source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2012

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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 "November">
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 literal or IPv4 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
1051<section title="Request Line" anchor="request.line">
1052  <x:anchor-alias value="Request"/>
1053  <x:anchor-alias value="request-line"/>
1054<t>
1055   A request-line begins with a method token, followed by a single
1056   space (SP), the request-target, another single space (SP), the
1057   protocol version, and ending with CRLF.
1058</t>
1059<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1060  <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>
1061</artwork></figure>
1062<t>
1063   A server &MUST; be able to parse any received message that begins
1064   with a request-line and matches the ABNF rule for HTTP-message.
1065</t>
1066<iref primary="true" item="method"/>
1067<t anchor="method">
1068   The method token indicates the request method to be performed on the
1069   target resource. The request method is case-sensitive.
1070</t>
1071<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1072  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1073</artwork></figure>
1074<t>
1075   The methods defined by this specification can be found in
1076   &methods;, along with information regarding the HTTP method registry
1077   and considerations for defining new methods.
1078</t>
1079<iref item="request-target"/>
1080<t>
1081   The request-target identifies the target resource upon which to apply
1082   the request, as defined in <xref target="request-target"/>.
1083</t>
1084<t>
1085   No whitespace is allowed inside the method, request-target, and
1086   protocol version.  Hence, recipients typically parse the request-line
1087   into its component parts by splitting on the SP characters.
1088</t>
1089<t>
1090   Unfortunately, some user agents fail to properly encode hypertext
1091   references that have embedded whitespace, sending the characters
1092   directly instead of properly percent-encoding the disallowed characters.
1093   Recipients of an invalid request-line &SHOULD; respond with either a
1094   <x:ref>400 (Bad Request)</x:ref> error or a <x:ref>301 (Moved Permanently)</x:ref>
1095   redirect with the request-target properly encoded.  Recipients &SHOULD-NOT;
1096   attempt to autocorrect and then process the request without a redirect,
1097   since the invalid request-line might be deliberately crafted to bypass
1098   security filters along the request chain.
1099</t>
1100<t>
1101   HTTP does not place a pre-defined limit on the length of a request-line.
1102   A server that receives a method longer than any that it implements
1103   &SHOULD; respond with either a <x:ref>405 (Method Not Allowed)</x:ref>, if it is an origin
1104   server, or a <x:ref>501 (Not Implemented)</x:ref> status code.
1105   A server &MUST; be prepared to receive URIs of unbounded length and
1106   respond with the <x:ref>414 (URI Too Long)</x:ref> status code if the received
1107   request-target would be longer than the server wishes to handle
1108   (see &status-414;).
1109</t>
1110<t>
1111   Various ad-hoc limitations on request-line length are found in practice.
1112   It is &RECOMMENDED; that all HTTP senders and recipients support, at a
1113   minimum, request-line lengths of up to 8000 octets.
1114</t>
1115</section>
1116
1117<section title="Status Line" anchor="status.line">
1118  <x:anchor-alias value="response"/>
1119  <x:anchor-alias value="status-line"/>
1120  <x:anchor-alias value="status-code"/>
1121  <x:anchor-alias value="reason-phrase"/>
1122<t>
1123   The first line of a response message is the status-line, consisting
1124   of the protocol version, a space (SP), the status code, another space,
1125   a possibly-empty textual phrase describing the status code, and
1126   ending with CRLF.
1127</t>
1128<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-line"/>
1129  <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>
1130</artwork></figure>
1131<t>
1132   A client &MUST; be able to parse any received message that begins
1133   with a status-line and matches the ABNF rule for HTTP-message.
1134</t>
1135<t>
1136   The status-code element is a 3-digit integer code describing the
1137   result of the server's attempt to understand and satisfy the client's
1138   corresponding request. The rest of the response message is to be
1139   interpreted in light of the semantics defined for that status code.
1140   See &status-codes; for information about the semantics of status codes,
1141   including the classes of status code (indicated by the first digit),
1142   the status codes defined by this specification, considerations for the
1143   definition of new status codes, and the IANA registry.
1144</t>
1145<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-code"/>
1146  <x:ref>status-code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1147</artwork></figure>
1148<t>   
1149   The reason-phrase element exists for the sole purpose of providing a
1150   textual description associated with the numeric status code, mostly
1151   out of deference to earlier Internet application protocols that were more
1152   frequently used with interactive text clients. A client &SHOULD; ignore
1153   the reason-phrase content.
1154</t>
1155<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
1156  <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> )
1157</artwork></figure>
1158</section>
1159</section>
1160
1161<section title="Header Fields" anchor="header.fields">
1162  <x:anchor-alias value="header-field"/>
1163  <x:anchor-alias value="field-content"/>
1164  <x:anchor-alias value="field-name"/>
1165  <x:anchor-alias value="field-value"/>
1166  <x:anchor-alias value="obs-fold"/>
1167<t>
1168   Each HTTP header field consists of a case-insensitive field name
1169   followed by a colon (":"), optional whitespace, and the field value.
1170</t>
1171<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"/>
1172  <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>
1173  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1174  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>obs-fold</x:ref> )
1175  <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> )
1176  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref> ( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1177                 ; obsolete line folding
1178                 ; see <xref target="field.parsing"/>
1179</artwork></figure>
1180<t>
1181   The field-name token labels the corresponding field-value as having the
1182   semantics defined by that header field.  For example, the <x:ref>Date</x:ref>
1183   header field is defined in &header-date; as containing the origination
1184   timestamp for the message in which it appears.
1185</t>
1186<t>
1187   HTTP header fields are fully extensible: there is no limit on the
1188   introduction of new field names, each presumably defining new semantics,
1189   or on the number of header fields used in a given message.  Existing
1190   fields are defined in each part of this specification and in many other
1191   specifications outside the standards process.
1192   New header fields can be introduced without changing the protocol version
1193   if their defined semantics allow them to be safely ignored by recipients
1194   that do not recognize them.
1195</t>
1196<t>
1197   New HTTP header fields &SHOULD; be registered with IANA in the
1198   Message Header Field Registry, as described in &iana-header-registry;.
1199   Unrecognized header fields &MUST; be forwarded by a proxy unless the
1200   field-name is listed in the <x:ref>Connection</x:ref> header field
1201   (<xref target="header.connection"/>) or the proxy is specifically
1202   configured to block or otherwise transform such fields.
1203   Unrecognized header fields &SHOULD; be ignored by other recipients.
1204</t>
1205<t>
1206   The order in which header fields with differing field names are
1207   received is not significant. However, it is "good practice" to send
1208   header fields that contain control data first, such as <x:ref>Host</x:ref>
1209   on requests and <x:ref>Date</x:ref> on responses, so that implementations
1210   can decide when not to handle a message as early as possible.  A server
1211   &MUST; wait until the entire header section is received before interpreting
1212   a request message, since later header fields might include conditionals,
1213   authentication credentials, or deliberately misleading duplicate
1214   header fields that would impact request processing.
1215</t>
1216<t>
1217   Multiple header fields with the same field name &MUST-NOT; be
1218   sent in a message unless the entire field value for that
1219   header field is defined as a comma-separated list [i.e., #(values)].
1220   Multiple header fields with the same field name can be combined into
1221   one "field-name: field-value" pair, without changing the semantics of the
1222   message, by appending each subsequent field value to the combined
1223   field value in order, separated by a comma. The order in which
1224   header fields with the same field name are received is therefore
1225   significant to the interpretation of the combined field value;
1226   a proxy &MUST-NOT; change the order of these field values when
1227   forwarding a message.
1228</t>
1229<x:note>
1230  <t>
1231   &Note; The "Set-Cookie" header field as implemented in
1232   practice can occur multiple times, but does not use the list syntax, and
1233   thus cannot be combined into a single line (<xref target="RFC6265"/>). (See Appendix A.2.3 of <xref target="Kri2001"/>
1234   for details.)
1235</t>
1236</x:note>
1237
1238<section title="Whitespace" anchor="whitespace">
1239<t anchor="rule.LWS">
1240   This specification uses three rules to denote the use of linear
1241   whitespace: OWS (optional whitespace), RWS (required whitespace), and
1242   BWS ("bad" whitespace).
1243</t>
1244<t anchor="rule.OWS">
1245   The OWS rule is used where zero or more linear whitespace octets might
1246   appear. OWS &SHOULD; either not be produced or be produced as a single
1247   SP. Multiple OWS octets that occur within field-content &SHOULD; either
1248   be replaced with a single SP or transformed to all SP octets (each
1249   octet other than SP replaced with SP) before interpreting the field value
1250   or forwarding the message downstream.
1251</t>
1252<t anchor="rule.RWS">
1253   RWS is used when at least one linear whitespace octet is required to
1254   separate field tokens. RWS &SHOULD; be produced as a single SP.
1255   Multiple RWS octets that occur within field-content &SHOULD; either
1256   be replaced with a single SP or transformed to all SP octets before
1257   interpreting the field value or forwarding the message downstream.
1258</t>
1259<t anchor="rule.BWS">
1260   BWS is used where the grammar allows optional whitespace, for historical
1261   reasons, but senders &SHOULD-NOT; produce it in messages;
1262   recipients &MUST; accept such bad optional whitespace and remove it before
1263   interpreting the field value or forwarding the message downstream.
1264</t>
1265<t anchor="rule.whitespace">
1266  <x:anchor-alias value="BWS"/>
1267  <x:anchor-alias value="OWS"/>
1268  <x:anchor-alias value="RWS"/>
1269</t>
1270<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"/>
1271  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1272                 ; optional whitespace
1273  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1274                 ; required whitespace
1275  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1276                 ; "bad" whitespace
1277</artwork></figure>
1278</section>
1279
1280<section title="Field Parsing" anchor="field.parsing">
1281<t>
1282   No whitespace is allowed between the header field-name and colon.
1283   In the past, differences in the handling of such whitespace have led to
1284   security vulnerabilities in request routing and response handling.
1285   Any received request message that contains whitespace between a header
1286   field-name and colon &MUST; be rejected with a response code of 400
1287   (Bad Request).  A proxy &MUST; remove any such whitespace from a response
1288   message before forwarding the message downstream.
1289</t>
1290<t>
1291   A field value is preceded by optional whitespace (OWS); a single SP is
1292   preferred. The field value does not include any leading or trailing white
1293   space: OWS occurring before the first non-whitespace octet of the
1294   field value or after the last non-whitespace octet of the field value
1295   is ignored and &SHOULD; be removed before further processing (as this does
1296   not change the meaning of the header field).
1297</t>
1298<t>
1299   Historically, HTTP header field values could be extended over multiple
1300   lines by preceding each extra line with at least one space or horizontal
1301   tab (obs-fold). This specification deprecates such line
1302   folding except within the message/http media type
1303   (<xref target="internet.media.type.message.http"/>).
1304   HTTP senders &MUST-NOT; produce messages that include line folding
1305   (i.e., that contain any field-value that matches the obs-fold rule) unless
1306   the message is intended for packaging within the message/http media type.
1307   HTTP recipients &SHOULD; accept line folding and replace any embedded
1308   obs-fold whitespace with either a single SP or a matching number of SP
1309   octets (to avoid buffer copying) prior to interpreting the field value or
1310   forwarding the message downstream.
1311</t>
1312<t>
1313   Historically, HTTP has allowed field content with text in the ISO-8859-1
1314   <xref target="ISO-8859-1"/> charset, supporting other charsets only
1315   through use of <xref target="RFC2047"/> encoding.
1316   In practice, most HTTP header field values use only a subset of the
1317   US-ASCII charset <xref target="USASCII"/>. Newly defined
1318   header fields &SHOULD; limit their field values to US-ASCII octets.
1319   Recipients &SHOULD; treat other octets in field content (obs-text) as
1320   opaque data.
1321</t>
1322</section>
1323
1324<section title="Field Length" anchor="field.length">
1325<t>
1326   HTTP does not place a pre-defined limit on the length of header fields,
1327   either in isolation or as a set. A server &MUST; be prepared to receive
1328   request header fields of unbounded length and respond with a <x:ref>4xx
1329   (Client Error)</x:ref> status code if the received header field(s) would be
1330   longer than the server wishes to handle.
1331</t>
1332<t>
1333   A client that receives response header fields that are longer than it wishes
1334   to handle can only treat it as a server error.
1335</t>
1336<t>
1337   Various ad-hoc limitations on header field length are found in practice. It
1338   is &RECOMMENDED; that all HTTP senders and recipients support messages whose
1339   combined header fields have 4000 or more octets.
1340</t>
1341</section>
1342
1343<section title="Field value components" anchor="field.components">
1344<t anchor="rule.token.separators">
1345  <x:anchor-alias value="tchar"/>
1346  <x:anchor-alias value="token"/>
1347  <x:anchor-alias value="special"/>
1348  <x:anchor-alias value="word"/>
1349   Many HTTP header field values consist of words (token or quoted-string)
1350   separated by whitespace or special characters. These special characters
1351   &MUST; be in a quoted string to be used within a parameter value (as defined
1352   in <xref target="transfer.codings"/>).
1353</t>
1354<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>
1355  <x:ref>word</x:ref>           = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1356
1357  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1358<!--
1359  IMPORTANT: when editing "tchar" make sure that "special" is updated accordingly!!!
1360 -->
1361  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1362                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1363                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1364                 ; any <x:ref>VCHAR</x:ref>, except <x:ref>special</x:ref>
1365
1366  <x:ref>special</x:ref>        = "(" / ")" / "&lt;" / ">" / "@" / ","
1367                 / ";" / ":" / "\" / DQUOTE / "/" / "["
1368                 / "]" / "?" / "=" / "{" / "}"
1369</artwork></figure>
1370<t anchor="rule.quoted-string">
1371  <x:anchor-alias value="quoted-string"/>
1372  <x:anchor-alias value="qdtext"/>
1373  <x:anchor-alias value="obs-text"/>
1374   A string of text is parsed as a single word if it is quoted using
1375   double-quote marks.
1376</t>
1377<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"/>
1378  <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>
1379  <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>
1380  <x:ref>obs-text</x:ref>       = %x80-FF
1381</artwork></figure>
1382<t anchor="rule.quoted-pair">
1383  <x:anchor-alias value="quoted-pair"/>
1384   The backslash octet ("\") can be used as a single-octet
1385   quoting mechanism within quoted-string constructs:
1386</t>
1387<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1388  <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> )
1389</artwork></figure>
1390<t>
1391   Recipients that process the value of the quoted-string &MUST; handle a
1392   quoted-pair as if it were replaced by the octet following the backslash.
1393</t>
1394<t>
1395   Senders &SHOULD-NOT; escape octets in quoted-strings that do not require
1396   escaping (i.e., other than DQUOTE and the backslash octet).
1397</t>
1398<t anchor="rule.comment">
1399  <x:anchor-alias value="comment"/>
1400  <x:anchor-alias value="ctext"/>
1401   Comments can be included in some HTTP header fields by surrounding
1402   the comment text with parentheses. Comments are only allowed in
1403   fields containing "comment" as part of their field value definition.
1404</t>
1405<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1406  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-cpair</x:ref> / <x:ref>comment</x:ref> ) ")"
1407  <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>
1408</artwork></figure>
1409<t anchor="rule.quoted-cpair">
1410  <x:anchor-alias value="quoted-cpair"/>
1411   The backslash octet ("\") can be used as a single-octet
1412   quoting mechanism within comment constructs:
1413</t>
1414<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-cpair"/>
1415  <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> )
1416</artwork></figure>
1417<t>
1418   Senders &SHOULD-NOT; escape octets in comments that do not require escaping
1419   (i.e., other than the backslash octet "\" and the parentheses "(" and ")").
1420</t>
1421</section>
1422
1423</section>
1424
1425<section title="Message Body" anchor="message.body">
1426  <x:anchor-alias value="message-body"/>
1427<t>
1428   The message body (if any) of an HTTP message is used to carry the
1429   payload body of that request or response.  The message body is
1430   identical to the payload body unless a transfer coding has been
1431   applied, as described in <xref target="header.transfer-encoding"/>.
1432</t>
1433<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1434  <x:ref>message-body</x:ref> = *OCTET
1435</artwork></figure>
1436<t>
1437   The rules for when a message body is allowed in a message differ for
1438   requests and responses.
1439</t>
1440<t>
1441   The presence of a message body in a request is signaled by a
1442   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1443   field. Request message framing is independent of method semantics,
1444   even if the method does not define any use for a message body.
1445</t>
1446<t>
1447   The presence of a message body in a response depends on both
1448   the request method to which it is responding and the response
1449   status code (<xref target="status.line"/>).
1450   Responses to the HEAD request method never include a message body
1451   because the associated response header fields (e.g.,
1452   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1453   if present, indicate only what their values would have been if the request
1454   method had been GET (&HEAD;).
1455   <x:ref>2xx (Successful)</x:ref> responses to CONNECT switch to tunnel
1456   mode instead of having a message body (&CONNECT;).
1457   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1458   <x:ref>304 (Not Modified)</x:ref> responses &MUST-NOT; include a message body.
1459   All other responses do include a message body, although the body
1460   &MAY; be of zero length.
1461</t>
1462
1463<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1464  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1465  <x:anchor-alias value="Transfer-Encoding"/>
1466<t>
1467   When one or more transfer codings are applied to a payload body in order
1468   to form the message body, a Transfer-Encoding header field &MUST; be sent
1469   in the message and &MUST; contain the list of corresponding
1470   transfer-coding names in the same order that they were applied.
1471   Transfer codings are defined in <xref target="transfer.codings"/>.
1472</t>
1473<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1474  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1475</artwork></figure>
1476<t>
1477   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1478   MIME, which was designed to enable safe transport of binary data over a
1479   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1480   However, safe transport has a different focus for an 8bit-clean transfer
1481   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1482   accurately delimit a dynamically generated payload and to distinguish
1483   payload encodings that are only applied for transport efficiency or
1484   security from those that are characteristics of the target resource.
1485</t>
1486<t>
1487   The "chunked" transfer-coding (<xref target="chunked.encoding"/>)
1488   &MUST; be implemented by all HTTP/1.1 recipients because it plays a
1489   crucial role in delimiting messages when the payload body size is not
1490   known in advance.
1491   When the "chunked" transfer-coding is used, it &MUST; be the last
1492   transfer-coding applied to form the message body and &MUST-NOT;
1493   be applied more than once in a message body.
1494   If any transfer-coding is applied to a request payload body,
1495   the final transfer-coding applied &MUST; be "chunked".
