source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2332

Last change on this file since 2332 was 2332, checked in by fielding@…, 9 years ago

Explain in more detail the conformance requirements related to specific roles, recipients, and workarounds; addresses #484

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 235.4 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 "July">
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 defined such that, when they are
733   understood by a recipient, they might override or enhance the
734   interpretation of previously defined header fields.  When an
735   implementation receives an unrecognized header field, the recipient
736   &MUST; ignore that header field for local processing regardless of
737   the message's HTTP version.  An unrecognized header field received
738   by a proxy &MUST; be forwarded downstream unless the header field's
739   field-name is listed in the message's <x:ref>Connection</x:ref> header field
740   (see <xref target="header.connection"/>).
741   These requirements allow HTTP's functionality to be enhanced without
742   requiring prior update of deployed intermediaries.
743</t>
744<t>
745   Intermediaries that process HTTP messages (i.e., all intermediaries
746   other than those acting as tunnels) &MUST; send their own HTTP-version
747   in forwarded messages.  In other words, they &MUST-NOT; blindly
748   forward the first line of an HTTP message without ensuring that the
749   protocol version in that message matches a version to which that
750   intermediary is conformant for both the receiving and
751   sending of messages.  Forwarding an HTTP message without rewriting
752   the HTTP-version might result in communication errors when downstream
753   recipients use the message sender's version to determine what features
754   are safe to use for later communication with that sender.
755</t>
756<t>
757   A client &SHOULD; send a request version equal to the highest
758   version to which the client is conformant and
759   whose major version is no higher than the highest version supported
760   by the server, if this is known.  A client &MUST-NOT; send a
761   version to which it is not conformant.
762</t>
763<t>
764   A client &MAY; send a lower request version if it is known that
765   the server incorrectly implements the HTTP specification, but only
766   after the client has attempted at least one normal request and determined
767   from the response status or header fields (e.g., <x:ref>Server</x:ref>) that
768   the server improperly handles higher request versions.
769</t>
770<t>
771   A server &SHOULD; send a response version equal to the highest
772   version to which the server is conformant and
773   whose major version is less than or equal to the one received in the
774   request.  A server &MUST-NOT; send a version to which it is not
775   conformant.  A server &MAY; send a <x:ref>505 (HTTP Version Not
776   Supported)</x:ref> response if it cannot send a response using the
777   major version used in the client's request.
778</t>
779<t>
780   A server &MAY; send an HTTP/1.0 response to a request
781   if it is known or suspected that the client incorrectly implements the
782   HTTP specification and is incapable of correctly processing later
783   version responses, such as when a client fails to parse the version
784   number correctly or when an intermediary is known to blindly forward
785   the HTTP-version even when it doesn't conform to the given minor
786   version of the protocol. Such protocol downgrades &SHOULD-NOT; be
787   performed unless triggered by specific client attributes, such as when
788   one or more of the request header fields (e.g., <x:ref>User-Agent</x:ref>)
789   uniquely match the values sent by a client known to be in error.
790</t>
791<t>
792   The intention of HTTP's versioning design is that the major number
793   will only be incremented if an incompatible message syntax is
794   introduced, and that the minor number will only be incremented when
795   changes made to the protocol have the effect of adding to the message
796   semantics or implying additional capabilities of the sender.  However,
797   the minor version was not incremented for the changes introduced between
798   <xref target="RFC2068"/> and <xref target="RFC2616"/>, and this revision
799   has specifically avoided any such changes to the protocol.
800</t>
801<t>
802   When an HTTP message is received with a major version number that the
803   recipient implements, but a higher minor version number than what the
804   recipient implements, the recipient &SHOULD; process the message as if it
805   were in the highest minor version within that major version to which the
806   recipient is conformant. A recipient can assume that a message with a
807   higher minor version, when sent to a recipient that has not yet indicated
808   support for that higher version, is sufficiently backwards-compatible to be
809   safely processed by any implementation of the same major version.
810</t>
811</section>
812
813<section title="Uniform Resource Identifiers" anchor="uri">
814<iref primary="true" item="resource"/>
815<t>
816   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used
817   throughout HTTP as the means for identifying resources (&resource;).
818   URI references are used to target requests, indicate redirects, and define
819   relationships.
820</t>
821  <x:anchor-alias value="URI-reference"/>
822  <x:anchor-alias value="absolute-URI"/>
823  <x:anchor-alias value="relative-part"/>
824  <x:anchor-alias value="authority"/>
825  <x:anchor-alias value="uri-host"/>
826  <x:anchor-alias value="port"/>
827  <x:anchor-alias value="path-abempty"/>
828  <x:anchor-alias value="segment"/>
829  <x:anchor-alias value="query"/>
830  <x:anchor-alias value="fragment"/>
831  <x:anchor-alias value="absolute-path"/>
832  <x:anchor-alias value="partial-URI"/>
833<t>
834   This specification adopts the definitions of "URI-reference",
835   "absolute-URI", "relative-part", "authority", "port", "host",
836   "path-abempty", "segment", "query", and "fragment" from the
837   URI generic syntax.
838   In addition, we define an "absolute-path" rule (that differs from
839   RFC 3986's "path-absolute" in that it allows a leading "//")
840   and a "partial-URI" rule for protocol elements
841   that allow a relative URI but not a fragment.
842</t>
843<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>
844  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
845  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
846  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
847  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
848  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
849  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
850  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
851  <x:ref>segment</x:ref>       = &lt;segment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
852  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
853  <x:ref>fragment</x:ref>      = &lt;fragment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.5"/>&gt;
854 
855  <x:ref>absolute-path</x:ref> = 1*( "/" segment )
856  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
857</artwork></figure>
858<t>
859   Each protocol element in HTTP that allows a URI reference will indicate
860   in its ABNF production whether the element allows any form of reference
861   (URI-reference), only a URI in absolute form (absolute-URI), only the
862   path and optional query components, or some combination of the above.
863   Unless otherwise indicated, URI references are parsed
864   relative to the effective request URI
865   (<xref target="effective.request.uri"/>).
866</t>
867
868<section title="http URI scheme" anchor="http.uri">
869  <x:anchor-alias value="http-URI"/>
870  <iref item="http URI scheme" primary="true"/>
871  <iref item="URI scheme" subitem="http" primary="true"/>
872<t>
873   The "http" URI scheme is hereby defined for the purpose of minting
874   identifiers according to their association with the hierarchical
875   namespace governed by a potential HTTP origin server listening for
876   TCP (<xref target="RFC0793"/>) connections on a given port.
877</t>
878<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"><!--terminal production--></iref>
879  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
880             [ "#" <x:ref>fragment</x:ref> ]
881</artwork></figure>
882<t>
883   The HTTP origin server is identified by the generic syntax's
884   <x:ref>authority</x:ref> component, which includes a host identifier
885   and optional TCP port (<xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>).
886   The remainder of the URI, consisting of both the hierarchical path
887   component and optional query component, serves as an identifier for
888   a potential resource within that origin server's name space.
889</t>
890<t>
891   If the host identifier is provided as an IP address,
892   then the origin server is any listener on the indicated TCP port at
893   that IP address. If host is a registered name, then that name is
894   considered an indirect identifier and the recipient might use a name
895   resolution service, such as DNS, to find the address of a listener
896   for that host.
897   The host &MUST-NOT; be empty; if an "http" URI is received with an
898   empty host, then it &MUST; be rejected as invalid.
899   If the port subcomponent is empty or not given, then TCP port 80 is
900   assumed (the default reserved port for WWW services).
901</t>
902<t>
903   Regardless of the form of host identifier, access to that host is not
904   implied by the mere presence of its name or address. The host might or might
905   not exist and, even when it does exist, might or might not be running an
906   HTTP server or listening to the indicated port. The "http" URI scheme
907   makes use of the delegated nature of Internet names and addresses to
908   establish a naming authority (whatever entity has the ability to place
909   an HTTP server at that Internet name or address) and allows that
910   authority to determine which names are valid and how they might be used.
911</t>
912<t>
913   When an "http" URI is used within a context that calls for access to the
914   indicated resource, a client &MAY; attempt access by resolving
915   the host to an IP address, establishing a TCP connection to that address
916   on the indicated port, and sending an HTTP request message
917   (<xref target="http.message"/>) containing the URI's identifying data
918   (<xref target="message.routing"/>) to the server.
919   If the server responds to that request with a non-interim HTTP response
920   message, as described in &status-codes;, then that response
921   is considered an authoritative answer to the client's request.
922</t>
923<t>
924   Although HTTP is independent of the transport protocol, the "http"
925   scheme is specific to TCP-based services because the name delegation
926   process depends on TCP for establishing authority.
927   An HTTP service based on some other underlying connection protocol
928   would presumably be identified using a different URI scheme, just as
929   the "https" scheme (below) is used for resources that require an
930   end-to-end secured connection. Other protocols might also be used to
931   provide access to "http" identified resources &mdash; it is only the
932   authoritative interface that is specific to TCP.
933</t>
934<t>
935   The URI generic syntax for authority also includes a deprecated
936   userinfo subcomponent (<xref target="RFC3986" x:fmt="," x:sec="3.2.1"/>)
937   for including user authentication information in the URI.  Some
938   implementations make use of the userinfo component for internal
939   configuration of authentication information, such as within command
940   invocation options, configuration files, or bookmark lists, even
941   though such usage might expose a user identifier or password.
942   Senders &MUST; exclude the userinfo subcomponent (and its "@"
943   delimiter) when an "http" URI is transmitted within a message as a
944   request target or header field value.
945   Recipients of an "http" URI reference &SHOULD; parse for userinfo and
946   treat its presence as an error, since it is likely being used to obscure
947   the authority for the sake of phishing attacks.
948</t>
949</section>
950
951<section title="https URI scheme" anchor="https.uri">
952   <x:anchor-alias value="https-URI"/>
953   <iref item="https URI scheme"/>
954   <iref item="URI scheme" subitem="https"/>
955<t>
956   The "https" URI scheme is hereby defined for the purpose of minting
957   identifiers according to their association with the hierarchical
958   namespace governed by a potential HTTP origin server listening to a
959   given TCP port for TLS-secured connections
960   (<xref target="RFC0793"/>, <xref target="RFC5246"/>).
961</t>
962<t>
963   All of the requirements listed above for the "http" scheme are also
964   requirements for the "https" scheme, except that a default TCP port
965   of 443 is assumed if the port subcomponent is empty or not given,
966   and the TCP connection &MUST; be secured, end-to-end, through the
967   use of strong encryption prior to sending the first HTTP request.
968</t>
969<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="https-URI"><!--terminal production--></iref>
970  <x:ref>https-URI</x:ref> = "https:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
971              [ "#" <x:ref>fragment</x:ref> ]
972</artwork></figure>
973<t>
974   Note that the "https" URI scheme depends on both TLS and TCP for
975   establishing authority.
976   Resources made available via the "https" scheme have no shared
977   identity with the "http" scheme even if their resource identifiers
978   indicate the same authority (the same host listening to the same
979   TCP port).  They are distinct name spaces and are considered to be
980   distinct origin servers.  However, an extension to HTTP that is
981   defined to apply to entire host domains, such as the Cookie protocol
982   <xref target="RFC6265"/>, can allow information
983   set by one service to impact communication with other services
984   within a matching group of host domains.
985</t>
986<t>
987   The process for authoritative access to an "https" identified
988   resource is defined in <xref target="RFC2818"/>.
989</t>
990</section>
991
992<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
993<t>
994   Since the "http" and "https" schemes conform to the URI generic syntax,
995   such URIs are normalized and compared according to the algorithm defined
996   in <xref target="RFC3986" x:fmt="," x:sec="6"/>, using the defaults
997   described above for each scheme.
998</t>
999<t>
1000   If the port is equal to the default port for a scheme, the normal form is
1001   to omit the port subcomponent. When not being used in absolute form as the
1002   request target of an OPTIONS request, an empty path component is equivalent
1003   to an absolute path of "/", so the normal form is to provide a path of "/"
1004   instead. The scheme and host are case-insensitive and normally provided in
1005   lowercase; all other components are compared in a case-sensitive manner.
1006   Characters other than those in the "reserved" set are equivalent to their
1007   percent-encoded octets (see <xref target="RFC3986" x:fmt=","
1008   x:sec="2.1"/>): the normal form is to not encode them.
1009</t>
1010<t>
1011   For example, the following three URIs are equivalent:
1012</t>
1013<figure><artwork type="example">
1014   http://example.com:80/~smith/home.html
1015   http://EXAMPLE.com/%7Esmith/home.html
1016   http://EXAMPLE.com:/%7esmith/home.html
1017</artwork></figure>
1018</section>
1019</section>
1020</section>
1021
1022<section title="Message Format" anchor="http.message">
1023<x:anchor-alias value="generic-message"/>
1024<x:anchor-alias value="message.types"/>
1025<x:anchor-alias value="HTTP-message"/>
1026<x:anchor-alias value="start-line"/>
1027<iref item="header section"/>
1028<iref item="headers"/>
1029<iref item="header field"/>
1030<t>
1031   All HTTP/1.1 messages consist of a start-line followed by a sequence of
1032   octets in a format similar to the Internet Message Format
1033   <xref target="RFC5322"/>: zero or more header fields (collectively
1034   referred to as the "headers" or the "header section"), an empty line
1035   indicating the end of the header section, and an optional message body.
1036</t>
1037<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"><!--terminal production--></iref>
1038  <x:ref>HTTP-message</x:ref>   = <x:ref>start-line</x:ref>
1039                   *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1040                   <x:ref>CRLF</x:ref>
1041                   [ <x:ref>message-body</x:ref> ]
1042</artwork></figure>
1043<t>
1044   The normal procedure for parsing an HTTP message is to read the
1045   start-line into a structure, read each header field into a hash
1046   table by field name until the empty line, and then use the parsed
1047   data to determine if a message body is expected.  If a message body
1048   has been indicated, then it is read as a stream until an amount
1049   of octets equal to the message body length is read or the connection
1050   is closed.
1051</t>
1052<t>
1053   Recipients &MUST; parse an HTTP message as a sequence of octets in an
1054   encoding that is a superset of US-ASCII <xref target="USASCII"/>.
1055   Parsing an HTTP message as a stream of Unicode characters, without regard
1056   for the specific encoding, creates security vulnerabilities due to the
1057   varying ways that string processing libraries handle invalid multibyte
1058   character sequences that contain the octet LF (%x0A).  String-based
1059   parsers can only be safely used within protocol elements after the element
1060   has been extracted from the message, such as within a header field-value
1061   after message parsing has delineated the individual fields.
1062</t>
1063<t>
1064   An HTTP message can be parsed as a stream for incremental processing or
1065   forwarding downstream.  However, recipients cannot rely on incremental
1066   delivery of partial messages, since some implementations will buffer or
1067   delay message forwarding for the sake of network efficiency, security
1068   checks, or payload transformations.
1069</t>
1070<t>
1071   A sender &MUST-NOT; send whitespace between the start-line and
1072   the first header field.
1073   A recipient that receives whitespace between the start-line and
1074   the first header field &MUST; either reject the message as invalid or
1075   consume each whitespace-preceded line without further processing of it
1076   (i.e., ignore the entire line, along with any subsequent lines preceded
1077   by whitespace, until a properly formed header field is received or the
1078   header block is terminated).
1079</t>
1080<t>
1081   The presence of such whitespace in a request
1082   might be an attempt to trick a server into ignoring that field or
1083   processing the line after it as a new request, either of which might
1084   result in a security vulnerability if other implementations within
1085   the request chain interpret the same message differently.
1086   Likewise, the presence of such whitespace in a response might be
1087   ignored by some clients or cause others to cease parsing.
1088</t>
1089
1090<section title="Start Line" anchor="start.line">
1091  <x:anchor-alias value="Start-Line"/>
1092<t>
1093   An HTTP message can either be a request from client to server or a
1094   response from server to client.  Syntactically, the two types of message
1095   differ only in the start-line, which is either a request-line (for requests)
1096   or a status-line (for responses), and in the algorithm for determining
1097   the length of the message body (<xref target="message.body"/>).
1098</t>
1099<t>
1100   In theory, a client could receive requests and a server could receive
1101   responses, distinguishing them by their different start-line formats,
1102   but in practice servers are implemented to only expect a request
1103   (a response is interpreted as an unknown or invalid request method)
1104   and clients are implemented to only expect a response.
1105</t>
1106<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="start-line"/>
1107  <x:ref>start-line</x:ref>     = <x:ref>request-line</x:ref> / <x:ref>status-line</x:ref>
1108</artwork></figure>
1109
1110<section title="Request Line" anchor="request.line">
1111  <x:anchor-alias value="Request"/>
1112  <x:anchor-alias value="request-line"/>
1113<t>
1114   A request-line begins with a method token, followed by a single
1115   space (SP), the request-target, another single space (SP), the
1116   protocol version, and ending with CRLF.
1117</t>
1118<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1119  <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>
1120</artwork></figure>
1121<iref primary="true" item="method"/>
1122<t anchor="method">
1123   The method token indicates the request method to be performed on the
1124   target resource. The request method is case-sensitive.
1125</t>
1126<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1127  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1128</artwork></figure>
1129<t>
1130   The methods defined by this specification can be found in
1131   &methods;, along with information regarding the HTTP method registry
1132   and considerations for defining new methods.
1133</t>
1134<iref item="request-target"/>
1135<t>
1136   The request-target identifies the target resource upon which to apply
1137   the request, as defined in <xref target="request-target"/>.
1138</t>
1139<t>
1140   Recipients typically parse the request-line into its component parts by
1141   splitting on whitespace (see <xref target="message.robustness"/>), since
1142   no whitespace is allowed in the three components.
1143   Unfortunately, some user agents fail to properly encode or exclude
1144   whitespace found in hypertext references, resulting in those disallowed
1145   characters being sent in a request-target.
1146</t>
1147<t>
1148   Recipients of an invalid request-line &SHOULD; respond with either a
1149   <x:ref>400 (Bad Request)</x:ref> error or a <x:ref>301 (Moved Permanently)</x:ref>
1150   redirect with the request-target properly encoded.  Recipients &SHOULD-NOT;
1151   attempt to autocorrect and then process the request without a redirect,
1152   since the invalid request-line might be deliberately crafted to bypass
1153   security filters along the request chain.
1154</t>
1155<t>
1156   HTTP does not place a pre-defined limit on the length of a request-line.
1157   A server that receives a method longer than any that it implements
1158   &SHOULD; respond with a <x:ref>501 (Not Implemented)</x:ref> status code.
1159   A server &MUST; be prepared to receive URIs of unbounded length and
1160   respond with the <x:ref>414 (URI Too Long)</x:ref> status code if the received
1161   request-target would be longer than the server wishes to handle
1162   (see &status-414;).
1163</t>
1164<t>
1165   Various ad-hoc limitations on request-line length are found in practice.
1166   It is &RECOMMENDED; that all HTTP senders and recipients support, at a
1167   minimum, request-line lengths of 8000 octets.
1168</t>
1169</section>
1170
1171<section title="Status Line" anchor="status.line">
1172  <x:anchor-alias value="response"/>
1173  <x:anchor-alias value="status-line"/>
1174  <x:anchor-alias value="status-code"/>
1175  <x:anchor-alias value="reason-phrase"/>
1176<t>
1177   The first line of a response message is the status-line, consisting
1178   of the protocol version, a space (SP), the status code, another space,
1179   a possibly-empty textual phrase describing the status code, and
1180   ending with CRLF.
1181</t>
1182<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-line"/>
1183  <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>
1184</artwork></figure>
1185<t>
1186   The status-code element is a 3-digit integer code describing the
1187   result of the server's attempt to understand and satisfy the client's
1188   corresponding request. The rest of the response message is to be
1189   interpreted in light of the semantics defined for that status code.
1190   See &status-codes; for information about the semantics of status codes,
1191   including the classes of status code (indicated by the first digit),
1192   the status codes defined by this specification, considerations for the
1193   definition of new status codes, and the IANA registry.
1194</t>
1195<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-code"/>
1196  <x:ref>status-code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1197</artwork></figure>
1198<t>  
1199   The reason-phrase element exists for the sole purpose of providing a
1200   textual description associated with the numeric status code, mostly
1201   out of deference to earlier Internet application protocols that were more
1202   frequently used with interactive text clients. A client &SHOULD; ignore
1203   the reason-phrase content.
1204</t>
1205<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
1206  <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> )
1207</artwork></figure>
1208</section>
1209</section>
1210
1211<section title="Header Fields" anchor="header.fields">
1212  <x:anchor-alias value="header-field"/>
1213  <x:anchor-alias value="field-content"/>
1214  <x:anchor-alias value="field-name"/>
1215  <x:anchor-alias value="field-value"/>
1216  <x:anchor-alias value="obs-fold"/>
1217<t>
1218   Each HTTP header field consists of a case-insensitive field name
1219   followed by a colon (":"), optional leading whitespace, the field value,
1220   and optional trailing whitespace.
1221</t>
1222<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"/>
1223  <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>
1224  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1225  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>obs-fold</x:ref> )
1226  <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> )
1227  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref> ( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1228                 ; obsolete line folding
1229                 ; see <xref target="field.parsing"/>
1230</artwork></figure>
1231<t>
1232   The field-name token labels the corresponding field-value as having the
1233   semantics defined by that header field.  For example, the <x:ref>Date</x:ref>
1234   header field is defined in &header-date; as containing the origination
1235   timestamp for the message in which it appears.
