source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2083

Last change on this file since 2083 was 2083, checked in by fielding@…, 9 years ago

Requirements are not allowed in appendices. They have been changed to prose.
Changes from RFC2616 have been rewritten for consistency and to remove changes
that are only editorial. Addresses #419

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 222.4 KB
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=''>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns=''>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns=''>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns=''>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns=''>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns=''>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns=''>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns=''>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns=''>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns=''>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "January">
16  <!ENTITY ID-YEAR "2013">
17  <!ENTITY mdash "&#8212;">
18  <!ENTITY Note "<x:h xmlns:x=''>Note:</x:h>">
19  <!ENTITY caching-overview       "<xref target='Part6' x:rel='#caching.overview' xmlns:x=''/>">
20  <!ENTITY cache-incomplete       "<xref target='Part6' x:rel='#response.cacheability' xmlns:x=''/>">
21  <!ENTITY payload                "<xref target='Part2' x:rel='#payload' xmlns:x=''/>">
22  <!ENTITY media-type            "<xref target='Part2' x:rel='#media.type' xmlns:x=''/>">
23  <!ENTITY content-codings        "<xref target='Part2' x:rel='#content.codings' xmlns:x=''/>">
24  <!ENTITY CONNECT                "<xref target='Part2' x:rel='#CONNECT' xmlns:x=''/>">
25  <!ENTITY content.negotiation    "<xref target='Part2' x:rel='#content.negotiation' xmlns:x=''/>">
26  <!ENTITY diff-mime              "<xref target='Part2' x:rel='#differences.between.http.and.mime' xmlns:x=''/>">
27  <!ENTITY representation         "<xref target='Part2' x:rel='#representations' xmlns:x=''/>">
28  <!ENTITY HEAD                   "<xref target='Part2' x:rel='#HEAD' xmlns:x=''/>">
29  <!ENTITY header-allow           "<xref target='Part2' x:rel='#header.allow' xmlns:x=''/>">
30  <!ENTITY header-cache-control   "<xref target='Part6' x:rel='#header.cache-control' xmlns:x=''/>">
31  <!ENTITY header-content-encoding    "<xref target='Part2' x:rel='#header.content-encoding' xmlns:x=''/>">
32  <!ENTITY header-content-location    "<xref target='Part2' x:rel='#header.content-location' xmlns:x=''/>">
33  <!ENTITY header-content-range   "<xref target='Part5' x:rel='#header.content-range' xmlns:x=''/>">
34  <!ENTITY header-content-type    "<xref target='Part2' x:rel='#header.content-type' xmlns:x=''/>">
35  <!ENTITY header-date            "<xref target='Part2' x:rel='' xmlns:x=''/>">
36  <!ENTITY header-etag            "<xref target='Part4' x:rel='#header.etag' xmlns:x=''/>">
37  <!ENTITY header-expires         "<xref target='Part6' x:rel='#header.expires' xmlns:x=''/>">
38  <!ENTITY header-last-modified   "<xref target='Part4' x:rel='#header.last-modified' xmlns:x=''/>">
39  <!ENTITY header-mime-version    "<xref target='Part2' x:rel='#mime-version' xmlns:x=''/>">
40  <!ENTITY header-pragma          "<xref target='Part6' x:rel='#header.pragma' xmlns:x=''/>">
41  <!ENTITY header-proxy-authenticate  "<xref target='Part7' x:rel='#header.proxy-authenticate' xmlns:x=''/>">
42  <!ENTITY header-proxy-authorization "<xref target='Part7' x:rel='#header.proxy-authorization' xmlns:x=''/>">
43  <!ENTITY header-server          "<xref target='Part2' x:rel='#header.server' xmlns:x=''/>">
44  <!ENTITY header-warning         "<xref target='Part6' x:rel='#header.warning' xmlns:x=''/>">
45  <!ENTITY idempotent-methods     "<xref target='Part2' x:rel='#idempotent.methods' xmlns:x=''/>">
46  <!ENTITY methods                "<xref target='Part2' x:rel='#methods' xmlns:x=''/>">
47  <!ENTITY OPTIONS                "<xref target='Part2' x:rel='#OPTIONS' xmlns:x=''/>">
48  <!ENTITY qvalue                 "<xref target='Part2' x:rel='#quality.values' xmlns:x=''/>">
49  <!ENTITY resource               "<xref target='Part2' x:rel='#resources' xmlns:x=''/>">
50  <!ENTITY selected-representation    "<xref target='Part2' x:rel='#selected.representation' xmlns:x=''/>">
51  <!ENTITY status-codes           "<xref target='Part2' x:rel='' xmlns:x=''/>">
52  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x=''/>">
53  <!ENTITY status-203             "<xref target='Part2' x:rel='#status.203' xmlns:x=''/>">
54  <!ENTITY status-3xx             "<xref target='Part2' x:rel='#status.3xx' xmlns:x=''/>">
55  <!ENTITY status-304             "<xref target='Part4' x:rel='#status.304' xmlns:x=''/>">
56  <!ENTITY status-4xx             "<xref target='Part2' x:rel='#status.4xx' xmlns:x=''/>">
57  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x=''/>">
58  <!ENTITY iana-header-registry   "<xref target='Part2' x:rel='#header.field.registry' xmlns:x=''/>">
60<?rfc toc="yes" ?>
61<?rfc symrefs="yes" ?>
62<?rfc sortrefs="yes" ?>
63<?rfc compact="yes"?>
64<?rfc subcompact="no" ?>
65<?rfc linkmailto="no" ?>
66<?rfc editing="no" ?>
67<?rfc comments="yes"?>
68<?rfc inline="yes"?>
69<?rfc rfcedstyle="yes"?>
70<?rfc-ext allow-markup-in-artwork="yes" ?>
71<?rfc-ext include-references-in-index="yes" ?>
72<rfc obsoletes="2145,2616" updates="2817,2818" category="std" x:maturity-level="proposed"
73     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
74     xmlns:x=''>
75<x:link rel="next" basename="p2-semantics"/>
76<x:feedback template="{docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
79  <title abbrev="HTTP/1.1 Message Syntax and Routing">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
81  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
82    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
83    <address>
84      <postal>
85        <street>345 Park Ave</street>
86        <city>San Jose</city>
87        <region>CA</region>
88        <code>95110</code>
89        <country>USA</country>
90      </postal>
91      <email></email>
92      <uri></uri>
93    </address>
94  </author>
96  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
97    <organization abbrev="greenbytes">greenbytes GmbH</organization>
98    <address>
99      <postal>
100        <street>Hafenweg 16</street>
101        <city>Muenster</city><region>NW</region><code>48155</code>
102        <country>Germany</country>
103      </postal>
104      <email></email>
105      <uri></uri>
106    </address>
107  </author>
109  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
110  <workgroup>HTTPbis Working Group</workgroup>
114   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
115   distributed, collaborative, hypertext information systems. HTTP has been in
116   use by the World Wide Web global information initiative since 1990.
117   This document provides an overview of HTTP architecture and its associated
118   terminology, defines the "http" and "https" Uniform Resource Identifier
119   (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements,
120   and describes general security concerns for implementations.
124<note title="Editorial Note (To be removed by RFC Editor)">
125  <t>
126    Discussion of this draft takes place on the HTTPBIS working group
127    mailing list (, which is archived at
128    <eref target=""/>.
129  </t>
130  <t>
131    The current issues list is at
132    <eref target=""/> and related
133    documents (including fancy diffs) can be found at
134    <eref target=""/>.
135  </t>
136  <t>
137    The changes in this draft are summarized in <xref target="changes.since.21"/>.
138  </t>
142<section title="Introduction" anchor="introduction">
144   The Hypertext Transfer Protocol (HTTP) is an application-level
145   request/response protocol that uses extensible semantics and self-descriptive
146   message payloads for flexible interaction with network-based hypertext
147   information systems. This document is the first in a series of documents
148   that collectively form the HTTP/1.1 specification:
149   <list style="empty">
150    <t>RFC xxx1: Message Syntax and Routing</t>
151    <t><xref target="Part2" x:fmt="none">RFC xxx2</xref>: Semantics and Content</t>
152    <t><xref target="Part4" x:fmt="none">RFC xxx3</xref>: Conditional Requests</t>
153    <t><xref target="Part5" x:fmt="none">RFC xxx4</xref>: Range Requests</t>
154    <t><xref target="Part6" x:fmt="none">RFC xxx5</xref>: Caching</t>
155    <t><xref target="Part7" x:fmt="none">RFC xxx6</xref>: Authentication</t>
156   </list>
159   This HTTP/1.1 specification obsoletes and moves to historic status
160   <xref target="RFC2616" x:fmt="none">RFC 2616</xref>, its predecessor
161   <xref target="RFC2068" x:fmt="none">RFC 2068</xref>, and
162   <xref target="RFC2145" x:fmt="none">RFC 2145</xref> (on HTTP versioning).
163   This specification also updates the use of CONNECT to establish a tunnel,
164   previously defined in <xref target="RFC2817" x:fmt="none">RFC 2817</xref>,
165   and defines the "https" URI scheme that was described informally in
166   <xref target="RFC2818" x:fmt="none">RFC 2818</xref>.
169   HTTP is a generic interface protocol for information systems. It is
170   designed to hide the details of how a service is implemented by presenting
171   a uniform interface to clients that is independent of the types of
172   resources provided. Likewise, servers do not need to be aware of each
173   client's purpose: an HTTP request can be considered in isolation rather
174   than being associated with a specific type of client or a predetermined
175   sequence of application steps. The result is a protocol that can be used
176   effectively in many different contexts and for which implementations can
177   evolve independently over time.
180   HTTP is also designed for use as an intermediation protocol for translating
181   communication to and from non-HTTP information systems.
182   HTTP proxies and gateways can provide access to alternative information
183   services by translating their diverse protocols into a hypertext
184   format that can be viewed and manipulated by clients in the same way
185   as HTTP services.
188   One consequence of this flexibility is that the protocol cannot be
189   defined in terms of what occurs behind the interface. Instead, we
190   are limited to defining the syntax of communication, the intent
191   of received communication, and the expected behavior of recipients.
192   If the communication is considered in isolation, then successful
193   actions ought to be reflected in corresponding changes to the
194   observable interface provided by servers. However, since multiple
195   clients might act in parallel and perhaps at cross-purposes, we
196   cannot require that such changes be observable beyond the scope
197   of a single response.
200   This document describes the architectural elements that are used or
201   referred to in HTTP, defines the "http" and "https" URI schemes,
202   describes overall network operation and connection management,
203   and defines HTTP message framing and forwarding requirements.
204   Our goal is to define all of the mechanisms necessary for HTTP message
205   handling that are independent of message semantics, thereby defining the
206   complete set of requirements for message parsers and
207   message-forwarding intermediaries.
211<section title="Requirement Notation" anchor="intro.requirements">
213   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
214   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
215   document are to be interpreted as described in <xref target="RFC2119"/>.
218   Conformance criteria and considerations regarding error handling
219   are defined in <xref target="conformance"/>.
223<section title="Syntax Notation" anchor="notation">
224<iref primary="true" item="Grammar" subitem="ALPHA"/>
225<iref primary="true" item="Grammar" subitem="CR"/>
226<iref primary="true" item="Grammar" subitem="CRLF"/>
227<iref primary="true" item="Grammar" subitem="CTL"/>
228<iref primary="true" item="Grammar" subitem="DIGIT"/>
229<iref primary="true" item="Grammar" subitem="DQUOTE"/>
230<iref primary="true" item="Grammar" subitem="HEXDIG"/>
231<iref primary="true" item="Grammar" subitem="HTAB"/>
232<iref primary="true" item="Grammar" subitem="LF"/>
233<iref primary="true" item="Grammar" subitem="OCTET"/>
234<iref primary="true" item="Grammar" subitem="SP"/>
235<iref primary="true" item="Grammar" subitem="VCHAR"/>
237   This specification uses the Augmented Backus-Naur Form (ABNF) notation
238   of <xref target="RFC5234"/> with the list rule extension defined in
239   <xref target="abnf.extension"/>.  <xref target="collected.abnf"/> shows
240   the collected ABNF with the list rule expanded.
242<t anchor="core.rules">
243  <x:anchor-alias value="ALPHA"/>
244  <x:anchor-alias value="CTL"/>
245  <x:anchor-alias value="CR"/>
246  <x:anchor-alias value="CRLF"/>
247  <x:anchor-alias value="DIGIT"/>
248  <x:anchor-alias value="DQUOTE"/>
249  <x:anchor-alias value="HEXDIG"/>
250  <x:anchor-alias value="HTAB"/>
251  <x:anchor-alias value="LF"/>
252  <x:anchor-alias value="OCTET"/>
253  <x:anchor-alias value="SP"/>
254  <x:anchor-alias value="VCHAR"/>
255   The following core rules are included by
256   reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
257   ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
258   DIGIT (decimal 0-9), DQUOTE (double quote),
259   HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed),
260   OCTET (any 8-bit sequence of data), SP (space), and
261   VCHAR (any visible <xref target="USASCII"/> character).
264   As a convention, ABNF rule names prefixed with "obs-" denote
265   "obsolete" grammar rules that appear for historical reasons.
270<section title="Architecture" anchor="architecture">
272   HTTP was created for the World Wide Web architecture
273   and has evolved over time to support the scalability needs of a worldwide
274   hypertext system. Much of that architecture is reflected in the terminology
275   and syntax productions used to define HTTP.
278<section title="Client/Server Messaging" anchor="operation">
279<iref primary="true" item="client"/>
280<iref primary="true" item="server"/>
281<iref primary="true" item="connection"/>
283   HTTP is a stateless request/response protocol that operates by exchanging
284   <x:dfn>messages</x:dfn> (<xref target="http.message"/>) across a reliable
285   transport or session-layer
286   "<x:dfn>connection</x:dfn>" (<xref target=""/>).
287   An HTTP "<x:dfn>client</x:dfn>" is a program that establishes a connection
288   to a server for the purpose of sending one or more HTTP requests.
289   An HTTP "<x:dfn>server</x:dfn>" is a program that accepts connections
290   in order to service HTTP requests by sending HTTP responses.
292<iref primary="true" item="user agent"/>
293<iref primary="true" item="origin server"/>
294<iref primary="true" item="browser"/>
295<iref primary="true" item="spider"/>
296<iref primary="true" item="sender"/>
297<iref primary="true" item="recipient"/>
299   The terms client and server refer only to the roles that
300   these programs perform for a particular connection.  The same program
301   might act as a client on some connections and a server on others.
302   We use the term "<x:dfn>user agent</x:dfn>" to refer to any of the various
303   client programs that initiate a request, including (but not limited to)
304   browsers, spiders (web-based robots), command-line tools, native
305   applications, and mobile apps.  The term "<x:dfn>origin server</x:dfn>" is
306   used to refer to the program that can originate authoritative responses to
307   a request. For general requirements, we use the terms
308   "<x:dfn>sender</x:dfn>" and "<x:dfn>recipient</x:dfn>" to refer to any
309   component that sends or receives, respectively, a given message.
312   HTTP relies upon the Uniform Resource Identifier (URI)
313   standard <xref target="RFC3986"/> to indicate the target resource
314   (<xref target="target-resource"/>) and relationships between resources.
315   Messages are passed in a format similar to that used by Internet mail
316   <xref target="RFC5322"/> and the Multipurpose Internet Mail Extensions
317   (MIME) <xref target="RFC2045"/> (see &diff-mime; for the differences
318   between HTTP and MIME messages).
321   Most HTTP communication consists of a retrieval request (GET) for
322   a representation of some resource identified by a URI.  In the
323   simplest case, this might be accomplished via a single bidirectional
324   connection (===) between the user agent (UA) and the origin server (O).
326<figure><artwork type="drawing">
327         request   &gt;
328    <x:highlight>UA</x:highlight> ======================================= <x:highlight>O</x:highlight>
329                                &lt;   response
331<iref primary="true" item="message"/>
332<iref primary="true" item="request"/>
333<iref primary="true" item="response"/>
335   A client sends an HTTP request to a server in the form of a <x:dfn>request</x:dfn>
336   message, beginning with a request-line that includes a method, URI, and
337   protocol version (<xref target="request.line"/>),
338   followed by header fields containing
339   request modifiers, client information, and representation metadata
340   (<xref target="header.fields"/>),
341   an empty line to indicate the end of the header section, and finally
342   a message body containing the payload body (if any,
343   <xref target="message.body"/>).
346   A server responds to a client's request by sending one or more HTTP
347   <x:dfn>response</x:dfn>
348   messages, each beginning with a status line that
349   includes the protocol version, a success or error code, and textual
350   reason phrase (<xref target="status.line"/>),
351   possibly followed by header fields containing server
352   information, resource metadata, and representation metadata
353   (<xref target="header.fields"/>),
354   an empty line to indicate the end of the header section, and finally
355   a message body containing the payload body (if any,
356   <xref target="message.body"/>).
359   A connection might be used for multiple request/response exchanges,
360   as defined in <xref target="persistent.connections"/>.
363   The following example illustrates a typical message exchange for a
364   GET request on the URI "":
367client request:
368</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
369GET /hello.txt HTTP/1.1
370User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
372Accept-Language: en, mi
376server response:
377</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
378HTTP/1.1 200 OK
379Date: Mon, 27 Jul 2009 12:28:53 GMT
380Server: Apache
381Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
382ETag: "34aa387-d-1568eb00"
383Accept-Ranges: bytes
384Content-Length: <x:length-of target="exbody"/>
385Vary: Accept-Encoding
386Content-Type: text/plain
388<x:span anchor="exbody">Hello World!
392<section title="Implementation Diversity" anchor="implementation-diversity">
394   When considering the design of HTTP, it is easy to fall into a trap of
395   thinking that all user agents are general-purpose browsers and all origin
396   servers are large public websites. That is not the case in practice.
397   Common HTTP user agents include household appliances, stereos, scales,
398   firmware update scripts, command-line programs, mobile apps,
399   and communication devices in a multitude of shapes and sizes.  Likewise,
400   common HTTP origin servers include home automation units, configurable
401   networking components, office machines, autonomous robots, news feeds,
402   traffic cameras, ad selectors, and video delivery platforms.
405   The term "user agent" does not imply that there is a human user directly
406   interacting with the software agent at the time of a request. In many
407   cases, a user agent is installed or configured to run in the background
408   and save its results for later inspection (or save only a subset of those
409   results that might be interesting or erroneous). Spiders, for example, are
410   typically given a start URI and configured to follow certain behavior while
411   crawling the Web as a hypertext graph.
414   The implementation diversity of HTTP means that we cannot assume the
415   user agent can make interactive suggestions to a user or provide adequate
416   warning for security or privacy options.  In the few cases where this
417   specification requires reporting of errors to the user, it is acceptable
418   for such reporting to only be observable in an error console or log file.
419   Likewise, requirements that an automated action be confirmed by the user
420   before proceeding can be met via advance configuration choices,
421   run-time options, or simply not proceeding with the unsafe action.
425<section title="Intermediaries" anchor="intermediaries">
426<iref primary="true" item="intermediary"/>
428   HTTP enables the use of intermediaries to satisfy requests through
429   a chain of connections.  There are three common forms of HTTP
430   <x:dfn>intermediary</x:dfn>: proxy, gateway, and tunnel.  In some cases,
431   a single intermediary might act as an origin server, proxy, gateway,
432   or tunnel, switching behavior based on the nature of each request.
434<figure><artwork type="drawing">
435         &gt;             &gt;             &gt;             &gt;
436    <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>
437               &lt;             &lt;             &lt;             &lt;
440   The figure above shows three intermediaries (A, B, and C) between the
441   user agent and origin server. A request or response message that
442   travels the whole chain will pass through four separate connections.
443   Some HTTP communication options
444   might apply only to the connection with the nearest, non-tunnel
445   neighbor, only to the end-points of the chain, or to all connections
446   along the chain. Although the diagram is linear, each participant might
447   be engaged in multiple, simultaneous communications. For example, B
448   might be receiving requests from many clients other than A, and/or
449   forwarding requests to servers other than C, at the same time that it
450   is handling A's request.
453<iref primary="true" item="upstream"/><iref primary="true" item="downstream"/>
454<iref primary="true" item="inbound"/><iref primary="true" item="outbound"/>
455   We use the terms "<x:dfn>upstream</x:dfn>" and "<x:dfn>downstream</x:dfn>"
456   to describe various requirements in relation to the directional flow of a
457   message: all messages flow from upstream to downstream.
458   Likewise, we use the terms inbound and outbound to refer to
459   directions in relation to the request path:
460   "<x:dfn>inbound</x:dfn>" means toward the origin server and
461   "<x:dfn>outbound</x:dfn>" means toward the user agent.
463<t><iref primary="true" item="proxy"/>
464   A "<x:dfn>proxy</x:dfn>" is a message forwarding agent that is selected by the
465   client, usually via local configuration rules, to receive requests
466   for some type(s) of absolute URI and attempt to satisfy those
467   requests via translation through the HTTP interface.  Some translations
468   are minimal, such as for proxy requests for "http" URIs, whereas
469   other requests might require translation to and from entirely different
470   application-level protocols. Proxies are often used to group an
471   organization's HTTP requests through a common intermediary for the
472   sake of security, annotation services, or shared caching.
475<iref primary="true" item="transforming proxy"/>
476<iref primary="true" item="non-transforming proxy"/>
477   An HTTP-to-HTTP proxy is called a "<x:dfn>transforming proxy</x:dfn>" if it is designed
478   or configured to modify request or response messages in a semantically
479   meaningful way (i.e., modifications, beyond those required by normal
480   HTTP processing, that change the message in a way that would be
481   significant to the original sender or potentially significant to
482   downstream recipients).  For example, a transforming proxy might be
483   acting as a shared annotation server (modifying responses to include
484   references to a local annotation database), a malware filter, a
485   format transcoder, or an intranet-to-Internet privacy filter.  Such
486   transformations are presumed to be desired by the client (or client
487   organization) that selected the proxy and are beyond the scope of
488   this specification.  However, when a proxy is not intended to transform
489   a given message, we use the term "<x:dfn>non-transforming proxy</x:dfn>" to target
490   requirements that preserve HTTP message semantics. See &status-203; and
491   &header-warning; for status and warning codes related to transformations.
