error
1866   response prior to closing the connection.
1867</t>
1868<t>
1869   A client that receives an incomplete response message, which can occur
1870   when a connection is closed prematurely or when decoding a supposedly
1871   chunked transfer coding fails, &MUST; record the message as incomplete.
1872   Cache requirements for incomplete responses are defined in
1873   &cache-incomplete;.
1874</t>
1875<t>
1876   If a response terminates in the middle of the header block (before the
1877   empty line is received) and the status code might rely on header fields to
1878   convey the full meaning of the response, then the client cannot assume
1879   that meaning has been conveyed; the client might need to repeat the
1880   request in order to determine what action to take next.
1881</t>
1882<t>
1883   A message body that uses the chunked transfer coding is
1884   incomplete if the zero-sized chunk that terminates the encoding has not
1885   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1886   incomplete if the size of the message body received (in octets) is less than
1887   the value given by Content-Length.  A response that has neither chunked
1888   transfer coding nor Content-Length is terminated by closure of the
1889   connection, and thus is considered complete regardless of the number of
1890   message body octets received, provided that the header block was received
1891   intact.
1892</t>
1893</section>
1894
1895<section title="Message Parsing Robustness" anchor="message.robustness">
1896<t>
1897   Older HTTP/1.0 user agent implementations might send an extra CRLF
1898   after a POST request as a workaround for some early server
1899   applications that failed to read message body content that was
1900   not terminated by a line-ending. An HTTP/1.1 user agent &MUST-NOT;
1901   preface or follow a request with an extra CRLF.  If terminating
1902   the request message body with a line-ending is desired, then the
1903   user agent &MUST; count the terminating CRLF octets as part of the
1904   message body length.
1905</t>
1906<t>
1907   In the interest of robustness, servers &SHOULD; ignore at least one
1908   empty line received where a request-line is expected. In other words, if
1909   a server is reading the protocol stream at the beginning of a
1910   message and receives a CRLF first, the server &SHOULD; ignore the CRLF.
1911</t>
1912<t>
1913   Although the line terminator for the start-line and header
1914   fields is the sequence CRLF, recipients &MAY; recognize a
1915   single LF as a line terminator and ignore any preceding CR.
1916</t>
1917<t>
1918   Although the request-line and status-line grammar rules require that each
1919   of the component elements be separated by a single SP octet, recipients
1920   &MAY; instead parse on whitespace-delimited word boundaries and, aside
1921   from the CRLF terminator, treat any form of whitespace as the SP separator
1922   while ignoring preceding or trailing whitespace;
1923   such whitespace includes one or more of the following octets:
1924   SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
1925</t>
1926<t>
1927   When a server listening only for HTTP request messages, or processing
1928   what appears from the start-line to be an HTTP request message,
1929   receives a sequence of octets that does not match the HTTP-message
1930   grammar aside from the robustness exceptions listed above, the
1931   server &SHOULD; respond with a <x:ref>400 (Bad Request)</x:ref> response. 
1932</t>
1933</section>
1934</section>
1935
1936<section title="Transfer Codings" anchor="transfer.codings">
1937  <x:anchor-alias value="transfer-coding"/>
1938  <x:anchor-alias value="transfer-extension"/>
1939<t>
1940   Transfer coding names are used to indicate an encoding
1941   transformation that has been, can be, or might need to be applied to a
1942   payload body in order to ensure "safe transport" through the network.
1943   This differs from a content coding in that the transfer coding is a
1944   property of the message rather than a property of the representation
1945   that is being transferred.
1946</t>
1947<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1948  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1949                     / "compress" ; <xref target="compress.coding"/>
1950                     / "deflate" ; <xref target="deflate.coding"/>
1951                     / "gzip" ; <xref target="gzip.coding"/>
1952                     / <x:ref>transfer-extension</x:ref>
1953  <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> )
1954</artwork></figure>
1955<t anchor="rule.parameter">
1956  <x:anchor-alias value="attribute"/>
1957  <x:anchor-alias value="transfer-parameter"/>
1958  <x:anchor-alias value="value"/>
1959   Parameters are in the form of attribute/value pairs.
1960</t>
1961<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"/>
1962  <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>
1963  <x:ref>attribute</x:ref>          = <x:ref>token</x:ref>
1964  <x:ref>value</x:ref>              = <x:ref>word</x:ref>
1965</artwork></figure>
1966<t>
1967   All transfer-coding names are case-insensitive and ought to be registered
1968   within the HTTP Transfer Coding registry, as defined in
1969   <xref target="transfer.coding.registry"/>.
1970   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1971   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1972   header fields.
1973</t>
1974
1975<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1976  <iref primary="true" item="chunked (Coding Format)"/>
1977  <x:anchor-alias value="chunk"/>
1978  <x:anchor-alias value="chunked-body"/>
1979  <x:anchor-alias value="chunk-data"/>
1980  <x:anchor-alias value="chunk-ext"/>
1981  <x:anchor-alias value="chunk-ext-name"/>
1982  <x:anchor-alias value="chunk-ext-val"/>
1983  <x:anchor-alias value="chunk-size"/>
1984  <x:anchor-alias value="last-chunk"/>
1985  <x:anchor-alias value="quoted-str-nf"/>
1986  <x:anchor-alias value="qdtext-nf"/>
1987<t>
1988   The chunked transfer coding modifies the body of a message in order to
1989   transfer it as a series of chunks, each with its own size indicator,
1990   followed by an &OPTIONAL; trailer containing header fields. This
1991   allows dynamically generated content to be transferred along with the
1992   information necessary for the recipient to verify that it has
1993   received the full message.
1994</t>
1995<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="false" item="Grammar" subitem="trailer-part"/><iref primary="true" item="Grammar" subitem="quoted-str-nf"/><iref primary="true" item="Grammar" subitem="qdtext-nf"/>
1996  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1997                   <x:ref>last-chunk</x:ref>
1998                   <x:ref>trailer-part</x:ref>
1999                   <x:ref>CRLF</x:ref>
2000 
2001  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
2002                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
2003  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
2004  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
2005 
2006  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
2007  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
2008  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-str-nf</x:ref>
2009  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
2010 
2011  <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>
2012                 ; like <x:ref>quoted-string</x:ref>, but disallowing line folding
2013  <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>
2014</artwork></figure>
2015<t>
2016   Chunk extensions within the chunked transfer coding are deprecated.
2017   Senders &SHOULD-NOT; send chunk-ext.
2018   Definition of new chunk extensions is discouraged.
2019</t>
2020<t>
2021   The chunk-size field is a string of hex digits indicating the size of
2022   the chunk-data in octets. The chunked transfer coding is complete when a
2023   chunk with a chunk-size of zero is received, possibly followed by a
2024   trailer, and finally terminated by an empty line.
2025</t>
2026
2027<section title="Chunked Trailer Part" anchor="chunked.trailer.part">
2028  <x:anchor-alias value="trailer-part"/>
2029<t>
2030   A trailer allows the sender to include additional fields at the end of a
2031   chunked message in order to supply metadata that might be dynamically
2032   generated while the message body is sent, such as a message integrity
2033   check, digital signature, or post-processing status. The trailer fields are
2034   identical to header fields, except they are sent in a chunked trailer
2035   instead of the message header block.
2036</t>
2037<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="trailer-part"/>
2038  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
2039</artwork></figure>
2040<t>
2041   A sender &MUST-NOT; generate a trailer that contains a field which needs to
2042   be known by the recipient before it can begin processing the message body.
2043   For example, most recipients need to know the values of
2044   <x:ref>Content-Encoding</x:ref> and <x:ref>Content-Type</x:ref> in order to
2045   select a content handler, so placing those fields in a trailer would force
2046   the recipient to buffer the entire body before it could begin, greatly
2047   increasing user-perceived latency and defeating one of the main advantages
2048   of using chunked to send data streams of unknown length.
2049   A sender &MUST-NOT; generate a trailer containing a
2050   <x:ref>Transfer-Encoding</x:ref>,
2051   <x:ref>Content-Length</x:ref>, or
2052   <x:ref>Trailer</x:ref> field.
2053</t>
2054<t>
2055   A server &MUST; generate an empty trailer with the chunked transfer coding
2056   unless at least one of the following is true:
2057  <list style="numbers">
2058    <t>the request included a <x:ref>TE</x:ref> header field that indicates
2059    "trailers" is acceptable in the transfer coding of the response, as
2060    described in <xref target="header.te"/>; or,</t>
2061     
2062    <t>the trailer fields consist entirely of optional metadata and the
2063    recipient could use the message (in a manner acceptable to the generating
2064    server) without receiving that metadata. In other words, the generating
2065    server is willing to accept the possibility that the trailer fields might
2066    be silently discarded along the path to the client.</t>
2067  </list>
2068</t>
2069<t>
2070   The above requirement prevents the need for an infinite buffer when a
2071   message is being received by an HTTP/1.1 (or later) proxy and forwarded to
2072   an HTTP/1.0 recipient.
2073</t>
2074</section>
2075
2076<section title="Decoding Chunked" anchor="decoding.chunked">
2077<t>
2078   A process for decoding the chunked transfer coding
2079   can be represented in pseudo-code as:
2080</t>
2081<figure><artwork type="code">
2082  length := 0
2083  read chunk-size, chunk-ext (if any), and CRLF
2084  while (chunk-size &gt; 0) {
2085     read chunk-data and CRLF
2086     append chunk-data to decoded-body
2087     length := length + chunk-size
2088     read chunk-size, chunk-ext (if any), and CRLF
2089  }
2090  read header-field
2091  while (header-field not empty) {
2092     append header-field to existing header fields
2093     read header-field
2094  }
2095  Content-Length := length
2096  Remove "chunked" from Transfer-Encoding
2097  Remove Trailer from existing header fields
2098</artwork></figure>
2099<t>
2100   All recipients &MUST; be able to parse and decode the
2101   chunked transfer coding and &MUST; ignore chunk-ext extensions
2102   they do not understand.
2103</t>
2104</section>
2105</section>
2106
2107<section title="Compression Codings" anchor="compression.codings">
2108<t>
2109   The codings defined below can be used to compress the payload of a
2110   message.
2111</t>
2112
2113<section title="Compress Coding" anchor="compress.coding">
2114<iref item="compress (Coding Format)"/>
2115<t>
2116   The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding
2117   <xref target="Welch"/> that is commonly produced by the UNIX file
2118   compression program "compress".
2119   Recipients &SHOULD; consider "x-compress" to be equivalent to "compress".
2120</t>
2121</section>
2122
2123<section title="Deflate Coding" anchor="deflate.coding">
2124<iref item="deflate (Coding Format)"/>
2125<t>
2126   The "deflate" coding is a "zlib" data format <xref target="RFC1950"/>
2127   containing a "deflate" compressed data stream <xref target="RFC1951"/>
2128   that uses a combination of the Lempel-Ziv (LZ77) compression algorithm and
2129   Huffman coding.
2130</t>
2131<x:note>
2132  <t>
2133    &Note; Some incorrect implementations send the "deflate"
2134    compressed data without the zlib wrapper.
2135   </t>
2136</x:note>
2137</section>
2138
2139<section title="Gzip Coding" anchor="gzip.coding">
2140<iref item="gzip (Coding Format)"/>
2141<t>
2142   The "gzip" coding is an LZ77 coding with a 32 bit CRC that is commonly
2143   produced by the gzip file compression program <xref target="RFC1952"/>.
2144   Recipients &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2145</t>
2146</section>
2147
2148</section>
2149
2150<section title="TE" anchor="header.te">
2151  <iref primary="true" item="TE header field" x:for-anchor=""/>
2152  <x:anchor-alias value="TE"/>
2153  <x:anchor-alias value="t-codings"/>
2154  <x:anchor-alias value="t-ranking"/>
2155  <x:anchor-alias value="rank"/>
2156<t>
2157   The "TE" header field in a request indicates what transfer codings,
2158   besides chunked, the client is willing to accept in response, and
2159   whether or not the client is willing to accept trailer fields in a
2160   chunked transfer coding.
2161</t>
2162<t>
2163   The TE field-value consists of a comma-separated list of transfer coding
2164   names, each allowing for optional parameters (as described in
2165   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2166   Clients &MUST-NOT; send the chunked transfer coding name in TE;
2167   chunked is always acceptable for HTTP/1.1 recipients.
2168</t>
2169<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"/>
2170  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2171  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2172  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2173  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2174             / ( "1" [ "." 0*3("0") ] )
2175</artwork></figure>
2176<t>
2177   Three examples of TE use are below.
2178</t>
2179<figure><artwork type="example">
2180  TE: deflate
2181  TE:
2182  TE: trailers, deflate;q=0.5
2183</artwork></figure>
2184<t>
2185   The presence of the keyword "trailers" indicates that the client is willing
2186   to accept trailer fields in a chunked transfer coding, as defined in
2187   <xref target="chunked.trailer.part"/>, on behalf of itself and any downstream
2188   clients. For requests from an intermediary, this implies that either:
2189   (a) all downstream clients are willing to accept trailer fields in the
2190   forwarded response; or,
2191   (b) the intermediary will attempt to buffer the response on behalf of
2192   downstream recipients.
2193   Note that HTTP/1.1 does not define any means to limit the size of a
2194   chunked response such that an intermediary can be assured of buffering the
2195   entire response.
2196</t>
2197<t>
2198   When multiple transfer codings are acceptable, the client &MAY; rank the
2199   codings by preference using a case-insensitive "q" parameter (similar to
2200   the qvalues used in content negotiation fields, &qvalue;). The rank value
2201   is a real number in the range 0 through 1, where 0.001 is the least
2202   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2203</t>
2204<t>
2205   If the TE field-value is empty or if no TE field is present, the only
2206   acceptable transfer coding is chunked. A message with no transfer coding
2207   is always acceptable.
2208</t>
2209<t>
2210   Since the TE header field only applies to the immediate connection,
2211   a sender of TE &MUST; also send a "TE" connection option within the
2212   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2213   in order to prevent the TE field from being forwarded by intermediaries
2214   that do not support its semantics.
2215</t>
2216</section>
2217
2218<section title="Trailer" anchor="header.trailer">
2219  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
2220  <x:anchor-alias value="Trailer"/>
2221<t>
2222   When a message includes a message body encoded with the chunked
2223   transfer coding and the sender desires to send metadata in the form of
2224   trailer fields at the end of the message, the sender &SHOULD; generate a
2225   <x:ref>Trailer</x:ref> header field before the message body to indicate
2226   which fields will be present in the trailers. This allows the recipient
2227   to prepare for receipt of that metadata before it starts processing the body,
2228   which is useful if the message is being streamed and the recipient wishes
2229   to confirm an integrity check on the fly.
2230</t>
2231<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
2232  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
2233</artwork></figure>
2234</section>
2235</section>
2236
2237<section title="Message Routing" anchor="message.routing">
2238<t>
2239   HTTP request message routing is determined by each client based on the
2240   target resource, the client's proxy configuration, and
2241   establishment or reuse of an inbound connection.  The corresponding
2242   response routing follows the same connection chain back to the client.
2243</t>
2244
2245<section title="Identifying a Target Resource" anchor="target-resource">
2246  <iref primary="true" item="target resource"/>
2247  <iref primary="true" item="target URI"/>
2248  <x:anchor-alias value="target resource"/>
2249  <x:anchor-alias value="target URI"/>
2250<t>
2251   HTTP is used in a wide variety of applications, ranging from
2252   general-purpose computers to home appliances.  In some cases,
2253   communication options are hard-coded in a client's configuration.
2254   However, most HTTP clients rely on the same resource identification
2255   mechanism and configuration techniques as general-purpose Web browsers.
2256</t>
2257<t>
2258   HTTP communication is initiated by a user agent for some purpose.
2259   The purpose is a combination of request semantics, which are defined in
2260   <xref target="Part2"/>, and a target resource upon which to apply those
2261   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2262   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2263   would resolve to its absolute form in order to obtain the
2264   "<x:dfn>target URI</x:dfn>".  The target URI
2265   excludes the reference's fragment component, if any,
2266   since fragment identifiers are reserved for client-side processing
2267   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2268</t>
2269</section>
2270
2271<section title="Connecting Inbound" anchor="connecting.inbound">
2272<t>
2273   Once the target URI is determined, a client needs to decide whether
2274   a network request is necessary to accomplish the desired semantics and,
2275   if so, where that request is to be directed.