1496   If any transfer-coding is applied to a response payload body, then either
1497   the final transfer-coding applied &MUST; be "chunked" or
1498   the message &MUST; be terminated by closing the connection.
1499</t>
1500<figure><preamble>
1501   For example,
1502</preamble><artwork type="example">
1503  Transfer-Encoding: gzip, chunked
1504</artwork><postamble>
1505   indicates that the payload body has been compressed using the gzip
1506   coding and then chunked using the chunked coding while forming the
1507   message body.
1508</postamble></figure>
1509<t>
1510   If more than one Transfer-Encoding header field is present in a message,
1511   the multiple field-values &MUST; be combined into one field-value,
1512   according to the algorithm defined in <xref target="header.fields"/>,
1513   before determining the message body length.
1514</t>
1515<t>
1516   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1517   Transfer-Encoding is a property of the message, not of the payload, and thus
1518   &MAY; be added or removed by any implementation along the request/response
1519   chain. Additional information about the encoding parameters &MAY; be
1520   provided by other header fields not defined by this specification.
1521</t>
1522<t>
1523   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1524   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1525   neither of which includes a message body,
1526   to indicate that the origin server would have applied a transfer coding
1527   to the message body if the request had been an unconditional GET.
1528   This indication is not required, however, because any recipient on
1529   the response chain (including the origin server) can remove transfer
1530   codings when they are not needed.
1531</t>
1532<t>
1533   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1534   implementations advertising only HTTP/1.0 support will not understand
1535   how to process a transfer-encoded payload.
1536   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1537   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1538   might be in the form of specific user configuration or by remembering the
1539   version of a prior received response.
1540   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1541   the corresponding request indicates HTTP/1.1 (or later).
1542</t>
1543<t>
1544   A server that receives a request message with a transfer-coding it does
1545   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref> and then
1546   close the connection.
1547</t>
1548</section>
1549
1550<section title="Content-Length" anchor="header.content-length">
1551  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1552  <x:anchor-alias value="Content-Length"/>
1553<t>
1554   When a message is allowed to contain a message body, does not have a
1555   <x:ref>Transfer-Encoding</x:ref> header field, and has a payload body
1556   length that is known to the sender before the message header section has
1557   been sent, the sender &SHOULD; send a Content-Length header field to
1558   indicate the length of the payload body as a decimal number of octets.
1559</t>
1560<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1561  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1562</artwork></figure>
1563<t>
1564   An example is
1565</t>
1566<figure><artwork type="example">
1567  Content-Length: 3495
1568</artwork></figure>
1569<t>
1570   A sender &MUST-NOT; send a Content-Length header field in any message that
1571   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1572</t>
1573<t>
1574   A server &MAY; send a Content-Length header field in a response to a HEAD
1575   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1576   response unless its field-value equals the decimal number of octets that
1577   would have been sent in the payload body of a response if the same
1578   request had used the GET method.
1579</t>
1580<t>
1581   A server &MAY; send a Content-Length header field in a
1582   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1583   (&status-304;); a server &MUST-NOT; send Content-Length in such a
1584   response unless its field-value equals the decimal number of octets that
1585   would have been sent in the payload body of a <x:ref>200 (OK)</x:ref>
1586   response to the same request.
1587</t>
1588<t>
1589   A server &MUST-NOT; send a Content-Length header field in any response
1590   with a status code of
1591   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1592   A server &SHOULD-NOT; send a Content-Length header field in any
1593   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1594</t>
1595<t>
1596   Any Content-Length field value greater than or equal to zero is valid.
1597   Since there is no predefined limit to the length of an HTTP payload,
1598   recipients &SHOULD; anticipate potentially large decimal numerals and
1599   prevent parsing errors due to integer conversion overflows
1600   (<xref target="attack.protocol.element.size.overflows"/>).
1601</t>
1602<t>
1603   If a message is received that has multiple Content-Length header fields
1604   with field-values consisting of the same decimal value, or a single
1605   Content-Length header field with a field value containing a list of
1606   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1607   duplicate Content-Length header fields have been generated or combined by an
1608   upstream message processor, then the recipient &MUST; either reject the
1609   message as invalid or replace the duplicated field-values with a single
1610   valid Content-Length field containing that decimal value prior to
1611   determining the message body length.
1612</t>
1613<x:note>
1614  <t>
1615   &Note; HTTP's use of Content-Length for message framing differs
1616   significantly from the same field's use in MIME, where it is an optional
1617   field used only within the "message/external-body" media-type.
1618  </t>
1619</x:note>
1620</section>
1621
1622<section title="Message Body Length" anchor="message.body.length">
1623<t>
1624   The length of a message body is determined by one of the following
1625   (in order of precedence):
1626</t>
1627<t>
1628  <list style="numbers">
1629    <x:lt><t>
1630     Any response to a HEAD request and any response with a
1631     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1632     <x:ref>304 (Not Modified)</x:ref> status code is always
1633     terminated by the first empty line after the header fields, regardless of
1634     the header fields present in the message, and thus cannot contain a
1635     message body.
1636    </t></x:lt>
1637    <x:lt><t>
1638     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1639     connection will become a tunnel immediately after the empty line that
1640     concludes the header fields.  A client &MUST; ignore any
1641     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1642     fields received in such a message.
1643    </t></x:lt>
1644    <x:lt><t>
1645     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1646     and the "chunked" transfer-coding (<xref target="chunked.encoding"/>)
1647     is the final encoding, the message body length is determined by reading
1648     and decoding the chunked data until the transfer-coding indicates the
1649     data is complete.
1650    </t>
1651    <t>
1652     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1653     response and the "chunked" transfer-coding is not the final encoding, the
1654     message body length is determined by reading the connection until it is
1655     closed by the server.
1656     If a Transfer-Encoding header field is present in a request and the
1657     "chunked" transfer-coding is not the final encoding, the message body
1658     length cannot be determined reliably; the server &MUST; respond with
1659     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1660    </t>
1661    <t>
1662     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1663     and a <x:ref>Content-Length</x:ref> header field, the
1664     Transfer-Encoding overrides the Content-Length.
1665     Such a message might indicate an attempt to perform request or response
1666     smuggling (bypass of security-related checks on message routing or content)
1667     and thus ought to be handled as an error.  The provided Content-Length &MUST;
1668     be removed, prior to forwarding the message downstream, or replaced with
1669     the real message body length after the transfer-coding is decoded.
1670    </t></x:lt>
1671    <x:lt><t>
1672     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1673     either multiple <x:ref>Content-Length</x:ref> header fields having
1674     differing field-values or a single Content-Length header field having an
1675     invalid value, then the message framing is invalid and &MUST; be treated
1676     as an error to prevent request or response smuggling.
1677     If this is a request message, the server &MUST; respond with
1678     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1679     If this is a response message received by a proxy, the proxy
1680     &MUST; discard the received response, send a <x:ref>502 (Bad Gateway)</x:ref>
1681     status code as its downstream response, and then close the connection.
1682     If this is a response message received by a user agent, it &MUST; be
1683     treated as an error by discarding the message and closing the connection.
1684    </t></x:lt>
1685    <x:lt><t>
1686     If a valid <x:ref>Content-Length</x:ref> header field is present without
1687     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1688     message body length in octets.  If the actual number of octets sent in
1689     the message is less than the indicated Content-Length, the recipient
1690     &MUST; consider the message to be incomplete and treat the connection
1691     as no longer usable.
1692     If the actual number of octets sent in the message is more than the indicated
1693     Content-Length, the recipient &MUST; only process the message body up to the
1694     field value's number of octets; the remainder of the message &MUST; either
1695     be discarded or treated as the next message in a pipeline.  For the sake of
1696     robustness, a user agent &MAY; attempt to detect and correct such an error
1697     in message framing if it is parsing the response to the last request on
1698     a connection and the connection has been closed by the server.
1699    </t></x:lt>
1700    <x:lt><t>
1701     If this is a request message and none of the above are true, then the
1702     message body length is zero (no message body is present).
1703    </t></x:lt>
1704    <x:lt><t>
1705     Otherwise, this is a response message without a declared message body
1706     length, so the message body length is determined by the number of octets
1707     received prior to the server closing the connection.
1708    </t></x:lt>
1709  </list>
1710</t>
1711<t>
1712   Since there is no way to distinguish a successfully completed,
1713   close-delimited message from a partially-received message interrupted
1714   by network failure, a server &SHOULD; use encoding or
1715   length-delimited messages whenever possible.  The close-delimiting
1716   feature exists primarily for backwards compatibility with HTTP/1.0.
1717</t>
1718<t>
1719   A server &MAY; reject a request that contains a message body but
1720   not a <x:ref>Content-Length</x:ref> by responding with
1721   <x:ref>411 (Length Required)</x:ref>.
1722</t>
1723<t>
1724   Unless a transfer-coding other than "chunked" has been applied,
1725   a client that sends a request containing a message body &SHOULD;
1726   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1727   length is known in advance, rather than the "chunked" encoding, since some
1728   existing services respond to "chunked" with a <x:ref>411 (Length Required)</x:ref>
1729   status code even though they understand the chunked encoding.  This
1730   is typically because such services are implemented via a gateway that
1731   requires a content-length in advance of being called and the server
1732   is unable or unwilling to buffer the entire request before processing.
1733</t>
1734<t>
1735   A client that sends a request containing a message body &MUST; include a
1736   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1737   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1738   the form of specific user configuration or by remembering the version of a
1739   prior received response.
1740</t>
1741</section>
1742</section>
1743
1744<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1745<t>
1746   Request messages that are prematurely terminated, possibly due to a
1747   canceled connection or a server-imposed time-out exception, &MUST;
1748   result in closure of the connection; sending an error response
1749   prior to closing the connection is &OPTIONAL;.
1750</t>
1751<t>
1752   Response messages that are prematurely terminated, usually by closure
1753   of the connection prior to receiving the expected number of octets or by
1754   failure to decode a transfer-encoded message body, &MUST; be recorded
1755   as incomplete.  A response that terminates in the middle of the header
1756   block (before the empty line is received) cannot be assumed to convey the
1757   full semantics of the response and &MUST; be treated as an error.
1758</t>
1759<t>
1760   A message body that uses the chunked transfer encoding is
1761   incomplete if the zero-sized chunk that terminates the encoding has not
1762   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1763   incomplete if the size of the message body received (in octets) is less than
1764   the value given by Content-Length.  A response that has neither chunked
1765   transfer encoding nor Content-Length is terminated by closure of the
1766   connection, and thus is considered complete regardless of the number of
1767   message body octets received, provided that the header block was received
1768   intact.
1769</t>
1770<t>
1771   A user agent &MUST-NOT; render an incomplete response message body as if
1772   it were complete (i.e., some indication needs to be given to the user that an
1773   error occurred).  Cache requirements for incomplete responses are defined
1774   in &cache-incomplete;.
1775</t>
1776<t>
1777   A server &MUST; read the entire request message body or close
1778   the connection after sending its response, since otherwise the
1779   remaining data on a persistent connection would be misinterpreted
1780   as the next request.  Likewise,
1781   a client &MUST; read the entire response message body if it intends
1782   to reuse the same connection for a subsequent request.  Pipelining
1783   multiple requests on a connection is described in <xref target="pipelining"/>.
1784</t>
1785</section>
1786
1787<section title="Message Parsing Robustness" anchor="message.robustness">
1788<t>
1789   Older HTTP/1.0 client implementations might send an extra CRLF
1790   after a POST request as a lame workaround for some early server
1791   applications that failed to read message body content that was
1792   not terminated by a line-ending. An HTTP/1.1 client &MUST-NOT;
1793   preface or follow a request with an extra CRLF.  If terminating
1794   the request message body with a line-ending is desired, then the
1795   client &MUST; include the terminating CRLF octets as part of the
1796   message body length.
1797</t>
1798<t>
1799   In the interest of robustness, servers &SHOULD; ignore at least one
1800   empty line received where a request-line is expected. In other words, if
1801   the server is reading the protocol stream at the beginning of a
1802   message and receives a CRLF first, it &SHOULD; ignore the CRLF.
1803   Likewise, although the line terminator for the start-line and header
1804   fields is the sequence CRLF, we recommend that recipients recognize a
1805   single LF as a line terminator and ignore any CR.
1806</t>
1807<t>
1808   When a server listening only for HTTP request messages, or processing
1809   what appears from the start-line to be an HTTP request message,
1810   receives a sequence of octets that does not match the HTTP-message
1811   grammar aside from the robustness exceptions listed above, the
1812   server &MUST; respond with an HTTP/1.1 <x:ref>400 (Bad Request)</x:ref> response. 
1813</t>
1814</section>
1815</section>
1816
1817<section title="Transfer Codings" anchor="transfer.codings">
1818  <x:anchor-alias value="transfer-coding"/>
1819  <x:anchor-alias value="transfer-extension"/>
1820<t>
1821   Transfer-coding values are used to indicate an encoding
1822   transformation that has been, can be, or might need to be applied to a
1823   payload body in order to ensure "safe transport" through the network.
1824   This differs from a content coding in that the transfer-coding is a
1825   property of the message rather than a property of the representation
1826   that is being transferred.
1827</t>
1828<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1829  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1830                     / "compress" ; <xref target="compress.coding"/>
1831                     / "deflate" ; <xref target="deflate.coding"/>
1832                     / "gzip" ; <xref target="gzip.coding"/>
1833                     / <x:ref>transfer-extension</x:ref>
1834  <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> )
1835</artwork></figure>
1836<t anchor="rule.parameter">
1837  <x:anchor-alias value="attribute"/>
1838  <x:anchor-alias value="transfer-parameter"/>
1839  <x:anchor-alias value="value"/>
1840   Parameters are in the form of attribute/value pairs.
1841</t>
1842<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"/>
1843  <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>
1844  <x:ref>attribute</x:ref>          = <x:ref>token</x:ref>
1845  <x:ref>value</x:ref>              = <x:ref>word</x:ref>
1846</artwork></figure>
1847<t>
1848   All transfer-coding values are case-insensitive and &SHOULD; be registered
1849   within the HTTP Transfer Coding registry, as defined in
1850   <xref target="transfer.coding.registry"/>.
1851   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1852   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1853   header fields.
1854</t>
1855
1856<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1857  <iref item="chunked (Coding Format)"/>
1858  <x:anchor-alias value="chunk"/>
1859  <x:anchor-alias value="chunked-body"/>
1860  <x:anchor-alias value="chunk-data"/>
1861  <x:anchor-alias value="chunk-ext"/>
1862  <x:anchor-alias value="chunk-ext-name"/>
1863  <x:anchor-alias value="chunk-ext-val"/>
1864  <x:anchor-alias value="chunk-size"/>
1865  <x:anchor-alias value="last-chunk"/>
1866  <x:anchor-alias value="trailer-part"/>
1867  <x:anchor-alias value="quoted-str-nf"/>
1868  <x:anchor-alias value="qdtext-nf"/>
1869<t>
1870   The chunked encoding modifies the body of a message in order to
1871   transfer it as a series of chunks, each with its own size indicator,
1872   followed by an &OPTIONAL; trailer containing header fields. This
1873   allows dynamically produced content to be transferred along with the
1874   information necessary for the recipient to verify that it has
1875   received the full message.
1876</t>
1877<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"/>
1878  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1879                   <x:ref>last-chunk</x:ref>
1880                   <x:ref>trailer-part</x:ref>
1881                   <x:ref>CRLF</x:ref>
1882 
1883  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1884                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1885  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1886  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1887 
1888  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
1889  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
1890  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-str-nf</x:ref>
1891  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1892  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1893 
1894  <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>
1895                 ; like <x:ref>quoted-string</x:ref>, but disallowing line folding
1896  <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>
1897</artwork></figure>
1898<t>
1899   Chunk extensions within the chucked encoding are deprecated.
1900   Senders &SHOULD-NOT; send chunk-ext.
1901   Definition of new chunk extensions is discouraged.
1902</t>
1903<t>
1904   The chunk-size field is a string of hex digits indicating the size of
1905   the chunk-data in octets. The chunked encoding is ended by any chunk whose size is
1906   zero, followed by the trailer, which is terminated by an empty line.
1907</t>
1908
1909<section title="Trailer" anchor="header.trailer">
1910  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
1911  <x:anchor-alias value="Trailer"/>
1912<t>
1913   A trailer allows the sender to include additional fields at the end of a
1914   chunked message in order to supply metadata that might be dynamically
1915   generated while the message body is sent, such as a message integrity
1916   check, digital signature, or post-processing status.
1917   The trailer &MUST-NOT; contain fields that need to be known before a
1918   recipient processes the body, such as <x:ref>Transfer-Encoding</x:ref>,
1919   <x:ref>Content-Length</x:ref>, and <x:ref>Trailer</x:ref>.
1920</t>
1921<t>
1922   When a message includes a message body encoded with the chunked
1923   transfer-coding and the sender desires to send metadata in the form of
1924   trailer fields at the end of the message, the sender &SHOULD; send a
1925   <x:ref>Trailer</x:ref> header field before the message body to indicate
1926   which fields will be present in the trailers. This allows the recipient
1927   to prepare for receipt of that metadata before it starts processing the body,
1928   which is useful if the message is being streamed and the recipient wishes
1929   to confirm an integrity check on the fly.
1930</t>
1931<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
1932  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
1933</artwork></figure>
1934<t>
1935   If no <x:ref>Trailer</x:ref> header field is present, the sender of a
1936   chunked message body &SHOULD; send an empty trailer.
1937</t>
1938<t>
1939   A server &MUST; send an empty trailer with the chunked transfer-coding
1940   unless at least one of the following is true:
1941  <list style="numbers">
1942    <t>the request included a <x:ref>TE</x:ref> header field that indicates
1943    "trailers" is acceptable in the transfer-coding of the response, as
1944    described in <xref target="header.te"/>; or,</t>
1945     
1946    <t>the trailer fields consist entirely of optional metadata and the
1947    recipient could use the message (in a manner acceptable to the server where
1948    the field originated) without receiving that metadata. In other words,
1949    the server that generated the header field is willing to accept the
1950    possibility that the trailer fields might be silently discarded along
1951    the path to the client.</t>
1952  </list>
1953</t>
1954<t>
1955   The above requirement prevents the need for an infinite buffer when a
1956   message is being received by an HTTP/1.1 (or later) proxy and forwarded to
1957   an HTTP/1.0 recipient.