1236</t>
1237
1238<section title="Field Extensibility" anchor="field.extensibility">
1239<t>
1240   HTTP header fields are fully extensible: there is no limit on the
1241   introduction of new field names, each presumably defining new semantics,
1242   nor on the number of header fields used in a given message.  Existing
1243   fields are defined in each part of this specification and in many other
1244   specifications outside the core standard.
1245   New header fields can be introduced without changing the protocol version
1246   if their defined semantics allow them to be safely ignored by recipients
1247   that do not recognize them.
1248</t>
1249<t>
1250   New HTTP header fields ought to be registered with IANA in the
1251   Message Header Field Registry, as described in &iana-header-registry;.
1252   A proxy &MUST; forward unrecognized header fields unless the
1253   field-name is listed in the <x:ref>Connection</x:ref> header field
1254   (<xref target="header.connection"/>) or the proxy is specifically
1255   configured to block, or otherwise transform, such fields.
1256   Other recipients &SHOULD; ignore unrecognized header fields.
1257</t>
1258</section>
1259
1260<section title="Field Order" anchor="field.order">
1261<t>
1262   The order in which header fields with differing field names are
1263   received is not significant. However, it is "good practice" to send
1264   header fields that contain control data first, such as <x:ref>Host</x:ref>
1265   on requests and <x:ref>Date</x:ref> on responses, so that implementations
1266   can decide when not to handle a message as early as possible.  A server
1267   &MUST; wait until the entire header section is received before interpreting
1268   a request message, since later header fields might include conditionals,
1269   authentication credentials, or deliberately misleading duplicate
1270   header fields that would impact request processing.
1271</t>
1272<t>
1273   A sender &MUST-NOT; generate multiple header fields with the same field
1274   name in a message unless either the entire field value for that
1275   header field is defined as a comma-separated list [i.e., #(values)]
1276   or the header field is a well-known exception (as noted below).
1277</t>
1278<t>
1279   Multiple header fields with the same field name can be combined into
1280   one "field-name: field-value" pair, without changing the semantics of the
1281   message, by appending each subsequent field value to the combined
1282   field value in order, separated by a comma. The order in which
1283   header fields with the same field name are received is therefore
1284   significant to the interpretation of the combined field value;
1285   a proxy &MUST-NOT; change the order of these field values when
1286   forwarding a message.
1287</t>
1288<x:note>
1289  <t>
1290   &Note; In practice, the "Set-Cookie" header field (<xref target="RFC6265"/>)
1291   often appears multiple times in a response message and does not use the
1292   list syntax, violating the above requirements on multiple header fields
1293   with the same name. Since it cannot be combined into a single field-value,
1294   recipients ought to handle "Set-Cookie" as a special case while processing
1295   header fields. (See Appendix A.2.3 of <xref target="Kri2001"/> for details.)
1296  </t>
1297</x:note>
1298</section>
1299
1300<section title="Whitespace" anchor="whitespace">
1301<t anchor="rule.LWS">
1302   This specification uses three rules to denote the use of linear
1303   whitespace: OWS (optional whitespace), RWS (required whitespace), and
1304   BWS ("bad" whitespace).
1305</t>
1306<t anchor="rule.OWS">
1307   The OWS rule is used where zero or more linear whitespace octets might
1308   appear. For protocol elements where optional whitespace is preferred to
1309   improve readability, a sender &SHOULD; generate the optional whitespace
1310   as a single SP; otherwise, a sender &SHOULD-NOT; generate optional
1311   whitespace except as needed to white-out invalid or unwanted protocol
1312   elements during in-place message filtering.
1313</t>
1314<t anchor="rule.RWS">
1315   The RWS rule is used when at least one linear whitespace octet is required
1316   to separate field tokens. A sender &SHOULD; generate RWS as a single SP.
1317</t>
1318<t anchor="rule.BWS">
1319   The BWS rule is used where the grammar allows optional whitespace only for
1320   historical reasons. A sender &MUST-NOT; generate BWS in messages.
1321   A recipient &MUST; parse for such bad whitespace and remove it before
1322   interpreting the protocol element.
1323</t>
1324<t anchor="rule.whitespace">
1325  <x:anchor-alias value="BWS"/>
1326  <x:anchor-alias value="OWS"/>
1327  <x:anchor-alias value="RWS"/>
1328</t>
1329<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"/>
1330  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1331                 ; optional whitespace
1332  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1333                 ; required whitespace
1334  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1335                 ; "bad" whitespace
1336</artwork></figure>
1337</section>
1338
1339<section title="Field Parsing" anchor="field.parsing">
1340<t>
1341   No whitespace is allowed between the header field-name and colon.
1342   In the past, differences in the handling of such whitespace have led to
1343   security vulnerabilities in request routing and response handling.
1344   A server &MUST; reject any received request message that contains
1345   whitespace between a header field-name and colon with a response code of
1346   <x:ref>400 (Bad Request)</x:ref>. A proxy &MUST; remove any such whitespace
1347   from a response message before forwarding the message downstream.
1348</t>
1349<t>
1350   A field value is preceded by optional whitespace (OWS); a single SP is
1351   preferred. The field value does not include any leading or trailing white
1352   space: OWS occurring before the first non-whitespace octet of the field
1353   value or after the last non-whitespace octet of the field value ought to be
1354   excluded by parsers when extracting the field value from a header field.
1355</t>
1356<t>
1357   A recipient of field-content containing multiple sequential octets of
1358   optional (OWS) or required (RWS) whitespace &SHOULD; either replace the
1359   sequence with a single SP or transform any non-SP octets in the sequence to
1360   SP octets before interpreting the field value or forwarding the message
1361   downstream.
1362</t>
1363<t>
1364   Historically, HTTP header field values could be extended over multiple
1365   lines by preceding each extra line with at least one space or horizontal
1366   tab (obs-fold). This specification deprecates such line folding except
1367   within the message/http media type
1368   (<xref target="internet.media.type.message.http"/>).
1369   Senders &MUST-NOT; generate messages that include line folding
1370   (i.e., that contain any field-value that contains a match to the
1371   <x:ref>obs-fold</x:ref> rule) unless the message is intended for packaging
1372   within the message/http media type.
1373</t>
1374<t>
1375   A server that receives an <x:ref>obs-fold</x:ref> in a request message that
1376   is not within a message/http container &MUST; either reject the message by
1377   sending a <x:ref>400 (Bad Request)</x:ref>, preferably with a
1378   representation explaining that obsolete line folding is unacceptable, or
1379   replace each received <x:ref>obs-fold</x:ref> with one or more
1380   <x:ref>SP</x:ref> octets prior to interpreting the field value or
1381   forwarding the message downstream.
1382</t>
1383<t>
1384   A proxy or gateway that receives an <x:ref>obs-fold</x:ref> in a response
1385   message that is not within a message/http container &MUST; either discard
1386   the message and replace it with a <x:ref>502 (Bad Gateway)</x:ref>
1387   response, preferably with a representation explaining that unacceptable
1388   line folding was received, or replace each received <x:ref>obs-fold</x:ref>
1389   with one or more <x:ref>SP</x:ref> octets prior to interpreting the field
1390   value or forwarding the message downstream.
1391</t>
1392<t>
1393   A user agent that receives an <x:ref>obs-fold</x:ref> in a response message
1394   that is not within a message/http container &MUST; replace each received
1395   <x:ref>obs-fold</x:ref> with one or more <x:ref>SP</x:ref> octets prior to
1396   interpreting the field value.
1397</t>
1398<t>
1399   Historically, HTTP has allowed field content with text in the ISO-8859-1
1400   <xref target="ISO-8859-1"/> charset, supporting other charsets only
1401   through use of <xref target="RFC2047"/> encoding.
1402   In practice, most HTTP header field values use only a subset of the
1403   US-ASCII charset <xref target="USASCII"/>. Newly defined
1404   header fields &SHOULD; limit their field values to US-ASCII octets.
1405   Recipients &SHOULD; treat other octets in field content (obs-text) as
1406   opaque data.
1407</t>
1408</section>
1409
1410<section title="Field Limits" anchor="field.limits">
1411<t>
1412   HTTP does not place a pre-defined limit on the length of each header field
1413   or on the length of the header block as a whole.  Various ad-hoc
1414   limitations on individual header field length are found in practice,
1415   often depending on the specific field semantics.
1416</t>
1417<t>
1418   A server &MUST; be prepared to receive request header fields of unbounded
1419   length and respond with an appropriate <x:ref>4xx (Client Error)</x:ref>
1420   status code if the received header field(s) are larger than the server
1421   wishes to process.
1422</t>
1423<t>
1424   A client &MUST; be prepared to receive response header fields of unbounded
1425   length. A client &MAY; discard or truncate received header fields that are
1426   larger than the client wishes to process if the field semantics are such
1427   that the dropped value(s) can be safely ignored without changing the
1428   response semantics.
1429</t>
1430</section>
1431
1432<section title="Field value components" anchor="field.components">
1433<t anchor="rule.token.separators">
1434  <x:anchor-alias value="tchar"/>
1435  <x:anchor-alias value="token"/>
1436  <x:anchor-alias value="special"/>
1437  <x:anchor-alias value="word"/>
1438   Many HTTP header field values consist of words (token or quoted-string)
1439   separated by whitespace or special characters. These special characters
1440   &MUST; be in a quoted string to be used within a parameter value (as defined
1441   in <xref target="transfer.codings"/>).
1442</t>
1443<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>
1444  <x:ref>word</x:ref>           = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1445
1446  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1447<!--
1448  IMPORTANT: when editing "tchar" make sure that "special" is updated accordingly!!!
1449 -->
1450  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1451                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1452                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1453                 ; any <x:ref>VCHAR</x:ref>, except <x:ref>special</x:ref>
1454
1455  <x:ref>special</x:ref>        = "(" / ")" / "&lt;" / ">" / "@" / ","
1456                 / ";" / ":" / "\" / DQUOTE / "/" / "["
1457                 / "]" / "?" / "=" / "{" / "}"
1458</artwork></figure>
1459<t anchor="rule.quoted-string">
1460  <x:anchor-alias value="quoted-string"/>
1461  <x:anchor-alias value="qdtext"/>
1462  <x:anchor-alias value="obs-text"/>
1463   A string of text is parsed as a single word if it is quoted using
1464   double-quote marks.
1465</t>
1466<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"/>
1467  <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>
1468  <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>
1469  <x:ref>obs-text</x:ref>       = %x80-FF
1470</artwork></figure>
1471<t anchor="rule.quoted-pair">
1472  <x:anchor-alias value="quoted-pair"/>
1473   The backslash octet ("\") can be used as a single-octet
1474   quoting mechanism within quoted-string constructs:
1475</t>
1476<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1477  <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> )
1478</artwork></figure>
1479<t>
1480   Recipients that process the value of a quoted-string &MUST; handle a
1481   quoted-pair as if it were replaced by the octet following the backslash.
1482</t>
1483<t>
1484   Senders &SHOULD-NOT; generate a quoted-pair in a quoted-string except where
1485   necessary to quote DQUOTE and backslash octets occurring within that string.
1486</t>
1487<t anchor="rule.comment">
1488  <x:anchor-alias value="comment"/>
1489  <x:anchor-alias value="ctext"/>
1490   Comments can be included in some HTTP header fields by surrounding
1491   the comment text with parentheses. Comments are only allowed in
1492   fields containing "comment" as part of their field value definition.
1493</t>
1494<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1495  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-cpair</x:ref> / <x:ref>comment</x:ref> ) ")"
1496  <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>
1497</artwork></figure>
1498<t anchor="rule.quoted-cpair">
1499  <x:anchor-alias value="quoted-cpair"/>
1500   The backslash octet ("\") can be used as a single-octet
1501   quoting mechanism within comment constructs:
1502</t>
1503<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-cpair"/>
1504  <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> )
1505</artwork></figure>
1506<t>
1507   Senders &SHOULD-NOT; escape octets in comments that do not require escaping
1508   (i.e., other than the backslash octet "\" and the parentheses "(" and ")").
1509</t>
1510</section>
1511
1512</section>
1513
1514<section title="Message Body" anchor="message.body">
1515  <x:anchor-alias value="message-body"/>
1516<t>
1517   The message body (if any) of an HTTP message is used to carry the
1518   payload body of that request or response.  The message body is
1519   identical to the payload body unless a transfer coding has been
1520   applied, as described in <xref target="header.transfer-encoding"/>.
1521</t>
1522<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1523  <x:ref>message-body</x:ref> = *OCTET
1524</artwork></figure>
1525<t>
1526   The rules for when a message body is allowed in a message differ for
1527   requests and responses.
1528</t>
1529<t>
1530   The presence of a message body in a request is signaled by a
1531   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1532   field. Request message framing is independent of method semantics,
1533   even if the method does not define any use for a message body.
1534</t>
1535<t>
1536   The presence of a message body in a response depends on both
1537   the request method to which it is responding and the response
1538   status code (<xref target="status.line"/>).
1539   Responses to the HEAD request method never include a message body
1540   because the associated response header fields (e.g.,
1541   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1542   if present, indicate only what their values would have been if the request
1543   method had been GET (&HEAD;).
1544   <x:ref>2xx (Successful)</x:ref> responses to CONNECT switch to tunnel
1545   mode instead of having a message body (&CONNECT;).
1546   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1547   <x:ref>304 (Not Modified)</x:ref> responses do not include a message body.
1548   All other responses do include a message body, although the body
1549   might be of zero length.
1550</t>
1551
1552<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1553  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1554  <iref item="chunked (Coding Format)"/>
1555  <x:anchor-alias value="Transfer-Encoding"/>
1556<t>
1557   The Transfer-Encoding header field lists the transfer coding names
1558   corresponding to the sequence of transfer codings that have been
1559   (or will be) applied to the payload body in order to form the message body.
1560   Transfer codings are defined in <xref target="transfer.codings"/>.
1561</t>
1562<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1563  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1564</artwork></figure>
1565<t>
1566   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1567   MIME, which was designed to enable safe transport of binary data over a
1568   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1569   However, safe transport has a different focus for an 8bit-clean transfer
1570   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1571   accurately delimit a dynamically generated payload and to distinguish
1572   payload encodings that are only applied for transport efficiency or
1573   security from those that are characteristics of the selected resource.
1574</t>
1575<t>
1576   All HTTP/1.1 recipients &MUST; implement the chunked transfer coding
1577   (<xref target="chunked.encoding"/>) because it plays a crucial role in
1578   framing messages when the payload body size is not known in advance.
1579   If chunked is applied to a payload body, the sender &MUST-NOT; apply
1580   chunked more than once (i.e., chunking an already chunked message is not
1581   allowed).
1582   If any transfer coding is applied to a request payload body, the
1583   sender &MUST; apply chunked as the final transfer coding to ensure that
1584   the message is properly framed.
1585   If any transfer coding is applied to a response payload body, the
1586   sender &MUST; either apply chunked as the final transfer coding or
1587   terminate the message by closing the connection.
1588</t>
1589<figure><preamble>
1590   For example,
1591</preamble><artwork type="example">
1592  Transfer-Encoding: gzip, chunked
1593</artwork><postamble>
1594   indicates that the payload body has been compressed using the gzip
1595   coding and then chunked using the chunked coding while forming the
1596   message body.
1597</postamble></figure>
1598<t>
1599   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1600   Transfer-Encoding is a property of the message, not of the representation, and
1601   any recipient along the request/response chain &MAY; decode the received
1602   transfer coding(s) or apply additional transfer coding(s) to the message
1603   body, assuming that corresponding changes are made to the Transfer-Encoding
1604   field-value. Additional information about the encoding parameters &MAY; be
1605   provided by other header fields not defined by this specification.
1606</t>
1607<t>
1608   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1609   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1610   neither of which includes a message body,
1611   to indicate that the origin server would have applied a transfer coding
1612   to the message body if the request had been an unconditional GET.
1613   This indication is not required, however, because any recipient on
1614   the response chain (including the origin server) can remove transfer
1615   codings when they are not needed.
1616</t>
1617<t>
1618   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1619   implementations advertising only HTTP/1.0 support will not understand
1620   how to process a transfer-encoded payload.
1621   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1622   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1623   might be in the form of specific user configuration or by remembering the
1624   version of a prior received response.
1625   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1626   the corresponding request indicates HTTP/1.1 (or later).
1627</t>
1628<t>
1629   A server that receives a request message with a transfer coding it does
1630   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref>.
1631</t>
1632</section>
1633
1634<section title="Content-Length" anchor="header.content-length">
1635  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1636  <x:anchor-alias value="Content-Length"/>
1637<t>
1638   When a message does not have a <x:ref>Transfer-Encoding</x:ref> header
1639   field, a Content-Length header field can provide the anticipated size,
1640   as a decimal number of octets, for a potential payload body.
1641   For messages that do include a payload body, the Content-Length field-value
1642   provides the framing information necessary for determining where the body
1643   (and message) ends.  For messages that do not include a payload body, the
1644   Content-Length indicates the size of the selected representation
1645   (&representation;).
1646</t>
1647<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1648  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1649</artwork></figure>
1650<t>
1651   An example is
1652</t>
1653<figure><artwork type="example">
1654  Content-Length: 3495
1655</artwork></figure>
1656<t>
1657   A sender &MUST-NOT; send a Content-Length header field in any message that
1658   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1659</t>
1660<t>
1661   A user agent &SHOULD; send a Content-Length in a request message when no
1662   <x:ref>Transfer-Encoding</x:ref> is sent and the request method defines
1663   a meaning for an enclosed payload body. For example, a Content-Length
1664   header field is normally sent in a POST request even when the value is
1665   0 (indicating an empty payload body).  A user agent &SHOULD-NOT; send a
1666   Content-Length header field when the request message does not contain a
1667   payload body and the method semantics do not anticipate such a body.
1668</t>
1669<t>
1670   A server &MAY; send a Content-Length header field in a response to a HEAD
1671   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1672   response unless its field-value equals the decimal number of octets that
1673   would have been sent in the payload body of a response if the same
1674   request had used the GET method.
1675</t>
1676<t>
1677   A server &MAY; send a Content-Length header field in a
1678   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1679   (&status-304;); a server &MUST-NOT; send Content-Length in such a
1680   response unless its field-value equals the decimal number of octets that
1681   would have been sent in the payload body of a <x:ref>200 (OK)</x:ref>
1682   response to the same request.
1683</t>
1684<t>
1685   A server &MUST-NOT; send a Content-Length header field in any response
1686   with a status code of
1687   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1688   A server &SHOULD-NOT; send a Content-Length header field in any
1689   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1690</t>
1691<t>
1692   Aside from the cases defined above, in the absence of Transfer-Encoding,
1693   an origin server &SHOULD; send a Content-Length header field when the
1694   payload body size is known prior to sending the complete header block.
1695   This will allow downstream recipients to measure transfer progress,
1696   know when a received message is complete, and potentially reuse the
1697   connection for additional requests.
1698</t>
1699<t>
1700   Any Content-Length field value greater than or equal to zero is valid.
1701   Since there is no predefined limit to the length of a payload,
1702   recipients &SHOULD; anticipate potentially large decimal numerals and
1703   prevent parsing errors due to integer conversion overflows
1704   (<xref target="attack.protocol.element.size.overflows"/>).
1705</t>
1706<t>
1707   If a message is received that has multiple Content-Length header fields
1708   with field-values consisting of the same decimal value, or a single
1709   Content-Length header field with a field value containing a list of
1710   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1711   duplicate Content-Length header fields have been generated or combined by an
1712   upstream message processor, then the recipient &MUST; either reject the
1713   message as invalid or replace the duplicated field-values with a single
1714   valid Content-Length field containing that decimal value prior to
1715   determining the message body length or forwarding the message.
1716</t>
1717<x:note>
1718  <t>
1719   &Note; HTTP's use of Content-Length for message framing differs
1720   significantly from the same field's use in MIME, where it is an optional
1721   field used only within the "message/external-body" media-type.
1722  </t>
1723</x:note>
1724</section>
1725
1726<section title="Message Body Length" anchor="message.body.length">
1727  <iref item="chunked (Coding Format)"/>
1728<t>
1729   The length of a message body is determined by one of the following
1730   (in order of precedence):
1731</t>
1732<t>
1733  <list style="numbers">
1734    <x:lt><t>
1735     Any response to a HEAD request and any response with a
1736     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1737     <x:ref>304 (Not Modified)</x:ref> status code is always
1738     terminated by the first empty line after the header fields, regardless of
1739     the header fields present in the message, and thus cannot contain a
1740     message body.
1741    </t></x:lt>
1742    <x:lt><t>
1743     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1744     connection will become a tunnel immediately after the empty line that
1745     concludes the header fields.  A client &MUST; ignore any
1746     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1747     fields received in such a message.
1748    </t></x:lt>
1749    <x:lt><t>
1750     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1751     and the chunked transfer coding (<xref target="chunked.encoding"/>)
1752     is the final encoding, the message body length is determined by reading
1753     and decoding the chunked data until the transfer coding indicates the
1754     data is complete.
1755    </t>
1756    <t>
1757     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1758     response and the chunked transfer coding is not the final encoding, the
1759     message body length is determined by reading the connection until it is
1760     closed by the server.
1761     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a request and the
1762     chunked transfer coding is not the final encoding, the message body
1763     length cannot be determined reliably; the server &MUST; respond with
1764     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1765    </t>
1766    <t>
1767     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1768     and a <x:ref>Content-Length</x:ref> header field, the Transfer-Encoding
1769     overrides the Content-Length. Such a message might indicate an attempt
1770     to perform request or response smuggling (bypass of security-related
1771     checks on message routing or content) and thus ought to be handled as
1772     an error.  A sender &MUST; remove the received Content-Length field
1773     prior to forwarding such a message downstream.
