source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2557

Last change on this file since 2557 was 2557, checked in by fielding@…, 9 years ago

(editorial) define WWW where World Wide Web first appears and remove redundant bits from the abstract and history; see #531 and [2555]

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 236.7 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 "2014">
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-expect          "<xref target='Part2' x:rel='#header.expect' xmlns:x=''/>">
38  <!ENTITY header-expires         "<xref target='Part6' x:rel='#header.expires' xmlns:x=''/>">
39  <!ENTITY header-last-modified   "<xref target='Part4' x:rel='#header.last-modified' xmlns:x=''/>">
40  <!ENTITY header-mime-version    "<xref target='Part2' x:rel='#mime-version' xmlns:x=''/>">
41  <!ENTITY header-pragma          "<xref target='Part6' x:rel='#header.pragma' xmlns:x=''/>">
42  <!ENTITY header-proxy-authenticate  "<xref target='Part7' x:rel='#header.proxy-authenticate' xmlns:x=''/>">
43  <!ENTITY header-proxy-authorization "<xref target='Part7' x:rel='#header.proxy-authorization' xmlns:x=''/>">
44  <!ENTITY header-server          "<xref target='Part2' x:rel='#header.server' xmlns:x=''/>">
45  <!ENTITY header-warning         "<xref target='Part6' x:rel='#header.warning' xmlns:x=''/>">
46  <!ENTITY idempotent-methods     "<xref target='Part2' x:rel='#idempotent.methods' xmlns:x=''/>">
47  <!ENTITY safe-methods           "<xref target='Part2' x:rel='#safe.methods' xmlns:x=''/>">
48  <!ENTITY methods                "<xref target='Part2' x:rel='#methods' xmlns:x=''/>">
49  <!ENTITY OPTIONS                "<xref target='Part2' x:rel='#OPTIONS' xmlns:x=''/>">
50  <!ENTITY qvalue                 "<xref target='Part2' x:rel='#quality.values' xmlns:x=''/>">
51  <!ENTITY resource               "<xref target='Part2' x:rel='#resources' xmlns:x=''/>">
52  <!ENTITY status-codes           "<xref target='Part2' x:rel='' xmlns:x=''/>">
53  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x=''/>">
54  <!ENTITY status-203             "<xref target='Part2' x:rel='#status.203' xmlns:x=''/>">
55  <!ENTITY status-3xx             "<xref target='Part2' x:rel='#status.3xx' xmlns:x=''/>">
56  <!ENTITY status-304             "<xref target='Part4' x:rel='#status.304' xmlns:x=''/>">
57  <!ENTITY status-4xx             "<xref target='Part2' x:rel='#status.4xx' xmlns:x=''/>">
58  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x=''/>">
59  <!ENTITY iana-header-registry   "<xref target='Part2' x:rel='#header.field.registry' xmlns:x=''/>">
61<?rfc toc="yes" ?>
62<?rfc symrefs="yes" ?>
63<?rfc sortrefs="yes" ?>
64<?rfc compact="yes"?>
65<?rfc subcompact="no" ?>
66<?rfc linkmailto="no" ?>
67<?rfc editing="no" ?>
68<?rfc comments="yes"?>
69<?rfc inline="yes"?>
70<?rfc rfcedstyle="yes"?>
71<?rfc-ext allow-markup-in-artwork="yes" ?>
72<?rfc-ext include-references-in-index="yes" ?>
73<rfc obsoletes="2145,2616" updates="2817,2818" category="std" x:maturity-level="proposed"
74     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
75     xmlns:x=''>
76<x:link rel="next" basename="p2-semantics"/>
77<x:feedback template="{docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
80  <title abbrev="HTTP/1.1 Message Syntax and Routing">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
82  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
83    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
84    <address>
85      <postal>
86        <street>345 Park Ave</street>
87        <city>San Jose</city>
88        <region>CA</region>
89        <code>95110</code>
90        <country>USA</country>
91      </postal>
92      <email></email>
93      <uri></uri>
94    </address>
95  </author>
97  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
98    <organization abbrev="greenbytes">greenbytes GmbH</organization>
99    <address>
100      <postal>
101        <street>Hafenweg 16</street>
102        <city>Muenster</city><region>NW</region><code>48155</code>
103        <country>Germany</country>
104      </postal>
105      <email></email>
106      <uri></uri>
107    </address>
108  </author>
110  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
111  <workgroup>HTTPbis Working Group</workgroup>
115   The Hypertext Transfer Protocol (HTTP) is a stateless application-level
116   protocol for distributed, collaborative, hypertext information systems.
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
120   requirements, and describes related 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.25"/>.
138  </t>
142<section title="Introduction" anchor="introduction">
144   The Hypertext Transfer Protocol (HTTP) is a stateless application-level
145   request/response protocol that uses extensible semantics and
146   self-descriptive message payloads for flexible interaction with
147   network-based hypertext information systems. This document is the first in
148   a series of documents 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
160   <xref target="RFC2616" x:fmt="none">RFC 2616</xref> and
161   <xref target="RFC2145" x:fmt="none">RFC 2145</xref> (on HTTP versioning).
162   This specification also updates the use of CONNECT to establish a tunnel,
163   previously defined in <xref target="RFC2817" x:fmt="none">RFC 2817</xref>,
164   and defines the "https" URI scheme that was described informally in
165   <xref target="RFC2818" x:fmt="none">RFC 2818</xref>.
168   HTTP is a generic interface protocol for information systems. It is
169   designed to hide the details of how a service is implemented by presenting
170   a uniform interface to clients that is independent of the types of
171   resources provided. Likewise, servers do not need to be aware of each
172   client's purpose: an HTTP request can be considered in isolation rather
173   than being associated with a specific type of client or a predetermined
174   sequence of application steps. The result is a protocol that can be used
175   effectively in many different contexts and for which implementations can
176   evolve independently over time.
179   HTTP is also designed for use as an intermediation protocol for translating
180   communication to and from non-HTTP information systems.
181   HTTP proxies and gateways can provide access to alternative information
182   services by translating their diverse protocols into a hypertext
183   format that can be viewed and manipulated by clients in the same way
184   as HTTP services.
187   One consequence of this flexibility is that the protocol cannot be
188   defined in terms of what occurs behind the interface. Instead, we
189   are limited to defining the syntax of communication, the intent
190   of received communication, and the expected behavior of recipients.
191   If the communication is considered in isolation, then successful
192   actions ought to be reflected in corresponding changes to the
193   observable interface provided by servers. However, since multiple
194   clients might act in parallel and perhaps at cross-purposes, we
195   cannot require that such changes be observable beyond the scope
196   of a single response.
199   This document describes the architectural elements that are used or
200   referred to in HTTP, defines the "http" and "https" URI schemes,
201   describes overall network operation and connection management,
202   and defines HTTP message framing and forwarding requirements.
203   Our goal is to define all of the mechanisms necessary for HTTP message
204   handling that are independent of message semantics, thereby defining the
205   complete set of requirements for message parsers and
206   message-forwarding intermediaries.
210<section title="Requirement Notation" anchor="intro.requirements">
212   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
213   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
214   document are to be interpreted as described in <xref target="RFC2119"/>.
217   Conformance criteria and considerations regarding error handling
218   are defined in <xref target="conformance"/>.
222<section title="Syntax Notation" anchor="notation">
223<iref primary="true" item="Grammar" subitem="ALPHA"/>
224<iref primary="true" item="Grammar" subitem="CR"/>
225<iref primary="true" item="Grammar" subitem="CRLF"/>
226<iref primary="true" item="Grammar" subitem="CTL"/>
227<iref primary="true" item="Grammar" subitem="DIGIT"/>
228<iref primary="true" item="Grammar" subitem="DQUOTE"/>
229<iref primary="true" item="Grammar" subitem="HEXDIG"/>
230<iref primary="true" item="Grammar" subitem="HTAB"/>
231<iref primary="true" item="Grammar" subitem="LF"/>
232<iref primary="true" item="Grammar" subitem="OCTET"/>
233<iref primary="true" item="Grammar" subitem="SP"/>
234<iref primary="true" item="Grammar" subitem="VCHAR"/>
236   This specification uses the Augmented Backus-Naur Form (ABNF) notation of
237   <xref target="RFC5234"/> with a list extension, defined in
238   <xref target="abnf.extension"/>, that allows for compact definition of
239   comma-separated lists using a '#' operator (similar to how the '*' operator
240   indicates repetition).
241   <xref target="collected.abnf"/> shows the collected grammar with all list
242   operators expanded to standard ABNF notation.
244<t anchor="core.rules">
245  <x:anchor-alias value="ALPHA"/>
246  <x:anchor-alias value="CTL"/>
247  <x:anchor-alias value="CR"/>
248  <x:anchor-alias value="CRLF"/>
249  <x:anchor-alias value="DIGIT"/>
250  <x:anchor-alias value="DQUOTE"/>
251  <x:anchor-alias value="HEXDIG"/>
252  <x:anchor-alias value="HTAB"/>
253  <x:anchor-alias value="LF"/>
254  <x:anchor-alias value="OCTET"/>
255  <x:anchor-alias value="SP"/>
256  <x:anchor-alias value="VCHAR"/>
257   The following core rules are included by
258   reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
259   ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
260   DIGIT (decimal 0-9), DQUOTE (double quote),
261   HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed),
262   OCTET (any 8-bit sequence of data), SP (space), and
263   VCHAR (any visible <xref target="USASCII"/> character).
266   As a convention, ABNF rule names prefixed with "obs-" denote
267   "obsolete" grammar rules that appear for historical reasons.
272<section title="Architecture" anchor="architecture">
274   HTTP was created for the World Wide Web (WWW) architecture
275   and has evolved over time to support the scalability needs of a worldwide
276   hypertext system. Much of that architecture is reflected in the terminology
277   and syntax productions used to define HTTP.
280<section title="Client/Server Messaging" anchor="operation">
281<iref primary="true" item="client"/>
282<iref primary="true" item="server"/>
283<iref primary="true" item="connection"/>
285   HTTP is a stateless request/response protocol that operates by exchanging
286   <x:dfn>messages</x:dfn> (<xref target="http.message"/>) across a reliable
287   transport or session-layer
288   "<x:dfn>connection</x:dfn>" (<xref target=""/>).
289   An HTTP "<x:dfn>client</x:dfn>" is a program that establishes a connection
290   to a server for the purpose of sending one or more HTTP requests.
291   An HTTP "<x:dfn>server</x:dfn>" is a program that accepts connections
292   in order to service HTTP requests by sending HTTP responses.
294<iref primary="true" item="user agent"/>
295<iref primary="true" item="origin server"/>
296<iref primary="true" item="browser"/>
297<iref primary="true" item="spider"/>
298<iref primary="true" item="sender"/>
299<iref primary="true" item="recipient"/>
301   The terms client and server refer only to the roles that
302   these programs perform for a particular connection.  The same program
303   might act as a client on some connections and a server on others.
304   We use the term "<x:dfn>user agent</x:dfn>" to refer to any of the various
305   client programs that initiate a request, including (but not limited to)
306   browsers, spiders (web-based robots), command-line tools, native
307   applications, and mobile apps.  The term "<x:dfn>origin server</x:dfn>" is
308   used to refer to the program that can originate authoritative responses to
309   a request. For general requirements, we use the terms
310   "<x:dfn>sender</x:dfn>" and "<x:dfn>recipient</x:dfn>" to refer to any
311   component that sends or receives, respectively, a given message.
314   HTTP relies upon the Uniform Resource Identifier (URI)
315   standard <xref target="RFC3986"/> to indicate the target resource
316   (<xref target="target-resource"/>) and relationships between resources.
317   Messages are passed in a format similar to that used by Internet mail
318   <xref target="RFC5322"/> and the Multipurpose Internet Mail Extensions
319   (MIME) <xref target="RFC2045"/> (see &diff-mime; for the differences
320   between HTTP and MIME messages).
323   Most HTTP communication consists of a retrieval request (GET) for
324   a representation of some resource identified by a URI.  In the
325   simplest case, this might be accomplished via a single bidirectional
326   connection (===) between the user agent (UA) and the origin server (O).
328<figure><artwork type="drawing">
329         request   &gt;
330    <x:highlight>UA</x:highlight> ======================================= <x:highlight>O</x:highlight>
331                                &lt;   response
333<iref primary="true" item="message"/>
334<iref primary="true" item="request"/>
335<iref primary="true" item="response"/>
337   A client sends an HTTP request to a server in the form of a <x:dfn>request</x:dfn>
338   message, beginning with a request-line that includes a method, URI, and
339   protocol version (<xref target="request.line"/>),
340   followed by header fields containing
341   request modifiers, client information, and representation metadata
342   (<xref target="header.fields"/>),
343   an empty line to indicate the end of the header section, and finally
344   a message body containing the payload body (if any,
345   <xref target="message.body"/>).
348   A server responds to a client's request by sending one or more HTTP
349   <x:dfn>response</x:dfn>
350   messages, each beginning with a status line that
351   includes the protocol version, a success or error code, and textual
352   reason phrase (<xref target="status.line"/>),
353   possibly followed by header fields containing server
354   information, resource metadata, and representation metadata
355   (<xref target="header.fields"/>),
356   an empty line to indicate the end of the header section, and finally
357   a message body containing the payload body (if any,
358   <xref target="message.body"/>).
361   A connection might be used for multiple request/response exchanges,
362   as defined in <xref target="persistent.connections"/>.
365   The following example illustrates a typical message exchange for a
366   GET request on the URI "":
369Client request:
370</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
371GET /hello.txt HTTP/1.1
372User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
374Accept-Language: en, mi
378Server response:
379</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
380HTTP/1.1 200 OK
381Date: Mon, 27 Jul 2009 12:28:53 GMT
382Server: Apache
383Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
384ETag: "34aa387-d-1568eb00"
385Accept-Ranges: bytes
386Content-Length: <x:length-of target="exbody"/>
387Vary: Accept-Encoding
388Content-Type: text/plain
390<x:span anchor="exbody">Hello World! My payload includes a trailing CRLF.
395<section title="Implementation Diversity" anchor="implementation-diversity">
397   When considering the design of HTTP, it is easy to fall into a trap of
398   thinking that all user agents are general-purpose browsers and all origin
399   servers are large public websites. That is not the case in practice.
400   Common HTTP user agents include household appliances, stereos, scales,
401   firmware update scripts, command-line programs, mobile apps,
402   and communication devices in a multitude of shapes and sizes.  Likewise,
403   common HTTP origin servers include home automation units, configurable
404   networking components, office machines, autonomous robots, news feeds,
405   traffic cameras, ad selectors, and video delivery platforms.
408   The term "user agent" does not imply that there is a human user directly
409   interacting with the software agent at the time of a request. In many
410   cases, a user agent is installed or configured to run in the background
411   and save its results for later inspection (or save only a subset of those
412   results that might be interesting or erroneous). Spiders, for example, are
413   typically given a start URI and configured to follow certain behavior while
414   crawling the Web as a hypertext graph.
417   The implementation diversity of HTTP means that we cannot assume the
418   user agent can make interactive suggestions to a user or provide adequate
419   warning for security or privacy options.  In the few cases where this
420   specification requires reporting of errors to the user, it is acceptable
421   for such reporting to only be observable in an error console or log file.
422   Likewise, requirements that an automated action be confirmed by the user
423   before proceeding might be met via advance configuration choices,
424   run-time options, or simple avoidance of the unsafe action; confirmation
425   does not imply any specific user interface or interruption of normal
426   processing if the user has already made that choice.
430<section title="Intermediaries" anchor="intermediaries">
431<iref primary="true" item="intermediary"/>
433   HTTP enables the use of intermediaries to satisfy requests through
434   a chain of connections.  There are three common forms of HTTP
435   <x:dfn>intermediary</x:dfn>: proxy, gateway, and tunnel.  In some cases,
436   a single intermediary might act as an origin server, proxy, gateway,
437   or tunnel, switching behavior based on the nature of each request.
439<figure><artwork type="drawing">
440         &gt;             &gt;             &gt;             &gt;
441    <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>
442               &lt;             &lt;             &lt;             &lt;
445   The figure above shows three intermediaries (A, B, and C) between the
446   user agent and origin server. A request or response message that
447   travels the whole chain will pass through four separate connections.
448   Some HTTP communication options
449   might apply only to the connection with the nearest, non-tunnel
450   neighbor, only to the end-points of the chain, or to all connections
451   along the chain. Although the diagram is linear, each participant might
452   be engaged in multiple, simultaneous communications. For example, B
453   might be receiving requests from many clients other than A, and/or
454   forwarding requests to servers other than C, at the same time that it
455   is handling A's request. Likewise, later requests might be sent through a
456   different path of connections, often based on dynamic configuration for
457   load balancing.   
460<iref primary="true" item="upstream"/><iref primary="true" item="downstream"/>
461<iref primary="true" item="inbound"/><iref primary="true" item="outbound"/>
462   We use the terms "<x:dfn>upstream</x:dfn>" and "<x:dfn>downstream</x:dfn>"
463   to describe various requirements in relation to the directional flow of a
464   message: all messages flow from upstream to downstream.
465   Likewise, we use the terms inbound and outbound to refer to
466   directions in relation to the request path:
467   "<x:dfn>inbound</x:dfn>" means toward the origin server and
468   "<x:dfn>outbound</x:dfn>" means toward the user agent.
470<t><iref primary="true" item="proxy"/>
471   A "<x:dfn>proxy</x:dfn>" is a message forwarding agent that is selected by the
472   client, usually via local configuration rules, to receive requests
473   for some type(s) of absolute URI and attempt to satisfy those
474   requests via translation through the HTTP interface.  Some translations
475   are minimal, such as for proxy requests for "http" URIs, whereas
476   other requests might require translation to and from entirely different
477   application-level protocols. Proxies are often used to group an
478   organization's HTTP requests through a common intermediary for the
479   sake of security, annotation services, or shared caching.
482<iref primary="true" item="transforming proxy"/>
483<iref primary="true" item="non-transforming proxy"/>
484   An HTTP-to-HTTP proxy is called a "<x:dfn>transforming proxy</x:dfn>" if it is designed
485   or configured to modify request or response messages in a semantically
486   meaningful way (i.e., modifications, beyond those required by normal
487   HTTP processing, that change the message in a way that would be
488   significant to the original sender or potentially significant to
489   downstream recipients).  For example, a transforming proxy might be
490   acting as a shared annotation server (modifying responses to include
491   references to a local annotation database), a malware filter, a
492   format transcoder, or an intranet-to-Internet privacy filter.  Such
493   transformations are presumed to be desired by the client (or client
494   organization) that selected the proxy and are beyond the scope of
495   this specification.  However, when a proxy is not intended to transform
496   a given message, we use the term "<x:dfn>non-transforming proxy</x:dfn>" to target
497   requirements that preserve HTTP message semantics. See &status-203; and
498   &header-warning; for status and warning codes related to transformations.
500<t><iref primary="true" item="gateway"/><iref primary="true" item="reverse proxy"/>
501<iref primary="true" item="accelerator"/>
502   A "<x:dfn>gateway</x:dfn>" (a.k.a., "<x:dfn>reverse proxy</x:dfn>") is an
503   intermediary that acts as an origin server for the outbound connection, but
504   translates received requests and forwards them inbound to another server or
505   servers. Gateways are often used to encapsulate legacy or untrusted
506   information services, to improve server performance through
507   "<x:dfn>accelerator</x:dfn>" caching, and to enable partitioning or load
508   balancing of HTTP services across multiple machines.
511   All HTTP requirements applicable to an origin server
512   also apply to the outbound communication of a gateway.
513   A gateway communicates with inbound servers using any protocol that
514   it desires, including private extensions to HTTP that are outside
515   the scope of this specification.  However, an HTTP-to-HTTP gateway
516   that wishes to interoperate with third-party HTTP servers ought to conform
517   to user agent requirements on the gateway's inbound connection.
519<t><iref primary="true" item="tunnel"/>
520   A "<x:dfn>tunnel</x:dfn>" acts as a blind relay between two connections
521   without changing the messages. Once active, a tunnel is not
522   considered a party to the HTTP communication, though the tunnel might
523   have been initiated by an HTTP request. A tunnel ceases to exist when
524   both ends of the relayed connection are closed. Tunnels are used to
525   extend a virtual connection through an intermediary, such as when
526   Transport Layer Security (TLS, <xref target="RFC5246"/>) is used to
527   establish confidential communication through a shared firewall proxy.
529<t><iref primary="true" item="interception proxy"/>
530<iref primary="true" item="transparent proxy"/>
531<iref primary="true" item="captive portal"/>
532   The above categories for intermediary only consider those acting as
533   participants in the HTTP communication.  There are also intermediaries
534   that can act on lower layers of the network protocol stack, filtering or
535   redirecting HTTP traffic without the knowledge or permission of message
536   senders. Network intermediaries often introduce security flaws or
537   interoperability problems by violating HTTP semantics.  For example, an
538   "<x:dfn>interception proxy</x:dfn>" <xref target="RFC3040"/> (also commonly
539   known as a "<x:dfn>transparent proxy</x:dfn>" <xref target="RFC1919"/> or
540   "<x:dfn>captive portal</x:dfn>")
541   differs from an HTTP proxy because it is not selected by the client.
542   Instead, an interception proxy filters or redirects outgoing TCP port 80
543   packets (and occasionally other common port traffic).
544   Interception proxies are commonly found on public network access points,
545   as a means of enforcing account subscription prior to allowing use of
546   non-local Internet services, and within corporate firewalls to enforce
547   network usage policies.
548   They are indistinguishable from a man-in-the-middle attack.
551   HTTP is defined as a stateless protocol, meaning that each request message
552   can be understood in isolation.  Many implementations depend on HTTP's
553   stateless design in order to reuse proxied connections or dynamically
554   load-balance requests across multiple servers.  Hence, a server &MUST-NOT;
555   assume that two requests on the same connection are from the same user
556   agent unless the connection is secured and specific to that agent.
557   Some non-standard HTTP extensions (e.g., <xref target="RFC4559"/>) have
558   been known to violate this requirement, resulting in security and
559   interoperability problems.
563<section title="Caches" anchor="caches">
564<iref primary="true" item="cache"/>
566   A "<x:dfn>cache</x:dfn>" is a local store of previous response messages and the
567   subsystem that controls its message storage, retrieval, and deletion.
568   A cache stores cacheable responses in order to reduce the response
569   time and network bandwidth consumption on future, equivalent
570   requests. Any client or server &MAY; employ a cache, though a cache
571   cannot be used by a server while it is acting as a tunnel.
574   The effect of a cache is that the request/response chain is shortened
575   if one of the participants along the chain has a cached response
576   applicable to that request. The following illustrates the resulting
577   chain if B has a cached copy of an earlier response from O (via C)
578   for a request that has not been cached by UA or A.
580<figure><artwork type="drawing">
581            &gt;             &gt;
582       <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>
583                  &lt;             &lt;
585<t><iref primary="true" item="cacheable"/>
586   A response is "<x:dfn>cacheable</x:dfn>" if a cache is allowed to store a copy of
587   the response message for use in answering subsequent requests.
588   Even when a response is cacheable, there might be additional
589   constraints placed by the client or by the origin server on when
590   that cached response can be used for a particular request. HTTP
591   requirements for cache behavior and cacheable responses are
592   defined in &caching-overview;. 
595   There are a wide variety of architectures and configurations
596   of caches deployed across the World Wide Web and
597   inside large organizations. These include national hierarchies
598   of proxy caches to save transoceanic bandwidth, collaborative systems that
599   broadcast or multicast cache entries, archives of pre-fetched cache
600   entries for use in off-line or high-latency environments, and so on.
