source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2334

Last change on this file since 2334 was 2334, checked in by fielding@…, 9 years ago

Many changes to properly target MUST requirements by role; addresses #484

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 235.9 KB
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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "August">
16  <!ENTITY ID-YEAR "2013">
17  <!ENTITY mdash "&#8212;">
18  <!ENTITY Note "<x:h xmlns:x='http://purl.org/net/xml2rfc/ext'>Note:</x:h>">
19  <!ENTITY caching-overview       "<xref target='Part6' x:rel='#caching.overview' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY cache-incomplete       "<xref target='Part6' x:rel='#response.cacheability' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY payload                "<xref target='Part2' x:rel='#payload' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY media-type            "<xref target='Part2' x:rel='#media.type' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY content-codings        "<xref target='Part2' x:rel='#content.codings' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY CONNECT                "<xref target='Part2' x:rel='#CONNECT' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY content.negotiation    "<xref target='Part2' x:rel='#content.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY diff-mime              "<xref target='Part2' x:rel='#differences.between.http.and.mime' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY representation         "<xref target='Part2' x:rel='#representations' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY HEAD                   "<xref target='Part2' x:rel='#HEAD' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-allow           "<xref target='Part2' x:rel='#header.allow' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-cache-control   "<xref target='Part6' x:rel='#header.cache-control' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY header-content-encoding    "<xref target='Part2' x:rel='#header.content-encoding' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY header-content-location    "<xref target='Part2' x:rel='#header.content-location' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY header-content-range   "<xref target='Part5' x:rel='#header.content-range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY header-content-type    "<xref target='Part2' x:rel='#header.content-type' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY header-date            "<xref target='Part2' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY header-etag            "<xref target='Part4' x:rel='#header.etag' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY header-expires         "<xref target='Part6' x:rel='#header.expires' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY header-last-modified   "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY header-mime-version    "<xref target='Part2' x:rel='#mime-version' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40  <!ENTITY header-pragma          "<xref target='Part6' x:rel='#header.pragma' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41  <!ENTITY header-proxy-authenticate  "<xref target='Part7' x:rel='#header.proxy-authenticate' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
42  <!ENTITY header-proxy-authorization "<xref target='Part7' x:rel='#header.proxy-authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
43  <!ENTITY header-server          "<xref target='Part2' x:rel='#header.server' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
44  <!ENTITY header-warning         "<xref target='Part6' x:rel='#header.warning' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
45  <!ENTITY idempotent-methods     "<xref target='Part2' x:rel='#idempotent.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
46  <!ENTITY safe-methods           "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
47  <!ENTITY methods                "<xref target='Part2' x:rel='#methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
48  <!ENTITY OPTIONS                "<xref target='Part2' x:rel='#OPTIONS' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
49  <!ENTITY qvalue                 "<xref target='Part2' x:rel='#quality.values' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
50  <!ENTITY resource               "<xref target='Part2' x:rel='#resources' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
51  <!ENTITY status-codes           "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
52  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
53  <!ENTITY status-203             "<xref target='Part2' x:rel='#status.203' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
54  <!ENTITY status-3xx             "<xref target='Part2' x:rel='#status.3xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
55  <!ENTITY status-304             "<xref target='Part4' x:rel='#status.304' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
56  <!ENTITY status-4xx             "<xref target='Part2' x:rel='#status.4xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
57  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
58  <!ENTITY iana-header-registry   "<xref target='Part2' x:rel='#header.field.registry' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
59]>
60<?rfc toc="yes" ?>
61<?rfc symrefs="yes" ?>
62<?rfc sortrefs="yes" ?>
63<?rfc compact="yes"?>
64<?rfc subcompact="no" ?>
65<?rfc linkmailto="no" ?>
66<?rfc editing="no" ?>
67<?rfc comments="yes"?>
68<?rfc inline="yes"?>
69<?rfc rfcedstyle="yes"?>
70<?rfc-ext allow-markup-in-artwork="yes" ?>
71<?rfc-ext include-references-in-index="yes" ?>
72<rfc obsoletes="2145,2616" updates="2817,2818" category="std" x:maturity-level="proposed"
73     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
74     xmlns:x='http://purl.org/net/xml2rfc/ext'>
75<x:link rel="next" basename="p2-semantics"/>
76<x:feedback template="mailto:ietf-http-wg@w3.org?subject={docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
77<front>
78
79  <title abbrev="HTTP/1.1 Message Syntax and Routing">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
80
81  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
82    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
83    <address>
84      <postal>
85        <street>345 Park Ave</street>
86        <city>San Jose</city>
87        <region>CA</region>
88        <code>95110</code>
89        <country>USA</country>
90      </postal>
91      <email>fielding@gbiv.com</email>
92      <uri>http://roy.gbiv.com/</uri>
93    </address>
94  </author>
95
96  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
97    <organization abbrev="greenbytes">greenbytes GmbH</organization>
98    <address>
99      <postal>
100        <street>Hafenweg 16</street>
101        <city>Muenster</city><region>NW</region><code>48155</code>
102        <country>Germany</country>
103      </postal>
104      <email>julian.reschke@greenbytes.de</email>
105      <uri>http://greenbytes.de/tech/webdav/</uri>
106    </address>
107  </author>
108
109  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
110  <workgroup>HTTPbis Working Group</workgroup>
111
112<abstract>
113<t>
114   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
115   distributed, collaborative, hypertext information systems. HTTP has been in
116   use by the World Wide Web global information initiative since 1990.
117   This document provides an overview of HTTP architecture and its associated
118   terminology, defines the "http" and "https" Uniform Resource Identifier
119   (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements,
120   and describes general security concerns for implementations.
121</t>   
122</abstract>
123
124<note title="Editorial Note (To be removed by RFC Editor)">
125  <t>
126    Discussion of this draft takes place on the HTTPBIS working group
127    mailing list (ietf-http-wg@w3.org), which is archived at
128    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
129  </t>
130  <t>
131    The current issues list is at
132    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
133    documents (including fancy diffs) can be found at
134    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
135  </t>
136  <t>
137    The changes in this draft are summarized in <xref target="changes.since.23"/>.
138  </t>
139</note>
140</front>
141<middle>
142<section title="Introduction" anchor="introduction">
143<t>
144   The Hypertext Transfer Protocol (HTTP) is an application-level
145   request/response protocol that uses extensible semantics and self-descriptive
146   message payloads for flexible interaction with network-based hypertext
147   information systems. This document is the first in a series of documents
148   that collectively form the HTTP/1.1 specification:
149   <list style="empty">
150    <t>RFC xxx1: Message Syntax and Routing</t>
151    <t><xref target="Part2" x:fmt="none">RFC xxx2</xref>: Semantics and Content</t>
152    <t><xref target="Part4" x:fmt="none">RFC xxx3</xref>: Conditional Requests</t>
153    <t><xref target="Part5" x:fmt="none">RFC xxx4</xref>: Range Requests</t>
154    <t><xref target="Part6" x:fmt="none">RFC xxx5</xref>: Caching</t>
155    <t><xref target="Part7" x:fmt="none">RFC xxx6</xref>: Authentication</t>
156   </list>
157</t>
158<t>
159   This HTTP/1.1 specification obsoletes and moves to historic status
160   <xref target="RFC2616" x:fmt="none">RFC 2616</xref>, its predecessor
161   <xref target="RFC2068" x:fmt="none">RFC 2068</xref>, and
162   <xref target="RFC2145" x:fmt="none">RFC 2145</xref> (on HTTP versioning).
163   This specification also updates the use of CONNECT to establish a tunnel,
164   previously defined in <xref target="RFC2817" x:fmt="none">RFC 2817</xref>,
165   and defines the "https" URI scheme that was described informally in
166   <xref target="RFC2818" x:fmt="none">RFC 2818</xref>.
167</t>
168<t>
169   HTTP is a generic interface protocol for information systems. It is
170   designed to hide the details of how a service is implemented by presenting
171   a uniform interface to clients that is independent of the types of
172   resources provided. Likewise, servers do not need to be aware of each
173   client's purpose: an HTTP request can be considered in isolation rather
174   than being associated with a specific type of client or a predetermined
175   sequence of application steps. The result is a protocol that can be used
176   effectively in many different contexts and for which implementations can
177   evolve independently over time.
178</t>
179<t>
180   HTTP is also designed for use as an intermediation protocol for translating
181   communication to and from non-HTTP information systems.
182   HTTP proxies and gateways can provide access to alternative information
183   services by translating their diverse protocols into a hypertext
184   format that can be viewed and manipulated by clients in the same way
185   as HTTP services.
186</t>
187<t>
188   One consequence of this flexibility is that the protocol cannot be
189   defined in terms of what occurs behind the interface. Instead, we
190   are limited to defining the syntax of communication, the intent
191   of received communication, and the expected behavior of recipients.
192   If the communication is considered in isolation, then successful
193   actions ought to be reflected in corresponding changes to the
194   observable interface provided by servers. However, since multiple
195   clients might act in parallel and perhaps at cross-purposes, we
196   cannot require that such changes be observable beyond the scope
197   of a single response.
198</t>
199<t>
200   This document describes the architectural elements that are used or
201   referred to in HTTP, defines the "http" and "https" URI schemes,
202   describes overall network operation and connection management,
203   and defines HTTP message framing and forwarding requirements.
204   Our goal is to define all of the mechanisms necessary for HTTP message
205   handling that are independent of message semantics, thereby defining the
206   complete set of requirements for message parsers and
207   message-forwarding intermediaries.
208</t>
209
210
211<section title="Requirement Notation" anchor="intro.requirements">
212<t>
213   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
214   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
215   document are to be interpreted as described in <xref target="RFC2119"/>.
216</t>
217<t>
218   Conformance criteria and considerations regarding error handling
219   are defined in <xref target="conformance"/>.
220</t>
221</section>
222
223<section title="Syntax Notation" anchor="notation">
224<iref primary="true" item="Grammar" subitem="ALPHA"/>
225<iref primary="true" item="Grammar" subitem="CR"/>
226<iref primary="true" item="Grammar" subitem="CRLF"/>
227<iref primary="true" item="Grammar" subitem="CTL"/>
228<iref primary="true" item="Grammar" subitem="DIGIT"/>
229<iref primary="true" item="Grammar" subitem="DQUOTE"/>
230<iref primary="true" item="Grammar" subitem="HEXDIG"/>
231<iref primary="true" item="Grammar" subitem="HTAB"/>
232<iref primary="true" item="Grammar" subitem="LF"/>
233<iref primary="true" item="Grammar" subitem="OCTET"/>
234<iref primary="true" item="Grammar" subitem="SP"/>
235<iref primary="true" item="Grammar" subitem="VCHAR"/>
236<t>
237   This specification uses the Augmented Backus-Naur Form (ABNF) notation
238   of <xref target="RFC5234"/> with the list rule extension defined in
239   <xref target="abnf.extension"/>.  <xref target="collected.abnf"/> shows
240   the collected ABNF with the list rule expanded.
241</t>
242<t anchor="core.rules">
243  <x:anchor-alias value="ALPHA"/>
244  <x:anchor-alias value="CTL"/>
245  <x:anchor-alias value="CR"/>
246  <x:anchor-alias value="CRLF"/>
247  <x:anchor-alias value="DIGIT"/>
248  <x:anchor-alias value="DQUOTE"/>
249  <x:anchor-alias value="HEXDIG"/>
250  <x:anchor-alias value="HTAB"/>
251  <x:anchor-alias value="LF"/>
252  <x:anchor-alias value="OCTET"/>
253  <x:anchor-alias value="SP"/>
254  <x:anchor-alias value="VCHAR"/>
255   The following core rules are included by
256   reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
257   ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
258   DIGIT (decimal 0-9), DQUOTE (double quote),
259   HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed),
260   OCTET (any 8-bit sequence of data), SP (space), and
261   VCHAR (any visible <xref target="USASCII"/> character).
262</t>
263<t>
264   As a convention, ABNF rule names prefixed with "obs-" denote
265   "obsolete" grammar rules that appear for historical reasons.
266</t>
267</section>
268</section>
269
270<section title="Architecture" anchor="architecture">
271<t>
272   HTTP was created for the World Wide Web architecture
273   and has evolved over time to support the scalability needs of a worldwide
274   hypertext system. Much of that architecture is reflected in the terminology
275   and syntax productions used to define HTTP.
276</t>
277
278<section title="Client/Server Messaging" anchor="operation">
279<iref primary="true" item="client"/>
280<iref primary="true" item="server"/>
281<iref primary="true" item="connection"/>
282<t>
283   HTTP is a stateless request/response protocol that operates by exchanging
284   <x:dfn>messages</x:dfn> (<xref target="http.message"/>) across a reliable
285   transport or session-layer
286   "<x:dfn>connection</x:dfn>" (<xref target="connection.management"/>).
287   An HTTP "<x:dfn>client</x:dfn>" is a program that establishes a connection
288   to a server for the purpose of sending one or more HTTP requests.
289   An HTTP "<x:dfn>server</x:dfn>" is a program that accepts connections
290   in order to service HTTP requests by sending HTTP responses.
291</t>
292<iref primary="true" item="user agent"/>
293<iref primary="true" item="origin server"/>
294<iref primary="true" item="browser"/>
295<iref primary="true" item="spider"/>
296<iref primary="true" item="sender"/>
297<iref primary="true" item="recipient"/>
298<t>
299   The terms client and server refer only to the roles that
300   these programs perform for a particular connection.  The same program
301   might act as a client on some connections and a server on others.
302   We use the term "<x:dfn>user agent</x:dfn>" to refer to any of the various
303   client programs that initiate a request, including (but not limited to)
304   browsers, spiders (web-based robots), command-line tools, native
305   applications, and mobile apps.  The term "<x:dfn>origin server</x:dfn>" is
306   used to refer to the program that can originate authoritative responses to
307   a request. For general requirements, we use the terms
308   "<x:dfn>sender</x:dfn>" and "<x:dfn>recipient</x:dfn>" to refer to any
309   component that sends or receives, respectively, a given message.
310</t>
311<t>
312   HTTP relies upon the Uniform Resource Identifier (URI)
313   standard <xref target="RFC3986"/> to indicate the target resource
314   (<xref target="target-resource"/>) and relationships between resources.
315   Messages are passed in a format similar to that used by Internet mail
316   <xref target="RFC5322"/> and the Multipurpose Internet Mail Extensions
317   (MIME) <xref target="RFC2045"/> (see &diff-mime; for the differences
318   between HTTP and MIME messages).
319</t>
320<t>
321   Most HTTP communication consists of a retrieval request (GET) for
322   a representation of some resource identified by a URI.  In the
323   simplest case, this might be accomplished via a single bidirectional
324   connection (===) between the user agent (UA) and the origin server (O).
325</t>
326<figure><artwork type="drawing">
327         request   &gt;
328    <x:highlight>UA</x:highlight> ======================================= <x:highlight>O</x:highlight>
329                                &lt;   response
330</artwork></figure>
331<iref primary="true" item="message"/>
332<iref primary="true" item="request"/>
333<iref primary="true" item="response"/>
334<t>
335   A client sends an HTTP request to a server in the form of a <x:dfn>request</x:dfn>
336   message, beginning with a request-line that includes a method, URI, and
337   protocol version (<xref target="request.line"/>),
338   followed by header fields containing
339   request modifiers, client information, and representation metadata
340   (<xref target="header.fields"/>),
341   an empty line to indicate the end of the header section, and finally
342   a message body containing the payload body (if any,
343   <xref target="message.body"/>).
344</t>
345<t>
346   A server responds to a client's request by sending one or more HTTP
347   <x:dfn>response</x:dfn>
348   messages, each beginning with a status line that
349   includes the protocol version, a success or error code, and textual
350   reason phrase (<xref target="status.line"/>),
351   possibly followed by header fields containing server
352   information, resource metadata, and representation metadata
353   (<xref target="header.fields"/>),
354   an empty line to indicate the end of the header section, and finally
355   a message body containing the payload body (if any,
356   <xref target="message.body"/>).
357</t>
358<t>
359   A connection might be used for multiple request/response exchanges,
360   as defined in <xref target="persistent.connections"/>.
361</t>
362<t>
363   The following example illustrates a typical message exchange for a
364   GET request on the URI "http://www.example.com/hello.txt":
365</t>
366<figure><preamble>
367client request:
368</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
369GET /hello.txt HTTP/1.1
370User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
371Host: www.example.com
372Accept-Language: en, mi
373
374</artwork></figure>
375<figure><preamble>
376server response:
377</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
378HTTP/1.1 200 OK
379Date: Mon, 27 Jul 2009 12:28:53 GMT
380Server: Apache
381Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
382ETag: "34aa387-d-1568eb00"
383Accept-Ranges: bytes
384Content-Length: <x:length-of target="exbody"/>
385Vary: Accept-Encoding
386Content-Type: text/plain
387
388<x:span anchor="exbody">Hello World! My payload includes a trailing CRLF.
389</x:span></artwork>
390</figure>
391</section>
392
393<section title="Implementation Diversity" anchor="implementation-diversity">
394<t>
395   When considering the design of HTTP, it is easy to fall into a trap of
396   thinking that all user agents are general-purpose browsers and all origin
397   servers are large public websites. That is not the case in practice.
398   Common HTTP user agents include household appliances, stereos, scales,
399   firmware update scripts, command-line programs, mobile apps,
400   and communication devices in a multitude of shapes and sizes.  Likewise,
401   common HTTP origin servers include home automation units, configurable
402   networking components, office machines, autonomous robots, news feeds,
403   traffic cameras, ad selectors, and video delivery platforms.
404</t>
405<t>
406   The term "user agent" does not imply that there is a human user directly
407   interacting with the software agent at the time of a request. In many
408   cases, a user agent is installed or configured to run in the background
409   and save its results for later inspection (or save only a subset of those
410   results that might be interesting or erroneous). Spiders, for example, are
411   typically given a start URI and configured to follow certain behavior while
412   crawling the Web as a hypertext graph.
413</t>
414<t>
415   The implementation diversity of HTTP means that we cannot assume the
416   user agent can make interactive suggestions to a user or provide adequate
417   warning for security or privacy options.  In the few cases where this
418   specification requires reporting of errors to the user, it is acceptable
419   for such reporting to only be observable in an error console or log file.
420   Likewise, requirements that an automated action be confirmed by the user
421   before proceeding might be met via advance configuration choices,
422   run-time options, or simple avoidance of the unsafe action; confirmation
423   does not imply any specific user interface or interruption of normal
424   processing if the user has already made that choice.
425</t>
426</section>
427
428<section title="Intermediaries" anchor="intermediaries">
429<iref primary="true" item="intermediary"/>
430<t>
431   HTTP enables the use of intermediaries to satisfy requests through
432   a chain of connections.  There are three common forms of HTTP
433   <x:dfn>intermediary</x:dfn>: proxy, gateway, and tunnel.  In some cases,
434   a single intermediary might act as an origin server, proxy, gateway,
435   or tunnel, switching behavior based on the nature of each request.
436</t>
437<figure><artwork type="drawing">
438         &gt;             &gt;             &gt;             &gt;
439    <x:highlight>UA</x:highlight> =========== <x:highlight>A</x:highlight> =========== <x:highlight>B</x:highlight> =========== <x:highlight>C</x:highlight> =========== <x:highlight>O</x:highlight>
440               &lt;             &lt;             &lt;             &lt;
441</artwork></figure>
442<t>
443   The figure above shows three intermediaries (A, B, and C) between the
444   user agent and origin server. A request or response message that
445   travels the whole chain will pass through four separate connections.
446   Some HTTP communication options
447   might apply only to the connection with the nearest, non-tunnel
448   neighbor, only to the end-points of the chain, or to all connections
449   along the chain. Although the diagram is linear, each participant might
450   be engaged in multiple, simultaneous communications. For example, B
451   might be receiving requests from many clients other than A, and/or
452   forwarding requests to servers other than C, at the same time that it
453   is handling A's request. Likewise, later requests might be sent through a
454   different path of connections, often based on dynamic configuration for
455   load balancing.   
456</t>
457<t>
458<iref primary="true" item="upstream"/><iref primary="true" item="downstream"/>
459<iref primary="true" item="inbound"/><iref primary="true" item="outbound"/>
460   We use the terms "<x:dfn>upstream</x:dfn>" and "<x:dfn>downstream</x:dfn>"
461   to describe various requirements in relation to the directional flow of a
462   message: all messages flow from upstream to downstream.
463   Likewise, we use the terms inbound and outbound to refer to
464   directions in relation to the request path:
465   "<x:dfn>inbound</x:dfn>" means toward the origin server and
466   "<x:dfn>outbound</x:dfn>" means toward the user agent.
467</t>
468<t><iref primary="true" item="proxy"/>
469   A "<x:dfn>proxy</x:dfn>" is a message forwarding agent that is selected by the
470   client, usually via local configuration rules, to receive requests
471   for some type(s) of absolute URI and attempt to satisfy those
472   requests via translation through the HTTP interface.  Some translations
473   are minimal, such as for proxy requests for "http" URIs, whereas
474   other requests might require translation to and from entirely different
475   application-level protocols. Proxies are often used to group an
476   organization's HTTP requests through a common intermediary for the
477   sake of security, annotation services, or shared caching.
478</t>
479<t>
480<iref primary="true" item="transforming proxy"/>
481<iref primary="true" item="non-transforming proxy"/>
482   An HTTP-to-HTTP proxy is called a "<x:dfn>transforming proxy</x:dfn>" if it is designed
483   or configured to modify request or response messages in a semantically
484   meaningful way (i.e., modifications, beyond those required by normal
485   HTTP processing, that change the message in a way that would be
486   significant to the original sender or potentially significant to
487   downstream recipients).  For example, a transforming proxy might be
488   acting as a shared annotation server (modifying responses to include
489   references to a local annotation database), a malware filter, a
490   format transcoder, or an intranet-to-Internet privacy filter.  Such
491   transformations are presumed to be desired by the client (or client
492   organization) that selected the proxy and are beyond the scope of
493   this specification.  However, when a proxy is not intended to transform
494   a given message, we use the term "<x:dfn>non-transforming proxy</x:dfn>" to target
495   requirements that preserve HTTP message semantics. See &status-203; and
496   &header-warning; for status and warning codes related to transformations.
497</t>
498<t><iref primary="true" item="gateway"/><iref primary="true" item="reverse proxy"/>
499<iref primary="true" item="accelerator"/>
500   A "<x:dfn>gateway</x:dfn>" (a.k.a., "<x:dfn>reverse proxy</x:dfn>") is an
501   intermediary that acts as an origin server for the outbound connection, but
502   translates received requests and forwards them inbound to another server or
503   servers. Gateways are often used to encapsulate legacy or untrusted
504   information services, to improve server performance through
505   "<x:dfn>accelerator</x:dfn>" caching, and to enable partitioning or load
506   balancing of HTTP services across multiple machines.
507</t>
508<t>
509   All HTTP requirements applicable to an origin server
510   also apply to the outbound communication of a gateway.
511   A gateway communicates with inbound servers using any protocol that
512   it desires, including private extensions to HTTP that are outside
513   the scope of this specification.  However, an HTTP-to-HTTP gateway
514   that wishes to interoperate with third-party HTTP servers ought to conform
515   to user agent requirements on the gateway's inbound connection.
516</t>
517<t><iref primary="true" item="tunnel"/>
518   A "<x:dfn>tunnel</x:dfn>" acts as a blind relay between two connections
519   without changing the messages. Once active, a tunnel is not
520   considered a party to the HTTP communication, though the tunnel might
521   have been initiated by an HTTP request. A tunnel ceases to exist when
522   both ends of the relayed connection are closed. Tunnels are used to
523   extend a virtual connection through an intermediary, such as when
524   Transport Layer Security (TLS, <xref target="RFC5246"/>) is used to
525   establish confidential communication through a shared firewall proxy.
526</t>
527<t><iref primary="true" item="interception proxy"/>
528<iref primary="true" item="transparent proxy"/>
529<iref primary="true" item="captive portal"/>
530   The above categories for intermediary only consider those acting as
531   participants in the HTTP communication.  There are also intermediaries
532   that can act on lower layers of the network protocol stack, filtering or
533   redirecting HTTP traffic without the knowledge or permission of message
534   senders. Network intermediaries often introduce security flaws or
535   interoperability problems by violating HTTP semantics.  For example, an
536   "<x:dfn>interception proxy</x:dfn>" <xref target="RFC3040"/> (also commonly
537   known as a "<x:dfn>transparent proxy</x:dfn>" <xref target="RFC1919"/> or
538   "<x:dfn>captive portal</x:dfn>")
539   differs from an HTTP proxy because it is not selected by the client.
540   Instead, an interception proxy filters or redirects outgoing TCP port 80
541   packets (and occasionally other common port traffic).
542   Interception proxies are commonly found on public network access points,
543   as a means of enforcing account subscription prior to allowing use of
544   non-local Internet services, and within corporate firewalls to enforce
545   network usage policies.
546   They are indistinguishable from a man-in-the-middle attack.
547</t>
548<t>
549   HTTP is defined as a stateless protocol, meaning that each request message
550   can be understood in isolation.  Many implementations depend on HTTP's
551   stateless design in order to reuse proxied connections or dynamically
552   load-balance requests across multiple servers.  Hence, servers &MUST-NOT;
553   assume that two requests on the same connection are from the same user
554   agent unless the connection is secured and specific to that agent.
555   Some non-standard HTTP extensions (e.g., <xref target="RFC4559"/>) have
556   been known to violate this requirement, resulting in security and
557   interoperability problems.
558</t>
559</section>
560
561<section title="Caches" anchor="caches">
562<iref primary="true" item="cache"/>
563<t>
564   A "<x:dfn>cache</x:dfn>" is a local store of previous response messages and the
565   subsystem that controls its message storage, retrieval, and deletion.
566   A cache stores cacheable responses in order to reduce the response
567   time and network bandwidth consumption on future, equivalent
568   requests. Any client or server &MAY; employ a cache, though a cache
569   cannot be used by a server while it is acting as a tunnel.
570</t>
571<t>
572   The effect of a cache is that the request/response chain is shortened
573   if one of the participants along the chain has a cached response
574   applicable to that request. The following illustrates the resulting
575   chain if B has a cached copy of an earlier response from O (via C)
576   for a request that has not been cached by UA or A.
