source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2569

Last change on this file since 2569 was 2569, checked in by fielding@…, 7 years ago

(editorial) OWASP only provides useful additional info for web application semantics and authentication; see #520 and #549

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 236.3 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 GET                    "<xref target='Part2' x:rel='#GET' xmlns:x=''/>">
29  <!ENTITY HEAD                   "<xref target='Part2' x:rel='#HEAD' xmlns:x=''/>">
30  <!ENTITY header-allow           "<xref target='Part2' x:rel='#header.allow' xmlns:x=''/>">
31  <!ENTITY header-cache-control   "<xref target='Part6' x:rel='#header.cache-control' xmlns:x=''/>">
32  <!ENTITY header-content-encoding    "<xref target='Part2' x:rel='#header.content-encoding' xmlns:x=''/>">
33  <!ENTITY header-content-location    "<xref target='Part2' x:rel='#header.content-location' xmlns:x=''/>">
34  <!ENTITY header-content-range   "<xref target='Part5' x:rel='#header.content-range' xmlns:x=''/>">
35  <!ENTITY header-content-type    "<xref target='Part2' x:rel='#header.content-type' xmlns:x=''/>">
36  <!ENTITY header-date            "<xref target='Part2' x:rel='' xmlns:x=''/>">
37  <!ENTITY header-etag            "<xref target='Part4' x:rel='#header.etag' xmlns:x=''/>">
38  <!ENTITY header-expect          "<xref target='Part2' x:rel='#header.expect' xmlns:x=''/>">
39  <!ENTITY header-expires         "<xref target='Part6' x:rel='#header.expires' xmlns:x=''/>">
40  <!ENTITY header-last-modified   "<xref target='Part4' x:rel='#header.last-modified' xmlns:x=''/>">
41  <!ENTITY header-mime-version    "<xref target='Part2' x:rel='#mime-version' xmlns:x=''/>">
42  <!ENTITY header-pragma          "<xref target='Part6' x:rel='#header.pragma' xmlns:x=''/>">
43  <!ENTITY header-proxy-authenticate  "<xref target='Part7' x:rel='#header.proxy-authenticate' xmlns:x=''/>">
44  <!ENTITY header-proxy-authorization "<xref target='Part7' x:rel='#header.proxy-authorization' xmlns:x=''/>">
45  <!ENTITY header-server          "<xref target='Part2' x:rel='#header.server' xmlns:x=''/>">
46  <!ENTITY header-warning         "<xref target='Part6' x:rel='#header.warning' xmlns:x=''/>">
47  <!ENTITY idempotent-methods     "<xref target='Part2' x:rel='#idempotent.methods' xmlns:x=''/>">
48  <!ENTITY safe-methods           "<xref target='Part2' x:rel='#safe.methods' xmlns:x=''/>">
49  <!ENTITY methods                "<xref target='Part2' x:rel='#methods' xmlns:x=''/>">
50  <!ENTITY OPTIONS                "<xref target='Part2' x:rel='#OPTIONS' xmlns:x=''/>">
51  <!ENTITY qvalue                 "<xref target='Part2' x:rel='#quality.values' xmlns:x=''/>">
52  <!ENTITY resource               "<xref target='Part2' x:rel='#resources' xmlns:x=''/>">
53  <!ENTITY semantics              "<xref target='Part2' xmlns:x=''/>">
54  <!ENTITY status-codes           "<xref target='Part2' x:rel='' xmlns:x=''/>">
55  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x=''/>">
56  <!ENTITY status-203             "<xref target='Part2' x:rel='#status.203' xmlns:x=''/>">
57  <!ENTITY status-3xx             "<xref target='Part2' x:rel='#status.3xx' xmlns:x=''/>">
58  <!ENTITY status-304             "<xref target='Part4' x:rel='#status.304' xmlns:x=''/>">
59  <!ENTITY status-4xx             "<xref target='Part2' x:rel='#status.4xx' xmlns:x=''/>">
60  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x=''/>">
61  <!ENTITY iana-header-registry   "<xref target='Part2' x:rel='#header.field.registry' xmlns:x=''/>">
63<?rfc toc="yes" ?>
64<?rfc symrefs="yes" ?>
65<?rfc sortrefs="yes" ?>
66<?rfc compact="yes"?>
67<?rfc subcompact="no" ?>
68<?rfc linkmailto="no" ?>
69<?rfc editing="no" ?>
70<?rfc comments="yes"?>
71<?rfc inline="yes"?>
72<?rfc rfcedstyle="yes"?>
73<?rfc-ext allow-markup-in-artwork="yes" ?>
74<?rfc-ext include-references-in-index="yes" ?>
75<rfc obsoletes="2145,2616" updates="2817,2818" category="std" x:maturity-level="proposed"
76     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
77     xmlns:x=''>
78<x:link rel="next" basename="p2-semantics"/>
79<x:feedback template="{docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
82  <title abbrev="HTTP/1.1 Message Syntax and Routing">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
84  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
85    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
86    <address>
87      <postal>
88        <street>345 Park Ave</street>
89        <city>San Jose</city>
90        <region>CA</region>
91        <code>95110</code>
92        <country>USA</country>
93      </postal>
94      <email></email>
95      <uri></uri>
96    </address>
97  </author>
99  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
100    <organization abbrev="greenbytes">greenbytes GmbH</organization>
101    <address>
102      <postal>
103        <street>Hafenweg 16</street>
104        <city>Muenster</city><region>NW</region><code>48155</code>
105        <country>Germany</country>
106      </postal>
107      <email></email>
108      <uri></uri>
109    </address>
110  </author>
112  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
113  <workgroup>HTTPbis Working Group</workgroup>
117   The Hypertext Transfer Protocol (HTTP) is a stateless application-level
118   protocol for distributed, collaborative, hypertext information systems.
119   This document provides an overview of HTTP architecture and its associated
120   terminology, defines the "http" and "https" Uniform Resource Identifier
121   (URI) schemes, defines the HTTP/1.1 message syntax and parsing
122   requirements, and describes related security concerns for implementations.
126<note title="Editorial Note (To be removed by RFC Editor)">
127  <t>
128    Discussion of this draft takes place on the HTTPBIS working group
129    mailing list (, which is archived at
130    <eref target=""/>.
131  </t>
132  <t>
133    The current issues list is at
134    <eref target=""/> and related
135    documents (including fancy diffs) can be found at
136    <eref target=""/>.
137  </t>
138  <t>
139    The changes in this draft are summarized in <xref target="changes.since.25"/>.
140  </t>
144<section title="Introduction" anchor="introduction">
146   The Hypertext Transfer Protocol (HTTP) is a stateless application-level
147   request/response protocol that uses extensible semantics and
148   self-descriptive message payloads for flexible interaction with
149   network-based hypertext information systems. This document is the first in
150   a series of documents that collectively form the HTTP/1.1 specification:
151   <list style="empty">
152    <t>RFC xxx1: Message Syntax and Routing</t>
153    <t><xref target="Part2" x:fmt="none">RFC xxx2</xref>: Semantics and Content</t>
154    <t><xref target="Part4" x:fmt="none">RFC xxx3</xref>: Conditional Requests</t>
155    <t><xref target="Part5" x:fmt="none">RFC xxx4</xref>: Range Requests</t>
156    <t><xref target="Part6" x:fmt="none">RFC xxx5</xref>: Caching</t>
157    <t><xref target="Part7" x:fmt="none">RFC xxx6</xref>: Authentication</t>
158   </list>
161   This HTTP/1.1 specification obsoletes
162   <xref target="RFC2616" x:fmt="none">RFC 2616</xref> and
163   <xref target="RFC2145" x:fmt="none">RFC 2145</xref> (on HTTP versioning).
164   This specification also updates the use of CONNECT to establish a tunnel,
165   previously defined in <xref target="RFC2817" x:fmt="none">RFC 2817</xref>,
166   and defines the "https" URI scheme that was described informally in
167   <xref target="RFC2818" x:fmt="none">RFC 2818</xref>.
170   HTTP is a generic interface protocol for information systems. It is
171   designed to hide the details of how a service is implemented by presenting
172   a uniform interface to clients that is independent of the types of
173   resources provided. Likewise, servers do not need to be aware of each
174   client's purpose: an HTTP request can be considered in isolation rather
175   than being associated with a specific type of client or a predetermined
176   sequence of application steps. The result is a protocol that can be used
177   effectively in many different contexts and for which implementations can
178   evolve independently over time.
181   HTTP is also designed for use as an intermediation protocol for translating
182   communication to and from non-HTTP information systems.
183   HTTP proxies and gateways can provide access to alternative information
184   services by translating their diverse protocols into a hypertext
185   format that can be viewed and manipulated by clients in the same way
186   as HTTP services.
189   One consequence of this flexibility is that the protocol cannot be
190   defined in terms of what occurs behind the interface. Instead, we
191   are limited to defining the syntax of communication, the intent
192   of received communication, and the expected behavior of recipients.
193   If the communication is considered in isolation, then successful
194   actions ought to be reflected in corresponding changes to the
195   observable interface provided by servers. However, since multiple
196   clients might act in parallel and perhaps at cross-purposes, we
197   cannot require that such changes be observable beyond the scope
198   of a single response.
201   This document describes the architectural elements that are used or
202   referred to in HTTP, defines the "http" and "https" URI schemes,
203   describes overall network operation and connection management,
204   and defines HTTP message framing and forwarding requirements.
205   Our goal is to define all of the mechanisms necessary for HTTP message
206   handling that are independent of message semantics, thereby defining the
207   complete set of requirements for message parsers and
208   message-forwarding intermediaries.
212<section title="Requirement Notation" anchor="intro.requirements">
214   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
215   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
216   document are to be interpreted as described in <xref target="RFC2119"/>.
219   Conformance criteria and considerations regarding error handling
220   are defined in <xref target="conformance"/>.
224<section title="Syntax Notation" anchor="notation">
225<iref primary="true" item="Grammar" subitem="ALPHA"/>
226<iref primary="true" item="Grammar" subitem="CR"/>
227<iref primary="true" item="Grammar" subitem="CRLF"/>
228<iref primary="true" item="Grammar" subitem="CTL"/>
229<iref primary="true" item="Grammar" subitem="DIGIT"/>
230<iref primary="true" item="Grammar" subitem="DQUOTE"/>
231<iref primary="true" item="Grammar" subitem="HEXDIG"/>
232<iref primary="true" item="Grammar" subitem="HTAB"/>
233<iref primary="true" item="Grammar" subitem="LF"/>
234<iref primary="true" item="Grammar" subitem="OCTET"/>
235<iref primary="true" item="Grammar" subitem="SP"/>
236<iref primary="true" item="Grammar" subitem="VCHAR"/>
238   This specification uses the Augmented Backus-Naur Form (ABNF) notation of
239   <xref target="RFC5234"/> with a list extension, defined in
240   <xref target="abnf.extension"/>, that allows for compact definition of
241   comma-separated lists using a '#' operator (similar to how the '*' operator
242   indicates repetition).
243   <xref target="collected.abnf"/> shows the collected grammar with all list
244   operators expanded to standard ABNF notation.
246<t anchor="core.rules">
247  <x:anchor-alias value="ALPHA"/>
248  <x:anchor-alias value="CTL"/>
249  <x:anchor-alias value="CR"/>
250  <x:anchor-alias value="CRLF"/>
251  <x:anchor-alias value="DIGIT"/>
252  <x:anchor-alias value="DQUOTE"/>
253  <x:anchor-alias value="HEXDIG"/>
254  <x:anchor-alias value="HTAB"/>
255  <x:anchor-alias value="LF"/>
256  <x:anchor-alias value="OCTET"/>
257  <x:anchor-alias value="SP"/>
258  <x:anchor-alias value="VCHAR"/>
259   The following core rules are included by
260   reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
261   ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
262   DIGIT (decimal 0-9), DQUOTE (double quote),
263   HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed),
264   OCTET (any 8-bit sequence of data), SP (space), and
265   VCHAR (any visible <xref target="USASCII"/> character).
268   As a convention, ABNF rule names prefixed with "obs-" denote
269   "obsolete" grammar rules that appear for historical reasons.
274<section title="Architecture" anchor="architecture">
276   HTTP was created for the World Wide Web (WWW) architecture
277   and has evolved over time to support the scalability needs of a worldwide
278   hypertext system. Much of that architecture is reflected in the terminology
279   and syntax productions used to define HTTP.
282<section title="Client/Server Messaging" anchor="operation">
283<iref primary="true" item="client"/>
284<iref primary="true" item="server"/>
285<iref primary="true" item="connection"/>
287   HTTP is a stateless request/response protocol that operates by exchanging
288   <x:dfn>messages</x:dfn> (<xref target="http.message"/>) across a reliable
289   transport or session-layer
290   "<x:dfn>connection</x:dfn>" (<xref target=""/>).
291   An HTTP "<x:dfn>client</x:dfn>" is a program that establishes a connection
292   to a server for the purpose of sending one or more HTTP requests.
293   An HTTP "<x:dfn>server</x:dfn>" is a program that accepts connections
294   in order to service HTTP requests by sending HTTP responses.
296<iref primary="true" item="user agent"/>
297<iref primary="true" item="origin server"/>
298<iref primary="true" item="browser"/>
299<iref primary="true" item="spider"/>
300<iref primary="true" item="sender"/>
301<iref primary="true" item="recipient"/>
303   The terms client and server refer only to the roles that
304   these programs perform for a particular connection.  The same program
305   might act as a client on some connections and a server on others.
306   The term "<x:dfn>user agent</x:dfn>" refers to any of the various
307   client programs that initiate a request, including (but not limited to)
308   browsers, spiders (web-based robots), command-line tools, custom
309   applications, and mobile apps.
310   The term "<x:dfn>origin server</x:dfn>" refers to the program that can
311   originate authoritative responses for a given target resource.
312   The terms "<x:dfn>sender</x:dfn>" and "<x:dfn>recipient</x:dfn>" refer to
313   any implementation that sends or receives a given message, respectively.
316   HTTP relies upon the Uniform Resource Identifier (URI)
317   standard <xref target="RFC3986"/> to indicate the target resource
318   (<xref target="target-resource"/>) and relationships between resources.
319   Messages are passed in a format similar to that used by Internet mail
320   <xref target="RFC5322"/> and the Multipurpose Internet Mail Extensions
321   (MIME) <xref target="RFC2045"/> (see &diff-mime; for the differences
322   between HTTP and MIME messages).