2276</t>
2277<t>
2278   If the client has a cache <xref target="Part6"/> and the request can be
2279   satisfied by it, then the request is
2280   usually directed there first.
2281</t>
2282<t>
2283   If the request is not satisfied by a cache, then a typical client will
2284   check its configuration to determine whether a proxy is to be used to
2285   satisfy the request.  Proxy configuration is implementation-dependent,
2286   but is often based on URI prefix matching, selective authority matching,
2287   or both, and the proxy itself is usually identified by an "http" or
2288   "https" URI.  If a proxy is applicable, the client connects inbound by
2289   establishing (or reusing) a connection to that proxy.
2290</t>
2291<t>
2292   If no proxy is applicable, a typical client will invoke a handler routine,
2293   usually specific to the target URI's scheme, to connect directly
2294   to an authority for the target resource.  How that is accomplished is
2295   dependent on the target URI scheme and defined by its associated
2296   specification, similar to how this specification defines origin server
2297   access for resolution of the "http" (<xref target="http.uri"/>) and
2298   "https" (<xref target="https.uri"/>) schemes.
2299</t>
2300<t>
2301   HTTP requirements regarding connection management are defined in
2302   <xref target="connection.management"/>.
2303</t>
2304</section>
2305
2306<section title="Request Target" anchor="request-target">
2307<t>
2308   Once an inbound connection is obtained,
2309   the client sends an HTTP request message (<xref target="http.message"/>)
2310   with a request-target derived from the target URI.
2311   There are four distinct formats for the request-target, depending on both
2312   the method being requested and whether the request is to a proxy.
2313</t>   
2314<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"/>
2315  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2316                 / <x:ref>absolute-form</x:ref>
2317                 / <x:ref>authority-form</x:ref>
2318                 / <x:ref>asterisk-form</x:ref>
2319
2320  <x:ref>origin-form</x:ref>    = <x:ref>absolute-path</x:ref> [ "?" <x:ref>query</x:ref> ]
2321  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2322  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2323  <x:ref>asterisk-form</x:ref>  = "*"
2324</artwork></figure>
2325<t anchor="origin-form"><iref item="origin-form (of request-target)"/>
2326  <x:h>origin-form</x:h>
2327</t>
2328<t>
2329   The most common form of request-target is the <x:dfn>origin-form</x:dfn>.
2330   When making a request directly to an origin server, other than a CONNECT
2331   or server-wide OPTIONS request (as detailed below),
2332   a client &MUST; send only the absolute path and query components of
2333   the target URI as the request-target.
2334   If the target URI's path component is empty, then the client &MUST; send
2335   "/" as the path within the origin-form of request-target.
2336   A <x:ref>Host</x:ref> header field is also sent, as defined in
2337   <xref target="header.host"/>.
2338</t>
2339<t>
2340   For example, a client wishing to retrieve a representation of the resource
2341   identified as
2342</t>
2343<figure><artwork x:indent-with="  " type="example">
2344http://www.example.org/where?q=now
2345</artwork></figure>
2346<t>
2347   directly from the origin server would open (or reuse) a TCP connection
2348   to port 80 of the host "www.example.org" and send the lines:
2349</t>
2350<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2351GET /where?q=now HTTP/1.1
2352Host: www.example.org
2353</artwork></figure>
2354<t>
2355   followed by the remainder of the request message.
2356</t>
2357<t anchor="absolute-form"><iref item="absolute-form (of request-target)"/>
2358  <x:h>absolute-form</x:h>
2359</t>
2360<t>
2361   When making a request to a proxy, other than a CONNECT or server-wide
2362   OPTIONS request (as detailed below), a client &MUST; send the target URI
2363   in <x:dfn>absolute-form</x:dfn> as the request-target.
2364   The proxy is requested to either service that request from a valid cache,
2365   if possible, or make the same request on the client's behalf to either
2366   the next inbound proxy server or directly to the origin server indicated
2367   by the request-target.  Requirements on such "forwarding" of messages are
2368   defined in <xref target="message.forwarding"/>.
2369</t>
2370<t>
2371   An example absolute-form of request-line would be:
2372</t>
2373<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2374GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
2375</artwork></figure>
2376<t>
2377   To allow for transition to the absolute-form for all requests in some
2378   future version of HTTP, HTTP/1.1 servers &MUST; accept the absolute-form
2379   in requests, even though HTTP/1.1 clients will only send them in requests
2380   to proxies.
2381</t>
2382<t anchor="authority-form"><iref item="authority-form (of request-target)"/>
2383  <x:h>authority-form</x:h>
2384</t>
2385<t>
2386   The <x:dfn>authority-form</x:dfn> of request-target is only used for
2387   CONNECT requests (&CONNECT;). When making a CONNECT request to establish a
2388   tunnel through one or more proxies, a client &MUST; send only the target
2389   URI's authority component (excluding any userinfo and its "@" delimiter) as
2390   the request-target. For example,
2391</t>
2392<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2393CONNECT www.example.com:80 HTTP/1.1
2394</artwork></figure>
2395<t anchor="asterisk-form"><iref item="asterisk-form (of request-target)"/>
2396  <x:h>asterisk-form</x:h>
2397</t>
2398<t>
2399   The <x:dfn>asterisk-form</x:dfn> of request-target is only used for a server-wide
2400   OPTIONS request (&OPTIONS;).  When a client wishes to request OPTIONS
2401   for the server as a whole, as opposed to a specific named resource of
2402   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2403   For example,
2404</t>
2405<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2406OPTIONS * HTTP/1.1
2407</artwork></figure>
2408<t>
2409   If a proxy receives an OPTIONS request with an absolute-form of
2410   request-target in which the URI has an empty path and no query component,
2411   then the last proxy on the request chain &MUST; send a request-target
2412   of "*" when it forwards the request to the indicated origin server.
2413</t>
2414<figure><preamble>  
2415   For example, the request
2416</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2417OPTIONS http://www.example.org:8001 HTTP/1.1
2418</artwork></figure>
2419<figure><preamble>  
2420  would be forwarded by the final proxy as
2421</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2422OPTIONS * HTTP/1.1
2423Host: www.example.org:8001
2424</artwork>
2425<postamble>
2426   after connecting to port 8001 of host "www.example.org".
2427</postamble>
2428</figure>
2429</section>
2430
2431<section title="Host" anchor="header.host">
2432  <iref primary="true" item="Host header field" x:for-anchor=""/>
2433  <x:anchor-alias value="Host"/>
2434<t>
2435   The "Host" header field in a request provides the host and port
2436   information from the target URI, enabling the origin
2437   server to distinguish among resources while servicing requests
2438   for multiple host names on a single IP address.
2439</t>
2440<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2441  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2442</artwork></figure>
2443<t>
2444   A client &MUST; send a Host header field in all HTTP/1.1 request messages.
2445   If the target URI includes an authority component, then a client &MUST;
2446   send a field-value for Host that is identical to that authority
2447   component, excluding any userinfo subcomponent and its "@" delimiter
2448   (<xref target="http.uri"/>).
2449   If the authority component is missing or undefined for the target URI,
2450   then a client &MUST; send a Host header field with an empty field-value.
2451</t>
2452<t>
2453   Since the Host field-value is critical information for handling a request,
2454   a user agent &SHOULD; generate Host as the first header field following the
2455   request-line.
2456</t>
2457<t>
2458   For example, a GET request to the origin server for
2459   &lt;http://www.example.org/pub/WWW/&gt; would begin with:
2460</t>
2461<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2462GET /pub/WWW/ HTTP/1.1
2463Host: www.example.org
2464</artwork></figure>
2465<t>
2466   A client &MUST; send a Host header field in an HTTP/1.1 request even
2467   if the request-target is in the absolute-form, since this
2468   allows the Host information to be forwarded through ancient HTTP/1.0
2469   proxies that might not have implemented Host.
2470</t>
2471<t>
2472   When a proxy receives a request with an absolute-form of
2473   request-target, the proxy &MUST; ignore the received
2474   Host header field (if any) and instead replace it with the host
2475   information of the request-target.  A proxy that forwards such a request
2476   &MUST; generate a new Host field-value based on the received
2477   request-target rather than forward the received Host field-value.
2478</t>
2479<t>
2480   Since the Host header field acts as an application-level routing
2481   mechanism, it is a frequent target for malware seeking to poison
2482   a shared cache or redirect a request to an unintended server.
2483   An interception proxy is particularly vulnerable if it relies on
2484   the Host field-value for redirecting requests to internal
2485   servers, or for use as a cache key in a shared cache, without
2486   first verifying that the intercepted connection is targeting a
2487   valid IP address for that host.
2488</t>
2489<t>
2490   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2491   to any HTTP/1.1 request message that lacks a Host header field and
2492   to any request message that contains more than one Host header field
2493   or a Host header field with an invalid field-value.
2494</t>
2495</section>
2496
2497<section title="Effective Request URI" anchor="effective.request.uri">
2498  <iref primary="true" item="effective request URI"/>
2499  <x:anchor-alias value="effective request URI"/>
2500<t>
2501   A server that receives an HTTP request message &MUST; reconstruct
2502   the user agent's original target URI, based on the pieces of information
2503   learned from the request-target, <x:ref>Host</x:ref> header field, and
2504   connection context, in order to identify the intended target resource and
2505   properly service the request. The URI derived from this reconstruction
2506   process is referred to as the "<x:dfn>effective request URI</x:dfn>".
2507</t>
2508<t>
2509   For a user agent, the effective request URI is the target URI.
2510</t>
2511<t>
2512   If the request-target is in absolute-form, then the effective request URI
2513   is the same as the request-target.  Otherwise, the effective request URI
2514   is constructed as follows.
2515</t>
2516<t>
2517   If the request is received over a TLS-secured TCP connection,
2518   then the effective request URI's scheme is "https"; otherwise, the
2519   scheme is "http".
2520</t>
2521<t>
2522   If the request-target is in authority-form, then the effective
2523   request URI's authority component is the same as the request-target.
2524   Otherwise, if a <x:ref>Host</x:ref> header field is supplied with a
2525   non-empty field-value, then the authority component is the same as the
2526   Host field-value. Otherwise, the authority component is the concatenation of
2527   the default host name configured for the server, a colon (":"), and the
2528   connection's incoming TCP port number in decimal form.
2529</t>
2530<t>
2531   If the request-target is in authority-form or asterisk-form, then the
2532   effective request URI's combined path and query component is empty.
2533   Otherwise, the combined path and query component is the same as the
2534   request-target.
2535</t>
2536<t>
2537   The components of the effective request URI, once determined as above,
2538   can be combined into absolute-URI form by concatenating the scheme,
2539   "://", authority, and combined path and query component.
2540</t>
2541<figure>
2542<preamble>
2543   Example 1: the following message received over an insecure TCP connection
2544</preamble>
2545<artwork type="example" x:indent-with="  ">
2546GET /pub/WWW/TheProject.html HTTP/1.1
2547Host: www.example.org:8080
2548</artwork>
2549</figure>
2550<figure>
2551<preamble>
2552  has an effective request URI of
2553</preamble>
2554<artwork type="example" x:indent-with="  ">
2555http://www.example.org:8080/pub/WWW/TheProject.html
2556</artwork>
2557</figure>
2558<figure>
2559<preamble>
2560   Example 2: the following message received over a TLS-secured TCP connection
2561</preamble>
2562<artwork type="example" x:indent-with="  ">
2563OPTIONS * HTTP/1.1
2564Host: www.example.org
2565</artwork>
2566</figure>
2567<figure>
2568<preamble>
2569  has an effective request URI of
2570</preamble>
2571<artwork type="example" x:indent-with="  ">
2572https://www.example.org
2573</artwork>
2574</figure>
2575<t>
2576   An origin server that does not allow resources to differ by requested
2577   host &MAY; ignore the <x:ref>Host</x:ref> field-value and instead replace it
2578   with a configured server name when constructing the effective request URI.
2579</t>
2580<t>
2581   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2582   field &MAY; attempt to use heuristics (e.g., examination of the URI path for
2583   something unique to a particular host) in order to guess the
2584   effective request URI's authority component.
2585</t>
2586</section>
2587
2588<section title="Associating a Response to a Request" anchor="associating.response.to.request">
2589<t>
2590   HTTP does not include a request identifier for associating a given
2591   request message with its corresponding one or more response messages.
2592   Hence, it relies on the order of response arrival to correspond exactly
2593   to the order in which requests are made on the same connection.
2594   More than one response message per request only occurs when one or more
2595   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a
2596   final response to the same request.
2597</t>
2598<t>
2599   A client that has more than one outstanding request on a connection &MUST;
2600   maintain a list of outstanding requests in the order sent and &MUST;
2601   associate each received response message on that connection to the highest
2602   ordered request that has not yet received a final (non-<x:ref>1xx</x:ref>)
2603   response.
2604</t>
2605</section>
2606
2607<section title="Message Forwarding" anchor="message.forwarding">
2608<t>
2609   As described in <xref target="intermediaries"/>, intermediaries can serve
2610   a variety of roles in the processing of HTTP requests and responses.
2611   Some intermediaries are used to improve performance or availability.
2612   Others are used for access control or to filter content.
2613   Since an HTTP stream has characteristics similar to a pipe-and-filter
2614   architecture, there are no inherent limits to the extent an intermediary
2615   can enhance (or interfere) with either direction of the stream.
2616</t>
2617<t>
2618   An intermediary not acting as a tunnel &MUST; implement the
2619   <x:ref>Connection</x:ref> header field, as specified in
2620   <xref target="header.connection"/>, and exclude fields from being forwarded
2621   that are only intended for the incoming connection.
2622</t>
2623<t>
2624   An intermediary &MUST-NOT; forward a message to itself unless it is
2625   protected from an infinite request loop. In general, an intermediary ought
2626   to recognize its own server names, including any aliases, local variations,
2627   or literal IP addresses, and respond to such requests directly.
2628</t>
2629
2630<section title="Via" anchor="header.via">
2631  <iref primary="true" item="Via header field" x:for-anchor=""/>
2632  <x:anchor-alias value="pseudonym"/>
2633  <x:anchor-alias value="received-by"/>
2634  <x:anchor-alias value="received-protocol"/>
2635  <x:anchor-alias value="Via"/>
2636<t>
2637   The "Via" header field indicates the presence of intermediate protocols and
2638   recipients between the user agent and the server (on requests) or between
2639   the origin server and the client (on responses), similar to the
2640   "Received" header field in email
2641   (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2642   Via can be used for tracking message forwards,
2643   avoiding request loops, and identifying the protocol capabilities of
2644   senders along the request/response chain.
2645</t>
2646<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"/>
2647  <x:ref>Via</x:ref> = 1#( <x:ref>received-protocol</x:ref> <x:ref>RWS</x:ref> <x:ref>received-by</x:ref> [ <x:ref>RWS</x:ref> <x:ref>comment</x:ref> ] )
2648
2649  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2650                      ; see <xref target="header.upgrade"/>
2651  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2652  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2653</artwork></figure>
2654<t>
2655   Multiple Via field values represent each proxy or gateway that has
2656   forwarded the message. Each intermediary appends its own information
2657   about how the message was received, such that the end result is ordered
2658   according to the sequence of forwarding recipients.
2659</t>
2660<t>
2661   A proxy &MUST; send an appropriate Via header field, as described below, in
2662   each message that it forwards.
2663   An HTTP-to-HTTP gateway &MUST; send an appropriate Via header field in
2664   each inbound request message and &MAY; send a Via header field in
2665   forwarded response messages.
2666</t>
2667<t>
2668   For each intermediary, the received-protocol indicates the protocol and
2669   protocol version used by the upstream sender of the message. Hence, the
2670   Via field value records the advertised protocol capabilities of the
2671   request/response chain such that they remain visible to downstream
2672   recipients; this can be useful for determining what backwards-incompatible
2673   features might be safe to use in response, or within a later request, as
2674   described in <xref target="http.version"/>. For brevity, the protocol-name
2675   is omitted when the received protocol is HTTP.
2676</t>
2677<t>
2678   The received-by field is normally the host and optional port number of a
2679   recipient server or client that subsequently forwarded the message.
