source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2405

Last change on this file since 2405 was 2405, checked in by julian.reschke@…, 8 years ago

Undo deprecation of chunk extensions (see #343)

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 237.3 KB
Line 
1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "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, a server &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 code 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   A sender &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   Before making use of an "http" URI reference received from an untrusted
941   source, a recipient ought to parse for userinfo and treat its presence as
942   an error; it is likely being used to obscure the authority for the sake of
943   phishing attacks.
944</t>
945</section>
946
947<section title="https URI scheme" anchor="https.uri">
948   <x:anchor-alias value="https-URI"/>
949   <iref item="https URI scheme"/>
950   <iref item="URI scheme" subitem="https"/>
951<t>
952   The "https" URI scheme is hereby defined for the purpose of minting
953   identifiers according to their association with the hierarchical
954   namespace governed by a potential HTTP origin server listening to a
955   given TCP port for TLS-secured connections
956   (<xref target="RFC0793"/>, <xref target="RFC5246"/>).
957</t>
958<t>
959   All of the requirements listed above for the "http" scheme are also
960   requirements for the "https" scheme, except that a default TCP port
961   of 443 is assumed if the port subcomponent is empty or not given,
962   and the user agent &MUST; ensure that its connection to the origin
963   server is secured through the use of strong encryption, end-to-end,
964   prior to sending the first HTTP request.
965</t>
966<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="https-URI"><!--terminal production--></iref>
967  <x:ref>https-URI</x:ref> = "https:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
968              [ "#" <x:ref>fragment</x:ref> ]
969</artwork></figure>
970<t>
971   Note that the "https" URI scheme depends on both TLS and TCP for
972   establishing authority.
973   Resources made available via the "https" scheme have no shared
974   identity with the "http" scheme even if their resource identifiers
975   indicate the same authority (the same host listening to the same
976   TCP port).  They are distinct name spaces and are considered to be
977   distinct origin servers.  However, an extension to HTTP that is
978   defined to apply to entire host domains, such as the Cookie protocol
979   <xref target="RFC6265"/>, can allow information
980   set by one service to impact communication with other services
981   within a matching group of host domains.
982</t>
983<t>
984   The process for authoritative access to an "https" identified
985   resource is defined in <xref target="RFC2818"/>.
986</t>
987</section>
988
989<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
990<t>
991   Since the "http" and "https" schemes conform to the URI generic syntax,
992   such URIs are normalized and compared according to the algorithm defined
993   in <xref target="RFC3986" x:fmt="," x:sec="6"/>, using the defaults
994   described above for each scheme.
995</t>
996<t>
997   If the port is equal to the default port for a scheme, the normal form is
998   to omit the port subcomponent. When not being used in absolute form as the
999   request target of an OPTIONS request, an empty path component is equivalent
1000   to an absolute path of "/", so the normal form is to provide a path of "/"
1001   instead. The scheme and host are case-insensitive and normally provided in
1002   lowercase; all other components are compared in a case-sensitive manner.
1003   Characters other than those in the "reserved" set are equivalent to their
1004   percent-encoded octets (see <xref target="RFC3986" x:fmt=","
1005   x:sec="2.1"/>): the normal form is to not encode them.
1006</t>
1007<t>
1008   For example, the following three URIs are equivalent:
1009</t>
1010<figure><artwork type="example">
1011   http://example.com:80/~smith/home.html
1012   http://EXAMPLE.com/%7Esmith/home.html
1013   http://EXAMPLE.com:/%7esmith/home.html
1014</artwork></figure>
1015</section>
1016</section>
1017</section>
1018
1019<section title="Message Format" anchor="http.message">
1020<x:anchor-alias value="generic-message"/>
1021<x:anchor-alias value="message.types"/>
1022<x:anchor-alias value="HTTP-message"/>
1023<x:anchor-alias value="start-line"/>
1024<iref item="header section"/>
1025<iref item="headers"/>
1026<iref item="header field"/>
1027<t>
1028   All HTTP/1.1 messages consist of a start-line followed by a sequence of
1029   octets in a format similar to the Internet Message Format
1030   <xref target="RFC5322"/>: zero or more header fields (collectively
1031   referred to as the "headers" or the "header section"), an empty line
1032   indicating the end of the header section, and an optional message body.
1033</t>
1034<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"><!--terminal production--></iref>
1035  <x:ref>HTTP-message</x:ref>   = <x:ref>start-line</x:ref>
1036                   *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1037                   <x:ref>CRLF</x:ref>
1038                   [ <x:ref>message-body</x:ref> ]
1039</artwork></figure>
1040<t>
1041   The normal procedure for parsing an HTTP message is to read the
1042   start-line into a structure, read each header field into a hash
1043   table by field name until the empty line, and then use the parsed
1044   data to determine if a message body is expected.  If a message body
1045   has been indicated, then it is read as a stream until an amount
1046   of octets equal to the message body length is read or the connection
1047   is closed.
1048</t>
1049<t>
1050   A recipient &MUST; parse an HTTP message as a sequence of octets in an
1051   encoding that is a superset of US-ASCII <xref target="USASCII"/>.
1052   Parsing an HTTP message as a stream of Unicode characters, without regard
1053   for the specific encoding, creates security vulnerabilities due to the
1054   varying ways that string processing libraries handle invalid multibyte
1055   character sequences that contain the octet LF (%x0A).  String-based
1056   parsers can only be safely used within protocol elements after the element
1057   has been extracted from the message, such as within a header field-value
1058   after message parsing has delineated the individual fields.
1059</t>
1060<t>
1061   An HTTP message can be parsed as a stream for incremental processing or
1062   forwarding downstream.  However, recipients cannot rely on incremental
1063   delivery of partial messages, since some implementations will buffer or
1064   delay message forwarding for the sake of network efficiency, security
1065   checks, or payload transformations.
1066</t>
1067<t>
1068   A sender &MUST-NOT; send whitespace between the start-line and
1069   the first header field.
1070   A recipient that receives whitespace between the start-line and
1071   the first header field &MUST; either reject the message as invalid or
1072   consume each whitespace-preceded line without further processing of it
1073   (i.e., ignore the entire line, along with any subsequent lines preceded
1074   by whitespace, until a properly formed header field is received or the
1075   header section is terminated).
1076</t>
1077<t>
1078   The presence of such whitespace in a request
1079   might be an attempt to trick a server into ignoring that field or
1080   processing the line after it as a new request, either of which might
1081   result in a security vulnerability if other implementations within
1082   the request chain interpret the same message differently.
1083   Likewise, the presence of such whitespace in a response might be
1084   ignored by some clients or cause others to cease parsing.
1085</t>
1086
1087<section title="Start Line" anchor="start.line">
1088  <x:anchor-alias value="Start-Line"/>
1089<t>
1090   An HTTP message can either be a request from client to server or a
1091   response from server to client.  Syntactically, the two types of message
1092   differ only in the start-line, which is either a request-line (for requests)
1093   or a status-line (for responses), and in the algorithm for determining
1094   the length of the message body (<xref target="message.body"/>).
1095</t>
1096<t>
1097   In theory, a client could receive requests and a server could receive
1098   responses, distinguishing them by their different start-line formats,
1099   but in practice servers are implemented to only expect a request
1100   (a response is interpreted as an unknown or invalid request method)
1101   and clients are implemented to only expect a response.
1102</t>
1103<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="start-line"/>
1104  <x:ref>start-line</x:ref>     = <x:ref>request-line</x:ref> / <x:ref>status-line</x:ref>
1105</artwork></figure>
1106
1107<section title="Request Line" anchor="request.line">
1108  <x:anchor-alias value="Request"/>
1109  <x:anchor-alias value="request-line"/>
1110<t>
1111   A request-line begins with a method token, followed by a single
1112   space (SP), the request-target, another single space (SP), the
1113   protocol version, and ending with CRLF.
1114</t>
1115<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1116  <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>
1117</artwork></figure>
1118<iref primary="true" item="method"/>
1119<t anchor="method">
1120   The method token indicates the request method to be performed on the
1121   target resource. The request method is case-sensitive.
1122</t>
1123<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1124  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1125</artwork></figure>
1126<t>
1127   The request methods defined by this specification can be found in
1128   &methods;, along with information regarding the HTTP method registry
1129   and considerations for defining new methods.
1130</t>
1131<iref item="request-target"/>
1132<t>
1133   The request-target identifies the target resource upon which to apply
1134   the request, as defined in <xref target="request-target"/>.
1135</t>
1136<t>
1137   Recipients typically parse the request-line into its component parts by
1138   splitting on whitespace (see <xref target="message.robustness"/>), since
1139   no whitespace is allowed in the three components.
1140   Unfortunately, some user agents fail to properly encode or exclude
1141   whitespace found in hypertext references, resulting in those disallowed
1142   characters being sent in a request-target.
1143</t>
1144<t>
1145   Recipients of an invalid request-line &SHOULD; respond with either a
1146   <x:ref>400 (Bad Request)</x:ref> error or a <x:ref>301 (Moved Permanently)</x:ref>
1147   redirect with the request-target properly encoded.  A recipient &SHOULD-NOT;
1148   attempt to autocorrect and then process the request without a redirect,
1149   since the invalid request-line might be deliberately crafted to bypass
1150   security filters along the request chain.
1151</t>
1152<t>
1153   HTTP does not place a pre-defined limit on the length of a request-line.
1154   A server that receives a method longer than any that it implements
1155   &SHOULD; respond with a <x:ref>501 (Not Implemented)</x:ref> status code.
1156   A server ought to be prepared to receive URIs of unbounded length, as
1157   described in <xref target="conformance"/>, and &MUST; respond with a
1158   <x:ref>414 (URI Too Long)</x:ref> status code if the received
1159   request-target is longer than the server wishes to parse (see &status-414;).
1160</t>
1161<t>
1162   Various ad-hoc limitations on request-line length are found in practice.
1163   It is &RECOMMENDED; that all HTTP senders and recipients support, at a
1164   minimum, request-line lengths of 8000 octets.
1165</t>
1166</section>
1167
1168<section title="Status Line" anchor="status.line">
1169  <x:anchor-alias value="response"/>
1170  <x:anchor-alias value="status-line"/>
1171  <x:anchor-alias value="status-code"/>
1172  <x:anchor-alias value="reason-phrase"/>
1173<t>
1174   The first line of a response message is the status-line, consisting
1175   of the protocol version, a space (SP), the status code, another space,
1176   a possibly-empty textual phrase describing the status code, and
1177   ending with CRLF.
1178</t>
1179<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-line"/>
1180  <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>
1181</artwork></figure>
1182<t>
1183   The status-code element is a 3-digit integer code describing the
1184   result of the server's attempt to understand and satisfy the client's
1185   corresponding request. The rest of the response message is to be
1186   interpreted in light of the semantics defined for that status code.
1187   See &status-codes; for information about the semantics of status codes,
1188   including the classes of status code (indicated by the first digit),
1189   the status codes defined by this specification, considerations for the
1190   definition of new status codes, and the IANA registry.
1191</t>
1192<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-code"/>
1193  <x:ref>status-code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1194</artwork></figure>
1195<t>  
1196   The reason-phrase element exists for the sole purpose of providing a
1197   textual description associated with the numeric status code, mostly
1198   out of deference to earlier Internet application protocols that were more
1199   frequently used with interactive text clients. A client &SHOULD; ignore
1200   the reason-phrase content.
1201</t>
1202<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
1203  <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> )
1204</artwork></figure>
1205</section>
1206</section>
1207
1208<section title="Header Fields" anchor="header.fields">
1209  <x:anchor-alias value="header-field"/>
1210  <x:anchor-alias value="field-content"/>
1211  <x:anchor-alias value="field-name"/>
1212  <x:anchor-alias value="field-value"/>
1213  <x:anchor-alias value="obs-fold"/>
1214<t>
1215   Each HTTP header field consists of a case-insensitive field name
1216   followed by a colon (":"), optional leading whitespace, the field value,
1217   and optional trailing whitespace.
1218</t>
1219<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"/>
1220  <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>
1221  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1222  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>obs-fold</x:ref> )
1223  <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> )
1224  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref> ( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1225                 ; obsolete line folding
1226                 ; see <xref target="field.parsing"/>
1227</artwork></figure>
1228<t>
1229   The field-name token labels the corresponding field-value as having the
1230   semantics defined by that header field.  For example, the <x:ref>Date</x:ref>
1231   header field is defined in &header-date; as containing the origination
1232   timestamp for the message in which it appears.
1233</t>
1234
1235<section title="Field Extensibility" anchor="field.extensibility">
1236<t>
1237   Header fields are fully extensible: there is no limit on the
1238   introduction of new field names, each presumably defining new semantics,
1239   nor on the number of header fields used in a given message.  Existing
1240   fields are defined in each part of this specification and in many other
1241   specifications outside the core standard.
1242</t>
1243<t>
1244   New header fields can be defined such that, when they are understood by a
1245   recipient, they might override or enhance the interpretation of previously
1246   defined header fields, define preconditions on request evaluation, or
1247   refine the meaning of responses.
1248</t>
1249<t>
1250   A proxy &MUST; forward unrecognized header fields unless the
1251   field-name is listed in the <x:ref>Connection</x:ref> header field
1252   (<xref target="header.connection"/>) or the proxy is specifically
1253   configured to block, or otherwise transform, such fields.
1254   Other recipients &SHOULD; ignore unrecognized header fields.
1255   These requirements allow HTTP's functionality to be enhanced without
1256   requiring prior update of deployed intermediaries.
1257</t>
1258<t>
1259   All defined header fields ought to be registered with IANA in the
1260   Message Header Field Registry, as described in &iana-header-registry;.
1261</t>
1262</section>
1263
1264<section title="Field Order" anchor="field.order">
1265<t>
1266   The order in which header fields with differing field names are
1267   received is not significant. However, it is "good practice" to send
1268   header fields that contain control data first, such as <x:ref>Host</x:ref>
1269   on requests and <x:ref>Date</x:ref> on responses, so that implementations
1270   can decide when not to handle a message as early as possible.  A server
1271   &MUST; wait until the entire header section is received before interpreting
1272   a request message, since later header fields might include conditionals,
1273   authentication credentials, or deliberately misleading duplicate
1274   header fields that would impact request processing.
1275</t>
1276<t>
1277   A sender &MUST-NOT; generate multiple header fields with the same field
1278   name in a message unless either the entire field value for that
1279   header field is defined as a comma-separated list [i.e., #(values)]
1280   or the header field is a well-known exception (as noted below).
1281</t>
1282<t>
1283   A recipient &MAY; combine multiple header fields with the same field name
1284   into one "field-name: field-value" pair, without changing the semantics of
1285   the message, by appending each subsequent field value to the combined
1286   field value in order, separated by a comma. The order in which
1287   header fields with the same field name are received is therefore
1288   significant to the interpretation of the combined field value;
1289   a proxy &MUST-NOT; change the order of these field values when
1290   forwarding a message.
1291</t>
1292<x:note>
1293  <t>
1294   &Note; In practice, the "Set-Cookie" header field (<xref target="RFC6265"/>)
1295   often appears multiple times in a response message and does not use the
1296   list syntax, violating the above requirements on multiple header fields
1297   with the same name. Since it cannot be combined into a single field-value,
1298   recipients ought to handle "Set-Cookie" as a special case while processing
1299   header fields. (See Appendix A.2.3 of <xref target="Kri2001"/> for details.)
1300  </t>
1301</x:note>
1302</section>
1303
1304<section title="Whitespace" anchor="whitespace">
1305<t anchor="rule.LWS">
1306   This specification uses three rules to denote the use of linear
1307   whitespace: OWS (optional whitespace), RWS (required whitespace), and
1308   BWS ("bad" whitespace).
1309</t>
1310<t anchor="rule.OWS">
1311   The OWS rule is used where zero or more linear whitespace octets might
1312   appear. For protocol elements where optional whitespace is preferred to
1313   improve readability, a sender &SHOULD; generate the optional whitespace
1314   as a single SP; otherwise, a sender &SHOULD-NOT; generate optional
1315   whitespace except as needed to white-out invalid or unwanted protocol
1316   elements during in-place message filtering.
1317</t>
1318<t anchor="rule.RWS">
1319   The RWS rule is used when at least one linear whitespace octet is required
1320   to separate field tokens. A sender &SHOULD; generate RWS as a single SP.
1321</t>
1322<t anchor="rule.BWS">
1323   The BWS rule is used where the grammar allows optional whitespace only for
1324   historical reasons. A sender &MUST-NOT; generate BWS in messages.
1325   A recipient &MUST; parse for such bad whitespace and remove it before
1326   interpreting the protocol element.
1327</t>
1328<t anchor="rule.whitespace">
1329  <x:anchor-alias value="BWS"/>
1330  <x:anchor-alias value="OWS"/>
1331  <x:anchor-alias value="RWS"/>
1332</t>
1333<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"/>
1334  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1335                 ; optional whitespace
1336  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1337                 ; required whitespace
1338  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1339                 ; "bad" whitespace
1340</artwork></figure>
1341</section>
1342
1343<section title="Field Parsing" anchor="field.parsing">
1344<t>
1345   No whitespace is allowed between the header field-name and colon.
1346   In the past, differences in the handling of such whitespace have led to
1347   security vulnerabilities in request routing and response handling.
1348   A server &MUST; reject any received request message that contains
1349   whitespace between a header field-name and colon with a response code of
1350   <x:ref>400 (Bad Request)</x:ref>. A proxy &MUST; remove any such whitespace
1351   from a response message before forwarding the message downstream.
1352</t>
1353<t>
1354   A field value is preceded by optional whitespace (OWS); a single SP is
1355   preferred. The field value does not include any leading or trailing white
1356   space: OWS occurring before the first non-whitespace octet of the field
1357   value or after the last non-whitespace octet of the field value ought to be
1358   excluded by parsers when extracting the field value from a header field.
1359</t>
1360<t>
1361   A recipient of field-content containing multiple sequential octets of
1362   optional (OWS) or required (RWS) whitespace &SHOULD; either replace the
1363   sequence with a single SP or transform any non-SP octets in the sequence to
1364   SP octets before interpreting the field value or forwarding the message
1365   downstream.
1366</t>
1367<t>
1368   Historically, HTTP header field values could be extended over multiple
1369   lines by preceding each extra line with at least one space or horizontal
1370   tab (obs-fold). This specification deprecates such line folding except
1371   within the message/http media type
1372   (<xref target="internet.media.type.message.http"/>).
1373   A sender &MUST-NOT; generate a message that includes line folding
1374   (i.e., that has any field-value that contains a match to the
1375   <x:ref>obs-fold</x:ref> rule) unless the message is intended for packaging
1376   within the message/http media type.
1377</t>
1378<t>
1379   A server that receives an <x:ref>obs-fold</x:ref> in a request message that
1380   is not within a message/http container &MUST; either reject the message by
1381   sending a <x:ref>400 (Bad Request)</x:ref>, preferably with a
1382   representation explaining that obsolete line folding is unacceptable, or
1383   replace each received <x:ref>obs-fold</x:ref> with one or more
1384   <x:ref>SP</x:ref> octets prior to interpreting the field value or
1385   forwarding the message downstream.
1386</t>
1387<t>
1388   A proxy or gateway that receives an <x:ref>obs-fold</x:ref> in a response
1389   message that is not within a message/http container &MUST; either discard
1390   the message and replace it with a <x:ref>502 (Bad Gateway)</x:ref>
1391   response, preferably with a representation explaining that unacceptable
1392   line folding was received, or replace each received <x:ref>obs-fold</x:ref>
1393   with one or more <x:ref>SP</x:ref> octets prior to interpreting the field
1394   value or forwarding the message downstream.
1395</t>
1396<t>
1397   A user agent that receives an <x:ref>obs-fold</x:ref> in a response message
1398   that is not within a message/http container &MUST; replace each received
1399   <x:ref>obs-fold</x:ref> with one or more <x:ref>SP</x:ref> octets prior to
1400   interpreting the field value.
1401</t>
1402<t>
1403   Historically, HTTP has allowed field content with text in the ISO-8859-1
1404   <xref target="ISO-8859-1"/> charset, supporting other charsets only
1405   through use of <xref target="RFC2047"/> encoding.
1406   In practice, most HTTP header field values use only a subset of the
1407   US-ASCII charset <xref target="USASCII"/>. Newly defined
1408   header fields &SHOULD; limit their field values to US-ASCII octets.
1409   A recipient &SHOULD; treat other octets in field content (obs-text) as
1410   opaque data.