604<section title="Conformance and Error Handling" anchor="conformance">
606   This specification targets conformance criteria according to the role of
607   a participant in HTTP communication.  Hence, HTTP requirements are placed
608   on senders, recipients, clients, servers, user agents, intermediaries,
609   origin servers, proxies, gateways, or caches, depending on what behavior
610   is being constrained by the requirement. Additional (social) requirements
611   are placed on implementations, resource owners, and protocol element
612   registrations when they apply beyond the scope of a single communication.
615   The verb "generate" is used instead of "send" where a requirement
616   differentiates between creating a protocol element and merely forwarding a
617   received element downstream.
620   An implementation is considered conformant if it complies with all of the
621   requirements associated with the roles it partakes in HTTP.
624   Conformance includes both the syntax and semantics of protocol
625   elements. A sender &MUST-NOT; generate protocol elements that convey a
626   meaning that is known by that sender to be false. A sender &MUST-NOT;
627   generate protocol elements that do not match the grammar defined by the
628   corresponding ABNF rules. Within a given message, a sender &MUST-NOT;
629   generate protocol elements or syntax alternatives that are only allowed to
630   be generated by participants in other roles (i.e., a role that the sender
631   does not have for that message).
634   When a received protocol element is parsed, the recipient &MUST; be able to
635   parse any value of reasonable length that is applicable to the recipient's
636   role and matches the grammar defined by the corresponding ABNF rules.
637   Note, however, that some received protocol elements might not be parsed.
638   For example, an intermediary forwarding a message might parse a
639   header-field into generic field-name and field-value components, but then
640   forward the header field without further parsing inside the field-value.
643   HTTP does not have specific length limitations for many of its protocol
644   elements because the lengths that might be appropriate will vary widely,
645   depending on the deployment context and purpose of the implementation.
646   Hence, interoperability between senders and recipients depends on shared
647   expectations regarding what is a reasonable length for each protocol
648   element. Furthermore, what is commonly understood to be a reasonable length
649   for some protocol elements has changed over the course of the past two
650   decades of HTTP use, and is expected to continue changing in the future.
653   At a minimum, a recipient &MUST; be able to parse and process protocol
654   element lengths that are at least as long as the values that it generates
655   for those same protocol elements in other messages. For example, an origin
656   server that publishes very long URI references to its own resources needs
657   to be able to parse and process those same references when received as a
658   request target.
661   A recipient &MUST; interpret a received protocol element according to the
662   semantics defined for it by this specification, including extensions to
663   this specification, unless the recipient has determined (through experience
664   or configuration) that the sender incorrectly implements what is implied by
665   those semantics.
666   For example, an origin server might disregard the contents of a received
667   <x:ref>Accept-Encoding</x:ref> header field if inspection of the
668   <x:ref>User-Agent</x:ref> header field indicates a specific implementation
669   version that is known to fail on receipt of certain content codings.
672   Unless noted otherwise, a recipient &MAY; attempt to recover a usable
673   protocol element from an invalid construct.  HTTP does not define
674   specific error handling mechanisms except when they have a direct impact
675   on security, since different applications of the protocol require
676   different error handling strategies.  For example, a Web browser might
677   wish to transparently recover from a response where the
678   <x:ref>Location</x:ref> header field doesn't parse according to the ABNF,
679   whereas a systems control client might consider any form of error recovery
680   to be dangerous.
684<section title="Protocol Versioning" anchor="http.version">
685  <x:anchor-alias value="HTTP-version"/>
686  <x:anchor-alias value="HTTP-name"/>
688   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate
689   versions of the protocol. This specification defines version "1.1".
690   The protocol version as a whole indicates the sender's conformance
691   with the set of requirements laid out in that version's corresponding
692   specification of HTTP.
695   The version of an HTTP message is indicated by an HTTP-version field
696   in the first line of the message. HTTP-version is case-sensitive.
698<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-version"/><iref primary="true" item="Grammar" subitem="HTTP-name"/>
699  <x:ref>HTTP-version</x:ref>  = <x:ref>HTTP-name</x:ref> "/" <x:ref>DIGIT</x:ref> "." <x:ref>DIGIT</x:ref>
700  <x:ref>HTTP-name</x:ref>     = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
703   The HTTP version number consists of two decimal digits separated by a "."
704   (period or decimal point).  The first digit ("major version") indicates the
705   HTTP messaging syntax, whereas the second digit ("minor version") indicates
706   the highest minor version within that major version to which the sender is
707   conformant and able to understand for future communication.  The minor
708   version advertises the sender's communication capabilities even when the
709   sender is only using a backwards-compatible subset of the protocol,
710   thereby letting the recipient know that more advanced features can
711   be used in response (by servers) or in future requests (by clients).
714   When an HTTP/1.1 message is sent to an HTTP/1.0 recipient
715   <xref target="RFC1945"/> or a recipient whose version is unknown,
716   the HTTP/1.1 message is constructed such that it can be interpreted
717   as a valid HTTP/1.0 message if all of the newer features are ignored.
718   This specification places recipient-version requirements on some
719   new features so that a conformant sender will only use compatible
720   features until it has determined, through configuration or the
721   receipt of a message, that the recipient supports HTTP/1.1.
724   The interpretation of a header field does not change between minor
725   versions of the same major HTTP version, though the default
726   behavior of a recipient in the absence of such a field can change.
727   Unless specified otherwise, header fields defined in HTTP/1.1 are
728   defined for all versions of HTTP/1.x.  In particular, the <x:ref>Host</x:ref>
729   and <x:ref>Connection</x:ref> header fields ought to be implemented by all
730   HTTP/1.x implementations whether or not they advertise conformance with
731   HTTP/1.1.
734   New header fields can be introduced without changing the protocol version
735   if their defined semantics allow them to be safely ignored by recipients
736   that do not recognize them. Header field extensibility is discussed in
737   <xref target="field.extensibility"/>.
740   Intermediaries that process HTTP messages (i.e., all intermediaries
741   other than those acting as tunnels) &MUST; send their own HTTP-version
742   in forwarded messages.  In other words, they are not allowed to blindly
743   forward the first line of an HTTP message without ensuring that the
744   protocol version in that message matches a version to which that
745   intermediary is conformant for both the receiving and
746   sending of messages.  Forwarding an HTTP message without rewriting
747   the HTTP-version might result in communication errors when downstream
748   recipients use the message sender's version to determine what features
749   are safe to use for later communication with that sender.
752   A client &SHOULD; send a request version equal to the highest
753   version to which the client is conformant and
754   whose major version is no higher than the highest version supported
755   by the server, if this is known.  A client &MUST-NOT; send a
756   version to which it is not conformant.
759   A client &MAY; send a lower request version if it is known that
760   the server incorrectly implements the HTTP specification, but only
761   after the client has attempted at least one normal request and determined
762   from the response status code or header fields (e.g., <x:ref>Server</x:ref>) that
763   the server improperly handles higher request versions.
766   A server &SHOULD; send a response version equal to the highest version to
767   which the server is conformant that has a major version less than or equal
768   to the one received in the request.
769   A server &MUST-NOT; send a version to which it is not conformant.
770   A server can send a <x:ref>505 (HTTP Version Not Supported)</x:ref>
771   response if it wishes, for any reason, to refuse service of the client's
772   major protocol version.
775   A server &MAY; send an HTTP/1.0 response to a request
776   if it is known or suspected that the client incorrectly implements the
777   HTTP specification and is incapable of correctly processing later
778   version responses, such as when a client fails to parse the version
779   number correctly or when an intermediary is known to blindly forward
780   the HTTP-version even when it doesn't conform to the given minor
781   version of the protocol. Such protocol downgrades &SHOULD-NOT; be
782   performed unless triggered by specific client attributes, such as when
783   one or more of the request header fields (e.g., <x:ref>User-Agent</x:ref>)
784   uniquely match the values sent by a client known to be in error.
787   The intention of HTTP's versioning design is that the major number
788   will only be incremented if an incompatible message syntax is
789   introduced, and that the minor number will only be incremented when
790   changes made to the protocol have the effect of adding to the message
791   semantics or implying additional capabilities of the sender.  However,
792   the minor version was not incremented for the changes introduced between
793   <xref target="RFC2068"/> and <xref target="RFC2616"/>, and this revision
794   has specifically avoided any such changes to the protocol.
797   When an HTTP message is received with a major version number that the
798   recipient implements, but a higher minor version number than what the
799   recipient implements, the recipient &SHOULD; process the message as if it
800   were in the highest minor version within that major version to which the
801   recipient is conformant. A recipient can assume that a message with a
802   higher minor version, when sent to a recipient that has not yet indicated
803   support for that higher version, is sufficiently backwards-compatible to be
804   safely processed by any implementation of the same major version.
808<section title="Uniform Resource Identifiers" anchor="uri">
809<iref primary="true" item="resource"/>
811   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used
812   throughout HTTP as the means for identifying resources (&resource;).
813   URI references are used to target requests, indicate redirects, and define
814   relationships.
816  <x:anchor-alias value="URI-reference"/>
817  <x:anchor-alias value="absolute-URI"/>
818  <x:anchor-alias value="relative-part"/>
819  <x:anchor-alias value="authority"/>
820  <x:anchor-alias value="uri-host"/>
821  <x:anchor-alias value="port"/>
822  <x:anchor-alias value="path-abempty"/>
823  <x:anchor-alias value="segment"/>
824  <x:anchor-alias value="query"/>
825  <x:anchor-alias value="fragment"/>
826  <x:anchor-alias value="absolute-path"/>
827  <x:anchor-alias value="partial-URI"/>
829   This specification adopts the definitions of "URI-reference",
830   "absolute-URI", "relative-part", "authority", "port", "host",
831   "path-abempty", "segment", "query", and "fragment" from the
832   URI generic syntax.
833   In addition, we define an "absolute-path" rule (that differs from
834   RFC 3986's "path-absolute" in that it allows a leading "//")
835   and a "partial-URI" rule for protocol elements
836   that allow a relative URI but not a fragment.
838<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="URI-reference"><!--exported production--></iref><iref primary="true" item="Grammar" subitem="absolute-URI"/><iref primary="true" item="Grammar" subitem="authority"/><iref primary="true" item="Grammar" subitem="absolute-path"/><iref primary="true" item="Grammar" subitem="port"/><iref primary="true" item="Grammar" subitem="query"/><iref primary="true" item="Grammar" subitem="fragment"/><iref primary="true" item="Grammar" subitem="segment"/><iref primary="true" item="Grammar" subitem="uri-host"/><iref primary="true" item="Grammar" subitem="partial-URI"><!--exported production--></iref>
839  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
840  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
841  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
842  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
843  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
844  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
845  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
846  <x:ref>segment</x:ref>       = &lt;segment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
847  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
848  <x:ref>fragment</x:ref>      = &lt;fragment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.5"/>&gt;
850  <x:ref>absolute-path</x:ref> = 1*( "/" segment )
851  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
854   Each protocol element in HTTP that allows a URI reference will indicate
855   in its ABNF production whether the element allows any form of reference
856   (URI-reference), only a URI in absolute form (absolute-URI), only the
857   path and optional query components, or some combination of the above.
858   Unless otherwise indicated, URI references are parsed
859   relative to the effective request URI
860   (<xref target="effective.request.uri"/>).
863<section title="http URI scheme" anchor="http.uri">
864  <x:anchor-alias value="http-URI"/>
865  <iref item="http URI scheme" primary="true"/>
866  <iref item="URI scheme" subitem="http" primary="true"/>
868   The "http" URI scheme is hereby defined for the purpose of minting
869   identifiers according to their association with the hierarchical
870   namespace governed by a potential HTTP origin server listening for
871   TCP (<xref target="RFC0793"/>) connections on a given port.
873<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"><!--terminal production--></iref>
874  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
875             [ "#" <x:ref>fragment</x:ref> ]
878   The HTTP origin server is identified by the generic syntax's
879   <x:ref>authority</x:ref> component, which includes a host identifier
880   and optional TCP port (<xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>).
881   The remainder of the URI, consisting of both the hierarchical path
882   component and optional query component, serves as an identifier for
883   a potential resource within that origin server's name space.
886   A sender &MUST-NOT; generate an "http" URI with an empty host identifier.
887   A recipient that processes such a URI reference &MUST; reject it as invalid.
890   If the host identifier is provided as an IP address,
891   then the origin server is any listener on the indicated TCP port at
892   that IP address. If host is a registered name, then that name is
893   considered an indirect identifier and the recipient might use a name
894   resolution service, such as DNS, to find the address of a listener
895   for that host.
896   If the port subcomponent is empty or not given, then TCP port 80 is
897   assumed (the default reserved port for WWW services).
900   Regardless of the form of host identifier, access to that host is not
901   implied by the mere presence of its name or address. The host might or might
902   not exist and, even when it does exist, might or might not be running an
903   HTTP server or listening to the indicated port. The "http" URI scheme
904   makes use of the delegated nature of Internet names and addresses to
905   establish a naming authority (whatever entity has the ability to place
906   an HTTP server at that Internet name or address) and allows that
907   authority to determine which names are valid and how they might be used.
910   When an "http" URI is used within a context that calls for access to the
911   indicated resource, a client &MAY; attempt access by resolving
912   the host to an IP address, establishing a TCP connection to that address
913   on the indicated port, and sending an HTTP request message
914   (<xref target="http.message"/>) containing the URI's identifying data
915   (<xref target="message.routing"/>) to the server.
916   If the server responds to that request with a non-interim HTTP response
917   message, as described in &status-codes;, then that response
918   is considered an authoritative answer to the client's request.
921   Although HTTP is independent of the transport protocol, the "http"
922   scheme is specific to TCP-based services because the name delegation
923   process depends on TCP for establishing authority.
924   An HTTP service based on some other underlying connection protocol
925   would presumably be identified using a different URI scheme, just as
926   the "https" scheme (below) is used for resources that require an
927   end-to-end secured connection. Other protocols might also be used to
928   provide access to "http" identified resources &mdash; it is only the
929   authoritative interface that is specific to TCP.
932   The URI generic syntax for authority also includes a deprecated
933   userinfo subcomponent (<xref target="RFC3986" x:fmt="," x:sec="3.2.1"/>)
934   for including user authentication information in the URI.  Some
935   implementations make use of the userinfo component for internal
936   configuration of authentication information, such as within command
937   invocation options, configuration files, or bookmark lists, even
938   though such usage might expose a user identifier or password.
939   A sender &MUST-NOT; generate the userinfo subcomponent (and its "@"
940   delimiter) when an "http" URI reference is generated within a message as a
941   request target or header field value.
942   Before making use of an "http" URI reference received from an untrusted
943   source, a recipient ought to parse for userinfo and treat its presence as
944   an error; it is likely being used to obscure the authority for the sake of
945   phishing attacks.
949<section title="https URI scheme" anchor="https.uri">
950   <x:anchor-alias value="https-URI"/>
951   <iref item="https URI scheme"/>
952   <iref item="URI scheme" subitem="https"/>
954   The "https" URI scheme is hereby defined for the purpose of minting
955   identifiers according to their association with the hierarchical
956   namespace governed by a potential HTTP origin server listening to a
957   given TCP port for TLS-secured connections
958   (<xref target="RFC0793"/>, <xref target="RFC5246"/>).
961   All of the requirements listed above for the "http" scheme are also
962   requirements for the "https" scheme, except that a default TCP port
963   of 443 is assumed if the port subcomponent is empty or not given,
964   and the user agent &MUST; ensure that its connection to the origin
965   server is secured through the use of strong encryption, end-to-end,
966   prior to sending the first HTTP request.
968<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="https-URI"><!--terminal production--></iref>
969  <x:ref>https-URI</x:ref> = "https:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
970              [ "#" <x:ref>fragment</x:ref> ]
973   Note that the "https" URI scheme depends on both TLS and TCP for
974   establishing authority.
975   Resources made available via the "https" scheme have no shared
976   identity with the "http" scheme even if their resource identifiers
977   indicate the same authority (the same host listening to the same
978   TCP port).  They are distinct name spaces and are considered to be
979   distinct origin servers.  However, an extension to HTTP that is
980   defined to apply to entire host domains, such as the Cookie protocol
981   <xref target="RFC6265"/>, can allow information
982   set by one service to impact communication with other services
983   within a matching group of host domains.
986   The process for authoritative access to an "https" identified
987   resource is defined in <xref target="RFC2818"/>.
991<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
993   Since the "http" and "https" schemes conform to the URI generic syntax,
994   such URIs are normalized and compared according to the algorithm defined
995   in <xref target="RFC3986" x:fmt="of" x:sec="6"/>, using the defaults
996   described above for each scheme.
999   If the port is equal to the default port for a scheme, the normal form is
1000   to omit the port subcomponent. When not being used in absolute form as the
1001   request target of an OPTIONS request, an empty path component is equivalent
1002   to an absolute path of "/", so the normal form is to provide a path of "/"
1003   instead. The scheme and host are case-insensitive and normally provided in
1004   lowercase; all other components are compared in a case-sensitive manner.
1005   Characters other than those in the "reserved" set are equivalent to their
1006   percent-encoded octets: the normal form is to not encode them
1007   (see Sections <xref target="RFC3986" x:fmt="number" x:sec="2.1"/> and
1008   <xref target="RFC3986" x:fmt="number" x:sec="2.2"/> of
1009   <xref target="RFC3986"/>).
1012   For example, the following three URIs are equivalent:
1014<figure><artwork type="example">
1023<section title="Message Format" anchor="http.message">
1024<x:anchor-alias value="generic-message"/>
1025<x:anchor-alias value="message.types"/>
1026<x:anchor-alias value="HTTP-message"/>
1027<x:anchor-alias value="start-line"/>
1028<iref item="header section"/>
1029<iref item="headers"/>
1030<iref item="header field"/>
1032   All HTTP/1.1 messages consist of a start-line followed by a sequence of
1033   octets in a format similar to the Internet Message Format
1034   <xref target="RFC5322"/>: zero or more header fields (collectively
1035   referred to as the "headers" or the "header section"), an empty line
1036   indicating the end of the header section, and an optional message body.
1038<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"><!--terminal production--></iref>
1039  <x:ref>HTTP-message</x:ref>   = <x:ref>start-line</x:ref>
1040                   *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1041                   <x:ref>CRLF</x:ref>
1042                   [ <x:ref>message-body</x:ref> ]
1045   The normal procedure for parsing an HTTP message is to read the
1046   start-line into a structure, read each header field into a hash
1047   table by field name until the empty line, and then use the parsed
1048   data to determine if a message body is expected.  If a message body
1049   has been indicated, then it is read as a stream until an amount
1050   of octets equal to the message body length is read or the connection
1051   is closed.
1054   A recipient &MUST; parse an HTTP message as a sequence of octets in an
1055   encoding that is a superset of US-ASCII <xref target="USASCII"/>.
1056   Parsing an HTTP message as a stream of Unicode characters, without regard
1057   for the specific encoding, creates security vulnerabilities due to the
1058   varying ways that string processing libraries handle invalid multibyte
1059   character sequences that contain the octet LF (%x0A).  String-based
1060   parsers can only be safely used within protocol elements after the element
1061   has been extracted from the message, such as within a header field-value
1062   after message parsing has delineated the individual fields.
1065   An HTTP message can be parsed as a stream for incremental processing or
1066   forwarding downstream.  However, recipients cannot rely on incremental
1067   delivery of partial messages, since some implementations will buffer or
1068   delay message forwarding for the sake of network efficiency, security
1069   checks, or payload transformations.
1072   A sender &MUST-NOT; send whitespace between the start-line and
1073   the first header field.
1074   A recipient that receives whitespace between the start-line and
1075   the first header field &MUST; either reject the message as invalid or
1076   consume each whitespace-preceded line without further processing of it
1077   (i.e., ignore the entire line, along with any subsequent lines preceded
1078   by whitespace, until a properly formed header field is received or the
1079   header section is terminated).
1082   The presence of such whitespace in a request
1083   might be an attempt to trick a server into ignoring that field or
1084   processing the line after it as a new request, either of which might
1085   result in a security vulnerability if other implementations within
1086   the request chain interpret the same message differently.
1087   Likewise, the presence of such whitespace in a response might be
1088   ignored by some clients or cause others to cease parsing.
1091<section title="Start Line" anchor="start.line">
1092  <x:anchor-alias value="Start-Line"/>
1094   An HTTP message can either be a request from client to server or a
1095   response from server to client.  Syntactically, the two types of message
1096   differ only in the start-line, which is either a request-line (for requests)
1097   or a status-line (for responses), and in the algorithm for determining
1098   the length of the message body (<xref target="message.body"/>).
1101   In theory, a client could receive requests and a server could receive
1102   responses, distinguishing them by their different start-line formats,
1103   but in practice servers are implemented to only expect a request
1104   (a response is interpreted as an unknown or invalid request method)
1105   and clients are implemented to only expect a response.
1107<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="start-line"/>
1108  <x:ref>start-line</x:ref>     = <x:ref>request-line</x:ref> / <x:ref>status-line</x:ref>
1111<section title="Request Line" anchor="request.line">
1112  <x:anchor-alias value="Request"/>
1113  <x:anchor-alias value="request-line"/>
1115   A request-line begins with a method token, followed by a single
1116   space (SP), the request-target, another single space (SP), the
1117   protocol version, and ending with CRLF.
1119<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1120  <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>
1122<iref primary="true" item="method"/>
1123<t anchor="method">
1124   The method token indicates the request method to be performed on the
1125   target resource. The request method is case-sensitive.
1127<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1128  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1131   The request methods defined by this specification can be found in
1132   &methods;, along with information regarding the HTTP method registry
1133   and considerations for defining new methods.
1135<iref item="request-target"/>
1137   The request-target identifies the target resource upon which to apply
1138   the request, as defined in <xref target="request-target"/>.
1141   Recipients typically parse the request-line into its component parts by
1142   splitting on whitespace (see <xref target="message.robustness"/>), since
1143   no whitespace is allowed in the three components.
1144   Unfortunately, some user agents fail to properly encode or exclude
1145   whitespace found in hypertext references, resulting in those disallowed
1146   characters being sent in a request-target.
1149   Recipients of an invalid request-line &SHOULD; respond with either a
1150   <x:ref>400 (Bad Request)</x:ref> error or a <x:ref>301 (Moved Permanently)</x:ref>
1151   redirect with the request-target properly encoded.  A recipient &SHOULD-NOT;
1152   attempt to autocorrect and then process the request without a redirect,
1153   since the invalid request-line might be deliberately crafted to bypass
1154   security filters along the request chain.
1157   HTTP does not place a pre-defined limit on the length of a request-line.
1158   A server that receives a method longer than any that it implements
1159   &SHOULD; respond with a <x:ref>501 (Not Implemented)</x:ref> status code.
1160   A server ought to be prepared to receive URIs of unbounded length, as
1161   described in <xref target="conformance"/>, and &MUST; respond with a
1162   <x:ref>414 (URI Too Long)</x:ref> status code if the received
1163   request-target is longer than the server wishes to parse (see &status-414;).
1166   Various ad-hoc limitations on request-line length are found in practice.
1167   It is &RECOMMENDED; that all HTTP senders and recipients support, at a
1168   minimum, request-line lengths of 8000 octets.
1172<section title="Status Line" anchor="status.line">
1173  <x:anchor-alias value="response"/>
1174  <x:anchor-alias value="status-line"/>
1175  <x:anchor-alias value="status-code"/>
1176  <x:anchor-alias value="reason-phrase"/>
1178   The first line of a response message is the status-line, consisting
1179   of the protocol version, a space (SP), the status code, another space,
1180   a possibly-empty textual phrase describing the status code, and
1181   ending with CRLF.