577</t>
578<figure><artwork type="drawing">
579            &gt;             &gt;
580       <x:highlight>UA</x:highlight> =========== <x:highlight>A</x:highlight> =========== <x:highlight>B</x:highlight> - - - - - - <x:highlight>C</x:highlight> - - - - - - <x:highlight>O</x:highlight>
581                  &lt;             &lt;
582</artwork></figure>
583<t><iref primary="true" item="cacheable"/>
584   A response is "<x:dfn>cacheable</x:dfn>" if a cache is allowed to store a copy of
585   the response message for use in answering subsequent requests.
586   Even when a response is cacheable, there might be additional
587   constraints placed by the client or by the origin server on when
588   that cached response can be used for a particular request. HTTP
589   requirements for cache behavior and cacheable responses are
590   defined in &caching-overview;. 
591</t>
592<t>
593   There are a wide variety of architectures and configurations
594   of caches deployed across the World Wide Web and
595   inside large organizations. These include national hierarchies
596   of proxy caches to save transoceanic bandwidth, collaborative systems that
597   broadcast or multicast cache entries, archives of pre-fetched cache
598   entries for use in off-line or high-latency environments, and so on.
599</t>
600</section>
601
602<section title="Conformance and Error Handling" anchor="conformance">
603<t>
604   This specification targets conformance criteria according to the role of
605   a participant in HTTP communication.  Hence, HTTP requirements are placed
606   on senders, recipients, clients, servers, user agents, intermediaries,
607   origin servers, proxies, gateways, or caches, depending on what behavior
608   is being constrained by the requirement. Additional (social) requirements
609   are placed on implementations, resource owners, and protocol element
610   registrations when they apply beyond the scope of a single communication.
611</t>
612<t>
613   The verb "generate" is used instead of "send" where a requirement
614   differentiates between creating a protocol element and merely forwarding a
615   received element downstream.
616</t>
617<t>
618   An implementation is considered conformant if it complies with all of the
619   requirements associated with the roles it partakes in HTTP.
620</t>
621<t>
622   Conformance includes both the syntax and semantics of HTTP protocol
623   elements. A sender &MUST-NOT; generate protocol elements that convey a
624   meaning that is known by that sender to be false. A sender &MUST-NOT;
625   generate protocol elements that do not match the grammar defined by the
626   corresponding ABNF rules. Within a given message, a sender &MUST-NOT;
627   generate protocol elements or syntax alternatives that are only allowed to
628   be generated by participants in other roles (i.e., a role that the sender
629   does not have for that message).
630</t>
631<t>
632   When a received protocol element is parsed, the recipient &MUST; be able to
633   parse any value of reasonable length that is applicable to the recipient's
634   role and matches the grammar defined by the corresponding ABNF rules.
635   Note, however, that some received protocol elements might not be parsed.
636   For example, an intermediary forwarding a message might parse a
637   header-field into generic field-name and field-value components, but then
638   forward the header field without further parsing inside the field-value.
639</t>
640<t>
641   HTTP does not have specific length limitations for many of its protocol
642   elements because the lengths that might be appropriate will vary widely,
643   depending on the deployment context and purpose of the implementation.
644   Hence, interoperability between senders and recipients depends on shared
645   expectations regarding what is a reasonable length for each protocol
646   element. Furthermore, what is commonly understood to be a reasonable length
647   for some protocol elements has changed over the course of the past two
648   decades of HTTP use, and is expected to continue changing in the future.
649</t>
650<t>
651   At a minimum, a recipient &MUST; be able to parse and process protocol
652   element lengths that are at least as long as the values that it generates
653   for those same protocol elements in other messages. For example, an origin
654   server that publishes very long URI references to its own resources needs
655   to be able to parse and process those same references when received as a
656   request target.
657</t>
658<t>
659   A recipient &MUST; interpret a received protocol element according to the
660   semantics defined for it by this specification, including extensions to
661   this specification, unless the recipient has determined (through experience
662   or configuration) that the sender incorrectly implements what is implied by
663   those semantics.
664   For example, an origin server might disregard the contents of a received
665   <x:ref>Accept-Encoding</x:ref> header field if inspection of the
666   <x:ref>User-Agent</x:ref> header field indicates a specific implementation
667   version that is known to fail on receipt of certain content codings.
668</t>
669<t>
670   Unless noted otherwise, a recipient &MAY; attempt to recover a usable
671   protocol element from an invalid construct.  HTTP does not define
672   specific error handling mechanisms except when they have a direct impact
673   on security, since different applications of the protocol require
674   different error handling strategies.  For example, a Web browser might
675   wish to transparently recover from a response where the
676   <x:ref>Location</x:ref> header field doesn't parse according to the ABNF,
677   whereas a systems control client might consider any form of error recovery
678   to be dangerous.
679</t>
680</section>
681
682<section title="Protocol Versioning" anchor="http.version">
683  <x:anchor-alias value="HTTP-version"/>
684  <x:anchor-alias value="HTTP-name"/>
685<t>
686   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate
687   versions of the protocol. This specification defines version "1.1".
688   The protocol version as a whole indicates the sender's conformance
689   with the set of requirements laid out in that version's corresponding
690   specification of HTTP.
691</t>
692<t>
693   The version of an HTTP message is indicated by an HTTP-version field
694   in the first line of the message. HTTP-version is case-sensitive.
695</t>
696<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-version"/><iref primary="true" item="Grammar" subitem="HTTP-name"/>
697  <x:ref>HTTP-version</x:ref>  = <x:ref>HTTP-name</x:ref> "/" <x:ref>DIGIT</x:ref> "." <x:ref>DIGIT</x:ref>
698  <x:ref>HTTP-name</x:ref>     = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
699</artwork></figure>
700<t>
701   The HTTP version number consists of two decimal digits separated by a "."
702   (period or decimal point).  The first digit ("major version") indicates the
703   HTTP messaging syntax, whereas the second digit ("minor version") indicates
704   the highest minor version within that major version to which the sender is
705   conformant and able to understand for future communication.  The minor
706   version advertises the sender's communication capabilities even when the
707   sender is only using a backwards-compatible subset of the protocol,
708   thereby letting the recipient know that more advanced features can
709   be used in response (by servers) or in future requests (by clients).
710</t>
711<t>
712   When an HTTP/1.1 message is sent to an HTTP/1.0 recipient
713   <xref target="RFC1945"/> or a recipient whose version is unknown,
714   the HTTP/1.1 message is constructed such that it can be interpreted
715   as a valid HTTP/1.0 message if all of the newer features are ignored.
716   This specification places recipient-version requirements on some
717   new features so that a conformant sender will only use compatible
718   features until it has determined, through configuration or the
719   receipt of a message, that the recipient supports HTTP/1.1.
720</t>
721<t>
722   The interpretation of a header field does not change between minor
723   versions of the same major HTTP version, though the default
724   behavior of a recipient in the absence of such a field can change.
725   Unless specified otherwise, header fields defined in HTTP/1.1 are
726   defined for all versions of HTTP/1.x.  In particular, the <x:ref>Host</x:ref>
727   and <x:ref>Connection</x:ref> header fields ought to be implemented by all
728   HTTP/1.x implementations whether or not they advertise conformance with
729   HTTP/1.1.
730</t>
731<t>
732   New header fields can be introduced without changing the protocol version
733   if their defined semantics allow them to be safely ignored by recipients
734   that do not recognize them. Header field extensibility is discussed in
735   <xref target="field.extensibility"/>.
736</t>
737<t>
738   Intermediaries that process HTTP messages (i.e., all intermediaries
739   other than those acting as tunnels) &MUST; send their own HTTP-version
740   in forwarded messages.  In other words, they &MUST-NOT; blindly
741   forward the first line of an HTTP message without ensuring that the
742   protocol version in that message matches a version to which that
743   intermediary is conformant for both the receiving and
744   sending of messages.  Forwarding an HTTP message without rewriting
745   the HTTP-version might result in communication errors when downstream
746   recipients use the message sender's version to determine what features
747   are safe to use for later communication with that sender.
748</t>
749<t>
750   A client &SHOULD; send a request version equal to the highest
751   version to which the client is conformant and
752   whose major version is no higher than the highest version supported
753   by the server, if this is known.  A client &MUST-NOT; send a
754   version to which it is not conformant.
755</t>
756<t>
757   A client &MAY; send a lower request version if it is known that
758   the server incorrectly implements the HTTP specification, but only
759   after the client has attempted at least one normal request and determined
760   from the response status or header fields (e.g., <x:ref>Server</x:ref>) that
761   the server improperly handles higher request versions.
762</t>
763<t>
764   A server &SHOULD; send a response version equal to the highest
765   version to which the server is conformant and
766   whose major version is less than or equal to the one received in the
767   request.  A server &MUST-NOT; send a version to which it is not
768   conformant.  A server &MAY; send a <x:ref>505 (HTTP Version Not
769   Supported)</x:ref> response if it cannot send a response using the
770   major version used in the client's request.
771</t>
772<t>
773   A server &MAY; send an HTTP/1.0 response to a request
774   if it is known or suspected that the client incorrectly implements the
775   HTTP specification and is incapable of correctly processing later
776   version responses, such as when a client fails to parse the version
777   number correctly or when an intermediary is known to blindly forward
778   the HTTP-version even when it doesn't conform to the given minor
779   version of the protocol. Such protocol downgrades &SHOULD-NOT; be
780   performed unless triggered by specific client attributes, such as when
781   one or more of the request header fields (e.g., <x:ref>User-Agent</x:ref>)
782   uniquely match the values sent by a client known to be in error.
783</t>
784<t>
785   The intention of HTTP's versioning design is that the major number
786   will only be incremented if an incompatible message syntax is
787   introduced, and that the minor number will only be incremented when
788   changes made to the protocol have the effect of adding to the message
789   semantics or implying additional capabilities of the sender.  However,
790   the minor version was not incremented for the changes introduced between
791   <xref target="RFC2068"/> and <xref target="RFC2616"/>, and this revision
792   has specifically avoided any such changes to the protocol.
793</t>
794<t>
795   When an HTTP message is received with a major version number that the
796   recipient implements, but a higher minor version number than what the
797   recipient implements, the recipient &SHOULD; process the message as if it
798   were in the highest minor version within that major version to which the
799   recipient is conformant. A recipient can assume that a message with a
800   higher minor version, when sent to a recipient that has not yet indicated
801   support for that higher version, is sufficiently backwards-compatible to be
802   safely processed by any implementation of the same major version.
803</t>
804</section>
805
806<section title="Uniform Resource Identifiers" anchor="uri">
807<iref primary="true" item="resource"/>
808<t>
809   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used
810   throughout HTTP as the means for identifying resources (&resource;).
811   URI references are used to target requests, indicate redirects, and define
812   relationships.
813</t>
814  <x:anchor-alias value="URI-reference"/>
815  <x:anchor-alias value="absolute-URI"/>
816  <x:anchor-alias value="relative-part"/>
817  <x:anchor-alias value="authority"/>
818  <x:anchor-alias value="uri-host"/>
819  <x:anchor-alias value="port"/>
820  <x:anchor-alias value="path-abempty"/>
821  <x:anchor-alias value="segment"/>
822  <x:anchor-alias value="query"/>
823  <x:anchor-alias value="fragment"/>
824  <x:anchor-alias value="absolute-path"/>
825  <x:anchor-alias value="partial-URI"/>
826<t>
827   This specification adopts the definitions of "URI-reference",
828   "absolute-URI", "relative-part", "authority", "port", "host",
829   "path-abempty", "segment", "query", and "fragment" from the
830   URI generic syntax.
831   In addition, we define an "absolute-path" rule (that differs from
832   RFC 3986's "path-absolute" in that it allows a leading "//")
833   and a "partial-URI" rule for protocol elements
834   that allow a relative URI but not a fragment.
835</t>
836<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="URI-reference"><!--exported production--></iref><iref primary="true" item="Grammar" subitem="absolute-URI"/><iref primary="true" item="Grammar" subitem="authority"/><iref primary="true" item="Grammar" subitem="absolute-path"/><iref primary="true" item="Grammar" subitem="port"/><iref primary="true" item="Grammar" subitem="query"/><iref primary="true" item="Grammar" subitem="fragment"/><iref primary="true" item="Grammar" subitem="segment"/><iref primary="true" item="Grammar" subitem="uri-host"/><iref primary="true" item="Grammar" subitem="partial-URI"><!--exported production--></iref>
837  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
838  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
839  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
840  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
841  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
842  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
843  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
844  <x:ref>segment</x:ref>       = &lt;segment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
845  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
846  <x:ref>fragment</x:ref>      = &lt;fragment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.5"/>&gt;
847 
848  <x:ref>absolute-path</x:ref> = 1*( "/" segment )
849  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
850</artwork></figure>
851<t>
852   Each protocol element in HTTP that allows a URI reference will indicate
853   in its ABNF production whether the element allows any form of reference
854   (URI-reference), only a URI in absolute form (absolute-URI), only the
855   path and optional query components, or some combination of the above.
856   Unless otherwise indicated, URI references are parsed
857   relative to the effective request URI
858   (<xref target="effective.request.uri"/>).
859</t>
860
861<section title="http URI scheme" anchor="http.uri">
862  <x:anchor-alias value="http-URI"/>
863  <iref item="http URI scheme" primary="true"/>
864  <iref item="URI scheme" subitem="http" primary="true"/>
865<t>
866   The "http" URI scheme is hereby defined for the purpose of minting
867   identifiers according to their association with the hierarchical
868   namespace governed by a potential HTTP origin server listening for
869   TCP (<xref target="RFC0793"/>) connections on a given port.
870</t>
871<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"><!--terminal production--></iref>
872  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
873             [ "#" <x:ref>fragment</x:ref> ]
874</artwork></figure>
875<t>
876   The HTTP origin server is identified by the generic syntax's
877   <x:ref>authority</x:ref> component, which includes a host identifier
878   and optional TCP port (<xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>).
879   The remainder of the URI, consisting of both the hierarchical path
880   component and optional query component, serves as an identifier for
881   a potential resource within that origin server's name space.
882</t>
883<t>
884   A sender &MUST-NOT; generate an "http" URI with an empty host identifier.
885   A recipient that processes such a URI reference &MUST; reject it as invalid.
886</t>
887<t>
888   If the host identifier is provided as an IP address,
889   then the origin server is any listener on the indicated TCP port at
890   that IP address. If host is a registered name, then that name is
891   considered an indirect identifier and the recipient might use a name
892   resolution service, such as DNS, to find the address of a listener
893   for that host.
894   If the port subcomponent is empty or not given, then TCP port 80 is
895   assumed (the default reserved port for WWW services).
896</t>
897<t>
898   Regardless of the form of host identifier, access to that host is not
899   implied by the mere presence of its name or address. The host might or might
900   not exist and, even when it does exist, might or might not be running an
901   HTTP server or listening to the indicated port. The "http" URI scheme
902   makes use of the delegated nature of Internet names and addresses to
903   establish a naming authority (whatever entity has the ability to place
904   an HTTP server at that Internet name or address) and allows that
905   authority to determine which names are valid and how they might be used.
906</t>
907<t>
908   When an "http" URI is used within a context that calls for access to the
909   indicated resource, a client &MAY; attempt access by resolving
910   the host to an IP address, establishing a TCP connection to that address
911   on the indicated port, and sending an HTTP request message
912   (<xref target="http.message"/>) containing the URI's identifying data
913   (<xref target="message.routing"/>) to the server.
914   If the server responds to that request with a non-interim HTTP response
915   message, as described in &status-codes;, then that response
916   is considered an authoritative answer to the client's request.
917</t>
918<t>
919   Although HTTP is independent of the transport protocol, the "http"
920   scheme is specific to TCP-based services because the name delegation
921   process depends on TCP for establishing authority.
922   An HTTP service based on some other underlying connection protocol
923   would presumably be identified using a different URI scheme, just as
924   the "https" scheme (below) is used for resources that require an
925   end-to-end secured connection. Other protocols might also be used to
926   provide access to "http" identified resources &mdash; it is only the
927   authoritative interface that is specific to TCP.
928</t>
929<t>
930   The URI generic syntax for authority also includes a deprecated
931   userinfo subcomponent (<xref target="RFC3986" x:fmt="," x:sec="3.2.1"/>)
932   for including user authentication information in the URI.  Some
933   implementations make use of the userinfo component for internal
934   configuration of authentication information, such as within command
935   invocation options, configuration files, or bookmark lists, even
936   though such usage might expose a user identifier or password.
937   Senders &MUST-NOT; generate the userinfo subcomponent (and its "@"
938   delimiter) when an "http" URI reference is generated within a message as a
939   request target or header field value.
940   Recipients of an "http" URI reference &SHOULD; parse for userinfo and
941   treat its presence as an error, since it is likely being used to obscure
942   the authority for the sake of phishing attacks.
943</t>
944</section>
945
946<section title="https URI scheme" anchor="https.uri">
947   <x:anchor-alias value="https-URI"/>
948   <iref item="https URI scheme"/>
949   <iref item="URI scheme" subitem="https"/>
950<t>
951   The "https" URI scheme is hereby defined for the purpose of minting
952   identifiers according to their association with the hierarchical
953   namespace governed by a potential HTTP origin server listening to a
954   given TCP port for TLS-secured connections
955   (<xref target="RFC0793"/>, <xref target="RFC5246"/>).
956</t>
957<t>
958   All of the requirements listed above for the "http" scheme are also
959   requirements for the "https" scheme, except that a default TCP port
960   of 443 is assumed if the port subcomponent is empty or not given,
961   and the user agent &MUST; ensure that its connection to the origin
962   server is secured through the use of strong encryption, end-to-end,
963   prior to sending the first HTTP request.
964</t>
965<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="https-URI"><!--terminal production--></iref>
966  <x:ref>https-URI</x:ref> = "https:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
967              [ "#" <x:ref>fragment</x:ref> ]
968</artwork></figure>
969<t>
970   Note that the "https" URI scheme depends on both TLS and TCP for
971   establishing authority.
972   Resources made available via the "https" scheme have no shared
973   identity with the "http" scheme even if their resource identifiers
974   indicate the same authority (the same host listening to the same
975   TCP port).  They are distinct name spaces and are considered to be
976   distinct origin servers.  However, an extension to HTTP that is
977   defined to apply to entire host domains, such as the Cookie protocol
978   <xref target="RFC6265"/>, can allow information
979   set by one service to impact communication with other services
980   within a matching group of host domains.
981</t>
982<t>
983   The process for authoritative access to an "https" identified
984   resource is defined in <xref target="RFC2818"/>.
985</t>
986</section>
987
988<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
989<t>
990   Since the "http" and "https" schemes conform to the URI generic syntax,
991   such URIs are normalized and compared according to the algorithm defined
992   in <xref target="RFC3986" x:fmt="," x:sec="6"/>, using the defaults
993   described above for each scheme.
994</t>
995<t>
996   If the port is equal to the default port for a scheme, the normal form is
997   to omit the port subcomponent. When not being used in absolute form as the
998   request target of an OPTIONS request, an empty path component is equivalent
999   to an absolute path of "/", so the normal form is to provide a path of "/"
1000   instead. The scheme and host are case-insensitive and normally provided in
1001   lowercase; all other components are compared in a case-sensitive manner.
1002   Characters other than those in the "reserved" set are equivalent to their
1003   percent-encoded octets (see <xref target="RFC3986" x:fmt=","
1004   x:sec="2.1"/>): the normal form is to not encode them.
1005</t>
1006<t>
1007   For example, the following three URIs are equivalent:
1008</t>
1009<figure><artwork type="example">
1010   http://example.com:80/~smith/home.html
1011   http://EXAMPLE.com/%7Esmith/home.html
1012   http://EXAMPLE.com:/%7esmith/home.html
1013</artwork></figure>
1014</section>
1015</section>
1016</section>
1017
1018<section title="Message Format" anchor="http.message">
1019<x:anchor-alias value="generic-message"/>
1020<x:anchor-alias value="message.types"/>
1021<x:anchor-alias value="HTTP-message"/>
1022<x:anchor-alias value="start-line"/>
1023<iref item="header section"/>
1024<iref item="headers"/>
1025<iref item="header field"/>
1026<t>
1027   All HTTP/1.1 messages consist of a start-line followed by a sequence of
1028   octets in a format similar to the Internet Message Format
1029   <xref target="RFC5322"/>: zero or more header fields (collectively
1030   referred to as the "headers" or the "header section"), an empty line
1031   indicating the end of the header section, and an optional message body.
1032</t>
1033<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"><!--terminal production--></iref>
1034  <x:ref>HTTP-message</x:ref>   = <x:ref>start-line</x:ref>
1035                   *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1036                   <x:ref>CRLF</x:ref>
1037                   [ <x:ref>message-body</x:ref> ]
1038</artwork></figure>
1039<t>
1040   The normal procedure for parsing an HTTP message is to read the
1041   start-line into a structure, read each header field into a hash
1042   table by field name until the empty line, and then use the parsed
1043   data to determine if a message body is expected.  If a message body
1044   has been indicated, then it is read as a stream until an amount
1045   of octets equal to the message body length is read or the connection
1046   is closed.
1047</t>
1048<t>
1049   Recipients &MUST; parse an HTTP message as a sequence of octets in an
1050   encoding that is a superset of US-ASCII <xref target="USASCII"/>.
1051   Parsing an HTTP message as a stream of Unicode characters, without regard
1052   for the specific encoding, creates security vulnerabilities due to the
1053   varying ways that string processing libraries handle invalid multibyte
1054   character sequences that contain the octet LF (%x0A).  String-based
1055   parsers can only be safely used within protocol elements after the element
1056   has been extracted from the message, such as within a header field-value
1057   after message parsing has delineated the individual fields.
1058</t>
1059<t>
1060   An HTTP message can be parsed as a stream for incremental processing or
1061   forwarding downstream.  However, recipients cannot rely on incremental
1062   delivery of partial messages, since some implementations will buffer or
1063   delay message forwarding for the sake of network efficiency, security
1064   checks, or payload transformations.
1065</t>
1066<t>
1067   A sender &MUST-NOT; send whitespace between the start-line and
1068   the first header field.
1069   A recipient that receives whitespace between the start-line and
1070   the first header field &MUST; either reject the message as invalid or
1071   consume each whitespace-preceded line without further processing of it
1072   (i.e., ignore the entire line, along with any subsequent lines preceded
1073   by whitespace, until a properly formed header field is received or the
1074   header block is terminated).
1075</t>
1076<t>
1077   The presence of such whitespace in a request
1078   might be an attempt to trick a server into ignoring that field or
1079   processing the line after it as a new request, either of which might
1080   result in a security vulnerability if other implementations within
1081   the request chain interpret the same message differently.
1082   Likewise, the presence of such whitespace in a response might be
1083   ignored by some clients or cause others to cease parsing.
1084</t>
1085
1086<section title="Start Line" anchor="start.line">
1087  <x:anchor-alias value="Start-Line"/>
1088<t>
1089   An HTTP message can either be a request from client to server or a
1090   response from server to client.  Syntactically, the two types of message
1091   differ only in the start-line, which is either a request-line (for requests)
1092   or a status-line (for responses), and in the algorithm for determining
1093   the length of the message body (<xref target="message.body"/>).
1094</t>
1095<t>
1096   In theory, a client could receive requests and a server could receive
1097   responses, distinguishing them by their different start-line formats,
1098   but in practice servers are implemented to only expect a request
1099   (a response is interpreted as an unknown or invalid request method)
1100   and clients are implemented to only expect a response.
1101</t>
1102<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="start-line"/>
1103  <x:ref>start-line</x:ref>     = <x:ref>request-line</x:ref> / <x:ref>status-line</x:ref>
1104</artwork></figure>
1105
1106<section title="Request Line" anchor="request.line">
1107  <x:anchor-alias value="Request"/>
1108  <x:anchor-alias value="request-line"/>
1109<t>
1110   A request-line begins with a method token, followed by a single
1111   space (SP), the request-target, another single space (SP), the
1112   protocol version, and ending with CRLF.
1113</t>
1114<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1115  <x:ref>request-line</x:ref>   = <x:ref>method</x:ref> <x:ref>SP</x:ref> <x:ref>request-target</x:ref> <x:ref>SP</x:ref> <x:ref>HTTP-version</x:ref> <x:ref>CRLF</x:ref>
1116</artwork></figure>
1117<iref primary="true" item="method"/>
1118<t anchor="method">
1119   The method token indicates the request method to be performed on the
1120   target resource. The request method is case-sensitive.
1121</t>
1122<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1123  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1124</artwork></figure>
1125<t>
1126   The methods defined by this specification can be found in
1127   &methods;, along with information regarding the HTTP method registry
1128   and considerations for defining new methods.
1129</t>
1130<iref item="request-target"/>
1131<t>
1132   The request-target identifies the target resource upon which to apply
1133   the request, as defined in <xref target="request-target"/>.
1134</t>
1135<t>
1136   Recipients typically parse the request-line into its component parts by
1137   splitting on whitespace (see <xref target="message.robustness"/>), since
1138   no whitespace is allowed in the three components.
1139   Unfortunately, some user agents fail to properly encode or exclude
1140   whitespace found in hypertext references, resulting in those disallowed
1141   characters being sent in a request-target.
1142</t>
1143<t>
1144   Recipients of an invalid request-line &SHOULD; respond with either a
1145   <x:ref>400 (Bad Request)</x:ref> error or a <x:ref>301 (Moved Permanently)</x:ref>
1146   redirect with the request-target properly encoded.  Recipients &SHOULD-NOT;
1147   attempt to autocorrect and then process the request without a redirect,
1148   since the invalid request-line might be deliberately crafted to bypass
1149   security filters along the request chain.
1150</t>
1151<t>
1152   HTTP does not place a pre-defined limit on the length of a request-line.
1153   A server that receives a method longer than any that it implements
1154   &SHOULD; respond with a <x:ref>501 (Not Implemented)</x:ref> status code.
1155   A server ought to be prepared to receive URIs of unbounded length, as
1156   described in <xref target="conformance"/>, and &MUST; respond with a
1157   <x:ref>414 (URI Too Long)</x:ref> status code if the received
1158   request-target is longer than the server wishes to parse (see &status-414;).
1159</t>
1160<t>
1161   Various ad-hoc limitations on request-line length are found in practice.
1162   It is &RECOMMENDED; that all HTTP senders and recipients support, at a
1163   minimum, request-line lengths of 8000 octets.
1164</t>
1165</section>
1166
1167<section title="Status Line" anchor="status.line">
1168  <x:anchor-alias value="response"/>
1169  <x:anchor-alias value="status-line"/>
1170  <x:anchor-alias value="status-code"/>
1171  <x:anchor-alias value="reason-phrase"/>
1172<t>
1173   The first line of a response message is the status-line, consisting
1174   of the protocol version, a space (SP), the status code, another space,
1175   a possibly-empty textual phrase describing the status code, and
1176   ending with CRLF.