325   Most HTTP communication consists of a retrieval request (GET) for
326   a representation of some resource identified by a URI.  In the
327   simplest case, this might be accomplished via a single bidirectional
328   connection (===) between the user agent (UA) and the origin server (O).
330<figure><artwork type="drawing">
331         request   &gt;
332    <x:highlight>UA</x:highlight> ======================================= <x:highlight>O</x:highlight>
333                                &lt;   response
335<iref primary="true" item="message"/>
336<iref primary="true" item="request"/>
337<iref primary="true" item="response"/>
339   A client sends an HTTP request to a server in the form of a <x:dfn>request</x:dfn>
340   message, beginning with a request-line that includes a method, URI, and
341   protocol version (<xref target="request.line"/>),
342   followed by header fields containing
343   request modifiers, client information, and representation metadata
344   (<xref target="header.fields"/>),
345   an empty line to indicate the end of the header section, and finally
346   a message body containing the payload body (if any,
347   <xref target="message.body"/>).
350   A server responds to a client's request by sending one or more HTTP
351   <x:dfn>response</x:dfn>
352   messages, each beginning with a status line that
353   includes the protocol version, a success or error code, and textual
354   reason phrase (<xref target="status.line"/>),
355   possibly followed by header fields containing server
356   information, resource metadata, and representation metadata
357   (<xref target="header.fields"/>),
358   an empty line to indicate the end of the header section, and finally
359   a message body containing the payload body (if any,
360   <xref target="message.body"/>).
363   A connection might be used for multiple request/response exchanges,
364   as defined in <xref target="persistent.connections"/>.
367   The following example illustrates a typical message exchange for a
368   GET request (&GET;) on the URI "":
371Client request:
372</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
373GET /hello.txt HTTP/1.1
374User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
376Accept-Language: en, mi
380Server response:
381</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
382HTTP/1.1 200 OK
383Date: Mon, 27 Jul 2009 12:28:53 GMT
384Server: Apache
385Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
386ETag: "34aa387-d-1568eb00"
387Accept-Ranges: bytes
388Content-Length: <x:length-of target="exbody"/>
389Vary: Accept-Encoding
390Content-Type: text/plain
392<x:span anchor="exbody">Hello World! My payload includes a trailing CRLF.
397<section title="Implementation Diversity" anchor="implementation-diversity">
399   When considering the design of HTTP, it is easy to fall into a trap of
400   thinking that all user agents are general-purpose browsers and all origin
401   servers are large public websites. That is not the case in practice.
402   Common HTTP user agents include household appliances, stereos, scales,
403   firmware update scripts, command-line programs, mobile apps,
404   and communication devices in a multitude of shapes and sizes.  Likewise,
405   common HTTP origin servers include home automation units, configurable
406   networking components, office machines, autonomous robots, news feeds,
407   traffic cameras, ad selectors, and video delivery platforms.
410   The term "user agent" does not imply that there is a human user directly
411   interacting with the software agent at the time of a request. In many
412   cases, a user agent is installed or configured to run in the background
413   and save its results for later inspection (or save only a subset of those
414   results that might be interesting or erroneous). Spiders, for example, are
415   typically given a start URI and configured to follow certain behavior while
416   crawling the Web as a hypertext graph.
419   The implementation diversity of HTTP means that not all user agents can
420   make interactive suggestions to their user or provide adequate warning for
421   security or privacy concerns. In the few cases where this
422   specification requires reporting of errors to the user, it is acceptable
423   for such reporting to only be observable in an error console or log file.
424   Likewise, requirements that an automated action be confirmed by the user
425   before proceeding might be met via advance configuration choices,
426   run-time options, or simple avoidance of the unsafe action; confirmation
427   does not imply any specific user interface or interruption of normal
428   processing if the user has already made that choice.
432<section title="Intermediaries" anchor="intermediaries">
433<iref primary="true" item="intermediary"/>
435   HTTP enables the use of intermediaries to satisfy requests through
436   a chain of connections.  There are three common forms of HTTP
437   <x:dfn>intermediary</x:dfn>: proxy, gateway, and tunnel.  In some cases,
438   a single intermediary might act as an origin server, proxy, gateway,
439   or tunnel, switching behavior based on the nature of each request.
441<figure><artwork type="drawing">
442         &gt;             &gt;             &gt;             &gt;
443    <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>
444               &lt;             &lt;             &lt;             &lt;
447   The figure above shows three intermediaries (A, B, and C) between the
448   user agent and origin server. A request or response message that
449   travels the whole chain will pass through four separate connections.
450   Some HTTP communication options
451   might apply only to the connection with the nearest, non-tunnel
452   neighbor, only to the end-points of the chain, or to all connections
453   along the chain. Although the diagram is linear, each participant might
454   be engaged in multiple, simultaneous communications. For example, B
455   might be receiving requests from many clients other than A, and/or
456   forwarding requests to servers other than C, at the same time that it
457   is handling A's request. Likewise, later requests might be sent through a
458   different path of connections, often based on dynamic configuration for
459   load balancing.   
462<iref primary="true" item="upstream"/><iref primary="true" item="downstream"/>
463<iref primary="true" item="inbound"/><iref primary="true" item="outbound"/>
464   The terms "<x:dfn>upstream</x:dfn>" and "<x:dfn>downstream</x:dfn>" are
465   used to describe directional requirements in relation to the message flow:
466   all messages flow from upstream to downstream.
467   The terms inbound and outbound are used to describe directional
468   requirements in relation to the request route:
469   "<x:dfn>inbound</x:dfn>" means toward the origin server and
470   "<x:dfn>outbound</x:dfn>" means toward the user agent.
472<t><iref primary="true" item="proxy"/>
473   A "<x:dfn>proxy</x:dfn>" is a message forwarding agent that is selected by the
474   client, usually via local configuration rules, to receive requests
475   for some type(s) of absolute URI and attempt to satisfy those
476   requests via translation through the HTTP interface.  Some translations
477   are minimal, such as for proxy requests for "http" URIs, whereas
478   other requests might require translation to and from entirely different
479   application-level protocols. Proxies are often used to group an
480   organization's HTTP requests through a common intermediary for the
481   sake of security, annotation services, or shared caching. Some proxies
482   are designed to apply transformations to selected messages or payloads
483   while they are being forwarded, as described in
484   <xref target="message.transformations"/>.
486<t><iref primary="true" item="gateway"/><iref primary="true" item="reverse proxy"/>
487<iref primary="true" item="accelerator"/>
488   A "<x:dfn>gateway</x:dfn>" (a.k.a., "<x:dfn>reverse proxy</x:dfn>") is an
489   intermediary that acts as an origin server for the outbound connection, but
490   translates received requests and forwards them inbound to another server or
491   servers. Gateways are often used to encapsulate legacy or untrusted
492   information services, to improve server performance through
493   "<x:dfn>accelerator</x:dfn>" caching, and to enable partitioning or load
494   balancing of HTTP services across multiple machines.
497   All HTTP requirements applicable to an origin server
498   also apply to the outbound communication of a gateway.
499   A gateway communicates with inbound servers using any protocol that
500   it desires, including private extensions to HTTP that are outside
501   the scope of this specification.  However, an HTTP-to-HTTP gateway
502   that wishes to interoperate with third-party HTTP servers ought to conform
503   to user agent requirements on the gateway's inbound connection.
505<t><iref primary="true" item="tunnel"/>
506   A "<x:dfn>tunnel</x:dfn>" acts as a blind relay between two connections
507   without changing the messages. Once active, a tunnel is not
508   considered a party to the HTTP communication, though the tunnel might
509   have been initiated by an HTTP request. A tunnel ceases to exist when
510   both ends of the relayed connection are closed. Tunnels are used to
511   extend a virtual connection through an intermediary, such as when
512   Transport Layer Security (TLS, <xref target="RFC5246"/>) is used to
513   establish confidential communication through a shared firewall proxy.
515<t><iref primary="true" item="interception proxy"/>
516<iref primary="true" item="transparent proxy"/>
517<iref primary="true" item="captive portal"/>
518   The above categories for intermediary only consider those acting as
519   participants in the HTTP communication.  There are also intermediaries
520   that can act on lower layers of the network protocol stack, filtering or
521   redirecting HTTP traffic without the knowledge or permission of message
522   senders. Network intermediaries often introduce security flaws or
523   interoperability problems by violating HTTP semantics.  For example, an
524   "<x:dfn>interception proxy</x:dfn>" <xref target="RFC3040"/> (also commonly
525   known as a "<x:dfn>transparent proxy</x:dfn>" <xref target="RFC1919"/> or
526   "<x:dfn>captive portal</x:dfn>")
527   differs from an HTTP proxy because it is not selected by the client.
528   Instead, an interception proxy filters or redirects outgoing TCP port 80
529   packets (and occasionally other common port traffic).
530   Interception proxies are commonly found on public network access points,
531   as a means of enforcing account subscription prior to allowing use of
532   non-local Internet services, and within corporate firewalls to enforce
533   network usage policies.
534   They are indistinguishable from a man-in-the-middle attack.
537   HTTP is defined as a stateless protocol, meaning that each request message
538   can be understood in isolation.  Many implementations depend on HTTP's
539   stateless design in order to reuse proxied connections or dynamically
540   load-balance requests across multiple servers.  Hence, a server &MUST-NOT;
541   assume that two requests on the same connection are from the same user
542   agent unless the connection is secured and specific to that agent.
543   Some non-standard HTTP extensions (e.g., <xref target="RFC4559"/>) have
544   been known to violate this requirement, resulting in security and
545   interoperability problems.
549<section title="Caches" anchor="caches">
550<iref primary="true" item="cache"/>
552   A "<x:dfn>cache</x:dfn>" is a local store of previous response messages and the
553   subsystem that controls its message storage, retrieval, and deletion.
554   A cache stores cacheable responses in order to reduce the response
555   time and network bandwidth consumption on future, equivalent
556   requests. Any client or server &MAY; employ a cache, though a cache
557   cannot be used by a server while it is acting as a tunnel.
560   The effect of a cache is that the request/response chain is shortened
561   if one of the participants along the chain has a cached response
562   applicable to that request. The following illustrates the resulting
563   chain if B has a cached copy of an earlier response from O (via C)
564   for a request that has not been cached by UA or A.
566<figure><artwork type="drawing">
567            &gt;             &gt;
568       <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>
569                  &lt;             &lt;
571<t><iref primary="true" item="cacheable"/>
572   A response is "<x:dfn>cacheable</x:dfn>" if a cache is allowed to store a copy of
573   the response message for use in answering subsequent requests.
574   Even when a response is cacheable, there might be additional
575   constraints placed by the client or by the origin server on when
576   that cached response can be used for a particular request. HTTP
577   requirements for cache behavior and cacheable responses are
578   defined in &caching-overview;. 
581   There are a wide variety of architectures and configurations
582   of caches deployed across the World Wide Web and
583   inside large organizations. These include national hierarchies
584   of proxy caches to save transoceanic bandwidth, collaborative systems that
585   broadcast or multicast cache entries, archives of pre-fetched cache
586   entries for use in off-line or high-latency environments, and so on.
590<section title="Conformance and Error Handling" anchor="conformance">
592   This specification targets conformance criteria according to the role of
593   a participant in HTTP communication.  Hence, HTTP requirements are placed
594   on senders, recipients, clients, servers, user agents, intermediaries,
595   origin servers, proxies, gateways, or caches, depending on what behavior
596   is being constrained by the requirement. Additional (social) requirements
597   are placed on implementations, resource owners, and protocol element
598   registrations when they apply beyond the scope of a single communication.
601   The verb "generate" is used instead of "send" where a requirement
602   differentiates between creating a protocol element and merely forwarding a
603   received element downstream.
606   An implementation is considered conformant if it complies with all of the
607   requirements associated with the roles it partakes in HTTP.
610   Conformance includes both the syntax and semantics of protocol
611   elements. A sender &MUST-NOT; generate protocol elements that convey a
612   meaning that is known by that sender to be false. A sender &MUST-NOT;
613   generate protocol elements that do not match the grammar defined by the
614   corresponding ABNF rules. Within a given message, a sender &MUST-NOT;
615   generate protocol elements or syntax alternatives that are only allowed to
616   be generated by participants in other roles (i.e., a role that the sender
617   does not have for that message).
620   When a received protocol element is parsed, the recipient &MUST; be able to
621   parse any value of reasonable length that is applicable to the recipient's
622   role and matches the grammar defined by the corresponding ABNF rules.
623   Note, however, that some received protocol elements might not be parsed.
624   For example, an intermediary forwarding a message might parse a
625   header-field into generic field-name and field-value components, but then
626   forward the header field without further parsing inside the field-value.
629   HTTP does not have specific length limitations for many of its protocol
630   elements because the lengths that might be appropriate will vary widely,
631   depending on the deployment context and purpose of the implementation.
632   Hence, interoperability between senders and recipients depends on shared
633   expectations regarding what is a reasonable length for each protocol
634   element. Furthermore, what is commonly understood to be a reasonable length
635   for some protocol elements has changed over the course of the past two
636   decades of HTTP use, and is expected to continue changing in the future.
639   At a minimum, a recipient &MUST; be able to parse and process protocol
640   element lengths that are at least as long as the values that it generates
641   for those same protocol elements in other messages. For example, an origin
642   server that publishes very long URI references to its own resources needs
643   to be able to parse and process those same references when received as a
644   request target.
647   A recipient &MUST; interpret a received protocol element according to the
648   semantics defined for it by this specification, including extensions to
649   this specification, unless the recipient has determined (through experience
650   or configuration) that the sender incorrectly implements what is implied by
651   those semantics.
652   For example, an origin server might disregard the contents of a received
653   <x:ref>Accept-Encoding</x:ref> header field if inspection of the
654   <x:ref>User-Agent</x:ref> header field indicates a specific implementation
655   version that is known to fail on receipt of certain content codings.
658   Unless noted otherwise, a recipient &MAY; attempt to recover a usable
659   protocol element from an invalid construct.  HTTP does not define
660   specific error handling mechanisms except when they have a direct impact
661   on security, since different applications of the protocol require
662   different error handling strategies.  For example, a Web browser might
663   wish to transparently recover from a response where the
664   <x:ref>Location</x:ref> header field doesn't parse according to the ABNF,
665   whereas a systems control client might consider any form of error recovery
666   to be dangerous.