2680   However, if the real host is considered to be sensitive information, a
2681   sender &MAY; replace it with a pseudonym. If a port is not provided,
2682   a recipient &MAY; interpret that as meaning it was received on the default
2683   TCP port, if any, for the received-protocol.
2684</t>
2685<t>
2686   A sender &MAY; generate comments in the Via header field to identify the
2687   software of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2688   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2689   are optional and a recipient &MAY; remove them prior to forwarding the
2690   message.
2691</t>
2692<t>
2693   For example, a request message could be sent from an HTTP/1.0 user
2694   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2695   forward the request to a public proxy at p.example.net, which completes
2696   the request by forwarding it to the origin server at www.example.com.
2697   The request received by www.example.com would then have the following
2698   Via header field:
2699</t>
2700<figure><artwork type="example">
2701  Via: 1.0 fred, 1.1 p.example.net
2702</artwork></figure>
2703<t>
2704   An intermediary used as a portal through a network firewall
2705   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2706   region unless it is explicitly enabled to do so. If not enabled, such an
2707   intermediary &SHOULD; replace each received-by host of any host behind the
2708   firewall by an appropriate pseudonym for that host.
2709</t>
2710<t>
2711   An intermediary &MAY; combine an ordered subsequence of Via header
2712   field entries into a single such entry if the entries have identical
2713   received-protocol values. For example,
2714</t>
2715<figure><artwork type="example">
2716  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2717</artwork></figure>
2718<t>
2719  could be collapsed to
2720</t>
2721<figure><artwork type="example">
2722  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2723</artwork></figure>
2724<t>
2725   Senders &SHOULD-NOT; combine multiple entries unless they are all
2726   under the same organizational control and the hosts have already been
2727   replaced by pseudonyms. Senders &MUST-NOT; combine entries that
2728   have different received-protocol values.
2729</t>
2730</section>
2731
2732<section title="Transformations" anchor="message.transformations">
2733<t>
2734   Some intermediaries include features for transforming messages and their
2735   payloads.  A transforming proxy might, for example, convert between image
2736   formats in order to save cache space or to reduce the amount of traffic on
2737   a slow link. However, operational problems might occur when these
2738   transformations are applied to payloads intended for critical applications,
2739   such as medical imaging or scientific data analysis, particularly when
2740   integrity checks or digital signatures are used to ensure that the payload
2741   received is identical to the original.
2742</t>
2743<t>
2744   If a proxy receives a request-target with a host name that is not a
2745   fully qualified domain name, it &MAY; add its own domain to the host name
2746   it received when forwarding the request.  A proxy &MUST-NOT; change the
2747   host name if it is a fully qualified domain name.
2748</t>
2749<t>
2750   A proxy &MUST-NOT; modify the "absolute-path" and "query" parts of the
2751   received request-target when forwarding it to the next inbound server,
2752   except as noted above to replace an empty path with "/" or "*".
2753</t>
2754<t>
2755   A proxy &MUST-NOT; modify header fields that provide information about the
2756   end points of the communication chain, the resource state, or the selected
2757   representation. A proxy &MAY; change the message body through application
2758   or removal of a transfer coding (<xref target="transfer.codings"/>).
2759</t>
2760<t>
2761   A non-transforming proxy &MUST-NOT; modify the message payload (&payload;).
2762   A transforming proxy &MUST-NOT; modify the payload of a message that
2763   contains the no-transform cache-control directive.
2764</t>
2765<t>
2766   A transforming proxy &MAY; transform the payload of a message
2767   that does not contain the no-transform cache-control directive;
2768   if the payload is transformed, the transforming proxy &MUST; add a
2769   Warning header field with the warn-code of 214 ("Transformation Applied")
2770   if one does not already appear in the message (see &header-warning;).
2771   If the payload of a <x:ref>200 (OK)</x:ref> response is transformed, the
2772   transforming proxy can also inform downstream recipients that a
2773   transformation has been applied by changing the response status code to
2774   <x:ref>203 (Non-Authoritative Information)</x:ref> (&status-203;).
2775</t>
2776</section>
2777</section>
2778</section>
2779
2780<section title="Connection Management" anchor="connection.management">
2781<t>
2782   HTTP messaging is independent of the underlying transport or
2783   session-layer connection protocol(s).  HTTP only presumes a reliable
2784   transport with in-order delivery of requests and the corresponding
2785   in-order delivery of responses.  The mapping of HTTP request and
2786   response structures onto the data units of an underlying transport
2787   protocol is outside the scope of this specification.
2788</t>
2789<t>
2790   As described in <xref target="connecting.inbound"/>, the specific
2791   connection protocols to be used for an HTTP interaction are determined by
2792   client configuration and the <x:ref>target URI</x:ref>.
2793   For example, the "http" URI scheme
2794   (<xref target="http.uri"/>) indicates a default connection of TCP
2795   over IP, with a default TCP port of 80, but the client might be
2796   configured to use a proxy via some other connection, port, or protocol.
2797</t>
2798<t>
2799   HTTP implementations are expected to engage in connection management,
2800   which includes maintaining the state of current connections,
2801   establishing a new connection or reusing an existing connection,
2802   processing messages received on a connection, detecting connection
2803   failures, and closing each connection.
2804   Most clients maintain multiple connections in parallel, including
2805   more than one connection per server endpoint.
2806   Most servers are designed to maintain thousands of concurrent connections,
2807   while controlling request queues to enable fair use and detect
2808   denial of service attacks.
2809</t>
2810
2811<section title="Connection" anchor="header.connection">
2812  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2813  <iref primary="true" item="close" x:for-anchor=""/>
2814  <x:anchor-alias value="Connection"/>
2815  <x:anchor-alias value="connection-option"/>
2816  <x:anchor-alias value="close"/>
2817<t>
2818   The "Connection" header field allows the sender to indicate desired
2819   control options for the current connection.  In order to avoid confusing
2820   downstream recipients, a proxy or gateway &MUST; remove or replace any
2821   received connection options before forwarding the message.
2822</t>
2823<t>
2824   When a header field aside from Connection is used to supply control
2825   information for or about the current connection, the sender &MUST; list
2826   the corresponding field-name within the "Connection" header field.
2827   A proxy or gateway &MUST; parse a received Connection
2828   header field before a message is forwarded and, for each
2829   connection-option in this field, remove any header field(s) from
2830   the message with the same name as the connection-option, and then
2831   remove the Connection header field itself (or replace it with the
2832   intermediary's own connection options for the forwarded message).
2833</t>
2834<t>
2835   Hence, the Connection header field provides a declarative way of
2836   distinguishing header fields that are only intended for the
2837   immediate recipient ("hop-by-hop") from those fields that are
2838   intended for all recipients on the chain ("end-to-end"), enabling the
2839   message to be self-descriptive and allowing future connection-specific
2840   extensions to be deployed without fear that they will be blindly
2841   forwarded by older intermediaries.
2842</t>
2843<t>
2844   The Connection header field's value has the following grammar:
2845</t>
2846<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2847  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2848  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2849</artwork></figure>
2850<t>
2851   Connection options are case-insensitive.
2852</t>
2853<t>
2854   A sender &MUST-NOT; send a connection option corresponding to a header
2855   field that is intended for all recipients of the payload.
2856   For example, <x:ref>Cache-Control</x:ref> is never appropriate as a
2857   connection option (&header-cache-control;).
2858</t>
2859<t>
2860   The connection options do not have to correspond to a header field
2861   present in the message, since a connection-specific header field
2862   might not be needed if there are no parameters associated with that
2863   connection option.  Recipients that trigger certain connection
2864   behavior based on the presence of connection options &MUST; do so
2865   based on the presence of the connection-option rather than only the
2866   presence of the optional header field.  In other words, if the
2867   connection option is received as a header field but not indicated
2868   within the Connection field-value, then the recipient &MUST; ignore
2869   the connection-specific header field because it has likely been
2870   forwarded by an intermediary that is only partially conformant.
2871</t>
2872<t>
2873   When defining new connection options, specifications ought to
2874   carefully consider existing deployed header fields and ensure
2875   that the new connection option does not share the same name as
2876   an unrelated header field that might already be deployed.
2877   Defining a new connection option essentially reserves that potential
2878   field-name for carrying additional information related to the
2879   connection option, since it would be unwise for senders to use
2880   that field-name for anything else.
2881</t>
2882<t>
2883   The "<x:dfn>close</x:dfn>" connection option is defined for a
2884   sender to signal that this connection will be closed after completion of
2885   the response. For example,
2886</t>
2887<figure><artwork type="example">
2888  Connection: close
2889</artwork></figure>
2890<t>
2891   in either the request or the response header fields indicates that the
2892   sender is going to close the connection after the current request/response
2893   is complete (<xref target="persistent.tear-down"/>).
2894</t>
2895<t>
2896   A client that does not support <x:ref>persistent connections</x:ref> &MUST;
2897   send the "close" connection option in every request message.
2898</t>
2899<t>
2900   A server that does not support <x:ref>persistent connections</x:ref> &MUST;
2901   send the "close" connection option in every response message that
2902   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2903</t>
2904</section>
2905
2906<section title="Establishment" anchor="persistent.establishment">
2907<t>
2908   It is beyond the scope of this specification to describe how connections
2909   are established via various transport or session-layer protocols.
2910   Each connection applies to only one transport link.
2911</t>
2912</section>
2913
2914<section title="Persistence" anchor="persistent.connections">
2915   <x:anchor-alias value="persistent connections"/>
2916<t>
2917   HTTP/1.1 defaults to the use of "<x:dfn>persistent connections</x:dfn>",
2918   allowing multiple requests and responses to be carried over a single
2919   connection. The "<x:ref>close</x:ref>" connection-option is used to signal
2920   that a connection will not persist after the current request/response.
2921   HTTP implementations &SHOULD; support persistent connections.
2922</t>
2923<t>
2924   A recipient determines whether a connection is persistent or not based on
2925   the most recently received message's protocol version and
2926   <x:ref>Connection</x:ref> header field (if any):
2927   <list style="symbols">
2928     <t>If the <x:ref>close</x:ref> connection option is present, the
2929        connection will not persist after the current response; else,</t>
2930     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2931        persist after the current response; else,</t>
2932     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2933        connection option is present, the recipient is not a proxy, and
2934        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2935        the connection will persist after the current response; otherwise,</t>
2936     <t>The connection will close after the current response.</t>
2937   </list>
2938</t>
2939<t>
2940   A server &MAY; assume that an HTTP/1.1 client intends to maintain a
2941   persistent connection until a <x:ref>close</x:ref> connection option
2942   is received in a request.
2943</t>
2944<t>
2945   A client &MAY; reuse a persistent connection until it sends or receives
2946   a <x:ref>close</x:ref> connection option or receives an HTTP/1.0 response
2947   without a "keep-alive" connection option.
2948</t>
2949<t>
2950   In order to remain persistent, all messages on a connection need to
2951   have a self-defined message length (i.e., one not defined by closure
2952   of the connection), as described in <xref target="message.body"/>.
2953   A server &MUST; read the entire request message body or close
2954   the connection after sending its response, since otherwise the
2955   remaining data on a persistent connection would be misinterpreted
2956   as the next request.  Likewise,
2957   a client &MUST; read the entire response message body if it intends
2958   to reuse the same connection for a subsequent request.
2959</t>
2960<t>
2961   A proxy server &MUST-NOT; maintain a persistent connection with an
2962   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
2963   information and discussion of the problems with the Keep-Alive header field
2964   implemented by many HTTP/1.0 clients).
2965</t>
2966<t>
2967   Clients and servers &SHOULD-NOT; assume that a persistent connection is
2968   maintained for HTTP versions less than 1.1 unless it is explicitly
2969   signaled.
2970   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
2971   for more information on backward compatibility with HTTP/1.0 clients.
2972</t>
2973
2974<section title="Retrying Requests" anchor="persistent.retrying.requests">
2975<t>
2976   Connections can be closed at any time, with or without intention.
2977   Implementations ought to anticipate the need to recover
2978   from asynchronous close events.
2979</t>
2980<t>
2981   When an inbound connection is closed prematurely, a client &MAY; open a new
2982   connection and automatically retransmit an aborted sequence of requests if
2983   all of those requests have idempotent methods (&idempotent-methods;).
2984   A proxy &MUST-NOT; automatically retry non-idempotent requests.
2985</t>
2986<t>
2987   A user agent &MUST-NOT; automatically retry a request with a non-idempotent
2988   method unless it has some means to know that the request semantics are
2989   actually idempotent, regardless of the method, or some means to detect that
2990   the original request was never applied. For example, a user agent that
2991   knows (through design or configuration) that a POST request to a given
2992   resource is safe can repeat that request automatically.
2993   Likewise, a user agent designed specifically to operate on a version
2994   control repository might be able to recover from partial failure conditions
2995   by checking the target resource revision(s) after a failed connection,
2996   reverting or fixing any changes that were partially applied, and then
2997   automatically retrying the requests that failed.
2998</t>
2999<t>
3000   A client &SHOULD-NOT; automatically retry a failed automatic retry.
3001</t>
3002</section>
3003
3004<section title="Pipelining" anchor="pipelining">
3005   <x:anchor-alias value="pipeline"/>
3006<t>
3007   A client that supports persistent connections &MAY; "<x:dfn>pipeline</x:dfn>"
3008   its requests (i.e., send multiple requests without waiting for each
3009   response). A server &MAY; process a sequence of pipelined requests in
3010   parallel if they all have safe methods (&safe-methods;), but &MUST; send
3011   the corresponding responses in the same order that the requests were
3012   received.
3013</t>
3014<t>
3015   A client that pipelines requests &SHOULD; retry unanswered requests if the
3016   connection closes before it receives all of the corresponding responses.
3017   When retrying pipelined requests after a failed connection (a connection
3018   not explicitly closed by the server in its last complete response), a
3019   client &MUST-NOT; pipeline immediately after connection establishment,
3020   since the first remaining request in the prior pipeline might have caused
3021   an error response that can be lost again if multiple requests are sent on a
3022   prematurely closed connection (see the TCP reset problem described in
3023   <xref target="persistent.tear-down"/>).
3024</t>
3025<t>
3026   Idempotent methods (&idempotent-methods;) are significant to pipelining
3027   because they can be automatically retried after a connection failure.
3028   A user agent &SHOULD-NOT; pipeline requests after a non-idempotent method,
3029   until the final response status code for that method has been received,
3030   unless the user agent has a means to detect and recover from partial
3031   failure conditions involving the pipelined sequence.
3032</t>
3033<t>
3034   An intermediary that receives pipelined requests &MAY; pipeline those
3035   requests when forwarding them inbound, since it can rely on the outbound
3036   user agent(s) to determine what requests can be safely pipelined. If the
3037   inbound connection fails before receiving a response, the pipelining
3038   intermediary &MAY; attempt to retry a sequence of requests that have yet
3039   to receive a response if the requests all have idempotent methods;
3040   otherwise, the pipelining intermediary &SHOULD; forward any received
3041   responses and then close the corresponding outbound connection(s) so that
3042   the outbound user agent(s) can recover accordingly.
3043</t>
3044</section>
3045</section>
3046   
3047<section title="Concurrency" anchor="persistent.concurrency">
3048<t>
3049   Clients &SHOULD; limit the number of simultaneous
3050   connections that they maintain to a given server.
3051</t>
3052<t>
3053   Previous revisions of HTTP gave a specific number of connections as a
3054   ceiling, but this was found to be impractical for many applications. As a
3055   result, this specification does not mandate a particular maximum number of
3056   connections, but instead encourages clients to be conservative when opening
3057   multiple connections.
3058</t>
3059<t>
3060   Multiple connections are typically used to avoid the "head-of-line
3061   blocking" problem, wherein a request that takes significant server-side
3062   processing and/or has a large payload blocks subsequent requests on the
3063   same connection. However, each connection consumes server resources.
3064   Furthermore, using multiple connections can cause undesirable side effects
3065   in congested networks.
3066</t>
3067<t>
3068   Note that servers might reject traffic that they deem abusive, including an
3069   excessive number of connections from a client.
3070</t>
3071</section>
3072
3073<section title="Failures and Time-outs" anchor="persistent.failures">
3074<t>
3075   Servers will usually have some time-out value beyond which they will
3076   no longer maintain an inactive connection. Proxy servers might make
3077   this a higher value since it is likely that the client will be making
3078   more connections through the same server. The use of persistent
3079   connections places no requirements on the length (or existence) of
3080   this time-out for either the client or the server.