493<t><iref primary="true" item="gateway"/><iref primary="true" item="reverse proxy"/>
494<iref primary="true" item="accelerator"/>
495   A "<x:dfn>gateway</x:dfn>" (a.k.a., "<x:dfn>reverse proxy</x:dfn>")
496   is a receiving agent that acts
497   as a layer above some other server(s) and translates the received
498   requests to the underlying server's protocol.  Gateways are often
499   used to encapsulate legacy or untrusted information services, to
500   improve server performance through "<x:dfn>accelerator</x:dfn>" caching, and to
501   enable partitioning or load-balancing of HTTP services across
502   multiple machines.
505   A gateway behaves as an origin server on its outbound connection and
506   as a user agent on its inbound connection.
507   All HTTP requirements applicable to an origin server
508   also apply to the outbound communication of a gateway.
509   A gateway communicates with inbound servers using any protocol that
510   it desires, including private extensions to HTTP that are outside
511   the scope of this specification.  However, an HTTP-to-HTTP gateway
512   that wishes to interoperate with third-party HTTP servers &MUST;
513   conform to HTTP user agent requirements on the gateway's inbound
514   connection and &MUST; implement the <x:ref>Connection</x:ref>
515   (<xref target="header.connection"/>) and <x:ref>Via</x:ref>
516   (<xref target="header.via"/>) header fields for both connections.
518<t><iref primary="true" item="tunnel"/>
519   A "<x:dfn>tunnel</x:dfn>" acts as a blind relay between two connections
520   without changing the messages. Once active, a tunnel is not
521   considered a party to the HTTP communication, though the tunnel might
522   have been initiated by an HTTP request. A tunnel ceases to exist when
523   both ends of the relayed connection are closed. Tunnels are used to
524   extend a virtual connection through an intermediary, such as when
525   Transport Layer Security (TLS, <xref target="RFC5246"/>) is used to
526   establish confidential communication through a shared firewall proxy.
528<t><iref primary="true" item="interception proxy"/>
529<iref primary="true" item="transparent proxy"/>
530<iref primary="true" item="captive portal"/>
531   The above categories for intermediary only consider those acting as
532   participants in the HTTP communication.  There are also intermediaries
533   that can act on lower layers of the network protocol stack, filtering or
534   redirecting HTTP traffic without the knowledge or permission of message
535   senders. Network intermediaries often introduce security flaws or
536   interoperability problems by violating HTTP semantics.  For example, an
537   "<x:dfn>interception proxy</x:dfn>" <xref target="RFC3040"/> (also commonly
538   known as a "<x:dfn>transparent proxy</x:dfn>" <xref target="RFC1919"/> or
539   "<x:dfn>captive portal</x:dfn>")
540   differs from an HTTP proxy because it is not selected by the client.
541   Instead, an interception proxy filters or redirects outgoing TCP port 80
542   packets (and occasionally other common port traffic).
543   Interception proxies are commonly found on public network access points,
544   as a means of enforcing account subscription prior to allowing use of
545   non-local Internet services, and within corporate firewalls to enforce
546   network usage policies.
547   They are indistinguishable from a man-in-the-middle attack.
550   HTTP is defined as a stateless protocol, meaning that each request message
551   can be understood in isolation.  Many implementations depend on HTTP's
552   stateless design in order to reuse proxied connections or dynamically
553   load-balance requests across multiple servers.  Hence, servers &MUST-NOT;
554   assume that two requests on the same connection are from the same user
555   agent unless the connection is secured and specific to that agent.
556   Some non-standard HTTP extensions (e.g., <xref target="RFC4559"/>) have
557   been known to violate this requirement, resulting in security and
558   interoperability problems.
562<section title="Caches" anchor="caches">
563<iref primary="true" item="cache"/>
565   A "<x:dfn>cache</x:dfn>" is a local store of previous response messages and the
566   subsystem that controls its message storage, retrieval, and deletion.
567   A cache stores cacheable responses in order to reduce the response
568   time and network bandwidth consumption on future, equivalent
569   requests. Any client or server &MAY; employ a cache, though a cache
570   cannot be used by a server while it is acting as a tunnel.
573   The effect of a cache is that the request/response chain is shortened
574   if one of the participants along the chain has a cached response
575   applicable to that request. The following illustrates the resulting
576   chain if B has a cached copy of an earlier response from O (via C)
577   for a request that has not been cached by UA or A.
579<figure><artwork type="drawing">
580            &gt;             &gt;
581       <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>
582                  &lt;             &lt;
584<t><iref primary="true" item="cacheable"/>
585   A response is "<x:dfn>cacheable</x:dfn>" if a cache is allowed to store a copy of
586   the response message for use in answering subsequent requests.
587   Even when a response is cacheable, there might be additional
588   constraints placed by the client or by the origin server on when
589   that cached response can be used for a particular request. HTTP
590   requirements for cache behavior and cacheable responses are
591   defined in &caching-overview;. 
594   There are a wide variety of architectures and configurations
595   of caches deployed across the World Wide Web and
596   inside large organizations. These include national hierarchies
597   of proxy caches to save transoceanic bandwidth, collaborative systems that
598   broadcast or multicast cache entries, archives of pre-fetched cache
599   entries for use in off-line or high-latency environments, and so on.
603<section title="Conformance and Error Handling" anchor="conformance">
605   This specification targets conformance criteria according to the role of
606   a participant in HTTP communication.  Hence, HTTP requirements are placed
607   on senders, recipients, clients, servers, user agents, intermediaries,
608   origin servers, proxies, gateways, or caches, depending on what behavior
609   is being constrained by the requirement. Additional (social) requirements
610   are placed on implementations, resource owners, and protocol element
611   registrations when they apply beyond the scope of a single communication.
614   The verb "generate" is used instead of "send" where a requirement
615   differentiates between creating a protocol element and merely forwarding a
616   received element downstream.
619   An implementation is considered conformant if it complies with all of the
620   requirements associated with the roles it partakes in HTTP. Note that
621   SHOULD-level requirements are relevant here, unless one of the documented
622   exceptions is applicable.
625   Conformance applies to both the syntax and semantics of HTTP protocol
626   elements. A sender &MUST-NOT; generate protocol elements that convey a
627   meaning that is known by that sender to be false. A sender &MUST-NOT;
628   generate protocol elements that do not match the grammar defined by the
629   ABNF rules for those protocol elements that are applicable to the sender's
630   role. If a received protocol element is processed, the recipient &MUST; be
631   able to parse any value that would match the ABNF rules for that protocol
632   element, excluding only those rules not applicable to the recipient's role.
635   Unless noted otherwise, a recipient &MAY; attempt to recover a usable
636   protocol element from an invalid construct.  HTTP does not define
637   specific error handling mechanisms except when they have a direct impact
638   on security, since different applications of the protocol require
639   different error handling strategies.  For example, a Web browser might
640   wish to transparently recover from a response where the
641   <x:ref>Location</x:ref> header field doesn't parse according to the ABNF,
642   whereas a systems control client might consider any form of error recovery
643   to be dangerous.
647<section title="Protocol Versioning" anchor="http.version">
648  <x:anchor-alias value="HTTP-version"/>
649  <x:anchor-alias value="HTTP-name"/>
651   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate
652   versions of the protocol. This specification defines version "1.1".
653   The protocol version as a whole indicates the sender's conformance
654   with the set of requirements laid out in that version's corresponding
655   specification of HTTP.
658   The version of an HTTP message is indicated by an HTTP-version field
659   in the first line of the message. HTTP-version is case-sensitive.
661<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-version"/><iref primary="true" item="Grammar" subitem="HTTP-name"/>
662  <x:ref>HTTP-version</x:ref>  = <x:ref>HTTP-name</x:ref> "/" <x:ref>DIGIT</x:ref> "." <x:ref>DIGIT</x:ref>
663  <x:ref>HTTP-name</x:ref>     = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
666   The HTTP version number consists of two decimal digits separated by a "."
667   (period or decimal point).  The first digit ("major version") indicates the
668   HTTP messaging syntax, whereas the second digit ("minor version") indicates
669   the highest minor version to which the sender is
670   conformant and able to understand for future communication.  The minor
671   version advertises the sender's communication capabilities even when the
672   sender is only using a backwards-compatible subset of the protocol,
673   thereby letting the recipient know that more advanced features can
674   be used in response (by servers) or in future requests (by clients).
677   When an HTTP/1.1 message is sent to an HTTP/1.0 recipient
678   <xref target="RFC1945"/> or a recipient whose version is unknown,
679   the HTTP/1.1 message is constructed such that it can be interpreted
680   as a valid HTTP/1.0 message if all of the newer features are ignored.
681   This specification places recipient-version requirements on some
682   new features so that a conformant sender will only use compatible
683   features until it has determined, through configuration or the
684   receipt of a message, that the recipient supports HTTP/1.1.
687   The interpretation of a header field does not change between minor
688   versions of the same major HTTP version, though the default
689   behavior of a recipient in the absence of such a field can change.
690   Unless specified otherwise, header fields defined in HTTP/1.1 are
691   defined for all versions of HTTP/1.x.  In particular, the <x:ref>Host</x:ref>
692   and <x:ref>Connection</x:ref> header fields ought to be implemented by all
693   HTTP/1.x implementations whether or not they advertise conformance with
694   HTTP/1.1.
697   New header fields can be defined such that, when they are
698   understood by a recipient, they might override or enhance the
699   interpretation of previously defined header fields.  When an
700   implementation receives an unrecognized header field, the recipient
701   &MUST; ignore that header field for local processing regardless of
702   the message's HTTP version.  An unrecognized header field received
703   by a proxy &MUST; be forwarded downstream unless the header field's
704   field-name is listed in the message's <x:ref>Connection</x:ref> header field
705   (see <xref target="header.connection"/>).
706   These requirements allow HTTP's functionality to be enhanced without
707   requiring prior update of deployed intermediaries.
710   Intermediaries that process HTTP messages (i.e., all intermediaries
711   other than those acting as tunnels) &MUST; send their own HTTP-version
712   in forwarded messages.  In other words, they &MUST-NOT; blindly
713   forward the first line of an HTTP message without ensuring that the
714   protocol version in that message matches a version to which that
715   intermediary is conformant for both the receiving and
716   sending of messages.  Forwarding an HTTP message without rewriting
717   the HTTP-version might result in communication errors when downstream
718   recipients use the message sender's version to determine what features
719   are safe to use for later communication with that sender.
722   An HTTP client &SHOULD; send a request version equal to the highest
723   version to which the client is conformant and
724   whose major version is no higher than the highest version supported
725   by the server, if this is known.  An HTTP client &MUST-NOT; send a
726   version to which it is not conformant.
729   An HTTP client &MAY; send a lower request version if it is known that
730   the server incorrectly implements the HTTP specification, but only
731   after the client has attempted at least one normal request and determined
732   from the response status or header fields (e.g., <x:ref>Server</x:ref>) that
733   the server improperly handles higher request versions.
736   An HTTP server &SHOULD; send a response version equal to the highest
737   version to which the server is conformant and
738   whose major version is less than or equal to the one received in the
739   request.  An HTTP server &MUST-NOT; send a version to which it is not
740   conformant.  A server &MAY; send a <x:ref>505 (HTTP Version Not
741   Supported)</x:ref> response if it cannot send a response using the
742   major version used in the client's request.
745   An HTTP server &MAY; send an HTTP/1.0 response to an HTTP/1.0 request
746   if it is known or suspected that the client incorrectly implements the
747   HTTP specification and is incapable of correctly processing later
748   version responses, such as when a client fails to parse the version
749   number correctly or when an intermediary is known to blindly forward
750   the HTTP-version even when it doesn't conform to the given minor
751   version of the protocol. Such protocol downgrades &SHOULD-NOT; be
752   performed unless triggered by specific client attributes, such as when
753   one or more of the request header fields (e.g., <x:ref>User-Agent</x:ref>)
754   uniquely match the values sent by a client known to be in error.
757   The intention of HTTP's versioning design is that the major number
758   will only be incremented if an incompatible message syntax is
759   introduced, and that the minor number will only be incremented when
760   changes made to the protocol have the effect of adding to the message
761   semantics or implying additional capabilities of the sender.  However,
762   the minor version was not incremented for the changes introduced between
763   <xref target="RFC2068"/> and <xref target="RFC2616"/>, and this revision
764   has specifically avoiding any such changes to the protocol.
768<section title="Uniform Resource Identifiers" anchor="uri">
769<iref primary="true" item="resource"/>
771   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used
772   throughout HTTP as the means for identifying resources (&resource;).
773   URI references are used to target requests, indicate redirects, and define
774   relationships.
776  <x:anchor-alias value="URI-reference"/>
777  <x:anchor-alias value="absolute-URI"/>
778  <x:anchor-alias value="relative-part"/>
779  <x:anchor-alias value="authority"/>
780  <x:anchor-alias value="path-abempty"/>
781  <x:anchor-alias value="path-absolute"/>
782  <x:anchor-alias value="port"/>
783  <x:anchor-alias value="query"/>
784  <x:anchor-alias value="uri-host"/>
785  <x:anchor-alias value="partial-URI"/>
787   This specification adopts the definitions of "URI-reference",
788   "absolute-URI", "relative-part", "port", "host",
789   "path-abempty", "path-absolute", "query", and "authority" from the
790   URI generic syntax.
791   In addition, we define a partial-URI rule for protocol elements
792   that allow a relative URI but not a fragment.
794<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>
795  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
796  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
797  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
798  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
799  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
800  <x:ref>path-absolute</x:ref> = &lt;path-absolute, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
801  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
802  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
803  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
805  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
808   Each protocol element in HTTP that allows a URI reference will indicate
809   in its ABNF production whether the element allows any form of reference
810   (URI-reference), only a URI in absolute form (absolute-URI), only the
811   path and optional query components, or some combination of the above.
812   Unless otherwise indicated, URI references are parsed
813   relative to the effective request URI
814   (<xref target="effective.request.uri"/>).
817<section title="http URI scheme" anchor="http.uri">
818  <x:anchor-alias value="http-URI"/>
819  <iref item="http URI scheme" primary="true"/>
820  <iref item="URI scheme" subitem="http" primary="true"/>
822   The "http" URI scheme is hereby defined for the purpose of minting
823   identifiers according to their association with the hierarchical
824   namespace governed by a potential HTTP origin server listening for
825   TCP connections on a given port.
827<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"><!--terminal production--></iref>
828  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
831   The HTTP origin server is identified by the generic syntax's
832   <x:ref>authority</x:ref> component, which includes a host identifier
833   and optional TCP port (<xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>).
834   The remainder of the URI, consisting of both the hierarchical path
835   component and optional query component, serves as an identifier for
836   a potential resource within that origin server's name space.
839   If the host identifier is provided as an IP address,
840   then the origin server is any listener on the indicated TCP port at
841   that IP address. If host is a registered name, then that name is
842   considered an indirect identifier and the recipient might use a name
843   resolution service, such as DNS, to find the address of a listener
844   for that host.
845   The host &MUST-NOT; be empty; if an "http" URI is received with an
846   empty host, then it &MUST; be rejected as invalid.
847   If the port subcomponent is empty or not given, then TCP port 80 is
848   assumed (the default reserved port for WWW services).
851   Regardless of the form of host identifier, access to that host is not
852   implied by the mere presence of its name or address. The host might or might
853   not exist and, even when it does exist, might or might not be running an
854   HTTP server or listening to the indicated port. The "http" URI scheme
855   makes use of the delegated nature of Internet names and addresses to
856   establish a naming authority (whatever entity has the ability to place
857   an HTTP server at that Internet name or address) and allows that
858   authority to determine which names are valid and how they might be used.
861   When an "http" URI is used within a context that calls for access to the
862   indicated resource, a client &MAY; attempt access by resolving
863   the host to an IP address, establishing a TCP connection to that address
864   on the indicated port, and sending an HTTP request message
865   (<xref target="http.message"/>) containing the URI's identifying data
866   (<xref target="message.routing"/>) to the server.
867   If the server responds to that request with a non-interim HTTP response
868   message, as described in &status-codes;, then that response
869   is considered an authoritative answer to the client's request.
872   Although HTTP is independent of the transport protocol, the "http"
873   scheme is specific to TCP-based services because the name delegation
874   process depends on TCP for establishing authority.
875   An HTTP service based on some other underlying connection protocol
876   would presumably be identified using a different URI scheme, just as
877   the "https" scheme (below) is used for resources that require an
878   end-to-end secured connection. Other protocols might also be used to
879   provide access to "http" identified resources &mdash; it is only the
880   authoritative interface used for mapping the namespace that is
881   specific to TCP.
884   The URI generic syntax for authority also includes a deprecated
885   userinfo subcomponent (<xref target="RFC3986" x:fmt="," x:sec="3.2.1"/>)
886   for including user authentication information in the URI.  Some
887   implementations make use of the userinfo component for internal
888   configuration of authentication information, such as within command
889   invocation options, configuration files, or bookmark lists, even
890   though such usage might expose a user identifier or password.
891   Senders &MUST; exclude the userinfo subcomponent (and its "@"
892   delimiter) when an "http" URI is transmitted within a message as a
893   request target or header field value.
894   Recipients of an "http" URI reference &SHOULD; parse for userinfo and
895   treat its presence as an error, since it is likely being used to obscure
896   the authority for the sake of phishing attacks.
900<section title="https URI scheme" anchor="https.uri">
901   <x:anchor-alias value="https-URI"/>
902   <iref item="https URI scheme"/>
903   <iref item="URI scheme" subitem="https"/>
905   The "https" URI scheme is hereby defined for the purpose of minting
906   identifiers according to their association with the hierarchical
907   namespace governed by a potential HTTP origin server listening to a
908   given TCP port for TLS-secured connections <xref target="RFC5246"/>.
911   All of the requirements listed above for the "http" scheme are also
912   requirements for the "https" scheme, except that a default TCP port
913   of 443 is assumed if the port subcomponent is empty or not given,
914   and the TCP connection &MUST; be secured, end-to-end, through the
915   use of strong encryption prior to sending the first HTTP request.
917<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="https-URI"><!--terminal production--></iref>
918  <x:ref>https-URI</x:ref> = "https:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
921   Resources made available via the "https" scheme have no shared
922   identity with the "http" scheme even if their resource identifiers
923   indicate the same authority (the same host listening to the same
924   TCP port).  They are distinct name spaces and are considered to be
925   distinct origin servers.  However, an extension to HTTP that is
926   defined to apply to entire host domains, such as the Cookie protocol
927   <xref target="RFC6265"/>, can allow information
928   set by one service to impact communication with other services
929   within a matching group of host domains.
932   The process for authoritative access to an "https" identified
933   resource is defined in <xref target="RFC2818"/>.
937<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
939   Since the "http" and "https" schemes conform to the URI generic syntax,
940   such URIs are normalized and compared according to the algorithm defined
941   in <xref target="RFC3986" x:fmt="," x:sec="6"/>, using the defaults
942   described above for each scheme.
945   If the port is equal to the default port for a scheme, the normal form is
946   to elide the port subcomponent. When not being used in absolute form as the
947   request target of an OPTIONS request, an empty path component is equivalent
948   to an absolute path of "/", so the normal form is to provide a path of "/"
949   instead. The scheme and host are case-insensitive and normally provided in
950   lowercase; all other components are compared in a case-sensitive manner.
951   Characters other than those in the "reserved" set are equivalent to their
952   percent-encoded octets (see <xref target="RFC3986" x:fmt=","
953   x:sec="2.1"/>): the normal form is to not encode them.
956   For example, the following three URIs are equivalent:
958<figure><artwork type="example">
967<section title="Message Format" anchor="http.message">
968<x:anchor-alias value="generic-message"/>
969<x:anchor-alias value="message.types"/>
970<x:anchor-alias value="HTTP-message"/>
971<x:anchor-alias value="start-line"/>
972<iref item="header section"/>
973<iref item="headers"/>
974<iref item="header field"/>
976   All HTTP/1.1 messages consist of a start-line followed by a sequence of
977   octets in a format similar to the Internet Message Format
978   <xref target="RFC5322"/>: zero or more header fields (collectively
979   referred to as the "headers" or the "header section"), an empty line
980   indicating the end of the header section, and an optional message body.
982<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"><!--terminal production--></iref>
983  <x:ref>HTTP-message</x:ref>   = <x:ref>start-line</x:ref>
984                   *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
985                   <x:ref>CRLF</x:ref>
986                   [ <x:ref>message-body</x:ref> ]
989   The normal procedure for parsing an HTTP message is to read the
990   start-line into a structure, read each header field into a hash
991   table by field name until the empty line, and then use the parsed
992   data to determine if a message body is expected.  If a message body
993   has been indicated, then it is read as a stream until an amount
994   of octets equal to the message body length is read or the connection
995   is closed.
998   Recipients &MUST; parse an HTTP message as a sequence of octets in an
999   encoding that is a superset of US-ASCII <xref target="USASCII"/>.
1000   Parsing an HTTP message as a stream of Unicode characters, without regard
1001   for the specific encoding, creates security vulnerabilities due to the
1002   varying ways that string processing libraries handle invalid multibyte
1003   character sequences that contain the octet LF (%x0A).  String-based
1004   parsers can only be safely used within protocol elements after the element
1005   has been extracted from the message, such as within a header field-value
1006   after message parsing has delineated the individual fields.
1009   An HTTP message can be parsed as a stream for incremental processing or
1010   forwarding downstream.  However, recipients cannot rely on incremental
1011   delivery of partial messages, since some implementations will buffer or
1012   delay message forwarding for the sake of network efficiency, security
1013   checks, or payload transformations.