1177</t>
1178<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-line"/>
1179  <x:ref>status-line</x:ref> = <x:ref>HTTP-version</x:ref> <x:ref>SP</x:ref> <x:ref>status-code</x:ref> <x:ref>SP</x:ref> <x:ref>reason-phrase</x:ref> <x:ref>CRLF</x:ref>
1180</artwork></figure>
1181<t>
1182   The status-code element is a 3-digit integer code describing the
1183   result of the server's attempt to understand and satisfy the client's
1184   corresponding request. The rest of the response message is to be
1185   interpreted in light of the semantics defined for that status code.
1186   See &status-codes; for information about the semantics of status codes,
1187   including the classes of status code (indicated by the first digit),
1188   the status codes defined by this specification, considerations for the
1189   definition of new status codes, and the IANA registry.
1190</t>
1191<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-code"/>
1192  <x:ref>status-code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1193</artwork></figure>
1194<t>  
1195   The reason-phrase element exists for the sole purpose of providing a
1196   textual description associated with the numeric status code, mostly
1197   out of deference to earlier Internet application protocols that were more
1198   frequently used with interactive text clients. A client &SHOULD; ignore
1199   the reason-phrase content.
1200</t>
1201<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
1202  <x:ref>reason-phrase</x:ref>  = *( <x:ref>HTAB</x:ref> / <x:ref>SP</x:ref> / <x:ref>VCHAR</x:ref> / <x:ref>obs-text</x:ref> )
1203</artwork></figure>
1204</section>
1205</section>
1206
1207<section title="Header Fields" anchor="header.fields">
1208  <x:anchor-alias value="header-field"/>
1209  <x:anchor-alias value="field-content"/>
1210  <x:anchor-alias value="field-name"/>
1211  <x:anchor-alias value="field-value"/>
1212  <x:anchor-alias value="obs-fold"/>
1213<t>
1214   Each HTTP header field consists of a case-insensitive field name
1215   followed by a colon (":"), optional leading whitespace, the field value,
1216   and optional trailing whitespace.
1217</t>
1218<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="header-field"/><iref primary="true" item="Grammar" subitem="field-name"/><iref primary="true" item="Grammar" subitem="field-value"/><iref primary="true" item="Grammar" subitem="field-content"/><iref primary="true" item="Grammar" subitem="obs-fold"/>
1219  <x:ref>header-field</x:ref>   = <x:ref>field-name</x:ref> ":" <x:ref>OWS</x:ref> <x:ref>field-value</x:ref> <x:ref>OWS</x:ref>
1220  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1221  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>obs-fold</x:ref> )
1222  <x:ref>field-content</x:ref>  = *( <x:ref>HTAB</x:ref> / <x:ref>SP</x:ref> / <x:ref>VCHAR</x:ref> / <x:ref>obs-text</x:ref> )
1223  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref> ( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1224                 ; obsolete line folding
1225                 ; see <xref target="field.parsing"/>
1226</artwork></figure>
1227<t>
1228   The field-name token labels the corresponding field-value as having the
1229   semantics defined by that header field.  For example, the <x:ref>Date</x:ref>
1230   header field is defined in &header-date; as containing the origination
1231   timestamp for the message in which it appears.
1232</t>
1233
1234<section title="Field Extensibility" anchor="field.extensibility">
1235<t>
1236   Header fields are fully extensible: there is no limit on the
1237   introduction of new field names, each presumably defining new semantics,
1238   nor on the number of header fields used in a given message.  Existing
1239   fields are defined in each part of this specification and in many other
1240   specifications outside the core standard.
1241</t>
1242<t>
1243   New header fields can be defined such that, when they are understood by a
1244   recipient, they might override or enhance the interpretation of previously
1245   defined header fields, define preconditions on request evaluation, or
1246   refine the meaning of responses.
1247</t>
1248<t>
1249   A proxy &MUST; forward unrecognized header fields unless the
1250   field-name is listed in the <x:ref>Connection</x:ref> header field
1251   (<xref target="header.connection"/>) or the proxy is specifically
1252   configured to block, or otherwise transform, such fields.
1253   Other recipients &SHOULD; ignore unrecognized header fields.
1254   These requirements allow HTTP's functionality to be enhanced without
1255   requiring prior update of deployed intermediaries.
1256</t>
1257<t>
1258   All defined header fields ought to be registered with IANA in the
1259   Message Header Field Registry, as described in &iana-header-registry;.
1260</t>
1261</section>
1262
1263<section title="Field Order" anchor="field.order">
1264<t>
1265   The order in which header fields with differing field names are
1266   received is not significant. However, it is "good practice" to send
1267   header fields that contain control data first, such as <x:ref>Host</x:ref>
1268   on requests and <x:ref>Date</x:ref> on responses, so that implementations
1269   can decide when not to handle a message as early as possible.  A server
1270   &MUST; wait until the entire header section is received before interpreting
1271   a request message, since later header fields might include conditionals,
1272   authentication credentials, or deliberately misleading duplicate
1273   header fields that would impact request processing.
1274</t>
1275<t>
1276   A sender &MUST-NOT; generate multiple header fields with the same field
1277   name in a message unless either the entire field value for that
1278   header field is defined as a comma-separated list [i.e., #(values)]
1279   or the header field is a well-known exception (as noted below).
1280</t>
1281<t>
1282   A recipient &MAY; combine multiple header fields with the same field name
1283   into one "field-name: field-value" pair, without changing the semantics of
1284   the message, by appending each subsequent field value to the combined
1285   field value in order, separated by a comma. The order in which
1286   header fields with the same field name are received is therefore
1287   significant to the interpretation of the combined field value;
1288   a proxy &MUST-NOT; change the order of these field values when
1289   forwarding a message.
1290</t>
1291<x:note>
1292  <t>
1293   &Note; In practice, the "Set-Cookie" header field (<xref target="RFC6265"/>)
1294   often appears multiple times in a response message and does not use the
1295   list syntax, violating the above requirements on multiple header fields
1296   with the same name. Since it cannot be combined into a single field-value,
1297   recipients ought to handle "Set-Cookie" as a special case while processing
1298   header fields. (See Appendix A.2.3 of <xref target="Kri2001"/> for details.)
1299  </t>
1300</x:note>
1301</section>
1302
1303<section title="Whitespace" anchor="whitespace">
1304<t anchor="rule.LWS">
1305   This specification uses three rules to denote the use of linear
1306   whitespace: OWS (optional whitespace), RWS (required whitespace), and
1307   BWS ("bad" whitespace).
1308</t>
1309<t anchor="rule.OWS">
1310   The OWS rule is used where zero or more linear whitespace octets might
1311   appear. For protocol elements where optional whitespace is preferred to
1312   improve readability, a sender &SHOULD; generate the optional whitespace
1313   as a single SP; otherwise, a sender &SHOULD-NOT; generate optional
1314   whitespace except as needed to white-out invalid or unwanted protocol
1315   elements during in-place message filtering.
1316</t>
1317<t anchor="rule.RWS">
1318   The RWS rule is used when at least one linear whitespace octet is required
1319   to separate field tokens. A sender &SHOULD; generate RWS as a single SP.
1320</t>
1321<t anchor="rule.BWS">
1322   The BWS rule is used where the grammar allows optional whitespace only for
1323   historical reasons. A sender &MUST-NOT; generate BWS in messages.
1324   A recipient &MUST; parse for such bad whitespace and remove it before
1325   interpreting the protocol element.
1326</t>
1327<t anchor="rule.whitespace">
1328  <x:anchor-alias value="BWS"/>
1329  <x:anchor-alias value="OWS"/>
1330  <x:anchor-alias value="RWS"/>
1331</t>
1332<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="OWS"/><iref primary="true" item="Grammar" subitem="RWS"/><iref primary="true" item="Grammar" subitem="BWS"/>
1333  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1334                 ; optional whitespace
1335  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1336                 ; required whitespace
1337  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1338                 ; "bad" whitespace
1339</artwork></figure>
1340</section>
1341
1342<section title="Field Parsing" anchor="field.parsing">
1343<t>
1344   No whitespace is allowed between the header field-name and colon.
1345   In the past, differences in the handling of such whitespace have led to
1346   security vulnerabilities in request routing and response handling.
1347   A server &MUST; reject any received request message that contains
1348   whitespace between a header field-name and colon with a response code of
1349   <x:ref>400 (Bad Request)</x:ref>. A proxy &MUST; remove any such whitespace
1350   from a response message before forwarding the message downstream.
1351</t>
1352<t>
1353   A field value is preceded by optional whitespace (OWS); a single SP is
1354   preferred. The field value does not include any leading or trailing white
1355   space: OWS occurring before the first non-whitespace octet of the field
1356   value or after the last non-whitespace octet of the field value ought to be
1357   excluded by parsers when extracting the field value from a header field.
1358</t>
1359<t>
1360   A recipient of field-content containing multiple sequential octets of
1361   optional (OWS) or required (RWS) whitespace &SHOULD; either replace the
1362   sequence with a single SP or transform any non-SP octets in the sequence to
1363   SP octets before interpreting the field value or forwarding the message
1364   downstream.
1365</t>
1366<t>
1367   Historically, HTTP header field values could be extended over multiple
1368   lines by preceding each extra line with at least one space or horizontal
1369   tab (obs-fold). This specification deprecates such line folding except
1370   within the message/http media type
1371   (<xref target="internet.media.type.message.http"/>).
1372   Senders &MUST-NOT; generate messages that include line folding
1373   (i.e., that contain any field-value that contains a match to the
1374   <x:ref>obs-fold</x:ref> rule) unless the message is intended for packaging
1375   within the message/http media type.
1376</t>
1377<t>
1378   A server that receives an <x:ref>obs-fold</x:ref> in a request message that
1379   is not within a message/http container &MUST; either reject the message by
1380   sending a <x:ref>400 (Bad Request)</x:ref>, preferably with a
1381   representation explaining that obsolete line folding is unacceptable, or
1382   replace each received <x:ref>obs-fold</x:ref> with one or more
1383   <x:ref>SP</x:ref> octets prior to interpreting the field value or
1384   forwarding the message downstream.
1385</t>
1386<t>
1387   A proxy or gateway that receives an <x:ref>obs-fold</x:ref> in a response
1388   message that is not within a message/http container &MUST; either discard
1389   the message and replace it with a <x:ref>502 (Bad Gateway)</x:ref>
1390   response, preferably with a representation explaining that unacceptable
1391   line folding was received, or replace each received <x:ref>obs-fold</x:ref>
1392   with one or more <x:ref>SP</x:ref> octets prior to interpreting the field
1393   value or forwarding the message downstream.
1394</t>
1395<t>
1396   A user agent that receives an <x:ref>obs-fold</x:ref> in a response message
1397   that is not within a message/http container &MUST; replace each received
1398   <x:ref>obs-fold</x:ref> with one or more <x:ref>SP</x:ref> octets prior to
1399   interpreting the field value.
1400</t>
1401<t>
1402   Historically, HTTP has allowed field content with text in the ISO-8859-1
1403   <xref target="ISO-8859-1"/> charset, supporting other charsets only
1404   through use of <xref target="RFC2047"/> encoding.
1405   In practice, most HTTP header field values use only a subset of the
1406   US-ASCII charset <xref target="USASCII"/>. Newly defined
1407   header fields &SHOULD; limit their field values to US-ASCII octets.
1408   Recipients &SHOULD; treat other octets in field content (obs-text) as
1409   opaque data.
1410</t>
1411</section>
1412
1413<section title="Field Limits" anchor="field.limits">
1414<t>
1415   HTTP does not place a pre-defined limit on the length of each header field
1416   or on the length of the header block as a whole, as described in
1417   <xref target="conformance"/>. Various ad-hoc limitations on individual
1418   header field length are found in practice, often depending on the specific
1419   field semantics.
1420</t>
1421<t>
1422   A server ought to be prepared to receive request header fields of unbounded
1423   length and &MUST; respond with an appropriate
1424   <x:ref>4xx (Client Error)</x:ref> status code if the received header
1425   field(s) are larger than the server wishes to process.
1426</t>
1427<t>
1428   A client ought to be prepared to receive response header fields of
1429   unbounded length.
1430   A client &MAY; discard or truncate received header fields that are larger
1431   than the client wishes to process if the field semantics are such that the
1432   dropped value(s) can be safely ignored without changing the
1433   message framing or response semantics.
1434</t>
1435</section>
1436
1437<section title="Field value components" anchor="field.components">
1438<t anchor="rule.token.separators">
1439  <x:anchor-alias value="tchar"/>
1440  <x:anchor-alias value="token"/>
1441  <x:anchor-alias value="special"/>
1442  <x:anchor-alias value="word"/>
1443   Many HTTP header field values consist of words (token or quoted-string)
1444   separated by whitespace or special characters.
1445</t>
1446<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="word"/><iref primary="true" item="Grammar" subitem="token"/><iref primary="true" item="Grammar" subitem="tchar"/><iref primary="true" item="Grammar" subitem="special"><!--unused production--></iref>
1447  <x:ref>word</x:ref>           = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1448
1449  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1450<!--
1451  IMPORTANT: when editing "tchar" make sure that "special" is updated accordingly!!!
1452 -->
1453  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1454                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1455                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1456                 ; any <x:ref>VCHAR</x:ref>, except <x:ref>special</x:ref>
1457
1458  <x:ref>special</x:ref>        = "(" / ")" / "&lt;" / ">" / "@" / ","
1459                 / ";" / ":" / "\" / DQUOTE / "/" / "["
1460                 / "]" / "?" / "=" / "{" / "}"
1461</artwork></figure>
1462<t anchor="rule.quoted-string">
1463  <x:anchor-alias value="quoted-string"/>
1464  <x:anchor-alias value="qdtext"/>
1465  <x:anchor-alias value="obs-text"/>
1466   A string of text is parsed as a single word if it is quoted using
1467   double-quote marks.
1468</t>
1469<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-string"/><iref primary="true" item="Grammar" subitem="qdtext"/><iref primary="true" item="Grammar" subitem="obs-text"/>
1470  <x:ref>quoted-string</x:ref>  = <x:ref>DQUOTE</x:ref> *( <x:ref>qdtext</x:ref> / <x:ref>quoted-pair</x:ref> ) <x:ref>DQUOTE</x:ref>
1471  <x:ref>qdtext</x:ref>         = <x:ref>HTAB</x:ref> / <x:ref>SP</x:ref> /%x21 / %x23-5B / %x5D-7E / <x:ref>obs-text</x:ref>
1472  <x:ref>obs-text</x:ref>       = %x80-FF
1473</artwork></figure>
1474<t anchor="rule.quoted-pair">
1475  <x:anchor-alias value="quoted-pair"/>
1476   The backslash octet ("\") can be used as a single-octet
1477   quoting mechanism within quoted-string constructs:
1478</t>
1479<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1480  <x:ref>quoted-pair</x:ref>    = "\" ( <x:ref>HTAB</x:ref> / <x:ref>SP</x:ref> / <x:ref>VCHAR</x:ref> / <x:ref>obs-text</x:ref> )
1481</artwork></figure>
1482<t>
1483   Recipients that process the value of a quoted-string &MUST; handle a
1484   quoted-pair as if it were replaced by the octet following the backslash.
1485</t>
1486<t>
1487   Senders &SHOULD-NOT; generate a quoted-pair in a quoted-string except where
1488   necessary to quote DQUOTE and backslash octets occurring within that string.
1489</t>
1490<t anchor="rule.comment">
1491  <x:anchor-alias value="comment"/>
1492  <x:anchor-alias value="ctext"/>
1493   Comments can be included in some HTTP header fields by surrounding
1494   the comment text with parentheses. Comments are only allowed in
1495   fields containing "comment" as part of their field value definition.
1496</t>
1497<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1498  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-cpair</x:ref> / <x:ref>comment</x:ref> ) ")"
1499  <x:ref>ctext</x:ref>          = <x:ref>HTAB</x:ref> / <x:ref>SP</x:ref> / %x21-27 / %x2A-5B / %x5D-7E / <x:ref>obs-text</x:ref>
1500</artwork></figure>
1501<t anchor="rule.quoted-cpair">
1502  <x:anchor-alias value="quoted-cpair"/>
1503   The backslash octet ("\") can be used as a single-octet
1504   quoting mechanism within comment constructs:
1505</t>
1506<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-cpair"/>
1507  <x:ref>quoted-cpair</x:ref>   = "\" ( <x:ref>HTAB</x:ref> / <x:ref>SP</x:ref> / <x:ref>VCHAR</x:ref> / <x:ref>obs-text</x:ref> )
1508</artwork></figure>
1509<t>
1510   Senders &SHOULD-NOT; escape octets in comments that do not require escaping
1511   (i.e., other than the backslash octet "\" and the parentheses "(" and ")").
1512</t>
1513</section>
1514
1515</section>
1516
1517<section title="Message Body" anchor="message.body">
1518  <x:anchor-alias value="message-body"/>
1519<t>
1520   The message body (if any) of an HTTP message is used to carry the
1521   payload body of that request or response.  The message body is
1522   identical to the payload body unless a transfer coding has been
1523   applied, as described in <xref target="header.transfer-encoding"/>.
1524</t>
1525<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1526  <x:ref>message-body</x:ref> = *OCTET
1527</artwork></figure>
1528<t>
1529   The rules for when a message body is allowed in a message differ for
1530   requests and responses.
1531</t>
1532<t>
1533   The presence of a message body in a request is signaled by a
1534   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1535   field. Request message framing is independent of method semantics,
1536   even if the method does not define any use for a message body.
1537</t>
1538<t>
1539   The presence of a message body in a response depends on both
1540   the request method to which it is responding and the response
1541   status code (<xref target="status.line"/>).
1542   Responses to the HEAD request method never include a message body
1543   because the associated response header fields (e.g.,
1544   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1545   if present, indicate only what their values would have been if the request
1546   method had been GET (&HEAD;).
1547   <x:ref>2xx (Successful)</x:ref> responses to CONNECT switch to tunnel
1548   mode instead of having a message body (&CONNECT;).
1549   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1550   <x:ref>304 (Not Modified)</x:ref> responses do not include a message body.
1551   All other responses do include a message body, although the body
1552   might be of zero length.
1553</t>
1554
1555<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1556  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1557  <iref item="chunked (Coding Format)"/>
1558  <x:anchor-alias value="Transfer-Encoding"/>
1559<t>
1560   The Transfer-Encoding header field lists the transfer coding names
1561   corresponding to the sequence of transfer codings that have been
1562   (or will be) applied to the payload body in order to form the message body.
1563   Transfer codings are defined in <xref target="transfer.codings"/>.
1564</t>
1565<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1566  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1567</artwork></figure>
1568<t>
1569   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1570   MIME, which was designed to enable safe transport of binary data over a
1571   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1572   However, safe transport has a different focus for an 8bit-clean transfer
1573   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1574   accurately delimit a dynamically generated payload and to distinguish
1575   payload encodings that are only applied for transport efficiency or
1576   security from those that are characteristics of the selected resource.
1577</t>
1578<t>
1579   All HTTP/1.1 recipients &MUST; be able to parse the chunked transfer coding
1580   (<xref target="chunked.encoding"/>) because it plays a crucial role in
1581   framing messages when the payload body size is not known in advance.
1582   A sender &MUST-NOT; apply chunked more than once to a message body
1583   (i.e., chunking an already chunked message is not allowed).
1584   If any transfer coding other than chunked is applied to a request payload
1585   body, the sender &MUST; apply chunked as the final transfer coding to
1586   ensure that the message is properly framed.
1587   If any transfer coding other than chunked is applied to a response payload
1588   body, the sender &MUST; either apply chunked as the final transfer coding
1589   or terminate the message by closing the connection.
1590</t>
1591<figure><preamble>
1592   For example,
1593</preamble><artwork type="example">
1594  Transfer-Encoding: gzip, chunked
1595</artwork><postamble>
1596   indicates that the payload body has been compressed using the gzip
1597   coding and then chunked using the chunked coding while forming the
1598   message body.
1599</postamble></figure>
1600<t>
1601   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1602   Transfer-Encoding is a property of the message, not of the representation, and
1603   any recipient along the request/response chain &MAY; decode the received
1604   transfer coding(s) or apply additional transfer coding(s) to the message
1605   body, assuming that corresponding changes are made to the Transfer-Encoding
1606   field-value. Additional information about the encoding parameters &MAY; be
1607   provided by other header fields not defined by this specification.
1608</t>
1609<t>
1610   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1611   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1612   neither of which includes a message body,
1613   to indicate that the origin server would have applied a transfer coding
1614   to the message body if the request had been an unconditional GET.
1615   This indication is not required, however, because any recipient on
1616   the response chain (including the origin server) can remove transfer
1617   codings when they are not needed.
1618</t>
1619<t>
1620   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1621   implementations advertising only HTTP/1.0 support will not understand
1622   how to process a transfer-encoded payload.
1623   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1624   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1625   might be in the form of specific user configuration or by remembering the
1626   version of a prior received response.
1627   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1628   the corresponding request indicates HTTP/1.1 (or later).
1629</t>
1630<t>
1631   A server that receives a request message with a transfer coding it does
1632   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref>.
1633</t>
1634</section>
1635
1636<section title="Content-Length" anchor="header.content-length">
1637  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1638  <x:anchor-alias value="Content-Length"/>
1639<t>
1640   When a message does not have a <x:ref>Transfer-Encoding</x:ref> header
1641   field, a Content-Length header field can provide the anticipated size,
1642   as a decimal number of octets, for a potential payload body.
1643   For messages that do include a payload body, the Content-Length field-value
1644   provides the framing information necessary for determining where the body
1645   (and message) ends.  For messages that do not include a payload body, the
1646   Content-Length indicates the size of the selected representation
1647   (&representation;).
1648</t>
1649<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1650  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1651</artwork></figure>
1652<t>
1653   An example is
1654</t>
1655<figure><artwork type="example">
1656  Content-Length: 3495
1657</artwork></figure>
1658<t>
1659   A sender &MUST-NOT; send a Content-Length header field in any message that
1660   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1661</t>
1662<t>
1663   A user agent &SHOULD; send a Content-Length in a request message when no
1664   <x:ref>Transfer-Encoding</x:ref> is sent and the request method defines
1665   a meaning for an enclosed payload body. For example, a Content-Length
1666   header field is normally sent in a POST request even when the value is
1667   0 (indicating an empty payload body).  A user agent &SHOULD-NOT; send a
1668   Content-Length header field when the request message does not contain a
1669   payload body and the method semantics do not anticipate such a body.
1670</t>
1671<t>
1672   A server &MAY; send a Content-Length header field in a response to a HEAD
1673   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1674   response unless its field-value equals the decimal number of octets that
1675   would have been sent in the payload body of a response if the same
1676   request had used the GET method.
1677</t>
1678<t>
1679   A server &MAY; send a Content-Length header field in a
1680   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1681   (&status-304;); 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 <x:ref>200 (OK)</x:ref>
1684   response to the same request.
1685</t>
1686<t>
1687   A server &MUST-NOT; send a Content-Length header field in any response
1688   with a status code of
1689   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1690   A server &SHOULD-NOT; send a Content-Length header field in any
1691   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1692</t>
1693<t>
1694   Aside from the cases defined above, in the absence of Transfer-Encoding,
1695   an origin server &SHOULD; send a Content-Length header field when the
1696   payload body size is known prior to sending the complete header block.
1697   This will allow downstream recipients to measure transfer progress,
1698   know when a received message is complete, and potentially reuse the
1699   connection for additional requests.
1700</t>
1701<t>
1702   Any Content-Length field value greater than or equal to zero is valid.
1703   Since there is no predefined limit to the length of a payload,
1704   recipients &SHOULD; anticipate potentially large decimal numerals and
1705   prevent parsing errors due to integer conversion overflows
1706   (<xref target="attack.protocol.element.size.overflows"/>).
1707</t>
1708<t>
1709   If a message is received that has multiple Content-Length header fields
1710   with field-values consisting of the same decimal value, or a single
1711   Content-Length header field with a field value containing a list of
1712   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1713   duplicate Content-Length header fields have been generated or combined by an
1714   upstream message processor, then the recipient &MUST; either reject the
1715   message as invalid or replace the duplicated field-values with a single
1716   valid Content-Length field containing that decimal value prior to
1717   determining the message body length or forwarding the message.
1718</t>
1719<x:note>
1720  <t>
1721   &Note; HTTP's use of Content-Length for message framing differs
1722   significantly from the same field's use in MIME, where it is an optional
1723   field used only within the "message/external-body" media-type.
1724  </t>
1725</x:note>
1726</section>
1727
1728<section title="Message Body Length" anchor="message.body.length">
1729  <iref item="chunked (Coding Format)"/>
1730<t>
1731   The length of a message body is determined by one of the following
1732   (in order of precedence):
1733</t>
1734<t>
1735  <list style="numbers">
1736    <x:lt><t>
1737     Any response to a HEAD request and any response with a
1738     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1739     <x:ref>304 (Not Modified)</x:ref> status code is always
1740     terminated by the first empty line after the header fields, regardless of
1741     the header fields present in the message, and thus cannot contain a
1742     message body.
1743    </t></x:lt>
1744    <x:lt><t>
1745     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1746     connection will become a tunnel immediately after the empty line that
1747     concludes the header fields.  A client &MUST; ignore any
1748     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1749     fields received in such a message.
1750    </t></x:lt>
1751    <x:lt><t>
1752     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1753     and the chunked transfer coding (<xref target="chunked.encoding"/>)
1754     is the final encoding, the message body length is determined by reading
1755     and decoding the chunked data until the transfer coding indicates the
1756     data is complete.
1757    </t>
1758    <t>
1759     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1760     response and the chunked transfer coding is not the final encoding, the
1761     message body length is determined by reading the connection until it is
1762     closed by the server.
1763     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a request and the
1764     chunked transfer coding is not the final encoding, the message body
1765     length cannot be determined reliably; the server &MUST; respond with
1766     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1767    </t>
1768    <t>
1769     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1770     and a <x:ref>Content-Length</x:ref> header field, the Transfer-Encoding
1771     overrides the Content-Length. Such a message might indicate an attempt
1772     to perform request or response smuggling (bypass of security-related
1773     checks on message routing or content) and thus ought to be handled as
1774     an error.  A sender &MUST; remove the received Content-Length field
1775     prior to forwarding such a message downstream.