1183<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-line"/>
1184  <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>
1187   The status-code element is a 3-digit integer code describing the
1188   result of the server's attempt to understand and satisfy the client's
1189   corresponding request. The rest of the response message is to be
1190   interpreted in light of the semantics defined for that status code.
1191   See &status-codes; for information about the semantics of status codes,
1192   including the classes of status code (indicated by the first digit),
1193   the status codes defined by this specification, considerations for the
1194   definition of new status codes, and the IANA registry.
1196<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-code"/>
1197  <x:ref>status-code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1200   The reason-phrase element exists for the sole purpose of providing a
1201   textual description associated with the numeric status code, mostly
1202   out of deference to earlier Internet application protocols that were more
1203   frequently used with interactive text clients. A client &SHOULD; ignore
1204   the reason-phrase content.
1206<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
1207  <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> )
1212<section title="Header Fields" anchor="header.fields">
1213  <x:anchor-alias value="header-field"/>
1214  <x:anchor-alias value="field-content"/>
1215  <x:anchor-alias value="field-name"/>
1216  <x:anchor-alias value="field-value"/>
1217  <x:anchor-alias value="obs-fold"/>
1219   Each header field consists of a case-insensitive field name
1220   followed by a colon (":"), optional leading whitespace, the field value,
1221   and optional trailing whitespace.
1223<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"/>
1224  <x:ref>header-field</x:ref>   = <x:ref>field-name</x:ref> ":" <x:ref>OWS</x:ref> <x:ref>field-value</x:ref> <x:ref>OWS</x:ref>
1225  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1226  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>obs-fold</x:ref> )
1227  <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> )
1228  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref> ( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1229                 ; obsolete line folding
1230                 ; see <xref target="field.parsing"/>
1233   The field-name token labels the corresponding field-value as having the
1234   semantics defined by that header field.  For example, the <x:ref>Date</x:ref>
1235   header field is defined in &header-date; as containing the origination
1236   timestamp for the message in which it appears.
1239<section title="Field Extensibility" anchor="field.extensibility">
1241   Header fields are fully extensible: there is no limit on the
1242   introduction of new field names, each presumably defining new semantics,
1243   nor on the number of header fields used in a given message.  Existing
1244   fields are defined in each part of this specification and in many other
1245   specifications outside the core standard.
1248   New header fields can be defined such that, when they are understood by a
1249   recipient, they might override or enhance the interpretation of previously
1250   defined header fields, define preconditions on request evaluation, or
1251   refine the meaning of responses.
1254   A proxy &MUST; forward unrecognized header fields unless the
1255   field-name is listed in the <x:ref>Connection</x:ref> header field
1256   (<xref target="header.connection"/>) or the proxy is specifically
1257   configured to block, or otherwise transform, such fields.
1258   Other recipients &SHOULD; ignore unrecognized header fields.
1259   These requirements allow HTTP's functionality to be enhanced without
1260   requiring prior update of deployed intermediaries.
1263   All defined header fields ought to be registered with IANA in the
1264   Message Header Field Registry, as described in &iana-header-registry;.
1268<section title="Field Order" anchor="field.order">
1270   The order in which header fields with differing field names are
1271   received is not significant. However, it is "good practice" to send
1272   header fields that contain control data first, such as <x:ref>Host</x:ref>
1273   on requests and <x:ref>Date</x:ref> on responses, so that implementations
1274   can decide when not to handle a message as early as possible.  A server
1275   &MUST; wait until the entire header section is received before interpreting
1276   a request message, since later header fields might include conditionals,
1277   authentication credentials, or deliberately misleading duplicate
1278   header fields that would impact request processing.
1281   A sender &MUST-NOT; generate multiple header fields with the same field
1282   name in a message unless either the entire field value for that
1283   header field is defined as a comma-separated list [i.e., #(values)]
1284   or the header field is a well-known exception (as noted below).
1287   A recipient &MAY; combine multiple header fields with the same field name
1288   into one "field-name: field-value" pair, without changing the semantics of
1289   the message, by appending each subsequent field value to the combined
1290   field value in order, separated by a comma. The order in which
1291   header fields with the same field name are received is therefore
1292   significant to the interpretation of the combined field value;
1293   a proxy &MUST-NOT; change the order of these field values when
1294   forwarding a message.
1297  <t>
1298   &Note; In practice, the "Set-Cookie" header field (<xref target="RFC6265"/>)
1299   often appears multiple times in a response message and does not use the
1300   list syntax, violating the above requirements on multiple header fields
1301   with the same name. Since it cannot be combined into a single field-value,
1302   recipients ought to handle "Set-Cookie" as a special case while processing
1303   header fields. (See Appendix A.2.3 of <xref target="Kri2001"/> for details.)
1304  </t>
1308<section title="Whitespace" anchor="whitespace">
1309<t anchor="rule.LWS">
1310   This specification uses three rules to denote the use of linear
1311   whitespace: OWS (optional whitespace), RWS (required whitespace), and
1312   BWS ("bad" whitespace).
1314<t anchor="rule.OWS">
1315   The OWS rule is used where zero or more linear whitespace octets might
1316   appear. For protocol elements where optional whitespace is preferred to
1317   improve readability, a sender &SHOULD; generate the optional whitespace
1318   as a single SP; otherwise, a sender &SHOULD-NOT; generate optional
1319   whitespace except as needed to white-out invalid or unwanted protocol
1320   elements during in-place message filtering.
1322<t anchor="rule.RWS">
1323   The RWS rule is used when at least one linear whitespace octet is required
1324   to separate field tokens. A sender &SHOULD; generate RWS as a single SP.
1326<t anchor="rule.BWS">
1327   The BWS rule is used where the grammar allows optional whitespace only for
1328   historical reasons. A sender &MUST-NOT; generate BWS in messages.
1329   A recipient &MUST; parse for such bad whitespace and remove it before
1330   interpreting the protocol element.
1332<t anchor="rule.whitespace">
1333  <x:anchor-alias value="BWS"/>
1334  <x:anchor-alias value="OWS"/>
1335  <x:anchor-alias value="RWS"/>
1337<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"/>
1338  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1339                 ; optional whitespace
1340  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1341                 ; required whitespace
1342  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1343                 ; "bad" whitespace
1347<section title="Field Parsing" anchor="field.parsing">
1349   Messages are parsed using a generic algorithm, independent of the
1350   individual header field names. The contents within a given field value are
1351   not parsed until a later stage of message interpretation (usually after the
1352   message's entire header section has been processed).
1353   Consequently, this specification does not use ABNF rules to define each
1354   "Field-Name: Field Value" pair, as was done in previous editions.
1355   Instead, this specification uses ABNF rules which are named according to
1356   each registered field name, wherein the rule defines the valid grammar for
1357   that field's corresponding field values (i.e., after the field-value
1358   has been extracted from the header section by a generic field parser).
1361   No whitespace is allowed between the header field-name and colon.
1362   In the past, differences in the handling of such whitespace have led to
1363   security vulnerabilities in request routing and response handling.
1364   A server &MUST; reject any received request message that contains
1365   whitespace between a header field-name and colon with a response code of
1366   <x:ref>400 (Bad Request)</x:ref>. A proxy &MUST; remove any such whitespace
1367   from a response message before forwarding the message downstream.
1370   A field value is preceded by optional whitespace (OWS); a single SP is
1371   preferred. The field value does not include any leading or trailing white
1372   space: OWS occurring before the first non-whitespace octet of the field
1373   value or after the last non-whitespace octet of the field value ought to be
1374   excluded by parsers when extracting the field value from a header field.
1377   A recipient of field-content containing multiple sequential octets of
1378   optional (OWS) or required (RWS) whitespace &SHOULD; either replace the
1379   sequence with a single SP or transform any non-SP octets in the sequence to
1380   SP octets before interpreting the field value or forwarding the message
1381   downstream.
1384   Historically, HTTP header field values could be extended over multiple
1385   lines by preceding each extra line with at least one space or horizontal
1386   tab (obs-fold). This specification deprecates such line folding except
1387   within the message/http media type
1388   (<xref target=""/>).
1389   A sender &MUST-NOT; generate a message that includes line folding
1390   (i.e., that has any field-value that contains a match to the
1391   <x:ref>obs-fold</x:ref> rule) unless the message is intended for packaging
1392   within the message/http media type.
1395   A server that receives an <x:ref>obs-fold</x:ref> in a request message that
1396   is not within a message/http container &MUST; either reject the message by
1397   sending a <x:ref>400 (Bad Request)</x:ref>, preferably with a
1398   representation explaining that obsolete line folding is unacceptable, or
1399   replace each received <x:ref>obs-fold</x:ref> with one or more
1400   <x:ref>SP</x:ref> octets prior to interpreting the field value or
1401   forwarding the message downstream.
1404   A proxy or gateway that receives an <x:ref>obs-fold</x:ref> in a response
1405   message that is not within a message/http container &MUST; either discard
1406   the message and replace it with a <x:ref>502 (Bad Gateway)</x:ref>
1407   response, preferably with a representation explaining that unacceptable
1408   line folding was received, or replace each received <x:ref>obs-fold</x:ref>
1409   with one or more <x:ref>SP</x:ref> octets prior to interpreting the field
1410   value or forwarding the message downstream.
1413   A user agent that receives an <x:ref>obs-fold</x:ref> in a response message
1414   that is not within a message/http container &MUST; replace each received
1415   <x:ref>obs-fold</x:ref> with one or more <x:ref>SP</x:ref> octets prior to
1416   interpreting the field value.
1419   Historically, HTTP has allowed field content with text in the ISO-8859-1
1420   <xref target="ISO-8859-1"/> charset, supporting other charsets only
1421   through use of <xref target="RFC2047"/> encoding.
1422   In practice, most HTTP header field values use only a subset of the
1423   US-ASCII charset <xref target="USASCII"/>. Newly defined
1424   header fields &SHOULD; limit their field values to US-ASCII octets.
1425   A recipient &SHOULD; treat other octets in field content (obs-text) as
1426   opaque data.
1430<section title="Field Limits" anchor="field.limits">
1432   HTTP does not place a pre-defined limit on the length of each header field
1433   or on the length of the header section as a whole, as described in
1434   <xref target="conformance"/>. Various ad-hoc limitations on individual
1435   header field length are found in practice, often depending on the specific
1436   field semantics.
1439   A server ought to be prepared to receive request header fields of unbounded
1440   length and &MUST; respond with an appropriate
1441   <x:ref>4xx (Client Error)</x:ref> status code if the received header
1442   field(s) are larger than the server wishes to process.
1445   A client ought to be prepared to receive response header fields of
1446   unbounded length.
1447   A client &MAY; discard or truncate received header fields that are larger
1448   than the client wishes to process if the field semantics are such that the
1449   dropped value(s) can be safely ignored without changing the
1450   message framing or response semantics.
1454<section title="Field value components" anchor="field.components">
1455<t anchor="rule.token.separators">
1456  <x:anchor-alias value="tchar"/>
1457  <x:anchor-alias value="token"/>
1458  <iref item="Delimiters"/>
1459   Most HTTP header field values are defined using common syntax components
1460   (token, quoted-string, and comment) separated by whitespace or specific
1461   delimiting characters. Delimiters are chosen from the set of US-ASCII
1462   visual characters not allowed in a <x:ref>token</x:ref>
1463   (DQUOTE and "(),/:;&lt;=>?@[\]{}").
1465<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="token"/><iref primary="true" item="Grammar" subitem="tchar"/>
1466  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1468  NOTE: the definition of tchar and the prose above about special characters need to match!
1469 -->
1470  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1471                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1472                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1473                 ; any <x:ref>VCHAR</x:ref>, except delimiters
1475<t anchor="rule.quoted-string">
1476  <x:anchor-alias value="quoted-string"/>
1477  <x:anchor-alias value="qdtext"/>
1478  <x:anchor-alias value="obs-text"/>
1479   A string of text is parsed as a single value if it is quoted using
1480   double-quote marks.
1482<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"/>
1483  <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>
1484  <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>
1485  <x:ref>obs-text</x:ref>       = %x80-FF
1487<t anchor="rule.comment">
1488  <x:anchor-alias value="comment"/>
1489  <x:anchor-alias value="ctext"/>
1490   Comments can be included in some HTTP header fields by surrounding
1491   the comment text with parentheses. Comments are only allowed in
1492   fields containing "comment" as part of their field value definition.
1494<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1495  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-pair</x:ref> / <x:ref>comment</x:ref> ) ")"
1496  <x:ref>ctext</x:ref>          = <x:ref>HTAB</x:ref> / <x:ref>SP</x:ref> / %x21-27 / %x2A-5B / %x5D-7E / <x:ref>obs-text</x:ref>
1498<t anchor="rule.quoted-pair">
1499  <x:anchor-alias value="quoted-pair"/>
1500   The backslash octet ("\") can be used as a single-octet
1501   quoting mechanism within quoted-string and comment constructs.
1502   Recipients that process the value of a quoted-string &MUST; handle a
1503   quoted-pair as if it were replaced by the octet following the backslash.
1505<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1506  <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> )
1509   A sender &SHOULD-NOT; generate a quoted-pair in a quoted-string except
1510   where necessary to quote DQUOTE and backslash octets occurring within that
1511   string.
1512   A sender &SHOULD-NOT; generate a quoted-pair in a comment except
1513   where necessary to quote parentheses ["(" and ")"] and backslash octets
1514   occurring within that comment.
1520<section title="Message Body" anchor="message.body">
1521  <x:anchor-alias value="message-body"/>
1523   The message body (if any) of an HTTP message is used to carry the
1524   payload body of that request or response.  The message body is
1525   identical to the payload body unless a transfer coding has been
1526   applied, as described in <xref target="header.transfer-encoding"/>.
1528<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1529  <x:ref>message-body</x:ref> = *OCTET
1532   The rules for when a message body is allowed in a message differ for
1533   requests and responses.
1536   The presence of a message body in a request is signaled by a
1537   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1538   field. Request message framing is independent of method semantics,
1539   even if the method does not define any use for a message body.
1542   The presence of a message body in a response depends on both
1543   the request method to which it is responding and the response
1544   status code (<xref target="status.line"/>).
1545   Responses to the HEAD request method never include a message body
1546   because the associated response header fields (e.g.,
1547   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1548   if present, indicate only what their values would have been if the request
1549   method had been GET (&HEAD;).
1550   <x:ref>2xx (Successful)</x:ref> responses to CONNECT switch to tunnel
1551   mode instead of having a message body (&CONNECT;).
1552   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1553   <x:ref>304 (Not Modified)</x:ref> responses do not include a message body.
1554   All other responses do include a message body, although the body
1555   might be of zero length.
1558<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1559  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1560  <iref item="chunked (Coding Format)"/>
1561  <x:anchor-alias value="Transfer-Encoding"/>
1563   The Transfer-Encoding header field lists the transfer coding names
1564   corresponding to the sequence of transfer codings that have been
1565   (or will be) applied to the payload body in order to form the message body.
1566   Transfer codings are defined in <xref target="transfer.codings"/>.
1568<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1569  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1572   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1573   MIME, which was designed to enable safe transport of binary data over a
1574   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1575   However, safe transport has a different focus for an 8bit-clean transfer
1576   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1577   accurately delimit a dynamically generated payload and to distinguish
1578   payload encodings that are only applied for transport efficiency or
1579   security from those that are characteristics of the selected resource.
1582   A recipient &MUST; be able to parse the chunked transfer coding
1583   (<xref target="chunked.encoding"/>) because it plays a crucial role in
1584   framing messages when the payload body size is not known in advance.
1585   A sender &MUST-NOT; apply chunked more than once to a message body
1586   (i.e., chunking an already chunked message is not allowed).
1587   If any transfer coding other than chunked is applied to a request payload
1588   body, the sender &MUST; apply chunked as the final transfer coding to
1589   ensure that the message is properly framed.
1590   If any transfer coding other than chunked is applied to a response payload
1591   body, the sender &MUST; either apply chunked as the final transfer coding
1592   or terminate the message by closing the connection.
1595   For example,
1596</preamble><artwork type="example">
1597  Transfer-Encoding: gzip, chunked
1599   indicates that the payload body has been compressed using the gzip
1600   coding and then chunked using the chunked coding while forming the
1601   message body.
1604   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1605   Transfer-Encoding is a property of the message, not of the representation, and
1606   any recipient along the request/response chain &MAY; decode the received
1607   transfer coding(s) or apply additional transfer coding(s) to the message
1608   body, assuming that corresponding changes are made to the Transfer-Encoding
1609   field-value. Additional information about the encoding parameters &MAY; be
1610   provided by other header fields not defined by this specification.
1613   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1614   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1615   neither of which includes a message body,
1616   to indicate that the origin server would have applied a transfer coding
1617   to the message body if the request had been an unconditional GET.
1618   This indication is not required, however, because any recipient on
1619   the response chain (including the origin server) can remove transfer
1620   codings when they are not needed.
1623   A server &MUST-NOT; send a Transfer-Encoding header field in any response
1624   with a status code of
1625   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1626   A server &MUST-NOT; send a Transfer-Encoding header field in any
1627   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1630   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1631   implementations advertising only HTTP/1.0 support will not understand
1632   how to process a transfer-encoded payload.
1633   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1634   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1635   might be in the form of specific user configuration or by remembering the
1636   version of a prior received response.
1637   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1638   the corresponding request indicates HTTP/1.1 (or later).
1641   A server that receives a request message with a transfer coding it does
1642   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref>.
1646<section title="Content-Length" anchor="header.content-length">
1647  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1648  <x:anchor-alias value="Content-Length"/>
1650   When a message does not have a <x:ref>Transfer-Encoding</x:ref> header
1651   field, a Content-Length header field can provide the anticipated size,
1652   as a decimal number of octets, for a potential payload body.
1653   For messages that do include a payload body, the Content-Length field-value
1654   provides the framing information necessary for determining where the body
1655   (and message) ends.  For messages that do not include a payload body, the
1656   Content-Length indicates the size of the selected representation
1657   (&representation;).
1659<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1660  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1663   An example is
1665<figure><artwork type="example">
1666  Content-Length: 3495
1669   A sender &MUST-NOT; send a Content-Length header field in any message that
1670   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1673   A user agent &SHOULD; send a Content-Length in a request message when no
1674   <x:ref>Transfer-Encoding</x:ref> is sent and the request method defines
1675   a meaning for an enclosed payload body. For example, a Content-Length
1676   header field is normally sent in a POST request even when the value is
1677   0 (indicating an empty payload body).  A user agent &SHOULD-NOT; send a
1678   Content-Length header field when the request message does not contain a
1679   payload body and the method semantics do not anticipate such a body.
1682   A server &MAY; send a Content-Length header field in a response to a HEAD
1683   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1684   response unless its field-value equals the decimal number of octets that
1685   would have been sent in the payload body of a response if the same
1686   request had used the GET method.
1689   A server &MAY; send a Content-Length header field in a
1690   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1691   (&status-304;); a server &MUST-NOT; send Content-Length in such a
1692   response unless its field-value equals the decimal number of octets that
1693   would have been sent in the payload body of a <x:ref>200 (OK)</x:ref>
1694   response to the same request.
1697   A server &MUST-NOT; send a Content-Length header field in any response
1698   with a status code of
1699   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1700   A server &MUST-NOT; send a Content-Length header field in any
1701   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1704   Aside from the cases defined above, in the absence of Transfer-Encoding,
1705   an origin server &SHOULD; send a Content-Length header field when the
1706   payload body size is known prior to sending the complete header section.
1707   This will allow downstream recipients to measure transfer progress,
1708   know when a received message is complete, and potentially reuse the
1709   connection for additional requests.
1712   Any Content-Length field value greater than or equal to zero is valid.
1713   Since there is no predefined limit to the length of a payload, a
1714   recipient &MUST; anticipate potentially large decimal numerals and
1715   prevent parsing errors due to integer conversion overflows
1716   (<xref target="attack.protocol.element.size.overflows"/>).
1719   If a message is received that has multiple Content-Length header fields
1720   with field-values consisting of the same decimal value, or a single
1721   Content-Length header field with a field value containing a list of
1722   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1723   duplicate Content-Length header fields have been generated or combined by an
1724   upstream message processor, then the recipient &MUST; either reject the
1725   message as invalid or replace the duplicated field-values with a single
1726   valid Content-Length field containing that decimal value prior to
1727   determining the message body length or forwarding the message.
1730  <t>
1731   &Note; HTTP's use of Content-Length for message framing differs
1732   significantly from the same field's use in MIME, where it is an optional
1733   field used only within the "message/external-body" media-type.
1734  </t>
1738<section title="Message Body Length" anchor="message.body.length">
1739  <iref item="chunked (Coding Format)"/>
1741   The length of a message body is determined by one of the following
1742   (in order of precedence):
1745  <list style="numbers">
1746    <x:lt><t>
1747     Any response to a HEAD request and any response with a
1748     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1749     <x:ref>304 (Not Modified)</x:ref> status code is always
1750     terminated by the first empty line after the header fields, regardless of
1751     the header fields present in the message, and thus cannot contain a
1752     message body.
1753    </t></x:lt>
1754    <x:lt><t>
1755     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1756     connection will become a tunnel immediately after the empty line that
1757     concludes the header fields.  A client &MUST; ignore any
1758     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1759     fields received in such a message.
1760    </t></x:lt>
1761    <x:lt><t>
1762     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1763     and the chunked transfer coding (<xref target="chunked.encoding"/>)
1764     is the final encoding, the message body length is determined by reading
1765     and decoding the chunked data until the transfer coding indicates the
1766     data is complete.
1767    </t>
1768    <t>
1769     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1770     response and the chunked transfer coding is not the final encoding, the
1771     message body length is determined by reading the connection until it is
1772     closed by the server.
1773     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a request and the
1774     chunked transfer coding is not the final encoding, the message body
1775     length cannot be determined reliably; the server &MUST; respond with
1776     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1777    </t>
1778    <t>
1779     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1780     and a <x:ref>Content-Length</x:ref> header field, the Transfer-Encoding
1781     overrides the Content-Length. Such a message might indicate an attempt
1782     to perform request or response smuggling (bypass of security-related
1783     checks on message routing or content) and thus ought to be handled as
1784     an error.  A sender &MUST; remove the received Content-Length field
1785     prior to forwarding such a message downstream.
1786    </t></x:lt>
1787    <x:lt><t>
1788     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1789     either multiple <x:ref>Content-Length</x:ref> header fields having
1790     differing field-values or a single Content-Length header field having an
1791     invalid value, then the message framing is invalid and
1792     the recipient &MUST; treat it as an unrecoverable error to prevent
1793     request or response smuggling.
1794     If this is a request message, the server &MUST; respond with
1795     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1796     If this is a response message received by a proxy,
1797     the proxy &MUST; close the connection to the server, discard the received
1798     response, and send a <x:ref>502 (Bad Gateway)</x:ref> response to the
1799     client.
1800     If this is a response message received by a user agent,
1801     the user agent &MUST; close the connection to the server and discard the
1802     received response.
1803    </t></x:lt>
1804    <x:lt><t>
1805     If a valid <x:ref>Content-Length</x:ref> header field is present without
1806     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1807     expected message body length in octets.
1808     If the sender closes the connection or the recipient times out before the
1809     indicated number of octets are received, the recipient &MUST; consider
1810     the message to be incomplete and close the connection.
1811    </t></x:lt>
1812    <x:lt><t>
1813     If this is a request message and none of the above are true, then the
1814     message body length is zero (no message body is present).
