source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2029

Last change on this file since 2029 was 2029, checked in by fielding@…, 8 years ago

(editorial) just refer to IP address since we don't distinguish IPv4 vs IPv6 anywhere else; addresses #390

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 222.3 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 "December">
16  <!ENTITY ID-YEAR "2012">
17  <!ENTITY mdash "&#8212;">
18  <!ENTITY Note "<x:h xmlns:x='http://purl.org/net/xml2rfc/ext'>Note:</x:h>">
19  <!ENTITY caching-overview       "<xref target='Part6' x:rel='#caching.overview' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY cache-incomplete       "<xref target='Part6' x:rel='#response.cacheability' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY payload                "<xref target='Part2' x:rel='#payload' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY media-type            "<xref target='Part2' x:rel='#media.type' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY content-codings        "<xref target='Part2' x:rel='#content.codings' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY CONNECT                "<xref target='Part2' x:rel='#CONNECT' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY content.negotiation    "<xref target='Part2' x:rel='#content.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY diff-mime              "<xref target='Part2' x:rel='#differences.between.http.and.mime' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY representation         "<xref target='Part2' x:rel='#representation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY HEAD                   "<xref target='Part2' x:rel='#HEAD' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-allow           "<xref target='Part2' x:rel='#header.allow' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-cache-control   "<xref target='Part6' x:rel='#header.cache-control' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY header-content-encoding    "<xref target='Part2' x:rel='#header.content-encoding' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY header-content-location    "<xref target='Part2' x:rel='#header.content-location' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY header-content-range   "<xref target='Part5' x:rel='#header.content-range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY header-content-type    "<xref target='Part2' x:rel='#header.content-type' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY header-date            "<xref target='Part2' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY header-etag            "<xref target='Part4' x:rel='#header.etag' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY header-expires         "<xref target='Part6' x:rel='#header.expires' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY header-last-modified   "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY header-mime-version    "<xref target='Part2' x:rel='#mime-version' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40  <!ENTITY header-pragma          "<xref target='Part6' x:rel='#header.pragma' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41  <!ENTITY header-proxy-authenticate  "<xref target='Part7' x:rel='#header.proxy-authenticate' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
42  <!ENTITY header-proxy-authorization "<xref target='Part7' x:rel='#header.proxy-authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
43  <!ENTITY header-server          "<xref target='Part2' x:rel='#header.server' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
44  <!ENTITY header-warning         "<xref target='Part6' x:rel='#header.warning' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
45  <!ENTITY idempotent-methods     "<xref target='Part2' x:rel='#idempotent.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
46  <!ENTITY methods                "<xref target='Part2' x:rel='#methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
47  <!ENTITY OPTIONS                "<xref target='Part2' x:rel='#OPTIONS' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
48  <!ENTITY qvalue                 "<xref target='Part2' x:rel='#quality.values' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
49  <!ENTITY resource               "<xref target='Part2' x:rel='#resource' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
50  <!ENTITY status-codes           "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
51  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
52  <!ENTITY status-203             "<xref target='Part2' x:rel='#status.203' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
53  <!ENTITY status-3xx             "<xref target='Part2' x:rel='#status.3xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
54  <!ENTITY status-304             "<xref target='Part4' x:rel='#status.304' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
55  <!ENTITY status-4xx             "<xref target='Part2' x:rel='#status.4xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
56  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
57  <!ENTITY iana-header-registry   "<xref target='Part2' x:rel='#header.field.registry' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
58]>
59<?rfc toc="yes" ?>
60<?rfc symrefs="yes" ?>
61<?rfc sortrefs="yes" ?>
62<?rfc compact="yes"?>
63<?rfc subcompact="no" ?>
64<?rfc linkmailto="no" ?>
65<?rfc editing="no" ?>
66<?rfc comments="yes"?>
67<?rfc inline="yes"?>
68<?rfc rfcedstyle="yes"?>
69<?rfc-ext allow-markup-in-artwork="yes" ?>
70<?rfc-ext include-references-in-index="yes" ?>
71<rfc obsoletes="2145,2616" updates="2817,2818" category="std" x:maturity-level="proposed"
72     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
73     xmlns:x='http://purl.org/net/xml2rfc/ext'>
74<x:link rel="next" basename="p2-semantics"/>
75<x:feedback template="mailto:ietf-http-wg@w3.org?subject={docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
76<front>
77
78  <title abbrev="HTTP/1.1 Message Syntax and Routing">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
79
80  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
81    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
82    <address>
83      <postal>
84        <street>345 Park Ave</street>
85        <city>San Jose</city>
86        <region>CA</region>
87        <code>95110</code>
88        <country>USA</country>
89      </postal>
90      <email>fielding@gbiv.com</email>
91      <uri>http://roy.gbiv.com/</uri>
92    </address>
93  </author>
94
95  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
96    <organization abbrev="greenbytes">greenbytes GmbH</organization>
97    <address>
98      <postal>
99        <street>Hafenweg 16</street>
100        <city>Muenster</city><region>NW</region><code>48155</code>
101        <country>Germany</country>
102      </postal>
103      <email>julian.reschke@greenbytes.de</email>
104      <uri>http://greenbytes.de/tech/webdav/</uri>
105    </address>
106  </author>
107
108  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
109  <workgroup>HTTPbis Working Group</workgroup>
110
111<abstract>
112<t>
113   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
114   distributed, collaborative, hypertext information systems. HTTP has been in
115   use by the World Wide Web global information initiative since 1990.
116   This document provides an overview of HTTP architecture and its associated
117   terminology, defines the "http" and "https" Uniform Resource Identifier
118   (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements,
119   and describes general security concerns for implementations.
120</t>   
121</abstract>
122
123<note title="Editorial Note (To be removed by RFC Editor)">
124  <t>
125    Discussion of this draft takes place on the HTTPBIS working group
126    mailing list (ietf-http-wg@w3.org), which is archived at
127    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
128  </t>
129  <t>
130    The current issues list is at
131    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
132    documents (including fancy diffs) can be found at
133    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
134  </t>
135  <t>
136    The changes in this draft are summarized in <xref target="changes.since.21"/>.
137  </t>
138</note>
139</front>
140<middle>
141<section title="Introduction" anchor="introduction">
142<t>
143   The Hypertext Transfer Protocol (HTTP) is an application-level
144   request/response protocol that uses extensible semantics and MIME-like
145   message payloads for flexible interaction with network-based hypertext
146   information systems. This document is the first in a series of documents
147   that collectively form the HTTP/1.1 specification:
148   <list style="empty">
149    <t>RFC xxx1: Message Syntax and Routing</t>
150    <t><xref target="Part2" x:fmt="none">RFC xxx2</xref>: Semantics and Content</t>
151    <t><xref target="Part4" x:fmt="none">RFC xxx3</xref>: Conditional Requests</t>
152    <t><xref target="Part5" x:fmt="none">RFC xxx4</xref>: Range Requests</t>
153    <t><xref target="Part6" x:fmt="none">RFC xxx5</xref>: Caching</t>
154    <t><xref target="Part7" x:fmt="none">RFC xxx6</xref>: Authentication</t>
155   </list>
156</t>
157<t>
158   This HTTP/1.1 specification obsoletes and moves to historic status
159   <xref target="RFC2616" x:fmt="none">RFC 2616</xref>, its predecessor
160   <xref target="RFC2068" x:fmt="none">RFC 2068</xref>, and
161   <xref target="RFC2145" x:fmt="none">RFC 2145</xref> (on HTTP versioning).
162   This specification also updates the use of CONNECT to establish a tunnel,
163   previously defined in <xref target="RFC2817" x:fmt="none">RFC 2817</xref>,
164   and defines the "https" URI scheme that was described informally in
165   <xref target="RFC2818" x:fmt="none">RFC 2818</xref>.
166</t>
167<t>
168   HTTP is a generic interface protocol for information systems. It is
169   designed to hide the details of how a service is implemented by presenting
170   a uniform interface to clients that is independent of the types of
171   resources provided. Likewise, servers do not need to be aware of each
172   client's purpose: an HTTP request can be considered in isolation rather
173   than being associated with a specific type of client or a predetermined
174   sequence of application steps. The result is a protocol that can be used
175   effectively in many different contexts and for which implementations can
176   evolve independently over time.
177</t>
178<t>
179   HTTP is also designed for use as an intermediation protocol for translating
180   communication to and from non-HTTP information systems.
181   HTTP proxies and gateways can provide access to alternative information
182   services by translating their diverse protocols into a hypertext
183   format that can be viewed and manipulated by clients in the same way
184   as HTTP services.
185</t>
186<t>
187   One consequence of HTTP flexibility is that the protocol cannot be
188   defined in terms of what occurs behind the interface. Instead, we
189   are limited to defining the syntax of communication, the intent
190   of received communication, and the expected behavior of recipients.
191   If the communication is considered in isolation, then successful
192   actions ought to be reflected in corresponding changes to the
193   observable interface provided by servers. However, since multiple
194   clients might act in parallel and perhaps at cross-purposes, we
195   cannot require that such changes be observable beyond the scope
196   of a single response.
197</t>
198<t>
199   This document describes the architectural elements that are used or
200   referred to in HTTP, defines the "http" and "https" URI schemes,
201   describes overall network operation and connection management,
202   and defines HTTP message framing and forwarding requirements.
203   Our goal is to define all of the mechanisms necessary for HTTP message
204   handling that are independent of message semantics, thereby defining the
205   complete set of requirements for message parsers and
206   message-forwarding intermediaries.
207</t>
208
209
210<section title="Requirement Notation" anchor="intro.requirements">
211<t>
212   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
213   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
214   document are to be interpreted as described in <xref target="RFC2119"/>.
215</t>
216<t>
217   Conformance criteria and considerations regarding error handling
218   are defined in <xref target="conformance"/>.
219</t>
220</section>
221
222<section title="Syntax Notation" anchor="notation">
223<iref primary="true" item="Grammar" subitem="ALPHA"/>
224<iref primary="true" item="Grammar" subitem="CR"/>
225<iref primary="true" item="Grammar" subitem="CRLF"/>
226<iref primary="true" item="Grammar" subitem="CTL"/>
227<iref primary="true" item="Grammar" subitem="DIGIT"/>
228<iref primary="true" item="Grammar" subitem="DQUOTE"/>
229<iref primary="true" item="Grammar" subitem="HEXDIG"/>
230<iref primary="true" item="Grammar" subitem="HTAB"/>
231<iref primary="true" item="Grammar" subitem="LF"/>
232<iref primary="true" item="Grammar" subitem="OCTET"/>
233<iref primary="true" item="Grammar" subitem="SP"/>
234<iref primary="true" item="Grammar" subitem="VCHAR"/>
235<t>
236   This specification uses the Augmented Backus-Naur Form (ABNF) notation
237   of <xref target="RFC5234"/> with the list rule extension defined in
238   <xref target="abnf.extension"/>.  <xref target="collected.abnf"/> shows
239   the collected ABNF with the list rule expanded.
240</t>
241<t anchor="core.rules">
242  <x:anchor-alias value="ALPHA"/>
243  <x:anchor-alias value="CTL"/>
244  <x:anchor-alias value="CR"/>
245  <x:anchor-alias value="CRLF"/>
246  <x:anchor-alias value="DIGIT"/>
247  <x:anchor-alias value="DQUOTE"/>
248  <x:anchor-alias value="HEXDIG"/>
249  <x:anchor-alias value="HTAB"/>
250  <x:anchor-alias value="LF"/>
251  <x:anchor-alias value="OCTET"/>
252  <x:anchor-alias value="SP"/>
253  <x:anchor-alias value="VCHAR"/>
254   The following core rules are included by
255   reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
256   ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
257   DIGIT (decimal 0-9), DQUOTE (double quote),
258   HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed),
259   OCTET (any 8-bit sequence of data), SP (space), and
260   VCHAR (any visible <xref target="USASCII"/> character).
261</t>
262<t>
263   As a convention, ABNF rule names prefixed with "obs-" denote
264   "obsolete" grammar rules that appear for historical reasons.
265</t>
266</section>
267</section>
268
269<section title="Architecture" anchor="architecture">
270<t>
271   HTTP was created for the World Wide Web architecture
272   and has evolved over time to support the scalability needs of a worldwide
273   hypertext system. Much of that architecture is reflected in the terminology
274   and syntax productions used to define HTTP.
275</t>
276
277<section title="Client/Server Messaging" anchor="operation">
278<iref primary="true" item="client"/>
279<iref primary="true" item="server"/>
280<iref primary="true" item="connection"/>
281<t>
282   HTTP is a stateless request/response protocol that operates by exchanging
283   <x:dfn>messages</x:dfn> (<xref target="http.message"/>) across a reliable
284   transport or session-layer
285   "<x:dfn>connection</x:dfn>" (<xref target="connection.management"/>).
286   An HTTP "<x:dfn>client</x:dfn>" is a program that establishes a connection
287   to a server for the purpose of sending one or more HTTP requests.
288   An HTTP "<x:dfn>server</x:dfn>" is a program that accepts connections
289   in order to service HTTP requests by sending HTTP responses.
290</t>
291<iref primary="true" item="user agent"/>
292<iref primary="true" item="origin server"/>
293<iref primary="true" item="browser"/>
294<iref primary="true" item="spider"/>
295<iref primary="true" item="sender"/>
296<iref primary="true" item="recipient"/>
297<t>
298   The terms client and server refer only to the roles that
299   these programs perform for a particular connection.  The same program
300   might act as a client on some connections and a server on others.  We use
301   the term "<x:dfn>user agent</x:dfn>" to refer to the program that initiates a request,
302   such as a WWW browser, editor, or spider (web-traversing robot), and
303   the term "<x:dfn>origin server</x:dfn>" to refer to the program that can originate
304   authoritative responses to a request.  For general requirements, we use
305   the term "<x:dfn>sender</x:dfn>" to refer to whichever component sent a given message
306   and the term "<x:dfn>recipient</x:dfn>" to refer to any component that receives the
307   message.
308</t>
309<t>
310   HTTP relies upon the Uniform Resource Identifier (URI)
311   standard <xref target="RFC3986"/> to indicate the target resource
312   (<xref target="target-resource"/>) and relationships between resources.
313   Messages are passed in a format similar to that used by Internet mail
314   <xref target="RFC5322"/> and the Multipurpose Internet Mail Extensions
315   (MIME) <xref target="RFC2045"/> (see &diff-mime; for the differences
316   between HTTP and MIME messages).
317</t>
318<t>
319   Most HTTP communication consists of a retrieval request (GET) for
320   a representation of some resource identified by a URI.  In the
321   simplest case, this might be accomplished via a single bidirectional
322   connection (===) between the user agent (UA) and the origin server (O).
323</t>
324<figure><artwork type="drawing">
325         request   &gt;
326    <x:highlight>UA</x:highlight> ======================================= <x:highlight>O</x:highlight>
327                                &lt;   response
328</artwork></figure>
329<iref primary="true" item="message"/>
330<iref primary="true" item="request"/>
331<iref primary="true" item="response"/>
332<t>
333   A client sends an HTTP request to a server in the form of a <x:dfn>request</x:dfn>
334   message, beginning with a request-line that includes a method, URI, and
335   protocol version (<xref target="request.line"/>),
336   followed by header fields containing
337   request modifiers, client information, and representation metadata
338   (<xref target="header.fields"/>),
339   an empty line to indicate the end of the header section, and finally
340   a message body containing the payload body (if any,
341   <xref target="message.body"/>).
342</t>
343<t>
344   A server responds to a client's request by sending one or more HTTP
345   <x:dfn>response</x:dfn>
346   messages, each beginning with a status line that
347   includes the protocol version, a success or error code, and textual
348   reason phrase (<xref target="status.line"/>),
349   possibly followed by header fields containing server
350   information, resource metadata, and representation metadata
351   (<xref target="header.fields"/>),
352   an empty line to indicate the end of the header section, and finally
353   a message body containing the payload body (if any,
354   <xref target="message.body"/>).
355</t>
356<t>
357   A connection might be used for multiple request/response exchanges,
358   as defined in <xref target="persistent.connections"/>.
359</t>
360<t>
361   The following example illustrates a typical message exchange for a
362   GET request on the URI "http://www.example.com/hello.txt":
363</t>
364<figure><preamble>
365client request:
366</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
367GET /hello.txt HTTP/1.1
368User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
369Host: www.example.com
370Accept-Language: en, mi
371
372</artwork></figure>
373<figure><preamble>
374server response:
375</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
376HTTP/1.1 200 OK
377Date: Mon, 27 Jul 2009 12:28:53 GMT
378Server: Apache
379Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
380ETag: "34aa387-d-1568eb00"
381Accept-Ranges: bytes
382Content-Length: <x:length-of target="exbody"/>
383Vary: Accept-Encoding
384Content-Type: text/plain
385
386<x:span anchor="exbody">Hello World!
387</x:span></artwork></figure>
388</section>
389
390<section title="Implementation Diversity" anchor="implementation-diversity">
391<t>
392   When considering the design of HTTP, it is easy to fall into a trap of
393   thinking that all user agents are general-purpose browsers and all origin
394   servers are large public websites. That is not the case in practice.
395   Common HTTP user agents include household appliances, stereos, scales,
396   firmware update scripts, command-line programs, mobile apps,
397   and communication devices in a multitude of shapes and sizes.  Likewise,
398   common HTTP origin servers include home automation units, configurable
399   networking components, office machines, autonomous robots, news feeds,
400   traffic cameras, ad selectors, and video delivery platforms.
401</t>
402<t>
403   The term "user agent" does not imply that there is a human user directly
404   interacting with the software agent at the time of a request. In many
405   cases, a user agent is installed or configured to run in the background
406   and save its results for later inspection (or save only a subset of those
407   results that might be interesting or erroneous). Spiders, for example, are
408   typically given a start URI and configured to follow certain behavior while
409   crawling the Web as a hypertext graph.
410</t>
411<t>
412   The implementation diversity of HTTP means that we cannot assume the
413   user agent can make interactive suggestions to a user or provide adequate
414   warning for security or privacy options.  In the few cases where this
415   specification requires reporting of errors to the user, it is acceptable
416   for such reporting to only be observable in an error console or log file.
417   Likewise, requirements that an automated action be confirmed by the user
418   before proceeding can be met via advance configuration choices,
419   run-time options, or simply not proceeding with the unsafe action.
420</t>
421</section>
422
423<section title="Intermediaries" anchor="intermediaries">
424<iref primary="true" item="intermediary"/>
425<t>
426   HTTP enables the use of intermediaries to satisfy requests through
427   a chain of connections.  There are three common forms of HTTP
428   <x:dfn>intermediary</x:dfn>: proxy, gateway, and tunnel.  In some cases,
429   a single intermediary might act as an origin server, proxy, gateway,
430   or tunnel, switching behavior based on the nature of each request.
431</t>
432<figure><artwork type="drawing">
433         &gt;             &gt;             &gt;             &gt;
434    <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>
435               &lt;             &lt;             &lt;             &lt;
436</artwork></figure>
437<t>
438   The figure above shows three intermediaries (A, B, and C) between the
439   user agent and origin server. A request or response message that
440   travels the whole chain will pass through four separate connections.
441   Some HTTP communication options
442   might apply only to the connection with the nearest, non-tunnel
443   neighbor, only to the end-points of the chain, or to all connections
444   along the chain. Although the diagram is linear, each participant might
445   be engaged in multiple, simultaneous communications. For example, B
446   might be receiving requests from many clients other than A, and/or
447   forwarding requests to servers other than C, at the same time that it
448   is handling A's request.
449</t>
450<t>
451<iref primary="true" item="upstream"/><iref primary="true" item="downstream"/>
452<iref primary="true" item="inbound"/><iref primary="true" item="outbound"/>
453   We use the terms "<x:dfn>upstream</x:dfn>" and "<x:dfn>downstream</x:dfn>"
454   to describe various requirements in relation to the directional flow of a
455   message: all messages flow from upstream to downstream.
456   Likewise, we use the terms inbound and outbound to refer to
457   directions in relation to the request path:
458   "<x:dfn>inbound</x:dfn>" means toward the origin server and
459   "<x:dfn>outbound</x:dfn>" means toward the user agent.
460</t>
461<t><iref primary="true" item="proxy"/>
462   A "<x:dfn>proxy</x:dfn>" is a message forwarding agent that is selected by the
463   client, usually via local configuration rules, to receive requests
464   for some type(s) of absolute URI and attempt to satisfy those
465   requests via translation through the HTTP interface.  Some translations
466   are minimal, such as for proxy requests for "http" URIs, whereas
467   other requests might require translation to and from entirely different
468   application-level protocols. Proxies are often used to group an
469   organization's HTTP requests through a common intermediary for the
470   sake of security, annotation services, or shared caching.
471</t>
472<t>
473<iref primary="true" item="transforming proxy"/>
474<iref primary="true" item="non-transforming proxy"/>
475   An HTTP-to-HTTP proxy is called a "<x:dfn>transforming proxy</x:dfn>" if it is designed
476   or configured to modify request or response messages in a semantically
477   meaningful way (i.e., modifications, beyond those required by normal
478   HTTP processing, that change the message in a way that would be
479   significant to the original sender or potentially significant to
480   downstream recipients).  For example, a transforming proxy might be
481   acting as a shared annotation server (modifying responses to include
482   references to a local annotation database), a malware filter, a
483   format transcoder, or an intranet-to-Internet privacy filter.  Such
484   transformations are presumed to be desired by the client (or client
485   organization) that selected the proxy and are beyond the scope of
486   this specification.  However, when a proxy is not intended to transform
487   a given message, we use the term "<x:dfn>non-transforming proxy</x:dfn>" to target
488   requirements that preserve HTTP message semantics. See &status-203; and
489   &header-warning; for status and warning codes related to transformations.
490</t>
491<t><iref primary="true" item="gateway"/><iref primary="true" item="reverse proxy"/>
492<iref primary="true" item="accelerator"/>
493   A "<x:dfn>gateway</x:dfn>" (a.k.a., "<x:dfn>reverse proxy</x:dfn>")
494   is a receiving agent that acts
495   as a layer above some other server(s) and translates the received
496   requests to the underlying server's protocol.  Gateways are often
497   used to encapsulate legacy or untrusted information services, to
498   improve server performance through "<x:dfn>accelerator</x:dfn>" caching, and to
499   enable partitioning or load-balancing of HTTP services across
500   multiple machines.
501</t>
502<t>
503   A gateway behaves as an origin server on its outbound connection and
504   as a user agent on its inbound connection.
505   All HTTP requirements applicable to an origin server
506   also apply to the outbound communication of a gateway.
507   A gateway communicates with inbound servers using any protocol that
508   it desires, including private extensions to HTTP that are outside
509   the scope of this specification.  However, an HTTP-to-HTTP gateway
510   that wishes to interoperate with third-party HTTP servers &MUST;
511   conform to HTTP user agent requirements on the gateway's inbound
512   connection and &MUST; implement the <x:ref>Connection</x:ref>
513   (<xref target="header.connection"/>) and <x:ref>Via</x:ref>
514   (<xref target="header.via"/>) header fields for both connections.
515</t>
516<t><iref primary="true" item="tunnel"/>
517   A "<x:dfn>tunnel</x:dfn>" acts as a blind relay between two connections
518   without changing the messages. Once active, a tunnel is not
519   considered a party to the HTTP communication, though the tunnel might
520   have been initiated by an HTTP request. A tunnel ceases to exist when
521   both ends of the relayed connection are closed. Tunnels are used to
522   extend a virtual connection through an intermediary, such as when
523   Transport Layer Security (TLS, <xref target="RFC5246"/>) is used to
524   establish confidential communication through a shared firewall proxy.
525</t>
526<t><iref primary="true" item="interception proxy"/>
527<iref primary="true" item="transparent proxy"/>
528<iref primary="true" item="captive portal"/>
529   The above categories for intermediary only consider those acting as
530   participants in the HTTP communication.  There are also intermediaries
531   that can act on lower layers of the network protocol stack, filtering or
532   redirecting HTTP traffic without the knowledge or permission of message
533   senders. Network intermediaries often introduce security flaws or
534   interoperability problems by violating HTTP semantics.  For example, an
535   "<x:dfn>interception proxy</x:dfn>" <xref target="RFC3040"/> (also commonly
536   known as a "<x:dfn>transparent proxy</x:dfn>" <xref target="RFC1919"/> or
537   "<x:dfn>captive portal</x:dfn>")
538   differs from an HTTP proxy because it is not selected by the client.