1958</t>
1959</section>
1960
1961<section title="Decoding chunked" anchor="decoding.chunked">
1962<t>
1963   A process for decoding the "chunked" transfer-coding
1964   can be represented in pseudo-code as:
1965</t>
1966<figure><artwork type="code">
1967  length := 0
1968  read chunk-size, chunk-ext (if any) and CRLF
1969  while (chunk-size &gt; 0) {
1970     read chunk-data and CRLF
1971     append chunk-data to decoded-body
1972     length := length + chunk-size
1973     read chunk-size and CRLF
1974  }
1975  read header-field
1976  while (header-field not empty) {
1977     append header-field to existing header fields
1978     read header-field
1979  }
1980  Content-Length := length
1981  Remove "chunked" from Transfer-Encoding
1982  Remove Trailer from existing header fields
1983</artwork></figure>
1984<t>
1985   All recipients &MUST; be able to receive and decode the
1986   "chunked" transfer-coding and &MUST; ignore chunk-ext extensions
1987   they do not understand.
1988</t>
1989</section>
1990</section>
1991
1992<section title="Compression Codings" anchor="compression.codings">
1993<t>
1994   The codings defined below can be used to compress the payload of a
1995   message.
1996</t>
1997
1998<section title="Compress Coding" anchor="compress.coding">
1999<iref item="compress (Coding Format)"/>
2000<t>
2001   The "compress" format is produced by the common UNIX file compression
2002   program "compress". This format is an adaptive Lempel-Ziv-Welch
2003   coding (LZW). Recipients &SHOULD; consider "x-compress" to be
2004   equivalent to "compress".
2005</t>
2006</section>
2007
2008<section title="Deflate Coding" anchor="deflate.coding">
2009<iref item="deflate (Coding Format)"/>
2010<t>
2011   The "deflate" format is defined as the "deflate" compression mechanism
2012   (described in <xref target="RFC1951"/>) used inside the "zlib"
2013   data format (<xref target="RFC1950"/>).
2014</t>
2015<x:note>
2016  <t>
2017    &Note; Some incorrect implementations send the "deflate"
2018    compressed data without the zlib wrapper.
2019   </t>
2020</x:note>
2021</section>
2022
2023<section title="Gzip Coding" anchor="gzip.coding">
2024<iref item="gzip (Coding Format)"/>
2025<t>
2026   The "gzip" format is produced by the file compression program
2027   "gzip" (GNU zip), as described in <xref target="RFC1952"/>. This format is a
2028   Lempel-Ziv coding (LZ77) with a 32 bit CRC.
2029   Recipients &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2030</t>
2031</section>
2032
2033</section>
2034
2035<section title="TE" anchor="header.te">
2036  <iref primary="true" item="TE header field" x:for-anchor=""/>
2037  <x:anchor-alias value="TE"/>
2038  <x:anchor-alias value="t-codings"/>
2039  <x:anchor-alias value="t-ranking"/>
2040  <x:anchor-alias value="rank"/>
2041<t>
2042   The "TE" header field in a request indicates what transfer-codings,
2043   besides "chunked", the client is willing to accept in response, and
2044   whether or not the client is willing to accept trailer fields in a
2045   chunked transfer-coding.
2046</t>
2047<t>
2048   The TE field-value consists of a comma-separated list of transfer-coding
2049   names, each allowing for optional parameters (as described in
2050   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2051   Clients &MUST-NOT; send the chunked transfer-coding name in TE;
2052   chunked is always acceptable for HTTP/1.1 recipients.
2053</t>
2054<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"/>
2055  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2056  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2057  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2058  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2059             / ( "1" [ "." 0*3("0") ] )
2060</artwork></figure>
2061<t>
2062   Three examples of TE use are below.
2063</t>
2064<figure><artwork type="example">
2065  TE: deflate
2066  TE:
2067  TE: trailers, deflate;q=0.5
2068</artwork></figure>
2069<t>
2070   The presence of the keyword "trailers" indicates that the client is
2071   willing to accept trailer fields in a chunked transfer-coding,
2072   as defined in <xref target="chunked.encoding"/>, on behalf of itself and
2073   any downstream clients. For chained requests, this implies that either:
2074   (a) all downstream clients are willing to accept trailer fields in the
2075   forwarded response; or,
2076   (b) the client will attempt to buffer the response on behalf of downstream
2077   recipients.
2078   Note that HTTP/1.1 does not define any means to limit the size of a
2079   chunked response such that a client can be assured of buffering the
2080   entire response.
2081</t>
2082<t>
2083   When multiple transfer-codings are acceptable, the client &MAY; rank the
2084   codings by preference using a case-insensitive "q" parameter (similar to
2085   the qvalues used in content negotiation fields, &qvalue;). The rank value
2086   is a real number in the range 0 through 1, where 0.001 is the least
2087   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2088</t>
2089<t>
2090   If the TE field-value is empty or if no TE field is present, the only
2091   acceptable transfer-coding is "chunked". A message with no transfer-coding
2092   is always acceptable.
2093</t>
2094<t>
2095   Since the TE header field only applies to the immediate connection,
2096   a sender of TE &MUST; also send a "TE" connection option within the
2097   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2098   in order to prevent the TE field from being forwarded by intermediaries
2099   that do not support its semantics.
2100</t>
2101</section>
2102</section>
2103
2104<section title="Message Routing" anchor="message.routing">
2105<t>
2106   HTTP request message routing is determined by each client based on the
2107   target resource, the client's proxy configuration, and
2108   establishment or reuse of an inbound connection.  The corresponding
2109   response routing follows the same connection chain back to the client.
2110</t>
2111
2112<section title="Identifying a Target Resource" anchor="target-resource">
2113  <iref primary="true" item="target resource"/>
2114  <iref primary="true" item="target URI"/>
2115  <x:anchor-alias value="target resource"/>
2116  <x:anchor-alias value="target URI"/>
2117<t>
2118   HTTP is used in a wide variety of applications, ranging from
2119   general-purpose computers to home appliances.  In some cases,
2120   communication options are hard-coded in a client's configuration.
2121   However, most HTTP clients rely on the same resource identification
2122   mechanism and configuration techniques as general-purpose Web browsers.
2123</t>
2124<t>
2125   HTTP communication is initiated by a user agent for some purpose.
2126   The purpose is a combination of request semantics, which are defined in
2127   <xref target="Part2"/>, and a target resource upon which to apply those
2128   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2129   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2130   would resolve to its absolute form in order to obtain the
2131   "<x:dfn>target URI</x:dfn>".  The target URI
2132   excludes the reference's fragment identifier component, if any,
2133   since fragment identifiers are reserved for client-side processing
2134   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2135</t>
2136</section>
2137
2138<section title="Connecting Inbound" anchor="connecting.inbound">
2139<t>
2140   Once the target URI is determined, a client needs to decide whether
2141   a network request is necessary to accomplish the desired semantics and,
2142   if so, where that request is to be directed.
2143</t>
2144<t>
2145   If the client has a response cache and the request semantics can be
2146   satisfied by a cache (<xref target="Part6"/>), then the request is
2147   usually directed to the cache first.
2148</t>
2149<t>
2150   If the request is not satisfied by a cache, then a typical client will
2151   check its configuration to determine whether a proxy is to be used to
2152   satisfy the request.  Proxy configuration is implementation-dependent,
2153   but is often based on URI prefix matching, selective authority matching,
2154   or both, and the proxy itself is usually identified by an "http" or
2155   "https" URI.  If a proxy is applicable, the client connects inbound by
2156   establishing (or reusing) a connection to that proxy.
2157</t>
2158<t>
2159   If no proxy is applicable, a typical client will invoke a handler routine,
2160   usually specific to the target URI's scheme, to connect directly
2161   to an authority for the target resource.  How that is accomplished is
2162   dependent on the target URI scheme and defined by its associated
2163   specification, similar to how this specification defines origin server
2164   access for resolution of the "http" (<xref target="http.uri"/>) and
2165   "https" (<xref target="https.uri"/>) schemes.
2166</t>
2167<t>
2168   HTTP requirements regarding connection management are defined in
2169   <xref target="connection.management"/>.
2170</t>
2171</section>
2172
2173<section title="Request Target" anchor="request-target">
2174<t>
2175   Once an inbound connection is obtained,
2176   the client sends an HTTP request message (<xref target="http.message"/>)
2177   with a request-target derived from the target URI.
2178   There are four distinct formats for the request-target, depending on both
2179   the method being requested and whether the request is to a proxy.
2180</t>   
2181<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"/>
2182  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2183                 / <x:ref>absolute-form</x:ref>
2184                 / <x:ref>authority-form</x:ref>
2185                 / <x:ref>asterisk-form</x:ref>
2186
2187  <x:ref>origin-form</x:ref>    = <x:ref>path-absolute</x:ref> [ "?" <x:ref>query</x:ref> ]
2188  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2189  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2190  <x:ref>asterisk-form</x:ref>  = "*"
2191</artwork></figure>
2192<t anchor="origin-form"><iref item="origin-form (of request-target)"/>
2193   The most common form of request-target is the origin-form.
2194   When making a request directly to an origin server, other than a CONNECT
2195   or server-wide OPTIONS request (as detailed below),
2196   a client &MUST; send only the absolute path and query components of
2197   the target URI as the request-target.
2198   If the target URI's path component is empty, then the client &MUST; send
2199   "/" as the path within the origin-form of request-target.
2200   A <x:ref>Host</x:ref> header field is also sent, as defined in
2201   <xref target="header.host"/>, containing the target URI's
2202   authority component (excluding any userinfo).
2203</t>
2204<t>
2205   For example, a client wishing to retrieve a representation of the resource
2206   identified as
2207</t>
2208<figure><artwork x:indent-with="  " type="example">
2209http://www.example.org/where?q=now
2210</artwork></figure>
2211<t>
2212   directly from the origin server would open (or reuse) a TCP connection
2213   to port 80 of the host "www.example.org" and send the lines:
2214</t>
2215<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2216GET /where?q=now HTTP/1.1
2217Host: www.example.org
2218</artwork></figure>
2219<t>
2220   followed by the remainder of the request message.
2221</t>
2222<t anchor="absolute-form"><iref item="absolute-form (of request-target)"/>
2223   When making a request to a proxy, other than a CONNECT or server-wide
2224   OPTIONS request (as detailed below), a client &MUST; send the target URI
2225   in absolute-form as the request-target.
2226   The proxy is requested to either service that request from a valid cache,
2227   if possible, or make the same request on the client's behalf to either
2228   the next inbound proxy server or directly to the origin server indicated
2229   by the request-target.  Requirements on such "forwarding" of messages are
2230   defined in <xref target="message.forwarding"/>.
2231</t>
2232<t>
2233   An example absolute-form of request-line would be:
2234</t>
2235<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2236GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
2237</artwork></figure>
2238<t>
2239   To allow for transition to the absolute-form for all requests in some
2240   future version of HTTP, HTTP/1.1 servers &MUST; accept the absolute-form
2241   in requests, even though HTTP/1.1 clients will only send them in requests
2242   to proxies.
2243</t>
2244<t anchor="authority-form"><iref item="authority-form (of request-target)"/>
2245   The authority-form of request-target is only used for CONNECT requests
2246   (&CONNECT;).  When making a CONNECT request to establish a tunnel through
2247   one or more proxies, a client &MUST; send only the target URI's
2248   authority component (excluding any userinfo) as the request-target.
2249   For example,
2250</t>
2251<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2252CONNECT www.example.com:80 HTTP/1.1
2253</artwork></figure>
2254<t anchor="asterisk-form"><iref item="asterisk-form (of request-target)"/>
2255   The asterisk-form of request-target is only used for a server-wide
2256   OPTIONS request (&OPTIONS;).  When a client wishes to request OPTIONS
2257   for the server as a whole, as opposed to a specific named resource of
2258   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2259   For example,
2260</t>
2261<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2262OPTIONS * HTTP/1.1
2263</artwork></figure>
2264<t>
2265   If a proxy receives an OPTIONS request with an absolute-form of
2266   request-target in which the URI has an empty path and no query component,
2267   then the last proxy on the request chain &MUST; send a request-target
2268   of "*" when it forwards the request to the indicated origin server.
2269</t>
2270<figure><preamble>   
2271   For example, the request
2272</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2273OPTIONS http://www.example.org:8001 HTTP/1.1
2274</artwork></figure>
2275<figure><preamble>   
2276  would be forwarded by the final proxy as
2277</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2278OPTIONS * HTTP/1.1
2279Host: www.example.org:8001
2280</artwork>
2281<postamble>
2282   after connecting to port 8001 of host "www.example.org".
2283</postamble>
2284</figure>
2285</section>
2286
2287<section title="Host" anchor="header.host">
2288  <iref primary="true" item="Host header field" x:for-anchor=""/>
2289  <x:anchor-alias value="Host"/>
2290<t>
2291   The "Host" header field in a request provides the host and port
2292   information from the target URI, enabling the origin
2293   server to distinguish among resources while servicing requests
2294   for multiple host names on a single IP address.  Since the Host
2295   field-value is critical information for handling a request, it
2296   &SHOULD; be sent as the first header field following the request-line.
2297</t>
2298<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2299  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2300</artwork></figure>
2301<t>
2302   A client &MUST; send a Host header field in all HTTP/1.1 request
2303   messages.  If the target URI includes an authority component, then
2304   the Host field-value &MUST; be identical to that authority component
2305   after excluding any userinfo (<xref target="http.uri"/>).
2306   If the authority component is missing or undefined for the target URI,
2307   then the Host header field &MUST; be sent with an empty field-value.
2308</t>
2309<t>
2310   For example, a GET request to the origin server for
2311   &lt;http://www.example.org/pub/WWW/&gt; would begin with:
2312</t>
2313<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2314GET /pub/WWW/ HTTP/1.1
2315Host: www.example.org
2316</artwork></figure>
2317<t>
2318   The Host header field &MUST; be sent in an HTTP/1.1 request even
2319   if the request-target is in the absolute-form, since this
2320   allows the Host information to be forwarded through ancient HTTP/1.0
2321   proxies that might not have implemented Host.
2322</t>
2323<t>
2324   When a proxy receives a request with an absolute-form of
2325   request-target, the proxy &MUST; ignore the received
2326   Host header field (if any) and instead replace it with the host
2327   information of the request-target.  If the proxy forwards the request,
2328   it &MUST; generate a new Host field-value based on the received
2329   request-target rather than forward the received Host field-value.
2330</t>
2331<t>
2332   Since the Host header field acts as an application-level routing
2333   mechanism, it is a frequent target for malware seeking to poison
2334   a shared cache or redirect a request to an unintended server.
2335   An interception proxy is particularly vulnerable if it relies on
2336   the Host field-value for redirecting requests to internal
2337   servers, or for use as a cache key in a shared cache, without
2338   first verifying that the intercepted connection is targeting a
2339   valid IP address for that host.
2340</t>
2341<t>
2342   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2343   to any HTTP/1.1 request message that lacks a Host header field and
2344   to any request message that contains more than one Host header field
2345   or a Host header field with an invalid field-value.
2346</t>
2347</section>
2348
2349<section title="Effective Request URI" anchor="effective.request.uri">
2350  <iref primary="true" item="effective request URI"/>
2351<t>
2352   A server that receives an HTTP request message &MUST; reconstruct
2353   the user agent's original target URI, based on the pieces of information
2354   learned from the request-target, <x:ref>Host</x:ref> header field, and
2355   connection context, in order to identify the intended target resource and
2356   properly service the request. The URI derived from this reconstruction
2357   process is referred to as the "<x:dfn>effective request URI</x:dfn>".
2358</t>
2359<t>
2360   For a user agent, the effective request URI is the target URI.
2361</t>
2362<t>
2363   If the request-target is in absolute-form, then the effective request URI
2364   is the same as the request-target.  Otherwise, the effective request URI
2365   is constructed as follows.
2366</t>
2367<t>
2368   If the request is received over a TLS-secured TCP connection,
2369   then the effective request URI's scheme is "https"; otherwise, the
2370   scheme is "http".
2371</t>
2372<t>
2373   If the request-target is in authority-form, then the effective
2374   request URI's authority component is the same as the request-target.
2375   Otherwise, if a <x:ref>Host</x:ref> header field is supplied with a
2376   non-empty field-value, then the authority component is the same as the
2377   Host field-value. Otherwise, the authority component is the concatenation of
2378   the default host name configured for the server, a colon (":"), and the
2379   connection's incoming TCP port number in decimal form.
2380</t>
2381<t>
2382   If the request-target is in authority-form or asterisk-form, then the
2383   effective request URI's combined path and query component is empty.
2384   Otherwise, the combined path and query component is the same as the
2385   request-target.
2386</t>
2387<t>
2388   The components of the effective request URI, once determined as above,
2389   can be combined into absolute-URI form by concatenating the scheme,
2390   "://", authority, and combined path and query component.
2391</t>
2392<figure>
2393<preamble>
2394   Example 1: the following message received over an insecure TCP connection
2395</preamble> 
2396<artwork type="example" x:indent-with="  ">
2397GET /pub/WWW/TheProject.html HTTP/1.1
2398Host: www.example.org:8080
2399</artwork>
2400</figure>
2401<figure>
2402<preamble>
2403  has an effective request URI of
2404</preamble>
2405<artwork type="example" x:indent-with="  ">
2406http://www.example.org:8080/pub/WWW/TheProject.html
2407</artwork>
2408</figure>
2409<figure>
2410<preamble>
2411   Example 2: the following message received over a TLS-secured TCP connection
2412</preamble> 
2413<artwork type="example" x:indent-with="  ">
2414OPTIONS * HTTP/1.1
2415Host: www.example.org
2416</artwork>
2417</figure>
2418<figure>
2419<preamble>
2420  has an effective request URI of
2421</preamble>
2422<artwork type="example" x:indent-with="  ">
2423https://www.example.org
2424</artwork>
2425</figure>
2426<t>
2427   An origin server that does not allow resources to differ by requested
2428   host &MAY; ignore the <x:ref>Host</x:ref> field-value and instead replace it
2429   with a configured server name when constructing the effective request URI.
2430</t>
2431<t>
2432   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2433   field &MAY; attempt to use heuristics (e.g., examination of the URI path for
2434   something unique to a particular host) in order to guess the
2435   effective request URI's authority component.