1411</t>
1412</section>
1413
1414<section title="Field Limits" anchor="field.limits">
1415<t>
1416   HTTP does not place a pre-defined limit on the length of each header field
1417   or on the length of the header section as a whole, as described in
1418   <xref target="conformance"/>. Various ad-hoc limitations on individual
1419   header field length are found in practice, often depending on the specific
1420   field semantics.
1421</t>
1422<t>
1423   A server ought to be prepared to receive request header fields of unbounded
1424   length and &MUST; respond with an appropriate
1425   <x:ref>4xx (Client Error)</x:ref> status code if the received header
1426   field(s) are larger than the server wishes to process.
1427</t>
1428<t>
1429   A client ought to be prepared to receive response header fields of
1430   unbounded length.
1431   A client &MAY; discard or truncate received header fields that are larger
1432   than the client wishes to process if the field semantics are such that the
1433   dropped value(s) can be safely ignored without changing the
1434   message framing or response semantics.
1435</t>
1436</section>
1437
1438<section title="Field value components" anchor="field.components">
1439<t anchor="rule.token.separators">
1440  <x:anchor-alias value="tchar"/>
1441  <x:anchor-alias value="token"/>
1442  <x:anchor-alias value="special"/>
1443  <x:anchor-alias value="word"/>
1444   Many HTTP header field values consist of words (token or quoted-string)
1445   separated by whitespace or special characters.
1446</t>
1447<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>
1448  <x:ref>word</x:ref>           = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1449
1450  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1451<!--
1452  IMPORTANT: when editing "tchar" make sure that "special" is updated accordingly!!!
1453 -->
1454  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1455                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1456                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1457                 ; any <x:ref>VCHAR</x:ref>, except <x:ref>special</x:ref>
1458
1459  <x:ref>special</x:ref>        = "(" / ")" / "&lt;" / ">" / "@" / ","
1460                 / ";" / ":" / "\" / DQUOTE / "/" / "["
1461                 / "]" / "?" / "=" / "{" / "}"
1462</artwork></figure>
1463<t anchor="rule.quoted-string">
1464  <x:anchor-alias value="quoted-string"/>
1465  <x:anchor-alias value="qdtext"/>
1466  <x:anchor-alias value="obs-text"/>
1467   A string of text is parsed as a single word if it is quoted using
1468   double-quote marks.
1469</t>
1470<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"/>
1471  <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>
1472  <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>
1473  <x:ref>obs-text</x:ref>       = %x80-FF
1474</artwork></figure>
1475<t anchor="rule.quoted-pair">
1476  <x:anchor-alias value="quoted-pair"/>
1477   The backslash octet ("\") can be used as a single-octet
1478   quoting mechanism within quoted-string constructs:
1479</t>
1480<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1481  <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> )
1482</artwork></figure>
1483<t>
1484   Recipients that process the value of a quoted-string &MUST; handle a
1485   quoted-pair as if it were replaced by the octet following the backslash.
1486</t>
1487<t>
1488   A sender &SHOULD-NOT; generate a quoted-pair in a quoted-string except where
1489   necessary to quote DQUOTE and backslash octets occurring within that string.
1490</t>
1491<t anchor="rule.comment">
1492  <x:anchor-alias value="comment"/>
1493  <x:anchor-alias value="ctext"/>
1494   Comments can be included in some HTTP header fields by surrounding
1495   the comment text with parentheses. Comments are only allowed in
1496   fields containing "comment" as part of their field value definition.
1497</t>
1498<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1499  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-cpair</x:ref> / <x:ref>comment</x:ref> ) ")"
1500  <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>
1501</artwork></figure>
1502<t anchor="rule.quoted-cpair">
1503  <x:anchor-alias value="quoted-cpair"/>
1504   The backslash octet ("\") can be used as a single-octet
1505   quoting mechanism within comment constructs:
1506</t>
1507<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-cpair"/>
1508  <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> )
1509</artwork></figure>
1510<t>
1511   A sender &SHOULD-NOT; escape octets in comments that do not require escaping
1512   (i.e., other than the backslash octet "\" and the parentheses "(" and ")").
1513</t>
1514</section>
1515
1516</section>
1517
1518<section title="Message Body" anchor="message.body">
1519  <x:anchor-alias value="message-body"/>
1520<t>
1521   The message body (if any) of an HTTP message is used to carry the
1522   payload body of that request or response.  The message body is
1523   identical to the payload body unless a transfer coding has been
1524   applied, as described in <xref target="header.transfer-encoding"/>.
1525</t>
1526<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1527  <x:ref>message-body</x:ref> = *OCTET
1528</artwork></figure>
1529<t>
1530   The rules for when a message body is allowed in a message differ for
1531   requests and responses.
1532</t>
1533<t>
1534   The presence of a message body in a request is signaled by a
1535   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1536   field. Request message framing is independent of method semantics,
1537   even if the method does not define any use for a message body.
1538</t>
1539<t>
1540   The presence of a message body in a response depends on both
1541   the request method to which it is responding and the response
1542   status code (<xref target="status.line"/>).
1543   Responses to the HEAD request method never include a message body
1544   because the associated response header fields (e.g.,
1545   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1546   if present, indicate only what their values would have been if the request
1547   method had been GET (&HEAD;).
1548   <x:ref>2xx (Successful)</x:ref> responses to CONNECT switch to tunnel
1549   mode instead of having a message body (&CONNECT;).
1550   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1551   <x:ref>304 (Not Modified)</x:ref> responses do not include a message body.
1552   All other responses do include a message body, although the body
1553   might be of zero length.
1554</t>
1555
1556<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1557  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1558  <iref item="chunked (Coding Format)"/>
1559  <x:anchor-alias value="Transfer-Encoding"/>
1560<t>
1561   The Transfer-Encoding header field lists the transfer coding names
1562   corresponding to the sequence of transfer codings that have been
1563   (or will be) applied to the payload body in order to form the message body.
1564   Transfer codings are defined in <xref target="transfer.codings"/>.
1565</t>
1566<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1567  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1568</artwork></figure>
1569<t>
1570   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1571   MIME, which was designed to enable safe transport of binary data over a
1572   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1573   However, safe transport has a different focus for an 8bit-clean transfer
1574   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1575   accurately delimit a dynamically generated payload and to distinguish
1576   payload encodings that are only applied for transport efficiency or
1577   security from those that are characteristics of the selected resource.
1578</t>
1579<t>
1580   A recipient &MUST; be able to parse the chunked transfer coding
1581   (<xref target="chunked.encoding"/>) because it plays a crucial role in
1582   framing messages when the payload body size is not known in advance.
1583   A sender &MUST-NOT; apply chunked more than once to a message body
1584   (i.e., chunking an already chunked message is not allowed).
1585   If any transfer coding other than chunked is applied to a request payload
1586   body, the sender &MUST; apply chunked as the final transfer coding to
1587   ensure that the message is properly framed.
1588   If any transfer coding other than chunked is applied to a response payload
1589   body, the sender &MUST; either apply chunked as the final transfer coding
1590   or terminate the message by closing the connection.
1591</t>
1592<figure><preamble>
1593   For example,
1594</preamble><artwork type="example">
1595  Transfer-Encoding: gzip, chunked
1596</artwork><postamble>
1597   indicates that the payload body has been compressed using the gzip
1598   coding and then chunked using the chunked coding while forming the
1599   message body.
1600</postamble></figure>
1601<t>
1602   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1603   Transfer-Encoding is a property of the message, not of the representation, and
1604   any recipient along the request/response chain &MAY; decode the received
1605   transfer coding(s) or apply additional transfer coding(s) to the message
1606   body, assuming that corresponding changes are made to the Transfer-Encoding
1607   field-value. Additional information about the encoding parameters &MAY; be
1608   provided by other header fields not defined by this specification.
1609</t>
1610<t>
1611   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1612   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1613   neither of which includes a message body,
1614   to indicate that the origin server would have applied a transfer coding
1615   to the message body if the request had been an unconditional GET.
1616   This indication is not required, however, because any recipient on
1617   the response chain (including the origin server) can remove transfer
1618   codings when they are not needed.
1619</t>
1620<t>
1621   A server &MUST-NOT; send a Transfer-Encoding header field in any response
1622   with a status code of
1623   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1624   A server &MUST-NOT; send a Transfer-Encoding header field in any
1625   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1626</t>
1627<t>
1628   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1629   implementations advertising only HTTP/1.0 support will not understand
1630   how to process a transfer-encoded payload.
1631   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1632   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1633   might be in the form of specific user configuration or by remembering the
1634   version of a prior received response.
1635   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1636   the corresponding request indicates HTTP/1.1 (or later).
1637</t>
1638<t>
1639   A server that receives a request message with a transfer coding it does
1640   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref>.
1641</t>
1642</section>
1643
1644<section title="Content-Length" anchor="header.content-length">
1645  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1646  <x:anchor-alias value="Content-Length"/>
1647<t>
1648   When a message does not have a <x:ref>Transfer-Encoding</x:ref> header
1649   field, a Content-Length header field can provide the anticipated size,
1650   as a decimal number of octets, for a potential payload body.
1651   For messages that do include a payload body, the Content-Length field-value
1652   provides the framing information necessary for determining where the body
1653   (and message) ends.  For messages that do not include a payload body, the
1654   Content-Length indicates the size of the selected representation
1655   (&representation;).
1656</t>
1657<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1658  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1659</artwork></figure>
1660<t>
1661   An example is
1662</t>
1663<figure><artwork type="example">
1664  Content-Length: 3495
1665</artwork></figure>
1666<t>
1667   A sender &MUST-NOT; send a Content-Length header field in any message that
1668   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1669</t>
1670<t>
1671   A user agent &SHOULD; send a Content-Length in a request message when no
1672   <x:ref>Transfer-Encoding</x:ref> is sent and the request method defines
1673   a meaning for an enclosed payload body. For example, a Content-Length
1674   header field is normally sent in a POST request even when the value is
1675   0 (indicating an empty payload body).  A user agent &SHOULD-NOT; send a
1676   Content-Length header field when the request message does not contain a
1677   payload body and the method semantics do not anticipate such a body.
1678</t>
1679<t>
1680   A server &MAY; send a Content-Length header field in a response to a HEAD
1681   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1682   response unless its field-value equals the decimal number of octets that
1683   would have been sent in the payload body of a response if the same
1684   request had used the GET method.
1685</t>
1686<t>
1687   A server &MAY; send a Content-Length header field in a
1688   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1689   (&status-304;); a server &MUST-NOT; send Content-Length in such a
1690   response unless its field-value equals the decimal number of octets that
1691   would have been sent in the payload body of a <x:ref>200 (OK)</x:ref>
1692   response to the same request.
1693</t>
1694<t>
1695   A server &MUST-NOT; send a Content-Length header field in any response
1696   with a status code of
1697   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1698   A server &MUST-NOT; send a Content-Length header field in any
1699   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1700</t>
1701<t>
1702   Aside from the cases defined above, in the absence of Transfer-Encoding,
1703   an origin server &SHOULD; send a Content-Length header field when the
1704   payload body size is known prior to sending the complete header section.
1705   This will allow downstream recipients to measure transfer progress,
1706   know when a received message is complete, and potentially reuse the
1707   connection for additional requests.
1708</t>
1709<t>
1710   Any Content-Length field value greater than or equal to zero is valid.
1711   Since there is no predefined limit to the length of a payload, a
1712   recipient &SHOULD; anticipate potentially large decimal numerals and
1713   prevent parsing errors due to integer conversion overflows
1714   (<xref target="attack.protocol.element.size.overflows"/>).
1715</t>
1716<t>
1717   If a message is received that has multiple Content-Length header fields
1718   with field-values consisting of the same decimal value, or a single
1719   Content-Length header field with a field value containing a list of
1720   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1721   duplicate Content-Length header fields have been generated or combined by an
1722   upstream message processor, then the recipient &MUST; either reject the
1723   message as invalid or replace the duplicated field-values with a single
1724   valid Content-Length field containing that decimal value prior to
1725   determining the message body length or forwarding the message.
1726</t>
1727<x:note>
1728  <t>
1729   &Note; HTTP's use of Content-Length for message framing differs
1730   significantly from the same field's use in MIME, where it is an optional
1731   field used only within the "message/external-body" media-type.
1732  </t>
1733</x:note>
1734</section>
1735
1736<section title="Message Body Length" anchor="message.body.length">
1737  <iref item="chunked (Coding Format)"/>
1738<t>
1739   The length of a message body is determined by one of the following
1740   (in order of precedence):
1741</t>
1742<t>
1743  <list style="numbers">
1744    <x:lt><t>
1745     Any response to a HEAD request and any response with a
1746     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1747     <x:ref>304 (Not Modified)</x:ref> status code is always
1748     terminated by the first empty line after the header fields, regardless of
1749     the header fields present in the message, and thus cannot contain a
1750     message body.
1751    </t></x:lt>
1752    <x:lt><t>
1753     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1754     connection will become a tunnel immediately after the empty line that
1755     concludes the header fields.  A client &MUST; ignore any
1756     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1757     fields received in such a message.
1758    </t></x:lt>
1759    <x:lt><t>
1760     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1761     and the chunked transfer coding (<xref target="chunked.encoding"/>)
1762     is the final encoding, the message body length is determined by reading
1763     and decoding the chunked data until the transfer coding indicates the
1764     data is complete.
1765    </t>
1766    <t>
1767     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1768     response and the chunked transfer coding is not the final encoding, the
1769     message body length is determined by reading the connection until it is
1770     closed by the server.
1771     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a request and the
1772     chunked transfer coding is not the final encoding, the message body
1773     length cannot be determined reliably; the server &MUST; respond with
1774     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1775    </t>
1776    <t>
1777     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1778     and a <x:ref>Content-Length</x:ref> header field, the Transfer-Encoding
1779     overrides the Content-Length. Such a message might indicate an attempt
1780     to perform request or response smuggling (bypass of security-related
1781     checks on message routing or content) and thus ought to be handled as
1782     an error.  A sender &MUST; remove the received Content-Length field
1783     prior to forwarding such a message downstream.
1784    </t></x:lt>
1785    <x:lt><t>
1786     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1787     either multiple <x:ref>Content-Length</x:ref> header fields having
1788     differing field-values or a single Content-Length header field having an
1789     invalid value, then the message framing is invalid and
1790     the recipient &MUST; treat it as an unrecoverable error to prevent
1791     request or response smuggling.
1792     If this is a request message, the server &MUST; respond with
1793     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1794     If this is a response message received by a proxy,
1795     the proxy &MUST; close the connection to the server, discard the received
1796     response, and send a <x:ref>502 (Bad Gateway)</x:ref> response to the
1797     client.
1798     If this is a response message received by a user agent,
1799     the user agent &MUST; close the connection to the server and discard the
1800     received response.
1801    </t></x:lt>
1802    <x:lt><t>
1803     If a valid <x:ref>Content-Length</x:ref> header field is present without
1804     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1805     expected message body length in octets.
1806     If the sender closes the connection or the recipient times out before the
1807     indicated number of octets are received, the recipient &MUST; consider
1808     the message to be incomplete and close the connection.
1809    </t></x:lt>
1810    <x:lt><t>
1811     If this is a request message and none of the above are true, then the
1812     message body length is zero (no message body is present).
1813    </t></x:lt>
1814    <x:lt><t>
1815     Otherwise, this is a response message without a declared message body
1816     length, so the message body length is determined by the number of octets
1817     received prior to the server closing the connection.
1818    </t></x:lt>
1819  </list>
1820</t>
1821<t>
1822   Since there is no way to distinguish a successfully completed,
1823   close-delimited message from a partially-received message interrupted
1824   by network failure, a server &SHOULD; generate encoding or
1825   length-delimited messages whenever possible.  The close-delimiting
1826   feature exists primarily for backwards compatibility with HTTP/1.0.
1827</t>
1828<t>
1829   A server &MAY; reject a request that contains a message body but
1830   not a <x:ref>Content-Length</x:ref> by responding with
1831   <x:ref>411 (Length Required)</x:ref>.
1832</t>
1833<t>
1834   Unless a transfer coding other than chunked has been applied,
1835   a client that sends a request containing a message body &SHOULD;
1836   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1837   length is known in advance, rather than the chunked transfer coding, since some
1838   existing services respond to chunked with a <x:ref>411 (Length Required)</x:ref>
1839   status code even though they understand the chunked transfer coding.  This
1840   is typically because such services are implemented via a gateway that
1841   requires a content-length in advance of being called and the server
1842   is unable or unwilling to buffer the entire request before processing.
1843</t>
1844<t>
1845   A user agent that sends a request containing a message body &MUST; send a
1846   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1847   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1848   the form of specific user configuration or by remembering the version of a
1849   prior received response.
1850</t>
1851<t>
1852   If the final response to the last request on a connection has been
1853   completely received and there remains additional data to read, a user agent
1854   &MAY; discard the remaining data or attempt to determine if that data
1855   belongs as part of the prior response body, which might be the case if the
1856   prior message's Content-Length value is incorrect. A client &MUST-NOT;
1857   process, cache, or forward such extra data as a separate response, since
1858   such behavior would be vulnerable to cache poisoning.
1859</t>
1860</section>
1861</section>
1862
1863<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1864<t>
1865   A server that receives an incomplete request message, usually due to a
1866   canceled request or a triggered time-out exception, &MAY; send an error
1867   response prior to closing the connection.
1868</t>
1869<t>
1870   A client that receives an incomplete response message, which can occur
1871   when a connection is closed prematurely or when decoding a supposedly
1872   chunked transfer coding fails, &MUST; record the message as incomplete.
1873   Cache requirements for incomplete responses are defined in
1874   &cache-incomplete;.
1875</t>
1876<t>
1877   If a response terminates in the middle of the header section (before the
1878   empty line is received) and the status code might rely on header fields to
1879   convey the full meaning of the response, then the client cannot assume
1880   that meaning has been conveyed; the client might need to repeat the
1881   request in order to determine what action to take next.
1882</t>
1883<t>
1884   A message body that uses the chunked transfer coding is
1885   incomplete if the zero-sized chunk that terminates the encoding has not
1886   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1887   incomplete if the size of the message body received (in octets) is less than
1888   the value given by Content-Length.  A response that has neither chunked
1889   transfer coding nor Content-Length is terminated by closure of the
1890   connection, and thus is considered complete regardless of the number of
1891   message body octets received, provided that the header section was received
1892   intact.
1893</t>
1894</section>
1895
1896<section title="Message Parsing Robustness" anchor="message.robustness">
1897<t>
1898   Older HTTP/1.0 user agent implementations might send an extra CRLF
1899   after a POST request as a workaround for some early server
1900   applications that failed to read message body content that was
1901   not terminated by a line-ending. An HTTP/1.1 user agent &MUST-NOT;
1902   preface or follow a request with an extra CRLF.  If terminating
1903   the request message body with a line-ending is desired, then the
1904   user agent &MUST; count the terminating CRLF octets as part of the
1905   message body length.
1906</t>
1907<t>
1908   In the interest of robustness, a server that is expecting to receive and
1909   parse a request-line &SHOULD; ignore at least one empty line (CRLF)
1910   received prior to the request-line.
1911</t>
1912<t>
1913   Although the line terminator for the start-line and header
1914   fields is the sequence CRLF, a recipient &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 need to be used carefully because use of chunking does not
2017   have any semantics related to the message; it is by definition hop-by-hop
2018   and thus can be removed/reformatted by any intermediary.
2019   Furthermore, few implementations expose information about their contents.
2020</t>
2021<t>
2022   The chunk-size field is a string of hex digits indicating the size of
2023   the chunk-data in octets. The chunked transfer coding is complete when a
2024   chunk with a chunk-size of zero is received, possibly followed by a
2025   trailer, and finally terminated by an empty line.
2026</t>
2027
2028<section title="Chunked Trailer Part" anchor="chunked.trailer.part">
2029  <x:anchor-alias value="trailer-part"/>
2030<t>
2031   A trailer allows the sender to include additional fields at the end of a
2032   chunked message in order to supply metadata that might be dynamically
2033   generated while the message body is sent, such as a message integrity
2034   check, digital signature, or post-processing status. The trailer fields are
2035   identical to header fields, except they are sent in a chunked trailer
2036   instead of the message's header section.
2037</t>
2038<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="trailer-part"/>
2039  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
2040</artwork></figure>
2041<t>
2042   A sender &MUST-NOT; generate a trailer that contains a field which needs to
2043   be known by the recipient before it can begin processing the message body.