1774    </t></x:lt>
1775    <x:lt><t>
1776     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1777     either multiple <x:ref>Content-Length</x:ref> header fields having
1778     differing field-values or a single Content-Length header field having an
1779     invalid value, then the message framing is invalid and &MUST; be treated
1780     as an error to prevent request or response smuggling.
1781     If this is a request message, the server &MUST; respond with
1782     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1783     If this is a response message received by a proxy, the proxy
1784     &MUST; close the connection to the server, discard the received response,
1785     and send a <x:ref>502 (Bad Gateway)</x:ref> response to the client.
1786     If this is a response message received by a user agent, it &MUST; be
1787     treated as an error by discarding the message and closing the connection.
1788    </t></x:lt>
1789    <x:lt><t>
1790     If a valid <x:ref>Content-Length</x:ref> header field is present without
1791     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1792     expected message body length in octets.
1793     If the sender closes the connection or the recipient times out before the
1794     indicated number of octets are received, the recipient &MUST; consider
1795     the message to be incomplete and close the connection.
1796    </t></x:lt>
1797    <x:lt><t>
1798     If this is a request message and none of the above are true, then the
1799     message body length is zero (no message body is present).
1800    </t></x:lt>
1801    <x:lt><t>
1802     Otherwise, this is a response message without a declared message body
1803     length, so the message body length is determined by the number of octets
1804     received prior to the server closing the connection.
1805    </t></x:lt>
1806  </list>
1807</t>
1808<t>
1809   Since there is no way to distinguish a successfully completed,
1810   close-delimited message from a partially-received message interrupted
1811   by network failure, a server &SHOULD; use encoding or
1812   length-delimited messages whenever possible.  The close-delimiting
1813   feature exists primarily for backwards compatibility with HTTP/1.0.
1814</t>
1815<t>
1816   A server &MAY; reject a request that contains a message body but
1817   not a <x:ref>Content-Length</x:ref> by responding with
1818   <x:ref>411 (Length Required)</x:ref>.
1819</t>
1820<t>
1821   Unless a transfer coding other than chunked has been applied,
1822   a client that sends a request containing a message body &SHOULD;
1823   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1824   length is known in advance, rather than the chunked transfer coding, since some
1825   existing services respond to chunked with a <x:ref>411 (Length Required)</x:ref>
1826   status code even though they understand the chunked transfer coding.  This
1827   is typically because such services are implemented via a gateway that
1828   requires a content-length in advance of being called and the server
1829   is unable or unwilling to buffer the entire request before processing.
1830</t>
1831<t>
1832   A user agent that sends a request containing a message body &MUST; send a
1833   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1834   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1835   the form of specific user configuration or by remembering the version of a
1836   prior received response.
1837</t>
1838<t>
1839   If the final response to the last request on a connection has been
1840   completely received and there remains additional data to read, a user agent
1841   &MAY; discard the remaining data or attempt to determine if that data
1842   belongs as part of the prior response body, which might be the case if the
1843   prior message's Content-Length value is incorrect. A client &MUST-NOT;
1844   process, cache, or forward such extra data as a separate response, since
1845   such behavior would be vulnerable to cache poisoning.
1846</t>
1847</section>
1848</section>
1849
1850<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1851<t>
1852   A server that receives an incomplete request message, usually due to a
1853   canceled request or a triggered time-out exception, &MAY; send an error
1854   response prior to closing the connection.
1855</t>
1856<t>
1857   A client that receives an incomplete response message, which can occur
1858   when a connection is closed prematurely or when decoding a supposedly
1859   chunked transfer coding fails, &MUST; record the message as incomplete.
1860   Cache requirements for incomplete responses are defined in
1861   &cache-incomplete;.
1862</t>
1863<t>
1864   If a response terminates in the middle of the header block (before the
1865   empty line is received) and the status code might rely on header fields to
1866   convey the full meaning of the response, then the client cannot assume
1867   that meaning has been conveyed; the client might need to repeat the
1868   request in order to determine what action to take next.
1869</t>
1870<t>
1871   A message body that uses the chunked transfer coding is
1872   incomplete if the zero-sized chunk that terminates the encoding has not
1873   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1874   incomplete if the size of the message body received (in octets) is less than
1875   the value given by Content-Length.  A response that has neither chunked
1876   transfer coding nor Content-Length is terminated by closure of the
1877   connection, and thus is considered complete regardless of the number of
1878   message body octets received, provided that the header block was received
1879   intact.
1880</t>
1881</section>
1882
1883<section title="Message Parsing Robustness" anchor="message.robustness">
1884<t>
1885   Older HTTP/1.0 user agent implementations might send an extra CRLF
1886   after a POST request as a workaround for some early server
1887   applications that failed to read message body content that was
1888   not terminated by a line-ending. An HTTP/1.1 user agent &MUST-NOT;
1889   preface or follow a request with an extra CRLF.  If terminating
1890   the request message body with a line-ending is desired, then the
1891   user agent &MUST; count the terminating CRLF octets as part of the
1892   message body length.
1893</t>
1894<t>
1895   In the interest of robustness, servers &SHOULD; ignore at least one
1896   empty line received where a request-line is expected. In other words, if
1897   a server is reading the protocol stream at the beginning of a
1898   message and receives a CRLF first, the server &SHOULD; ignore the CRLF.
1899</t>
1900<t>
1901   Although the line terminator for the start-line and header
1902   fields is the sequence CRLF, recipients &MAY; recognize a
1903   single LF as a line terminator and ignore any preceding CR.
1904</t>
1905<t>
1906   Although the request-line and status-line grammar rules require that each
1907   of the component elements be separated by a single SP octet, recipients
1908   &MAY; instead parse on whitespace-delimited word boundaries and, aside
1909   from the CRLF terminator, treat any form of whitespace as the SP separator
1910   while ignoring preceding or trailing whitespace;
1911   such whitespace includes one or more of the following octets:
1912   SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
1913</t>
1914<t>
1915   When a server listening only for HTTP request messages, or processing
1916   what appears from the start-line to be an HTTP request message,
1917   receives a sequence of octets that does not match the HTTP-message
1918   grammar aside from the robustness exceptions listed above, the
1919   server &SHOULD; respond with a <x:ref>400 (Bad Request)</x:ref> response. 
1920</t>
1921</section>
1922</section>
1923
1924<section title="Transfer Codings" anchor="transfer.codings">
1925  <x:anchor-alias value="transfer-coding"/>
1926  <x:anchor-alias value="transfer-extension"/>
1927<t>
1928   Transfer coding names are used to indicate an encoding
1929   transformation that has been, can be, or might need to be applied to a
1930   payload body in order to ensure "safe transport" through the network.
1931   This differs from a content coding in that the transfer coding is a
1932   property of the message rather than a property of the representation
1933   that is being transferred.
1934</t>
1935<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1936  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1937                     / "compress" ; <xref target="compress.coding"/>
1938                     / "deflate" ; <xref target="deflate.coding"/>
1939                     / "gzip" ; <xref target="gzip.coding"/>
1940                     / <x:ref>transfer-extension</x:ref>
1941  <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> )
1942</artwork></figure>
1943<t anchor="rule.parameter">
1944  <x:anchor-alias value="attribute"/>
1945  <x:anchor-alias value="transfer-parameter"/>
1946  <x:anchor-alias value="value"/>
1947   Parameters are in the form of attribute/value pairs.
1948</t>
1949<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"/>
1950  <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>
1951  <x:ref>attribute</x:ref>          = <x:ref>token</x:ref>
1952  <x:ref>value</x:ref>              = <x:ref>word</x:ref>
1953</artwork></figure>
1954<t>
1955   All transfer-coding names are case-insensitive and ought to be registered
1956   within the HTTP Transfer Coding registry, as defined in
1957   <xref target="transfer.coding.registry"/>.
1958   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1959   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1960   header fields.
1961</t>
1962
1963<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1964  <iref primary="true" item="chunked (Coding Format)"/>
1965  <x:anchor-alias value="chunk"/>
1966  <x:anchor-alias value="chunked-body"/>
1967  <x:anchor-alias value="chunk-data"/>
1968  <x:anchor-alias value="chunk-ext"/>
1969  <x:anchor-alias value="chunk-ext-name"/>
1970  <x:anchor-alias value="chunk-ext-val"/>
1971  <x:anchor-alias value="chunk-size"/>
1972  <x:anchor-alias value="last-chunk"/>
1973  <x:anchor-alias value="trailer-part"/>
1974  <x:anchor-alias value="quoted-str-nf"/>
1975  <x:anchor-alias value="qdtext-nf"/>
1976<t>
1977   The chunked transfer coding modifies the body of a message in order to
1978   transfer it as a series of chunks, each with its own size indicator,
1979   followed by an &OPTIONAL; trailer containing header fields. This
1980   allows dynamically generated content to be transferred along with the
1981   information necessary for the recipient to verify that it has
1982   received the full message.
1983</t>
1984<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"/>
1985  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1986                   <x:ref>last-chunk</x:ref>
1987                   <x:ref>trailer-part</x:ref>
1988                   <x:ref>CRLF</x:ref>
1989 
1990  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1991                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1992  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1993  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1994 
1995  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
1996  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
1997  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-str-nf</x:ref>
1998  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1999  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
2000 
2001  <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>
2002                 ; like <x:ref>quoted-string</x:ref>, but disallowing line folding
2003  <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>
2004</artwork></figure>
2005<t>
2006   Chunk extensions within the chunked transfer coding are deprecated.
2007   Senders &SHOULD-NOT; send chunk-ext.
2008   Definition of new chunk extensions is discouraged.
2009</t>
2010<t>
2011   The chunk-size field is a string of hex digits indicating the size of
2012   the chunk-data in octets. The chunked transfer coding is complete when a
2013   chunk with a chunk-size of zero is received, possibly followed by a
2014   trailer, and finally terminated by an empty line.
2015</t>
2016
2017<section title="Trailer" anchor="header.trailer">
2018  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
2019  <x:anchor-alias value="Trailer"/>
2020<t>
2021   A trailer allows the sender to include additional fields at the end of a
2022   chunked message in order to supply metadata that might be dynamically
2023   generated while the message body is sent, such as a message integrity
2024   check, digital signature, or post-processing status.
2025   The trailer &MUST-NOT; contain fields that need to be known before a
2026   recipient processes the body, such as <x:ref>Transfer-Encoding</x:ref>,
2027   <x:ref>Content-Length</x:ref>, and <x:ref>Trailer</x:ref>.
2028</t>
2029<t>
2030   When a message includes a message body encoded with the chunked
2031   transfer coding and the sender desires to send metadata in the form of
2032   trailer fields at the end of the message, the sender &SHOULD; send a
2033   <x:ref>Trailer</x:ref> header field before the message body to indicate
2034   which fields will be present in the trailers. This allows the recipient
2035   to prepare for receipt of that metadata before it starts processing the body,
2036   which is useful if the message is being streamed and the recipient wishes
2037   to confirm an integrity check on the fly.
2038</t>
2039<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
2040  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
2041</artwork></figure>
2042<t>
2043   If no <x:ref>Trailer</x:ref> header field is present, the sender of a
2044   chunked message body &SHOULD; send an empty trailer.
2045</t>
2046<t>
2047   A server &MUST; send an empty trailer with the chunked transfer coding
2048   unless at least one of the following is true:
2049  <list style="numbers">
2050    <t>the request included a <x:ref>TE</x:ref> header field that indicates
2051    "trailers" is acceptable in the transfer coding of the response, as
2052    described in <xref target="header.te"/>; or,</t>
2053     
2054    <t>the trailer fields consist entirely of optional metadata and the
2055    recipient could use the message (in a manner acceptable to the server where
2056    the field originated) without receiving that metadata. In other words,
2057    the server that generated the header field is willing to accept the
2058    possibility that the trailer fields might be silently discarded along
2059    the path to the client.</t>
2060  </list>
2061</t>
2062<t>
2063   The above requirement prevents the need for an infinite buffer when a
2064   message is being received by an HTTP/1.1 (or later) proxy and forwarded to
2065   an HTTP/1.0 recipient.
2066</t>
2067</section>
2068
2069<section title="Decoding chunked" anchor="decoding.chunked">
2070<t>
2071   A process for decoding the chunked transfer coding
2072   can be represented in pseudo-code as:
2073</t>
2074<figure><artwork type="code">
2075  length := 0
2076  read chunk-size, chunk-ext (if any), and CRLF
2077  while (chunk-size &gt; 0) {
2078     read chunk-data and CRLF
2079     append chunk-data to decoded-body
2080     length := length + chunk-size
2081     read chunk-size, chunk-ext (if any), and CRLF
2082  }
2083  read header-field
2084  while (header-field not empty) {
2085     append header-field to existing header fields
2086     read header-field
2087  }
2088  Content-Length := length
2089  Remove "chunked" from Transfer-Encoding
2090  Remove Trailer from existing header fields
2091</artwork></figure>
2092<t>
2093   All recipients &MUST; be able to receive and decode the
2094   chunked transfer coding and &MUST; ignore chunk-ext extensions
2095   they do not understand.
2096</t>
2097</section>
2098</section>
2099
2100<section title="Compression Codings" anchor="compression.codings">
2101<t>
2102   The codings defined below can be used to compress the payload of a
2103   message.
2104</t>
2105
2106<section title="Compress Coding" anchor="compress.coding">
2107<iref item="compress (Coding Format)"/>
2108<t>
2109   The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding
2110   <xref target="Welch"/> that is commonly produced by the UNIX file
2111   compression program "compress".
2112   Recipients &SHOULD; consider "x-compress" to be equivalent to "compress".
2113</t>
2114</section>
2115
2116<section title="Deflate Coding" anchor="deflate.coding">
2117<iref item="deflate (Coding Format)"/>
2118<t>
2119   The "deflate" coding is a "zlib" data format <xref target="RFC1950"/>
2120   containing a "deflate" compressed data stream <xref target="RFC1951"/>
2121   that uses a combination of the Lempel-Ziv (LZ77) compression algorithm and
2122   Huffman coding.
2123</t>
2124<x:note>
2125  <t>
2126    &Note; Some incorrect implementations send the "deflate"
2127    compressed data without the zlib wrapper.
2128   </t>
2129</x:note>
2130</section>
2131
2132<section title="Gzip Coding" anchor="gzip.coding">
2133<iref item="gzip (Coding Format)"/>
2134<t>
2135   The "gzip" coding is an LZ77 coding with a 32 bit CRC that is commonly
2136   produced by the gzip file compression program <xref target="RFC1952"/>.
2137   Recipients &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2138</t>
2139</section>
2140
2141</section>
2142
2143<section title="TE" anchor="header.te">
2144  <iref primary="true" item="TE header field" x:for-anchor=""/>
2145  <x:anchor-alias value="TE"/>
2146  <x:anchor-alias value="t-codings"/>
2147  <x:anchor-alias value="t-ranking"/>
2148  <x:anchor-alias value="rank"/>
2149<t>
2150   The "TE" header field in a request indicates what transfer codings,
2151   besides chunked, the client is willing to accept in response, and
2152   whether or not the client is willing to accept trailer fields in a
2153   chunked transfer coding.
2154</t>
2155<t>
2156   The TE field-value consists of a comma-separated list of transfer coding
2157   names, each allowing for optional parameters (as described in
2158   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2159   Clients &MUST-NOT; send the chunked transfer coding name in TE;
2160   chunked is always acceptable for HTTP/1.1 recipients.
2161</t>
2162<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"/>
2163  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2164  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2165  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2166  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2167             / ( "1" [ "." 0*3("0") ] )
2168</artwork></figure>
2169<t>
2170   Three examples of TE use are below.
2171</t>
2172<figure><artwork type="example">
2173  TE: deflate
2174  TE:
2175  TE: trailers, deflate;q=0.5
2176</artwork></figure>
2177<t>
2178   The presence of the keyword "trailers" indicates that the client is willing
2179   to accept trailer fields in a chunked transfer coding, as defined in
2180   <xref target="chunked.encoding"/>, on behalf of itself and any downstream
2181   clients. For requests from an intermediary, this implies that either:
2182   (a) all downstream clients are willing to accept trailer fields in the
2183   forwarded response; or,
2184   (b) the intermediary will attempt to buffer the response on behalf of
2185   downstream recipients.
2186   Note that HTTP/1.1 does not define any means to limit the size of a
2187   chunked response such that an intermediary can be assured of buffering the
2188   entire response.
2189</t>
2190<t>
2191   When multiple transfer codings are acceptable, the client &MAY; rank the
2192   codings by preference using a case-insensitive "q" parameter (similar to
2193   the qvalues used in content negotiation fields, &qvalue;). The rank value
2194   is a real number in the range 0 through 1, where 0.001 is the least
2195   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2196</t>
2197<t>
2198   If the TE field-value is empty or if no TE field is present, the only
2199   acceptable transfer coding is chunked. A message with no transfer coding
2200   is always acceptable.
2201</t>
2202<t>
2203   Since the TE header field only applies to the immediate connection,
2204   a sender of TE &MUST; also send a "TE" connection option within the
2205   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2206   in order to prevent the TE field from being forwarded by intermediaries
2207   that do not support its semantics.
2208</t>
2209</section>
2210</section>
2211
2212<section title="Message Routing" anchor="message.routing">
2213<t>
2214   HTTP request message routing is determined by each client based on the
2215   target resource, the client's proxy configuration, and
2216   establishment or reuse of an inbound connection.  The corresponding
2217   response routing follows the same connection chain back to the client.
2218</t>
2219
2220<section title="Identifying a Target Resource" anchor="target-resource">
2221  <iref primary="true" item="target resource"/>
2222  <iref primary="true" item="target URI"/>
2223  <x:anchor-alias value="target resource"/>
2224  <x:anchor-alias value="target URI"/>
2225<t>
2226   HTTP is used in a wide variety of applications, ranging from
2227   general-purpose computers to home appliances.  In some cases,
2228   communication options are hard-coded in a client's configuration.
2229   However, most HTTP clients rely on the same resource identification
2230   mechanism and configuration techniques as general-purpose Web browsers.
2231</t>
2232<t>
2233   HTTP communication is initiated by a user agent for some purpose.
2234   The purpose is a combination of request semantics, which are defined in
2235   <xref target="Part2"/>, and a target resource upon which to apply those
2236   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2237   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2238   would resolve to its absolute form in order to obtain the
2239   "<x:dfn>target URI</x:dfn>".  The target URI
2240   excludes the reference's fragment component, if any,
2241   since fragment identifiers are reserved for client-side processing
2242   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2243</t>
2244</section>
2245
2246<section title="Connecting Inbound" anchor="connecting.inbound">
2247<t>
2248   Once the target URI is determined, a client needs to decide whether
2249   a network request is necessary to accomplish the desired semantics and,
2250   if so, where that request is to be directed.
2251</t>
2252<t>
2253   If the client has a cache <xref target="Part6"/> and the request can be
2254   satisfied by it, then the request is
2255   usually directed there first.
2256</t>
2257<t>
2258   If the request is not satisfied by a cache, then a typical client will
2259   check its configuration to determine whether a proxy is to be used to
2260   satisfy the request.  Proxy configuration is implementation-dependent,
2261   but is often based on URI prefix matching, selective authority matching,
2262   or both, and the proxy itself is usually identified by an "http" or
2263   "https" URI.  If a proxy is applicable, the client connects inbound by
2264   establishing (or reusing) a connection to that proxy.
2265</t>
2266<t>
2267   If no proxy is applicable, a typical client will invoke a handler routine,
2268   usually specific to the target URI's scheme, to connect directly
2269   to an authority for the target resource.  How that is accomplished is
2270   dependent on the target URI scheme and defined by its associated
2271   specification, similar to how this specification defines origin server
2272   access for resolution of the "http" (<xref target="http.uri"/>) and
2273   "https" (<xref target="https.uri"/>) schemes.
2274</t>
2275<t>
2276   HTTP requirements regarding connection management are defined in
2277   <xref target="connection.management"/>.
2278</t>
2279</section>
2280
2281<section title="Request Target" anchor="request-target">
2282<t>
2283   Once an inbound connection is obtained,
2284   the client sends an HTTP request message (<xref target="http.message"/>)
2285   with a request-target derived from the target URI.
2286   There are four distinct formats for the request-target, depending on both
2287   the method being requested and whether the request is to a proxy.
2288</t>   
2289<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"/>
2290  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2291                 / <x:ref>absolute-form</x:ref>
2292                 / <x:ref>authority-form</x:ref>
2293                 / <x:ref>asterisk-form</x:ref>
2294
2295  <x:ref>origin-form</x:ref>    = <x:ref>absolute-path</x:ref> [ "?" <x:ref>query</x:ref> ]
2296  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2297  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2298  <x:ref>asterisk-form</x:ref>  = "*"
2299</artwork></figure>
2300<t anchor="origin-form"><iref item="origin-form (of request-target)"/>
2301  <x:h>origin-form</x:h>
2302</t>
2303<t>
2304   The most common form of request-target is the <x:dfn>origin-form</x:dfn>.
2305   When making a request directly to an origin server, other than a CONNECT
2306   or server-wide OPTIONS request (as detailed below),
2307   a client &MUST; send only the absolute path and query components of
2308   the target URI as the request-target.