670<section title="Protocol Versioning" anchor="http.version">
671  <x:anchor-alias value="HTTP-version"/>
672  <x:anchor-alias value="HTTP-name"/>
674   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate
675   versions of the protocol. This specification defines version "1.1".
676   The protocol version as a whole indicates the sender's conformance
677   with the set of requirements laid out in that version's corresponding
678   specification of HTTP.
681   The version of an HTTP message is indicated by an HTTP-version field
682   in the first line of the message. HTTP-version is case-sensitive.
684<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-version"/><iref primary="true" item="Grammar" subitem="HTTP-name"/>
685  <x:ref>HTTP-version</x:ref>  = <x:ref>HTTP-name</x:ref> "/" <x:ref>DIGIT</x:ref> "." <x:ref>DIGIT</x:ref>
686  <x:ref>HTTP-name</x:ref>     = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
689   The HTTP version number consists of two decimal digits separated by a "."
690   (period or decimal point).  The first digit ("major version") indicates the
691   HTTP messaging syntax, whereas the second digit ("minor version") indicates
692   the highest minor version within that major version to which the sender is
693   conformant and able to understand for future communication.  The minor
694   version advertises the sender's communication capabilities even when the
695   sender is only using a backwards-compatible subset of the protocol,
696   thereby letting the recipient know that more advanced features can
697   be used in response (by servers) or in future requests (by clients).
700   When an HTTP/1.1 message is sent to an HTTP/1.0 recipient
701   <xref target="RFC1945"/> or a recipient whose version is unknown,
702   the HTTP/1.1 message is constructed such that it can be interpreted
703   as a valid HTTP/1.0 message if all of the newer features are ignored.
704   This specification places recipient-version requirements on some
705   new features so that a conformant sender will only use compatible
706   features until it has determined, through configuration or the
707   receipt of a message, that the recipient supports HTTP/1.1.
710   The interpretation of a header field does not change between minor
711   versions of the same major HTTP version, though the default
712   behavior of a recipient in the absence of such a field can change.
713   Unless specified otherwise, header fields defined in HTTP/1.1 are
714   defined for all versions of HTTP/1.x.  In particular, the <x:ref>Host</x:ref>
715   and <x:ref>Connection</x:ref> header fields ought to be implemented by all
716   HTTP/1.x implementations whether or not they advertise conformance with
717   HTTP/1.1.
720   New header fields can be introduced without changing the protocol version
721   if their defined semantics allow them to be safely ignored by recipients
722   that do not recognize them. Header field extensibility is discussed in
723   <xref target="field.extensibility"/>.
726   Intermediaries that process HTTP messages (i.e., all intermediaries
727   other than those acting as tunnels) &MUST; send their own HTTP-version
728   in forwarded messages.  In other words, they are not allowed to blindly
729   forward the first line of an HTTP message without ensuring that the
730   protocol version in that message matches a version to which that
731   intermediary is conformant for both the receiving and
732   sending of messages.  Forwarding an HTTP message without rewriting
733   the HTTP-version might result in communication errors when downstream
734   recipients use the message sender's version to determine what features
735   are safe to use for later communication with that sender.
738   A client &SHOULD; send a request version equal to the highest
739   version to which the client is conformant and
740   whose major version is no higher than the highest version supported
741   by the server, if this is known.  A client &MUST-NOT; send a
742   version to which it is not conformant.
745   A client &MAY; send a lower request version if it is known that
746   the server incorrectly implements the HTTP specification, but only
747   after the client has attempted at least one normal request and determined
748   from the response status code or header fields (e.g., <x:ref>Server</x:ref>) that
749   the server improperly handles higher request versions.
752   A server &SHOULD; send a response version equal to the highest version to
753   which the server is conformant that has a major version less than or equal
754   to the one received in the request.
755   A server &MUST-NOT; send a version to which it is not conformant.
756   A server can send a <x:ref>505 (HTTP Version Not Supported)</x:ref>
757   response if it wishes, for any reason, to refuse service of the client's
758   major protocol version.
761   A server &MAY; send an HTTP/1.0 response to a request
762   if it is known or suspected that the client incorrectly implements the
763   HTTP specification and is incapable of correctly processing later
764   version responses, such as when a client fails to parse the version
765   number correctly or when an intermediary is known to blindly forward
766   the HTTP-version even when it doesn't conform to the given minor
767   version of the protocol. Such protocol downgrades &SHOULD-NOT; be
768   performed unless triggered by specific client attributes, such as when
769   one or more of the request header fields (e.g., <x:ref>User-Agent</x:ref>)
770   uniquely match the values sent by a client known to be in error.
773   The intention of HTTP's versioning design is that the major number
774   will only be incremented if an incompatible message syntax is
775   introduced, and that the minor number will only be incremented when
776   changes made to the protocol have the effect of adding to the message
777   semantics or implying additional capabilities of the sender.  However,
778   the minor version was not incremented for the changes introduced between
779   <xref target="RFC2068"/> and <xref target="RFC2616"/>, and this revision
780   has specifically avoided any such changes to the protocol.
783   When an HTTP message is received with a major version number that the
784   recipient implements, but a higher minor version number than what the
785   recipient implements, the recipient &SHOULD; process the message as if it
786   were in the highest minor version within that major version to which the
787   recipient is conformant. A recipient can assume that a message with a
788   higher minor version, when sent to a recipient that has not yet indicated
789   support for that higher version, is sufficiently backwards-compatible to be
790   safely processed by any implementation of the same major version.
794<section title="Uniform Resource Identifiers" anchor="uri">
795<iref primary="true" item="resource"/>
797   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used
798   throughout HTTP as the means for identifying resources (&resource;).
799   URI references are used to target requests, indicate redirects, and define
800   relationships.
802  <x:anchor-alias value="URI-reference"/>
803  <x:anchor-alias value="absolute-URI"/>
804  <x:anchor-alias value="relative-part"/>
805  <x:anchor-alias value="authority"/>
806  <x:anchor-alias value="uri-host"/>
807  <x:anchor-alias value="port"/>
808  <x:anchor-alias value="path-abempty"/>
809  <x:anchor-alias value="segment"/>
810  <x:anchor-alias value="query"/>
811  <x:anchor-alias value="fragment"/>
812  <x:anchor-alias value="absolute-path"/>
813  <x:anchor-alias value="partial-URI"/>
815   The definitions of "URI-reference",
816   "absolute-URI", "relative-part", "authority", "port", "host",
817   "path-abempty", "segment", "query", and "fragment" are adopted from the
818   URI generic syntax.
819   An "absolute-path" rule is defined, differing slightly from
820   RFC 3986's "path-absolute" in that it allows a leading "//".
821   A "partial-URI" rule is defined for protocol elements
822   that allow a relative URI but not a fragment.
824<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>
825  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
826  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
827  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
828  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
829  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
830  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
831  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
832  <x:ref>segment</x:ref>       = &lt;segment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
833  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
834  <x:ref>fragment</x:ref>      = &lt;fragment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.5"/>&gt;
836  <x:ref>absolute-path</x:ref> = 1*( "/" segment )
837  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
840   Each protocol element in HTTP that allows a URI reference will indicate
841   in its ABNF production whether the element allows any form of reference
842   (URI-reference), only a URI in absolute form (absolute-URI), only the
843   path and optional query components, or some combination of the above.
844   Unless otherwise indicated, URI references are parsed
845   relative to the effective request URI
846   (<xref target="effective.request.uri"/>).
849<section title="http URI scheme" anchor="http.uri">
850  <x:anchor-alias value="http-URI"/>
851  <iref item="http URI scheme" primary="true"/>
852  <iref item="URI scheme" subitem="http" primary="true"/>
854   The "http" URI scheme is hereby defined for the purpose of minting
855   identifiers according to their association with the hierarchical
856   namespace governed by a potential HTTP origin server listening for
857   TCP (<xref target="RFC0793"/>) connections on a given port.
859<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"><!--terminal production--></iref>
860  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
861             [ "#" <x:ref>fragment</x:ref> ]
864   The HTTP origin server is identified by the generic syntax's
865   <x:ref>authority</x:ref> component, which includes a host identifier
866   and optional TCP port (<xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>).
867   The remainder of the URI, consisting of both the hierarchical path
868   component and optional query component, serves as an identifier for
869   a potential resource within that origin server's name space.
872   A sender &MUST-NOT; generate an "http" URI with an empty host identifier.
873   A recipient that processes such a URI reference &MUST; reject it as invalid.
876   If the host identifier is provided as an IP address,
877   then the origin server is any listener on the indicated TCP port at
878   that IP address. If host is a registered name, then that name is
879   considered an indirect identifier and the recipient might use a name
880   resolution service, such as DNS, to find the address of a listener
881   for that host.
882   If the port subcomponent is empty or not given, then TCP port 80 is
883   assumed (the default reserved port for WWW services).
886   Regardless of the form of host identifier, access to that host is not
887   implied by the mere presence of its name or address. The host might or might
888   not exist and, even when it does exist, might or might not be running an
889   HTTP server or listening to the indicated port. The "http" URI scheme
890   makes use of the delegated nature of Internet names and addresses to
891   establish a naming authority (whatever entity has the ability to place
892   an HTTP server at that Internet name or address) and allows that
893   authority to determine which names are valid and how they might be used.
896   When an "http" URI is used within a context that calls for access to the
897   indicated resource, a client &MAY; attempt access by resolving
898   the host to an IP address, establishing a TCP connection to that address
899   on the indicated port, and sending an HTTP request message
900   (<xref target="http.message"/>) containing the URI's identifying data
901   (<xref target="message.routing"/>) to the server.
902   If the server responds to that request with a non-interim HTTP response
903   message, as described in &status-codes;, then that response
904   is considered an authoritative answer to the client's request.
907   Although HTTP is independent of the transport protocol, the "http"
908   scheme is specific to TCP-based services because the name delegation
909   process depends on TCP for establishing authority.
910   An HTTP service based on some other underlying connection protocol
911   would presumably be identified using a different URI scheme, just as
912   the "https" scheme (below) is used for resources that require an
913   end-to-end secured connection. Other protocols might also be used to
914   provide access to "http" identified resources &mdash; it is only the
915   authoritative interface that is specific to TCP.
918   The URI generic syntax for authority also includes a deprecated
919   userinfo subcomponent (<xref target="RFC3986" x:fmt="," x:sec="3.2.1"/>)
920   for including user authentication information in the URI.  Some
921   implementations make use of the userinfo component for internal
922   configuration of authentication information, such as within command
923   invocation options, configuration files, or bookmark lists, even
924   though such usage might expose a user identifier or password.
925   A sender &MUST-NOT; generate the userinfo subcomponent (and its "@"
926   delimiter) when an "http" URI reference is generated within a message as a
927   request target or header field value.
928   Before making use of an "http" URI reference received from an untrusted
929   source, a recipient ought to parse for userinfo and treat its presence as
930   an error; it is likely being used to obscure the authority for the sake of
931   phishing attacks.
935<section title="https URI scheme" anchor="https.uri">
936   <x:anchor-alias value="https-URI"/>
937   <iref item="https URI scheme"/>
938   <iref item="URI scheme" subitem="https"/>
940   The "https" URI scheme is hereby defined for the purpose of minting
941   identifiers according to their association with the hierarchical
942   namespace governed by a potential HTTP origin server listening to a
943   given TCP port for TLS-secured connections
944   (<xref target="RFC0793"/>, <xref target="RFC5246"/>).
947   All of the requirements listed above for the "http" scheme are also
948   requirements for the "https" scheme, except that a default TCP port
949   of 443 is assumed if the port subcomponent is empty or not given,
950   and the user agent &MUST; ensure that its connection to the origin
951   server is secured through the use of strong encryption, end-to-end,
952   prior to sending the first HTTP request.
954<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="https-URI"><!--terminal production--></iref>
955  <x:ref>https-URI</x:ref> = "https:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
956              [ "#" <x:ref>fragment</x:ref> ]
959   Note that the "https" URI scheme depends on both TLS and TCP for
960   establishing authority.
961   Resources made available via the "https" scheme have no shared
962   identity with the "http" scheme even if their resource identifiers
963   indicate the same authority (the same host listening to the same
964   TCP port).  They are distinct name spaces and are considered to be
965   distinct origin servers.  However, an extension to HTTP that is
966   defined to apply to entire host domains, such as the Cookie protocol
967   <xref target="RFC6265"/>, can allow information
968   set by one service to impact communication with other services
969   within a matching group of host domains.
972   The process for authoritative access to an "https" identified
973   resource is defined in <xref target="RFC2818"/>.
977<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
979   Since the "http" and "https" schemes conform to the URI generic syntax,
980   such URIs are normalized and compared according to the algorithm defined
981   in <xref target="RFC3986" x:fmt="of" x:sec="6"/>, using the defaults
982   described above for each scheme.
985   If the port is equal to the default port for a scheme, the normal form is
986   to omit the port subcomponent. When not being used in absolute form as the
987   request target of an OPTIONS request, an empty path component is equivalent
988   to an absolute path of "/", so the normal form is to provide a path of "/"
989   instead. The scheme and host are case-insensitive and normally provided in
990   lowercase; all other components are compared in a case-sensitive manner.
991   Characters other than those in the "reserved" set are equivalent to their
992   percent-encoded octets: the normal form is to not encode them
993   (see Sections <xref target="RFC3986" x:fmt="number" x:sec="2.1"/> and
994   <xref target="RFC3986" x:fmt="number" x:sec="2.2"/> of
995   <xref target="RFC3986"/>).
998   For example, the following three URIs are equivalent:
1000<figure><artwork type="example">
1009<section title="Message Format" anchor="http.message">
1010<x:anchor-alias value="generic-message"/>
1011<x:anchor-alias value="message.types"/>
1012<x:anchor-alias value="HTTP-message"/>
1013<x:anchor-alias value="start-line"/>
1014<iref item="header section"/>
1015<iref item="headers"/>
1016<iref item="header field"/>
1018   All HTTP/1.1 messages consist of a start-line followed by a sequence of
1019   octets in a format similar to the Internet Message Format
1020   <xref target="RFC5322"/>: zero or more header fields (collectively
1021   referred to as the "headers" or the "header section"), an empty line
1022   indicating the end of the header section, and an optional message body.