2044   For example, most recipients need to know the values of
2045   <x:ref>Content-Encoding</x:ref> and <x:ref>Content-Type</x:ref> in order to
2046   select a content handler, so placing those fields in a trailer would force
2047   the recipient to buffer the entire body before it could begin, greatly
2048   increasing user-perceived latency and defeating one of the main advantages
2049   of using chunked to send data streams of unknown length.
2050   A sender &MUST-NOT; generate a trailer containing a
2051   <x:ref>Transfer-Encoding</x:ref>,
2052   <x:ref>Content-Length</x:ref>, or
2053   <x:ref>Trailer</x:ref> field.
2054</t>
2055<t>
2056   A server &MUST; generate an empty trailer with the chunked transfer coding
2057   unless at least one of the following is true:
2058  <list style="numbers">
2059    <t>the request included a <x:ref>TE</x:ref> header field that indicates
2060    "trailers" is acceptable in the transfer coding of the response, as
2061    described in <xref target="header.te"/>; or,</t>
2062     
2063    <t>the trailer fields consist entirely of optional metadata and the
2064    recipient could use the message (in a manner acceptable to the generating
2065    server) without receiving that metadata. In other words, the generating
2066    server is willing to accept the possibility that the trailer fields might
2067    be silently discarded along the path to the client.</t>
2068  </list>
2069</t>
2070<t>
2071   The above requirement prevents the need for an infinite buffer when a
2072   message is being received by an HTTP/1.1 (or later) proxy and forwarded to
2073   an HTTP/1.0 recipient.
2074</t>
2075</section>
2076
2077<section title="Decoding Chunked" anchor="decoding.chunked">
2078<t>
2079   A process for decoding the chunked transfer coding
2080   can be represented in pseudo-code as:
2081</t>
2082<figure><artwork type="code">
2083  length := 0
2084  read chunk-size, chunk-ext (if any), and CRLF
2085  while (chunk-size &gt; 0) {
2086     read chunk-data and CRLF
2087     append chunk-data to decoded-body
2088     length := length + chunk-size
2089     read chunk-size, chunk-ext (if any), and CRLF
2090  }
2091  read header-field
2092  while (header-field not empty) {
2093     append header-field to existing header fields
2094     read header-field
2095  }
2096  Content-Length := length
2097  Remove "chunked" from Transfer-Encoding
2098  Remove Trailer from existing header fields
2099</artwork></figure>
2100<t>
2101   A recipient &MUST; be able to parse and decode the
2102   chunked transfer coding and &MUST; ignore chunk-ext extensions
2103   they do not understand.
2104</t>
2105</section>
2106</section>
2107
2108<section title="Compression Codings" anchor="compression.codings">
2109<t>
2110   The codings defined below can be used to compress the payload of a
2111   message.
2112</t>
2113
2114<section title="Compress Coding" anchor="compress.coding">
2115<iref item="compress (Coding Format)"/>
2116<t>
2117   The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding
2118   <xref target="Welch"/> that is commonly produced by the UNIX file
2119   compression program "compress".
2120   A recipient &SHOULD; consider "x-compress" to be equivalent to "compress".
2121</t>
2122</section>
2123
2124<section title="Deflate Coding" anchor="deflate.coding">
2125<iref item="deflate (Coding Format)"/>
2126<t>
2127   The "deflate" coding is a "zlib" data format <xref target="RFC1950"/>
2128   containing a "deflate" compressed data stream <xref target="RFC1951"/>
2129   that uses a combination of the Lempel-Ziv (LZ77) compression algorithm and
2130   Huffman coding.
2131</t>
2132<x:note>
2133  <t>
2134    &Note; Some incorrect implementations send the "deflate"
2135    compressed data without the zlib wrapper.
2136   </t>
2137</x:note>
2138</section>
2139
2140<section title="Gzip Coding" anchor="gzip.coding">
2141<iref item="gzip (Coding Format)"/>
2142<t>
2143   The "gzip" coding is an LZ77 coding with a 32 bit CRC that is commonly
2144   produced by the gzip file compression program <xref target="RFC1952"/>.
2145   A recipient &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2146</t>
2147</section>
2148
2149</section>
2150
2151<section title="TE" anchor="header.te">
2152  <iref primary="true" item="TE header field" x:for-anchor=""/>
2153  <x:anchor-alias value="TE"/>
2154  <x:anchor-alias value="t-codings"/>
2155  <x:anchor-alias value="t-ranking"/>
2156  <x:anchor-alias value="rank"/>
2157<t>
2158   The "TE" header field in a request indicates what transfer codings,
2159   besides chunked, the client is willing to accept in response, and
2160   whether or not the client is willing to accept trailer fields in a
2161   chunked transfer coding.
2162</t>
2163<t>
2164   The TE field-value consists of a comma-separated list of transfer coding
2165   names, each allowing for optional parameters (as described in
2166   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2167   A client &MUST-NOT; send the chunked transfer coding name in TE;
2168   chunked is always acceptable for HTTP/1.1 recipients.
2169</t>
2170<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"/>
2171  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2172  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2173  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2174  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2175             / ( "1" [ "." 0*3("0") ] )
2176</artwork></figure>
2177<t>
2178   Three examples of TE use are below.
2179</t>
2180<figure><artwork type="example">
2181  TE: deflate
2182  TE:
2183  TE: trailers, deflate;q=0.5
2184</artwork></figure>
2185<t>
2186   The presence of the keyword "trailers" indicates that the client is willing
2187   to accept trailer fields in a chunked transfer coding, as defined in
2188   <xref target="chunked.trailer.part"/>, on behalf of itself and any downstream
2189   clients. For requests from an intermediary, this implies that either:
2190   (a) all downstream clients are willing to accept trailer fields in the
2191   forwarded response; or,
2192   (b) the intermediary will attempt to buffer the response on behalf of
2193   downstream recipients.
2194   Note that HTTP/1.1 does not define any means to limit the size of a
2195   chunked response such that an intermediary can be assured of buffering the
2196   entire response.
2197</t>
2198<t>
2199   When multiple transfer codings are acceptable, the client &MAY; rank the
2200   codings by preference using a case-insensitive "q" parameter (similar to
2201   the qvalues used in content negotiation fields, &qvalue;). The rank value
2202   is a real number in the range 0 through 1, where 0.001 is the least
2203   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2204</t>
2205<t>
2206   If the TE field-value is empty or if no TE field is present, the only
2207   acceptable transfer coding is chunked. A message with no transfer coding
2208   is always acceptable.
2209</t>
2210<t>
2211   Since the TE header field only applies to the immediate connection,
2212   a sender of TE &MUST; also send a "TE" connection option within the
2213   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2214   in order to prevent the TE field from being forwarded by intermediaries
2215   that do not support its semantics.
2216</t>
2217</section>
2218
2219<section title="Trailer" anchor="header.trailer">
2220  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
2221  <x:anchor-alias value="Trailer"/>
2222<t>
2223   When a message includes a message body encoded with the chunked
2224   transfer coding and the sender desires to send metadata in the form of
2225   trailer fields at the end of the message, the sender &SHOULD; generate a
2226   <x:ref>Trailer</x:ref> header field before the message body to indicate
2227   which fields will be present in the trailers. This allows the recipient
2228   to prepare for receipt of that metadata before it starts processing the body,
2229   which is useful if the message is being streamed and the recipient wishes
2230   to confirm an integrity check on the fly.
2231</t>
2232<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
2233  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
2234</artwork></figure>
2235</section>
2236</section>
2237
2238<section title="Message Routing" anchor="message.routing">
2239<t>
2240   HTTP request message routing is determined by each client based on the
2241   target resource, the client's proxy configuration, and
2242   establishment or reuse of an inbound connection.  The corresponding
2243   response routing follows the same connection chain back to the client.
2244</t>
2245
2246<section title="Identifying a Target Resource" anchor="target-resource">
2247  <iref primary="true" item="target resource"/>
2248  <iref primary="true" item="target URI"/>
2249  <x:anchor-alias value="target resource"/>
2250  <x:anchor-alias value="target URI"/>
2251<t>
2252   HTTP is used in a wide variety of applications, ranging from
2253   general-purpose computers to home appliances.  In some cases,
2254   communication options are hard-coded in a client's configuration.
2255   However, most HTTP clients rely on the same resource identification
2256   mechanism and configuration techniques as general-purpose Web browsers.
2257</t>
2258<t>
2259   HTTP communication is initiated by a user agent for some purpose.
2260   The purpose is a combination of request semantics, which are defined in
2261   <xref target="Part2"/>, and a target resource upon which to apply those
2262   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2263   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2264   would resolve to its absolute form in order to obtain the
2265   "<x:dfn>target URI</x:dfn>".  The target URI
2266   excludes the reference's fragment component, if any,
2267   since fragment identifiers are reserved for client-side processing
2268   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2269</t>
2270</section>
2271
2272<section title="Connecting Inbound" anchor="connecting.inbound">
2273<t>
2274   Once the target URI is determined, a client needs to decide whether
2275   a network request is necessary to accomplish the desired semantics and,
2276   if so, where that request is to be directed.
2277</t>
2278<t>
2279   If the client has a cache <xref target="Part6"/> and the request can be
2280   satisfied by it, then the request is
2281   usually directed there first.
2282</t>
2283<t>
2284   If the request is not satisfied by a cache, then a typical client will
2285   check its configuration to determine whether a proxy is to be used to
2286   satisfy the request.  Proxy configuration is implementation-dependent,
2287   but is often based on URI prefix matching, selective authority matching,
2288   or both, and the proxy itself is usually identified by an "http" or
2289   "https" URI.  If a proxy is applicable, the client connects inbound by
2290   establishing (or reusing) a connection to that proxy.
2291</t>
2292<t>
2293   If no proxy is applicable, a typical client will invoke a handler routine,
2294   usually specific to the target URI's scheme, to connect directly
2295   to an authority for the target resource.  How that is accomplished is
2296   dependent on the target URI scheme and defined by its associated
2297   specification, similar to how this specification defines origin server
2298   access for resolution of the "http" (<xref target="http.uri"/>) and
2299   "https" (<xref target="https.uri"/>) schemes.
2300</t>
2301<t>
2302   HTTP requirements regarding connection management are defined in
2303   <xref target="connection.management"/>.
2304</t>
2305</section>
2306
2307<section title="Request Target" anchor="request-target">
2308<t>
2309   Once an inbound connection is obtained,
2310   the client sends an HTTP request message (<xref target="http.message"/>)
2311   with a request-target derived from the target URI.
2312   There are four distinct formats for the request-target, depending on both
2313   the method being requested and whether the request is to a proxy.
2314</t>   
2315<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"/>
2316  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2317                 / <x:ref>absolute-form</x:ref>
2318                 / <x:ref>authority-form</x:ref>
2319                 / <x:ref>asterisk-form</x:ref>
2320
2321  <x:ref>origin-form</x:ref>    = <x:ref>absolute-path</x:ref> [ "?" <x:ref>query</x:ref> ]
2322  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2323  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2324  <x:ref>asterisk-form</x:ref>  = "*"
2325</artwork></figure>
2326<t anchor="origin-form"><iref item="origin-form (of request-target)"/>
2327  <x:h>origin-form</x:h>
2328</t>
2329<t>
2330   The most common form of request-target is the <x:dfn>origin-form</x:dfn>.
2331   When making a request directly to an origin server, other than a CONNECT
2332   or server-wide OPTIONS request (as detailed below),
2333   a client &MUST; send only the absolute path and query components of
2334   the target URI as the request-target.
2335   If the target URI's path component is empty, then the client &MUST; send
2336   "/" as the path within the origin-form of request-target.
2337   A <x:ref>Host</x:ref> header field is also sent, as defined in
2338   <xref target="header.host"/>.
2339</t>
2340<t>
2341   For example, a client wishing to retrieve a representation of the resource
2342   identified as
2343</t>
2344<figure><artwork x:indent-with="  " type="example">
2345http://www.example.org/where?q=now
2346</artwork></figure>
2347<t>
2348   directly from the origin server would open (or reuse) a TCP connection
2349   to port 80 of the host "www.example.org" and send the lines:
2350</t>
2351<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2352GET /where?q=now HTTP/1.1
2353Host: www.example.org
2354</artwork></figure>
2355<t>
2356   followed by the remainder of the request message.
2357</t>
2358<t anchor="absolute-form"><iref item="absolute-form (of request-target)"/>
2359  <x:h>absolute-form</x:h>
2360</t>
2361<t>
2362   When making a request to a proxy, other than a CONNECT or server-wide
2363   OPTIONS request (as detailed below), a client &MUST; send the target URI
2364   in <x:dfn>absolute-form</x:dfn> as the request-target.
2365   The proxy is requested to either service that request from a valid cache,
2366   if possible, or make the same request on the client's behalf to either
2367   the next inbound proxy server or directly to the origin server indicated
2368   by the request-target.  Requirements on such "forwarding" of messages are
2369   defined in <xref target="message.forwarding"/>.
2370</t>
2371<t>
2372   An example absolute-form of request-line would be:
2373</t>
2374<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2375GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
2376</artwork></figure>
2377<t>
2378   To allow for transition to the absolute-form for all requests in some
2379   future version of HTTP, a server &MUST; accept the absolute-form
2380   in requests, even though HTTP/1.1 clients will only send them in requests
2381   to proxies.
2382</t>
2383<t anchor="authority-form"><iref item="authority-form (of request-target)"/>
2384  <x:h>authority-form</x:h>
2385</t>
2386<t>
2387   The <x:dfn>authority-form</x:dfn> of request-target is only used for
2388   CONNECT requests (&CONNECT;). When making a CONNECT request to establish a
2389   tunnel through one or more proxies, a client &MUST; send only the target
2390   URI's authority component (excluding any userinfo and its "@" delimiter) as
2391   the request-target. For example,
2392</t>
2393<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2394CONNECT www.example.com:80 HTTP/1.1
2395</artwork></figure>
2396<t anchor="asterisk-form"><iref item="asterisk-form (of request-target)"/>
2397  <x:h>asterisk-form</x:h>
2398</t>
2399<t>
2400   The <x:dfn>asterisk-form</x:dfn> of request-target is only used for a server-wide
2401   OPTIONS request (&OPTIONS;).  When a client wishes to request OPTIONS
2402   for the server as a whole, as opposed to a specific named resource of
2403   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2404   For example,
2405</t>
2406<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2407OPTIONS * HTTP/1.1
2408</artwork></figure>
2409<t>
2410   If a proxy receives an OPTIONS request with an absolute-form of
2411   request-target in which the URI has an empty path and no query component,
2412   then the last proxy on the request chain &MUST; send a request-target
2413   of "*" when it forwards the request to the indicated origin server.
2414</t>
2415<figure><preamble>  
2416   For example, the request
2417</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2418OPTIONS http://www.example.org:8001 HTTP/1.1
2419</artwork></figure>
2420<figure><preamble>  
2421  would be forwarded by the final proxy as
2422</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2423OPTIONS * HTTP/1.1
2424Host: www.example.org:8001
2425</artwork>
2426<postamble>
2427   after connecting to port 8001 of host "www.example.org".
2428</postamble>
2429</figure>
2430</section>
2431
2432<section title="Host" anchor="header.host">
2433  <iref primary="true" item="Host header field" x:for-anchor=""/>
2434  <x:anchor-alias value="Host"/>
2435<t>
2436   The "Host" header field in a request provides the host and port
2437   information from the target URI, enabling the origin
2438   server to distinguish among resources while servicing requests
2439   for multiple host names on a single IP address.
2440</t>
2441<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2442  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2443</artwork></figure>
2444<t>
2445   A client &MUST; send a Host header field in all HTTP/1.1 request messages.
2446   If the target URI includes an authority component, then a client &MUST;
2447   send a field-value for Host that is identical to that authority
2448   component, excluding any userinfo subcomponent and its "@" delimiter
2449   (<xref target="http.uri"/>).
2450   If the authority component is missing or undefined for the target URI,
2451   then a client &MUST; send a Host header field with an empty field-value.
2452</t>
2453<t>
2454   Since the Host field-value is critical information for handling a request,
2455   a user agent &SHOULD; generate Host as the first header field following the
2456   request-line.
2457</t>
2458<t>
2459   For example, a GET request to the origin server for
2460   &lt;http://www.example.org/pub/WWW/&gt; would begin with:
2461</t>
2462<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2463GET /pub/WWW/ HTTP/1.1
2464Host: www.example.org
2465</artwork></figure>
2466<t>
2467   A client &MUST; send a Host header field in an HTTP/1.1 request even
2468   if the request-target is in the absolute-form, since this
2469   allows the Host information to be forwarded through ancient HTTP/1.0
2470   proxies that might not have implemented Host.
2471</t>
2472<t>
2473   When a proxy receives a request with an absolute-form of
2474   request-target, the proxy &MUST; ignore the received
2475   Host header field (if any) and instead replace it with the host
2476   information of the request-target.  A proxy that forwards such a request
2477   &MUST; generate a new Host field-value based on the received
2478   request-target rather than forward the received Host field-value.
2479</t>
2480<t>
2481   Since the Host header field acts as an application-level routing
2482   mechanism, it is a frequent target for malware seeking to poison
2483   a shared cache or redirect a request to an unintended server.
2484   An interception proxy is particularly vulnerable if it relies on
2485   the Host field-value for redirecting requests to internal
2486   servers, or for use as a cache key in a shared cache, without
2487   first verifying that the intercepted connection is targeting a
2488   valid IP address for that host.
2489</t>
2490<t>
2491   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2492   to any HTTP/1.1 request message that lacks a Host header field and
2493   to any request message that contains more than one Host header field
2494   or a Host header field with an invalid field-value.
2495</t>
2496</section>
2497
2498<section title="Effective Request URI" anchor="effective.request.uri">
2499  <iref primary="true" item="effective request URI"/>
2500  <x:anchor-alias value="effective request URI"/>
2501<t>
2502   A server that receives an HTTP request message &MUST; reconstruct
2503   the user agent's original target URI, based on the pieces of information
2504   learned from the request-target, <x:ref>Host</x:ref> header field, and
2505   connection context, in order to identify the intended target resource and
2506   properly service the request. The URI derived from this reconstruction
2507   process is referred to as the "<x:dfn>effective request URI</x:dfn>".
2508</t>
2509<t>
2510   For a user agent, the effective request URI is the target URI.
2511</t>
2512<t>
2513   If the request-target is in absolute-form, then the effective request URI
2514   is the same as the request-target.  Otherwise, the effective request URI
2515   is constructed as follows.
2516</t>
2517<t>
2518   If the request is received over a TLS-secured TCP connection,
2519   then the effective request URI's scheme is "https"; otherwise, the
2520   scheme is "http".
2521</t>
2522<t>
2523   If the request-target is in authority-form, then the effective
2524   request URI's authority component is the same as the request-target.
2525   Otherwise, if a <x:ref>Host</x:ref> header field is supplied with a
2526   non-empty field-value, then the authority component is the same as the
2527   Host field-value. Otherwise, the authority component is the concatenation of
2528   the default host name configured for the server, a colon (":"), and the
2529   connection's incoming TCP port number in decimal form.
2530</t>
2531<t>
2532   If the request-target is in authority-form or asterisk-form, then the
2533   effective request URI's combined path and query component is empty.
2534   Otherwise, the combined path and query component is the same as the
2535   request-target.
2536</t>
2537<t>
2538   The components of the effective request URI, once determined as above,
2539   can be combined into absolute-URI form by concatenating the scheme,
2540   "://", authority, and combined path and query component.
2541</t>
2542<figure>
2543<preamble>
2544   Example 1: the following message received over an insecure TCP connection
2545</preamble>
2546<artwork type="example" x:indent-with="  ">
2547GET /pub/WWW/TheProject.html HTTP/1.1
2548Host: www.example.org:8080
2549</artwork>
2550</figure>
2551<figure>
2552<preamble>
2553  has an effective request URI of
2554</preamble>
2555<artwork type="example" x:indent-with="  ">
2556http://www.example.org:8080/pub/WWW/TheProject.html
2557</artwork>
2558</figure>
2559<figure>
2560<preamble>
2561   Example 2: the following message received over a TLS-secured TCP connection
2562</preamble>
2563<artwork type="example" x:indent-with="  ">
2564OPTIONS * HTTP/1.1
2565Host: www.example.org
2566</artwork>
2567</figure>
2568<figure>
2569<preamble>
2570  has an effective request URI of
2571</preamble>
2572<artwork type="example" x:indent-with="  ">
2573https://www.example.org
2574</artwork>
2575</figure>
2576<t>
2577   An origin server that does not allow resources to differ by requested
2578   host &MAY; ignore the <x:ref>Host</x:ref> field-value and instead replace it
2579   with a configured server name when constructing the effective request URI.