1776    </t></x:lt>
1777    <x:lt><t>
1778     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1779     either multiple <x:ref>Content-Length</x:ref> header fields having
1780     differing field-values or a single Content-Length header field having an
1781     invalid value, then the message framing is invalid and
1782     the recipient &MUST; treat it as an unrecoverable error to prevent
1783     request or response smuggling.
1784     If this is a request message, the server &MUST; respond with
1785     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1786     If this is a response message received by a proxy,
1787     the proxy &MUST; close the connection to the server, discard the received
1788     response, and send a <x:ref>502 (Bad Gateway)</x:ref> response to the
1789     client.
1790     If this is a response message received by a user agent,
1791     the user agent &MUST; close the connection to the server and discard the
1792     received response.
1793    </t></x:lt>
1794    <x:lt><t>
1795     If a valid <x:ref>Content-Length</x:ref> header field is present without
1796     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1797     expected message body length in octets.
1798     If the sender closes the connection or the recipient times out before the
1799     indicated number of octets are received, the recipient &MUST; consider
1800     the message to be incomplete and close the connection.
1801    </t></x:lt>
1802    <x:lt><t>
1803     If this is a request message and none of the above are true, then the
1804     message body length is zero (no message body is present).
1805    </t></x:lt>
1806    <x:lt><t>
1807     Otherwise, this is a response message without a declared message body
1808     length, so the message body length is determined by the number of octets
1809     received prior to the server closing the connection.
1810    </t></x:lt>
1811  </list>
1812</t>
1813<t>
1814   Since there is no way to distinguish a successfully completed,
1815   close-delimited message from a partially-received message interrupted
1816   by network failure, a server &SHOULD; use encoding or
1817   length-delimited messages whenever possible.  The close-delimiting
1818   feature exists primarily for backwards compatibility with HTTP/1.0.
1819</t>
1820<t>
1821   A server &MAY; reject a request that contains a message body but
1822   not a <x:ref>Content-Length</x:ref> by responding with
1823   <x:ref>411 (Length Required)</x:ref>.
1824</t>
1825<t>
1826   Unless a transfer coding other than chunked has been applied,
1827   a client that sends a request containing a message body &SHOULD;
1828   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1829   length is known in advance, rather than the chunked transfer coding, since some
1830   existing services respond to chunked with a <x:ref>411 (Length Required)</x:ref>
1831   status code even though they understand the chunked transfer coding.  This
1832   is typically because such services are implemented via a gateway that
1833   requires a content-length in advance of being called and the server
1834   is unable or unwilling to buffer the entire request before processing.
1835</t>
1836<t>
1837   A user agent that sends a request containing a message body &MUST; send a
1838   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1839   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1840   the form of specific user configuration or by remembering the version of a
1841   prior received response.
1842</t>
1843<t>
1844   If the final response to the last request on a connection has been
1845   completely received and there remains additional data to read, a user agent
1846   &MAY; discard the remaining data or attempt to determine if that data
1847   belongs as part of the prior response body, which might be the case if the
1848   prior message's Content-Length value is incorrect. A client &MUST-NOT;
1849   process, cache, or forward such extra data as a separate response, since
1850   such behavior would be vulnerable to cache poisoning.
1851</t>
1852</section>
1853</section>
1854
1855<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1856<t>
1857   A server that receives an incomplete request message, usually due to a
1858   canceled request or a triggered time-out exception, &MAY; send an error
1859   response prior to closing the connection.
1860</t>
1861<t>
1862   A client that receives an incomplete response message, which can occur
1863   when a connection is closed prematurely or when decoding a supposedly
1864   chunked transfer coding fails, &MUST; record the message as incomplete.
1865   Cache requirements for incomplete responses are defined in
1866   &cache-incomplete;.
1867</t>
1868<t>
1869   If a response terminates in the middle of the header block (before the
1870   empty line is received) and the status code might rely on header fields to
1871   convey the full meaning of the response, then the client cannot assume
1872   that meaning has been conveyed; the client might need to repeat the
1873   request in order to determine what action to take next.
1874</t>
1875<t>
1876   A message body that uses the chunked transfer coding is
1877   incomplete if the zero-sized chunk that terminates the encoding has not
1878   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1879   incomplete if the size of the message body received (in octets) is less than
1880   the value given by Content-Length.  A response that has neither chunked
1881   transfer coding nor Content-Length is terminated by closure of the
1882   connection, and thus is considered complete regardless of the number of
1883   message body octets received, provided that the header block was received
1884   intact.
1885</t>
1886</section>
1887
1888<section title="Message Parsing Robustness" anchor="message.robustness">
1889<t>
1890   Older HTTP/1.0 user agent implementations might send an extra CRLF
1891   after a POST request as a workaround for some early server
1892   applications that failed to read message body content that was
1893   not terminated by a line-ending. An HTTP/1.1 user agent &MUST-NOT;
1894   preface or follow a request with an extra CRLF.  If terminating
1895   the request message body with a line-ending is desired, then the
1896   user agent &MUST; count the terminating CRLF octets as part of the
1897   message body length.
1898</t>
1899<t>
1900   In the interest of robustness, servers &SHOULD; ignore at least one
1901   empty line received where a request-line is expected. In other words, if
1902   a server is reading the protocol stream at the beginning of a
1903   message and receives a CRLF first, the server &SHOULD; ignore the CRLF.
1904</t>
1905<t>
1906   Although the line terminator for the start-line and header
1907   fields is the sequence CRLF, recipients &MAY; recognize a
1908   single LF as a line terminator and ignore any preceding CR.
1909</t>
1910<t>
1911   Although the request-line and status-line grammar rules require that each
1912   of the component elements be separated by a single SP octet, recipients
1913   &MAY; instead parse on whitespace-delimited word boundaries and, aside
1914   from the CRLF terminator, treat any form of whitespace as the SP separator
1915   while ignoring preceding or trailing whitespace;
1916   such whitespace includes one or more of the following octets:
1917   SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
1918</t>
1919<t>
1920   When a server listening only for HTTP request messages, or processing
1921   what appears from the start-line to be an HTTP request message,
1922   receives a sequence of octets that does not match the HTTP-message
1923   grammar aside from the robustness exceptions listed above, the
1924   server &SHOULD; respond with a <x:ref>400 (Bad Request)</x:ref> response. 
1925</t>
1926</section>
1927</section>
1928
1929<section title="Transfer Codings" anchor="transfer.codings">
1930  <x:anchor-alias value="transfer-coding"/>
1931  <x:anchor-alias value="transfer-extension"/>
1932<t>
1933   Transfer coding names are used to indicate an encoding
1934   transformation that has been, can be, or might need to be applied to a
1935   payload body in order to ensure "safe transport" through the network.
1936   This differs from a content coding in that the transfer coding is a
1937   property of the message rather than a property of the representation
1938   that is being transferred.
1939</t>
1940<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1941  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1942                     / "compress" ; <xref target="compress.coding"/>
1943                     / "deflate" ; <xref target="deflate.coding"/>
1944                     / "gzip" ; <xref target="gzip.coding"/>
1945                     / <x:ref>transfer-extension</x:ref>
1946  <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> )
1947</artwork></figure>
1948<t anchor="rule.parameter">
1949  <x:anchor-alias value="attribute"/>
1950  <x:anchor-alias value="transfer-parameter"/>
1951  <x:anchor-alias value="value"/>
1952   Parameters are in the form of attribute/value pairs.
1953</t>
1954<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"/>
1955  <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>
1956  <x:ref>attribute</x:ref>          = <x:ref>token</x:ref>
1957  <x:ref>value</x:ref>              = <x:ref>word</x:ref>
1958</artwork></figure>
1959<t>
1960   All transfer-coding names are case-insensitive and ought to be registered
1961   within the HTTP Transfer Coding registry, as defined in
1962   <xref target="transfer.coding.registry"/>.
1963   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1964   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1965   header fields.
1966</t>
1967
1968<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1969  <iref primary="true" item="chunked (Coding Format)"/>
1970  <x:anchor-alias value="chunk"/>
1971  <x:anchor-alias value="chunked-body"/>
1972  <x:anchor-alias value="chunk-data"/>
1973  <x:anchor-alias value="chunk-ext"/>
1974  <x:anchor-alias value="chunk-ext-name"/>
1975  <x:anchor-alias value="chunk-ext-val"/>
1976  <x:anchor-alias value="chunk-size"/>
1977  <x:anchor-alias value="last-chunk"/>
1978  <x:anchor-alias value="trailer-part"/>
1979  <x:anchor-alias value="quoted-str-nf"/>
1980  <x:anchor-alias value="qdtext-nf"/>
1981<t>
1982   The chunked transfer coding modifies the body of a message in order to
1983   transfer it as a series of chunks, each with its own size indicator,
1984   followed by an &OPTIONAL; trailer containing header fields. This
1985   allows dynamically generated content to be transferred along with the
1986   information necessary for the recipient to verify that it has
1987   received the full message.
1988</t>
1989<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="chunked-body"><!--terminal production--></iref><iref primary="true" item="Grammar" subitem="chunk"/><iref primary="true" item="Grammar" subitem="chunk-size"/><iref primary="true" item="Grammar" subitem="last-chunk"/><iref primary="true" item="Grammar" subitem="chunk-ext"/><iref primary="true" item="Grammar" subitem="chunk-ext-name"/><iref primary="true" item="Grammar" subitem="chunk-ext-val"/><iref primary="true" item="Grammar" subitem="chunk-data"/><iref primary="true" item="Grammar" subitem="trailer-part"/><iref primary="true" item="Grammar" subitem="quoted-str-nf"/><iref primary="true" item="Grammar" subitem="qdtext-nf"/>
1990  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1991                   <x:ref>last-chunk</x:ref>
1992                   <x:ref>trailer-part</x:ref>
1993                   <x:ref>CRLF</x:ref>
1994 
1995  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1996                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1997  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1998  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1999 
2000  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
2001  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
2002  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-str-nf</x:ref>
2003  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
2004  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
2005 
2006  <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>
2007                 ; like <x:ref>quoted-string</x:ref>, but disallowing line folding
2008  <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>
2009</artwork></figure>
2010<t>
2011   Chunk extensions within the chunked transfer coding are deprecated.
2012   Senders &SHOULD-NOT; send chunk-ext.
2013   Definition of new chunk extensions is discouraged.
2014</t>
2015<t>
2016   The chunk-size field is a string of hex digits indicating the size of
2017   the chunk-data in octets. The chunked transfer coding is complete when a
2018   chunk with a chunk-size of zero is received, possibly followed by a
2019   trailer, and finally terminated by an empty line.
2020</t>
2021
2022<section title="Trailer" anchor="header.trailer">
2023  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
2024  <x:anchor-alias value="Trailer"/>
2025<t>
2026   A trailer allows the sender to include additional fields at the end of a
2027   chunked message in order to supply metadata that might be dynamically
2028   generated while the message body is sent, such as a message integrity
2029   check, digital signature, or post-processing status.
2030   A sender &MUST-NOT; generate a trailer containing a field that needs to be
2031   known before a recipient processes the body, such as
2032   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, and
2033   <x:ref>Trailer</x:ref>.
2034</t>
2035<t>
2036   When a message includes a message body encoded with the chunked
2037   transfer coding and the sender desires to send metadata in the form of
2038   trailer fields at the end of the message, the sender &SHOULD; generate a
2039   <x:ref>Trailer</x:ref> header field before the message body to indicate
2040   which fields will be present in the trailers. This allows the recipient
2041   to prepare for receipt of that metadata before it starts processing the body,
2042   which is useful if the message is being streamed and the recipient wishes
2043   to confirm an integrity check on the fly.
2044</t>
2045<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
2046  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
2047</artwork></figure>
2048<t>
2049   A server &MUST; generate an empty trailer with the chunked transfer coding
2050   unless at least one of the following is true:
2051  <list style="numbers">
2052    <t>the request included a <x:ref>TE</x:ref> header field that indicates
2053    "trailers" is acceptable in the transfer coding of the response, as
2054    described in <xref target="header.te"/>; or,</t>
2055     
2056    <t>the trailer fields consist entirely of optional metadata and the
2057    recipient could use the message (in a manner acceptable to the generating
2058    server) without receiving that metadata. In other words, the generating
2059    server is willing to accept the possibility that the trailer fields might
2060    be silently discarded along the path to the client.</t>
2061  </list>
2062</t>
2063<t>
2064   The above requirement prevents the need for an infinite buffer when a
2065   message is being received by an HTTP/1.1 (or later) proxy and forwarded to
2066   an HTTP/1.0 recipient.
2067</t>
2068</section>
2069
2070<section title="Decoding chunked" anchor="decoding.chunked">
2071<t>
2072   A process for decoding the chunked transfer coding
2073   can be represented in pseudo-code as:
2074</t>
2075<figure><artwork type="code">
2076  length := 0
2077  read chunk-size, chunk-ext (if any), and CRLF
2078  while (chunk-size &gt; 0) {
2079     read chunk-data and CRLF
2080     append chunk-data to decoded-body
2081     length := length + chunk-size
2082     read chunk-size, chunk-ext (if any), and CRLF
2083  }
2084  read header-field
2085  while (header-field not empty) {
2086     append header-field to existing header fields
2087     read header-field
2088  }
2089  Content-Length := length
2090  Remove "chunked" from Transfer-Encoding
2091  Remove Trailer from existing header fields
2092</artwork></figure>
2093<t>
2094   All recipients &MUST; be able to parse and decode the
2095   chunked transfer coding and &MUST; ignore chunk-ext extensions
2096   they do not understand.
2097</t>
2098</section>
2099</section>
2100
2101<section title="Compression Codings" anchor="compression.codings">
2102<t>
2103   The codings defined below can be used to compress the payload of a
2104   message.
2105</t>
2106
2107<section title="Compress Coding" anchor="compress.coding">
2108<iref item="compress (Coding Format)"/>
2109<t>
2110   The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding
2111   <xref target="Welch"/> that is commonly produced by the UNIX file
2112   compression program "compress".
2113   Recipients &SHOULD; consider "x-compress" to be equivalent to "compress".
2114</t>
2115</section>
2116
2117<section title="Deflate Coding" anchor="deflate.coding">
2118<iref item="deflate (Coding Format)"/>
2119<t>
2120   The "deflate" coding is a "zlib" data format <xref target="RFC1950"/>
2121   containing a "deflate" compressed data stream <xref target="RFC1951"/>
2122   that uses a combination of the Lempel-Ziv (LZ77) compression algorithm and
2123   Huffman coding.
2124</t>
2125<x:note>
2126  <t>
2127    &Note; Some incorrect implementations send the "deflate"
2128    compressed data without the zlib wrapper.
2129   </t>
2130</x:note>
2131</section>
2132
2133<section title="Gzip Coding" anchor="gzip.coding">
2134<iref item="gzip (Coding Format)"/>
2135<t>
2136   The "gzip" coding is an LZ77 coding with a 32 bit CRC that is commonly
2137   produced by the gzip file compression program <xref target="RFC1952"/>.
2138   Recipients &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2139</t>
2140</section>
2141
2142</section>
2143
2144<section title="TE" anchor="header.te">
2145  <iref primary="true" item="TE header field" x:for-anchor=""/>
2146  <x:anchor-alias value="TE"/>
2147  <x:anchor-alias value="t-codings"/>
2148  <x:anchor-alias value="t-ranking"/>
2149  <x:anchor-alias value="rank"/>
2150<t>
2151   The "TE" header field in a request indicates what transfer codings,
2152   besides chunked, the client is willing to accept in response, and
2153   whether or not the client is willing to accept trailer fields in a
2154   chunked transfer coding.
2155</t>
2156<t>
2157   The TE field-value consists of a comma-separated list of transfer coding
2158   names, each allowing for optional parameters (as described in
2159   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2160   Clients &MUST-NOT; send the chunked transfer coding name in TE;
2161   chunked is always acceptable for HTTP/1.1 recipients.
2162</t>
2163<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"/>
2164  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2165  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2166  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2167  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2168             / ( "1" [ "." 0*3("0") ] )
2169</artwork></figure>
2170<t>
2171   Three examples of TE use are below.
2172</t>
2173<figure><artwork type="example">
2174  TE: deflate
2175  TE:
2176  TE: trailers, deflate;q=0.5
2177</artwork></figure>
2178<t>
2179   The presence of the keyword "trailers" indicates that the client is willing
2180   to accept trailer fields in a chunked transfer coding, as defined in
2181   <xref target="chunked.encoding"/>, on behalf of itself and any downstream
2182   clients. For requests from an intermediary, this implies that either:
2183   (a) all downstream clients are willing to accept trailer fields in the
2184   forwarded response; or,
2185   (b) the intermediary will attempt to buffer the response on behalf of
2186   downstream recipients.
2187   Note that HTTP/1.1 does not define any means to limit the size of a
2188   chunked response such that an intermediary can be assured of buffering the
2189   entire response.
2190</t>
2191<t>
2192   When multiple transfer codings are acceptable, the client &MAY; rank the
2193   codings by preference using a case-insensitive "q" parameter (similar to
2194   the qvalues used in content negotiation fields, &qvalue;). The rank value
2195   is a real number in the range 0 through 1, where 0.001 is the least
2196   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2197</t>
2198<t>
2199   If the TE field-value is empty or if no TE field is present, the only
2200   acceptable transfer coding is chunked. A message with no transfer coding
2201   is always acceptable.
2202</t>
2203<t>
2204   Since the TE header field only applies to the immediate connection,
2205   a sender of TE &MUST; also send a "TE" connection option within the
2206   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2207   in order to prevent the TE field from being forwarded by intermediaries
2208   that do not support its semantics.
2209</t>
2210</section>
2211</section>
2212
2213<section title="Message Routing" anchor="message.routing">
2214<t>
2215   HTTP request message routing is determined by each client based on the
2216   target resource, the client's proxy configuration, and
2217   establishment or reuse of an inbound connection.  The corresponding
2218   response routing follows the same connection chain back to the client.
2219</t>
2220
2221<section title="Identifying a Target Resource" anchor="target-resource">
2222  <iref primary="true" item="target resource"/>
2223  <iref primary="true" item="target URI"/>
2224  <x:anchor-alias value="target resource"/>
2225  <x:anchor-alias value="target URI"/>
2226<t>
2227   HTTP is used in a wide variety of applications, ranging from
2228   general-purpose computers to home appliances.  In some cases,
2229   communication options are hard-coded in a client's configuration.
2230   However, most HTTP clients rely on the same resource identification
2231   mechanism and configuration techniques as general-purpose Web browsers.
2232</t>
2233<t>
2234   HTTP communication is initiated by a user agent for some purpose.
2235   The purpose is a combination of request semantics, which are defined in
2236   <xref target="Part2"/>, and a target resource upon which to apply those
2237   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2238   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2239   would resolve to its absolute form in order to obtain the
2240   "<x:dfn>target URI</x:dfn>".  The target URI
2241   excludes the reference's fragment component, if any,
2242   since fragment identifiers are reserved for client-side processing
2243   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2244</t>
2245</section>
2246
2247<section title="Connecting Inbound" anchor="connecting.inbound">
2248<t>
2249   Once the target URI is determined, a client needs to decide whether
2250   a network request is necessary to accomplish the desired semantics and,
2251   if so, where that request is to be directed.
2252</t>
2253<t>
2254   If the client has a cache <xref target="Part6"/> and the request can be
2255   satisfied by it, then the request is
2256   usually directed there first.
2257</t>
2258<t>
2259   If the request is not satisfied by a cache, then a typical client will
2260   check its configuration to determine whether a proxy is to be used to
2261   satisfy the request.  Proxy configuration is implementation-dependent,
2262   but is often based on URI prefix matching, selective authority matching,
2263   or both, and the proxy itself is usually identified by an "http" or
2264   "https" URI.  If a proxy is applicable, the client connects inbound by
2265   establishing (or reusing) a connection to that proxy.
2266</t>
2267<t>
2268   If no proxy is applicable, a typical client will invoke a handler routine,
2269   usually specific to the target URI's scheme, to connect directly
2270   to an authority for the target resource.  How that is accomplished is
2271   dependent on the target URI scheme and defined by its associated
2272   specification, similar to how this specification defines origin server
2273   access for resolution of the "http" (<xref target="http.uri"/>) and
2274   "https" (<xref target="https.uri"/>) schemes.
2275</t>
2276<t>
2277   HTTP requirements regarding connection management are defined in
2278   <xref target="connection.management"/>.
2279</t>
2280</section>
2281
2282<section title="Request Target" anchor="request-target">
2283<t>
2284   Once an inbound connection is obtained,
2285   the client sends an HTTP request message (<xref target="http.message"/>)
2286   with a request-target derived from the target URI.
2287   There are four distinct formats for the request-target, depending on both
2288   the method being requested and whether the request is to a proxy.
2289</t>   
2290<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"/>
2291  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2292                 / <x:ref>absolute-form</x:ref>
2293                 / <x:ref>authority-form</x:ref>
2294                 / <x:ref>asterisk-form</x:ref>
2295
2296  <x:ref>origin-form</x:ref>    = <x:ref>absolute-path</x:ref> [ "?" <x:ref>query</x:ref> ]
2297  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2298  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2299  <x:ref>asterisk-form</x:ref>  = "*"
2300</artwork></figure>
2301<t anchor="origin-form"><iref item="origin-form (of request-target)"/>
2302  <x:h>origin-form</x:h>
2303</t>
2304<t>
2305   The most common form of request-target is the <x:dfn>origin-form</x:dfn>.
2306   When making a request directly to an origin server, other than a CONNECT
2307   or server-wide OPTIONS request (as detailed below),
2308   a client &MUST; send only the absolute path and query components of
2309   the target URI as the request-target.
2310   If the target URI's path component is empty, then the client &MUST; send
2311   "/" as the path within the origin-form of request-target.
2312   A <x:ref>Host</x:ref> header field is also sent, as defined in
2313   <xref target="header.host"/>.
2314</t>
2315<t>
2316   For example, a client wishing to retrieve a representation of the resource
2317   identified as
2318</t>
2319<figure><artwork x:indent-with="  " type="example">
2320http://www.example.org/where?q=now
2321</artwork></figure>
2322<t>
2323   directly from the origin server would open (or reuse) a TCP connection
2324   to port 80 of the host "www.example.org" and send the lines:
2325</t>
2326<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2327GET /where?q=now HTTP/1.1
2328Host: www.example.org
2329</artwork></figure>
2330<t>
2331   followed by the remainder of the request message.
2332</t>
2333<t anchor="absolute-form"><iref item="absolute-form (of request-target)"/>
2334  <x:h>absolute-form</x:h>
2335</t>
2336<t>
2337   When making a request to a proxy, other than a CONNECT or server-wide
2338   OPTIONS request (as detailed below), a client &MUST; send the target URI
2339   in <x:dfn>absolute-form</x:dfn> as the request-target.
2340   The proxy is requested to either service that request from a valid cache,
2341   if possible, or make the same request on the client's behalf to either
2342   the next inbound proxy server or directly to the origin server indicated
2343   by the request-target.  Requirements on such "forwarding" of messages are
2344   defined in <xref target="message.forwarding"/>.
2345</t>
2346<t>
2347   An example absolute-form of request-line would be:
2348</t>
2349<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2350GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
2351</artwork></figure>
2352<t>
2353   To allow for transition to the absolute-form for all requests in some
2354   future version of HTTP, HTTP/1.1 servers &MUST; accept the absolute-form
2355   in requests, even though HTTP/1.1 clients will only send them in requests
2356   to proxies.
2357</t>
2358<t anchor="authority-form"><iref item="authority-form (of request-target)"/>
2359  <x:h>authority-form</x:h>
2360</t>
2361<t>
2362   The <x:dfn>authority-form</x:dfn> of request-target is only used for
2363   CONNECT requests (&CONNECT;). When making a CONNECT request to establish a
2364   tunnel through one or more proxies, a client &MUST; send only the target
2365   URI's authority component (excluding any userinfo and its "@" delimiter) as
2366   the request-target. For example,
2367</t>
2368<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2369CONNECT www.example.com:80 HTTP/1.1
2370</artwork></figure>
2371<t anchor="asterisk-form"><iref item="asterisk-form (of request-target)"/>
2372  <x:h>asterisk-form</x:h>
2373</t>
2374<t>
2375   The <x:dfn>asterisk-form</x:dfn> of request-target is only used for a server-wide
2376   OPTIONS request (&OPTIONS;).  When a client wishes to request OPTIONS
2377   for the server as a whole, as opposed to a specific named resource of
2378   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2379   For example,
2380</t>
2381<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2382OPTIONS * HTTP/1.1
2383</artwork></figure>
2384<t>
2385   If a proxy receives an OPTIONS request with an absolute-form of
2386   request-target in which the URI has an empty path and no query component,
2387   then the last proxy on the request chain &MUST; send a request-target
2388   of "*" when it forwards the request to the indicated origin server.
2389</t>
2390<figure><preamble>  
2391   For example, the request
2392</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2393OPTIONS http://www.example.org:8001 HTTP/1.1
2394</artwork></figure>
2395<figure><preamble>  
2396  would be forwarded by the final proxy as
2397</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2398OPTIONS * HTTP/1.1
2399Host: www.example.org:8001
2400</artwork>
2401<postamble>
2402   after connecting to port 8001 of host "www.example.org".
2403</postamble>
2404</figure>
2405</section>
2406
2407<section title="Host" anchor="header.host">
2408  <iref primary="true" item="Host header field" x:for-anchor=""/>
2409  <x:anchor-alias value="Host"/>
2410<t>
2411   The "Host" header field in a request provides the host and port
2412   information from the target URI, enabling the origin
2413   server to distinguish among resources while servicing requests
2414   for multiple host names on a single IP address.
2415</t>
2416<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2417  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2418</artwork></figure>
2419<t>
2420   A client &MUST; send a Host header field in all HTTP/1.1 request messages.
2421   If the target URI includes an authority component, then a client &MUST;
2422   send a field-value for Host that is identical to that authority
2423   component, excluding any userinfo subcomponent and its "@" delimiter
2424   (<xref target="http.uri"/>).
2425   If the authority component is missing or undefined for the target URI,
2426   then a client &MUST; send a Host header field with an empty field-value.
2427</t>
2428<t>
2429   Since the Host field-value is critical information for handling a request,
2430   a user agent &SHOULD; generate Host as the first header field following the
2431   request-line.