1024<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"><!--terminal production--></iref>
1025  <x:ref>HTTP-message</x:ref>   = <x:ref>start-line</x:ref>
1026                   *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1027                   <x:ref>CRLF</x:ref>
1028                   [ <x:ref>message-body</x:ref> ]
1031   The normal procedure for parsing an HTTP message is to read the
1032   start-line into a structure, read each header field into a hash
1033   table by field name until the empty line, and then use the parsed
1034   data to determine if a message body is expected.  If a message body
1035   has been indicated, then it is read as a stream until an amount
1036   of octets equal to the message body length is read or the connection
1037   is closed.
1040   A recipient &MUST; parse an HTTP message as a sequence of octets in an
1041   encoding that is a superset of US-ASCII <xref target="USASCII"/>.
1042   Parsing an HTTP message as a stream of Unicode characters, without regard
1043   for the specific encoding, creates security vulnerabilities due to the
1044   varying ways that string processing libraries handle invalid multibyte
1045   character sequences that contain the octet LF (%x0A).  String-based
1046   parsers can only be safely used within protocol elements after the element
1047   has been extracted from the message, such as within a header field-value
1048   after message parsing has delineated the individual fields.
1051   An HTTP message can be parsed as a stream for incremental processing or
1052   forwarding downstream.  However, recipients cannot rely on incremental
1053   delivery of partial messages, since some implementations will buffer or
1054   delay message forwarding for the sake of network efficiency, security
1055   checks, or payload transformations.
1058   A sender &MUST-NOT; send whitespace between the start-line and
1059   the first header field.
1060   A recipient that receives whitespace between the start-line and
1061   the first header field &MUST; either reject the message as invalid or
1062   consume each whitespace-preceded line without further processing of it
1063   (i.e., ignore the entire line, along with any subsequent lines preceded
1064   by whitespace, until a properly formed header field is received or the
1065   header section is terminated).
1068   The presence of such whitespace in a request
1069   might be an attempt to trick a server into ignoring that field or
1070   processing the line after it as a new request, either of which might
1071   result in a security vulnerability if other implementations within
1072   the request chain interpret the same message differently.
1073   Likewise, the presence of such whitespace in a response might be
1074   ignored by some clients or cause others to cease parsing.
1077<section title="Start Line" anchor="start.line">
1078  <x:anchor-alias value="Start-Line"/>
1080   An HTTP message can either be a request from client to server or a
1081   response from server to client.  Syntactically, the two types of message
1082   differ only in the start-line, which is either a request-line (for requests)
1083   or a status-line (for responses), and in the algorithm for determining
1084   the length of the message body (<xref target="message.body"/>).
1087   In theory, a client could receive requests and a server could receive
1088   responses, distinguishing them by their different start-line formats,
1089   but in practice servers are implemented to only expect a request
1090   (a response is interpreted as an unknown or invalid request method)
1091   and clients are implemented to only expect a response.
1093<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="start-line"/>
1094  <x:ref>start-line</x:ref>     = <x:ref>request-line</x:ref> / <x:ref>status-line</x:ref>
1097<section title="Request Line" anchor="request.line">
1098  <x:anchor-alias value="Request"/>
1099  <x:anchor-alias value="request-line"/>
1101   A request-line begins with a method token, followed by a single
1102   space (SP), the request-target, another single space (SP), the
1103   protocol version, and ending with CRLF.
1105<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1106  <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>
1108<iref primary="true" item="method"/>
1109<t anchor="method">
1110   The method token indicates the request method to be performed on the
1111   target resource. The request method is case-sensitive.
1113<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1114  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1117   The request methods defined by this specification can be found in
1118   &methods;, along with information regarding the HTTP method registry
1119   and considerations for defining new methods.
1121<iref item="request-target"/>
1123   The request-target identifies the target resource upon which to apply
1124   the request, as defined in <xref target="request-target"/>.
1127   Recipients typically parse the request-line into its component parts by
1128   splitting on whitespace (see <xref target="message.robustness"/>), since
1129   no whitespace is allowed in the three components.
1130   Unfortunately, some user agents fail to properly encode or exclude
1131   whitespace found in hypertext references, resulting in those disallowed
1132   characters being sent in a request-target.
1135   Recipients of an invalid request-line &SHOULD; respond with either a
1136   <x:ref>400 (Bad Request)</x:ref> error or a <x:ref>301 (Moved Permanently)</x:ref>
1137   redirect with the request-target properly encoded.  A recipient &SHOULD-NOT;
1138   attempt to autocorrect and then process the request without a redirect,
1139   since the invalid request-line might be deliberately crafted to bypass
1140   security filters along the request chain.
1143   HTTP does not place a pre-defined limit on the length of a request-line.
1144   A server that receives a method longer than any that it implements
1145   &SHOULD; respond with a <x:ref>501 (Not Implemented)</x:ref> status code.
1146   A server ought to be prepared to receive URIs of unbounded length, as
1147   described in <xref target="conformance"/>, and &MUST; respond with a
1148   <x:ref>414 (URI Too Long)</x:ref> status code if the received
1149   request-target is longer than the server wishes to parse (see &status-414;).
1152   Various ad-hoc limitations on request-line length are found in practice.
1153   It is &RECOMMENDED; that all HTTP senders and recipients support, at a
1154   minimum, request-line lengths of 8000 octets.
1158<section title="Status Line" anchor="status.line">
1159  <x:anchor-alias value="response"/>
1160  <x:anchor-alias value="status-line"/>
1161  <x:anchor-alias value="status-code"/>
1162  <x:anchor-alias value="reason-phrase"/>
1164   The first line of a response message is the status-line, consisting
1165   of the protocol version, a space (SP), the status code, another space,
1166   a possibly-empty textual phrase describing the status code, and
1167   ending with CRLF.
1169<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-line"/>
1170  <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>
1173   The status-code element is a 3-digit integer code describing the
1174   result of the server's attempt to understand and satisfy the client's
1175   corresponding request. The rest of the response message is to be
1176   interpreted in light of the semantics defined for that status code.
1177   See &status-codes; for information about the semantics of status codes,
1178   including the classes of status code (indicated by the first digit),
1179   the status codes defined by this specification, considerations for the
1180   definition of new status codes, and the IANA registry.
1182<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-code"/>
1183  <x:ref>status-code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1186   The reason-phrase element exists for the sole purpose of providing a
1187   textual description associated with the numeric status code, mostly
1188   out of deference to earlier Internet application protocols that were more
1189   frequently used with interactive text clients. A client &SHOULD; ignore
1190   the reason-phrase content.
1192<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
1193  <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> )
1198<section title="Header Fields" anchor="header.fields">
1199  <x:anchor-alias value="header-field"/>
1200  <x:anchor-alias value="field-content"/>
1201  <x:anchor-alias value="field-name"/>
1202  <x:anchor-alias value="field-value"/>
1203  <x:anchor-alias value="obs-fold"/>
1205   Each header field consists of a case-insensitive field name
1206   followed by a colon (":"), optional leading whitespace, the field value,
1207   and optional trailing whitespace.
1209<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"/>
1210  <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>
1211  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1212  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>obs-fold</x:ref> )
1213  <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> )
1214  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref> ( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1215                 ; obsolete line folding
1216                 ; see <xref target="field.parsing"/>
1219   The field-name token labels the corresponding field-value as having the
1220   semantics defined by that header field.  For example, the <x:ref>Date</x:ref>
1221   header field is defined in &header-date; as containing the origination
1222   timestamp for the message in which it appears.
1225<section title="Field Extensibility" anchor="field.extensibility">
1227   Header fields are fully extensible: there is no limit on the
1228   introduction of new field names, each presumably defining new semantics,
1229   nor on the number of header fields used in a given message.  Existing
1230   fields are defined in each part of this specification and in many other
1231   specifications outside the core standard.
1234   New header fields can be defined such that, when they are understood by a
1235   recipient, they might override or enhance the interpretation of previously
1236   defined header fields, define preconditions on request evaluation, or
1237   refine the meaning of responses.
1240   A proxy &MUST; forward unrecognized header fields unless the
1241   field-name is listed in the <x:ref>Connection</x:ref> header field
1242   (<xref target="header.connection"/>) or the proxy is specifically
1243   configured to block, or otherwise transform, such fields.
1244   Other recipients &SHOULD; ignore unrecognized header fields.
1245   These requirements allow HTTP's functionality to be enhanced without
1246   requiring prior update of deployed intermediaries.
1249   All defined header fields ought to be registered with IANA in the
1250   Message Header Field Registry, as described in &iana-header-registry;.
1254<section title="Field Order" anchor="field.order">
1256   The order in which header fields with differing field names are
1257   received is not significant. However, it is "good practice" to send
1258   header fields that contain control data first, such as <x:ref>Host</x:ref>
1259   on requests and <x:ref>Date</x:ref> on responses, so that implementations
1260   can decide when not to handle a message as early as possible.  A server
1261   &MUST; wait until the entire header section is received before interpreting
1262   a request message, since later header fields might include conditionals,
1263   authentication credentials, or deliberately misleading duplicate
1264   header fields that would impact request processing.
1267   A sender &MUST-NOT; generate multiple header fields with the same field
1268   name in a message unless either the entire field value for that
1269   header field is defined as a comma-separated list [i.e., #(values)]
1270   or the header field is a well-known exception (as noted below).
1273   A recipient &MAY; combine multiple header fields with the same field name
1274   into one "field-name: field-value" pair, without changing the semantics of
1275   the message, by appending each subsequent field value to the combined
1276   field value in order, separated by a comma. The order in which
1277   header fields with the same field name are received is therefore
1278   significant to the interpretation of the combined field value;
1279   a proxy &MUST-NOT; change the order of these field values when
1280   forwarding a message.
1283  <t>
1284   &Note; In practice, the "Set-Cookie" header field (<xref target="RFC6265"/>)
1285   often appears multiple times in a response message and does not use the
1286   list syntax, violating the above requirements on multiple header fields
1287   with the same name. Since it cannot be combined into a single field-value,
1288   recipients ought to handle "Set-Cookie" as a special case while processing
1289   header fields. (See Appendix A.2.3 of <xref target="Kri2001"/> for details.)
1290  </t>
1294<section title="Whitespace" anchor="whitespace">
1295<t anchor="rule.LWS">
1296   This specification uses three rules to denote the use of linear
1297   whitespace: OWS (optional whitespace), RWS (required whitespace), and
1298   BWS ("bad" whitespace).
1300<t anchor="rule.OWS">
1301   The OWS rule is used where zero or more linear whitespace octets might
1302   appear. For protocol elements where optional whitespace is preferred to
1303   improve readability, a sender &SHOULD; generate the optional whitespace
1304   as a single SP; otherwise, a sender &SHOULD-NOT; generate optional
1305   whitespace except as needed to white-out invalid or unwanted protocol
1306   elements during in-place message filtering.
1308<t anchor="rule.RWS">
1309   The RWS rule is used when at least one linear whitespace octet is required
1310   to separate field tokens. A sender &SHOULD; generate RWS as a single SP.
1312<t anchor="rule.BWS">
1313   The BWS rule is used where the grammar allows optional whitespace only for
1314   historical reasons. A sender &MUST-NOT; generate BWS in messages.
1315   A recipient &MUST; parse for such bad whitespace and remove it before
1316   interpreting the protocol element.
1318<t anchor="rule.whitespace">
1319  <x:anchor-alias value="BWS"/>
1320  <x:anchor-alias value="OWS"/>
1321  <x:anchor-alias value="RWS"/>
1323<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"/>
1324  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1325                 ; optional whitespace
1326  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1327                 ; required whitespace
1328  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1329                 ; "bad" whitespace
1333<section title="Field Parsing" anchor="field.parsing">
1335   Messages are parsed using a generic algorithm, independent of the
1336   individual header field names. The contents within a given field value are
1337   not parsed until a later stage of message interpretation (usually after the
1338   message's entire header section has been processed).
1339   Consequently, this specification does not use ABNF rules to define each
1340   "Field-Name: Field Value" pair, as was done in previous editions.
1341   Instead, this specification uses ABNF rules which are named according to
1342   each registered field name, wherein the rule defines the valid grammar for
1343   that field's corresponding field values (i.e., after the field-value
1344   has been extracted from the header section by a generic field parser).
1347   No whitespace is allowed between the header field-name and colon.
1348   In the past, differences in the handling of such whitespace have led to
1349   security vulnerabilities in request routing and response handling.
1350   A server &MUST; reject any received request message that contains
1351   whitespace between a header field-name and colon with a response code of
1352   <x:ref>400 (Bad Request)</x:ref>. A proxy &MUST; remove any such whitespace
1353   from a response message before forwarding the message downstream.
1356   A field value is preceded by optional whitespace (OWS); a single SP is
1357   preferred. The field value does not include any leading or trailing white
1358   space: OWS occurring before the first non-whitespace octet of the field
1359   value or after the last non-whitespace octet of the field value ought to be
1360   excluded by parsers when extracting the field value from a header field.
1363   A recipient of field-content containing multiple sequential octets of
1364   optional (OWS) or required (RWS) whitespace &SHOULD; either replace the
1365   sequence with a single SP or transform any non-SP octets in the sequence to
1366   SP octets before interpreting the field value or forwarding the message
1367   downstream.
1370   Historically, HTTP header field values could be extended over multiple
1371   lines by preceding each extra line with at least one space or horizontal
1372   tab (obs-fold). This specification deprecates such line folding except
1373   within the message/http media type
1374   (<xref target=""/>).
1375   A sender &MUST-NOT; generate a message that includes line folding
1376   (i.e., that has any field-value that contains a match to the
1377   <x:ref>obs-fold</x:ref> rule) unless the message is intended for packaging
1378   within the message/http media type.
1381   A server that receives an <x:ref>obs-fold</x:ref> in a request message that
1382   is not within a message/http container &MUST; either reject the message by
1383   sending a <x:ref>400 (Bad Request)</x:ref>, preferably with a
1384   representation explaining that obsolete line folding is unacceptable, or
1385   replace each received <x:ref>obs-fold</x:ref> with one or more
1386   <x:ref>SP</x:ref> octets prior to interpreting the field value or
1387   forwarding the message downstream.