1815    </t></x:lt>
1816    <x:lt><t>
1817     Otherwise, this is a response message without a declared message body
1818     length, so the message body length is determined by the number of octets
1819     received prior to the server closing the connection.
1820    </t></x:lt>
1821  </list>
1824   Since there is no way to distinguish a successfully completed,
1825   close-delimited message from a partially-received message interrupted
1826   by network failure, a server &SHOULD; generate encoding or
1827   length-delimited messages whenever possible.  The close-delimiting
1828   feature exists primarily for backwards compatibility with HTTP/1.0.
1831   A server &MAY; reject a request that contains a message body but
1832   not a <x:ref>Content-Length</x:ref> by responding with
1833   <x:ref>411 (Length Required)</x:ref>.
1836   Unless a transfer coding other than chunked has been applied,
1837   a client that sends a request containing a message body &SHOULD;
1838   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1839   length is known in advance, rather than the chunked transfer coding, since some
1840   existing services respond to chunked with a <x:ref>411 (Length Required)</x:ref>
1841   status code even though they understand the chunked transfer coding.  This
1842   is typically because such services are implemented via a gateway that
1843   requires a content-length in advance of being called and the server
1844   is unable or unwilling to buffer the entire request before processing.
1847   A user agent that sends a request containing a message body &MUST; send a
1848   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1849   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1850   the form of specific user configuration or by remembering the version of a
1851   prior received response.
1854   If the final response to the last request on a connection has been
1855   completely received and there remains additional data to read, a user agent
1856   &MAY; discard the remaining data or attempt to determine if that data
1857   belongs as part of the prior response body, which might be the case if the
1858   prior message's Content-Length value is incorrect. A client &MUST-NOT;
1859   process, cache, or forward such extra data as a separate response, since
1860   such behavior would be vulnerable to cache poisoning.
1865<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1867   A server that receives an incomplete request message, usually due to a
1868   canceled request or a triggered time-out exception, &MAY; send an error
1869   response prior to closing the connection.
1872   A client that receives an incomplete response message, which can occur
1873   when a connection is closed prematurely or when decoding a supposedly
1874   chunked transfer coding fails, &MUST; record the message as incomplete.
1875   Cache requirements for incomplete responses are defined in
1876   &cache-incomplete;.
1879   If a response terminates in the middle of the header section (before the
1880   empty line is received) and the status code might rely on header fields to
1881   convey the full meaning of the response, then the client cannot assume
1882   that meaning has been conveyed; the client might need to repeat the
1883   request in order to determine what action to take next.
1886   A message body that uses the chunked transfer coding is
1887   incomplete if the zero-sized chunk that terminates the encoding has not
1888   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1889   incomplete if the size of the message body received (in octets) is less than
1890   the value given by Content-Length.  A response that has neither chunked
1891   transfer coding nor Content-Length is terminated by closure of the
1892   connection, and thus is considered complete regardless of the number of
1893   message body octets received, provided that the header section was received
1894   intact.
1898<section title="Message Parsing Robustness" anchor="message.robustness">
1900   Older HTTP/1.0 user agent implementations might send an extra CRLF
1901   after a POST request as a workaround for some early server
1902   applications that failed to read message body content that was
1903   not terminated by a line-ending. An HTTP/1.1 user agent &MUST-NOT;
1904   preface or follow a request with an extra CRLF.  If terminating
1905   the request message body with a line-ending is desired, then the
1906   user agent &MUST; count the terminating CRLF octets as part of the
1907   message body length.
1910   In the interest of robustness, a server that is expecting to receive and
1911   parse a request-line &SHOULD; ignore at least one empty line (CRLF)
1912   received prior to the request-line.
1915   Although the line terminator for the start-line and header
1916   fields is the sequence CRLF, a recipient &MAY; recognize a
1917   single LF as a line terminator and ignore any preceding CR.
1920   Although the request-line and status-line grammar rules require that each
1921   of the component elements be separated by a single SP octet, recipients
1922   &MAY; instead parse on whitespace-delimited word boundaries and, aside
1923   from the CRLF terminator, treat any form of whitespace as the SP separator
1924   while ignoring preceding or trailing whitespace;
1925   such whitespace includes one or more of the following octets:
1926   SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
1929   When a server listening only for HTTP request messages, or processing
1930   what appears from the start-line to be an HTTP request message,
1931   receives a sequence of octets that does not match the HTTP-message
1932   grammar aside from the robustness exceptions listed above, the
1933   server &SHOULD; respond with a <x:ref>400 (Bad Request)</x:ref> response. 
1938<section title="Transfer Codings" anchor="transfer.codings">
1939  <x:anchor-alias value="transfer-coding"/>
1940  <x:anchor-alias value="transfer-extension"/>
1942   Transfer coding names are used to indicate an encoding
1943   transformation that has been, can be, or might need to be applied to a
1944   payload body in order to ensure "safe transport" through the network.
1945   This differs from a content coding in that the transfer coding is a
1946   property of the message rather than a property of the representation
1947   that is being transferred.
1949<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1950  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1951                     / "compress" ; <xref target="compress.coding"/>
1952                     / "deflate" ; <xref target="deflate.coding"/>
1953                     / "gzip" ; <xref target="gzip.coding"/>
1954                     / <x:ref>transfer-extension</x:ref>
1955  <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> )
1957<t anchor="rule.parameter">
1958  <x:anchor-alias value="transfer-parameter"/>
1959   Parameters are in the form of a name or name=value pair.
1961<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-parameter"/>
1962  <x:ref>transfer-parameter</x:ref> = <x:ref>token</x:ref> <x:ref>BWS</x:ref> "=" <x:ref>BWS</x:ref> ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> )
1965   All transfer-coding names are case-insensitive and ought to be registered
1966   within the HTTP Transfer Coding registry, as defined in
1967   <xref target="transfer.coding.registry"/>.
1968   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1969   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1970   header fields.
1973<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1974  <iref primary="true" item="chunked (Coding Format)"/>
1975  <x:anchor-alias value="chunk"/>
1976  <x:anchor-alias value="chunked-body"/>
1977  <x:anchor-alias value="chunk-data"/>
1978  <x:anchor-alias value="chunk-size"/>
1979  <x:anchor-alias value="last-chunk"/>
1981   The chunked transfer coding wraps the payload body in order to transfer it
1982   as a series of chunks, each with its own size indicator, followed by an
1983   &OPTIONAL; trailer containing header fields. Chunked enables content
1984   streams of unknown size to be transferred as a sequence of length-delimited
1985   buffers, which enables the sender to retain connection persistence and the
1986   recipient to know when it has received the entire message.
1988<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="false" item="Grammar" subitem="trailer-part"/>
1989  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1990                   <x:ref>last-chunk</x:ref>
1991                   <x:ref>trailer-part</x:ref>
1992                   <x:ref>CRLF</x:ref>
1994  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1995                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1996  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1997  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1999  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
2002   The chunk-size field is a string of hex digits indicating the size of
2003   the chunk-data in octets. The chunked transfer coding is complete when a
2004   chunk with a chunk-size of zero is received, possibly followed by a
2005   trailer, and finally terminated by an empty line.
2008   A recipient &MUST; be able to parse and decode the chunked transfer coding.
2011<section title="Chunk Extensions" anchor="chunked.extension">
2012  <x:anchor-alias value="chunk-ext"/>
2013  <x:anchor-alias value="chunk-ext-name"/>
2014  <x:anchor-alias value="chunk-ext-val"/>
2016   The chunked encoding allows each chunk to include zero or more chunk
2017   extensions, immediately following the <x:ref>chunk-size</x:ref>, for the
2018   sake of supplying per-chunk metadata (such as a signature or hash),
2019   mid-message control information, or randomization of message body size.
2021<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="false" item="Grammar" subitem="trailer-part"/><iref primary="true" item="Grammar" subitem="quoted-str-nf"/><iref primary="true" item="Grammar" subitem="qdtext-nf"/>
2022  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
2024  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
2025  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
2028   The chunked encoding is specific to each connection and is likely to be
2029   removed or recoded by each recipient (including intermediaries) before any
2030   higher-level application would have a chance to inspect the extensions.
2031   Hence, use of chunk extensions is generally limited to specialized HTTP
2032   services such as "long polling" (where client and server can have shared
2033   expectations regarding the use of chunk extensions) or for padding within
2034   an end-to-end secured connection.
2037   A recipient &MUST; ignore unrecognized chunk extensions.
2038   A server ought to limit the total length of chunk extensions received in a
2039   request to an amount reasonable for the services provided, in the same way
2040   that it applies length limitations and timeouts for other parts of a
2041   message, and generate an appropriate <x:ref>4xx (Client Error)</x:ref>
2042   response if that amount is exceeded.
2046<section title="Chunked Trailer Part" anchor="chunked.trailer.part">
2047  <x:anchor-alias value="trailer-part"/>
2049   A trailer allows the sender to include additional fields at the end of a
2050   chunked message in order to supply metadata that might be dynamically
2051   generated while the message body is sent, such as a message integrity
2052   check, digital signature, or post-processing status. The trailer fields are
2053   identical to header fields, except they are sent in a chunked trailer
2054   instead of the message's header section.
2056<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="trailer-part"/>
2057  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
2060   A sender &MUST-NOT; generate a trailer that contains a field which needs to
2061   be known by the recipient before it can begin processing the message body.
2062   For example, most recipients need to know the values of
2063   <x:ref>Content-Encoding</x:ref> and <x:ref>Content-Type</x:ref> in order to
2064   select a content handler, so placing those fields in a trailer would force
2065   the recipient to buffer the entire body before it could begin, greatly
2066   increasing user-perceived latency and defeating one of the main advantages
2067   of using chunked to send data streams of unknown length.
2068   A sender &MUST-NOT; generate a trailer containing a
2069   <x:ref>Transfer-Encoding</x:ref>,
2070   <x:ref>Content-Length</x:ref>, or
2071   <x:ref>Trailer</x:ref> field.
2074   A server &MUST; generate an empty trailer with the chunked transfer coding
2075   unless at least one of the following is true:
2076  <list style="numbers">
2077    <t>the request included a <x:ref>TE</x:ref> header field that indicates
2078    "trailers" is acceptable in the transfer coding of the response, as
2079    described in <xref target="header.te"/>; or,</t>
2081    <t>the trailer fields consist entirely of optional metadata and the
2082    recipient could use the message (in a manner acceptable to the generating
2083    server) without receiving that metadata. In other words, the generating
2084    server is willing to accept the possibility that the trailer fields might
2085    be silently discarded along the path to the client.</t>
2086  </list>
2089   The above requirement prevents the need for an infinite buffer when a
2090   message is being received by an HTTP/1.1 (or later) proxy and forwarded to
2091   an HTTP/1.0 recipient.
2095<section title="Decoding Chunked" anchor="decoding.chunked">
2097   A process for decoding the chunked transfer coding
2098   can be represented in pseudo-code as:
2100<figure><artwork type="code">
2101  length := 0
2102  read chunk-size, chunk-ext (if any), and CRLF
2103  while (chunk-size &gt; 0) {
2104     read chunk-data and CRLF
2105     append chunk-data to decoded-body
2106     length := length + chunk-size
2107     read chunk-size, chunk-ext (if any), and CRLF
2108  }
2109  read header-field
2110  while (header-field not empty) {
2111     append header-field to existing header fields
2112     read header-field
2113  }
2114  Content-Length := length
2115  Remove "chunked" from Transfer-Encoding
2116  Remove Trailer from existing header fields
2121<section title="Compression Codings" anchor="compression.codings">
2123   The codings defined below can be used to compress the payload of a
2124   message.
2127<section title="Compress Coding" anchor="compress.coding">
2128<iref item="compress (Coding Format)"/>
2130   The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding
2131   <xref target="Welch"/> that is commonly produced by the UNIX file
2132   compression program "compress".
2133   A recipient &SHOULD; consider "x-compress" to be equivalent to "compress".
2137<section title="Deflate Coding" anchor="deflate.coding">
2138<iref item="deflate (Coding Format)"/>
2140   The "deflate" coding is a "zlib" data format <xref target="RFC1950"/>
2141   containing a "deflate" compressed data stream <xref target="RFC1951"/>
2142   that uses a combination of the Lempel-Ziv (LZ77) compression algorithm and
2143   Huffman coding.
2146  <t>
2147    &Note; Some incorrect implementations send the "deflate"
2148    compressed data without the zlib wrapper.
2149   </t>
2153<section title="Gzip Coding" anchor="gzip.coding">
2154<iref item="gzip (Coding Format)"/>
2156   The "gzip" coding is an LZ77 coding with a 32 bit CRC that is commonly
2157   produced by the gzip file compression program <xref target="RFC1952"/>.
2158   A recipient &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2164<section title="TE" anchor="header.te">
2165  <iref primary="true" item="TE header field" x:for-anchor=""/>
2166  <x:anchor-alias value="TE"/>
2167  <x:anchor-alias value="t-codings"/>
2168  <x:anchor-alias value="t-ranking"/>
2169  <x:anchor-alias value="rank"/>
2171   The "TE" header field in a request indicates what transfer codings,
2172   besides chunked, the client is willing to accept in response, and
2173   whether or not the client is willing to accept trailer fields in a
2174   chunked transfer coding.
2177   The TE field-value consists of a comma-separated list of transfer coding
2178   names, each allowing for optional parameters (as described in
2179   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2180   A client &MUST-NOT; send the chunked transfer coding name in TE;
2181   chunked is always acceptable for HTTP/1.1 recipients.
2183<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"/>
2184  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2185  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2186  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2187  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2188             / ( "1" [ "." 0*3("0") ] )
2191   Three examples of TE use are below.
2193<figure><artwork type="example">
2194  TE: deflate
2195  TE:
2196  TE: trailers, deflate;q=0.5
2199   The presence of the keyword "trailers" indicates that the client is willing
2200   to accept trailer fields in a chunked transfer coding, as defined in
2201   <xref target="chunked.trailer.part"/>, on behalf of itself and any downstream
2202   clients. For requests from an intermediary, this implies that either:
2203   (a) all downstream clients are willing to accept trailer fields in the
2204   forwarded response; or,
2205   (b) the intermediary will attempt to buffer the response on behalf of
2206   downstream recipients.
2207   Note that HTTP/1.1 does not define any means to limit the size of a
2208   chunked response such that an intermediary can be assured of buffering the
2209   entire response.
2212   When multiple transfer codings are acceptable, the client &MAY; rank the
2213   codings by preference using a case-insensitive "q" parameter (similar to
2214   the qvalues used in content negotiation fields, &qvalue;). The rank value
2215   is a real number in the range 0 through 1, where 0.001 is the least
2216   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2219   If the TE field-value is empty or if no TE field is present, the only
2220   acceptable transfer coding is chunked. A message with no transfer coding
2221   is always acceptable.
2224   Since the TE header field only applies to the immediate connection,
2225   a sender of TE &MUST; also send a "TE" connection option within the
2226   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2227   in order to prevent the TE field from being forwarded by intermediaries
2228   that do not support its semantics.
2232<section title="Trailer" anchor="header.trailer">
2233  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
2234  <x:anchor-alias value="Trailer"/>
2236   When a message includes a message body encoded with the chunked
2237   transfer coding and the sender desires to send metadata in the form of
2238   trailer fields at the end of the message, the sender &SHOULD; generate a
2239   <x:ref>Trailer</x:ref> header field before the message body to indicate
2240   which fields will be present in the trailers. This allows the recipient
2241   to prepare for receipt of that metadata before it starts processing the body,
2242   which is useful if the message is being streamed and the recipient wishes
2243   to confirm an integrity check on the fly.
2245<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
2246  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
2251<section title="Message Routing" anchor="message.routing">
2253   HTTP request message routing is determined by each client based on the
2254   target resource, the client's proxy configuration, and
2255   establishment or reuse of an inbound connection.  The corresponding
2256   response routing follows the same connection chain back to the client.
2259<section title="Identifying a Target Resource" anchor="target-resource">
2260  <iref primary="true" item="target resource"/>
2261  <iref primary="true" item="target URI"/>
2262  <x:anchor-alias value="target resource"/>
2263  <x:anchor-alias value="target URI"/>
2265   HTTP is used in a wide variety of applications, ranging from
2266   general-purpose computers to home appliances.  In some cases,
2267   communication options are hard-coded in a client's configuration.
2268   However, most HTTP clients rely on the same resource identification
2269   mechanism and configuration techniques as general-purpose Web browsers.
2272   HTTP communication is initiated by a user agent for some purpose.
2273   The purpose is a combination of request semantics, which are defined in
2274   <xref target="Part2"/>, and a target resource upon which to apply those
2275   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2276   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2277   would resolve to its absolute form in order to obtain the
2278   "<x:dfn>target URI</x:dfn>".  The target URI
2279   excludes the reference's fragment component, if any,
2280   since fragment identifiers are reserved for client-side processing
2281   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2285<section title="Connecting Inbound" anchor="connecting.inbound">
2287   Once the target URI is determined, a client needs to decide whether
2288   a network request is necessary to accomplish the desired semantics and,
2289   if so, where that request is to be directed.
2292   If the client has a cache <xref target="Part6"/> and the request can be
2293   satisfied by it, then the request is
2294   usually directed there first.
2297   If the request is not satisfied by a cache, then a typical client will
2298   check its configuration to determine whether a proxy is to be used to
2299   satisfy the request.  Proxy configuration is implementation-dependent,
2300   but is often based on URI prefix matching, selective authority matching,
2301   or both, and the proxy itself is usually identified by an "http" or
2302   "https" URI.  If a proxy is applicable, the client connects inbound by
2303   establishing (or reusing) a connection to that proxy.
2306   If no proxy is applicable, a typical client will invoke a handler routine,
2307   usually specific to the target URI's scheme, to connect directly
2308   to an authority for the target resource.  How that is accomplished is
2309   dependent on the target URI scheme and defined by its associated
2310   specification, similar to how this specification defines origin server
2311   access for resolution of the "http" (<xref target="http.uri"/>) and
2312   "https" (<xref target="https.uri"/>) schemes.
2315   HTTP requirements regarding connection management are defined in
2316   <xref target=""/>.
2320<section title="Request Target" anchor="request-target">
2322   Once an inbound connection is obtained,
2323   the client sends an HTTP request message (<xref target="http.message"/>)
2324   with a request-target derived from the target URI.
2325   There are four distinct formats for the request-target, depending on both
2326   the method being requested and whether the request is to a proxy.
2328<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"/>
2329  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2330                 / <x:ref>absolute-form</x:ref>
2331                 / <x:ref>authority-form</x:ref>
2332                 / <x:ref>asterisk-form</x:ref>
2334  <x:ref>origin-form</x:ref>    = <x:ref>absolute-path</x:ref> [ "?" <x:ref>query</x:ref> ]
2335  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2336  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2337  <x:ref>asterisk-form</x:ref>  = "*"
2339<t anchor="origin-form"><iref item="origin-form (of request-target)"/>
2340  <x:h>origin-form</x:h>
2343   The most common form of request-target is the <x:dfn>origin-form</x:dfn>.
2344   When making a request directly to an origin server, other than a CONNECT
2345   or server-wide OPTIONS request (as detailed below),
2346   a client &MUST; send only the absolute path and query components of
2347   the target URI as the request-target.
2348   If the target URI's path component is empty, then the client &MUST; send
2349   "/" as the path within the origin-form of request-target.
2350   A <x:ref>Host</x:ref> header field is also sent, as defined in
2351   <xref target=""/>.
2354   For example, a client wishing to retrieve a representation of the resource
2355   identified as
2357<figure><artwork x:indent-with="  " type="example">
2361   directly from the origin server would open (or reuse) a TCP connection
2362   to port 80 of the host "" and send the lines:
2364<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2365GET /where?q=now HTTP/1.1
2369   followed by the remainder of the request message.
2371<t anchor="absolute-form"><iref item="absolute-form (of request-target)"/>
2372  <x:h>absolute-form</x:h>
2375   When making a request to a proxy, other than a CONNECT or server-wide
2376   OPTIONS request (as detailed below), a client &MUST; send the target URI
2377   in <x:dfn>absolute-form</x:dfn> as the request-target.
2378   The proxy is requested to either service that request from a valid cache,
2379   if possible, or make the same request on the client's behalf to either
2380   the next inbound proxy server or directly to the origin server indicated
2381   by the request-target.  Requirements on such "forwarding" of messages are
2382   defined in <xref target="message.forwarding"/>.
2385   An example absolute-form of request-line would be:
2387<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2388GET HTTP/1.1
2391   To allow for transition to the absolute-form for all requests in some
2392   future version of HTTP, a server &MUST; accept the absolute-form
2393   in requests, even though HTTP/1.1 clients will only send them in requests
2394   to proxies.
2396<t anchor="authority-form"><iref item="authority-form (of request-target)"/>
2397  <x:h>authority-form</x:h>
2400   The <x:dfn>authority-form</x:dfn> of request-target is only used for
2401   CONNECT requests (&CONNECT;). When making a CONNECT request to establish a
2402   tunnel through one or more proxies, a client &MUST; send only the target
2403   URI's authority component (excluding any userinfo and its "@" delimiter) as
2404   the request-target. For example,
2406<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2409<t anchor="asterisk-form"><iref item="asterisk-form (of request-target)"/>
2410  <x:h>asterisk-form</x:h>
2413   The <x:dfn>asterisk-form</x:dfn> of request-target is only used for a server-wide
2414   OPTIONS request (&OPTIONS;).  When a client wishes to request OPTIONS
2415   for the server as a whole, as opposed to a specific named resource of
2416   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2417   For example,
2419<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2420OPTIONS * HTTP/1.1
2423   If a proxy receives an OPTIONS request with an absolute-form of
2424   request-target in which the URI has an empty path and no query component,
2425   then the last proxy on the request chain &MUST; send a request-target
2426   of "*" when it forwards the request to the indicated origin server.
2429   For example, the request
2430</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2434  would be forwarded by the final proxy as
2435</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2436OPTIONS * HTTP/1.1
2440   after connecting to port 8001 of host "".
2445<section title="Host" anchor="">
2446  <iref primary="true" item="Host header field" x:for-anchor=""/>
2447  <x:anchor-alias value="Host"/>
2449   The "Host" header field in a request provides the host and port
2450   information from the target URI, enabling the origin
2451   server to distinguish among resources while servicing requests
2452   for multiple host names on a single IP address.
2454<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2455  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2458   A client &MUST; send a Host header field in all HTTP/1.1 request messages.
2459   If the target URI includes an authority component, then a client &MUST;
2460   send a field-value for Host that is identical to that authority
2461   component, excluding any userinfo subcomponent and its "@" delimiter
2462   (<xref target="http.uri"/>).
2463   If the authority component is missing or undefined for the target URI,
2464   then a client &MUST; send a Host header field with an empty field-value.
2467   Since the Host field-value is critical information for handling a request,
2468   a user agent &SHOULD; generate Host as the first header field following the
2469   request-line.
2472   For example, a GET request to the origin server for
2473   &lt;; would begin with:
2475<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2476GET /pub/WWW/ HTTP/1.1
2480   A client &MUST; send a Host header field in an HTTP/1.1 request even
2481   if the request-target is in the absolute-form, since this
2482   allows the Host information to be forwarded through ancient HTTP/1.0
2483   proxies that might not have implemented Host.
2486   When a proxy receives a request with an absolute-form of
2487   request-target, the proxy &MUST; ignore the received
2488   Host header field (if any) and instead replace it with the host
2489   information of the request-target.  A proxy that forwards such a request
2490   &MUST; generate a new Host field-value based on the received
2491   request-target rather than forward the received Host field-value.