2580</t>
2581<t>
2582   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2583   field &MAY; attempt to use heuristics (e.g., examination of the URI path for
2584   something unique to a particular host) in order to guess the
2585   effective request URI's authority component.
2586</t>
2587</section>
2588
2589<section title="Associating a Response to a Request" anchor="associating.response.to.request">
2590<t>
2591   HTTP does not include a request identifier for associating a given
2592   request message with its corresponding one or more response messages.
2593   Hence, it relies on the order of response arrival to correspond exactly
2594   to the order in which requests are made on the same connection.
2595   More than one response message per request only occurs when one or more
2596   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a
2597   final response to the same request.
2598</t>
2599<t>
2600   A client that has more than one outstanding request on a connection &MUST;
2601   maintain a list of outstanding requests in the order sent and &MUST;
2602   associate each received response message on that connection to the highest
2603   ordered request that has not yet received a final (non-<x:ref>1xx</x:ref>)
2604   response.
2605</t>
2606</section>
2607
2608<section title="Message Forwarding" anchor="message.forwarding">
2609<t>
2610   As described in <xref target="intermediaries"/>, intermediaries can serve
2611   a variety of roles in the processing of HTTP requests and responses.
2612   Some intermediaries are used to improve performance or availability.
2613   Others are used for access control or to filter content.
2614   Since an HTTP stream has characteristics similar to a pipe-and-filter
2615   architecture, there are no inherent limits to the extent an intermediary
2616   can enhance (or interfere) with either direction of the stream.
2617</t>
2618<t>
2619   An intermediary not acting as a tunnel &MUST; implement the
2620   <x:ref>Connection</x:ref> header field, as specified in
2621   <xref target="header.connection"/>, and exclude fields from being forwarded
2622   that are only intended for the incoming connection.
2623</t>
2624<t>
2625   An intermediary &MUST-NOT; forward a message to itself unless it is
2626   protected from an infinite request loop. In general, an intermediary ought
2627   to recognize its own server names, including any aliases, local variations,
2628   or literal IP addresses, and respond to such requests directly.
2629</t>
2630
2631<section title="Via" anchor="header.via">
2632  <iref primary="true" item="Via header field" x:for-anchor=""/>
2633  <x:anchor-alias value="pseudonym"/>
2634  <x:anchor-alias value="received-by"/>
2635  <x:anchor-alias value="received-protocol"/>
2636  <x:anchor-alias value="Via"/>
2637<t>
2638   The "Via" header field indicates the presence of intermediate protocols and
2639   recipients between the user agent and the server (on requests) or between
2640   the origin server and the client (on responses), similar to the
2641   "Received" header field in email
2642   (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2643   Via can be used for tracking message forwards,
2644   avoiding request loops, and identifying the protocol capabilities of
2645   senders along the request/response chain.
2646</t>
2647<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"/>
2648  <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> ] )
2649
2650  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2651                      ; see <xref target="header.upgrade"/>
2652  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2653  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2654</artwork></figure>
2655<t>
2656   Multiple Via field values represent each proxy or gateway that has
2657   forwarded the message. Each intermediary appends its own information
2658   about how the message was received, such that the end result is ordered
2659   according to the sequence of forwarding recipients.
2660</t>
2661<t>
2662   A proxy &MUST; send an appropriate Via header field, as described below, in
2663   each message that it forwards.
2664   An HTTP-to-HTTP gateway &MUST; send an appropriate Via header field in
2665   each inbound request message and &MAY; send a Via header field in
2666   forwarded response messages.
2667</t>
2668<t>
2669   For each intermediary, the received-protocol indicates the protocol and
2670   protocol version used by the upstream sender of the message. Hence, the
2671   Via field value records the advertised protocol capabilities of the
2672   request/response chain such that they remain visible to downstream
2673   recipients; this can be useful for determining what backwards-incompatible
2674   features might be safe to use in response, or within a later request, as
2675   described in <xref target="http.version"/>. For brevity, the protocol-name
2676   is omitted when the received protocol is HTTP.
2677</t>
2678<t>
2679   The received-by field is normally the host and optional port number of a
2680   recipient server or client that subsequently forwarded the message.
2681   However, if the real host is considered to be sensitive information, a
2682   sender &MAY; replace it with a pseudonym. If a port is not provided,
2683   a recipient &MAY; interpret that as meaning it was received on the default
2684   TCP port, if any, for the received-protocol.
2685</t>
2686<t>
2687   A sender &MAY; generate comments in the Via header field to identify the
2688   software of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2689   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2690   are optional and a recipient &MAY; remove them prior to forwarding the
2691   message.
2692</t>
2693<t>
2694   For example, a request message could be sent from an HTTP/1.0 user
2695   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2696   forward the request to a public proxy at p.example.net, which completes
2697   the request by forwarding it to the origin server at www.example.com.
2698   The request received by www.example.com would then have the following
2699   Via header field:
2700</t>
2701<figure><artwork type="example">
2702  Via: 1.0 fred, 1.1 p.example.net
2703</artwork></figure>
2704<t>
2705   An intermediary used as a portal through a network firewall
2706   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2707   region unless it is explicitly enabled to do so. If not enabled, such an
2708   intermediary &SHOULD; replace each received-by host of any host behind the
2709   firewall by an appropriate pseudonym for that host.
2710</t>
2711<t>
2712   An intermediary &MAY; combine an ordered subsequence of Via header
2713   field entries into a single such entry if the entries have identical
2714   received-protocol values. For example,
2715</t>
2716<figure><artwork type="example">
2717  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2718</artwork></figure>
2719<t>
2720  could be collapsed to
2721</t>
2722<figure><artwork type="example">
2723  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2724</artwork></figure>
2725<t>
2726   A sender &SHOULD-NOT; combine multiple entries unless they are all
2727   under the same organizational control and the hosts have already been
2728   replaced by pseudonyms. A sender &MUST-NOT; combine entries that
2729   have different received-protocol values.
2730</t>
2731</section>
2732
2733<section title="Transformations" anchor="message.transformations">
2734<t>
2735   Some intermediaries include features for transforming messages and their
2736   payloads.  A transforming proxy might, for example, convert between image
2737   formats in order to save cache space or to reduce the amount of traffic on
2738   a slow link. However, operational problems might occur when these
2739   transformations are applied to payloads intended for critical applications,
2740   such as medical imaging or scientific data analysis, particularly when
2741   integrity checks or digital signatures are used to ensure that the payload
2742   received is identical to the original.
2743</t>
2744<t>
2745   If a proxy receives a request-target with a host name that is not a
2746   fully qualified domain name, it &MAY; add its own domain to the host name
2747   it received when forwarding the request.  A proxy &MUST-NOT; change the
2748   host name if it is a fully qualified domain name.
2749</t>
2750<t>
2751   A proxy &MUST-NOT; modify the "absolute-path" and "query" parts of the
2752   received request-target when forwarding it to the next inbound server,
2753   except as noted above to replace an empty path with "/" or "*".
2754</t>
2755<t>
2756   A proxy &MUST-NOT; modify header fields that provide information about the
2757   end points of the communication chain, the resource state, or the selected
2758   representation. A proxy &MAY; change the message body through application
2759   or removal of a transfer coding (<xref target="transfer.codings"/>).
2760</t>
2761<t>
2762   A non-transforming proxy &MUST-NOT; modify the message payload (&payload;).
2763   A transforming proxy &MUST-NOT; modify the payload of a message that
2764   contains the no-transform cache-control directive.
2765</t>
2766<t>
2767   A transforming proxy &MAY; transform the payload of a message
2768   that does not contain the no-transform cache-control directive;
2769   if the payload is transformed, the transforming proxy &MUST; add a
2770   Warning header field with the warn-code of 214 ("Transformation Applied")
2771   if one does not already appear in the message (see &header-warning;).
2772   If the payload of a <x:ref>200 (OK)</x:ref> response is transformed, the
2773   transforming proxy can also inform downstream recipients that a
2774   transformation has been applied by changing the response status code to
2775   <x:ref>203 (Non-Authoritative Information)</x:ref> (&status-203;).
2776</t>
2777</section>
2778</section>
2779</section>
2780
2781<section title="Connection Management" anchor="connection.management">
2782<t>
2783   HTTP messaging is independent of the underlying transport or
2784   session-layer connection protocol(s).  HTTP only presumes a reliable
2785   transport with in-order delivery of requests and the corresponding
2786   in-order delivery of responses.  The mapping of HTTP request and
2787   response structures onto the data units of an underlying transport
2788   protocol is outside the scope of this specification.
2789</t>
2790<t>
2791   As described in <xref target="connecting.inbound"/>, the specific
2792   connection protocols to be used for an HTTP interaction are determined by
2793   client configuration and the <x:ref>target URI</x:ref>.
2794   For example, the "http" URI scheme
2795   (<xref target="http.uri"/>) indicates a default connection of TCP
2796   over IP, with a default TCP port of 80, but the client might be
2797   configured to use a proxy via some other connection, port, or protocol.
2798</t>
2799<t>
2800   HTTP implementations are expected to engage in connection management,
2801   which includes maintaining the state of current connections,
2802   establishing a new connection or reusing an existing connection,
2803   processing messages received on a connection, detecting connection
2804   failures, and closing each connection.
2805   Most clients maintain multiple connections in parallel, including
2806   more than one connection per server endpoint.
2807   Most servers are designed to maintain thousands of concurrent connections,
2808   while controlling request queues to enable fair use and detect
2809   denial of service attacks.
2810</t>
2811
2812<section title="Connection" anchor="header.connection">
2813  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2814  <iref primary="true" item="close" x:for-anchor=""/>
2815  <x:anchor-alias value="Connection"/>
2816  <x:anchor-alias value="connection-option"/>
2817  <x:anchor-alias value="close"/>
2818<t>
2819   The "Connection" header field allows the sender to indicate desired
2820   control options for the current connection.  In order to avoid confusing
2821   downstream recipients, a proxy or gateway &MUST; remove or replace any
2822   received connection options before forwarding the message.
2823</t>
2824<t>
2825   When a header field aside from Connection is used to supply control
2826   information for or about the current connection, the sender &MUST; list
2827   the corresponding field-name within the "Connection" header field.
2828   A proxy or gateway &MUST; parse a received Connection
2829   header field before a message is forwarded and, for each
2830   connection-option in this field, remove any header field(s) from
2831   the message with the same name as the connection-option, and then
2832   remove the Connection header field itself (or replace it with the
2833   intermediary's own connection options for the forwarded message).
2834</t>
2835<t>
2836   Hence, the Connection header field provides a declarative way of
2837   distinguishing header fields that are only intended for the
2838   immediate recipient ("hop-by-hop") from those fields that are
2839   intended for all recipients on the chain ("end-to-end"), enabling the
2840   message to be self-descriptive and allowing future connection-specific
2841   extensions to be deployed without fear that they will be blindly
2842   forwarded by older intermediaries.
2843</t>
2844<t>
2845   The Connection header field's value has the following grammar:
2846</t>
2847<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2848  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2849  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2850</artwork></figure>
2851<t>
2852   Connection options are case-insensitive.
2853</t>
2854<t>
2855   A sender &MUST-NOT; send a connection option corresponding to a header
2856   field that is intended for all recipients of the payload.
2857   For example, <x:ref>Cache-Control</x:ref> is never appropriate as a
2858   connection option (&header-cache-control;).
2859</t>
2860<t>
2861   The connection options do not have to correspond to a header field
2862   present in the message, since a connection-specific header field
2863   might not be needed if there are no parameters associated with that
2864   connection option.  Recipients that trigger certain connection
2865   behavior based on the presence of connection options &MUST; do so
2866   based on the presence of the connection-option rather than only the
2867   presence of the optional header field.  In other words, if the
2868   connection option is received as a header field but not indicated
2869   within the Connection field-value, then the recipient &MUST; ignore
2870   the connection-specific header field because it has likely been
2871   forwarded by an intermediary that is only partially conformant.
2872</t>
2873<t>
2874   When defining new connection options, specifications ought to
2875   carefully consider existing deployed header fields and ensure
2876   that the new connection option does not share the same name as
2877   an unrelated header field that might already be deployed.
2878   Defining a new connection option essentially reserves that potential
2879   field-name for carrying additional information related to the
2880   connection option, since it would be unwise for senders to use
2881   that field-name for anything else.
2882</t>
2883<t>
2884   The "<x:dfn>close</x:dfn>" connection option is defined for a
2885   sender to signal that this connection will be closed after completion of
2886   the response. For example,
2887</t>
2888<figure><artwork type="example">
2889  Connection: close
2890</artwork></figure>
2891<t>
2892   in either the request or the response header fields indicates that the
2893   sender is going to close the connection after the current request/response
2894   is complete (<xref target="persistent.tear-down"/>).
2895</t>
2896<t>
2897   A client that does not support <x:ref>persistent connections</x:ref> &MUST;
2898   send the "close" connection option in every request message.
2899</t>
2900<t>
2901   A server that does not support <x:ref>persistent connections</x:ref> &MUST;
2902   send the "close" connection option in every response message that
2903   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2904</t>
2905</section>
2906
2907<section title="Establishment" anchor="persistent.establishment">
2908<t>
2909   It is beyond the scope of this specification to describe how connections
2910   are established via various transport or session-layer protocols.
2911   Each connection applies to only one transport link.
2912</t>
2913</section>
2914
2915<section title="Persistence" anchor="persistent.connections">
2916   <x:anchor-alias value="persistent connections"/>
2917<t>
2918   HTTP/1.1 defaults to the use of "<x:dfn>persistent connections</x:dfn>",
2919   allowing multiple requests and responses to be carried over a single
2920   connection. The "<x:ref>close</x:ref>" connection-option is used to signal
2921   that a connection will not persist after the current request/response.
2922   HTTP implementations &SHOULD; support persistent connections.
2923</t>
2924<t>
2925   A recipient determines whether a connection is persistent or not based on
2926   the most recently received message's protocol version and
2927   <x:ref>Connection</x:ref> header field (if any):
2928   <list style="symbols">
2929     <t>If the <x:ref>close</x:ref> connection option is present, the
2930        connection will not persist after the current response; else,</t>
2931     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2932        persist after the current response; else,</t>
2933     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2934        connection option is present, the recipient is not a proxy, and
2935        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2936        the connection will persist after the current response; otherwise,</t>
2937     <t>The connection will close after the current response.</t>
2938   </list>
2939</t>
2940<t>
2941   A server &MAY; assume that an HTTP/1.1 client intends to maintain a
2942   persistent connection until a <x:ref>close</x:ref> connection option
2943   is received in a request.
2944</t>
2945<t>
2946   A client &MAY; reuse a persistent connection until it sends or receives
2947   a <x:ref>close</x:ref> connection option or receives an HTTP/1.0 response
2948   without a "keep-alive" connection option.
2949</t>
2950<t>
2951   In order to remain persistent, all messages on a connection need to
2952   have a self-defined message length (i.e., one not defined by closure
2953   of the connection), as described in <xref target="message.body"/>.
2954   A server &MUST; read the entire request message body or close
2955   the connection after sending its response, since otherwise the
2956   remaining data on a persistent connection would be misinterpreted
2957   as the next request.  Likewise,
2958   a client &MUST; read the entire response message body if it intends
2959   to reuse the same connection for a subsequent request.
2960</t>
2961<t>
2962   A proxy server &MUST-NOT; maintain a persistent connection with an
2963   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
2964   information and discussion of the problems with the Keep-Alive header field
2965   implemented by many HTTP/1.0 clients).
2966</t>
2967<t>
2968   Clients and servers &SHOULD-NOT; assume that a persistent connection is
2969   maintained for HTTP versions less than 1.1 unless it is explicitly
2970   signaled.
2971   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
2972   for more information on backward compatibility with HTTP/1.0 clients.
2973</t>
2974
2975<section title="Retrying Requests" anchor="persistent.retrying.requests">
2976<t>
2977   Connections can be closed at any time, with or without intention.
2978   Implementations ought to anticipate the need to recover
2979   from asynchronous close events.
2980</t>
2981<t>
2982   When an inbound connection is closed prematurely, a client &MAY; open a new
2983   connection and automatically retransmit an aborted sequence of requests if
2984   all of those requests have idempotent methods (&idempotent-methods;).
2985   A proxy &MUST-NOT; automatically retry non-idempotent requests.
2986</t>
2987<t>
2988   A user agent &MUST-NOT; automatically retry a request with a non-idempotent
2989   method unless it has some means to know that the request semantics are
2990   actually idempotent, regardless of the method, or some means to detect that
2991   the original request was never applied. For example, a user agent that
2992   knows (through design or configuration) that a POST request to a given
2993   resource is safe can repeat that request automatically.
2994   Likewise, a user agent designed specifically to operate on a version
2995   control repository might be able to recover from partial failure conditions
2996   by checking the target resource revision(s) after a failed connection,
2997   reverting or fixing any changes that were partially applied, and then
2998   automatically retrying the requests that failed.
2999</t>
3000<t>
3001   A client &SHOULD-NOT; automatically retry a failed automatic retry.
3002</t>
3003</section>
3004
3005<section title="Pipelining" anchor="pipelining">
3006   <x:anchor-alias value="pipeline"/>
3007<t>
3008   A client that supports persistent connections &MAY; "<x:dfn>pipeline</x:dfn>"
3009   its requests (i.e., send multiple requests without waiting for each
3010   response). A server &MAY; process a sequence of pipelined requests in
3011   parallel if they all have safe methods (&safe-methods;), but &MUST; send
3012   the corresponding responses in the same order that the requests were
3013   received.
3014</t>
3015<t>
3016   A client that pipelines requests &SHOULD; retry unanswered requests if the
3017   connection closes before it receives all of the corresponding responses.
3018   When retrying pipelined requests after a failed connection (a connection
3019   not explicitly closed by the server in its last complete response), a
3020   client &MUST-NOT; pipeline immediately after connection establishment,
3021   since the first remaining request in the prior pipeline might have caused
3022   an error response that can be lost again if multiple requests are sent on a
3023   prematurely closed connection (see the TCP reset problem described in
3024   <xref target="persistent.tear-down"/>).
3025</t>
3026<t>
3027   Idempotent methods (&idempotent-methods;) are significant to pipelining
3028   because they can be automatically retried after a connection failure.
3029   A user agent &SHOULD-NOT; pipeline requests after a non-idempotent method,
3030   until the final response status code for that method has been received,
3031   unless the user agent has a means to detect and recover from partial
3032   failure conditions involving the pipelined sequence.
3033</t>
3034<t>
3035   An intermediary that receives pipelined requests &MAY; pipeline those
3036   requests when forwarding them inbound, since it can rely on the outbound
3037   user agent(s) to determine what requests can be safely pipelined. If the
3038   inbound connection fails before receiving a response, the pipelining
3039   intermediary &MAY; attempt to retry a sequence of requests that have yet
3040   to receive a response if the requests all have idempotent methods;
3041   otherwise, the pipelining intermediary &SHOULD; forward any received
3042   responses and then close the corresponding outbound connection(s) so that
3043   the outbound user agent(s) can recover accordingly.
3044</t>
3045</section>
3046</section>
3047   
3048<section title="Concurrency" anchor="persistent.concurrency">
3049<t>
3050   A client &SHOULD; limit the number of simultaneous open
3051   connections that it maintains to a given server.
3052</t>
3053<t>
3054   Previous revisions of HTTP gave a specific number of connections as a
3055   ceiling, but this was found to be impractical for many applications. As a
3056   result, this specification does not mandate a particular maximum number of
3057   connections, but instead encourages clients to be conservative when opening
3058   multiple connections.
3059</t>
3060<t>
3061   Multiple connections are typically used to avoid the "head-of-line
3062   blocking" problem, wherein a request that takes significant server-side
3063   processing and/or has a large payload blocks subsequent requests on the
3064   same connection. However, each connection consumes server resources.
3065   Furthermore, using multiple connections can cause undesirable side effects
3066   in congested networks.
3067</t>
3068<t>
3069   Note that servers might reject traffic that they deem abusive, including an
3070   excessive number of connections from a client.