2309   If the target URI's path component is empty, then the client &MUST; send
2310   "/" as the path within the origin-form of request-target.
2311   A <x:ref>Host</x:ref> header field is also sent, as defined in
2312   <xref target="header.host"/>, containing the target URI's
2313   authority component (excluding any userinfo).
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 CONNECT requests
2363   (&CONNECT;).  When making a CONNECT request to establish a tunnel through
2364   one or more proxies, a client &MUST; send only the target URI's
2365   authority component (excluding any userinfo) as the request-target.
2366   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.  Since the Host
2415   field-value is critical information for handling a request, it
2416   &SHOULD; be sent as the first header field following the request-line.
2417</t>
2418<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2419  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2420</artwork></figure>
2421<t>
2422   A client &MUST; send a Host header field in all HTTP/1.1 request
2423   messages.  If the target URI includes an authority component, then
2424   the Host field-value &MUST; be identical to that authority component
2425   after excluding any userinfo (<xref target="http.uri"/>).
2426   If the authority component is missing or undefined for the target URI,
2427   then the Host header field &MUST; be sent with an empty field-value.
2428</t>
2429<t>
2430   For example, a GET request to the origin server for
2431   &lt;http://www.example.org/pub/WWW/&gt; would begin with:
2432</t>
2433<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2434GET /pub/WWW/ HTTP/1.1
2435Host: www.example.org
2436</artwork></figure>
2437<t>
2438   The Host header field &MUST; be sent in an HTTP/1.1 request even
2439   if the request-target is in the absolute-form, since this
2440   allows the Host information to be forwarded through ancient HTTP/1.0
2441   proxies that might not have implemented Host.
2442</t>
2443<t>
2444   When a proxy receives a request with an absolute-form of
2445   request-target, the proxy &MUST; ignore the received
2446   Host header field (if any) and instead replace it with the host
2447   information of the request-target.  If the proxy forwards the request,
2448   it &MUST; generate a new Host field-value based on the received
2449   request-target rather than forward the received Host field-value.
2450</t>
2451<t>
2452   Since the Host header field acts as an application-level routing
2453   mechanism, it is a frequent target for malware seeking to poison
2454   a shared cache or redirect a request to an unintended server.
2455   An interception proxy is particularly vulnerable if it relies on
2456   the Host field-value for redirecting requests to internal
2457   servers, or for use as a cache key in a shared cache, without
2458   first verifying that the intercepted connection is targeting a
2459   valid IP address for that host.
2460</t>
2461<t>
2462   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2463   to any HTTP/1.1 request message that lacks a Host header field and
2464   to any request message that contains more than one Host header field
2465   or a Host header field with an invalid field-value.
2466</t>
2467</section>
2468
2469<section title="Effective Request URI" anchor="effective.request.uri">
2470  <iref primary="true" item="effective request URI"/>
2471  <x:anchor-alias value="effective request URI"/>
2472<t>
2473   A server that receives an HTTP request message &MUST; reconstruct
2474   the user agent's original target URI, based on the pieces of information
2475   learned from the request-target, <x:ref>Host</x:ref> header field, and
2476   connection context, in order to identify the intended target resource and
2477   properly service the request. The URI derived from this reconstruction
2478   process is referred to as the "<x:dfn>effective request URI</x:dfn>".
2479</t>
2480<t>
2481   For a user agent, the effective request URI is the target URI.
2482</t>
2483<t>
2484   If the request-target is in absolute-form, then the effective request URI
2485   is the same as the request-target.  Otherwise, the effective request URI
2486   is constructed as follows.
2487</t>
2488<t>
2489   If the request is received over a TLS-secured TCP connection,
2490   then the effective request URI's scheme is "https"; otherwise, the
2491   scheme is "http".
2492</t>
2493<t>
2494   If the request-target is in authority-form, then the effective
2495   request URI's authority component is the same as the request-target.
2496   Otherwise, if a <x:ref>Host</x:ref> header field is supplied with a
2497   non-empty field-value, then the authority component is the same as the
2498   Host field-value. Otherwise, the authority component is the concatenation of
2499   the default host name configured for the server, a colon (":"), and the
2500   connection's incoming TCP port number in decimal form.
2501</t>
2502<t>
2503   If the request-target is in authority-form or asterisk-form, then the
2504   effective request URI's combined path and query component is empty.
2505   Otherwise, the combined path and query component is the same as the
2506   request-target.
2507</t>
2508<t>
2509   The components of the effective request URI, once determined as above,
2510   can be combined into absolute-URI form by concatenating the scheme,
2511   "://", authority, and combined path and query component.
2512</t>
2513<figure>
2514<preamble>
2515   Example 1: the following message received over an insecure TCP connection
2516</preamble>
2517<artwork type="example" x:indent-with="  ">
2518GET /pub/WWW/TheProject.html HTTP/1.1
2519Host: www.example.org:8080
2520</artwork>
2521</figure>
2522<figure>
2523<preamble>
2524  has an effective request URI of
2525</preamble>
2526<artwork type="example" x:indent-with="  ">
2527http://www.example.org:8080/pub/WWW/TheProject.html
2528</artwork>
2529</figure>
2530<figure>
2531<preamble>
2532   Example 2: the following message received over a TLS-secured TCP connection
2533</preamble>
2534<artwork type="example" x:indent-with="  ">
2535OPTIONS * HTTP/1.1
2536Host: www.example.org
2537</artwork>
2538</figure>
2539<figure>
2540<preamble>
2541  has an effective request URI of
2542</preamble>
2543<artwork type="example" x:indent-with="  ">
2544https://www.example.org
2545</artwork>
2546</figure>
2547<t>
2548   An origin server that does not allow resources to differ by requested
2549   host &MAY; ignore the <x:ref>Host</x:ref> field-value and instead replace it
2550   with a configured server name when constructing the effective request URI.
2551</t>
2552<t>
2553   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2554   field &MAY; attempt to use heuristics (e.g., examination of the URI path for
2555   something unique to a particular host) in order to guess the
2556   effective request URI's authority component.
2557</t>
2558</section>
2559
2560<section title="Associating a Response to a Request" anchor="associating.response.to.request">
2561<t>
2562   HTTP does not include a request identifier for associating a given
2563   request message with its corresponding one or more response messages.
2564   Hence, it relies on the order of response arrival to correspond exactly
2565   to the order in which requests are made on the same connection.
2566   More than one response message per request only occurs when one or more
2567   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a
2568   final response to the same request.
2569</t>
2570<t>
2571   A client that has more than one outstanding request on a connection &MUST;
2572   maintain a list of outstanding requests in the order sent and &MUST;
2573   associate each received response message on that connection to the highest
2574   ordered request that has not yet received a final (non-<x:ref>1xx</x:ref>)
2575   response.
2576</t>
2577</section>
2578
2579<section title="Message Forwarding" anchor="message.forwarding">
2580<t>
2581   As described in <xref target="intermediaries"/>, intermediaries can serve
2582   a variety of roles in the processing of HTTP requests and responses.
2583   Some intermediaries are used to improve performance or availability.
2584   Others are used for access control or to filter content.
2585   Since an HTTP stream has characteristics similar to a pipe-and-filter
2586   architecture, there are no inherent limits to the extent an intermediary
2587   can enhance (or interfere) with either direction of the stream.
2588</t>
2589<t>
2590   Intermediaries that forward a message &MUST; implement the
2591   <x:ref>Connection</x:ref> header field, as specified in
2592   <xref target="header.connection"/>, to exclude fields that are only
2593   intended for the incoming connection.
2594</t>
2595<t>
2596   In order to avoid request loops, a proxy that forwards requests to other
2597   proxies &MUST; be able to recognize and exclude all of its own server
2598   names, including any aliases, local variations, or literal IP addresses.
2599</t>
2600
2601<section title="Via" anchor="header.via">
2602  <iref primary="true" item="Via header field" x:for-anchor=""/>
2603  <x:anchor-alias value="pseudonym"/>
2604  <x:anchor-alias value="received-by"/>
2605  <x:anchor-alias value="received-protocol"/>
2606  <x:anchor-alias value="Via"/>
2607<t>
2608   The "Via" header field indicates the presence of intermediate protocols and
2609   recipients between the user agent and the server (on requests) or between
2610   the origin server and the client (on responses), similar to the
2611   "Received" header field in email
2612   (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2613   Via can be used for tracking message forwards,
2614   avoiding request loops, and identifying the protocol capabilities of
2615   senders along the request/response chain.
2616</t>
2617<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"/>
2618  <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> ] )
2619
2620  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2621                      ; see <xref target="header.upgrade"/>
2622  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2623  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2624</artwork></figure>
2625<t>
2626   Multiple Via field values represent each proxy or gateway that has
2627   forwarded the message. Each intermediary appends its own information
2628   about how the message was received, such that the end result is ordered
2629   according to the sequence of forwarding recipients.
2630</t>
2631<t>
2632   A proxy &MUST; send an appropriate Via header field, as described below, in
2633   each message that it forwards.
2634   An HTTP-to-HTTP gateway &MUST; send an appropriate Via header field in
2635   each inbound request message and &MAY; send a Via header field in
2636   forwarded response messages.
2637</t>
2638<t>
2639   For each intermediary, the received-protocol indicates the protocol and
2640   protocol version used by the upstream sender of the message. Hence, the
2641   Via field value records the advertised protocol capabilities of the
2642   request/response chain such that they remain visible to downstream
2643   recipients; this can be useful for determining what backwards-incompatible
2644   features might be safe to use in response, or within a later request, as
2645   described in <xref target="http.version"/>. For brevity, the protocol-name
2646   is omitted when the received protocol is HTTP.
2647</t>
2648<t>
2649   The received-by field is normally the host and optional port number of a
2650   recipient server or client that subsequently forwarded the message.
2651   However, if the real host is considered to be sensitive information, it
2652   &MAY; be replaced by a pseudonym. If the port is not given, it &MAY; be
2653   assumed to be the default port of the received-protocol.
2654</t>
2655<t>
2656   Comments &MAY; be used in the Via header field to identify the software
2657   of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2658   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2659   are optional and &MAY; be removed by any recipient prior to forwarding the
2660   message.
2661</t>
2662<t>
2663   For example, a request message could be sent from an HTTP/1.0 user
2664   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2665   forward the request to a public proxy at p.example.net, which completes
2666   the request by forwarding it to the origin server at www.example.com.
2667   The request received by www.example.com would then have the following
2668   Via header field:
2669</t>
2670<figure><artwork type="example">
2671  Via: 1.0 fred, 1.1 p.example.net
2672</artwork></figure>
2673<t>
2674   A proxy or gateway used as a portal through a network firewall
2675   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2676   region unless it is explicitly enabled to do so. If not enabled, the
2677   received-by host of any host behind the firewall &SHOULD; be replaced
2678   by an appropriate pseudonym for that host.
2679</t>
2680<t>
2681   A proxy or gateway &MAY; combine an ordered subsequence of Via header
2682   field entries into a single such entry if the entries have identical
2683   received-protocol values. For example,
2684</t>
2685<figure><artwork type="example">
2686  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2687</artwork></figure>
2688<t>
2689  could be collapsed to
2690</t>
2691<figure><artwork type="example">
2692  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2693</artwork></figure>
2694<t>
2695   Senders &SHOULD-NOT; combine multiple entries unless they are all
2696   under the same organizational control and the hosts have already been
2697   replaced by pseudonyms. Senders &MUST-NOT; combine entries that
2698   have different received-protocol values.
2699</t>
2700</section>
2701
2702<section title="Transformations" anchor="message.transformations">
2703<t>
2704   Some intermediaries include features for transforming messages and their
2705   payloads.  A transforming proxy might, for example, convert between image
2706   formats in order to save cache space or to reduce the amount of traffic on
2707   a slow link. However, operational problems might occur when these
2708   transformations are applied to payloads intended for critical applications,
2709   such as medical imaging or scientific data analysis, particularly when
2710   integrity checks or digital signatures are used to ensure that the payload
2711   received is identical to the original.
2712</t>
2713<t>
2714   If a proxy receives a request-target with a host name that is not a
2715   fully qualified domain name, it &MAY; add its own domain to the host name
2716   it received when forwarding the request.  A proxy &MUST-NOT; change the
2717   host name if it is a fully qualified domain name.
2718</t>
2719<t>
2720   A proxy &MUST-NOT; modify the "absolute-path" and "query" parts of the
2721   received request-target when forwarding it to the next inbound server,
2722   except as noted above to replace an empty path with "/" or "*".
2723</t>
2724<t>
2725   A proxy &MUST-NOT; modify header fields that provide information about the
2726   end points of the communication chain, the resource state, or the selected
2727   representation. A proxy &MAY; change the message body through application
2728   or removal of a transfer coding (<xref target="transfer.codings"/>).
2729</t>
2730<t>
2731   A non-transforming proxy &MUST-NOT; modify the message payload (&payload;).
2732   A transforming proxy &MUST-NOT; modify the payload of a message that
2733   contains the no-transform cache-control directive.
2734</t>
2735<t>
2736   A transforming proxy &MAY; transform the payload of a message
2737   that does not contain the no-transform cache-control directive;
2738   if the payload is transformed, the transforming proxy &MUST; add a
2739   Warning header field with the warn-code of 214 ("Transformation Applied")
2740   if one does not already appear in the message (see &header-warning;).
2741   If the payload of a <x:ref>200 (OK)</x:ref> response is transformed, the
2742   transforming proxy can also inform downstream recipients that a
2743   transformation has been applied by changing the response status code to
2744   <x:ref>203 (Non-Authoritative Information)</x:ref> (&status-203;).
2745</t>
2746</section>
2747</section>
2748</section>
2749
2750<section title="Connection Management" anchor="connection.management">
2751<t>
2752   HTTP messaging is independent of the underlying transport or
2753   session-layer connection protocol(s).  HTTP only presumes a reliable
2754   transport with in-order delivery of requests and the corresponding
2755   in-order delivery of responses.  The mapping of HTTP request and
2756   response structures onto the data units of an underlying transport
2757   protocol is outside the scope of this specification.
2758</t>
2759<t>
2760   As described in <xref target="connecting.inbound"/>, the specific
2761   connection protocols to be used for an HTTP interaction are determined by
2762   client configuration and the <x:ref>target URI</x:ref>.
2763   For example, the "http" URI scheme
2764   (<xref target="http.uri"/>) indicates a default connection of TCP
2765   over IP, with a default TCP port of 80, but the client might be
2766   configured to use a proxy via some other connection, port, or protocol.
2767</t>
2768<t>
2769   HTTP implementations are expected to engage in connection management,
2770   which includes maintaining the state of current connections,
2771   establishing a new connection or reusing an existing connection,
2772   processing messages received on a connection, detecting connection
2773   failures, and closing each connection.
2774   Most clients maintain multiple connections in parallel, including
2775   more than one connection per server endpoint.
2776   Most servers are designed to maintain thousands of concurrent connections,
2777   while controlling request queues to enable fair use and detect
2778   denial of service attacks.
2779</t>
2780
2781<section title="Connection" anchor="header.connection">
2782  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2783  <iref primary="true" item="close" x:for-anchor=""/>
2784  <x:anchor-alias value="Connection"/>
2785  <x:anchor-alias value="connection-option"/>
2786  <x:anchor-alias value="close"/>
2787<t>
2788   The "Connection" header field allows the sender to indicate desired
2789   control options for the current connection.  In order to avoid confusing
2790   downstream recipients, a proxy or gateway &MUST; remove or replace any
2791   received connection options before forwarding the message.
2792</t>
2793<t>
2794   When a header field aside from Connection is used to supply control
2795   information for or about the current connection, the sender &MUST; list
2796   the corresponding field-name within the "Connection" header field.
2797   A proxy or gateway &MUST; parse a received Connection
2798   header field before a message is forwarded and, for each
2799   connection-option in this field, remove any header field(s) from
2800   the message with the same name as the connection-option, and then
2801   remove the Connection header field itself (or replace it with the
2802   intermediary's own connection options for the forwarded message).
2803</t>
2804<t>
2805   Hence, the Connection header field provides a declarative way of
2806   distinguishing header fields that are only intended for the
2807   immediate recipient ("hop-by-hop") from those fields that are
2808   intended for all recipients on the chain ("end-to-end"), enabling the
2809   message to be self-descriptive and allowing future connection-specific
2810   extensions to be deployed without fear that they will be blindly
2811   forwarded by older intermediaries.
2812</t>
2813<t>
2814   The Connection header field's value has the following grammar:
2815</t>
2816<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2817  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2818  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2819</artwork></figure>
2820<t>
2821   Connection options are case-insensitive.
2822</t>
2823<t>
2824   A sender &MUST-NOT; send a connection option corresponding to a header
2825   field that is intended for all recipients of the payload.
2826   For example, <x:ref>Cache-Control</x:ref> is never appropriate as a
2827   connection option (&header-cache-control;).
2828</t>
2829<t>
2830   The connection options do not have to correspond to a header field
2831   present in the message, since a connection-specific header field
2832   might not be needed if there are no parameters associated with that
2833   connection option.  Recipients that trigger certain connection
2834   behavior based on the presence of connection options &MUST; do so
2835   based on the presence of the connection-option rather than only the
2836   presence of the optional header field.  In other words, if the
2837   connection option is received as a header field but not indicated
2838   within the Connection field-value, then the recipient &MUST; ignore
2839   the connection-specific header field because it has likely been
2840   forwarded by an intermediary that is only partially conformant.
2841</t>
2842<t>
2843   When defining new connection options, specifications ought to
2844   carefully consider existing deployed header fields and ensure
2845   that the new connection option does not share the same name as
2846   an unrelated header field that might already be deployed.
2847   Defining a new connection option essentially reserves that potential
2848   field-name for carrying additional information related to the
2849   connection option, since it would be unwise for senders to use
2850   that field-name for anything else.
2851</t>
2852<t>
2853   The "<x:dfn>close</x:dfn>" connection option is defined for a
2854   sender to signal that this connection will be closed after completion of
2855   the response. For example,
2856</t>
2857<figure><artwork type="example">
2858  Connection: close
2859</artwork></figure>
2860<t>
2861   in either the request or the response header fields indicates that
2862   the connection &MUST; be closed after the current request/response
2863   is complete (<xref target="persistent.tear-down"/>).
2864</t>
2865<t>
2866   A client that does not support <x:ref>persistent connections</x:ref> &MUST;
2867   send the "close" connection option in every request message.
2868</t>
2869<t>
2870   A server that does not support <x:ref>persistent connections</x:ref> &MUST;
2871   send the "close" connection option in every response message that
2872   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2873</t>
2874</section>
2875
2876<section title="Establishment" anchor="persistent.establishment">
2877<t>
2878   It is beyond the scope of this specification to describe how connections
2879   are established via various transport or session-layer protocols.
2880   Each connection applies to only one transport link.
2881</t>
2882</section>
2883
2884<section title="Persistence" anchor="persistent.connections">
2885   <x:anchor-alias value="persistent connections"/>
2886<t>
2887   HTTP/1.1 defaults to the use of "<x:dfn>persistent connections</x:dfn>",
2888   allowing multiple requests and responses to be carried over a single
2889   connection. The "<x:ref>close</x:ref>" connection-option is used to signal
2890   that a connection will not persist after the current request/response.
2891   HTTP implementations &SHOULD; support persistent connections.
2892</t>
2893<t>
2894   A recipient determines whether a connection is persistent or not based on
2895   the most recently received message's protocol version and
2896   <x:ref>Connection</x:ref> header field (if any):
2897   <list style="symbols">
2898     <t>If the <x:ref>close</x:ref> connection option is present, the
2899        connection will not persist after the current response; else,</t>
2900     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2901        persist after the current response; else,</t>
2902     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2903        connection option is present, the recipient is not a proxy, and
2904        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2905        the connection will persist after the current response; otherwise,</t>
2906     <t>The connection will close after the current response.</t>
2907   </list>
2908</t>
2909<t>
2910   A server &MAY; assume that an HTTP/1.1 client intends to maintain a
2911   persistent connection until a <x:ref>close</x:ref> connection option
2912   is received in a request.
2913</t>
2914<t>
2915   A client &MAY; reuse a persistent connection until it sends or receives
2916   a <x:ref>close</x:ref> connection option or receives an HTTP/1.0 response
2917   without a "keep-alive" connection option.
2918</t>
2919<t>
2920   In order to remain persistent, all messages on a connection &MUST;
2921   have a self-defined message length (i.e., one not defined by closure
2922   of the connection), as described in <xref target="message.body"/>.
2923   A server &MUST; read the entire request message body or close
2924   the connection after sending its response, since otherwise the
2925   remaining data on a persistent connection would be misinterpreted
2926   as the next request.  Likewise,
2927   a client &MUST; read the entire response message body if it intends
2928   to reuse the same connection for a subsequent request.
2929</t>
2930<t>
2931   A proxy server &MUST-NOT; maintain a persistent connection with an
2932   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
2933   information and discussion of the problems with the Keep-Alive header field
2934   implemented by many HTTP/1.0 clients).
2935</t>
2936<t>
2937   Clients and servers &SHOULD-NOT; assume that a persistent connection is
2938   maintained for HTTP versions less than 1.1 unless it is explicitly
2939   signaled.
2940   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
2941   for more information on backward compatibility with HTTP/1.0 clients.
2942</t>
2943
2944<section title="Retrying Requests" anchor="persistent.retrying.requests">
2945<t>
2946   Connections can be closed at any time, with or without intention.
2947   Implementations ought to anticipate the need to recover
2948   from asynchronous close events.