2494   Since the Host header field acts as an application-level routing
2495   mechanism, it is a frequent target for malware seeking to poison
2496   a shared cache or redirect a request to an unintended server.
2497   An interception proxy is particularly vulnerable if it relies on
2498   the Host field-value for redirecting requests to internal
2499   servers, or for use as a cache key in a shared cache, without
2500   first verifying that the intercepted connection is targeting a
2501   valid IP address for that host.
2504   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2505   to any HTTP/1.1 request message that lacks a Host header field and
2506   to any request message that contains more than one Host header field
2507   or a Host header field with an invalid field-value.
2511<section title="Effective Request URI" anchor="effective.request.uri">
2512  <iref primary="true" item="effective request URI"/>
2513  <x:anchor-alias value="effective request URI"/>
2515   A server that receives an HTTP request message &MUST; reconstruct
2516   the user agent's original target URI, based on the pieces of information
2517   learned from the request-target, <x:ref>Host</x:ref> header field, and
2518   connection context, in order to identify the intended target resource and
2519   properly service the request. The URI derived from this reconstruction
2520   process is referred to as the "<x:dfn>effective request URI</x:dfn>".
2523   For a user agent, the effective request URI is the target URI.
2526   If the request-target is in absolute-form, then the effective request URI
2527   is the same as the request-target.  Otherwise, the effective request URI
2528   is constructed as follows.
2531   If the request is received over a TLS-secured TCP connection,
2532   then the effective request URI's scheme is "https"; otherwise, the
2533   scheme is "http".
2536   If the request-target is in authority-form, then the effective
2537   request URI's authority component is the same as the request-target.
2538   Otherwise, if a <x:ref>Host</x:ref> header field is supplied with a
2539   non-empty field-value, then the authority component is the same as the
2540   Host field-value. Otherwise, the authority component is the concatenation of
2541   the default host name configured for the server, a colon (":"), and the
2542   connection's incoming TCP port number in decimal form.
2545   If the request-target is in authority-form or asterisk-form, then the
2546   effective request URI's combined path and query component is empty.
2547   Otherwise, the combined path and query component is the same as the
2548   request-target.
2551   The components of the effective request URI, once determined as above,
2552   can be combined into absolute-URI form by concatenating the scheme,
2553   "://", authority, and combined path and query component.
2557   Example 1: the following message received over an insecure TCP connection
2559<artwork type="example" x:indent-with="  ">
2560GET /pub/WWW/TheProject.html HTTP/1.1
2566  has an effective request URI of
2568<artwork type="example" x:indent-with="  ">
2574   Example 2: the following message received over a TLS-secured TCP connection
2576<artwork type="example" x:indent-with="  ">
2577OPTIONS * HTTP/1.1
2583  has an effective request URI of
2585<artwork type="example" x:indent-with="  ">
2590   An origin server that does not allow resources to differ by requested
2591   host &MAY; ignore the <x:ref>Host</x:ref> field-value and instead replace it
2592   with a configured server name when constructing the effective request URI.
2595   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2596   field &MAY; attempt to use heuristics (e.g., examination of the URI path for
2597   something unique to a particular host) in order to guess the
2598   effective request URI's authority component.
2602<section title="Associating a Response to a Request" anchor="">
2604   HTTP does not include a request identifier for associating a given
2605   request message with its corresponding one or more response messages.
2606   Hence, it relies on the order of response arrival to correspond exactly
2607   to the order in which requests are made on the same connection.
2608   More than one response message per request only occurs when one or more
2609   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a
2610   final response to the same request.
2613   A client that has more than one outstanding request on a connection &MUST;
2614   maintain a list of outstanding requests in the order sent and &MUST;
2615   associate each received response message on that connection to the highest
2616   ordered request that has not yet received a final (non-<x:ref>1xx</x:ref>)
2617   response.
2621<section title="Message Forwarding" anchor="message.forwarding">
2623   As described in <xref target="intermediaries"/>, intermediaries can serve
2624   a variety of roles in the processing of HTTP requests and responses.
2625   Some intermediaries are used to improve performance or availability.
2626   Others are used for access control or to filter content.
2627   Since an HTTP stream has characteristics similar to a pipe-and-filter
2628   architecture, there are no inherent limits to the extent an intermediary
2629   can enhance (or interfere) with either direction of the stream.
2632   An intermediary not acting as a tunnel &MUST; implement the
2633   <x:ref>Connection</x:ref> header field, as specified in
2634   <xref target="header.connection"/>, and exclude fields from being forwarded
2635   that are only intended for the incoming connection.
2638   An intermediary &MUST-NOT; forward a message to itself unless it is
2639   protected from an infinite request loop. In general, an intermediary ought
2640   to recognize its own server names, including any aliases, local variations,
2641   or literal IP addresses, and respond to such requests directly.
2644<section title="Via" anchor="header.via">
2645  <iref primary="true" item="Via header field" x:for-anchor=""/>
2646  <x:anchor-alias value="pseudonym"/>
2647  <x:anchor-alias value="received-by"/>
2648  <x:anchor-alias value="received-protocol"/>
2649  <x:anchor-alias value="Via"/>
2651   The "Via" header field indicates the presence of intermediate protocols and
2652   recipients between the user agent and the server (on requests) or between
2653   the origin server and the client (on responses), similar to the
2654   "Received" header field in email
2655   (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2656   Via can be used for tracking message forwards,
2657   avoiding request loops, and identifying the protocol capabilities of
2658   senders along the request/response chain.
2660<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"/>
2661  <x:ref>Via</x:ref> = 1#( <x:ref>received-protocol</x:ref> <x:ref>RWS</x:ref> <x:ref>received-by</x:ref> [ <x:ref>RWS</x:ref> <x:ref>comment</x:ref> ] )
2663  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2664                      ; see <xref target="header.upgrade"/>
2665  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2666  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2669   Multiple Via field values represent each proxy or gateway that has
2670   forwarded the message. Each intermediary appends its own information
2671   about how the message was received, such that the end result is ordered
2672   according to the sequence of forwarding recipients.
2675   A proxy &MUST; send an appropriate Via header field, as described below, in
2676   each message that it forwards.
2677   An HTTP-to-HTTP gateway &MUST; send an appropriate Via header field in
2678   each inbound request message and &MAY; send a Via header field in
2679   forwarded response messages.
2682   For each intermediary, the received-protocol indicates the protocol and
2683   protocol version used by the upstream sender of the message. Hence, the
2684   Via field value records the advertised protocol capabilities of the
2685   request/response chain such that they remain visible to downstream
2686   recipients; this can be useful for determining what backwards-incompatible
2687   features might be safe to use in response, or within a later request, as
2688   described in <xref target="http.version"/>. For brevity, the protocol-name
2689   is omitted when the received protocol is HTTP.
2692   The received-by portion of the field value is normally the host and optional
2693   port number of a recipient server or client that subsequently forwarded the
2694   message.
2695   However, if the real host is considered to be sensitive information, a
2696   sender &MAY; replace it with a pseudonym. If a port is not provided,
2697   a recipient &MAY; interpret that as meaning it was received on the default
2698   TCP port, if any, for the received-protocol.
2701   A sender &MAY; generate comments in the Via header field to identify the
2702   software of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2703   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2704   are optional and a recipient &MAY; remove them prior to forwarding the
2705   message.
2708   For example, a request message could be sent from an HTTP/1.0 user
2709   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2710   forward the request to a public proxy at, which completes
2711   the request by forwarding it to the origin server at
2712   The request received by would then have the following
2713   Via header field:
2715<figure><artwork type="example">
2716  Via: 1.0 fred, 1.1
2719   An intermediary used as a portal through a network firewall
2720   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2721   region unless it is explicitly enabled to do so. If not enabled, such an
2722   intermediary &SHOULD; replace each received-by host of any host behind the
2723   firewall by an appropriate pseudonym for that host.
2726   An intermediary &MAY; combine an ordered subsequence of Via header
2727   field entries into a single such entry if the entries have identical
2728   received-protocol values. For example,
2730<figure><artwork type="example">
2731  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2734  could be collapsed to
2736<figure><artwork type="example">
2737  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2740   A sender &SHOULD-NOT; combine multiple entries unless they are all
2741   under the same organizational control and the hosts have already been
2742   replaced by pseudonyms. A sender &MUST-NOT; combine entries that
2743   have different received-protocol values.
2747<section title="Transformations" anchor="message.transformations">
2749   Some intermediaries include features for transforming messages and their
2750   payloads.  A transforming proxy might, for example, convert between image
2751   formats in order to save cache space or to reduce the amount of traffic on
2752   a slow link. However, operational problems might occur when these
2753   transformations are applied to payloads intended for critical applications,
2754   such as medical imaging or scientific data analysis, particularly when
2755   integrity checks or digital signatures are used to ensure that the payload
2756   received is identical to the original.
2759   If a proxy receives a request-target with a host name that is not a
2760   fully qualified domain name, it &MAY; add its own domain to the host name
2761   it received when forwarding the request.  A proxy &MUST-NOT; change the
2762   host name if it is a fully qualified domain name.
2765   A proxy &MUST-NOT; modify the "absolute-path" and "query" parts of the
2766   received request-target when forwarding it to the next inbound server,
2767   except as noted above to replace an empty path with "/" or "*".
2770   A proxy &MUST-NOT; modify header fields that provide information about the
2771   end points of the communication chain, the resource state, or the selected
2772   representation. A proxy &MAY; change the message body through application
2773   or removal of a transfer coding (<xref target="transfer.codings"/>).
2776   A non-transforming proxy &MUST-NOT; modify the message payload (&payload;).
2777   A transforming proxy &MUST-NOT; modify the payload of a message that
2778   contains the no-transform cache-control directive (&header-cache-control;).
2781   A transforming proxy &MAY; transform the payload of a message
2782   that does not contain the no-transform cache-control directive;
2783   if the payload is transformed, the transforming proxy &MUST; add a
2784   Warning header field with the warn-code of 214 ("Transformation Applied")
2785   if one does not already appear in the message (see &header-warning;).
2786   If the payload of a <x:ref>200 (OK)</x:ref> response is transformed, the
2787   transforming proxy can also inform downstream recipients that a
2788   transformation has been applied by changing the response status code to
2789   <x:ref>203 (Non-Authoritative Information)</x:ref> (&status-203;).
2795<section title="Connection Management" anchor="">
2797   HTTP messaging is independent of the underlying transport or
2798   session-layer connection protocol(s).  HTTP only presumes a reliable
2799   transport with in-order delivery of requests and the corresponding
2800   in-order delivery of responses.  The mapping of HTTP request and
2801   response structures onto the data units of an underlying transport
2802   protocol is outside the scope of this specification.
2805   As described in <xref target="connecting.inbound"/>, the specific
2806   connection protocols to be used for an HTTP interaction are determined by
2807   client configuration and the <x:ref>target URI</x:ref>.
2808   For example, the "http" URI scheme
2809   (<xref target="http.uri"/>) indicates a default connection of TCP
2810   over IP, with a default TCP port of 80, but the client might be
2811   configured to use a proxy via some other connection, port, or protocol.
2814   HTTP implementations are expected to engage in connection management,
2815   which includes maintaining the state of current connections,
2816   establishing a new connection or reusing an existing connection,
2817   processing messages received on a connection, detecting connection
2818   failures, and closing each connection.
2819   Most clients maintain multiple connections in parallel, including
2820   more than one connection per server endpoint.
2821   Most servers are designed to maintain thousands of concurrent connections,
2822   while controlling request queues to enable fair use and detect
2823   denial of service attacks.
2826<section title="Connection" anchor="header.connection">
2827  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2828  <iref primary="true" item="close" x:for-anchor=""/>
2829  <x:anchor-alias value="Connection"/>
2830  <x:anchor-alias value="connection-option"/>
2831  <x:anchor-alias value="close"/>
2833   The "Connection" header field allows the sender to indicate desired
2834   control options for the current connection.  In order to avoid confusing
2835   downstream recipients, a proxy or gateway &MUST; remove or replace any
2836   received connection options before forwarding the message.
2839   When a header field aside from Connection is used to supply control
2840   information for or about the current connection, the sender &MUST; list
2841   the corresponding field-name within the "Connection" header field.
2842   A proxy or gateway &MUST; parse a received Connection
2843   header field before a message is forwarded and, for each
2844   connection-option in this field, remove any header field(s) from
2845   the message with the same name as the connection-option, and then
2846   remove the Connection header field itself (or replace it with the
2847   intermediary's own connection options for the forwarded message).
2850   Hence, the Connection header field provides a declarative way of
2851   distinguishing header fields that are only intended for the
2852   immediate recipient ("hop-by-hop") from those fields that are
2853   intended for all recipients on the chain ("end-to-end"), enabling the
2854   message to be self-descriptive and allowing future connection-specific
2855   extensions to be deployed without fear that they will be blindly
2856   forwarded by older intermediaries.
2859   The Connection header field's value has the following grammar:
2861<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2862  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2863  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2866   Connection options are case-insensitive.
2869   A sender &MUST-NOT; send a connection option corresponding to a header
2870   field that is intended for all recipients of the payload.
2871   For example, <x:ref>Cache-Control</x:ref> is never appropriate as a
2872   connection option (&header-cache-control;).
2875   The connection options do not always correspond to a header field
2876   present in the message, since a connection-specific header field
2877   might not be needed if there are no parameters associated with a
2878   connection option. In contrast, a connection-specific header field that
2879   is received without a corresponding connection option usually indicates
2880   that the field has been improperly forwarded by an intermediary and
2881   ought to be ignored by the recipient.
2884   When defining new connection options, specification authors ought to survey
2885   existing header field names and ensure that the new connection option does
2886   not share the same name as an already deployed header field.
2887   Defining a new connection option essentially reserves that potential
2888   field-name for carrying additional information related to the
2889   connection option, since it would be unwise for senders to use
2890   that field-name for anything else.
2893   The "<x:dfn>close</x:dfn>" connection option is defined for a
2894   sender to signal that this connection will be closed after completion of
2895   the response. For example,
2897<figure><artwork type="example">
2898  Connection: close
2901   in either the request or the response header fields indicates that the
2902   sender is going to close the connection after the current request/response
2903   is complete (<xref target="persistent.tear-down"/>).
2906   A client that does not support <x:ref>persistent connections</x:ref> &MUST;
2907   send the "close" connection option in every request message.
2910   A server that does not support <x:ref>persistent connections</x:ref> &MUST;
2911   send the "close" connection option in every response message that
2912   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2916<section title="Establishment" anchor="persistent.establishment">
2918   It is beyond the scope of this specification to describe how connections
2919   are established via various transport or session-layer protocols.
2920   Each connection applies to only one transport link.
2924<section title="Persistence" anchor="persistent.connections">
2925   <x:anchor-alias value="persistent connections"/>
2927   HTTP/1.1 defaults to the use of "<x:dfn>persistent connections</x:dfn>",
2928   allowing multiple requests and responses to be carried over a single
2929   connection. The "<x:ref>close</x:ref>" connection-option is used to signal
2930   that a connection will not persist after the current request/response.
2931   HTTP implementations &SHOULD; support persistent connections.
2934   A recipient determines whether a connection is persistent or not based on
2935   the most recently received message's protocol version and
2936   <x:ref>Connection</x:ref> header field (if any):
2937   <list style="symbols">
2938     <t>If the <x:ref>close</x:ref> connection option is present, the
2939        connection will not persist after the current response; else,</t>
2940     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2941        persist after the current response; else,</t>
2942     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2943        connection option is present, the recipient is not a proxy, and
2944        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2945        the connection will persist after the current response; otherwise,</t>
2946     <t>The connection will close after the current response.</t>
2947   </list>
2950   A server &MAY; assume that an HTTP/1.1 client intends to maintain a
2951   persistent connection until a <x:ref>close</x:ref> connection option
2952   is received in a request.
2955   A client &MAY; reuse a persistent connection until it sends or receives
2956   a <x:ref>close</x:ref> connection option or receives an HTTP/1.0 response
2957   without a "keep-alive" connection option.
2960   In order to remain persistent, all messages on a connection need to
2961   have a self-defined message length (i.e., one not defined by closure
2962   of the connection), as described in <xref target="message.body"/>.
2963   A server &MUST; read the entire request message body or close
2964   the connection after sending its response, since otherwise the
2965   remaining data on a persistent connection would be misinterpreted
2966   as the next request.  Likewise,
2967   a client &MUST; read the entire response message body if it intends
2968   to reuse the same connection for a subsequent request.
2971   A proxy server &MUST-NOT; maintain a persistent connection with an
2972   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
2973   information and discussion of the problems with the Keep-Alive header field
2974   implemented by many HTTP/1.0 clients).
2977   Clients and servers &SHOULD-NOT; assume that a persistent connection is
2978   maintained for HTTP versions less than 1.1 unless it is explicitly
2979   signaled.
2980   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
2981   for more information on backward compatibility with HTTP/1.0 clients.
2984<section title="Retrying Requests" anchor="persistent.retrying.requests">
2986   Connections can be closed at any time, with or without intention.
2987   Implementations ought to anticipate the need to recover
2988   from asynchronous close events.
2991   When an inbound connection is closed prematurely, a client &MAY; open a new
2992   connection and automatically retransmit an aborted sequence of requests if
2993   all of those requests have idempotent methods (&idempotent-methods;).
2994   A proxy &MUST-NOT; automatically retry non-idempotent requests.
2997   A user agent &MUST-NOT; automatically retry a request with a non-idempotent
2998   method unless it has some means to know that the request semantics are
2999   actually idempotent, regardless of the method, or some means to detect that
3000   the original request was never applied. For example, a user agent that
3001   knows (through design or configuration) that a POST request to a given
3002   resource is safe can repeat that request automatically.
3003   Likewise, a user agent designed specifically to operate on a version
3004   control repository might be able to recover from partial failure conditions
3005   by checking the target resource revision(s) after a failed connection,
3006   reverting or fixing any changes that were partially applied, and then
3007   automatically retrying the requests that failed.
3010   A client &SHOULD-NOT; automatically retry a failed automatic retry.
3014<section title="Pipelining" anchor="pipelining">
3015   <x:anchor-alias value="pipeline"/>
3017   A client that supports persistent connections &MAY; "<x:dfn>pipeline</x:dfn>"
3018   its requests (i.e., send multiple requests without waiting for each
3019   response). A server &MAY; process a sequence of pipelined requests in
3020   parallel if they all have safe methods (&safe-methods;), but &MUST; send
3021   the corresponding responses in the same order that the requests were
3022   received.
3025   A client that pipelines requests &SHOULD; retry unanswered requests if the
3026   connection closes before it receives all of the corresponding responses.
3027   When retrying pipelined requests after a failed connection (a connection
3028   not explicitly closed by the server in its last complete response), a
3029   client &MUST-NOT; pipeline immediately after connection establishment,
3030   since the first remaining request in the prior pipeline might have caused
3031   an error response that can be lost again if multiple requests are sent on a
3032   prematurely closed connection (see the TCP reset problem described in
3033   <xref target="persistent.tear-down"/>).
3036   Idempotent methods (&idempotent-methods;) are significant to pipelining
3037   because they can be automatically retried after a connection failure.
3038   A user agent &SHOULD-NOT; pipeline requests after a non-idempotent method,
3039   until the final response status code for that method has been received,
3040   unless the user agent has a means to detect and recover from partial
3041   failure conditions involving the pipelined sequence.
3044   An intermediary that receives pipelined requests &MAY; pipeline those
3045   requests when forwarding them inbound, since it can rely on the outbound
3046   user agent(s) to determine what requests can be safely pipelined. If the
3047   inbound connection fails before receiving a response, the pipelining
3048   intermediary &MAY; attempt to retry a sequence of requests that have yet
3049   to receive a response if the requests all have idempotent methods;
3050   otherwise, the pipelining intermediary &SHOULD; forward any received
3051   responses and then close the corresponding outbound connection(s) so that
3052   the outbound user agent(s) can recover accordingly.
3057<section title="Concurrency" anchor="persistent.concurrency">
3059   A client &SHOULD; limit the number of simultaneous open
3060   connections that it maintains to a given server.
3063   Previous revisions of HTTP gave a specific number of connections as a
3064   ceiling, but this was found to be impractical for many applications. As a
3065   result, this specification does not mandate a particular maximum number of
3066   connections, but instead encourages clients to be conservative when opening
3067   multiple connections.
3070   Multiple connections are typically used to avoid the "head-of-line
3071   blocking" problem, wherein a request that takes significant server-side
3072   processing and/or has a large payload blocks subsequent requests on the
3073   same connection. However, each connection consumes server resources.
3074   Furthermore, using multiple connections can cause undesirable side effects
3075   in congested networks.
3078   Note that servers might reject traffic that they deem abusive, including an
3079   excessive number of connections from a client.
3083<section title="Failures and Time-outs" anchor="persistent.failures">
3085   Servers will usually have some time-out value beyond which they will
3086   no longer maintain an inactive connection. Proxy servers might make
3087   this a higher value since it is likely that the client will be making
3088   more connections through the same proxy server. The use of persistent
3089   connections places no requirements on the length (or existence) of
3090   this time-out for either the client or the server.
3093   A client or server that wishes to time-out &SHOULD; issue a graceful close
3094   on the connection. Implementations &SHOULD; constantly monitor open
3095   connections for a received closure signal and respond to it as appropriate,
3096   since prompt closure of both sides of a connection enables allocated system
3097   resources to be reclaimed.
3100   A client, server, or proxy &MAY; close the transport connection at any
3101   time. For example, a client might have started to send a new request
3102   at the same time that the server has decided to close the "idle"
3103   connection. From the server's point of view, the connection is being
3104   closed while it was idle, but from the client's point of view, a
3105   request is in progress.
3108   A server &SHOULD; sustain persistent connections, when possible, and allow
3109   the underlying
3110   transport's flow control mechanisms to resolve temporary overloads, rather
3111   than terminate connections with the expectation that clients will retry.
3112   The latter technique can exacerbate network congestion.
3115   A client sending a message body &SHOULD; monitor
3116   the network connection for an error response while it is transmitting
3117   the request. If the client sees a response that indicates the server does
3118   not wish to receive the message body and is closing the connection, the
3119   client &SHOULD; immediately cease transmitting the body and close its side
3120   of the connection.
3124<section title="Tear-down" anchor="persistent.tear-down">
3125  <iref primary="false" item="Connection header field" x:for-anchor=""/>
3126  <iref primary="false" item="close" x:for-anchor=""/>
3128   The <x:ref>Connection</x:ref> header field
3129   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
3130   connection option that a sender &SHOULD; send when it wishes to close
3131   the connection after the current request/response pair.
3134   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
3135   send further requests on that connection (after the one containing
3136   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
3137   final response message corresponding to this request.
3140   A server that receives a <x:ref>close</x:ref> connection option &MUST;
3141   initiate a close of the connection (see below) after it sends the
3142   final response to the request that contained <x:ref>close</x:ref>.
3143   The server &SHOULD; send a <x:ref>close</x:ref> connection option
3144   in its final response on that connection. The server &MUST-NOT; process
3145   any further requests received on that connection.
3148   A server that sends a <x:ref>close</x:ref> connection option &MUST;
3149   initiate a close of the connection (see below) after it sends the
3150   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
3151   any further requests received on that connection.
3154   A client that receives a <x:ref>close</x:ref> connection option &MUST;
3155   cease sending requests on that connection and close the connection
3156   after reading the response message containing the close; if additional
3157   pipelined requests had been sent on the connection, the client &SHOULD-NOT;
3158   assume that they will be processed by the server.