2436</t>
2437</section>
2438
2439<section title="Message Forwarding" anchor="message.forwarding">
2440<t>
2441   As described in <xref target="intermediaries"/>, intermediaries can serve
2442   a variety of roles in the processing of HTTP requests and responses.
2443   Some intermediaries are used to improve performance or availability.
2444   Others are used for access control or to filter content.
2445   Since an HTTP stream has characteristics similar to a pipe-and-filter
2446   architecture, there are no inherent limits to the extent an intermediary
2447   can enhance (or interfere) with either direction of the stream.
2448</t>
2449<t>
2450   Intermediaries that forward a message &MUST; implement the
2451   <x:ref>Connection</x:ref> header field, as specified in
2452   <xref target="header.connection"/>, to exclude fields that are only
2453   intended for the incoming connection.
2454</t>
2455<t>
2456   In order to avoid request loops, a proxy that forwards requests to other
2457   proxies &MUST; be able to recognize and exclude all of its own server
2458   names, including any aliases, local variations, or literal IP addresses.
2459</t>
2460</section>
2461
2462<section title="Via" anchor="header.via">
2463  <iref primary="true" item="Via header field" x:for-anchor=""/>
2464  <x:anchor-alias value="pseudonym"/>
2465  <x:anchor-alias value="received-by"/>
2466  <x:anchor-alias value="received-protocol"/>
2467  <x:anchor-alias value="Via"/>
2468<t>
2469   The "Via" header field &MUST; be sent by a proxy or gateway in forwarded
2470   messages to indicate the intermediate protocols and recipients between the
2471   user agent and the server on requests, and between the origin server and
2472   the client on responses. It is analogous to the "Received" field
2473   used by email systems (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2474   Via is used in HTTP for tracking message forwards,
2475   avoiding request loops, and identifying the protocol capabilities of
2476   all senders along the request/response chain.
2477</t>
2478<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"/>
2479  <x:ref>Via</x:ref>               = 1#( <x:ref>received-protocol</x:ref> <x:ref>RWS</x:ref> <x:ref>received-by</x:ref>
2480                          [ <x:ref>RWS</x:ref> <x:ref>comment</x:ref> ] )
2481  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2482                      ; see <xref target="header.upgrade"/>
2483  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2484  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2485</artwork></figure>
2486<t>
2487   The received-protocol indicates the protocol version of the message
2488   received by the server or client along each segment of the
2489   request/response chain. The received-protocol version is appended to
2490   the Via field value when the message is forwarded so that information
2491   about the protocol capabilities of upstream applications remains
2492   visible to all recipients.
2493</t>
2494<t>
2495   The protocol-name is excluded if and only if it would be "HTTP". The
2496   received-by field is normally the host and optional port number of a
2497   recipient server or client that subsequently forwarded the message.
2498   However, if the real host is considered to be sensitive information,
2499   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2500   be assumed to be the default port of the received-protocol.
2501</t>
2502<t>
2503   Multiple Via field values represent each proxy or gateway that has
2504   forwarded the message. Each recipient &MUST; append its information
2505   such that the end result is ordered according to the sequence of
2506   forwarding applications.
2507</t>
2508<t>
2509   Comments &MAY; be used in the Via header field to identify the software
2510   of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2511   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2512   are optional and &MAY; be removed by any recipient prior to forwarding the
2513   message.
2514</t>
2515<t>
2516   For example, a request message could be sent from an HTTP/1.0 user
2517   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2518   forward the request to a public proxy at p.example.net, which completes
2519   the request by forwarding it to the origin server at www.example.com.
2520   The request received by www.example.com would then have the following
2521   Via header field:
2522</t>
2523<figure><artwork type="example">
2524  Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
2525</artwork></figure>
2526<t>
2527   A proxy or gateway used as a portal through a network firewall
2528   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2529   region unless it is explicitly enabled to do so. If not enabled, the
2530   received-by host of any host behind the firewall &SHOULD; be replaced
2531   by an appropriate pseudonym for that host.
2532</t>
2533<t>
2534   A proxy or gateway &MAY; combine an ordered subsequence of Via header
2535   field entries into a single such entry if the entries have identical
2536   received-protocol values. For example,
2537</t>
2538<figure><artwork type="example">
2539  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2540</artwork></figure>
2541<t>
2542  could be collapsed to
2543</t>
2544<figure><artwork type="example">
2545  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2546</artwork></figure>
2547<t>
2548   Senders &SHOULD-NOT; combine multiple entries unless they are all
2549   under the same organizational control and the hosts have already been
2550   replaced by pseudonyms. Senders &MUST-NOT; combine entries which
2551   have different received-protocol values.
2552</t>
2553</section>
2554
2555<section title="Message Transforming" anchor="message.transforming">
2556<t>
2557   If a proxy receives a request-target with a host name that is not a
2558   fully qualified domain name, it &MAY; add its own domain to the host name
2559   it received when forwarding the request.  A proxy &MUST-NOT; change the
2560   host name if it is a fully qualified domain name.
2561</t>
2562<t>
2563   A non-transforming proxy &MUST-NOT; modify the "path-absolute" and "query"
2564   parts of the received request-target when forwarding it to the next inbound
2565   server, except as noted above to replace an empty path with "/" or "*".
2566</t>
2567<t>
2568   A non-transforming proxy &MUST; preserve the message payload (&payload;),
2569   though it &MAY; change the message body through application or removal
2570   of a transfer-coding (<xref target="transfer.codings"/>).
2571</t>
2572<t>
2573   A non-transforming proxy &SHOULD-NOT; modify header fields that provide
2574   information about the end points of the communication chain, the resource
2575   state, or the selected representation.
2576</t>
2577<t>
2578   A non-transforming proxy &MUST-NOT; modify any of the following fields in a
2579   request or response, and it &MUST-NOT; add any of these fields if not
2580   already present:
2581  <list style="symbols">
2582    <t><x:ref>Allow</x:ref> (&header-allow;)</t>
2583    <t><x:ref>Content-Location</x:ref> (&header-content-location;)</t>
2584    <t>Content-MD5 (<xref target="RFC2616" x:fmt="of" x:sec="14.15"/>)</t>
2585    <t><x:ref>ETag</x:ref> (&header-etag;)</t>
2586    <t><x:ref>Last-Modified</x:ref> (&header-last-modified;)</t>
2587    <t><x:ref>Server</x:ref> (&header-server;)</t>
2588  </list>
2589</t>
2590<t>
2591   A non-transforming proxy &MUST-NOT; modify an <x:ref>Expires</x:ref>
2592   header field (&header-expires;) if already present in a response, but
2593   it &MAY; add an <x:ref>Expires</x:ref> header field with a field-value
2594   identical to that of the <x:ref>Date</x:ref> header field.
2595</t>
2596<t>
2597   A proxy &MUST-NOT; modify or add any of the following fields in a
2598   message that contains the no-transform cache-control directive:
2599  <list style="symbols">
2600    <t><x:ref>Content-Encoding</x:ref> (&header-content-encoding;)</t>
2601    <t><x:ref>Content-Range</x:ref> (&header-content-range;)</t>
2602    <t><x:ref>Content-Type</x:ref> (&header-content-type;)</t>
2603  </list>
2604</t>
2605<t>
2606   A transforming proxy &MAY; modify or add these fields to a message
2607   that does not include no-transform, but if it does so, it &MUST; add a
2608   Warning 214 (Transformation applied) if one does not already appear
2609   in the message (see &header-warning;).
2610</t>
2611<x:note>
2612  <t>
2613    <x:h>Warning:</x:h> Unnecessary modification of header fields might
2614    cause authentication failures if stronger authentication
2615    mechanisms are introduced in later versions of HTTP. Such
2616    authentication mechanisms &MAY; rely on the values of header fields
2617    not listed here.
2618  </t>
2619</x:note>
2620</section>
2621
2622<section title="Associating a Response to a Request" anchor="associating.response.to.request">
2623<t>
2624   HTTP does not include a request identifier for associating a given
2625   request message with its corresponding one or more response messages.
2626   Hence, it relies on the order of response arrival to correspond exactly
2627   to the order in which requests are made on the same connection.
2628   More than one response message per request only occurs when one or more
2629   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a final response
2630   to the same request.
2631</t>
2632<t>
2633   A client that uses persistent connections and sends more than one request
2634   per connection &MUST; maintain a list of outstanding requests in the
2635   order sent on that connection and &MUST; associate each received response
2636   message to the highest ordered request that has not yet received a final
2637   (non-<x:ref>1xx</x:ref>) response.
2638</t>
2639</section>
2640</section>
2641
2642<section title="Connection Management" anchor="connection.management">
2643<t>
2644   HTTP messaging is independent of the underlying transport or
2645   session-layer connection protocol(s).  HTTP only presumes a reliable
2646   transport with in-order delivery of requests and the corresponding
2647   in-order delivery of responses.  The mapping of HTTP request and
2648   response structures onto the data units of an underlying transport
2649   protocol is outside the scope of this specification.
2650</t>
2651<t>
2652   As described in <xref target="connecting.inbound"/>, the specific
2653   connection protocols to be used for an HTTP interaction are determined by
2654   client configuration and the <x:ref>target URI</x:ref>.
2655   For example, the "http" URI scheme
2656   (<xref target="http.uri"/>) indicates a default connection of TCP
2657   over IP, with a default TCP port of 80, but the client might be
2658   configured to use a proxy via some other connection, port, or protocol.
2659</t>
2660<t>
2661   HTTP implementations are expected to engage in connection management,
2662   which includes maintaining the state of current connections,
2663   establishing a new connection or reusing an existing connection,
2664   processing messages received on a connection, detecting connection
2665   failures, and closing each connection.
2666   Most clients maintain multiple connections in parallel, including
2667   more than one connection per server endpoint.
2668   Most servers are designed to maintain thousands of concurrent connections,
2669   while controlling request queues to enable fair use and detect
2670   denial of service attacks.
2671</t>
2672
2673<section title="Connection" anchor="header.connection">
2674  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2675  <iref primary="true" item="close" x:for-anchor=""/>
2676  <x:anchor-alias value="Connection"/>
2677  <x:anchor-alias value="connection-option"/>
2678  <x:anchor-alias value="close"/>
2679<t>
2680   The "Connection" header field allows the sender to indicate desired
2681   control options for the current connection.  In order to avoid confusing
2682   downstream recipients, a proxy or gateway &MUST; remove or replace any
2683   received connection options before forwarding the message.
2684</t>
2685<t>
2686   When a header field is used to supply control information for or about
2687   the current connection, the sender &SHOULD; list the corresponding
2688   field-name within the "Connection" header field.
2689   A proxy or gateway &MUST; parse a received Connection
2690   header field before a message is forwarded and, for each
2691   connection-option in this field, remove any header field(s) from
2692   the message with the same name as the connection-option, and then
2693   remove the Connection header field itself (or replace it with the
2694   intermediary's own connection options for the forwarded message).
2695</t>
2696<t>
2697   Hence, the Connection header field provides a declarative way of
2698   distinguishing header fields that are only intended for the
2699   immediate recipient ("hop-by-hop") from those fields that are
2700   intended for all recipients on the chain ("end-to-end"), enabling the
2701   message to be self-descriptive and allowing future connection-specific
2702   extensions to be deployed without fear that they will be blindly
2703   forwarded by older intermediaries.
2704</t>
2705<t>
2706   The Connection header field's value has the following grammar:
2707</t>
2708<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2709  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2710  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2711</artwork></figure>
2712<t>
2713   Connection options are case-insensitive.
2714</t>
2715<t>
2716   A sender &MUST-NOT; include field-names in the Connection header
2717   field-value for fields that are defined as expressing constraints
2718   for all recipients in the request or response chain, such as the
2719   Cache-Control header field (&header-cache-control;).
2720</t>
2721<t>
2722   The connection options do not have to correspond to a header field
2723   present in the message, since a connection-specific header field
2724   might not be needed if there are no parameters associated with that
2725   connection option.  Recipients that trigger certain connection
2726   behavior based on the presence of connection options &MUST; do so
2727   based on the presence of the connection-option rather than only the
2728   presence of the optional header field.  In other words, if the
2729   connection option is received as a header field but not indicated
2730   within the Connection field-value, then the recipient &MUST; ignore
2731   the connection-specific header field because it has likely been
2732   forwarded by an intermediary that is only partially conformant.
2733</t>
2734<t>
2735   When defining new connection options, specifications ought to
2736   carefully consider existing deployed header fields and ensure
2737   that the new connection option does not share the same name as
2738   an unrelated header field that might already be deployed.
2739   Defining a new connection option essentially reserves that potential
2740   field-name for carrying additional information related to the
2741   connection option, since it would be unwise for senders to use
2742   that field-name for anything else.
2743</t>
2744<t>
2745   The "<x:dfn>close</x:dfn>" connection option is defined for a
2746   sender to signal that this connection will be closed after completion of
2747   the response. For example,
2748</t>
2749<figure><artwork type="example">
2750  Connection: close
2751</artwork></figure>
2752<t>
2753   in either the request or the response header fields indicates that
2754   the connection &SHOULD; be closed after the current request/response
2755   is complete (<xref target="persistent.tear-down"/>).
2756</t>
2757<t>
2758   A client that does not support persistent connections &MUST;
2759   send the "close" connection option in every request message.
2760</t>
2761<t>
2762   A server that does not support persistent connections &MUST;
2763   send the "close" connection option in every response message that
2764   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2765</t>
2766</section>
2767
2768<section title="Persistent Connections" anchor="persistent.connections">
2769  <x:anchor-alias value="persistent connections"/>
2770<t>
2771   HTTP was originally designed to use a separate connection for each
2772   request/response pair. As the Web evolved and embedded requests became
2773   common for inline images, the connection establishment overhead was
2774   a significant drain on performance and a concern for Internet congestion.
2775   Message framing (via <x:ref>Content-Length</x:ref>) and optional
2776   long-lived connections (via Keep-Alive) were added to HTTP/1.0 in order
2777   to improve performance for some requests. However, these extensions were
2778   insufficient for dynamically generated responses and difficult to use
2779   with intermediaries.
2780</t>
2781<t>
2782   HTTP/1.1 defaults to the use of "<x:ref>persistent connections</x:ref>",
2783   which allow multiple requests and responses to be carried over a single
2784   connection. The "<x:ref>close</x:ref>" connection-option is used to
2785   signal that a connection will close after the current request/response.
2786   Persistent connections have a number of advantages:
2787  <list style="symbols">
2788      <t>
2789        By opening and closing fewer connections, CPU time is saved
2790        in routers and hosts (clients, servers, proxies, gateways,
2791        tunnels, or caches), and memory used for protocol control
2792        blocks can be saved in hosts.
2793      </t>
2794      <t>
2795        Most requests and responses can be pipelined on a connection.
2796        Pipelining allows a client to make multiple requests without
2797        waiting for each response, allowing a single connection to
2798        be used much more efficiently and with less overall latency.
2799      </t>
2800      <t>
2801        For TCP connections, network congestion is reduced by eliminating the
2802        packets associated with the three way handshake and graceful close
2803        procedures, and by allowing sufficient time to determine the
2804        congestion state of the network.
2805      </t>
2806      <t>
2807        Latency on subsequent requests is reduced since there is no time
2808        spent in the connection opening handshake.
2809      </t>
2810      <t>
2811        HTTP can evolve more gracefully, since most errors can be reported
2812        without the penalty of closing the connection. Clients using
2813        future versions of HTTP might optimistically try a new feature,
2814        but if communicating with an older server, retry with old
2815        semantics after an error is reported.
2816      </t>
2817    </list>
2818</t>
2819<t>
2820   HTTP implementations &SHOULD; implement persistent connections.
2821</t>
2822   
2823<section title="Establishment" anchor="persistent.establishment">
2824<t>
2825   It is beyond the scope of this specification to describe how connections
2826   are established via various transport or session-layer protocols.
2827   Each connection applies to only one transport link.
2828</t>
2829<t>
2830   A recipient determines whether a connection is persistent or not based on
2831   the most recently received message's protocol version and
2832   <x:ref>Connection</x:ref> header field (if any):
2833   <list style="symbols">
2834     <t>If the <x:ref>close</x:ref> connection option is present, the
2835        connection will not persist after the current response; else,</t>
2836     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2837        persist after the current response; else,</t>
2838     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2839        connection option is present, the recipient is not a proxy, and
2840        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2841        the connection will persist after the current response; otherwise,</t>
2842     <t>The connection will close after the current response.</t>
2843   </list>
2844</t>
2845<t>
2846   A proxy server &MUST-NOT; maintain a persistent connection with an
2847   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
2848   information and discussion of the problems with the Keep-Alive header field
2849   implemented by many HTTP/1.0 clients).
2850</t>
2851</section>
2852
2853<section title="Reuse" anchor="persistent.reuse">
2854<t>
2855   In order to remain persistent, all messages on a connection &MUST;
2856   have a self-defined message length (i.e., one not defined by closure
2857   of the connection), as described in <xref target="message.body"/>.
2858</t>
2859<t>
2860   A server &MAY; assume that an HTTP/1.1 client intends to maintain a
2861   persistent connection until a <x:ref>close</x:ref> connection option
2862   is received in a request.
2863</t>
2864<t>
2865   A client &MAY; reuse a persistent connection until it sends or receives
2866   a <x:ref>close</x:ref> connection option or receives an HTTP/1.0 response
2867   without a "keep-alive" connection option.
2868</t>
2869<t>
2870   Clients and servers &SHOULD-NOT; assume that a persistent connection is
2871   maintained for HTTP versions less than 1.1 unless it is explicitly
2872   signaled.
2873   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
2874   for more information on backward compatibility with HTTP/1.0 clients.
2875</t>
2876
2877<section title="Pipelining" anchor="pipelining">
2878<t>
2879   A client that supports persistent connections &MAY; "pipeline" its
2880   requests (i.e., send multiple requests without waiting for each
2881   response). A server &MUST; send its responses to those requests in the
2882   same order that the requests were received.
2883</t>
2884<t>
2885   Clients which assume persistent connections and pipeline immediately
2886   after connection establishment &SHOULD; be prepared to retry their
2887   connection if the first pipelined attempt fails. If a client does
2888   such a retry, it &MUST-NOT; pipeline before it knows the connection is
2889   persistent. Clients &MUST; also be prepared to resend their requests if
2890   the server closes the connection before sending all of the
2891   corresponding responses.
2892</t>
2893<t>
2894   Clients &SHOULD-NOT; pipeline requests using non-idempotent request methods
2895   or non-idempotent sequences of request methods (see &idempotent-methods;).