2432</t>
2433<t>
2434   For example, a GET request to the origin server for
2435   &lt;http://www.example.org/pub/WWW/&gt; would begin with:
2436</t>
2437<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2438GET /pub/WWW/ HTTP/1.1
2439Host: www.example.org
2440</artwork></figure>
2441<t>
2442   A client &MUST; send a Host header field in an HTTP/1.1 request even
2443   if the request-target is in the absolute-form, since this
2444   allows the Host information to be forwarded through ancient HTTP/1.0
2445   proxies that might not have implemented Host.
2446</t>
2447<t>
2448   When a proxy receives a request with an absolute-form of
2449   request-target, the proxy &MUST; ignore the received
2450   Host header field (if any) and instead replace it with the host
2451   information of the request-target.  A proxy that forwards such a request
2452   &MUST; generate a new Host field-value based on the received
2453   request-target rather than forward the received Host field-value.
2454</t>
2455<t>
2456   Since the Host header field acts as an application-level routing
2457   mechanism, it is a frequent target for malware seeking to poison
2458   a shared cache or redirect a request to an unintended server.
2459   An interception proxy is particularly vulnerable if it relies on
2460   the Host field-value for redirecting requests to internal
2461   servers, or for use as a cache key in a shared cache, without
2462   first verifying that the intercepted connection is targeting a
2463   valid IP address for that host.
2464</t>
2465<t>
2466   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2467   to any HTTP/1.1 request message that lacks a Host header field and
2468   to any request message that contains more than one Host header field
2469   or a Host header field with an invalid field-value.
2470</t>
2471</section>
2472
2473<section title="Effective Request URI" anchor="effective.request.uri">
2474  <iref primary="true" item="effective request URI"/>
2475  <x:anchor-alias value="effective request URI"/>
2476<t>
2477   A server that receives an HTTP request message &MUST; reconstruct
2478   the user agent's original target URI, based on the pieces of information
2479   learned from the request-target, <x:ref>Host</x:ref> header field, and
2480   connection context, in order to identify the intended target resource and
2481   properly service the request. The URI derived from this reconstruction
2482   process is referred to as the "<x:dfn>effective request URI</x:dfn>".
2483</t>
2484<t>
2485   For a user agent, the effective request URI is the target URI.
2486</t>
2487<t>
2488   If the request-target is in absolute-form, then the effective request URI
2489   is the same as the request-target.  Otherwise, the effective request URI
2490   is constructed as follows.
2491</t>
2492<t>
2493   If the request is received over a TLS-secured TCP connection,
2494   then the effective request URI's scheme is "https"; otherwise, the
2495   scheme is "http".
2496</t>
2497<t>
2498   If the request-target is in authority-form, then the effective
2499   request URI's authority component is the same as the request-target.
2500   Otherwise, if a <x:ref>Host</x:ref> header field is supplied with a
2501   non-empty field-value, then the authority component is the same as the
2502   Host field-value. Otherwise, the authority component is the concatenation of
2503   the default host name configured for the server, a colon (":"), and the
2504   connection's incoming TCP port number in decimal form.
2505</t>
2506<t>
2507   If the request-target is in authority-form or asterisk-form, then the
2508   effective request URI's combined path and query component is empty.
2509   Otherwise, the combined path and query component is the same as the
2510   request-target.
2511</t>
2512<t>
2513   The components of the effective request URI, once determined as above,
2514   can be combined into absolute-URI form by concatenating the scheme,
2515   "://", authority, and combined path and query component.
2516</t>
2517<figure>
2518<preamble>
2519   Example 1: the following message received over an insecure TCP connection
2520</preamble>
2521<artwork type="example" x:indent-with="  ">
2522GET /pub/WWW/TheProject.html HTTP/1.1
2523Host: www.example.org:8080
2524</artwork>
2525</figure>
2526<figure>
2527<preamble>
2528  has an effective request URI of
2529</preamble>
2530<artwork type="example" x:indent-with="  ">
2531http://www.example.org:8080/pub/WWW/TheProject.html
2532</artwork>
2533</figure>
2534<figure>
2535<preamble>
2536   Example 2: the following message received over a TLS-secured TCP connection
2537</preamble>
2538<artwork type="example" x:indent-with="  ">
2539OPTIONS * HTTP/1.1
2540Host: www.example.org
2541</artwork>
2542</figure>
2543<figure>
2544<preamble>
2545  has an effective request URI of
2546</preamble>
2547<artwork type="example" x:indent-with="  ">
2548https://www.example.org
2549</artwork>
2550</figure>
2551<t>
2552   An origin server that does not allow resources to differ by requested
2553   host &MAY; ignore the <x:ref>Host</x:ref> field-value and instead replace it
2554   with a configured server name when constructing the effective request URI.
2555</t>
2556<t>
2557   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2558   field &MAY; attempt to use heuristics (e.g., examination of the URI path for
2559   something unique to a particular host) in order to guess the
2560   effective request URI's authority component.
2561</t>
2562</section>
2563
2564<section title="Associating a Response to a Request" anchor="associating.response.to.request">
2565<t>
2566   HTTP does not include a request identifier for associating a given
2567   request message with its corresponding one or more response messages.
2568   Hence, it relies on the order of response arrival to correspond exactly
2569   to the order in which requests are made on the same connection.
2570   More than one response message per request only occurs when one or more
2571   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a
2572   final response to the same request.
2573</t>
2574<t>
2575   A client that has more than one outstanding request on a connection &MUST;
2576   maintain a list of outstanding requests in the order sent and &MUST;
2577   associate each received response message on that connection to the highest
2578   ordered request that has not yet received a final (non-<x:ref>1xx</x:ref>)
2579   response.
2580</t>
2581</section>
2582
2583<section title="Message Forwarding" anchor="message.forwarding">
2584<t>
2585   As described in <xref target="intermediaries"/>, intermediaries can serve
2586   a variety of roles in the processing of HTTP requests and responses.
2587   Some intermediaries are used to improve performance or availability.
2588   Others are used for access control or to filter content.
2589   Since an HTTP stream has characteristics similar to a pipe-and-filter
2590   architecture, there are no inherent limits to the extent an intermediary
2591   can enhance (or interfere) with either direction of the stream.
2592</t>
2593<t>
2594   An intermediary not acting as a tunnel &MUST; implement the
2595   <x:ref>Connection</x:ref> header field, as specified in
2596   <xref target="header.connection"/>, and exclude fields from being forwarded
2597   that are only intended for the incoming connection.
2598</t>
2599<t>
2600   An intermediary &MUST-NOT; forward a message to itself unless it is
2601   protected from an infinite request loop. In general, an intermediary ought
2602   to recognize its own server names, including any aliases, local variations,
2603   or literal IP addresses, and respond to such requests directly.
2604</t>
2605
2606<section title="Via" anchor="header.via">
2607  <iref primary="true" item="Via header field" x:for-anchor=""/>
2608  <x:anchor-alias value="pseudonym"/>
2609  <x:anchor-alias value="received-by"/>
2610  <x:anchor-alias value="received-protocol"/>
2611  <x:anchor-alias value="Via"/>
2612<t>
2613   The "Via" header field indicates the presence of intermediate protocols and
2614   recipients between the user agent and the server (on requests) or between
2615   the origin server and the client (on responses), similar to the
2616   "Received" header field in email
2617   (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2618   Via can be used for tracking message forwards,
2619   avoiding request loops, and identifying the protocol capabilities of
2620   senders along the request/response chain.
2621</t>
2622<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"/>
2623  <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> ] )
2624
2625  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2626                      ; see <xref target="header.upgrade"/>
2627  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2628  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2629</artwork></figure>
2630<t>
2631   Multiple Via field values represent each proxy or gateway that has
2632   forwarded the message. Each intermediary appends its own information
2633   about how the message was received, such that the end result is ordered
2634   according to the sequence of forwarding recipients.
2635</t>
2636<t>
2637   A proxy &MUST; send an appropriate Via header field, as described below, in
2638   each message that it forwards.
2639   An HTTP-to-HTTP gateway &MUST; send an appropriate Via header field in
2640   each inbound request message and &MAY; send a Via header field in
2641   forwarded response messages.
2642</t>
2643<t>
2644   For each intermediary, the received-protocol indicates the protocol and
2645   protocol version used by the upstream sender of the message. Hence, the
2646   Via field value records the advertised protocol capabilities of the
2647   request/response chain such that they remain visible to downstream
2648   recipients; this can be useful for determining what backwards-incompatible
2649   features might be safe to use in response, or within a later request, as
2650   described in <xref target="http.version"/>. For brevity, the protocol-name
2651   is omitted when the received protocol is HTTP.
2652</t>
2653<t>
2654   The received-by field is normally the host and optional port number of a
2655   recipient server or client that subsequently forwarded the message.
2656   However, if the real host is considered to be sensitive information, a
2657   sender &MAY; replace it with a pseudonym. If a port is not provided,
2658   a recipient &MAY; interpret that as meaning it was received on the default
2659   TCP port, if any, for the received-protocol.
2660</t>
2661<t>
2662   A sender &MAY; generate comments in the Via header field to identify the
2663   software of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2664   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2665   are optional and a recipient &MAY; remove them prior to forwarding the
2666   message.
2667</t>
2668<t>
2669   For example, a request message could be sent from an HTTP/1.0 user
2670   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2671   forward the request to a public proxy at p.example.net, which completes
2672   the request by forwarding it to the origin server at www.example.com.
2673   The request received by www.example.com would then have the following
2674   Via header field:
2675</t>
2676<figure><artwork type="example">
2677  Via: 1.0 fred, 1.1 p.example.net
2678</artwork></figure>
2679<t>
2680   An intermediary used as a portal through a network firewall
2681   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2682   region unless it is explicitly enabled to do so. If not enabled, such an
2683   intermediary &SHOULD; replace each received-by host of any host behind the
2684   firewall by an appropriate pseudonym for that host.
2685</t>
2686<t>
2687   An intermediary &MAY; combine an ordered subsequence of Via header
2688   field entries into a single such entry if the entries have identical
2689   received-protocol values. For example,
2690</t>
2691<figure><artwork type="example">
2692  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2693</artwork></figure>
2694<t>
2695  could be collapsed to
2696</t>
2697<figure><artwork type="example">
2698  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2699</artwork></figure>
2700<t>
2701   Senders &SHOULD-NOT; combine multiple entries unless they are all
2702   under the same organizational control and the hosts have already been
2703   replaced by pseudonyms. Senders &MUST-NOT; combine entries that
2704   have different received-protocol values.
2705</t>
2706</section>
2707
2708<section title="Transformations" anchor="message.transformations">
2709<t>
2710   Some intermediaries include features for transforming messages and their
2711   payloads.  A transforming proxy might, for example, convert between image
2712   formats in order to save cache space or to reduce the amount of traffic on
2713   a slow link. However, operational problems might occur when these
2714   transformations are applied to payloads intended for critical applications,
2715   such as medical imaging or scientific data analysis, particularly when
2716   integrity checks or digital signatures are used to ensure that the payload
2717   received is identical to the original.
2718</t>
2719<t>
2720   If a proxy receives a request-target with a host name that is not a
2721   fully qualified domain name, it &MAY; add its own domain to the host name
2722   it received when forwarding the request.  A proxy &MUST-NOT; change the
2723   host name if it is a fully qualified domain name.
2724</t>
2725<t>
2726   A proxy &MUST-NOT; modify the "absolute-path" and "query" parts of the
2727   received request-target when forwarding it to the next inbound server,
2728   except as noted above to replace an empty path with "/" or "*".
2729</t>
2730<t>
2731   A proxy &MUST-NOT; modify header fields that provide information about the
2732   end points of the communication chain, the resource state, or the selected
2733   representation. A proxy &MAY; change the message body through application
2734   or removal of a transfer coding (<xref target="transfer.codings"/>).
2735</t>
2736<t>
2737   A non-transforming proxy &MUST-NOT; modify the message payload (&payload;).
2738   A transforming proxy &MUST-NOT; modify the payload of a message that
2739   contains the no-transform cache-control directive.
2740</t>
2741<t>
2742   A transforming proxy &MAY; transform the payload of a message
2743   that does not contain the no-transform cache-control directive;
2744   if the payload is transformed, the transforming proxy &MUST; add a
2745   Warning header field with the warn-code of 214 ("Transformation Applied")
2746   if one does not already appear in the message (see &header-warning;).
2747   If the payload of a <x:ref>200 (OK)</x:ref> response is transformed, the
2748   transforming proxy can also inform downstream recipients that a
2749   transformation has been applied by changing the response status code to
2750   <x:ref>203 (Non-Authoritative Information)</x:ref> (&status-203;).
2751</t>
2752</section>
2753</section>
2754</section>
2755
2756<section title="Connection Management" anchor="connection.management">
2757<t>
2758   HTTP messaging is independent of the underlying transport or
2759   session-layer connection protocol(s).  HTTP only presumes a reliable
2760   transport with in-order delivery of requests and the corresponding
2761   in-order delivery of responses.  The mapping of HTTP request and
2762   response structures onto the data units of an underlying transport
2763   protocol is outside the scope of this specification.
2764</t>
2765<t>
2766   As described in <xref target="connecting.inbound"/>, the specific
2767   connection protocols to be used for an HTTP interaction are determined by
2768   client configuration and the <x:ref>target URI</x:ref>.
2769   For example, the "http" URI scheme
2770   (<xref target="http.uri"/>) indicates a default connection of TCP
2771   over IP, with a default TCP port of 80, but the client might be
2772   configured to use a proxy via some other connection, port, or protocol.
2773</t>
2774<t>
2775   HTTP implementations are expected to engage in connection management,
2776   which includes maintaining the state of current connections,
2777   establishing a new connection or reusing an existing connection,
2778   processing messages received on a connection, detecting connection
2779   failures, and closing each connection.
2780   Most clients maintain multiple connections in parallel, including
2781   more than one connection per server endpoint.
2782   Most servers are designed to maintain thousands of concurrent connections,
2783   while controlling request queues to enable fair use and detect
2784   denial of service attacks.
2785</t>
2786
2787<section title="Connection" anchor="header.connection">
2788  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2789  <iref primary="true" item="close" x:for-anchor=""/>
2790  <x:anchor-alias value="Connection"/>
2791  <x:anchor-alias value="connection-option"/>
2792  <x:anchor-alias value="close"/>
2793<t>
2794   The "Connection" header field allows the sender to indicate desired
2795   control options for the current connection.  In order to avoid confusing
2796   downstream recipients, a proxy or gateway &MUST; remove or replace any
2797   received connection options before forwarding the message.
2798</t>
2799<t>
2800   When a header field aside from Connection is used to supply control
2801   information for or about the current connection, the sender &MUST; list
2802   the corresponding field-name within the "Connection" header field.
2803   A proxy or gateway &MUST; parse a received Connection
2804   header field before a message is forwarded and, for each
2805   connection-option in this field, remove any header field(s) from
2806   the message with the same name as the connection-option, and then
2807   remove the Connection header field itself (or replace it with the
2808   intermediary's own connection options for the forwarded message).
2809</t>
2810<t>
2811   Hence, the Connection header field provides a declarative way of
2812   distinguishing header fields that are only intended for the
2813   immediate recipient ("hop-by-hop") from those fields that are
2814   intended for all recipients on the chain ("end-to-end"), enabling the
2815   message to be self-descriptive and allowing future connection-specific
2816   extensions to be deployed without fear that they will be blindly
2817   forwarded by older intermediaries.
2818</t>
2819<t>
2820   The Connection header field's value has the following grammar:
2821</t>
2822<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2823  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2824  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2825</artwork></figure>
2826<t>
2827   Connection options are case-insensitive.
2828</t>
2829<t>
2830   A sender &MUST-NOT; send a connection option corresponding to a header
2831   field that is intended for all recipients of the payload.
2832   For example, <x:ref>Cache-Control</x:ref> is never appropriate as a
2833   connection option (&header-cache-control;).
2834</t>
2835<t>
2836   The connection options do not have to correspond to a header field
2837   present in the message, since a connection-specific header field
2838   might not be needed if there are no parameters associated with that
2839   connection option.  Recipients that trigger certain connection
2840   behavior based on the presence of connection options &MUST; do so
2841   based on the presence of the connection-option rather than only the
2842   presence of the optional header field.  In other words, if the
2843   connection option is received as a header field but not indicated
2844   within the Connection field-value, then the recipient &MUST; ignore
2845   the connection-specific header field because it has likely been
2846   forwarded by an intermediary that is only partially conformant.
2847</t>
2848<t>
2849   When defining new connection options, specifications ought to
2850   carefully consider existing deployed header fields and ensure
2851   that the new connection option does not share the same name as
2852   an unrelated header field that might already be deployed.
2853   Defining a new connection option essentially reserves that potential
2854   field-name for carrying additional information related to the
2855   connection option, since it would be unwise for senders to use
2856   that field-name for anything else.
2857</t>
2858<t>
2859   The "<x:dfn>close</x:dfn>" connection option is defined for a
2860   sender to signal that this connection will be closed after completion of
2861   the response. For example,
2862</t>
2863<figure><artwork type="example">
2864  Connection: close
2865</artwork></figure>
2866<t>
2867   in either the request or the response header fields indicates that the
2868   sender is going to close the connection after the current request/response
2869   is complete (<xref target="persistent.tear-down"/>).
2870</t>
2871<t>
2872   A client that does not support <x:ref>persistent connections</x:ref> &MUST;
2873   send the "close" connection option in every request message.
2874</t>
2875<t>
2876   A server that does not support <x:ref>persistent connections</x:ref> &MUST;
2877   send the "close" connection option in every response message that
2878   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2879</t>
2880</section>
2881
2882<section title="Establishment" anchor="persistent.establishment">
2883<t>
2884   It is beyond the scope of this specification to describe how connections
2885   are established via various transport or session-layer protocols.
2886   Each connection applies to only one transport link.
2887</t>
2888</section>
2889
2890<section title="Persistence" anchor="persistent.connections">
2891   <x:anchor-alias value="persistent connections"/>
2892<t>
2893   HTTP/1.1 defaults to the use of "<x:dfn>persistent connections</x:dfn>",
2894   allowing multiple requests and responses to be carried over a single
2895   connection. The "<x:ref>close</x:ref>" connection-option is used to signal
2896   that a connection will not persist after the current request/response.
2897   HTTP implementations &SHOULD; support persistent connections.
2898</t>
2899<t>
2900   A recipient determines whether a connection is persistent or not based on
2901   the most recently received message's protocol version and
2902   <x:ref>Connection</x:ref> header field (if any):
2903   <list style="symbols">
2904     <t>If the <x:ref>close</x:ref> connection option is present, the
2905        connection will not persist after the current response; else,</t>
2906     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2907        persist after the current response; else,</t>
2908     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2909        connection option is present, the recipient is not a proxy, and
2910        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2911        the connection will persist after the current response; otherwise,</t>
2912     <t>The connection will close after the current response.</t>
2913   </list>
2914</t>
2915<t>
2916   A server &MAY; assume that an HTTP/1.1 client intends to maintain a
2917   persistent connection until a <x:ref>close</x:ref> connection option
2918   is received in a request.
2919</t>
2920<t>
2921   A client &MAY; reuse a persistent connection until it sends or receives
2922   a <x:ref>close</x:ref> connection option or receives an HTTP/1.0 response
2923   without a "keep-alive" connection option.
2924</t>
2925<t>
2926   In order to remain persistent, all messages on a connection need to
2927   have a self-defined message length (i.e., one not defined by closure
2928   of the connection), as described in <xref target="message.body"/>.
2929   A server &MUST; read the entire request message body or close
2930   the connection after sending its response, since otherwise the
2931   remaining data on a persistent connection would be misinterpreted
2932   as the next request.  Likewise,
2933   a client &MUST; read the entire response message body if it intends
2934   to reuse the same connection for a subsequent request.
2935</t>
2936<t>
2937   A proxy server &MUST-NOT; maintain a persistent connection with an
2938   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
2939   information and discussion of the problems with the Keep-Alive header field
2940   implemented by many HTTP/1.0 clients).
2941</t>
2942<t>
2943   Clients and servers &SHOULD-NOT; assume that a persistent connection is
2944   maintained for HTTP versions less than 1.1 unless it is explicitly
2945   signaled.
2946   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
2947   for more information on backward compatibility with HTTP/1.0 clients.
2948</t>
2949
2950<section title="Retrying Requests" anchor="persistent.retrying.requests">
2951<t>
2952   Connections can be closed at any time, with or without intention.
2953   Implementations ought to anticipate the need to recover
2954   from asynchronous close events.
2955</t>
2956<t>
2957   When an inbound connection is closed prematurely, a client &MAY; open a new
2958   connection and automatically retransmit an aborted sequence of requests if
2959   all of those requests have idempotent methods (&idempotent-methods;).
2960   A proxy &MUST-NOT; automatically retry non-idempotent requests.
2961</t>
2962<t>
2963   A user agent &MUST-NOT; automatically retry a request with a non-idempotent
2964   method unless it has some means to know that the request semantics are
2965   actually idempotent, regardless of the method, or some means to detect that
2966   the original request was never applied. For example, a user agent that
2967   knows (through design or configuration) that a POST request to a given
2968   resource is safe can repeat that request automatically.
2969   Likewise, a user agent designed specifically to operate on a version
2970   control repository might be able to recover from partial failure conditions
2971   by checking the target resource revision(s) after a failed connection,
2972   reverting or fixing any changes that were partially applied, and then
2973   automatically retrying the requests that failed.
2974</t>
2975<t>
2976   A client &SHOULD-NOT; automatically retry a failed automatic retry.
2977</t>
2978</section>
2979
2980<section title="Pipelining" anchor="pipelining">
2981   <x:anchor-alias value="pipeline"/>
2982<t>
2983   A client that supports persistent connections &MAY; "<x:dfn>pipeline</x:dfn>"
2984   its requests (i.e., send multiple requests without waiting for each
2985   response). A server &MAY; process a sequence of pipelined requests in
2986   parallel if they all have safe methods (&safe-methods;), but &MUST; send
2987   the corresponding responses in the same order that the requests were
2988   received.
2989</t>
2990<t>
2991   A client that pipelines requests &SHOULD; retry unanswered requests if the
2992   connection closes before it receives all of the corresponding responses.
2993   When retrying pipelined requests after a failed connection (a connection
2994   not explicitly closed by the server in its last complete response), a
2995   client &MUST-NOT; pipeline immediately after connection establishment,
2996   since the first remaining request in the prior pipeline might have caused
2997   an error response that can be lost again if multiple requests are sent on a
2998   prematurely closed connection (see the TCP reset problem described in
2999   <xref target="persistent.tear-down"/>).
3000</t>
3001<t>
3002   Idempotent methods (&idempotent-methods;) are significant to pipelining
3003   because they can be automatically retried after a connection failure.
3004   A user agent &SHOULD-NOT; pipeline requests after a non-idempotent method,
3005   until the final response status code for that method has been received,
3006   unless the user agent has a means to detect and recover from partial
3007   failure conditions involving the pipelined sequence.
3008</t>
3009<t>
3010   An intermediary that receives pipelined requests &MAY; pipeline those
3011   requests when forwarding them inbound, since it can rely on the outbound
3012   user agent(s) to determine what requests can be safely pipelined. If the
3013   inbound connection fails before receiving a response, the pipelining
3014   intermediary &MAY; attempt to retry a sequence of requests that have yet
3015   to receive a response if the requests all have idempotent methods;
3016   otherwise, the pipelining intermediary &SHOULD; forward any received
3017   responses and then close the corresponding outbound connection(s) so that
3018   the outbound user agent(s) can recover accordingly.
3019</t>
3020</section>
3021</section>
3022   
3023<section title="Concurrency" anchor="persistent.concurrency">
3024<t>
3025   Clients &SHOULD; limit the number of simultaneous
3026   connections that they maintain to a given server.
3027</t>
3028<t>
3029   Previous revisions of HTTP gave a specific number of connections as a
3030   ceiling, but this was found to be impractical for many applications. As a
3031   result, this specification does not mandate a particular maximum number of
3032   connections, but instead encourages clients to be conservative when opening
3033   multiple connections.
3034</t>
3035<t>
3036   Multiple connections are typically used to avoid the "head-of-line
3037   blocking" problem, wherein a request that takes significant server-side
3038   processing and/or has a large payload blocks subsequent requests on the
3039   same connection. However, each connection consumes server resources.
3040   Furthermore, using multiple connections can cause undesirable side effects
3041   in congested networks.
3042</t>
3043<t>
3044   Note that servers might reject traffic that they deem abusive, including an
3045   excessive number of connections from a client.
3046</t>
3047</section>
3048
3049<section title="Failures and Time-outs" anchor="persistent.failures">
3050<t>
3051   Servers will usually have some time-out value beyond which they will
3052   no longer maintain an inactive connection. Proxy servers might make
3053   this a higher value since it is likely that the client will be making
3054   more connections through the same server. The use of persistent
3055   connections places no requirements on the length (or existence) of
3056   this time-out for either the client or the server.
3057</t>
3058<t>
3059   When a client or server wishes to time-out it &SHOULD; issue a graceful
3060   close on the transport connection. Clients and servers &SHOULD; both
3061   constantly watch for the other side of the transport close, and
3062   respond to it as appropriate. If a client or server does not detect
3063   the other side's close promptly it could cause unnecessary resource
3064   drain on the network.
3065</t>
3066<t>
3067   A client, server, or proxy &MAY; close the transport connection at any
3068   time. For example, a client might have started to send a new request
3069   at the same time that the server has decided to close the "idle"
3070   connection. From the server's point of view, the connection is being
3071   closed while it was idle, but from the client's point of view, a
3072   request is in progress.
3073</t>
3074<t>
3075   Servers &SHOULD; maintain persistent connections and allow the underlying
3076   transport's flow control mechanisms to resolve temporary overloads, rather
3077   than terminate connections with the expectation that clients will retry.
3078   The latter technique can exacerbate network congestion.
3079</t>
3080<t>
3081   A client sending a message body &SHOULD; monitor
3082   the network connection for an error response while it is transmitting
3083   the request. If the client sees an error response, it &SHOULD;
3084   immediately cease transmitting the body and close the connection.
3085</t>
3086</section>
3087   
3088<section title="Tear-down" anchor="persistent.tear-down">
3089  <iref primary="false" item="Connection header field" x:for-anchor=""/>
3090  <iref primary="false" item="close" x:for-anchor=""/>
3091<t>
3092   The <x:ref>Connection</x:ref> header field
3093   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
3094   connection option that a sender &SHOULD; send when it wishes to close
3095   the connection after the current request/response pair.
3096</t>
3097<t>
3098   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
3099   send further requests on that connection (after the one containing
3100   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
3101   final response message corresponding to this request.