1016<section title="Start Line" anchor="start.line">
1017  <x:anchor-alias value="Start-Line"/>
1019   An HTTP message can either be a request from client to server or a
1020   response from server to client.  Syntactically, the two types of message
1021   differ only in the start-line, which is either a request-line (for requests)
1022   or a status-line (for responses), and in the algorithm for determining
1023   the length of the message body (<xref target="message.body"/>).
1026   In theory, a client could receive requests and a server could receive
1027   responses, distinguishing them by their different start-line formats,
1028   but in practice servers are implemented to only expect a request
1029   (a response is interpreted as an unknown or invalid request method)
1030   and clients are implemented to only expect a response.
1032<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="start-line"/>
1033  <x:ref>start-line</x:ref>     = <x:ref>request-line</x:ref> / <x:ref>status-line</x:ref>
1036   A sender &MUST-NOT; send whitespace between the start-line and
1037   the first header field. The presence of such whitespace in a request
1038   might be an attempt to trick a server into ignoring that field or
1039   processing the line after it as a new request, either of which might
1040   result in a security vulnerability if other implementations within
1041   the request chain interpret the same message differently.
1042   Likewise, the presence of such whitespace in a response might be
1043   ignored by some clients or cause others to cease parsing.
1046   A recipient that receives whitespace between the start-line and
1047   the first header field &MUST; either reject the message as invalid or
1048   consume each whitespace-preceded line without further processing of it
1049   (i.e., ignore the entire line, along with any subsequent lines preceded
1050   by whitespace, until a properly formed header field is received or the
1051   header block is terminated).
1054<section title="Request Line" anchor="request.line">
1055  <x:anchor-alias value="Request"/>
1056  <x:anchor-alias value="request-line"/>
1058   A request-line begins with a method token, followed by a single
1059   space (SP), the request-target, another single space (SP), the
1060   protocol version, and ending with CRLF.
1062<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1063  <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>
1065<iref primary="true" item="method"/>
1066<t anchor="method">
1067   The method token indicates the request method to be performed on the
1068   target resource. The request method is case-sensitive.
1070<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1071  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1074   The methods defined by this specification can be found in
1075   &methods;, along with information regarding the HTTP method registry
1076   and considerations for defining new methods.
1078<iref item="request-target"/>
1080   The request-target identifies the target resource upon which to apply
1081   the request, as defined in <xref target="request-target"/>.
1084   No whitespace is allowed inside the method, request-target, and
1085   protocol version.  Hence, recipients typically parse the request-line
1086   into its component parts by splitting on whitespace
1087   (see <xref target="message.robustness"/>).
1090   Unfortunately, some user agents fail to properly encode hypertext
1091   references that have embedded whitespace, sending the characters directly
1092   instead of properly encoding or excluding 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.
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;).
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 8000 octets.
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"/>
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.
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>
1132   The status-code element is a 3-digit integer code describing the
1133   result of the server's attempt to understand and satisfy the client's
1134   corresponding request. The rest of the response message is to be
1135   interpreted in light of the semantics defined for that status code.
1136   See &status-codes; for information about the semantics of status codes,
1137   including the classes of status code (indicated by the first digit),
1138   the status codes defined by this specification, considerations for the
1139   definition of new status codes, and the IANA registry.
1141<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-code"/>
1142  <x:ref>status-code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1145   The reason-phrase element exists for the sole purpose of providing a
1146   textual description associated with the numeric status code, mostly
1147   out of deference to earlier Internet application protocols that were more
1148   frequently used with interactive text clients. A client &SHOULD; ignore
1149   the reason-phrase content.
1151<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
1152  <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<section title="Header Fields" anchor="header.fields">
1158  <x:anchor-alias value="header-field"/>
1159  <x:anchor-alias value="field-content"/>
1160  <x:anchor-alias value="field-name"/>
1161  <x:anchor-alias value="field-value"/>
1162  <x:anchor-alias value="obs-fold"/>
1164   Each HTTP header field consists of a case-insensitive field name
1165   followed by a colon (":"), optional whitespace, and the field value.
1167<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"/>
1168  <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>
1169  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1170  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>obs-fold</x:ref> )
1171  <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> )
1172  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref> ( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1173                 ; obsolete line folding
1174                 ; see <xref target="field.parsing"/>
1177   The field-name token labels the corresponding field-value as having the
1178   semantics defined by that header field.  For example, the <x:ref>Date</x:ref>
1179   header field is defined in &header-date; as containing the origination
1180   timestamp for the message in which it appears.
1183<section title="Field Extensibility" anchor="field.extensibility">
1185   HTTP header fields are fully extensible: there is no limit on the
1186   introduction of new field names, each presumably defining new semantics,
1187   nor on the number of header fields used in a given message.  Existing
1188   fields are defined in each part of this specification and in many other
1189   specifications outside the core standard.
1190   New header fields can be introduced without changing the protocol version
1191   if their defined semantics allow them to be safely ignored by recipients
1192   that do not recognize them.
1195   New HTTP header fields ought to be be registered with IANA in the
1196   Message Header Field Registry, as described in &iana-header-registry;.
1197   A proxy &MUST; forward unrecognized header fields unless the
1198   field-name is listed in the <x:ref>Connection</x:ref> header field
1199   (<xref target="header.connection"/>) or the proxy is specifically
1200   configured to block, or otherwise transform, such fields.
1201   Other recipients &SHOULD; ignore unrecognized header fields.
1205<section title="Field Order" anchor="field.order">
1207   The order in which header fields with differing field names are
1208   received is not significant. However, it is "good practice" to send
1209   header fields that contain control data first, such as <x:ref>Host</x:ref>
1210   on requests and <x:ref>Date</x:ref> on responses, so that implementations
1211   can decide when not to handle a message as early as possible.  A server
1212   &MUST; wait until the entire header section is received before interpreting
1213   a request message, since later header fields might include conditionals,
1214   authentication credentials, or deliberately misleading duplicate
1215   header fields that would impact request processing.
1218   A sender &MUST-NOT; generate multiple header fields with the same field
1219   name in a message unless either the entire field value for that
1220   header field is defined as a comma-separated list [i.e., #(values)]
1221   or the header field is a well-known exception (as noted below).
1224   Multiple header fields with the same field name can be combined into
1225   one "field-name: field-value" pair, without changing the semantics of the
1226   message, by appending each subsequent field value to the combined
1227   field value in order, separated by a comma. The order in which
1228   header fields with the same field name are received is therefore
1229   significant to the interpretation of the combined field value;
1230   a proxy &MUST-NOT; change the order of these field values when
1231   forwarding a message.
1234  <t>
1235   &Note; In practice, the "Set-Cookie" header field (<xref target="RFC6265"/>)
1236   often appears multiple times in a response message and does not use the
1237   list syntax, violating the above requirements on multiple header fields
1238   with the same name. Since it cannot be combined into a single field-value,
1239   recipients ought to handle "Set-Cookie" as a special case while processing
1240   header fields. (See Appendix A.2.3 of <xref target="Kri2001"/> for details.)
1241  </t>
1245<section title="Whitespace" anchor="whitespace">
1246<t anchor="rule.LWS">
1247   This specification uses three rules to denote the use of linear
1248   whitespace: OWS (optional whitespace), RWS (required whitespace), and
1249   BWS ("bad" whitespace).
1251<t anchor="rule.OWS">
1252   The OWS rule is used where zero or more linear whitespace octets might
1253   appear. OWS &SHOULD; either not be generated or be generated as a single
1254   SP. Multiple OWS octets that occur within field-content &SHOULD; either
1255   be replaced with a single SP or transformed to all SP octets (each
1256   octet other than SP replaced with SP) before interpreting the field value
1257   or forwarding the message downstream.
1259<t anchor="rule.RWS">
1260   RWS is used when at least one linear whitespace octet is required to
1261   separate field tokens. RWS &SHOULD; be generated as a single SP.
1262   Multiple RWS octets that occur within field-content &SHOULD; either
1263   be replaced with a single SP or transformed to all SP octets before
1264   interpreting the field value or forwarding the message downstream.
1266<t anchor="rule.BWS">
1267   BWS is used where the grammar allows optional whitespace, for historical
1268   reasons, but senders &SHOULD-NOT; generate it in messages;
1269   recipients &MUST; accept such bad optional whitespace and remove it before
1270   interpreting the field value or forwarding the message downstream.
1272<t anchor="rule.whitespace">
1273  <x:anchor-alias value="BWS"/>
1274  <x:anchor-alias value="OWS"/>
1275  <x:anchor-alias value="RWS"/>
1277<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"/>
1278  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1279                 ; optional whitespace
1280  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1281                 ; required whitespace
1282  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1283                 ; "bad" whitespace
1287<section title="Field Parsing" anchor="field.parsing">
1289   No whitespace is allowed between the header field-name and colon.
1290   In the past, differences in the handling of such whitespace have led to
1291   security vulnerabilities in request routing and response handling.
1292   A server &MUST; reject any received request message that contains
1293   whitespace between a header field-name and colon with a response code of
1294   <x:ref>400 (Bad Request)</x:ref>. A proxy &MUST; remove any such whitespace
1295   from a response message before forwarding the message downstream.
1298   A field value is preceded by optional whitespace (OWS); a single SP is
1299   preferred. The field value does not include any leading or trailing white
1300   space: OWS occurring before the first non-whitespace octet of the
1301   field value or after the last non-whitespace octet of the field value
1302   is ignored and &SHOULD; be removed before further processing (as this does
1303   not change the meaning of the header field).
1306   Historically, HTTP header field values could be extended over multiple
1307   lines by preceding each extra line with at least one space or horizontal
1308   tab (obs-fold). This specification deprecates such line
1309   folding except within the message/http media type
1310   (<xref target=""/>).
1311   Senders &MUST-NOT; generate messages that include line folding
1312   (i.e., that contain any field-value that matches the obs-fold rule) unless
1313   the message is intended for packaging within the message/http media type.
1314   Recipients &MUST; accept line folding and replace any embedded
1315   obs-fold whitespace with either a single SP or a matching number of SP
1316   octets (to avoid buffer copying) prior to interpreting the field value or
1317   forwarding the message downstream.
1320   Historically, HTTP has allowed field content with text in the ISO-8859-1
1321   <xref target="ISO-8859-1"/> charset, supporting other charsets only
1322   through use of <xref target="RFC2047"/> encoding.
1323   In practice, most HTTP header field values use only a subset of the
1324   US-ASCII charset <xref target="USASCII"/>. Newly defined
1325   header fields &SHOULD; limit their field values to US-ASCII octets.
1326   Recipients &SHOULD; treat other octets in field content (obs-text) as
1327   opaque data.
1331<section title="Field Limits" anchor="field.limits">
1333   HTTP does not place a pre-defined limit on the length of each header field
1334   or on the length of the header block as a whole.  Various ad-hoc
1335   limitations on individual header field length are found in practice,
1336   often depending on the specific field semantics.
1339   A server &MUST; be prepared to receive request header fields of unbounded
1340   length and respond with an appropriate <x:ref>4xx (Client Error)</x:ref>
1341   status code if the received header field(s) are larger than the server
1342   wishes to process.
1345   A client &MUST; be prepared to receive response header fields of unbounded
1346   length. A client &MAY; discard or truncate received header fields that are
1347   larger than the client wishes to process if the field semantics are such
1348   that the dropped value(s) can be safely ignored without changing the
1349   response semantics.
1353<section title="Field value components" anchor="field.components">
1354<t anchor="rule.token.separators">
1355  <x:anchor-alias value="tchar"/>
1356  <x:anchor-alias value="token"/>
1357  <x:anchor-alias value="special"/>
1358  <x:anchor-alias value="word"/>
1359   Many HTTP header field values consist of words (token or quoted-string)
1360   separated by whitespace or special characters. These special characters
1361   &MUST; be in a quoted string to be used within a parameter value (as defined
1362   in <xref target="transfer.codings"/>).
1364<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>
1365  <x:ref>word</x:ref>           = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1367  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1369  IMPORTANT: when editing "tchar" make sure that "special" is updated accordingly!!!
1370 -->
1371  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1372                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1373                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1374                 ; any <x:ref>VCHAR</x:ref>, except <x:ref>special</x:ref>
1376  <x:ref>special</x:ref>        = "(" / ")" / "&lt;" / ">" / "@" / ","
1377                 / ";" / ":" / "\" / DQUOTE / "/" / "["
1378                 / "]" / "?" / "=" / "{" / "}"
1380<t anchor="rule.quoted-string">
1381  <x:anchor-alias value="quoted-string"/>
1382  <x:anchor-alias value="qdtext"/>
1383  <x:anchor-alias value="obs-text"/>
1384   A string of text is parsed as a single word if it is quoted using
1385   double-quote marks.
1387<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"/>
1388  <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>
1389  <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>
1390  <x:ref>obs-text</x:ref>       = %x80-FF
1392<t anchor="rule.quoted-pair">
1393  <x:anchor-alias value="quoted-pair"/>
1394   The backslash octet ("\") can be used as a single-octet
1395   quoting mechanism within quoted-string constructs:
1397<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1398  <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> )
1401   Recipients that process the value of a quoted-string &MUST; handle a
1402   quoted-pair as if it were replaced by the octet following the backslash.
1405   Senders &SHOULD-NOT; generate a quoted-pair in a quoted-string except where
1406   necessary to quote DQUOTE and backslash octets occurring within that string.
1408<t anchor="rule.comment">
1409  <x:anchor-alias value="comment"/>
1410  <x:anchor-alias value="ctext"/>
1411   Comments can be included in some HTTP header fields by surrounding
1412   the comment text with parentheses. Comments are only allowed in
1413   fields containing "comment" as part of their field value definition.
1415<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1416  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-cpair</x:ref> / <x:ref>comment</x:ref> ) ")"
1417  <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>
1419<t anchor="rule.quoted-cpair">
1420  <x:anchor-alias value="quoted-cpair"/>
1421   The backslash octet ("\") can be used as a single-octet
1422   quoting mechanism within comment constructs:
1424<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-cpair"/>
1425  <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> )
1428   Senders &SHOULD-NOT; escape octets in comments that do not require escaping
1429   (i.e., other than the backslash octet "\" and the parentheses "(" and ")").
1435<section title="Message Body" anchor="message.body">
1436  <x:anchor-alias value="message-body"/>
1438   The message body (if any) of an HTTP message is used to carry the
1439   payload body of that request or response.  The message body is
1440   identical to the payload body unless a transfer coding has been
1441   applied, as described in <xref target="header.transfer-encoding"/>.
1443<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1444  <x:ref>message-body</x:ref> = *OCTET
1447   The rules for when a message body is allowed in a message differ for
1448   requests and responses.
1451   The presence of a message body in a request is signaled by a
1452   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1453   field. Request message framing is independent of method semantics,
1454   even if the method does not define any use for a message body.
1457   The presence of a message body in a response depends on both
1458   the request method to which it is responding and the response
1459   status code (<xref target="status.line"/>).
1460   Responses to the HEAD request method never include a message body
1461   because the associated response header fields (e.g.,
1462   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1463   if present, indicate only what their values would have been if the request
1464   method had been GET (&HEAD;).
1465   <x:ref>2xx (Successful)</x:ref> responses to CONNECT switch to tunnel
1466   mode instead of having a message body (&CONNECT;).
1467   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1468   <x:ref>304 (Not Modified)</x:ref> responses do not include a message body.
1469   All other responses do include a message body, although the body
1470   might be of zero length.
1473<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1474  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1475  <iref item="chunked (Coding Format)"/>
1476  <x:anchor-alias value="Transfer-Encoding"/>
1478   The Transfer-Encoding header field lists the transfer coding names
1479   corresponding to the sequence of transfer codings that have been
1480   (or will be) applied to the payload body in order to form the message body.
1481   Transfer codings are defined in <xref target="transfer.codings"/>.
1483<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1484  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1487   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1488   MIME, which was designed to enable safe transport of binary data over a
1489   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1490   However, safe transport has a different focus for an 8bit-clean transfer
1491   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1492   accurately delimit a dynamically generated payload and to distinguish
1493   payload encodings that are only applied for transport efficiency or
1494   security from those that are characteristics of the selected resource.
1497   All HTTP/1.1 recipients &MUST; implement the chunked transfer coding
1498   (<xref target="chunked.encoding"/>) because it plays a crucial role in
1499   framing messages when the payload body size is not known in advance.
1500   If chunked is applied to a payload body, the sender &MUST-NOT; apply
1501   chunked more than once (i.e., chunking an already chunked message is not
1502   allowed).
1503   If any transfer coding is applied to a request payload body, the
1504   sender &MUST; apply chunked as the final transfer coding to ensure that
1505   the message is properly framed.
1506   If any transfer coding is applied to a response payload body, the
1507   sender &MUST; either apply chunked as the final transfer coding or
1508   terminate the message by closing the connection.
1511   For example,
1512</preamble><artwork type="example">
1513  Transfer-Encoding: gzip, chunked
1515   indicates that the payload body has been compressed using the gzip
1516   coding and then chunked using the chunked coding while forming the
1517   message body.
1520   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1521   Transfer-Encoding is a property of the message, not of the payload, and
1522   any recipient along the request/response chain &MAY; decode the received
1523   transfer coding(s) or apply additional transfer coding(s) to the message
1524   body, assuming that corresponding changes are made to the Transfer-Encoding
1525   field-value. Additional information about the encoding parameters &MAY; be
1526   provided by other header fields not defined by this specification.
1529   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1530   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1531   neither of which includes a message body,
1532   to indicate that the origin server would have applied a transfer coding
1533   to the message body if the request had been an unconditional GET.
1534   This indication is not required, however, because any recipient on
1535   the response chain (including the origin server) can remove transfer
1536   codings when they are not needed.
1539   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1540   implementations advertising only HTTP/1.0 support will not understand
1541   how to process a transfer-encoded payload.
1542   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1543   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1544   might be in the form of specific user configuration or by remembering the
1545   version of a prior received response.
1546   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1547   the corresponding request indicates HTTP/1.1 (or later).
1550   A server that receives a request message with a transfer coding it does
1551   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref>.
1555<section title="Content-Length" anchor="header.content-length">
1556  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1557  <x:anchor-alias value="Content-Length"/>
1559   When a message does not have a <x:ref>Transfer-Encoding</x:ref> header
1560   field, a Content-Length header field can provide the anticipated size,
1561   as a decimal number of octets, for a potential payload body.
1562   For messages that do include a payload body, the Content-Length field-value
1563   provides the framing information necessary for determining where the body
1564   (and message) ends.  For messages that do not include a payload body, the
1565   Content-Length indicates the size of the selected representation
1566   (&selected-representation;).
1568<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1569  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1572   An example is
1574<figure><artwork type="example">
1575  Content-Length: 3495
1578   A sender &MUST-NOT; send a Content-Length header field in any message that
1579   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1582   A user agent &SHOULD; send a Content-Length in a request message when no
1583   <x:ref>Transfer-Encoding</x:ref> is sent and the request method defines
1584   a meaning for an enclosed payload body. For example, a Content-Length
1585   header field is normally sent in a POST request even when the value is
1586   0 (indicating an empty payload body).  A user agent &SHOULD-NOT; send a
1587   Content-Length header field when the request message does not contain a
1588   payload body and the method semantics do not anticipate such a body.
1591   A server &MAY; send a Content-Length header field in a response to a HEAD
1592   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1593   response unless its field-value equals the decimal number of octets that
1594   would have been sent in the payload body of a response if the same
1595   request had used the GET method.
1598   A server &MAY; send a Content-Length header field in a
1599   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1600   (&status-304;); a server &MUST-NOT; send Content-Length in such a
1601   response unless its field-value equals the decimal number of octets that
1602   would have been sent in the payload body of a <x:ref>200 (OK)</x:ref>
1603   response to the same request.
1606   A server &MUST-NOT; send a Content-Length header field in any response
1607   with a status code of
1608   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1609   A server &SHOULD-NOT; send a Content-Length header field in any
1610   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1613   Aside from the cases defined above, in the absence of Transfer-Encoding,
1614   an origin server &SHOULD; send a Content-Length header field when the
1615   payload body size is known prior to sending the complete header block.
1616   This will allow downstream recipients to measure transfer progress,
1617   know when a received message is complete, and potentially reuse the
1618   connection for additional requests.
1621   Any Content-Length field value greater than or equal to zero is valid.
1622   Since there is no predefined limit to the length of an HTTP payload,
1623   recipients &SHOULD; anticipate potentially large decimal numerals and
1624   prevent parsing errors due to integer conversion overflows
1625   (<xref target="attack.protocol.element.size.overflows"/>).
1628   If a message is received that has multiple Content-Length header fields
1629   with field-values consisting of the same decimal value, or a single
1630   Content-Length header field with a field value containing a list of
1631   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1632   duplicate Content-Length header fields have been generated or combined by an
1633   upstream message processor, then the recipient &MUST; either reject the
1634   message as invalid or replace the duplicated field-values with a single
1635   valid Content-Length field containing that decimal value prior to
1636   determining the message body length.
1639  <t>
1640   &Note; HTTP's use of Content-Length for message framing differs
1641   significantly from the same field's use in MIME, where it is an optional
1642   field used only within the "message/external-body" media-type.
1643  </t>
1647<section title="Message Body Length" anchor="message.body.length">
1648  <iref item="chunked (Coding Format)"/>
1650   The length of a message body is determined by one of the following
1651   (in order of precedence):
1654  <list style="numbers">
1655    <x:lt><t>
1656     Any response to a HEAD request and any response with a
1657     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1658     <x:ref>304 (Not Modified)</x:ref> status code is always
1659     terminated by the first empty line after the header fields, regardless of
1660     the header fields present in the message, and thus cannot contain a
1661     message body.
1662    </t></x:lt>
1663    <x:lt><t>
1664     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1665     connection will become a tunnel immediately after the empty line that
1666     concludes the header fields.  A client &MUST; ignore any
1667     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1668     fields received in such a message.
1669    </t></x:lt>
1670    <x:lt><t>
1671     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1672     and the chunked transfer coding (<xref target="chunked.encoding"/>)
1673     is the final encoding, the message body length is determined by reading
1674     and decoding the chunked data until the transfer coding indicates the
1675     data is complete.