539   Instead, an interception proxy filters or redirects outgoing TCP port 80
540   packets (and occasionally other common port traffic).
541   Interception proxies are commonly found on public network access points,
542   as a means of enforcing account subscription prior to allowing use of
543   non-local Internet services, and within corporate firewalls to enforce
544   network usage policies.
545   They are indistinguishable from a man-in-the-middle attack.
546</t>
547<t>
548   HTTP is defined as a stateless protocol, meaning that each request message
549   can be understood in isolation.  Many implementations depend on HTTP's
550   stateless design in order to reuse proxied connections or dynamically
551   load-balance requests across multiple servers.  Hence, servers &MUST-NOT;
552   assume that two requests on the same connection are from the same user
553   agent unless the connection is secured and specific to that agent.
554   Some non-standard HTTP extensions (e.g., <xref target="RFC4559"/>) have
555   been known to violate this requirement, resulting in security and
556   interoperability problems.
557</t>
558</section>
559
560<section title="Caches" anchor="caches">
561<iref primary="true" item="cache"/>
562<t>
563   A "<x:dfn>cache</x:dfn>" is a local store of previous response messages and the
564   subsystem that controls its message storage, retrieval, and deletion.
565   A cache stores cacheable responses in order to reduce the response
566   time and network bandwidth consumption on future, equivalent
567   requests. Any client or server &MAY; employ a cache, though a cache
568   cannot be used by a server while it is acting as a tunnel.
569</t>
570<t>
571   The effect of a cache is that the request/response chain is shortened
572   if one of the participants along the chain has a cached response
573   applicable to that request. The following illustrates the resulting
574   chain if B has a cached copy of an earlier response from O (via C)
575   for a request which has not been cached by UA or A.
576</t>
577<figure><artwork type="drawing">
578            &gt;             &gt;
579       <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>
580                  &lt;             &lt;
581</artwork></figure>
582<t><iref primary="true" item="cacheable"/>
583   A response is "<x:dfn>cacheable</x:dfn>" if a cache is allowed to store a copy of
584   the response message for use in answering subsequent requests.
585   Even when a response is cacheable, there might be additional
586   constraints placed by the client or by the origin server on when
587   that cached response can be used for a particular request. HTTP
588   requirements for cache behavior and cacheable responses are
589   defined in &caching-overview;. 
590</t>
591<t>
592   There are a wide variety of architectures and configurations
593   of caches deployed across the World Wide Web and
594   inside large organizations. These include national hierarchies
595   of proxy caches to save transoceanic bandwidth, collaborative systems that
596   broadcast or multicast cache entries, archives of pre-fetched cache
597   entries for use in off-line or high-latency environments, and so on.
598</t>
599</section>
600
601<section title="Conformance and Error Handling" anchor="conformance">
602<t>
603   This specification targets conformance criteria according to the role of
604   a participant in HTTP communication.  Hence, HTTP requirements are placed
605   on senders, recipients, clients, servers, user agents, intermediaries,
606   origin servers, proxies, gateways, or caches, depending on what behavior
607   is being constrained by the requirement. Additional (social) requirements
608   are placed on implementations, resource owners, and protocol element
609   registrations when they apply beyond the scope of a single communication.
610</t>
611<t>
612   The verb "generate" is used instead of "send" where a requirement
613   differentiates between creating a protocol element and merely forwarding a
614   received element downstream.
615</t>
616<t>
617   An implementation is considered conformant if it complies with all of the
618   requirements associated with the roles it partakes in HTTP. Note that
619   SHOULD-level requirements are relevant here, unless one of the documented
620   exceptions is applicable.
621</t>
622<t>
623   Conformance applies to both the syntax and semantics of HTTP protocol
624   elements. A sender &MUST-NOT; generate protocol elements that convey a
625   meaning that is known by that sender to be false. A sender &MUST-NOT;
626   generate protocol elements that do not match the grammar defined by the
627   ABNF rules for those protocol elements that are applicable to the sender's
628   role. If a received protocol element is processed, the recipient &MUST; be
629   able to parse any value that would match the ABNF rules for that protocol
630   element, excluding only those rules not applicable to the recipient's role.
631</t>
632<t>
633   Unless noted otherwise, a recipient &MAY; attempt to recover a usable
634   protocol element from an invalid construct.  HTTP does not define
635   specific error handling mechanisms except when they have a direct impact
636   on security, since different applications of the protocol require
637   different error handling strategies.  For example, a Web browser might
638   wish to transparently recover from a response where the
639   <x:ref>Location</x:ref> header field doesn't parse according to the ABNF,
640   whereas a systems control client might consider any form of error recovery
641   to be dangerous.
642</t>
643</section>
644
645<section title="Protocol Versioning" anchor="http.version">
646  <x:anchor-alias value="HTTP-version"/>
647  <x:anchor-alias value="HTTP-name"/>
648<t>
649   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate
650   versions of the protocol. This specification defines version "1.1".
651   The protocol version as a whole indicates the sender's conformance
652   with the set of requirements laid out in that version's corresponding
653   specification of HTTP.
654</t>
655<t>
656   The version of an HTTP message is indicated by an HTTP-version field
657   in the first line of the message. HTTP-version is case-sensitive.
658</t>
659<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-version"/><iref primary="true" item="Grammar" subitem="HTTP-name"/>
660  <x:ref>HTTP-version</x:ref>  = <x:ref>HTTP-name</x:ref> "/" <x:ref>DIGIT</x:ref> "." <x:ref>DIGIT</x:ref>
661  <x:ref>HTTP-name</x:ref>     = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
662</artwork></figure>
663<t>
664   The HTTP version number consists of two decimal digits separated by a "."
665   (period or decimal point).  The first digit ("major version") indicates the
666   HTTP messaging syntax, whereas the second digit ("minor version") indicates
667   the highest minor version to which the sender is
668   conformant and able to understand for future communication.  The minor
669   version advertises the sender's communication capabilities even when the
670   sender is only using a backwards-compatible subset of the protocol,
671   thereby letting the recipient know that more advanced features can
672   be used in response (by servers) or in future requests (by clients).
673</t>
674<t>
675   When an HTTP/1.1 message is sent to an HTTP/1.0 recipient
676   <xref target="RFC1945"/> or a recipient whose version is unknown,
677   the HTTP/1.1 message is constructed such that it can be interpreted
678   as a valid HTTP/1.0 message if all of the newer features are ignored.
679   This specification places recipient-version requirements on some
680   new features so that a conformant sender will only use compatible
681   features until it has determined, through configuration or the
682   receipt of a message, that the recipient supports HTTP/1.1.
683</t>
684<t>
685   The interpretation of a header field does not change between minor
686   versions of the same major HTTP version, though the default
687   behavior of a recipient in the absence of such a field can change.
688   Unless specified otherwise, header fields defined in HTTP/1.1 are
689   defined for all versions of HTTP/1.x.  In particular, the <x:ref>Host</x:ref>
690   and <x:ref>Connection</x:ref> header fields ought to be implemented by all
691   HTTP/1.x implementations whether or not they advertise conformance with
692   HTTP/1.1.
693</t>
694<t>
695   New header fields can be defined such that, when they are
696   understood by a recipient, they might override or enhance the
697   interpretation of previously defined header fields.  When an
698   implementation receives an unrecognized header field, the recipient
699   &MUST; ignore that header field for local processing regardless of
700   the message's HTTP version.  An unrecognized header field received
701   by a proxy &MUST; be forwarded downstream unless the header field's
702   field-name is listed in the message's <x:ref>Connection</x:ref> header field
703   (see <xref target="header.connection"/>).
704   These requirements allow HTTP's functionality to be enhanced without
705   requiring prior update of deployed intermediaries.
706</t>
707<t>
708   Intermediaries that process HTTP messages (i.e., all intermediaries
709   other than those acting as tunnels) &MUST; send their own HTTP-version
710   in forwarded messages.  In other words, they &MUST-NOT; blindly
711   forward the first line of an HTTP message without ensuring that the
712   protocol version in that message matches a version to which that
713   intermediary is conformant for both the receiving and
714   sending of messages.  Forwarding an HTTP message without rewriting
715   the HTTP-version might result in communication errors when downstream
716   recipients use the message sender's version to determine what features
717   are safe to use for later communication with that sender.
718</t>
719<t>
720   An HTTP client &SHOULD; send a request version equal to the highest
721   version to which the client is conformant and
722   whose major version is no higher than the highest version supported
723   by the server, if this is known.  An HTTP client &MUST-NOT; send a
724   version to which it is not conformant.
725</t>
726<t>
727   An HTTP client &MAY; send a lower request version if it is known that
728   the server incorrectly implements the HTTP specification, but only
729   after the client has attempted at least one normal request and determined
730   from the response status or header fields (e.g., <x:ref>Server</x:ref>) that
731   the server improperly handles higher request versions.
732</t>
733<t>
734   An HTTP server &SHOULD; send a response version equal to the highest
735   version to which the server is conformant and
736   whose major version is less than or equal to the one received in the
737   request.  An HTTP server &MUST-NOT; send a version to which it is not
738   conformant.  A server &MAY; send a <x:ref>505 (HTTP Version Not
739   Supported)</x:ref> response if it cannot send a response using the
740   major version used in the client's request.
741</t>
742<t>
743   An HTTP server &MAY; send an HTTP/1.0 response to an HTTP/1.0 request
744   if it is known or suspected that the client incorrectly implements the
745   HTTP specification and is incapable of correctly processing later
746   version responses, such as when a client fails to parse the version
747   number correctly or when an intermediary is known to blindly forward
748   the HTTP-version even when it doesn't conform to the given minor
749   version of the protocol. Such protocol downgrades &SHOULD-NOT; be
750   performed unless triggered by specific client attributes, such as when
751   one or more of the request header fields (e.g., <x:ref>User-Agent</x:ref>)
752   uniquely match the values sent by a client known to be in error.
753</t>
754<t>
755   The intention of HTTP's versioning design is that the major number
756   will only be incremented if an incompatible message syntax is
757   introduced, and that the minor number will only be incremented when
758   changes made to the protocol have the effect of adding to the message
759   semantics or implying additional capabilities of the sender.  However,
760   the minor version was not incremented for the changes introduced between
761   <xref target="RFC2068"/> and <xref target="RFC2616"/>, and this revision
762   has specifically avoiding any such changes to the protocol.
763</t>
764</section>
765
766<section title="Uniform Resource Identifiers" anchor="uri">
767<iref primary="true" item="resource"/>
768<t>
769   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used
770   throughout HTTP as the means for identifying resources (&resource;).
771   URI references are used to target requests, indicate redirects, and define
772   relationships.
773</t>
774  <x:anchor-alias value="URI-reference"/>
775  <x:anchor-alias value="absolute-URI"/>
776  <x:anchor-alias value="relative-part"/>
777  <x:anchor-alias value="authority"/>
778  <x:anchor-alias value="path-abempty"/>
779  <x:anchor-alias value="path-absolute"/>
780  <x:anchor-alias value="port"/>
781  <x:anchor-alias value="query"/>
782  <x:anchor-alias value="uri-host"/>
783  <x:anchor-alias value="partial-URI"/>
784<t>
785   This specification adopts the definitions of "URI-reference",
786   "absolute-URI", "relative-part", "port", "host",
787   "path-abempty", "path-absolute", "query", and "authority" from the
788   URI generic syntax.
789   In addition, we define a partial-URI rule for protocol elements
790   that allow a relative URI but not a fragment.
791</t>
792<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="URI-reference"><!--exported production--></iref><iref primary="true" item="Grammar" subitem="absolute-URI"/><iref primary="true" item="Grammar" subitem="authority"/><iref primary="true" item="Grammar" subitem="path-absolute"/><iref primary="true" item="Grammar" subitem="port"/><iref primary="true" item="Grammar" subitem="query"/><iref primary="true" item="Grammar" subitem="uri-host"/><iref primary="true" item="Grammar" subitem="partial-URI"><!--exported production--></iref>
793  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
794  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
795  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
796  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
797  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
798  <x:ref>path-absolute</x:ref> = &lt;path-absolute, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
799  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
800  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
801  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
802 
803  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
804</artwork></figure>
805<t>
806   Each protocol element in HTTP that allows a URI reference will indicate
807   in its ABNF production whether the element allows any form of reference
808   (URI-reference), only a URI in absolute form (absolute-URI), only the
809   path and optional query components, or some combination of the above.
810   Unless otherwise indicated, URI references are parsed
811   relative to the effective request URI
812   (<xref target="effective.request.uri"/>).
813</t>
814
815<section title="http URI scheme" anchor="http.uri">
816  <x:anchor-alias value="http-URI"/>
817  <iref item="http URI scheme" primary="true"/>
818  <iref item="URI scheme" subitem="http" primary="true"/>
819<t>
820   The "http" URI scheme is hereby defined for the purpose of minting
821   identifiers according to their association with the hierarchical
822   namespace governed by a potential HTTP origin server listening for
823   TCP connections on a given port.
824</t>
825<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"><!--terminal production--></iref>
826  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
827</artwork></figure>
828<t>
829   The HTTP origin server is identified by the generic syntax's
830   <x:ref>authority</x:ref> component, which includes a host identifier
831   and optional TCP port (<xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>).
832   The remainder of the URI, consisting of both the hierarchical path
833   component and optional query component, serves as an identifier for
834   a potential resource within that origin server's name space.
835</t>
836<t>
837   If the host identifier is provided as an IP address,
838   then the origin server is any listener on the indicated TCP port at
839   that IP address. If host is a registered name, then that name is
840   considered an indirect identifier and the recipient might use a name
841   resolution service, such as DNS, to find the address of a listener
842   for that host.
843   The host &MUST-NOT; be empty; if an "http" URI is received with an
844   empty host, then it &MUST; be rejected as invalid.
845   If the port subcomponent is empty or not given, then TCP port 80 is
846   assumed (the default reserved port for WWW services).
847</t>
848<t>
849   Regardless of the form of host identifier, access to that host is not
850   implied by the mere presence of its name or address. The host might or might
851   not exist and, even when it does exist, might or might not be running an
852   HTTP server or listening to the indicated port. The "http" URI scheme
853   makes use of the delegated nature of Internet names and addresses to
854   establish a naming authority (whatever entity has the ability to place
855   an HTTP server at that Internet name or address) and allows that
856   authority to determine which names are valid and how they might be used.
857</t>
858<t>
859   When an "http" URI is used within a context that calls for access to the
860   indicated resource, a client &MAY; attempt access by resolving
861   the host to an IP address, establishing a TCP connection to that address
862   on the indicated port, and sending an HTTP request message
863   (<xref target="http.message"/>) containing the URI's identifying data
864   (<xref target="message.routing"/>) to the server.
865   If the server responds to that request with a non-interim HTTP response
866   message, as described in &status-codes;, then that response
867   is considered an authoritative answer to the client's request.
868</t>
869<t>
870   Although HTTP is independent of the transport protocol, the "http"
871   scheme is specific to TCP-based services because the name delegation
872   process depends on TCP for establishing authority.
873   An HTTP service based on some other underlying connection protocol
874   would presumably be identified using a different URI scheme, just as
875   the "https" scheme (below) is used for resources that require an
876   end-to-end secured connection. Other protocols might also be used to
877   provide access to "http" identified resources &mdash; it is only the
878   authoritative interface used for mapping the namespace that is
879   specific to TCP.
880</t>
881<t>
882   The URI generic syntax for authority also includes a deprecated
883   userinfo subcomponent (<xref target="RFC3986" x:fmt="," x:sec="3.2.1"/>)
884   for including user authentication information in the URI.  Some
885   implementations make use of the userinfo component for internal
886   configuration of authentication information, such as within command
887   invocation options, configuration files, or bookmark lists, even
888   though such usage might expose a user identifier or password.
889   Senders &MUST-NOT; include a userinfo subcomponent (and its "@"
890   delimiter) when transmitting an "http" URI in a message.  Recipients
891   of HTTP messages that contain a URI reference &SHOULD; parse for the
892   existence of userinfo and treat its presence as an error, likely
893   indicating that the deprecated subcomponent is being used to obscure
894   the authority for the sake of phishing attacks.
895</t>
896</section>
897
898<section title="https URI scheme" anchor="https.uri">
899   <x:anchor-alias value="https-URI"/>
900   <iref item="https URI scheme"/>
901   <iref item="URI scheme" subitem="https"/>
902<t>
903   The "https" URI scheme is hereby defined for the purpose of minting
904   identifiers according to their association with the hierarchical
905   namespace governed by a potential HTTP origin server listening to a
906   given TCP port for TLS-secured connections <xref target="RFC5246"/>.
907</t>
908<t>
909   All of the requirements listed above for the "http" scheme are also
910   requirements for the "https" scheme, except that a default TCP port
911   of 443 is assumed if the port subcomponent is empty or not given,
912   and the TCP connection &MUST; be secured, end-to-end, through the
913   use of strong encryption prior to sending the first HTTP request.
914</t>
915<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="https-URI"><!--terminal production--></iref>
916  <x:ref>https-URI</x:ref> = "https:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
917</artwork></figure>
918<t>
919   Unlike the "http" scheme, responses to "https" identified requests
920   are never "public" and thus &MUST-NOT; be reused for shared caching.
921   They can, however, be reused in a private cache if the message is
922   cacheable by default in HTTP or specifically indicated as such by
923   the Cache-Control header field (&header-cache-control;).
924</t>
925<t>
926   Resources made available via the "https" scheme have no shared
927   identity with the "http" scheme even if their resource identifiers
928   indicate the same authority (the same host listening to the same
929   TCP port).  They are distinct name spaces and are considered to be
930   distinct origin servers.  However, an extension to HTTP that is
931   defined to apply to entire host domains, such as the Cookie protocol
932   <xref target="RFC6265"/>, can allow information
933   set by one service to impact communication with other services
934   within a matching group of host domains.
935</t>
936<t>
937   The process for authoritative access to an "https" identified
938   resource is defined in <xref target="RFC2818"/>.
939</t>
940</section>
941
942<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
943<t>
944   Since the "http" and "https" schemes conform to the URI generic syntax,
945   such URIs are normalized and compared according to the algorithm defined
946   in <xref target="RFC3986" x:fmt="," x:sec="6"/>, using the defaults
947   described above for each scheme.
948</t>
949<t>
950   If the port is equal to the default port for a scheme, the normal
951   form is to elide the port subcomponent. Likewise, an empty path
952   component is equivalent to an absolute path of "/", so the normal
953   form is to provide a path of "/" instead. The scheme and host
954   are case-insensitive and normally provided in lowercase; all
955   other components are compared in a case-sensitive manner.
956   Characters other than those in the "reserved" set are equivalent
957   to their percent-encoded octets (see <xref target="RFC3986"
958   x:fmt="," x:sec="2.1"/>): the normal form is to not encode them.
959</t>
960<t>
961   For example, the following three URIs are equivalent:
962</t>
963<figure><artwork type="example">
964   http://example.com:80/~smith/home.html
965   http://EXAMPLE.com/%7Esmith/home.html
966   http://EXAMPLE.com:/%7esmith/home.html
967</artwork></figure>
968</section>
969</section>
970</section>
971
972<section title="Message Format" anchor="http.message">
973<x:anchor-alias value="generic-message"/>
974<x:anchor-alias value="message.types"/>
975<x:anchor-alias value="HTTP-message"/>
976<x:anchor-alias value="start-line"/>
977<iref item="header section"/>
978<iref item="headers"/>
979<iref item="header field"/>
980<t>
981   All HTTP/1.1 messages consist of a start-line followed by a sequence of
982   octets in a format similar to the Internet Message Format
983   <xref target="RFC5322"/>: zero or more header fields (collectively
984   referred to as the "headers" or the "header section"), an empty line
985   indicating the end of the header section, and an optional message body.
986</t>
987<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"><!--terminal production--></iref>
988  <x:ref>HTTP-message</x:ref>   = <x:ref>start-line</x:ref>
989                   *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
990                   <x:ref>CRLF</x:ref>
991                   [ <x:ref>message-body</x:ref> ]
992</artwork></figure>
993<t>
994   The normal procedure for parsing an HTTP message is to read the
995   start-line into a structure, read each header field into a hash
996   table by field name until the empty line, and then use the parsed
997   data to determine if a message body is expected.  If a message body
998   has been indicated, then it is read as a stream until an amount
999   of octets equal to the message body length is read or the connection
1000   is closed.
1001</t>
1002<t>
1003   Recipients &MUST; parse an HTTP message as a sequence of octets in an
1004   encoding that is a superset of US-ASCII <xref target="USASCII"/>.
1005   Parsing an HTTP message as a stream of Unicode characters, without regard
1006   for the specific encoding, creates security vulnerabilities due to the
1007   varying ways that string processing libraries handle invalid multibyte
1008   character sequences that contain the octet LF (%x0A).  String-based
1009   parsers can only be safely used within protocol elements after the element
1010   has been extracted from the message, such as within a header field-value
1011   after message parsing has delineated the individual fields.
1012</t>
1013<t>
1014   An HTTP message can be parsed as a stream for incremental processing or
1015   forwarding downstream.  However, recipients cannot rely on incremental
1016   delivery of partial messages, since some implementations will buffer or
1017   delay message forwarding for the sake of network efficiency, security
1018   checks, or payload transformations.
1019</t>
1020
1021<section title="Start Line" anchor="start.line">
1022  <x:anchor-alias value="Start-Line"/>
1023<t>
1024   An HTTP message can either be a request from client to server or a
1025   response from server to client.  Syntactically, the two types of message
1026   differ only in the start-line, which is either a request-line (for requests)
1027   or a status-line (for responses), and in the algorithm for determining
1028   the length of the message body (<xref target="message.body"/>).
1029</t>
1030<t>
1031   In theory, a client could receive requests and a server could receive
1032   responses, distinguishing them by their different start-line formats,
1033   but in practice servers are implemented to only expect a request
1034   (a response is interpreted as an unknown or invalid request method)
1035   and clients are implemented to only expect a response.
1036</t>
1037<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="start-line"/>
1038  <x:ref>start-line</x:ref>     = <x:ref>request-line</x:ref> / <x:ref>status-line</x:ref>
1039</artwork></figure>
1040<t>
1041   A sender &MUST-NOT; send whitespace between the start-line and
1042   the first header field. The presence of such whitespace in a request
1043   might be an attempt to trick a server into ignoring that field or
1044   processing the line after it as a new request, either of which might
1045   result in a security vulnerability if other implementations within
1046   the request chain interpret the same message differently.
1047   Likewise, the presence of such whitespace in a response might be
1048   ignored by some clients or cause others to cease parsing.
1049</t>
1050<t>
1051   A recipient that receives whitespace between the start-line and
1052   the first header field &MUST; either reject the message as invalid or
1053   consume each whitespace-preceded line without further processing of it
1054   (i.e., ignore the entire line, along with any subsequent lines preceded
1055   by whitespace, until a properly formed header field is received or the
1056   header block is terminated).
1057</t>
1058
1059<section title="Request Line" anchor="request.line">
1060  <x:anchor-alias value="Request"/>
1061  <x:anchor-alias value="request-line"/>
1062<t>
1063   A request-line begins with a method token, followed by a single
1064   space (SP), the request-target, another single space (SP), the
1065   protocol version, and ending with CRLF.
1066</t>
1067<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1068  <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>
1069</artwork></figure>
1070<t>
1071   A server &MUST; be able to parse any received message that begins
1072   with a request-line and matches the ABNF rule for HTTP-message.
1073</t>
1074<iref primary="true" item="method"/>
1075<t anchor="method">
1076   The method token indicates the request method to be performed on the
1077   target resource. The request method is case-sensitive.
1078</t>
1079<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1080  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1081</artwork></figure>
1082<t>
1083   The methods defined by this specification can be found in
1084   &methods;, along with information regarding the HTTP method registry
1085   and considerations for defining new methods.
1086</t>
1087<iref item="request-target"/>
1088<t>
1089   The request-target identifies the target resource upon which to apply
1090   the request, as defined in <xref target="request-target"/>.
1091</t>
1092<t>
1093   No whitespace is allowed inside the method, request-target, and
1094   protocol version.  Hence, recipients typically parse the request-line
1095   into its component parts by splitting on whitespace
1096   (see <xref target="message.robustness"/>).
1097</t>
1098<t>
1099   Unfortunately, some user agents fail to properly encode hypertext
1100   references that have embedded whitespace, sending the characters directly
1101   instead of properly encoding or excluding the disallowed characters.