3161   If a server performs an immediate close of a TCP connection, there is a
3162   significant risk that the client will not be able to read the last HTTP
3163   response.  If the server receives additional data from the client on a
3164   fully-closed connection, such as another request that was sent by the
3165   client before receiving the server's response, the server's TCP stack will
3166   send a reset packet to the client; unfortunately, the reset packet might
3167   erase the client's unacknowledged input buffers before they can be read
3168   and interpreted by the client's HTTP parser.
3171   To avoid the TCP reset problem, servers typically close a connection in
3172   stages. First, the server performs a half-close by closing only the write
3173   side of the read/write connection. The server then continues to read from
3174   the connection until it receives a corresponding close by the client, or
3175   until the server is reasonably certain that its own TCP stack has received
3176   the client's acknowledgement of the packet(s) containing the server's last
3177   response. Finally, the server fully closes the connection.
3180   It is unknown whether the reset problem is exclusive to TCP or might also
3181   be found in other transport connection protocols.
3185<section title="Upgrade" anchor="header.upgrade">
3186  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3187  <x:anchor-alias value="Upgrade"/>
3188  <x:anchor-alias value="protocol"/>
3189  <x:anchor-alias value="protocol-name"/>
3190  <x:anchor-alias value="protocol-version"/>
3192   The "Upgrade" header field is intended to provide a simple mechanism
3193   for transitioning from HTTP/1.1 to some other protocol on the same
3194   connection.  A client &MAY; send a list of protocols in the Upgrade
3195   header field of a request to invite the server to switch to one or
3196   more of those protocols, in order of descending preference, before sending
3197   the final response. A server &MAY; ignore a received Upgrade header field
3198   if it wishes to continue using the current protocol on that connection.
3200<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3201  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3203  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3204  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3205  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3208   A server that sends a <x:ref>101 (Switching Protocols)</x:ref> response
3209   &MUST; send an Upgrade header field to indicate the new protocol(s) to
3210   which the connection is being switched; if multiple protocol layers are
3211   being switched, the sender &MUST; list the protocols in layer-ascending
3212   order. A server &MUST-NOT; switch to a protocol that was not indicated by
3213   the client in the corresponding request's Upgrade header field.
3214   A server &MAY; choose to ignore the order of preference indicated by the
3215   client and select the new protocol(s) based on other factors, such as the
3216   nature of the request or the current load on the server.
3219   A server that sends a <x:ref>426 (Upgrade Required)</x:ref> response
3220   &MUST; send an Upgrade header field to indicate the acceptable protocols,
3221   in order of descending preference.
3224   A server &MAY; send an Upgrade header field in any other response to
3225   advertise that it implements support for upgrading to the listed protocols,
3226   in order of descending preference, when appropriate for a future request.
3229   The following is a hypothetical example sent by a client:
3230</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
3231GET /hello.txt HTTP/1.1
3233Connection: upgrade
3234Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3238   Upgrade cannot be used to insist on a protocol change; its acceptance and
3239   use by the server is optional. The capabilities and nature of the
3240   application-level communication after the protocol change is entirely
3241   dependent upon the new protocol(s) chosen. However, immediately after
3242   sending the 101 response, the server is expected to continue responding to
3243   the original request as if it had received its equivalent within the new
3244   protocol (i.e., the server still has an outstanding request to satisfy
3245   after the protocol has been changed, and is expected to do so without
3246   requiring the request to be repeated).
3249   For example, if the Upgrade header field is received in a GET request
3250   and the server decides to switch protocols, it first responds
3251   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3252   then immediately follows that with the new protocol's equivalent of a
3253   response to a GET on the target resource.  This allows a connection to be
3254   upgraded to protocols with the same semantics as HTTP without the
3255   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3256   protocols unless the received message semantics can be honored by the new
3257   protocol; an OPTIONS request can be honored by any protocol.
3260   The following is an example response to the above hypothetical request:
3261</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
3262HTTP/1.1 101 Switching Protocols
3263Connection: upgrade
3264Upgrade: HTTP/2.0
3266[... data stream switches to HTTP/2.0 with an appropriate response
3267(as defined by new protocol) to the "GET /hello.txt" request ...]
3270   When Upgrade is sent, the sender &MUST; also send a
3271   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3272   that contains an "upgrade" connection option, in order to prevent Upgrade
3273   from being accidentally forwarded by intermediaries that might not implement
3274   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3275   is received in an HTTP/1.0 request.
3278   A client cannot begin using an upgraded protocol on the connection until
3279   it has completely sent the request message (i.e., the client can't change
3280   the protocol it is sending in the middle of a message).
3281   If a server receives both Upgrade and an <x:ref>Expect</x:ref> header field
3282   with the "100-continue" expectation (&header-expect;), the
3283   server &MUST; send a <x:ref>100 (Continue)</x:ref> response before sending
3284   a <x:ref>101 (Switching Protocols)</x:ref> response.
3287   The Upgrade header field only applies to switching protocols on top of the
3288   existing connection; it cannot be used to switch the underlying connection
3289   (transport) protocol, nor to switch the existing communication to a
3290   different connection. For those purposes, it is more appropriate to use a
3291   <x:ref>3xx (Redirection)</x:ref> response (&status-3xx;).
3294   This specification only defines the protocol name "HTTP" for use by
3295   the family of Hypertext Transfer Protocols, as defined by the HTTP
3296   version rules of <xref target="http.version"/> and future updates to this
3297   specification. Additional tokens ought to be registered with IANA using the
3298   registration procedure defined in <xref target="upgrade.token.registry"/>.
3303<section title="ABNF list extension: #rule" anchor="abnf.extension">
3305  A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
3306  improve readability in the definitions of some header field values.
3309  A construct "#" is defined, similar to "*", for defining comma-delimited
3310  lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
3311  at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
3312  comma (",") and optional whitespace (OWS).   
3315  Thus, a sender &MUST; expand the list construct as follows:
3316</preamble><artwork type="example">
3317  1#element =&gt; element *( OWS "," OWS element )
3320  and:
3321</preamble><artwork type="example">
3322  #element =&gt; [ 1#element ]
3325  and for n &gt;= 1 and m &gt; 1:
3326</preamble><artwork type="example">
3327  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
3330  For compatibility with legacy list rules, a recipient &MUST; parse and ignore
3331  a reasonable number of empty list elements: enough to handle common mistakes
3332  by senders that merge values, but not so much that they could be used as a
3333  denial of service mechanism. In other words, a recipient &MUST; expand the
3334  list construct as follows:
3336<figure><artwork type="example">
3337  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
3339  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
3342  Empty elements do not contribute to the count of elements present.
3343  For example, given these ABNF productions:
3345<figure><artwork type="example">
3346  example-list      = 1#example-list-elmt
3347  example-list-elmt = token ; see <xref target="field.components"/>
3350  Then the following are valid values for example-list (not including the
3351  double quotes, which are present for delimitation only):
3353<figure><artwork type="example">
3354  "foo,bar"
3355  "foo ,bar,"
3356  "foo , ,bar,charlie   "
3359  In contrast, the following values would be invalid, since at least one
3360  non-empty element is required by the example-list production:
3362<figure><artwork type="example">
3363  ""
3364  ","
3365  ",   ,"
3368  <xref target="collected.abnf"/> shows the collected ABNF after the list
3369  constructs have been expanded, as described above, for recipients.
3373<section title="IANA Considerations" anchor="IANA.considerations">
3375<section title="Header Field Registration" anchor="header.field.registration">
3377   HTTP header fields are registered within the Message Header Field Registry
3378   maintained at
3379   <eref target=""/>.
3382   This document defines the following HTTP header fields, so their
3383   associated registry entries shall be updated according to the permanent
3384   registrations below (see <xref target="BCP90"/>):
3386<?BEGININC p1-messaging.iana-headers ?>
3387<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3388<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3389   <ttcol>Header Field Name</ttcol>
3390   <ttcol>Protocol</ttcol>
3391   <ttcol>Status</ttcol>
3392   <ttcol>Reference</ttcol>
3394   <c>Connection</c>
3395   <c>http</c>
3396   <c>standard</c>
3397   <c>
3398      <xref target="header.connection"/>
3399   </c>
3400   <c>Content-Length</c>
3401   <c>http</c>
3402   <c>standard</c>
3403   <c>
3404      <xref target="header.content-length"/>
3405   </c>
3406   <c>Host</c>
3407   <c>http</c>
3408   <c>standard</c>
3409   <c>
3410      <xref target=""/>
3411   </c>
3412   <c>TE</c>
3413   <c>http</c>
3414   <c>standard</c>
3415   <c>
3416      <xref target="header.te"/>
3417   </c>
3418   <c>Trailer</c>
3419   <c>http</c>
3420   <c>standard</c>
3421   <c>
3422      <xref target="header.trailer"/>
3423   </c>
3424   <c>Transfer-Encoding</c>
3425   <c>http</c>
3426   <c>standard</c>
3427   <c>
3428      <xref target="header.transfer-encoding"/>
3429   </c>
3430   <c>Upgrade</c>
3431   <c>http</c>
3432   <c>standard</c>
3433   <c>
3434      <xref target="header.upgrade"/>
3435   </c>
3436   <c>Via</c>
3437   <c>http</c>
3438   <c>standard</c>
3439   <c>
3440      <xref target="header.via"/>
3441   </c>
3444<?ENDINC p1-messaging.iana-headers ?>
3446   Furthermore, the header field-name "Close" shall be registered as
3447   "reserved", since using that name as an HTTP header field might
3448   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3449   header field (<xref target="header.connection"/>).
3451<texttable align="left" suppress-title="true">
3452   <ttcol>Header Field Name</ttcol>
3453   <ttcol>Protocol</ttcol>
3454   <ttcol>Status</ttcol>
3455   <ttcol>Reference</ttcol>
3457   <c>Close</c>
3458   <c>http</c>
3459   <c>reserved</c>
3460   <c>
3461      <xref target="header.field.registration"/>
3462   </c>
3465   The change controller is: "IETF ( - Internet Engineering Task Force".
3469<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3471   IANA maintains the registry of URI Schemes <xref target="BCP115"/> at
3472   <eref target=""/>.
3475   This document defines the following URI schemes, so their
3476   associated registry entries shall be updated according to the permanent
3477   registrations below:
3479<texttable align="left" suppress-title="true">
3480   <ttcol>URI Scheme</ttcol>
3481   <ttcol>Description</ttcol>
3482   <ttcol>Reference</ttcol>
3484   <c>http</c>
3485   <c>Hypertext Transfer Protocol</c>
3486   <c><xref target="http.uri"/></c>
3488   <c>https</c>
3489   <c>Hypertext Transfer Protocol Secure</c>
3490   <c><xref target="https.uri"/></c>
3494<section title="Internet Media Type Registration" anchor="">
3496   IANA maintains the registry of Internet media types <xref target="BCP13"/> at
3497   <eref target=""/>.
3500   This document serves as the specification for the Internet media types
3501   "message/http" and "application/http". The following is to be registered with
3502   IANA.
3504<section title="Internet Media Type message/http" anchor="">
3505<iref item="Media Type" subitem="message/http" primary="true"/>
3506<iref item="message/http Media Type" primary="true"/>
3508   The message/http type can be used to enclose a single HTTP request or
3509   response message, provided that it obeys the MIME restrictions for all
3510   "message" types regarding line length and encodings.
3513  <list style="hanging" x:indent="12em">
3514    <t hangText="Type name:">
3515      message
3516    </t>
3517    <t hangText="Subtype name:">
3518      http
3519    </t>
3520    <t hangText="Required parameters:">
3521      N/A
3522    </t>
3523    <t hangText="Optional parameters:">
3524      version, msgtype
3525      <list style="hanging">
3526        <t hangText="version:">
3527          The HTTP-version number of the enclosed message
3528          (e.g., "1.1"). If not present, the version can be
3529          determined from the first line of the body.
3530        </t>
3531        <t hangText="msgtype:">
3532          The message type &mdash; "request" or "response". If not
3533          present, the type can be determined from the first
3534          line of the body.
3535        </t>
3536      </list>
3537    </t>
3538    <t hangText="Encoding considerations:">
3539      only "7bit", "8bit", or "binary" are permitted
3540    </t>
3541    <t hangText="Security considerations:">
3542      see <xref target="security.considerations"/>
3543    </t>
3544    <t hangText="Interoperability considerations:">
3545      N/A
3546    </t>
3547    <t hangText="Published specification:">
3548      This specification (see <xref target=""/>).
3549    </t>
3550    <t hangText="Applications that use this media type:">
3551      N/A
3552    </t>
3553    <t hangText="Fragment identifier considerations:">
3554      N/A
3555    </t>
3556    <t hangText="Additional information:">
3557      <list style="hanging">
3558        <t hangText="Magic number(s):">N/A</t>
3559        <t hangText="Deprecated alias names for this type:">N/A</t>
3560        <t hangText="File extension(s):">N/A</t>
3561        <t hangText="Macintosh file type code(s):">N/A</t>
3562      </list>
3563    </t>
3564    <t hangText="Person and email address to contact for further information:">
3565      See Authors Section.
3566    </t>
3567    <t hangText="Intended usage:">
3568      COMMON
3569    </t>
3570    <t hangText="Restrictions on usage:">
3571      N/A
3572    </t>
3573    <t hangText="Author:">
3574      See Authors Section.
3575    </t>
3576    <t hangText="Change controller:">
3577      IESG
3578    </t>
3579  </list>
3582<section title="Internet Media Type application/http" anchor="">
3583<iref item="Media Type" subitem="application/http" primary="true"/>
3584<iref item="application/http Media Type" primary="true"/>
3586   The application/http type can be used to enclose a pipeline of one or more
3587   HTTP request or response messages (not intermixed).
3590  <list style="hanging" x:indent="12em">
3591    <t hangText="Type name:">
3592      application
3593    </t>
3594    <t hangText="Subtype name:">
3595      http
3596    </t>
3597    <t hangText="Required parameters:">
3598      N/A
3599    </t>
3600    <t hangText="Optional parameters:">
3601      version, msgtype
3602      <list style="hanging">
3603        <t hangText="version:">
3604          The HTTP-version number of the enclosed messages
3605          (e.g., "1.1"). If not present, the version can be
3606          determined from the first line of the body.
3607        </t>
3608        <t hangText="msgtype:">
3609          The message type &mdash; "request" or "response". If not
3610          present, the type can be determined from the first
3611          line of the body.
3612        </t>
3613      </list>
3614    </t>
3615    <t hangText="Encoding considerations:">
3616      HTTP messages enclosed by this type
3617      are in "binary" format; use of an appropriate
3618      Content-Transfer-Encoding is required when
3619      transmitted via E-mail.
3620    </t>
3621    <t hangText="Security considerations:">
3622      see <xref target="security.considerations"/>
3623    </t>
3624    <t hangText="Interoperability considerations:">
3625      N/A
3626    </t>
3627    <t hangText="Published specification:">
3628      This specification (see <xref target=""/>).
3629    </t>
3630    <t hangText="Applications that use this media type:">
3631      N/A
3632    </t>
3633    <t hangText="Fragment identifier considerations:">
3634      N/A
3635    </t>
3636    <t hangText="Additional information:">
3637      <list style="hanging">
3638        <t hangText="Deprecated alias names for this type:">N/A</t>
3639        <t hangText="Magic number(s):">N/A</t>
3640        <t hangText="File extension(s):">N/A</t>
3641        <t hangText="Macintosh file type code(s):">N/A</t>
3642      </list>
3643    </t>
3644    <t hangText="Person and email address to contact for further information:">
3645      See Authors Section.
3646    </t>
3647    <t hangText="Intended usage:">
3648      COMMON
3649    </t>
3650    <t hangText="Restrictions on usage:">
3651      N/A
3652    </t>
3653    <t hangText="Author:">
3654      See Authors Section.
3655    </t>
3656    <t hangText="Change controller:">
3657      IESG
3658    </t>
3659  </list>
3664<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3666   The HTTP Transfer Coding Registry defines the name space for transfer
3667   coding names. It is maintained at <eref target=""/>.
3670<section title="Procedure" anchor="transfer.coding.registry.procedure">
3672   Registrations &MUST; include the following fields:
3673   <list style="symbols">
3674     <t>Name</t>
3675     <t>Description</t>
3676     <t>Pointer to specification text</t>
3677   </list>
3680   Names of transfer codings &MUST-NOT; overlap with names of content codings
3681   (&content-codings;) unless the encoding transformation is identical, as
3682   is the case for the compression codings defined in
3683   <xref target="compression.codings"/>.
3686   Values to be added to this name space require IETF Review (see
3687   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3688   conform to the purpose of transfer coding defined in this specification.
3691   Use of program names for the identification of encoding formats
3692   is not desirable and is discouraged for future encodings.
3696<section title="Registration" anchor="transfer.coding.registration">
3698   The HTTP Transfer Coding Registry shall be updated with the registrations
3699   below:
3701<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3702   <ttcol>Name</ttcol>
3703   <ttcol>Description</ttcol>
3704   <ttcol>Reference</ttcol>
3705   <c>chunked</c>
3706   <c>Transfer in a series of chunks</c>
3707   <c>
3708      <xref target="chunked.encoding"/>
3709   </c>
3710   <c>compress</c>
3711   <c>UNIX "compress" data format <xref target="Welch"/></c>
3712   <c>
3713      <xref target="compress.coding"/>
3714   </c>
3715   <c>deflate</c>
3716   <c>"deflate" compressed data (<xref target="RFC1951"/>) inside
3717   the "zlib" data format (<xref target="RFC1950"/>)
3718   </c>
3719   <c>
3720      <xref target="deflate.coding"/>
3721   </c>
3722   <c>gzip</c>
3723   <c>GZIP file format <xref target="RFC1952"/></c>
3724   <c>
3725      <xref target="gzip.coding"/>
3726   </c>
3727   <c>x-compress</c>
3728   <c>Deprecated (alias for compress)</c>
3729   <c>
3730      <xref target="compress.coding"/>
3731   </c>
3732   <c>x-gzip</c>
3733   <c>Deprecated (alias for gzip)</c>
3734   <c>
3735      <xref target="gzip.coding"/>
3736   </c>
3741<section title="Content Coding Registration" anchor="content.coding.registration">
3743   IANA maintains the registry of HTTP Content Codings at
3744   <eref target=""/>.
3747   The HTTP Content Codings Registry shall be updated with the registrations
3748   below:
3750<texttable align="left" suppress-title="true" anchor="iana.content.coding.registration.table">
3751   <ttcol>Name</ttcol>
3752   <ttcol>Description</ttcol>
3753   <ttcol>Reference</ttcol>
3754   <c>compress</c>
3755   <c>UNIX "compress" data format <xref target="Welch"/></c>
3756   <c>
3757      <xref target="compress.coding"/>
3758   </c>
3759   <c>deflate</c>
3760   <c>"deflate" compressed data (<xref target="RFC1951"/>) inside
3761   the "zlib" data format (<xref target="RFC1950"/>)</c>
3762   <c>
3763      <xref target="deflate.coding"/>
3764   </c>
3765   <c>gzip</c>
3766   <c>GZIP file format <xref target="RFC1952"/></c>
3767   <c>
3768      <xref target="gzip.coding"/>
3769   </c>
3770   <c>x-compress</c>
3771   <c>Deprecated (alias for compress)</c>
3772   <c>
3773      <xref target="compress.coding"/>
3774   </c>
3775   <c>x-gzip</c>
3776   <c>Deprecated (alias for gzip)</c>
3777   <c>
3778      <xref target="gzip.coding"/>
3779   </c>
3783<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3785   The HTTP Upgrade Token Registry defines the name space for protocol-name
3786   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3787   field. The registry is maintained at <eref target=""/>.
3790<section title="Procedure" anchor="upgrade.token.registry.procedure">  
3792   Each registered protocol name is associated with contact information
3793   and an optional set of specifications that details how the connection
3794   will be processed after it has been upgraded.
3797   Registrations happen on a "First Come First Served" basis (see
3798   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3799   following rules:
3800  <list style="numbers">
3801    <t>A protocol-name token, once registered, stays registered forever.</t>
3802    <t>The registration &MUST; name a responsible party for the
3803       registration.</t>
3804    <t>The registration &MUST; name a point of contact.</t>
3805    <t>The registration &MAY; name a set of specifications associated with
3806       that token. Such specifications need not be publicly available.</t>
3807    <t>The registration &SHOULD; name a set of expected "protocol-version"
3808       tokens associated with that token at the time of registration.</t>
3809    <t>The responsible party &MAY; change the registration at any time.
3810       The IANA will keep a record of all such changes, and make them
3811       available upon request.</t>
3812    <t>The IESG &MAY; reassign responsibility for a protocol token.
3813       This will normally only be used in the case when a
3814       responsible party cannot be contacted.</t>
3815  </list>
3818   This registration procedure for HTTP Upgrade Tokens replaces that
3819   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3823<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3825   The "HTTP" entry in the HTTP Upgrade Token Registry shall be updated with
3826   the registration below:
3828<texttable align="left" suppress-title="true">
3829   <ttcol>Value</ttcol>
3830   <ttcol>Description</ttcol>
3831   <ttcol>Expected Version Tokens</ttcol>
3832   <ttcol>Reference</ttcol>
3834   <c>HTTP</c>
3835   <c>Hypertext Transfer Protocol</c>
3836   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3837   <c><xref target="http.version"/></c>
3840   The responsible party is: "IETF ( - Internet Engineering Task Force".
3847<section title="Security Considerations" anchor="security.considerations">
3849   This section is meant to inform developers, information providers, and
3850   users of known security concerns relevant to HTTP/1.1 message syntax,
3851   parsing, and routing.
3854   The list of considerations below is not exhaustive &mdash; security
3855   analysis in an ongoing activity. Various organizations, such as the
3856   "Open Web Application Security Project" (OWASP,
3857   <eref target=""/>), provide information about current
3858   research.
3861<section title="DNS-related Attacks" anchor="dns.related.attacks">
3863   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3864   generally prone to security attacks based on the deliberate misassociation
3865   of IP addresses and DNS names not protected by DNSSEC. Clients need to be
3866   cautious in assuming the validity of an IP number/DNS name association unless
3867   the response is protected by DNSSEC (<xref target="RFC4033"/>).
3871<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3873   By their very nature, HTTP intermediaries are men-in-the-middle, and
3874   represent an opportunity for man-in-the-middle attacks. Compromise of
3875   the systems on which the intermediaries run can result in serious security
3876   and privacy problems. Intermediaries have access to security-related
3877   information, personal information about individual users and
3878   organizations, and proprietary information belonging to users and
3879   content providers. A compromised intermediary, or an intermediary
3880   implemented or configured without regard to security and privacy
3881   considerations, might be used in the commission of a wide range of
3882   potential attacks.
3885   Intermediaries that contain a shared cache are especially vulnerable
3886   to cache poisoning attacks.
3889   Implementers need to consider the privacy and security
3890   implications of their design and coding decisions, and of the
3891   configuration options they provide to operators (especially the
3892   default configuration).
3895   Users need to be aware that intermediaries are no more trustworthy than
3896   the people who run them; HTTP itself cannot solve this problem.
3900<section title="Buffer Overflows" anchor="attack.protocol.element.size.overflows">
3902   Because HTTP uses mostly textual, character-delimited fields, attackers can
3903   overflow buffers in implementations, and/or perform a Denial of Service
3904   against implementations that accept fields with unlimited lengths.