1676    </t>
1677    <t>
1678     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1679     response and the chunked transfer coding is not the final encoding, the
1680     message body length is determined by reading the connection until it is
1681     closed by the server.
1682     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a request and the
1683     chunked transfer coding is not the final encoding, the message body
1684     length cannot be determined reliably; the server &MUST; respond with
1685     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1686    </t>
1687    <t>
1688     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1689     and a <x:ref>Content-Length</x:ref> header field, the Transfer-Encoding
1690     overrides the Content-Length. Such a message might indicate an attempt
1691     to perform request or response smuggling (bypass of security-related
1692     checks on message routing or content) and thus ought to be handled as
1693     an error.  A sender &MUST; remove the received Content-Length field
1694     prior to forwarding such a message downstream.
1695    </t></x:lt>
1696    <x:lt><t>
1697     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1698     either multiple <x:ref>Content-Length</x:ref> header fields having
1699     differing field-values or a single Content-Length header field having an
1700     invalid value, then the message framing is invalid and &MUST; be treated
1701     as an error to prevent request or response smuggling.
1702     If this is a request message, the server &MUST; respond with
1703     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1704     If this is a response message received by a proxy, the proxy
1705     &MUST; discard the received response, send a <x:ref>502 (Bad Gateway)</x:ref>
1706     status code as its downstream response, and then close the connection.
1707     If this is a response message received by a user agent, it &MUST; be
1708     treated as an error by discarding the message and closing the connection.
1709    </t></x:lt>
1710    <x:lt><t>
1711     If a valid <x:ref>Content-Length</x:ref> header field is present without
1712     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1713     expected message body length in octets.
1714     If the sender closes the connection or the recipient times out before the
1715     indicated number of octets are received, the recipient &MUST; consider
1716     the message to be incomplete and close the connection.
1717    </t></x:lt>
1718    <x:lt><t>
1719     If this is a request message and none of the above are true, then the
1720     message body length is zero (no message body is present).
1721    </t></x:lt>
1722    <x:lt><t>
1723     Otherwise, this is a response message without a declared message body
1724     length, so the message body length is determined by the number of octets
1725     received prior to the server closing the connection.
1726    </t></x:lt>
1727  </list>
1730   Since there is no way to distinguish a successfully completed,
1731   close-delimited message from a partially-received message interrupted
1732   by network failure, a server &SHOULD; use encoding or
1733   length-delimited messages whenever possible.  The close-delimiting
1734   feature exists primarily for backwards compatibility with HTTP/1.0.
1737   A server &MAY; reject a request that contains a message body but
1738   not a <x:ref>Content-Length</x:ref> by responding with
1739   <x:ref>411 (Length Required)</x:ref>.
1742   Unless a transfer coding other than chunked has been applied,
1743   a client that sends a request containing a message body &SHOULD;
1744   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1745   length is known in advance, rather than the chunked transfer coding, since some
1746   existing services respond to chunked with a <x:ref>411 (Length Required)</x:ref>
1747   status code even though they understand the chunked transfer coding.  This
1748   is typically because such services are implemented via a gateway that
1749   requires a content-length in advance of being called and the server
1750   is unable or unwilling to buffer the entire request before processing.
1753   A user agent that sends a request containing a message body &MUST; send a
1754   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1755   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1756   the form of specific user configuration or by remembering the version of a
1757   prior received response.
1760   If the final response to the last request on a connection has been
1761   completely received and there remains additional data to read, a user agent
1762   &MAY; discard the remaining data or attempt to determine if that data
1763   belongs as part of the prior response body, which might be the case if the
1764   prior message's Content-Length value is incorrect. A client &MUST-NOT;
1765   process, cache, or forward such extra data as a separate response, since
1766   such behavior would be vulnerable to cache poisoning.
1771<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1773   A server that receives an incomplete request message, usually due to a
1774   canceled request or a triggered time-out exception, &MAY; send an error
1775   response prior to closing the connection.
1778   A client that receives an incomplete response message, which can occur
1779   when a connection is closed prematurely or when decoding a supposedly
1780   chunked transfer coding fails, &MUST; record the message as incomplete.
1781   Cache requirements for incomplete responses are defined in
1782   &cache-incomplete;.
1785   If a response terminates in the middle of the header block (before the
1786   empty line is received) and the status code might rely on header fields to
1787   convey the full meaning of the response, then the client cannot assume
1788   that meaning has been conveyed; the client might need to repeat the
1789   request in order to determine what action to take next.
1792   A message body that uses the chunked transfer coding is
1793   incomplete if the zero-sized chunk that terminates the encoding has not
1794   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1795   incomplete if the size of the message body received (in octets) is less than
1796   the value given by Content-Length.  A response that has neither chunked
1797   transfer coding nor Content-Length is terminated by closure of the
1798   connection, and thus is considered complete regardless of the number of
1799   message body octets received, provided that the header block was received
1800   intact.
1804<section title="Message Parsing Robustness" anchor="message.robustness">
1806   Older HTTP/1.0 user agent implementations might send an extra CRLF
1807   after a POST request as a lame workaround for some early server
1808   applications that failed to read message body content that was
1809   not terminated by a line-ending. An HTTP/1.1 user agent &MUST-NOT;
1810   preface or follow a request with an extra CRLF.  If terminating
1811   the request message body with a line-ending is desired, then the
1812   user agent &MUST; count the terminating CRLF octets as part of the
1813   message body length.
1816   In the interest of robustness, servers &SHOULD; ignore at least one
1817   empty line received where a request-line is expected. In other words, if
1818   a server is reading the protocol stream at the beginning of a
1819   message and receives a CRLF first, the server &SHOULD; ignore the CRLF.
1822   Although the line terminator for the start-line and header
1823   fields is the sequence CRLF, recipients &MAY; recognize a
1824   single LF as a line terminator and ignore any preceding CR.
1827   Although the request-line and status-line grammar rules require that each
1828   of the component elements be separated by a single SP octet, recipients
1829   &MAY; instead parse on whitespace-delimited word boundaries and, aside
1830   from the CRLF terminator, treat any form of whitespace as the SP separator
1831   while ignoring preceding or trailing whitespace;
1832   such whitespace includes one or more of the following octets:
1833   SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
1836   When a server listening only for HTTP request messages, or processing
1837   what appears from the start-line to be an HTTP request message,
1838   receives a sequence of octets that does not match the HTTP-message
1839   grammar aside from the robustness exceptions listed above, the
1840   server &SHOULD; respond with a <x:ref>400 (Bad Request)</x:ref> response. 
1845<section title="Transfer Codings" anchor="transfer.codings">
1846  <x:anchor-alias value="transfer-coding"/>
1847  <x:anchor-alias value="transfer-extension"/>
1849   Transfer coding names are used to indicate an encoding
1850   transformation that has been, can be, or might need to be applied to a
1851   payload body in order to ensure "safe transport" through the network.
1852   This differs from a content coding in that the transfer coding is a
1853   property of the message rather than a property of the representation
1854   that is being transferred.
1856<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1857  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1858                     / "compress" ; <xref target="compress.coding"/>
1859                     / "deflate" ; <xref target="deflate.coding"/>
1860                     / "gzip" ; <xref target="gzip.coding"/>
1861                     / <x:ref>transfer-extension</x:ref>
1862  <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> )
1864<t anchor="rule.parameter">
1865  <x:anchor-alias value="attribute"/>
1866  <x:anchor-alias value="transfer-parameter"/>
1867  <x:anchor-alias value="value"/>
1868   Parameters are in the form of attribute/value pairs.
1870<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"/>
1871  <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>
1872  <x:ref>attribute</x:ref>          = <x:ref>token</x:ref>
1873  <x:ref>value</x:ref>              = <x:ref>word</x:ref>
1876   All transfer-coding names are case-insensitive and ought to be registered
1877   within the HTTP Transfer Coding registry, as defined in
1878   <xref target="transfer.coding.registry"/>.
1879   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1880   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1881   header fields.
1884<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1885  <iref primary="true" item="chunked (Coding Format)"/>
1886  <x:anchor-alias value="chunk"/>
1887  <x:anchor-alias value="chunked-body"/>
1888  <x:anchor-alias value="chunk-data"/>
1889  <x:anchor-alias value="chunk-ext"/>
1890  <x:anchor-alias value="chunk-ext-name"/>
1891  <x:anchor-alias value="chunk-ext-val"/>
1892  <x:anchor-alias value="chunk-size"/>
1893  <x:anchor-alias value="last-chunk"/>
1894  <x:anchor-alias value="trailer-part"/>
1895  <x:anchor-alias value="quoted-str-nf"/>
1896  <x:anchor-alias value="qdtext-nf"/>
1898   The chunked transfer coding modifies the body of a message in order to
1899   transfer it as a series of chunks, each with its own size indicator,
1900   followed by an &OPTIONAL; trailer containing header fields. This
1901   allows dynamically generated content to be transferred along with the
1902   information necessary for the recipient to verify that it has
1903   received the full message.
1905<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"/>
1906  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1907                   <x:ref>last-chunk</x:ref>
1908                   <x:ref>trailer-part</x:ref>
1909                   <x:ref>CRLF</x:ref>
1911  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1912                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1913  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1914  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1916  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
1917  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
1918  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-str-nf</x:ref>
1919  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1920  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1922  <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>
1923                 ; like <x:ref>quoted-string</x:ref>, but disallowing line folding
1924  <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>
1927   Chunk extensions within the chunked transfer coding are deprecated.
1928   Senders &SHOULD-NOT; send chunk-ext.
1929   Definition of new chunk extensions is discouraged.
1932   The chunk-size field is a string of hex digits indicating the size of
1933   the chunk-data in octets. The chunked transfer coding is complete when a
1934   chunk with a chunk-size of zero is received, possibly followed by a
1935   trailer, and finally terminated by an empty line.
1938<section title="Trailer" anchor="header.trailer">
1939  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
1940  <x:anchor-alias value="Trailer"/>
1942   A trailer allows the sender to include additional fields at the end of a
1943   chunked message in order to supply metadata that might be dynamically
1944   generated while the message body is sent, such as a message integrity
1945   check, digital signature, or post-processing status.
1946   The trailer &MUST-NOT; contain fields that need to be known before a
1947   recipient processes the body, such as <x:ref>Transfer-Encoding</x:ref>,
1948   <x:ref>Content-Length</x:ref>, and <x:ref>Trailer</x:ref>.
1951   When a message includes a message body encoded with the chunked
1952   transfer coding and the sender desires to send metadata in the form of
1953   trailer fields at the end of the message, the sender &SHOULD; send a
1954   <x:ref>Trailer</x:ref> header field before the message body to indicate
1955   which fields will be present in the trailers. This allows the recipient
1956   to prepare for receipt of that metadata before it starts processing the body,
1957   which is useful if the message is being streamed and the recipient wishes
1958   to confirm an integrity check on the fly.
1960<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
1961  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
1964   If no <x:ref>Trailer</x:ref> header field is present, the sender of a
1965   chunked message body &SHOULD; send an empty trailer.
1968   A server &MUST; send an empty trailer with the chunked transfer coding
1969   unless at least one of the following is true:
1970  <list style="numbers">
1971    <t>the request included a <x:ref>TE</x:ref> header field that indicates
1972    "trailers" is acceptable in the transfer coding of the response, as
1973    described in <xref target="header.te"/>; or,</t>
1975    <t>the trailer fields consist entirely of optional metadata and the
1976    recipient could use the message (in a manner acceptable to the server where
1977    the field originated) without receiving that metadata. In other words,
1978    the server that generated the header field is willing to accept the
1979    possibility that the trailer fields might be silently discarded along
1980    the path to the client.</t>
1981  </list>
1984   The above requirement prevents the need for an infinite buffer when a
1985   message is being received by an HTTP/1.1 (or later) proxy and forwarded to
1986   an HTTP/1.0 recipient.
1990<section title="Decoding chunked" anchor="decoding.chunked">
1992   A process for decoding the chunked transfer coding
1993   can be represented in pseudo-code as:
1995<figure><artwork type="code">
1996  length := 0
1997  read chunk-size, chunk-ext (if any) and CRLF
1998  while (chunk-size &gt; 0) {
1999     read chunk-data and CRLF
2000     append chunk-data to decoded-body
2001     length := length + chunk-size
2002     read chunk-size and CRLF
2003  }
2004  read header-field
2005  while (header-field not empty) {
2006     append header-field to existing header fields
2007     read header-field
2008  }
2009  Content-Length := length
2010  Remove "chunked" from Transfer-Encoding
2011  Remove Trailer from existing header fields
2014   All recipients &MUST; be able to receive and decode the
2015   chunked transfer coding and &MUST; ignore chunk-ext extensions
2016   they do not understand.
2021<section title="Compression Codings" anchor="compression.codings">
2023   The codings defined below can be used to compress the payload of a
2024   message.
2027<section title="Compress Coding" anchor="compress.coding">
2028<iref item="compress (Coding Format)"/>
2030   The "compress" format is produced by the common UNIX file compression
2031   program "compress". This format is an adaptive Lempel-Ziv-Welch
2032   coding (LZW). Recipients &SHOULD; consider "x-compress" to be
2033   equivalent to "compress".
2037<section title="Deflate Coding" anchor="deflate.coding">
2038<iref item="deflate (Coding Format)"/>
2040   The "deflate" format is defined as the "deflate" compression mechanism
2041   (described in <xref target="RFC1951"/>) used inside the "zlib"
2042   data format (<xref target="RFC1950"/>).
2045  <t>
2046    &Note; Some incorrect implementations send the "deflate"
2047    compressed data without the zlib wrapper.
2048   </t>
2052<section title="Gzip Coding" anchor="gzip.coding">
2053<iref item="gzip (Coding Format)"/>
2055   The "gzip" format is produced by the file compression program
2056   "gzip" (GNU zip), as described in <xref target="RFC1952"/>. This format is a
2057   Lempel-Ziv coding (LZ77) with a 32 bit CRC.
2058   Recipients &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2064<section title="TE" anchor="header.te">
2065  <iref primary="true" item="TE header field" x:for-anchor=""/>
2066  <x:anchor-alias value="TE"/>
2067  <x:anchor-alias value="t-codings"/>
2068  <x:anchor-alias value="t-ranking"/>
2069  <x:anchor-alias value="rank"/>
2071   The "TE" header field in a request indicates what transfer codings,
2072   besides chunked, the client is willing to accept in response, and
2073   whether or not the client is willing to accept trailer fields in a
2074   chunked transfer coding.
2077   The TE field-value consists of a comma-separated list of transfer coding
2078   names, each allowing for optional parameters (as described in
2079   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2080   Clients &MUST-NOT; send the chunked transfer coding name in TE;
2081   chunked is always acceptable for HTTP/1.1 recipients.
2083<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"/>
2084  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2085  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2086  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2087  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2088             / ( "1" [ "." 0*3("0") ] )
2091   Three examples of TE use are below.
2093<figure><artwork type="example">
2094  TE: deflate
2095  TE:
2096  TE: trailers, deflate;q=0.5
2099   The presence of the keyword "trailers" indicates that the client is
2100   willing to accept trailer fields in a chunked transfer coding,
2101   as defined in <xref target="chunked.encoding"/>, on behalf of itself and
2102   any downstream clients. For chained requests, this implies that either:
2103   (a) all downstream clients are willing to accept trailer fields in the
2104   forwarded response; or,
2105   (b) the client will attempt to buffer the response on behalf of downstream
2106   recipients.
2107   Note that HTTP/1.1 does not define any means to limit the size of a
2108   chunked response such that a client can be assured of buffering the
2109   entire response.
2112   When multiple transfer codings are acceptable, the client &MAY; rank the
2113   codings by preference using a case-insensitive "q" parameter (similar to
2114   the qvalues used in content negotiation fields, &qvalue;). The rank value
2115   is a real number in the range 0 through 1, where 0.001 is the least
2116   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2119   If the TE field-value is empty or if no TE field is present, the only
2120   acceptable transfer coding is chunked. A message with no transfer coding
2121   is always acceptable.
2124   Since the TE header field only applies to the immediate connection,
2125   a sender of TE &MUST; also send a "TE" connection option within the
2126   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2127   in order to prevent the TE field from being forwarded by intermediaries
2128   that do not support its semantics.
2133<section title="Message Routing" anchor="message.routing">
2135   HTTP request message routing is determined by each client based on the
2136   target resource, the client's proxy configuration, and
2137   establishment or reuse of an inbound connection.  The corresponding
2138   response routing follows the same connection chain back to the client.
2141<section title="Identifying a Target Resource" anchor="target-resource">
2142  <iref primary="true" item="target resource"/>
2143  <iref primary="true" item="target URI"/>
2144  <x:anchor-alias value="target resource"/>
2145  <x:anchor-alias value="target URI"/>
2147   HTTP is used in a wide variety of applications, ranging from
2148   general-purpose computers to home appliances.  In some cases,
2149   communication options are hard-coded in a client's configuration.
2150   However, most HTTP clients rely on the same resource identification
2151   mechanism and configuration techniques as general-purpose Web browsers.
2154   HTTP communication is initiated by a user agent for some purpose.
2155   The purpose is a combination of request semantics, which are defined in
2156   <xref target="Part2"/>, and a target resource upon which to apply those
2157   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2158   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2159   would resolve to its absolute form in order to obtain the
2160   "<x:dfn>target URI</x:dfn>".  The target URI
2161   excludes the reference's fragment identifier component, if any,
2162   since fragment identifiers are reserved for client-side processing
2163   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2167<section title="Connecting Inbound" anchor="connecting.inbound">
2169   Once the target URI is determined, a client needs to decide whether
2170   a network request is necessary to accomplish the desired semantics and,
2171   if so, where that request is to be directed.
2174   If the client has a response cache and the request semantics can be
2175   satisfied by a cache (<xref target="Part6"/>), then the request is
2176   usually directed to the cache first.
2179   If the request is not satisfied by a cache, then a typical client will
2180   check its configuration to determine whether a proxy is to be used to
2181   satisfy the request.  Proxy configuration is implementation-dependent,
2182   but is often based on URI prefix matching, selective authority matching,
2183   or both, and the proxy itself is usually identified by an "http" or
2184   "https" URI.  If a proxy is applicable, the client connects inbound by
2185   establishing (or reusing) a connection to that proxy.
2188   If no proxy is applicable, a typical client will invoke a handler routine,
2189   usually specific to the target URI's scheme, to connect directly
2190   to an authority for the target resource.  How that is accomplished is
2191   dependent on the target URI scheme and defined by its associated
2192   specification, similar to how this specification defines origin server
2193   access for resolution of the "http" (<xref target="http.uri"/>) and
2194   "https" (<xref target="https.uri"/>) schemes.
2197   HTTP requirements regarding connection management are defined in
2198   <xref target=""/>.
2202<section title="Request Target" anchor="request-target">
2204   Once an inbound connection is obtained,
2205   the client sends an HTTP request message (<xref target="http.message"/>)
2206   with a request-target derived from the target URI.
2207   There are four distinct formats for the request-target, depending on both
2208   the method being requested and whether the request is to a proxy.
2210<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"/>
2211  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2212                 / <x:ref>absolute-form</x:ref>
2213                 / <x:ref>authority-form</x:ref>
2214                 / <x:ref>asterisk-form</x:ref>
2216  <x:ref>origin-form</x:ref>    = <x:ref>path-absolute</x:ref> [ "?" <x:ref>query</x:ref> ]
2217  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2218  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2219  <x:ref>asterisk-form</x:ref>  = "*"
2221<t anchor="origin-form"><iref item="origin-form (of request-target)"/>
2222   The most common form of request-target is the origin-form.
2223   When making a request directly to an origin server, other than a CONNECT
2224   or server-wide OPTIONS request (as detailed below),
2225   a client &MUST; send only the absolute path and query components of
2226   the target URI as the request-target.
2227   If the target URI's path component is empty, then the client &MUST; send
2228   "/" as the path within the origin-form of request-target.
2229   A <x:ref>Host</x:ref> header field is also sent, as defined in
2230   <xref target=""/>, containing the target URI's
2231   authority component (excluding any userinfo).
2234   For example, a client wishing to retrieve a representation of the resource
2235   identified as
2237<figure><artwork x:indent-with="  " type="example">
2241   directly from the origin server would open (or reuse) a TCP connection
2242   to port 80 of the host "" and send the lines:
2244<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2245GET /where?q=now HTTP/1.1
2249   followed by the remainder of the request message.
2251<t anchor="absolute-form"><iref item="absolute-form (of request-target)"/>
2252   When making a request to a proxy, other than a CONNECT or server-wide
2253   OPTIONS request (as detailed below), a client &MUST; send the target URI
2254   in absolute-form as the request-target.
2255   The proxy is requested to either service that request from a valid cache,
2256   if possible, or make the same request on the client's behalf to either
2257   the next inbound proxy server or directly to the origin server indicated
2258   by the request-target.  Requirements on such "forwarding" of messages are
2259   defined in <xref target="message.forwarding"/>.
2262   An example absolute-form of request-line would be:
2264<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2265GET HTTP/1.1
2268   To allow for transition to the absolute-form for all requests in some
2269   future version of HTTP, HTTP/1.1 servers &MUST; accept the absolute-form
2270   in requests, even though HTTP/1.1 clients will only send them in requests
2271   to proxies.
2273<t anchor="authority-form"><iref item="authority-form (of request-target)"/>
2274   The authority-form of request-target is only used for CONNECT requests
2275   (&CONNECT;).  When making a CONNECT request to establish a tunnel through
2276   one or more proxies, a client &MUST; send only the target URI's
2277   authority component (excluding any userinfo) as the request-target.