2949</t>
2950<t>
2951   When an inbound connection is closed prematurely, a client &MAY; open a new
2952   connection and automatically retransmit an aborted sequence of requests if
2953   all of those requests have idempotent methods (&idempotent-methods;).
2954   A proxy &MUST-NOT; automatically retry non-idempotent requests.
2955</t>
2956<t>
2957   A user agent &MUST-NOT; automatically retry a request with a non-idempotent
2958   method unless it has some means to know that the request semantics are
2959   actually idempotent, regardless of the method, or some means to detect that
2960   the original request was never applied. For example, a user agent that
2961   knows (through design or configuration) that a POST request to a given
2962   resource is safe can repeat that request automatically.
2963   Likewise, a user agent designed specifically to operate on a version
2964   control repository might be able to recover from partial failure conditions
2965   by checking the target resource revision(s) after a failed connection,
2966   reverting or fixing any changes that were partially applied, and then
2967   automatically retrying the requests that failed.
2968</t>
2969<t>
2970   An automatic retry &SHOULD-NOT; be repeated if it fails.
2971</t>
2972</section>
2973
2974<section title="Pipelining" anchor="pipelining">
2975   <x:anchor-alias value="pipeline"/>
2976<t>
2977   A client that supports persistent connections &MAY; "<x:dfn>pipeline</x:dfn>"
2978   its requests (i.e., send multiple requests without waiting for each
2979   response). A server &MAY; process a sequence of pipelined requests in
2980   parallel if they all have safe methods (&safe-methods;), but &MUST; send
2981   the corresponding responses in the same order that the requests were
2982   received.
2983</t>
2984<t>
2985   A client that pipelines requests &MUST; be prepared to retry those
2986   requests if the connection closes before it receives all of the
2987   corresponding responses. A client that assumes a persistent connection and
2988   pipelines immediately after connection establishment &MUST-NOT; pipeline
2989   on a retry connection until it knows the connection is persistent.
2990</t>
2991<t>
2992   Idempotent methods (&idempotent-methods;) are significant to pipelining
2993   because they can be automatically retried after a connection failure.
2994   A user agent &SHOULD-NOT; pipeline requests after a non-idempotent method
2995   until the final response status code for that method has been received,
2996   unless the user agent has a means to detect and recover from partial
2997   failure conditions involving the pipelined sequence.
2998</t>
2999<t>
3000   An intermediary that receives pipelined requests &MAY; pipeline those
3001   requests when forwarding them inbound, since it can rely on the outbound
3002   user agent(s) to determine what requests can be safely pipelined. If the
3003   inbound connection fails before receiving a response, the pipelining
3004   intermediary &MAY; attempt to retry a sequence of requests that have yet
3005   to receive a response if the requests all have idempotent methods;
3006   otherwise, the pipelining intermediary &SHOULD; forward any received
3007   responses and then close the corresponding outbound connection(s) so that
3008   the outbound user agent(s) can recover accordingly.
3009</t>
3010</section>
3011</section>
3012   
3013<section title="Concurrency" anchor="persistent.concurrency">
3014<t>
3015   Clients &SHOULD; limit the number of simultaneous
3016   connections that they maintain to a given server.
3017</t>
3018<t>
3019   Previous revisions of HTTP gave a specific number of connections as a
3020   ceiling, but this was found to be impractical for many applications. As a
3021   result, this specification does not mandate a particular maximum number of
3022   connections, but instead encourages clients to be conservative when opening
3023   multiple connections.
3024</t>
3025<t>
3026   Multiple connections are typically used to avoid the "head-of-line
3027   blocking" problem, wherein a request that takes significant server-side
3028   processing and/or has a large payload blocks subsequent requests on the
3029   same connection. However, each connection consumes server resources.
3030   Furthermore, using multiple connections can cause undesirable side effects
3031   in congested networks.
3032</t>
3033<t>
3034   Note that servers might reject traffic that they deem abusive, including an
3035   excessive number of connections from a client.
3036</t>
3037</section>
3038
3039<section title="Failures and Time-outs" anchor="persistent.failures">
3040<t>
3041   Servers will usually have some time-out value beyond which they will
3042   no longer maintain an inactive connection. Proxy servers might make
3043   this a higher value since it is likely that the client will be making
3044   more connections through the same server. The use of persistent
3045   connections places no requirements on the length (or existence) of
3046   this time-out for either the client or the server.
3047</t>
3048<t>
3049   When a client or server wishes to time-out it &SHOULD; issue a graceful
3050   close on the transport connection. Clients and servers &SHOULD; both
3051   constantly watch for the other side of the transport close, and
3052   respond to it as appropriate. If a client or server does not detect
3053   the other side's close promptly it could cause unnecessary resource
3054   drain on the network.
3055</t>
3056<t>
3057   A client, server, or proxy &MAY; close the transport connection at any
3058   time. For example, a client might have started to send a new request
3059   at the same time that the server has decided to close the "idle"
3060   connection. From the server's point of view, the connection is being
3061   closed while it was idle, but from the client's point of view, a
3062   request is in progress.
3063</t>
3064<t>
3065   Servers &SHOULD; maintain persistent connections and allow the underlying
3066   transport's flow control mechanisms to resolve temporary overloads, rather
3067   than terminate connections with the expectation that clients will retry.
3068   The latter technique can exacerbate network congestion.
3069</t>
3070<t>
3071   A client sending a message body &SHOULD; monitor
3072   the network connection for an error response while it is transmitting
3073   the request. If the client sees an error response, it &SHOULD;
3074   immediately cease transmitting the body and close the connection.
3075</t>
3076</section>
3077   
3078<section title="Tear-down" anchor="persistent.tear-down">
3079  <iref primary="false" item="Connection header field" x:for-anchor=""/>
3080  <iref primary="false" item="close" x:for-anchor=""/>
3081<t>
3082   The <x:ref>Connection</x:ref> header field
3083   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
3084   connection option that a sender &SHOULD; send when it wishes to close
3085   the connection after the current request/response pair.
3086</t>
3087<t>
3088   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
3089   send further requests on that connection (after the one containing
3090   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
3091   final response message corresponding to this request.
3092</t>
3093<t>
3094   A server that receives a <x:ref>close</x:ref> connection option &MUST;
3095   initiate a close of the connection (see below) after it sends the
3096   final response to the request that contained <x:ref>close</x:ref>.
3097   The server &SHOULD; send a <x:ref>close</x:ref> connection option
3098   in its final response on that connection. The server &MUST-NOT; process
3099   any further requests received on that connection.
3100</t>
3101<t>
3102   A server that sends a <x:ref>close</x:ref> connection option &MUST;
3103   initiate a close of the connection (see below) after it sends the
3104   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
3105   any further requests received on that connection.
3106</t>
3107<t>
3108   A client that receives a <x:ref>close</x:ref> connection option &MUST;
3109   cease sending requests on that connection and close the connection
3110   after reading the response message containing the close; if additional
3111   pipelined requests had been sent on the connection, the client &SHOULD-NOT;
3112   assume that they will be processed by the server.
3113</t>
3114<t>
3115   If a server performs an immediate close of a TCP connection, there is a
3116   significant risk that the client will not be able to read the last HTTP
3117   response.  If the server receives additional data from the client on a
3118   fully-closed connection, such as another request that was sent by the
3119   client before receiving the server's response, the server's TCP stack will
3120   send a reset packet to the client; unfortunately, the reset packet might
3121   erase the client's unacknowledged input buffers before they can be read
3122   and interpreted by the client's HTTP parser.
3123</t>
3124<t>
3125   To avoid the TCP reset problem, servers typically close a connection in
3126   stages. First, the server performs a half-close by closing only the write
3127   side of the read/write connection. The server then continues to read from
3128   the connection until it receives a corresponding close by the client, or
3129   until the server is reasonably certain that its own TCP stack has received
3130   the client's acknowledgement of the packet(s) containing the server's last
3131   response. Finally, the server fully closes the connection.
3132</t>
3133<t>
3134   It is unknown whether the reset problem is exclusive to TCP or might also
3135   be found in other transport connection protocols.
3136</t>
3137</section>
3138
3139<section title="Upgrade" anchor="header.upgrade">
3140  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3141  <x:anchor-alias value="Upgrade"/>
3142  <x:anchor-alias value="protocol"/>
3143  <x:anchor-alias value="protocol-name"/>
3144  <x:anchor-alias value="protocol-version"/>
3145<t>
3146   The "Upgrade" header field is intended to provide a simple mechanism
3147   for transitioning from HTTP/1.1 to some other protocol on the same
3148   connection.  A client &MAY; send a list of protocols in the Upgrade
3149   header field of a request to invite the server to switch to one or
3150   more of those protocols, in order of descending preference, before sending
3151   the final response. A server &MAY; ignore a received Upgrade header field
3152   if it wishes to continue using the current protocol on that connection.
3153</t>
3154<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3155  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3156
3157  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3158  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3159  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3160</artwork></figure>
3161<t>
3162   A server that sends a <x:ref>101 (Switching Protocols)</x:ref> response
3163   &MUST; send an Upgrade header field to indicate the new protocol(s) to
3164   which the connection is being switched; if multiple protocol layers are
3165   being switched, the new protocols &MUST; be listed in layer-ascending
3166   order. A server &MUST-NOT; switch to a protocol that was not indicated by
3167   the client in the corresponding request's Upgrade header field.
3168   A server &MAY; choose to ignore the order of preference indicated by the
3169   client and select the new protocol(s) based on other factors, such as the
3170   nature of the request or the current load on the server.
3171</t>
3172<t>
3173   A server that sends a <x:ref>426 (Upgrade Required)</x:ref> response
3174   &MUST; send an Upgrade header field to indicate the acceptable protocols,
3175   in order of descending preference.
3176</t>
3177<t>
3178   A server &MAY; send an Upgrade header field in any other response to
3179   advertise that it implements support for upgrading to the listed protocols,
3180   in order of descending preference, when appropriate for a future request.
3181</t>
3182<figure><preamble>
3183   The following is a hypothetical example sent by a client:
3184</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
3185GET /hello.txt HTTP/1.1
3186Host: www.example.com
3187Connection: upgrade
3188Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3189
3190</artwork></figure>
3191<t>
3192   Upgrade cannot be used to insist on a protocol change; its acceptance and
3193   use by the server is optional. The capabilities and nature of the
3194   application-level communication after the protocol change is entirely
3195   dependent upon the new protocol(s) chosen, although the first action
3196   after changing the protocol &MUST; be a response to the initial HTTP
3197   request that contained the Upgrade header field.
3198</t>
3199<t>
3200   For example, if the Upgrade header field is received in a GET request
3201   and the server decides to switch protocols, it first responds
3202   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3203   then immediately follows that with the new protocol's equivalent of a
3204   response to a GET on the target resource.  This allows a connection to be
3205   upgraded to protocols with the same semantics as HTTP without the
3206   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3207   protocols unless the received message semantics can be honored by the new
3208   protocol; an OPTIONS request can be honored by any protocol.
3209</t>
3210<figure><preamble>
3211   The following is an example response to the above hypothetical request:
3212</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
3213HTTP/1.1 101 Switching Protocols
3214Connection: upgrade
3215Upgrade: HTTP/2.0
3216
3217[... data stream switches to HTTP/2.0 with an appropriate response
3218(as defined by new protocol) to the "GET /hello.txt" request ...]
3219</artwork></figure>
3220<t>
3221   When Upgrade is sent, the sender &MUST; also send a
3222   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3223   that contains an "upgrade" connection option, in order to prevent Upgrade
3224   from being accidentally forwarded by intermediaries that might not implement
3225   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3226   is received in an HTTP/1.0 request.
3227</t>
3228<t>
3229   The Upgrade header field only applies to switching protocols on top of the
3230   existing connection; it cannot be used to switch the underlying connection
3231   (transport) protocol, nor to switch the existing communication to a
3232   different connection. For those purposes, it is more appropriate to use a
3233   <x:ref>3xx (Redirection)</x:ref> response (&status-3xx;).
3234</t>
3235<t>
3236   This specification only defines the protocol name "HTTP" for use by
3237   the family of Hypertext Transfer Protocols, as defined by the HTTP
3238   version rules of <xref target="http.version"/> and future updates to this
3239   specification. Additional tokens ought to be registered with IANA using the
3240   registration procedure defined in <xref target="upgrade.token.registry"/>.
3241</t>
3242</section>
3243</section>
3244
3245<section title="IANA Considerations" anchor="IANA.considerations">
3246
3247<section title="Header Field Registration" anchor="header.field.registration">
3248<t>
3249   HTTP header fields are registered within the Message Header Field Registry
3250   maintained at
3251   <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
3252</t>
3253<t>
3254   This document defines the following HTTP header fields, so their
3255   associated registry entries shall be updated according to the permanent
3256   registrations below (see <xref target="BCP90"/>):
3257</t>
3258<?BEGININC p1-messaging.iana-headers ?>
3259<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3260<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3261   <ttcol>Header Field Name</ttcol>
3262   <ttcol>Protocol</ttcol>
3263   <ttcol>Status</ttcol>
3264   <ttcol>Reference</ttcol>
3265
3266   <c>Connection</c>
3267   <c>http</c>
3268   <c>standard</c>
3269   <c>
3270      <xref target="header.connection"/>
3271   </c>
3272   <c>Content-Length</c>
3273   <c>http</c>
3274   <c>standard</c>
3275   <c>
3276      <xref target="header.content-length"/>
3277   </c>
3278   <c>Host</c>
3279   <c>http</c>
3280   <c>standard</c>
3281   <c>
3282      <xref target="header.host"/>
3283   </c>
3284   <c>TE</c>
3285   <c>http</c>
3286   <c>standard</c>
3287   <c>
3288      <xref target="header.te"/>
3289   </c>
3290   <c>Trailer</c>
3291   <c>http</c>
3292   <c>standard</c>
3293   <c>
3294      <xref target="header.trailer"/>
3295   </c>
3296   <c>Transfer-Encoding</c>
3297   <c>http</c>
3298   <c>standard</c>
3299   <c>
3300      <xref target="header.transfer-encoding"/>
3301   </c>
3302   <c>Upgrade</c>
3303   <c>http</c>
3304   <c>standard</c>
3305   <c>
3306      <xref target="header.upgrade"/>
3307   </c>
3308   <c>Via</c>
3309   <c>http</c>
3310   <c>standard</c>
3311   <c>
3312      <xref target="header.via"/>
3313   </c>
3314</texttable>
3315<!--(END)-->
3316<?ENDINC p1-messaging.iana-headers ?>
3317<t>
3318   Furthermore, the header field-name "Close" shall be registered as
3319   "reserved", since using that name as an HTTP header field might
3320   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3321   header field (<xref target="header.connection"/>).
3322</t>
3323<texttable align="left" suppress-title="true">
3324   <ttcol>Header Field Name</ttcol>
3325   <ttcol>Protocol</ttcol>
3326   <ttcol>Status</ttcol>
3327   <ttcol>Reference</ttcol>
3328
3329   <c>Close</c>
3330   <c>http</c>
3331   <c>reserved</c>
3332   <c>
3333      <xref target="header.field.registration"/>
3334   </c>
3335</texttable>
3336<t>
3337   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3338</t>
3339</section>
3340
3341<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3342<t>
3343   IANA maintains the registry of URI Schemes <xref target="BCP115"/> at
3344   <eref target="http://www.iana.org/assignments/uri-schemes.html"/>.
3345</t>
3346<t>
3347   This document defines the following URI schemes, so their
3348   associated registry entries shall be updated according to the permanent
3349   registrations below:
3350</t>
3351<texttable align="left" suppress-title="true">
3352   <ttcol>URI Scheme</ttcol>
3353   <ttcol>Description</ttcol>
3354   <ttcol>Reference</ttcol>
3355
3356   <c>http</c>
3357   <c>Hypertext Transfer Protocol</c>
3358   <c><xref target="http.uri"/></c>
3359
3360   <c>https</c>
3361   <c>Hypertext Transfer Protocol Secure</c>
3362   <c><xref target="https.uri"/></c>
3363</texttable>
3364</section>
3365
3366<section title="Internet Media Type Registration" anchor="internet.media.type.http">
3367<t>
3368   This document serves as the specification for the Internet media types
3369   "message/http" and "application/http". The following is to be registered with
3370   IANA (see <xref target="BCP13"/>).
3371</t>
3372<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
3373<iref item="Media Type" subitem="message/http" primary="true"/>
3374<iref item="message/http Media Type" primary="true"/>
3375<t>
3376   The message/http type can be used to enclose a single HTTP request or
3377   response message, provided that it obeys the MIME restrictions for all
3378   "message" types regarding line length and encodings.
3379</t>
3380<t>
3381  <list style="hanging" x:indent="12em">
3382    <t hangText="Type name:">
3383      message
3384    </t>
3385    <t hangText="Subtype name:">
3386      http
3387    </t>
3388    <t hangText="Required parameters:">
3389      none
3390    </t>
3391    <t hangText="Optional parameters:">
3392      version, msgtype
3393      <list style="hanging">
3394        <t hangText="version:">
3395          The HTTP-version number of the enclosed message
3396          (e.g., "1.1"). If not present, the version can be
3397          determined from the first line of the body.
3398        </t>
3399        <t hangText="msgtype:">
3400          The message type &mdash; "request" or "response". If not
3401          present, the type can be determined from the first
3402          line of the body.
3403        </t>
3404      </list>
3405    </t>
3406    <t hangText="Encoding considerations:">
3407      only "7bit", "8bit", or "binary" are permitted
3408    </t>
3409    <t hangText="Security considerations:">
3410      none
3411    </t>
3412    <t hangText="Interoperability considerations:">
3413      none
3414    </t>
3415    <t hangText="Published specification:">
3416      This specification (see <xref target="internet.media.type.message.http"/>).
3417    </t>
3418    <t hangText="Applications that use this media type:">
3419    </t>
3420    <t hangText="Additional information:">
3421      <list style="hanging">
3422        <t hangText="Magic number(s):">none</t>
3423        <t hangText="File extension(s):">none</t>
3424        <t hangText="Macintosh file type code(s):">none</t>
3425      </list>
3426    </t>
3427    <t hangText="Person and email address to contact for further information:">
3428      See Authors Section.
3429    </t>
3430    <t hangText="Intended usage:">
3431      COMMON
3432    </t>
3433    <t hangText="Restrictions on usage:">
3434      none
3435    </t>
3436    <t hangText="Author:">
3437      See Authors Section.
3438    </t>
3439    <t hangText="Change controller:">
3440      IESG
3441    </t>
3442  </list>
3443</t>
3444</section>
3445<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
3446<iref item="Media Type" subitem="application/http" primary="true"/>
3447<iref item="application/http Media Type" primary="true"/>
3448<t>
3449   The application/http type can be used to enclose a pipeline of one or more
3450   HTTP request or response messages (not intermixed).
3451</t>
3452<t>
3453  <list style="hanging" x:indent="12em">
3454    <t hangText="Type name:">
3455      application
3456    </t>
3457    <t hangText="Subtype name:">
3458      http
3459    </t>
3460    <t hangText="Required parameters:">
3461      none
3462    </t>
3463    <t hangText="Optional parameters:">
3464      version, msgtype
3465      <list style="hanging">
3466        <t hangText="version:">
3467          The HTTP-version number of the enclosed messages
3468          (e.g., "1.1"). If not present, the version can be
3469          determined from the first line of the body.
3470        </t>
3471        <t hangText="msgtype:">
3472          The message type &mdash; "request" or "response". If not
3473          present, the type can be determined from the first
3474          line of the body.
3475        </t>
3476      </list>
3477    </t>
3478    <t hangText="Encoding considerations:">
3479      HTTP messages enclosed by this type
3480      are in "binary" format; use of an appropriate
3481      Content-Transfer-Encoding is required when
3482      transmitted via E-mail.
3483    </t>
3484    <t hangText="Security considerations:">
3485      none
3486    </t>
3487    <t hangText="Interoperability considerations:">
3488      none
3489    </t>
3490    <t hangText="Published specification:">
3491      This specification (see <xref target="internet.media.type.application.http"/>).
3492    </t>
3493    <t hangText="Applications that use this media type:">
3494    </t>
3495    <t hangText="Additional information:">
3496      <list style="hanging">
3497        <t hangText="Magic number(s):">none</t>
3498        <t hangText="File extension(s):">none</t>
3499        <t hangText="Macintosh file type code(s):">none</t>
3500      </list>
3501    </t>
3502    <t hangText="Person and email address to contact for further information:">
3503      See Authors Section.
3504    </t>
3505    <t hangText="Intended usage:">
3506      COMMON
3507    </t>
3508    <t hangText="Restrictions on usage:">
3509      none
3510    </t>
3511    <t hangText="Author:">
3512      See Authors Section.
3513    </t>
3514    <t hangText="Change controller:">
3515      IESG
3516    </t>
3517  </list>
3518</t>
3519</section>
3520</section>
3521
3522<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3523<t>
3524   The HTTP Transfer Coding Registry defines the name space for transfer
3525   coding names. It is maintained at <eref target="http://www.iana.org/assignments/http-parameters"/>.