3071</t>
3072</section>
3073
3074<section title="Failures and Time-outs" anchor="persistent.failures">
3075<t>
3076   Servers will usually have some time-out value beyond which they will
3077   no longer maintain an inactive connection. Proxy servers might make
3078   this a higher value since it is likely that the client will be making
3079   more connections through the same server. The use of persistent
3080   connections places no requirements on the length (or existence) of
3081   this time-out for either the client or the server.
3082</t>
3083<t>
3084   A client or server that wishes to time-out &SHOULD; issue a graceful close
3085   on the connection. Implementations &SHOULD; constantly monitor open
3086   connections for a received closure signal and respond to it as appropriate,
3087   since prompt closure of both sides of a connection enables allocated system
3088   resources to be reclaimed.
3089</t>
3090<t>
3091   A client, server, or proxy &MAY; close the transport connection at any
3092   time. For example, a client might have started to send a new request
3093   at the same time that the server has decided to close the "idle"
3094   connection. From the server's point of view, the connection is being
3095   closed while it was idle, but from the client's point of view, a
3096   request is in progress.
3097</t>
3098<t>
3099   A server &SHOULD; sustain persistent connections, when possible, and allow
3100   the underlying
3101   transport's flow control mechanisms to resolve temporary overloads, rather
3102   than terminate connections with the expectation that clients will retry.
3103   The latter technique can exacerbate network congestion.
3104</t>
3105<t>
3106   A client sending a message body &SHOULD; monitor
3107   the network connection for an error response while it is transmitting
3108   the request. If the client sees a response that indicates the server does
3109   not wish to receive the message body and is closing the connection, the
3110   client &SHOULD; immediately cease transmitting the body and close its side
3111   of the connection.
3112</t>
3113</section>
3114   
3115<section title="Tear-down" anchor="persistent.tear-down">
3116  <iref primary="false" item="Connection header field" x:for-anchor=""/>
3117  <iref primary="false" item="close" x:for-anchor=""/>
3118<t>
3119   The <x:ref>Connection</x:ref> header field
3120   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
3121   connection option that a sender &SHOULD; send when it wishes to close
3122   the connection after the current request/response pair.
3123</t>
3124<t>
3125   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
3126   send further requests on that connection (after the one containing
3127   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
3128   final response message corresponding to this request.
3129</t>
3130<t>
3131   A server that receives a <x:ref>close</x:ref> connection option &MUST;
3132   initiate a close of the connection (see below) after it sends the
3133   final response to the request that contained <x:ref>close</x:ref>.
3134   The server &SHOULD; send a <x:ref>close</x:ref> connection option
3135   in its final response on that connection. The server &MUST-NOT; process
3136   any further requests received on that connection.
3137</t>
3138<t>
3139   A server that sends a <x:ref>close</x:ref> connection option &MUST;
3140   initiate a close of the connection (see below) after it sends the
3141   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
3142   any further requests received on that connection.
3143</t>
3144<t>
3145   A client that receives a <x:ref>close</x:ref> connection option &MUST;
3146   cease sending requests on that connection and close the connection
3147   after reading the response message containing the close; if additional
3148   pipelined requests had been sent on the connection, the client &SHOULD-NOT;
3149   assume that they will be processed by the server.
3150</t>
3151<t>
3152   If a server performs an immediate close of a TCP connection, there is a
3153   significant risk that the client will not be able to read the last HTTP
3154   response.  If the server receives additional data from the client on a
3155   fully-closed connection, such as another request that was sent by the
3156   client before receiving the server's response, the server's TCP stack will
3157   send a reset packet to the client; unfortunately, the reset packet might
3158   erase the client's unacknowledged input buffers before they can be read
3159   and interpreted by the client's HTTP parser.
3160</t>
3161<t>
3162   To avoid the TCP reset problem, servers typically close a connection in
3163   stages. First, the server performs a half-close by closing only the write
3164   side of the read/write connection. The server then continues to read from
3165   the connection until it receives a corresponding close by the client, or
3166   until the server is reasonably certain that its own TCP stack has received
3167   the client's acknowledgement of the packet(s) containing the server's last
3168   response. Finally, the server fully closes the connection.
3169</t>
3170<t>
3171   It is unknown whether the reset problem is exclusive to TCP or might also
3172   be found in other transport connection protocols.
3173</t>
3174</section>
3175
3176<section title="Upgrade" anchor="header.upgrade">
3177  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3178  <x:anchor-alias value="Upgrade"/>
3179  <x:anchor-alias value="protocol"/>
3180  <x:anchor-alias value="protocol-name"/>
3181  <x:anchor-alias value="protocol-version"/>
3182<t>
3183   The "Upgrade" header field is intended to provide a simple mechanism
3184   for transitioning from HTTP/1.1 to some other protocol on the same
3185   connection.  A client &MAY; send a list of protocols in the Upgrade
3186   header field of a request to invite the server to switch to one or
3187   more of those protocols, in order of descending preference, before sending
3188   the final response. A server &MAY; ignore a received Upgrade header field
3189   if it wishes to continue using the current protocol on that connection.
3190</t>
3191<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3192  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3193
3194  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3195  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3196  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3197</artwork></figure>
3198<t>
3199   A server that sends a <x:ref>101 (Switching Protocols)</x:ref> response
3200   &MUST; send an Upgrade header field to indicate the new protocol(s) to
3201   which the connection is being switched; if multiple protocol layers are
3202   being switched, the sender &MUST; list the protocols in layer-ascending
3203   order. A server &MUST-NOT; switch to a protocol that was not indicated by
3204   the client in the corresponding request's Upgrade header field.
3205   A server &MAY; choose to ignore the order of preference indicated by the
3206   client and select the new protocol(s) based on other factors, such as the
3207   nature of the request or the current load on the server.
3208</t>
3209<t>
3210   A server that sends a <x:ref>426 (Upgrade Required)</x:ref> response
3211   &MUST; send an Upgrade header field to indicate the acceptable protocols,
3212   in order of descending preference.
3213</t>
3214<t>
3215   A server &MAY; send an Upgrade header field in any other response to
3216   advertise that it implements support for upgrading to the listed protocols,
3217   in order of descending preference, when appropriate for a future request.
3218</t>
3219<figure><preamble>
3220   The following is a hypothetical example sent by a client:
3221</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
3222GET /hello.txt HTTP/1.1
3223Host: www.example.com
3224Connection: upgrade
3225Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3226
3227</artwork></figure>
3228<t>
3229   Upgrade cannot be used to insist on a protocol change; its acceptance and
3230   use by the server is optional. The capabilities and nature of the
3231   application-level communication after the protocol change is entirely
3232   dependent upon the new protocol(s) chosen. However, immediately after
3233   sending the 101 response, the server is expected to continue responding to
3234   the original request as if it had received its equivalent within the new
3235   protocol (i.e., the server still has an outstanding request to satisfy
3236   after the protocol has been changed, and is expected to do so without
3237   requiring the request to be repeated).
3238</t>
3239<t>
3240   For example, if the Upgrade header field is received in a GET request
3241   and the server decides to switch protocols, it first responds
3242   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3243   then immediately follows that with the new protocol's equivalent of a
3244   response to a GET on the target resource.  This allows a connection to be
3245   upgraded to protocols with the same semantics as HTTP without the
3246   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3247   protocols unless the received message semantics can be honored by the new
3248   protocol; an OPTIONS request can be honored by any protocol.
3249</t>
3250<figure><preamble>
3251   The following is an example response to the above hypothetical request:
3252</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
3253HTTP/1.1 101 Switching Protocols
3254Connection: upgrade
3255Upgrade: HTTP/2.0
3256
3257[... data stream switches to HTTP/2.0 with an appropriate response
3258(as defined by new protocol) to the "GET /hello.txt" request ...]
3259</artwork></figure>
3260<t>
3261   When Upgrade is sent, the sender &MUST; also send a
3262   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3263   that contains an "upgrade" connection option, in order to prevent Upgrade
3264   from being accidentally forwarded by intermediaries that might not implement
3265   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3266   is received in an HTTP/1.0 request.
3267</t>
3268<t>
3269   The Upgrade header field only applies to switching protocols on top of the
3270   existing connection; it cannot be used to switch the underlying connection
3271   (transport) protocol, nor to switch the existing communication to a
3272   different connection. For those purposes, it is more appropriate to use a
3273   <x:ref>3xx (Redirection)</x:ref> response (&status-3xx;).
3274</t>
3275<t>
3276   This specification only defines the protocol name "HTTP" for use by
3277   the family of Hypertext Transfer Protocols, as defined by the HTTP
3278   version rules of <xref target="http.version"/> and future updates to this
3279   specification. Additional tokens ought to be registered with IANA using the
3280   registration procedure defined in <xref target="upgrade.token.registry"/>.
3281</t>
3282</section>
3283</section>
3284
3285<section title="ABNF list extension: #rule" anchor="abnf.extension">
3286<t>
3287  A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
3288  improve readability in the definitions of some header field values.
3289</t>
3290<t>
3291  A construct "#" is defined, similar to "*", for defining comma-delimited
3292  lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
3293  at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
3294  comma (",") and optional whitespace (OWS).   
3295</t>
3296<figure><preamble>
3297  Thus, a sender &MUST; expand the list construct as follows:
3298</preamble><artwork type="example">
3299  1#element =&gt; element *( OWS "," OWS element )
3300</artwork></figure>
3301<figure><preamble>
3302  and:
3303</preamble><artwork type="example">
3304  #element =&gt; [ 1#element ]
3305</artwork></figure>
3306<figure><preamble>
3307  and for n &gt;= 1 and m &gt; 1:
3308</preamble><artwork type="example">
3309  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
3310</artwork></figure>
3311<t>
3312  For compatibility with legacy list rules, a recipient &MUST; parse and ignore
3313  a reasonable number of empty list elements: enough to handle common mistakes
3314  by senders that merge values, but not so much that they could be used as a
3315  denial of service mechanism. In other words, a recipient &MUST; expand the
3316  list construct as follows:
3317</t>
3318<figure><artwork type="example">
3319  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
3320 
3321  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
3322</artwork></figure>
3323<t>
3324  Empty elements do not contribute to the count of elements present.
3325  For example, given these ABNF productions:
3326</t>
3327<figure><artwork type="example">
3328  example-list      = 1#example-list-elmt
3329  example-list-elmt = token ; see <xref target="field.components"/>
3330</artwork></figure>
3331<t>
3332  Then the following are valid values for example-list (not including the
3333  double quotes, which are present for delimitation only):
3334</t>
3335<figure><artwork type="example">
3336  "foo,bar"
3337  "foo ,bar,"
3338  "foo , ,bar,charlie   "
3339</artwork></figure>
3340<t>
3341  In contrast, the following values would be invalid, since at least one
3342  non-empty element is required by the example-list production:
3343</t>
3344<figure><artwork type="example">
3345  ""
3346  ","
3347  ",   ,"
3348</artwork></figure>
3349<t>
3350  <xref target="collected.abnf"/> shows the collected ABNF after the list
3351  constructs have been expanded, as described above, for recipients.
3352</t>
3353</section>
3354
3355<section title="IANA Considerations" anchor="IANA.considerations">
3356
3357<section title="Header Field Registration" anchor="header.field.registration">
3358<t>
3359   HTTP header fields are registered within the Message Header Field Registry
3360   maintained at
3361   <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
3362</t>
3363<t>
3364   This document defines the following HTTP header fields, so their
3365   associated registry entries shall be updated according to the permanent
3366   registrations below (see <xref target="BCP90"/>):
3367</t>
3368<?BEGININC p1-messaging.iana-headers ?>
3369<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3370<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3371   <ttcol>Header Field Name</ttcol>
3372   <ttcol>Protocol</ttcol>
3373   <ttcol>Status</ttcol>
3374   <ttcol>Reference</ttcol>
3375
3376   <c>Connection</c>
3377   <c>http</c>
3378   <c>standard</c>
3379   <c>
3380      <xref target="header.connection"/>
3381   </c>
3382   <c>Content-Length</c>
3383   <c>http</c>
3384   <c>standard</c>
3385   <c>
3386      <xref target="header.content-length"/>
3387   </c>
3388   <c>Host</c>
3389   <c>http</c>
3390   <c>standard</c>
3391   <c>
3392      <xref target="header.host"/>
3393   </c>
3394   <c>TE</c>
3395   <c>http</c>
3396   <c>standard</c>
3397   <c>
3398      <xref target="header.te"/>
3399   </c>
3400   <c>Trailer</c>
3401   <c>http</c>
3402   <c>standard</c>
3403   <c>
3404      <xref target="header.trailer"/>
3405   </c>
3406   <c>Transfer-Encoding</c>
3407   <c>http</c>
3408   <c>standard</c>
3409   <c>
3410      <xref target="header.transfer-encoding"/>
3411   </c>
3412   <c>Upgrade</c>
3413   <c>http</c>
3414   <c>standard</c>
3415   <c>
3416      <xref target="header.upgrade"/>
3417   </c>
3418   <c>Via</c>
3419   <c>http</c>
3420   <c>standard</c>
3421   <c>
3422      <xref target="header.via"/>
3423   </c>
3424</texttable>
3425<!--(END)-->
3426<?ENDINC p1-messaging.iana-headers ?>
3427<t>
3428   Furthermore, the header field-name "Close" shall be registered as
3429   "reserved", since using that name as an HTTP header field might
3430   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3431   header field (<xref target="header.connection"/>).
3432</t>
3433<texttable align="left" suppress-title="true">
3434   <ttcol>Header Field Name</ttcol>
3435   <ttcol>Protocol</ttcol>
3436   <ttcol>Status</ttcol>
3437   <ttcol>Reference</ttcol>
3438
3439   <c>Close</c>
3440   <c>http</c>
3441   <c>reserved</c>
3442   <c>
3443      <xref target="header.field.registration"/>
3444   </c>
3445</texttable>
3446<t>
3447   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3448</t>
3449</section>
3450
3451<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3452<t>
3453   IANA maintains the registry of URI Schemes <xref target="BCP115"/> at
3454   <eref target="http://www.iana.org/assignments/uri-schemes.html"/>.
3455</t>
3456<t>
3457   This document defines the following URI schemes, so their
3458   associated registry entries shall be updated according to the permanent
3459   registrations below:
3460</t>
3461<texttable align="left" suppress-title="true">
3462   <ttcol>URI Scheme</ttcol>
3463   <ttcol>Description</ttcol>
3464   <ttcol>Reference</ttcol>
3465
3466   <c>http</c>
3467   <c>Hypertext Transfer Protocol</c>
3468   <c><xref target="http.uri"/></c>
3469
3470   <c>https</c>
3471   <c>Hypertext Transfer Protocol Secure</c>
3472   <c><xref target="https.uri"/></c>
3473</texttable>
3474</section>
3475
3476<section title="Internet Media Type Registration" anchor="internet.media.type.http">
3477<t>
3478   This document serves as the specification for the Internet media types
3479   "message/http" and "application/http". The following is to be registered with
3480   IANA (see <xref target="BCP13"/>).
3481</t>
3482<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
3483<iref item="Media Type" subitem="message/http" primary="true"/>
3484<iref item="message/http Media Type" primary="true"/>
3485<t>
3486   The message/http type can be used to enclose a single HTTP request or
3487   response message, provided that it obeys the MIME restrictions for all
3488   "message" types regarding line length and encodings.
3489</t>
3490<t>
3491  <list style="hanging" x:indent="12em">
3492    <t hangText="Type name:">
3493      message
3494    </t>
3495    <t hangText="Subtype name:">
3496      http
3497    </t>
3498    <t hangText="Required parameters:">
3499      none
3500    </t>
3501    <t hangText="Optional parameters:">
3502      version, msgtype
3503      <list style="hanging">
3504        <t hangText="version:">
3505          The HTTP-version number of the enclosed message
3506          (e.g., "1.1"). If not present, the version can be
3507          determined from the first line of the body.
3508        </t>
3509        <t hangText="msgtype:">
3510          The message type &mdash; "request" or "response". If not
3511          present, the type can be determined from the first
3512          line of the body.
3513        </t>
3514      </list>
3515    </t>
3516    <t hangText="Encoding considerations:">
3517      only "7bit", "8bit", or "binary" are permitted
3518    </t>
3519    <t hangText="Security considerations:">
3520      none
3521    </t>
3522    <t hangText="Interoperability considerations:">
3523      none
3524    </t>
3525    <t hangText="Published specification:">
3526      This specification (see <xref target="internet.media.type.message.http"/>).
3527    </t>
3528    <t hangText="Applications that use this media type:">
3529    </t>
3530    <t hangText="Additional information:">
3531      <list style="hanging">
3532        <t hangText="Magic number(s):">none</t>
3533        <t hangText="File extension(s):">none</t>
3534        <t hangText="Macintosh file type code(s):">none</t>
3535      </list>
3536    </t>
3537    <t hangText="Person and email address to contact for further information:">
3538      See Authors Section.
3539    </t>
3540    <t hangText="Intended usage:">
3541      COMMON
3542    </t>
3543    <t hangText="Restrictions on usage:">
3544      none
3545    </t>
3546    <t hangText="Author:">
3547      See Authors Section.
3548    </t>
3549    <t hangText="Change controller:">
3550      IESG
3551    </t>
3552  </list>
3553</t>
3554</section>
3555<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
3556<iref item="Media Type" subitem="application/http" primary="true"/>
3557<iref item="application/http Media Type" primary="true"/>
3558<t>
3559   The application/http type can be used to enclose a pipeline of one or more
3560   HTTP request or response messages (not intermixed).
3561</t>
3562<t>
3563  <list style="hanging" x:indent="12em">
3564    <t hangText="Type name:">
3565      application
3566    </t>
3567    <t hangText="Subtype name:">
3568      http
3569    </t>
3570    <t hangText="Required parameters:">
3571      none
3572    </t>
3573    <t hangText="Optional parameters:">
3574      version, msgtype
3575      <list style="hanging">
3576        <t hangText="version:">
3577          The HTTP-version number of the enclosed messages
3578          (e.g., "1.1"). If not present, the version can be
3579          determined from the first line of the body.
3580        </t>
3581        <t hangText="msgtype:">
3582          The message type &mdash; "request" or "response". If not
3583          present, the type can be determined from the first
3584          line of the body.
3585        </t>
3586      </list>
3587    </t>
3588    <t hangText="Encoding considerations:">
3589      HTTP messages enclosed by this type
3590      are in "binary" format; use of an appropriate
3591      Content-Transfer-Encoding is required when
3592      transmitted via E-mail.
3593    </t>
3594    <t hangText="Security considerations:">
3595      none
3596    </t>
3597    <t hangText="Interoperability considerations:">
3598      none
3599    </t>
3600    <t hangText="Published specification:">
3601      This specification (see <xref target="internet.media.type.application.http"/>).
3602    </t>
3603    <t hangText="Applications that use this media type:">
3604    </t>
3605    <t hangText="Additional information:">
3606      <list style="hanging">
3607        <t hangText="Magic number(s):">none</t>
3608        <t hangText="File extension(s):">none</t>
3609        <t hangText="Macintosh file type code(s):">none</t>
3610      </list>
3611    </t>
3612    <t hangText="Person and email address to contact for further information:">
3613      See Authors Section.
3614    </t>
3615    <t hangText="Intended usage:">
3616      COMMON
3617    </t>
3618    <t hangText="Restrictions on usage:">
3619      none
3620    </t>
3621    <t hangText="Author:">
3622      See Authors Section.
3623    </t>
3624    <t hangText="Change controller:">
3625      IESG
3626    </t>
3627  </list>
3628</t>
3629</section>
3630</section>
3631
3632<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3633<t>
3634   The HTTP Transfer Coding Registry defines the name space for transfer
3635   coding names. It is maintained at <eref target="http://www.iana.org/assignments/http-parameters"/>.
3636</t>
3637
3638<section title="Procedure" anchor="transfer.coding.registry.procedure">
3639<t>
3640   Registrations &MUST; include the following fields:
3641   <list style="symbols">
3642     <t>Name</t>
3643     <t>Description</t>
3644     <t>Pointer to specification text</t>
3645   </list>
3646</t>
3647<t>
3648   Names of transfer codings &MUST-NOT; overlap with names of content codings
3649   (&content-codings;) unless the encoding transformation is identical, as
3650   is the case for the compression codings defined in
3651   <xref target="compression.codings"/>.
3652</t>
3653<t>
3654   Values to be added to this name space require IETF Review (see
3655   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3656   conform to the purpose of transfer coding defined in this specification.