1102   Recipients of an invalid request-line &SHOULD; respond with either a
1103   <x:ref>400 (Bad Request)</x:ref> error or a <x:ref>301 (Moved Permanently)</x:ref>
1104   redirect with the request-target properly encoded.  Recipients &SHOULD-NOT;
1105   attempt to autocorrect and then process the request without a redirect,
1106   since the invalid request-line might be deliberately crafted to bypass
1107   security filters along the request chain.
1108</t>
1109<t>
1110   HTTP does not place a pre-defined limit on the length of a request-line.
1111   A server that receives a method longer than any that it implements
1112   &SHOULD; respond with either a <x:ref>405 (Method Not Allowed)</x:ref>, if it is an origin
1113   server, or a <x:ref>501 (Not Implemented)</x:ref> status code.
1114   A server &MUST; be prepared to receive URIs of unbounded length and
1115   respond with the <x:ref>414 (URI Too Long)</x:ref> status code if the received
1116   request-target would be longer than the server wishes to handle
1117   (see &status-414;).
1118</t>
1119<t>
1120   Various ad-hoc limitations on request-line length are found in practice.
1121   It is &RECOMMENDED; that all HTTP senders and recipients support, at a
1122   minimum, request-line lengths of up to 8000 octets.
1123</t>
1124</section>
1125
1126<section title="Status Line" anchor="status.line">
1127  <x:anchor-alias value="response"/>
1128  <x:anchor-alias value="status-line"/>
1129  <x:anchor-alias value="status-code"/>
1130  <x:anchor-alias value="reason-phrase"/>
1131<t>
1132   The first line of a response message is the status-line, consisting
1133   of the protocol version, a space (SP), the status code, another space,
1134   a possibly-empty textual phrase describing the status code, and
1135   ending with CRLF.
1136</t>
1137<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-line"/>
1138  <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>
1139</artwork></figure>
1140<t>
1141   A client &MUST; be able to parse any received message that begins
1142   with a status-line and matches the ABNF rule for HTTP-message.
1143</t>
1144<t>
1145   The status-code element is a 3-digit integer code describing the
1146   result of the server's attempt to understand and satisfy the client's
1147   corresponding request. The rest of the response message is to be
1148   interpreted in light of the semantics defined for that status code.
1149   See &status-codes; for information about the semantics of status codes,
1150   including the classes of status code (indicated by the first digit),
1151   the status codes defined by this specification, considerations for the
1152   definition of new status codes, and the IANA registry.
1153</t>
1154<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-code"/>
1155  <x:ref>status-code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1156</artwork></figure>
1157<t>  
1158   The reason-phrase element exists for the sole purpose of providing a
1159   textual description associated with the numeric status code, mostly
1160   out of deference to earlier Internet application protocols that were more
1161   frequently used with interactive text clients. A client &SHOULD; ignore
1162   the reason-phrase content.
1163</t>
1164<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
1165  <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> )
1166</artwork></figure>
1167</section>
1168</section>
1169
1170<section title="Header Fields" anchor="header.fields">
1171  <x:anchor-alias value="header-field"/>
1172  <x:anchor-alias value="field-content"/>
1173  <x:anchor-alias value="field-name"/>
1174  <x:anchor-alias value="field-value"/>
1175  <x:anchor-alias value="obs-fold"/>
1176<t>
1177   Each HTTP header field consists of a case-insensitive field name
1178   followed by a colon (":"), optional whitespace, and the field value.
1179</t>
1180<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"/>
1181  <x:ref>header-field</x:ref>   = <x:ref>field-name</x:ref> ":" <x:ref>OWS</x:ref> <x:ref>field-value</x:ref> <x:ref>BWS</x:ref>
1182  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1183  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>obs-fold</x:ref> )
1184  <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> )
1185  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref> ( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1186                 ; obsolete line folding
1187                 ; see <xref target="field.parsing"/>
1188</artwork></figure>
1189<t>
1190   The field-name token labels the corresponding field-value as having the
1191   semantics defined by that header field.  For example, the <x:ref>Date</x:ref>
1192   header field is defined in &header-date; as containing the origination
1193   timestamp for the message in which it appears.
1194</t>
1195<t>
1196   HTTP header fields are fully extensible: there is no limit on the
1197   introduction of new field names, each presumably defining new semantics,
1198   or on the number of header fields used in a given message.  Existing
1199   fields are defined in each part of this specification and in many other
1200   specifications outside the standards process.
1201   New header fields can be introduced without changing the protocol version
1202   if their defined semantics allow them to be safely ignored by recipients
1203   that do not recognize them.
1204</t>
1205<t>
1206   New HTTP header fields &SHOULD; be registered with IANA in the
1207   Message Header Field Registry, as described in &iana-header-registry;.
1208   Unrecognized header fields &MUST; be forwarded by a proxy unless the
1209   field-name is listed in the <x:ref>Connection</x:ref> header field
1210   (<xref target="header.connection"/>) or the proxy is specifically
1211   configured to block or otherwise transform such fields.
1212   Unrecognized header fields &SHOULD; be ignored by other recipients.
1213</t>
1214<t>
1215   The order in which header fields with differing field names are
1216   received is not significant. However, it is "good practice" to send
1217   header fields that contain control data first, such as <x:ref>Host</x:ref>
1218   on requests and <x:ref>Date</x:ref> on responses, so that implementations
1219   can decide when not to handle a message as early as possible.  A server
1220   &MUST; wait until the entire header section is received before interpreting
1221   a request message, since later header fields might include conditionals,
1222   authentication credentials, or deliberately misleading duplicate
1223   header fields that would impact request processing.
1224</t>
1225<t>
1226   Multiple header fields with the same field name &MUST-NOT; be
1227   sent in a message unless the entire field value for that
1228   header field is defined as a comma-separated list [i.e., #(values)].
1229   Multiple header fields with the same field name can be combined into
1230   one "field-name: field-value" pair, without changing the semantics of the
1231   message, by appending each subsequent field value to the combined
1232   field value in order, separated by a comma. The order in which
1233   header fields with the same field name are received is therefore
1234   significant to the interpretation of the combined field value;
1235   a proxy &MUST-NOT; change the order of these field values when
1236   forwarding a message.
1237</t>
1238<x:note>
1239  <t>
1240   &Note; The "Set-Cookie" header field as implemented in
1241   practice can occur multiple times, but does not use the list syntax, and
1242   thus cannot be combined into a single line (<xref target="RFC6265"/>). (See Appendix A.2.3 of <xref target="Kri2001"/>
1243   for details.)
1244</t>
1245</x:note>
1246
1247<section title="Whitespace" anchor="whitespace">
1248<t anchor="rule.LWS">
1249   This specification uses three rules to denote the use of linear
1250   whitespace: OWS (optional whitespace), RWS (required whitespace), and
1251   BWS ("bad" whitespace).
1252</t>
1253<t anchor="rule.OWS">
1254   The OWS rule is used where zero or more linear whitespace octets might
1255   appear. OWS &SHOULD; either not be produced or be produced as a single
1256   SP. Multiple OWS octets that occur within field-content &SHOULD; either
1257   be replaced with a single SP or transformed to all SP octets (each
1258   octet other than SP replaced with SP) before interpreting the field value
1259   or forwarding the message downstream.
1260</t>
1261<t anchor="rule.RWS">
1262   RWS is used when at least one linear whitespace octet is required to
1263   separate field tokens. RWS &SHOULD; be produced as a single SP.
1264   Multiple RWS octets that occur within field-content &SHOULD; either
1265   be replaced with a single SP or transformed to all SP octets before
1266   interpreting the field value or forwarding the message downstream.
1267</t>
1268<t anchor="rule.BWS">
1269   BWS is used where the grammar allows optional whitespace, for historical
1270   reasons, but senders &SHOULD-NOT; produce it in messages;
1271   recipients &MUST; accept such bad optional whitespace and remove it before
1272   interpreting the field value or forwarding the message downstream.
1273</t>
1274<t anchor="rule.whitespace">
1275  <x:anchor-alias value="BWS"/>
1276  <x:anchor-alias value="OWS"/>
1277  <x:anchor-alias value="RWS"/>
1278</t>
1279<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"/>
1280  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1281                 ; optional whitespace
1282  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1283                 ; required whitespace
1284  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1285                 ; "bad" whitespace
1286</artwork></figure>
1287</section>
1288
1289<section title="Field Parsing" anchor="field.parsing">
1290<t>
1291   No whitespace is allowed between the header field-name and colon.
1292   In the past, differences in the handling of such whitespace have led to
1293   security vulnerabilities in request routing and response handling.
1294   Any received request message that contains whitespace between a header
1295   field-name and colon &MUST; be rejected with a response code of 400
1296   (Bad Request).  A proxy &MUST; remove any such whitespace from a response
1297   message before forwarding the message downstream.
1298</t>
1299<t>
1300   A field value is preceded by optional whitespace (OWS); a single SP is
1301   preferred. The field value does not include any leading or trailing white
1302   space: OWS occurring before the first non-whitespace octet of the
1303   field value or after the last non-whitespace octet of the field value
1304   is ignored and &SHOULD; be removed before further processing (as this does
1305   not change the meaning of the header field).
1306</t>
1307<t>
1308   Historically, HTTP header field values could be extended over multiple
1309   lines by preceding each extra line with at least one space or horizontal
1310   tab (obs-fold). This specification deprecates such line
1311   folding except within the message/http media type
1312   (<xref target="internet.media.type.message.http"/>).
1313   HTTP senders &MUST-NOT; produce messages that include line folding
1314   (i.e., that contain any field-value that matches the obs-fold rule) unless
1315   the message is intended for packaging within the message/http media type.
1316   HTTP recipients &SHOULD; accept line folding and replace any embedded
1317   obs-fold whitespace with either a single SP or a matching number of SP
1318   octets (to avoid buffer copying) prior to interpreting the field value or
1319   forwarding the message downstream.
1320</t>
1321<t>
1322   Historically, HTTP has allowed field content with text in the ISO-8859-1
1323   <xref target="ISO-8859-1"/> charset, supporting other charsets only
1324   through use of <xref target="RFC2047"/> encoding.
1325   In practice, most HTTP header field values use only a subset of the
1326   US-ASCII charset <xref target="USASCII"/>. Newly defined
1327   header fields &SHOULD; limit their field values to US-ASCII octets.
1328   Recipients &SHOULD; treat other octets in field content (obs-text) as
1329   opaque data.
1330</t>
1331</section>
1332
1333<section title="Field Length" anchor="field.length">
1334<t>
1335   HTTP does not place a pre-defined limit on the length of header fields,
1336   either in isolation or as a set. A server &MUST; be prepared to receive
1337   request header fields of unbounded length and respond with a <x:ref>4xx
1338   (Client Error)</x:ref> status code if the received header field(s) would be
1339   longer than the server wishes to handle.
1340</t>
1341<t>
1342   A client that receives response header fields that are longer than it wishes
1343   to handle can only treat it as a server error.
1344</t>
1345<t>
1346   Various ad-hoc limitations on header field length are found in practice. It
1347   is &RECOMMENDED; that all HTTP senders and recipients support messages whose
1348   combined header fields have 4000 or more octets.
1349</t>
1350</section>
1351
1352<section title="Field value components" anchor="field.components">
1353<t anchor="rule.token.separators">
1354  <x:anchor-alias value="tchar"/>
1355  <x:anchor-alias value="token"/>
1356  <x:anchor-alias value="special"/>
1357  <x:anchor-alias value="word"/>
1358   Many HTTP header field values consist of words (token or quoted-string)
1359   separated by whitespace or special characters. These special characters
1360   &MUST; be in a quoted string to be used within a parameter value (as defined
1361   in <xref target="transfer.codings"/>).
1362</t>
1363<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>
1364  <x:ref>word</x:ref>           = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1365
1366  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1367<!--
1368  IMPORTANT: when editing "tchar" make sure that "special" is updated accordingly!!!
1369 -->
1370  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1371                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1372                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1373                 ; any <x:ref>VCHAR</x:ref>, except <x:ref>special</x:ref>
1374
1375  <x:ref>special</x:ref>        = "(" / ")" / "&lt;" / ">" / "@" / ","
1376                 / ";" / ":" / "\" / DQUOTE / "/" / "["
1377                 / "]" / "?" / "=" / "{" / "}"
1378</artwork></figure>
1379<t anchor="rule.quoted-string">
1380  <x:anchor-alias value="quoted-string"/>
1381  <x:anchor-alias value="qdtext"/>
1382  <x:anchor-alias value="obs-text"/>
1383   A string of text is parsed as a single word if it is quoted using
1384   double-quote marks.
1385</t>
1386<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"/>
1387  <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>
1388  <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>
1389  <x:ref>obs-text</x:ref>       = %x80-FF
1390</artwork></figure>
1391<t anchor="rule.quoted-pair">
1392  <x:anchor-alias value="quoted-pair"/>
1393   The backslash octet ("\") can be used as a single-octet
1394   quoting mechanism within quoted-string constructs:
1395</t>
1396<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1397  <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> )
1398</artwork></figure>
1399<t>
1400   Recipients that process the value of the quoted-string &MUST; handle a
1401   quoted-pair as if it were replaced by the octet following the backslash.
1402</t>
1403<t>
1404   Senders &SHOULD-NOT; escape octets in quoted-strings that do not require
1405   escaping (i.e., other than DQUOTE and the backslash octet).
1406</t>
1407<t anchor="rule.comment">
1408  <x:anchor-alias value="comment"/>
1409  <x:anchor-alias value="ctext"/>
1410   Comments can be included in some HTTP header fields by surrounding
1411   the comment text with parentheses. Comments are only allowed in
1412   fields containing "comment" as part of their field value definition.
1413</t>
1414<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1415  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-cpair</x:ref> / <x:ref>comment</x:ref> ) ")"
1416  <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>
1417</artwork></figure>
1418<t anchor="rule.quoted-cpair">
1419  <x:anchor-alias value="quoted-cpair"/>
1420   The backslash octet ("\") can be used as a single-octet
1421   quoting mechanism within comment constructs:
1422</t>
1423<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-cpair"/>
1424  <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> )
1425</artwork></figure>
1426<t>
1427   Senders &SHOULD-NOT; escape octets in comments that do not require escaping
1428   (i.e., other than the backslash octet "\" and the parentheses "(" and ")").
1429</t>
1430</section>
1431
1432</section>
1433
1434<section title="Message Body" anchor="message.body">
1435  <x:anchor-alias value="message-body"/>
1436<t>
1437   The message body (if any) of an HTTP message is used to carry the
1438   payload body of that request or response.  The message body is
1439   identical to the payload body unless a transfer coding has been
1440   applied, as described in <xref target="header.transfer-encoding"/>.
1441</t>
1442<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1443  <x:ref>message-body</x:ref> = *OCTET
1444</artwork></figure>
1445<t>
1446   The rules for when a message body is allowed in a message differ for
1447   requests and responses.
1448</t>
1449<t>
1450   The presence of a message body in a request is signaled by a
1451   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1452   field. Request message framing is independent of method semantics,
1453   even if the method does not define any use for a message body.
1454</t>
1455<t>
1456   The presence of a message body in a response depends on both
1457   the request method to which it is responding and the response
1458   status code (<xref target="status.line"/>).
1459   Responses to the HEAD request method never include a message body
1460   because the associated response header fields (e.g.,
1461   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1462   if present, indicate only what their values would have been if the request
1463   method had been GET (&HEAD;).
1464   <x:ref>2xx (Successful)</x:ref> responses to CONNECT switch to tunnel
1465   mode instead of having a message body (&CONNECT;).
1466   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1467   <x:ref>304 (Not Modified)</x:ref> responses &MUST-NOT; include a message body.
1468   All other responses do include a message body, although the body
1469   &MAY; be of zero length.
1470</t>
1471
1472<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1473  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1474  <x:anchor-alias value="Transfer-Encoding"/>
1475<t>
1476   When one or more transfer codings are applied to a payload body in order
1477   to form the message body, a Transfer-Encoding header field &MUST; be sent
1478   in the message and &MUST; contain the list of corresponding
1479   transfer-coding names in the same order that they were applied.
1480   Transfer codings are defined in <xref target="transfer.codings"/>.
1481</t>
1482<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1483  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1484</artwork></figure>
1485<t>
1486   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1487   MIME, which was designed to enable safe transport of binary data over a
1488   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1489   However, safe transport has a different focus for an 8bit-clean transfer
1490   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1491   accurately delimit a dynamically generated payload and to distinguish
1492   payload encodings that are only applied for transport efficiency or
1493   security from those that are characteristics of the target resource.
1494</t>
1495<t>
1496   The "chunked" transfer-coding (<xref target="chunked.encoding"/>)
1497   &MUST; be implemented by all HTTP/1.1 recipients because it plays a
1498   crucial role in delimiting messages when the payload body size is not
1499   known in advance.
1500   When the "chunked" transfer-coding is used, it &MUST; be the last
1501   transfer-coding applied to form the message body and &MUST-NOT;
1502   be applied more than once in a message body.
1503   If any transfer-coding is applied to a request payload body,
1504   the final transfer-coding applied &MUST; be "chunked".
1505   If any transfer-coding is applied to a response payload body, then either
1506   the final transfer-coding applied &MUST; be "chunked" or
1507   the message &MUST; be terminated by closing the connection.
1508</t>
1509<figure><preamble>
1510   For example,
1511</preamble><artwork type="example">
1512  Transfer-Encoding: gzip, chunked
1513</artwork><postamble>
1514   indicates that the payload body has been compressed using the gzip
1515   coding and then chunked using the chunked coding while forming the
1516   message body.
1517</postamble></figure>
1518<t>
1519   If more than one Transfer-Encoding header field is present in a message,
1520   the multiple field-values &MUST; be combined into one field-value,
1521   according to the algorithm defined in <xref target="header.fields"/>,
1522   before determining the message body length.
1523</t>
1524<t>
1525   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1526   Transfer-Encoding is a property of the message, not of the payload, and thus
1527   &MAY; be added or removed by any implementation along the request/response
1528   chain. Additional information about the encoding parameters &MAY; be
1529   provided by other header fields not defined by this specification.
1530</t>
1531<t>
1532   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1533   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1534   neither of which includes a message body,
1535   to indicate that the origin server would have applied a transfer coding
1536   to the message body if the request had been an unconditional GET.
1537   This indication is not required, however, because any recipient on
1538   the response chain (including the origin server) can remove transfer
1539   codings when they are not needed.
1540</t>
1541<t>
1542   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1543   implementations advertising only HTTP/1.0 support will not understand
1544   how to process a transfer-encoded payload.
1545   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1546   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1547   might be in the form of specific user configuration or by remembering the
1548   version of a prior received response.
1549   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1550   the corresponding request indicates HTTP/1.1 (or later).
1551</t>
1552<t>
1553   A server that receives a request message with a transfer-coding it does
1554   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref> and then
1555   close the connection.
1556</t>
1557</section>
1558
1559<section title="Content-Length" anchor="header.content-length">
1560  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1561  <x:anchor-alias value="Content-Length"/>
1562<t>
1563   When a message is allowed to contain a message body, does not have a
1564   <x:ref>Transfer-Encoding</x:ref> header field, and has a payload body
1565   length that is known to the sender before the message header section has
1566   been sent, the sender &SHOULD; send a Content-Length header field to
1567   indicate the length of the payload body as a decimal number of octets.
1568</t>
1569<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1570  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1571</artwork></figure>
1572<t>
1573   An example is
1574</t>
1575<figure><artwork type="example">
1576  Content-Length: 3495
1577</artwork></figure>
1578<t>
1579   A sender &MUST-NOT; send a Content-Length header field in any message that
1580   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1581</t>
1582<t>
1583   A server &MAY; send a Content-Length header field in a response to a HEAD
1584   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1585   response unless its field-value equals the decimal number of octets that
1586   would have been sent in the payload body of a response if the same
1587   request had used the GET method.
1588</t>
1589<t>
1590   A server &MAY; send a Content-Length header field in a
1591   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1592   (&status-304;); a server &MUST-NOT; send Content-Length in such a
1593   response unless its field-value equals the decimal number of octets that
1594   would have been sent in the payload body of a <x:ref>200 (OK)</x:ref>
1595   response to the same request.
1596</t>
1597<t>
1598   A server &MUST-NOT; send a Content-Length header field in any response
1599   with a status code of
1600   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1601   A server &SHOULD-NOT; send a Content-Length header field in any
1602   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1603</t>
1604<t>
1605   Any Content-Length field value greater than or equal to zero is valid.
1606   Since there is no predefined limit to the length of an HTTP payload,
1607   recipients &SHOULD; anticipate potentially large decimal numerals and
1608   prevent parsing errors due to integer conversion overflows
1609   (<xref target="attack.protocol.element.size.overflows"/>).
1610</t>
1611<t>
1612   If a message is received that has multiple Content-Length header fields
1613   with field-values consisting of the same decimal value, or a single
1614   Content-Length header field with a field value containing a list of
1615   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1616   duplicate Content-Length header fields have been generated or combined by an
1617   upstream message processor, then the recipient &MUST; either reject the
1618   message as invalid or replace the duplicated field-values with a single
1619   valid Content-Length field containing that decimal value prior to
1620   determining the message body length.
1621</t>
1622<x:note>
1623  <t>
1624   &Note; HTTP's use of Content-Length for message framing differs
1625   significantly from the same field's use in MIME, where it is an optional
1626   field used only within the "message/external-body" media-type.
1627  </t>
1628</x:note>
1629</section>
1630
1631<section title="Message Body Length" anchor="message.body.length">
1632<t>
1633   The length of a message body is determined by one of the following
1634   (in order of precedence):
1635</t>
1636<t>
1637  <list style="numbers">
1638    <x:lt><t>
1639     Any response to a HEAD request and any response with a
1640     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1641     <x:ref>304 (Not Modified)</x:ref> status code is always
1642     terminated by the first empty line after the header fields, regardless of
1643     the header fields present in the message, and thus cannot contain a
1644     message body.
1645    </t></x:lt>
1646    <x:lt><t>
1647     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1648     connection will become a tunnel immediately after the empty line that
1649     concludes the header fields.  A client &MUST; ignore any
1650     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1651     fields received in such a message.
1652    </t></x:lt>
1653    <x:lt><t>
1654     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1655     and the "chunked" transfer-coding (<xref target="chunked.encoding"/>)
1656     is the final encoding, the message body length is determined by reading
1657     and decoding the chunked data until the transfer-coding indicates the
1658     data is complete.
1659    </t>
1660    <t>
1661     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1662     response and the "chunked" transfer-coding is not the final encoding, the
1663     message body length is determined by reading the connection until it is
1664     closed by the server.
1665     If a Transfer-Encoding header field is present in a request and the
1666     "chunked" transfer-coding is not the final encoding, the message body
1667     length cannot be determined reliably; the server &MUST; respond with
1668     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1669    </t>
1670    <t>
1671     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1672     and a <x:ref>Content-Length</x:ref> header field, the
1673     Transfer-Encoding overrides the Content-Length.
1674     Such a message might indicate an attempt to perform request or response
1675     smuggling (bypass of security-related checks on message routing or content)
1676     and thus ought to be handled as an error.  The provided Content-Length &MUST;
1677     be removed, prior to forwarding the message downstream, or replaced with
1678     the real message body length after the transfer-coding is decoded.
1679    </t></x:lt>
1680    <x:lt><t>
1681     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1682     either multiple <x:ref>Content-Length</x:ref> header fields having
1683     differing field-values or a single Content-Length header field having an
1684     invalid value, then the message framing is invalid and &MUST; be treated
1685     as an error to prevent request or response smuggling.
1686     If this is a request message, the server &MUST; respond with
1687     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1688     If this is a response message received by a proxy, the proxy
1689     &MUST; discard the received response, send a <x:ref>502 (Bad Gateway)</x:ref>
1690     status code as its downstream response, and then close the connection.
1691     If this is a response message received by a user agent, it &MUST; be
1692     treated as an error by discarding the message and closing the connection.
1693    </t></x:lt>
1694    <x:lt><t>
1695     If a valid <x:ref>Content-Length</x:ref> header field is present without
1696     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1697     message body length in octets.  If the actual number of octets sent in
1698     the message is less than the indicated Content-Length, the recipient
1699     &MUST; consider the message to be incomplete and treat the connection
1700     as no longer usable.
1701     If the actual number of octets sent in the message is more than the indicated
1702     Content-Length, the recipient &MUST; only process the message body up to the
1703     field value's number of octets; the remainder of the message &MUST; either
1704     be discarded or treated as the next message in a pipeline.  For the sake of
1705     robustness, a user agent &MAY; attempt to detect and correct such an error
1706     in message framing if it is parsing the response to the last request on
1707     a connection and the connection has been closed by the server.