3102</t>
3103<t>
3104   A server that receives a <x:ref>close</x:ref> connection option &MUST;
3105   initiate a close of the connection (see below) after it sends the
3106   final response to the request that contained <x:ref>close</x:ref>.
3107   The server &SHOULD; send a <x:ref>close</x:ref> connection option
3108   in its final response on that connection. The server &MUST-NOT; process
3109   any further requests received on that connection.
3110</t>
3111<t>
3112   A server that sends a <x:ref>close</x:ref> connection option &MUST;
3113   initiate a close of the connection (see below) after it sends the
3114   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
3115   any further requests received on that connection.
3116</t>
3117<t>
3118   A client that receives a <x:ref>close</x:ref> connection option &MUST;
3119   cease sending requests on that connection and close the connection
3120   after reading the response message containing the close; if additional
3121   pipelined requests had been sent on the connection, the client &SHOULD-NOT;
3122   assume that they will be processed by the server.
3123</t>
3124<t>
3125   If a server performs an immediate close of a TCP connection, there is a
3126   significant risk that the client will not be able to read the last HTTP
3127   response.  If the server receives additional data from the client on a
3128   fully-closed connection, such as another request that was sent by the
3129   client before receiving the server's response, the server's TCP stack will
3130   send a reset packet to the client; unfortunately, the reset packet might
3131   erase the client's unacknowledged input buffers before they can be read
3132   and interpreted by the client's HTTP parser.
3133</t>
3134<t>
3135   To avoid the TCP reset problem, servers typically close a connection in
3136   stages. First, the server performs a half-close by closing only the write
3137   side of the read/write connection. The server then continues to read from
3138   the connection until it receives a corresponding close by the client, or
3139   until the server is reasonably certain that its own TCP stack has received
3140   the client's acknowledgement of the packet(s) containing the server's last
3141   response. Finally, the server fully closes the connection.
3142</t>
3143<t>
3144   It is unknown whether the reset problem is exclusive to TCP or might also
3145   be found in other transport connection protocols.
3146</t>
3147</section>
3148
3149<section title="Upgrade" anchor="header.upgrade">
3150  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3151  <x:anchor-alias value="Upgrade"/>
3152  <x:anchor-alias value="protocol"/>
3153  <x:anchor-alias value="protocol-name"/>
3154  <x:anchor-alias value="protocol-version"/>
3155<t>
3156   The "Upgrade" header field is intended to provide a simple mechanism
3157   for transitioning from HTTP/1.1 to some other protocol on the same
3158   connection.  A client &MAY; send a list of protocols in the Upgrade
3159   header field of a request to invite the server to switch to one or
3160   more of those protocols, in order of descending preference, before sending
3161   the final response. A server &MAY; ignore a received Upgrade header field
3162   if it wishes to continue using the current protocol on that connection.
3163</t>
3164<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3165  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3166
3167  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3168  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3169  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3170</artwork></figure>
3171<t>
3172   A server that sends a <x:ref>101 (Switching Protocols)</x:ref> response
3173   &MUST; send an Upgrade header field to indicate the new protocol(s) to
3174   which the connection is being switched; if multiple protocol layers are
3175   being switched, the sender &MUST; list the protocols in layer-ascending
3176   order. A server &MUST-NOT; switch to a protocol that was not indicated by
3177   the client in the corresponding request's Upgrade header field.
3178   A server &MAY; choose to ignore the order of preference indicated by the
3179   client and select the new protocol(s) based on other factors, such as the
3180   nature of the request or the current load on the server.
3181</t>
3182<t>
3183   A server that sends a <x:ref>426 (Upgrade Required)</x:ref> response
3184   &MUST; send an Upgrade header field to indicate the acceptable protocols,
3185   in order of descending preference.
3186</t>
3187<t>
3188   A server &MAY; send an Upgrade header field in any other response to
3189   advertise that it implements support for upgrading to the listed protocols,
3190   in order of descending preference, when appropriate for a future request.
3191</t>
3192<figure><preamble>
3193   The following is a hypothetical example sent by a client:
3194</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
3195GET /hello.txt HTTP/1.1
3196Host: www.example.com
3197Connection: upgrade
3198Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3199
3200</artwork></figure>
3201<t>
3202   Upgrade cannot be used to insist on a protocol change; its acceptance and
3203   use by the server is optional. The capabilities and nature of the
3204   application-level communication after the protocol change is entirely
3205   dependent upon the new protocol(s) chosen. However, immediately after
3206   sending the 101 response, the server is expected to continue responding to
3207   the original request as if it had received its equivalent within the new
3208   protocol (i.e., the server still has an outstanding request to satisfy
3209   after the protocol has been changed, and is expected to do so without
3210   requiring the request to be repeated).
3211</t>
3212<t>
3213   For example, if the Upgrade header field is received in a GET request
3214   and the server decides to switch protocols, it first responds
3215   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3216   then immediately follows that with the new protocol's equivalent of a
3217   response to a GET on the target resource.  This allows a connection to be
3218   upgraded to protocols with the same semantics as HTTP without the
3219   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3220   protocols unless the received message semantics can be honored by the new
3221   protocol; an OPTIONS request can be honored by any protocol.
3222</t>
3223<figure><preamble>
3224   The following is an example response to the above hypothetical request:
3225</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
3226HTTP/1.1 101 Switching Protocols
3227Connection: upgrade
3228Upgrade: HTTP/2.0
3229
3230[... data stream switches to HTTP/2.0 with an appropriate response
3231(as defined by new protocol) to the "GET /hello.txt" request ...]
3232</artwork></figure>
3233<t>
3234   When Upgrade is sent, the sender &MUST; also send a
3235   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3236   that contains an "upgrade" connection option, in order to prevent Upgrade
3237   from being accidentally forwarded by intermediaries that might not implement
3238   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3239   is received in an HTTP/1.0 request.
3240</t>
3241<t>
3242   The Upgrade header field only applies to switching protocols on top of the
3243   existing connection; it cannot be used to switch the underlying connection
3244   (transport) protocol, nor to switch the existing communication to a
3245   different connection. For those purposes, it is more appropriate to use a
3246   <x:ref>3xx (Redirection)</x:ref> response (&status-3xx;).
3247</t>
3248<t>
3249   This specification only defines the protocol name "HTTP" for use by
3250   the family of Hypertext Transfer Protocols, as defined by the HTTP
3251   version rules of <xref target="http.version"/> and future updates to this
3252   specification. Additional tokens ought to be registered with IANA using the
3253   registration procedure defined in <xref target="upgrade.token.registry"/>.
3254</t>
3255</section>
3256</section>
3257
3258<section title="IANA Considerations" anchor="IANA.considerations">
3259
3260<section title="Header Field Registration" anchor="header.field.registration">
3261<t>
3262   HTTP header fields are registered within the Message Header Field Registry
3263   maintained at
3264   <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
3265</t>
3266<t>
3267   This document defines the following HTTP header fields, so their
3268   associated registry entries shall be updated according to the permanent
3269   registrations below (see <xref target="BCP90"/>):
3270</t>
3271<?BEGININC p1-messaging.iana-headers ?>
3272<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3273<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3274   <ttcol>Header Field Name</ttcol>
3275   <ttcol>Protocol</ttcol>
3276   <ttcol>Status</ttcol>
3277   <ttcol>Reference</ttcol>
3278
3279   <c>Connection</c>
3280   <c>http</c>
3281   <c>standard</c>
3282   <c>
3283      <xref target="header.connection"/>
3284   </c>
3285   <c>Content-Length</c>
3286   <c>http</c>
3287   <c>standard</c>
3288   <c>
3289      <xref target="header.content-length"/>
3290   </c>
3291   <c>Host</c>
3292   <c>http</c>
3293   <c>standard</c>
3294   <c>
3295      <xref target="header.host"/>
3296   </c>
3297   <c>TE</c>
3298   <c>http</c>
3299   <c>standard</c>
3300   <c>
3301      <xref target="header.te"/>
3302   </c>
3303   <c>Trailer</c>
3304   <c>http</c>
3305   <c>standard</c>
3306   <c>
3307      <xref target="header.trailer"/>
3308   </c>
3309   <c>Transfer-Encoding</c>
3310   <c>http</c>
3311   <c>standard</c>
3312   <c>
3313      <xref target="header.transfer-encoding"/>
3314   </c>
3315   <c>Upgrade</c>
3316   <c>http</c>
3317   <c>standard</c>
3318   <c>
3319      <xref target="header.upgrade"/>
3320   </c>
3321   <c>Via</c>
3322   <c>http</c>
3323   <c>standard</c>
3324   <c>
3325      <xref target="header.via"/>
3326   </c>
3327</texttable>
3328<!--(END)-->
3329<?ENDINC p1-messaging.iana-headers ?>
3330<t>
3331   Furthermore, the header field-name "Close" shall be registered as
3332   "reserved", since using that name as an HTTP header field might
3333   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3334   header field (<xref target="header.connection"/>).
3335</t>
3336<texttable align="left" suppress-title="true">
3337   <ttcol>Header Field Name</ttcol>
3338   <ttcol>Protocol</ttcol>
3339   <ttcol>Status</ttcol>
3340   <ttcol>Reference</ttcol>
3341
3342   <c>Close</c>
3343   <c>http</c>
3344   <c>reserved</c>
3345   <c>
3346      <xref target="header.field.registration"/>
3347   </c>
3348</texttable>
3349<t>
3350   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3351</t>
3352</section>
3353
3354<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3355<t>
3356   IANA maintains the registry of URI Schemes <xref target="BCP115"/> at
3357   <eref target="http://www.iana.org/assignments/uri-schemes.html"/>.
3358</t>
3359<t>
3360   This document defines the following URI schemes, so their
3361   associated registry entries shall be updated according to the permanent
3362   registrations below:
3363</t>
3364<texttable align="left" suppress-title="true">
3365   <ttcol>URI Scheme</ttcol>
3366   <ttcol>Description</ttcol>
3367   <ttcol>Reference</ttcol>
3368
3369   <c>http</c>
3370   <c>Hypertext Transfer Protocol</c>
3371   <c><xref target="http.uri"/></c>
3372
3373   <c>https</c>
3374   <c>Hypertext Transfer Protocol Secure</c>
3375   <c><xref target="https.uri"/></c>
3376</texttable>
3377</section>
3378
3379<section title="Internet Media Type Registration" anchor="internet.media.type.http">
3380<t>
3381   This document serves as the specification for the Internet media types
3382   "message/http" and "application/http". The following is to be registered with
3383   IANA (see <xref target="BCP13"/>).
3384</t>
3385<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
3386<iref item="Media Type" subitem="message/http" primary="true"/>
3387<iref item="message/http Media Type" primary="true"/>
3388<t>
3389   The message/http type can be used to enclose a single HTTP request or
3390   response message, provided that it obeys the MIME restrictions for all
3391   "message" types regarding line length and encodings.
3392</t>
3393<t>
3394  <list style="hanging" x:indent="12em">
3395    <t hangText="Type name:">
3396      message
3397    </t>
3398    <t hangText="Subtype name:">
3399      http
3400    </t>
3401    <t hangText="Required parameters:">
3402      none
3403    </t>
3404    <t hangText="Optional parameters:">
3405      version, msgtype
3406      <list style="hanging">
3407        <t hangText="version:">
3408          The HTTP-version number of the enclosed message
3409          (e.g., "1.1"). If not present, the version can be
3410          determined from the first line of the body.
3411        </t>
3412        <t hangText="msgtype:">
3413          The message type &mdash; "request" or "response". If not
3414          present, the type can be determined from the first
3415          line of the body.
3416        </t>
3417      </list>
3418    </t>
3419    <t hangText="Encoding considerations:">
3420      only "7bit", "8bit", or "binary" are permitted
3421    </t>
3422    <t hangText="Security considerations:">
3423      none
3424    </t>
3425    <t hangText="Interoperability considerations:">
3426      none
3427    </t>
3428    <t hangText="Published specification:">
3429      This specification (see <xref target="internet.media.type.message.http"/>).
3430    </t>
3431    <t hangText="Applications that use this media type:">
3432    </t>
3433    <t hangText="Additional information:">
3434      <list style="hanging">
3435        <t hangText="Magic number(s):">none</t>
3436        <t hangText="File extension(s):">none</t>
3437        <t hangText="Macintosh file type code(s):">none</t>
3438      </list>
3439    </t>
3440    <t hangText="Person and email address to contact for further information:">
3441      See Authors Section.
3442    </t>
3443    <t hangText="Intended usage:">
3444      COMMON
3445    </t>
3446    <t hangText="Restrictions on usage:">
3447      none
3448    </t>
3449    <t hangText="Author:">
3450      See Authors Section.
3451    </t>
3452    <t hangText="Change controller:">
3453      IESG
3454    </t>
3455  </list>
3456</t>
3457</section>
3458<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
3459<iref item="Media Type" subitem="application/http" primary="true"/>
3460<iref item="application/http Media Type" primary="true"/>
3461<t>
3462   The application/http type can be used to enclose a pipeline of one or more
3463   HTTP request or response messages (not intermixed).
3464</t>
3465<t>
3466  <list style="hanging" x:indent="12em">
3467    <t hangText="Type name:">
3468      application
3469    </t>
3470    <t hangText="Subtype name:">
3471      http
3472    </t>
3473    <t hangText="Required parameters:">
3474      none
3475    </t>
3476    <t hangText="Optional parameters:">
3477      version, msgtype
3478      <list style="hanging">
3479        <t hangText="version:">
3480          The HTTP-version number of the enclosed messages
3481          (e.g., "1.1"). If not present, the version can be
3482          determined from the first line of the body.
3483        </t>
3484        <t hangText="msgtype:">
3485          The message type &mdash; "request" or "response". If not
3486          present, the type can be determined from the first
3487          line of the body.
3488        </t>
3489      </list>
3490    </t>
3491    <t hangText="Encoding considerations:">
3492      HTTP messages enclosed by this type
3493      are in "binary" format; use of an appropriate
3494      Content-Transfer-Encoding is required when
3495      transmitted via E-mail.
3496    </t>
3497    <t hangText="Security considerations:">
3498      none
3499    </t>
3500    <t hangText="Interoperability considerations:">
3501      none
3502    </t>
3503    <t hangText="Published specification:">
3504      This specification (see <xref target="internet.media.type.application.http"/>).
3505    </t>
3506    <t hangText="Applications that use this media type:">
3507    </t>
3508    <t hangText="Additional information:">
3509      <list style="hanging">
3510        <t hangText="Magic number(s):">none</t>
3511        <t hangText="File extension(s):">none</t>
3512        <t hangText="Macintosh file type code(s):">none</t>
3513      </list>
3514    </t>
3515    <t hangText="Person and email address to contact for further information:">
3516      See Authors Section.
3517    </t>
3518    <t hangText="Intended usage:">
3519      COMMON
3520    </t>
3521    <t hangText="Restrictions on usage:">
3522      none
3523    </t>
3524    <t hangText="Author:">
3525      See Authors Section.
3526    </t>
3527    <t hangText="Change controller:">
3528      IESG
3529    </t>
3530  </list>
3531</t>
3532</section>
3533</section>
3534
3535<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3536<t>
3537   The HTTP Transfer Coding Registry defines the name space for transfer
3538   coding names. It is maintained at <eref target="http://www.iana.org/assignments/http-parameters"/>.
3539</t>
3540
3541<section title="Procedure" anchor="transfer.coding.registry.procedure">
3542<t>
3543   Registrations &MUST; include the following fields:
3544   <list style="symbols">
3545     <t>Name</t>
3546     <t>Description</t>
3547     <t>Pointer to specification text</t>
3548   </list>
3549</t>
3550<t>
3551   Names of transfer codings &MUST-NOT; overlap with names of content codings
3552   (&content-codings;) unless the encoding transformation is identical, as
3553   is the case for the compression codings defined in
3554   <xref target="compression.codings"/>.
3555</t>
3556<t>
3557   Values to be added to this name space require IETF Review (see
3558   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3559   conform to the purpose of transfer coding defined in this specification.
3560</t>
3561<t>
3562   Use of program names for the identification of encoding formats
3563   is not desirable and is discouraged for future encodings.
3564</t>
3565</section>
3566
3567<section title="Registration" anchor="transfer.coding.registration">
3568<t>
3569   The HTTP Transfer Coding Registry shall be updated with the registrations
3570   below:
3571</t>
3572<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3573   <ttcol>Name</ttcol>
3574   <ttcol>Description</ttcol>
3575   <ttcol>Reference</ttcol>
3576   <c>chunked</c>
3577   <c>Transfer in a series of chunks</c>
3578   <c>
3579      <xref target="chunked.encoding"/>
3580   </c>
3581   <c>compress</c>
3582   <c>UNIX "compress" data format <xref target="Welch"/></c>
3583   <c>
3584      <xref target="compress.coding"/>
3585   </c>
3586   <c>deflate</c>
3587   <c>"deflate" compressed data (<xref target="RFC1951"/>) inside
3588   the "zlib" data format (<xref target="RFC1950"/>)
3589   </c>
3590   <c>
3591      <xref target="deflate.coding"/>
3592   </c>
3593   <c>gzip</c>
3594   <c>GZIP file format <xref target="RFC1952"/></c>
3595   <c>
3596      <xref target="gzip.coding"/>
3597   </c>
3598   <c>x-compress</c>
3599   <c>Deprecated (alias for compress)</c>
3600   <c>
3601      <xref target="compress.coding"/>
3602   </c>
3603   <c>x-gzip</c>
3604   <c>Deprecated (alias for gzip)</c>
3605   <c>
3606      <xref target="gzip.coding"/>
3607   </c>
3608</texttable>
3609</section>
3610</section>
3611
3612<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3613<t>
3614   The HTTP Upgrade Token Registry defines the name space for protocol-name
3615   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3616   field. The registry is maintained at <eref target="http://www.iana.org/assignments/http-upgrade-tokens"/>.
3617</t>
3618
3619<section title="Procedure" anchor="upgrade.token.registry.procedure">  
3620<t>
3621   Each registered protocol name is associated with contact information
3622   and an optional set of specifications that details how the connection
3623   will be processed after it has been upgraded.
3624</t>
3625<t>
3626   Registrations happen on a "First Come First Served" basis (see
3627   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3628   following rules:
3629  <list style="numbers">
3630    <t>A protocol-name token, once registered, stays registered forever.</t>
3631    <t>The registration &MUST; name a responsible party for the
3632       registration.</t>
3633    <t>The registration &MUST; name a point of contact.</t>
3634    <t>The registration &MAY; name a set of specifications associated with
3635       that token. Such specifications need not be publicly available.</t>
3636    <t>The registration &SHOULD; name a set of expected "protocol-version"
3637       tokens associated with that token at the time of registration.</t>
3638    <t>The responsible party &MAY; change the registration at any time.
3639       The IANA will keep a record of all such changes, and make them
3640       available upon request.</t>
3641    <t>The IESG &MAY; reassign responsibility for a protocol token.
3642       This will normally only be used in the case when a
3643       responsible party cannot be contacted.</t>
3644  </list>
3645</t>
3646<t>
3647   This registration procedure for HTTP Upgrade Tokens replaces that
3648   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3649</t>
3650</section>
3651
3652<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3653<t>
3654   The HTTP Upgrade Token Registry shall be updated with the registration
3655   below:
3656</t>
3657<texttable align="left" suppress-title="true">
3658   <ttcol>Value</ttcol>
3659   <ttcol>Description</ttcol>
3660   <ttcol>Expected Version Tokens</ttcol>
3661   <ttcol>Reference</ttcol>
3662
3663   <c>HTTP</c>
3664   <c>Hypertext Transfer Protocol</c>
3665   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3666   <c><xref target="http.version"/></c>
3667</texttable>
3668<t>
3669   The responsible party is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3670</t>
3671</section>
3672</section>
3673
3674</section>
3675
3676<section title="Security Considerations" anchor="security.considerations">
3677<t>
3678   This section is meant to inform developers, information providers, and
3679   users of known security concerns relevant to HTTP/1.1 message syntax,
3680   parsing, and routing.
3681</t>
3682
3683<section title="DNS-related Attacks" anchor="dns.related.attacks">
3684<t>
3685   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3686   generally prone to security attacks based on the deliberate misassociation
3687   of IP addresses and DNS names not protected by DNSSEC. Clients need to be
3688   cautious in assuming the validity of an IP number/DNS name association unless
3689   the response is protected by DNSSEC (<xref target="RFC4033"/>).
3690</t>
3691</section>
3692
3693<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3694<t>
3695   By their very nature, HTTP intermediaries are men-in-the-middle, and
3696   represent an opportunity for man-in-the-middle attacks. Compromise of
3697   the systems on which the intermediaries run can result in serious security
3698   and privacy problems. Intermediaries have access to security-related
3699   information, personal information about individual users and
3700   organizations, and proprietary information belonging to users and
3701   content providers. A compromised intermediary, or an intermediary
3702   implemented or configured without regard to security and privacy
3703   considerations, might be used in the commission of a wide range of
3704   potential attacks.
3705</t>
3706<t>
3707   Intermediaries that contain a shared cache are especially vulnerable
3708   to cache poisoning attacks.
3709</t>
3710<t>
3711   Implementers need to consider the privacy and security
3712   implications of their design and coding decisions, and of the
3713   configuration options they provide to operators (especially the
3714   default configuration).
3715</t>
3716<t>
3717   Users need to be aware that intermediaries are no more trustworthy than
3718   the people who run them; HTTP itself cannot solve this problem.
3719</t>
3720</section>
3721
3722<section title="Buffer Overflows" anchor="attack.protocol.element.size.overflows">
3723<t>
3724   Because HTTP uses mostly textual, character-delimited fields, attackers can
3725   overflow buffers in implementations, and/or perform a Denial of Service
3726   against implementations that accept fields with unlimited lengths.
3727</t>
3728<t>
3729   To promote interoperability, this specification makes specific
3730   recommendations for minimum size limits on request-line
3731   (<xref target="request.line"/>)
3732   and blocks of header fields (<xref target="header.fields"/>). These are
3733   minimum recommendations, chosen to be supportable even by implementations
3734   with limited resources; it is expected that most implementations will
3735   choose substantially higher limits.
3736</t>
3737<t>
3738   This specification also provides a way for servers to reject messages that
3739   have request-targets that are too long (&status-414;) or request entities
3740   that are too large (&status-4xx;). Additional status codes related to
3741   capacity limits have been defined by extensions to HTTP
3742   <xref target="RFC6585"/>.
3743</t>
3744<t>
3745   Recipients &SHOULD; carefully limit the extent to which they read other
3746   fields, including (but not limited to) request methods, response status
3747   phrases, header field-names, and body chunks, so as to avoid denial of
3748   service attacks without impeding interoperability.
3749</t>
3750</section>
3751
3752<section title="Message Integrity" anchor="message.integrity">
3753<t>
3754   HTTP does not define a specific mechanism for ensuring message integrity,
3755   instead relying on the error-detection ability of underlying transport
3756   protocols and the use of length or chunk-delimited framing to detect
3757   completeness. Additional integrity mechanisms, such as hash functions or
3758   digital signatures applied to the content, can be selectively added to
3759   messages via extensible metadata header fields. Historically, the lack of
3760   a single integrity mechanism has been justified by the informal nature of
3761   most HTTP communication.  However, the prevalence of HTTP as an information
3762   access mechanism has resulted in its increasing use within environments
3763   where verification of message integrity is crucial.
3764</t>
3765<t>
3766   User agents are encouraged to implement configurable means for detecting
3767   and reporting failures of message integrity such that those means can be
3768   enabled within environments for which integrity is necessary. For example,
3769   a browser being used to view medical history or drug interaction
3770   information needs to indicate to the user when such information is detected
3771   by the protocol to be incomplete, expired, or corrupted during transfer.
3772   Such mechanisms might be selectively enabled via user agent extensions or
3773   the presence of message integrity metadata in a response.
3774   At a minimum, user agents ought to provide some indication that allows a
3775   user to distinguish between a complete and incomplete response message
3776   (<xref target="incomplete.messages"/>) when such verification is desired.
3777</t>
3778</section>
3779
3780<section title="Server Log Information" anchor="abuse.of.server.log.information">
3781<t>
3782   A server is in the position to save personal data about a user's requests
3783   over time, which might identify their reading patterns or subjects of
3784   interest.  In particular, log information gathered at an intermediary
3785   often contains a history of user agent interaction, across a multitude
3786   of sites, that can be traced to individual users.
3787</t>
3788<t>
3789   HTTP log information is confidential in nature; its handling is often
3790   constrained by laws and regulations.  Log information needs to be securely
3791   stored and appropriate guidelines followed for its analysis.
3792   Anonymization of personal information within individual entries helps,
3793   but is generally not sufficient to prevent real log traces from being
3794   re-identified based on correlation with other access characteristics.
3795   As such, access traces that are keyed to a specific client should not
3796   be published even if the key is pseudonymous.
3797</t>
3798<t>
3799   To minimize the risk of theft or accidental publication, log information
3800   should be purged of personally identifiable information, including
3801   user identifiers, IP addresses, and user-provided query parameters,
3802   as soon as that information is no longer necessary to support operational
3803   needs for security, auditing, or fraud control.