3657</t>
3658<t>
3659   Use of program names for the identification of encoding formats
3660   is not desirable and is discouraged for future encodings.
3661</t>
3662</section>
3663
3664<section title="Registration" anchor="transfer.coding.registration">
3665<t>
3666   The HTTP Transfer Coding Registry shall be updated with the registrations
3667   below:
3668</t>
3669<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3670   <ttcol>Name</ttcol>
3671   <ttcol>Description</ttcol>
3672   <ttcol>Reference</ttcol>
3673   <c>chunked</c>
3674   <c>Transfer in a series of chunks</c>
3675   <c>
3676      <xref target="chunked.encoding"/>
3677   </c>
3678   <c>compress</c>
3679   <c>UNIX "compress" data format <xref target="Welch"/></c>
3680   <c>
3681      <xref target="compress.coding"/>
3682   </c>
3683   <c>deflate</c>
3684   <c>"deflate" compressed data (<xref target="RFC1951"/>) inside
3685   the "zlib" data format (<xref target="RFC1950"/>)
3686   </c>
3687   <c>
3688      <xref target="deflate.coding"/>
3689   </c>
3690   <c>gzip</c>
3691   <c>GZIP file format <xref target="RFC1952"/></c>
3692   <c>
3693      <xref target="gzip.coding"/>
3694   </c>
3695   <c>x-compress</c>
3696   <c>Deprecated (alias for compress)</c>
3697   <c>
3698      <xref target="compress.coding"/>
3699   </c>
3700   <c>x-gzip</c>
3701   <c>Deprecated (alias for gzip)</c>
3702   <c>
3703      <xref target="gzip.coding"/>
3704   </c>
3705</texttable>
3706</section>
3707</section>
3708
3709<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3710<t>
3711   The HTTP Upgrade Token Registry defines the name space for protocol-name
3712   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3713   field. The registry is maintained at <eref target="http://www.iana.org/assignments/http-upgrade-tokens"/>.
3714</t>
3715
3716<section title="Procedure" anchor="upgrade.token.registry.procedure">  
3717<t>
3718   Each registered protocol name is associated with contact information
3719   and an optional set of specifications that details how the connection
3720   will be processed after it has been upgraded.
3721</t>
3722<t>
3723   Registrations happen on a "First Come First Served" basis (see
3724   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3725   following rules:
3726  <list style="numbers">
3727    <t>A protocol-name token, once registered, stays registered forever.</t>
3728    <t>The registration &MUST; name a responsible party for the
3729       registration.</t>
3730    <t>The registration &MUST; name a point of contact.</t>
3731    <t>The registration &MAY; name a set of specifications associated with
3732       that token. Such specifications need not be publicly available.</t>
3733    <t>The registration &SHOULD; name a set of expected "protocol-version"
3734       tokens associated with that token at the time of registration.</t>
3735    <t>The responsible party &MAY; change the registration at any time.
3736       The IANA will keep a record of all such changes, and make them
3737       available upon request.</t>
3738    <t>The IESG &MAY; reassign responsibility for a protocol token.
3739       This will normally only be used in the case when a
3740       responsible party cannot be contacted.</t>
3741  </list>
3742</t>
3743<t>
3744   This registration procedure for HTTP Upgrade Tokens replaces that
3745   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3746</t>
3747</section>
3748
3749<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3750<t>
3751   The HTTP Upgrade Token Registry shall be updated with the registration
3752   below:
3753</t>
3754<texttable align="left" suppress-title="true">
3755   <ttcol>Value</ttcol>
3756   <ttcol>Description</ttcol>
3757   <ttcol>Expected Version Tokens</ttcol>
3758   <ttcol>Reference</ttcol>
3759
3760   <c>HTTP</c>
3761   <c>Hypertext Transfer Protocol</c>
3762   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3763   <c><xref target="http.version"/></c>
3764</texttable>
3765<t>
3766   The responsible party is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3767</t>
3768</section>
3769</section>
3770
3771</section>
3772
3773<section title="Security Considerations" anchor="security.considerations">
3774<t>
3775   This section is meant to inform developers, information providers, and
3776   users of known security concerns relevant to HTTP/1.1 message syntax,
3777   parsing, and routing.
3778</t>
3779
3780<section title="DNS-related Attacks" anchor="dns.related.attacks">
3781<t>
3782   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3783   generally prone to security attacks based on the deliberate misassociation
3784   of IP addresses and DNS names not protected by DNSSEC. Clients need to be
3785   cautious in assuming the validity of an IP number/DNS name association unless
3786   the response is protected by DNSSEC (<xref target="RFC4033"/>).
3787</t>
3788</section>
3789
3790<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3791<t>
3792   By their very nature, HTTP intermediaries are men-in-the-middle, and
3793   represent an opportunity for man-in-the-middle attacks. Compromise of
3794   the systems on which the intermediaries run can result in serious security
3795   and privacy problems. Intermediaries have access to security-related
3796   information, personal information about individual users and
3797   organizations, and proprietary information belonging to users and
3798   content providers. A compromised intermediary, or an intermediary
3799   implemented or configured without regard to security and privacy
3800   considerations, might be used in the commission of a wide range of
3801   potential attacks.
3802</t>
3803<t>
3804   Intermediaries that contain a shared cache are especially vulnerable
3805   to cache poisoning attacks.
3806</t>
3807<t>
3808   Implementers need to consider the privacy and security
3809   implications of their design and coding decisions, and of the
3810   configuration options they provide to operators (especially the
3811   default configuration).
3812</t>
3813<t>
3814   Users need to be aware that intermediaries are no more trustworthy than
3815   the people who run them; HTTP itself cannot solve this problem.
3816</t>
3817</section>
3818
3819<section title="Buffer Overflows" anchor="attack.protocol.element.size.overflows">
3820<t>
3821   Because HTTP uses mostly textual, character-delimited fields, attackers can
3822   overflow buffers in implementations, and/or perform a Denial of Service
3823   against implementations that accept fields with unlimited lengths.
3824</t>
3825<t>
3826   To promote interoperability, this specification makes specific
3827   recommendations for minimum size limits on request-line
3828   (<xref target="request.line"/>)
3829   and header fields (<xref target="header.fields"/>). These are
3830   minimum recommendations, chosen to be supportable even by implementations
3831   with limited resources; it is expected that most implementations will
3832   choose substantially higher limits.
3833</t>
3834<t>
3835   This specification also provides a way for servers to reject messages that
3836   have request-targets that are too long (&status-414;) or request entities
3837   that are too large (&status-4xx;). Additional status codes related to
3838   capacity limits have been defined by extensions to HTTP
3839   <xref target="RFC6585"/>.
3840</t>
3841<t>
3842   Recipients ought to carefully limit the extent to which they read other
3843   fields, including (but not limited to) request methods, response status
3844   phrases, header field-names, and body chunks, so as to avoid denial of
3845   service attacks without impeding interoperability.
3846</t>
3847</section>
3848
3849<section title="Message Integrity" anchor="message.integrity">
3850<t>
3851   HTTP does not define a specific mechanism for ensuring message integrity,
3852   instead relying on the error-detection ability of underlying transport
3853   protocols and the use of length or chunk-delimited framing to detect
3854   completeness. Additional integrity mechanisms, such as hash functions or
3855   digital signatures applied to the content, can be selectively added to
3856   messages via extensible metadata header fields. Historically, the lack of
3857   a single integrity mechanism has been justified by the informal nature of
3858   most HTTP communication.  However, the prevalence of HTTP as an information
3859   access mechanism has resulted in its increasing use within environments
3860   where verification of message integrity is crucial.
3861</t>
3862<t>
3863   User agents are encouraged to implement configurable means for detecting
3864   and reporting failures of message integrity such that those means can be
3865   enabled within environments for which integrity is necessary. For example,
3866   a browser being used to view medical history or drug interaction
3867   information needs to indicate to the user when such information is detected
3868   by the protocol to be incomplete, expired, or corrupted during transfer.
3869   Such mechanisms might be selectively enabled via user agent extensions or
3870   the presence of message integrity metadata in a response.
3871   At a minimum, user agents ought to provide some indication that allows a
3872   user to distinguish between a complete and incomplete response message
3873   (<xref target="incomplete.messages"/>) when such verification is desired.
3874</t>
3875</section>
3876
3877<section title="Server Log Information" anchor="abuse.of.server.log.information">
3878<t>
3879   A server is in the position to save personal data about a user's requests
3880   over time, which might identify their reading patterns or subjects of
3881   interest.  In particular, log information gathered at an intermediary
3882   often contains a history of user agent interaction, across a multitude
3883   of sites, that can be traced to individual users.
3884</t>
3885<t>
3886   HTTP log information is confidential in nature; its handling is often
3887   constrained by laws and regulations.  Log information needs to be securely
3888   stored and appropriate guidelines followed for its analysis.
3889   Anonymization of personal information within individual entries helps,
3890   but is generally not sufficient to prevent real log traces from being
3891   re-identified based on correlation with other access characteristics.
3892   As such, access traces that are keyed to a specific client are unsafe to
3893   publish even if the key is pseudonymous.
3894</t>
3895<t>
3896   To minimize the risk of theft or accidental publication, log information
3897   ought to be purged of personally identifiable information, including
3898   user identifiers, IP addresses, and user-provided query parameters,
3899   as soon as that information is no longer necessary to support operational
3900   needs for security, auditing, or fraud control.
3901</t>
3902</section>
3903</section>
3904
3905<section title="Acknowledgments" anchor="acks">
3906<t>
3907   This edition of HTTP/1.1 builds on the many contributions that went into
3908   <xref target="RFC1945" format="none">RFC 1945</xref>,
3909   <xref target="RFC2068" format="none">RFC 2068</xref>,
3910   <xref target="RFC2145" format="none">RFC 2145</xref>, and
3911   <xref target="RFC2616" format="none">RFC 2616</xref>, including
3912   substantial contributions made by the previous authors, editors, and
3913   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
3914   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
3915   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
3916</t>
3917<t>
3918   Since 1999, the following contributors have helped improve the HTTP
3919   specification by reporting bugs, asking smart questions, drafting or
3920   reviewing text, and evaluating open issues:
3921</t>
3922<?BEGININC acks ?>
3923<t>Adam Barth,
3924Adam Roach,
3925Addison Phillips,
3926Adrian Chadd,
3927Adrien W. de Croy,
3928Alan Ford,
3929Alan Ruttenberg,
3930Albert Lunde,
3931Alek Storm,
3932Alex Rousskov,
3933Alexandre Morgaut,
3934Alexey Melnikov,
3935Alisha Smith,
3936Amichai Rothman,
3937Amit Klein,
3938Amos Jeffries,
3939Andreas Maier,
3940Andreas Petersson,
3941Anil Sharma,
3942Anne van Kesteren,
3943Anthony Bryan,
3944Asbjorn Ulsberg,
3945Ashok Kumar,
3946Balachander Krishnamurthy,
3947Barry Leiba,
3948Ben Laurie,
3949Benjamin Carlyle,
3950Benjamin Niven-Jenkins,
3951Bil Corry,
3952Bill Burke,
3953Bjoern Hoehrmann,
3954Bob Scheifler,
3955Boris Zbarsky,
3956Brett Slatkin,
3957Brian Kell,
3958Brian McBarron,
3959Brian Pane,
3960Brian Raymor,
3961Brian Smith,
3962Bryce Nesbitt,
3963Cameron Heavon-Jones,
3964Carl Kugler,
3965Carsten Bormann,
3966Charles Fry,
3967Chris Newman,
3968Cyrus Daboo,
3969Dale Robert Anderson,
3970Dan Wing,
3971Dan Winship,
3972Daniel Stenberg,
3973Darrel Miller,
3974Dave Cridland,
3975Dave Crocker,
3976Dave Kristol,
3977Dave Thaler,
3978David Booth,
3979David Singer,
3980David W. Morris,
3981Diwakar Shetty,
3982Dmitry Kurochkin,
3983Drummond Reed,
3984Duane Wessels,
3985Edward Lee,
3986Eitan Adler,
3987Eliot Lear,
3988Eran Hammer-Lahav,
3989Eric D. Williams,
3990Eric J. Bowman,
3991Eric Lawrence,
3992Eric Rescorla,
3993Erik Aronesty,
3994Evan Prodromou,
3995Felix Geisendoerfer,
3996Florian Weimer,
3997Frank Ellermann,
3998Fred Akalin,
3999Fred Bohle,
4000Frederic Kayser,
4001Gabor Molnar,
4002Gabriel Montenegro,
4003Geoffrey Sneddon,
4004Gervase Markham,
4005Gili Tzabari,
4006Grahame Grieve,
4007Greg Wilkins,
4008Grzegorz Calkowski,
4009Harald Tveit Alvestrand,
4010Harry Halpin,
4011Helge Hess,
4012Henrik Nordstrom,
4013Henry S. Thompson,
4014Henry Story,
4015Herbert van de Sompel,
4016Herve Ruellan,
4017Howard Melman,
4018Hugo Haas,
4019Ian Fette,
4020Ian Hickson,
4021Ido Safruti,
4022Ilari Liusvaara,
4023Ilya Grigorik,
4024Ingo Struck,
4025J. Ross Nicoll,
4026James Cloos,
4027James H. Manger,
4028James Lacey,
4029James M. Snell,
4030Jamie Lokier,
4031Jan Algermissen,
4032Jeff Hodges (who came up with the term 'effective Request-URI'),
4033Jeff Pinner,
4034Jeff Walden,
4035Jim Luther,
4036Jitu Padhye,
4037Joe D. Williams,
4038Joe Gregorio,
4039Joe Orton,
4040John C. Klensin,
4041John C. Mallery,
4042John Cowan,
4043John Kemp,
4044John Panzer,
4045John Schneider,
4046John Stracke,
4047John Sullivan,
4048Jonas Sicking,
4049Jonathan A. Rees,
4050Jonathan Billington,
4051Jonathan Moore,
4052Jonathan Silvera,
4053Jordi Ros,
4054Joris Dobbelsteen,
4055Josh Cohen,
4056Julien Pierre,
4057Jungshik Shin,
4058Justin Chapweske,
4059Justin Erenkrantz,
4060Justin James,
4061Kalvinder Singh,
4062Karl Dubost,
4063Keith Hoffman,
4064Keith Moore,
4065Ken Murchison,
4066Koen Holtman,
4067Konstantin Voronkov,
4068Kris Zyp,
4069Lisa Dusseault,
4070Maciej Stachowiak,
4071Manu Sporny,
4072Marc Schneider,
4073Marc Slemko,
4074Mark Baker,
4075Mark Pauley,
4076Mark Watson,
4077Markus Isomaki,
4078Markus Lanthaler,
4079Martin J. Duerst,
4080Martin Musatov,
4081Martin Nilsson,
4082Martin Thomson,
4083Matt Lynch,
4084Matthew Cox,
4085Max Clark,
4086Michael Burrows,
4087Michael Hausenblas,
4088Michael Sweet,
4089Michael Tuexen,
4090Michael Welzl,
4091Mike Amundsen,
4092Mike Belshe,
4093Mike Bishop,
4094Mike Kelly,
4095Mike Schinkel,
4096Miles Sabin,
4097Murray S. Kucherawy,
4098Mykyta Yevstifeyev,
4099Nathan Rixham,
4100Nicholas Shanks,
4101Nico Williams,
4102Nicolas Alvarez,
4103Nicolas Mailhot,
4104Noah Slater,
4105Osama Mazahir,
4106Pablo Castro,
4107Pat Hayes,
4108Patrick R. McManus,
4109Paul E. Jones,
4110Paul Hoffman,
4111Paul Marquess,
4112Peter Lepeska,
4113Peter Occil,
4114Peter Saint-Andre,
4115Peter Watkins,
4116Phil Archer,
4117Philippe Mougin,
4118Phillip Hallam-Baker,
4119Piotr Dobrogost,
4120Poul-Henning Kamp,
4121Preethi Natarajan,
4122Rajeev Bector,
4123Ray Polk,
4124Reto Bachmann-Gmuer,
4125Richard Cyganiak,
4126Robby Simpson,
4127Robert Brewer,
4128Robert Collins,
4129Robert Mattson,
4130Robert O'Callahan,
4131Robert Olofsson,
4132Robert Sayre,
4133Robert Siemer,
4134Robert de Wilde,
4135Roberto Javier Godoy,
4136Roberto Peon,
4137Roland Zink,
4138Ronny Widjaja,
4139Ryan Hamilton,
4140S. Mike Dierken,
4141Salvatore Loreto,
4142Sam Johnston,
4143Sam Pullara,
4144Sam Ruby,
4145Scott Lawrence (who maintained the original issues list),
4146Sean B. Palmer,
4147Sebastien Barnoud,
4148Shane McCarron,
4149Shigeki Ohtsu,
4150Stefan Eissing,
4151Stefan Tilkov,
4152Stefanos Harhalakis,
4153Stephane Bortzmeyer,
4154Stephen Farrell,
4155Stephen Ludin,
4156Stuart Williams,
4157Subbu Allamaraju,
4158Sylvain Hellegouarch,
4159Tapan Divekar,
4160Tatsuhiro Tsujikawa,
4161Tatsuya Hayashi,
4162Ted Hardie,
4163Thomas Broyer,
4164Thomas Fossati,
4165Thomas Maslen,
4166Thomas Nordin,
4167Thomas Roessler,
4168Tim Bray,
4169Tim Morgan,
4170Tim Olsen,
4171Tom Zhou,
4172Travis Snoozy,
4173Tyler Close,
4174Vincent Murphy,
4175Wenbo Zhu,
4176Werner Baumann,
4177Wilbur Streett,
4178Wilfredo Sanchez Vega,
4179William A. Rowe Jr.,
4180William Chan,
4181Willy Tarreau,
4182Xiaoshu Wang,
4183Yaron Goland,
4184Yngve Nysaeter Pettersen,
4185Yoav Nir,
4186Yogesh Bang,
4187Yuchung Cheng,
4188Yutaka Oiwa,
4189Yves Lafon (long-time member of the editor team),
4190Zed A. Shaw, and
4191Zhong Yu.
4192</t>
4193<?ENDINC acks ?>
4194<t>
4195   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
4196   acknowledgements from prior revisions.