3081</t>
3082<t>
3083   A client or server that wishes to time-out &SHOULD; issue a graceful close
3084   on the connection. Implementations &SHOULD; constantly monitor open
3085   connections for a received closure signal and respond to it as appropriate,
3086   since prompt closure of both sides of a connection enables allocated system
3087   resources to be reclaimed.
3088</t>
3089<t>
3090   A client, server, or proxy &MAY; close the transport connection at any
3091   time. For example, a client might have started to send a new request
3092   at the same time that the server has decided to close the "idle"
3093   connection. From the server's point of view, the connection is being
3094   closed while it was idle, but from the client's point of view, a
3095   request is in progress.
3096</t>
3097<t>
3098   Servers &SHOULD; maintain persistent connections and allow the underlying
3099   transport's flow control mechanisms to resolve temporary overloads, rather
3100   than terminate connections with the expectation that clients will retry.
3101   The latter technique can exacerbate network congestion.
3102</t>
3103<t>
3104   A client sending a message body &SHOULD; monitor
3105   the network connection for an error response while it is transmitting
3106   the request. If the client sees an error response, it &SHOULD;
3107   immediately cease transmitting the body and close the connection.
3108</t>
3109</section>
3110   
3111<section title="Tear-down" anchor="persistent.tear-down">
3112  <iref primary="false" item="Connection header field" x:for-anchor=""/>
3113  <iref primary="false" item="close" x:for-anchor=""/>
3114<t>
3115   The <x:ref>Connection</x:ref> header field
3116   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
3117   connection option that a sender &SHOULD; send when it wishes to close
3118   the connection after the current request/response pair.
3119</t>
3120<t>
3121   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
3122   send further requests on that connection (after the one containing
3123   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
3124   final response message corresponding to this request.
3125</t>
3126<t>
3127   A server that receives a <x:ref>close</x:ref> connection option &MUST;
3128   initiate a close of the connection (see below) after it sends the
3129   final response to the request that contained <x:ref>close</x:ref>.
3130   The server &SHOULD; send a <x:ref>close</x:ref> connection option
3131   in its final response on that connection. The server &MUST-NOT; process
3132   any further requests received on that connection.
3133</t>
3134<t>
3135   A server that sends a <x:ref>close</x:ref> connection option &MUST;
3136   initiate a close of the connection (see below) after it sends the
3137   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
3138   any further requests received on that connection.
3139</t>
3140<t>
3141   A client that receives a <x:ref>close</x:ref> connection option &MUST;
3142   cease sending requests on that connection and close the connection
3143   after reading the response message containing the close; if additional
3144   pipelined requests had been sent on the connection, the client &SHOULD-NOT;
3145   assume that they will be processed by the server.
3146</t>
3147<t>
3148   If a server performs an immediate close of a TCP connection, there is a
3149   significant risk that the client will not be able to read the last HTTP
3150   response.  If the server receives additional data from the client on a
3151   fully-closed connection, such as another request that was sent by the
3152   client before receiving the server's response, the server's TCP stack will
3153   send a reset packet to the client; unfortunately, the reset packet might
3154   erase the client's unacknowledged input buffers before they can be read
3155   and interpreted by the client's HTTP parser.
3156</t>
3157<t>
3158   To avoid the TCP reset problem, servers typically close a connection in
3159   stages. First, the server performs a half-close by closing only the write
3160   side of the read/write connection. The server then continues to read from
3161   the connection until it receives a corresponding close by the client, or
3162   until the server is reasonably certain that its own TCP stack has received
3163   the client's acknowledgement of the packet(s) containing the server's last
3164   response. Finally, the server fully closes the connection.
3165</t>
3166<t>
3167   It is unknown whether the reset problem is exclusive to TCP or might also
3168   be found in other transport connection protocols.
3169</t>
3170</section>
3171
3172<section title="Upgrade" anchor="header.upgrade">
3173  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3174  <x:anchor-alias value="Upgrade"/>
3175  <x:anchor-alias value="protocol"/>
3176  <x:anchor-alias value="protocol-name"/>
3177  <x:anchor-alias value="protocol-version"/>
3178<t>
3179   The "Upgrade" header field is intended to provide a simple mechanism
3180   for transitioning from HTTP/1.1 to some other protocol on the same
3181   connection.  A client &MAY; send a list of protocols in the Upgrade
3182   header field of a request to invite the server to switch to one or
3183   more of those protocols, in order of descending preference, before sending
3184   the final response. A server &MAY; ignore a received Upgrade header field
3185   if it wishes to continue using the current protocol on that connection.
3186</t>
3187<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3188  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3189
3190  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3191  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3192  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3193</artwork></figure>
3194<t>
3195   A server that sends a <x:ref>101 (Switching Protocols)</x:ref> response
3196   &MUST; send an Upgrade header field to indicate the new protocol(s) to
3197   which the connection is being switched; if multiple protocol layers are
3198   being switched, the sender &MUST; list the protocols in layer-ascending
3199   order. A server &MUST-NOT; switch to a protocol that was not indicated by
3200   the client in the corresponding request's Upgrade header field.
3201   A server &MAY; choose to ignore the order of preference indicated by the
3202   client and select the new protocol(s) based on other factors, such as the
3203   nature of the request or the current load on the server.
3204</t>
3205<t>
3206   A server that sends a <x:ref>426 (Upgrade Required)</x:ref> response
3207   &MUST; send an Upgrade header field to indicate the acceptable protocols,
3208   in order of descending preference.
3209</t>
3210<t>
3211   A server &MAY; send an Upgrade header field in any other response to
3212   advertise that it implements support for upgrading to the listed protocols,
3213   in order of descending preference, when appropriate for a future request.
3214</t>
3215<figure><preamble>
3216   The following is a hypothetical example sent by a client:
3217</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
3218GET /hello.txt HTTP/1.1
3219Host: www.example.com
3220Connection: upgrade
3221Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3222
3223</artwork></figure>
3224<t>
3225   Upgrade cannot be used to insist on a protocol change; its acceptance and
3226   use by the server is optional. The capabilities and nature of the
3227   application-level communication after the protocol change is entirely
3228   dependent upon the new protocol(s) chosen. However, immediately after
3229   sending the 101 response, the server is expected to continue responding to
3230   the original request as if it had received its equivalent within the new
3231   protocol (i.e., the server still has an outstanding request to satisfy
3232   after the protocol has been changed, and is expected to do so without
3233   requiring the request to be repeated).
3234</t>
3235<t>
3236   For example, if the Upgrade header field is received in a GET request
3237   and the server decides to switch protocols, it first responds
3238   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3239   then immediately follows that with the new protocol's equivalent of a
3240   response to a GET on the target resource.  This allows a connection to be
3241   upgraded to protocols with the same semantics as HTTP without the
3242   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3243   protocols unless the received message semantics can be honored by the new
3244   protocol; an OPTIONS request can be honored by any protocol.
3245</t>
3246<figure><preamble>
3247   The following is an example response to the above hypothetical request:
3248</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
3249HTTP/1.1 101 Switching Protocols
3250Connection: upgrade
3251Upgrade: HTTP/2.0
3252
3253[... data stream switches to HTTP/2.0 with an appropriate response
3254(as defined by new protocol) to the "GET /hello.txt" request ...]
3255</artwork></figure>
3256<t>
3257   When Upgrade is sent, the sender &MUST; also send a
3258   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3259   that contains an "upgrade" connection option, in order to prevent Upgrade
3260   from being accidentally forwarded by intermediaries that might not implement
3261   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3262   is received in an HTTP/1.0 request.
3263</t>
3264<t>
3265   The Upgrade header field only applies to switching protocols on top of the
3266   existing connection; it cannot be used to switch the underlying connection
3267   (transport) protocol, nor to switch the existing communication to a
3268   different connection. For those purposes, it is more appropriate to use a
3269   <x:ref>3xx (Redirection)</x:ref> response (&status-3xx;).
3270</t>
3271<t>
3272   This specification only defines the protocol name "HTTP" for use by
3273   the family of Hypertext Transfer Protocols, as defined by the HTTP
3274   version rules of <xref target="http.version"/> and future updates to this
3275   specification. Additional tokens ought to be registered with IANA using the
3276   registration procedure defined in <xref target="upgrade.token.registry"/>.
3277</t>
3278</section>
3279</section>
3280
3281<section title="ABNF list extension: #rule" anchor="abnf.extension">
3282<t>
3283  A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
3284  improve readability in the definitions of some header field values.
3285</t>
3286<t>
3287  A construct "#" is defined, similar to "*", for defining comma-delimited
3288  lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
3289  at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
3290  comma (",") and optional whitespace (OWS).   
3291</t>
3292<figure><preamble>
3293  Thus, a sender &MUST; expand the list construct as follows:
3294</preamble><artwork type="example">
3295  1#element =&gt; element *( OWS "," OWS element )
3296</artwork></figure>
3297<figure><preamble>
3298  and:
3299</preamble><artwork type="example">
3300  #element =&gt; [ 1#element ]
3301</artwork></figure>
3302<figure><preamble>
3303  and for n &gt;= 1 and m &gt; 1:
3304</preamble><artwork type="example">
3305  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
3306</artwork></figure>
3307<t>
3308  For compatibility with legacy list rules, recipients &MUST; parse and ignore
3309  a reasonable number of empty list elements: enough to handle common mistakes
3310  by senders that merge values, but not so much that they could be used as a
3311  denial of service mechanism. In other words, recipients &MUST; expand the
3312  list construct as follows:
3313</t>
3314<figure><artwork type="example">
3315  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
3316 
3317  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
3318</artwork></figure>
3319<t>
3320  Empty elements do not contribute to the count of elements present.
3321  For example, given these ABNF productions:
3322</t>
3323<figure><artwork type="example">
3324  example-list      = 1#example-list-elmt
3325  example-list-elmt = token ; see <xref target="field.components"/>
3326</artwork></figure>
3327<t>
3328  Then the following are valid values for example-list (not including the
3329  double quotes, which are present for delimitation only):
3330</t>
3331<figure><artwork type="example">
3332  "foo,bar"
3333  "foo ,bar,"
3334  "foo , ,bar,charlie   "
3335</artwork></figure>
3336<t>
3337  In contrast, the following values would be invalid, since at least one
3338  non-empty element is required by the example-list production:
3339</t>
3340<figure><artwork type="example">
3341  ""
3342  ","
3343  ",   ,"
3344</artwork></figure>
3345<t>
3346  <xref target="collected.abnf"/> shows the collected ABNF after the list
3347  constructs have been expanded, as described above, for recipients.
3348</t>
3349</section>
3350
3351<section title="IANA Considerations" anchor="IANA.considerations">
3352
3353<section title="Header Field Registration" anchor="header.field.registration">
3354<t>
3355   HTTP header fields are registered within the Message Header Field Registry
3356   maintained at
3357   <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
3358</t>
3359<t>
3360   This document defines the following HTTP header fields, so their
3361   associated registry entries shall be updated according to the permanent
3362   registrations below (see <xref target="BCP90"/>):
3363</t>
3364<?BEGININC p1-messaging.iana-headers ?>
3365<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3366<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3367   <ttcol>Header Field Name</ttcol>
3368   <ttcol>Protocol</ttcol>
3369   <ttcol>Status</ttcol>
3370   <ttcol>Reference</ttcol>
3371
3372   <c>Connection</c>
3373   <c>http</c>
3374   <c>standard</c>
3375   <c>
3376      <xref target="header.connection"/>
3377   </c>
3378   <c>Content-Length</c>
3379   <c>http</c>
3380   <c>standard</c>
3381   <c>
3382      <xref target="header.content-length"/>
3383   </c>
3384   <c>Host</c>
3385   <c>http</c>
3386   <c>standard</c>
3387   <c>
3388      <xref target="header.host"/>
3389   </c>
3390   <c>TE</c>
3391   <c>http</c>
3392   <c>standard</c>
3393   <c>
3394      <xref target="header.te"/>
3395   </c>
3396   <c>Trailer</c>
3397   <c>http</c>
3398   <c>standard</c>
3399   <c>
3400      <xref target="header.trailer"/>
3401   </c>
3402   <c>Transfer-Encoding</c>
3403   <c>http</c>
3404   <c>standard</c>
3405   <c>
3406      <xref target="header.transfer-encoding"/>
3407   </c>
3408   <c>Upgrade</c>
3409   <c>http</c>
3410   <c>standard</c>
3411   <c>
3412      <xref target="header.upgrade"/>
3413   </c>
3414   <c>Via</c>
3415   <c>http</c>
3416   <c>standard</c>
3417   <c>
3418      <xref target="header.via"/>
3419   </c>
3420</texttable>
3421<!--(END)-->
3422<?ENDINC p1-messaging.iana-headers ?>
3423<t>
3424   Furthermore, the header field-name "Close" shall be registered as
3425   "reserved", since using that name as an HTTP header field might
3426   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3427   header field (<xref target="header.connection"/>).
3428</t>
3429<texttable align="left" suppress-title="true">
3430   <ttcol>Header Field Name</ttcol>
3431   <ttcol>Protocol</ttcol>
3432   <ttcol>Status</ttcol>
3433   <ttcol>Reference</ttcol>
3434
3435   <c>Close</c>
3436   <c>http</c>
3437   <c>reserved</c>
3438   <c>
3439      <xref target="header.field.registration"/>
3440   </c>
3441</texttable>
3442<t>
3443   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3444</t>
3445</section>
3446
3447<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3448<t>
3449   IANA maintains the registry of URI Schemes <xref target="BCP115"/> at
3450   <eref target="http://www.iana.org/assignments/uri-schemes.html"/>.
3451</t>
3452<t>
3453   This document defines the following URI schemes, so their
3454   associated registry entries shall be updated according to the permanent
3455   registrations below:
3456</t>
3457<texttable align="left" suppress-title="true">
3458   <ttcol>URI Scheme</ttcol>
3459   <ttcol>Description</ttcol>
3460   <ttcol>Reference</ttcol>
3461
3462   <c>http</c>
3463   <c>Hypertext Transfer Protocol</c>
3464   <c><xref target="http.uri"/></c>
3465
3466   <c>https</c>
3467   <c>Hypertext Transfer Protocol Secure</c>
3468   <c><xref target="https.uri"/></c>
3469</texttable>
3470</section>
3471
3472<section title="Internet Media Type Registration" anchor="internet.media.type.http">
3473<t>
3474   This document serves as the specification for the Internet media types
3475   "message/http" and "application/http". The following is to be registered with
3476   IANA (see <xref target="BCP13"/>).
3477</t>
3478<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
3479<iref item="Media Type" subitem="message/http" primary="true"/>
3480<iref item="message/http Media Type" primary="true"/>
3481<t>
3482   The message/http type can be used to enclose a single HTTP request or
3483   response message, provided that it obeys the MIME restrictions for all
3484   "message" types regarding line length and encodings.
3485</t>
3486<t>
3487  <list style="hanging" x:indent="12em">
3488    <t hangText="Type name:">
3489      message
3490    </t>
3491    <t hangText="Subtype name:">
3492      http
3493    </t>
3494    <t hangText="Required parameters:">
3495      none
3496    </t>
3497    <t hangText="Optional parameters:">
3498      version, msgtype
3499      <list style="hanging">
3500        <t hangText="version:">
3501          The HTTP-version number of the enclosed message
3502          (e.g., "1.1"). If not present, the version can be
3503          determined from the first line of the body.
3504        </t>
3505        <t hangText="msgtype:">
3506          The message type &mdash; "request" or "response". If not
3507          present, the type can be determined from the first
3508          line of the body.