2896   Otherwise, a premature termination of the transport connection could lead
2897   to indeterminate results. A client wishing to send a non-idempotent
2898   request &SHOULD; wait to send that request until it has received the
2899   response status line for the previous request.
2900</t>
2901</section>
2902
2903<section title="Retrying Requests" anchor="persistent.retrying.requests">
2904<t>
2905   Connections can be closed at any time, with or without intention.
2906   Implementations ought to anticipate the need to recover
2907   from asynchronous close events.
2908   A client &MAY; open a new connection and retransmit an aborted sequence
2909   of requests without user interaction so long as the request sequence is
2910   idempotent (see &idempotent-methods;).
2911   A client &MUST-NOT; automatically retry non-idempotent request sequences,
2912   although user agents &MAY; offer a human operator the choice of retrying
2913   the request(s). Confirmation by
2914   user agent software with semantic understanding of the application
2915   &MAY; substitute for user confirmation. An automatic retry &SHOULD-NOT; 
2916   be repeated if a second sequence of requests fails.
2917</t>
2918</section>
2919</section>
2920   
2921<section title="Concurrency" anchor="persistent.concurrency">
2922<t>
2923   Clients &SHOULD; limit the number of simultaneous
2924   connections that they maintain to a given server.
2925</t>
2926<t>
2927   Previous revisions of HTTP gave a specific number of connections as a
2928   ceiling, but this was found to be impractical for many applications. As a
2929   result, this specification does not mandate a particular maximum number of
2930   connections, but instead encourages clients to be conservative when opening
2931   multiple connections.
2932</t>
2933<t>
2934   Multiple connections are typically used to avoid the "head-of-line
2935   blocking" problem, wherein a request that takes significant server-side
2936   processing and/or has a large payload blocks subsequent requests on the
2937   same connection. However, each connection consumes server resources.
2938   Furthermore, using multiple connections can cause undesirable side effects
2939   in congested networks.
2940</t>
2941<t>
2942   Note that servers might reject traffic that they deem abusive, including an
2943   excessive number of connections from a client.
2944</t>
2945</section>
2946
2947<section title="Failures and Time-outs" anchor="persistent.failures">
2948<t>
2949   Servers will usually have some time-out value beyond which they will
2950   no longer maintain an inactive connection. Proxy servers might make
2951   this a higher value since it is likely that the client will be making
2952   more connections through the same server. The use of persistent
2953   connections places no requirements on the length (or existence) of
2954   this time-out for either the client or the server.
2955</t>
2956<t>
2957   When a client or server wishes to time-out it &SHOULD; issue a graceful
2958   close on the transport connection. Clients and servers &SHOULD; both
2959   constantly watch for the other side of the transport close, and
2960   respond to it as appropriate. If a client or server does not detect
2961   the other side's close promptly it could cause unnecessary resource
2962   drain on the network.
2963</t>
2964<t>
2965   A client, server, or proxy &MAY; close the transport connection at any
2966   time. For example, a client might have started to send a new request
2967   at the same time that the server has decided to close the "idle"
2968   connection. From the server's point of view, the connection is being
2969   closed while it was idle, but from the client's point of view, a
2970   request is in progress.
2971</t>
2972<t>
2973   Servers &SHOULD; maintain persistent connections and allow the underlying
2974   transport's flow control mechanisms to resolve temporary overloads, rather
2975   than terminate connections with the expectation that clients will retry.
2976   The latter technique can exacerbate network congestion.
2977</t>
2978<t>
2979   A client sending a message body &SHOULD; monitor
2980   the network connection for an error status code while it is transmitting
2981   the request. If the client sees an error status code, it &SHOULD;
2982   immediately cease transmitting the body and close the connection.
2983</t>
2984</section>
2985   
2986<section title="Tear-down" anchor="persistent.tear-down">
2987  <iref primary="false" item="Connection header field" x:for-anchor=""/>
2988  <iref primary="false" item="close" x:for-anchor=""/>
2989<t>
2990   The <x:ref>Connection</x:ref> header field
2991   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
2992   connection option that a sender &SHOULD; send when it wishes to close
2993   the connection after the current request/response pair.
2994</t>
2995<t>
2996   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
2997   send further requests on that connection (after the one containing
2998   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
2999   final response message corresponding to this request.
3000</t>
3001<t>
3002   A server that receives a <x:ref>close</x:ref> connection option &MUST;
3003   initiate a lingering close (see below) of the connection after it sends the
3004   final response to the request that contained <x:ref>close</x:ref>.
3005   The server &SHOULD; include a <x:ref>close</x:ref> connection option
3006   in its final response on that connection. The server &MUST-NOT; process
3007   any further requests received on that connection.
3008</t>
3009<t>
3010   A server that sends a <x:ref>close</x:ref> connection option &MUST;
3011   initiate a lingering close of the connection after it sends the
3012   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
3013   any further requests received on that connection.
3014</t>
3015<t>
3016   A client that receives a <x:ref>close</x:ref> connection option &MUST;
3017   cease sending requests on that connection and close the connection
3018   after reading the response message containing the close; if additional
3019   pipelined requests had been sent on the connection, the client &SHOULD;
3020   assume that they will not be processed by the server.
3021</t>
3022<t>
3023   If a server performs an immediate close of a TCP connection, there is a
3024   significant risk that the client will not be able to read the last HTTP
3025   response.  If the server receives additional data from the client on a
3026   fully-closed connection, such as another request that was sent by the
3027   client before receiving the server's response, the server's TCP stack will
3028   send a reset packet to the client; unfortunately, the reset packet might
3029   erase the client's unacknowledged input buffers before they can be read
3030   and interpreted by the client's HTTP parser.
3031</t>
3032<t>
3033   To avoid the TCP reset problem, a server can perform a lingering close on a
3034   connection by closing only the write side of the read/write connection
3035   (a half-close) and continuing to read from the connection until the
3036   connection is closed by the client or the server is reasonably certain
3037   that its own TCP stack has received the client's acknowledgement of the
3038   packet(s) containing the server's last response. It is then safe for the
3039   server to fully close the connection.
3040</t>
3041<t>
3042   It is unknown whether the reset problem is exclusive to TCP or might also
3043   be found in other transport connection protocols.
3044</t>
3045</section>
3046</section>
3047
3048<section title="Upgrade" anchor="header.upgrade">
3049  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3050  <x:anchor-alias value="Upgrade"/>
3051  <x:anchor-alias value="protocol"/>
3052  <x:anchor-alias value="protocol-name"/>
3053  <x:anchor-alias value="protocol-version"/>
3054<t>
3055   The "Upgrade" header field is intended to provide a simple mechanism
3056   for transitioning from HTTP/1.1 to some other protocol on the same
3057   connection.  A client &MAY; send a list of protocols in the Upgrade
3058   header field of a request to invite the server to switch to one or
3059   more of those protocols before sending the final response.
3060   A server &MUST; send an Upgrade header field in <x:ref>101 (Switching
3061   Protocols)</x:ref> responses to indicate which protocol(s) are being
3062   switched to, and &MUST; send it in <x:ref>426 (Upgrade Required)</x:ref>
3063   responses to indicate acceptable protocols.
3064   A server &MAY; send an Upgrade header field in any other response to
3065   indicate that they might be willing to upgrade to one of the
3066   specified protocols for a future request.
3067</t>
3068<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3069  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3070
3071  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3072  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3073  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3074</artwork></figure>
3075<t>
3076   For example,
3077</t>
3078<figure><artwork type="example">
3079  Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3080</artwork></figure>
3081<t>
3082   Upgrade eases the difficult transition between incompatible protocols by
3083   allowing the client to initiate a request in the more commonly
3084   supported protocol while indicating to the server that it would like
3085   to use a "better" protocol if available (where "better" is determined
3086   by the server, possibly according to the nature of the request method
3087   or target resource).
3088</t>
3089<t>
3090   Upgrade cannot be used to insist on a protocol change; its acceptance and
3091   use by the server is optional. The capabilities and nature of the
3092   application-level communication after the protocol change is entirely
3093   dependent upon the new protocol chosen, although the first action
3094   after changing the protocol &MUST; be a response to the initial HTTP
3095   request that contained the Upgrade header field.
3096</t>
3097<t>
3098   For example, if the Upgrade header field is received in a GET request
3099   and the server decides to switch protocols, then it &MUST; first respond
3100   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3101   then immediately follow that with the new protocol's equivalent of a
3102   response to a GET on the target resource.  This allows a connection to be
3103   upgraded to protocols with the same semantics as HTTP without the
3104   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3105   protocols unless the received message semantics can be honored by the new
3106   protocol; an OPTIONS request can be honored by any protocol.
3107</t>
3108<t>
3109   When Upgrade is sent, a sender &MUST; also send a
3110   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3111   that contains the "upgrade" connection option, in order to prevent Upgrade
3112   from being accidentally forwarded by intermediaries that might not implement
3113   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3114   is received in an HTTP/1.0 request.
3115</t>
3116<t>
3117   The Upgrade header field only applies to switching application-level
3118   protocols on the existing connection; it cannot be used
3119   to switch to a protocol on a different connection. For that purpose, it is
3120   more appropriate to use a <x:ref>3xx (Redirection)</x:ref> response
3121   (&status-3xx;).
3122</t>
3123<t>
3124   This specification only defines the protocol name "HTTP" for use by
3125   the family of Hypertext Transfer Protocols, as defined by the HTTP
3126   version rules of <xref target="http.version"/> and future updates to this
3127   specification. Additional tokens can be registered with IANA using the
3128   registration procedure defined in <xref target="upgrade.token.registry"/>.
3129</t>
3130</section>
3131</section>
3132
3133<section title="IANA Considerations" anchor="IANA.considerations">
3134
3135<section title="Header Field Registration" anchor="header.field.registration">
3136<t>
3137   HTTP header fields are registered within the Message Header Field Registry
3138   <xref target="RFC3864"/> maintained by IANA at
3139   <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
3140</t>
3141<t>
3142   This document defines the following HTTP header fields, so their
3143   associated registry entries shall be updated according to the permanent
3144   registrations below:
3145</t>
3146<?BEGININC p1-messaging.iana-headers ?>
3147<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3148<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3149   <ttcol>Header Field Name</ttcol>
3150   <ttcol>Protocol</ttcol>
3151   <ttcol>Status</ttcol>
3152   <ttcol>Reference</ttcol>
3153
3154   <c>Connection</c>
3155   <c>http</c>
3156   <c>standard</c>
3157   <c>
3158      <xref target="header.connection"/>
3159   </c>
3160   <c>Content-Length</c>
3161   <c>http</c>
3162   <c>standard</c>
3163   <c>
3164      <xref target="header.content-length"/>
3165   </c>
3166   <c>Host</c>
3167   <c>http</c>
3168   <c>standard</c>
3169   <c>
3170      <xref target="header.host"/>
3171   </c>
3172   <c>TE</c>
3173   <c>http</c>
3174   <c>standard</c>
3175   <c>
3176      <xref target="header.te"/>
3177   </c>
3178   <c>Trailer</c>
3179   <c>http</c>
3180   <c>standard</c>
3181   <c>
3182      <xref target="header.trailer"/>
3183   </c>
3184   <c>Transfer-Encoding</c>
3185   <c>http</c>
3186   <c>standard</c>
3187   <c>
3188      <xref target="header.transfer-encoding"/>
3189   </c>
3190   <c>Upgrade</c>
3191   <c>http</c>
3192   <c>standard</c>
3193   <c>
3194      <xref target="header.upgrade"/>
3195   </c>
3196   <c>Via</c>
3197   <c>http</c>
3198   <c>standard</c>
3199   <c>
3200      <xref target="header.via"/>
3201   </c>
3202</texttable>
3203<!--(END)-->
3204<?ENDINC p1-messaging.iana-headers ?>
3205<t>
3206   Furthermore, the header field-name "Close" shall be registered as
3207   "reserved", since using that name as an HTTP header field might
3208   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3209   header field (<xref target="header.connection"/>).
3210</t>
3211<texttable align="left" suppress-title="true">
3212   <ttcol>Header Field Name</ttcol>
3213   <ttcol>Protocol</ttcol>
3214   <ttcol>Status</ttcol>
3215   <ttcol>Reference</ttcol>
3216
3217   <c>Close</c>
3218   <c>http</c>
3219   <c>reserved</c>
3220   <c>
3221      <xref target="header.field.registration"/>
3222   </c>
3223</texttable>
3224<t>
3225   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3226</t>
3227</section>
3228
3229<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3230<t>
3231   IANA maintains the registry of URI Schemes <xref target="RFC4395"/> at
3232   <eref target="http://www.iana.org/assignments/uri-schemes.html"/>.
3233</t>
3234<t>
3235   This document defines the following URI schemes, so their
3236   associated registry entries shall be updated according to the permanent
3237   registrations below:
3238</t>
3239<texttable align="left" suppress-title="true">
3240   <ttcol>URI Scheme</ttcol>
3241   <ttcol>Description</ttcol>
3242   <ttcol>Reference</ttcol>
3243
3244   <c>http</c>
3245   <c>Hypertext Transfer Protocol</c>
3246   <c><xref target="http.uri"/></c>
3247
3248   <c>https</c>
3249   <c>Hypertext Transfer Protocol Secure</c>
3250   <c><xref target="https.uri"/></c>
3251</texttable>
3252</section>
3253
3254<section title="Internet Media Type Registrations" anchor="internet.media.type.http">
3255<t>
3256   This document serves as the specification for the Internet media types
3257   "message/http" and "application/http". The following is to be registered with
3258   IANA (see <xref target="RFC4288"/>).
3259</t>
3260<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
3261<iref item="Media Type" subitem="message/http" primary="true"/>
3262<iref item="message/http Media Type" primary="true"/>
3263<t>
3264   The message/http type can be used to enclose a single HTTP request or
3265   response message, provided that it obeys the MIME restrictions for all
3266   "message" types regarding line length and encodings.
3267</t>
3268<t>
3269  <list style="hanging" x:indent="12em">
3270    <t hangText="Type name:">
3271      message
3272    </t>
3273    <t hangText="Subtype name:">
3274      http
3275    </t>
3276    <t hangText="Required parameters:">
3277      none
3278    </t>
3279    <t hangText="Optional parameters:">
3280      version, msgtype
3281      <list style="hanging">
3282        <t hangText="version:">
3283          The HTTP-version number of the enclosed message
3284          (e.g., "1.1"). If not present, the version can be
3285          determined from the first line of the body.
3286        </t>
3287        <t hangText="msgtype:">
3288          The message type &mdash; "request" or "response". If not
3289          present, the type can be determined from the first
3290          line of the body.
3291        </t>
3292      </list>
3293    </t>
3294    <t hangText="Encoding considerations:">
3295      only "7bit", "8bit", or "binary" are permitted
3296    </t>
3297    <t hangText="Security considerations:">
3298      none
3299    </t>
3300    <t hangText="Interoperability considerations:">
3301      none
3302    </t>
3303    <t hangText="Published specification:">
3304      This specification (see <xref target="internet.media.type.message.http"/>).
3305    </t>
3306    <t hangText="Applications that use this media type:">
3307    </t>
3308    <t hangText="Additional information:">
3309      <list style="hanging">
3310        <t hangText="Magic number(s):">none</t>
3311        <t hangText="File extension(s):">none</t>
3312        <t hangText="Macintosh file type code(s):">none</t>
3313      </list>
3314    </t>
3315    <t hangText="Person and email address to contact for further information:">
3316      See Authors Section.
3317    </t>
3318    <t hangText="Intended usage:">
3319      COMMON
3320    </t>
3321    <t hangText="Restrictions on usage:">
3322      none
3323    </t>
3324    <t hangText="Author/Change controller:">
3325      IESG
3326    </t>
3327  </list>
3328</t>
3329</section>
3330<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
3331<iref item="Media Type" subitem="application/http" primary="true"/>
3332<iref item="application/http Media Type" primary="true"/>
3333<t>
3334   The application/http type can be used to enclose a pipeline of one or more
3335   HTTP request or response messages (not intermixed).
3336</t>
3337<t>
3338  <list style="hanging" x:indent="12em">
3339    <t hangText="Type name:">
3340      application
3341    </t>
3342    <t hangText="Subtype name:">
3343      http
3344    </t>
3345    <t hangText="Required parameters:">
3346      none
3347    </t>
3348    <t hangText="Optional parameters:">
3349      version, msgtype
3350      <list style="hanging">
3351        <t hangText="version:">
3352          The HTTP-version number of the enclosed messages
3353          (e.g., "1.1"). If not present, the version can be
3354          determined from the first line of the body.
3355        </t>
3356        <t hangText="msgtype:">
3357          The message type &mdash; "request" or "response". If not
3358          present, the type can be determined from the first
3359          line of the body.
3360        </t>
3361      </list>
3362    </t>
3363    <t hangText="Encoding considerations:">
3364      HTTP messages enclosed by this type
3365      are in "binary" format; use of an appropriate
3366      Content-Transfer-Encoding is required when
3367      transmitted via E-mail.
3368    </t>
3369    <t hangText="Security considerations:">
3370      none
3371    </t>
3372    <t hangText="Interoperability considerations:">
3373      none
3374    </t>
3375    <t hangText="Published specification:">
3376      This specification (see <xref target="internet.media.type.application.http"/>).
3377    </t>
3378    <t hangText="Applications that use this media type:">
3379    </t>
3380    <t hangText="Additional information:">
3381      <list style="hanging">
3382        <t hangText="Magic number(s):">none</t>
3383        <t hangText="File extension(s):">none</t>
3384        <t hangText="Macintosh file type code(s):">none</t>
3385      </list>
3386    </t>
3387    <t hangText="Person and email address to contact for further information:">
3388      See Authors Section.
3389    </t>
3390    <t hangText="Intended usage:">
3391      COMMON
3392    </t>
3393    <t hangText="Restrictions on usage:">
3394      none
3395    </t>
3396    <t hangText="Author/Change controller:">
3397      IESG
3398    </t>
3399  </list>
3400</t>
3401</section>
3402</section>
3403
3404<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3405<t>
3406   The HTTP Transfer Coding Registry defines the name space for transfer
3407   coding names.