1708    </t></x:lt>
1709    <x:lt><t>
1710     If this is a request message and none of the above are true, then the
1711     message body length is zero (no message body is present).
1712    </t></x:lt>
1713    <x:lt><t>
1714     Otherwise, this is a response message without a declared message body
1715     length, so the message body length is determined by the number of octets
1716     received prior to the server closing the connection.
1717    </t></x:lt>
1718  </list>
1719</t>
1720<t>
1721   Since there is no way to distinguish a successfully completed,
1722   close-delimited message from a partially-received message interrupted
1723   by network failure, a server &SHOULD; use encoding or
1724   length-delimited messages whenever possible.  The close-delimiting
1725   feature exists primarily for backwards compatibility with HTTP/1.0.
1726</t>
1727<t>
1728   A server &MAY; reject a request that contains a message body but
1729   not a <x:ref>Content-Length</x:ref> by responding with
1730   <x:ref>411 (Length Required)</x:ref>.
1731</t>
1732<t>
1733   Unless a transfer-coding other than "chunked" has been applied,
1734   a client that sends a request containing a message body &SHOULD;
1735   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1736   length is known in advance, rather than the "chunked" encoding, since some
1737   existing services respond to "chunked" with a <x:ref>411 (Length Required)</x:ref>
1738   status code even though they understand the chunked encoding.  This
1739   is typically because such services are implemented via a gateway that
1740   requires a content-length in advance of being called and the server
1741   is unable or unwilling to buffer the entire request before processing.
1742</t>
1743<t>
1744   A client that sends a request containing a message body &MUST; include a
1745   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1746   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1747   the form of specific user configuration or by remembering the version of a
1748   prior received response.
1749</t>
1750</section>
1751</section>
1752
1753<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1754<t>
1755   Request messages that are prematurely terminated, possibly due to a
1756   canceled connection or a server-imposed time-out exception, &MUST;
1757   result in closure of the connection; sending an error response
1758   prior to closing the connection is &OPTIONAL;.
1759</t>
1760<t>
1761   Response messages that are prematurely terminated, usually by closure
1762   of the connection prior to receiving the expected number of octets or by
1763   failure to decode a transfer-encoded message body, &MUST; be recorded
1764   as incomplete.  A response that terminates in the middle of the header
1765   block (before the empty line is received) cannot be assumed to convey the
1766   full semantics of the response and &MUST; be treated as an error.
1767</t>
1768<t>
1769   A message body that uses the chunked transfer encoding is
1770   incomplete if the zero-sized chunk that terminates the encoding has not
1771   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1772   incomplete if the size of the message body received (in octets) is less than
1773   the value given by Content-Length.  A response that has neither chunked
1774   transfer encoding nor Content-Length is terminated by closure of the
1775   connection, and thus is considered complete regardless of the number of
1776   message body octets received, provided that the header block was received
1777   intact.
1778</t>
1779<t>
1780   A user agent &MUST-NOT; render an incomplete response message body as if
1781   it were complete (i.e., some indication needs to be given to the user that an
1782   error occurred).  Cache requirements for incomplete responses are defined
1783   in &cache-incomplete;.
1784</t>
1785<t>
1786   A server &MUST; read the entire request message body or close
1787   the connection after sending its response, since otherwise the
1788   remaining data on a persistent connection would be misinterpreted
1789   as the next request.  Likewise,
1790   a client &MUST; read the entire response message body if it intends
1791   to reuse the same connection for a subsequent request.  Pipelining
1792   multiple requests on a connection is described in <xref target="pipelining"/>.
1793</t>
1794</section>
1795
1796<section title="Message Parsing Robustness" anchor="message.robustness">
1797<t>
1798   Older HTTP/1.0 client implementations might send an extra CRLF
1799   after a POST request as a lame workaround for some early server
1800   applications that failed to read message body content that was
1801   not terminated by a line-ending. An HTTP/1.1 client &MUST-NOT;
1802   preface or follow a request with an extra CRLF.  If terminating
1803   the request message body with a line-ending is desired, then the
1804   client &MUST; include the terminating CRLF octets as part of the
1805   message body length.
1806</t>
1807<t>
1808   In the interest of robustness, servers &SHOULD; ignore at least one
1809   empty line received where a request-line is expected. In other words, if
1810   the server is reading the protocol stream at the beginning of a
1811   message and receives a CRLF first, it &SHOULD; ignore the CRLF.
1812</t>
1813<t>
1814   Although the line terminator for the start-line and header
1815   fields is the sequence CRLF, recipients &MAY; recognize a
1816   single LF as a line terminator and ignore any preceding CR.
1817</t>
1818<t>
1819   Although the request-line and status-line grammar rules require that each
1820   of the component elements be separated by a single SP octet, recipients
1821   &MAY; instead parse on whitespace-delimited word boundaries and, aside
1822   from the CRLF terminator, treat any form of whitespace as the SP separator
1823   while ignoring preceding or trailing whitespace;
1824   such whitespace includes one or more of the following octets:
1825   SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
1826</t>
1827<t>
1828   When a server listening only for HTTP request messages, or processing
1829   what appears from the start-line to be an HTTP request message,
1830   receives a sequence of octets that does not match the HTTP-message
1831   grammar aside from the robustness exceptions listed above, the
1832   server &SHOULD; respond with a <x:ref>400 (Bad Request)</x:ref> response. 
1833</t>
1834</section>
1835</section>
1836
1837<section title="Transfer Codings" anchor="transfer.codings">
1838  <x:anchor-alias value="transfer-coding"/>
1839  <x:anchor-alias value="transfer-extension"/>
1840<t>
1841   Transfer-coding values are used to indicate an encoding
1842   transformation that has been, can be, or might need to be applied to a
1843   payload body in order to ensure "safe transport" through the network.
1844   This differs from a content coding in that the transfer-coding is a
1845   property of the message rather than a property of the representation
1846   that is being transferred.
1847</t>
1848<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1849  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1850                     / "compress" ; <xref target="compress.coding"/>
1851                     / "deflate" ; <xref target="deflate.coding"/>
1852                     / "gzip" ; <xref target="gzip.coding"/>
1853                     / <x:ref>transfer-extension</x:ref>
1854  <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> )
1855</artwork></figure>
1856<t anchor="rule.parameter">
1857  <x:anchor-alias value="attribute"/>
1858  <x:anchor-alias value="transfer-parameter"/>
1859  <x:anchor-alias value="value"/>
1860   Parameters are in the form of attribute/value pairs.
1861</t>
1862<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"/>
1863  <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>
1864  <x:ref>attribute</x:ref>          = <x:ref>token</x:ref>
1865  <x:ref>value</x:ref>              = <x:ref>word</x:ref>
1866</artwork></figure>
1867<t>
1868   All transfer-coding values are case-insensitive and &SHOULD; be registered
1869   within the HTTP Transfer Coding registry, as defined in
1870   <xref target="transfer.coding.registry"/>.
1871   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1872   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1873   header fields.
1874</t>
1875
1876<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1877  <iref item="chunked (Coding Format)"/>
1878  <x:anchor-alias value="chunk"/>
1879  <x:anchor-alias value="chunked-body"/>
1880  <x:anchor-alias value="chunk-data"/>
1881  <x:anchor-alias value="chunk-ext"/>
1882  <x:anchor-alias value="chunk-ext-name"/>
1883  <x:anchor-alias value="chunk-ext-val"/>
1884  <x:anchor-alias value="chunk-size"/>
1885  <x:anchor-alias value="last-chunk"/>
1886  <x:anchor-alias value="trailer-part"/>
1887  <x:anchor-alias value="quoted-str-nf"/>
1888  <x:anchor-alias value="qdtext-nf"/>
1889<t>
1890   The chunked encoding modifies the body of a message in order to
1891   transfer it as a series of chunks, each with its own size indicator,
1892   followed by an &OPTIONAL; trailer containing header fields. This
1893   allows dynamically produced content to be transferred along with the
1894   information necessary for the recipient to verify that it has
1895   received the full message.
1896</t>
1897<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"/>
1898  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1899                   <x:ref>last-chunk</x:ref>
1900                   <x:ref>trailer-part</x:ref>
1901                   <x:ref>CRLF</x:ref>
1902 
1903  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1904                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1905  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1906  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1907 
1908  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
1909  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
1910  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-str-nf</x:ref>
1911  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1912  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1913 
1914  <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>
1915                 ; like <x:ref>quoted-string</x:ref>, but disallowing line folding
1916  <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>
1917</artwork></figure>
1918<t>
1919   Chunk extensions within the chucked encoding are deprecated.
1920   Senders &SHOULD-NOT; send chunk-ext.
1921   Definition of new chunk extensions is discouraged.
1922</t>
1923<t>
1924   The chunk-size field is a string of hex digits indicating the size of
1925   the chunk-data in octets. The chunked encoding is ended by any chunk whose size is
1926   zero, followed by the trailer, which is terminated by an empty line.
1927</t>
1928
1929<section title="Trailer" anchor="header.trailer">
1930  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
1931  <x:anchor-alias value="Trailer"/>
1932<t>
1933   A trailer allows the sender to include additional fields at the end of a
1934   chunked message in order to supply metadata that might be dynamically
1935   generated while the message body is sent, such as a message integrity
1936   check, digital signature, or post-processing status.
1937   The trailer &MUST-NOT; contain fields that need to be known before a
1938   recipient processes the body, such as <x:ref>Transfer-Encoding</x:ref>,
1939   <x:ref>Content-Length</x:ref>, and <x:ref>Trailer</x:ref>.
1940</t>
1941<t>
1942   When a message includes a message body encoded with the chunked
1943   transfer-coding and the sender desires to send metadata in the form of
1944   trailer fields at the end of the message, the sender &SHOULD; send a
1945   <x:ref>Trailer</x:ref> header field before the message body to indicate
1946   which fields will be present in the trailers. This allows the recipient
1947   to prepare for receipt of that metadata before it starts processing the body,
1948   which is useful if the message is being streamed and the recipient wishes
1949   to confirm an integrity check on the fly.
1950</t>
1951<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
1952  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
1953</artwork></figure>
1954<t>
1955   If no <x:ref>Trailer</x:ref> header field is present, the sender of a
1956   chunked message body &SHOULD; send an empty trailer.
1957</t>
1958<t>
1959   A server &MUST; send an empty trailer with the chunked transfer-coding
1960   unless at least one of the following is true:
1961  <list style="numbers">
1962    <t>the request included a <x:ref>TE</x:ref> header field that indicates
1963    "trailers" is acceptable in the transfer-coding of the response, as
1964    described in <xref target="header.te"/>; or,</t>
1965     
1966    <t>the trailer fields consist entirely of optional metadata and the
1967    recipient could use the message (in a manner acceptable to the server where
1968    the field originated) without receiving that metadata. In other words,
1969    the server that generated the header field is willing to accept the
1970    possibility that the trailer fields might be silently discarded along
1971    the path to the client.</t>
1972  </list>
1973</t>
1974<t>
1975   The above requirement prevents the need for an infinite buffer when a
1976   message is being received by an HTTP/1.1 (or later) proxy and forwarded to
1977   an HTTP/1.0 recipient.
1978</t>
1979</section>
1980
1981<section title="Decoding chunked" anchor="decoding.chunked">
1982<t>
1983   A process for decoding the "chunked" transfer-coding
1984   can be represented in pseudo-code as:
1985</t>
1986<figure><artwork type="code">
1987  length := 0
1988  read chunk-size, chunk-ext (if any) and CRLF
1989  while (chunk-size &gt; 0) {
1990     read chunk-data and CRLF
1991     append chunk-data to decoded-body
1992     length := length + chunk-size
1993     read chunk-size and CRLF
1994  }
1995  read header-field
1996  while (header-field not empty) {
1997     append header-field to existing header fields
1998     read header-field
1999  }
2000  Content-Length := length
2001  Remove "chunked" from Transfer-Encoding
2002  Remove Trailer from existing header fields
2003</artwork></figure>
2004<t>
2005   All recipients &MUST; be able to receive and decode the
2006   "chunked" transfer-coding and &MUST; ignore chunk-ext extensions
2007   they do not understand.
2008</t>
2009</section>
2010</section>
2011
2012<section title="Compression Codings" anchor="compression.codings">
2013<t>
2014   The codings defined below can be used to compress the payload of a
2015   message.
2016</t>
2017
2018<section title="Compress Coding" anchor="compress.coding">
2019<iref item="compress (Coding Format)"/>
2020<t>
2021   The "compress" format is produced by the common UNIX file compression
2022   program "compress". This format is an adaptive Lempel-Ziv-Welch
2023   coding (LZW). Recipients &SHOULD; consider "x-compress" to be
2024   equivalent to "compress".
2025</t>
2026</section>
2027
2028<section title="Deflate Coding" anchor="deflate.coding">
2029<iref item="deflate (Coding Format)"/>
2030<t>
2031   The "deflate" format is defined as the "deflate" compression mechanism
2032   (described in <xref target="RFC1951"/>) used inside the "zlib"
2033   data format (<xref target="RFC1950"/>).
2034</t>
2035<x:note>
2036  <t>
2037    &Note; Some incorrect implementations send the "deflate"
2038    compressed data without the zlib wrapper.
2039   </t>
2040</x:note>
2041</section>
2042
2043<section title="Gzip Coding" anchor="gzip.coding">
2044<iref item="gzip (Coding Format)"/>
2045<t>
2046   The "gzip" format is produced by the file compression program
2047   "gzip" (GNU zip), as described in <xref target="RFC1952"/>. This format is a
2048   Lempel-Ziv coding (LZ77) with a 32 bit CRC.
2049   Recipients &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2050</t>
2051</section>
2052
2053</section>
2054
2055<section title="TE" anchor="header.te">
2056  <iref primary="true" item="TE header field" x:for-anchor=""/>
2057  <x:anchor-alias value="TE"/>
2058  <x:anchor-alias value="t-codings"/>
2059  <x:anchor-alias value="t-ranking"/>
2060  <x:anchor-alias value="rank"/>
2061<t>
2062   The "TE" header field in a request indicates what transfer-codings,
2063   besides "chunked", the client is willing to accept in response, and
2064   whether or not the client is willing to accept trailer fields in a
2065   chunked transfer-coding.
2066</t>
2067<t>
2068   The TE field-value consists of a comma-separated list of transfer-coding
2069   names, each allowing for optional parameters (as described in
2070   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2071   Clients &MUST-NOT; send the chunked transfer-coding name in TE;
2072   chunked is always acceptable for HTTP/1.1 recipients.
2073</t>
2074<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"/>
2075  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2076  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2077  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2078  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2079             / ( "1" [ "." 0*3("0") ] )
2080</artwork></figure>
2081<t>
2082   Three examples of TE use are below.
2083</t>
2084<figure><artwork type="example">
2085  TE: deflate
2086  TE:
2087  TE: trailers, deflate;q=0.5
2088</artwork></figure>
2089<t>
2090   The presence of the keyword "trailers" indicates that the client is
2091   willing to accept trailer fields in a chunked transfer-coding,
2092   as defined in <xref target="chunked.encoding"/>, on behalf of itself and
2093   any downstream clients. For chained requests, this implies that either:
2094   (a) all downstream clients are willing to accept trailer fields in the
2095   forwarded response; or,
2096   (b) the client will attempt to buffer the response on behalf of downstream
2097   recipients.
2098   Note that HTTP/1.1 does not define any means to limit the size of a
2099   chunked response such that a client can be assured of buffering the
2100   entire response.
2101</t>
2102<t>
2103   When multiple transfer-codings are acceptable, the client &MAY; rank the
2104   codings by preference using a case-insensitive "q" parameter (similar to
2105   the qvalues used in content negotiation fields, &qvalue;). The rank value
2106   is a real number in the range 0 through 1, where 0.001 is the least
2107   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2108</t>
2109<t>
2110   If the TE field-value is empty or if no TE field is present, the only
2111   acceptable transfer-coding is "chunked". A message with no transfer-coding
2112   is always acceptable.
2113</t>
2114<t>
2115   Since the TE header field only applies to the immediate connection,
2116   a sender of TE &MUST; also send a "TE" connection option within the
2117   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2118   in order to prevent the TE field from being forwarded by intermediaries
2119   that do not support its semantics.
2120</t>
2121</section>
2122</section>
2123
2124<section title="Message Routing" anchor="message.routing">
2125<t>
2126   HTTP request message routing is determined by each client based on the
2127   target resource, the client's proxy configuration, and
2128   establishment or reuse of an inbound connection.  The corresponding
2129   response routing follows the same connection chain back to the client.
2130</t>
2131
2132<section title="Identifying a Target Resource" anchor="target-resource">
2133  <iref primary="true" item="target resource"/>
2134  <iref primary="true" item="target URI"/>
2135  <x:anchor-alias value="target resource"/>
2136  <x:anchor-alias value="target URI"/>
2137<t>
2138   HTTP is used in a wide variety of applications, ranging from
2139   general-purpose computers to home appliances.  In some cases,
2140   communication options are hard-coded in a client's configuration.
2141   However, most HTTP clients rely on the same resource identification
2142   mechanism and configuration techniques as general-purpose Web browsers.
2143</t>
2144<t>
2145   HTTP communication is initiated by a user agent for some purpose.
2146   The purpose is a combination of request semantics, which are defined in
2147   <xref target="Part2"/>, and a target resource upon which to apply those
2148   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2149   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2150   would resolve to its absolute form in order to obtain the
2151   "<x:dfn>target URI</x:dfn>".  The target URI
2152   excludes the reference's fragment identifier component, if any,
2153   since fragment identifiers are reserved for client-side processing
2154   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2155</t>
2156</section>
2157
2158<section title="Connecting Inbound" anchor="connecting.inbound">
2159<t>
2160   Once the target URI is determined, a client needs to decide whether
2161   a network request is necessary to accomplish the desired semantics and,
2162   if so, where that request is to be directed.
2163</t>
2164<t>
2165   If the client has a response cache and the request semantics can be
2166   satisfied by a cache (<xref target="Part6"/>), then the request is
2167   usually directed to the cache first.
2168</t>
2169<t>
2170   If the request is not satisfied by a cache, then a typical client will
2171   check its configuration to determine whether a proxy is to be used to
2172   satisfy the request.  Proxy configuration is implementation-dependent,
2173   but is often based on URI prefix matching, selective authority matching,
2174   or both, and the proxy itself is usually identified by an "http" or
2175   "https" URI.  If a proxy is applicable, the client connects inbound by
2176   establishing (or reusing) a connection to that proxy.
2177</t>
2178<t>
2179   If no proxy is applicable, a typical client will invoke a handler routine,
2180   usually specific to the target URI's scheme, to connect directly
2181   to an authority for the target resource.  How that is accomplished is
2182   dependent on the target URI scheme and defined by its associated
2183   specification, similar to how this specification defines origin server
2184   access for resolution of the "http" (<xref target="http.uri"/>) and
2185   "https" (<xref target="https.uri"/>) schemes.
2186</t>
2187<t>
2188   HTTP requirements regarding connection management are defined in
2189   <xref target="connection.management"/>.
2190</t>
2191</section>
2192
2193<section title="Request Target" anchor="request-target">
2194<t>
2195   Once an inbound connection is obtained,
2196   the client sends an HTTP request message (<xref target="http.message"/>)
2197   with a request-target derived from the target URI.
2198   There are four distinct formats for the request-target, depending on both
2199   the method being requested and whether the request is to a proxy.
2200</t>   
2201<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"/>
2202  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2203                 / <x:ref>absolute-form</x:ref>
2204                 / <x:ref>authority-form</x:ref>
2205                 / <x:ref>asterisk-form</x:ref>
2206
2207  <x:ref>origin-form</x:ref>    = <x:ref>path-absolute</x:ref> [ "?" <x:ref>query</x:ref> ]
2208  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2209  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2210  <x:ref>asterisk-form</x:ref>  = "*"
2211</artwork></figure>
2212<t anchor="origin-form"><iref item="origin-form (of request-target)"/>
2213   The most common form of request-target is the origin-form.
2214   When making a request directly to an origin server, other than a CONNECT
2215   or server-wide OPTIONS request (as detailed below),
2216   a client &MUST; send only the absolute path and query components of
2217   the target URI as the request-target.
2218   If the target URI's path component is empty, then the client &MUST; send
2219   "/" as the path within the origin-form of request-target.
2220   A <x:ref>Host</x:ref> header field is also sent, as defined in
2221   <xref target="header.host"/>, containing the target URI's
2222   authority component (excluding any userinfo).
2223</t>
2224<t>
2225   For example, a client wishing to retrieve a representation of the resource
2226   identified as
2227</t>
2228<figure><artwork x:indent-with="  " type="example">
2229http://www.example.org/where?q=now
2230</artwork></figure>
2231<t>
2232   directly from the origin server would open (or reuse) a TCP connection
2233   to port 80 of the host "www.example.org" and send the lines:
2234</t>
2235<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2236GET /where?q=now HTTP/1.1
2237Host: www.example.org
2238</artwork></figure>
2239<t>
2240   followed by the remainder of the request message.
2241</t>
2242<t anchor="absolute-form"><iref item="absolute-form (of request-target)"/>
2243   When making a request to a proxy, other than a CONNECT or server-wide
2244   OPTIONS request (as detailed below), a client &MUST; send the target URI
2245   in absolute-form as the request-target.
2246   The proxy is requested to either service that request from a valid cache,
2247   if possible, or make the same request on the client's behalf to either
2248   the next inbound proxy server or directly to the origin server indicated
2249   by the request-target.  Requirements on such "forwarding" of messages are
2250   defined in <xref target="message.forwarding"/>.
2251</t>
2252<t>
2253   An example absolute-form of request-line would be:
2254</t>
2255<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2256GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
2257</artwork></figure>
2258<t>
2259   To allow for transition to the absolute-form for all requests in some
2260   future version of HTTP, HTTP/1.1 servers &MUST; accept the absolute-form
2261   in requests, even though HTTP/1.1 clients will only send them in requests
2262   to proxies.
2263</t>
2264<t anchor="authority-form"><iref item="authority-form (of request-target)"/>
2265   The authority-form of request-target is only used for CONNECT requests
2266   (&CONNECT;).  When making a CONNECT request to establish a tunnel through
2267   one or more proxies, a client &MUST; send only the target URI's
2268   authority component (excluding any userinfo) as the request-target.
2269   For example,
2270</t>
2271<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2272CONNECT www.example.com:80 HTTP/1.1
2273</artwork></figure>
2274<t anchor="asterisk-form"><iref item="asterisk-form (of request-target)"/>
2275   The asterisk-form of request-target is only used for a server-wide
2276   OPTIONS request (&OPTIONS;).  When a client wishes to request OPTIONS
2277   for the server as a whole, as opposed to a specific named resource of
2278   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2279   For example,
2280</t>
2281<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2282OPTIONS * HTTP/1.1
2283</artwork></figure>
2284<t>
2285   If a proxy receives an OPTIONS request with an absolute-form of
2286   request-target in which the URI has an empty path and no query component,
2287   then the last proxy on the request chain &MUST; send a request-target
2288   of "*" when it forwards the request to the indicated origin server.
2289</t>
2290<figure><preamble>  
2291   For example, the request
2292</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2293OPTIONS http://www.example.org:8001 HTTP/1.1
2294</artwork></figure>
2295<figure><preamble>  
2296  would be forwarded by the final proxy as
2297</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2298OPTIONS * HTTP/1.1
2299Host: www.example.org:8001
2300</artwork>
2301<postamble>
2302   after connecting to port 8001 of host "www.example.org".
2303</postamble>
2304</figure>
2305</section>
2306
2307<section title="Host" anchor="header.host">
2308  <iref primary="true" item="Host header field" x:for-anchor=""/>
2309  <x:anchor-alias value="Host"/>
2310<t>
2311   The "Host" header field in a request provides the host and port
2312   information from the target URI, enabling the origin
2313   server to distinguish among resources while servicing requests
2314   for multiple host names on a single IP address.  Since the Host
2315   field-value is critical information for handling a request, it
2316   &SHOULD; be sent as the first header field following the request-line.
2317</t>
2318<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2319  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2320</artwork></figure>
2321<t>
2322   A client &MUST; send a Host header field in all HTTP/1.1 request
2323   messages.  If the target URI includes an authority component, then
2324   the Host field-value &MUST; be identical to that authority component
2325   after excluding any userinfo (<xref target="http.uri"/>).
2326   If the authority component is missing or undefined for the target URI,
2327   then the Host header field &MUST; be sent with an empty field-value.