3907   To promote interoperability, this specification makes specific
3908   recommendations for minimum size limits on request-line
3909   (<xref target="request.line"/>)
3910   and header fields (<xref target="header.fields"/>). These are
3911   minimum recommendations, chosen to be supportable even by implementations
3912   with limited resources; it is expected that most implementations will
3913   choose substantially higher limits.
3916   This specification also provides a way for servers to reject messages that
3917   have request-targets that are too long (&status-414;) or request entities
3918   that are too large (&status-4xx;). Additional status codes related to
3919   capacity limits have been defined by extensions to HTTP
3920   <xref target="RFC6585"/>.
3923   Recipients ought to carefully limit the extent to which they read other
3924   fields, including (but not limited to) request methods, response status
3925   phrases, header field-names, and body chunks, so as to avoid denial of
3926   service attacks without impeding interoperability.
3930<section title="Message Integrity" anchor="message.integrity">
3932   HTTP does not define a specific mechanism for ensuring message integrity,
3933   instead relying on the error-detection ability of underlying transport
3934   protocols and the use of length or chunk-delimited framing to detect
3935   completeness. Additional integrity mechanisms, such as hash functions or
3936   digital signatures applied to the content, can be selectively added to
3937   messages via extensible metadata header fields. Historically, the lack of
3938   a single integrity mechanism has been justified by the informal nature of
3939   most HTTP communication.  However, the prevalence of HTTP as an information
3940   access mechanism has resulted in its increasing use within environments
3941   where verification of message integrity is crucial.
3944   User agents are encouraged to implement configurable means for detecting
3945   and reporting failures of message integrity such that those means can be
3946   enabled within environments for which integrity is necessary. For example,
3947   a browser being used to view medical history or drug interaction
3948   information needs to indicate to the user when such information is detected
3949   by the protocol to be incomplete, expired, or corrupted during transfer.
3950   Such mechanisms might be selectively enabled via user agent extensions or
3951   the presence of message integrity metadata in a response.
3952   At a minimum, user agents ought to provide some indication that allows a
3953   user to distinguish between a complete and incomplete response message
3954   (<xref target="incomplete.messages"/>) when such verification is desired.
3958<section title="Server Log Information" anchor="abuse.of.server.log.information">
3960   A server is in the position to save personal data about a user's requests
3961   over time, which might identify their reading patterns or subjects of
3962   interest.  In particular, log information gathered at an intermediary
3963   often contains a history of user agent interaction, across a multitude
3964   of sites, that can be traced to individual users.
3967   HTTP log information is confidential in nature; its handling is often
3968   constrained by laws and regulations.  Log information needs to be securely
3969   stored and appropriate guidelines followed for its analysis.
3970   Anonymization of personal information within individual entries helps,
3971   but is generally not sufficient to prevent real log traces from being
3972   re-identified based on correlation with other access characteristics.
3973   As such, access traces that are keyed to a specific client are unsafe to
3974   publish even if the key is pseudonymous.
3977   To minimize the risk of theft or accidental publication, log information
3978   ought to be purged of personally identifiable information, including
3979   user identifiers, IP addresses, and user-provided query parameters,
3980   as soon as that information is no longer necessary to support operational
3981   needs for security, auditing, or fraud control.
3986<section title="Acknowledgments" anchor="acks">
3988   This edition of HTTP/1.1 builds on the many contributions that went into
3989   <xref target="RFC1945" format="none">RFC 1945</xref>,
3990   <xref target="RFC2068" format="none">RFC 2068</xref>,
3991   <xref target="RFC2145" format="none">RFC 2145</xref>, and
3992   <xref target="RFC2616" format="none">RFC 2616</xref>, including
3993   substantial contributions made by the previous authors, editors, and
3994   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
3995   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
3996   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
3999   Since 1999, the following contributors have helped improve the HTTP
4000   specification by reporting bugs, asking smart questions, drafting or
4001   reviewing text, and evaluating open issues:
4003<?BEGININC acks ?>
4004<t>Adam Barth,
4005Adam Roach,
4006Addison Phillips,
4007Adrian Chadd,
4008Adrian Cole,
4009Adrien W. de Croy,
4010Alan Ford,
4011Alan Ruttenberg,
4012Albert Lunde,
4013Alek Storm,
4014Alex Rousskov,
4015Alexandre Morgaut,
4016Alexey Melnikov,
4017Alisha Smith,
4018Amichai Rothman,
4019Amit Klein,
4020Amos Jeffries,
4021Andreas Maier,
4022Andreas Petersson,
4023Andrei Popov,
4024Anil Sharma,
4025Anne van Kesteren,
4026Anthony Bryan,
4027Asbjorn Ulsberg,
4028Ashok Kumar,
4029Balachander Krishnamurthy,
4030Barry Leiba,
4031Ben Laurie,
4032Benjamin Carlyle,
4033Benjamin Niven-Jenkins,
4034Benoit Claise,
4035Bil Corry,
4036Bill Burke,
4037Bjoern Hoehrmann,
4038Bob Scheifler,
4039Boris Zbarsky,
4040Brett Slatkin,
4041Brian Kell,
4042Brian McBarron,
4043Brian Pane,
4044Brian Raymor,
4045Brian Smith,
4046Bruce Perens,
4047Bryce Nesbitt,
4048Cameron Heavon-Jones,
4049Carl Kugler,
4050Carsten Bormann,
4051Charles Fry,
4052Chris Burdess,
4053Chris Newman,
4054Christian Huitema,
4055Cyrus Daboo,
4056Dale Robert Anderson,
4057Dan Wing,
4058Dan Winship,
4059Daniel Stenberg,
4060Darrel Miller,
4061Dave Cridland,
4062Dave Crocker,
4063Dave Kristol,
4064Dave Thaler,
4065David Booth,
4066David Singer,
4067David W. Morris,
4068Diwakar Shetty,
4069Dmitry Kurochkin,
4070Drummond Reed,
4071Duane Wessels,
4072Edward Lee,
4073Eitan Adler,
4074Eliot Lear,
4075Emile Stephan,
4076Eran Hammer-Lahav,
4077Eric D. Williams,
4078Eric J. Bowman,
4079Eric Lawrence,
4080Eric Rescorla,
4081Erik Aronesty,
4082EungJun Yi,
4083Evan Prodromou,
4084Felix Geisendoerfer,
4085Florian Weimer,
4086Frank Ellermann,
4087Fred Akalin,
4088Fred Bohle,
4089Frederic Kayser,
4090Gabor Molnar,
4091Gabriel Montenegro,
4092Geoffrey Sneddon,
4093Gervase Markham,
4094Gili Tzabari,
4095Grahame Grieve,
4096Greg Slepak,
4097Greg Wilkins,
4098Grzegorz Calkowski,
4099Harald Tveit Alvestrand,
4100Harry Halpin,
4101Helge Hess,
4102Henrik Nordstrom,
4103Henry S. Thompson,
4104Henry Story,
4105Herbert van de Sompel,
4106Herve Ruellan,
4107Howard Melman,
4108Hugo Haas,
4109Ian Fette,
4110Ian Hickson,
4111Ido Safruti,
4112Ilari Liusvaara,
4113Ilya Grigorik,
4114Ingo Struck,
4115J. Ross Nicoll,
4116James Cloos,
4117James H. Manger,
4118James Lacey,
4119James M. Snell,
4120Jamie Lokier,
4121Jan Algermissen,
4122Jari Arkko,
4123Jeff Hodges (who came up with the term 'effective Request-URI'),
4124Jeff Pinner,
4125Jeff Walden,
4126Jim Luther,
4127Jitu Padhye,
4128Joe D. Williams,
4129Joe Gregorio,
4130Joe Orton,
4131Joel Jaeggli,
4132John C. Klensin,
4133John C. Mallery,
4134John Cowan,
4135John Kemp,
4136John Panzer,
4137John Schneider,
4138John Stracke,
4139John Sullivan,
4140Jonas Sicking,
4141Jonathan A. Rees,
4142Jonathan Billington,
4143Jonathan Moore,
4144Jonathan Silvera,
4145Jordi Ros,
4146Joris Dobbelsteen,
4147Josh Cohen,
4148Julien Pierre,
4149Jungshik Shin,
4150Justin Chapweske,
4151Justin Erenkrantz,
4152Justin James,
4153Kalvinder Singh,
4154Karl Dubost,
4155Kathleen Moriarty,
4156Keith Hoffman,
4157Keith Moore,
4158Ken Murchison,
4159Koen Holtman,
4160Konstantin Voronkov,
4161Kris Zyp,
4162Leif Hedstrom,
4163Lionel Morand,
4164Lisa Dusseault,
4165Maciej Stachowiak,
4166Manu Sporny,
4167Marc Schneider,
4168Marc Slemko,
4169Mark Baker,
4170Mark Pauley,
4171Mark Watson,
4172Markus Isomaki,
4173Markus Lanthaler,
4174Martin J. Duerst,
4175Martin Musatov,
4176Martin Nilsson,
4177Martin Thomson,
4178Matt Lynch,
4179Matthew Cox,
4180Matthew Kerwin,
4181Max Clark,
4182Menachem Dodge,
4183Meral Shirazipour,
4184Michael Burrows,
4185Michael Hausenblas,
4186Michael Scharf,
4187Michael Sweet,
4188Michael Tuexen,
4189Michael Welzl,
4190Mike Amundsen,
4191Mike Belshe,
4192Mike Bishop,
4193Mike Kelly,
4194Mike Schinkel,
4195Miles Sabin,
4196Murray S. Kucherawy,
4197Mykyta Yevstifeyev,
4198Nathan Rixham,
4199Nicholas Shanks,
4200Nico Williams,
4201Nicolas Alvarez,
4202Nicolas Mailhot,
4203Noah Slater,
4204Osama Mazahir,
4205Pablo Castro,
4206Pat Hayes,
4207Patrick R. McManus,
4208Paul E. Jones,
4209Paul Hoffman,
4210Paul Marquess,
4211Pete Resnick,
4212Peter Lepeska,
4213Peter Occil,
4214Peter Saint-Andre,
4215Peter Watkins,
4216Phil Archer,
4217Phil Hunt,
4218Philippe Mougin,
4219Phillip Hallam-Baker,
4220Piotr Dobrogost,
4221Poul-Henning Kamp,
4222Preethi Natarajan,
4223Rajeev Bector,
4224Ray Polk,
4225Reto Bachmann-Gmuer,
4226Richard Barnes,
4227Richard Cyganiak,
4228Rob Trace,
4229Robby Simpson,
4230Robert Brewer,
4231Robert Collins,
4232Robert Mattson,
4233Robert O'Callahan,
4234Robert Olofsson,
4235Robert Sayre,
4236Robert Siemer,
4237Robert de Wilde,
4238Roberto Javier Godoy,
4239Roberto Peon,
4240Roland Zink,
4241Ronny Widjaja,
4242Ryan Hamilton,
4243S. Mike Dierken,
4244Salvatore Loreto,
4245Sam Johnston,
4246Sam Pullara,
4247Sam Ruby,
4248Saurabh Kulkarni,
4249Scott Lawrence (who maintained the original issues list),
4250Sean B. Palmer,
4251Sean Turner,
4252Sebastien Barnoud,
4253Shane McCarron,
4254Shigeki Ohtsu,
4255Simon Yarde,
4256Stefan Eissing,
4257Stefan Tilkov,
4258Stefanos Harhalakis,
4259Stephane Bortzmeyer,
4260Stephen Farrell,
4261Stephen Kent,
4262Stephen Ludin,
4263Stuart Williams,
4264Subbu Allamaraju,
4265Subramanian Moonesamy,
4266Susan Hares,
4267Sylvain Hellegouarch,
4268Tapan Divekar,
4269Tatsuhiro Tsujikawa,
4270Tatsuya Hayashi,
4271Ted Hardie,
4272Ted Lemon,
4273Thomas Broyer,
4274Thomas Fossati,
4275Thomas Maslen,
4276Thomas Nadeau,
4277Thomas Nordin,
4278Thomas Roessler,
4279Tim Bray,
4280Tim Morgan,
4281Tim Olsen,
4282Tom Zhou,
4283Travis Snoozy,
4284Tyler Close,
4285Vincent Murphy,
4286Wenbo Zhu,
4287Werner Baumann,
4288Wilbur Streett,
4289Wilfredo Sanchez Vega,
4290William A. Rowe Jr.,
4291William Chan,
4292Willy Tarreau,
4293Xiaoshu Wang,
4294Yaron Goland,
4295Yngve Nysaeter Pettersen,
4296Yoav Nir,
4297Yogesh Bang,
4298Yuchung Cheng,
4299Yutaka Oiwa,
4300Yves Lafon (long-time member of the editor team),
4301Zed A. Shaw, and
4302Zhong Yu.
4304<?ENDINC acks ?>
4306   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
4307   acknowledgements from prior revisions.
4314<references title="Normative References">
4316<reference anchor="Part2">
4317  <front>
4318    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
4319    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4320      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4321      <address><email></email></address>
4322    </author>
4323    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4324      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4325      <address><email></email></address>
4326    </author>
4327    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4328  </front>
4329  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
4330  <x:source href="p2-semantics.xml" basename="p2-semantics">
4331    <x:defines>1xx (Informational)</x:defines>
4332    <x:defines>1xx</x:defines>
4333    <x:defines>100 (Continue)</x:defines>
4334    <x:defines>101 (Switching Protocols)</x:defines>
4335    <x:defines>2xx (Successful)</x:defines>
4336    <x:defines>2xx</x:defines>
4337    <x:defines>200 (OK)</x:defines>
4338    <x:defines>203 (Non-Authoritative Information)</x:defines>
4339    <x:defines>204 (No Content)</x:defines>
4340    <x:defines>3xx (Redirection)</x:defines>
4341    <x:defines>3xx</x:defines>
4342    <x:defines>301 (Moved Permanently)</x:defines>
4343    <x:defines>4xx (Client Error)</x:defines>
4344    <x:defines>4xx</x:defines>
4345    <x:defines>400 (Bad Request)</x:defines>
4346    <x:defines>411 (Length Required)</x:defines>
4347    <x:defines>414 (URI Too Long)</x:defines>
4348    <x:defines>417 (Expectation Failed)</x:defines>
4349    <x:defines>426 (Upgrade Required)</x:defines>
4350    <x:defines>501 (Not Implemented)</x:defines>
4351    <x:defines>502 (Bad Gateway)</x:defines>
4352    <x:defines>505 (HTTP Version Not Supported)</x:defines>
4353    <x:defines>Accept-Encoding</x:defines>
4354    <x:defines>Allow</x:defines>
4355    <x:defines>Content-Encoding</x:defines>
4356    <x:defines>Content-Location</x:defines>
4357    <x:defines>Content-Type</x:defines>
4358    <x:defines>Date</x:defines>
4359    <x:defines>Expect</x:defines>
4360    <x:defines>Location</x:defines>
4361    <x:defines>Server</x:defines>
4362    <x:defines>User-Agent</x:defines>
4363  </x:source>
4366<reference anchor="Part4">
4367  <front>
4368    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
4369    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
4370      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4371      <address><email></email></address>
4372    </author>
4373    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
4374      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4375      <address><email></email></address>
4376    </author>
4377    <date month="&ID-MONTH;" year="&ID-YEAR;" />
4378  </front>
4379  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
4380  <x:source basename="p4-conditional" href="p4-conditional.xml">
4381    <x:defines>304 (Not Modified)</x:defines>
4382    <x:defines>ETag</x:defines>
4383    <x:defines>Last-Modified</x:defines>
4384  </x:source>
4387<reference anchor="Part5">
4388  <front>
4389    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
4390    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4391      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4392      <address><email></email></address>
4393    </author>
4394    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
4395      <organization abbrev="W3C">World Wide Web Consortium</organization>
4396      <address><email></email></address>
4397    </author>
4398    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4399      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4400      <address><email></email></address>
4401    </author>
4402    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4403  </front>
4404  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4405  <x:source href="p5-range.xml" basename="p5-range">
4406    <x:defines>Content-Range</x:defines>
4407  </x:source>
4410<reference anchor="Part6">
4411  <front>
4412    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4413    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4414      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4415      <address><email></email></address>
4416    </author>
4417    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4418      <organization>Akamai</organization>
4419      <address><email></email></address>
4420    </author>
4421    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4422      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4423      <address><email></email></address>
4424    </author>
4425    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4426  </front>
4427  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4428  <x:source href="p6-cache.xml" basename="p6-cache">
4429    <x:defines>Cache-Control</x:defines>
4430    <x:defines>Expires</x:defines>
4431  </x:source>
4434<reference anchor="Part7">
4435  <front>
4436    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4437    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4438      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4439      <address><email></email></address>
4440    </author>
4441    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4442      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4443      <address><email></email></address>
4444    </author>
4445    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4446  </front>
4447  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4448  <x:source href="p7-auth.xml" basename="p7-auth">
4449    <x:defines>Proxy-Authenticate</x:defines>
4450    <x:defines>Proxy-Authorization</x:defines>
4451  </x:source>
4454<reference anchor="RFC5234">
4455  <front>
4456    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4457    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4458      <organization>Brandenburg InternetWorking</organization>
4459      <address>
4460        <email></email>
4461      </address> 
4462    </author>
4463    <author initials="P." surname="Overell" fullname="Paul Overell">
4464      <organization>THUS plc.</organization>
4465      <address>
4466        <email></email>
4467      </address>
4468    </author>
4469    <date month="January" year="2008"/>
4470  </front>
4471  <seriesInfo name="STD" value="68"/>
4472  <seriesInfo name="RFC" value="5234"/>
4475<reference anchor="RFC2119">
4476  <front>
4477    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4478    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4479      <organization>Harvard University</organization>
4480      <address><email></email></address>
4481    </author>
4482    <date month="March" year="1997"/>
4483  </front>
4484  <seriesInfo name="BCP" value="14"/>
4485  <seriesInfo name="RFC" value="2119"/>
4488<reference anchor="RFC3986">
4489 <front>
4490  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4491  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4492    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4493    <address>
4494       <email></email>
4495       <uri></uri>
4496    </address>
4497  </author>
4498  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4499    <organization abbrev="Day Software">Day Software</organization>
4500    <address>
4501      <email></email>
4502      <uri></uri>
4503    </address>
4504  </author>
4505  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4506    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4507    <address>
4508      <email></email>
4509      <uri></uri>
4510    </address>
4511  </author>
4512  <date month='January' year='2005'></date>
4513 </front>
4514 <seriesInfo name="STD" value="66"/>
4515 <seriesInfo name="RFC" value="3986"/>
4518<reference anchor="RFC0793">
4519  <front>
4520    <title>Transmission Control Protocol</title>
4521    <author initials='J.' surname='Postel' fullname='Jon Postel'>
4522      <organization>University of Southern California (USC)/Information Sciences Institute</organization>
4523    </author>
4524    <date year='1981' month='September' />
4525  </front>
4526  <seriesInfo name='STD' value='7' />
4527  <seriesInfo name='RFC' value='793' />
4530<reference anchor="USASCII">
4531  <front>
4532    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4533    <author>
4534      <organization>American National Standards Institute</organization>
4535    </author>
4536    <date year="1986"/>
4537  </front>
4538  <seriesInfo name="ANSI" value="X3.4"/>
4541<reference anchor="RFC1950">
4542  <front>
4543    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4544    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4545      <organization>Aladdin Enterprises</organization>
4546      <address><email></email></address>
4547    </author>
4548    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4549    <date month="May" year="1996"/>
4550  </front>
4551  <seriesInfo name="RFC" value="1950"/>
4552  <!--<annotation>
4553    RFC 1950 is an Informational RFC, thus it might be less stable than
4554    this specification. On the other hand, this downward reference was
4555    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4556    therefore it is unlikely to cause problems in practice. See also
4557    <xref target="BCP97"/>.
4558  </annotation>-->
4561<reference anchor="RFC1951">
4562  <front>
4563    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4564    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4565      <organization>Aladdin Enterprises</organization>
4566      <address><email></email></address>
4567    </author>
4568    <date month="May" year="1996"/>
4569  </front>
4570  <seriesInfo name="RFC" value="1951"/>
4571  <!--<annotation>
4572    RFC 1951 is an Informational RFC, thus it might be less stable than
4573    this specification. On the other hand, this downward reference was
4574    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4575    therefore it is unlikely to cause problems in practice. See also
4576    <xref target="BCP97"/>.
4577  </annotation>-->
4580<reference anchor="RFC1952">
4581  <front>
4582    <title>GZIP file format specification version 4.3</title>
4583    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4584      <organization>Aladdin Enterprises</organization>
4585      <address><email></email></address>
4586    </author>
4587    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4588      <address><email></email></address>
4589    </author>
4590    <author initials="M." surname="Adler" fullname="Mark Adler">
4591      <address><email></email></address>
4592    </author>
4593    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4594      <address><email></email></address>
4595    </author>
4596    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4597      <address><email></email></address>
4598    </author>
4599    <date month="May" year="1996"/>
4600  </front>
4601  <seriesInfo name="RFC" value="1952"/>
4602  <!--<annotation>
4603    RFC 1952 is an Informational RFC, thus it might be less stable than
4604    this specification. On the other hand, this downward reference was
4605    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4606    therefore it is unlikely to cause problems in practice. See also
4607    <xref target="BCP97"/>.