2278   For example,
2280<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2283<t anchor="asterisk-form"><iref item="asterisk-form (of request-target)"/>
2284   The asterisk-form of request-target is only used for a server-wide
2285   OPTIONS request (&OPTIONS;).  When a client wishes to request OPTIONS
2286   for the server as a whole, as opposed to a specific named resource of
2287   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2288   For example,
2290<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2291OPTIONS * HTTP/1.1
2294   If a proxy receives an OPTIONS request with an absolute-form of
2295   request-target in which the URI has an empty path and no query component,
2296   then the last proxy on the request chain &MUST; send a request-target
2297   of "*" when it forwards the request to the indicated origin server.
2300   For example, the request
2301</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2305  would be forwarded by the final proxy as
2306</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2307OPTIONS * HTTP/1.1
2311   after connecting to port 8001 of host "".
2316<section title="Host" anchor="">
2317  <iref primary="true" item="Host header field" x:for-anchor=""/>
2318  <x:anchor-alias value="Host"/>
2320   The "Host" header field in a request provides the host and port
2321   information from the target URI, enabling the origin
2322   server to distinguish among resources while servicing requests
2323   for multiple host names on a single IP address.  Since the Host
2324   field-value is critical information for handling a request, it
2325   &SHOULD; be sent as the first header field following the request-line.
2327<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2328  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2331   A client &MUST; send a Host header field in all HTTP/1.1 request
2332   messages.  If the target URI includes an authority component, then
2333   the Host field-value &MUST; be identical to that authority component
2334   after excluding any userinfo (<xref target="http.uri"/>).
2335   If the authority component is missing or undefined for the target URI,
2336   then the Host header field &MUST; be sent with an empty field-value.
2339   For example, a GET request to the origin server for
2340   &lt;; would begin with:
2342<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2343GET /pub/WWW/ HTTP/1.1
2347   The Host header field &MUST; be sent in an HTTP/1.1 request even
2348   if the request-target is in the absolute-form, since this
2349   allows the Host information to be forwarded through ancient HTTP/1.0
2350   proxies that might not have implemented Host.
2353   When a proxy receives a request with an absolute-form of
2354   request-target, the proxy &MUST; ignore the received
2355   Host header field (if any) and instead replace it with the host
2356   information of the request-target.  If the proxy forwards the request,
2357   it &MUST; generate a new Host field-value based on the received
2358   request-target rather than forward the received Host field-value.
2361   Since the Host header field acts as an application-level routing
2362   mechanism, it is a frequent target for malware seeking to poison
2363   a shared cache or redirect a request to an unintended server.
2364   An interception proxy is particularly vulnerable if it relies on
2365   the Host field-value for redirecting requests to internal
2366   servers, or for use as a cache key in a shared cache, without
2367   first verifying that the intercepted connection is targeting a
2368   valid IP address for that host.
2371   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2372   to any HTTP/1.1 request message that lacks a Host header field and
2373   to any request message that contains more than one Host header field
2374   or a Host header field with an invalid field-value.
2378<section title="Effective Request URI" anchor="effective.request.uri">
2379  <iref primary="true" item="effective request URI"/>
2380  <x:anchor-alias value="effective request URI"/>
2382   A server that receives an HTTP request message &MUST; reconstruct
2383   the user agent's original target URI, based on the pieces of information
2384   learned from the request-target, <x:ref>Host</x:ref> header field, and
2385   connection context, in order to identify the intended target resource and
2386   properly service the request. The URI derived from this reconstruction
2387   process is referred to as the "<x:dfn>effective request URI</x:dfn>".
2390   For a user agent, the effective request URI is the target URI.
2393   If the request-target is in absolute-form, then the effective request URI
2394   is the same as the request-target.  Otherwise, the effective request URI
2395   is constructed as follows.
2398   If the request is received over a TLS-secured TCP connection,
2399   then the effective request URI's scheme is "https"; otherwise, the
2400   scheme is "http".
2403   If the request-target is in authority-form, then the effective
2404   request URI's authority component is the same as the request-target.
2405   Otherwise, if a <x:ref>Host</x:ref> header field is supplied with a
2406   non-empty field-value, then the authority component is the same as the
2407   Host field-value. Otherwise, the authority component is the concatenation of
2408   the default host name configured for the server, a colon (":"), and the
2409   connection's incoming TCP port number in decimal form.
2412   If the request-target is in authority-form or asterisk-form, then the
2413   effective request URI's combined path and query component is empty.
2414   Otherwise, the combined path and query component is the same as the
2415   request-target.
2418   The components of the effective request URI, once determined as above,
2419   can be combined into absolute-URI form by concatenating the scheme,
2420   "://", authority, and combined path and query component.
2424   Example 1: the following message received over an insecure TCP connection
2426<artwork type="example" x:indent-with="  ">
2427GET /pub/WWW/TheProject.html HTTP/1.1
2433  has an effective request URI of
2435<artwork type="example" x:indent-with="  ">
2441   Example 2: the following message received over a TLS-secured TCP connection
2443<artwork type="example" x:indent-with="  ">
2444OPTIONS * HTTP/1.1
2450  has an effective request URI of
2452<artwork type="example" x:indent-with="  ">
2457   An origin server that does not allow resources to differ by requested
2458   host &MAY; ignore the <x:ref>Host</x:ref> field-value and instead replace it
2459   with a configured server name when constructing the effective request URI.
2462   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2463   field &MAY; attempt to use heuristics (e.g., examination of the URI path for
2464   something unique to a particular host) in order to guess the
2465   effective request URI's authority component.
2469<section title="Associating a Response to a Request" anchor="">
2471   HTTP does not include a request identifier for associating a given
2472   request message with its corresponding one or more response messages.
2473   Hence, it relies on the order of response arrival to correspond exactly
2474   to the order in which requests are made on the same connection.
2475   More than one response message per request only occurs when one or more
2476   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a
2477   final response to the same request.
2480   A client that has more than one outstanding request on a connection &MUST;
2481   maintain a list of outstanding requests in the order sent and &MUST;
2482   associate each received response message on that connection to the highest
2483   ordered request that has not yet received a final (non-<x:ref>1xx</x:ref>)
2484   response.
2488<section title="Message Forwarding" anchor="message.forwarding">
2490   As described in <xref target="intermediaries"/>, intermediaries can serve
2491   a variety of roles in the processing of HTTP requests and responses.
2492   Some intermediaries are used to improve performance or availability.
2493   Others are used for access control or to filter content.
2494   Since an HTTP stream has characteristics similar to a pipe-and-filter
2495   architecture, there are no inherent limits to the extent an intermediary
2496   can enhance (or interfere) with either direction of the stream.
2499   Intermediaries that forward a message &MUST; implement the
2500   <x:ref>Connection</x:ref> header field, as specified in
2501   <xref target="header.connection"/>, to exclude fields that are only
2502   intended for the incoming connection.
2505   In order to avoid request loops, a proxy that forwards requests to other
2506   proxies &MUST; be able to recognize and exclude all of its own server
2507   names, including any aliases, local variations, or literal IP addresses.
2510<section title="Via" anchor="header.via">
2511  <iref primary="true" item="Via header field" x:for-anchor=""/>
2512  <x:anchor-alias value="pseudonym"/>
2513  <x:anchor-alias value="received-by"/>
2514  <x:anchor-alias value="received-protocol"/>
2515  <x:anchor-alias value="Via"/>
2517   The "Via" header field &MUST; be sent by a proxy or gateway in forwarded
2518   messages to indicate the intermediate protocols and recipients between the
2519   user agent and the server on requests, and between the origin server and
2520   the client on responses. It is analogous to the "Received" field
2521   used by email systems (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2522   Via is used in HTTP for tracking message forwards,
2523   avoiding request loops, and identifying the protocol capabilities of
2524   all senders along the request/response chain.
2526<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"/>
2527  <x:ref>Via</x:ref>               = 1#( <x:ref>received-protocol</x:ref> <x:ref>RWS</x:ref> <x:ref>received-by</x:ref>
2528                          [ <x:ref>RWS</x:ref> <x:ref>comment</x:ref> ] )
2529  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2530                      ; see <xref target="header.upgrade"/>
2531  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2532  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2535   The received-protocol indicates the protocol version of the message
2536   received by the server or client along each segment of the
2537   request/response chain. The received-protocol version is appended to
2538   the Via field value when the message is forwarded so that information
2539   about the protocol capabilities of upstream applications remains
2540   visible to all recipients.
2543   The protocol-name is excluded if and only if it would be "HTTP". The
2544   received-by field is normally the host and optional port number of a
2545   recipient server or client that subsequently forwarded the message.
2546   However, if the real host is considered to be sensitive information,
2547   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2548   be assumed to be the default port of the received-protocol.
2551   Multiple Via field values represent each proxy or gateway that has
2552   forwarded the message. Each recipient &MUST; append its information
2553   such that the end result is ordered according to the sequence of
2554   forwarding applications.
2557   Comments &MAY; be used in the Via header field to identify the software
2558   of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2559   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2560   are optional and &MAY; be removed by any recipient prior to forwarding the
2561   message.
2564   For example, a request message could be sent from an HTTP/1.0 user
2565   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2566   forward the request to a public proxy at, which completes
2567   the request by forwarding it to the origin server at
2568   The request received by would then have the following
2569   Via header field:
2571<figure><artwork type="example">
2572  Via: 1.0 fred, 1.1 (Apache/1.1)
2575   A proxy or gateway used as a portal through a network firewall
2576   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2577   region unless it is explicitly enabled to do so. If not enabled, the
2578   received-by host of any host behind the firewall &SHOULD; be replaced
2579   by an appropriate pseudonym for that host.
2582   A proxy or gateway &MAY; combine an ordered subsequence of Via header
2583   field entries into a single such entry if the entries have identical
2584   received-protocol values. For example,
2586<figure><artwork type="example">
2587  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2590  could be collapsed to
2592<figure><artwork type="example">
2593  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2596   Senders &SHOULD-NOT; combine multiple entries unless they are all
2597   under the same organizational control and the hosts have already been
2598   replaced by pseudonyms. Senders &MUST-NOT; combine entries that
2599   have different received-protocol values.
2603<section title="Transformations" anchor="message.transformations">
2605   Some intermediaries include features for transforming messages and their
2606   payloads.  A transforming proxy might, for example, convert between image
2607   formats in order to save cache space or to reduce the amount of traffic on
2608   a slow link. However, operational problems might occur when these
2609   transformations are applied to payloads intended for critical applications,
2610   such as medical imaging or scientific data analysis, particularly when
2611   integrity checks or digital signatures are used to ensure that the payload
2612   received is identical to the original.
2615   If a proxy receives a request-target with a host name that is not a
2616   fully qualified domain name, it &MAY; add its own domain to the host name
2617   it received when forwarding the request.  A proxy &MUST-NOT; change the
2618   host name if it is a fully qualified domain name.
2621   A proxy &MUST-NOT; modify the "path-absolute" and "query" parts of the
2622   received request-target when forwarding it to the next inbound server,
2623   except as noted above to replace an empty path with "/" or "*".
2626   A proxy &MUST-NOT; modify header fields that provide information about the
2627   end points of the communication chain, the resource state, or the selected
2628   representation. A proxy &MAY; change the message body through application
2629   or removal of a transfer coding (<xref target="transfer.codings"/>).
2632   A non-transforming proxy &MUST; preserve the message payload (&payload;).
2633   A transforming proxy &MUST; preserve the payload of a message that
2634   contains the no-transform cache-control directive.
2637   A transforming proxy &MAY; transform the payload of a message
2638   that does not contain the no-transform cache-control directive;
2639   if the payload is transformed, the transforming proxy &MUST; add a
2640   Warning 214 (Transformation applied) header field if one does not
2641   already appear in the message (see &header-warning;).
2647<section title="Connection Management" anchor="">
2649   HTTP messaging is independent of the underlying transport or
2650   session-layer connection protocol(s).  HTTP only presumes a reliable
2651   transport with in-order delivery of requests and the corresponding
2652   in-order delivery of responses.  The mapping of HTTP request and
2653   response structures onto the data units of an underlying transport
2654   protocol is outside the scope of this specification.
2657   As described in <xref target="connecting.inbound"/>, the specific
2658   connection protocols to be used for an HTTP interaction are determined by
2659   client configuration and the <x:ref>target URI</x:ref>.
2660   For example, the "http" URI scheme
2661   (<xref target="http.uri"/>) indicates a default connection of TCP
2662   over IP, with a default TCP port of 80, but the client might be
2663   configured to use a proxy via some other connection, port, or protocol.
2666   HTTP implementations are expected to engage in connection management,
2667   which includes maintaining the state of current connections,
2668   establishing a new connection or reusing an existing connection,
2669   processing messages received on a connection, detecting connection
2670   failures, and closing each connection.
2671   Most clients maintain multiple connections in parallel, including
2672   more than one connection per server endpoint.
2673   Most servers are designed to maintain thousands of concurrent connections,
2674   while controlling request queues to enable fair use and detect
2675   denial of service attacks.
2678<section title="Connection" anchor="header.connection">
2679  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2680  <iref primary="true" item="close" x:for-anchor=""/>
2681  <x:anchor-alias value="Connection"/>
2682  <x:anchor-alias value="connection-option"/>
2683  <x:anchor-alias value="close"/>
2685   The "Connection" header field allows the sender to indicate desired
2686   control options for the current connection.  In order to avoid confusing
2687   downstream recipients, a proxy or gateway &MUST; remove or replace any
2688   received connection options before forwarding the message.
2691   When a header field aside from Connection is used to supply control
2692   information for or about the current connection, the sender &MUST; list
2693   the corresponding field-name within the "Connection" header field.
2694   A proxy or gateway &MUST; parse a received Connection
2695   header field before a message is forwarded and, for each
2696   connection-option in this field, remove any header field(s) from
2697   the message with the same name as the connection-option, and then
2698   remove the Connection header field itself (or replace it with the
2699   intermediary's own connection options for the forwarded message).
2702   Hence, the Connection header field provides a declarative way of
2703   distinguishing header fields that are only intended for the
2704   immediate recipient ("hop-by-hop") from those fields that are
2705   intended for all recipients on the chain ("end-to-end"), enabling the
2706   message to be self-descriptive and allowing future connection-specific
2707   extensions to be deployed without fear that they will be blindly
2708   forwarded by older intermediaries.
2711   The Connection header field's value has the following grammar:
2713<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2714  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2715  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2718   Connection options are case-insensitive.
2721   A sender &MUST-NOT; send a connection option corresponding to a header
2722   field that is intended for all recipients of the payload.
2723   For example, <x:ref>Cache-Control</x:ref> is never appropriate as a
2724   connection option (&header-cache-control;).
2727   The connection options do not have to correspond to a header field
2728   present in the message, since a connection-specific header field
2729   might not be needed if there are no parameters associated with that
2730   connection option.  Recipients that trigger certain connection
2731   behavior based on the presence of connection options &MUST; do so
2732   based on the presence of the connection-option rather than only the
2733   presence of the optional header field.  In other words, if the
2734   connection option is received as a header field but not indicated
2735   within the Connection field-value, then the recipient &MUST; ignore
2736   the connection-specific header field because it has likely been
2737   forwarded by an intermediary that is only partially conformant.
2740   When defining new connection options, specifications ought to
2741   carefully consider existing deployed header fields and ensure
2742   that the new connection option does not share the same name as
2743   an unrelated header field that might already be deployed.
2744   Defining a new connection option essentially reserves that potential
2745   field-name for carrying additional information related to the
2746   connection option, since it would be unwise for senders to use
2747   that field-name for anything else.
2750   The "<x:dfn>close</x:dfn>" connection option is defined for a
2751   sender to signal that this connection will be closed after completion of
2752   the response. For example,
2754<figure><artwork type="example">
2755  Connection: close
2758   in either the request or the response header fields indicates that
2759   the connection &MUST; be closed after the current request/response
2760   is complete (<xref target="persistent.tear-down"/>).
2763   A client that does not support <x:ref>persistent connections</x:ref> &MUST;
2764   send the "close" connection option in every request message.
2767   A server that does not support <x:ref>persistent connections</x:ref> &MUST;
2768   send the "close" connection option in every response message that
2769   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2773<section title="Establishment" anchor="persistent.establishment">
2775   It is beyond the scope of this specification to describe how connections
2776   are established via various transport or session-layer protocols.
2777   Each connection applies to only one transport link.
2781<section title="Persistence" anchor="persistent.connections">
2782   <x:anchor-alias value="persistent connections"/>
2784   HTTP/1.1 defaults to the use of "<x:dfn>persistent connections</x:dfn>",
2785   allowing multiple requests and responses to be carried over a single
2786   connection. The "<x:ref>close</x:ref>" connection-option is used to signal
2787   that a connection will not persist after the current request/response.
2788   HTTP implementations &SHOULD; support persistent connections.
2791   A recipient determines whether a connection is persistent or not based on
2792   the most recently received message's protocol version and
2793   <x:ref>Connection</x:ref> header field (if any):
2794   <list style="symbols">
2795     <t>If the <x:ref>close</x:ref> connection option is present, the
2796        connection will not persist after the current response; else,</t>
2797     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2798        persist after the current response; else,</t>
2799     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2800        connection option is present, the recipient is not a proxy, and
2801        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2802        the connection will persist after the current response; otherwise,</t>
2803     <t>The connection will close after the current response.</t>
2804   </list>
2807   A server &MAY; assume that an HTTP/1.1 client intends to maintain a
2808   persistent connection until a <x:ref>close</x:ref> connection option
2809   is received in a request.
2812   A client &MAY; reuse a persistent connection until it sends or receives
2813   a <x:ref>close</x:ref> connection option or receives an HTTP/1.0 response
2814   without a "keep-alive" connection option.
2817   In order to remain persistent, all messages on a connection &MUST;
2818   have a self-defined message length (i.e., one not defined by closure
2819   of the connection), as described in <xref target="message.body"/>.
2820   A server &MUST; read the entire request message body or close
2821   the connection after sending its response, since otherwise the
2822   remaining data on a persistent connection would be misinterpreted
2823   as the next request.  Likewise,
2824   a client &MUST; read the entire response message body if it intends
2825   to reuse the same connection for a subsequent request.
2828   A proxy server &MUST-NOT; maintain a persistent connection with an
2829   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
2830   information and discussion of the problems with the Keep-Alive header field
2831   implemented by many HTTP/1.0 clients).
2834   Clients and servers &SHOULD-NOT; assume that a persistent connection is
2835   maintained for HTTP versions less than 1.1 unless it is explicitly
2836   signaled.
2837   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
2838   for more information on backward compatibility with HTTP/1.0 clients.
2841<section title="Pipelining" anchor="pipelining">
2843   A client that supports persistent connections &MAY; "pipeline" its
2844   requests (i.e., send multiple requests without waiting for each
2845   response). A server &MUST; send its responses to those requests in the
2846   same order that the requests were received.
2849   Clients that assume persistent connections and pipeline immediately
2850   after connection establishment &SHOULD; be prepared to retry their
2851   connection if the first pipelined attempt fails. If a client does
2852   such a retry, it &MUST-NOT; pipeline before it knows the connection is
2853   persistent. Clients &MUST; also be prepared to resend their requests if
2854   the server closes the connection before sending all of the
2855   corresponding responses.
2858   Clients &SHOULD-NOT; pipeline requests using non-idempotent request methods
2859   or non-idempotent sequences of request methods (see &idempotent-methods;).
2860   Otherwise, a premature termination of the transport connection could lead
2861   to indeterminate results. A client wishing to send a non-idempotent
2862   request &SHOULD; wait to send that request until it has received the
2863   response status line for the previous request.
2867<section title="Retrying Requests" anchor="persistent.retrying.requests">
2869   Connections can be closed at any time, with or without intention.
2870   Implementations ought to anticipate the need to recover
2871   from asynchronous close events.
2872   A client &MAY; open a new connection and retransmit an aborted sequence
2873   of requests without user interaction so long as the request sequence is
2874   idempotent (see &idempotent-methods;).
2875   A client &MUST-NOT; automatically retry non-idempotent request sequences,
2876   although user agents &MAY; offer a human operator the choice of retrying
2877   the request(s). Confirmation by
2878   user agent software with semantic understanding of the application
2879   &MAY; substitute for user confirmation. An automatic retry &SHOULD-NOT;
2880   be repeated if a second sequence of requests fails.
2885<section title="Concurrency" anchor="persistent.concurrency">
2887   Clients &SHOULD; limit the number of simultaneous
2888   connections that they maintain to a given server.
2891   Previous revisions of HTTP gave a specific number of connections as a
2892   ceiling, but this was found to be impractical for many applications. As a
2893   result, this specification does not mandate a particular maximum number of
2894   connections, but instead encourages clients to be conservative when opening
2895   multiple connections.
2898   Multiple connections are typically used to avoid the "head-of-line
2899   blocking" problem, wherein a request that takes significant server-side
2900   processing and/or has a large payload blocks subsequent requests on the
2901   same connection. However, each connection consumes server resources.
2902   Furthermore, using multiple connections can cause undesirable side effects
2903   in congested networks.
2906   Note that servers might reject traffic that they deem abusive, including an
2907   excessive number of connections from a client.
2911<section title="Failures and Time-outs" anchor="persistent.failures">
2913   Servers will usually have some time-out value beyond which they will
2914   no longer maintain an inactive connection. Proxy servers might make
2915   this a higher value since it is likely that the client will be making
2916   more connections through the same server. The use of persistent
2917   connections places no requirements on the length (or existence) of
2918   this time-out for either the client or the server.
2921   When a client or server wishes to time-out it &SHOULD; issue a graceful
2922   close on the transport connection. Clients and servers &SHOULD; both
2923   constantly watch for the other side of the transport close, and
2924   respond to it as appropriate. If a client or server does not detect
2925   the other side's close promptly it could cause unnecessary resource
2926   drain on the network.
2929   A client, server, or proxy &MAY; close the transport connection at any
2930   time. For example, a client might have started to send a new request
2931   at the same time that the server has decided to close the "idle"
2932   connection. From the server's point of view, the connection is being
2933   closed while it was idle, but from the client's point of view, a
2934   request is in progress.
2937   Servers &SHOULD; maintain persistent connections and allow the underlying
2938   transport's flow control mechanisms to resolve temporary overloads, rather
2939   than terminate connections with the expectation that clients will retry.