2328</t>
2329<t>
2330   For example, a GET request to the origin server for
2331   &lt;http://www.example.org/pub/WWW/&gt; would begin with:
2332</t>
2333<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2334GET /pub/WWW/ HTTP/1.1
2335Host: www.example.org
2336</artwork></figure>
2337<t>
2338   The Host header field &MUST; be sent in an HTTP/1.1 request even
2339   if the request-target is in the absolute-form, since this
2340   allows the Host information to be forwarded through ancient HTTP/1.0
2341   proxies that might not have implemented Host.
2342</t>
2343<t>
2344   When a proxy receives a request with an absolute-form of
2345   request-target, the proxy &MUST; ignore the received
2346   Host header field (if any) and instead replace it with the host
2347   information of the request-target.  If the proxy forwards the request,
2348   it &MUST; generate a new Host field-value based on the received
2349   request-target rather than forward the received Host field-value.
2350</t>
2351<t>
2352   Since the Host header field acts as an application-level routing
2353   mechanism, it is a frequent target for malware seeking to poison
2354   a shared cache or redirect a request to an unintended server.
2355   An interception proxy is particularly vulnerable if it relies on
2356   the Host field-value for redirecting requests to internal
2357   servers, or for use as a cache key in a shared cache, without
2358   first verifying that the intercepted connection is targeting a
2359   valid IP address for that host.
2360</t>
2361<t>
2362   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2363   to any HTTP/1.1 request message that lacks a Host header field and
2364   to any request message that contains more than one Host header field
2365   or a Host header field with an invalid field-value.
2366</t>
2367</section>
2368
2369<section title="Effective Request URI" anchor="effective.request.uri">
2370  <iref primary="true" item="effective request URI"/>
2371<t>
2372   A server that receives an HTTP request message &MUST; reconstruct
2373   the user agent's original target URI, based on the pieces of information
2374   learned from the request-target, <x:ref>Host</x:ref> header field, and
2375   connection context, in order to identify the intended target resource and
2376   properly service the request. The URI derived from this reconstruction
2377   process is referred to as the "<x:dfn>effective request URI</x:dfn>".
2378</t>
2379<t>
2380   For a user agent, the effective request URI is the target URI.
2381</t>
2382<t>
2383   If the request-target is in absolute-form, then the effective request URI
2384   is the same as the request-target.  Otherwise, the effective request URI
2385   is constructed as follows.
2386</t>
2387<t>
2388   If the request is received over a TLS-secured TCP connection,
2389   then the effective request URI's scheme is "https"; otherwise, the
2390   scheme is "http".
2391</t>
2392<t>
2393   If the request-target is in authority-form, then the effective
2394   request URI's authority component is the same as the request-target.
2395   Otherwise, if a <x:ref>Host</x:ref> header field is supplied with a
2396   non-empty field-value, then the authority component is the same as the
2397   Host field-value. Otherwise, the authority component is the concatenation of
2398   the default host name configured for the server, a colon (":"), and the
2399   connection's incoming TCP port number in decimal form.
2400</t>
2401<t>
2402   If the request-target is in authority-form or asterisk-form, then the
2403   effective request URI's combined path and query component is empty.
2404   Otherwise, the combined path and query component is the same as the
2405   request-target.
2406</t>
2407<t>
2408   The components of the effective request URI, once determined as above,
2409   can be combined into absolute-URI form by concatenating the scheme,
2410   "://", authority, and combined path and query component.
2411</t>
2412<figure>
2413<preamble>
2414   Example 1: the following message received over an insecure TCP connection
2415</preamble>
2416<artwork type="example" x:indent-with="  ">
2417GET /pub/WWW/TheProject.html HTTP/1.1
2418Host: www.example.org:8080
2419</artwork>
2420</figure>
2421<figure>
2422<preamble>
2423  has an effective request URI of
2424</preamble>
2425<artwork type="example" x:indent-with="  ">
2426http://www.example.org:8080/pub/WWW/TheProject.html
2427</artwork>
2428</figure>
2429<figure>
2430<preamble>
2431   Example 2: the following message received over a TLS-secured TCP connection
2432</preamble>
2433<artwork type="example" x:indent-with="  ">
2434OPTIONS * HTTP/1.1
2435Host: www.example.org
2436</artwork>
2437</figure>
2438<figure>
2439<preamble>
2440  has an effective request URI of
2441</preamble>
2442<artwork type="example" x:indent-with="  ">
2443https://www.example.org
2444</artwork>
2445</figure>
2446<t>
2447   An origin server that does not allow resources to differ by requested
2448   host &MAY; ignore the <x:ref>Host</x:ref> field-value and instead replace it
2449   with a configured server name when constructing the effective request URI.
2450</t>
2451<t>
2452   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2453   field &MAY; attempt to use heuristics (e.g., examination of the URI path for
2454   something unique to a particular host) in order to guess the
2455   effective request URI's authority component.
2456</t>
2457</section>
2458
2459<section title="Message Forwarding" anchor="message.forwarding">
2460<t>
2461   As described in <xref target="intermediaries"/>, intermediaries can serve
2462   a variety of roles in the processing of HTTP requests and responses.
2463   Some intermediaries are used to improve performance or availability.
2464   Others are used for access control or to filter content.
2465   Since an HTTP stream has characteristics similar to a pipe-and-filter
2466   architecture, there are no inherent limits to the extent an intermediary
2467   can enhance (or interfere) with either direction of the stream.
2468</t>
2469<t>
2470   Intermediaries that forward a message &MUST; implement the
2471   <x:ref>Connection</x:ref> header field, as specified in
2472   <xref target="header.connection"/>, to exclude fields that are only
2473   intended for the incoming connection.
2474</t>
2475<t>
2476   In order to avoid request loops, a proxy that forwards requests to other
2477   proxies &MUST; be able to recognize and exclude all of its own server
2478   names, including any aliases, local variations, or literal IP addresses.
2479</t>
2480</section>
2481
2482<section title="Via" anchor="header.via">
2483  <iref primary="true" item="Via header field" x:for-anchor=""/>
2484  <x:anchor-alias value="pseudonym"/>
2485  <x:anchor-alias value="received-by"/>
2486  <x:anchor-alias value="received-protocol"/>
2487  <x:anchor-alias value="Via"/>
2488<t>
2489   The "Via" header field &MUST; be sent by a proxy or gateway in forwarded
2490   messages to indicate the intermediate protocols and recipients between the
2491   user agent and the server on requests, and between the origin server and
2492   the client on responses. It is analogous to the "Received" field
2493   used by email systems (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2494   Via is used in HTTP for tracking message forwards,
2495   avoiding request loops, and identifying the protocol capabilities of
2496   all senders along the request/response chain.
2497</t>
2498<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"/>
2499  <x:ref>Via</x:ref>               = 1#( <x:ref>received-protocol</x:ref> <x:ref>RWS</x:ref> <x:ref>received-by</x:ref>
2500                          [ <x:ref>RWS</x:ref> <x:ref>comment</x:ref> ] )
2501  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2502                      ; see <xref target="header.upgrade"/>
2503  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2504  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2505</artwork></figure>
2506<t>
2507   The received-protocol indicates the protocol version of the message
2508   received by the server or client along each segment of the
2509   request/response chain. The received-protocol version is appended to
2510   the Via field value when the message is forwarded so that information
2511   about the protocol capabilities of upstream applications remains
2512   visible to all recipients.
2513</t>
2514<t>
2515   The protocol-name is excluded if and only if it would be "HTTP". The
2516   received-by field is normally the host and optional port number of a
2517   recipient server or client that subsequently forwarded the message.
2518   However, if the real host is considered to be sensitive information,
2519   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2520   be assumed to be the default port of the received-protocol.
2521</t>
2522<t>
2523   Multiple Via field values represent each proxy or gateway that has
2524   forwarded the message. Each recipient &MUST; append its information
2525   such that the end result is ordered according to the sequence of
2526   forwarding applications.
2527</t>
2528<t>
2529   Comments &MAY; be used in the Via header field to identify the software
2530   of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2531   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2532   are optional and &MAY; be removed by any recipient prior to forwarding the
2533   message.
2534</t>
2535<t>
2536   For example, a request message could be sent from an HTTP/1.0 user
2537   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2538   forward the request to a public proxy at p.example.net, which completes
2539   the request by forwarding it to the origin server at www.example.com.
2540   The request received by www.example.com would then have the following
2541   Via header field:
2542</t>
2543<figure><artwork type="example">
2544  Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
2545</artwork></figure>
2546<t>
2547   A proxy or gateway used as a portal through a network firewall
2548   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2549   region unless it is explicitly enabled to do so. If not enabled, the
2550   received-by host of any host behind the firewall &SHOULD; be replaced
2551   by an appropriate pseudonym for that host.
2552</t>
2553<t>
2554   A proxy or gateway &MAY; combine an ordered subsequence of Via header
2555   field entries into a single such entry if the entries have identical
2556   received-protocol values. For example,
2557</t>
2558<figure><artwork type="example">
2559  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2560</artwork></figure>
2561<t>
2562  could be collapsed to
2563</t>
2564<figure><artwork type="example">
2565  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2566</artwork></figure>
2567<t>
2568   Senders &SHOULD-NOT; combine multiple entries unless they are all
2569   under the same organizational control and the hosts have already been
2570   replaced by pseudonyms. Senders &MUST-NOT; combine entries which
2571   have different received-protocol values.
2572</t>
2573</section>
2574
2575<section title="Message Transforming" anchor="message.transforming">
2576<t>
2577   If a proxy receives a request-target with a host name that is not a
2578   fully qualified domain name, it &MAY; add its own domain to the host name
2579   it received when forwarding the request.  A proxy &MUST-NOT; change the
2580   host name if it is a fully qualified domain name.
2581</t>
2582<t>
2583   A non-transforming proxy &MUST-NOT; modify the "path-absolute" and "query"
2584   parts of the received request-target when forwarding it to the next inbound
2585   server, except as noted above to replace an empty path with "/" or "*".
2586</t>
2587<t>
2588   A non-transforming proxy &MUST; preserve the message payload (&payload;),
2589   though it &MAY; change the message body through application or removal
2590   of a transfer-coding (<xref target="transfer.codings"/>).
2591</t>
2592<t>
2593   A non-transforming proxy &SHOULD-NOT; modify header fields that provide
2594   information about the end points of the communication chain, the resource
2595   state, or the selected representation.
2596</t>
2597<t>
2598   A non-transforming proxy &MUST-NOT; modify any of the following fields in a
2599   request or response, and it &MUST-NOT; add any of these fields if not
2600   already present:
2601  <list style="symbols">
2602    <t><x:ref>Allow</x:ref> (&header-allow;)</t>
2603    <t><x:ref>Content-Location</x:ref> (&header-content-location;)</t>
2604    <t>Content-MD5 (<xref target="RFC2616" x:fmt="of" x:sec="14.15"/>)</t>
2605    <t><x:ref>ETag</x:ref> (&header-etag;)</t>
2606    <t><x:ref>Last-Modified</x:ref> (&header-last-modified;)</t>
2607    <t><x:ref>Server</x:ref> (&header-server;)</t>
2608  </list>
2609</t>
2610<t>
2611   A non-transforming proxy &MUST-NOT; modify an <x:ref>Expires</x:ref>
2612   header field (&header-expires;) if already present in a response, but
2613   it &MAY; add an <x:ref>Expires</x:ref> header field with a field-value
2614   identical to that of the <x:ref>Date</x:ref> header field.
2615</t>
2616<t>
2617   A proxy &MUST-NOT; modify or add any of the following fields in a
2618   message that contains the no-transform cache-control directive:
2619  <list style="symbols">
2620    <t><x:ref>Content-Encoding</x:ref> (&header-content-encoding;)</t>
2621    <t><x:ref>Content-Range</x:ref> (&header-content-range;)</t>
2622    <t><x:ref>Content-Type</x:ref> (&header-content-type;)</t>
2623  </list>
2624</t>
2625<t>
2626   A transforming proxy &MAY; modify or add these fields to a message
2627   that does not include no-transform, but if it does so, it &MUST; add a
2628   Warning 214 (Transformation applied) if one does not already appear
2629   in the message (see &header-warning;).
2630</t>
2631<x:note>
2632  <t>
2633    <x:h>Warning:</x:h> Unnecessary modification of header fields might
2634    cause authentication failures if stronger authentication
2635    mechanisms are introduced in later versions of HTTP. Such
2636    authentication mechanisms &MAY; rely on the values of header fields
2637    not listed here.
2638  </t>
2639</x:note>
2640</section>
2641
2642<section title="Associating a Response to a Request" anchor="associating.response.to.request">
2643<t>
2644   HTTP does not include a request identifier for associating a given
2645   request message with its corresponding one or more response messages.
2646   Hence, it relies on the order of response arrival to correspond exactly
2647   to the order in which requests are made on the same connection.
2648   More than one response message per request only occurs when one or more
2649   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a final response
2650   to the same request.
2651</t>
2652<t>
2653   A client that uses persistent connections and sends more than one request
2654   per connection &MUST; maintain a list of outstanding requests in the
2655   order sent on that connection and &MUST; associate each received response
2656   message to the highest ordered request that has not yet received a final
2657   (non-<x:ref>1xx</x:ref>) response.
2658</t>
2659</section>
2660</section>
2661
2662<section title="Connection Management" anchor="connection.management">
2663<t>
2664   HTTP messaging is independent of the underlying transport or
2665   session-layer connection protocol(s).  HTTP only presumes a reliable
2666   transport with in-order delivery of requests and the corresponding
2667   in-order delivery of responses.  The mapping of HTTP request and
2668   response structures onto the data units of an underlying transport
2669   protocol is outside the scope of this specification.
2670</t>
2671<t>
2672   As described in <xref target="connecting.inbound"/>, the specific
2673   connection protocols to be used for an HTTP interaction are determined by
2674   client configuration and the <x:ref>target URI</x:ref>.
2675   For example, the "http" URI scheme
2676   (<xref target="http.uri"/>) indicates a default connection of TCP
2677   over IP, with a default TCP port of 80, but the client might be
2678   configured to use a proxy via some other connection, port, or protocol.
2679</t>
2680<t>
2681   HTTP implementations are expected to engage in connection management,
2682   which includes maintaining the state of current connections,
2683   establishing a new connection or reusing an existing connection,
2684   processing messages received on a connection, detecting connection
2685   failures, and closing each connection.
2686   Most clients maintain multiple connections in parallel, including
2687   more than one connection per server endpoint.
2688   Most servers are designed to maintain thousands of concurrent connections,
2689   while controlling request queues to enable fair use and detect
2690   denial of service attacks.
2691</t>
2692
2693<section title="Connection" anchor="header.connection">
2694  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2695  <iref primary="true" item="close" x:for-anchor=""/>
2696  <x:anchor-alias value="Connection"/>
2697  <x:anchor-alias value="connection-option"/>
2698  <x:anchor-alias value="close"/>
2699<t>
2700   The "Connection" header field allows the sender to indicate desired
2701   control options for the current connection.  In order to avoid confusing
2702   downstream recipients, a proxy or gateway &MUST; remove or replace any
2703   received connection options before forwarding the message.
2704</t>
2705<t>
2706   When a header field is used to supply control information for or about
2707   the current connection, the sender &SHOULD; list the corresponding
2708   field-name within the "Connection" header field.
2709   A proxy or gateway &MUST; parse a received Connection
2710   header field before a message is forwarded and, for each
2711   connection-option in this field, remove any header field(s) from
2712   the message with the same name as the connection-option, and then
2713   remove the Connection header field itself (or replace it with the
2714   intermediary's own connection options for the forwarded message).
2715</t>
2716<t>
2717   Hence, the Connection header field provides a declarative way of
2718   distinguishing header fields that are only intended for the
2719   immediate recipient ("hop-by-hop") from those fields that are
2720   intended for all recipients on the chain ("end-to-end"), enabling the
2721   message to be self-descriptive and allowing future connection-specific
2722   extensions to be deployed without fear that they will be blindly
2723   forwarded by older intermediaries.
2724</t>
2725<t>
2726   The Connection header field's value has the following grammar:
2727</t>
2728<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2729  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2730  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2731</artwork></figure>
2732<t>
2733   Connection options are case-insensitive.
2734</t>
2735<t>
2736   A sender &MUST-NOT; include field-names in the Connection header
2737   field-value for fields that are defined as expressing constraints
2738   for all recipients in the request or response chain, such as the
2739   Cache-Control header field (&header-cache-control;).
2740</t>
2741<t>
2742   The connection options do not have to correspond to a header field
2743   present in the message, since a connection-specific header field
2744   might not be needed if there are no parameters associated with that
2745   connection option.  Recipients that trigger certain connection
2746   behavior based on the presence of connection options &MUST; do so
2747   based on the presence of the connection-option rather than only the
2748   presence of the optional header field.  In other words, if the
2749   connection option is received as a header field but not indicated
2750   within the Connection field-value, then the recipient &MUST; ignore
2751   the connection-specific header field because it has likely been
2752   forwarded by an intermediary that is only partially conformant.
2753</t>
2754<t>
2755   When defining new connection options, specifications ought to
2756   carefully consider existing deployed header fields and ensure
2757   that the new connection option does not share the same name as
2758   an unrelated header field that might already be deployed.
2759   Defining a new connection option essentially reserves that potential
2760   field-name for carrying additional information related to the
2761   connection option, since it would be unwise for senders to use
2762   that field-name for anything else.
2763</t>
2764<t>
2765   The "<x:dfn>close</x:dfn>" connection option is defined for a
2766   sender to signal that this connection will be closed after completion of
2767   the response. For example,
2768</t>
2769<figure><artwork type="example">
2770  Connection: close
2771</artwork></figure>
2772<t>
2773   in either the request or the response header fields indicates that
2774   the connection &SHOULD; be closed after the current request/response
2775   is complete (<xref target="persistent.tear-down"/>).
2776</t>
2777<t>
2778   A client that does not support persistent connections &MUST;
2779   send the "close" connection option in every request message.
2780</t>
2781<t>
2782   A server that does not support persistent connections &MUST;
2783   send the "close" connection option in every response message that
2784   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2785</t>
2786</section>
2787
2788<section title="Persistent Connections" anchor="persistent.connections">
2789  <x:anchor-alias value="persistent connections"/>
2790<t>
2791   HTTP was originally designed to use a separate connection for each
2792   request/response pair. As the Web evolved and embedded requests became
2793   common for inline images, the connection establishment overhead was
2794   a significant drain on performance and a concern for Internet congestion.
2795   Message framing (via <x:ref>Content-Length</x:ref>) and optional
2796   long-lived connections (via Keep-Alive) were added to HTTP/1.0 in order
2797   to improve performance for some requests. However, these extensions were
2798   insufficient for dynamically generated responses and difficult to use
2799   with intermediaries.
2800</t>
2801<t>
2802   HTTP/1.1 defaults to the use of "<x:ref>persistent connections</x:ref>",
2803   which allow multiple requests and responses to be carried over a single
2804   connection. The "<x:ref>close</x:ref>" connection-option is used to
2805   signal that a connection will close after the current request/response.
2806   Persistent connections have a number of advantages:
2807  <list style="symbols">
2808      <t>
2809        By opening and closing fewer connections, CPU time is saved
2810        in routers and hosts (clients, servers, proxies, gateways,
2811        tunnels, or caches), and memory used for protocol control
2812        blocks can be saved in hosts.
2813      </t>
2814      <t>
2815        Most requests and responses can be pipelined on a connection.
2816        Pipelining allows a client to make multiple requests without
2817        waiting for each response, allowing a single connection to
2818        be used much more efficiently and with less overall latency.
2819      </t>
2820      <t>
2821        For TCP connections, network congestion is reduced by eliminating the
2822        packets associated with the three way handshake and graceful close
2823        procedures, and by allowing sufficient time to determine the
2824        congestion state of the network.
2825      </t>
2826      <t>
2827        Latency on subsequent requests is reduced since there is no time
2828        spent in the connection opening handshake.
2829      </t>
2830      <t>
2831        HTTP can evolve more gracefully, since most errors can be reported
2832        without the penalty of closing the connection. Clients using
2833        future versions of HTTP might optimistically try a new feature,
2834        but if communicating with an older server, retry with old
2835        semantics after an error is reported.
2836      </t>
2837    </list>
2838</t>
2839<t>
2840   HTTP implementations &SHOULD; implement persistent connections.
2841</t>
2842   
2843<section title="Establishment" anchor="persistent.establishment">
2844<t>
2845   It is beyond the scope of this specification to describe how connections
2846   are established via various transport or session-layer protocols.
2847   Each connection applies to only one transport link.
2848</t>
2849<t>
2850   A recipient determines whether a connection is persistent or not based on
2851   the most recently received message's protocol version and
2852   <x:ref>Connection</x:ref> header field (if any):
2853   <list style="symbols">
2854     <t>If the <x:ref>close</x:ref> connection option is present, the
2855        connection will not persist after the current response; else,</t>
2856     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2857        persist after the current response; else,</t>
2858     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2859        connection option is present, the recipient is not a proxy, and
2860        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2861        the connection will persist after the current response; otherwise,</t>
2862     <t>The connection will close after the current response.</t>
2863   </list>
2864</t>
2865<t>
2866   A proxy server &MUST-NOT; maintain a persistent connection with an
2867   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
2868   information and discussion of the problems with the Keep-Alive header field
2869   implemented by many HTTP/1.0 clients).
2870</t>
2871</section>
2872
2873<section title="Reuse" anchor="persistent.reuse">
2874<t>
2875   In order to remain persistent, all messages on a connection &MUST;
2876   have a self-defined message length (i.e., one not defined by closure
2877   of the connection), as described in <xref target="message.body"/>.
2878</t>
2879<t>
2880   A server &MAY; assume that an HTTP/1.1 client intends to maintain a
2881   persistent connection until a <x:ref>close</x:ref> connection option
2882   is received in a request.
2883</t>
2884<t>
2885   A client &MAY; reuse a persistent connection until it sends or receives
2886   a <x:ref>close</x:ref> connection option or receives an HTTP/1.0 response
2887   without a "keep-alive" connection option.
2888</t>
2889<t>
2890   Clients and servers &SHOULD-NOT; assume that a persistent connection is
2891   maintained for HTTP versions less than 1.1 unless it is explicitly
2892   signaled.
2893   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
2894   for more information on backward compatibility with HTTP/1.0 clients.
2895</t>
2896
2897<section title="Pipelining" anchor="pipelining">
2898<t>
2899   A client that supports persistent connections &MAY; "pipeline" its
2900   requests (i.e., send multiple requests without waiting for each
2901   response). A server &MUST; send its responses to those requests in the
2902   same order that the requests were received.
2903</t>
2904<t>
2905   Clients which assume persistent connections and pipeline immediately
2906   after connection establishment &SHOULD; be prepared to retry their
2907   connection if the first pipelined attempt fails. If a client does
2908   such a retry, it &MUST-NOT; pipeline before it knows the connection is
2909   persistent. Clients &MUST; also be prepared to resend their requests if
2910   the server closes the connection before sending all of the
2911   corresponding responses.
2912</t>
2913<t>
2914   Clients &SHOULD-NOT; pipeline requests using non-idempotent request methods
2915   or non-idempotent sequences of request methods (see &idempotent-methods;).
2916   Otherwise, a premature termination of the transport connection could lead
2917   to indeterminate results. A client wishing to send a non-idempotent
2918   request &SHOULD; wait to send that request until it has received the
2919   response status line for the previous request.
2920</t>
2921</section>
2922
2923<section title="Retrying Requests" anchor="persistent.retrying.requests">
2924<t>
2925   Connections can be closed at any time, with or without intention.
2926   Implementations ought to anticipate the need to recover
2927   from asynchronous close events.
2928   A client &MAY; open a new connection and retransmit an aborted sequence
2929   of requests without user interaction so long as the request sequence is
2930   idempotent (see &idempotent-methods;).
2931   A client &MUST-NOT; automatically retry non-idempotent request sequences,
2932   although user agents &MAY; offer a human operator the choice of retrying
2933   the request(s). Confirmation by
2934   user agent software with semantic understanding of the application
2935   &MAY; substitute for user confirmation. An automatic retry &SHOULD-NOT;
2936   be repeated if a second sequence of requests fails.
2937</t>
2938</section>
2939</section>
2940   
2941<section title="Concurrency" anchor="persistent.concurrency">
2942<t>
2943   Clients &SHOULD; limit the number of simultaneous
2944   connections that they maintain to a given server.
2945</t>
2946<t>
2947   Previous revisions of HTTP gave a specific number of connections as a
2948   ceiling, but this was found to be impractical for many applications. As a
2949   result, this specification does not mandate a particular maximum number of
2950   connections, but instead encourages clients to be conservative when opening
2951   multiple connections.