4197</t>
4198</section>
4199
4200</middle>
4201<back>
4202
4203<references title="Normative References">
4204
4205<reference anchor="Part2">
4206  <front>
4207    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
4208    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4209      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4210      <address><email>fielding@gbiv.com</email></address>
4211    </author>
4212    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4213      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4214      <address><email>julian.reschke@greenbytes.de</email></address>
4215    </author>
4216    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4217  </front>
4218  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
4219  <x:source href="p2-semantics.xml" basename="p2-semantics">
4220    <x:defines>1xx (Informational)</x:defines>
4221    <x:defines>1xx</x:defines>
4222    <x:defines>100 (Continue)</x:defines>
4223    <x:defines>101 (Switching Protocols)</x:defines>
4224    <x:defines>2xx (Successful)</x:defines>
4225    <x:defines>2xx</x:defines>
4226    <x:defines>200 (OK)</x:defines>
4227    <x:defines>203 (Non-Authoritative Information)</x:defines>
4228    <x:defines>204 (No Content)</x:defines>
4229    <x:defines>3xx (Redirection)</x:defines>
4230    <x:defines>3xx</x:defines>
4231    <x:defines>301 (Moved Permanently)</x:defines>
4232    <x:defines>4xx (Client Error)</x:defines>
4233    <x:defines>4xx</x:defines>
4234    <x:defines>400 (Bad Request)</x:defines>
4235    <x:defines>411 (Length Required)</x:defines>
4236    <x:defines>414 (URI Too Long)</x:defines>
4237    <x:defines>417 (Expectation Failed)</x:defines>
4238    <x:defines>426 (Upgrade Required)</x:defines>
4239    <x:defines>501 (Not Implemented)</x:defines>
4240    <x:defines>502 (Bad Gateway)</x:defines>
4241    <x:defines>505 (HTTP Version Not Supported)</x:defines>
4242    <x:defines>Accept-Encoding</x:defines>
4243    <x:defines>Allow</x:defines>
4244    <x:defines>Content-Encoding</x:defines>
4245    <x:defines>Content-Location</x:defines>
4246    <x:defines>Content-Type</x:defines>
4247    <x:defines>Date</x:defines>
4248    <x:defines>Expect</x:defines>
4249    <x:defines>Location</x:defines>
4250    <x:defines>Server</x:defines>
4251    <x:defines>User-Agent</x:defines>
4252  </x:source>
4253</reference>
4254
4255<reference anchor="Part4">
4256  <front>
4257    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
4258    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
4259      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4260      <address><email>fielding@gbiv.com</email></address>
4261    </author>
4262    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
4263      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4264      <address><email>julian.reschke@greenbytes.de</email></address>
4265    </author>
4266    <date month="&ID-MONTH;" year="&ID-YEAR;" />
4267  </front>
4268  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
4269  <x:source basename="p4-conditional" href="p4-conditional.xml">
4270    <x:defines>304 (Not Modified)</x:defines>
4271    <x:defines>ETag</x:defines>
4272    <x:defines>Last-Modified</x:defines>
4273  </x:source>
4274</reference>
4275
4276<reference anchor="Part5">
4277  <front>
4278    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
4279    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4280      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4281      <address><email>fielding@gbiv.com</email></address>
4282    </author>
4283    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
4284      <organization abbrev="W3C">World Wide Web Consortium</organization>
4285      <address><email>ylafon@w3.org</email></address>
4286    </author>
4287    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4288      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4289      <address><email>julian.reschke@greenbytes.de</email></address>
4290    </author>
4291    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4292  </front>
4293  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4294  <x:source href="p5-range.xml" basename="p5-range">
4295    <x:defines>Content-Range</x:defines>
4296  </x:source>
4297</reference>
4298
4299<reference anchor="Part6">
4300  <front>
4301    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4302    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4303      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4304      <address><email>fielding@gbiv.com</email></address>
4305    </author>
4306    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4307      <organization>Akamai</organization>
4308      <address><email>mnot@mnot.net</email></address>
4309    </author>
4310    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4311      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4312      <address><email>julian.reschke@greenbytes.de</email></address>
4313    </author>
4314    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4315  </front>
4316  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4317  <x:source href="p6-cache.xml" basename="p6-cache">
4318    <x:defines>Cache-Control</x:defines>
4319    <x:defines>Expires</x:defines>
4320  </x:source>
4321</reference>
4322
4323<reference anchor="Part7">
4324  <front>
4325    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4326    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4327      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4328      <address><email>fielding@gbiv.com</email></address>
4329    </author>
4330    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4331      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4332      <address><email>julian.reschke@greenbytes.de</email></address>
4333    </author>
4334    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4335  </front>
4336  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4337  <x:source href="p7-auth.xml" basename="p7-auth">
4338    <x:defines>Proxy-Authenticate</x:defines>
4339    <x:defines>Proxy-Authorization</x:defines>
4340  </x:source>
4341</reference>
4342
4343<reference anchor="RFC5234">
4344  <front>
4345    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4346    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4347      <organization>Brandenburg InternetWorking</organization>
4348      <address>
4349        <email>dcrocker@bbiw.net</email>
4350      </address> 
4351    </author>
4352    <author initials="P." surname="Overell" fullname="Paul Overell">
4353      <organization>THUS plc.</organization>
4354      <address>
4355        <email>paul.overell@thus.net</email>
4356      </address>
4357    </author>
4358    <date month="January" year="2008"/>
4359  </front>
4360  <seriesInfo name="STD" value="68"/>
4361  <seriesInfo name="RFC" value="5234"/>
4362</reference>
4363
4364<reference anchor="RFC2119">
4365  <front>
4366    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4367    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4368      <organization>Harvard University</organization>
4369      <address><email>sob@harvard.edu</email></address>
4370    </author>
4371    <date month="March" year="1997"/>
4372  </front>
4373  <seriesInfo name="BCP" value="14"/>
4374  <seriesInfo name="RFC" value="2119"/>
4375</reference>
4376
4377<reference anchor="RFC3986">
4378 <front>
4379  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4380  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4381    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4382    <address>
4383       <email>timbl@w3.org</email>
4384       <uri>http://www.w3.org/People/Berners-Lee/</uri>
4385    </address>
4386  </author>
4387  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4388    <organization abbrev="Day Software">Day Software</organization>
4389    <address>
4390      <email>fielding@gbiv.com</email>
4391      <uri>http://roy.gbiv.com/</uri>
4392    </address>
4393  </author>
4394  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4395    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4396    <address>
4397      <email>LMM@acm.org</email>
4398      <uri>http://larry.masinter.net/</uri>
4399    </address>
4400  </author>
4401  <date month='January' year='2005'></date>
4402 </front>
4403 <seriesInfo name="STD" value="66"/>
4404 <seriesInfo name="RFC" value="3986"/>
4405</reference>
4406
4407<reference anchor="RFC0793">
4408  <front>
4409    <title>Transmission Control Protocol</title>
4410    <author initials='J.' surname='Postel' fullname='Jon Postel'>
4411      <organization>University of Southern California (USC)/Information Sciences Institute</organization>
4412    </author>
4413    <date year='1981' month='September' />
4414  </front>
4415  <seriesInfo name='STD' value='7' />
4416  <seriesInfo name='RFC' value='793' />
4417</reference>
4418
4419<reference anchor="USASCII">
4420  <front>
4421    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4422    <author>
4423      <organization>American National Standards Institute</organization>
4424    </author>
4425    <date year="1986"/>
4426  </front>
4427  <seriesInfo name="ANSI" value="X3.4"/>
4428</reference>
4429
4430<reference anchor="RFC1950">
4431  <front>
4432    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4433    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4434      <organization>Aladdin Enterprises</organization>
4435      <address><email>ghost@aladdin.com</email></address>
4436    </author>
4437    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4438    <date month="May" year="1996"/>
4439  </front>
4440  <seriesInfo name="RFC" value="1950"/>
4441  <!--<annotation>
4442    RFC 1950 is an Informational RFC, thus it might be less stable than
4443    this specification. On the other hand, this downward reference was
4444    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4445    therefore it is unlikely to cause problems in practice. See also
4446    <xref target="BCP97"/>.
4447  </annotation>-->
4448</reference>
4449
4450<reference anchor="RFC1951">
4451  <front>
4452    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4453    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4454      <organization>Aladdin Enterprises</organization>
4455      <address><email>ghost@aladdin.com</email></address>
4456    </author>
4457    <date month="May" year="1996"/>
4458  </front>
4459  <seriesInfo name="RFC" value="1951"/>
4460  <!--<annotation>
4461    RFC 1951 is an Informational RFC, thus it might be less stable than
4462    this specification. On the other hand, this downward reference was
4463    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4464    therefore it is unlikely to cause problems in practice. See also
4465    <xref target="BCP97"/>.
4466  </annotation>-->
4467</reference>
4468
4469<reference anchor="RFC1952">
4470  <front>
4471    <title>GZIP file format specification version 4.3</title>
4472    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4473      <organization>Aladdin Enterprises</organization>
4474      <address><email>ghost@aladdin.com</email></address>
4475    </author>
4476    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4477      <address><email>gzip@prep.ai.mit.edu</email></address>
4478    </author>
4479    <author initials="M." surname="Adler" fullname="Mark Adler">
4480      <address><email>madler@alumni.caltech.edu</email></address>
4481    </author>
4482    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4483      <address><email>ghost@aladdin.com</email></address>
4484    </author>
4485    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4486      <address><email>randeg@alumni.rpi.edu</email></address>
4487    </author>
4488    <date month="May" year="1996"/>
4489  </front>
4490  <seriesInfo name="RFC" value="1952"/>
4491  <!--<annotation>
4492    RFC 1952 is an Informational RFC, thus it might be less stable than
4493    this specification. On the other hand, this downward reference was
4494    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4495    therefore it is unlikely to cause problems in practice. See also
4496    <xref target="BCP97"/>.
4497  </annotation>-->
4498</reference>
4499
4500<reference anchor="Welch">
4501  <front>
4502    <title>A Technique for High Performance Data Compression</title>
4503    <author initials="T.A." surname="Welch" fullname="Terry A. Welch"/>
4504    <date month="June" year="1984"/>
4505  </front>
4506  <seriesInfo name="IEEE Computer" value="17(6)"/>
4507</reference>
4508
4509</references>
4510
4511<references title="Informative References">
4512
4513<reference anchor="ISO-8859-1">
4514  <front>
4515    <title>
4516     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4517    </title>
4518    <author>
4519      <organization>International Organization for Standardization</organization>
4520    </author>
4521    <date year="1998"/>
4522  </front>
4523  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4524</reference>
4525
4526<reference anchor='RFC1919'>
4527  <front>
4528    <title>Classical versus Transparent IP Proxies</title>
4529    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4530      <address><email>mchatel@pax.eunet.ch</email></address>
4531    </author>
4532    <date year='1996' month='March' />
4533  </front>
4534  <seriesInfo name='RFC' value='1919' />
4535</reference>
4536
4537<reference anchor="RFC1945">
4538  <front>
4539    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4540    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4541      <organization>MIT, Laboratory for Computer Science</organization>
4542      <address><email>timbl@w3.org</email></address>
4543    </author>
4544    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4545      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4546      <address><email>fielding@ics.uci.edu</email></address>
4547    </author>
4548    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4549      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4550      <address><email>frystyk@w3.org</email></address>
4551    </author>
4552    <date month="May" year="1996"/>
4553  </front>
4554  <seriesInfo name="RFC" value="1945"/>
4555</reference>
4556
4557<reference anchor="RFC2045">
4558  <front>
4559    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4560    <author initials="N." surname="Freed" fullname="Ned Freed">
4561      <organization>Innosoft International, Inc.</organization>
4562      <address><email>ned@innosoft.com</email></address>
4563    </author>
4564    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4565      <organization>First Virtual Holdings</organization>
4566      <address><email>nsb@nsb.fv.com</email></address>
4567    </author>
4568    <date month="November" year="1996"/>
4569  </front>
4570  <seriesInfo name="RFC" value="2045"/>
4571</reference>
4572
4573<reference anchor="RFC2047">
4574  <front>
4575    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4576    <author initials="K." surname="Moore" fullname="Keith Moore">
4577      <organization>University of Tennessee</organization>
4578      <address><email>moore@cs.utk.edu</email></address>
4579    </author>
4580    <date month="November" year="1996"/>
4581  </front>
4582  <seriesInfo name="RFC" value="2047"/>
4583</reference>
4584
4585<reference anchor="RFC2068">
4586  <front>
4587    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4588    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
4589      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4590      <address><email>fielding@ics.uci.edu</email></address>
4591    </author>
4592    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4593      <organization>MIT Laboratory for Computer Science</organization>
4594      <address><email>jg@w3.org</email></address>
4595    </author>
4596    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4597      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4598      <address><email>mogul@wrl.dec.com</email></address>
4599    </author>
4600    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4601      <organization>MIT Laboratory for Computer Science</organization>
4602      <address><email>frystyk@w3.org</email></address>
4603    </author>
4604    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4605      <organization>MIT Laboratory for Computer Science</organization>
4606      <address><email>timbl@w3.org</email></address>
4607    </author>
4608    <date month="January" year="1997"/>
4609  </front>
4610  <seriesInfo name="RFC" value="2068"/>
4611</reference>
4612
4613<reference anchor="RFC2145">
4614  <front>
4615    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4616    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4617      <organization>Western Research Laboratory</organization>
4618      <address><email>mogul@wrl.dec.com</email></address>
4619    </author>
4620    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4621      <organization>Department of Information and Computer Science</organization>
4622      <address><email>fielding@ics.uci.edu</email></address>
4623    </author>
4624    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4625      <organization>MIT Laboratory for Computer Science</organization>
4626      <address><email>jg@w3.org</email></address>
4627    </author>
4628    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4629      <organization>W3 Consortium</organization>
4630      <address><email>frystyk@w3.org</email></address>
4631    </author>
4632    <date month="May" year="1997"/>
4633  </front>
4634  <seriesInfo name="RFC" value="2145"/>
4635</reference>
4636
4637<reference anchor="RFC2616">
4638  <front>
4639    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4640    <author initials="R." surname="Fielding" fullname="R. Fielding">
4641      <organization>University of California, Irvine</organization>
4642      <address><email>fielding@ics.uci.edu</email></address>
4643    </author>
4644    <author initials="J." surname="Gettys" fullname="J. Gettys">
4645      <organization>W3C</organization>
4646      <address><email>jg@w3.org</email></address>
4647    </author>
4648    <author initials="J." surname="Mogul" fullname="J. Mogul">
4649      <organization>Compaq Computer Corporation</organization>
4650      <address><email>mogul@wrl.dec.com</email></address>
4651    </author>
4652    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4653      <organization>MIT Laboratory for Computer Science</organization>
4654      <address><email>frystyk@w3.org</email></address>
4655    </author>
4656    <author initials="L." surname="Masinter" fullname="L. Masinter">
4657      <organization>Xerox Corporation</organization>
4658      <address><email>masinter@parc.xerox.com</email></address>
4659    </author>
4660    <author initials="P." surname="Leach" fullname="P. Leach">
4661      <organization>Microsoft Corporation</organization>
4662      <address><email>paulle@microsoft.com</email></address>
4663    </author>
4664    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4665      <organization>W3C</organization>
4666      <address><email>timbl@w3.org</email></address>
4667    </author>
4668    <date month="June" year="1999"/>
4669  </front>
4670  <seriesInfo name="RFC" value="2616"/>
4671</reference>
4672
4673<reference anchor='RFC2817'>
4674  <front>
4675    <title>Upgrading to TLS Within HTTP/1.1</title>
4676    <author initials='R.' surname='Khare' fullname='R. Khare'>
4677      <organization>4K Associates / UC Irvine</organization>
4678      <address><email>rohit@4K-associates.com</email></address>
4679    </author>
4680    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4681      <organization>Agranat Systems, Inc.</organization>
4682      <address><email>lawrence@agranat.com</email></address>
4683    </author>
4684    <date year='2000' month='May' />
4685  </front>
4686  <seriesInfo name='RFC' value='2817' />
4687</reference>
4688
4689<reference anchor='RFC2818'>
4690  <front>
4691    <title>HTTP Over TLS</title>
4692    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4693      <organization>RTFM, Inc.</organization>
4694      <address><email>ekr@rtfm.com</email></address>
4695    </author>
4696    <date year='2000' month='May' />
4697  </front>
4698  <seriesInfo name='RFC' value='2818' />
4699</reference>
4700
4701<reference anchor='RFC3040'>
4702  <front>
4703    <title>Internet Web Replication and Caching Taxonomy</title>
4704    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4705      <organization>Equinix, Inc.</organization>
4706    </author>
4707    <author initials='I.' surname='Melve' fullname='I. Melve'>
4708      <organization>UNINETT</organization>
4709    </author>
4710    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4711      <organization>CacheFlow Inc.</organization>
4712    </author>
4713    <date year='2001' month='January' />
4714  </front>
4715  <seriesInfo name='RFC' value='3040' />
4716</reference>
4717
4718<reference anchor='BCP90'>
4719  <front>
4720    <title>Registration Procedures for Message Header Fields</title>
4721    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4722      <organization>Nine by Nine</organization>
4723      <address><email>GK-IETF@ninebynine.org</email></address>
4724    </author>
4725    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4726      <organization>BEA Systems</organization>
4727      <address><email>mnot@pobox.com</email></address>
4728    </author>
4729    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4730      <organization>HP Labs</organization>
4731      <address><email>JeffMogul@acm.org</email></address>
4732    </author>
4733    <date year='2004' month='September' />
4734  </front>
4735  <seriesInfo name='BCP' value='90' />
4736  <seriesInfo name='RFC' value='3864' />
4737</reference>
4738
4739<reference anchor='RFC4033'>
4740  <front>
4741    <title>DNS Security Introduction and Requirements</title>
4742    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4743    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4744    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4745    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4746    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4747    <date year='2005' month='March' />
4748  </front>
4749  <seriesInfo name='RFC' value='4033' />
4750</reference>
4751
4752<reference anchor="BCP13">
4753  <front>
4754    <title>Media Type Specifications and Registration Procedures</title>
4755    <author initials="N." surname="Freed" fullname="Ned Freed">
4756      <organization>Oracle</organization>
4757      <address>
4758        <email>ned+ietf@mrochek.com</email>
4759      </address>
4760    </author>
4761    <author initials="J." surname="Klensin" fullname="John C. Klensin">
4762      <address>
4763        <email>john+ietf@jck.com</email>
4764      </address>
4765    </author>
4766    <author initials="T." surname="Hansen" fullname="Tony Hansen">
4767      <organization>AT&amp;T Laboratories</organization>
4768      <address>
4769        <email>tony+mtsuffix@maillennium.att.com</email>
4770      </address>
4771    </author>
4772    <date year="2013" month="January"/>
4773  </front>
4774  <seriesInfo name="BCP" value="13"/>
4775  <seriesInfo name="RFC" value="6838"/>
4776</reference>
4777
4778<reference anchor='BCP115'>
4779  <front>
4780    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4781    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4782      <organization>AT&amp;T Laboratories</organization>
4783      <address>
4784        <email>tony+urireg@maillennium.att.com</email>
4785      </address>
4786    </author>
4787    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4788      <organization>Qualcomm, Inc.</organization>
4789      <address>
4790        <email>hardie@qualcomm.com</email>
4791      </address>
4792    </author>
4793    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4794      <organization>Adobe Systems</organization>
4795      <address>
4796        <email>LMM@acm.org</email>
4797      </address>
4798    </author>
4799    <date year='2006' month='February' />
4800  </front>
4801  <seriesInfo name='BCP' value='115' />
4802  <seriesInfo name='RFC' value='4395' />
4803</reference>
4804
4805<reference anchor='RFC4559'>
4806  <front>
4807    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4808    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4809    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4810    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4811    <date year='2006' month='June' />
4812  </front>
4813  <seriesInfo name='RFC' value='4559' />
4814</reference>
4815
4816<reference anchor='RFC5226'>
4817  <front>
4818    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4819    <author initials='T.' surname='Narten' fullname='T. Narten'>
4820      <organization>IBM</organization>
4821      <address><email>narten@us.ibm.com</email></address>
4822    </author>
4823    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4824      <organization>Google</organization>
4825      <address><email>Harald@Alvestrand.no</email></address>
4826    </author>
4827    <date year='2008' month='May' />
4828  </front>
4829  <seriesInfo name='BCP' value='26' />
4830  <seriesInfo name='RFC' value='5226' />
4831</reference>
4832
4833<reference anchor='RFC5246'>
4834   <front>
4835      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4836      <author initials='T.' surname='Dierks' fullname='T. Dierks'>
4837         <organization />
4838      </author>
4839      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4840         <organization>RTFM, Inc.</organization>
4841      </author>
4842      <date year='2008' month='August' />
4843   </front>
4844   <seriesInfo name='RFC' value='5246' />
4845</reference>
4846
4847<reference anchor="RFC5322">
4848  <front>
4849    <title>Internet Message Format</title>
4850    <author initials="P." surname="Resnick" fullname="P. Resnick">
4851      <organization>Qualcomm Incorporated</organization>
4852    </author>
4853    <date year="2008" month="October"/>
4854  </front>
4855  <seriesInfo name="RFC" value="5322"/>
4856</reference>
4857
4858<reference anchor="RFC6265">
4859  <front>
4860    <title>HTTP State Management Mechanism</title>
4861    <author initials="A." surname="Barth" fullname="Adam Barth">
4862      <organization abbrev="U.C. Berkeley">
4863        University of California, Berkeley
4864      </organization>
4865      <address><email>abarth@eecs.berkeley.edu</email></address>
4866    </author>
4867    <date year="2011" month="April" />
4868  </front>
4869  <seriesInfo name="RFC" value="6265"/>
4870</reference>
4871
4872<reference anchor='RFC6585'>
4873  <front>
4874    <title>Additional HTTP Status Codes</title>
4875    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4876      <organization>Rackspace</organization>
4877    </author>
4878    <author initials='R.' surname='Fielding' fullname='R. Fielding'>
4879      <organization>Adobe</organization>
4880    </author>
4881    <date year='2012' month='April' />
4882   </front>
4883   <seriesInfo name='RFC' value='6585' />
4884</reference>
4885
4886<!--<reference anchor='BCP97'>
4887  <front>
4888    <title>Handling Normative References to Standards-Track Documents</title>
4889    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
4890      <address>
4891        <email>klensin+ietf@jck.com</email>
4892      </address>
4893    </author>
4894    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
4895      <organization>MIT</organization>
4896      <address>
4897        <email>hartmans-ietf@mit.edu</email>
4898      </address>
4899    </author>
4900    <date year='2007' month='June' />
4901  </front>
4902  <seriesInfo name='BCP' value='97' />
4903  <seriesInfo name='RFC' value='4897' />
4904</reference>-->
4905
4906<reference anchor="Kri2001" target="http://arxiv.org/abs/cs.SE/0105018">
4907  <front>
4908    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
4909    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
4910    <date year="2001" month="November"/>
4911  </front>
4912  <seriesInfo name="ACM Transactions on Internet Technology" value="1(2)"/>
4913</reference>
4914
4915</references>
4916
4917
4918<section title="HTTP Version History" anchor="compatibility">
4919<t>
4920   HTTP has been in use by the World-Wide Web global information initiative
4921   since 1990. The first version of HTTP, later referred to as HTTP/0.9,
4922   was a simple protocol for hypertext data transfer across the Internet
4923   with only a single request method (GET) and no metadata.