2940   The latter technique can exacerbate network congestion.
2943   A client sending a message body &SHOULD; monitor
2944   the network connection for an error status code while it is transmitting
2945   the request. If the client sees an error status code, it &SHOULD;
2946   immediately cease transmitting the body and close the connection.
2950<section title="Tear-down" anchor="persistent.tear-down">
2951  <iref primary="false" item="Connection header field" x:for-anchor=""/>
2952  <iref primary="false" item="close" x:for-anchor=""/>
2954   The <x:ref>Connection</x:ref> header field
2955   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
2956   connection option that a sender &SHOULD; send when it wishes to close
2957   the connection after the current request/response pair.
2960   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
2961   send further requests on that connection (after the one containing
2962   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
2963   final response message corresponding to this request.
2966   A server that receives a <x:ref>close</x:ref> connection option &MUST;
2967   initiate a lingering close (see below) of the connection after it sends the
2968   final response to the request that contained <x:ref>close</x:ref>.
2969   The server &SHOULD; send a <x:ref>close</x:ref> connection option
2970   in its final response on that connection. The server &MUST-NOT; process
2971   any further requests received on that connection.
2974   A server that sends a <x:ref>close</x:ref> connection option &MUST;
2975   initiate a lingering close of the connection after it sends the
2976   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
2977   any further requests received on that connection.
2980   A client that receives a <x:ref>close</x:ref> connection option &MUST;
2981   cease sending requests on that connection and close the connection
2982   after reading the response message containing the close; if additional
2983   pipelined requests had been sent on the connection, the client &SHOULD;
2984   assume that they will not be processed by the server.
2987   If a server performs an immediate close of a TCP connection, there is a
2988   significant risk that the client will not be able to read the last HTTP
2989   response.  If the server receives additional data from the client on a
2990   fully-closed connection, such as another request that was sent by the
2991   client before receiving the server's response, the server's TCP stack will
2992   send a reset packet to the client; unfortunately, the reset packet might
2993   erase the client's unacknowledged input buffers before they can be read
2994   and interpreted by the client's HTTP parser.
2997   To avoid the TCP reset problem, a server can perform a lingering close on a
2998   connection by closing only the write side of the read/write connection
2999   (a half-close) and continuing to read from the connection until the
3000   connection is closed by the client or the server is reasonably certain
3001   that its own TCP stack has received the client's acknowledgement of the
3002   packet(s) containing the server's last response. It is then safe for the
3003   server to fully close the connection.
3006   It is unknown whether the reset problem is exclusive to TCP or might also
3007   be found in other transport connection protocols.
3011<section title="Upgrade" anchor="header.upgrade">
3012  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3013  <x:anchor-alias value="Upgrade"/>
3014  <x:anchor-alias value="protocol"/>
3015  <x:anchor-alias value="protocol-name"/>
3016  <x:anchor-alias value="protocol-version"/>
3018   The "Upgrade" header field is intended to provide a simple mechanism
3019   for transitioning from HTTP/1.1 to some other protocol on the same
3020   connection.  A client &MAY; send a list of protocols in the Upgrade
3021   header field of a request to invite the server to switch to one or
3022   more of those protocols before sending the final response.
3023   A server &MUST; send an Upgrade header field in <x:ref>101 (Switching
3024   Protocols)</x:ref> responses to indicate which protocol(s) are being
3025   switched to, and &MUST; send it in <x:ref>426 (Upgrade Required)</x:ref>
3026   responses to indicate acceptable protocols.
3027   A server &MAY; send an Upgrade header field in any other response to
3028   indicate that they might be willing to upgrade to one of the
3029   specified protocols for a future request.
3031<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3032  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3034  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3035  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3036  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3039   For example,
3041<figure><artwork type="example">
3042  Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3045   Upgrade eases the difficult transition between incompatible protocols by
3046   allowing the client to initiate a request in the more commonly
3047   supported protocol while indicating to the server that it would like
3048   to use a "better" protocol if available (where "better" is determined
3049   by the server, possibly according to the nature of the request method
3050   or target resource).
3053   Upgrade cannot be used to insist on a protocol change; its acceptance and
3054   use by the server is optional. The capabilities and nature of the
3055   application-level communication after the protocol change is entirely
3056   dependent upon the new protocol chosen, although the first action
3057   after changing the protocol &MUST; be a response to the initial HTTP
3058   request that contained the Upgrade header field.
3061   For example, if the Upgrade header field is received in a GET request
3062   and the server decides to switch protocols, then it &MUST; first respond
3063   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3064   then immediately follow that with the new protocol's equivalent of a
3065   response to a GET on the target resource.  This allows a connection to be
3066   upgraded to protocols with the same semantics as HTTP without the
3067   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3068   protocols unless the received message semantics can be honored by the new
3069   protocol; an OPTIONS request can be honored by any protocol.
3072   When Upgrade is sent, a sender &MUST; also send a
3073   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3074   that contains the "upgrade" connection option, in order to prevent Upgrade
3075   from being accidentally forwarded by intermediaries that might not implement
3076   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3077   is received in an HTTP/1.0 request.
3080   The Upgrade header field only applies to switching application-level
3081   protocols on the existing connection; it cannot be used
3082   to switch to a protocol on a different connection. For that purpose, it is
3083   more appropriate to use a <x:ref>3xx (Redirection)</x:ref> response
3084   (&status-3xx;).
3087   This specification only defines the protocol name "HTTP" for use by
3088   the family of Hypertext Transfer Protocols, as defined by the HTTP
3089   version rules of <xref target="http.version"/> and future updates to this
3090   specification. Additional tokens ought to be registered with IANA using the
3091   registration procedure defined in <xref target="upgrade.token.registry"/>.
3096<section title="IANA Considerations" anchor="IANA.considerations">
3098<section title="Header Field Registration" anchor="header.field.registration">
3100   HTTP header fields are registered within the Message Header Field Registry
3101   <xref target="BCP90"/> maintained by IANA at
3102   <eref target=""/>.
3105   This document defines the following HTTP header fields, so their
3106   associated registry entries shall be updated according to the permanent
3107   registrations below:
3109<?BEGININC p1-messaging.iana-headers ?>
3110<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3111<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3112   <ttcol>Header Field Name</ttcol>
3113   <ttcol>Protocol</ttcol>
3114   <ttcol>Status</ttcol>
3115   <ttcol>Reference</ttcol>
3117   <c>Connection</c>
3118   <c>http</c>
3119   <c>standard</c>
3120   <c>
3121      <xref target="header.connection"/>
3122   </c>
3123   <c>Content-Length</c>
3124   <c>http</c>
3125   <c>standard</c>
3126   <c>
3127      <xref target="header.content-length"/>
3128   </c>
3129   <c>Host</c>
3130   <c>http</c>
3131   <c>standard</c>
3132   <c>
3133      <xref target=""/>
3134   </c>
3135   <c>TE</c>
3136   <c>http</c>
3137   <c>standard</c>
3138   <c>
3139      <xref target="header.te"/>
3140   </c>
3141   <c>Trailer</c>
3142   <c>http</c>
3143   <c>standard</c>
3144   <c>
3145      <xref target="header.trailer"/>
3146   </c>
3147   <c>Transfer-Encoding</c>
3148   <c>http</c>
3149   <c>standard</c>
3150   <c>
3151      <xref target="header.transfer-encoding"/>
3152   </c>
3153   <c>Upgrade</c>
3154   <c>http</c>
3155   <c>standard</c>
3156   <c>
3157      <xref target="header.upgrade"/>
3158   </c>
3159   <c>Via</c>
3160   <c>http</c>
3161   <c>standard</c>
3162   <c>
3163      <xref target="header.via"/>
3164   </c>
3167<?ENDINC p1-messaging.iana-headers ?>
3169   Furthermore, the header field-name "Close" shall be registered as
3170   "reserved", since using that name as an HTTP header field might
3171   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3172   header field (<xref target="header.connection"/>).
3174<texttable align="left" suppress-title="true">
3175   <ttcol>Header Field Name</ttcol>
3176   <ttcol>Protocol</ttcol>
3177   <ttcol>Status</ttcol>
3178   <ttcol>Reference</ttcol>
3180   <c>Close</c>
3181   <c>http</c>
3182   <c>reserved</c>
3183   <c>
3184      <xref target="header.field.registration"/>
3185   </c>
3188   The change controller is: "IETF ( - Internet Engineering Task Force".
3192<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3194   IANA maintains the registry of URI Schemes <xref target="BCP115"/> at
3195   <eref target=""/>.
3198   This document defines the following URI schemes, so their
3199   associated registry entries shall be updated according to the permanent
3200   registrations below:
3202<texttable align="left" suppress-title="true">
3203   <ttcol>URI Scheme</ttcol>
3204   <ttcol>Description</ttcol>
3205   <ttcol>Reference</ttcol>
3207   <c>http</c>
3208   <c>Hypertext Transfer Protocol</c>
3209   <c><xref target="http.uri"/></c>
3211   <c>https</c>
3212   <c>Hypertext Transfer Protocol Secure</c>
3213   <c><xref target="https.uri"/></c>
3217<section title="Internet Media Type Registration" anchor="">
3219   This document serves as the specification for the Internet media types
3220   "message/http" and "application/http". The following is to be registered with
3221   IANA (see <xref target="BCP13"/>).
3223<section title="Internet Media Type message/http" anchor="">
3224<iref item="Media Type" subitem="message/http" primary="true"/>
3225<iref item="message/http Media Type" primary="true"/>
3227   The message/http type can be used to enclose a single HTTP request or
3228   response message, provided that it obeys the MIME restrictions for all
3229   "message" types regarding line length and encodings.
3232  <list style="hanging" x:indent="12em">
3233    <t hangText="Type name:">
3234      message
3235    </t>
3236    <t hangText="Subtype name:">
3237      http
3238    </t>
3239    <t hangText="Required parameters:">
3240      none
3241    </t>
3242    <t hangText="Optional parameters:">
3243      version, msgtype
3244      <list style="hanging">
3245        <t hangText="version:">
3246          The HTTP-version number of the enclosed message
3247          (e.g., "1.1"). If not present, the version can be
3248          determined from the first line of the body.
3249        </t>
3250        <t hangText="msgtype:">
3251          The message type &mdash; "request" or "response". If not
3252          present, the type can be determined from the first
3253          line of the body.
3254        </t>
3255      </list>
3256    </t>
3257    <t hangText="Encoding considerations:">
3258      only "7bit", "8bit", or "binary" are permitted
3259    </t>
3260    <t hangText="Security considerations:">
3261      none
3262    </t>
3263    <t hangText="Interoperability considerations:">
3264      none
3265    </t>
3266    <t hangText="Published specification:">
3267      This specification (see <xref target=""/>).
3268    </t>
3269    <t hangText="Applications that use this media type:">
3270    </t>
3271    <t hangText="Additional information:">
3272      <list style="hanging">
3273        <t hangText="Magic number(s):">none</t>
3274        <t hangText="File extension(s):">none</t>
3275        <t hangText="Macintosh file type code(s):">none</t>
3276      </list>
3277    </t>
3278    <t hangText="Person and email address to contact for further information:">
3279      See Authors Section.
3280    </t>
3281    <t hangText="Intended usage:">
3282      COMMON
3283    </t>
3284    <t hangText="Restrictions on usage:">
3285      none
3286    </t>
3287    <t hangText="Author/Change controller:">
3288      IESG
3289    </t>
3290  </list>
3293<section title="Internet Media Type application/http" anchor="">
3294<iref item="Media Type" subitem="application/http" primary="true"/>
3295<iref item="application/http Media Type" primary="true"/>
3297   The application/http type can be used to enclose a pipeline of one or more
3298   HTTP request or response messages (not intermixed).
3301  <list style="hanging" x:indent="12em">
3302    <t hangText="Type name:">
3303      application
3304    </t>
3305    <t hangText="Subtype name:">
3306      http
3307    </t>
3308    <t hangText="Required parameters:">
3309      none
3310    </t>
3311    <t hangText="Optional parameters:">
3312      version, msgtype
3313      <list style="hanging">
3314        <t hangText="version:">
3315          The HTTP-version number of the enclosed messages
3316          (e.g., "1.1"). If not present, the version can be
3317          determined from the first line of the body.
3318        </t>
3319        <t hangText="msgtype:">
3320          The message type &mdash; "request" or "response". If not
3321          present, the type can be determined from the first
3322          line of the body.
3323        </t>
3324      </list>
3325    </t>
3326    <t hangText="Encoding considerations:">
3327      HTTP messages enclosed by this type
3328      are in "binary" format; use of an appropriate
3329      Content-Transfer-Encoding is required when
3330      transmitted via E-mail.
3331    </t>
3332    <t hangText="Security considerations:">
3333      none
3334    </t>
3335    <t hangText="Interoperability considerations:">
3336      none
3337    </t>
3338    <t hangText="Published specification:">
3339      This specification (see <xref target=""/>).
3340    </t>
3341    <t hangText="Applications that use this media type:">
3342    </t>
3343    <t hangText="Additional information:">
3344      <list style="hanging">
3345        <t hangText="Magic number(s):">none</t>
3346        <t hangText="File extension(s):">none</t>
3347        <t hangText="Macintosh file type code(s):">none</t>
3348      </list>
3349    </t>
3350    <t hangText="Person and email address to contact for further information:">
3351      See Authors Section.
3352    </t>
3353    <t hangText="Intended usage:">
3354      COMMON
3355    </t>
3356    <t hangText="Restrictions on usage:">
3357      none
3358    </t>
3359    <t hangText="Author/Change controller:">
3360      IESG
3361    </t>
3362  </list>
3367<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3369   The HTTP Transfer Coding Registry defines the name space for transfer
3370   coding names.
3373   Registrations &MUST; include the following fields:
3374   <list style="symbols">
3375     <t>Name</t>
3376     <t>Description</t>
3377     <t>Pointer to specification text</t>
3378   </list>
3381   Names of transfer codings &MUST-NOT; overlap with names of content codings
3382   (&content-codings;) unless the encoding transformation is identical, as
3383   is the case for the compression codings defined in
3384   <xref target="compression.codings"/>.
3387   Values to be added to this name space require IETF Review (see
3388   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3389   conform to the purpose of transfer coding defined in this section.
3390   Use of program names for the identification of encoding formats
3391   is not desirable and is discouraged for future encodings.
3394   The registry itself is maintained at
3395   <eref target=""/>.
3399<section title="Transfer Coding Registration" anchor="transfer.coding.registration">
3401   The HTTP Transfer Coding Registry shall be updated with the registrations
3402   below:
3404<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3405   <ttcol>Name</ttcol>
3406   <ttcol>Description</ttcol>
3407   <ttcol>Reference</ttcol>
3408   <c>chunked</c>
3409   <c>Transfer in a series of chunks</c>
3410   <c>
3411      <xref target="chunked.encoding"/>
3412   </c>
3413   <c>compress</c>
3414   <c>UNIX "compress" program method</c>
3415   <c>
3416      <xref target="compress.coding"/>
3417   </c>
3418   <c>deflate</c>
3419   <c>"deflate" compression mechanism (<xref target="RFC1951"/>) used inside
3420   the "zlib" data format (<xref target="RFC1950"/>)
3421   </c>
3422   <c>
3423      <xref target="deflate.coding"/>
3424   </c>
3425   <c>gzip</c>
3426   <c>Same as GNU zip <xref target="RFC1952"/></c>
3427   <c>
3428      <xref target="gzip.coding"/>
3429   </c>
3433<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3435   The HTTP Upgrade Token Registry defines the name space for protocol-name
3436   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3437   field. Each registered protocol name is associated with contact information
3438   and an optional set of specifications that details how the connection
3439   will be processed after it has been upgraded.
3442   Registrations happen on a "First Come First Served" basis (see
3443   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3444   following rules:
3445  <list style="numbers">
3446    <t>A protocol-name token, once registered, stays registered forever.</t>
3447    <t>The registration &MUST; name a responsible party for the
3448       registration.</t>
3449    <t>The registration &MUST; name a point of contact.</t>
3450    <t>The registration &MAY; name a set of specifications associated with
3451       that token. Such specifications need not be publicly available.</t>
3452    <t>The registration &SHOULD; name a set of expected "protocol-version"
3453       tokens associated with that token at the time of registration.</t>
3454    <t>The responsible party &MAY; change the registration at any time.
3455       The IANA will keep a record of all such changes, and make them
3456       available upon request.</t>
3457    <t>The IESG &MAY; reassign responsibility for a protocol token.
3458       This will normally only be used in the case when a
3459       responsible party cannot be contacted.</t>
3460  </list>
3463   This registration procedure for HTTP Upgrade Tokens replaces that
3464   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3468<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3470   The HTTP Upgrade Token Registry shall be updated with the registration
3471   below:
3473<texttable align="left" suppress-title="true">
3474   <ttcol>Value</ttcol>
3475   <ttcol>Description</ttcol>
3476   <ttcol>Expected Version Tokens</ttcol>
3477   <ttcol>Reference</ttcol>
3479   <c>HTTP</c>
3480   <c>Hypertext Transfer Protocol</c>
3481   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3482   <c><xref target="http.version"/></c>
3485   The responsible party is: "IETF ( - Internet Engineering Task Force".
3491<section title="Security Considerations" anchor="security.considerations">
3493   This section is meant to inform developers, information providers, and
3494   users of known security concerns relevant to HTTP/1.1 message syntax,
3495   parsing, and routing.
3498<section title="DNS-related Attacks" anchor="dns.related.attacks">
3500   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3501   generally prone to security attacks based on the deliberate misassociation
3502   of IP addresses and DNS names not protected by DNSSEC. Clients need to be
3503   cautious in assuming the validity of an IP number/DNS name association unless
3504   the response is protected by DNSSEC (<xref target="RFC4033"/>).
3508<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3510   By their very nature, HTTP intermediaries are men-in-the-middle, and
3511   represent an opportunity for man-in-the-middle attacks. Compromise of
3512   the systems on which the intermediaries run can result in serious security
3513   and privacy problems. Intermediaries have access to security-related
3514   information, personal information about individual users and
3515   organizations, and proprietary information belonging to users and
3516   content providers. A compromised intermediary, or an intermediary
3517   implemented or configured without regard to security and privacy
3518   considerations, might be used in the commission of a wide range of
3519   potential attacks.
3522   Intermediaries that contain a shared cache are especially vulnerable
3523   to cache poisoning attacks.
3526   Implementers need to consider the privacy and security
3527   implications of their design and coding decisions, and of the
3528   configuration options they provide to operators (especially the
3529   default configuration).
3532   Users need to be aware that intermediaries are no more trustworthy than
3533   the people who run them; HTTP itself cannot solve this problem.
3537<section title="Buffer Overflows" anchor="attack.protocol.element.size.overflows">
3539   Because HTTP uses mostly textual, character-delimited fields, attackers can
3540   overflow buffers in implementations, and/or perform a Denial of Service
3541   against implementations that accept fields with unlimited lengths.
3544   To promote interoperability, this specification makes specific
3545   recommendations for minimum size limits on request-line
3546   (<xref target="request.line"/>)
3547   and blocks of header fields (<xref target="header.fields"/>). These are
3548   minimum recommendations, chosen to be supportable even by implementations
3549   with limited resources; it is expected that most implementations will
3550   choose substantially higher limits.
3553   This specification also provides a way for servers to reject messages that
3554   have request-targets that are too long (&status-414;) or request entities
3555   that are too large (&status-4xx;).
3558   Recipients &SHOULD; carefully limit the extent to which they read other
3559   fields, including (but not limited to) request methods, response status
3560   phrases, header field-names, and body chunks, so as to avoid denial of
3561   service attacks without impeding interoperability.
3565<section title="Message Integrity" anchor="message.integrity">
3567   HTTP does not define a specific mechanism for ensuring message integrity,
3568   instead relying on the error-detection ability of underlying transport
3569   protocols and the use of length or chunk-delimited framing to detect
3570   completeness. Additional integrity mechanisms, such as hash functions or
3571   digital signatures applied to the content, can be selectively added to
3572   messages via extensible metadata header fields. Historically, the lack of
3573   a single integrity mechanism has been justified by the informal nature of
3574   most HTTP communication.  However, the prevalence of HTTP as an information
3575   access mechanism has resulted in its increasing use within environments
3576   where verification of message integrity is crucial.
3579   User agents are encouraged to implement configurable means for detecting
3580   and reporting failures of message integrity such that those means can be
3581   enabled within environments for which integrity is necessary. For example,
3582   a browser being used to view medical history or drug interaction
3583   information needs to indicate to the user when such information is detected
3584   by the protocol to be incomplete, expired, or corrupted during transfer.
3585   Such mechanisms might be selectively enabled via user agent extensions or
3586   the presence of message integrity metadata in a response.
3587   At a minimum, user agents ought to provide some indication that allows a
3588   user to distinguish between a complete and incomplete response message
3589   (<xref target="incomplete.messages"/>) when such verification is desired.
3593<section title="Server Log Information" anchor="abuse.of.server.log.information">
3595   A server is in the position to save personal data about a user's requests
3596   over time, which might identify their reading patterns or subjects of
3597   interest.  In particular, log information gathered at an intermediary
3598   often contains a history of user agent interaction, across a multitude
3599   of sites, that can be traced to individual users.
3602   HTTP log information is confidential in nature; its handling is often
3603   constrained by laws and regulations.  Log information needs to be securely
3604   stored and appropriate guidelines followed for its analysis.
3605   Anonymization of personal information within individual entries helps,
3606   but is generally not sufficient to prevent real log traces from being
3607   re-identified based on correlation with other access characteristics.
3608   As such, access traces that are keyed to a specific client should not
3609   be published even if the key is pseudonymous.
3612   To minimize the risk of theft or accidental publication, log information
3613   should be purged of personally identifiable information, including
3614   user identifiers, IP addresses, and user-provided query parameters,
3615   as soon as that information is no longer necessary to support operational
3616   needs for security, auditing, or fraud control.