2952</t>
2953<t>
2954   Multiple connections are typically used to avoid the "head-of-line
2955   blocking" problem, wherein a request that takes significant server-side
2956   processing and/or has a large payload blocks subsequent requests on the
2957   same connection. However, each connection consumes server resources.
2958   Furthermore, using multiple connections can cause undesirable side effects
2959   in congested networks.
2960</t>
2961<t>
2962   Note that servers might reject traffic that they deem abusive, including an
2963   excessive number of connections from a client.
2964</t>
2965</section>
2966
2967<section title="Failures and Time-outs" anchor="persistent.failures">
2968<t>
2969   Servers will usually have some time-out value beyond which they will
2970   no longer maintain an inactive connection. Proxy servers might make
2971   this a higher value since it is likely that the client will be making
2972   more connections through the same server. The use of persistent
2973   connections places no requirements on the length (or existence) of
2974   this time-out for either the client or the server.
2975</t>
2976<t>
2977   When a client or server wishes to time-out it &SHOULD; issue a graceful
2978   close on the transport connection. Clients and servers &SHOULD; both
2979   constantly watch for the other side of the transport close, and
2980   respond to it as appropriate. If a client or server does not detect
2981   the other side's close promptly it could cause unnecessary resource
2982   drain on the network.
2983</t>
2984<t>
2985   A client, server, or proxy &MAY; close the transport connection at any
2986   time. For example, a client might have started to send a new request
2987   at the same time that the server has decided to close the "idle"
2988   connection. From the server's point of view, the connection is being
2989   closed while it was idle, but from the client's point of view, a
2990   request is in progress.
2991</t>
2992<t>
2993   Servers &SHOULD; maintain persistent connections and allow the underlying
2994   transport's flow control mechanisms to resolve temporary overloads, rather
2995   than terminate connections with the expectation that clients will retry.
2996   The latter technique can exacerbate network congestion.
2997</t>
2998<t>
2999   A client sending a message body &SHOULD; monitor
3000   the network connection for an error status code while it is transmitting
3001   the request. If the client sees an error status code, it &SHOULD;
3002   immediately cease transmitting the body and close the connection.
3003</t>
3004</section>
3005   
3006<section title="Tear-down" anchor="persistent.tear-down">
3007  <iref primary="false" item="Connection header field" x:for-anchor=""/>
3008  <iref primary="false" item="close" x:for-anchor=""/>
3009<t>
3010   The <x:ref>Connection</x:ref> header field
3011   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
3012   connection option that a sender &SHOULD; send when it wishes to close
3013   the connection after the current request/response pair.
3014</t>
3015<t>
3016   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
3017   send further requests on that connection (after the one containing
3018   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
3019   final response message corresponding to this request.
3020</t>
3021<t>
3022   A server that receives a <x:ref>close</x:ref> connection option &MUST;
3023   initiate a lingering close (see below) of the connection after it sends the
3024   final response to the request that contained <x:ref>close</x:ref>.
3025   The server &SHOULD; include a <x:ref>close</x:ref> connection option
3026   in its final response on that connection. The server &MUST-NOT; process
3027   any further requests received on that connection.
3028</t>
3029<t>
3030   A server that sends a <x:ref>close</x:ref> connection option &MUST;
3031   initiate a lingering close of the connection after it sends the
3032   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
3033   any further requests received on that connection.
3034</t>
3035<t>
3036   A client that receives a <x:ref>close</x:ref> connection option &MUST;
3037   cease sending requests on that connection and close the connection
3038   after reading the response message containing the close; if additional
3039   pipelined requests had been sent on the connection, the client &SHOULD;
3040   assume that they will not be processed by the server.
3041</t>
3042<t>
3043   If a server performs an immediate close of a TCP connection, there is a
3044   significant risk that the client will not be able to read the last HTTP
3045   response.  If the server receives additional data from the client on a
3046   fully-closed connection, such as another request that was sent by the
3047   client before receiving the server's response, the server's TCP stack will
3048   send a reset packet to the client; unfortunately, the reset packet might
3049   erase the client's unacknowledged input buffers before they can be read
3050   and interpreted by the client's HTTP parser.
3051</t>
3052<t>
3053   To avoid the TCP reset problem, a server can perform a lingering close on a
3054   connection by closing only the write side of the read/write connection
3055   (a half-close) and continuing to read from the connection until the
3056   connection is closed by the client or the server is reasonably certain
3057   that its own TCP stack has received the client's acknowledgement of the
3058   packet(s) containing the server's last response. It is then safe for the
3059   server to fully close the connection.
3060</t>
3061<t>
3062   It is unknown whether the reset problem is exclusive to TCP or might also
3063   be found in other transport connection protocols.
3064</t>
3065</section>
3066</section>
3067
3068<section title="Upgrade" anchor="header.upgrade">
3069  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3070  <x:anchor-alias value="Upgrade"/>
3071  <x:anchor-alias value="protocol"/>
3072  <x:anchor-alias value="protocol-name"/>
3073  <x:anchor-alias value="protocol-version"/>
3074<t>
3075   The "Upgrade" header field is intended to provide a simple mechanism
3076   for transitioning from HTTP/1.1 to some other protocol on the same
3077   connection.  A client &MAY; send a list of protocols in the Upgrade
3078   header field of a request to invite the server to switch to one or
3079   more of those protocols before sending the final response.
3080   A server &MUST; send an Upgrade header field in <x:ref>101 (Switching
3081   Protocols)</x:ref> responses to indicate which protocol(s) are being
3082   switched to, and &MUST; send it in <x:ref>426 (Upgrade Required)</x:ref>
3083   responses to indicate acceptable protocols.
3084   A server &MAY; send an Upgrade header field in any other response to
3085   indicate that they might be willing to upgrade to one of the
3086   specified protocols for a future request.
3087</t>
3088<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3089  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3090
3091  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3092  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3093  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3094</artwork></figure>
3095<t>
3096   For example,
3097</t>
3098<figure><artwork type="example">
3099  Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3100</artwork></figure>
3101<t>
3102   Upgrade eases the difficult transition between incompatible protocols by
3103   allowing the client to initiate a request in the more commonly
3104   supported protocol while indicating to the server that it would like
3105   to use a "better" protocol if available (where "better" is determined
3106   by the server, possibly according to the nature of the request method
3107   or target resource).
3108</t>
3109<t>
3110   Upgrade cannot be used to insist on a protocol change; its acceptance and
3111   use by the server is optional. The capabilities and nature of the
3112   application-level communication after the protocol change is entirely
3113   dependent upon the new protocol chosen, although the first action
3114   after changing the protocol &MUST; be a response to the initial HTTP
3115   request that contained the Upgrade header field.
3116</t>
3117<t>
3118   For example, if the Upgrade header field is received in a GET request
3119   and the server decides to switch protocols, then it &MUST; first respond
3120   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3121   then immediately follow that with the new protocol's equivalent of a
3122   response to a GET on the target resource.  This allows a connection to be
3123   upgraded to protocols with the same semantics as HTTP without the
3124   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3125   protocols unless the received message semantics can be honored by the new
3126   protocol; an OPTIONS request can be honored by any protocol.
3127</t>
3128<t>
3129   When Upgrade is sent, a sender &MUST; also send a
3130   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3131   that contains the "upgrade" connection option, in order to prevent Upgrade
3132   from being accidentally forwarded by intermediaries that might not implement
3133   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3134   is received in an HTTP/1.0 request.
3135</t>
3136<t>
3137   The Upgrade header field only applies to switching application-level
3138   protocols on the existing connection; it cannot be used
3139   to switch to a protocol on a different connection. For that purpose, it is
3140   more appropriate to use a <x:ref>3xx (Redirection)</x:ref> response
3141   (&status-3xx;).
3142</t>
3143<t>
3144   This specification only defines the protocol name "HTTP" for use by
3145   the family of Hypertext Transfer Protocols, as defined by the HTTP
3146   version rules of <xref target="http.version"/> and future updates to this
3147   specification. Additional tokens can be registered with IANA using the
3148   registration procedure defined in <xref target="upgrade.token.registry"/>.
3149</t>
3150</section>
3151</section>
3152
3153<section title="IANA Considerations" anchor="IANA.considerations">
3154
3155<section title="Header Field Registration" anchor="header.field.registration">
3156<t>
3157   HTTP header fields are registered within the Message Header Field Registry
3158   <xref target="RFC3864"/> maintained by IANA at
3159   <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
3160</t>
3161<t>
3162   This document defines the following HTTP header fields, so their
3163   associated registry entries shall be updated according to the permanent
3164   registrations below:
3165</t>
3166<?BEGININC p1-messaging.iana-headers ?>
3167<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3168<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3169   <ttcol>Header Field Name</ttcol>
3170   <ttcol>Protocol</ttcol>
3171   <ttcol>Status</ttcol>
3172   <ttcol>Reference</ttcol>
3173
3174   <c>Connection</c>
3175   <c>http</c>
3176   <c>standard</c>
3177   <c>
3178      <xref target="header.connection"/>
3179   </c>
3180   <c>Content-Length</c>
3181   <c>http</c>
3182   <c>standard</c>
3183   <c>
3184      <xref target="header.content-length"/>
3185   </c>
3186   <c>Host</c>
3187   <c>http</c>
3188   <c>standard</c>
3189   <c>
3190      <xref target="header.host"/>
3191   </c>
3192   <c>TE</c>
3193   <c>http</c>
3194   <c>standard</c>
3195   <c>
3196      <xref target="header.te"/>
3197   </c>
3198   <c>Trailer</c>
3199   <c>http</c>
3200   <c>standard</c>
3201   <c>
3202      <xref target="header.trailer"/>
3203   </c>
3204   <c>Transfer-Encoding</c>
3205   <c>http</c>
3206   <c>standard</c>
3207   <c>
3208      <xref target="header.transfer-encoding"/>
3209   </c>
3210   <c>Upgrade</c>
3211   <c>http</c>
3212   <c>standard</c>
3213   <c>
3214      <xref target="header.upgrade"/>
3215   </c>
3216   <c>Via</c>
3217   <c>http</c>
3218   <c>standard</c>
3219   <c>
3220      <xref target="header.via"/>
3221   </c>
3222</texttable>
3223<!--(END)-->
3224<?ENDINC p1-messaging.iana-headers ?>
3225<t>
3226   Furthermore, the header field-name "Close" shall be registered as
3227   "reserved", since using that name as an HTTP header field might
3228   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3229   header field (<xref target="header.connection"/>).
3230</t>
3231<texttable align="left" suppress-title="true">
3232   <ttcol>Header Field Name</ttcol>
3233   <ttcol>Protocol</ttcol>
3234   <ttcol>Status</ttcol>
3235   <ttcol>Reference</ttcol>
3236
3237   <c>Close</c>
3238   <c>http</c>
3239   <c>reserved</c>
3240   <c>
3241      <xref target="header.field.registration"/>
3242   </c>
3243</texttable>
3244<t>
3245   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3246</t>
3247</section>
3248
3249<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3250<t>
3251   IANA maintains the registry of URI Schemes <xref target="RFC4395"/> at
3252   <eref target="http://www.iana.org/assignments/uri-schemes.html"/>.
3253</t>
3254<t>
3255   This document defines the following URI schemes, so their
3256   associated registry entries shall be updated according to the permanent
3257   registrations below:
3258</t>
3259<texttable align="left" suppress-title="true">
3260   <ttcol>URI Scheme</ttcol>
3261   <ttcol>Description</ttcol>
3262   <ttcol>Reference</ttcol>
3263
3264   <c>http</c>
3265   <c>Hypertext Transfer Protocol</c>
3266   <c><xref target="http.uri"/></c>
3267
3268   <c>https</c>
3269   <c>Hypertext Transfer Protocol Secure</c>
3270   <c><xref target="https.uri"/></c>
3271</texttable>
3272</section>
3273
3274<section title="Internet Media Type Registrations" anchor="internet.media.type.http">
3275<t>
3276   This document serves as the specification for the Internet media types
3277   "message/http" and "application/http". The following is to be registered with
3278   IANA (see <xref target="RFC4288"/>).
3279</t>
3280<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
3281<iref item="Media Type" subitem="message/http" primary="true"/>
3282<iref item="message/http Media Type" primary="true"/>
3283<t>
3284   The message/http type can be used to enclose a single HTTP request or
3285   response message, provided that it obeys the MIME restrictions for all
3286   "message" types regarding line length and encodings.
3287</t>
3288<t>
3289  <list style="hanging" x:indent="12em">
3290    <t hangText="Type name:">
3291      message
3292    </t>
3293    <t hangText="Subtype name:">
3294      http
3295    </t>
3296    <t hangText="Required parameters:">
3297      none
3298    </t>
3299    <t hangText="Optional parameters:">
3300      version, msgtype
3301      <list style="hanging">
3302        <t hangText="version:">
3303          The HTTP-version number of the enclosed message
3304          (e.g., "1.1"). If not present, the version can be
3305          determined from the first line of the body.
3306        </t>
3307        <t hangText="msgtype:">
3308          The message type &mdash; "request" or "response". If not
3309          present, the type can be determined from the first
3310          line of the body.
3311        </t>
3312      </list>
3313    </t>
3314    <t hangText="Encoding considerations:">
3315      only "7bit", "8bit", or "binary" are permitted
3316    </t>
3317    <t hangText="Security considerations:">
3318      none
3319    </t>
3320    <t hangText="Interoperability considerations:">
3321      none
3322    </t>
3323    <t hangText="Published specification:">
3324      This specification (see <xref target="internet.media.type.message.http"/>).
3325    </t>
3326    <t hangText="Applications that use this media type:">
3327    </t>
3328    <t hangText="Additional information:">
3329      <list style="hanging">
3330        <t hangText="Magic number(s):">none</t>
3331        <t hangText="File extension(s):">none</t>
3332        <t hangText="Macintosh file type code(s):">none</t>
3333      </list>
3334    </t>
3335    <t hangText="Person and email address to contact for further information:">
3336      See Authors Section.
3337    </t>
3338    <t hangText="Intended usage:">
3339      COMMON
3340    </t>
3341    <t hangText="Restrictions on usage:">
3342      none
3343    </t>
3344    <t hangText="Author/Change controller:">
3345      IESG
3346    </t>
3347  </list>
3348</t>
3349</section>
3350<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
3351<iref item="Media Type" subitem="application/http" primary="true"/>
3352<iref item="application/http Media Type" primary="true"/>
3353<t>
3354   The application/http type can be used to enclose a pipeline of one or more
3355   HTTP request or response messages (not intermixed).
3356</t>
3357<t>
3358  <list style="hanging" x:indent="12em">
3359    <t hangText="Type name:">
3360      application
3361    </t>
3362    <t hangText="Subtype name:">
3363      http
3364    </t>
3365    <t hangText="Required parameters:">
3366      none
3367    </t>
3368    <t hangText="Optional parameters:">
3369      version, msgtype
3370      <list style="hanging">
3371        <t hangText="version:">
3372          The HTTP-version number of the enclosed messages
3373          (e.g., "1.1"). If not present, the version can be
3374          determined from the first line of the body.
3375        </t>
3376        <t hangText="msgtype:">
3377          The message type &mdash; "request" or "response". If not
3378          present, the type can be determined from the first
3379          line of the body.
3380        </t>
3381      </list>
3382    </t>
3383    <t hangText="Encoding considerations:">
3384      HTTP messages enclosed by this type
3385      are in "binary" format; use of an appropriate
3386      Content-Transfer-Encoding is required when
3387      transmitted via E-mail.
3388    </t>
3389    <t hangText="Security considerations:">
3390      none
3391    </t>
3392    <t hangText="Interoperability considerations:">
3393      none
3394    </t>
3395    <t hangText="Published specification:">
3396      This specification (see <xref target="internet.media.type.application.http"/>).
3397    </t>
3398    <t hangText="Applications that use this media type:">
3399    </t>
3400    <t hangText="Additional information:">
3401      <list style="hanging">
3402        <t hangText="Magic number(s):">none</t>
3403        <t hangText="File extension(s):">none</t>
3404        <t hangText="Macintosh file type code(s):">none</t>
3405      </list>
3406    </t>
3407    <t hangText="Person and email address to contact for further information:">
3408      See Authors Section.
3409    </t>
3410    <t hangText="Intended usage:">
3411      COMMON
3412    </t>
3413    <t hangText="Restrictions on usage:">
3414      none
3415    </t>
3416    <t hangText="Author/Change controller:">
3417      IESG
3418    </t>
3419  </list>
3420</t>
3421</section>
3422</section>
3423
3424<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3425<t>
3426   The HTTP Transfer Coding Registry defines the name space for transfer
3427   coding names.
3428</t>
3429<t>
3430   Registrations &MUST; include the following fields:
3431   <list style="symbols">
3432     <t>Name</t>
3433     <t>Description</t>
3434     <t>Pointer to specification text</t>
3435   </list>
3436</t>
3437<t>
3438   Names of transfer codings &MUST-NOT; overlap with names of content codings
3439   (&content-codings;) unless the encoding transformation is identical, as
3440   is the case for the compression codings defined in
3441   <xref target="compression.codings"/>.
3442</t>
3443<t>
3444   Values to be added to this name space require IETF Review (see
3445   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3446   conform to the purpose of transfer coding defined in this section.
3447   Use of program names for the identification of encoding formats
3448   is not desirable and is discouraged for future encodings.
3449</t>
3450<t>
3451   The registry itself is maintained at
3452   <eref target="http://www.iana.org/assignments/http-parameters"/>.
3453</t>
3454</section>
3455
3456<section title="Transfer Coding Registrations" anchor="transfer.coding.registration">
3457<t>
3458   The HTTP Transfer Coding Registry shall be updated with the registrations
3459   below:
3460</t>
3461<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3462   <ttcol>Name</ttcol>
3463   <ttcol>Description</ttcol>
3464   <ttcol>Reference</ttcol>
3465   <c>chunked</c>
3466   <c>Transfer in a series of chunks</c>
3467   <c>
3468      <xref target="chunked.encoding"/>
3469   </c>
3470   <c>compress</c>
3471   <c>UNIX "compress" program method</c>
3472   <c>
3473      <xref target="compress.coding"/>
3474   </c>
3475   <c>deflate</c>
3476   <c>"deflate" compression mechanism (<xref target="RFC1951"/>) used inside
3477   the "zlib" data format (<xref target="RFC1950"/>)
3478   </c>
3479   <c>
3480      <xref target="deflate.coding"/>
3481   </c>
3482   <c>gzip</c>
3483   <c>Same as GNU zip <xref target="RFC1952"/></c>
3484   <c>
3485      <xref target="gzip.coding"/>
3486   </c>
3487</texttable>
3488</section>
3489
3490<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3491<t>
3492   The HTTP Upgrade Token Registry defines the name space for protocol-name
3493   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3494   field. Each registered protocol name is associated with contact information
3495   and an optional set of specifications that details how the connection
3496   will be processed after it has been upgraded.
3497</t>
3498<t>
3499   Registrations happen on a "First Come First Served" basis (see
3500   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3501   following rules:
3502  <list style="numbers">
3503    <t>A protocol-name token, once registered, stays registered forever.</t>
3504    <t>The registration &MUST; name a responsible party for the
3505       registration.</t>
3506    <t>The registration &MUST; name a point of contact.</t>
3507    <t>The registration &MAY; name a set of specifications associated with
3508       that token. Such specifications need not be publicly available.</t>
3509    <t>The registration &SHOULD; name a set of expected "protocol-version"
3510       tokens associated with that token at the time of registration.</t>
3511    <t>The responsible party &MAY; change the registration at any time.
3512       The IANA will keep a record of all such changes, and make them
3513       available upon request.</t>
3514    <t>The IESG &MAY; reassign responsibility for a protocol token.
3515       This will normally only be used in the case when a
3516       responsible party cannot be contacted.</t>
3517  </list>
3518</t>
3519<t>
3520   This registration procedure for HTTP Upgrade Tokens replaces that
3521   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3522</t>
3523</section>
3524
3525<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3526<t>
3527   The HTTP Upgrade Token Registry shall be updated with the registration
3528   below:
3529</t>
3530<texttable align="left" suppress-title="true">
3531   <ttcol>Value</ttcol>
3532   <ttcol>Description</ttcol>
3533   <ttcol>Expected Version Tokens</ttcol>
3534   <ttcol>Reference</ttcol>
3535
3536   <c>HTTP</c>
3537   <c>Hypertext Transfer Protocol</c>
3538   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3539   <c><xref target="http.version"/></c>
3540</texttable>
3541<t>
3542   The responsible party is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3543</t>
3544</section>
3545
3546</section>
3547
3548<section title="Security Considerations" anchor="security.considerations">
3549<t>
3550   This section is meant to inform application developers, information
3551   providers, and users of the security limitations in HTTP/1.1 as
3552   described by this document. The discussion does not include
3553   definitive solutions to the problems revealed, though it does make
3554   some suggestions for reducing security risks.
3555</t>
3556
3557<section title="Personal Information" anchor="personal.information">
3558<t>
3559   HTTP clients are often privy to large amounts of personal information,
3560   including both information provided by the user to interact with resources
3561   (e.g., the user's name, location, mail address, passwords, encryption
3562   keys, etc.) and information about the user's browsing activity over
3563   time (e.g., history, bookmarks, etc.). HTTP implementations need to
3564   prevent unintentional leakage of this information.
3565</t>
3566</section>
3567
3568<section title="Abuse of Server Log Information" anchor="abuse.of.server.log.information">
3569<t>
3570   A server is in the position to save personal data about a user's
3571   requests which might identify their reading patterns or subjects of
3572   interest.  In particular, log information gathered at an intermediary
3573   often contains a history of user agent interaction, across a multitude
3574   of sites, that can be traced to individual users.
3575</t>
3576<t>
3577   HTTP log information is confidential in nature; its handling is often
3578   constrained by laws and regulations.  Log information needs to be securely
3579   stored and appropriate guidelines followed for its analysis.
3580   Anonymization of personal information within individual entries helps,
3581   but is generally not sufficient to prevent real log traces from being
3582   re-identified based on correlation with other access characteristics.
3583   As such, access traces that are keyed to a specific client should not
3584   be published even if the key is pseudonymous.
3585</t>
3586<t>
3587   To minimize the risk of theft or accidental publication, log information
3588   should be purged of personally identifiable information, including
3589   user identifiers, IP addresses, and user-provided query parameters,
3590   as soon as that information is no longer necessary to support operational
3591   needs for security, auditing, or fraud control.
3592</t>
3593</section>
3594
3595<section title="Attacks Based On File and Path Names" anchor="attack.pathname">
3596<t>
3597   Origin servers &SHOULD; be careful to restrict
3598   the documents returned by HTTP requests to be only those that were
3599   intended by the server administrators. If an HTTP server translates
3600   HTTP URIs directly into file system calls, the server &MUST; take
3601   special care not to serve files that were not intended to be
3602   delivered to HTTP clients. For example, UNIX, Microsoft Windows, and
3603   other operating systems use ".." as a path component to indicate a
3604   directory level above the current one. On such a system, an HTTP
3605   server &MUST; disallow any such construct in the request-target if it
3606   would otherwise allow access to a resource outside those intended to
3607   be accessible via the HTTP server. Similarly, files intended for
3608   reference only internally to the server (such as access control
3609   files, configuration files, and script code) &MUST; be protected from
3610   inappropriate retrieval, since they might contain sensitive
3611   information.
3612</t>
3613</section>
3614
3615<section title="DNS-related Attacks" anchor="dns.related.attacks">
3616<t>
3617   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3618   generally prone to security attacks based on the deliberate misassociation
3619   of IP addresses and DNS names not protected by DNSSEC. Clients need to be
3620   cautious in assuming the validity of an IP number/DNS name association unless
3621   the response is protected by DNSSEC (<xref target="RFC4033"/>).
3622</t>
3623</section>
3624
3625<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3626<t>
3627   By their very nature, HTTP intermediaries are men-in-the-middle, and
3628   represent an opportunity for man-in-the-middle attacks. Compromise of
3629   the systems on which the intermediaries run can result in serious security
3630   and privacy problems. Intermediaries have access to security-related
3631   information, personal information about individual users and
3632   organizations, and proprietary information belonging to users and
3633   content providers. A compromised intermediary, or an intermediary
3634   implemented or configured without regard to security and privacy
3635   considerations, might be used in the commission of a wide range of
3636   potential attacks.
3637</t>
3638<t>
3639   Intermediaries that contain a shared cache are especially vulnerable
3640   to cache poisoning attacks.