3509        </t>
3510      </list>
3511    </t>
3512    <t hangText="Encoding considerations:">
3513      only "7bit", "8bit", or "binary" are permitted
3514    </t>
3515    <t hangText="Security considerations:">
3516      none
3517    </t>
3518    <t hangText="Interoperability considerations:">
3519      none
3520    </t>
3521    <t hangText="Published specification:">
3522      This specification (see <xref target="internet.media.type.message.http"/>).
3523    </t>
3524    <t hangText="Applications that use this media type:">
3525    </t>
3526    <t hangText="Additional information:">
3527      <list style="hanging">
3528        <t hangText="Magic number(s):">none</t>
3529        <t hangText="File extension(s):">none</t>
3530        <t hangText="Macintosh file type code(s):">none</t>
3531      </list>
3532    </t>
3533    <t hangText="Person and email address to contact for further information:">
3534      See Authors Section.
3535    </t>
3536    <t hangText="Intended usage:">
3537      COMMON
3538    </t>
3539    <t hangText="Restrictions on usage:">
3540      none
3541    </t>
3542    <t hangText="Author:">
3543      See Authors Section.
3544    </t>
3545    <t hangText="Change controller:">
3546      IESG
3547    </t>
3548  </list>
3549</t>
3550</section>
3551<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
3552<iref item="Media Type" subitem="application/http" primary="true"/>
3553<iref item="application/http Media Type" primary="true"/>
3554<t>
3555   The application/http type can be used to enclose a pipeline of one or more
3556   HTTP request or response messages (not intermixed).
3557</t>
3558<t>
3559  <list style="hanging" x:indent="12em">
3560    <t hangText="Type name:">
3561      application
3562    </t>
3563    <t hangText="Subtype name:">
3564      http
3565    </t>
3566    <t hangText="Required parameters:">
3567      none
3568    </t>
3569    <t hangText="Optional parameters:">
3570      version, msgtype
3571      <list style="hanging">
3572        <t hangText="version:">
3573          The HTTP-version number of the enclosed messages
3574          (e.g., "1.1"). If not present, the version can be
3575          determined from the first line of the body.
3576        </t>
3577        <t hangText="msgtype:">
3578          The message type &mdash; "request" or "response". If not
3579          present, the type can be determined from the first
3580          line of the body.
3581        </t>
3582      </list>
3583    </t>
3584    <t hangText="Encoding considerations:">
3585      HTTP messages enclosed by this type
3586      are in "binary" format; use of an appropriate
3587      Content-Transfer-Encoding is required when
3588      transmitted via E-mail.
3589    </t>
3590    <t hangText="Security considerations:">
3591      none
3592    </t>
3593    <t hangText="Interoperability considerations:">
3594      none
3595    </t>
3596    <t hangText="Published specification:">
3597      This specification (see <xref target="internet.media.type.application.http"/>).
3598    </t>
3599    <t hangText="Applications that use this media type:">
3600    </t>
3601    <t hangText="Additional information:">
3602      <list style="hanging">
3603        <t hangText="Magic number(s):">none</t>
3604        <t hangText="File extension(s):">none</t>
3605        <t hangText="Macintosh file type code(s):">none</t>
3606      </list>
3607    </t>
3608    <t hangText="Person and email address to contact for further information:">
3609      See Authors Section.
3610    </t>
3611    <t hangText="Intended usage:">
3612      COMMON
3613    </t>
3614    <t hangText="Restrictions on usage:">
3615      none
3616    </t>
3617    <t hangText="Author:">
3618      See Authors Section.
3619    </t>
3620    <t hangText="Change controller:">
3621      IESG
3622    </t>
3623  </list>
3624</t>
3625</section>
3626</section>
3627
3628<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3629<t>
3630   The HTTP Transfer Coding Registry defines the name space for transfer
3631   coding names. It is maintained at <eref target="http://www.iana.org/assignments/http-parameters"/>.
3632</t>
3633
3634<section title="Procedure" anchor="transfer.coding.registry.procedure">
3635<t>
3636   Registrations &MUST; include the following fields:
3637   <list style="symbols">
3638     <t>Name</t>
3639     <t>Description</t>
3640     <t>Pointer to specification text</t>
3641   </list>
3642</t>
3643<t>
3644   Names of transfer codings &MUST-NOT; overlap with names of content codings
3645   (&content-codings;) unless the encoding transformation is identical, as
3646   is the case for the compression codings defined in
3647   <xref target="compression.codings"/>.
3648</t>
3649<t>
3650   Values to be added to this name space require IETF Review (see
3651   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3652   conform to the purpose of transfer coding defined in this specification.
3653</t>
3654<t>
3655   Use of program names for the identification of encoding formats
3656   is not desirable and is discouraged for future encodings.
3657</t>
3658</section>
3659
3660<section title="Registration" anchor="transfer.coding.registration">
3661<t>
3662   The HTTP Transfer Coding Registry shall be updated with the registrations
3663   below:
3664</t>
3665<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3666   <ttcol>Name</ttcol>
3667   <ttcol>Description</ttcol>
3668   <ttcol>Reference</ttcol>
3669   <c>chunked</c>
3670   <c>Transfer in a series of chunks</c>
3671   <c>
3672      <xref target="chunked.encoding"/>
3673   </c>
3674   <c>compress</c>
3675   <c>UNIX "compress" data format <xref target="Welch"/></c>
3676   <c>
3677      <xref target="compress.coding"/>
3678   </c>
3679   <c>deflate</c>
3680   <c>"deflate" compressed data (<xref target="RFC1951"/>) inside
3681   the "zlib" data format (<xref target="RFC1950"/>)
3682   </c>
3683   <c>
3684      <xref target="deflate.coding"/>
3685   </c>
3686   <c>gzip</c>
3687   <c>GZIP file format <xref target="RFC1952"/></c>
3688   <c>
3689      <xref target="gzip.coding"/>
3690   </c>
3691   <c>x-compress</c>
3692   <c>Deprecated (alias for compress)</c>
3693   <c>
3694      <xref target="compress.coding"/>
3695   </c>
3696   <c>x-gzip</c>
3697   <c>Deprecated (alias for gzip)</c>
3698   <c>
3699      <xref target="gzip.coding"/>
3700   </c>
3701</texttable>
3702</section>
3703</section>
3704
3705<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3706<t>
3707   The HTTP Upgrade Token Registry defines the name space for protocol-name
3708   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3709   field. The registry is maintained at <eref target="http://www.iana.org/assignments/http-upgrade-tokens"/>.
3710</t>
3711
3712<section title="Procedure" anchor="upgrade.token.registry.procedure">  
3713<t>
3714   Each registered protocol name is associated with contact information
3715   and an optional set of specifications that details how the connection
3716   will be processed after it has been upgraded.
3717</t>
3718<t>
3719   Registrations happen on a "First Come First Served" basis (see
3720   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3721   following rules:
3722  <list style="numbers">
3723    <t>A protocol-name token, once registered, stays registered forever.</t>
3724    <t>The registration &MUST; name a responsible party for the
3725       registration.</t>
3726    <t>The registration &MUST; name a point of contact.</t>
3727    <t>The registration &MAY; name a set of specifications associated with
3728       that token. Such specifications need not be publicly available.</t>
3729    <t>The registration &SHOULD; name a set of expected "protocol-version"
3730       tokens associated with that token at the time of registration.</t>
3731    <t>The responsible party &MAY; change the registration at any time.
3732       The IANA will keep a record of all such changes, and make them
3733       available upon request.</t>
3734    <t>The IESG &MAY; reassign responsibility for a protocol token.
3735       This will normally only be used in the case when a
3736       responsible party cannot be contacted.</t>
3737  </list>
3738</t>
3739<t>
3740   This registration procedure for HTTP Upgrade Tokens replaces that
3741   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3742</t>
3743</section>
3744
3745<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3746<t>
3747   The HTTP Upgrade Token Registry shall be updated with the registration
3748   below:
3749</t>
3750<texttable align="left" suppress-title="true">
3751   <ttcol>Value</ttcol>
3752   <ttcol>Description</ttcol>
3753   <ttcol>Expected Version Tokens</ttcol>
3754   <ttcol>Reference</ttcol>
3755
3756   <c>HTTP</c>
3757   <c>Hypertext Transfer Protocol</c>
3758   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3759   <c><xref target="http.version"/></c>
3760</texttable>
3761<t>
3762   The responsible party is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3763</t>
3764</section>
3765</section>
3766
3767</section>
3768
3769<section title="Security Considerations" anchor="security.considerations">
3770<t>
3771   This section is meant to inform developers, information providers, and
3772   users of known security concerns relevant to HTTP/1.1 message syntax,
3773   parsing, and routing.
3774</t>
3775
3776<section title="DNS-related Attacks" anchor="dns.related.attacks">
3777<t>
3778   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3779   generally prone to security attacks based on the deliberate misassociation
3780   of IP addresses and DNS names not protected by DNSSEC. Clients need to be
3781   cautious in assuming the validity of an IP number/DNS name association unless
3782   the response is protected by DNSSEC (<xref target="RFC4033"/>).
3783</t>
3784</section>
3785
3786<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3787<t>
3788   By their very nature, HTTP intermediaries are men-in-the-middle, and
3789   represent an opportunity for man-in-the-middle attacks. Compromise of
3790   the systems on which the intermediaries run can result in serious security
3791   and privacy problems. Intermediaries have access to security-related
3792   information, personal information about individual users and
3793   organizations, and proprietary information belonging to users and
3794   content providers. A compromised intermediary, or an intermediary
3795   implemented or configured without regard to security and privacy
3796   considerations, might be used in the commission of a wide range of
3797   potential attacks.
3798</t>
3799<t>
3800   Intermediaries that contain a shared cache are especially vulnerable
3801   to cache poisoning attacks.
3802</t>
3803<t>
3804   Implementers need to consider the privacy and security
3805   implications of their design and coding decisions, and of the
3806   configuration options they provide to operators (especially the
3807   default configuration).
3808</t>
3809<t>
3810   Users need to be aware that intermediaries are no more trustworthy than
3811   the people who run them; HTTP itself cannot solve this problem.
3812</t>
3813</section>
3814
3815<section title="Buffer Overflows" anchor="attack.protocol.element.size.overflows">
3816<t>
3817   Because HTTP uses mostly textual, character-delimited fields, attackers can
3818   overflow buffers in implementations, and/or perform a Denial of Service
3819   against implementations that accept fields with unlimited lengths.
3820</t>
3821<t>
3822   To promote interoperability, this specification makes specific
3823   recommendations for minimum size limits on request-line
3824   (<xref target="request.line"/>)
3825   and blocks of header fields (<xref target="header.fields"/>). These are
3826   minimum recommendations, chosen to be supportable even by implementations
3827   with limited resources; it is expected that most implementations will
3828   choose substantially higher limits.
3829</t>
3830<t>
3831   This specification also provides a way for servers to reject messages that
3832   have request-targets that are too long (&status-414;) or request entities
3833   that are too large (&status-4xx;). Additional status codes related to
3834   capacity limits have been defined by extensions to HTTP
3835   <xref target="RFC6585"/>.
3836</t>
3837<t>
3838   Recipients &SHOULD; carefully limit the extent to which they read other
3839   fields, including (but not limited to) request methods, response status
3840   phrases, header field-names, and body chunks, so as to avoid denial of
3841   service attacks without impeding interoperability.
3842</t>
3843</section>
3844
3845<section title="Message Integrity" anchor="message.integrity">
3846<t>
3847   HTTP does not define a specific mechanism for ensuring message integrity,
3848   instead relying on the error-detection ability of underlying transport
3849   protocols and the use of length or chunk-delimited framing to detect
3850   completeness. Additional integrity mechanisms, such as hash functions or
3851   digital signatures applied to the content, can be selectively added to
3852   messages via extensible metadata header fields. Historically, the lack of
3853   a single integrity mechanism has been justified by the informal nature of
3854   most HTTP communication.  However, the prevalence of HTTP as an information
3855   access mechanism has resulted in its increasing use within environments
3856   where verification of message integrity is crucial.
3857</t>
3858<t>
3859   User agents are encouraged to implement configurable means for detecting
3860   and reporting failures of message integrity such that those means can be
3861   enabled within environments for which integrity is necessary. For example,
3862   a browser being used to view medical history or drug interaction
3863   information needs to indicate to the user when such information is detected
3864   by the protocol to be incomplete, expired, or corrupted during transfer.
3865   Such mechanisms might be selectively enabled via user agent extensions or
3866   the presence of message integrity metadata in a response.
3867   At a minimum, user agents ought to provide some indication that allows a
3868   user to distinguish between a complete and incomplete response message
3869   (<xref target="incomplete.messages"/>) when such verification is desired.
3870</t>
3871</section>
3872
3873<section title="Server Log Information" anchor="abuse.of.server.log.information">
3874<t>
3875   A server is in the position to save personal data about a user's requests
3876   over time, which might identify their reading patterns or subjects of
3877   interest.  In particular, log information gathered at an intermediary
3878   often contains a history of user agent interaction, across a multitude
3879   of sites, that can be traced to individual users.
3880</t>
3881<t>
3882   HTTP log information is confidential in nature; its handling is often
3883   constrained by laws and regulations.  Log information needs to be securely
3884   stored and appropriate guidelines followed for its analysis.
3885   Anonymization of personal information within individual entries helps,
3886   but is generally not sufficient to prevent real log traces from being
3887   re-identified based on correlation with other access characteristics.
3888   As such, access traces that are keyed to a specific client should not
3889   be published even if the key is pseudonymous.
3890</t>
3891<t>
3892   To minimize the risk of theft or accidental publication, log information
3893   should be purged of personally identifiable information, including
3894   user identifiers, IP addresses, and user-provided query parameters,
3895   as soon as that information is no longer necessary to support operational
3896   needs for security, auditing, or fraud control.