3621<section title="Acknowledgments" anchor="acks">
3623   This edition of HTTP/1.1 builds on the many contributions that went into
3624   <xref target="RFC1945" format="none">RFC 1945</xref>,
3625   <xref target="RFC2068" format="none">RFC 2068</xref>,
3626   <xref target="RFC2145" format="none">RFC 2145</xref>, and
3627   <xref target="RFC2616" format="none">RFC 2616</xref>, including
3628   substantial contributions made by the previous authors, editors, and
3629   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
3630   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
3631   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
3634   Since 1999, the following contributors have helped improve the HTTP
3635   specification by reporting bugs, asking smart questions, drafting or
3636   reviewing text, and evaluating open issues:
3638<?BEGININC acks ?>
3639<t>Adam Barth,
3640Adam Roach,
3641Addison Phillips,
3642Adrian Chadd,
3643Adrien W. de Croy,
3644Alan Ford,
3645Alan Ruttenberg,
3646Albert Lunde,
3647Alek Storm,
3648Alex Rousskov,
3649Alexandre Morgaut,
3650Alexey Melnikov,
3651Alisha Smith,
3652Amichai Rothman,
3653Amit Klein,
3654Amos Jeffries,
3655Andreas Maier,
3656Andreas Petersson,
3657Anil Sharma,
3658Anne van Kesteren,
3659Anthony Bryan,
3660Asbjorn Ulsberg,
3661Ashok Kumar,
3662Balachander Krishnamurthy,
3663Barry Leiba,
3664Ben Laurie,
3665Benjamin Niven-Jenkins,
3666Bil Corry,
3667Bill Burke,
3668Bjoern Hoehrmann,
3669Bob Scheifler,
3670Boris Zbarsky,
3671Brett Slatkin,
3672Brian Kell,
3673Brian McBarron,
3674Brian Pane,
3675Brian Smith,
3676Bryce Nesbitt,
3677Cameron Heavon-Jones,
3678Carl Kugler,
3679Carsten Bormann,
3680Charles Fry,
3681Chris Newman,
3682Chris Weber,
3683Cyrus Daboo,
3684Dale Robert Anderson,
3685Dan Wing,
3686Dan Winship,
3687Daniel Stenberg,
3688Darrel Miller,
3689Dave Cridland,
3690Dave Crocker,
3691Dave Kristol,
3692David Booth,
3693David Singer,
3694David W. Morris,
3695Diwakar Shetty,
3696Dmitry Kurochkin,
3697Drummond Reed,
3698Duane Wessels,
3699Duncan Cragg,
3700Edward Lee,
3701Eliot Lear,
3702Eran Hammer-Lahav,
3703Eric D. Williams,
3704Eric J. Bowman,
3705Eric Lawrence,
3706Eric Rescorla,
3707Erik Aronesty,
3708Evan Prodromou,
3709Florian Weimer,
3710Frank Ellermann,
3711Fred Bohle,
3712Gabriel Montenegro,
3713Geoffrey Sneddon,
3714Gervase Markham,
3715Grahame Grieve,
3716Greg Wilkins,
3717Harald Tveit Alvestrand,
3718Harry Halpin,
3719Helge Hess,
3720Henrik Nordstrom,
3721Henry S. Thompson,
3722Henry Story,
3723Herbert van de Sompel,
3724Howard Melman,
3725Hugo Haas,
3726Ian Fette,
3727Ian Hickson,
3728Ido Safruti,
3729Ilya Grigorik,
3730Ingo Struck,
3731J. Ross Nicoll,
3732James H. Manger,
3733James Lacey,
3734James M. Snell,
3735Jamie Lokier,
3736Jan Algermissen,
3737Jeff Hodges (who came up with the term 'effective Request-URI'),
3738Jeff Walden,
3739Jeroen de Borst,
3740Jim Luther,
3741Joe D. Williams,
3742Joe Gregorio,
3743Joe Orton,
3744John C. Klensin,
3745John C. Mallery,
3746John Cowan,
3747John Kemp,
3748John Panzer,
3749John Schneider,
3750John Stracke,
3751John Sullivan,
3752Jonas Sicking,
3753Jonathan A. Rees,
3754Jonathan Billington,
3755Jonathan Moore,
3756Jonathan Rees,
3757Jonathan Silvera,
3758Jordi Ros,
3759Joris Dobbelsteen,
3760Josh Cohen,
3761Julien Pierre,
3762Jungshik Shin,
3763Justin Chapweske,
3764Justin Erenkrantz,
3765Justin James,
3766Kalvinder Singh,
3767Karl Dubost,
3768Keith Hoffman,
3769Keith Moore,
3770Ken Murchison,
3771Koen Holtman,
3772Konstantin Voronkov,
3773Kris Zyp,
3774Lisa Dusseault,
3775Maciej Stachowiak,
3776Marc Schneider,
3777Marc Slemko,
3778Mark Baker,
3779Mark Pauley,
3780Mark Watson,
3781Markus Isomaki,
3782Markus Lanthaler,
3783Martin J. Duerst,
3784Martin Musatov,
3785Martin Nilsson,
3786Martin Thomson,
3787Matt Lynch,
3788Matthew Cox,
3789Max Clark,
3790Michael Burrows,
3791Michael Hausenblas,
3792Mike Amundsen,
3793Mike Belshe,
3794Mike Kelly,
3795Mike Schinkel,
3796Miles Sabin,
3797Murray S. Kucherawy,
3798Mykyta Yevstifeyev,
3799Nathan Rixham,
3800Nicholas Shanks,
3801Nico Williams,
3802Nicolas Alvarez,
3803Nicolas Mailhot,
3804Noah Slater,
3805Pablo Castro,
3806Pat Hayes,
3807Patrick R. McManus,
3808Patrik Faltstrom,
3809Paul E. Jones,
3810Paul Hoffman,
3811Paul Marquess,
3812Peter Lepeska,
3813Peter Saint-Andre,
3814Peter Watkins,
3815Phil Archer,
3816Philippe Mougin,
3817Phillip Hallam-Baker,
3818Poul-Henning Kamp,
3819Preethi Natarajan,
3820Rajeev Bector,
3821Ray Polk,
3822Reto Bachmann-Gmuer,
3823Richard Cyganiak,
3824Robert Brewer,
3825Robert Collins,
3826Robert O'Callahan,
3827Robert Olofsson,
3828Robert Sayre,
3829Robert Siemer,
3830Robert de Wilde,
3831Roberto Javier Godoy,
3832Roberto Peon,
3833Roland Zink,
3834Ronny Widjaja,
3835S. Mike Dierken,
3836Salvatore Loreto,
3837Sam Johnston,
3838Sam Ruby,
3839Scott Lawrence (who maintained the original issues list),
3840Sean B. Palmer,
3841Shane McCarron,
3842Stefan Eissing,
3843Stefan Tilkov,
3844Stefanos Harhalakis,
3845Stephane Bortzmeyer,
3846Stephen Farrell,
3847Stephen Ludin,
3848Stuart Williams,
3849Subbu Allamaraju,
3850Subramanian Moonesamy,
3851Sylvain Hellegouarch,
3852Tapan Divekar,
3853Tatsuya Hayashi,
3854Ted Hardie,
3855Thomas Broyer,
3856Thomas Fossati,
3857Thomas Nordin,
3858Thomas Roessler,
3859Tim Bray,
3860Tim Morgan,
3861Tim Olsen,
3862Tobias Oberstein,
3863Tom Zhou,
3864Travis Snoozy,
3865Tyler Close,
3866Vincent Murphy,
3867Wenbo Zhu,
3868Werner Baumann,
3869Wilbur Streett,
3870Wilfredo Sanchez Vega,
3871William A. Rowe Jr.,
3872William Chan,
3873Willy Tarreau,
3874Xiaoshu Wang,
3875Yaron Goland,
3876Yngve Nysaeter Pettersen,
3877Yoav Nir,
3878Yogesh Bang,
3879Yutaka Oiwa,
3880Yves Lafon (long-time member of the editor team),
3881Zed A. Shaw, and
3882Zhong Yu.
3884<?ENDINC acks ?>
3886   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
3887   acknowledgements from prior revisions.
3894<references title="Normative References">
3896<reference anchor="Part2">
3897  <front>
3898    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
3899    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3900      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
3901      <address><email></email></address>
3902    </author>
3903    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3904      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3905      <address><email></email></address>
3906    </author>
3907    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3908  </front>
3909  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
3910  <x:source href="p2-semantics.xml" basename="p2-semantics">
3911    <x:defines>1xx (Informational)</x:defines>
3912    <x:defines>1xx</x:defines>
3913    <x:defines>100 (Continue)</x:defines>
3914    <x:defines>101 (Switching Protocols)</x:defines>
3915    <x:defines>2xx (Successful)</x:defines>
3916    <x:defines>2xx</x:defines>
3917    <x:defines>200 (OK)</x:defines>
3918    <x:defines>204 (No Content)</x:defines>
3919    <x:defines>3xx (Redirection)</x:defines>
3920    <x:defines>3xx</x:defines>
3921    <x:defines>301 (Moved Permanently)</x:defines>
3922    <x:defines>4xx (Client Error)</x:defines>
3923    <x:defines>4xx</x:defines>
3924    <x:defines>400 (Bad Request)</x:defines>
3925    <x:defines>405 (Method Not Allowed)</x:defines>
3926    <x:defines>411 (Length Required)</x:defines>
3927    <x:defines>414 (URI Too Long)</x:defines>
3928    <x:defines>417 (Expectation Failed)</x:defines>
3929    <x:defines>426 (Upgrade Required)</x:defines>
3930    <x:defines>501 (Not Implemented)</x:defines>
3931    <x:defines>502 (Bad Gateway)</x:defines>
3932    <x:defines>505 (HTTP Version Not Supported)</x:defines>
3933    <x:defines>Allow</x:defines>
3934    <x:defines>Content-Encoding</x:defines>
3935    <x:defines>Content-Location</x:defines>
3936    <x:defines>Content-Type</x:defines>
3937    <x:defines>Date</x:defines>
3938    <x:defines>Expect</x:defines>
3939    <x:defines>Location</x:defines>
3940    <x:defines>Server</x:defines>
3941    <x:defines>User-Agent</x:defines>
3942  </x:source>
3945<reference anchor="Part4">
3946  <front>
3947    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
3948    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
3949      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
3950      <address><email></email></address>
3951    </author>
3952    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
3953      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3954      <address><email></email></address>
3955    </author>
3956    <date month="&ID-MONTH;" year="&ID-YEAR;" />
3957  </front>
3958  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
3959  <x:source basename="p4-conditional" href="p4-conditional.xml">
3960    <x:defines>304 (Not Modified)</x:defines>
3961    <x:defines>ETag</x:defines>
3962    <x:defines>Last-Modified</x:defines>
3963  </x:source>
3966<reference anchor="Part5">
3967  <front>
3968    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
3969    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3970      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
3971      <address><email></email></address>
3972    </author>
3973    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3974      <organization abbrev="W3C">World Wide Web Consortium</organization>
3975      <address><email></email></address>
3976    </author>
3977    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3978      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3979      <address><email></email></address>
3980    </author>
3981    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3982  </front>
3983  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
3984  <x:source href="p5-range.xml" basename="p5-range">
3985    <x:defines>Content-Range</x:defines>
3986  </x:source>
3989<reference anchor="Part6">
3990  <front>
3991    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
3992    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3993      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
3994      <address><email></email></address>
3995    </author>
3996    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
3997      <organization>Akamai</organization>
3998      <address><email></email></address>
3999    </author>
4000    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4001      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4002      <address><email></email></address>
4003    </author>
4004    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4005  </front>
4006  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4007  <x:source href="p6-cache.xml" basename="p6-cache">
4008    <x:defines>Cache-Control</x:defines>
4009    <x:defines>Expires</x:defines>
4010  </x:source>
4013<reference anchor="Part7">
4014  <front>
4015    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4016    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4017      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4018      <address><email></email></address>
4019    </author>
4020    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4021      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4022      <address><email></email></address>
4023    </author>
4024    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4025  </front>
4026  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4027  <x:source href="p7-auth.xml" basename="p7-auth">
4028    <x:defines>Proxy-Authenticate</x:defines>
4029    <x:defines>Proxy-Authorization</x:defines>
4030  </x:source>
4033<reference anchor="RFC5234">
4034  <front>
4035    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4036    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4037      <organization>Brandenburg InternetWorking</organization>
4038      <address>
4039        <email></email>
4040      </address> 
4041    </author>
4042    <author initials="P." surname="Overell" fullname="Paul Overell">
4043      <organization>THUS plc.</organization>
4044      <address>
4045        <email></email>
4046      </address>
4047    </author>
4048    <date month="January" year="2008"/>
4049  </front>
4050  <seriesInfo name="STD" value="68"/>
4051  <seriesInfo name="RFC" value="5234"/>
4054<reference anchor="RFC2119">
4055  <front>
4056    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4057    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4058      <organization>Harvard University</organization>
4059      <address><email></email></address>
4060    </author>
4061    <date month="March" year="1997"/>
4062  </front>
4063  <seriesInfo name="BCP" value="14"/>
4064  <seriesInfo name="RFC" value="2119"/>
4067<reference anchor="RFC3986">
4068 <front>
4069  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4070  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4071    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4072    <address>
4073       <email></email>
4074       <uri></uri>
4075    </address>
4076  </author>
4077  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4078    <organization abbrev="Day Software">Day Software</organization>
4079    <address>
4080      <email></email>
4081      <uri></uri>
4082    </address>
4083  </author>
4084  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4085    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4086    <address>
4087      <email></email>
4088      <uri></uri>
4089    </address>
4090  </author>
4091  <date month='January' year='2005'></date>
4092 </front>
4093 <seriesInfo name="STD" value="66"/>
4094 <seriesInfo name="RFC" value="3986"/>
4097<reference anchor="USASCII">
4098  <front>
4099    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4100    <author>
4101      <organization>American National Standards Institute</organization>
4102    </author>
4103    <date year="1986"/>
4104  </front>
4105  <seriesInfo name="ANSI" value="X3.4"/>
4108<reference anchor="RFC1950">
4109  <front>
4110    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4111    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4112      <organization>Aladdin Enterprises</organization>
4113      <address><email></email></address>
4114    </author>
4115    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4116    <date month="May" year="1996"/>
4117  </front>
4118  <seriesInfo name="RFC" value="1950"/>
4119  <!--<annotation>
4120    RFC 1950 is an Informational RFC, thus it might be less stable than
4121    this specification. On the other hand, this downward reference was
4122    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4123    therefore it is unlikely to cause problems in practice. See also
4124    <xref target="BCP97"/>.
4125  </annotation>-->
4128<reference anchor="RFC1951">
4129  <front>
4130    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4131    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4132      <organization>Aladdin Enterprises</organization>
4133      <address><email></email></address>
4134    </author>
4135    <date month="May" year="1996"/>
4136  </front>
4137  <seriesInfo name="RFC" value="1951"/>
4138  <!--<annotation>
4139    RFC 1951 is an Informational RFC, thus it might be less stable than
4140    this specification. On the other hand, this downward reference was
4141    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4142    therefore it is unlikely to cause problems in practice. See also
4143    <xref target="BCP97"/>.
4144  </annotation>-->
4147<reference anchor="RFC1952">
4148  <front>
4149    <title>GZIP file format specification version 4.3</title>
4150    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4151      <organization>Aladdin Enterprises</organization>
4152      <address><email></email></address>
4153    </author>
4154    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4155      <address><email></email></address>
4156    </author>
4157    <author initials="M." surname="Adler" fullname="Mark Adler">
4158      <address><email></email></address>
4159    </author>
4160    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4161      <address><email></email></address>
4162    </author>
4163    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4164      <address><email></email></address>
4165    </author>
4166    <date month="May" year="1996"/>
4167  </front>
4168  <seriesInfo name="RFC" value="1952"/>
4169  <!--<annotation>
4170    RFC 1952 is an Informational RFC, thus it might be less stable than
4171    this specification. On the other hand, this downward reference was
4172    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4173    therefore it is unlikely to cause problems in practice. See also
4174    <xref target="BCP97"/>.
4175  </annotation>-->
4180<references title="Informative References">
4182<reference anchor="ISO-8859-1">
4183  <front>
4184    <title>
4185     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4186    </title>
4187    <author>
4188      <organization>International Organization for Standardization</organization>
4189    </author>
4190    <date year="1998"/>
4191  </front>
4192  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4195<reference anchor='RFC1919'>
4196  <front>
4197    <title>Classical versus Transparent IP Proxies</title>
4198    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4199      <address><email></email></address>
4200    </author>
4201    <date year='1996' month='March' />
4202  </front>
4203  <seriesInfo name='RFC' value='1919' />
4206<reference anchor="RFC1945">
4207  <front>
4208    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4209    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4210      <organization>MIT, Laboratory for Computer Science</organization>
4211      <address><email></email></address>
4212    </author>
4213    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4214      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4215      <address><email></email></address>
4216    </author>
4217    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4218      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4219      <address><email></email></address>
4220    </author>
4221    <date month="May" year="1996"/>
4222  </front>
4223  <seriesInfo name="RFC" value="1945"/>
4226<reference anchor="RFC2045">
4227  <front>
4228    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4229    <author initials="N." surname="Freed" fullname="Ned Freed">
4230      <organization>Innosoft International, Inc.</organization>
4231      <address><email></email></address>
4232    </author>
4233    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4234      <organization>First Virtual Holdings</organization>
4235      <address><email></email></address>
4236    </author>
4237    <date month="November" year="1996"/>
4238  </front>
4239  <seriesInfo name="RFC" value="2045"/>
4242<reference anchor="RFC2047">
4243  <front>
4244    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4245    <author initials="K." surname="Moore" fullname="Keith Moore">
4246      <organization>University of Tennessee</organization>
4247      <address><email></email></address>
4248    </author>
4249    <date month="November" year="1996"/>
4250  </front>
4251  <seriesInfo name="RFC" value="2047"/>
4254<reference anchor="RFC2068">
4255  <front>
4256    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4257    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
4258      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4259      <address><email></email></address>
4260    </author>
4261    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4262      <organization>MIT Laboratory for Computer Science</organization>
4263      <address><email></email></address>
4264    </author>
4265    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4266      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4267      <address><email></email></address>
4268    </author>
4269    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4270      <organization>MIT Laboratory for Computer Science</organization>
4271      <address><email></email></address>
4272    </author>
4273    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4274      <organization>MIT Laboratory for Computer Science</organization>
4275      <address><email></email></address>
4276    </author>
4277    <date month="January" year="1997"/>
4278  </front>
4279  <seriesInfo name="RFC" value="2068"/>
4282<reference anchor="RFC2145">
4283  <front>
4284    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4285    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4286      <organization>Western Research Laboratory</organization>
4287      <address><email></email></address>
4288    </author>
4289    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4290      <organization>Department of Information and Computer Science</organization>
4291      <address><email></email></address>
4292    </author>
4293    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4294      <organization>MIT Laboratory for Computer Science</organization>
4295      <address><email></email></address>
4296    </author>
4297    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4298      <organization>W3 Consortium</organization>
4299      <address><email></email></address>
4300    </author>
4301    <date month="May" year="1997"/>
4302  </front>
4303  <seriesInfo name="RFC" value="2145"/>
4306<reference anchor="RFC2616">
4307  <front>
4308    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4309    <author initials="R." surname="Fielding" fullname="R. Fielding">
4310      <organization>University of California, Irvine</organization>
4311      <address><email></email></address>
4312    </author>
4313    <author initials="J." surname="Gettys" fullname="J. Gettys">
4314      <organization>W3C</organization>
4315      <address><email></email></address>
4316    </author>
4317    <author initials="J." surname="Mogul" fullname="J. Mogul">
4318      <organization>Compaq Computer Corporation</organization>
4319      <address><email></email></address>
4320    </author>
4321    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4322      <organization>MIT Laboratory for Computer Science</organization>
4323      <address><email></email></address>
4324    </author>
4325    <author initials="L." surname="Masinter" fullname="L. Masinter">
4326      <organization>Xerox Corporation</organization>
4327      <address><email></email></address>
4328    </author>
4329    <author initials="P." surname="Leach" fullname="P. Leach">
4330      <organization>Microsoft Corporation</organization>
4331      <address><email></email></address>
4332    </author>
4333    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4334      <organization>W3C</organization>
4335      <address><email></email></address>
4336    </author>
4337    <date month="June" year="1999"/>
4338  </front>
4339  <seriesInfo name="RFC" value="2616"/>
4342<reference anchor='RFC2817'>
4343  <front>
4344    <title>Upgrading to TLS Within HTTP/1.1</title>
4345    <author initials='R.' surname='Khare' fullname='R. Khare'>
4346      <organization>4K Associates / UC Irvine</organization>
4347      <address><email></email></address>
4348    </author>
4349    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4350      <organization>Agranat Systems, Inc.</organization>
4351      <address><email></email></address>
4352    </author>
4353    <date year='2000' month='May' />
4354  </front>
4355  <seriesInfo name='RFC' value='2817' />
4358<reference anchor='RFC2818'>
4359  <front>
4360    <title>HTTP Over TLS</title>
4361    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4362      <organization>RTFM, Inc.</organization>
4363      <address><email></email></address>
4364    </author>
4365    <date year='2000' month='May' />
4366  </front>
4367  <seriesInfo name='RFC' value='2818' />
4370<reference anchor='RFC3040'>
4371  <front>
4372    <title>Internet Web Replication and Caching Taxonomy</title>
4373    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4374      <organization>Equinix, Inc.</organization>
4375    </author>
4376    <author initials='I.' surname='Melve' fullname='I. Melve'>
4377      <organization>UNINETT</organization>
4378    </author>
4379    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4380      <organization>CacheFlow Inc.</organization>
4381    </author>
4382    <date year='2001' month='January' />
4383  </front>
4384  <seriesInfo name='RFC' value='3040' />
4387<reference anchor='BCP90'>
4388  <front>
4389    <title>Registration Procedures for Message Header Fields</title>
4390    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4391      <organization>Nine by Nine</organization>
4392      <address><email></email></address>
4393    </author>
4394    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4395      <organization>BEA Systems</organization>
4396      <address><email></email></address>
4397    </author>
4398    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4399      <organization>HP Labs</organization>
4400      <address><email></email></address>
4401    </author>
4402    <date year='2004' month='September' />
4403  </front>
4404  <seriesInfo name='BCP' value='90' />
4405  <seriesInfo name='RFC' value='3864' />
4408<reference anchor='RFC4033'>
4409  <front>
4410    <title>DNS Security Introduction and Requirements</title>
4411    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4412    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4413    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4414    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4415    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4416    <date year='2005' month='March' />
4417  </front>
4418  <seriesInfo name='RFC' value='4033' />
4421<reference anchor="BCP13">
4422  <front>
4423    <title>Media Type Specifications and Registration Procedures</title>
4424    <author initials="N." surname="Freed" fullname="N. Freed">
4425      <organization>Sun Microsystems</organization>
4426      <address>
4427        <email></email>
4428      </address>
4429    </author>
4430    <author initials="J." surname="Klensin" fullname="J. Klensin">
4431      <address>
4432        <email></email>
4433      </address>
4434    </author>
4435    <date year="2005" month="December"/>
4436  </front>
4437  <seriesInfo name="BCP" value="13"/>
4438  <seriesInfo name="RFC" value="4288"/>
4441<reference anchor='BCP115'>
4442  <front>
4443    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4444    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4445      <organization>AT&amp;T Laboratories</organization>
4446      <address>
4447        <email></email>
4448      </address>
4449    </author>
4450    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4451      <organization>Qualcomm, Inc.</organization>
4452      <address>
4453        <email></email>
4454      </address>
4455    </author>
4456    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4457      <organization>Adobe Systems</organization>
4458      <address>
4459        <email></email>
4460      </address>
4461    </author>
4462    <date year='2006' month='February' />
4463  </front>
4464  <seriesInfo name='BCP' value='115' />
4465  <seriesInfo name='RFC' value='4395' />
4468<reference anchor='RFC4559'>
4469  <front>
4470    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4471    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4472    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4473    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4474    <date year='2006' month='June' />
4475  </front>
4476  <seriesInfo name='RFC' value='4559' />
4479<reference anchor='RFC5226'>
4480  <front>
4481    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4482    <author initials='T.' surname='Narten' fullname='T. Narten'>
4483      <organization>IBM</organization>
4484      <address><email></email></address>
4485    </author>
4486    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4487      <organization>Google</organization>
4488      <address><email></email></address>
4489    </author>
4490    <date year='2008' month='May' />
4491  </front>
4492  <seriesInfo name='BCP' value='26' />
4493  <seriesInfo name='RFC' value='5226' />
4496<reference anchor='RFC5246'>
4497   <front>
4498      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4499      <author initials='T.' surname='Dierks' fullname='T. Dierks'>
4500         <organization />
4501      </author>
4502      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4503         <organization>RTFM, Inc.</organization>
4504      </author>
4505      <date year='2008' month='August' />
4506   </front>
4507   <seriesInfo name='RFC' value='5246' />
4510<reference anchor="RFC5322">
4511  <front>
4512    <title>Internet Message Format</title>
4513    <author initials="P." surname="Resnick" fullname="P. Resnick">
4514      <organization>Qualcomm Incorporated</organization>
4515    </author>
4516    <date year="2008" month="October"/>
4517  </front>
4518  <seriesInfo name="RFC" value="5322"/>
4521<reference anchor="RFC6265">
4522  <front>
4523    <title>HTTP State Management Mechanism</title>
4524    <author initials="A." surname="Barth" fullname="Adam Barth">
4525      <organization abbrev="U.C. Berkeley">
4526        University of California, Berkeley
4527      </organization>
4528      <address><email></email></address>
4529    </author>
4530    <date year="2011" month="April" />
4531  </front>
4532  <seriesInfo name="RFC" value="6265"/>
4535<!--<reference anchor='BCP97'>
4536  <front>
4537    <title>Handling Normative References to Standards-Track Documents</title>
4538    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
4539      <address>
4540        <email></email>
4541      </address>
4542    </author>
4543    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
4544      <organization>MIT</organization>
4545      <address>
4546        <email></email>
4547      </address>
4548    </author>
4549    <date year='2007' month='June' />
4550  </front>
4551  <seriesInfo name='BCP' value='97' />
4552  <seriesInfo name='RFC' value='4897' />
4555<reference anchor="Kri2001" target="">
4556  <front>
4557    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
4558    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
4559    <date year="2001" month="November"/>
4560  </front>
4561  <seriesInfo name="ACM Transactions on Internet Technology" value="Vol. 1, #2"/>
4567<section title="HTTP Version History" anchor="compatibility">
4569   HTTP has been in use by the World-Wide Web global information initiative
4570   since 1990. The first version of HTTP, later referred to as HTTP/0.9,
4571   was a simple protocol for hypertext data transfer across the Internet
4572   with only a single request method (GET) and no metadata.