3641</t>
3642<t>
3643   Implementers need to consider the privacy and security
3644   implications of their design and coding decisions, and of the
3645   configuration options they provide to operators (especially the
3646   default configuration).
3647</t>
3648<t>
3649   Users need to be aware that intermediaries are no more trustworthy than
3650   the people who run them; HTTP itself cannot solve this problem.
3651</t>
3652</section>
3653
3654<section title="Protocol Element Size Overflows" anchor="attack.protocol.element.size.overflows">
3655<t>
3656   Because HTTP uses mostly textual, character-delimited fields, attackers can
3657   overflow buffers in implementations, and/or perform a Denial of Service
3658   against implementations that accept fields with unlimited lengths.
3659</t>
3660<t>
3661   To promote interoperability, this specification makes specific
3662   recommendations for minimum size limits on request-line
3663   (<xref target="request.line"/>)
3664   and blocks of header fields (<xref target="header.fields"/>). These are
3665   minimum recommendations, chosen to be supportable even by implementations
3666   with limited resources; it is expected that most implementations will
3667   choose substantially higher limits.
3668</t>
3669<t>
3670   This specification also provides a way for servers to reject messages that
3671   have request-targets that are too long (&status-414;) or request entities
3672   that are too large (&status-4xx;).
3673</t>
3674<t>
3675   Recipients &SHOULD; carefully limit the extent to which they read other
3676   fields, including (but not limited to) request methods, response status
3677   phrases, header field-names, and body chunks, so as to avoid denial of
3678   service attacks without impeding interoperability.
3679</t>
3680</section>
3681</section>
3682
3683<section title="Acknowledgments" anchor="acks">
3684<t>
3685   This edition of HTTP/1.1 builds on the many contributions that went into
3686   <xref target="RFC1945" format="none">RFC 1945</xref>,
3687   <xref target="RFC2068" format="none">RFC 2068</xref>,
3688   <xref target="RFC2145" format="none">RFC 2145</xref>, and
3689   <xref target="RFC2616" format="none">RFC 2616</xref>, including
3690   substantial contributions made by the previous authors, editors, and
3691   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
3692   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
3693   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
3694</t>
3695<t>
3696   Since 1999, the following contributors have helped improve the HTTP
3697   specification by reporting bugs, asking smart questions, drafting or
3698   reviewing text, and evaluating open issues:
3699</t>
3700<?BEGININC acks ?>
3701<t>Adam Barth,
3702Adam Roach,
3703Addison Phillips,
3704Adrian Chadd,
3705Adrien W. de Croy,
3706Alan Ford,
3707Alan Ruttenberg,
3708Albert Lunde,
3709Alek Storm,
3710Alex Rousskov,
3711Alexandre Morgaut,
3712Alexey Melnikov,
3713Alisha Smith,
3714Amichai Rothman,
3715Amit Klein,
3716Amos Jeffries,
3717Andreas Maier,
3718Andreas Petersson,
3719Anil Sharma,
3720Anne van Kesteren,
3721Anthony Bryan,
3722Asbjorn Ulsberg,
3723Ashok Kumar,
3724Balachander Krishnamurthy,
3725Barry Leiba,
3726Ben Laurie,
3727Benjamin Niven-Jenkins,
3728Bil Corry,
3729Bill Burke,
3730Bjoern Hoehrmann,
3731Bob Scheifler,
3732Boris Zbarsky,
3733Brett Slatkin,
3734Brian Kell,
3735Brian McBarron,
3736Brian Pane,
3737Brian Smith,
3738Bryce Nesbitt,
3739Cameron Heavon-Jones,
3740Carl Kugler,
3741Carsten Bormann,
3742Charles Fry,
3743Chris Newman,
3744Cyrus Daboo,
3745Dale Robert Anderson,
3746Dan Wing,
3747Dan Winship,
3748Daniel Stenberg,
3749Dave Cridland,
3750Dave Crocker,
3751Dave Kristol,
3752David Booth,
3753David Singer,
3754David W. Morris,
3755Diwakar Shetty,
3756Dmitry Kurochkin,
3757Drummond Reed,
3758Duane Wessels,
3759Edward Lee,
3760Eliot Lear,
3761Eran Hammer-Lahav,
3762Eric D. Williams,
3763Eric J. Bowman,
3764Eric Lawrence,
3765Eric Rescorla,
3766Erik Aronesty,
3767Evan Prodromou,
3768Florian Weimer,
3769Frank Ellermann,
3770Fred Bohle,
3771Gabriel Montenegro,
3772Geoffrey Sneddon,
3773Gervase Markham,
3774Grahame Grieve,
3775Greg Wilkins,
3776Harald Tveit Alvestrand,
3777Harry Halpin,
3778Helge Hess,
3779Henrik Nordstrom,
3780Henry S. Thompson,
3781Henry Story,
3782Herbert van de Sompel,
3783Howard Melman,
3784Hugo Haas,
3785Ian Fette,
3786Ian Hickson,
3787Ido Safruti,
3788Ilya Grigorik,
3789Ingo Struck,
3790J. Ross Nicoll,
3791James H. Manger,
3792James Lacey,
3793James M. Snell,
3794Jamie Lokier,
3795Jan Algermissen,
3796Jeff Hodges (who came up with the term 'effective Request-URI'),
3797Jeff Walden,
3798Jim Luther,
3799Joe D. Williams,
3800Joe Gregorio,
3801Joe Orton,
3802John C. Klensin,
3803John C. Mallery,
3804John Cowan,
3805John Kemp,
3806John Panzer,
3807John Schneider,
3808John Stracke,
3809John Sullivan,
3810Jonas Sicking,
3811Jonathan Billington,
3812Jonathan Moore,
3813Jonathan Rees,
3814Jonathan Silvera,
3815Jordi Ros,
3816Joris Dobbelsteen,
3817Josh Cohen,
3818Julien Pierre,
3819Jungshik Shin,
3820Justin Chapweske,
3821Justin Erenkrantz,
3822Justin James,
3823Kalvinder Singh,
3824Karl Dubost,
3825Keith Hoffman,
3826Keith Moore,
3827Ken Murchison,
3828Koen Holtman,
3829Konstantin Voronkov,
3830Kris Zyp,
3831Lisa Dusseault,
3832Maciej Stachowiak,
3833Marc Schneider,
3834Marc Slemko,
3835Mark Baker,
3836Mark Pauley,
3837Mark Watson,
3838Markus Isomaki,
3839Markus Lanthaler,
3840Martin J. Duerst,
3841Martin Musatov,
3842Martin Nilsson,
3843Martin Thomson,
3844Matt Lynch,
3845Matthew Cox,
3846Max Clark,
3847Michael Burrows,
3848Michael Hausenblas,
3849Mike Amundsen,
3850Mike Belshe,
3851Mike Kelly,
3852Mike Schinkel,
3853Miles Sabin,
3854Murray S. Kucherawy,
3855Mykyta Yevstifeyev,
3856Nathan Rixham,
3857Nicholas Shanks,
3858Nico Williams,
3859Nicolas Alvarez,
3860Nicolas Mailhot,
3861Noah Slater,
3862Pablo Castro,
3863Pat Hayes,
3864Patrick R. McManus,
3865Paul E. Jones,
3866Paul Hoffman,
3867Paul Marquess,
3868Peter Lepeska,
3869Peter Saint-Andre,
3870Peter Watkins,
3871Phil Archer,
3872Philippe Mougin,
3873Phillip Hallam-Baker,
3874Poul-Henning Kamp,
3875Preethi Natarajan,
3876Rajeev Bector,
3877Ray Polk,
3878Reto Bachmann-Gmuer,
3879Richard Cyganiak,
3880Robert Brewer,
3881Robert Collins,
3882Robert O'Callahan,
3883Robert Olofsson,
3884Robert Sayre,
3885Robert Siemer,
3886Robert de Wilde,
3887Roberto Javier Godoy,
3888Roberto Peon,
3889Roland Zink,
3890Ronny Widjaja,
3891S. Mike Dierken,
3892Salvatore Loreto,
3893Sam Johnston,
3894Sam Ruby,
3895Scott Lawrence (who maintained the original issues list),
3896Sean B. Palmer,
3897Shane McCarron,
3898Stefan Eissing,
3899Stefan Tilkov,
3900Stefanos Harhalakis,
3901Stephane Bortzmeyer,
3902Stephen Farrell,
3903Stephen Ludin,
3904Stuart Williams,
3905Subbu Allamaraju,
3906Sylvain Hellegouarch,
3907Tapan Divekar,
3908Tatsuya Hayashi,
3909Ted Hardie,
3910Thomas Broyer,
3911Thomas Fossati,
3912Thomas Nordin,
3913Thomas Roessler,
3914Tim Bray,
3915Tim Morgan,
3916Tim Olsen,
3917Tom Zhou,
3918Travis Snoozy,
3919Tyler Close,
3920Vincent Murphy,
3921Wenbo Zhu,
3922Werner Baumann,
3923Wilbur Streett,
3924Wilfredo Sanchez Vega,
3925William A. Rowe Jr.,
3926William Chan,
3927Willy Tarreau,
3928Xiaoshu Wang,
3929Yaron Goland,
3930Yngve Nysaeter Pettersen,
3931Yoav Nir,
3932Yogesh Bang,
3933Yutaka Oiwa,
3934Yves Lafon (long-time member of the editor team),
3935Zed A. Shaw, and
3936Zhong Yu.
3937</t>
3938<?ENDINC acks ?>
3939<t>
3940   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
3941   acknowledgements from prior revisions.
3942</t>
3943</section>
3944
3945</middle>
3946<back>
3947
3948<references title="Normative References">
3949
3950<reference anchor="Part2">
3951  <front>
3952    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
3953    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3954      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
3955      <address><email>fielding@gbiv.com</email></address>
3956    </author>
3957    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3958      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3959      <address><email>julian.reschke@greenbytes.de</email></address>
3960    </author>
3961    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3962  </front>
3963  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
3964  <x:source href="p2-semantics.xml" basename="p2-semantics">
3965    <x:defines>1xx (Informational)</x:defines>
3966    <x:defines>1xx</x:defines>
3967    <x:defines>100 (Continue)</x:defines>
3968    <x:defines>101 (Switching Protocols)</x:defines>
3969    <x:defines>2xx (Successful)</x:defines>
3970    <x:defines>2xx</x:defines>
3971    <x:defines>200 (OK)</x:defines>
3972    <x:defines>204 (No Content)</x:defines>
3973    <x:defines>3xx (Redirection)</x:defines>
3974    <x:defines>3xx</x:defines>
3975    <x:defines>301 (Moved Permanently)</x:defines>
3976    <x:defines>4xx (Client Error)</x:defines>
3977    <x:defines>4xx</x:defines>
3978    <x:defines>400 (Bad Request)</x:defines>
3979    <x:defines>405 (Method Not Allowed)</x:defines>
3980    <x:defines>411 (Length Required)</x:defines>
3981    <x:defines>414 (URI Too Long)</x:defines>
3982    <x:defines>417 (Expectation Failed)</x:defines>
3983    <x:defines>426 (Upgrade Required)</x:defines>
3984    <x:defines>501 (Not Implemented)</x:defines>
3985    <x:defines>502 (Bad Gateway)</x:defines>
3986    <x:defines>505 (HTTP Version Not Supported)</x:defines>
3987    <x:defines>Allow</x:defines>
3988    <x:defines>Content-Encoding</x:defines>
3989    <x:defines>Content-Location</x:defines>
3990    <x:defines>Content-Type</x:defines>
3991    <x:defines>Date</x:defines>
3992    <x:defines>Expect</x:defines>
3993    <x:defines>Location</x:defines>
3994    <x:defines>Server</x:defines>
3995    <x:defines>User-Agent</x:defines>
3996  </x:source>
3997</reference>
3998
3999<reference anchor="Part4">
4000  <front>
4001    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
4002    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
4003      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4004      <address><email>fielding@gbiv.com</email></address>
4005    </author>
4006    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
4007      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4008      <address><email>julian.reschke@greenbytes.de</email></address>
4009    </author>
4010    <date month="&ID-MONTH;" year="&ID-YEAR;" />
4011  </front>
4012  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
4013  <x:source basename="p4-conditional" href="p4-conditional.xml">
4014    <x:defines>304 (Not Modified)</x:defines>
4015    <x:defines>ETag</x:defines>
4016    <x:defines>Last-Modified</x:defines>
4017  </x:source>
4018</reference>
4019
4020<reference anchor="Part5">
4021  <front>
4022    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
4023    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4024      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4025      <address><email>fielding@gbiv.com</email></address>
4026    </author>
4027    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
4028      <organization abbrev="W3C">World Wide Web Consortium</organization>
4029      <address><email>ylafon@w3.org</email></address>
4030    </author>
4031    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4032      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4033      <address><email>julian.reschke@greenbytes.de</email></address>
4034    </author>
4035    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4036  </front>
4037  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4038  <x:source href="p5-range.xml" basename="p5-range">
4039    <x:defines>Content-Range</x:defines>
4040  </x:source>
4041</reference>
4042
4043<reference anchor="Part6">
4044  <front>
4045    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4046    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4047      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4048      <address><email>fielding@gbiv.com</email></address>
4049    </author>
4050    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4051      <organization>Akamai</organization>
4052      <address><email>mnot@mnot.net</email></address>
4053    </author>
4054    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4055      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4056      <address><email>julian.reschke@greenbytes.de</email></address>
4057    </author>
4058    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4059  </front>
4060  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4061  <x:source href="p6-cache.xml" basename="p6-cache">
4062    <x:defines>Expires</x:defines>
4063  </x:source>
4064</reference>
4065
4066<reference anchor="Part7">
4067  <front>
4068    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4069    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4070      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4071      <address><email>fielding@gbiv.com</email></address>
4072    </author>
4073    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4074      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4075      <address><email>julian.reschke@greenbytes.de</email></address>
4076    </author>
4077    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4078  </front>
4079  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4080  <x:source href="p7-auth.xml" basename="p7-auth">
4081    <x:defines>Proxy-Authenticate</x:defines>
4082    <x:defines>Proxy-Authorization</x:defines>
4083  </x:source>
4084</reference>
4085
4086<reference anchor="RFC5234">
4087  <front>
4088    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4089    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4090      <organization>Brandenburg InternetWorking</organization>
4091      <address>
4092        <email>dcrocker@bbiw.net</email>
4093      </address> 
4094    </author>
4095    <author initials="P." surname="Overell" fullname="Paul Overell">
4096      <organization>THUS plc.</organization>
4097      <address>
4098        <email>paul.overell@thus.net</email>
4099      </address>
4100    </author>
4101    <date month="January" year="2008"/>
4102  </front>
4103  <seriesInfo name="STD" value="68"/>
4104  <seriesInfo name="RFC" value="5234"/>
4105</reference>
4106
4107<reference anchor="RFC2119">
4108  <front>
4109    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4110    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4111      <organization>Harvard University</organization>
4112      <address><email>sob@harvard.edu</email></address>
4113    </author>
4114    <date month="March" year="1997"/>
4115  </front>
4116  <seriesInfo name="BCP" value="14"/>
4117  <seriesInfo name="RFC" value="2119"/>
4118</reference>
4119
4120<reference anchor="RFC3986">
4121 <front>
4122  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4123  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4124    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4125    <address>
4126       <email>timbl@w3.org</email>
4127       <uri>http://www.w3.org/People/Berners-Lee/</uri>
4128    </address>
4129  </author>
4130  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4131    <organization abbrev="Day Software">Day Software</organization>
4132    <address>
4133      <email>fielding@gbiv.com</email>
4134      <uri>http://roy.gbiv.com/</uri>
4135    </address>
4136  </author>
4137  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4138    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4139    <address>
4140      <email>LMM@acm.org</email>
4141      <uri>http://larry.masinter.net/</uri>
4142    </address>
4143  </author>
4144  <date month='January' year='2005'></date>
4145 </front>
4146 <seriesInfo name="STD" value="66"/>
4147 <seriesInfo name="RFC" value="3986"/>
4148</reference>
4149
4150<reference anchor="USASCII">
4151  <front>
4152    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4153    <author>
4154      <organization>American National Standards Institute</organization>
4155    </author>
4156    <date year="1986"/>
4157  </front>
4158  <seriesInfo name="ANSI" value="X3.4"/>
4159</reference>
4160
4161<reference anchor="RFC1950">
4162  <front>
4163    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4164    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4165      <organization>Aladdin Enterprises</organization>
4166      <address><email>ghost@aladdin.com</email></address>
4167    </author>
4168    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4169    <date month="May" year="1996"/>
4170  </front>
4171  <seriesInfo name="RFC" value="1950"/>
4172  <!--<annotation>
4173    RFC 1950 is an Informational RFC, thus it might be less stable than
4174    this specification. On the other hand, this downward reference was
4175    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4176    therefore it is unlikely to cause problems in practice. See also
4177    <xref target="BCP97"/>.
4178  </annotation>-->
4179</reference>
4180
4181<reference anchor="RFC1951">
4182  <front>
4183    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4184    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4185      <organization>Aladdin Enterprises</organization>
4186      <address><email>ghost@aladdin.com</email></address>
4187    </author>
4188    <date month="May" year="1996"/>
4189  </front>
4190  <seriesInfo name="RFC" value="1951"/>
4191  <!--<annotation>
4192    RFC 1951 is an Informational RFC, thus it might be less stable than
4193    this specification. On the other hand, this downward reference was
4194    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4195    therefore it is unlikely to cause problems in practice. See also
4196    <xref target="BCP97"/>.
4197  </annotation>-->
4198</reference>
4199
4200<reference anchor="RFC1952">
4201  <front>
4202    <title>GZIP file format specification version 4.3</title>
4203    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4204      <organization>Aladdin Enterprises</organization>
4205      <address><email>ghost@aladdin.com</email></address>
4206    </author>
4207    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4208      <address><email>gzip@prep.ai.mit.edu</email></address>
4209    </author>
4210    <author initials="M." surname="Adler" fullname="Mark Adler">
4211      <address><email>madler@alumni.caltech.edu</email></address>
4212    </author>
4213    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4214      <address><email>ghost@aladdin.com</email></address>
4215    </author>
4216    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4217      <address><email>randeg@alumni.rpi.edu</email></address>
4218    </author>
4219    <date month="May" year="1996"/>
4220  </front>
4221  <seriesInfo name="RFC" value="1952"/>
4222  <!--<annotation>
4223    RFC 1952 is an Informational RFC, thus it might be less stable than
4224    this specification. On the other hand, this downward reference was
4225    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4226    therefore it is unlikely to cause problems in practice. See also
4227    <xref target="BCP97"/>.