3526</t>
3527
3528<section title="Procedure" anchor="transfer.coding.registry.procedure">
3529<t>
3530   Registrations &MUST; include the following fields:
3531   <list style="symbols">
3532     <t>Name</t>
3533     <t>Description</t>
3534     <t>Pointer to specification text</t>
3535   </list>
3536</t>
3537<t>
3538   Names of transfer codings &MUST-NOT; overlap with names of content codings
3539   (&content-codings;) unless the encoding transformation is identical, as
3540   is the case for the compression codings defined in
3541   <xref target="compression.codings"/>.
3542</t>
3543<t>
3544   Values to be added to this name space require IETF Review (see
3545   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3546   conform to the purpose of transfer coding defined in this specification.
3547</t>
3548<t>
3549   Use of program names for the identification of encoding formats
3550   is not desirable and is discouraged for future encodings.
3551</t>
3552</section>
3553
3554<section title="Registration" anchor="transfer.coding.registration">
3555<t>
3556   The HTTP Transfer Coding Registry shall be updated with the registrations
3557   below:
3558</t>
3559<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3560   <ttcol>Name</ttcol>
3561   <ttcol>Description</ttcol>
3562   <ttcol>Reference</ttcol>
3563   <c>chunked</c>
3564   <c>Transfer in a series of chunks</c>
3565   <c>
3566      <xref target="chunked.encoding"/>
3567   </c>
3568   <c>compress</c>
3569   <c>UNIX "compress" data format <xref target="Welch"/></c>
3570   <c>
3571      <xref target="compress.coding"/>
3572   </c>
3573   <c>deflate</c>
3574   <c>"deflate" compressed data (<xref target="RFC1951"/>) inside
3575   the "zlib" data format (<xref target="RFC1950"/>)
3576   </c>
3577   <c>
3578      <xref target="deflate.coding"/>
3579   </c>
3580   <c>gzip</c>
3581   <c>GZIP file format <xref target="RFC1952"/></c>
3582   <c>
3583      <xref target="gzip.coding"/>
3584   </c>
3585   <c>x-compress</c>
3586   <c>Deprecated (alias for compress)</c>
3587   <c>
3588      <xref target="compress.coding"/>
3589   </c>
3590   <c>x-gzip</c>
3591   <c>Deprecated (alias for gzip)</c>
3592   <c>
3593      <xref target="gzip.coding"/>
3594   </c>
3595</texttable>
3596</section>
3597</section>
3598
3599<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3600<t>
3601   The HTTP Upgrade Token Registry defines the name space for protocol-name
3602   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3603   field. The registry is maintained at <eref target="http://www.iana.org/assignments/http-upgrade-tokens"/>.
3604</t>
3605
3606<section title="Procedure" anchor="upgrade.token.registry.procedure">  
3607<t>
3608   Each registered protocol name is associated with contact information
3609   and an optional set of specifications that details how the connection
3610   will be processed after it has been upgraded.
3611</t>
3612<t>
3613   Registrations happen on a "First Come First Served" basis (see
3614   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3615   following rules:
3616  <list style="numbers">
3617    <t>A protocol-name token, once registered, stays registered forever.</t>
3618    <t>The registration &MUST; name a responsible party for the
3619       registration.</t>
3620    <t>The registration &MUST; name a point of contact.</t>
3621    <t>The registration &MAY; name a set of specifications associated with
3622       that token. Such specifications need not be publicly available.</t>
3623    <t>The registration &SHOULD; name a set of expected "protocol-version"
3624       tokens associated with that token at the time of registration.</t>
3625    <t>The responsible party &MAY; change the registration at any time.
3626       The IANA will keep a record of all such changes, and make them
3627       available upon request.</t>
3628    <t>The IESG &MAY; reassign responsibility for a protocol token.
3629       This will normally only be used in the case when a
3630       responsible party cannot be contacted.</t>
3631  </list>
3632</t>
3633<t>
3634   This registration procedure for HTTP Upgrade Tokens replaces that
3635   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3636</t>
3637</section>
3638
3639<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3640<t>
3641   The HTTP Upgrade Token Registry shall be updated with the registration
3642   below:
3643</t>
3644<texttable align="left" suppress-title="true">
3645   <ttcol>Value</ttcol>
3646   <ttcol>Description</ttcol>
3647   <ttcol>Expected Version Tokens</ttcol>
3648   <ttcol>Reference</ttcol>
3649
3650   <c>HTTP</c>
3651   <c>Hypertext Transfer Protocol</c>
3652   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3653   <c><xref target="http.version"/></c>
3654</texttable>
3655<t>
3656   The responsible party is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3657</t>
3658</section>
3659</section>
3660
3661</section>
3662
3663<section title="Security Considerations" anchor="security.considerations">
3664<t>
3665   This section is meant to inform developers, information providers, and
3666   users of known security concerns relevant to HTTP/1.1 message syntax,
3667   parsing, and routing.
3668</t>
3669
3670<section title="DNS-related Attacks" anchor="dns.related.attacks">
3671<t>
3672   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3673   generally prone to security attacks based on the deliberate misassociation
3674   of IP addresses and DNS names not protected by DNSSEC. Clients need to be
3675   cautious in assuming the validity of an IP number/DNS name association unless
3676   the response is protected by DNSSEC (<xref target="RFC4033"/>).
3677</t>
3678</section>
3679
3680<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3681<t>
3682   By their very nature, HTTP intermediaries are men-in-the-middle, and
3683   represent an opportunity for man-in-the-middle attacks. Compromise of
3684   the systems on which the intermediaries run can result in serious security
3685   and privacy problems. Intermediaries have access to security-related
3686   information, personal information about individual users and
3687   organizations, and proprietary information belonging to users and
3688   content providers. A compromised intermediary, or an intermediary
3689   implemented or configured without regard to security and privacy
3690   considerations, might be used in the commission of a wide range of
3691   potential attacks.
3692</t>
3693<t>
3694   Intermediaries that contain a shared cache are especially vulnerable
3695   to cache poisoning attacks.
3696</t>
3697<t>
3698   Implementers need to consider the privacy and security
3699   implications of their design and coding decisions, and of the
3700   configuration options they provide to operators (especially the
3701   default configuration).
3702</t>
3703<t>
3704   Users need to be aware that intermediaries are no more trustworthy than
3705   the people who run them; HTTP itself cannot solve this problem.
3706</t>
3707</section>
3708
3709<section title="Buffer Overflows" anchor="attack.protocol.element.size.overflows">
3710<t>
3711   Because HTTP uses mostly textual, character-delimited fields, attackers can
3712   overflow buffers in implementations, and/or perform a Denial of Service
3713   against implementations that accept fields with unlimited lengths.
3714</t>
3715<t>
3716   To promote interoperability, this specification makes specific
3717   recommendations for minimum size limits on request-line
3718   (<xref target="request.line"/>)
3719   and blocks of header fields (<xref target="header.fields"/>). These are
3720   minimum recommendations, chosen to be supportable even by implementations
3721   with limited resources; it is expected that most implementations will
3722   choose substantially higher limits.
3723</t>
3724<t>
3725   This specification also provides a way for servers to reject messages that
3726   have request-targets that are too long (&status-414;) or request entities
3727   that are too large (&status-4xx;). Additional status codes related to
3728   capacity limits have been defined by extensions to HTTP
3729   <xref target="RFC6585"/>.
3730</t>
3731<t>
3732   Recipients &SHOULD; carefully limit the extent to which they read other
3733   fields, including (but not limited to) request methods, response status
3734   phrases, header field-names, and body chunks, so as to avoid denial of
3735   service attacks without impeding interoperability.
3736</t>
3737</section>
3738
3739<section title="Message Integrity" anchor="message.integrity">
3740<t>
3741   HTTP does not define a specific mechanism for ensuring message integrity,
3742   instead relying on the error-detection ability of underlying transport
3743   protocols and the use of length or chunk-delimited framing to detect
3744   completeness. Additional integrity mechanisms, such as hash functions or
3745   digital signatures applied to the content, can be selectively added to
3746   messages via extensible metadata header fields. Historically, the lack of
3747   a single integrity mechanism has been justified by the informal nature of
3748   most HTTP communication.  However, the prevalence of HTTP as an information
3749   access mechanism has resulted in its increasing use within environments
3750   where verification of message integrity is crucial.
3751</t>
3752<t>
3753   User agents are encouraged to implement configurable means for detecting
3754   and reporting failures of message integrity such that those means can be
3755   enabled within environments for which integrity is necessary. For example,
3756   a browser being used to view medical history or drug interaction
3757   information needs to indicate to the user when such information is detected
3758   by the protocol to be incomplete, expired, or corrupted during transfer.
3759   Such mechanisms might be selectively enabled via user agent extensions or
3760   the presence of message integrity metadata in a response.
3761   At a minimum, user agents ought to provide some indication that allows a
3762   user to distinguish between a complete and incomplete response message
3763   (<xref target="incomplete.messages"/>) when such verification is desired.
3764</t>
3765</section>
3766
3767<section title="Server Log Information" anchor="abuse.of.server.log.information">
3768<t>
3769   A server is in the position to save personal data about a user's requests
3770   over time, which might identify their reading patterns or subjects of
3771   interest.  In particular, log information gathered at an intermediary
3772   often contains a history of user agent interaction, across a multitude
3773   of sites, that can be traced to individual users.
3774</t>
3775<t>
3776   HTTP log information is confidential in nature; its handling is often
3777   constrained by laws and regulations.  Log information needs to be securely
3778   stored and appropriate guidelines followed for its analysis.
3779   Anonymization of personal information within individual entries helps,
3780   but is generally not sufficient to prevent real log traces from being
3781   re-identified based on correlation with other access characteristics.
3782   As such, access traces that are keyed to a specific client should not
3783   be published even if the key is pseudonymous.
3784</t>
3785<t>
3786   To minimize the risk of theft or accidental publication, log information
3787   should be purged of personally identifiable information, including
3788   user identifiers, IP addresses, and user-provided query parameters,
3789   as soon as that information is no longer necessary to support operational
3790   needs for security, auditing, or fraud control.
3791</t>
3792</section>
3793</section>
3794
3795<section title="Acknowledgments" anchor="acks">
3796<t>
3797   This edition of HTTP/1.1 builds on the many contributions that went into
3798   <xref target="RFC1945" format="none">RFC 1945</xref>,
3799   <xref target="RFC2068" format="none">RFC 2068</xref>,
3800   <xref target="RFC2145" format="none">RFC 2145</xref>, and
3801   <xref target="RFC2616" format="none">RFC 2616</xref>, including
3802   substantial contributions made by the previous authors, editors, and
3803   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
3804   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
3805   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
3806</t>
3807<t>
3808   Since 1999, the following contributors have helped improve the HTTP
3809   specification by reporting bugs, asking smart questions, drafting or
3810   reviewing text, and evaluating open issues:
3811</t>
3812<?BEGININC acks ?>
3813<t>Adam Barth,
3814Adam Roach,
3815Addison Phillips,
3816Adrian Chadd,
3817Adrien W. de Croy,
3818Alan Ford,
3819Alan Ruttenberg,
3820Albert Lunde,
3821Alek Storm,
3822Alex Rousskov,
3823Alexandre Morgaut,
3824Alexey Melnikov,
3825Alisha Smith,
3826Amichai Rothman,
3827Amit Klein,
3828Amos Jeffries,
3829Andreas Maier,
3830Andreas Petersson,
3831Anil Sharma,
3832Anne van Kesteren,
3833Anthony Bryan,
3834Asbjorn Ulsberg,
3835Ashok Kumar,
3836Balachander Krishnamurthy,
3837Barry Leiba,
3838Ben Laurie,
3839Benjamin Carlyle,
3840Benjamin Niven-Jenkins,
3841Bil Corry,
3842Bill Burke,
3843Bjoern Hoehrmann,
3844Bob Scheifler,
3845Boris Zbarsky,
3846Brett Slatkin,
3847Brian Kell,
3848Brian McBarron,
3849Brian Pane,
3850Brian Raymor,
3851Brian Smith,
3852Bryce Nesbitt,
3853Cameron Heavon-Jones,
3854Carl Kugler,
3855Carsten Bormann,
3856Charles Fry,
3857Chris Newman,
3858Cyrus Daboo,
3859Dale Robert Anderson,
3860Dan Wing,
3861Dan Winship,
3862Daniel Stenberg,
3863Darrel Miller,
3864Dave Cridland,
3865Dave Crocker,
3866Dave Kristol,
3867Dave Thaler,
3868David Booth,
3869David Singer,
3870David W. Morris,
3871Diwakar Shetty,
3872Dmitry Kurochkin,
3873Drummond Reed,
3874Duane Wessels,
3875Edward Lee,
3876Eitan Adler,
3877Eliot Lear,
3878Eran Hammer-Lahav,
3879Eric D. Williams,
3880Eric J. Bowman,
3881Eric Lawrence,
3882Eric Rescorla,
3883Erik Aronesty,
3884Evan Prodromou,
3885Felix Geisendoerfer,
3886Florian Weimer,
3887Frank Ellermann,
3888Fred Akalin,
3889Fred Bohle,
3890Frederic Kayser,
3891Gabor Molnar,
3892Gabriel Montenegro,
3893Geoffrey Sneddon,
3894Gervase Markham,
3895Gili Tzabari,
3896Grahame Grieve,
3897Greg Wilkins,
3898Grzegorz Calkowski,
3899Harald Tveit Alvestrand,
3900Harry Halpin,
3901Helge Hess,
3902Henrik Nordstrom,
3903Henry S. Thompson,
3904Henry Story,
3905Herbert van de Sompel,
3906Herve Ruellan,
3907Howard Melman,
3908Hugo Haas,
3909Ian Fette,
3910Ian Hickson,
3911Ido Safruti,
3912Ilari Liusvaara,
3913Ilya Grigorik,
3914Ingo Struck,
3915J. Ross Nicoll,
3916James Cloos,
3917James H. Manger,
3918James Lacey,
3919James M. Snell,
3920Jamie Lokier,
3921Jan Algermissen,
3922Jeff Hodges (who came up with the term 'effective Request-URI'),
3923Jeff Pinner,
3924Jeff Walden,
3925Jim Luther,
3926Jitu Padhye,
3927Joe D. Williams,
3928Joe Gregorio,
3929Joe Orton,
3930John C. Klensin,
3931John C. Mallery,
3932John Cowan,
3933John Kemp,
3934John Panzer,
3935John Schneider,
3936John Stracke,
3937John Sullivan,
3938Jonas Sicking,
3939Jonathan A. Rees,
3940Jonathan Billington,
3941Jonathan Moore,
3942Jonathan Silvera,
3943Jordi Ros,
3944Joris Dobbelsteen,
3945Josh Cohen,
3946Julien Pierre,
3947Jungshik Shin,
3948Justin Chapweske,
3949Justin Erenkrantz,
3950Justin James,
3951Kalvinder Singh,
3952Karl Dubost,
3953Keith Hoffman,
3954Keith Moore,
3955Ken Murchison,
3956Koen Holtman,
3957Konstantin Voronkov,
3958Kris Zyp,
3959Lisa Dusseault,
3960Maciej Stachowiak,
3961Manu Sporny,
3962Marc Schneider,
3963Marc Slemko,
3964Mark Baker,
3965Mark Pauley,
3966Mark Watson,
3967Markus Isomaki,
3968Markus Lanthaler,
3969Martin J. Duerst,
3970Martin Musatov,
3971Martin Nilsson,
3972Martin Thomson,
3973Matt Lynch,
3974Matthew Cox,
3975Max Clark,
3976Michael Burrows,
3977Michael Hausenblas,
3978Michael Sweet,
3979Mike Amundsen,
3980Mike Belshe,
3981Mike Bishop,
3982Mike Kelly,
3983Mike Schinkel,
3984Miles Sabin,
3985Murray S. Kucherawy,
3986Mykyta Yevstifeyev,
3987Nathan Rixham,
3988Nicholas Shanks,
3989Nico Williams,
3990Nicolas Alvarez,
3991Nicolas Mailhot,
3992Noah Slater,
3993Osama Mazahir,
3994Pablo Castro,
3995Pat Hayes,
3996Patrick R. McManus,
3997Paul E. Jones,
3998Paul Hoffman,
3999Paul Marquess,
4000Peter Lepeska,
4001Peter Occil,
4002Peter Saint-Andre,
4003Peter Watkins,
4004Phil Archer,
4005Philippe Mougin,
4006Phillip Hallam-Baker,
4007Piotr Dobrogost,
4008Poul-Henning Kamp,
4009Preethi Natarajan,
4010Rajeev Bector,
4011Ray Polk,
4012Reto Bachmann-Gmuer,
4013Richard Cyganiak,
4014Robby Simpson,
4015Robert Brewer,
4016Robert Collins,
4017Robert Mattson,
4018Robert O'Callahan,
4019Robert Olofsson,
4020Robert Sayre,
4021Robert Siemer,
4022Robert de Wilde,
4023Roberto Javier Godoy,
4024Roberto Peon,
4025Roland Zink,
4026Ronny Widjaja,
4027S. Mike Dierken,
4028Salvatore Loreto,
4029Sam Johnston,
4030Sam Pullara,
4031Sam Ruby,
4032Scott Lawrence (who maintained the original issues list),
4033Sean B. Palmer,
4034Shane McCarron,
4035Shigeki Ohtsu,
4036Stefan Eissing,
4037Stefan Tilkov,
4038Stefanos Harhalakis,
4039Stephane Bortzmeyer,
4040Stephen Farrell,
4041Stephen Ludin,
4042Stuart Williams,
4043Subbu Allamaraju,
4044Sylvain Hellegouarch,
4045Tapan Divekar,
4046Tatsuhiro Tsujikawa,
4047Tatsuya Hayashi,
4048Ted Hardie,
4049Thomas Broyer,
4050Thomas Fossati,
4051Thomas Maslen,
4052Thomas Nordin,
4053Thomas Roessler,
4054Tim Bray,
4055Tim Morgan,
4056Tim Olsen,
4057Tom Zhou,
4058Travis Snoozy,
4059Tyler Close,
4060Vincent Murphy,
4061Wenbo Zhu,
4062Werner Baumann,
4063Wilbur Streett,
4064Wilfredo Sanchez Vega,
4065William A. Rowe Jr.,
4066William Chan,
4067Willy Tarreau,
4068Xiaoshu Wang,
4069Yaron Goland,
4070Yngve Nysaeter Pettersen,
4071Yoav Nir,
4072Yogesh Bang,
4073Yuchung Cheng,
4074Yutaka Oiwa,
4075Yves Lafon (long-time member of the editor team),
4076Zed A. Shaw, and
4077Zhong Yu.
4078</t>
4079<?ENDINC acks ?>
4080<t>
4081   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
4082   acknowledgements from prior revisions.