3897</t>
3898</section>
3899</section>
3900
3901<section title="Acknowledgments" anchor="acks">
3902<t>
3903   This edition of HTTP/1.1 builds on the many contributions that went into
3904   <xref target="RFC1945" format="none">RFC 1945</xref>,
3905   <xref target="RFC2068" format="none">RFC 2068</xref>,
3906   <xref target="RFC2145" format="none">RFC 2145</xref>, and
3907   <xref target="RFC2616" format="none">RFC 2616</xref>, including
3908   substantial contributions made by the previous authors, editors, and
3909   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
3910   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
3911   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
3912</t>
3913<t>
3914   Since 1999, the following contributors have helped improve the HTTP
3915   specification by reporting bugs, asking smart questions, drafting or
3916   reviewing text, and evaluating open issues:
3917</t>
3918<?BEGININC acks ?>
3919<t>Adam Barth,
3920Adam Roach,
3921Addison Phillips,
3922Adrian Chadd,
3923Adrien W. de Croy,
3924Alan Ford,
3925Alan Ruttenberg,
3926Albert Lunde,
3927Alek Storm,
3928Alex Rousskov,
3929Alexandre Morgaut,
3930Alexey Melnikov,
3931Alisha Smith,
3932Amichai Rothman,
3933Amit Klein,
3934Amos Jeffries,
3935Andreas Maier,
3936Andreas Petersson,
3937Anil Sharma,
3938Anne van Kesteren,
3939Anthony Bryan,
3940Asbjorn Ulsberg,
3941Ashok Kumar,
3942Balachander Krishnamurthy,
3943Barry Leiba,
3944Ben Laurie,
3945Benjamin Carlyle,
3946Benjamin Niven-Jenkins,
3947Bil Corry,
3948Bill Burke,
3949Bjoern Hoehrmann,
3950Bob Scheifler,
3951Boris Zbarsky,
3952Brett Slatkin,
3953Brian Kell,
3954Brian McBarron,
3955Brian Pane,
3956Brian Raymor,
3957Brian Smith,
3958Bryce Nesbitt,
3959Cameron Heavon-Jones,
3960Carl Kugler,
3961Carsten Bormann,
3962Charles Fry,
3963Chris Newman,
3964Cyrus Daboo,
3965Dale Robert Anderson,
3966Dan Wing,
3967Dan Winship,
3968Daniel Stenberg,
3969Darrel Miller,
3970Dave Cridland,
3971Dave Crocker,
3972Dave Kristol,
3973Dave Thaler,
3974David Booth,
3975David Singer,
3976David W. Morris,
3977Diwakar Shetty,
3978Dmitry Kurochkin,
3979Drummond Reed,
3980Duane Wessels,
3981Edward Lee,
3982Eitan Adler,
3983Eliot Lear,
3984Eran Hammer-Lahav,
3985Eric D. Williams,
3986Eric J. Bowman,
3987Eric Lawrence,
3988Eric Rescorla,
3989Erik Aronesty,
3990Evan Prodromou,
3991Felix Geisendoerfer,
3992Florian Weimer,
3993Frank Ellermann,
3994Fred Akalin,
3995Fred Bohle,
3996Frederic Kayser,
3997Gabor Molnar,
3998Gabriel Montenegro,
3999Geoffrey Sneddon,
4000Gervase Markham,
4001Gili Tzabari,
4002Grahame Grieve,
4003Greg Wilkins,
4004Grzegorz Calkowski,
4005Harald Tveit Alvestrand,
4006Harry Halpin,
4007Helge Hess,
4008Henrik Nordstrom,
4009Henry S. Thompson,
4010Henry Story,
4011Herbert van de Sompel,
4012Herve Ruellan,
4013Howard Melman,
4014Hugo Haas,
4015Ian Fette,
4016Ian Hickson,
4017Ido Safruti,
4018Ilari Liusvaara,
4019Ilya Grigorik,
4020Ingo Struck,
4021J. Ross Nicoll,
4022James Cloos,
4023James H. Manger,
4024James Lacey,
4025James M. Snell,
4026Jamie Lokier,
4027Jan Algermissen,
4028Jeff Hodges (who came up with the term 'effective Request-URI'),
4029Jeff Pinner,
4030Jeff Walden,
4031Jim Luther,
4032Jitu Padhye,
4033Joe D. Williams,
4034Joe Gregorio,
4035Joe Orton,
4036John C. Klensin,
4037John C. Mallery,
4038John Cowan,
4039John Kemp,
4040John Panzer,
4041John Schneider,
4042John Stracke,
4043John Sullivan,
4044Jonas Sicking,
4045Jonathan A. Rees,
4046Jonathan Billington,
4047Jonathan Moore,
4048Jonathan Silvera,
4049Jordi Ros,
4050Joris Dobbelsteen,
4051Josh Cohen,
4052Julien Pierre,
4053Jungshik Shin,
4054Justin Chapweske,
4055Justin Erenkrantz,
4056Justin James,
4057Kalvinder Singh,
4058Karl Dubost,
4059Keith Hoffman,
4060Keith Moore,
4061Ken Murchison,
4062Koen Holtman,
4063Konstantin Voronkov,
4064Kris Zyp,
4065Lisa Dusseault,
4066Maciej Stachowiak,
4067Manu Sporny,
4068Marc Schneider,
4069Marc Slemko,
4070Mark Baker,
4071Mark Pauley,
4072Mark Watson,
4073Markus Isomaki,
4074Markus Lanthaler,
4075Martin J. Duerst,
4076Martin Musatov,
4077Martin Nilsson,
4078Martin Thomson,
4079Matt Lynch,
4080Matthew Cox,
4081Max Clark,
4082Michael Burrows,
4083Michael Hausenblas,
4084Michael Sweet,
4085Michael Tuexen,
4086Michael Welzl,
4087Mike Amundsen,
4088Mike Belshe,
4089Mike Bishop,
4090Mike Kelly,
4091Mike Schinkel,
4092Miles Sabin,
4093Murray S. Kucherawy,
4094Mykyta Yevstifeyev,
4095Nathan Rixham,
4096Nicholas Shanks,
4097Nico Williams,
4098Nicolas Alvarez,
4099Nicolas Mailhot,
4100Noah Slater,
4101Osama Mazahir,
4102Pablo Castro,
4103Pat Hayes,
4104Patrick R. McManus,
4105Paul E. Jones,
4106Paul Hoffman,
4107Paul Marquess,
4108Peter Lepeska,
4109Peter Occil,
4110Peter Saint-Andre,
4111Peter Watkins,
4112Phil Archer,
4113Philippe Mougin,
4114Phillip Hallam-Baker,
4115Piotr Dobrogost,
4116Poul-Henning Kamp,
4117Preethi Natarajan,
4118Rajeev Bector,
4119Ray Polk,
4120Reto Bachmann-Gmuer,
4121Richard Cyganiak,
4122Robby Simpson,
4123Robert Brewer,
4124Robert Collins,
4125Robert Mattson,
4126Robert O'Callahan,
4127Robert Olofsson,
4128Robert Sayre,
4129Robert Siemer,
4130Robert de Wilde,
4131Roberto Javier Godoy,
4132Roberto Peon,
4133Roland Zink,
4134Ronny Widjaja,
4135Ryan Hamilton,
4136S. Mike Dierken,
4137Salvatore Loreto,
4138Sam Johnston,
4139Sam Pullara,
4140Sam Ruby,
4141Scott Lawrence (who maintained the original issues list),
4142Sean B. Palmer,
4143Sebastien Barnoud,
4144Shane McCarron,
4145Shigeki Ohtsu,
4146Stefan Eissing,
4147Stefan Tilkov,
4148Stefanos Harhalakis,
4149Stephane Bortzmeyer,
4150Stephen Farrell,
4151Stephen Ludin,
4152Stuart Williams,
4153Subbu Allamaraju,
4154Sylvain Hellegouarch,
4155Tapan Divekar,
4156Tatsuhiro Tsujikawa,
4157Tatsuya Hayashi,
4158Ted Hardie,
4159Thomas Broyer,
4160Thomas Fossati,
4161Thomas Maslen,
4162Thomas Nordin,
4163Thomas Roessler,
4164Tim Bray,
4165Tim Morgan,
4166Tim Olsen,
4167Tom Zhou,
4168Travis Snoozy,
4169Tyler Close,
4170Vincent Murphy,
4171Wenbo Zhu,
4172Werner Baumann,
4173Wilbur Streett,
4174Wilfredo Sanchez Vega,
4175William A. Rowe Jr.,
4176William Chan,
4177Willy Tarreau,
4178Xiaoshu Wang,
4179Yaron Goland,
4180Yngve Nysaeter Pettersen,
4181Yoav Nir,
4182Yogesh Bang,
4183Yuchung Cheng,
4184Yutaka Oiwa,
4185Yves Lafon (long-time member of the editor team),
4186Zed A. Shaw, and
4187Zhong Yu.
4188</t>
4189<?ENDINC acks ?>
4190<t>
4191   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
4192   acknowledgements from prior revisions.
4193</t>
4194</section>
4195
4196</middle>
4197<back>
4198
4199<references title="Normative References">
4200
4201<reference anchor="Part2">
4202  <front>
4203    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
4204    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4205      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4206      <address><email>fielding@gbiv.com</email></address>
4207    </author>
4208    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4209      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4210      <address><email>julian.reschke@greenbytes.de</email></address>
4211    </author>
4212    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4213  </front>
4214  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
4215  <x:source href="p2-semantics.xml" basename="p2-semantics">
4216    <x:defines>1xx (Informational)</x:defines>
4217    <x:defines>1xx</x:defines>
4218    <x:defines>100 (Continue)</x:defines>
4219    <x:defines>101 (Switching Protocols)</x:defines>
4220    <x:defines>2xx (Successful)</x:defines>
4221    <x:defines>2xx</x:defines>
4222    <x:defines>200 (OK)</x:defines>
4223    <x:defines>203 (Non-Authoritative Information)</x:defines>
4224    <x:defines>204 (No Content)</x:defines>
4225    <x:defines>3xx (Redirection)</x:defines>
4226    <x:defines>3xx</x:defines>
4227    <x:defines>301 (Moved Permanently)</x:defines>
4228    <x:defines>4xx (Client Error)</x:defines>
4229    <x:defines>4xx</x:defines>
4230    <x:defines>400 (Bad Request)</x:defines>
4231    <x:defines>411 (Length Required)</x:defines>
4232    <x:defines>414 (URI Too Long)</x:defines>
4233    <x:defines>417 (Expectation Failed)</x:defines>
4234    <x:defines>426 (Upgrade Required)</x:defines>
4235    <x:defines>501 (Not Implemented)</x:defines>
4236    <x:defines>502 (Bad Gateway)</x:defines>
4237    <x:defines>505 (HTTP Version Not Supported)</x:defines>
4238    <x:defines>Accept-Encoding</x:defines>
4239    <x:defines>Allow</x:defines>
4240    <x:defines>Content-Encoding</x:defines>
4241    <x:defines>Content-Location</x:defines>
4242    <x:defines>Content-Type</x:defines>
4243    <x:defines>Date</x:defines>
4244    <x:defines>Expect</x:defines>
4245    <x:defines>Location</x:defines>
4246    <x:defines>Server</x:defines>
4247    <x:defines>User-Agent</x:defines>
4248  </x:source>
4249</reference>
4250
4251<reference anchor="Part4">
4252  <front>
4253    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
4254    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
4255      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4256      <address><email>fielding@gbiv.com</email></address>
4257    </author>
4258    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
4259      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4260      <address><email>julian.reschke@greenbytes.de</email></address>
4261    </author>
4262    <date month="&ID-MONTH;" year="&ID-YEAR;" />
4263  </front>
4264  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
4265  <x:source basename="p4-conditional" href="p4-conditional.xml">
4266    <x:defines>304 (Not Modified)</x:defines>
4267    <x:defines>ETag</x:defines>
4268    <x:defines>Last-Modified</x:defines>
4269  </x:source>
4270</reference>
4271
4272<reference anchor="Part5">
4273  <front>
4274    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
4275    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4276      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4277      <address><email>fielding@gbiv.com</email></address>
4278    </author>
4279    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
4280      <organization abbrev="W3C">World Wide Web Consortium</organization>
4281      <address><email>ylafon@w3.org</email></address>
4282    </author>
4283    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4284      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4285      <address><email>julian.reschke@greenbytes.de</email></address>
4286    </author>
4287    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4288  </front>
4289  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4290  <x:source href="p5-range.xml" basename="p5-range">
4291    <x:defines>Content-Range</x:defines>
4292  </x:source>
4293</reference>
4294
4295<reference anchor="Part6">
4296  <front>
4297    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4298    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4299      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4300      <address><email>fielding@gbiv.com</email></address>
4301    </author>
4302    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4303      <organization>Akamai</organization>
4304      <address><email>mnot@mnot.net</email></address>
4305    </author>
4306    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4307      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4308      <address><email>julian.reschke@greenbytes.de</email></address>
4309    </author>
4310    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4311  </front>
4312  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4313  <x:source href="p6-cache.xml" basename="p6-cache">
4314    <x:defines>Cache-Control</x:defines>
4315    <x:defines>Expires</x:defines>
4316  </x:source>
4317</reference>
4318
4319<reference anchor="Part7">
4320  <front>
4321    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4322    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4323      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4324      <address><email>fielding@gbiv.com</email></address>
4325    </author>
4326    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4327      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4328      <address><email>julian.reschke@greenbytes.de</email></address>
4329    </author>
4330    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4331  </front>
4332  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4333  <x:source href="p7-auth.xml" basename="p7-auth">
4334    <x:defines>Proxy-Authenticate</x:defines>
4335    <x:defines>Proxy-Authorization</x:defines>
4336  </x:source>
4337</reference>
4338
4339<reference anchor="RFC5234">
4340  <front>
4341    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4342    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4343      <organization>Brandenburg InternetWorking</organization>
4344      <address>
4345        <email>dcrocker@bbiw.net</email>
4346      </address> 
4347    </author>
4348    <author initials="P." surname="Overell" fullname="Paul Overell">
4349      <organization>THUS plc.</organization>
4350      <address>
4351        <email>paul.overell@thus.net</email>
4352      </address>
4353    </author>
4354    <date month="January" year="2008"/>
4355  </front>
4356  <seriesInfo name="STD" value="68"/>
4357  <seriesInfo name="RFC" value="5234"/>
4358</reference>
4359
4360<reference anchor="RFC2119">
4361  <front>
4362    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4363    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4364      <organization>Harvard University</organization>
4365      <address><email>sob@harvard.edu</email></address>
4366    </author>
4367    <date month="March" year="1997"/>
4368  </front>
4369  <seriesInfo name="BCP" value="14"/>
4370  <seriesInfo name="RFC" value="2119"/>
4371</reference>
4372
4373<reference anchor="RFC3986">
4374 <front>
4375  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4376  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4377    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4378    <address>
4379       <email>timbl@w3.org</email>
4380       <uri>http://www.w3.org/People/Berners-Lee/</uri>
4381    </address>
4382  </author>
4383  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4384    <organization abbrev="Day Software">Day Software</organization>
4385    <address>
4386      <email>fielding@gbiv.com</email>
4387      <uri>http://roy.gbiv.com/</uri>
4388    </address>
4389  </author>
4390  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4391    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4392    <address>
4393      <email>LMM@acm.org</email>
4394      <uri>http://larry.masinter.net/</uri>
4395    </address>
4396  </author>
4397  <date month='January' year='2005'></date>
4398 </front>
4399 <seriesInfo name="STD" value="66"/>
4400 <seriesInfo name="RFC" value="3986"/>
4401</reference>
4402
4403<reference anchor="RFC0793">
4404  <front>
4405    <title>Transmission Control Protocol</title>
4406    <author initials='J.' surname='Postel' fullname='Jon Postel'>
4407      <organization>University of Southern California (USC)/Information Sciences Institute</organization>
4408    </author>
4409    <date year='1981' month='September' />
4410  </front>
4411  <seriesInfo name='STD' value='7' />
4412  <seriesInfo name='RFC' value='793' />
4413</reference>
4414
4415<reference anchor="USASCII">
4416  <front>
4417    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4418    <author>
4419      <organization>American National Standards Institute</organization>
4420    </author>
4421    <date year="1986"/>
4422  </front>
4423  <seriesInfo name="ANSI" value="X3.4"/>
4424</reference>
4425
4426<reference anchor="RFC1950">
4427  <front>
4428    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4429    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4430      <organization>Aladdin Enterprises</organization>
4431      <address><email>ghost@aladdin.com</email></address>
4432    </author>
4433    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4434    <date month="May" year="1996"/>
4435  </front>
4436  <seriesInfo name="RFC" value="1950"/>
4437  <!--<annotation>
4438    RFC 1950 is an Informational RFC, thus it might be less stable than
4439    this specification. On the other hand, this downward reference was
4440    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4441    therefore it is unlikely to cause problems in practice. See also
4442    <xref target="BCP97"/>.
4443  </annotation>-->
4444</reference>
4445
4446<reference anchor="RFC1951">
4447  <front>
4448    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4449    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4450      <organization>Aladdin Enterprises</organization>
4451      <address><email>ghost@aladdin.com</email></address>
4452    </author>
4453    <date month="May" year="1996"/>
4454  </front>
4455  <seriesInfo name="RFC" value="1951"/>
4456  <!--<annotation>
4457    RFC 1951 is an Informational RFC, thus it might be less stable than
4458    this specification. On the other hand, this downward reference was
4459    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4460    therefore it is unlikely to cause problems in practice. See also
4461    <xref target="BCP97"/>.
4462  </annotation>-->
4463</reference>
4464
4465<reference anchor="RFC1952">
4466  <front>
4467    <title>GZIP file format specification version 4.3</title>
4468    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4469      <organization>Aladdin Enterprises</organization>
4470      <address><email>ghost@aladdin.com</email></address>
4471    </author>
4472    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4473      <address><email>gzip@prep.ai.mit.edu</email></address>
4474    </author>
4475    <author initials="M." surname="Adler" fullname="Mark Adler">
4476      <address><email>madler@alumni.caltech.edu</email></address>
4477    </author>
4478    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4479      <address><email>ghost@aladdin.com</email></address>
4480    </author>
4481    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4482      <address><email>randeg@alumni.rpi.edu</email></address>
4483    </author>
4484    <date month="May" year="1996"/>
4485  </front>
4486  <seriesInfo name="RFC" value="1952"/>
4487  <!--<annotation>
4488    RFC 1952 is an Informational RFC, thus it might be less stable than
4489    this specification. On the other hand, this downward reference was
4490    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4491    therefore it is unlikely to cause problems in practice. See also
4492    <xref target="BCP97"/>.