4228  </annotation>-->
4229</reference>
4230
4231</references>
4232
4233<references title="Informative References">
4234
4235<reference anchor="ISO-8859-1">
4236  <front>
4237    <title>
4238     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4239    </title>
4240    <author>
4241      <organization>International Organization for Standardization</organization>
4242    </author>
4243    <date year="1998"/>
4244  </front>
4245  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4246</reference>
4247
4248<reference anchor='RFC1919'>
4249  <front>
4250    <title>Classical versus Transparent IP Proxies</title>
4251    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4252      <address><email>mchatel@pax.eunet.ch</email></address>
4253    </author>
4254    <date year='1996' month='March' />
4255  </front>
4256  <seriesInfo name='RFC' value='1919' />
4257</reference>
4258
4259<reference anchor="RFC1945">
4260  <front>
4261    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4262    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4263      <organization>MIT, Laboratory for Computer Science</organization>
4264      <address><email>timbl@w3.org</email></address>
4265    </author>
4266    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4267      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4268      <address><email>fielding@ics.uci.edu</email></address>
4269    </author>
4270    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4271      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4272      <address><email>frystyk@w3.org</email></address>
4273    </author>
4274    <date month="May" year="1996"/>
4275  </front>
4276  <seriesInfo name="RFC" value="1945"/>
4277</reference>
4278
4279<reference anchor="RFC2045">
4280  <front>
4281    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4282    <author initials="N." surname="Freed" fullname="Ned Freed">
4283      <organization>Innosoft International, Inc.</organization>
4284      <address><email>ned@innosoft.com</email></address>
4285    </author>
4286    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4287      <organization>First Virtual Holdings</organization>
4288      <address><email>nsb@nsb.fv.com</email></address>
4289    </author>
4290    <date month="November" year="1996"/>
4291  </front>
4292  <seriesInfo name="RFC" value="2045"/>
4293</reference>
4294
4295<reference anchor="RFC2047">
4296  <front>
4297    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4298    <author initials="K." surname="Moore" fullname="Keith Moore">
4299      <organization>University of Tennessee</organization>
4300      <address><email>moore@cs.utk.edu</email></address>
4301    </author>
4302    <date month="November" year="1996"/>
4303  </front>
4304  <seriesInfo name="RFC" value="2047"/>
4305</reference>
4306
4307<reference anchor="RFC2068">
4308  <front>
4309    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4310    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
4311      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4312      <address><email>fielding@ics.uci.edu</email></address>
4313    </author>
4314    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4315      <organization>MIT Laboratory for Computer Science</organization>
4316      <address><email>jg@w3.org</email></address>
4317    </author>
4318    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4319      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4320      <address><email>mogul@wrl.dec.com</email></address>
4321    </author>
4322    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4323      <organization>MIT Laboratory for Computer Science</organization>
4324      <address><email>frystyk@w3.org</email></address>
4325    </author>
4326    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4327      <organization>MIT Laboratory for Computer Science</organization>
4328      <address><email>timbl@w3.org</email></address>
4329    </author>
4330    <date month="January" year="1997"/>
4331  </front>
4332  <seriesInfo name="RFC" value="2068"/>
4333</reference>
4334
4335<reference anchor="RFC2145">
4336  <front>
4337    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4338    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4339      <organization>Western Research Laboratory</organization>
4340      <address><email>mogul@wrl.dec.com</email></address>
4341    </author>
4342    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4343      <organization>Department of Information and Computer Science</organization>
4344      <address><email>fielding@ics.uci.edu</email></address>
4345    </author>
4346    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4347      <organization>MIT Laboratory for Computer Science</organization>
4348      <address><email>jg@w3.org</email></address>
4349    </author>
4350    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4351      <organization>W3 Consortium</organization>
4352      <address><email>frystyk@w3.org</email></address>
4353    </author>
4354    <date month="May" year="1997"/>
4355  </front>
4356  <seriesInfo name="RFC" value="2145"/>
4357</reference>
4358
4359<reference anchor="RFC2616">
4360  <front>
4361    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4362    <author initials="R." surname="Fielding" fullname="R. Fielding">
4363      <organization>University of California, Irvine</organization>
4364      <address><email>fielding@ics.uci.edu</email></address>
4365    </author>
4366    <author initials="J." surname="Gettys" fullname="J. Gettys">
4367      <organization>W3C</organization>
4368      <address><email>jg@w3.org</email></address>
4369    </author>
4370    <author initials="J." surname="Mogul" fullname="J. Mogul">
4371      <organization>Compaq Computer Corporation</organization>
4372      <address><email>mogul@wrl.dec.com</email></address>
4373    </author>
4374    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4375      <organization>MIT Laboratory for Computer Science</organization>
4376      <address><email>frystyk@w3.org</email></address>
4377    </author>
4378    <author initials="L." surname="Masinter" fullname="L. Masinter">
4379      <organization>Xerox Corporation</organization>
4380      <address><email>masinter@parc.xerox.com</email></address>
4381    </author>
4382    <author initials="P." surname="Leach" fullname="P. Leach">
4383      <organization>Microsoft Corporation</organization>
4384      <address><email>paulle@microsoft.com</email></address>
4385    </author>
4386    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4387      <organization>W3C</organization>
4388      <address><email>timbl@w3.org</email></address>
4389    </author>
4390    <date month="June" year="1999"/>
4391  </front>
4392  <seriesInfo name="RFC" value="2616"/>
4393</reference>
4394
4395<reference anchor='RFC2817'>
4396  <front>
4397    <title>Upgrading to TLS Within HTTP/1.1</title>
4398    <author initials='R.' surname='Khare' fullname='R. Khare'>
4399      <organization>4K Associates / UC Irvine</organization>
4400      <address><email>rohit@4K-associates.com</email></address>
4401    </author>
4402    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4403      <organization>Agranat Systems, Inc.</organization>
4404      <address><email>lawrence@agranat.com</email></address>
4405    </author>
4406    <date year='2000' month='May' />
4407  </front>
4408  <seriesInfo name='RFC' value='2817' />
4409</reference>
4410
4411<reference anchor='RFC2818'>
4412  <front>
4413    <title>HTTP Over TLS</title>
4414    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4415      <organization>RTFM, Inc.</organization>
4416      <address><email>ekr@rtfm.com</email></address>
4417    </author>
4418    <date year='2000' month='May' />
4419  </front>
4420  <seriesInfo name='RFC' value='2818' />
4421</reference>
4422
4423<reference anchor='RFC3040'>
4424  <front>
4425    <title>Internet Web Replication and Caching Taxonomy</title>
4426    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4427      <organization>Equinix, Inc.</organization>
4428    </author>
4429    <author initials='I.' surname='Melve' fullname='I. Melve'>
4430      <organization>UNINETT</organization>
4431    </author>
4432    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4433      <organization>CacheFlow Inc.</organization>
4434    </author>
4435    <date year='2001' month='January' />
4436  </front>
4437  <seriesInfo name='RFC' value='3040' />
4438</reference>
4439
4440<reference anchor='RFC3864'>
4441  <front>
4442    <title>Registration Procedures for Message Header Fields</title>
4443    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4444      <organization>Nine by Nine</organization>
4445      <address><email>GK-IETF@ninebynine.org</email></address>
4446    </author>
4447    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4448      <organization>BEA Systems</organization>
4449      <address><email>mnot@pobox.com</email></address>
4450    </author>
4451    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4452      <organization>HP Labs</organization>
4453      <address><email>JeffMogul@acm.org</email></address>
4454    </author>
4455    <date year='2004' month='September' />
4456  </front>
4457  <seriesInfo name='BCP' value='90' />
4458  <seriesInfo name='RFC' value='3864' />
4459</reference>
4460
4461<reference anchor='RFC4033'>
4462  <front>
4463    <title>DNS Security Introduction and Requirements</title>
4464    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4465    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4466    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4467    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4468    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4469    <date year='2005' month='March' />
4470  </front>
4471  <seriesInfo name='RFC' value='4033' />
4472</reference>
4473
4474<reference anchor="RFC4288">
4475  <front>
4476    <title>Media Type Specifications and Registration Procedures</title>
4477    <author initials="N." surname="Freed" fullname="N. Freed">
4478      <organization>Sun Microsystems</organization>
4479      <address>
4480        <email>ned.freed@mrochek.com</email>
4481      </address>
4482    </author>
4483    <author initials="J." surname="Klensin" fullname="J. Klensin">
4484      <address>
4485        <email>klensin+ietf@jck.com</email>
4486      </address>
4487    </author>
4488    <date year="2005" month="December"/>
4489  </front>
4490  <seriesInfo name="BCP" value="13"/>
4491  <seriesInfo name="RFC" value="4288"/>
4492</reference>
4493
4494<reference anchor='RFC4395'>
4495  <front>
4496    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4497    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4498      <organization>AT&amp;T Laboratories</organization>
4499      <address>
4500        <email>tony+urireg@maillennium.att.com</email>
4501      </address>
4502    </author>
4503    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4504      <organization>Qualcomm, Inc.</organization>
4505      <address>
4506        <email>hardie@qualcomm.com</email>
4507      </address>
4508    </author>
4509    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4510      <organization>Adobe Systems</organization>
4511      <address>
4512        <email>LMM@acm.org</email>
4513      </address>
4514    </author>
4515    <date year='2006' month='February' />
4516  </front>
4517  <seriesInfo name='BCP' value='115' />
4518  <seriesInfo name='RFC' value='4395' />
4519</reference>
4520
4521<reference anchor='RFC4559'>
4522  <front>
4523    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4524    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4525    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4526    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4527    <date year='2006' month='June' />
4528  </front>
4529  <seriesInfo name='RFC' value='4559' />
4530</reference>
4531
4532<reference anchor='RFC5226'>
4533  <front>
4534    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4535    <author initials='T.' surname='Narten' fullname='T. Narten'>
4536      <organization>IBM</organization>
4537      <address><email>narten@us.ibm.com</email></address>
4538    </author>
4539    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4540      <organization>Google</organization>
4541      <address><email>Harald@Alvestrand.no</email></address>
4542    </author>
4543    <date year='2008' month='May' />
4544  </front>
4545  <seriesInfo name='BCP' value='26' />
4546  <seriesInfo name='RFC' value='5226' />
4547</reference>
4548
4549<reference anchor='RFC5246'>
4550   <front>
4551      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4552      <author initials='T.' surname='Dierks' fullname='T. Dierks'>
4553         <organization />
4554      </author>
4555      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4556         <organization>RTFM, Inc.</organization>
4557      </author>
4558      <date year='2008' month='August' />
4559   </front>
4560   <seriesInfo name='RFC' value='5246' />
4561</reference>
4562
4563<reference anchor="RFC5322">
4564  <front>
4565    <title>Internet Message Format</title>
4566    <author initials="P." surname="Resnick" fullname="P. Resnick">
4567      <organization>Qualcomm Incorporated</organization>
4568    </author>
4569    <date year="2008" month="October"/>
4570  </front>
4571  <seriesInfo name="RFC" value="5322"/>
4572</reference>
4573
4574<reference anchor="RFC6265">
4575  <front>
4576    <title>HTTP State Management Mechanism</title>
4577    <author initials="A." surname="Barth" fullname="Adam Barth">
4578      <organization abbrev="U.C. Berkeley">
4579        University of California, Berkeley
4580      </organization>
4581      <address><email>abarth@eecs.berkeley.edu</email></address>
4582    </author>
4583    <date year="2011" month="April" />
4584  </front>
4585  <seriesInfo name="RFC" value="6265"/>
4586</reference>
4587
4588<!--<reference anchor='BCP97'>
4589  <front>
4590    <title>Handling Normative References to Standards-Track Documents</title>
4591    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
4592      <address>
4593        <email>klensin+ietf@jck.com</email>
4594      </address>
4595    </author>
4596    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
4597      <organization>MIT</organization>
4598      <address>
4599        <email>hartmans-ietf@mit.edu</email>
4600      </address>
4601    </author>
4602    <date year='2007' month='June' />
4603  </front>
4604  <seriesInfo name='BCP' value='97' />
4605  <seriesInfo name='RFC' value='4897' />
4606</reference>-->
4607
4608<reference anchor="Kri2001" target="http://arxiv.org/abs/cs.SE/0105018">
4609  <front>
4610    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
4611    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
4612    <date year="2001" month="November"/>
4613  </front>
4614  <seriesInfo name="ACM Transactions on Internet Technology" value="Vol. 1, #2"/>
4615</reference>
4616
4617</references>
4618
4619
4620<section title="HTTP Version History" anchor="compatibility">
4621<t>
4622   HTTP has been in use by the World-Wide Web global information initiative
4623   since 1990. The first version of HTTP, later referred to as HTTP/0.9,
4624   was a simple protocol for hypertext data transfer across the Internet
4625   with only a single request method (GET) and no metadata.
4626   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
4627   methods and MIME-like messaging that could include metadata about the data
4628   transferred and modifiers on the request/response semantics. However,
4629   HTTP/1.0 did not sufficiently take into consideration the effects of
4630   hierarchical proxies, caching, the need for persistent connections, or
4631   name-based virtual hosts. The proliferation of incompletely-implemented
4632   applications calling themselves "HTTP/1.0" further necessitated a
4633   protocol version change in order for two communicating applications
4634   to determine each other's true capabilities.
4635</t>
4636<t>
4637   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
4638   requirements that enable reliable implementations, adding only
4639   those new features that will either be safely ignored by an HTTP/1.0
4640   recipient or only sent when communicating with a party advertising
4641   conformance with HTTP/1.1.
4642</t>
4643<t>
4644   It is beyond the scope of a protocol specification to mandate
4645   conformance with previous versions. HTTP/1.1 was deliberately
4646   designed, however, to make supporting previous versions easy.
4647   We would expect a general-purpose HTTP/1.1 server to understand
4648   any valid request in the format of HTTP/1.0 and respond appropriately
4649   with an HTTP/1.1 message that only uses features understood (or
4650   safely ignored) by HTTP/1.0 clients.  Likewise, we would expect
4651   an HTTP/1.1 client to understand any valid HTTP/1.0 response.
4652</t>
4653<t>
4654   Since HTTP/0.9 did not support header fields in a request,
4655   there is no mechanism for it to support name-based virtual
4656   hosts (selection of resource by inspection of the <x:ref>Host</x:ref> header
4657   field).  Any server that implements name-based virtual hosts
4658   ought to disable support for HTTP/0.9.  Most requests that
4659   appear to be HTTP/0.9 are, in fact, badly constructed HTTP/1.x
4660   requests wherein a buggy client failed to properly encode
4661   linear whitespace found in a URI reference and placed in
4662   the request-target.
4663</t>
4664
4665<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
4666<t>
4667   This section summarizes major differences between versions HTTP/1.0
4668   and HTTP/1.1.
4669</t>
4670
4671<section title="Multi-homed Web Servers" anchor="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">
4672<t>
4673   The requirements that clients and servers support the <x:ref>Host</x:ref>
4674   header field (<xref target="header.host"/>), report an error if it is
4675   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
4676   are among the most important changes defined by HTTP/1.1.
4677</t>
4678<t>
4679   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4680   addresses and servers; there was no other established mechanism for
4681   distinguishing the intended server of a request than the IP address
4682   to which that request was directed. The <x:ref>Host</x:ref> header field was
4683   introduced during the development of HTTP/1.1 and, though it was
4684   quickly implemented by most HTTP/1.0 browsers, additional requirements
4685   were placed on all HTTP/1.1 requests in order to ensure complete
4686   adoption.  At the time of this writing, most HTTP-based services
4687   are dependent upon the Host header field for targeting requests.
4688</t>
4689</section>
4690
4691<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4692<t>
4693   In HTTP/1.0, each connection is established by the client prior to the
4694   request and closed by the server after sending the response. However, some
4695   implementations implement the explicitly negotiated ("Keep-Alive") version
4696   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
4697   target="RFC2068"/>.
4698</t>
4699<t>
4700   Some clients and servers might wish to be compatible with these previous
4701   approaches to persistent connections, by explicitly negotiating for them
4702   with a "Connection: keep-alive" request header field. However, some
4703   experimental implementations of HTTP/1.0 persistent connections are faulty;
4704   for example, if a HTTP/1.0 proxy server doesn't understand
4705   <x:ref>Connection</x:ref>, it will erroneously forward that header field
4706   to the next inbound server, which would result in a hung connection.
4707</t>
4708<t>
4709   One attempted solution was the introduction of a Proxy-Connection header
4710   field, targeted specifically at proxies. In practice, this was also
4711   unworkable, because proxies are often deployed in multiple layers, bringing
4712   about the same problem discussed above.
4713</t>
4714<t>
4715   As a result, clients are encouraged not to send the Proxy-Connection header
4716   field in any requests.
4717</t>
4718<t>
4719   Clients are also encouraged to consider the use of Connection: keep-alive
4720   in requests carefully; while they can enable persistent connections with
4721   HTTP/1.0 servers, clients using them need will need to monitor the
4722   connection for "hung" requests (which indicate that the client ought stop
4723   sending the header field), and this mechanism ought not be used by clients
4724   at all when a proxy is being used.
4725</t>
4726</section>
4727
4728<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
4729<t>
4730   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
4731   (<xref target="header.transfer-encoding"/>). Proxies/gateways &MUST; remove
4732   any transfer-coding prior to forwarding a message via a MIME-compliant
4733   protocol.
4734</t>
4735</section>
4736
4737</section>
4738
4739<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4740<t>
4741  HTTP's approach to error handling has been explained.
4742  (<xref target="conformance"/>)
4743</t>
4744<t>
4745  The expectation to support HTTP/0.9 requests has been removed.
4746</t>
4747<t>
4748  The term "Effective Request URI" has been introduced.
4749  (<xref target="effective.request.uri" />)
4750</t>
4751<t>
4752  HTTP messages can be (and often are) buffered by implementations; despite
4753  it sometimes being available as a stream, HTTP is fundamentally a
4754  message-oriented protocol.
4755  (<xref target="http.message" />)
4756</t>
4757<t>
4758  Minimum supported sizes for various protocol elements have been
4759  suggested, to improve interoperability.
4760</t>
4761<t>
4762  Header fields that span multiple lines ("line folding") are deprecated.
4763  (<xref target="field.parsing" />)
4764</t>
4765<t>
4766  The HTTP-version ABNF production has been clarified to be case-sensitive.
4767  Additionally, version numbers has been restricted to single digits, due
4768  to the fact that implementations are known to handle multi-digit version
4769  numbers incorrectly.
4770  (<xref target="http.version"/>)
4771</t>
4772<t>
4773  The HTTPS URI scheme is now defined by this specification; previously,
4774  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
4775  (<xref target="https.uri"/>)
4776</t>
4777<t>
4778  The HTTPS URI scheme implies end-to-end security.
4779  (<xref target="https.uri"/>)
4780</t>
4781<t>
4782  Userinfo (i.e., username and password) are now disallowed in HTTP and
4783  HTTPS URIs, because of security issues related to their transmission on the
4784  wire.
4785  (<xref target="http.uri" />)
4786</t>
4787<t>
4788  Invalid whitespace around field-names is now required to be rejected,
4789  because accepting it represents a security vulnerability.
4790  (<xref target="header.fields"/>)
4791</t>
4792<t>
4793  The ABNF productions defining header fields now only list the field value.
4794  (<xref target="header.fields"/>)
4795</t>
4796<t>
4797  Rules about implicit linear whitespace between certain grammar productions
4798  have been removed; now whitespace is only allowed where specifically
4799  defined in the ABNF.
4800  (<xref target="whitespace"/>)
4801</t>
4802<t> 
4803  The NUL octet is no longer allowed in comment and quoted-string text, and
4804  handling of backslash-escaping in them has been clarified.
4805  (<xref target="field.components"/>)
4806</t> 
4807<t>
4808  The quoted-pair rule no longer allows escaping control characters other than
4809  HTAB.
4810  (<xref target="field.components"/>)
4811</t>
4812<t>
4813  Non-ASCII content in header fields and the reason phrase has been obsoleted
4814  and made opaque (the TEXT rule was removed).
4815  (<xref target="field.components"/>)
4816</t>
4817<t>
4818  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
4819  handled as errors by recipients.
4820  (<xref target="header.content-length"/>)
4821</t>
4822<t>
4823  The "identity" transfer-coding value token has been removed.
4824  (Sections <xref format="counter" target="message.body"/> and
4825  <xref format="counter" target="transfer.codings"/>)
4826</t>
4827<t>
4828  The algorithm for determining the message body length has been clarified
4829  to indicate all of the special cases (e.g., driven by methods or status
4830  codes) that affect it, and that new protocol elements cannot define such
4831  special cases.
4832  (<xref target="message.body.length"/>)
4833</t>
4834<t>
4835  "multipart/byteranges" is no longer a way of determining message body length
4836  detection.
4837  (<xref target="message.body.length"/>)
4838</t>
4839<t>
4840  CONNECT is a new, special case in determining message body length.
4841  (<xref target="message.body.length"/>)
4842</t>
4843<t>
4844  Chunk length does not include the count of the octets in the
4845  chunk header and trailer.
4846  (<xref target="chunked.encoding"/>)
4847</t>
4848<t>
4849  Use of chunk extensions is deprecated, and line folding in them is
4850  disallowed.
4851  (<xref target="chunked.encoding"/>)
4852</t>
4853<t>
4854  The path-absolute + query components of RFC3986 have been used to define the
4855  request-target, instead of abs_path from RFC 1808.
4856  (<xref target="request-target"/>)
4857</t>
4858<t>
4859  The asterisk form of the request-target is only allowed in the OPTIONS
4860  method.
4861  (<xref target="request-target"/>)
4862</t>
4863<t>
4864  Exactly when "close" connection options have to be sent has been clarified.
4865  (<xref target="header.connection"/>)
4866</t>
4867<t> 
4868  "hop-by-hop" header fields are required to appear in the Connection header
4869  field; just because they're defined as hop-by-hop in this specification
4870  doesn't exempt them.
4871  (<xref target="header.connection"/>)
4872</t>
4873<t>
4874  The limit of two connections per server has been removed.
4875  (<xref target="persistent.connections"/>)
4876</t>
4877<t>
4878  An idempotent sequence of requests is no longer required to be retried.
4879  (<xref target="persistent.connections"/>)
4880</t>
4881<t>
4882  The requirement to retry requests under certain circumstances when the
4883  server prematurely closes the connection has been removed.
4884  (<xref target="persistent.reuse"/>)
4885</t>
4886<t>
4887  Some extraneous requirements about when servers are allowed to close
4888  connections prematurely have been removed.
4889  (<xref target="persistent.connections"/>)
4890</t>
4891<t>
4892  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
4893  responses other than 101 (this was incorporated from <xref
4894  target="RFC2817"/>).
4895  (<xref target="header.upgrade"/>)
4896</t>
4897<t>
4898  Registration of Transfer Codings now requires IETF Review
4899  (<xref target="transfer.coding.registry"/>)
4900</t>
4901<t>
4902  The meaning of the "deflate" content coding has been clarified.
4903  (<xref target="deflate.coding" />)
4904</t>
4905<t>
4906  This specification now defines the Upgrade Token Registry, previously
4907  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
4908  (<xref target="upgrade.token.registry"/>)
4909</t>
4910<t>
4911  Empty list elements in list productions (e.g., a list header containing
4912  ", ,") have been deprecated.
4913  (<xref target="abnf.extension"/>)
4914</t>
4915<t>
4916  Issues with the Keep-Alive and Proxy-Connection headers in requests
4917  are pointed out, with use of the latter being discouraged altogether.
4918  (<xref target="compatibility.with.http.1.0.persistent.connections" />)
4919</t>
4920</section>
4921</section>
4922
4923<section title="ABNF list extension: #rule" anchor="abnf.extension">
4924<t>
4925  A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
4926  improve readability in the definitions of some header field values.
4927</t>
4928<t>
4929  A construct "#" is defined, similar to "*", for defining comma-delimited
4930  lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
4931  at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
4932  comma (",") and optional whitespace (OWS).   
4933</t>
4934<figure><preamble>
4935  Thus,
4936</preamble><artwork type="example">
4937  1#element =&gt; element *( OWS "," OWS element )
4938</artwork></figure>
4939<figure><preamble>
4940  and:
4941</preamble><artwork type="example">
4942  #element =&gt; [ 1#element ]
4943</artwork></figure>
4944<figure><preamble>
4945  and for n &gt;= 1 and m &gt; 1:
4946</preamble><artwork type="example">
4947  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
4948</artwork></figure>
4949<t>
4950  For compatibility with legacy list rules, recipients &SHOULD; accept empty
4951  list elements. In other words, consumers would follow the list productions:
4952</t>
4953<figure><artwork type="example">
4954  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
4955 
4956  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
4957</artwork></figure>
4958<t>
4959  Note that empty elements do not contribute to the count of elements present,
4960  though.
4961</t>
4962<t>
4963  For example, given these ABNF productions:
4964</t>
4965<figure><artwork type="example">
4966  example-list      = 1#example-list-elmt
4967  example-list-elmt = token ; see <xref target="field.components"/>
4968</artwork></figure>
4969<t>
4970  Then these are valid values for example-list (not including the double
4971  quotes, which are present for delimitation only):
4972</t>
4973<figure><artwork type="example">
4974  "foo,bar"
4975  "foo ,bar,"
4976  "foo , ,bar,charlie   "
4977</artwork></figure>
4978<t>
4979  But these values would be invalid, as at least one non-empty element is
4980  required:
4981</t>
4982<figure><artwork type="example">
4983  ""
4984  ","
4985  ",   ,"
4986</artwork></figure>
4987<t>
4988  <xref target="collected.abnf"/> shows the collected ABNF, with the list rules
4989  expanded as explained above.
4990</t>
4991</section>
4992
4993<?BEGININC p1-messaging.abnf-appendix ?>
4994<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
4995<figure>
4996<artwork type="abnf" name="p1-messaging.parsed-abnf">
4997<x:ref>BWS</x:ref> = OWS
4998
4999<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
5000 connection-option ] )
5001<x:ref>Content-Length</x:ref> = 1*DIGIT
5002
5003<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
5004 ]
5005<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
5006<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
5007<x:ref>Host</x:ref> = uri-host [ ":" port ]
5008
5009<x:ref>OWS</x:ref> = *( SP / HTAB )
5010
5011<x:ref>RWS</x:ref> = 1*( SP / HTAB )
5012
5013<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
5014<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
5015<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
5016 transfer-coding ] )
5017
5018<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
5019<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
5020
5021<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
5022 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
5023 comment ] ) ] )
5024
5025<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
5026<x:ref>absolute-form</x:ref> = absolute-URI
5027<x:ref>asterisk-form</x:ref> = "*"
5028<x:ref>attribute</x:ref> = token
5029<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
5030<x:ref>authority-form</x:ref> = authority
5031
5032<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
5033<x:ref>chunk-data</x:ref> = 1*OCTET
5034<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
5035<x:ref>chunk-ext-name</x:ref> = token
5036<x:ref>chunk-ext-val</x:ref> = token / quoted-str-nf
5037<x:ref>chunk-size</x:ref> = 1*HEXDIG
5038<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
5039<x:ref>comment</x:ref> = "(" *( ctext / quoted-cpair / comment ) ")"
5040<x:ref>connection-option</x:ref> = token
5041<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
5042 / %x2A-5B ; '*'-'['
5043 / %x5D-7E ; ']'-'~'
5044 / obs-text
5045
5046<x:ref>field-content</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5047<x:ref>field-name</x:ref> = token
5048<x:ref>field-value</x:ref> = *( field-content / obs-fold )
5049
5050<x:ref>header-field</x:ref> = field-name ":" OWS field-value BWS
5051<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ]
5052<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ]
5053
5054<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
5055
5056<x:ref>message-body</x:ref> = *OCTET
5057<x:ref>method</x:ref> = token
5058
5059<x:ref>obs-fold</x:ref> = CRLF ( SP / HTAB )
5060<x:ref>obs-text</x:ref> = %x80-FF
5061<x:ref>origin-form</x:ref> = path-absolute [ "?" query ]
5062
5063<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
5064<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
5065<x:ref>path-absolute</x:ref> = &lt;path-absolute, defined in [RFC3986], Section 3.3&gt;
5066<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
5067<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
5068<x:ref>protocol-name</x:ref> = token
5069<x:ref>protocol-version</x:ref> = token
5070<x:ref>pseudonym</x:ref> = token
5071
5072<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5073 / %x5D-7E ; ']'-'~'
5074 / obs-text
5075<x:ref>qdtext-nf</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5076 / %x5D-7E ; ']'-'~'
5077 / obs-text
5078<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5079<x:ref>quoted-cpair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5080<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5081<x:ref>quoted-str-nf</x:ref> = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
5082<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5083
5084<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5085<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5086<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5087<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5088<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5089<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5090<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5091 asterisk-form
5092
5093<x:ref>special</x:ref> = "(" / ")" / "&lt;" / "&gt;" / "@" / "," / ";" / ":" / "\" /
5094 DQUOTE / "/" / "[" / "]" / "?" / "=" / "{" / "}"
5095<x:ref>start-line</x:ref> = request-line / status-line
5096<x:ref>status-code</x:ref> = 3DIGIT
5097<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5098
5099<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5100<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5101<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5102 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5103<x:ref>token</x:ref> = 1*tchar
5104<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5105<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5106 transfer-extension
5107<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5108<x:ref>transfer-parameter</x:ref> = attribute BWS "=" BWS value
5109
5110<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5111
5112<x:ref>value</x:ref> = word
5113
5114<x:ref>word</x:ref> = token / quoted-string
5115</artwork>
5116</figure>
5117</section>
5118<?ENDINC p1-messaging.abnf-appendix ?>
5119
5120<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5121
5122<section title="Since RFC 2616">
5123<t>
5124  Changes up to the first Working Group Last Call draft are summarized
5125  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-21#appendix-D"/>.
5126</t>
5127</section>
5128
5129<section title="Since draft-ietf-httpbis-p1-messaging-21" anchor="changes.since.21">
5130<t>
5131  Closed issues:
5132  <list style="symbols">
5133    <t>
5134      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/128"/>:
5135      "Cite HTTPS URI scheme definition" (the spec now includes the HTTPs
5136      scheme definition and thus updates RFC 2818)
5137    </t>
5138    <t>
5139      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/389"/>:
5140      "mention of 'proxies' in section about caches"
5141    </t>
5142  </list>
5143</t>
5144</section>
5145
5146</section>
5147
5148</back>
5149</rfc>
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