4083</t>
4084</section>
4085
4086</middle>
4087<back>
4088
4089<references title="Normative References">
4090
4091<reference anchor="Part2">
4092  <front>
4093    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
4094    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4095      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4096      <address><email>fielding@gbiv.com</email></address>
4097    </author>
4098    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4099      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4100      <address><email>julian.reschke@greenbytes.de</email></address>
4101    </author>
4102    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4103  </front>
4104  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
4105  <x:source href="p2-semantics.xml" basename="p2-semantics">
4106    <x:defines>1xx (Informational)</x:defines>
4107    <x:defines>1xx</x:defines>
4108    <x:defines>100 (Continue)</x:defines>
4109    <x:defines>101 (Switching Protocols)</x:defines>
4110    <x:defines>2xx (Successful)</x:defines>
4111    <x:defines>2xx</x:defines>
4112    <x:defines>200 (OK)</x:defines>
4113    <x:defines>203 (Non-Authoritative Information)</x:defines>
4114    <x:defines>204 (No Content)</x:defines>
4115    <x:defines>3xx (Redirection)</x:defines>
4116    <x:defines>3xx</x:defines>
4117    <x:defines>301 (Moved Permanently)</x:defines>
4118    <x:defines>4xx (Client Error)</x:defines>
4119    <x:defines>4xx</x:defines>
4120    <x:defines>400 (Bad Request)</x:defines>
4121    <x:defines>411 (Length Required)</x:defines>
4122    <x:defines>414 (URI Too Long)</x:defines>
4123    <x:defines>417 (Expectation Failed)</x:defines>
4124    <x:defines>426 (Upgrade Required)</x:defines>
4125    <x:defines>501 (Not Implemented)</x:defines>
4126    <x:defines>502 (Bad Gateway)</x:defines>
4127    <x:defines>505 (HTTP Version Not Supported)</x:defines>
4128    <x:defines>Accept-Encoding</x:defines>
4129    <x:defines>Allow</x:defines>
4130    <x:defines>Content-Encoding</x:defines>
4131    <x:defines>Content-Location</x:defines>
4132    <x:defines>Content-Type</x:defines>
4133    <x:defines>Date</x:defines>
4134    <x:defines>Expect</x:defines>
4135    <x:defines>Location</x:defines>
4136    <x:defines>Server</x:defines>
4137    <x:defines>User-Agent</x:defines>
4138  </x:source>
4139</reference>
4140
4141<reference anchor="Part4">
4142  <front>
4143    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
4144    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
4145      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4146      <address><email>fielding@gbiv.com</email></address>
4147    </author>
4148    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
4149      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4150      <address><email>julian.reschke@greenbytes.de</email></address>
4151    </author>
4152    <date month="&ID-MONTH;" year="&ID-YEAR;" />
4153  </front>
4154  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
4155  <x:source basename="p4-conditional" href="p4-conditional.xml">
4156    <x:defines>304 (Not Modified)</x:defines>
4157    <x:defines>ETag</x:defines>
4158    <x:defines>Last-Modified</x:defines>
4159  </x:source>
4160</reference>
4161
4162<reference anchor="Part5">
4163  <front>
4164    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
4165    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4166      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4167      <address><email>fielding@gbiv.com</email></address>
4168    </author>
4169    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
4170      <organization abbrev="W3C">World Wide Web Consortium</organization>
4171      <address><email>ylafon@w3.org</email></address>
4172    </author>
4173    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4174      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4175      <address><email>julian.reschke@greenbytes.de</email></address>
4176    </author>
4177    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4178  </front>
4179  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4180  <x:source href="p5-range.xml" basename="p5-range">
4181    <x:defines>Content-Range</x:defines>
4182  </x:source>
4183</reference>
4184
4185<reference anchor="Part6">
4186  <front>
4187    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4188    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4189      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4190      <address><email>fielding@gbiv.com</email></address>
4191    </author>
4192    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4193      <organization>Akamai</organization>
4194      <address><email>mnot@mnot.net</email></address>
4195    </author>
4196    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4197      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4198      <address><email>julian.reschke@greenbytes.de</email></address>
4199    </author>
4200    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4201  </front>
4202  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4203  <x:source href="p6-cache.xml" basename="p6-cache">
4204    <x:defines>Cache-Control</x:defines>
4205    <x:defines>Expires</x:defines>
4206  </x:source>
4207</reference>
4208
4209<reference anchor="Part7">
4210  <front>
4211    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4212    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4213      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4214      <address><email>fielding@gbiv.com</email></address>
4215    </author>
4216    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4217      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4218      <address><email>julian.reschke@greenbytes.de</email></address>
4219    </author>
4220    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4221  </front>
4222  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4223  <x:source href="p7-auth.xml" basename="p7-auth">
4224    <x:defines>Proxy-Authenticate</x:defines>
4225    <x:defines>Proxy-Authorization</x:defines>
4226  </x:source>
4227</reference>
4228
4229<reference anchor="RFC5234">
4230  <front>
4231    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4232    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4233      <organization>Brandenburg InternetWorking</organization>
4234      <address>
4235        <email>dcrocker@bbiw.net</email>
4236      </address> 
4237    </author>
4238    <author initials="P." surname="Overell" fullname="Paul Overell">
4239      <organization>THUS plc.</organization>
4240      <address>
4241        <email>paul.overell@thus.net</email>
4242      </address>
4243    </author>
4244    <date month="January" year="2008"/>
4245  </front>
4246  <seriesInfo name="STD" value="68"/>
4247  <seriesInfo name="RFC" value="5234"/>
4248</reference>
4249
4250<reference anchor="RFC2119">
4251  <front>
4252    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4253    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4254      <organization>Harvard University</organization>
4255      <address><email>sob@harvard.edu</email></address>
4256    </author>
4257    <date month="March" year="1997"/>
4258  </front>
4259  <seriesInfo name="BCP" value="14"/>
4260  <seriesInfo name="RFC" value="2119"/>
4261</reference>
4262
4263<reference anchor="RFC3986">
4264 <front>
4265  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4266  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4267    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4268    <address>
4269       <email>timbl@w3.org</email>
4270       <uri>http://www.w3.org/People/Berners-Lee/</uri>
4271    </address>
4272  </author>
4273  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4274    <organization abbrev="Day Software">Day Software</organization>
4275    <address>
4276      <email>fielding@gbiv.com</email>
4277      <uri>http://roy.gbiv.com/</uri>
4278    </address>
4279  </author>
4280  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4281    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4282    <address>
4283      <email>LMM@acm.org</email>
4284      <uri>http://larry.masinter.net/</uri>
4285    </address>
4286  </author>
4287  <date month='January' year='2005'></date>
4288 </front>
4289 <seriesInfo name="STD" value="66"/>
4290 <seriesInfo name="RFC" value="3986"/>
4291</reference>
4292
4293<reference anchor="RFC0793">
4294  <front>
4295    <title>Transmission Control Protocol</title>
4296    <author initials='J.' surname='Postel' fullname='Jon Postel'>
4297      <organization>University of Southern California (USC)/Information Sciences Institute</organization>
4298    </author>
4299    <date year='1981' month='September' />
4300  </front>
4301  <seriesInfo name='STD' value='7' />
4302  <seriesInfo name='RFC' value='793' />
4303</reference>
4304
4305<reference anchor="USASCII">
4306  <front>
4307    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4308    <author>
4309      <organization>American National Standards Institute</organization>
4310    </author>
4311    <date year="1986"/>
4312  </front>
4313  <seriesInfo name="ANSI" value="X3.4"/>
4314</reference>
4315
4316<reference anchor="RFC1950">
4317  <front>
4318    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4319    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4320      <organization>Aladdin Enterprises</organization>
4321      <address><email>ghost@aladdin.com</email></address>
4322    </author>
4323    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4324    <date month="May" year="1996"/>
4325  </front>
4326  <seriesInfo name="RFC" value="1950"/>
4327  <!--<annotation>
4328    RFC 1950 is an Informational RFC, thus it might be less stable than
4329    this specification. On the other hand, this downward reference was
4330    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4331    therefore it is unlikely to cause problems in practice. See also
4332    <xref target="BCP97"/>.
4333  </annotation>-->
4334</reference>
4335
4336<reference anchor="RFC1951">
4337  <front>
4338    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4339    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4340      <organization>Aladdin Enterprises</organization>
4341      <address><email>ghost@aladdin.com</email></address>
4342    </author>
4343    <date month="May" year="1996"/>
4344  </front>
4345  <seriesInfo name="RFC" value="1951"/>
4346  <!--<annotation>
4347    RFC 1951 is an Informational RFC, thus it might be less stable than
4348    this specification. On the other hand, this downward reference was
4349    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4350    therefore it is unlikely to cause problems in practice. See also
4351    <xref target="BCP97"/>.
4352  </annotation>-->
4353</reference>
4354
4355<reference anchor="RFC1952">
4356  <front>
4357    <title>GZIP file format specification version 4.3</title>
4358    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4359      <organization>Aladdin Enterprises</organization>
4360      <address><email>ghost@aladdin.com</email></address>
4361    </author>
4362    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4363      <address><email>gzip@prep.ai.mit.edu</email></address>
4364    </author>
4365    <author initials="M." surname="Adler" fullname="Mark Adler">
4366      <address><email>madler@alumni.caltech.edu</email></address>
4367    </author>
4368    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4369      <address><email>ghost@aladdin.com</email></address>
4370    </author>
4371    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4372      <address><email>randeg@alumni.rpi.edu</email></address>
4373    </author>
4374    <date month="May" year="1996"/>
4375  </front>
4376  <seriesInfo name="RFC" value="1952"/>
4377  <!--<annotation>
4378    RFC 1952 is an Informational RFC, thus it might be less stable than
4379    this specification. On the other hand, this downward reference was
4380    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4381    therefore it is unlikely to cause problems in practice. See also
4382    <xref target="BCP97"/>.
4383  </annotation>-->
4384</reference>
4385
4386<reference anchor="Welch">
4387  <front>
4388    <title>A Technique for High Performance Data Compression</title>
4389    <author initials="T.A." surname="Welch" fullname="Terry A. Welch"/>
4390    <date month="June" year="1984"/>
4391  </front>
4392  <seriesInfo name="IEEE Computer" value="17(6)"/>
4393</reference>
4394
4395</references>
4396
4397<references title="Informative References">
4398
4399<reference anchor="ISO-8859-1">
4400  <front>
4401    <title>
4402     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4403    </title>
4404    <author>
4405      <organization>International Organization for Standardization</organization>
4406    </author>
4407    <date year="1998"/>
4408  </front>
4409  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4410</reference>
4411
4412<reference anchor='RFC1919'>
4413  <front>
4414    <title>Classical versus Transparent IP Proxies</title>
4415    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4416      <address><email>mchatel@pax.eunet.ch</email></address>
4417    </author>
4418    <date year='1996' month='March' />
4419  </front>
4420  <seriesInfo name='RFC' value='1919' />
4421</reference>
4422
4423<reference anchor="RFC1945">
4424  <front>
4425    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4426    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4427      <organization>MIT, Laboratory for Computer Science</organization>
4428      <address><email>timbl@w3.org</email></address>
4429    </author>
4430    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4431      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4432      <address><email>fielding@ics.uci.edu</email></address>
4433    </author>
4434    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4435      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4436      <address><email>frystyk@w3.org</email></address>
4437    </author>
4438    <date month="May" year="1996"/>
4439  </front>
4440  <seriesInfo name="RFC" value="1945"/>
4441</reference>
4442
4443<reference anchor="RFC2045">
4444  <front>
4445    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4446    <author initials="N." surname="Freed" fullname="Ned Freed">
4447      <organization>Innosoft International, Inc.</organization>
4448      <address><email>ned@innosoft.com</email></address>
4449    </author>
4450    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4451      <organization>First Virtual Holdings</organization>
4452      <address><email>nsb@nsb.fv.com</email></address>
4453    </author>
4454    <date month="November" year="1996"/>
4455  </front>
4456  <seriesInfo name="RFC" value="2045"/>
4457</reference>
4458
4459<reference anchor="RFC2047">
4460  <front>
4461    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4462    <author initials="K." surname="Moore" fullname="Keith Moore">
4463      <organization>University of Tennessee</organization>
4464      <address><email>moore@cs.utk.edu</email></address>
4465    </author>
4466    <date month="November" year="1996"/>
4467  </front>
4468  <seriesInfo name="RFC" value="2047"/>
4469</reference>
4470
4471<reference anchor="RFC2068">
4472  <front>
4473    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4474    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
4475      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4476      <address><email>fielding@ics.uci.edu</email></address>
4477    </author>
4478    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4479      <organization>MIT Laboratory for Computer Science</organization>
4480      <address><email>jg@w3.org</email></address>
4481    </author>
4482    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4483      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4484      <address><email>mogul@wrl.dec.com</email></address>
4485    </author>
4486    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4487      <organization>MIT Laboratory for Computer Science</organization>
4488      <address><email>frystyk@w3.org</email></address>
4489    </author>
4490    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4491      <organization>MIT Laboratory for Computer Science</organization>
4492      <address><email>timbl@w3.org</email></address>
4493    </author>
4494    <date month="January" year="1997"/>
4495  </front>
4496  <seriesInfo name="RFC" value="2068"/>
4497</reference>
4498
4499<reference anchor="RFC2145">
4500  <front>
4501    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4502    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4503      <organization>Western Research Laboratory</organization>
4504      <address><email>mogul@wrl.dec.com</email></address>
4505    </author>
4506    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4507      <organization>Department of Information and Computer Science</organization>
4508      <address><email>fielding@ics.uci.edu</email></address>
4509    </author>
4510    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4511      <organization>MIT Laboratory for Computer Science</organization>
4512      <address><email>jg@w3.org</email></address>
4513    </author>
4514    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4515      <organization>W3 Consortium</organization>
4516      <address><email>frystyk@w3.org</email></address>
4517    </author>
4518    <date month="May" year="1997"/>
4519  </front>
4520  <seriesInfo name="RFC" value="2145"/>
4521</reference>
4522
4523<reference anchor="RFC2616">
4524  <front>
4525    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4526    <author initials="R." surname="Fielding" fullname="R. Fielding">
4527      <organization>University of California, Irvine</organization>
4528      <address><email>fielding@ics.uci.edu</email></address>
4529    </author>
4530    <author initials="J." surname="Gettys" fullname="J. Gettys">
4531      <organization>W3C</organization>
4532      <address><email>jg@w3.org</email></address>
4533    </author>
4534    <author initials="J." surname="Mogul" fullname="J. Mogul">
4535      <organization>Compaq Computer Corporation</organization>
4536      <address><email>mogul@wrl.dec.com</email></address>
4537    </author>
4538    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4539      <organization>MIT Laboratory for Computer Science</organization>
4540      <address><email>frystyk@w3.org</email></address>
4541    </author>
4542    <author initials="L." surname="Masinter" fullname="L. Masinter">
4543      <organization>Xerox Corporation</organization>
4544      <address><email>masinter@parc.xerox.com</email></address>
4545    </author>
4546    <author initials="P." surname="Leach" fullname="P. Leach">
4547      <organization>Microsoft Corporation</organization>
4548      <address><email>paulle@microsoft.com</email></address>
4549    </author>
4550    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4551      <organization>W3C</organization>
4552      <address><email>timbl@w3.org</email></address>
4553    </author>
4554    <date month="June" year="1999"/>
4555  </front>
4556  <seriesInfo name="RFC" value="2616"/>
4557</reference>
4558
4559<reference anchor='RFC2817'>
4560  <front>
4561    <title>Upgrading to TLS Within HTTP/1.1</title>
4562    <author initials='R.' surname='Khare' fullname='R. Khare'>
4563      <organization>4K Associates / UC Irvine</organization>
4564      <address><email>rohit@4K-associates.com</email></address>
4565    </author>
4566    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4567      <organization>Agranat Systems, Inc.</organization>
4568      <address><email>lawrence@agranat.com</email></address>
4569    </author>
4570    <date year='2000' month='May' />
4571  </front>
4572  <seriesInfo name='RFC' value='2817' />
4573</reference>
4574
4575<reference anchor='RFC2818'>
4576  <front>
4577    <title>HTTP Over TLS</title>
4578    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4579      <organization>RTFM, Inc.</organization>
4580      <address><email>ekr@rtfm.com</email></address>
4581    </author>
4582    <date year='2000' month='May' />
4583  </front>
4584  <seriesInfo name='RFC' value='2818' />
4585</reference>
4586
4587<reference anchor='RFC3040'>
4588  <front>
4589    <title>Internet Web Replication and Caching Taxonomy</title>
4590    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4591      <organization>Equinix, Inc.</organization>
4592    </author>
4593    <author initials='I.' surname='Melve' fullname='I. Melve'>
4594      <organization>UNINETT</organization>
4595    </author>
4596    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4597      <organization>CacheFlow Inc.</organization>
4598    </author>
4599    <date year='2001' month='January' />
4600  </front>
4601  <seriesInfo name='RFC' value='3040' />
4602</reference>
4603
4604<reference anchor='BCP90'>
4605  <front>
4606    <title>Registration Procedures for Message Header Fields</title>
4607    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4608      <organization>Nine by Nine</organization>
4609      <address><email>GK-IETF@ninebynine.org</email></address>
4610    </author>
4611    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4612      <organization>BEA Systems</organization>
4613      <address><email>mnot@pobox.com</email></address>
4614    </author>
4615    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4616      <organization>HP Labs</organization>
4617      <address><email>JeffMogul@acm.org</email></address>
4618    </author>
4619    <date year='2004' month='September' />
4620  </front>
4621  <seriesInfo name='BCP' value='90' />
4622  <seriesInfo name='RFC' value='3864' />
4623</reference>
4624
4625<reference anchor='RFC4033'>
4626  <front>
4627    <title>DNS Security Introduction and Requirements</title>
4628    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4629    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4630    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4631    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4632    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4633    <date year='2005' month='March' />
4634  </front>
4635  <seriesInfo name='RFC' value='4033' />
4636</reference>
4637
4638<reference anchor="BCP13">
4639  <front>
4640    <title>Media Type Specifications and Registration Procedures</title>
4641    <author initials="N." surname="Freed" fullname="Ned Freed">
4642      <organization>Oracle</organization>
4643      <address>
4644        <email>ned+ietf@mrochek.com</email>
4645      </address>
4646    </author>
4647    <author initials="J." surname="Klensin" fullname="John C. Klensin">
4648      <address>
4649        <email>john+ietf@jck.com</email>
4650      </address>
4651    </author>
4652    <author initials="T." surname="Hansen" fullname="Tony Hansen">
4653      <organization>AT&amp;T Laboratories</organization>
4654      <address>
4655        <email>tony+mtsuffix@maillennium.att.com</email>
4656      </address>
4657    </author>
4658    <date year="2013" month="January"/>
4659  </front>
4660  <seriesInfo name="BCP" value="13"/>
4661  <seriesInfo name="RFC" value="6838"/>
4662</reference>
4663
4664<reference anchor='BCP115'>
4665  <front>
4666    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4667    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4668      <organization>AT&amp;T Laboratories</organization>
4669      <address>
4670        <email>tony+urireg@maillennium.att.com</email>
4671      </address>
4672    </author>
4673    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4674      <organization>Qualcomm, Inc.</organization>
4675      <address>
4676        <email>hardie@qualcomm.com</email>
4677      </address>
4678    </author>
4679    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4680      <organization>Adobe Systems</organization>
4681      <address>
4682        <email>LMM@acm.org</email>
4683      </address>
4684    </author>
4685    <date year='2006' month='February' />
4686  </front>
4687  <seriesInfo name='BCP' value='115' />
4688  <seriesInfo name='RFC' value='4395' />
4689</reference>
4690
4691<reference anchor='RFC4559'>
4692  <front>
4693    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4694    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4695    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4696    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4697    <date year='2006' month='June' />
4698  </front>
4699  <seriesInfo name='RFC' value='4559' />
4700</reference>
4701
4702<reference anchor='RFC5226'>
4703  <front>
4704    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4705    <author initials='T.' surname='Narten' fullname='T. Narten'>
4706      <organization>IBM</organization>
4707      <address><email>narten@us.ibm.com</email></address>
4708    </author>
4709    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4710      <organization>Google</organization>
4711      <address><email>Harald@Alvestrand.no</email></address>
4712    </author>
4713    <date year='2008' month='May' />
4714  </front>
4715  <seriesInfo name='BCP' value='26' />
4716  <seriesInfo name='RFC' value='5226' />
4717</reference>
4718
4719<reference anchor='RFC5246'>
4720   <front>
4721      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4722      <author initials='T.' surname='Dierks' fullname='T. Dierks'>
4723         <organization />
4724      </author>
4725      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4726         <organization>RTFM, Inc.</organization>
4727      </author>
4728      <date year='2008' month='August' />
4729   </front>
4730   <seriesInfo name='RFC' value='5246' />
4731</reference>
4732
4733<reference anchor="RFC5322">
4734  <front>
4735    <title>Internet Message Format</title>
4736    <author initials="P." surname="Resnick" fullname="P. Resnick">
4737      <organization>Qualcomm Incorporated</organization>
4738    </author>
4739    <date year="2008" month="October"/>
4740  </front>
4741  <seriesInfo name="RFC" value="5322"/>
4742</reference>
4743
4744<reference anchor="RFC6265">
4745  <front>
4746    <title>HTTP State Management Mechanism</title>
4747    <author initials="A." surname="Barth" fullname="Adam Barth">
4748      <organization abbrev="U.C. Berkeley">
4749        University of California, Berkeley
4750      </organization>
4751      <address><email>abarth@eecs.berkeley.edu</email></address>
4752    </author>
4753    <date year="2011" month="April" />
4754  </front>
4755  <seriesInfo name="RFC" value="6265"/>
4756</reference>
4757
4758<reference anchor='RFC6585'>
4759  <front>
4760    <title>Additional HTTP Status Codes</title>
4761    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4762      <organization>Rackspace</organization>
4763    </author>
4764    <author initials='R.' surname='Fielding' fullname='R. Fielding'>
4765      <organization>Adobe</organization>
4766    </author>
4767    <date year='2012' month='April' />
4768   </front>
4769   <seriesInfo name='RFC' value='6585' />
4770</reference>
4771
4772<!--<reference anchor='BCP97'>
4773  <front>
4774    <title>Handling Normative References to Standards-Track Documents</title>
4775    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
4776      <address>
4777        <email>klensin+ietf@jck.com</email>
4778      </address>
4779    </author>
4780    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
4781      <organization>MIT</organization>
4782      <address>
4783        <email>hartmans-ietf@mit.edu</email>
4784      </address>
4785    </author>
4786    <date year='2007' month='June' />
4787  </front>
4788  <seriesInfo name='BCP' value='97' />
4789  <seriesInfo name='RFC' value='4897' />
4790</reference>-->
4791
4792<reference anchor="Kri2001" target="http://arxiv.org/abs/cs.SE/0105018">
4793  <front>
4794    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
4795    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
4796    <date year="2001" month="November"/>
4797  </front>
4798  <seriesInfo name="ACM Transactions on Internet Technology" value="1(2)"/>
4799</reference>
4800
4801</references>
4802
4803
4804<section title="HTTP Version History" anchor="compatibility">
4805<t>
4806   HTTP has been in use by the World-Wide Web global information initiative
4807   since 1990. The first version of HTTP, later referred to as HTTP/0.9,
4808   was a simple protocol for hypertext data transfer across the Internet
4809   with only a single request method (GET) and no metadata.
4810   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
4811   methods and MIME-like messaging that could include metadata about the data
4812   transferred and modifiers on the request/response semantics. However,
4813   HTTP/1.0 did not sufficiently take into consideration the effects of
4814   hierarchical proxies, caching, the need for persistent connections, or
4815   name-based virtual hosts. The proliferation of incompletely-implemented
4816   applications calling themselves "HTTP/1.0" further necessitated a
4817   protocol version change in order for two communicating applications
4818   to determine each other's true capabilities.
4819</t>
4820<t>
4821   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
4822   requirements that enable reliable implementations, adding only
4823   those new features that will either be safely ignored by an HTTP/1.0
4824   recipient or only sent when communicating with a party advertising
4825   conformance with HTTP/1.1.
4826</t>
4827<t>
4828   It is beyond the scope of a protocol specification to mandate
4829   conformance with previous versions. HTTP/1.1 was deliberately
4830   designed, however, to make supporting previous versions easy.