3804</t>
3805</section>
3806</section>
3807
3808<section title="Acknowledgments" anchor="acks">
3809<t>
3810   This edition of HTTP/1.1 builds on the many contributions that went into
3811   <xref target="RFC1945" format="none">RFC 1945</xref>,
3812   <xref target="RFC2068" format="none">RFC 2068</xref>,
3813   <xref target="RFC2145" format="none">RFC 2145</xref>, and
3814   <xref target="RFC2616" format="none">RFC 2616</xref>, including
3815   substantial contributions made by the previous authors, editors, and
3816   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
3817   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
3818   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
3819</t>
3820<t>
3821   Since 1999, the following contributors have helped improve the HTTP
3822   specification by reporting bugs, asking smart questions, drafting or
3823   reviewing text, and evaluating open issues:
3824</t>
3825<?BEGININC acks ?>
3826<t>Adam Barth,
3827Adam Roach,
3828Addison Phillips,
3829Adrian Chadd,
3830Adrien W. de Croy,
3831Alan Ford,
3832Alan Ruttenberg,
3833Albert Lunde,
3834Alek Storm,
3835Alex Rousskov,
3836Alexandre Morgaut,
3837Alexey Melnikov,
3838Alisha Smith,
3839Amichai Rothman,
3840Amit Klein,
3841Amos Jeffries,
3842Andreas Maier,
3843Andreas Petersson,
3844Anil Sharma,
3845Anne van Kesteren,
3846Anthony Bryan,
3847Asbjorn Ulsberg,
3848Ashok Kumar,
3849Balachander Krishnamurthy,
3850Barry Leiba,
3851Ben Laurie,
3852Benjamin Carlyle,
3853Benjamin Niven-Jenkins,
3854Bil Corry,
3855Bill Burke,
3856Bjoern Hoehrmann,
3857Bob Scheifler,
3858Boris Zbarsky,
3859Brett Slatkin,
3860Brian Kell,
3861Brian McBarron,
3862Brian Pane,
3863Brian Raymor,
3864Brian Smith,
3865Bryce Nesbitt,
3866Cameron Heavon-Jones,
3867Carl Kugler,
3868Carsten Bormann,
3869Charles Fry,
3870Chris Newman,
3871Cyrus Daboo,
3872Dale Robert Anderson,
3873Dan Wing,
3874Dan Winship,
3875Daniel Stenberg,
3876Darrel Miller,
3877Dave Cridland,
3878Dave Crocker,
3879Dave Kristol,
3880Dave Thaler,
3881David Booth,
3882David Singer,
3883David W. Morris,
3884Diwakar Shetty,
3885Dmitry Kurochkin,
3886Drummond Reed,
3887Duane Wessels,
3888Edward Lee,
3889Eitan Adler,
3890Eliot Lear,
3891Eran Hammer-Lahav,
3892Eric D. Williams,
3893Eric J. Bowman,
3894Eric Lawrence,
3895Eric Rescorla,
3896Erik Aronesty,
3897Evan Prodromou,
3898Felix Geisendoerfer,
3899Florian Weimer,
3900Frank Ellermann,
3901Fred Akalin,
3902Fred Bohle,
3903Frederic Kayser,
3904Gabor Molnar,
3905Gabriel Montenegro,
3906Geoffrey Sneddon,
3907Gervase Markham,
3908Gili Tzabari,
3909Grahame Grieve,
3910Greg Wilkins,
3911Grzegorz Calkowski,
3912Harald Tveit Alvestrand,
3913Harry Halpin,
3914Helge Hess,
3915Henrik Nordstrom,
3916Henry S. Thompson,
3917Henry Story,
3918Herbert van de Sompel,
3919Herve Ruellan,
3920Howard Melman,
3921Hugo Haas,
3922Ian Fette,
3923Ian Hickson,
3924Ido Safruti,
3925Ilari Liusvaara,
3926Ilya Grigorik,
3927Ingo Struck,
3928J. Ross Nicoll,
3929James Cloos,
3930James H. Manger,
3931James Lacey,
3932James M. Snell,
3933Jamie Lokier,
3934Jan Algermissen,
3935Jeff Hodges (who came up with the term 'effective Request-URI'),
3936Jeff Pinner,
3937Jeff Walden,
3938Jim Luther,
3939Jitu Padhye,
3940Joe D. Williams,
3941Joe Gregorio,
3942Joe Orton,
3943John C. Klensin,
3944John C. Mallery,
3945John Cowan,
3946John Kemp,
3947John Panzer,
3948John Schneider,
3949John Stracke,
3950John Sullivan,
3951Jonas Sicking,
3952Jonathan A. Rees,
3953Jonathan Billington,
3954Jonathan Moore,
3955Jonathan Silvera,
3956Jordi Ros,
3957Joris Dobbelsteen,
3958Josh Cohen,
3959Julien Pierre,
3960Jungshik Shin,
3961Justin Chapweske,
3962Justin Erenkrantz,
3963Justin James,
3964Kalvinder Singh,
3965Karl Dubost,
3966Keith Hoffman,
3967Keith Moore,
3968Ken Murchison,
3969Koen Holtman,
3970Konstantin Voronkov,
3971Kris Zyp,
3972Lisa Dusseault,
3973Maciej Stachowiak,
3974Manu Sporny,
3975Marc Schneider,
3976Marc Slemko,
3977Mark Baker,
3978Mark Pauley,
3979Mark Watson,
3980Markus Isomaki,
3981Markus Lanthaler,
3982Martin J. Duerst,
3983Martin Musatov,
3984Martin Nilsson,
3985Martin Thomson,
3986Matt Lynch,
3987Matthew Cox,
3988Max Clark,
3989Michael Burrows,
3990Michael Hausenblas,
3991Michael Sweet,
3992Mike Amundsen,
3993Mike Belshe,
3994Mike Bishop,
3995Mike Kelly,
3996Mike Schinkel,
3997Miles Sabin,
3998Murray S. Kucherawy,
3999Mykyta Yevstifeyev,
4000Nathan Rixham,
4001Nicholas Shanks,
4002Nico Williams,
4003Nicolas Alvarez,
4004Nicolas Mailhot,
4005Noah Slater,
4006Osama Mazahir,
4007Pablo Castro,
4008Pat Hayes,
4009Patrick R. McManus,
4010Paul E. Jones,
4011Paul Hoffman,
4012Paul Marquess,
4013Peter Lepeska,
4014Peter Occil,
4015Peter Saint-Andre,
4016Peter Watkins,
4017Phil Archer,
4018Philippe Mougin,
4019Phillip Hallam-Baker,
4020Piotr Dobrogost,
4021Poul-Henning Kamp,
4022Preethi Natarajan,
4023Rajeev Bector,
4024Ray Polk,
4025Reto Bachmann-Gmuer,
4026Richard Cyganiak,
4027Robby Simpson,
4028Robert Brewer,
4029Robert Collins,
4030Robert Mattson,
4031Robert O'Callahan,
4032Robert Olofsson,
4033Robert Sayre,
4034Robert Siemer,
4035Robert de Wilde,
4036Roberto Javier Godoy,
4037Roberto Peon,
4038Roland Zink,
4039Ronny Widjaja,
4040S. Mike Dierken,
4041Salvatore Loreto,
4042Sam Johnston,
4043Sam Pullara,
4044Sam Ruby,
4045Scott Lawrence (who maintained the original issues list),
4046Sean B. Palmer,
4047Shane McCarron,
4048Shigeki Ohtsu,
4049Stefan Eissing,
4050Stefan Tilkov,
4051Stefanos Harhalakis,
4052Stephane Bortzmeyer,
4053Stephen Farrell,
4054Stephen Ludin,
4055Stuart Williams,
4056Subbu Allamaraju,
4057Sylvain Hellegouarch,
4058Tapan Divekar,
4059Tatsuhiro Tsujikawa,
4060Tatsuya Hayashi,
4061Ted Hardie,
4062Thomas Broyer,
4063Thomas Fossati,
4064Thomas Maslen,
4065Thomas Nordin,
4066Thomas Roessler,
4067Tim Bray,
4068Tim Morgan,
4069Tim Olsen,
4070Tom Zhou,
4071Travis Snoozy,
4072Tyler Close,
4073Vincent Murphy,
4074Wenbo Zhu,
4075Werner Baumann,
4076Wilbur Streett,
4077Wilfredo Sanchez Vega,
4078William A. Rowe Jr.,
4079William Chan,
4080Willy Tarreau,
4081Xiaoshu Wang,
4082Yaron Goland,
4083Yngve Nysaeter Pettersen,
4084Yoav Nir,
4085Yogesh Bang,
4086Yuchung Cheng,
4087Yutaka Oiwa,
4088Yves Lafon (long-time member of the editor team),
4089Zed A. Shaw, and
4090Zhong Yu.
4091</t>
4092<?ENDINC acks ?>
4093<t>
4094   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
4095   acknowledgements from prior revisions.
4096</t>
4097</section>
4098
4099</middle>
4100<back>
4101
4102<references title="Normative References">
4103
4104<reference anchor="Part2">
4105  <front>
4106    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
4107    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4108      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4109      <address><email>fielding@gbiv.com</email></address>
4110    </author>
4111    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4112      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4113      <address><email>julian.reschke@greenbytes.de</email></address>
4114    </author>
4115    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4116  </front>
4117  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
4118  <x:source href="p2-semantics.xml" basename="p2-semantics">
4119    <x:defines>1xx (Informational)</x:defines>
4120    <x:defines>1xx</x:defines>
4121    <x:defines>100 (Continue)</x:defines>
4122    <x:defines>101 (Switching Protocols)</x:defines>
4123    <x:defines>2xx (Successful)</x:defines>
4124    <x:defines>2xx</x:defines>
4125    <x:defines>200 (OK)</x:defines>
4126    <x:defines>203 (Non-Authoritative Information)</x:defines>
4127    <x:defines>204 (No Content)</x:defines>
4128    <x:defines>3xx (Redirection)</x:defines>
4129    <x:defines>3xx</x:defines>
4130    <x:defines>301 (Moved Permanently)</x:defines>
4131    <x:defines>4xx (Client Error)</x:defines>
4132    <x:defines>4xx</x:defines>
4133    <x:defines>400 (Bad Request)</x:defines>
4134    <x:defines>411 (Length Required)</x:defines>
4135    <x:defines>414 (URI Too Long)</x:defines>
4136    <x:defines>417 (Expectation Failed)</x:defines>
4137    <x:defines>426 (Upgrade Required)</x:defines>
4138    <x:defines>501 (Not Implemented)</x:defines>
4139    <x:defines>502 (Bad Gateway)</x:defines>
4140    <x:defines>505 (HTTP Version Not Supported)</x:defines>
4141    <x:defines>Accept-Encoding</x:defines>
4142    <x:defines>Allow</x:defines>
4143    <x:defines>Content-Encoding</x:defines>
4144    <x:defines>Content-Location</x:defines>
4145    <x:defines>Content-Type</x:defines>
4146    <x:defines>Date</x:defines>
4147    <x:defines>Expect</x:defines>
4148    <x:defines>Location</x:defines>
4149    <x:defines>Server</x:defines>
4150    <x:defines>User-Agent</x:defines>
4151  </x:source>
4152</reference>
4153
4154<reference anchor="Part4">
4155  <front>
4156    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
4157    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
4158      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4159      <address><email>fielding@gbiv.com</email></address>
4160    </author>
4161    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
4162      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4163      <address><email>julian.reschke@greenbytes.de</email></address>
4164    </author>
4165    <date month="&ID-MONTH;" year="&ID-YEAR;" />
4166  </front>
4167  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
4168  <x:source basename="p4-conditional" href="p4-conditional.xml">
4169    <x:defines>304 (Not Modified)</x:defines>
4170    <x:defines>ETag</x:defines>
4171    <x:defines>Last-Modified</x:defines>
4172  </x:source>
4173</reference>
4174
4175<reference anchor="Part5">
4176  <front>
4177    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
4178    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4179      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4180      <address><email>fielding@gbiv.com</email></address>
4181    </author>
4182    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
4183      <organization abbrev="W3C">World Wide Web Consortium</organization>
4184      <address><email>ylafon@w3.org</email></address>
4185    </author>
4186    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4187      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4188      <address><email>julian.reschke@greenbytes.de</email></address>
4189    </author>
4190    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4191  </front>
4192  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4193  <x:source href="p5-range.xml" basename="p5-range">
4194    <x:defines>Content-Range</x:defines>
4195  </x:source>
4196</reference>
4197
4198<reference anchor="Part6">
4199  <front>
4200    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4201    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4202      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4203      <address><email>fielding@gbiv.com</email></address>
4204    </author>
4205    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4206      <organization>Akamai</organization>
4207      <address><email>mnot@mnot.net</email></address>
4208    </author>
4209    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4210      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4211      <address><email>julian.reschke@greenbytes.de</email></address>
4212    </author>
4213    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4214  </front>
4215  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4216  <x:source href="p6-cache.xml" basename="p6-cache">
4217    <x:defines>Cache-Control</x:defines>
4218    <x:defines>Expires</x:defines>
4219  </x:source>
4220</reference>
4221
4222<reference anchor="Part7">
4223  <front>
4224    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4225    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4226      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4227      <address><email>fielding@gbiv.com</email></address>
4228    </author>
4229    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4230      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4231      <address><email>julian.reschke@greenbytes.de</email></address>
4232    </author>
4233    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4234  </front>
4235  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4236  <x:source href="p7-auth.xml" basename="p7-auth">
4237    <x:defines>Proxy-Authenticate</x:defines>
4238    <x:defines>Proxy-Authorization</x:defines>
4239  </x:source>
4240</reference>
4241
4242<reference anchor="RFC5234">
4243  <front>
4244    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4245    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4246      <organization>Brandenburg InternetWorking</organization>
4247      <address>
4248        <email>dcrocker@bbiw.net</email>
4249      </address> 
4250    </author>
4251    <author initials="P." surname="Overell" fullname="Paul Overell">
4252      <organization>THUS plc.</organization>
4253      <address>
4254        <email>paul.overell@thus.net</email>
4255      </address>
4256    </author>
4257    <date month="January" year="2008"/>
4258  </front>
4259  <seriesInfo name="STD" value="68"/>
4260  <seriesInfo name="RFC" value="5234"/>
4261</reference>
4262
4263<reference anchor="RFC2119">
4264  <front>
4265    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4266    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4267      <organization>Harvard University</organization>
4268      <address><email>sob@harvard.edu</email></address>
4269    </author>
4270    <date month="March" year="1997"/>
4271  </front>
4272  <seriesInfo name="BCP" value="14"/>
4273  <seriesInfo name="RFC" value="2119"/>
4274</reference>
4275
4276<reference anchor="RFC3986">
4277 <front>
4278  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4279  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4280    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4281    <address>
4282       <email>timbl@w3.org</email>
4283       <uri>http://www.w3.org/People/Berners-Lee/</uri>
4284    </address>
4285  </author>
4286  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4287    <organization abbrev="Day Software">Day Software</organization>
4288    <address>
4289      <email>fielding@gbiv.com</email>
4290      <uri>http://roy.gbiv.com/</uri>
4291    </address>
4292  </author>
4293  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4294    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4295    <address>
4296      <email>LMM@acm.org</email>
4297      <uri>http://larry.masinter.net/</uri>
4298    </address>
4299  </author>
4300  <date month='January' year='2005'></date>
4301 </front>
4302 <seriesInfo name="STD" value="66"/>
4303 <seriesInfo name="RFC" value="3986"/>
4304</reference>
4305
4306<reference anchor="RFC0793">
4307  <front>
4308    <title>Transmission Control Protocol</title>
4309    <author initials='J.' surname='Postel' fullname='Jon Postel'>
4310      <organization>University of Southern California (USC)/Information Sciences Institute</organization>
4311    </author>
4312    <date year='1981' month='September' />
4313  </front>
4314  <seriesInfo name='STD' value='7' />
4315  <seriesInfo name='RFC' value='793' />
4316</reference>
4317
4318<reference anchor="USASCII">
4319  <front>
4320    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4321    <author>
4322      <organization>American National Standards Institute</organization>
4323    </author>
4324    <date year="1986"/>
4325  </front>
4326  <seriesInfo name="ANSI" value="X3.4"/>
4327</reference>
4328
4329<reference anchor="RFC1950">
4330  <front>
4331    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4332    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4333      <organization>Aladdin Enterprises</organization>
4334      <address><email>ghost@aladdin.com</email></address>
4335    </author>
4336    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4337    <date month="May" year="1996"/>
4338  </front>
4339  <seriesInfo name="RFC" value="1950"/>
4340  <!--<annotation>
4341    RFC 1950 is an Informational RFC, thus it might be less stable than
4342    this specification. On the other hand, this downward reference was
4343    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4344    therefore it is unlikely to cause problems in practice. See also
4345    <xref target="BCP97"/>.
4346  </annotation>-->
4347</reference>
4348
4349<reference anchor="RFC1951">
4350  <front>
4351    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4352    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4353      <organization>Aladdin Enterprises</organization>
4354      <address><email>ghost@aladdin.com</email></address>
4355    </author>
4356    <date month="May" year="1996"/>
4357  </front>
4358  <seriesInfo name="RFC" value="1951"/>
4359  <!--<annotation>
4360    RFC 1951 is an Informational RFC, thus it might be less stable than
4361    this specification. On the other hand, this downward reference was
4362    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4363    therefore it is unlikely to cause problems in practice. See also
4364    <xref target="BCP97"/>.
4365  </annotation>-->
4366</reference>
4367
4368<reference anchor="RFC1952">
4369  <front>
4370    <title>GZIP file format specification version 4.3</title>
4371    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4372      <organization>Aladdin Enterprises</organization>
4373      <address><email>ghost@aladdin.com</email></address>
4374    </author>
4375    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4376      <address><email>gzip@prep.ai.mit.edu</email></address>
4377    </author>
4378    <author initials="M." surname="Adler" fullname="Mark Adler">
4379      <address><email>madler@alumni.caltech.edu</email></address>
4380    </author>
4381    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4382      <address><email>ghost@aladdin.com</email></address>
4383    </author>
4384    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4385      <address><email>randeg@alumni.rpi.edu</email></address>
4386    </author>
4387    <date month="May" year="1996"/>
4388  </front>
4389  <seriesInfo name="RFC" value="1952"/>
4390  <!--<annotation>
4391    RFC 1952 is an Informational RFC, thus it might be less stable than
4392    this specification. On the other hand, this downward reference was
4393    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4394    therefore it is unlikely to cause problems in practice. See also
4395    <xref target="BCP97"/>.
4396  </annotation>-->
4397</reference>
4398
4399<reference anchor="Welch">
4400  <front>
4401    <title>A Technique for High Performance Data Compression</title>
4402    <author initials="T.A." surname="Welch" fullname="Terry A. Welch"/>
4403    <date month="June" year="1984"/>
4404  </front>
4405  <seriesInfo name="IEEE Computer" value="17(6)"/>
4406</reference>
4407
4408</references>
4409
4410<references title="Informative References">
4411
4412<reference anchor="ISO-8859-1">
4413  <front>
4414    <title>
4415     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4416    </title>
4417    <author>
4418      <organization>International Organization for Standardization</organization>
4419    </author>
4420    <date year="1998"/>
4421  </front>
4422  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4423</reference>
4424
4425<reference anchor='RFC1919'>
4426  <front>
4427    <title>Classical versus Transparent IP Proxies</title>
4428    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4429      <address><email>mchatel@pax.eunet.ch</email></address>
4430    </author>
4431    <date year='1996' month='March' />
4432  </front>
4433  <seriesInfo name='RFC' value='1919' />
4434</reference>
4435
4436<reference anchor="RFC1945">
4437  <front>
4438    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4439    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4440      <organization>MIT, Laboratory for Computer Science</organization>
4441      <address><email>timbl@w3.org</email></address>
4442    </author>
4443    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4444      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4445      <address><email>fielding@ics.uci.edu</email></address>
4446    </author>
4447    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4448      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4449      <address><email>frystyk@w3.org</email></address>
4450    </author>
4451    <date month="May" year="1996"/>
4452  </front>
4453  <seriesInfo name="RFC" value="1945"/>
4454</reference>
4455
4456<reference anchor="RFC2045">
4457  <front>
4458    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4459    <author initials="N." surname="Freed" fullname="Ned Freed">
4460      <organization>Innosoft International, Inc.</organization>
4461      <address><email>ned@innosoft.com</email></address>
4462    </author>
4463    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4464      <organization>First Virtual Holdings</organization>
4465      <address><email>nsb@nsb.fv.com</email></address>
4466    </author>
4467    <date month="November" year="1996"/>
4468  </front>
4469  <seriesInfo name="RFC" value="2045"/>
4470</reference>
4471
4472<reference anchor="RFC2047">
4473  <front>
4474    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4475    <author initials="K." surname="Moore" fullname="Keith Moore">
4476      <organization>University of Tennessee</organization>
4477      <address><email>moore@cs.utk.edu</email></address>
4478    </author>
4479    <date month="November" year="1996"/>
4480  </front>
4481  <seriesInfo name="RFC" value="2047"/>
4482</reference>
4483
4484<reference anchor="RFC2068">
4485  <front>
4486    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4487    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
4488      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4489      <address><email>fielding@ics.uci.edu</email></address>
4490    </author>
4491    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4492      <organization>MIT Laboratory for Computer Science</organization>
4493      <address><email>jg@w3.org</email></address>
4494    </author>
4495    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4496      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4497      <address><email>mogul@wrl.dec.com</email></address>
4498    </author>
4499    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4500      <organization>MIT Laboratory for Computer Science</organization>
4501      <address><email>frystyk@w3.org</email></address>
4502    </author>
4503    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4504      <organization>MIT Laboratory for Computer Science</organization>
4505      <address><email>timbl@w3.org</email></address>
4506    </author>
4507    <date month="January" year="1997"/>
4508  </front>
4509  <seriesInfo name="RFC" value="2068"/>
4510</reference>
4511
4512<reference anchor="RFC2145">
4513  <front>
4514    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4515    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4516      <organization>Western Research Laboratory</organization>
4517      <address><email>mogul@wrl.dec.com</email></address>
4518    </author>
4519    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4520      <organization>Department of Information and Computer Science</organization>
4521      <address><email>fielding@ics.uci.edu</email></address>
4522    </author>
4523    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4524      <organization>MIT Laboratory for Computer Science</organization>
4525      <address><email>jg@w3.org</email></address>
4526    </author>
4527    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4528      <organization>W3 Consortium</organization>
4529      <address><email>frystyk@w3.org</email></address>
4530    </author>
4531    <date month="May" year="1997"/>
4532  </front>
4533  <seriesInfo name="RFC" value="2145"/>
4534</reference>
4535
4536<reference anchor="RFC2616">
4537  <front>
4538    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4539    <author initials="R." surname="Fielding" fullname="R. Fielding">
4540      <organization>University of California, Irvine</organization>
4541      <address><email>fielding@ics.uci.edu</email></address>
4542    </author>
4543    <author initials="J." surname="Gettys" fullname="J. Gettys">
4544      <organization>W3C</organization>
4545      <address><email>jg@w3.org</email></address>
4546    </author>
4547    <author initials="J." surname="Mogul" fullname="J. Mogul">
4548      <organization>Compaq Computer Corporation</organization>
4549      <address><email>mogul@wrl.dec.com</email></address>
4550    </author>
4551    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4552      <organization>MIT Laboratory for Computer Science</organization>
4553      <address><email>frystyk@w3.org</email></address>
4554    </author>
4555    <author initials="L." surname="Masinter" fullname="L. Masinter">
4556      <organization>Xerox Corporation</organization>
4557      <address><email>masinter@parc.xerox.com</email></address>
4558    </author>
4559    <author initials="P." surname="Leach" fullname="P. Leach">
4560      <organization>Microsoft Corporation</organization>
4561      <address><email>paulle@microsoft.com</email></address>
4562    </author>
4563    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4564      <organization>W3C</organization>
4565      <address><email>timbl@w3.org</email></address>
4566    </author>
4567    <date month="June" year="1999"/>
4568  </front>
4569  <seriesInfo name="RFC" value="2616"/>
4570</reference>
4571
4572<reference anchor='RFC2817'>
4573  <front>
4574    <title>Upgrading to TLS Within HTTP/1.1</title>
4575    <author initials='R.' surname='Khare' fullname='R. Khare'>
4576      <organization>4K Associates / UC Irvine</organization>
4577      <address><email>rohit@4K-associates.com</email></address>
4578    </author>
4579    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4580      <organization>Agranat Systems, Inc.</organization>
4581      <address><email>lawrence@agranat.com</email></address>
4582    </author>
4583    <date year='2000' month='May' />
4584  </front>
4585  <seriesInfo name='RFC' value='2817' />
4586</reference>
4587
4588<reference anchor='RFC2818'>
4589  <front>
4590    <title>HTTP Over TLS</title>
4591    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4592      <organization>RTFM, Inc.</organization>
4593      <address><email>ekr@rtfm.com</email></address>
4594    </author>
4595    <date year='2000' month='May' />
4596  </front>
4597  <seriesInfo name='RFC' value='2818' />
4598</reference>
4599
4600<reference anchor='RFC3040'>
4601  <front>
4602    <title>Internet Web Replication and Caching Taxonomy</title>
4603    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4604      <organization>Equinix, Inc.</organization>
4605    </author>
4606    <author initials='I.' surname='Melve' fullname='I. Melve'>
4607      <organization>UNINETT</organization>
4608    </author>
4609    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4610      <organization>CacheFlow Inc.</organization>
4611    </author>
4612    <date year='2001' month='January' />
4613  </front>
4614  <seriesInfo name='RFC' value='3040' />
4615</reference>
4616
4617<reference anchor='BCP90'>
4618  <front>
4619    <title>Registration Procedures for Message Header Fields</title>
4620    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4621      <organization>Nine by Nine</organization>
4622      <address><email>GK-IETF@ninebynine.org</email></address>
4623    </author>
4624    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4625      <organization>BEA Systems</organization>
4626      <address><email>mnot@pobox.com</email></address>
4627    </author>
4628    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4629      <organization>HP Labs</organization>
4630      <address><email>JeffMogul@acm.org</email></address>
4631    </author>
4632    <date year='2004' month='September' />
4633  </front>
4634  <seriesInfo name='BCP' value='90' />
4635  <seriesInfo name='RFC' value='3864' />
4636</reference>
4637
4638<reference anchor='RFC4033'>
4639  <front>
4640    <title>DNS Security Introduction and Requirements</title>
4641    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4642    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4643    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4644    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4645    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4646    <date year='2005' month='March' />
4647  </front>
4648  <seriesInfo name='RFC' value='4033' />
4649</reference>
4650
4651<reference anchor="BCP13">
4652  <front>
4653    <title>Media Type Specifications and Registration Procedures</title>
4654    <author initials="N." surname="Freed" fullname="Ned Freed">
4655      <organization>Oracle</organization>
4656      <address>
4657        <email>ned+ietf@mrochek.com</email>
4658      </address>
4659    </author>
4660    <author initials="J." surname="Klensin" fullname="John C. Klensin">
4661      <address>
4662        <email>john+ietf@jck.com</email>
4663      </address>
4664    </author>
4665    <author initials="T." surname="Hansen" fullname="Tony Hansen">
4666      <organization>AT&amp;T Laboratories</organization>
4667      <address>
4668        <email>tony+mtsuffix@maillennium.att.com</email>
4669      </address>
4670    </author>
4671    <date year="2013" month="January"/>
4672  </front>
4673  <seriesInfo name="BCP" value="13"/>
4674  <seriesInfo name="RFC" value="6838"/>
4675</reference>
4676
4677<reference anchor='BCP115'>
4678  <front>
4679    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4680    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4681      <organization>AT&amp;T Laboratories</organization>
4682      <address>
4683        <email>tony+urireg@maillennium.att.com</email>
4684      </address>
4685    </author>
4686    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4687      <organization>Qualcomm, Inc.</organization>
4688      <address>
4689        <email>hardie@qualcomm.com</email>
4690      </address>
4691    </author>
4692    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4693      <organization>Adobe Systems</organization>
4694      <address>
4695        <email>LMM@acm.org</email>
4696      </address>
4697    </author>
4698    <date year='2006' month='February' />
4699  </front>
4700  <seriesInfo name='BCP' value='115' />
4701  <seriesInfo name='RFC' value='4395' />
4702</reference>
4703
4704<reference anchor='RFC4559'>
4705  <front>
4706    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4707    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4708    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4709    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4710    <date year='2006' month='June' />
4711  </front>
4712  <seriesInfo name='RFC' value='4559' />
4713</reference>
4714
4715<reference anchor='RFC5226'>
4716  <front>
4717    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4718    <author initials='T.' surname='Narten' fullname='T. Narten'>
4719      <organization>IBM</organization>
4720      <address><email>narten@us.ibm.com</email></address>
4721    </author>
4722    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4723      <organization>Google</organization>
4724      <address><email>Harald@Alvestrand.no</email></address>
4725    </author>
4726    <date year='2008' month='May' />
4727  </front>
4728  <seriesInfo name='BCP' value='26' />
4729  <seriesInfo name='RFC' value='5226' />
4730</reference>
4731
4732<reference anchor='RFC5246'>
4733   <front>
4734      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4735      <author initials='T.' surname='Dierks' fullname='T. Dierks'>
4736         <organization />
4737      </author>
4738      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4739         <organization>RTFM, Inc.</organization>
4740      </author>
4741      <date year='2008' month='August' />
4742   </front>
4743   <seriesInfo name='RFC' value='5246' />
4744</reference>
4745
4746<reference anchor="RFC5322">
4747  <front>
4748    <title>Internet Message Format</title>
4749    <author initials="P." surname="Resnick" fullname="P. Resnick">
4750      <organization>Qualcomm Incorporated</organization>
4751    </author>
4752    <date year="2008" month="October"/>
4753  </front>
4754  <seriesInfo name="RFC" value="5322"/>
4755</reference>
4756
4757<reference anchor="RFC6265">
4758  <front>
4759    <title>HTTP State Management Mechanism</title>
4760    <author initials="A." surname="Barth" fullname="Adam Barth">
4761      <organization abbrev="U.C. Berkeley">
4762        University of California, Berkeley
4763      </organization>
4764      <address><email>abarth@eecs.berkeley.edu</email></address>
4765    </author>
4766    <date year="2011" month="April" />
4767  </front>
4768  <seriesInfo name="RFC" value="6265"/>
4769</reference>
4770
4771<reference anchor='RFC6585'>
4772  <front>
4773    <title>Additional HTTP Status Codes</title>
4774    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4775      <organization>Rackspace</organization>
4776    </author>
4777    <author initials='R.' surname='Fielding' fullname='R. Fielding'>
4778      <organization>Adobe</organization>
4779    </author>
4780    <date year='2012' month='April' />
4781   </front>
4782   <seriesInfo name='RFC' value='6585' />
4783</reference>
4784
4785<!--<reference anchor='BCP97'>
4786  <front>
4787    <title>Handling Normative References to Standards-Track Documents</title>
4788    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
4789      <address>
4790        <email>klensin+ietf@jck.com</email>
4791      </address>
4792    </author>
4793    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
4794      <organization>MIT</organization>
4795      <address>
4796        <email>hartmans-ietf@mit.edu</email>
4797      </address>
4798    </author>
4799    <date year='2007' month='June' />
4800  </front>
4801  <seriesInfo name='BCP' value='97' />
4802  <seriesInfo name='RFC' value='4897' />
4803</reference>-->
4804
4805<reference anchor="Kri2001" target="http://arxiv.org/abs/cs.SE/0105018">
4806  <front>
4807    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
4808    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
4809    <date year="2001" month="November"/>
4810  </front>
4811  <seriesInfo name="ACM Transactions on Internet Technology" value="1(2)"/>
4812</reference>
4813
4814</references>
4815
4816
4817<section title="HTTP Version History" anchor="compatibility">
4818<t>
4819   HTTP has been in use by the World-Wide Web global information initiative
4820   since 1990. The first version of HTTP, later referred to as HTTP/0.9,
4821   was a simple protocol for hypertext data transfer across the Internet
4822   with only a single request method (GET) and no metadata.