3408</t>
3409<t>
3410   Registrations &MUST; include the following fields:
3411   <list style="symbols">
3412     <t>Name</t>
3413     <t>Description</t>
3414     <t>Pointer to specification text</t>
3415   </list>
3416</t>
3417<t>
3418   Names of transfer codings &MUST-NOT; overlap with names of content codings
3419   (&content-codings;) unless the encoding transformation is identical, as
3420   is the case for the compression codings defined in
3421   <xref target="compression.codings"/>.
3422</t>
3423<t>
3424   Values to be added to this name space require IETF Review (see
3425   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3426   conform to the purpose of transfer coding defined in this section.
3427   Use of program names for the identification of encoding formats
3428   is not desirable and is discouraged for future encodings.
3429</t>
3430<t>
3431   The registry itself is maintained at
3432   <eref target="http://www.iana.org/assignments/http-parameters"/>.
3433</t>
3434</section>
3435
3436<section title="Transfer Coding Registrations" anchor="transfer.coding.registration">
3437<t>
3438   The HTTP Transfer Coding Registry shall be updated with the registrations
3439   below:
3440</t>
3441<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3442   <ttcol>Name</ttcol>
3443   <ttcol>Description</ttcol>
3444   <ttcol>Reference</ttcol>
3445   <c>chunked</c>
3446   <c>Transfer in a series of chunks</c>
3447   <c>
3448      <xref target="chunked.encoding"/>
3449   </c>
3450   <c>compress</c>
3451   <c>UNIX "compress" program method</c>
3452   <c>
3453      <xref target="compress.coding"/>
3454   </c>
3455   <c>deflate</c>
3456   <c>"deflate" compression mechanism (<xref target="RFC1951"/>) used inside
3457   the "zlib" data format (<xref target="RFC1950"/>)
3458   </c>
3459   <c>
3460      <xref target="deflate.coding"/>
3461   </c>
3462   <c>gzip</c>
3463   <c>Same as GNU zip <xref target="RFC1952"/></c>
3464   <c>
3465      <xref target="gzip.coding"/>
3466   </c>
3467</texttable>
3468</section>
3469
3470<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3471<t>
3472   The HTTP Upgrade Token Registry defines the name space for protocol-name
3473   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3474   field. Each registered protocol name is associated with contact information
3475   and an optional set of specifications that details how the connection
3476   will be processed after it has been upgraded.
3477</t>
3478<t>
3479   Registrations happen on a "First Come First Served" basis (see
3480   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3481   following rules:
3482  <list style="numbers">
3483    <t>A protocol-name token, once registered, stays registered forever.</t>
3484    <t>The registration &MUST; name a responsible party for the
3485       registration.</t>
3486    <t>The registration &MUST; name a point of contact.</t>
3487    <t>The registration &MAY; name a set of specifications associated with
3488       that token. Such specifications need not be publicly available.</t>
3489    <t>The registration &SHOULD; name a set of expected "protocol-version"
3490       tokens associated with that token at the time of registration.</t>
3491    <t>The responsible party &MAY; change the registration at any time.
3492       The IANA will keep a record of all such changes, and make them
3493       available upon request.</t>
3494    <t>The IESG &MAY; reassign responsibility for a protocol token.
3495       This will normally only be used in the case when a
3496       responsible party cannot be contacted.</t>
3497  </list>
3498</t>
3499<t>
3500   This registration procedure for HTTP Upgrade Tokens replaces that
3501   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3502</t>
3503</section>
3504
3505<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3506<t>
3507   The HTTP Upgrade Token Registry shall be updated with the registration
3508   below:
3509</t>
3510<texttable align="left" suppress-title="true">
3511   <ttcol>Value</ttcol>
3512   <ttcol>Description</ttcol>
3513   <ttcol>Expected Version Tokens</ttcol>
3514   <ttcol>Reference</ttcol>
3515
3516   <c>HTTP</c>
3517   <c>Hypertext Transfer Protocol</c>
3518   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3519   <c><xref target="http.version"/></c>
3520</texttable>
3521<t>
3522   The responsible party is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
3523</t>
3524</section>
3525
3526</section>
3527
3528<section title="Security Considerations" anchor="security.considerations">
3529<t>
3530   This section is meant to inform application developers, information
3531   providers, and users of the security limitations in HTTP/1.1 as
3532   described by this document. The discussion does not include
3533   definitive solutions to the problems revealed, though it does make
3534   some suggestions for reducing security risks.
3535</t>
3536
3537<section title="Personal Information" anchor="personal.information">
3538<t>
3539   HTTP clients are often privy to large amounts of personal information,
3540   including both information provided by the user to interact with resources
3541   (e.g., the user's name, location, mail address, passwords, encryption
3542   keys, etc.) and information about the user's browsing activity over
3543   time (e.g., history, bookmarks, etc.). HTTP implementations need to
3544   prevent unintentional leakage of this information.
3545</t>
3546</section>
3547
3548<section title="Abuse of Server Log Information" anchor="abuse.of.server.log.information">
3549<t>
3550   A server is in the position to save personal data about a user's
3551   requests which might identify their reading patterns or subjects of
3552   interest.  In particular, log information gathered at an intermediary
3553   often contains a history of user agent interaction, across a multitude
3554   of sites, that can be traced to individual users.
3555</t>
3556<t>
3557   HTTP log information is confidential in nature; its handling is often
3558   constrained by laws and regulations.  Log information needs to be securely
3559   stored and appropriate guidelines followed for its analysis.
3560   Anonymization of personal information within individual entries helps,
3561   but is generally not sufficient to prevent real log traces from being
3562   re-identified based on correlation with other access characteristics.
3563   As such, access traces that are keyed to a specific client should not
3564   be published even if the key is pseudonymous.
3565</t>
3566<t>
3567   To minimize the risk of theft or accidental publication, log information
3568   should be purged of personally identifiable information, including
3569   user identifiers, IP addresses, and user-provided query parameters,
3570   as soon as that information is no longer necessary to support operational
3571   needs for security, auditing, or fraud control.
3572</t>
3573</section>
3574
3575<section title="Attacks Based On File and Path Names" anchor="attack.pathname">
3576<t>
3577   Origin servers &SHOULD; be careful to restrict
3578   the documents returned by HTTP requests to be only those that were
3579   intended by the server administrators. If an HTTP server translates
3580   HTTP URIs directly into file system calls, the server &MUST; take
3581   special care not to serve files that were not intended to be
3582   delivered to HTTP clients. For example, UNIX, Microsoft Windows, and
3583   other operating systems use ".." as a path component to indicate a
3584   directory level above the current one. On such a system, an HTTP
3585   server &MUST; disallow any such construct in the request-target if it
3586   would otherwise allow access to a resource outside those intended to
3587   be accessible via the HTTP server. Similarly, files intended for
3588   reference only internally to the server (such as access control
3589   files, configuration files, and script code) &MUST; be protected from
3590   inappropriate retrieval, since they might contain sensitive
3591   information.
3592</t>
3593</section>
3594
3595<section title="DNS-related Attacks" anchor="dns.related.attacks">
3596<t>
3597   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3598   generally prone to security attacks based on the deliberate misassociation
3599   of IP addresses and DNS names not protected by DNSSEC. Clients need to be
3600   cautious in assuming the validity of an IP number/DNS name association unless
3601   the response is protected by DNSSEC (<xref target="RFC4033"/>).
3602</t>
3603</section>
3604
3605<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3606<t>
3607   By their very nature, HTTP intermediaries are men-in-the-middle, and
3608   represent an opportunity for man-in-the-middle attacks. Compromise of
3609   the systems on which the intermediaries run can result in serious security
3610   and privacy problems. Intermediaries have access to security-related
3611   information, personal information about individual users and
3612   organizations, and proprietary information belonging to users and
3613   content providers. A compromised intermediary, or an intermediary
3614   implemented or configured without regard to security and privacy
3615   considerations, might be used in the commission of a wide range of
3616   potential attacks.
3617</t>
3618<t>
3619   Intermediaries that contain a shared cache are especially vulnerable
3620   to cache poisoning attacks.
3621</t>
3622<t>
3623   Implementers need to consider the privacy and security
3624   implications of their design and coding decisions, and of the
3625   configuration options they provide to operators (especially the
3626   default configuration).
3627</t>
3628<t>
3629   Users need to be aware that intermediaries are no more trustworthy than
3630   the people who run them; HTTP itself cannot solve this problem.
3631</t>
3632</section>
3633
3634<section title="Protocol Element Size Overflows" anchor="attack.protocol.element.size.overflows">
3635<t>
3636   Because HTTP uses mostly textual, character-delimited fields, attackers can
3637   overflow buffers in implementations, and/or perform a Denial of Service
3638   against implementations that accept fields with unlimited lengths.
3639</t>
3640<t>
3641   To promote interoperability, this specification makes specific
3642   recommendations for minimum size limits on request-line
3643   (<xref target="request.line"/>)
3644   and blocks of header fields (<xref target="header.fields"/>). These are
3645   minimum recommendations, chosen to be supportable even by implementations
3646   with limited resources; it is expected that most implementations will
3647   choose substantially higher limits.
3648</t>
3649<t>
3650   This specification also provides a way for servers to reject messages that
3651   have request-targets that are too long (&status-414;) or request entities
3652   that are too large (&status-4xx;).
3653</t>
3654<t>
3655   Recipients &SHOULD; carefully limit the extent to which they read other
3656   fields, including (but not limited to) request methods, response status
3657   phrases, header field-names, and body chunks, so as to avoid denial of
3658   service attacks without impeding interoperability.
3659</t>
3660</section>
3661</section>
3662
3663<section title="Acknowledgments" anchor="acks">
3664<t>
3665   This edition of HTTP/1.1 builds on the many contributions that went into
3666   <xref target="RFC1945" format="none">RFC 1945</xref>,
3667   <xref target="RFC2068" format="none">RFC 2068</xref>,
3668   <xref target="RFC2145" format="none">RFC 2145</xref>, and
3669   <xref target="RFC2616" format="none">RFC 2616</xref>, including
3670   substantial contributions made by the previous authors, editors, and
3671   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
3672   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
3673   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
3674</t>
3675<t>
3676   Since 1999, the following contributors have helped improve the HTTP
3677   specification by reporting bugs, asking smart questions, drafting or
3678   reviewing text, and evaluating open issues:
3679</t>
3680<?BEGININC acks ?>
3681<t>Adam Barth,
3682Adam Roach,
3683Addison Phillips,
3684Adrian Chadd,
3685Adrien W. de Croy,
3686Alan Ford,
3687Alan Ruttenberg,
3688Albert Lunde,
3689Alek Storm,
3690Alex Rousskov,
3691Alexandre Morgaut,
3692Alexey Melnikov,
3693Alisha Smith,
3694Amichai Rothman,
3695Amit Klein,
3696Amos Jeffries,
3697Andreas Maier,
3698Andreas Petersson,
3699Anil Sharma,
3700Anne van Kesteren,
3701Anthony Bryan,
3702Asbjorn Ulsberg,
3703Ashok Kumar,
3704Balachander Krishnamurthy,
3705Barry Leiba,
3706Ben Laurie,
3707Benjamin Niven-Jenkins,
3708Bil Corry,
3709Bill Burke,
3710Bjoern Hoehrmann,
3711Bob Scheifler,
3712Boris Zbarsky,
3713Brett Slatkin,
3714Brian Kell,
3715Brian McBarron,
3716Brian Pane,
3717Brian Smith,
3718Bryce Nesbitt,
3719Cameron Heavon-Jones,
3720Carl Kugler,
3721Carsten Bormann,
3722Charles Fry,
3723Chris Newman,
3724Cyrus Daboo,
3725Dale Robert Anderson,
3726Dan Wing,
3727Dan Winship,
3728Daniel Stenberg,
3729Dave Cridland,
3730Dave Crocker,
3731Dave Kristol,
3732David Booth,
3733David Singer,
3734David W. Morris,
3735Diwakar Shetty,
3736Dmitry Kurochkin,
3737Drummond Reed,
3738Duane Wessels,
3739Edward Lee,
3740Eliot Lear,
3741Eran Hammer-Lahav,
3742Eric D. Williams,
3743Eric J. Bowman,
3744Eric Lawrence,
3745Eric Rescorla,
3746Erik Aronesty,
3747Evan Prodromou,
3748Florian Weimer,
3749Frank Ellermann,
3750Fred Bohle,
3751Gabriel Montenegro,
3752Geoffrey Sneddon,
3753Gervase Markham,
3754Grahame Grieve,
3755Greg Wilkins,
3756Harald Tveit Alvestrand,
3757Harry Halpin,
3758Helge Hess,
3759Henrik Nordstrom,
3760Henry S. Thompson,
3761Henry Story,
3762Herbert van de Sompel,
3763Howard Melman,
3764Hugo Haas,
3765Ian Fette,
3766Ian Hickson,
3767Ido Safruti,
3768Ilya Grigorik,
3769Ingo Struck,
3770J. Ross Nicoll,
3771James H. Manger,
3772James Lacey,
3773James M. Snell,
3774Jamie Lokier,
3775Jan Algermissen,
3776Jeff Hodges (who came up with the term 'effective Request-URI'),
3777Jeff Walden,
3778Jim Luther,
3779Joe D. Williams,
3780Joe Gregorio,
3781Joe Orton,
3782John C. Klensin,
3783John C. Mallery,
3784John Cowan,
3785John Kemp,
3786John Panzer,
3787John Schneider,
3788John Stracke,
3789John Sullivan,
3790Jonas Sicking,
3791Jonathan Billington,
3792Jonathan Moore,
3793Jonathan Rees,
3794Jonathan Silvera,
3795Jordi Ros,
3796Joris Dobbelsteen,
3797Josh Cohen,
3798Julien Pierre,
3799Jungshik Shin,
3800Justin Chapweske,
3801Justin Erenkrantz,
3802Justin James,
3803Kalvinder Singh,
3804Karl Dubost,
3805Keith Hoffman,
3806Keith Moore,
3807Ken Murchison,
3808Koen Holtman,
3809Konstantin Voronkov,
3810Kris Zyp,
3811Lisa Dusseault,
3812Maciej Stachowiak,
3813Marc Schneider,
3814Marc Slemko,
3815Mark Baker,
3816Mark Pauley,
3817Mark Watson,
3818Markus Isomaki,
3819Markus Lanthaler,
3820Martin J. Duerst,
3821Martin Musatov,
3822Martin Nilsson,
3823Martin Thomson,
3824Matt Lynch,
3825Matthew Cox,
3826Max Clark,
3827Michael Burrows,
3828Michael Hausenblas,
3829Mike Amundsen,
3830Mike Belshe,
3831Mike Kelly,
3832Mike Schinkel,
3833Miles Sabin,
3834Murray S. Kucherawy,
3835Mykyta Yevstifeyev,
3836Nathan Rixham,
3837Nicholas Shanks,
3838Nico Williams,
3839Nicolas Alvarez,
3840Nicolas Mailhot,
3841Noah Slater,
3842Pablo Castro,
3843Pat Hayes,
3844Patrick R. McManus,
3845Paul E. Jones,
3846Paul Hoffman,
3847Paul Marquess,
3848Peter Lepeska,
3849Peter Saint-Andre,
3850Peter Watkins,
3851Phil Archer,
3852Philippe Mougin,
3853Phillip Hallam-Baker,
3854Poul-Henning Kamp,
3855Preethi Natarajan,
3856Rajeev Bector,
3857Ray Polk,
3858Reto Bachmann-Gmuer,
3859Richard Cyganiak,
3860Robert Brewer,
3861Robert Collins,
3862Robert O'Callahan,
3863Robert Olofsson,
3864Robert Sayre,
3865Robert Siemer,
3866Robert de Wilde,
3867Roberto Javier Godoy,
3868Roberto Peon,
3869Roland Zink,
3870Ronny Widjaja,
3871S. Mike Dierken,
3872Salvatore Loreto,
3873Sam Johnston,
3874Sam Ruby,
3875Scott Lawrence (who maintained the original issues list),
3876Sean B. Palmer,
3877Shane McCarron,
3878Stefan Eissing,
3879Stefan Tilkov,
3880Stefanos Harhalakis,
3881Stephane Bortzmeyer,
3882Stephen Farrell,
3883Stephen Ludin,
3884Stuart Williams,
3885Subbu Allamaraju,
3886Sylvain Hellegouarch,
3887Tapan Divekar,
3888Tatsuya Hayashi,
3889Ted Hardie,
3890Thomas Broyer,
3891Thomas Fossati,
3892Thomas Nordin,
3893Thomas Roessler,
3894Tim Bray,
3895Tim Morgan,
3896Tim Olsen,
3897Tom Zhou,
3898Travis Snoozy,
3899Tyler Close,
3900Vincent Murphy,
3901Wenbo Zhu,
3902Werner Baumann,
3903Wilbur Streett,
3904Wilfredo Sanchez Vega,
3905William A. Rowe Jr.,
3906William Chan,
3907Willy Tarreau,
3908Xiaoshu Wang,
3909Yaron Goland,
3910Yngve Nysaeter Pettersen,
3911Yoav Nir,
3912Yogesh Bang,
3913Yutaka Oiwa,
3914Yves Lafon (long-time member of the editor team),
3915Zed A. Shaw, and
3916Zhong Yu.
3917</t>
3918<?ENDINC acks ?>
3919<t>
3920   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
3921   acknowledgements from prior revisions.