4608  </annotation>-->
4611<reference anchor="Welch">
4612  <front>
4613    <title>A Technique for High Performance Data Compression</title>
4614    <author initials="T.A." surname="Welch" fullname="Terry A. Welch"/>
4615    <date month="June" year="1984"/>
4616  </front>
4617  <seriesInfo name="IEEE Computer" value="17(6)"/>
4622<references title="Informative References">
4624<reference anchor="ISO-8859-1">
4625  <front>
4626    <title>
4627     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4628    </title>
4629    <author>
4630      <organization>International Organization for Standardization</organization>
4631    </author>
4632    <date year="1998"/>
4633  </front>
4634  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4637<reference anchor='RFC1919'>
4638  <front>
4639    <title>Classical versus Transparent IP Proxies</title>
4640    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4641      <address><email></email></address>
4642    </author>
4643    <date year='1996' month='March' />
4644  </front>
4645  <seriesInfo name='RFC' value='1919' />
4648<reference anchor="RFC1945">
4649  <front>
4650    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4651    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4652      <organization>MIT, Laboratory for Computer Science</organization>
4653      <address><email></email></address>
4654    </author>
4655    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4656      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4657      <address><email></email></address>
4658    </author>
4659    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4660      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4661      <address><email></email></address>
4662    </author>
4663    <date month="May" year="1996"/>
4664  </front>
4665  <seriesInfo name="RFC" value="1945"/>
4668<reference anchor="RFC2045">
4669  <front>
4670    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4671    <author initials="N." surname="Freed" fullname="Ned Freed">
4672      <organization>Innosoft International, Inc.</organization>
4673      <address><email></email></address>
4674    </author>
4675    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4676      <organization>First Virtual Holdings</organization>
4677      <address><email></email></address>
4678    </author>
4679    <date month="November" year="1996"/>
4680  </front>
4681  <seriesInfo name="RFC" value="2045"/>
4684<reference anchor="RFC2047">
4685  <front>
4686    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4687    <author initials="K." surname="Moore" fullname="Keith Moore">
4688      <organization>University of Tennessee</organization>
4689      <address><email></email></address>
4690    </author>
4691    <date month="November" year="1996"/>
4692  </front>
4693  <seriesInfo name="RFC" value="2047"/>
4696<reference anchor="RFC2068">
4697  <front>
4698    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4699    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
4700      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4701      <address><email></email></address>
4702    </author>
4703    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4704      <organization>MIT Laboratory for Computer Science</organization>
4705      <address><email></email></address>
4706    </author>
4707    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4708      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4709      <address><email></email></address>
4710    </author>
4711    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4712      <organization>MIT Laboratory for Computer Science</organization>
4713      <address><email></email></address>
4714    </author>
4715    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4716      <organization>MIT Laboratory for Computer Science</organization>
4717      <address><email></email></address>
4718    </author>
4719    <date month="January" year="1997"/>
4720  </front>
4721  <seriesInfo name="RFC" value="2068"/>
4724<reference anchor="RFC2145">
4725  <front>
4726    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4727    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4728      <organization>Western Research Laboratory</organization>
4729      <address><email></email></address>
4730    </author>
4731    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4732      <organization>Department of Information and Computer Science</organization>
4733      <address><email></email></address>
4734    </author>
4735    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4736      <organization>MIT Laboratory for Computer Science</organization>
4737      <address><email></email></address>
4738    </author>
4739    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4740      <organization>W3 Consortium</organization>
4741      <address><email></email></address>
4742    </author>
4743    <date month="May" year="1997"/>
4744  </front>
4745  <seriesInfo name="RFC" value="2145"/>
4748<reference anchor="RFC2616">
4749  <front>
4750    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4751    <author initials="R." surname="Fielding" fullname="R. Fielding">
4752      <organization>University of California, Irvine</organization>
4753      <address><email></email></address>
4754    </author>
4755    <author initials="J." surname="Gettys" fullname="J. Gettys">
4756      <organization>W3C</organization>
4757      <address><email></email></address>
4758    </author>
4759    <author initials="J." surname="Mogul" fullname="J. Mogul">
4760      <organization>Compaq Computer Corporation</organization>
4761      <address><email></email></address>
4762    </author>
4763    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4764      <organization>MIT Laboratory for Computer Science</organization>
4765      <address><email></email></address>
4766    </author>
4767    <author initials="L." surname="Masinter" fullname="L. Masinter">
4768      <organization>Xerox Corporation</organization>
4769      <address><email></email></address>
4770    </author>
4771    <author initials="P." surname="Leach" fullname="P. Leach">
4772      <organization>Microsoft Corporation</organization>
4773      <address><email></email></address>
4774    </author>
4775    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4776      <organization>W3C</organization>
4777      <address><email></email></address>
4778    </author>
4779    <date month="June" year="1999"/>
4780  </front>
4781  <seriesInfo name="RFC" value="2616"/>
4784<reference anchor='RFC2817'>
4785  <front>
4786    <title>Upgrading to TLS Within HTTP/1.1</title>
4787    <author initials='R.' surname='Khare' fullname='R. Khare'>
4788      <organization>4K Associates / UC Irvine</organization>
4789      <address><email></email></address>
4790    </author>
4791    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4792      <organization>Agranat Systems, Inc.</organization>
4793      <address><email></email></address>
4794    </author>
4795    <date year='2000' month='May' />
4796  </front>
4797  <seriesInfo name='RFC' value='2817' />
4800<reference anchor='RFC2818'>
4801  <front>
4802    <title>HTTP Over TLS</title>
4803    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4804      <organization>RTFM, Inc.</organization>
4805      <address><email></email></address>
4806    </author>
4807    <date year='2000' month='May' />
4808  </front>
4809  <seriesInfo name='RFC' value='2818' />
4812<reference anchor='RFC3040'>
4813  <front>
4814    <title>Internet Web Replication and Caching Taxonomy</title>
4815    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4816      <organization>Equinix, Inc.</organization>
4817    </author>
4818    <author initials='I.' surname='Melve' fullname='I. Melve'>
4819      <organization>UNINETT</organization>
4820    </author>
4821    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4822      <organization>CacheFlow Inc.</organization>
4823    </author>
4824    <date year='2001' month='January' />
4825  </front>
4826  <seriesInfo name='RFC' value='3040' />
4829<reference anchor='BCP90'>
4830  <front>
4831    <title>Registration Procedures for Message Header Fields</title>
4832    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4833      <organization>Nine by Nine</organization>
4834      <address><email></email></address>
4835    </author>
4836    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4837      <organization>BEA Systems</organization>
4838      <address><email></email></address>
4839    </author>
4840    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4841      <organization>HP Labs</organization>
4842      <address><email></email></address>
4843    </author>
4844    <date year='2004' month='September' />
4845  </front>
4846  <seriesInfo name='BCP' value='90' />
4847  <seriesInfo name='RFC' value='3864' />
4850<reference anchor='RFC4033'>
4851  <front>
4852    <title>DNS Security Introduction and Requirements</title>
4853    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4854    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4855    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4856    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4857    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4858    <date year='2005' month='March' />
4859  </front>
4860  <seriesInfo name='RFC' value='4033' />
4863<reference anchor="BCP13">
4864  <front>
4865    <title>Media Type Specifications and Registration Procedures</title>
4866    <author initials="N." surname="Freed" fullname="Ned Freed">
4867      <organization>Oracle</organization>
4868      <address>
4869        <email></email>
4870      </address>
4871    </author>
4872    <author initials="J." surname="Klensin" fullname="John C. Klensin">
4873      <address>
4874        <email></email>
4875      </address>
4876    </author>
4877    <author initials="T." surname="Hansen" fullname="Tony Hansen">
4878      <organization>AT&amp;T Laboratories</organization>
4879      <address>
4880        <email></email>
4881      </address>
4882    </author>
4883    <date year="2013" month="January"/>
4884  </front>
4885  <seriesInfo name="BCP" value="13"/>
4886  <seriesInfo name="RFC" value="6838"/>
4889<reference anchor='BCP115'>
4890  <front>
4891    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4892    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4893      <organization>AT&amp;T Laboratories</organization>
4894      <address>
4895        <email></email>
4896      </address>
4897    </author>
4898    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4899      <organization>Qualcomm, Inc.</organization>
4900      <address>
4901        <email></email>
4902      </address>
4903    </author>
4904    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4905      <organization>Adobe Systems</organization>
4906      <address>
4907        <email></email>
4908      </address>
4909    </author>
4910    <date year='2006' month='February' />
4911  </front>
4912  <seriesInfo name='BCP' value='115' />
4913  <seriesInfo name='RFC' value='4395' />
4916<reference anchor='RFC4559'>
4917  <front>
4918    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4919    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4920    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4921    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4922    <date year='2006' month='June' />
4923  </front>
4924  <seriesInfo name='RFC' value='4559' />
4927<reference anchor='RFC5226'>
4928  <front>
4929    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4930    <author initials='T.' surname='Narten' fullname='T. Narten'>
4931      <organization>IBM</organization>
4932      <address><email></email></address>
4933    </author>
4934    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4935      <organization>Google</organization>
4936      <address><email></email></address>
4937    </author>
4938    <date year='2008' month='May' />
4939  </front>
4940  <seriesInfo name='BCP' value='26' />
4941  <seriesInfo name='RFC' value='5226' />
4944<reference anchor='RFC5246'>
4945   <front>
4946      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4947      <author initials='T.' surname='Dierks' fullname='T. Dierks'>
4948         <organization />
4949      </author>
4950      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4951         <organization>RTFM, Inc.</organization>
4952      </author>
4953      <date year='2008' month='August' />
4954   </front>
4955   <seriesInfo name='RFC' value='5246' />
4958<reference anchor="RFC5322">
4959  <front>
4960    <title>Internet Message Format</title>
4961    <author initials="P." surname="Resnick" fullname="P. Resnick">
4962      <organization>Qualcomm Incorporated</organization>
4963    </author>
4964    <date year="2008" month="October"/>
4965  </front>
4966  <seriesInfo name="RFC" value="5322"/>
4969<reference anchor="RFC6265">
4970  <front>
4971    <title>HTTP State Management Mechanism</title>
4972    <author initials="A." surname="Barth" fullname="Adam Barth">
4973      <organization abbrev="U.C. Berkeley">
4974        University of California, Berkeley
4975      </organization>
4976      <address><email></email></address>
4977    </author>
4978    <date year="2011" month="April" />
4979  </front>
4980  <seriesInfo name="RFC" value="6265"/>
4983<reference anchor='RFC6585'>
4984  <front>
4985    <title>Additional HTTP Status Codes</title>
4986    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4987      <organization>Rackspace</organization>
4988    </author>
4989    <author initials='R.' surname='Fielding' fullname='R. Fielding'>
4990      <organization>Adobe</organization>
4991    </author>
4992    <date year='2012' month='April' />
4993   </front>
4994   <seriesInfo name='RFC' value='6585' />
4997<!--<reference anchor='BCP97'>
4998  <front>
4999    <title>Handling Normative References to Standards-Track Documents</title>
5000    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
5001      <address>
5002        <email></email>
5003      </address>
5004    </author>
5005    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
5006      <organization>MIT</organization>
5007      <address>
5008        <email></email>
5009      </address>
5010    </author>
5011    <date year='2007' month='June' />
5012  </front>
5013  <seriesInfo name='BCP' value='97' />
5014  <seriesInfo name='RFC' value='4897' />
5017<reference anchor="Kri2001" target="">
5018  <front>
5019    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
5020    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
5021    <date year="2001" month="November"/>
5022  </front>
5023  <seriesInfo name="ACM Transactions on Internet Technology" value="1(2)"/>
5029<section title="HTTP Version History" anchor="compatibility">
5031   HTTP has been in use since 1990. The first version, later referred to as
5032   HTTP/0.9, was a simple protocol for hypertext data transfer across the
5033   Internet, using only a single request method (GET) and no metadata.
5034   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
5035   methods and MIME-like messaging, allowing for metadata to be transferred
5036   and modifiers placed on the request/response semantics. However,
5037   HTTP/1.0 did not sufficiently take into consideration the effects of
5038   hierarchical proxies, caching, the need for persistent connections, or
5039   name-based virtual hosts. The proliferation of incompletely-implemented
5040   applications calling themselves "HTTP/1.0" further necessitated a
5041   protocol version change in order for two communicating applications
5042   to determine each other's true capabilities.
5045   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
5046   requirements that enable reliable implementations, adding only
5047   those features that can either be safely ignored by an HTTP/1.0
5048   recipient or only sent when communicating with a party advertising
5049   conformance with HTTP/1.1.
5052   HTTP/1.1 has been designed to make supporting previous versions easy.
5053   A general-purpose HTTP/1.1 server ought to be able to understand any valid
5054   request in the format of HTTP/1.0, responding appropriately with an
5055   HTTP/1.1 message that only uses features understood (or safely ignored) by
5056   HTTP/1.0 clients. Likewise, an HTTP/1.1 client can be expected to
5057   understand any valid HTTP/1.0 response.
5060   Since HTTP/0.9 did not support header fields in a request, there is no
5061   mechanism for it to support name-based virtual hosts (selection of resource
5062   by inspection of the <x:ref>Host</x:ref> header field).
5063   Any server that implements name-based virtual hosts ought to disable
5064   support for HTTP/0.9. Most requests that appear to be HTTP/0.9 are, in
5065   fact, badly constructed HTTP/1.x requests caused by a client failing to
5066   properly encode the request-target.
5069<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
5071   This section summarizes major differences between versions HTTP/1.0
5072   and HTTP/1.1.
5075<section title="Multi-homed Web Servers" anchor="">
5077   The requirements that clients and servers support the <x:ref>Host</x:ref>
5078   header field (<xref target=""/>), report an error if it is
5079   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
5080   are among the most important changes defined by HTTP/1.1.
5083   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
5084   addresses and servers; there was no other established mechanism for
5085   distinguishing the intended server of a request than the IP address
5086   to which that request was directed. The <x:ref>Host</x:ref> header field was
5087   introduced during the development of HTTP/1.1 and, though it was
5088   quickly implemented by most HTTP/1.0 browsers, additional requirements
5089   were placed on all HTTP/1.1 requests in order to ensure complete
5090   adoption.  At the time of this writing, most HTTP-based services
5091   are dependent upon the Host header field for targeting requests.
5095<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
5097   In HTTP/1.0, each connection is established by the client prior to the
5098   request and closed by the server after sending the response. However, some
5099   implementations implement the explicitly negotiated ("Keep-Alive") version
5100   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
5101   target="RFC2068"/>.
5104   Some clients and servers might wish to be compatible with these previous
5105   approaches to persistent connections, by explicitly negotiating for them
5106   with a "Connection: keep-alive" request header field. However, some
5107   experimental implementations of HTTP/1.0 persistent connections are faulty;
5108   for example, if an HTTP/1.0 proxy server doesn't understand
5109   <x:ref>Connection</x:ref>, it will erroneously forward that header field
5110   to the next inbound server, which would result in a hung connection.
5113   One attempted solution was the introduction of a Proxy-Connection header
5114   field, targeted specifically at proxies. In practice, this was also
5115   unworkable, because proxies are often deployed in multiple layers, bringing
5116   about the same problem discussed above.
5119   As a result, clients are encouraged not to send the Proxy-Connection header
5120   field in any requests.
5123   Clients are also encouraged to consider the use of Connection: keep-alive
5124   in requests carefully; while they can enable persistent connections with
5125   HTTP/1.0 servers, clients using them will need to monitor the
5126   connection for "hung" requests (which indicate that the client ought stop
5127   sending the header field), and this mechanism ought not be used by clients
5128   at all when a proxy is being used.
5132<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
5134   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
5135   (<xref target="header.transfer-encoding"/>).
5136   Transfer codings need to be decoded prior to forwarding an HTTP message
5137   over a MIME-compliant protocol.
5143<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
5145  HTTP's approach to error handling has been explained.
5146  (<xref target="conformance" />)
5149  The HTTP-version ABNF production has been clarified to be case-sensitive.
5150  Additionally, version numbers has been restricted to single digits, due
5151  to the fact that implementations are known to handle multi-digit version
5152  numbers incorrectly.
5153  (<xref target="http.version"/>)
5156  Userinfo (i.e., username and password) are now disallowed in HTTP and
5157  HTTPS URIs, because of security issues related to their transmission on the
5158  wire.
5159  (<xref target="http.uri" />)
5162  The HTTPS URI scheme is now defined by this specification; previously,
5163  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
5164  Furthermore, it implies end-to-end security.
5165  (<xref target="https.uri"/>)
5168  HTTP messages can be (and often are) buffered by implementations; despite
5169  it sometimes being available as a stream, HTTP is fundamentally a
5170  message-oriented protocol.
5171  Minimum supported sizes for various protocol elements have been
5172  suggested, to improve interoperability.
5173  (<xref target="http.message" />)
5176  Invalid whitespace around field-names is now required to be rejected,
5177  because accepting it represents a security vulnerability.
5178  The ABNF productions defining header fields now only list the field value.
5179  (<xref target="header.fields"/>)
5182  Rules about implicit linear whitespace between certain grammar productions
5183  have been removed; now whitespace is only allowed where specifically
5184  defined in the ABNF.
5185  (<xref target="whitespace"/>)
5188  Header fields that span multiple lines ("line folding") are deprecated.
5189  (<xref target="field.parsing" />)
5192  The NUL octet is no longer allowed in comment and quoted-string text, and
5193  handling of backslash-escaping in them has been clarified.
5194  The quoted-pair rule no longer allows escaping control characters other than
5195  HTAB.
5196  Non-ASCII content in header fields and the reason phrase has been obsoleted
5197  and made opaque (the TEXT rule was removed).
5198  (<xref target="field.components"/>)
5201  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
5202  handled as errors by recipients.
5203  (<xref target="header.content-length"/>)
5206  The algorithm for determining the message body length has been clarified
5207  to indicate all of the special cases (e.g., driven by methods or status
5208  codes) that affect it, and that new protocol elements cannot define such
5209  special cases.
5210  CONNECT is a new, special case in determining message body length.
5211  "multipart/byteranges" is no longer a way of determining message body length
5212  detection.
5213  (<xref target="message.body.length"/>)
5216  The "identity" transfer coding token has been removed.
5217  (Sections <xref format="counter" target="message.body"/> and
5218  <xref format="counter" target="transfer.codings"/>)
5221  Chunk length does not include the count of the octets in the
5222  chunk header and trailer.
5223  Line folding in chunk extensions is  disallowed.
5224  (<xref target="chunked.encoding"/>)
5227  The meaning of the "deflate" content coding has been clarified.
5228  (<xref target="deflate.coding" />)
5231  The segment + query components of RFC 3986 have been used to define the
5232  request-target, instead of abs_path from RFC 1808.
5233  The asterisk-form of the request-target is only allowed with the OPTIONS
5234  method.
5235  (<xref target="request-target"/>)
5238  The term "Effective Request URI" has been introduced.
5239  (<xref target="effective.request.uri" />)
5242  Gateways do not need to generate <x:ref>Via</x:ref> header fields anymore.
5243  (<xref target="header.via"/>)
5246  Exactly when "close" connection options have to be sent has been clarified.
5247  Also, "hop-by-hop" header fields are required to appear in the Connection header
5248  field; just because they're defined as hop-by-hop in this specification
5249  doesn't exempt them.
5250  (<xref target="header.connection"/>)
5253  The limit of two connections per server has been removed.
5254  An idempotent sequence of requests is no longer required to be retried.
5255  The requirement to retry requests under certain circumstances when the
5256  server prematurely closes the connection has been removed.
5257  Also, some extraneous requirements about when servers are allowed to close
5258  connections prematurely have been removed.
5259  (<xref target="persistent.connections"/>)
5262  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
5263  responses other than 101 (this was incorporated from <xref
5264  target="RFC2817"/>). Furthermore, the ordering in the field value is now
5265  significant.
5266  (<xref target="header.upgrade"/>)
5269  Empty list elements in list productions (e.g., a list header field containing
5270  ", ,") have been deprecated.
5271  (<xref target="abnf.extension"/>)
5274  Registration of Transfer Codings now requires IETF Review
5275  (<xref target="transfer.coding.registry"/>)
5278  This specification now defines the Upgrade Token Registry, previously
5279  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
5280  (<xref target="upgrade.token.registry"/>)
5283  The expectation to support HTTP/0.9 requests has been removed.
5284  (<xref target="compatibility"/>)
5287  Issues with the Keep-Alive and Proxy-Connection header fields in requests
5288  are pointed out, with use of the latter being discouraged altogether.
5289  (<xref target="compatibility.with.http.1.0.persistent.connections" />)
5294<?BEGININC p1-messaging.abnf-appendix ?>
5295<section xmlns:x="" title="Collected ABNF" anchor="collected.abnf">
5297<artwork type="abnf" name="p1-messaging.parsed-abnf">
5298<x:ref>BWS</x:ref> = OWS
5300<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
5301 connection-option ] )
5302<x:ref>Content-Length</x:ref> = 1*DIGIT
5304<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
5305 ]
5306<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
5307<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
5308<x:ref>Host</x:ref> = uri-host [ ":" port ]
5310<x:ref>OWS</x:ref> = *( SP / HTAB )
5312<x:ref>RWS</x:ref> = 1*( SP / HTAB )
5314<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
5315<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
5316<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
5317 transfer-coding ] )
5319<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
5320<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
5322<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
5323 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
5324 comment ] ) ] )
5326<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
5327<x:ref>absolute-form</x:ref> = absolute-URI
5328<x:ref>absolute-path</x:ref> = 1*( "/" segment )
5329<x:ref>asterisk-form</x:ref> = "*"
5330<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
5331<x:ref>authority-form</x:ref> = authority
5333<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
5334<x:ref>chunk-data</x:ref> = 1*OCTET
5335<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
5336<x:ref>chunk-ext-name</x:ref> = token
5337<x:ref>chunk-ext-val</x:ref> = token / quoted-string
5338<x:ref>chunk-size</x:ref> = 1*HEXDIG
5339<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
5340<x:ref>comment</x:ref> = "(" *( ctext / quoted-pair / comment ) ")"
5341<x:ref>connection-option</x:ref> = token
5342<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
5343 / %x2A-5B ; '*'-'['
5344 / %x5D-7E ; ']'-'~'
5345 / obs-text
5347<x:ref>field-content</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5348<x:ref>field-name</x:ref> = token
5349<x:ref>field-value</x:ref> = *( field-content / obs-fold )
5350<x:ref>fragment</x:ref> = &lt;fragment, defined in [RFC3986], Section 3.5&gt;
5352<x:ref>header-field</x:ref> = field-name ":" OWS field-value OWS
5353<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ] [ "#"
5354 fragment ]
5355<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ] [ "#"
5356 fragment ]
5358<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
5360<x:ref>message-body</x:ref> = *OCTET
5361<x:ref>method</x:ref> = token
5363<x:ref>obs-fold</x:ref> = CRLF ( SP / HTAB )
5364<x:ref>obs-text</x:ref> = %x80-FF
5365<x:ref>origin-form</x:ref> = absolute-path [ "?" query ]
5367<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
5368<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
5369<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
5370<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
5371<x:ref>protocol-name</x:ref> = token
5372<x:ref>protocol-version</x:ref> = token
5373<x:ref>pseudonym</x:ref> = token
5375<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5376 / %x5D-7E ; ']'-'~'
5377 / obs-text
5378<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5379<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5380<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5382<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5383<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5384<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5385<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5386<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5387<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5388<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5389 asterisk-form
5391<x:ref>segment</x:ref> = &lt;segment, defined in [RFC3986], Section 3.3&gt;
5392<x:ref>start-line</x:ref> = request-line / status-line
5393<x:ref>status-code</x:ref> = 3DIGIT
5394<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5396<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5397<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5398<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5399 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5400<x:ref>token</x:ref> = 1*tchar
5401<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5402<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5403 transfer-extension
5404<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5405<x:ref>transfer-parameter</x:ref> = token BWS "=" BWS ( token / quoted-string )
5407<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5411<?ENDINC p1-messaging.abnf-appendix ?>
5413<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5415<section title="Since RFC 2616">
5417  Changes up to the IETF Last Call draft are summarized
5418  in <eref target=""/>.
5422<section title="Since draft-ietf-httpbis-p1-messaging-24" anchor="changes.since.24">
5424  Closed issues:
5425  <list style="symbols">
5426    <t>
5427      <eref target=""/>:
5428      "APPSDIR review of draft-ietf-httpbis-p1-messaging-24"
5429    </t>
5430    <t>
5431      <eref target=""/>:
5432      "integer value parsing"
5433    </t>
5434    <t>
5435      <eref target=""/>:
5436      "move IANA registrations to correct draft"
5437    </t>
5438  </list>
5442<section title="Since draft-ietf-httpbis-p1-messaging-25" anchor="changes.since.25">
5444  Closed issues:
5445  <list style="symbols">
5446    <t>
5447      <eref target=""/>:
5448      "check media type registration templates"
5449    </t>
5450    <t>
5451      <eref target=""/>:
5452      "Redundant rule quoted-str-nf"
5453    </t>
5454    <t>
5455      <eref target=""/>:
5456      "add 'stateless' to Abstract"
5457    </t>
5458    <t>
5459      <eref target=""/>:
5460      "clarify ABNF layering"
5461    </t>
5462    <t>
5463      <eref target=""/>:
5464      "use of 'word' ABNF production"
5465    </t>
5466    <t>
5467      <eref target=""/>:
5468      "improve introduction of list rule"
5469    </t>
5470    <t>
5471      <eref target=""/>:
5472      "moving 2616/2068/2145 to historic"
5473    </t>
5474    <t>
5475      <eref target=""/>:
5476      "augment security considerations with pointers to current research"
5477    </t>
5478  </list>
5481  Partly resolved issues:
5482  <list style="symbols">
5483    <t>
5484      <eref target=""/>:
5485      "IESG ballot on draft-ietf-httpbis-p1-messaging-25"
5486    </t>
5487  </list>
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