4823   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
4824   methods and MIME-like messaging that could include metadata about the data
4825   transferred and modifiers on the request/response semantics. However,
4826   HTTP/1.0 did not sufficiently take into consideration the effects of
4827   hierarchical proxies, caching, the need for persistent connections, or
4828   name-based virtual hosts. The proliferation of incompletely-implemented
4829   applications calling themselves "HTTP/1.0" further necessitated a
4830   protocol version change in order for two communicating applications
4831   to determine each other's true capabilities.
4832</t>
4833<t>
4834   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
4835   requirements that enable reliable implementations, adding only
4836   those new features that will either be safely ignored by an HTTP/1.0
4837   recipient or only sent when communicating with a party advertising
4838   conformance with HTTP/1.1.
4839</t>
4840<t>
4841   It is beyond the scope of a protocol specification to mandate
4842   conformance with previous versions. HTTP/1.1 was deliberately
4843   designed, however, to make supporting previous versions easy.
4844   We would expect a general-purpose HTTP/1.1 server to understand
4845   any valid request in the format of HTTP/1.0 and respond appropriately
4846   with an HTTP/1.1 message that only uses features understood (or
4847   safely ignored) by HTTP/1.0 clients.  Likewise, we would expect
4848   an HTTP/1.1 client to understand any valid HTTP/1.0 response.
4849</t>
4850<t>
4851   Since HTTP/0.9 did not support header fields in a request,
4852   there is no mechanism for it to support name-based virtual
4853   hosts (selection of resource by inspection of the <x:ref>Host</x:ref> header
4854   field).  Any server that implements name-based virtual hosts
4855   ought to disable support for HTTP/0.9.  Most requests that
4856   appear to be HTTP/0.9 are, in fact, badly constructed HTTP/1.x
4857   requests wherein a buggy client failed to properly encode
4858   linear whitespace found in a URI reference and placed in
4859   the request-target.
4860</t>
4861
4862<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
4863<t>
4864   This section summarizes major differences between versions HTTP/1.0
4865   and HTTP/1.1.
4866</t>
4867
4868<section title="Multi-homed Web Servers" anchor="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">
4869<t>
4870   The requirements that clients and servers support the <x:ref>Host</x:ref>
4871   header field (<xref target="header.host"/>), report an error if it is
4872   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
4873   are among the most important changes defined by HTTP/1.1.
4874</t>
4875<t>
4876   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4877   addresses and servers; there was no other established mechanism for
4878   distinguishing the intended server of a request than the IP address
4879   to which that request was directed. The <x:ref>Host</x:ref> header field was
4880   introduced during the development of HTTP/1.1 and, though it was
4881   quickly implemented by most HTTP/1.0 browsers, additional requirements
4882   were placed on all HTTP/1.1 requests in order to ensure complete
4883   adoption.  At the time of this writing, most HTTP-based services
4884   are dependent upon the Host header field for targeting requests.
4885</t>
4886</section>
4887
4888<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4889<t>
4890   In HTTP/1.0, each connection is established by the client prior to the
4891   request and closed by the server after sending the response. However, some
4892   implementations implement the explicitly negotiated ("Keep-Alive") version
4893   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
4894   target="RFC2068"/>.
4895</t>
4896<t>
4897   Some clients and servers might wish to be compatible with these previous
4898   approaches to persistent connections, by explicitly negotiating for them
4899   with a "Connection: keep-alive" request header field. However, some
4900   experimental implementations of HTTP/1.0 persistent connections are faulty;
4901   for example, if an HTTP/1.0 proxy server doesn't understand
4902   <x:ref>Connection</x:ref>, it will erroneously forward that header field
4903   to the next inbound server, which would result in a hung connection.
4904</t>
4905<t>
4906   One attempted solution was the introduction of a Proxy-Connection header
4907   field, targeted specifically at proxies. In practice, this was also
4908   unworkable, because proxies are often deployed in multiple layers, bringing
4909   about the same problem discussed above.
4910</t>
4911<t>
4912   As a result, clients are encouraged not to send the Proxy-Connection header
4913   field in any requests.
4914</t>
4915<t>
4916   Clients are also encouraged to consider the use of Connection: keep-alive
4917   in requests carefully; while they can enable persistent connections with
4918   HTTP/1.0 servers, clients using them will need to monitor the
4919   connection for "hung" requests (which indicate that the client ought stop
4920   sending the header field), and this mechanism ought not be used by clients
4921   at all when a proxy is being used.
4922</t>
4923</section>
4924
4925<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
4926<t>
4927   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
4928   (<xref target="header.transfer-encoding"/>).
4929   Transfer codings need to be decoded prior to forwarding an HTTP message
4930   over a MIME-compliant protocol.
4931</t>
4932</section>
4933
4934</section>
4935
4936<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4937<t>
4938  HTTP's approach to error handling has been explained.
4939  (<xref target="conformance"/>)
4940</t>
4941<t>
4942  The expectation to support HTTP/0.9 requests has been removed.
4943</t>
4944<t>
4945  The term "Effective Request URI" has been introduced.
4946  (<xref target="effective.request.uri" />)
4947</t>
4948<t>
4949  HTTP messages can be (and often are) buffered by implementations; despite
4950  it sometimes being available as a stream, HTTP is fundamentally a
4951  message-oriented protocol.
4952  (<xref target="http.message" />)
4953</t>
4954<t>
4955  Minimum supported sizes for various protocol elements have been
4956  suggested, to improve interoperability.
4957</t>
4958<t>
4959  Header fields that span multiple lines ("line folding") are deprecated.
4960  (<xref target="field.parsing" />)
4961</t>
4962<t>
4963  The HTTP-version ABNF production has been clarified to be case-sensitive.
4964  Additionally, version numbers has been restricted to single digits, due
4965  to the fact that implementations are known to handle multi-digit version
4966  numbers incorrectly.
4967  (<xref target="http.version"/>)
4968</t>
4969<t>
4970  The HTTPS URI scheme is now defined by this specification; previously,
4971  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
4972  (<xref target="https.uri"/>)
4973</t>
4974<t>
4975  The HTTPS URI scheme implies end-to-end security.
4976  (<xref target="https.uri"/>)
4977</t>
4978<t>
4979  Userinfo (i.e., username and password) are now disallowed in HTTP and
4980  HTTPS URIs, because of security issues related to their transmission on the
4981  wire.
4982  (<xref target="http.uri" />)
4983</t>
4984<t>
4985  Invalid whitespace around field-names is now required to be rejected,
4986  because accepting it represents a security vulnerability.
4987  (<xref target="header.fields"/>)
4988</t>
4989<t>
4990  The ABNF productions defining header fields now only list the field value.
4991  (<xref target="header.fields"/>)
4992</t>
4993<t>
4994  Rules about implicit linear whitespace between certain grammar productions
4995  have been removed; now whitespace is only allowed where specifically
4996  defined in the ABNF.
4997  (<xref target="whitespace"/>)
4998</t>
4999<t> 
5000  The NUL octet is no longer allowed in comment and quoted-string text, and
5001  handling of backslash-escaping in them has been clarified.
5002  (<xref target="field.components"/>)
5003</t> 
5004<t>
5005  The quoted-pair rule no longer allows escaping control characters other than
5006  HTAB.
5007  (<xref target="field.components"/>)
5008</t>
5009<t>
5010  Non-ASCII content in header fields and the reason phrase has been obsoleted
5011  and made opaque (the TEXT rule was removed).
5012  (<xref target="field.components"/>)
5013</t>
5014<t>
5015  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
5016  handled as errors by recipients.
5017  (<xref target="header.content-length"/>)
5018</t>
5019<t>
5020  The "identity" transfer coding token has been removed.
5021  (Sections <xref format="counter" target="message.body"/> and
5022  <xref format="counter" target="transfer.codings"/>)
5023</t>
5024<t>
5025  The algorithm for determining the message body length has been clarified
5026  to indicate all of the special cases (e.g., driven by methods or status
5027  codes) that affect it, and that new protocol elements cannot define such
5028  special cases.
5029  (<xref target="message.body.length"/>)
5030</t>
5031<t>
5032  "multipart/byteranges" is no longer a way of determining message body length
5033  detection.
5034  (<xref target="message.body.length"/>)
5035</t>
5036<t>
5037  CONNECT is a new, special case in determining message body length.
5038  (<xref target="message.body.length"/>)
5039</t>
5040<t>
5041  Chunk length does not include the count of the octets in the
5042  chunk header and trailer.
5043  (<xref target="chunked.encoding"/>)
5044</t>
5045<t>
5046  Use of chunk extensions is deprecated, and line folding in them is
5047  disallowed.
5048  (<xref target="chunked.encoding"/>)
5049</t>
5050<t>
5051  The segment + query components of RFC 3986 have been used to define the
5052  request-target, instead of abs_path from RFC 1808.
5053  (<xref target="request-target"/>)
5054</t>
5055<t>
5056  The asterisk-form of the request-target is only allowed in the OPTIONS
5057  method.
5058  (<xref target="request-target"/>)
5059</t>
5060<t>
5061  Gateways do not need to generate <x:ref>Via</x:ref> header fields anymore.
5062  (<xref target="header.via"/>)
5063</t>
5064<t>
5065  Exactly when "close" connection options have to be sent has been clarified.
5066  (<xref target="header.connection"/>)
5067</t>
5068<t> 
5069  "hop-by-hop" header fields are required to appear in the Connection header
5070  field; just because they're defined as hop-by-hop in this specification
5071  doesn't exempt them.
5072  (<xref target="header.connection"/>)
5073</t>
5074<t>
5075  The limit of two connections per server has been removed.
5076  (<xref target="persistent.connections"/>)
5077</t>
5078<t>
5079  An idempotent sequence of requests is no longer required to be retried.
5080  (<xref target="persistent.connections"/>)
5081</t>
5082<t>
5083  The requirement to retry requests under certain circumstances when the
5084  server prematurely closes the connection has been removed.
5085  (<xref target="persistent.connections"/>)
5086</t>
5087<t>
5088  Some extraneous requirements about when servers are allowed to close
5089  connections prematurely have been removed.
5090  (<xref target="persistent.connections"/>)
5091</t>
5092<t>
5093  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
5094  responses other than 101 (this was incorporated from <xref
5095  target="RFC2817"/>). Furthermore, the ordering in the field value is now
5096  significant.
5097  (<xref target="header.upgrade"/>)
5098</t>
5099<t>
5100  Registration of Transfer Codings now requires IETF Review
5101  (<xref target="transfer.coding.registry"/>)
5102</t>
5103<t>
5104  The meaning of the "deflate" content coding has been clarified.
5105  (<xref target="deflate.coding" />)
5106</t>
5107<t>
5108  This specification now defines the Upgrade Token Registry, previously
5109  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
5110  (<xref target="upgrade.token.registry"/>)
5111</t>
5112<t>
5113  Issues with the Keep-Alive and Proxy-Connection header fields in requests
5114  are pointed out, with use of the latter being discouraged altogether.
5115  (<xref target="compatibility.with.http.1.0.persistent.connections" />)
5116</t>
5117<t>
5118  Empty list elements in list productions (e.g., a list header field containing
5119  ", ,") have been deprecated.
5120  (<xref target="abnf.extension"/>)
5121</t>
5122</section>
5123</section>
5124
5125<section title="ABNF list extension: #rule" anchor="abnf.extension">
5126<t>
5127  A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
5128  improve readability in the definitions of some header field values.
5129</t>
5130<t>
5131  A construct "#" is defined, similar to "*", for defining comma-delimited
5132  lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
5133  at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
5134  comma (",") and optional whitespace (OWS).   
5135</t>
5136<figure><preamble>
5137  Thus,
5138</preamble><artwork type="example">
5139  1#element =&gt; element *( OWS "," OWS element )
5140</artwork></figure>
5141<figure><preamble>
5142  and:
5143</preamble><artwork type="example">
5144  #element =&gt; [ 1#element ]
5145</artwork></figure>
5146<figure><preamble>
5147  and for n &gt;= 1 and m &gt; 1:
5148</preamble><artwork type="example">
5149  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
5150</artwork></figure>
5151<t>
5152  For compatibility with legacy list rules, recipients &SHOULD; accept empty
5153  list elements. In other words, consumers would follow the list productions:
5154</t>
5155<figure><artwork type="example">
5156  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
5157 
5158  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
5159</artwork></figure>
5160<t>
5161  Note that empty elements do not contribute to the count of elements present,
5162  though.
5163</t>
5164<t>
5165  For example, given these ABNF productions:
5166</t>
5167<figure><artwork type="example">
5168  example-list      = 1#example-list-elmt
5169  example-list-elmt = token ; see <xref target="field.components"/>
5170</artwork></figure>
5171<t>
5172  Then these are valid values for example-list (not including the double
5173  quotes, which are present for delimitation only):
5174</t>
5175<figure><artwork type="example">
5176  "foo,bar"
5177  "foo ,bar,"
5178  "foo , ,bar,charlie   "
5179</artwork></figure>
5180<t>
5181  But these values would be invalid, as at least one non-empty element is
5182  required:
5183</t>
5184<figure><artwork type="example">
5185  ""
5186  ","
5187  ",   ,"
5188</artwork></figure>
5189<t>
5190  <xref target="collected.abnf"/> shows the collected ABNF, with the list rules
5191  expanded as explained above.
5192</t>
5193</section>
5194
5195<?BEGININC p1-messaging.abnf-appendix ?>
5196<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
5197<figure>
5198<artwork type="abnf" name="p1-messaging.parsed-abnf">
5199<x:ref>BWS</x:ref> = OWS
5200
5201<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
5202 connection-option ] )
5203<x:ref>Content-Length</x:ref> = 1*DIGIT
5204
5205<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
5206 ]
5207<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
5208<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
5209<x:ref>Host</x:ref> = uri-host [ ":" port ]
5210
5211<x:ref>OWS</x:ref> = *( SP / HTAB )
5212
5213<x:ref>RWS</x:ref> = 1*( SP / HTAB )
5214
5215<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
5216<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
5217<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
5218 transfer-coding ] )
5219
5220<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
5221<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
5222
5223<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
5224 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
5225 comment ] ) ] )
5226
5227<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
5228<x:ref>absolute-form</x:ref> = absolute-URI
5229<x:ref>absolute-path</x:ref> = 1*( "/" segment )
5230<x:ref>asterisk-form</x:ref> = "*"
5231<x:ref>attribute</x:ref> = token
5232<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
5233<x:ref>authority-form</x:ref> = authority
5234
5235<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
5236<x:ref>chunk-data</x:ref> = 1*OCTET
5237<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
5238<x:ref>chunk-ext-name</x:ref> = token
5239<x:ref>chunk-ext-val</x:ref> = token / quoted-str-nf
5240<x:ref>chunk-size</x:ref> = 1*HEXDIG
5241<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
5242<x:ref>comment</x:ref> = "(" *( ctext / quoted-cpair / comment ) ")"
5243<x:ref>connection-option</x:ref> = token
5244<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
5245 / %x2A-5B ; '*'-'['
5246 / %x5D-7E ; ']'-'~'
5247 / obs-text
5248
5249<x:ref>field-content</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5250<x:ref>field-name</x:ref> = token
5251<x:ref>field-value</x:ref> = *( field-content / obs-fold )
5252<x:ref>fragment</x:ref> = &lt;fragment, defined in [RFC3986], Section 3.5&gt;
5253
5254<x:ref>header-field</x:ref> = field-name ":" OWS field-value OWS
5255<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ] [ "#"
5256 fragment ]
5257<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ] [ "#"
5258 fragment ]
5259
5260<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
5261
5262<x:ref>message-body</x:ref> = *OCTET
5263<x:ref>method</x:ref> = token
5264
5265<x:ref>obs-fold</x:ref> = CRLF ( SP / HTAB )
5266<x:ref>obs-text</x:ref> = %x80-FF
5267<x:ref>origin-form</x:ref> = absolute-path [ "?" query ]
5268
5269<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
5270<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
5271<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
5272<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
5273<x:ref>protocol-name</x:ref> = token
5274<x:ref>protocol-version</x:ref> = token
5275<x:ref>pseudonym</x:ref> = token
5276
5277<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5278 / %x5D-7E ; ']'-'~'
5279 / obs-text
5280<x:ref>qdtext-nf</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5281 / %x5D-7E ; ']'-'~'
5282 / obs-text
5283<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5284<x:ref>quoted-cpair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5285<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5286<x:ref>quoted-str-nf</x:ref> = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
5287<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5288
5289<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5290<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5291<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5292<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5293<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5294<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5295<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5296 asterisk-form
5297
5298<x:ref>segment</x:ref> = &lt;segment, defined in [RFC3986], Section 3.3&gt;
5299<x:ref>special</x:ref> = "(" / ")" / "&lt;" / "&gt;" / "@" / "," / ";" / ":" / "\" /
5300 DQUOTE / "/" / "[" / "]" / "?" / "=" / "{" / "}"
5301<x:ref>start-line</x:ref> = request-line / status-line
5302<x:ref>status-code</x:ref> = 3DIGIT
5303<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5304
5305<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5306<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5307<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5308 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5309<x:ref>token</x:ref> = 1*tchar
5310<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5311<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5312 transfer-extension
5313<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5314<x:ref>transfer-parameter</x:ref> = attribute BWS "=" BWS value
5315
5316<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5317
5318<x:ref>value</x:ref> = word
5319
5320<x:ref>word</x:ref> = token / quoted-string
5321</artwork>
5322</figure>
5323</section>
5324<?ENDINC p1-messaging.abnf-appendix ?>
5325
5326<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5327
5328<section title="Since RFC 2616">
5329<t>
5330  Changes up to the first Working Group Last Call draft are summarized
5331  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-21#appendix-D"/>.
5332</t>
5333</section>
5334
5335<section title="Since draft-ietf-httpbis-p1-messaging-21" anchor="changes.since.21">
5336<t>
5337  Closed issues:
5338  <list style="symbols">
5339    <t>
5340      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/128"/>:
5341      "Cite HTTPS URI scheme definition" (the spec now includes the HTTPs
5342      scheme definition and thus updates RFC 2818)
5343    </t>
5344    <t>
5345      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/389"/>:
5346      "mention of 'proxies' in section about caches"
5347    </t>
5348    <t>
5349      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/390"/>:
5350      "use of ABNF terms from RFC 3986"
5351    </t>
5352    <t>
5353      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/391"/>:
5354      "transferring URIs with userinfo in payload"
5355    </t>
5356    <t>
5357      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/392"/>:
5358      "editorial improvements to message length definition"
5359    </t>
5360    <t>
5361      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/395"/>:
5362      "Connection header field MUST vs SHOULD"
5363    </t>
5364    <t>
5365      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/396"/>:
5366      "editorial improvements to persistent connections section"
5367    </t>
5368    <t>
5369      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/397"/>:
5370      "URI normalization vs empty path"
5371    </t>
5372    <t>
5373      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/408"/>:
5374      "p1 feedback"
5375    </t>
5376    <t>
5377      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/409"/>:
5378      "is parsing OBS-FOLD mandatory?"
5379    </t>
5380    <t>
5381      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/410"/>:
5382      "HTTPS and Shared Caching"
5383    </t>
5384    <t>
5385      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/411"/>:
5386      "Requirements for recipients of ws between start-line and first header field"
5387    </t>
5388    <t>
5389      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/412"/>:
5390      "SP and HT when being tolerant"
5391    </t>
5392    <t>
5393      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/414"/>:
5394      "Message Parsing Strictness"
5395    </t>
5396    <t>
5397      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/415"/>:
5398      "'Render'"
5399    </t>
5400    <t>
5401      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/418"/>:
5402      "No-Transform"
5403    </t>
5404    <t>
5405      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/419"/>:
5406      "p2 editorial feedback"
5407    </t>
5408    <t>
5409      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/420"/>:
5410      "Content-Length SHOULD be sent"
5411    </t>
5412    <t>
5413      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/431"/>:
5414      "origin-form does not allow path starting with "//""
5415    </t>
5416    <t>
5417      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/433"/>:
5418      "ambiguity in part 1 example"
5419    </t>
5420  </list>
5421</t>
5422</section>
5423
5424<section title="Since draft-ietf-httpbis-p1-messaging-22" anchor="changes.since.22">
5425<t>
5426  Closed issues:
5427  <list style="symbols">
5428    <t>
5429      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/435"/>:
5430      "Part1 should have a reference to TCP (RFC 793)"
5431    </t>
5432    <t>
5433      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/438"/>:
5434      "media type registration template issues"
5435    </t>
5436    <t>
5437      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/441"/>:
5438      P1 editorial nits
5439    </t>
5440    <t>
5441      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/442"/>:
5442      "BWS" (vs conformance)
5443    </t>
5444    <t>
5445      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/444"/>:
5446      "obs-fold language"
5447    </t>
5448    <t>
5449      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/445"/>:
5450      "Ordering in Upgrade"
5451    </t>
5452    <t>
5453      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/446"/>:
5454      "p1 editorial feedback"
5455    </t>
5456    <t>
5457      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/447"/>:
5458      "HTTP and TCP name delegation"
5459    </t>
5460    <t>
5461      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/449"/>:
5462      "Receiving a higher minor HTTP version number"
5463    </t>
5464    <t>
5465      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/451"/>:
5466      "HTTP(S) URIs and fragids"
5467    </t>
5468    <t>
5469      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/457"/>:
5470      "Registering x-gzip and x-deflate"
5471    </t>
5472    <t>
5473      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/460"/>:
5474      "Via and gateways"
5475    </t>
5476    <t>
5477      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/465"/>:
5478      "Mention 203 Non-Authoritative Information in p1"
5479    </t>
5480    <t>
5481      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/476"/>:
5482      "SHOULD and conformance"
5483    </t>
5484    <t>
5485      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/477"/>:
5486      "Pipelining language"
5487    </t>
5488    <t>
5489      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/482"/>:
5490      "proxy handling of a really bad Content-Length"
5491    </t>
5492  </list>
5493</t>
5494</section>
5495
5496<section title="Since draft-ietf-httpbis-p1-messaging-23" anchor="changes.since.23">
5497<t>
5498  Closed issues:
5499  <list style="symbols">
5500    <t>
5501      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/483"/>:
5502      "MUST fix Content-Length?"
5503    </t>
5504  </list>
5505</t>
5506</section>
5507</section>
5508
5509</back>
5510</rfc>
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