1390   A proxy or gateway that receives an <x:ref>obs-fold</x:ref> in a response
1391   message that is not within a message/http container &MUST; either discard
1392   the message and replace it with a <x:ref>502 (Bad Gateway)</x:ref>
1393   response, preferably with a representation explaining that unacceptable
1394   line folding was received, or replace each received <x:ref>obs-fold</x:ref>
1395   with one or more <x:ref>SP</x:ref> octets prior to interpreting the field
1396   value or forwarding the message downstream.
1399   A user agent that receives an <x:ref>obs-fold</x:ref> in a response message
1400   that is not within a message/http container &MUST; replace each received
1401   <x:ref>obs-fold</x:ref> with one or more <x:ref>SP</x:ref> octets prior to
1402   interpreting the field value.
1405   Historically, HTTP has allowed field content with text in the ISO-8859-1
1406   <xref target="ISO-8859-1"/> charset, supporting other charsets only
1407   through use of <xref target="RFC2047"/> encoding.
1408   In practice, most HTTP header field values use only a subset of the
1409   US-ASCII charset <xref target="USASCII"/>. Newly defined
1410   header fields &SHOULD; limit their field values to US-ASCII octets.
1411   A recipient &SHOULD; treat other octets in field content (obs-text) as
1412   opaque data.
1416<section title="Field Limits" anchor="field.limits">
1418   HTTP does not place a pre-defined limit on the length of each header field
1419   or on the length of the header section as a whole, as described in
1420   <xref target="conformance"/>. Various ad-hoc limitations on individual
1421   header field length are found in practice, often depending on the specific
1422   field semantics.
1425   A server ought to be prepared to receive request header fields of unbounded
1426   length and &MUST; respond with an appropriate
1427   <x:ref>4xx (Client Error)</x:ref> status code if the received header
1428   field(s) are larger than the server wishes to process.
1431   A client ought to be prepared to receive response header fields of
1432   unbounded length.
1433   A client &MAY; discard or truncate received header fields that are larger
1434   than the client wishes to process if the field semantics are such that the
1435   dropped value(s) can be safely ignored without changing the
1436   message framing or response semantics.
1440<section title="Field value components" anchor="field.components">
1441<t anchor="rule.token.separators">
1442  <x:anchor-alias value="tchar"/>
1443  <x:anchor-alias value="token"/>
1444  <iref item="Delimiters"/>
1445   Most HTTP header field values are defined using common syntax components
1446   (token, quoted-string, and comment) separated by whitespace or specific
1447   delimiting characters. Delimiters are chosen from the set of US-ASCII
1448   visual characters not allowed in a <x:ref>token</x:ref>
1449   (DQUOTE and "(),/:;&lt;=>?@[\]{}").
1451<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="token"/><iref primary="true" item="Grammar" subitem="tchar"/>
1452  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1454  NOTE: the definition of tchar and the prose above about special characters need to match!
1455 -->
1456  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1457                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1458                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1459                 ; any <x:ref>VCHAR</x:ref>, except delimiters
1461<t anchor="rule.quoted-string">
1462  <x:anchor-alias value="quoted-string"/>
1463  <x:anchor-alias value="qdtext"/>
1464  <x:anchor-alias value="obs-text"/>
1465   A string of text is parsed as a single value if it is quoted using
1466   double-quote marks.
1468<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"/>
1469  <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>
1470  <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>
1471  <x:ref>obs-text</x:ref>       = %x80-FF
1473<t anchor="rule.comment">
1474  <x:anchor-alias value="comment"/>
1475  <x:anchor-alias value="ctext"/>
1476   Comments can be included in some HTTP header fields by surrounding
1477   the comment text with parentheses. Comments are only allowed in
1478   fields containing "comment" as part of their field value definition.
1480<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1481  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-pair</x:ref> / <x:ref>comment</x:ref> ) ")"
1482  <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>
1484<t anchor="rule.quoted-pair">
1485  <x:anchor-alias value="quoted-pair"/>
1486   The backslash octet ("\") can be used as a single-octet
1487   quoting mechanism within quoted-string and comment constructs.
1488   Recipients that process the value of a quoted-string &MUST; handle a
1489   quoted-pair as if it were replaced by the octet following the backslash.
1491<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1492  <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> )
1495   A sender &SHOULD-NOT; generate a quoted-pair in a quoted-string except
1496   where necessary to quote DQUOTE and backslash octets occurring within that
1497   string.
1498   A sender &SHOULD-NOT; generate a quoted-pair in a comment except
1499   where necessary to quote parentheses ["(" and ")"] and backslash octets
1500   occurring within that comment.
1506<section title="Message Body" anchor="message.body">
1507  <x:anchor-alias value="message-body"/>
1509   The message body (if any) of an HTTP message is used to carry the
1510   payload body of that request or response.  The message body is
1511   identical to the payload body unless a transfer coding has been
1512   applied, as described in <xref target="header.transfer-encoding"/>.
1514<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1515  <x:ref>message-body</x:ref> = *OCTET
1518   The rules for when a message body is allowed in a message differ for
1519   requests and responses.
1522   The presence of a message body in a request is signaled by a
1523   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1524   field. Request message framing is independent of method semantics,
1525   even if the method does not define any use for a message body.
1528   The presence of a message body in a response depends on both
1529   the request method to which it is responding and the response
1530   status code (<xref target="status.line"/>).
1531   Responses to the HEAD request method (&HEAD;) never include a message body
1532   because the associated response header fields (e.g.,
1533   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1534   if present, indicate only what their values would have been if the request
1535   method had been GET (&GET;).
1536   <x:ref>2xx (Successful)</x:ref> responses to a CONNECT request method
1537   (&CONNECT;) switch to tunnel mode instead of having a message body.
1538   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1539   <x:ref>304 (Not Modified)</x:ref> responses do not include a message body.
1540   All other responses do include a message body, although the body
1541   might be of zero length.
1544<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1545  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1546  <iref item="chunked (Coding Format)"/>
1547  <x:anchor-alias value="Transfer-Encoding"/>
1549   The Transfer-Encoding header field lists the transfer coding names
1550   corresponding to the sequence of transfer codings that have been
1551   (or will be) applied to the payload body in order to form the message body.
1552   Transfer codings are defined in <xref target="transfer.codings"/>.
1554<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1555  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1558   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1559   MIME, which was designed to enable safe transport of binary data over a
1560   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1561   However, safe transport has a different focus for an 8bit-clean transfer
1562   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1563   accurately delimit a dynamically generated payload and to distinguish
1564   payload encodings that are only applied for transport efficiency or
1565   security from those that are characteristics of the selected resource.
1568   A recipient &MUST; be able to parse the chunked transfer coding
1569   (<xref target="chunked.encoding"/>) because it plays a crucial role in
1570   framing messages when the payload body size is not known in advance.
1571   A sender &MUST-NOT; apply chunked more than once to a message body
1572   (i.e., chunking an already chunked message is not allowed).
1573   If any transfer coding other than chunked is applied to a request payload
1574   body, the sender &MUST; apply chunked as the final transfer coding to
1575   ensure that the message is properly framed.
1576   If any transfer coding other than chunked is applied to a response payload
1577   body, the sender &MUST; either apply chunked as the final transfer coding
1578   or terminate the message by closing the connection.
1581   For example,
1582</preamble><artwork type="example">
1583  Transfer-Encoding: gzip, chunked
1585   indicates that the payload body has been compressed using the gzip
1586   coding and then chunked using the chunked coding while forming the
1587   message body.
1590   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1591   Transfer-Encoding is a property of the message, not of the representation, and
1592   any recipient along the request/response chain &MAY; decode the received
1593   transfer coding(s) or apply additional transfer coding(s) to the message
1594   body, assuming that corresponding changes are made to the Transfer-Encoding
1595   field-value. Additional information about the encoding parameters &MAY; be
1596   provided by other header fields not defined by this specification.
1599   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1600   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1601   neither of which includes a message body,
1602   to indicate that the origin server would have applied a transfer coding
1603   to the message body if the request had been an unconditional GET.
1604   This indication is not required, however, because any recipient on
1605   the response chain (including the origin server) can remove transfer
1606   codings when they are not needed.
1609   A server &MUST-NOT; send a Transfer-Encoding header field in any response
1610   with a status code of
1611   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1612   A server &MUST-NOT; send a Transfer-Encoding header field in any
1613   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1616   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1617   implementations advertising only HTTP/1.0 support will not understand
1618   how to process a transfer-encoded payload.
1619   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1620   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1621   might be in the form of specific user configuration or by remembering the
1622   version of a prior received response.
1623   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1624   the corresponding request indicates HTTP/1.1 (or later).
1627   A server that receives a request message with a transfer coding it does
1628   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref>.
1632<section title="Content-Length" anchor="header.content-length">
1633  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1634  <x:anchor-alias value="Content-Length"/>
1636   When a message does not have a <x:ref>Transfer-Encoding</x:ref> header
1637   field, a Content-Length header field can provide the anticipated size,
1638   as a decimal number of octets, for a potential payload body.
1639   For messages that do include a payload body, the Content-Length field-value
1640   provides the framing information necessary for determining where the body
1641   (and message) ends.  For messages that do not include a payload body, the
1642   Content-Length indicates the size of the selected representation
1643   (&representation;).
1645<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1646  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1649   An example is
1651<figure><artwork type="example">
1652  Content-Length: 3495
1655   A sender &MUST-NOT; send a Content-Length header field in any message that
1656   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1659   A user agent &SHOULD; send a Content-Length in a request message when no
1660   <x:ref>Transfer-Encoding</x:ref> is sent and the request method defines
1661   a meaning for an enclosed payload body. For example, a Content-Length
1662   header field is normally sent in a POST request even when the value is
1663   0 (indicating an empty payload body).  A user agent &SHOULD-NOT; send a
1664   Content-Length header field when the request message does not contain a
1665   payload body and the method semantics do not anticipate such a body.
1668   A server &MAY; send a Content-Length header field in a response to a HEAD
1669   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1670   response unless its field-value equals the decimal number of octets that
1671   would have been sent in the payload body of a response if the same
1672   request had used the GET method.
1675   A server &MAY; send a Content-Length header field in a
1676   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1677   (&status-304;); a server &MUST-NOT; send Content-Length in such a
1678   response unless its field-value equals the decimal number of octets that
1679   would have been sent in the payload body of a <x:ref>200 (OK)</x:ref>
1680   response to the same request.
1683   A server &MUST-NOT; send a Content-Length header field in any response
1684   with a status code of
1685   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1686   A server &MUST-NOT; send a Content-Length header field in any
1687   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1690   Aside from the cases defined above, in the absence of Transfer-Encoding,
1691   an origin server &SHOULD; send a Content-Length header field when the
1692   payload body size is known prior to sending the complete header section.
1693   This will allow downstream recipients to measure transfer progress,
1694   know when a received message is complete, and potentially reuse the
1695   connection for additional requests.
1698   Any Content-Length field value greater than or equal to zero is valid.
1699   Since there is no predefined limit to the length of a payload, a
1700   recipient &MUST; anticipate potentially large decimal numerals and
1701   prevent parsing errors due to integer conversion overflows
1702   (<xref target="attack.protocol.element.size.overflows"/>).
1705   If a message is received that has multiple Content-Length header fields
1706   with field-values consisting of the same decimal value, or a single
1707   Content-Length header field with a field value containing a list of
1708   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1709   duplicate Content-Length header fields have been generated or combined by an
1710   upstream message processor, then the recipient &MUST; either reject the
1711   message as invalid or replace the duplicated field-values with a single
1712   valid Content-Length field containing that decimal value prior to
1713   determining the message body length or forwarding the message.
1716  <t>
1717   &Note; HTTP's use of Content-Length for message framing differs
1718   significantly from the same field's use in MIME, where it is an optional
1719   field used only within the "message/external-body" media-type.
1720  </t>
1724<section title="Message Body Length" anchor="message.body.length">
1725  <iref item="chunked (Coding Format)"/>
1727   The length of a message body is determined by one of the following
1728   (in order of precedence):
1731  <list style="numbers">
1732    <x:lt><t>
1733     Any response to a HEAD request and any response with a
1734     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1735     <x:ref>304 (Not Modified)</x:ref> status code is always
1736     terminated by the first empty line after the header fields, regardless of
1737     the header fields present in the message, and thus cannot contain a
1738     message body.
1739    </t></x:lt>
1740    <x:lt><t>
1741     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1742     connection will become a tunnel immediately after the empty line that
1743     concludes the header fields.  A client &MUST; ignore any
1744     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1745     fields received in such a message.
1746    </t></x:lt>
1747    <x:lt><t>
1748     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1749     and the chunked transfer coding (<xref target="chunked.encoding"/>)
1750     is the final encoding, the message body length is determined by reading
1751     and decoding the chunked data until the transfer coding indicates the
1752     data is complete.
1753    </t>
1754    <t>
1755     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1756     response and the chunked transfer coding is not the final encoding, the
1757     message body length is determined by reading the connection until it is
1758     closed by the server.
1759     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a request and the
1760     chunked transfer coding is not the final encoding, the message body
1761     length cannot be determined reliably; the server &MUST; respond with
1762     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1763    </t>
1764    <t>
1765     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1766     and a <x:ref>Content-Length</x:ref> header field, the Transfer-Encoding
1767     overrides the Content-Length. Such a message might indicate an attempt
1768     to perform request or response smuggling (bypass of security-related
1769     checks on message routing or content) and thus ought to be handled as
1770     an error.  A sender &MUST; remove the received Content-Length field
1771     prior to forwarding such a message downstream.
1772    </t></x:lt>
1773    <x:lt><t>
1774     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1775     either multiple <x:ref>Content-Length</x:ref> header fields having
1776     differing field-values or a single Content-Length header field having an
1777     invalid value, then the message framing is invalid and
1778     the recipient &MUST; treat it as an unrecoverable error to prevent
1779     request or response smuggling.
1780     If this is a request message, the server &MUST; respond with
1781     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1782     If this is a response message received by a proxy,
1783     the proxy &MUST; close the connection to the server, discard the received
1784     response, and send a <x:ref>502 (Bad Gateway)</x:ref> response to the
1785     client.
1786     If this is a response message received by a user agent,
1787     the user agent &MUST; close the connection to the server and discard the
1788     received response.
1789    </t></x:lt>
1790    <x:lt><t>
1791     If a valid <x:ref>Content-Length</x:ref> header field is present without
1792     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1793     expected message body length in octets.
1794     If the sender closes the connection or the recipient times out before the
1795     indicated number of octets are received, the recipient &MUST; consider
1796     the message to be incomplete and close the connection.