3922</t>
3923</section>
3924
3925</middle>
3926<back>
3927
3928<references title="Normative References">
3929
3930<reference anchor="Part2">
3931  <front>
3932    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
3933    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3934      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
3935      <address><email>fielding@gbiv.com</email></address>
3936    </author>
3937    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3938      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3939      <address><email>julian.reschke@greenbytes.de</email></address>
3940    </author>
3941    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3942  </front>
3943  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
3944  <x:source href="p2-semantics.xml" basename="p2-semantics">
3945    <x:defines>1xx (Informational)</x:defines>
3946    <x:defines>1xx</x:defines>
3947    <x:defines>100 (Continue)</x:defines>
3948    <x:defines>101 (Switching Protocols)</x:defines>
3949    <x:defines>2xx (Successful)</x:defines>
3950    <x:defines>2xx</x:defines>
3951    <x:defines>200 (OK)</x:defines>
3952    <x:defines>204 (No Content)</x:defines>
3953    <x:defines>3xx (Redirection)</x:defines>
3954    <x:defines>3xx</x:defines>
3955    <x:defines>301 (Moved Permanently)</x:defines>
3956    <x:defines>4xx (Client Error)</x:defines>
3957    <x:defines>4xx</x:defines>
3958    <x:defines>400 (Bad Request)</x:defines>
3959    <x:defines>405 (Method Not Allowed)</x:defines>
3960    <x:defines>411 (Length Required)</x:defines>
3961    <x:defines>414 (URI Too Long)</x:defines>
3962    <x:defines>417 (Expectation Failed)</x:defines>
3963    <x:defines>426 (Upgrade Required)</x:defines>
3964    <x:defines>501 (Not Implemented)</x:defines>
3965    <x:defines>502 (Bad Gateway)</x:defines>
3966    <x:defines>505 (HTTP Version Not Supported)</x:defines>
3967    <x:defines>Allow</x:defines>
3968    <x:defines>Content-Encoding</x:defines>
3969    <x:defines>Content-Location</x:defines>
3970    <x:defines>Content-Type</x:defines>
3971    <x:defines>Date</x:defines>
3972    <x:defines>Expect</x:defines>
3973    <x:defines>Location</x:defines>
3974    <x:defines>Server</x:defines>
3975    <x:defines>User-Agent</x:defines>
3976  </x:source>
3977</reference>
3978
3979<reference anchor="Part4">
3980  <front>
3981    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
3982    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
3983      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
3984      <address><email>fielding@gbiv.com</email></address>
3985    </author>
3986    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
3987      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3988      <address><email>julian.reschke@greenbytes.de</email></address>
3989    </author>
3990    <date month="&ID-MONTH;" year="&ID-YEAR;" />
3991  </front>
3992  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
3993  <x:source basename="p4-conditional" href="p4-conditional.xml">
3994    <x:defines>304 (Not Modified)</x:defines>
3995    <x:defines>ETag</x:defines>
3996    <x:defines>Last-Modified</x:defines>
3997  </x:source>
3998</reference>
3999
4000<reference anchor="Part5">
4001  <front>
4002    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
4003    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4004      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4005      <address><email>fielding@gbiv.com</email></address>
4006    </author>
4007    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
4008      <organization abbrev="W3C">World Wide Web Consortium</organization>
4009      <address><email>ylafon@w3.org</email></address>
4010    </author>
4011    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4012      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4013      <address><email>julian.reschke@greenbytes.de</email></address>
4014    </author>
4015    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4016  </front>
4017  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4018  <x:source href="p5-range.xml" basename="p5-range">
4019    <x:defines>Content-Range</x:defines>
4020  </x:source>
4021</reference>
4022
4023<reference anchor="Part6">
4024  <front>
4025    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4026    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4027      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4028      <address><email>fielding@gbiv.com</email></address>
4029    </author>
4030    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4031      <organization>Akamai</organization>
4032      <address><email>mnot@mnot.net</email></address>
4033    </author>
4034    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4035      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4036      <address><email>julian.reschke@greenbytes.de</email></address>
4037    </author>
4038    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4039  </front>
4040  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4041  <x:source href="p6-cache.xml" basename="p6-cache">
4042    <x:defines>Expires</x:defines>
4043  </x:source>
4044</reference>
4045
4046<reference anchor="Part7">
4047  <front>
4048    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4049    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4050      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4051      <address><email>fielding@gbiv.com</email></address>
4052    </author>
4053    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4054      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4055      <address><email>julian.reschke@greenbytes.de</email></address>
4056    </author>
4057    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4058  </front>
4059  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4060  <x:source href="p7-auth.xml" basename="p7-auth">
4061    <x:defines>Proxy-Authenticate</x:defines>
4062    <x:defines>Proxy-Authorization</x:defines>
4063  </x:source>
4064</reference>
4065
4066<reference anchor="RFC5234">
4067  <front>
4068    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4069    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4070      <organization>Brandenburg InternetWorking</organization>
4071      <address>
4072        <email>dcrocker@bbiw.net</email>
4073      </address> 
4074    </author>
4075    <author initials="P." surname="Overell" fullname="Paul Overell">
4076      <organization>THUS plc.</organization>
4077      <address>
4078        <email>paul.overell@thus.net</email>
4079      </address>
4080    </author>
4081    <date month="January" year="2008"/>
4082  </front>
4083  <seriesInfo name="STD" value="68"/>
4084  <seriesInfo name="RFC" value="5234"/>
4085</reference>
4086
4087<reference anchor="RFC2119">
4088  <front>
4089    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4090    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4091      <organization>Harvard University</organization>
4092      <address><email>sob@harvard.edu</email></address>
4093    </author>
4094    <date month="March" year="1997"/>
4095  </front>
4096  <seriesInfo name="BCP" value="14"/>
4097  <seriesInfo name="RFC" value="2119"/>
4098</reference>
4099
4100<reference anchor="RFC3986">
4101 <front>
4102  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4103  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4104    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4105    <address>
4106       <email>timbl@w3.org</email>
4107       <uri>http://www.w3.org/People/Berners-Lee/</uri>
4108    </address>
4109  </author>
4110  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4111    <organization abbrev="Day Software">Day Software</organization>
4112    <address>
4113      <email>fielding@gbiv.com</email>
4114      <uri>http://roy.gbiv.com/</uri>
4115    </address>
4116  </author>
4117  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4118    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4119    <address>
4120      <email>LMM@acm.org</email>
4121      <uri>http://larry.masinter.net/</uri>
4122    </address>
4123  </author>
4124  <date month='January' year='2005'></date>
4125 </front>
4126 <seriesInfo name="STD" value="66"/>
4127 <seriesInfo name="RFC" value="3986"/>
4128</reference>
4129
4130<reference anchor="USASCII">
4131  <front>
4132    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4133    <author>
4134      <organization>American National Standards Institute</organization>
4135    </author>
4136    <date year="1986"/>
4137  </front>
4138  <seriesInfo name="ANSI" value="X3.4"/>
4139</reference>
4140
4141<reference anchor="RFC1950">
4142  <front>
4143    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4144    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4145      <organization>Aladdin Enterprises</organization>
4146      <address><email>ghost@aladdin.com</email></address>
4147    </author>
4148    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4149    <date month="May" year="1996"/>
4150  </front>
4151  <seriesInfo name="RFC" value="1950"/>
4152  <!--<annotation>
4153    RFC 1950 is an Informational RFC, thus it might be less stable than
4154    this specification. On the other hand, this downward reference was
4155    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4156    therefore it is unlikely to cause problems in practice. See also
4157    <xref target="BCP97"/>.
4158  </annotation>-->
4159</reference>
4160
4161<reference anchor="RFC1951">
4162  <front>
4163    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4164    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4165      <organization>Aladdin Enterprises</organization>
4166      <address><email>ghost@aladdin.com</email></address>
4167    </author>
4168    <date month="May" year="1996"/>
4169  </front>
4170  <seriesInfo name="RFC" value="1951"/>
4171  <!--<annotation>
4172    RFC 1951 is an Informational RFC, thus it might be less stable than
4173    this specification. On the other hand, this downward reference was
4174    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4175    therefore it is unlikely to cause problems in practice. See also
4176    <xref target="BCP97"/>.
4177  </annotation>-->
4178</reference>
4179
4180<reference anchor="RFC1952">
4181  <front>
4182    <title>GZIP file format specification version 4.3</title>
4183    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4184      <organization>Aladdin Enterprises</organization>
4185      <address><email>ghost@aladdin.com</email></address>
4186    </author>
4187    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4188      <address><email>gzip@prep.ai.mit.edu</email></address>
4189    </author>
4190    <author initials="M." surname="Adler" fullname="Mark Adler">
4191      <address><email>madler@alumni.caltech.edu</email></address>
4192    </author>
4193    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4194      <address><email>ghost@aladdin.com</email></address>
4195    </author>
4196    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4197      <address><email>randeg@alumni.rpi.edu</email></address>
4198    </author>
4199    <date month="May" year="1996"/>
4200  </front>
4201  <seriesInfo name="RFC" value="1952"/>
4202  <!--<annotation>
4203    RFC 1952 is an Informational RFC, thus it might be less stable than
4204    this specification. On the other hand, this downward reference was
4205    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4206    therefore it is unlikely to cause problems in practice. See also
4207    <xref target="BCP97"/>.
4208  </annotation>-->
4209</reference>
4210
4211</references>
4212
4213<references title="Informative References">
4214
4215<reference anchor="ISO-8859-1">
4216  <front>
4217    <title>
4218     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4219    </title>
4220    <author>
4221      <organization>International Organization for Standardization</organization>
4222    </author>
4223    <date year="1998"/>
4224  </front>
4225  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4226</reference>
4227
4228<reference anchor='RFC1919'>
4229  <front>
4230    <title>Classical versus Transparent IP Proxies</title>
4231    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4232      <address><email>mchatel@pax.eunet.ch</email></address>
4233    </author>
4234    <date year='1996' month='March' />
4235  </front>
4236  <seriesInfo name='RFC' value='1919' />
4237</reference>
4238
4239<reference anchor="RFC1945">
4240  <front>
4241    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4242    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4243      <organization>MIT, Laboratory for Computer Science</organization>
4244      <address><email>timbl@w3.org</email></address>
4245    </author>
4246    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4247      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4248      <address><email>fielding@ics.uci.edu</email></address>
4249    </author>
4250    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4251      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4252      <address><email>frystyk@w3.org</email></address>
4253    </author>
4254    <date month="May" year="1996"/>
4255  </front>
4256  <seriesInfo name="RFC" value="1945"/>
4257</reference>
4258
4259<reference anchor="RFC2045">
4260  <front>
4261    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4262    <author initials="N." surname="Freed" fullname="Ned Freed">
4263      <organization>Innosoft International, Inc.</organization>
4264      <address><email>ned@innosoft.com</email></address>
4265    </author>
4266    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4267      <organization>First Virtual Holdings</organization>
4268      <address><email>nsb@nsb.fv.com</email></address>
4269    </author>
4270    <date month="November" year="1996"/>
4271  </front>
4272  <seriesInfo name="RFC" value="2045"/>
4273</reference>
4274
4275<reference anchor="RFC2047">
4276  <front>
4277    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4278    <author initials="K." surname="Moore" fullname="Keith Moore">
4279      <organization>University of Tennessee</organization>
4280      <address><email>moore@cs.utk.edu</email></address>
4281    </author>
4282    <date month="November" year="1996"/>
4283  </front>
4284  <seriesInfo name="RFC" value="2047"/>
4285</reference>
4286
4287<reference anchor="RFC2068">
4288  <front>
4289    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4290    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
4291      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4292      <address><email>fielding@ics.uci.edu</email></address>
4293    </author>
4294    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4295      <organization>MIT Laboratory for Computer Science</organization>
4296      <address><email>jg@w3.org</email></address>
4297    </author>
4298    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4299      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4300      <address><email>mogul@wrl.dec.com</email></address>
4301    </author>
4302    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4303      <organization>MIT Laboratory for Computer Science</organization>
4304      <address><email>frystyk@w3.org</email></address>
4305    </author>
4306    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4307      <organization>MIT Laboratory for Computer Science</organization>
4308      <address><email>timbl@w3.org</email></address>
4309    </author>
4310    <date month="January" year="1997"/>
4311  </front>
4312  <seriesInfo name="RFC" value="2068"/>
4313</reference>
4314
4315<reference anchor="RFC2145">
4316  <front>
4317    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4318    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4319      <organization>Western Research Laboratory</organization>
4320      <address><email>mogul@wrl.dec.com</email></address>
4321    </author>
4322    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4323      <organization>Department of Information and Computer Science</organization>
4324      <address><email>fielding@ics.uci.edu</email></address>
4325    </author>
4326    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4327      <organization>MIT Laboratory for Computer Science</organization>
4328      <address><email>jg@w3.org</email></address>
4329    </author>
4330    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4331      <organization>W3 Consortium</organization>
4332      <address><email>frystyk@w3.org</email></address>
4333    </author>
4334    <date month="May" year="1997"/>
4335  </front>
4336  <seriesInfo name="RFC" value="2145"/>
4337</reference>
4338
4339<reference anchor="RFC2616">
4340  <front>
4341    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4342    <author initials="R." surname="Fielding" fullname="R. Fielding">
4343      <organization>University of California, Irvine</organization>
4344      <address><email>fielding@ics.uci.edu</email></address>
4345    </author>
4346    <author initials="J." surname="Gettys" fullname="J. Gettys">
4347      <organization>W3C</organization>
4348      <address><email>jg@w3.org</email></address>
4349    </author>
4350    <author initials="J." surname="Mogul" fullname="J. Mogul">
4351      <organization>Compaq Computer Corporation</organization>
4352      <address><email>mogul@wrl.dec.com</email></address>
4353    </author>
4354    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4355      <organization>MIT Laboratory for Computer Science</organization>
4356      <address><email>frystyk@w3.org</email></address>
4357    </author>
4358    <author initials="L." surname="Masinter" fullname="L. Masinter">
4359      <organization>Xerox Corporation</organization>
4360      <address><email>masinter@parc.xerox.com</email></address>
4361    </author>
4362    <author initials="P." surname="Leach" fullname="P. Leach">
4363      <organization>Microsoft Corporation</organization>
4364      <address><email>paulle@microsoft.com</email></address>
4365    </author>
4366    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4367      <organization>W3C</organization>
4368      <address><email>timbl@w3.org</email></address>
4369    </author>
4370    <date month="June" year="1999"/>
4371  </front>
4372  <seriesInfo name="RFC" value="2616"/>
4373</reference>
4374
4375<reference anchor='RFC2817'>
4376  <front>
4377    <title>Upgrading to TLS Within HTTP/1.1</title>
4378    <author initials='R.' surname='Khare' fullname='R. Khare'>
4379      <organization>4K Associates / UC Irvine</organization>
4380      <address><email>rohit@4K-associates.com</email></address>
4381    </author>
4382    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4383      <organization>Agranat Systems, Inc.</organization>
4384      <address><email>lawrence@agranat.com</email></address>
4385    </author>
4386    <date year='2000' month='May' />
4387  </front>
4388  <seriesInfo name='RFC' value='2817' />
4389</reference>
4390
4391<reference anchor='RFC2818'>
4392  <front>
4393    <title>HTTP Over TLS</title>
4394    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4395      <organization>RTFM, Inc.</organization>
4396      <address><email>ekr@rtfm.com</email></address>
4397    </author>
4398    <date year='2000' month='May' />
4399  </front>
4400  <seriesInfo name='RFC' value='2818' />
4401</reference>
4402
4403<reference anchor='RFC3040'>
4404  <front>
4405    <title>Internet Web Replication and Caching Taxonomy</title>
4406    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4407      <organization>Equinix, Inc.</organization>
4408    </author>
4409    <author initials='I.' surname='Melve' fullname='I. Melve'>
4410      <organization>UNINETT</organization>
4411    </author>
4412    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4413      <organization>CacheFlow Inc.</organization>
4414    </author>
4415    <date year='2001' month='January' />
4416  </front>
4417  <seriesInfo name='RFC' value='3040' />
4418</reference>
4419
4420<reference anchor='RFC3864'>
4421  <front>
4422    <title>Registration Procedures for Message Header Fields</title>
4423    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4424      <organization>Nine by Nine</organization>
4425      <address><email>GK-IETF@ninebynine.org</email></address>
4426    </author>
4427    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4428      <organization>BEA Systems</organization>
4429      <address><email>mnot@pobox.com</email></address>
4430    </author>
4431    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4432      <organization>HP Labs</organization>
4433      <address><email>JeffMogul@acm.org</email></address>
4434    </author>
4435    <date year='2004' month='September' />
4436  </front>
4437  <seriesInfo name='BCP' value='90' />
4438  <seriesInfo name='RFC' value='3864' />
4439</reference>
4440
4441<reference anchor='RFC4033'>
4442  <front>
4443    <title>DNS Security Introduction and Requirements</title>
4444    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4445    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4446    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4447    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4448    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4449    <date year='2005' month='March' />
4450  </front>
4451  <seriesInfo name='RFC' value='4033' />
4452</reference>
4453
4454<reference anchor="RFC4288">
4455  <front>
4456    <title>Media Type Specifications and Registration Procedures</title>
4457    <author initials="N." surname="Freed" fullname="N. Freed">
4458      <organization>Sun Microsystems</organization>
4459      <address>
4460        <email>ned.freed@mrochek.com</email>
4461      </address>
4462    </author>
4463    <author initials="J." surname="Klensin" fullname="J. Klensin">
4464      <address>
4465        <email>klensin+ietf@jck.com</email>
4466      </address>
4467    </author>
4468    <date year="2005" month="December"/>
4469  </front>
4470  <seriesInfo name="BCP" value="13"/>
4471  <seriesInfo name="RFC" value="4288"/>
4472</reference>
4473
4474<reference anchor='RFC4395'>
4475  <front>
4476    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4477    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4478      <organization>AT&amp;T Laboratories</organization>
4479      <address>
4480        <email>tony+urireg@maillennium.att.com</email>
4481      </address>
4482    </author>
4483    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4484      <organization>Qualcomm, Inc.</organization>
4485      <address>
4486        <email>hardie@qualcomm.com</email>
4487      </address>
4488    </author>
4489    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4490      <organization>Adobe Systems</organization>
4491      <address>
4492        <email>LMM@acm.org</email>
4493      </address>
4494    </author>
4495    <date year='2006' month='February' />
4496  </front>
4497  <seriesInfo name='BCP' value='115' />
4498  <seriesInfo name='RFC' value='4395' />
4499</reference>
4500
4501<reference anchor='RFC4559'>
4502  <front>
4503    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4504    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4505    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4506    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4507    <date year='2006' month='June' />
4508  </front>
4509  <seriesInfo name='RFC' value='4559' />
4510</reference>
4511
4512<reference anchor='RFC5226'>
4513  <front>
4514    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4515    <author initials='T.' surname='Narten' fullname='T. Narten'>
4516      <organization>IBM</organization>
4517      <address><email>narten@us.ibm.com</email></address>
4518    </author>
4519    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4520      <organization>Google</organization>
4521      <address><email>Harald@Alvestrand.no</email></address>
4522    </author>
4523    <date year='2008' month='May' />
4524  </front>
4525  <seriesInfo name='BCP' value='26' />
4526  <seriesInfo name='RFC' value='5226' />
4527</reference>
4528
4529<reference anchor='RFC5246'>
4530   <front>
4531      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4532      <author initials='T.' surname='Dierks' fullname='T. Dierks'>
4533         <organization />
4534      </author>
4535      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4536         <organization>RTFM, Inc.</organization>
4537      </author>
4538      <date year='2008' month='August' />
4539   </front>
4540   <seriesInfo name='RFC' value='5246' />
4541</reference>
4542
4543<reference anchor="RFC5322">
4544  <front>
4545    <title>Internet Message Format</title>
4546    <author initials="P." surname="Resnick" fullname="P. Resnick">
4547      <organization>Qualcomm Incorporated</organization>
4548    </author>
4549    <date year="2008" month="October"/>
4550  </front> 
4551  <seriesInfo name="RFC" value="5322"/>
4552</reference>
4553
4554<reference anchor="RFC6265">
4555  <front>
4556    <title>HTTP State Management Mechanism</title>
4557    <author initials="A." surname="Barth" fullname="Adam Barth">
4558      <organization abbrev="U.C. Berkeley">
4559        University of California, Berkeley
4560      </organization>
4561      <address><email>abarth@eecs.berkeley.edu</email></address>
4562    </author>
4563    <date year="2011" month="April" />
4564  </front>
4565  <seriesInfo name="RFC" value="6265"/>
4566</reference>
4567
4568<!--<reference anchor='BCP97'>
4569  <front>
4570    <title>Handling Normative References to Standards-Track Documents</title>
4571    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
4572      <address>
4573        <email>klensin+ietf@jck.com</email>
4574      </address>
4575    </author>
4576    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
4577      <organization>MIT</organization>
4578      <address>
4579        <email>hartmans-ietf@mit.edu</email>
4580      </address>
4581    </author>
4582    <date year='2007' month='June' />
4583  </front>
4584  <seriesInfo name='BCP' value='97' />
4585  <seriesInfo name='RFC' value='4897' />
4586</reference>-->
4587
4588<reference anchor="Kri2001" target="http://arxiv.org/abs/cs.SE/0105018">
4589  <front>
4590    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
4591    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
4592    <date year="2001" month="November"/>
4593  </front>
4594  <seriesInfo name="ACM Transactions on Internet Technology" value="Vol. 1, #2"/>
4595</reference>
4596
4597</references>
4598
4599
4600<section title="HTTP Version History" anchor="compatibility">
4601<t>
4602   HTTP has been in use by the World-Wide Web global information initiative
4603   since 1990. The first version of HTTP, later referred to as HTTP/0.9,
4604   was a simple protocol for hypertext data transfer across the Internet
4605   with only a single request method (GET) and no metadata.