4493  </annotation>-->
4494</reference>
4495
4496<reference anchor="Welch">
4497  <front>
4498    <title>A Technique for High Performance Data Compression</title>
4499    <author initials="T.A." surname="Welch" fullname="Terry A. Welch"/>
4500    <date month="June" year="1984"/>
4501  </front>
4502  <seriesInfo name="IEEE Computer" value="17(6)"/>
4503</reference>
4504
4505</references>
4506
4507<references title="Informative References">
4508
4509<reference anchor="ISO-8859-1">
4510  <front>
4511    <title>
4512     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4513    </title>
4514    <author>
4515      <organization>International Organization for Standardization</organization>
4516    </author>
4517    <date year="1998"/>
4518  </front>
4519  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4520</reference>
4521
4522<reference anchor='RFC1919'>
4523  <front>
4524    <title>Classical versus Transparent IP Proxies</title>
4525    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4526      <address><email>mchatel@pax.eunet.ch</email></address>
4527    </author>
4528    <date year='1996' month='March' />
4529  </front>
4530  <seriesInfo name='RFC' value='1919' />
4531</reference>
4532
4533<reference anchor="RFC1945">
4534  <front>
4535    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4536    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4537      <organization>MIT, Laboratory for Computer Science</organization>
4538      <address><email>timbl@w3.org</email></address>
4539    </author>
4540    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4541      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4542      <address><email>fielding@ics.uci.edu</email></address>
4543    </author>
4544    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4545      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4546      <address><email>frystyk@w3.org</email></address>
4547    </author>
4548    <date month="May" year="1996"/>
4549  </front>
4550  <seriesInfo name="RFC" value="1945"/>
4551</reference>
4552
4553<reference anchor="RFC2045">
4554  <front>
4555    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4556    <author initials="N." surname="Freed" fullname="Ned Freed">
4557      <organization>Innosoft International, Inc.</organization>
4558      <address><email>ned@innosoft.com</email></address>
4559    </author>
4560    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4561      <organization>First Virtual Holdings</organization>
4562      <address><email>nsb@nsb.fv.com</email></address>
4563    </author>
4564    <date month="November" year="1996"/>
4565  </front>
4566  <seriesInfo name="RFC" value="2045"/>
4567</reference>
4568
4569<reference anchor="RFC2047">
4570  <front>
4571    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4572    <author initials="K." surname="Moore" fullname="Keith Moore">
4573      <organization>University of Tennessee</organization>
4574      <address><email>moore@cs.utk.edu</email></address>
4575    </author>
4576    <date month="November" year="1996"/>
4577  </front>
4578  <seriesInfo name="RFC" value="2047"/>
4579</reference>
4580
4581<reference anchor="RFC2068">
4582  <front>
4583    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4584    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
4585      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4586      <address><email>fielding@ics.uci.edu</email></address>
4587    </author>
4588    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4589      <organization>MIT Laboratory for Computer Science</organization>
4590      <address><email>jg@w3.org</email></address>
4591    </author>
4592    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4593      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4594      <address><email>mogul@wrl.dec.com</email></address>
4595    </author>
4596    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4597      <organization>MIT Laboratory for Computer Science</organization>
4598      <address><email>frystyk@w3.org</email></address>
4599    </author>
4600    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4601      <organization>MIT Laboratory for Computer Science</organization>
4602      <address><email>timbl@w3.org</email></address>
4603    </author>
4604    <date month="January" year="1997"/>
4605  </front>
4606  <seriesInfo name="RFC" value="2068"/>
4607</reference>
4608
4609<reference anchor="RFC2145">
4610  <front>
4611    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4612    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4613      <organization>Western Research Laboratory</organization>
4614      <address><email>mogul@wrl.dec.com</email></address>
4615    </author>
4616    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4617      <organization>Department of Information and Computer Science</organization>
4618      <address><email>fielding@ics.uci.edu</email></address>
4619    </author>
4620    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4621      <organization>MIT Laboratory for Computer Science</organization>
4622      <address><email>jg@w3.org</email></address>
4623    </author>
4624    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4625      <organization>W3 Consortium</organization>
4626      <address><email>frystyk@w3.org</email></address>
4627    </author>
4628    <date month="May" year="1997"/>
4629  </front>
4630  <seriesInfo name="RFC" value="2145"/>
4631</reference>
4632
4633<reference anchor="RFC2616">
4634  <front>
4635    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4636    <author initials="R." surname="Fielding" fullname="R. Fielding">
4637      <organization>University of California, Irvine</organization>
4638      <address><email>fielding@ics.uci.edu</email></address>
4639    </author>
4640    <author initials="J." surname="Gettys" fullname="J. Gettys">
4641      <organization>W3C</organization>
4642      <address><email>jg@w3.org</email></address>
4643    </author>
4644    <author initials="J." surname="Mogul" fullname="J. Mogul">
4645      <organization>Compaq Computer Corporation</organization>
4646      <address><email>mogul@wrl.dec.com</email></address>
4647    </author>
4648    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4649      <organization>MIT Laboratory for Computer Science</organization>
4650      <address><email>frystyk@w3.org</email></address>
4651    </author>
4652    <author initials="L." surname="Masinter" fullname="L. Masinter">
4653      <organization>Xerox Corporation</organization>
4654      <address><email>masinter@parc.xerox.com</email></address>
4655    </author>
4656    <author initials="P." surname="Leach" fullname="P. Leach">
4657      <organization>Microsoft Corporation</organization>
4658      <address><email>paulle@microsoft.com</email></address>
4659    </author>
4660    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4661      <organization>W3C</organization>
4662      <address><email>timbl@w3.org</email></address>
4663    </author>
4664    <date month="June" year="1999"/>
4665  </front>
4666  <seriesInfo name="RFC" value="2616"/>
4667</reference>
4668
4669<reference anchor='RFC2817'>
4670  <front>
4671    <title>Upgrading to TLS Within HTTP/1.1</title>
4672    <author initials='R.' surname='Khare' fullname='R. Khare'>
4673      <organization>4K Associates / UC Irvine</organization>
4674      <address><email>rohit@4K-associates.com</email></address>
4675    </author>
4676    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4677      <organization>Agranat Systems, Inc.</organization>
4678      <address><email>lawrence@agranat.com</email></address>
4679    </author>
4680    <date year='2000' month='May' />
4681  </front>
4682  <seriesInfo name='RFC' value='2817' />
4683</reference>
4684
4685<reference anchor='RFC2818'>
4686  <front>
4687    <title>HTTP Over TLS</title>
4688    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4689      <organization>RTFM, Inc.</organization>
4690      <address><email>ekr@rtfm.com</email></address>
4691    </author>
4692    <date year='2000' month='May' />
4693  </front>
4694  <seriesInfo name='RFC' value='2818' />
4695</reference>
4696
4697<reference anchor='RFC3040'>
4698  <front>
4699    <title>Internet Web Replication and Caching Taxonomy</title>
4700    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4701      <organization>Equinix, Inc.</organization>
4702    </author>
4703    <author initials='I.' surname='Melve' fullname='I. Melve'>
4704      <organization>UNINETT</organization>
4705    </author>
4706    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4707      <organization>CacheFlow Inc.</organization>
4708    </author>
4709    <date year='2001' month='January' />
4710  </front>
4711  <seriesInfo name='RFC' value='3040' />
4712</reference>
4713
4714<reference anchor='BCP90'>
4715  <front>
4716    <title>Registration Procedures for Message Header Fields</title>
4717    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4718      <organization>Nine by Nine</organization>
4719      <address><email>GK-IETF@ninebynine.org</email></address>
4720    </author>
4721    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4722      <organization>BEA Systems</organization>
4723      <address><email>mnot@pobox.com</email></address>
4724    </author>
4725    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4726      <organization>HP Labs</organization>
4727      <address><email>JeffMogul@acm.org</email></address>
4728    </author>
4729    <date year='2004' month='September' />
4730  </front>
4731  <seriesInfo name='BCP' value='90' />
4732  <seriesInfo name='RFC' value='3864' />
4733</reference>
4734
4735<reference anchor='RFC4033'>
4736  <front>
4737    <title>DNS Security Introduction and Requirements</title>
4738    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4739    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4740    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4741    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4742    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4743    <date year='2005' month='March' />
4744  </front>
4745  <seriesInfo name='RFC' value='4033' />
4746</reference>
4747
4748<reference anchor="BCP13">
4749  <front>
4750    <title>Media Type Specifications and Registration Procedures</title>
4751    <author initials="N." surname="Freed" fullname="Ned Freed">
4752      <organization>Oracle</organization>
4753      <address>
4754        <email>ned+ietf@mrochek.com</email>
4755      </address>
4756    </author>
4757    <author initials="J." surname="Klensin" fullname="John C. Klensin">
4758      <address>
4759        <email>john+ietf@jck.com</email>
4760      </address>
4761    </author>
4762    <author initials="T." surname="Hansen" fullname="Tony Hansen">
4763      <organization>AT&amp;T Laboratories</organization>
4764      <address>
4765        <email>tony+mtsuffix@maillennium.att.com</email>
4766      </address>
4767    </author>
4768    <date year="2013" month="January"/>
4769  </front>
4770  <seriesInfo name="BCP" value="13"/>
4771  <seriesInfo name="RFC" value="6838"/>
4772</reference>
4773
4774<reference anchor='BCP115'>
4775  <front>
4776    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4777    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4778      <organization>AT&amp;T Laboratories</organization>
4779      <address>
4780        <email>tony+urireg@maillennium.att.com</email>
4781      </address>
4782    </author>
4783    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4784      <organization>Qualcomm, Inc.</organization>
4785      <address>
4786        <email>hardie@qualcomm.com</email>
4787      </address>
4788    </author>
4789    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4790      <organization>Adobe Systems</organization>
4791      <address>
4792        <email>LMM@acm.org</email>
4793      </address>
4794    </author>
4795    <date year='2006' month='February' />
4796  </front>
4797  <seriesInfo name='BCP' value='115' />
4798  <seriesInfo name='RFC' value='4395' />
4799</reference>
4800
4801<reference anchor='RFC4559'>
4802  <front>
4803    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4804    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4805    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4806    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4807    <date year='2006' month='June' />
4808  </front>
4809  <seriesInfo name='RFC' value='4559' />
4810</reference>
4811
4812<reference anchor='RFC5226'>
4813  <front>
4814    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4815    <author initials='T.' surname='Narten' fullname='T. Narten'>
4816      <organization>IBM</organization>
4817      <address><email>narten@us.ibm.com</email></address>
4818    </author>
4819    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4820      <organization>Google</organization>
4821      <address><email>Harald@Alvestrand.no</email></address>
4822    </author>
4823    <date year='2008' month='May' />
4824  </front>
4825  <seriesInfo name='BCP' value='26' />
4826  <seriesInfo name='RFC' value='5226' />
4827</reference>
4828
4829<reference anchor='RFC5246'>
4830   <front>
4831      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4832      <author initials='T.' surname='Dierks' fullname='T. Dierks'>
4833         <organization />
4834      </author>
4835      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4836         <organization>RTFM, Inc.</organization>
4837      </author>
4838      <date year='2008' month='August' />
4839   </front>
4840   <seriesInfo name='RFC' value='5246' />
4841</reference>
4842
4843<reference anchor="RFC5322">
4844  <front>
4845    <title>Internet Message Format</title>
4846    <author initials="P." surname="Resnick" fullname="P. Resnick">
4847      <organization>Qualcomm Incorporated</organization>
4848    </author>
4849    <date year="2008" month="October"/>
4850  </front>
4851  <seriesInfo name="RFC" value="5322"/>
4852</reference>
4853
4854<reference anchor="RFC6265">
4855  <front>
4856    <title>HTTP State Management Mechanism</title>
4857    <author initials="A." surname="Barth" fullname="Adam Barth">
4858      <organization abbrev="U.C. Berkeley">
4859        University of California, Berkeley
4860      </organization>
4861      <address><email>abarth@eecs.berkeley.edu</email></address>
4862    </author>
4863    <date year="2011" month="April" />
4864  </front>
4865  <seriesInfo name="RFC" value="6265"/>
4866</reference>
4867
4868<reference anchor='RFC6585'>
4869  <front>
4870    <title>Additional HTTP Status Codes</title>
4871    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4872      <organization>Rackspace</organization>
4873    </author>
4874    <author initials='R.' surname='Fielding' fullname='R. Fielding'>
4875      <organization>Adobe</organization>
4876    </author>
4877    <date year='2012' month='April' />
4878   </front>
4879   <seriesInfo name='RFC' value='6585' />
4880</reference>
4881
4882<!--<reference anchor='BCP97'>
4883  <front>
4884    <title>Handling Normative References to Standards-Track Documents</title>
4885    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
4886      <address>
4887        <email>klensin+ietf@jck.com</email>
4888      </address>
4889    </author>
4890    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
4891      <organization>MIT</organization>
4892      <address>
4893        <email>hartmans-ietf@mit.edu</email>
4894      </address>
4895    </author>
4896    <date year='2007' month='June' />
4897  </front>
4898  <seriesInfo name='BCP' value='97' />
4899  <seriesInfo name='RFC' value='4897' />
4900</reference>-->
4901
4902<reference anchor="Kri2001" target="http://arxiv.org/abs/cs.SE/0105018">
4903  <front>
4904    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
4905    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
4906    <date year="2001" month="November"/>
4907  </front>
4908  <seriesInfo name="ACM Transactions on Internet Technology" value="1(2)"/>
4909</reference>
4910
4911</references>
4912
4913
4914<section title="HTTP Version History" anchor="compatibility">
4915<t>
4916   HTTP has been in use by the World-Wide Web global information initiative
4917   since 1990. The first version of HTTP, later referred to as HTTP/0.9,
4918   was a simple protocol for hypertext data transfer across the Internet
4919   with only a single request method (GET) and no metadata.
4920   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
4921   methods and MIME-like messaging that could include metadata about the data
4922   transferred and modifiers on the request/response semantics. However,
4923   HTTP/1.0 did not sufficiently take into consideration the effects of
4924   hierarchical proxies, caching, the need for persistent connections, or
4925   name-based virtual hosts. The proliferation of incompletely-implemented
4926   applications calling themselves "HTTP/1.0" further necessitated a
4927   protocol version change in order for two communicating applications
4928   to determine each other's true capabilities.
4929</t>
4930<t>
4931   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
4932   requirements that enable reliable implementations, adding only
4933   those new features that will either be safely ignored by an HTTP/1.0
4934   recipient or only sent when communicating with a party advertising
4935   conformance with HTTP/1.1.
4936</t>
4937<t>
4938   It is beyond the scope of a protocol specification to mandate
4939   conformance with previous versions. HTTP/1.1 was deliberately
4940   designed, however, to make supporting previous versions easy.
4941   We would expect a general-purpose HTTP/1.1 server to understand
4942   any valid request in the format of HTTP/1.0 and respond appropriately
4943   with an HTTP/1.1 message that only uses features understood (or
4944   safely ignored) by HTTP/1.0 clients.  Likewise, we would expect
4945   an HTTP/1.1 client to understand any valid HTTP/1.0 response.
4946</t>
4947<t>
4948   Since HTTP/0.9 did not support header fields in a request,
4949   there is no mechanism for it to support name-based virtual
4950   hosts (selection of resource by inspection of the <x:ref>Host</x:ref> header
4951   field).  Any server that implements name-based virtual hosts
4952   ought to disable support for HTTP/0.9.  Most requests that
4953   appear to be HTTP/0.9 are, in fact, badly constructed HTTP/1.x
4954   requests wherein a buggy client failed to properly encode
4955   linear whitespace found in a URI reference and placed in
4956   the request-target.
4957</t>
4958
4959<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
4960<t>
4961   This section summarizes major differences between versions HTTP/1.0
4962   and HTTP/1.1.
4963</t>
4964
4965<section title="Multi-homed Web Servers" anchor="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">
4966<t>
4967   The requirements that clients and servers support the <x:ref>Host</x:ref>
4968   header field (<xref target="header.host"/>), report an error if it is
4969   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
4970   are among the most important changes defined by HTTP/1.1.
4971</t>
4972<t>
4973   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4974   addresses and servers; there was no other established mechanism for
4975   distinguishing the intended server of a request than the IP address
4976   to which that request was directed. The <x:ref>Host</x:ref> header field was
4977   introduced during the development of HTTP/1.1 and, though it was
4978   quickly implemented by most HTTP/1.0 browsers, additional requirements
4979   were placed on all HTTP/1.1 requests in order to ensure complete
4980   adoption.  At the time of this writing, most HTTP-based services
4981   are dependent upon the Host header field for targeting requests.
4982</t>
4983</section>
4984
4985<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4986<t>
4987   In HTTP/1.0, each connection is established by the client prior to the
4988   request and closed by the server after sending the response. However, some
4989   implementations implement the explicitly negotiated ("Keep-Alive") version
4990   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
4991   target="RFC2068"/>.