1797    </t></x:lt>
1798    <x:lt><t>
1799     If this is a request message and none of the above are true, then the
1800     message body length is zero (no message body is present).
1801    </t></x:lt>
1802    <x:lt><t>
1803     Otherwise, this is a response message without a declared message body
1804     length, so the message body length is determined by the number of octets
1805     received prior to the server closing the connection.
1806    </t></x:lt>
1807  </list>
1810   Since there is no way to distinguish a successfully completed,
1811   close-delimited message from a partially-received message interrupted
1812   by network failure, a server &SHOULD; generate encoding or
1813   length-delimited messages whenever possible.  The close-delimiting
1814   feature exists primarily for backwards compatibility with HTTP/1.0.
1817   A server &MAY; reject a request that contains a message body but
1818   not a <x:ref>Content-Length</x:ref> by responding with
1819   <x:ref>411 (Length Required)</x:ref>.
1822   Unless a transfer coding other than chunked has been applied,
1823   a client that sends a request containing a message body &SHOULD;
1824   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1825   length is known in advance, rather than the chunked transfer coding, since some
1826   existing services respond to chunked with a <x:ref>411 (Length Required)</x:ref>
1827   status code even though they understand the chunked transfer coding.  This
1828   is typically because such services are implemented via a gateway that
1829   requires a content-length in advance of being called and the server
1830   is unable or unwilling to buffer the entire request before processing.
1833   A user agent that sends a request containing a message body &MUST; send a
1834   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1835   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1836   the form of specific user configuration or by remembering the version of a
1837   prior received response.
1840   If the final response to the last request on a connection has been
1841   completely received and there remains additional data to read, a user agent
1842   &MAY; discard the remaining data or attempt to determine if that data
1843   belongs as part of the prior response body, which might be the case if the
1844   prior message's Content-Length value is incorrect. A client &MUST-NOT;
1845   process, cache, or forward such extra data as a separate response, since
1846   such behavior would be vulnerable to cache poisoning.
1851<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1853   A server that receives an incomplete request message, usually due to a
1854   canceled request or a triggered time-out exception, &MAY; send an error
1855   response prior to closing the connection.
1858   A client that receives an incomplete response message, which can occur
1859   when a connection is closed prematurely or when decoding a supposedly
1860   chunked transfer coding fails, &MUST; record the message as incomplete.
1861   Cache requirements for incomplete responses are defined in
1862   &cache-incomplete;.
1865   If a response terminates in the middle of the header section (before the
1866   empty line is received) and the status code might rely on header fields to
1867   convey the full meaning of the response, then the client cannot assume
1868   that meaning has been conveyed; the client might need to repeat the
1869   request in order to determine what action to take next.
1872   A message body that uses the chunked transfer coding is
1873   incomplete if the zero-sized chunk that terminates the encoding has not
1874   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1875   incomplete if the size of the message body received (in octets) is less than
1876   the value given by Content-Length.  A response that has neither chunked
1877   transfer coding nor Content-Length is terminated by closure of the
1878   connection, and thus is considered complete regardless of the number of
1879   message body octets received, provided that the header section was received
1880   intact.
1884<section title="Message Parsing Robustness" anchor="message.robustness">
1886   Older HTTP/1.0 user agent implementations might send an extra CRLF
1887   after a POST request as a workaround for some early server
1888   applications that failed to read message body content that was
1889   not terminated by a line-ending. An HTTP/1.1 user agent &MUST-NOT;
1890   preface or follow a request with an extra CRLF.  If terminating
1891   the request message body with a line-ending is desired, then the
1892   user agent &MUST; count the terminating CRLF octets as part of the
1893   message body length.
1896   In the interest of robustness, a server that is expecting to receive and
1897   parse a request-line &SHOULD; ignore at least one empty line (CRLF)
1898   received prior to the request-line.
1901   Although the line terminator for the start-line and header
1902   fields is the sequence CRLF, a recipient &MAY; recognize a
1903   single LF as a line terminator and ignore any preceding CR.
1906   Although the request-line and status-line grammar rules require that each
1907   of the component elements be separated by a single SP octet, recipients
1908   &MAY; instead parse on whitespace-delimited word boundaries and, aside
1909   from the CRLF terminator, treat any form of whitespace as the SP separator
1910   while ignoring preceding or trailing whitespace;
1911   such whitespace includes one or more of the following octets:
1912   SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
1915   When a server listening only for HTTP request messages, or processing
1916   what appears from the start-line to be an HTTP request message,
1917   receives a sequence of octets that does not match the HTTP-message
1918   grammar aside from the robustness exceptions listed above, the
1919   server &SHOULD; respond with a <x:ref>400 (Bad Request)</x:ref> response. 
1924<section title="Transfer Codings" anchor="transfer.codings">
1925  <x:anchor-alias value="transfer-coding"/>
1926  <x:anchor-alias value="transfer-extension"/>
1928   Transfer coding names are used to indicate an encoding
1929   transformation that has been, can be, or might need to be applied to a
1930   payload body in order to ensure "safe transport" through the network.
1931   This differs from a content coding in that the transfer coding is a
1932   property of the message rather than a property of the representation
1933   that is being transferred.
1935<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1936  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1937                     / "compress" ; <xref target="compress.coding"/>
1938                     / "deflate" ; <xref target="deflate.coding"/>
1939                     / "gzip" ; <xref target="gzip.coding"/>
1940                     / <x:ref>transfer-extension</x:ref>
1941  <x:ref>transfer-extension</x:ref> = <x:ref>token</x:ref> *( <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> <x:ref>transfer-parameter</x:ref> )
1943<t anchor="rule.parameter">
1944  <x:anchor-alias value="transfer-parameter"/>
1945   Parameters are in the form of a name or name=value pair.
1947<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-parameter"/>
1948  <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> )
1951   All transfer-coding names are case-insensitive and ought to be registered
1952   within the HTTP Transfer Coding registry, as defined in
1953   <xref target="transfer.coding.registry"/>.
1954   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1955   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1956   header fields.
1959<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1960  <iref primary="true" item="chunked (Coding Format)"/>
1961  <x:anchor-alias value="chunk"/>
1962  <x:anchor-alias value="chunked-body"/>
1963  <x:anchor-alias value="chunk-data"/>
1964  <x:anchor-alias value="chunk-size"/>
1965  <x:anchor-alias value="last-chunk"/>
1967   The chunked transfer coding wraps the payload body in order to transfer it
1968   as a series of chunks, each with its own size indicator, followed by an
1969   &OPTIONAL; trailer containing header fields. Chunked enables content
1970   streams of unknown size to be transferred as a sequence of length-delimited
1971   buffers, which enables the sender to retain connection persistence and the
1972   recipient to know when it has received the entire message.
1974<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"/>
1975  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1976                   <x:ref>last-chunk</x:ref>
1977                   <x:ref>trailer-part</x:ref>
1978                   <x:ref>CRLF</x:ref>
1980  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1981                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1982  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1983  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1985  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1988   The chunk-size field is a string of hex digits indicating the size of
1989   the chunk-data in octets. The chunked transfer coding is complete when a
1990   chunk with a chunk-size of zero is received, possibly followed by a
1991   trailer, and finally terminated by an empty line.
1994   A recipient &MUST; be able to parse and decode the chunked transfer coding.
1997<section title="Chunk Extensions" anchor="chunked.extension">
1998  <x:anchor-alias value="chunk-ext"/>
1999  <x:anchor-alias value="chunk-ext-name"/>
2000  <x:anchor-alias value="chunk-ext-val"/>
2002   The chunked encoding allows each chunk to include zero or more chunk
2003   extensions, immediately following the <x:ref>chunk-size</x:ref>, for the
2004   sake of supplying per-chunk metadata (such as a signature or hash),
2005   mid-message control information, or randomization of message body size.
2007<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"/>
2008  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
2010  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
2011  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
2014   The chunked encoding is specific to each connection and is likely to be
2015   removed or recoded by each recipient (including intermediaries) before any
2016   higher-level application would have a chance to inspect the extensions.
2017   Hence, use of chunk extensions is generally limited to specialized HTTP
2018   services such as "long polling" (where client and server can have shared
2019   expectations regarding the use of chunk extensions) or for padding within
2020   an end-to-end secured connection.
2023   A recipient &MUST; ignore unrecognized chunk extensions.
2024   A server ought to limit the total length of chunk extensions received in a
2025   request to an amount reasonable for the services provided, in the same way
2026   that it applies length limitations and timeouts for other parts of a
2027   message, and generate an appropriate <x:ref>4xx (Client Error)</x:ref>
2028   response if that amount is exceeded.
2032<section title="Chunked Trailer Part" anchor="chunked.trailer.part">
2033  <x:anchor-alias value="trailer-part"/>
2035   A trailer allows the sender to include additional fields at the end of a
2036   chunked message in order to supply metadata that might be dynamically
2037   generated while the message body is sent, such as a message integrity
2038   check, digital signature, or post-processing status. The trailer fields are
2039   identical to header fields, except they are sent in a chunked trailer
2040   instead of the message's header section.
2042<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="trailer-part"/>
2043  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
2046   A sender &MUST-NOT; generate a trailer that contains a field which needs to
2047   be known by the recipient before it can begin processing the message body.
2048   For example, most recipients need to know the values of
2049   <x:ref>Content-Encoding</x:ref> and <x:ref>Content-Type</x:ref> in order to
2050   select a content handler, so placing those fields in a trailer would force
2051   the recipient to buffer the entire body before it could begin, greatly
2052   increasing user-perceived latency and defeating one of the main advantages
2053   of using chunked to send data streams of unknown length.
2054   A sender &MUST-NOT; generate a trailer containing a
2055   <x:ref>Transfer-Encoding</x:ref>,
2056   <x:ref>Content-Length</x:ref>, or
2057   <x:ref>Trailer</x:ref> field.
2060   A server &MUST; generate an empty trailer with the chunked transfer coding
2061   unless at least one of the following is true:
2062  <list style="numbers">
2063    <t>the request included a <x:ref>TE</x:ref> header field that indicates
2064    "trailers" is acceptable in the transfer coding of the response, as
2065    described in <xref target="header.te"/>; or,</t>
2067    <t>the trailer fields consist entirely of optional metadata and the
2068    recipient could use the message (in a manner acceptable to the generating
2069    server) without receiving that metadata. In other words, the generating
2070    server is willing to accept the possibility that the trailer fields might
2071    be silently discarded along the path to the client.</t>
2072  </list>
2075   The above requirement prevents the need for an infinite buffer when a
2076   message is being received by an HTTP/1.1 (or later) proxy and forwarded to
2077   an HTTP/1.0 recipient.
2081<section title="Decoding Chunked" anchor="decoding.chunked">
2083   A process for decoding the chunked transfer coding
2084   can be represented in pseudo-code as:
2086<figure><artwork type="code">
2087  length := 0
2088  read chunk-size, chunk-ext (if any), and CRLF
2089  while (chunk-size &gt; 0) {
2090     read chunk-data and CRLF
2091     append chunk-data to decoded-body
2092     length := length + chunk-size
2093     read chunk-size, chunk-ext (if any), and CRLF
2094  }
2095  read header-field
2096  while (header-field not empty) {
2097     append header-field to existing header fields
2098     read header-field
2099  }
2100  Content-Length := length
2101  Remove "chunked" from Transfer-Encoding
2102  Remove Trailer from existing header fields
2107<section title="Compression Codings" anchor="compression.codings">
2109   The codings defined below can be used to compress the payload of a
2110   message.
2113<section title="Compress Coding" anchor="compress.coding">
2114<iref item="compress (Coding Format)"/>
2116   The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding
2117   <xref target="Welch"/> that is commonly produced by the UNIX file
2118   compression program "compress".
2119   A recipient &SHOULD; consider "x-compress" to be equivalent to "compress".
2123<section title="Deflate Coding" anchor="deflate.coding">
2124<iref item="deflate (Coding Format)"/>
2126   The "deflate" coding is a "zlib" data format <xref target="RFC1950"/>
2127   containing a "deflate" compressed data stream <xref target="RFC1951"/>
2128   that uses a combination of the Lempel-Ziv (LZ77) compression algorithm and
2129   Huffman coding.
2132  <t>
2133    &Note; Some incorrect implementations send the "deflate"
2134    compressed data without the zlib wrapper.
2135   </t>
2139<section title="Gzip Coding" anchor="gzip.coding">
2140<iref item="gzip (Coding Format)"/>
2142   The "gzip" coding is an LZ77 coding with a 32 bit CRC that is commonly
2143   produced by the gzip file compression program <xref target="RFC1952"/>.
2144   A recipient &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2150<section title="TE" anchor="header.te">
2151  <iref primary="true" item="TE header field" x:for-anchor=""/>
2152  <x:anchor-alias value="TE"/>
2153  <x:anchor-alias value="t-codings"/>
2154  <x:anchor-alias value="t-ranking"/>
2155  <x:anchor-alias value="rank"/>
2157   The "TE" header field in a request indicates what transfer codings,
2158   besides chunked, the client is willing to accept in response, and
2159   whether or not the client is willing to accept trailer fields in a
2160   chunked transfer coding.
2163   The TE field-value consists of a comma-separated list of transfer coding
2164   names, each allowing for optional parameters (as described in
2165   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2166   A client &MUST-NOT; send the chunked transfer coding name in TE;
2167   chunked is always acceptable for HTTP/1.1 recipients.
2169<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"/>
2170  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2171  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2172  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2173  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2174             / ( "1" [ "." 0*3("0") ] )
2177   Three examples of TE use are below.
2179<figure><artwork type="example">
2180  TE: deflate
2181  TE:
2182  TE: trailers, deflate;q=0.5
2185   The presence of the keyword "trailers" indicates that the client is willing
2186   to accept trailer fields in a chunked transfer coding, as defined in
2187   <xref target="chunked.trailer.part"/>, on behalf of itself and any downstream
2188   clients. For requests from an intermediary, this implies that either:
2189   (a) all downstream clients are willing to accept trailer fields in the
2190   forwarded response; or,
2191   (b) the intermediary will attempt to buffer the response on behalf of
2192   downstream recipients.
2193   Note that HTTP/1.1 does not define any means to limit the size of a
2194   chunked response such that an intermediary can be assured of buffering the
2195   entire response.