4924   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
4925   methods and MIME-like messaging that could include metadata about the data
4926   transferred and modifiers on the request/response semantics. However,
4927   HTTP/1.0 did not sufficiently take into consideration the effects of
4928   hierarchical proxies, caching, the need for persistent connections, or
4929   name-based virtual hosts. The proliferation of incompletely-implemented
4930   applications calling themselves "HTTP/1.0" further necessitated a
4931   protocol version change in order for two communicating applications
4932   to determine each other's true capabilities.
4933</t>
4934<t>
4935   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
4936   requirements that enable reliable implementations, adding only
4937   those new features that will either be safely ignored by an HTTP/1.0
4938   recipient or only sent when communicating with a party advertising
4939   conformance with HTTP/1.1.
4940</t>
4941<t>
4942   It is beyond the scope of a protocol specification to mandate
4943   conformance with previous versions. HTTP/1.1 was deliberately
4944   designed, however, to make supporting previous versions easy.
4945   We would expect a general-purpose HTTP/1.1 server to understand
4946   any valid request in the format of HTTP/1.0 and respond appropriately
4947   with an HTTP/1.1 message that only uses features understood (or
4948   safely ignored) by HTTP/1.0 clients.  Likewise, we would expect
4949   an HTTP/1.1 client to understand any valid HTTP/1.0 response.
4950</t>
4951<t>
4952   Since HTTP/0.9 did not support header fields in a request,
4953   there is no mechanism for it to support name-based virtual
4954   hosts (selection of resource by inspection of the <x:ref>Host</x:ref> header
4955   field).  Any server that implements name-based virtual hosts
4956   ought to disable support for HTTP/0.9.  Most requests that
4957   appear to be HTTP/0.9 are, in fact, badly constructed HTTP/1.x
4958   requests wherein a buggy client failed to properly encode
4959   linear whitespace found in a URI reference and placed in
4960   the request-target.
4961</t>
4962
4963<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
4964<t>
4965   This section summarizes major differences between versions HTTP/1.0
4966   and HTTP/1.1.
4967</t>
4968
4969<section title="Multi-homed Web Servers" anchor="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">
4970<t>
4971   The requirements that clients and servers support the <x:ref>Host</x:ref>
4972   header field (<xref target="header.host"/>), report an error if it is
4973   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
4974   are among the most important changes defined by HTTP/1.1.
4975</t>
4976<t>
4977   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4978   addresses and servers; there was no other established mechanism for
4979   distinguishing the intended server of a request than the IP address
4980   to which that request was directed. The <x:ref>Host</x:ref> header field was
4981   introduced during the development of HTTP/1.1 and, though it was
4982   quickly implemented by most HTTP/1.0 browsers, additional requirements
4983   were placed on all HTTP/1.1 requests in order to ensure complete
4984   adoption.  At the time of this writing, most HTTP-based services
4985   are dependent upon the Host header field for targeting requests.
4986</t>
4987</section>
4988
4989<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4990<t>
4991   In HTTP/1.0, each connection is established by the client prior to the
4992   request and closed by the server after sending the response. However, some
4993   implementations implement the explicitly negotiated ("Keep-Alive") version
4994   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
4995   target="RFC2068"/>.
4996</t>
4997<t>
4998   Some clients and servers might wish to be compatible with these previous
4999   approaches to persistent connections, by explicitly negotiating for them
5000   with a "Connection: keep-alive" request header field. However, some
5001   experimental implementations of HTTP/1.0 persistent connections are faulty;
5002   for example, if an HTTP/1.0 proxy server doesn't understand
5003   <x:ref>Connection</x:ref>, it will erroneously forward that header field
5004   to the next inbound server, which would result in a hung connection.
5005</t>
5006<t>
5007   One attempted solution was the introduction of a Proxy-Connection header
5008   field, targeted specifically at proxies. In practice, this was also
5009   unworkable, because proxies are often deployed in multiple layers, bringing
5010   about the same problem discussed above.
5011</t>
5012<t>
5013   As a result, clients are encouraged not to send the Proxy-Connection header
5014   field in any requests.
5015</t>
5016<t>
5017   Clients are also encouraged to consider the use of Connection: keep-alive
5018   in requests carefully; while they can enable persistent connections with
5019   HTTP/1.0 servers, clients using them will need to monitor the
5020   connection for "hung" requests (which indicate that the client ought stop
5021   sending the header field), and this mechanism ought not be used by clients
5022   at all when a proxy is being used.
5023</t>
5024</section>
5025
5026<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
5027<t>
5028   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
5029   (<xref target="header.transfer-encoding"/>).
5030   Transfer codings need to be decoded prior to forwarding an HTTP message
5031   over a MIME-compliant protocol.
5032</t>
5033</section>
5034
5035</section>
5036
5037<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
5038<t>
5039  HTTP's approach to error handling has been explained.
5040  (<xref target="conformance" />)
5041</t>
5042<t>
5043  The HTTP-version ABNF production has been clarified to be case-sensitive.
5044  Additionally, version numbers has been restricted to single digits, due
5045  to the fact that implementations are known to handle multi-digit version
5046  numbers incorrectly.
5047  (<xref target="http.version"/>)
5048</t>
5049<t>
5050  Userinfo (i.e., username and password) are now disallowed in HTTP and
5051  HTTPS URIs, because of security issues related to their transmission on the
5052  wire.
5053  (<xref target="http.uri" />)
5054</t>
5055<t>
5056  The HTTPS URI scheme is now defined by this specification; previously,
5057  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
5058  Furthermore, it implies end-to-end security.
5059  (<xref target="https.uri"/>)
5060</t>
5061<t>
5062  HTTP messages can be (and often are) buffered by implementations; despite
5063  it sometimes being available as a stream, HTTP is fundamentally a
5064  message-oriented protocol.
5065  Minimum supported sizes for various protocol elements have been
5066  suggested, to improve interoperability.
5067  (<xref target="http.message" />)
5068</t>
5069<t>
5070  Invalid whitespace around field-names is now required to be rejected,
5071  because accepting it represents a security vulnerability.
5072  The ABNF productions defining header fields now only list the field value.
5073  (<xref target="header.fields"/>)
5074</t>
5075<t>
5076  Rules about implicit linear whitespace between certain grammar productions
5077  have been removed; now whitespace is only allowed where specifically
5078  defined in the ABNF.
5079  (<xref target="whitespace"/>)
5080</t>
5081<t>
5082  Header fields that span multiple lines ("line folding") are deprecated.
5083  (<xref target="field.parsing" />)
5084</t>
5085<t> 
5086  The NUL octet is no longer allowed in comment and quoted-string text, and
5087  handling of backslash-escaping in them has been clarified.
5088  The quoted-pair rule no longer allows escaping control characters other than
5089  HTAB.
5090  Non-ASCII content in header fields and the reason phrase has been obsoleted
5091  and made opaque (the TEXT rule was removed).
5092  (<xref target="field.components"/>)
5093</t> 
5094<t>
5095  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
5096  handled as errors by recipients.
5097  (<xref target="header.content-length"/>)
5098</t>
5099<t>
5100  The algorithm for determining the message body length has been clarified
5101  to indicate all of the special cases (e.g., driven by methods or status
5102  codes) that affect it, and that new protocol elements cannot define such
5103  special cases.
5104  CONNECT is a new, special case in determining message body length.
5105  "multipart/byteranges" is no longer a way of determining message body length
5106  detection.
5107  (<xref target="message.body.length"/>)
5108</t>
5109<t>
5110  The "identity" transfer coding token has been removed.
5111  (Sections <xref format="counter" target="message.body"/> and
5112  <xref format="counter" target="transfer.codings"/>)
5113</t>
5114<t>
5115  Chunk length does not include the count of the octets in the
5116  chunk header and trailer.
5117  Line folding in chunk extensions is  disallowed.
5118  (<xref target="chunked.encoding"/>)
5119</t>
5120<t>
5121  The meaning of the "deflate" content coding has been clarified.
5122  (<xref target="deflate.coding" />)
5123</t>
5124<t>
5125  The segment + query components of RFC 3986 have been used to define the
5126  request-target, instead of abs_path from RFC 1808.
5127  The asterisk-form of the request-target is only allowed with the OPTIONS
5128  method.
5129  (<xref target="request-target"/>)
5130</t>
5131<t>
5132  The term "Effective Request URI" has been introduced.
5133  (<xref target="effective.request.uri" />)
5134</t>
5135<t>
5136  Gateways do not need to generate <x:ref>Via</x:ref> header fields anymore.
5137  (<xref target="header.via"/>)
5138</t>
5139<t>
5140  Exactly when "close" connection options have to be sent has been clarified.
5141  Also, "hop-by-hop" header fields are required to appear in the Connection header
5142  field; just because they're defined as hop-by-hop in this specification
5143  doesn't exempt them.
5144  (<xref target="header.connection"/>)
5145</t>
5146<t>
5147  The limit of two connections per server has been removed.
5148  An idempotent sequence of requests is no longer required to be retried.
5149  The requirement to retry requests under certain circumstances when the
5150  server prematurely closes the connection has been removed.
5151  Also, some extraneous requirements about when servers are allowed to close
5152  connections prematurely have been removed.
5153  (<xref target="persistent.connections"/>)
5154</t>
5155<t>
5156  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
5157  responses other than 101 (this was incorporated from <xref
5158  target="RFC2817"/>). Furthermore, the ordering in the field value is now
5159  significant.
5160  (<xref target="header.upgrade"/>)
5161</t>
5162<t>
5163  Empty list elements in list productions (e.g., a list header field containing
5164  ", ,") have been deprecated.
5165  (<xref target="abnf.extension"/>)
5166</t>
5167<t>
5168  Registration of Transfer Codings now requires IETF Review
5169  (<xref target="transfer.coding.registry"/>)
5170</t>
5171<t>
5172  This specification now defines the Upgrade Token Registry, previously
5173  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
5174  (<xref target="upgrade.token.registry"/>)
5175</t>
5176<t>
5177  The expectation to support HTTP/0.9 requests has been removed.
5178  (<xref target="compatibility"/>)
5179</t>
5180<t>
5181  Issues with the Keep-Alive and Proxy-Connection header fields in requests
5182  are pointed out, with use of the latter being discouraged altogether.
5183  (<xref target="compatibility.with.http.1.0.persistent.connections" />)
5184</t>
5185</section>
5186</section>
5187
5188<?BEGININC p1-messaging.abnf-appendix ?>
5189<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
5190<figure>
5191<artwork type="abnf" name="p1-messaging.parsed-abnf">
5192<x:ref>BWS</x:ref> = OWS
5193
5194<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
5195 connection-option ] )
5196<x:ref>Content-Length</x:ref> = 1*DIGIT
5197
5198<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
5199 ]
5200<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
5201<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
5202<x:ref>Host</x:ref> = uri-host [ ":" port ]
5203
5204<x:ref>OWS</x:ref> = *( SP / HTAB )
5205
5206<x:ref>RWS</x:ref> = 1*( SP / HTAB )
5207
5208<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
5209<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
5210<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
5211 transfer-coding ] )
5212
5213<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
5214<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
5215
5216<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
5217 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
5218 comment ] ) ] )
5219
5220<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
5221<x:ref>absolute-form</x:ref> = absolute-URI
5222<x:ref>absolute-path</x:ref> = 1*( "/" segment )
5223<x:ref>asterisk-form</x:ref> = "*"
5224<x:ref>attribute</x:ref> = token
5225<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
5226<x:ref>authority-form</x:ref> = authority
5227
5228<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
5229<x:ref>chunk-data</x:ref> = 1*OCTET
5230<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
5231<x:ref>chunk-ext-name</x:ref> = token
5232<x:ref>chunk-ext-val</x:ref> = token / quoted-str-nf
5233<x:ref>chunk-size</x:ref> = 1*HEXDIG
5234<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
5235<x:ref>comment</x:ref> = "(" *( ctext / quoted-cpair / comment ) ")"
5236<x:ref>connection-option</x:ref> = token
5237<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
5238 / %x2A-5B ; '*'-'['
5239 / %x5D-7E ; ']'-'~'
5240 / obs-text
5241
5242<x:ref>field-content</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5243<x:ref>field-name</x:ref> = token
5244<x:ref>field-value</x:ref> = *( field-content / obs-fold )
5245<x:ref>fragment</x:ref> = &lt;fragment, defined in [RFC3986], Section 3.5&gt;
5246
5247<x:ref>header-field</x:ref> = field-name ":" OWS field-value OWS
5248<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ] [ "#"
5249 fragment ]
5250<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ] [ "#"
5251 fragment ]
5252
5253<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
5254
5255<x:ref>message-body</x:ref> = *OCTET
5256<x:ref>method</x:ref> = token
5257
5258<x:ref>obs-fold</x:ref> = CRLF ( SP / HTAB )
5259<x:ref>obs-text</x:ref> = %x80-FF
5260<x:ref>origin-form</x:ref> = absolute-path [ "?" query ]
5261
5262<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
5263<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
5264<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
5265<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
5266<x:ref>protocol-name</x:ref> = token
5267<x:ref>protocol-version</x:ref> = token
5268<x:ref>pseudonym</x:ref> = token
5269
5270<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5271 / %x5D-7E ; ']'-'~'
5272 / obs-text
5273<x:ref>qdtext-nf</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5274 / %x5D-7E ; ']'-'~'
5275 / obs-text
5276<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5277<x:ref>quoted-cpair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5278<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5279<x:ref>quoted-str-nf</x:ref> = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
5280<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5281
5282<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5283<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5284<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5285<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5286<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5287<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5288<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5289 asterisk-form
5290
5291<x:ref>segment</x:ref> = &lt;segment, defined in [RFC3986], Section 3.3&gt;
5292<x:ref>special</x:ref> = "(" / ")" / "&lt;" / "&gt;" / "@" / "," / ";" / ":" / "\" /
5293 DQUOTE / "/" / "[" / "]" / "?" / "=" / "{" / "}"
5294<x:ref>start-line</x:ref> = request-line / status-line
5295<x:ref>status-code</x:ref> = 3DIGIT
5296<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5297
5298<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5299<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5300<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5301 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5302<x:ref>token</x:ref> = 1*tchar
5303<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5304<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5305 transfer-extension
5306<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5307<x:ref>transfer-parameter</x:ref> = attribute BWS "=" BWS value
5308
5309<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5310
5311<x:ref>value</x:ref> = word
5312
5313<x:ref>word</x:ref> = token / quoted-string
5314</artwork>
5315</figure>
5316</section>
5317<?ENDINC p1-messaging.abnf-appendix ?>
5318
5319<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5320
5321<section title="Since RFC 2616">
5322<t>
5323  Changes up to the first Working Group Last Call draft are summarized
5324  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-21#appendix-D"/>.
5325</t>
5326</section>
5327
5328<section title="Since draft-ietf-httpbis-p1-messaging-21" anchor="changes.since.21">
5329<t>
5330  Closed issues:
5331  <list style="symbols">
5332    <t>
5333      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/128"/>:
5334      "Cite HTTPS URI scheme definition" (the spec now includes the HTTPs
5335      scheme definition and thus updates RFC 2818)
5336    </t>
5337    <t>
5338      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/389"/>:
5339      "mention of 'proxies' in section about caches"
5340    </t>
5341    <t>
5342      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/390"/>:
5343      "use of ABNF terms from RFC 3986"
5344    </t>
5345    <t>
5346      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/391"/>:
5347      "transferring URIs with userinfo in payload"
5348    </t>
5349    <t>
5350      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/392"/>:
5351      "editorial improvements to message length definition"
5352    </t>
5353    <t>
5354      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/395"/>:
5355      "Connection header field MUST vs SHOULD"
5356    </t>
5357    <t>
5358      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/396"/>:
5359      "editorial improvements to persistent connections section"
5360    </t>
5361    <t>
5362      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/397"/>:
5363      "URI normalization vs empty path"
5364    </t>
5365    <t>
5366      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/408"/>:
5367      "p1 feedback"
5368    </t>
5369    <t>
5370      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/409"/>:
5371      "is parsing OBS-FOLD mandatory?"
5372    </t>
5373    <t>
5374      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/410"/>:
5375      "HTTPS and Shared Caching"
5376    </t>
5377    <t>
5378      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/411"/>:
5379      "Requirements for recipients of ws between start-line and first header field"
5380    </t>
5381    <t>
5382      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/412"/>:
5383      "SP and HT when being tolerant"
5384    </t>
5385    <t>
5386      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/414"/>:
5387      "Message Parsing Strictness"
5388    </t>
5389    <t>
5390      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/415"/>:
5391      "'Render'"
5392    </t>
5393    <t>
5394      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/418"/>:
5395      "No-Transform"
5396    </t>
5397    <t>
5398      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/419"/>:
5399      "p2 editorial feedback"
5400    </t>
5401    <t>
5402      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/420"/>:
5403      "Content-Length SHOULD be sent"
5404    </t>
5405    <t>
5406      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/431"/>:
5407      "origin-form does not allow path starting with "//""
5408    </t>
5409    <t>
5410      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/433"/>:
5411      "ambiguity in part 1 example"
5412    </t>
5413  </list>
5414</t>
5415</section>
5416
5417<section title="Since draft-ietf-httpbis-p1-messaging-22" anchor="changes.since.22">
5418<t>
5419  Closed issues:
5420  <list style="symbols">
5421    <t>
5422      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/435"/>:
5423      "Part1 should have a reference to TCP (RFC 793)"
5424    </t>
5425    <t>
5426      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/438"/>:
5427      "media type registration template issues"
5428    </t>
5429    <t>
5430      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/441"/>:
5431      P1 editorial nits
5432    </t>
5433    <t>
5434      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/442"/>:
5435      "BWS" (vs conformance)
5436    </t>
5437    <t>
5438      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/444"/>:
5439      "obs-fold language"
5440    </t>
5441    <t>
5442      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/445"/>:
5443      "Ordering in Upgrade"
5444    </t>
5445    <t>
5446      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/446"/>:
5447      "p1 editorial feedback"
5448    </t>
5449    <t>
5450      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/447"/>:
5451      "HTTP and TCP name delegation"
5452    </t>
5453    <t>
5454      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/449"/>:
5455      "Receiving a higher minor HTTP version number"
5456    </t>
5457    <t>
5458      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/451"/>:
5459      "HTTP(S) URIs and fragids"
5460    </t>
5461    <t>
5462      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/457"/>:
5463      "Registering x-gzip and x-deflate"
5464    </t>
5465    <t>
5466      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/460"/>:
5467      "Via and gateways"
5468    </t>
5469    <t>
5470      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/465"/>:
5471      "Mention 203 Non-Authoritative Information in p1"
5472    </t>
5473    <t>
5474      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/476"/>:
5475      "SHOULD and conformance"
5476    </t>
5477    <t>
5478      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/477"/>:
5479      "Pipelining language"
5480    </t>
5481    <t>
5482      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/482"/>:
5483      "proxy handling of a really bad Content-Length"
5484    </t>
5485  </list>
5486</t>
5487</section>
5488
5489<section title="Since draft-ietf-httpbis-p1-messaging-23" anchor="changes.since.23">
5490<t>
5491  Closed issues:
5492  <list style="symbols">
5493    <t>
5494      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/343"/>:
5495      "chunk-extensions" (removed the earlier deprecation)
5496    </t>
5497    <t>
5498      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/483"/>:
5499      "MUST fix Content-Length?"
5500    </t>
5501    <t>
5502      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/492"/>:
5503      "list notation defined in appendix"
5504    </t>
5505  </list>
5506</t>
5507</section>
5508</section>
5509
5510</back>
5511</rfc>
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