4831   We would expect a general-purpose HTTP/1.1 server to understand
4832   any valid request in the format of HTTP/1.0 and respond appropriately
4833   with an HTTP/1.1 message that only uses features understood (or
4834   safely ignored) by HTTP/1.0 clients.  Likewise, we would expect
4835   an HTTP/1.1 client to understand any valid HTTP/1.0 response.
4836</t>
4837<t>
4838   Since HTTP/0.9 did not support header fields in a request,
4839   there is no mechanism for it to support name-based virtual
4840   hosts (selection of resource by inspection of the <x:ref>Host</x:ref> header
4841   field).  Any server that implements name-based virtual hosts
4842   ought to disable support for HTTP/0.9.  Most requests that
4843   appear to be HTTP/0.9 are, in fact, badly constructed HTTP/1.x
4844   requests wherein a buggy client failed to properly encode
4845   linear whitespace found in a URI reference and placed in
4846   the request-target.
4847</t>
4848
4849<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
4850<t>
4851   This section summarizes major differences between versions HTTP/1.0
4852   and HTTP/1.1.
4853</t>
4854
4855<section title="Multi-homed Web Servers" anchor="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">
4856<t>
4857   The requirements that clients and servers support the <x:ref>Host</x:ref>
4858   header field (<xref target="header.host"/>), report an error if it is
4859   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
4860   are among the most important changes defined by HTTP/1.1.
4861</t>
4862<t>
4863   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4864   addresses and servers; there was no other established mechanism for
4865   distinguishing the intended server of a request than the IP address
4866   to which that request was directed. The <x:ref>Host</x:ref> header field was
4867   introduced during the development of HTTP/1.1 and, though it was
4868   quickly implemented by most HTTP/1.0 browsers, additional requirements
4869   were placed on all HTTP/1.1 requests in order to ensure complete
4870   adoption.  At the time of this writing, most HTTP-based services
4871   are dependent upon the Host header field for targeting requests.
4872</t>
4873</section>
4874
4875<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4876<t>
4877   In HTTP/1.0, each connection is established by the client prior to the
4878   request and closed by the server after sending the response. However, some
4879   implementations implement the explicitly negotiated ("Keep-Alive") version
4880   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
4881   target="RFC2068"/>.
4882</t>
4883<t>
4884   Some clients and servers might wish to be compatible with these previous
4885   approaches to persistent connections, by explicitly negotiating for them
4886   with a "Connection: keep-alive" request header field. However, some
4887   experimental implementations of HTTP/1.0 persistent connections are faulty;
4888   for example, if an HTTP/1.0 proxy server doesn't understand
4889   <x:ref>Connection</x:ref>, it will erroneously forward that header field
4890   to the next inbound server, which would result in a hung connection.
4891</t>
4892<t>
4893   One attempted solution was the introduction of a Proxy-Connection header
4894   field, targeted specifically at proxies. In practice, this was also
4895   unworkable, because proxies are often deployed in multiple layers, bringing
4896   about the same problem discussed above.
4897</t>
4898<t>
4899   As a result, clients are encouraged not to send the Proxy-Connection header
4900   field in any requests.
4901</t>
4902<t>
4903   Clients are also encouraged to consider the use of Connection: keep-alive
4904   in requests carefully; while they can enable persistent connections with
4905   HTTP/1.0 servers, clients using them will need to monitor the
4906   connection for "hung" requests (which indicate that the client ought stop
4907   sending the header field), and this mechanism ought not be used by clients
4908   at all when a proxy is being used.
4909</t>
4910</section>
4911
4912<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
4913<t>
4914   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
4915   (<xref target="header.transfer-encoding"/>).
4916   Transfer codings need to be decoded prior to forwarding an HTTP message
4917   over a MIME-compliant protocol.
4918</t>
4919</section>
4920
4921</section>
4922
4923<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4924<t>
4925  HTTP's approach to error handling has been explained.
4926  (<xref target="conformance"/>)
4927</t>
4928<t>
4929  The expectation to support HTTP/0.9 requests has been removed.
4930</t>
4931<t>
4932  The term "Effective Request URI" has been introduced.
4933  (<xref target="effective.request.uri" />)
4934</t>
4935<t>
4936  HTTP messages can be (and often are) buffered by implementations; despite
4937  it sometimes being available as a stream, HTTP is fundamentally a
4938  message-oriented protocol.
4939  (<xref target="http.message" />)
4940</t>
4941<t>
4942  Minimum supported sizes for various protocol elements have been
4943  suggested, to improve interoperability.
4944</t>
4945<t>
4946  Header fields that span multiple lines ("line folding") are deprecated.
4947  (<xref target="field.parsing" />)
4948</t>
4949<t>
4950  The HTTP-version ABNF production has been clarified to be case-sensitive.
4951  Additionally, version numbers has been restricted to single digits, due
4952  to the fact that implementations are known to handle multi-digit version
4953  numbers incorrectly.
4954  (<xref target="http.version"/>)
4955</t>
4956<t>
4957  The HTTPS URI scheme is now defined by this specification; previously,
4958  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
4959  (<xref target="https.uri"/>)
4960</t>
4961<t>
4962  The HTTPS URI scheme implies end-to-end security.
4963  (<xref target="https.uri"/>)
4964</t>
4965<t>
4966  Userinfo (i.e., username and password) are now disallowed in HTTP and
4967  HTTPS URIs, because of security issues related to their transmission on the
4968  wire.
4969  (<xref target="http.uri" />)
4970</t>
4971<t>
4972  Invalid whitespace around field-names is now required to be rejected,
4973  because accepting it represents a security vulnerability.
4974  (<xref target="header.fields"/>)
4975</t>
4976<t>
4977  The ABNF productions defining header fields now only list the field value.
4978  (<xref target="header.fields"/>)
4979</t>
4980<t>
4981  Rules about implicit linear whitespace between certain grammar productions
4982  have been removed; now whitespace is only allowed where specifically
4983  defined in the ABNF.
4984  (<xref target="whitespace"/>)
4985</t>
4986<t> 
4987  The NUL octet is no longer allowed in comment and quoted-string text, and
4988  handling of backslash-escaping in them has been clarified.
4989  (<xref target="field.components"/>)
4990</t> 
4991<t>
4992  The quoted-pair rule no longer allows escaping control characters other than
4993  HTAB.
4994  (<xref target="field.components"/>)
4995</t>
4996<t>
4997  Non-ASCII content in header fields and the reason phrase has been obsoleted
4998  and made opaque (the TEXT rule was removed).
4999  (<xref target="field.components"/>)
5000</t>
5001<t>
5002  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
5003  handled as errors by recipients.
5004  (<xref target="header.content-length"/>)
5005</t>
5006<t>
5007  The "identity" transfer coding token has been removed.
5008  (Sections <xref format="counter" target="message.body"/> and
5009  <xref format="counter" target="transfer.codings"/>)
5010</t>
5011<t>
5012  The algorithm for determining the message body length has been clarified
5013  to indicate all of the special cases (e.g., driven by methods or status
5014  codes) that affect it, and that new protocol elements cannot define such
5015  special cases.
5016  (<xref target="message.body.length"/>)
5017</t>
5018<t>
5019  "multipart/byteranges" is no longer a way of determining message body length
5020  detection.
5021  (<xref target="message.body.length"/>)
5022</t>
5023<t>
5024  CONNECT is a new, special case in determining message body length.
5025  (<xref target="message.body.length"/>)
5026</t>
5027<t>
5028  Chunk length does not include the count of the octets in the
5029  chunk header and trailer.
5030  (<xref target="chunked.encoding"/>)
5031</t>
5032<t>
5033  Use of chunk extensions is deprecated, and line folding in them is
5034  disallowed.
5035  (<xref target="chunked.encoding"/>)
5036</t>
5037<t>
5038  The segment + query components of RFC 3986 have been used to define the
5039  request-target, instead of abs_path from RFC 1808.
5040  (<xref target="request-target"/>)
5041</t>
5042<t>
5043  The asterisk-form of the request-target is only allowed in the OPTIONS
5044  method.
5045  (<xref target="request-target"/>)
5046</t>
5047<t>
5048  Gateways do not need to generate <x:ref>Via</x:ref> header fields anymore.
5049  (<xref target="header.via"/>)
5050</t>
5051<t>
5052  Exactly when "close" connection options have to be sent has been clarified.
5053  (<xref target="header.connection"/>)
5054</t>
5055<t> 
5056  "hop-by-hop" header fields are required to appear in the Connection header
5057  field; just because they're defined as hop-by-hop in this specification
5058  doesn't exempt them.
5059  (<xref target="header.connection"/>)
5060</t>
5061<t>
5062  The limit of two connections per server has been removed.
5063  (<xref target="persistent.connections"/>)
5064</t>
5065<t>
5066  An idempotent sequence of requests is no longer required to be retried.
5067  (<xref target="persistent.connections"/>)
5068</t>
5069<t>
5070  The requirement to retry requests under certain circumstances when the
5071  server prematurely closes the connection has been removed.
5072  (<xref target="persistent.connections"/>)
5073</t>
5074<t>
5075  Some extraneous requirements about when servers are allowed to close
5076  connections prematurely have been removed.
5077  (<xref target="persistent.connections"/>)
5078</t>
5079<t>
5080  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
5081  responses other than 101 (this was incorporated from <xref
5082  target="RFC2817"/>). Furthermore, the ordering in the field value is now
5083  significant.
5084  (<xref target="header.upgrade"/>)
5085</t>
5086<t>
5087  Registration of Transfer Codings now requires IETF Review
5088  (<xref target="transfer.coding.registry"/>)
5089</t>
5090<t>
5091  The meaning of the "deflate" content coding has been clarified.
5092  (<xref target="deflate.coding" />)
5093</t>
5094<t>
5095  This specification now defines the Upgrade Token Registry, previously
5096  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
5097  (<xref target="upgrade.token.registry"/>)
5098</t>
5099<t>
5100  Issues with the Keep-Alive and Proxy-Connection header fields in requests
5101  are pointed out, with use of the latter being discouraged altogether.
5102  (<xref target="compatibility.with.http.1.0.persistent.connections" />)
5103</t>
5104<t>
5105  Empty list elements in list productions (e.g., a list header field containing
5106  ", ,") have been deprecated.
5107  (<xref target="abnf.extension"/>)
5108</t>
5109</section>
5110</section>
5111
5112<section title="ABNF list extension: #rule" anchor="abnf.extension">
5113<t>
5114  A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
5115  improve readability in the definitions of some header field values.
5116</t>
5117<t>
5118  A construct "#" is defined, similar to "*", for defining comma-delimited
5119  lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
5120  at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
5121  comma (",") and optional whitespace (OWS).   
5122</t>
5123<figure><preamble>
5124  Thus,
5125</preamble><artwork type="example">
5126  1#element =&gt; element *( OWS "," OWS element )
5127</artwork></figure>
5128<figure><preamble>
5129  and:
5130</preamble><artwork type="example">
5131  #element =&gt; [ 1#element ]
5132</artwork></figure>
5133<figure><preamble>
5134  and for n &gt;= 1 and m &gt; 1:
5135</preamble><artwork type="example">
5136  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
5137</artwork></figure>
5138<t>
5139  For compatibility with legacy list rules, recipients &SHOULD; accept empty
5140  list elements. In other words, consumers would follow the list productions:
5141</t>
5142<figure><artwork type="example">
5143  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
5144 
5145  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
5146</artwork></figure>
5147<t>
5148  Note that empty elements do not contribute to the count of elements present,
5149  though.
5150</t>
5151<t>
5152  For example, given these ABNF productions:
5153</t>
5154<figure><artwork type="example">
5155  example-list      = 1#example-list-elmt
5156  example-list-elmt = token ; see <xref target="field.components"/>
5157</artwork></figure>
5158<t>
5159  Then these are valid values for example-list (not including the double
5160  quotes, which are present for delimitation only):
5161</t>
5162<figure><artwork type="example">
5163  "foo,bar"
5164  "foo ,bar,"
5165  "foo , ,bar,charlie   "
5166</artwork></figure>
5167<t>
5168  But these values would be invalid, as at least one non-empty element is
5169  required:
5170</t>
5171<figure><artwork type="example">
5172  ""
5173  ","
5174  ",   ,"
5175</artwork></figure>
5176<t>
5177  <xref target="collected.abnf"/> shows the collected ABNF, with the list rules
5178  expanded as explained above.
5179</t>
5180</section>
5181
5182<?BEGININC p1-messaging.abnf-appendix ?>
5183<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
5184<figure>
5185<artwork type="abnf" name="p1-messaging.parsed-abnf">
5186<x:ref>BWS</x:ref> = OWS
5187
5188<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
5189 connection-option ] )
5190<x:ref>Content-Length</x:ref> = 1*DIGIT
5191
5192<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
5193 ]
5194<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
5195<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
5196<x:ref>Host</x:ref> = uri-host [ ":" port ]
5197
5198<x:ref>OWS</x:ref> = *( SP / HTAB )
5199
5200<x:ref>RWS</x:ref> = 1*( SP / HTAB )
5201
5202<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
5203<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
5204<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
5205 transfer-coding ] )
5206
5207<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
5208<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
5209
5210<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
5211 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
5212 comment ] ) ] )
5213
5214<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
5215<x:ref>absolute-form</x:ref> = absolute-URI
5216<x:ref>absolute-path</x:ref> = 1*( "/" segment )
5217<x:ref>asterisk-form</x:ref> = "*"
5218<x:ref>attribute</x:ref> = token
5219<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
5220<x:ref>authority-form</x:ref> = authority
5221
5222<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
5223<x:ref>chunk-data</x:ref> = 1*OCTET
5224<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
5225<x:ref>chunk-ext-name</x:ref> = token
5226<x:ref>chunk-ext-val</x:ref> = token / quoted-str-nf
5227<x:ref>chunk-size</x:ref> = 1*HEXDIG
5228<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
5229<x:ref>comment</x:ref> = "(" *( ctext / quoted-cpair / comment ) ")"
5230<x:ref>connection-option</x:ref> = token
5231<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
5232 / %x2A-5B ; '*'-'['
5233 / %x5D-7E ; ']'-'~'
5234 / obs-text
5235
5236<x:ref>field-content</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5237<x:ref>field-name</x:ref> = token
5238<x:ref>field-value</x:ref> = *( field-content / obs-fold )
5239<x:ref>fragment</x:ref> = &lt;fragment, defined in [RFC3986], Section 3.5&gt;
5240
5241<x:ref>header-field</x:ref> = field-name ":" OWS field-value OWS
5242<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ] [ "#"
5243 fragment ]
5244<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ] [ "#"
5245 fragment ]
5246
5247<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
5248
5249<x:ref>message-body</x:ref> = *OCTET
5250<x:ref>method</x:ref> = token
5251
5252<x:ref>obs-fold</x:ref> = CRLF ( SP / HTAB )
5253<x:ref>obs-text</x:ref> = %x80-FF
5254<x:ref>origin-form</x:ref> = absolute-path [ "?" query ]
5255
5256<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
5257<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
5258<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
5259<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
5260<x:ref>protocol-name</x:ref> = token
5261<x:ref>protocol-version</x:ref> = token
5262<x:ref>pseudonym</x:ref> = token
5263
5264<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5265 / %x5D-7E ; ']'-'~'
5266 / obs-text
5267<x:ref>qdtext-nf</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5268 / %x5D-7E ; ']'-'~'
5269 / obs-text
5270<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5271<x:ref>quoted-cpair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5272<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5273<x:ref>quoted-str-nf</x:ref> = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
5274<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5275
5276<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5277<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5278<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5279<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5280<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5281<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5282<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5283 asterisk-form
5284
5285<x:ref>segment</x:ref> = &lt;segment, defined in [RFC3986], Section 3.3&gt;
5286<x:ref>special</x:ref> = "(" / ")" / "&lt;" / "&gt;" / "@" / "," / ";" / ":" / "\" /
5287 DQUOTE / "/" / "[" / "]" / "?" / "=" / "{" / "}"
5288<x:ref>start-line</x:ref> = request-line / status-line
5289<x:ref>status-code</x:ref> = 3DIGIT
5290<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5291
5292<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5293<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5294<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5295 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5296<x:ref>token</x:ref> = 1*tchar
5297<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5298<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5299 transfer-extension
5300<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5301<x:ref>transfer-parameter</x:ref> = attribute BWS "=" BWS value
5302
5303<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5304
5305<x:ref>value</x:ref> = word
5306
5307<x:ref>word</x:ref> = token / quoted-string
5308</artwork>
5309</figure>
5310</section>
5311<?ENDINC p1-messaging.abnf-appendix ?>
5312
5313<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5314
5315<section title="Since RFC 2616">
5316<t>
5317  Changes up to the first Working Group Last Call draft are summarized
5318  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-21#appendix-D"/>.
5319</t>
5320</section>
5321
5322<section title="Since draft-ietf-httpbis-p1-messaging-21" anchor="changes.since.21">
5323<t>
5324  Closed issues:
5325  <list style="symbols">
5326    <t>
5327      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/128"/>:
5328      "Cite HTTPS URI scheme definition" (the spec now includes the HTTPs
5329      scheme definition and thus updates RFC 2818)
5330    </t>
5331    <t>
5332      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/389"/>:
5333      "mention of 'proxies' in section about caches"
5334    </t>
5335    <t>
5336      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/390"/>:
5337      "use of ABNF terms from RFC 3986"
5338    </t>
5339    <t>
5340      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/391"/>:
5341      "transferring URIs with userinfo in payload"
5342    </t>
5343    <t>
5344      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/392"/>:
5345      "editorial improvements to message length definition"
5346    </t>
5347    <t>
5348      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/395"/>:
5349      "Connection header field MUST vs SHOULD"
5350    </t>
5351    <t>
5352      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/396"/>:
5353      "editorial improvements to persistent connections section"
5354    </t>
5355    <t>
5356      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/397"/>:
5357      "URI normalization vs empty path"
5358    </t>
5359    <t>
5360      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/408"/>:
5361      "p1 feedback"
5362    </t>
5363    <t>
5364      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/409"/>:
5365      "is parsing OBS-FOLD mandatory?"
5366    </t>
5367    <t>
5368      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/410"/>:
5369      "HTTPS and Shared Caching"
5370    </t>
5371    <t>
5372      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/411"/>:
5373      "Requirements for recipients of ws between start-line and first header field"
5374    </t>
5375    <t>
5376      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/412"/>:
5377      "SP and HT when being tolerant"
5378    </t>
5379    <t>
5380      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/414"/>:
5381      "Message Parsing Strictness"
5382    </t>
5383    <t>
5384      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/415"/>:
5385      "'Render'"
5386    </t>
5387    <t>
5388      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/418"/>:
5389      "No-Transform"
5390    </t>
5391    <t>
5392      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/419"/>:
5393      "p2 editorial feedback"
5394    </t>
5395    <t>
5396      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/420"/>:
5397      "Content-Length SHOULD be sent"
5398    </t>
5399    <t>
5400      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/431"/>:
5401      "origin-form does not allow path starting with "//""
5402    </t>
5403    <t>
5404      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/433"/>:
5405      "ambiguity in part 1 example"
5406    </t>
5407  </list>
5408</t>
5409</section>
5410
5411<section title="Since draft-ietf-httpbis-p1-messaging-22" anchor="changes.since.22">
5412<t>
5413  Closed issues:
5414  <list style="symbols">
5415    <t>
5416      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/435"/>:
5417      "Part1 should have a reference to TCP (RFC 793)"
5418    </t>
5419    <t>
5420      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/438"/>:
5421      "media type registration template issues"
5422    </t>
5423    <t>
5424      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/441"/>:
5425      P1 editorial nits
5426    </t>
5427    <t>
5428      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/442"/>:
5429      "BWS" (vs conformance)
5430    </t>
5431    <t>
5432      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/444"/>:
5433      "obs-fold language"
5434    </t>
5435    <t>
5436      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/445"/>:
5437      "Ordering in Upgrade"
5438    </t>
5439    <t>
5440      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/446"/>:
5441      "p1 editorial feedback"
5442    </t>
5443    <t>
5444      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/447"/>:
5445      "HTTP and TCP name delegation"
5446    </t>
5447    <t>
5448      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/449"/>:
5449      "Receiving a higher minor HTTP version number"
5450    </t>
5451    <t>
5452      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/451"/>:
5453      "HTTP(S) URIs and fragids"
5454    </t>
5455    <t>
5456      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/457"/>:
5457      "Registering x-gzip and x-deflate"
5458    </t>
5459    <t>
5460      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/460"/>:
5461      "Via and gateways"
5462    </t>
5463    <t>
5464      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/465"/>:
5465      "Mention 203 Non-Authoritative Information in p1"
5466    </t>
5467    <t>
5468      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/476"/>:
5469      "SHOULD and conformance"
5470    </t>
5471    <t>
5472      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/477"/>:
5473      "Pipelining language"
5474    </t>
5475    <t>
5476      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/482"/>:
5477      "proxy handling of a really bad Content-Length"
5478    </t>
5479  </list>
5480</t>
5481</section>
5482
5483<section title="Since draft-ietf-httpbis-p1-messaging-23" anchor="changes.since.23">
5484<t>
5485  Closed issues:
5486  <list style="symbols">
5487    <t>
5488      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/483"/>:
5489      "MUST fix Content-Length?"
5490    </t>
5491  </list>
5492</t>
5493</section>
5494</section>
5495
5496</back>
5497</rfc>
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