2198   When multiple transfer codings are acceptable, the client &MAY; rank the
2199   codings by preference using a case-insensitive "q" parameter (similar to
2200   the qvalues used in content negotiation fields, &qvalue;). The rank value
2201   is a real number in the range 0 through 1, where 0.001 is the least
2202   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2205   If the TE field-value is empty or if no TE field is present, the only
2206   acceptable transfer coding is chunked. A message with no transfer coding
2207   is always acceptable.
2210   Since the TE header field only applies to the immediate connection,
2211   a sender of TE &MUST; also send a "TE" connection option within the
2212   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2213   in order to prevent the TE field from being forwarded by intermediaries
2214   that do not support its semantics.
2218<section title="Trailer" anchor="header.trailer">
2219  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
2220  <x:anchor-alias value="Trailer"/>
2222   When a message includes a message body encoded with the chunked
2223   transfer coding and the sender desires to send metadata in the form of
2224   trailer fields at the end of the message, the sender &SHOULD; generate a
2225   <x:ref>Trailer</x:ref> header field before the message body to indicate
2226   which fields will be present in the trailers. This allows the recipient
2227   to prepare for receipt of that metadata before it starts processing the body,
2228   which is useful if the message is being streamed and the recipient wishes
2229   to confirm an integrity check on the fly.
2231<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
2232  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
2237<section title="Message Routing" anchor="message.routing">
2239   HTTP request message routing is determined by each client based on the
2240   target resource, the client's proxy configuration, and
2241   establishment or reuse of an inbound connection.  The corresponding
2242   response routing follows the same connection chain back to the client.
2245<section title="Identifying a Target Resource" anchor="target-resource">
2246  <iref primary="true" item="target resource"/>
2247  <iref primary="true" item="target URI"/>
2248  <x:anchor-alias value="target resource"/>
2249  <x:anchor-alias value="target URI"/>
2251   HTTP is used in a wide variety of applications, ranging from
2252   general-purpose computers to home appliances.  In some cases,
2253   communication options are hard-coded in a client's configuration.
2254   However, most HTTP clients rely on the same resource identification
2255   mechanism and configuration techniques as general-purpose Web browsers.
2258   HTTP communication is initiated by a user agent for some purpose.
2259   The purpose is a combination of request semantics, which are defined in
2260   <xref target="Part2"/>, and a target resource upon which to apply those
2261   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2262   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2263   would resolve to its absolute form in order to obtain the
2264   "<x:dfn>target URI</x:dfn>".  The target URI
2265   excludes the reference's fragment component, if any,
2266   since fragment identifiers are reserved for client-side processing
2267   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2271<section title="Connecting Inbound" anchor="connecting.inbound">
2273   Once the target URI is determined, a client needs to decide whether
2274   a network request is necessary to accomplish the desired semantics and,
2275   if so, where that request is to be directed.
2278   If the client has a cache <xref target="Part6"/> and the request can be
2279   satisfied by it, then the request is
2280   usually directed there first.
2283   If the request is not satisfied by a cache, then a typical client will
2284   check its configuration to determine whether a proxy is to be used to
2285   satisfy the request.  Proxy configuration is implementation-dependent,
2286   but is often based on URI prefix matching, selective authority matching,
2287   or both, and the proxy itself is usually identified by an "http" or
2288   "https" URI.  If a proxy is applicable, the client connects inbound by
2289   establishing (or reusing) a connection to that proxy.
2292   If no proxy is applicable, a typical client will invoke a handler routine,
2293   usually specific to the target URI's scheme, to connect directly
2294   to an authority for the target resource.  How that is accomplished is
2295   dependent on the target URI scheme and defined by its associated
2296   specification, similar to how this specification defines origin server
2297   access for resolution of the "http" (<xref target="http.uri"/>) and
2298   "https" (<xref target="https.uri"/>) schemes.
2301   HTTP requirements regarding connection management are defined in
2302   <xref target=""/>.
2306<section title="Request Target" anchor="request-target">
2308   Once an inbound connection is obtained,
2309   the client sends an HTTP request message (<xref target="http.message"/>)
2310   with a request-target derived from the target URI.
2311   There are four distinct formats for the request-target, depending on both
2312   the method being requested and whether the request is to a proxy.
2314<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"/>
2315  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2316                 / <x:ref>absolute-form</x:ref>
2317                 / <x:ref>authority-form</x:ref>
2318                 / <x:ref>asterisk-form</x:ref>
2320  <x:ref>origin-form</x:ref>    = <x:ref>absolute-path</x:ref> [ "?" <x:ref>query</x:ref> ]
2321  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2322  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2323  <x:ref>asterisk-form</x:ref>  = "*"
2325<t anchor="origin-form"><iref item="origin-form (of request-target)"/>
2326  <x:h>origin-form</x:h>
2329   The most common form of request-target is the <x:dfn>origin-form</x:dfn>.
2330   When making a request directly to an origin server, other than a CONNECT
2331   or server-wide OPTIONS request (as detailed below),
2332   a client &MUST; send only the absolute path and query components of
2333   the target URI as the request-target.
2334   If the target URI's path component is empty, then the client &MUST; send
2335   "/" as the path within the origin-form of request-target.
2336   A <x:ref>Host</x:ref> header field is also sent, as defined in
2337   <xref target=""/>.
2340   For example, a client wishing to retrieve a representation of the resource
2341   identified as
2343<figure><artwork x:indent-with="  " type="example">
2347   directly from the origin server would open (or reuse) a TCP connection
2348   to port 80 of the host "" and send the lines:
2350<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2351GET /where?q=now HTTP/1.1
2355   followed by the remainder of the request message.
2357<t anchor="absolute-form"><iref item="absolute-form (of request-target)"/>
2358  <x:h>absolute-form</x:h>
2361   When making a request to a proxy, other than a CONNECT or server-wide
2362   OPTIONS request (as detailed below), a client &MUST; send the target URI
2363   in <x:dfn>absolute-form</x:dfn> as the request-target.
2364   The proxy is requested to either service that request from a valid cache,
2365   if possible, or make the same request on the client's behalf to either
2366   the next inbound proxy server or directly to the origin server indicated
2367   by the request-target.  Requirements on such "forwarding" of messages are
2368   defined in <xref target="message.forwarding"/>.
2371   An example absolute-form of request-line would be:
2373<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2374GET HTTP/1.1
2377   To allow for transition to the absolute-form for all requests in some
2378   future version of HTTP, a server &MUST; accept the absolute-form
2379   in requests, even though HTTP/1.1 clients will only send them in requests
2380   to proxies.
2382<t anchor="authority-form"><iref item="authority-form (of request-target)"/>
2383  <x:h>authority-form</x:h>
2386   The <x:dfn>authority-form</x:dfn> of request-target is only used for
2387   CONNECT requests (&CONNECT;). When making a CONNECT request to establish a
2388   tunnel through one or more proxies, a client &MUST; send only the target
2389   URI's authority component (excluding any userinfo and its "@" delimiter) as
2390   the request-target. For example,
2392<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2395<t anchor="asterisk-form"><iref item="asterisk-form (of request-target)"/>
2396  <x:h>asterisk-form</x:h>
2399   The <x:dfn>asterisk-form</x:dfn> of request-target is only used for a server-wide
2400   OPTIONS request (&OPTIONS;).  When a client wishes to request OPTIONS
2401   for the server as a whole, as opposed to a specific named resource of
2402   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2403   For example,
2405<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2406OPTIONS * HTTP/1.1
2409   If a proxy receives an OPTIONS request with an absolute-form of
2410   request-target in which the URI has an empty path and no query component,
2411   then the last proxy on the request chain &MUST; send a request-target
2412   of "*" when it forwards the request to the indicated origin server.
2415   For example, the request
2416</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2420  would be forwarded by the final proxy as
2421</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2422OPTIONS * HTTP/1.1
2426   after connecting to port 8001 of host "".
2431<section title="Host" anchor="">
2432  <iref primary="true" item="Host header field" x:for-anchor=""/>
2433  <x:anchor-alias value="Host"/>
2435   The "Host" header field in a request provides the host and port
2436   information from the target URI, enabling the origin
2437   server to distinguish among resources while servicing requests
2438   for multiple host names on a single IP address.
2440<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2441  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2444   A client &MUST; send a Host header field in all HTTP/1.1 request messages.
2445   If the target URI includes an authority component, then a client &MUST;
2446   send a field-value for Host that is identical to that authority
2447   component, excluding any userinfo subcomponent and its "@" delimiter
2448   (<xref target="http.uri"/>).
2449   If the authority component is missing or undefined for the target URI,
2450   then a client &MUST; send a Host header field with an empty field-value.
2453   Since the Host field-value is critical information for handling a request,
2454   a user agent &SHOULD; generate Host as the first header field following the
2455   request-line.
2458   For example, a GET request to the origin server for
2459   &lt;; would begin with:
2461<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2462GET /pub/WWW/ HTTP/1.1
2466   A client &MUST; send a Host header field in an HTTP/1.1 request even
2467   if the request-target is in the absolute-form, since this
2468   allows the Host information to be forwarded through ancient HTTP/1.0
2469   proxies that might not have implemented Host.
2472   When a proxy receives a request with an absolute-form of
2473   request-target, the proxy &MUST; ignore the received
2474   Host header field (if any) and instead replace it with the host
2475   information of the request-target.  A proxy that forwards such a request
2476   &MUST; generate a new Host field-value based on the received
2477   request-target rather than forward the received Host field-value.
2480   Since the Host header field acts as an application-level routing
2481   mechanism, it is a frequent target for malware seeking to poison
2482   a shared cache or redirect a request to an unintended server.
2483   An interception proxy is particularly vulnerable if it relies on
2484   the Host field-value for redirecting requests to internal
2485   servers, or for use as a cache key in a shared cache, without
2486   first verifying that the intercepted connection is targeting a
2487   valid IP address for that host.
2490   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2491   to any HTTP/1.1 request message that lacks a Host header field and
2492   to any request message that contains more than one Host header field
2493   or a Host header field with an invalid field-value.
2497<section title="Effective Request URI" anchor="effective.request.uri">
2498  <iref primary="true" item="effective request URI"/>
2499  <x:anchor-alias value="effective request URI"/>
2501   A server that receives an HTTP request message &MUST; reconstruct
2502   the user agent's original target URI, based on the pieces of information
2503   learned from the request-target, <x:ref>Host</x:ref> header field, and
2504   connection context, in order to identify the intended target resource and
2505   properly service the request. The URI derived from this reconstruction
2506   process is referred to as the "<x:dfn>effective request URI</x:dfn>".
2509   For a user agent, the effective request URI is the target URI.
2512   If the request-target is in absolute-form, then the effective request URI
2513   is the same as the request-target.  Otherwise, the effective request URI
2514   is constructed as follows.
2517   If the request is received over a TLS-secured TCP connection,
2518   then the effective request URI's scheme is "https"; otherwise, the
2519   scheme is "http".
2522   If the request-target is in authority-form, then the effective
2523   request URI's authority component is the same as the request-target.
2524   Otherwise, if a <x:ref>Host</x:ref> header field is supplied with a
2525   non-empty field-value, then the authority component is the same as the
2526   Host field-value. Otherwise, the authority component is the concatenation of
2527   the default host name configured for the server, a colon (":"), and the
2528   connection's incoming TCP port number in decimal form.
2531   If the request-target is in authority-form or asterisk-form, then the
2532   effective request URI's combined path and query component is empty.
2533   Otherwise, the combined path and query component is the same as the
2534   request-target.
2537   The components of the effective request URI, once determined as above,
2538   can be combined into absolute-URI form by concatenating the scheme,
2539   "://", authority, and combined path and query component.
2543   Example 1: the following message received over an insecure TCP connection
2545<artwork type="example" x:indent-with="  ">
2546GET /pub/WWW/TheProject.html HTTP/1.1
2552  has an effective request URI of
2554<artwork type="example" x:indent-with="  ">
2560   Example 2: the following message received over a TLS-secured TCP connection
2562<artwork type="example" x:indent-with="  ">
2563OPTIONS * HTTP/1.1
2569  has an effective request URI of
2571<artwork type="example" x:indent-with="  ">
2576   An origin server that does not allow resources to differ by requested
2577   host &MAY; ignore the <x:ref>Host</x:ref> field-value and instead replace it
2578   with a configured server name when constructing the effective request URI.
2581   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2582   field &MAY; attempt to use heuristics (e.g., examination of the URI path for
2583   something unique to a particular host) in order to guess the
2584   effective request URI's authority component.
2588<section title="Associating a Response to a Request" anchor="">
2590   HTTP does not include a request identifier for associating a given
2591   request message with its corresponding one or more response messages.
2592   Hence, it relies on the order of response arrival to correspond exactly
2593   to the order in which requests are made on the same connection.
2594   More than one response message per request only occurs when one or more
2595   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a
2596   final response to the same request.
2599   A client that has more than one outstanding request on a connection &MUST;
2600   maintain a list of outstanding requests in the order sent and &MUST;
2601   associate each received response message on that connection to the highest
2602   ordered request that has not yet received a final (non-<x:ref>1xx</x:ref>)
2603   response.
2607<section title="Message Forwarding" anchor="message.forwarding">
2609   As described in <xref target="intermediaries"/>, intermediaries can serve
2610   a variety of roles in the processing of HTTP requests and responses.
2611   Some intermediaries are used to improve performance or availability.
2612   Others are used for access control or to filter content.
2613   Since an HTTP stream has characteristics similar to a pipe-and-filter
2614   architecture, there are no inherent limits to the extent an intermediary
2615   can enhance (or interfere) with either direction of the stream.
2618   An intermediary not acting as a tunnel &MUST; implement the
2619   <x:ref>Connection</x:ref> header field, as specified in
2620   <xref target="header.connection"/>, and exclude fields from being forwarded
2621   that are only intended for the incoming connection.
2624   An intermediary &MUST-NOT; forward a message to itself unless it is
2625   protected from an infinite request loop. In general, an intermediary ought
2626   to recognize its own server names, including any aliases, local variations,
2627   or literal IP addresses, and respond to such requests directly.
2630<section title="Via" anchor="header.via">
2631  <iref primary="true" item="Via header field" x:for-anchor=""/>
2632  <x:anchor-alias value="pseudonym"/>
2633  <x:anchor-alias value="received-by"/>
2634  <x:anchor-alias value="received-protocol"/>
2635  <x:anchor-alias value="Via"/>
2637   The "Via" header field indicates the presence of intermediate protocols and
2638   recipients between the user agent and the server (on requests) or between
2639   the origin server and the client (on responses), similar to the
2640   "Received" header field in email
2641   (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2642   Via can be used for tracking message forwards,
2643   avoiding request loops, and identifying the protocol capabilities of
2644   senders along the request/response chain.