4573   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
4574   methods and MIME-like messaging that could include metadata about the data
4575   transferred and modifiers on the request/response semantics. However,
4576   HTTP/1.0 did not sufficiently take into consideration the effects of
4577   hierarchical proxies, caching, the need for persistent connections, or
4578   name-based virtual hosts. The proliferation of incompletely-implemented
4579   applications calling themselves "HTTP/1.0" further necessitated a
4580   protocol version change in order for two communicating applications
4581   to determine each other's true capabilities.
4584   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
4585   requirements that enable reliable implementations, adding only
4586   those new features that will either be safely ignored by an HTTP/1.0
4587   recipient or only sent when communicating with a party advertising
4588   conformance with HTTP/1.1.
4591   It is beyond the scope of a protocol specification to mandate
4592   conformance with previous versions. HTTP/1.1 was deliberately
4593   designed, however, to make supporting previous versions easy.
4594   We would expect a general-purpose HTTP/1.1 server to understand
4595   any valid request in the format of HTTP/1.0 and respond appropriately
4596   with an HTTP/1.1 message that only uses features understood (or
4597   safely ignored) by HTTP/1.0 clients.  Likewise, we would expect
4598   an HTTP/1.1 client to understand any valid HTTP/1.0 response.
4601   Since HTTP/0.9 did not support header fields in a request,
4602   there is no mechanism for it to support name-based virtual
4603   hosts (selection of resource by inspection of the <x:ref>Host</x:ref> header
4604   field).  Any server that implements name-based virtual hosts
4605   ought to disable support for HTTP/0.9.  Most requests that
4606   appear to be HTTP/0.9 are, in fact, badly constructed HTTP/1.x
4607   requests wherein a buggy client failed to properly encode
4608   linear whitespace found in a URI reference and placed in
4609   the request-target.
4612<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
4614   This section summarizes major differences between versions HTTP/1.0
4615   and HTTP/1.1.
4618<section title="Multi-homed Web Servers" anchor="">
4620   The requirements that clients and servers support the <x:ref>Host</x:ref>
4621   header field (<xref target=""/>), report an error if it is
4622   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
4623   are among the most important changes defined by HTTP/1.1.
4626   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
4627   addresses and servers; there was no other established mechanism for
4628   distinguishing the intended server of a request than the IP address
4629   to which that request was directed. The <x:ref>Host</x:ref> header field was
4630   introduced during the development of HTTP/1.1 and, though it was
4631   quickly implemented by most HTTP/1.0 browsers, additional requirements
4632   were placed on all HTTP/1.1 requests in order to ensure complete
4633   adoption.  At the time of this writing, most HTTP-based services
4634   are dependent upon the Host header field for targeting requests.
4638<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
4640   In HTTP/1.0, each connection is established by the client prior to the
4641   request and closed by the server after sending the response. However, some
4642   implementations implement the explicitly negotiated ("Keep-Alive") version
4643   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
4644   target="RFC2068"/>.
4647   Some clients and servers might wish to be compatible with these previous
4648   approaches to persistent connections, by explicitly negotiating for them
4649   with a "Connection: keep-alive" request header field. However, some
4650   experimental implementations of HTTP/1.0 persistent connections are faulty;
4651   for example, if an HTTP/1.0 proxy server doesn't understand
4652   <x:ref>Connection</x:ref>, it will erroneously forward that header field
4653   to the next inbound server, which would result in a hung connection.
4656   One attempted solution was the introduction of a Proxy-Connection header
4657   field, targeted specifically at proxies. In practice, this was also
4658   unworkable, because proxies are often deployed in multiple layers, bringing
4659   about the same problem discussed above.
4662   As a result, clients are encouraged not to send the Proxy-Connection header
4663   field in any requests.
4666   Clients are also encouraged to consider the use of Connection: keep-alive
4667   in requests carefully; while they can enable persistent connections with
4668   HTTP/1.0 servers, clients using them need will need to monitor the
4669   connection for "hung" requests (which indicate that the client ought stop
4670   sending the header field), and this mechanism ought not be used by clients
4671   at all when a proxy is being used.
4675<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
4677   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
4678   (<xref target="header.transfer-encoding"/>).
4679   Transfer codings need to be decoded prior to forwarding an HTTP message
4680   over a MIME-compliant protocol.
4686<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4688  HTTP's approach to error handling has been explained.
4689  (<xref target="conformance"/>)
4692  The expectation to support HTTP/0.9 requests has been removed.
4695  The term "Effective Request URI" has been introduced.
4696  (<xref target="effective.request.uri" />)
4699  HTTP messages can be (and often are) buffered by implementations; despite
4700  it sometimes being available as a stream, HTTP is fundamentally a
4701  message-oriented protocol.
4702  (<xref target="http.message" />)
4705  Minimum supported sizes for various protocol elements have been
4706  suggested, to improve interoperability.
4709  Header fields that span multiple lines ("line folding") are deprecated.
4710  (<xref target="field.parsing" />)
4713  The HTTP-version ABNF production has been clarified to be case-sensitive.
4714  Additionally, version numbers has been restricted to single digits, due
4715  to the fact that implementations are known to handle multi-digit version
4716  numbers incorrectly.
4717  (<xref target="http.version"/>)
4720  The HTTPS URI scheme is now defined by this specification; previously,
4721  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
4722  (<xref target="https.uri"/>)
4725  The HTTPS URI scheme implies end-to-end security.
4726  (<xref target="https.uri"/>)
4729  Userinfo (i.e., username and password) are now disallowed in HTTP and
4730  HTTPS URIs, because of security issues related to their transmission on the
4731  wire.
4732  (<xref target="http.uri" />)
4735  Invalid whitespace around field-names is now required to be rejected,
4736  because accepting it represents a security vulnerability.
4737  (<xref target="header.fields"/>)
4740  The ABNF productions defining header fields now only list the field value.
4741  (<xref target="header.fields"/>)
4744  Rules about implicit linear whitespace between certain grammar productions
4745  have been removed; now whitespace is only allowed where specifically
4746  defined in the ABNF.
4747  (<xref target="whitespace"/>)
4750  The NUL octet is no longer allowed in comment and quoted-string text, and
4751  handling of backslash-escaping in them has been clarified.
4752  (<xref target="field.components"/>)
4755  The quoted-pair rule no longer allows escaping control characters other than
4756  HTAB.
4757  (<xref target="field.components"/>)
4760  Non-ASCII content in header fields and the reason phrase has been obsoleted
4761  and made opaque (the TEXT rule was removed).
4762  (<xref target="field.components"/>)
4765  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
4766  handled as errors by recipients.
4767  (<xref target="header.content-length"/>)
4770  The "identity" transfer coding token has been removed.
4771  (Sections <xref format="counter" target="message.body"/> and
4772  <xref format="counter" target="transfer.codings"/>)
4775  The algorithm for determining the message body length has been clarified
4776  to indicate all of the special cases (e.g., driven by methods or status
4777  codes) that affect it, and that new protocol elements cannot define such
4778  special cases.
4779  (<xref target="message.body.length"/>)
4782  "multipart/byteranges" is no longer a way of determining message body length
4783  detection.
4784  (<xref target="message.body.length"/>)
4787  CONNECT is a new, special case in determining message body length.
4788  (<xref target="message.body.length"/>)
4791  Chunk length does not include the count of the octets in the
4792  chunk header and trailer.
4793  (<xref target="chunked.encoding"/>)
4796  Use of chunk extensions is deprecated, and line folding in them is
4797  disallowed.
4798  (<xref target="chunked.encoding"/>)
4801  The path-absolute + query components of RFC3986 have been used to define the
4802  request-target, instead of abs_path from RFC 1808.
4803  (<xref target="request-target"/>)
4806  The asterisk form of the request-target is only allowed in the OPTIONS
4807  method.
4808  (<xref target="request-target"/>)
4811  Exactly when "close" connection options have to be sent has been clarified.
4812  (<xref target="header.connection"/>)
4815  "hop-by-hop" header fields are required to appear in the Connection header
4816  field; just because they're defined as hop-by-hop in this specification
4817  doesn't exempt them.
4818  (<xref target="header.connection"/>)
4821  The limit of two connections per server has been removed.
4822  (<xref target="persistent.connections"/>)
4825  An idempotent sequence of requests is no longer required to be retried.
4826  (<xref target="persistent.connections"/>)
4829  The requirement to retry requests under certain circumstances when the
4830  server prematurely closes the connection has been removed.
4831  (<xref target="persistent.connections"/>)
4834  Some extraneous requirements about when servers are allowed to close
4835  connections prematurely have been removed.
4836  (<xref target="persistent.connections"/>)
4839  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
4840  responses other than 101 (this was incorporated from <xref
4841  target="RFC2817"/>).
4842  (<xref target="header.upgrade"/>)
4845  Registration of Transfer Codings now requires IETF Review
4846  (<xref target="transfer.coding.registry"/>)
4849  The meaning of the "deflate" content coding has been clarified.
4850  (<xref target="deflate.coding" />)
4853  This specification now defines the Upgrade Token Registry, previously
4854  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
4855  (<xref target="upgrade.token.registry"/>)
4858  Empty list elements in list productions (e.g., a list header containing
4859  ", ,") have been deprecated.
4860  (<xref target="abnf.extension"/>)
4863  Issues with the Keep-Alive and Proxy-Connection headers in requests
4864  are pointed out, with use of the latter being discouraged altogether.
4865  (<xref target="compatibility.with.http.1.0.persistent.connections" />)
4870<section title="ABNF list extension: #rule" anchor="abnf.extension">
4872  A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
4873  improve readability in the definitions of some header field values.
4876  A construct "#" is defined, similar to "*", for defining comma-delimited
4877  lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
4878  at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
4879  comma (",") and optional whitespace (OWS).   
4882  Thus,
4883</preamble><artwork type="example">
4884  1#element =&gt; element *( OWS "," OWS element )
4887  and:
4888</preamble><artwork type="example">
4889  #element =&gt; [ 1#element ]
4892  and for n &gt;= 1 and m &gt; 1:
4893</preamble><artwork type="example">
4894  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
4897  For compatibility with legacy list rules, recipients &SHOULD; accept empty
4898  list elements. In other words, consumers would follow the list productions:
4900<figure><artwork type="example">
4901  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
4903  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
4906  Note that empty elements do not contribute to the count of elements present,
4907  though.
4910  For example, given these ABNF productions:
4912<figure><artwork type="example">
4913  example-list      = 1#example-list-elmt
4914  example-list-elmt = token ; see <xref target="field.components"/>
4917  Then these are valid values for example-list (not including the double
4918  quotes, which are present for delimitation only):
4920<figure><artwork type="example">
4921  "foo,bar"
4922  "foo ,bar,"
4923  "foo , ,bar,charlie   "
4926  But these values would be invalid, as at least one non-empty element is
4927  required:
4929<figure><artwork type="example">
4930  ""
4931  ","
4932  ",   ,"
4935  <xref target="collected.abnf"/> shows the collected ABNF, with the list rules
4936  expanded as explained above.
4940<?BEGININC p1-messaging.abnf-appendix ?>
4941<section xmlns:x="" title="Collected ABNF" anchor="collected.abnf">
4943<artwork type="abnf" name="p1-messaging.parsed-abnf">
4944<x:ref>BWS</x:ref> = OWS
4946<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
4947 connection-option ] )
4948<x:ref>Content-Length</x:ref> = 1*DIGIT
4950<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
4951 ]
4952<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
4953<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
4954<x:ref>Host</x:ref> = uri-host [ ":" port ]
4956<x:ref>OWS</x:ref> = *( SP / HTAB )
4958<x:ref>RWS</x:ref> = 1*( SP / HTAB )
4960<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
4961<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
4962<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
4963 transfer-coding ] )
4965<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
4966<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
4968<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
4969 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
4970 comment ] ) ] )
4972<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
4973<x:ref>absolute-form</x:ref> = absolute-URI
4974<x:ref>asterisk-form</x:ref> = "*"
4975<x:ref>attribute</x:ref> = token
4976<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
4977<x:ref>authority-form</x:ref> = authority
4979<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
4980<x:ref>chunk-data</x:ref> = 1*OCTET
4981<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
4982<x:ref>chunk-ext-name</x:ref> = token
4983<x:ref>chunk-ext-val</x:ref> = token / quoted-str-nf
4984<x:ref>chunk-size</x:ref> = 1*HEXDIG
4985<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
4986<x:ref>comment</x:ref> = "(" *( ctext / quoted-cpair / comment ) ")"
4987<x:ref>connection-option</x:ref> = token
4988<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
4989 / %x2A-5B ; '*'-'['
4990 / %x5D-7E ; ']'-'~'
4991 / obs-text
4993<x:ref>field-content</x:ref> = *( HTAB / SP / VCHAR / obs-text )
4994<x:ref>field-name</x:ref> = token
4995<x:ref>field-value</x:ref> = *( field-content / obs-fold )
4997<x:ref>header-field</x:ref> = field-name ":" OWS field-value BWS
4998<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ]
4999<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ]
5001<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
5003<x:ref>message-body</x:ref> = *OCTET
5004<x:ref>method</x:ref> = token
5006<x:ref>obs-fold</x:ref> = CRLF ( SP / HTAB )
5007<x:ref>obs-text</x:ref> = %x80-FF
5008<x:ref>origin-form</x:ref> = path-absolute [ "?" query ]
5010<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
5011<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
5012<x:ref>path-absolute</x:ref> = &lt;path-absolute, defined in [RFC3986], Section 3.3&gt;
5013<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
5014<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
5015<x:ref>protocol-name</x:ref> = token
5016<x:ref>protocol-version</x:ref> = token
5017<x:ref>pseudonym</x:ref> = token
5019<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5020 / %x5D-7E ; ']'-'~'
5021 / obs-text
5022<x:ref>qdtext-nf</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5023 / %x5D-7E ; ']'-'~'
5024 / obs-text
5025<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5026<x:ref>quoted-cpair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5027<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5028<x:ref>quoted-str-nf</x:ref> = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
5029<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5031<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5032<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5033<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5034<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5035<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5036<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5037<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5038 asterisk-form
5040<x:ref>special</x:ref> = "(" / ")" / "&lt;" / "&gt;" / "@" / "," / ";" / ":" / "\" /
5041 DQUOTE / "/" / "[" / "]" / "?" / "=" / "{" / "}"
5042<x:ref>start-line</x:ref> = request-line / status-line
5043<x:ref>status-code</x:ref> = 3DIGIT
5044<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5046<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5047<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5048<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5049 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5050<x:ref>token</x:ref> = 1*tchar
5051<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5052<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5053 transfer-extension
5054<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5055<x:ref>transfer-parameter</x:ref> = attribute BWS "=" BWS value
5057<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5059<x:ref>value</x:ref> = word
5061<x:ref>word</x:ref> = token / quoted-string
5065<?ENDINC p1-messaging.abnf-appendix ?>
5067<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5069<section title="Since RFC 2616">
5071  Changes up to the first Working Group Last Call draft are summarized
5072  in <eref target=""/>.
5076<section title="Since draft-ietf-httpbis-p1-messaging-21" anchor="changes.since.21">
5078  Closed issues:
5079  <list style="symbols">
5080    <t>
5081      <eref target=""/>:
5082      "Cite HTTPS URI scheme definition" (the spec now includes the HTTPs
5083      scheme definition and thus updates RFC 2818)
5084    </t>
5085    <t>
5086      <eref target=""/>:
5087      "mention of 'proxies' in section about caches"
5088    </t>
5089    <t>
5090      <eref target=""/>:
5091      "use of ABNF terms from RFC 3986"
5092    </t>
5093    <t>
5094      <eref target=""/>:
5095      "editorial improvements to message length definition"
5096    </t>
5097    <t>
5098      <eref target=""/>:
5099      "Connection header field MUST vs SHOULD"
5100    </t>
5101    <t>
5102      <eref target=""/>:
5103      "editorial improvements to persistent connections section"
5104    </t>
5105    <t>
5106      <eref target=""/>:
5107      "URI normalization vs empty path"
5108    </t>
5109    <t>
5110      <eref target=""/>:
5111      "p1 feedback"
5112    </t>
5113    <t>
5114      <eref target=""/>:
5115      "is parsing OBS-FOLD mandatory?"
5116    </t>
5117    <t>
5118      <eref target=""/>:
5119      "HTTPS and Shared Caching"
5120    </t>
5121    <t>
5122      <eref target=""/>:
5123      "Requirements for recipients of ws between start-line and first header field"
5124    </t>
5125    <t>
5126      <eref target=""/>:
5127      "SP and HT when being tolerant"
5128    </t>
5129    <t>
5130      <eref target=""/>:
5131      "Message Parsing Strictness"
5132    </t>
5133    <t>
5134      <eref target=""/>:
5135      "'Render'"
5136    </t>
5137    <t>
5138      <eref target=""/>:
5139      "No-Transform"
5140    </t>
5141    <t>
5142      <eref target=""/>:
5143      "p2 editorial feedback"
5144    </t>
5145    <t>
5146      <eref target=""/>:
5147      "Content-Length SHOULD be sent"
5148    </t>
5149  </list>
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