4992</t>
4993<t>
4994   Some clients and servers might wish to be compatible with these previous
4995   approaches to persistent connections, by explicitly negotiating for them
4996   with a "Connection: keep-alive" request header field. However, some
4997   experimental implementations of HTTP/1.0 persistent connections are faulty;
4998   for example, if an HTTP/1.0 proxy server doesn't understand
4999   <x:ref>Connection</x:ref>, it will erroneously forward that header field
5000   to the next inbound server, which would result in a hung connection.
5001</t>
5002<t>
5003   One attempted solution was the introduction of a Proxy-Connection header
5004   field, targeted specifically at proxies. In practice, this was also
5005   unworkable, because proxies are often deployed in multiple layers, bringing
5006   about the same problem discussed above.
5007</t>
5008<t>
5009   As a result, clients are encouraged not to send the Proxy-Connection header
5010   field in any requests.
5011</t>
5012<t>
5013   Clients are also encouraged to consider the use of Connection: keep-alive
5014   in requests carefully; while they can enable persistent connections with
5015   HTTP/1.0 servers, clients using them will need to monitor the
5016   connection for "hung" requests (which indicate that the client ought stop
5017   sending the header field), and this mechanism ought not be used by clients
5018   at all when a proxy is being used.
5019</t>
5020</section>
5021
5022<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
5023<t>
5024   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
5025   (<xref target="header.transfer-encoding"/>).
5026   Transfer codings need to be decoded prior to forwarding an HTTP message
5027   over a MIME-compliant protocol.
5028</t>
5029</section>
5030
5031</section>
5032
5033<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
5034<t>
5035  HTTP's approach to error handling has been explained.
5036  (<xref target="conformance" />)
5037</t>
5038<t>
5039  The HTTP-version ABNF production has been clarified to be case-sensitive.
5040  Additionally, version numbers has been restricted to single digits, due
5041  to the fact that implementations are known to handle multi-digit version
5042  numbers incorrectly.
5043  (<xref target="http.version"/>)
5044</t>
5045<t>
5046  Userinfo (i.e., username and password) are now disallowed in HTTP and
5047  HTTPS URIs, because of security issues related to their transmission on the
5048  wire.
5049  (<xref target="http.uri" />)
5050</t>
5051<t>
5052  The HTTPS URI scheme is now defined by this specification; previously,
5053  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
5054  Furthermore, it implies end-to-end security.
5055  (<xref target="https.uri"/>)
5056</t>
5057<t>
5058  HTTP messages can be (and often are) buffered by implementations; despite
5059  it sometimes being available as a stream, HTTP is fundamentally a
5060  message-oriented protocol.
5061  Minimum supported sizes for various protocol elements have been
5062  suggested, to improve interoperability.
5063  (<xref target="http.message" />)
5064</t>
5065<t>
5066  Invalid whitespace around field-names is now required to be rejected,
5067  because accepting it represents a security vulnerability.
5068  The ABNF productions defining header fields now only list the field value.
5069  (<xref target="header.fields"/>)
5070</t>
5071<t>
5072  Rules about implicit linear whitespace between certain grammar productions
5073  have been removed; now whitespace is only allowed where specifically
5074  defined in the ABNF.
5075  (<xref target="whitespace"/>)
5076</t>
5077<t>
5078  Header fields that span multiple lines ("line folding") are deprecated.
5079  (<xref target="field.parsing" />)
5080</t>
5081<t> 
5082  The NUL octet is no longer allowed in comment and quoted-string text, and
5083  handling of backslash-escaping in them has been clarified.
5084  The quoted-pair rule no longer allows escaping control characters other than
5085  HTAB.
5086  Non-ASCII content in header fields and the reason phrase has been obsoleted
5087  and made opaque (the TEXT rule was removed).
5088  (<xref target="field.components"/>)
5089</t> 
5090<t>
5091  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
5092  handled as errors by recipients.
5093  (<xref target="header.content-length"/>)
5094</t>
5095<t>
5096  The algorithm for determining the message body length has been clarified
5097  to indicate all of the special cases (e.g., driven by methods or status
5098  codes) that affect it, and that new protocol elements cannot define such
5099  special cases.
5100  CONNECT is a new, special case in determining message body length.
5101  "multipart/byteranges" is no longer a way of determining message body length
5102  detection.
5103  (<xref target="message.body.length"/>)
5104</t>
5105<t>
5106  The "identity" transfer coding token has been removed.
5107  (Sections <xref format="counter" target="message.body"/> and
5108  <xref format="counter" target="transfer.codings"/>)
5109</t>
5110<t>
5111  Chunk length does not include the count of the octets in the
5112  chunk header and trailer.
5113  Use of chunk extensions is deprecated, and line folding in them is
5114  disallowed.
5115  (<xref target="chunked.encoding"/>)
5116</t>
5117<t>
5118  The meaning of the "deflate" content coding has been clarified.
5119  (<xref target="deflate.coding" />)
5120</t>
5121<t>
5122  The segment + query components of RFC 3986 have been used to define the
5123  request-target, instead of abs_path from RFC 1808.
5124  The asterisk-form of the request-target is only allowed in the OPTIONS
5125  method.
5126  (<xref target="request-target"/>)
5127</t>
5128<t>
5129  The term "Effective Request URI" has been introduced.
5130  (<xref target="effective.request.uri" />)
5131</t>
5132<t>
5133  Gateways do not need to generate <x:ref>Via</x:ref> header fields anymore.
5134  (<xref target="header.via"/>)
5135</t>
5136<t>
5137  Exactly when "close" connection options have to be sent has been clarified.
5138  Also, "hop-by-hop" header fields are required to appear in the Connection header
5139  field; just because they're defined as hop-by-hop in this specification
5140  doesn't exempt them.
5141  (<xref target="header.connection"/>)
5142</t>
5143<t>
5144  The limit of two connections per server has been removed.
5145  An idempotent sequence of requests is no longer required to be retried.
5146  The requirement to retry requests under certain circumstances when the
5147  server prematurely closes the connection has been removed.
5148  Also, some extraneous requirements about when servers are allowed to close
5149  connections prematurely have been removed.
5150  (<xref target="persistent.connections"/>)
5151</t>
5152<t>
5153  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
5154  responses other than 101 (this was incorporated from <xref
5155  target="RFC2817"/>). Furthermore, the ordering in the field value is now
5156  significant.
5157  (<xref target="header.upgrade"/>)
5158</t>
5159<t>
5160  Empty list elements in list productions (e.g., a list header field containing
5161  ", ,") have been deprecated.
5162  (<xref target="abnf.extension"/>)
5163</t>
5164<t>
5165  Registration of Transfer Codings now requires IETF Review
5166  (<xref target="transfer.coding.registry"/>)
5167</t>
5168<t>
5169  This specification now defines the Upgrade Token Registry, previously
5170  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
5171  (<xref target="upgrade.token.registry"/>)
5172</t>
5173<t>
5174  The expectation to support HTTP/0.9 requests has been removed.
5175  (<xref target="compatibility"/>)
5176</t>
5177<t>
5178  Issues with the Keep-Alive and Proxy-Connection header fields in requests
5179  are pointed out, with use of the latter being discouraged altogether.
5180  (<xref target="compatibility.with.http.1.0.persistent.connections" />)
5181</t>
5182</section>
5183</section>
5184
5185<?BEGININC p1-messaging.abnf-appendix ?>
5186<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
5187<figure>
5188<artwork type="abnf" name="p1-messaging.parsed-abnf">
5189<x:ref>BWS</x:ref> = OWS
5190
5191<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
5192 connection-option ] )
5193<x:ref>Content-Length</x:ref> = 1*DIGIT
5194
5195<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
5196 ]
5197<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
5198<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
5199<x:ref>Host</x:ref> = uri-host [ ":" port ]
5200
5201<x:ref>OWS</x:ref> = *( SP / HTAB )
5202
5203<x:ref>RWS</x:ref> = 1*( SP / HTAB )
5204
5205<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
5206<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
5207<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
5208 transfer-coding ] )
5209
5210<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
5211<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
5212
5213<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
5214 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
5215 comment ] ) ] )
5216
5217<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
5218<x:ref>absolute-form</x:ref> = absolute-URI
5219<x:ref>absolute-path</x:ref> = 1*( "/" segment )
5220<x:ref>asterisk-form</x:ref> = "*"
5221<x:ref>attribute</x:ref> = token
5222<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
5223<x:ref>authority-form</x:ref> = authority
5224
5225<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
5226<x:ref>chunk-data</x:ref> = 1*OCTET
5227<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
5228<x:ref>chunk-ext-name</x:ref> = token
5229<x:ref>chunk-ext-val</x:ref> = token / quoted-str-nf
5230<x:ref>chunk-size</x:ref> = 1*HEXDIG
5231<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
5232<x:ref>comment</x:ref> = "(" *( ctext / quoted-cpair / comment ) ")"
5233<x:ref>connection-option</x:ref> = token
5234<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
5235 / %x2A-5B ; '*'-'['
5236 / %x5D-7E ; ']'-'~'
5237 / obs-text
5238
5239<x:ref>field-content</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5240<x:ref>field-name</x:ref> = token
5241<x:ref>field-value</x:ref> = *( field-content / obs-fold )
5242<x:ref>fragment</x:ref> = &lt;fragment, defined in [RFC3986], Section 3.5&gt;
5243
5244<x:ref>header-field</x:ref> = field-name ":" OWS field-value OWS
5245<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ] [ "#"
5246 fragment ]
5247<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ] [ "#"
5248 fragment ]
5249
5250<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
5251
5252<x:ref>message-body</x:ref> = *OCTET
5253<x:ref>method</x:ref> = token
5254
5255<x:ref>obs-fold</x:ref> = CRLF ( SP / HTAB )
5256<x:ref>obs-text</x:ref> = %x80-FF
5257<x:ref>origin-form</x:ref> = absolute-path [ "?" query ]
5258
5259<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
5260<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
5261<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
5262<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
5263<x:ref>protocol-name</x:ref> = token
5264<x:ref>protocol-version</x:ref> = token
5265<x:ref>pseudonym</x:ref> = token
5266
5267<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5268 / %x5D-7E ; ']'-'~'
5269 / obs-text
5270<x:ref>qdtext-nf</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5271 / %x5D-7E ; ']'-'~'
5272 / obs-text
5273<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5274<x:ref>quoted-cpair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5275<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5276<x:ref>quoted-str-nf</x:ref> = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
5277<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5278
5279<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5280<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5281<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5282<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5283<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5284<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5285<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5286 asterisk-form
5287
5288<x:ref>segment</x:ref> = &lt;segment, defined in [RFC3986], Section 3.3&gt;
5289<x:ref>special</x:ref> = "(" / ")" / "&lt;" / "&gt;" / "@" / "," / ";" / ":" / "\" /
5290 DQUOTE / "/" / "[" / "]" / "?" / "=" / "{" / "}"
5291<x:ref>start-line</x:ref> = request-line / status-line
5292<x:ref>status-code</x:ref> = 3DIGIT
5293<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5294
5295<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5296<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5297<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5298 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5299<x:ref>token</x:ref> = 1*tchar
5300<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5301<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5302 transfer-extension
5303<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5304<x:ref>transfer-parameter</x:ref> = attribute BWS "=" BWS value
5305
5306<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5307
5308<x:ref>value</x:ref> = word
5309
5310<x:ref>word</x:ref> = token / quoted-string
5311</artwork>
5312</figure>
5313</section>
5314<?ENDINC p1-messaging.abnf-appendix ?>
5315
5316<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5317
5318<section title="Since RFC 2616">
5319<t>
5320  Changes up to the first Working Group Last Call draft are summarized
5321  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-21#appendix-D"/>.
5322</t>
5323</section>
5324
5325<section title="Since draft-ietf-httpbis-p1-messaging-21" anchor="changes.since.21">
5326<t>
5327  Closed issues:
5328  <list style="symbols">
5329    <t>
5330      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/128"/>:
5331      "Cite HTTPS URI scheme definition" (the spec now includes the HTTPs
5332      scheme definition and thus updates RFC 2818)
5333    </t>
5334    <t>
5335      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/389"/>:
5336      "mention of 'proxies' in section about caches"
5337    </t>
5338    <t>
5339      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/390"/>:
5340      "use of ABNF terms from RFC 3986"
5341    </t>
5342    <t>
5343      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/391"/>:
5344      "transferring URIs with userinfo in payload"
5345    </t>
5346    <t>
5347      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/392"/>:
5348      "editorial improvements to message length definition"
5349    </t>
5350    <t>
5351      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/395"/>:
5352      "Connection header field MUST vs SHOULD"
5353    </t>
5354    <t>
5355      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/396"/>:
5356      "editorial improvements to persistent connections section"
5357    </t>
5358    <t>
5359      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/397"/>:
5360      "URI normalization vs empty path"
5361    </t>
5362    <t>
5363      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/408"/>:
5364      "p1 feedback"
5365    </t>
5366    <t>
5367      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/409"/>:
5368      "is parsing OBS-FOLD mandatory?"
5369    </t>
5370    <t>
5371      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/410"/>:
5372      "HTTPS and Shared Caching"
5373    </t>
5374    <t>
5375      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/411"/>:
5376      "Requirements for recipients of ws between start-line and first header field"
5377    </t>
5378    <t>
5379      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/412"/>:
5380      "SP and HT when being tolerant"
5381    </t>
5382    <t>
5383      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/414"/>:
5384      "Message Parsing Strictness"
5385    </t>
5386    <t>
5387      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/415"/>:
5388      "'Render'"
5389    </t>
5390    <t>
5391      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/418"/>:
5392      "No-Transform"
5393    </t>
5394    <t>
5395      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/419"/>:
5396      "p2 editorial feedback"
5397    </t>
5398    <t>
5399      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/420"/>:
5400      "Content-Length SHOULD be sent"
5401    </t>
5402    <t>
5403      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/431"/>:
5404      "origin-form does not allow path starting with "//""
5405    </t>
5406    <t>
5407      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/433"/>:
5408      "ambiguity in part 1 example"
5409    </t>
5410  </list>
5411</t>
5412</section>
5413
5414<section title="Since draft-ietf-httpbis-p1-messaging-22" anchor="changes.since.22">
5415<t>
5416  Closed issues:
5417  <list style="symbols">
5418    <t>
5419      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/435"/>:
5420      "Part1 should have a reference to TCP (RFC 793)"
5421    </t>
5422    <t>
5423      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/438"/>:
5424      "media type registration template issues"
5425    </t>
5426    <t>
5427      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/441"/>:
5428      P1 editorial nits
5429    </t>
5430    <t>
5431      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/442"/>:
5432      "BWS" (vs conformance)
5433    </t>
5434    <t>
5435      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/444"/>:
5436      "obs-fold language"
5437    </t>
5438    <t>
5439      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/445"/>:
5440      "Ordering in Upgrade"
5441    </t>
5442    <t>
5443      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/446"/>:
5444      "p1 editorial feedback"
5445    </t>
5446    <t>
5447      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/447"/>:
5448      "HTTP and TCP name delegation"
5449    </t>
5450    <t>
5451      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/449"/>:
5452      "Receiving a higher minor HTTP version number"
5453    </t>
5454    <t>
5455      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/451"/>:
5456      "HTTP(S) URIs and fragids"
5457    </t>
5458    <t>
5459      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/457"/>:
5460      "Registering x-gzip and x-deflate"
5461    </t>
5462    <t>
5463      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/460"/>:
5464      "Via and gateways"
5465    </t>
5466    <t>
5467      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/465"/>:
5468      "Mention 203 Non-Authoritative Information in p1"
5469    </t>
5470    <t>
5471      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/476"/>:
5472      "SHOULD and conformance"
5473    </t>
5474    <t>
5475      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/477"/>:
5476      "Pipelining language"
5477    </t>
5478    <t>
5479      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/482"/>:
5480      "proxy handling of a really bad Content-Length"
5481    </t>
5482  </list>
5483</t>
5484</section>
5485
5486<section title="Since draft-ietf-httpbis-p1-messaging-23" anchor="changes.since.23">
5487<t>
5488  Closed issues:
5489  <list style="symbols">
5490    <t>
5491      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/483"/>:
5492      "MUST fix Content-Length?"
5493    </t>
5494    <t>
5495      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/492"/>:
5496      "list notation defined in appendix"
5497    </t>
5498  </list>
5499</t>
5500</section>
5501</section>
5502
5503</back>
5504</rfc>
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