4606   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
4607   methods and MIME-like messaging that could include metadata about the data
4608   transferred and modifiers on the request/response semantics. However,
4609   HTTP/1.0 did not sufficiently take into consideration the effects of
4610   hierarchical proxies, caching, the need for persistent connections, or
4611   name-based virtual hosts. The proliferation of incompletely-implemented
4612   applications calling themselves "HTTP/1.0" further necessitated a
4613   protocol version change in order for two communicating applications
4614   to determine each other's true capabilities.
4615</t>
4616<t>
4617   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
4618   requirements that enable reliable implementations, adding only
4619   those new features that will either be safely ignored by an HTTP/1.0
4620   recipient or only sent when communicating with a party advertising
4621   conformance with HTTP/1.1.
4622</t>
4623<t>
4624   It is beyond the scope of a protocol specification to mandate
4625   conformance with previous versions. HTTP/1.1 was deliberately
4626   designed, however, to make supporting previous versions easy.
4627   We would expect a general-purpose HTTP/1.1 server to understand
4628   any valid request in the format of HTTP/1.0 and respond appropriately
4629   with an HTTP/1.1 message that only uses features understood (or
4630   safely ignored) by HTTP/1.0 clients.  Likewise, we would expect
4631   an HTTP/1.1 client to understand any valid HTTP/1.0 response.
4632</t>
4633<t>
4634   Since HTTP/0.9 did not support header fields in a request,
4635   there is no mechanism for it to support name-based virtual
4636   hosts (selection of resource by inspection of the <x:ref>Host</x:ref> header
4637   field).  Any server that implements name-based virtual hosts
4638   ought to disable support for HTTP/0.9.  Most requests that
4639   appear to be HTTP/0.9 are, in fact, badly constructed HTTP/1.x
4640   requests wherein a buggy client failed to properly encode
4641   linear whitespace found in a URI reference and placed in
4642   the request-target.
4643</t>
4644
4645<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
4646<t>
4647   This section summarizes major differences between versions HTTP/1.0
4648   and HTTP/1.1.
4649</t>
4650
4651<section title="Multi-homed Web Servers" anchor="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">
4652<t>
4653   The requirements that clients and servers support the <x:ref>Host</x:ref>
4654   header field (<xref target="header.host"/>), report an error if it is
4655   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
4656   are among the most important changes defined by HTTP/1.1.
4657</t>
4658<t>
4659   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4660   addresses and servers; there was no other established mechanism for
4661   distinguishing the intended server of a request than the IP address
4662   to which that request was directed. The <x:ref>Host</x:ref> header field was
4663   introduced during the development of HTTP/1.1 and, though it was
4664   quickly implemented by most HTTP/1.0 browsers, additional requirements
4665   were placed on all HTTP/1.1 requests in order to ensure complete
4666   adoption.  At the time of this writing, most HTTP-based services
4667   are dependent upon the Host header field for targeting requests.
4668</t>
4669</section>
4670
4671<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4672<t>
4673   In HTTP/1.0, each connection is established by the client prior to the
4674   request and closed by the server after sending the response. However, some
4675   implementations implement the explicitly negotiated ("Keep-Alive") version
4676   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
4677   target="RFC2068"/>.
4678</t>
4679<t>
4680   Some clients and servers might wish to be compatible with these previous
4681   approaches to persistent connections, by explicitly negotiating for them
4682   with a "Connection: keep-alive" request header field. However, some
4683   experimental implementations of HTTP/1.0 persistent connections are faulty;
4684   for example, if a HTTP/1.0 proxy server doesn't understand
4685   <x:ref>Connection</x:ref>, it will erroneously forward that header field
4686   to the next inbound server, which would result in a hung connection.
4687</t>
4688<t>
4689   One attempted solution was the introduction of a Proxy-Connection header
4690   field, targeted specifically at proxies. In practice, this was also
4691   unworkable, because proxies are often deployed in multiple layers, bringing
4692   about the same problem discussed above.
4693</t>
4694<t>
4695   As a result, clients are encouraged not to send the Proxy-Connection header
4696   field in any requests.
4697</t>
4698<t>
4699   Clients are also encouraged to consider the use of Connection: keep-alive
4700   in requests carefully; while they can enable persistent connections with
4701   HTTP/1.0 servers, clients using them need will need to monitor the
4702   connection for "hung" requests (which indicate that the client ought stop
4703   sending the header field), and this mechanism ought not be used by clients
4704   at all when a proxy is being used.
4705</t>
4706</section>
4707
4708<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
4709<t>
4710   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
4711   (<xref target="header.transfer-encoding"/>). Proxies/gateways &MUST; remove
4712   any transfer-coding prior to forwarding a message via a MIME-compliant
4713   protocol.
4714</t>
4715</section>
4716
4717</section>
4718
4719<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4720<t>
4721  The HTTP-version ABNF production has been clarified to be case-sensitive.
4722  Additionally, version numbers has been restricted to single digits, due
4723  to the fact that implementations are known to handle multi-digit version
4724  numbers incorrectly.
4725  (<xref target="http.version"/>)
4726</t>
4727<t>
4728  The "HTTPS" URI scheme is now defined by this specification; previously,
4729  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
4730  (<xref target="https.uri"/>)
4731</t>
4732<t>
4733  Invalid whitespace around field-names is now required to be rejected,
4734  because accepting it represents a security vulnerability.
4735  (<xref target="header.fields"/>)
4736</t>
4737<t>
4738  The ABNF productions defining header fields now only list the field value.
4739  (<xref target="header.fields"/>)
4740</t>
4741<t>
4742  Rules about implicit linear whitespace between certain grammar productions
4743  have been removed; now whitespace is only allowed where specifically
4744  defined in the ABNF.
4745  (<xref target="whitespace"/>)
4746</t>
4747<t> 
4748  The NUL octet is no longer allowed in comment and quoted-string text.
4749  (<xref target="field.components"/>)
4750</t> 
4751<t>
4752  The quoted-pair rule no longer allows escaping control characters other than
4753  HTAB.
4754  (<xref target="field.components"/>)
4755</t>
4756<t>
4757  Non-ASCII content in header fields and the reason phrase has been obsoleted
4758  and made opaque (the TEXT rule was removed).
4759  (<xref target="field.components"/>)
4760</t>
4761<t>
4762  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
4763  handled as errors by recipients.
4764  (<xref target="header.content-length"/>)
4765</t>
4766<t>
4767  The "identity" transfer-coding value token has been removed.
4768  (Sections <xref format="counter" target="message.body"/> and
4769  <xref format="counter" target="transfer.codings"/>)
4770</t>
4771<t>
4772  "multipart/byteranges" is no longer a way of determining message body length
4773  detection.
4774  (<xref target="message.body.length"/>)
4775</t>
4776<t>
4777  CONNECT is a new, special case in determining message body length.
4778  (<xref target="message.body.length"/>)
4779</t>
4780<t>
4781  Chunk length does not include the count of the octets in the
4782  chunk header and trailer.
4783  (<xref target="chunked.encoding"/>)
4784</t>
4785<t>
4786  Use of chunk extensions is deprecated, and line folding in them is
4787  disallowed.
4788  (<xref target="chunked.encoding"/>)
4789</t>
4790<t>
4791  The path-absolute + query components of RFC3986 have been used to define the
4792  request-target, instead of abs_path from RFC 1808.
4793  (<xref target="request-target"/>)
4794</t>
4795<t>
4796  The asterisk form of the request-target is only allowed in the OPTIONS
4797  method.
4798  (<xref target="request-target"/>)
4799</t>
4800<t>
4801  Exactly when "close" connection options have to be sent has been clarified.
4802  (<xref target="header.connection"/>)
4803</t>
4804<t> 
4805  "hop-by-hop" header fields are required to appear in the Connection header
4806  field; just because they're defined as hop-by-hop in this specification
4807  doesn't exempt them.
4808  (<xref target="header.connection"/>)
4809</t>
4810<t>
4811  The limit of two connections per server has been removed.
4812  (<xref target="persistent.connections"/>)
4813</t>
4814<t>
4815  An idempotent sequence of requests is no longer required to be retried.
4816  (<xref target="persistent.connections"/>)
4817</t>
4818<t>
4819  The requirement to retry requests under certain circumstances when the
4820  server prematurely closes the connection has been removed.
4821  (<xref target="persistent.reuse"/>)
4822</t>
4823<t>
4824  Some extraneous requirements about when servers are allowed to close
4825  connections prematurely have been removed.
4826  (<xref target="persistent.connections"/>)
4827</t>
4828<t>
4829  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
4830  responses other than 101 (this was incorporated from <xref
4831  target="RFC2817"/>).
4832  (<xref target="header.upgrade"/>)
4833</t>
4834<t>
4835  Registration of Transfer Codings now requires IETF Review
4836  (<xref target="transfer.coding.registry"/>)
4837</t>
4838<t>
4839  This specification now defines the Upgrade Token Registry, previously
4840  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
4841  (<xref target="upgrade.token.registry"/>)
4842</t>
4843<t>
4844  Empty list elements in list productions have been deprecated.
4845  (<xref target="abnf.extension"/>)
4846</t>
4847</section>
4848</section>
4849
4850<section title="ABNF list extension: #rule" anchor="abnf.extension">
4851<t>
4852  A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
4853  improve readability in the definitions of some header field values.
4854</t>
4855<t>
4856  A construct "#" is defined, similar to "*", for defining comma-delimited
4857  lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
4858  at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
4859  comma (",") and optional whitespace (OWS).   
4860</t>
4861<figure><preamble>
4862  Thus,
4863</preamble><artwork type="example">
4864  1#element =&gt; element *( OWS "," OWS element )
4865</artwork></figure>
4866<figure><preamble>
4867  and:
4868</preamble><artwork type="example">
4869  #element =&gt; [ 1#element ]
4870</artwork></figure>
4871<figure><preamble>
4872  and for n &gt;= 1 and m &gt; 1:
4873</preamble><artwork type="example">
4874  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
4875</artwork></figure>
4876<t>
4877  For compatibility with legacy list rules, recipients &SHOULD; accept empty
4878  list elements. In other words, consumers would follow the list productions:
4879</t>
4880<figure><artwork type="example">
4881  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
4882 
4883  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
4884</artwork></figure>
4885<t>
4886  Note that empty elements do not contribute to the count of elements present,
4887  though.
4888</t>
4889<t>
4890  For example, given these ABNF productions:
4891</t>
4892<figure><artwork type="example">
4893  example-list      = 1#example-list-elmt
4894  example-list-elmt = token ; see <xref target="field.components"/> 
4895</artwork></figure>
4896<t>
4897  Then these are valid values for example-list (not including the double
4898  quotes, which are present for delimitation only):
4899</t>
4900<figure><artwork type="example">
4901  "foo,bar"
4902  "foo ,bar,"
4903  "foo , ,bar,charlie   "
4904</artwork></figure>
4905<t>
4906  But these values would be invalid, as at least one non-empty element is
4907  required:
4908</t>
4909<figure><artwork type="example">
4910  ""
4911  ","
4912  ",   ,"
4913</artwork></figure>
4914<t>
4915  <xref target="collected.abnf"/> shows the collected ABNF, with the list rules
4916  expanded as explained above.
4917</t>
4918</section>
4919
4920<?BEGININC p1-messaging.abnf-appendix ?>
4921<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
4922<figure>
4923<artwork type="abnf" name="p1-messaging.parsed-abnf">
4924<x:ref>BWS</x:ref> = OWS
4925
4926<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
4927 connection-option ] )
4928<x:ref>Content-Length</x:ref> = 1*DIGIT
4929
4930<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
4931 ]
4932<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
4933<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
4934<x:ref>Host</x:ref> = uri-host [ ":" port ]
4935
4936<x:ref>OWS</x:ref> = *( SP / HTAB )
4937
4938<x:ref>RWS</x:ref> = 1*( SP / HTAB )
4939
4940<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
4941<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
4942<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
4943 transfer-coding ] )
4944
4945<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
4946<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
4947
4948<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
4949 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
4950 comment ] ) ] )
4951
4952<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
4953<x:ref>absolute-form</x:ref> = absolute-URI
4954<x:ref>asterisk-form</x:ref> = "*"
4955<x:ref>attribute</x:ref> = token
4956<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
4957<x:ref>authority-form</x:ref> = authority
4958
4959<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
4960<x:ref>chunk-data</x:ref> = 1*OCTET
4961<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
4962<x:ref>chunk-ext-name</x:ref> = token
4963<x:ref>chunk-ext-val</x:ref> = token / quoted-str-nf
4964<x:ref>chunk-size</x:ref> = 1*HEXDIG
4965<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
4966<x:ref>comment</x:ref> = "(" *( ctext / quoted-cpair / comment ) ")"
4967<x:ref>connection-option</x:ref> = token
4968<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
4969 / %x2A-5B ; '*'-'['
4970 / %x5D-7E ; ']'-'~'
4971 / obs-text
4972
4973<x:ref>field-content</x:ref> = *( HTAB / SP / VCHAR / obs-text )
4974<x:ref>field-name</x:ref> = token
4975<x:ref>field-value</x:ref> = *( field-content / obs-fold )
4976
4977<x:ref>header-field</x:ref> = field-name ":" OWS field-value BWS
4978<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ]
4979<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ]
4980
4981<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
4982
4983<x:ref>message-body</x:ref> = *OCTET
4984<x:ref>method</x:ref> = token
4985
4986<x:ref>obs-fold</x:ref> = CRLF ( SP / HTAB )
4987<x:ref>obs-text</x:ref> = %x80-FF
4988<x:ref>origin-form</x:ref> = path-absolute [ "?" query ]
4989
4990<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
4991<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
4992<x:ref>path-absolute</x:ref> = &lt;path-absolute, defined in [RFC3986], Section 3.3&gt;
4993<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
4994<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
4995<x:ref>protocol-name</x:ref> = token
4996<x:ref>protocol-version</x:ref> = token
4997<x:ref>pseudonym</x:ref> = token
4998
4999<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5000 / %x5D-7E ; ']'-'~'
5001 / obs-text
5002<x:ref>qdtext-nf</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5003 / %x5D-7E ; ']'-'~'
5004 / obs-text
5005<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5006<x:ref>quoted-cpair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5007<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5008<x:ref>quoted-str-nf</x:ref> = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
5009<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5010
5011<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5012<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5013<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5014<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5015<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5016<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5017<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5018 asterisk-form
5019
5020<x:ref>special</x:ref> = "(" / ")" / "&lt;" / "&gt;" / "@" / "," / ";" / ":" / "\" /
5021 DQUOTE / "/" / "[" / "]" / "?" / "=" / "{" / "}"
5022<x:ref>start-line</x:ref> = request-line / status-line
5023<x:ref>status-code</x:ref> = 3DIGIT
5024<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5025
5026<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5027<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5028<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5029 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5030<x:ref>token</x:ref> = 1*tchar
5031<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5032<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5033 transfer-extension
5034<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5035<x:ref>transfer-parameter</x:ref> = attribute BWS "=" BWS value
5036
5037<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5038
5039<x:ref>value</x:ref> = word
5040
5041<x:ref>word</x:ref> = token / quoted-string
5042</artwork>
5043</figure>
5044</section>
5045<?ENDINC p1-messaging.abnf-appendix ?>
5046
5047<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5048
5049<section title="Since RFC 2616">
5050<t>
5051  Changes up to the first Working Group Last Call draft are summarized
5052  in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-21#appendix-D"/>.
5053</t>
5054</section>
5055
5056<section title="Since draft-ietf-httpbis-p1-messaging-21" anchor="changes.since.21">
5057<t>
5058  Closed issues:
5059  <list style="symbols">
5060    <t>
5061      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/128"/>:
5062      "Cite HTTPS URI scheme definition" (the spec now includes the HTTPs
5063      scheme definition and thus updates RFC 2818)
5064    </t>
5065    <t>
5066      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/389"/>:
5067      "mention of 'proxies' in section about caches"
5068    </t>
5069  </list>
5070</t>
5071</section>
5072
5073</section>
5074
5075</back>
5076</rfc>
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