2646<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"/>
2647  <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> ] )
2649  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2650                      ; see <xref target="header.upgrade"/>
2651  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2652  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2655   Multiple Via field values represent each proxy or gateway that has
2656   forwarded the message. Each intermediary appends its own information
2657   about how the message was received, such that the end result is ordered
2658   according to the sequence of forwarding recipients.
2661   A proxy &MUST; send an appropriate Via header field, as described below, in
2662   each message that it forwards.
2663   An HTTP-to-HTTP gateway &MUST; send an appropriate Via header field in
2664   each inbound request message and &MAY; send a Via header field in
2665   forwarded response messages.
2668   For each intermediary, the received-protocol indicates the protocol and
2669   protocol version used by the upstream sender of the message. Hence, the
2670   Via field value records the advertised protocol capabilities of the
2671   request/response chain such that they remain visible to downstream
2672   recipients; this can be useful for determining what backwards-incompatible
2673   features might be safe to use in response, or within a later request, as
2674   described in <xref target="http.version"/>. For brevity, the protocol-name
2675   is omitted when the received protocol is HTTP.
2678   The received-by portion of the field value is normally the host and optional
2679   port number of a recipient server or client that subsequently forwarded the
2680   message.
2681   However, if the real host is considered to be sensitive information, a
2682   sender &MAY; replace it with a pseudonym. If a port is not provided,
2683   a recipient &MAY; interpret that as meaning it was received on the default
2684   TCP port, if any, for the received-protocol.
2687   A sender &MAY; generate comments in the Via header field to identify the
2688   software of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2689   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2690   are optional and a recipient &MAY; remove them prior to forwarding the
2691   message.
2694   For example, a request message could be sent from an HTTP/1.0 user
2695   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2696   forward the request to a public proxy at, which completes
2697   the request by forwarding it to the origin server at
2698   The request received by would then have the following
2699   Via header field:
2701<figure><artwork type="example">
2702  Via: 1.0 fred, 1.1
2705   An intermediary used as a portal through a network firewall
2706   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2707   region unless it is explicitly enabled to do so. If not enabled, such an
2708   intermediary &SHOULD; replace each received-by host of any host behind the
2709   firewall by an appropriate pseudonym for that host.
2712   An intermediary &MAY; combine an ordered subsequence of Via header
2713   field entries into a single such entry if the entries have identical
2714   received-protocol values. For example,
2716<figure><artwork type="example">
2717  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2720  could be collapsed to
2722<figure><artwork type="example">
2723  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2726   A sender &SHOULD-NOT; combine multiple entries unless they are all
2727   under the same organizational control and the hosts have already been
2728   replaced by pseudonyms. A sender &MUST-NOT; combine entries that
2729   have different received-protocol values.
2733<section title="Transformations" anchor="message.transformations">
2734   <iref primary="true" item="transforming proxy"/>
2735   <iref primary="true" item="non-transforming proxy"/>
2737   Some intermediaries include features for transforming messages and their
2738   payloads. A proxy might, for example, convert between image formats in
2739   order to save cache space or to reduce the amount of traffic on a slow
2740   link. However, operational problems might occur when these transformations
2741   are applied to payloads intended for critical applications, such as medical
2742   imaging or scientific data analysis, particularly when integrity checks or
2743   digital signatures are used to ensure that the payload received is
2744   identical to the original.
2747   An HTTP-to-HTTP proxy is called a "<x:dfn>transforming proxy</x:dfn>"
2748   if it is designed or configured to modify messages in a semantically
2749   meaningful way (i.e., modifications, beyond those required by normal
2750   HTTP processing, that change the message in a way that would be
2751   significant to the original sender or potentially significant to
2752   downstream recipients).  For example, a transforming proxy might be
2753   acting as a shared annotation server (modifying responses to include
2754   references to a local annotation database), a malware filter, a
2755   format transcoder, or a privacy filter. Such transformations are presumed
2756   to be desired by whichever client (or client organization) selected the
2757   proxy.
2760   If a proxy receives a request-target with a host name that is not a
2761   fully qualified domain name, it &MAY; add its own domain to the host name
2762   it received when forwarding the request.  A proxy &MUST-NOT; change the
2763   host name if it is a fully qualified domain name.
2766   A proxy &MUST-NOT; modify the "absolute-path" and "query" parts of the
2767   received request-target when forwarding it to the next inbound server,
2768   except as noted above to replace an empty path with "/" or "*".
2771   A proxy &MUST-NOT; modify header fields that provide information about the
2772   end points of the communication chain, the resource state, or the selected
2773   representation. A proxy &MAY; change the message body through application
2774   or removal of a transfer coding (<xref target="transfer.codings"/>).
2777   A proxy &MUST-NOT; modify the payload (&payload;) of a message that
2778   contains a no-transform cache-control directive (&header-cache-control;).
2781   A proxy &MAY; transform the payload of a message
2782   that does not contain a no-transform cache-control directive.
2783   A proxy that transforms a payload &MUST; add a
2784   Warning header field with the warn-code of 214 ("Transformation Applied")
2785   if one is not already in the message (see &header-warning;).
2786   A proxy that transforms the payload of a <x:ref>200 (OK)</x:ref> response
2787   can further inform downstream recipients that a transformation has been
2788   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 considerations relevant to HTTP message syntax,
3851   parsing, and routing. Security considerations about HTTP semantics and
3852   payloads are addressed in &semantics;.
3855<section title="DNS-related Attacks" anchor="dns.related.attacks">
3857   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3858   generally prone to security attacks based on the deliberate misassociation
3859   of IP addresses and DNS names not protected by DNSSEC. Clients need to be
3860   cautious in assuming the validity of an IP number/DNS name association unless
3861   the response is protected by DNSSEC (<xref target="RFC4033"/>).
3865<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3867   By their very nature, HTTP intermediaries are men-in-the-middle, and
3868   represent an opportunity for man-in-the-middle attacks. Compromise of
3869   the systems on which the intermediaries run can result in serious security
3870   and privacy problems. Intermediaries have access to security-related
3871   information, personal information about individual users and
3872   organizations, and proprietary information belonging to users and
3873   content providers. A compromised intermediary, or an intermediary
3874   implemented or configured without regard to security and privacy
3875   considerations, might be used in the commission of a wide range of
3876   potential attacks.
3879   Intermediaries that contain a shared cache are especially vulnerable
3880   to cache poisoning attacks.
3883   Implementers need to consider the privacy and security
3884   implications of their design and coding decisions, and of the
3885   configuration options they provide to operators (especially the
3886   default configuration).
3889   Users need to be aware that intermediaries are no more trustworthy than
3890   the people who run them; HTTP itself cannot solve this problem.
3894<section title="Buffer Overflows" anchor="attack.protocol.element.size.overflows">
3896   Because HTTP uses mostly textual, character-delimited fields, attackers can
3897   overflow buffers in implementations, and/or perform a Denial of Service
3898   against implementations that accept fields with unlimited lengths.
3901   To promote interoperability, this specification makes specific
3902   recommendations for minimum size limits on request-line
3903   (<xref target="request.line"/>)
3904   and header fields (<xref target="header.fields"/>). These are
3905   minimum recommendations, chosen to be supportable even by implementations
3906   with limited resources; it is expected that most implementations will
3907   choose substantially higher limits.
3910   This specification also provides a way for servers to reject messages that
3911   have request-targets that are too long (&status-414;) or request entities
3912   that are too large (&status-4xx;). Additional status codes related to
3913   capacity limits have been defined by extensions to HTTP
3914   <xref target="RFC6585"/>.
3917   Recipients ought to carefully limit the extent to which they read other
3918   fields, including (but not limited to) request methods, response status
3919   phrases, header field-names, and body chunks, so as to avoid denial of
3920   service attacks without impeding interoperability.
3924<section title="Message Integrity" anchor="message.integrity">
3926   HTTP does not define a specific mechanism for ensuring message integrity,
3927   instead relying on the error-detection ability of underlying transport
3928   protocols and the use of length or chunk-delimited framing to detect
3929   completeness. Additional integrity mechanisms, such as hash functions or
3930   digital signatures applied to the content, can be selectively added to
3931   messages via extensible metadata header fields. Historically, the lack of
3932   a single integrity mechanism has been justified by the informal nature of
3933   most HTTP communication.  However, the prevalence of HTTP as an information
3934   access mechanism has resulted in its increasing use within environments
3935   where verification of message integrity is crucial.
3938   User agents are encouraged to implement configurable means for detecting
3939   and reporting failures of message integrity such that those means can be
3940   enabled within environments for which integrity is necessary. For example,
3941   a browser being used to view medical history or drug interaction
3942   information needs to indicate to the user when such information is detected
3943   by the protocol to be incomplete, expired, or corrupted during transfer.
3944   Such mechanisms might be selectively enabled via user agent extensions or
3945   the presence of message integrity metadata in a response.
3946   At a minimum, user agents ought to provide some indication that allows a
3947   user to distinguish between a complete and incomplete response message
3948   (<xref target="incomplete.messages"/>) when such verification is desired.
3952<section title="Server Log Information" anchor="abuse.of.server.log.information">
3954   A server is in the position to save personal data about a user's requests
3955   over time, which might identify their reading patterns or subjects of
3956   interest.  In particular, log information gathered at an intermediary
3957   often contains a history of user agent interaction, across a multitude
3958   of sites, that can be traced to individual users.
3961   HTTP log information is confidential in nature; its handling is often
3962   constrained by laws and regulations.  Log information needs to be securely
3963   stored and appropriate guidelines followed for its analysis.
3964   Anonymization of personal information within individual entries helps,
3965   but is generally not sufficient to prevent real log traces from being
3966   re-identified based on correlation with other access characteristics.
3967   As such, access traces that are keyed to a specific client are unsafe to
3968   publish even if the key is pseudonymous.
3971   To minimize the risk of theft or accidental publication, log information
3972   ought to be purged of personally identifiable information, including
3973   user identifiers, IP addresses, and user-provided query parameters,
3974   as soon as that information is no longer necessary to support operational
3975   needs for security, auditing, or fraud control.
3980<section title="Acknowledgments" anchor="acks">
3982   This edition of HTTP/1.1 builds on the many contributions that went into
3983   <xref target="RFC1945" format="none">RFC 1945</xref>,
3984   <xref target="RFC2068" format="none">RFC 2068</xref>,
3985   <xref target="RFC2145" format="none">RFC 2145</xref>, and
3986   <xref target="RFC2616" format="none">RFC 2616</xref>, including
3987   substantial contributions made by the previous authors, editors, and
3988   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
3989   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
3990   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
3993   Since 1999, the following contributors have helped improve the HTTP
3994   specification by reporting bugs, asking smart questions, drafting or
3995   reviewing text, and evaluating open issues:
3997<?BEGININC acks ?>
3998<t>Adam Barth,
3999Adam Roach,
4000Addison Phillips,
4001Adrian Chadd,
4002Adrian Cole,
4003Adrien W. de Croy,
4004Alan Ford,
4005Alan Ruttenberg,
4006Albert Lunde,
4007Alek Storm,
4008Alex Rousskov,
4009Alexandre Morgaut,
4010Alexey Melnikov,
4011Alisha Smith,
4012Amichai Rothman,
4013Amit Klein,
4014Amos Jeffries,
4015Andreas Maier,
4016Andreas Petersson,
4017Andrei Popov,
4018Anil Sharma,
4019Anne van Kesteren,
4020Anthony Bryan,
4021Asbjorn Ulsberg,
4022Ashok Kumar,
4023Balachander Krishnamurthy,
4024Barry Leiba,
4025Ben Laurie,
4026Benjamin Carlyle,
4027Benjamin Niven-Jenkins,
4028Benoit Claise,
4029Bil Corry,
4030Bill Burke,
4031Bjoern Hoehrmann,
4032Bob Scheifler,
4033Boris Zbarsky,
4034Brett Slatkin,
4035Brian Kell,
4036Brian McBarron,
4037Brian Pane,
4038Brian Raymor,
4039Brian Smith,
4040Bruce Perens,
4041Bryce Nesbitt,
4042Cameron Heavon-Jones,
4043Carl Kugler,
4044Carsten Bormann,
4045Charles Fry,
4046Chris Burdess,
4047Chris Newman,
4048Christian Huitema,
4049Cyrus Daboo,
4050Dale Robert Anderson,
4051Dan Wing,
4052Dan Winship,
4053Daniel Stenberg,
4054Darrel Miller,
4055Dave Cridland,
4056Dave Crocker,
4057Dave Kristol,
4058Dave Thaler,
4059David Booth,
4060David Singer,
4061David W. Morris,
4062Diwakar Shetty,
4063Dmitry Kurochkin,
4064Drummond Reed,
4065Duane Wessels,
4066Edward Lee,
4067Eitan Adler,
4068Eliot Lear,
4069Emile Stephan,
4070Eran Hammer-Lahav,
4071Eric D. Williams,
4072Eric J. Bowman,
4073Eric Lawrence,
4074Eric Rescorla,
4075Erik Aronesty,
4076EungJun Yi,
4077Evan Prodromou,
4078Felix Geisendoerfer,
4079Florian Weimer,
4080Frank Ellermann,
4081Fred Akalin,
4082Fred Bohle,
4083Frederic Kayser,
4084Gabor Molnar,
4085Gabriel Montenegro,
4086Geoffrey Sneddon,
4087Gervase Markham,
4088Gili Tzabari,
4089Grahame Grieve,
4090Greg Slepak,
4091Greg Wilkins,
4092Grzegorz Calkowski,
4093Harald Tveit Alvestrand,
4094Harry Halpin,
4095Helge Hess,
4096Henrik Nordstrom,
4097Henry S. Thompson,
4098Henry Story,
4099Herbert van de Sompel,
4100Herve Ruellan,
4101Howard Melman,
4102Hugo Haas,
4103Ian Fette,
4104Ian Hickson,
4105Ido Safruti,
4106Ilari Liusvaara,
4107Ilya Grigorik,
4108Ingo Struck,
4109J. Ross Nicoll,
4110James Cloos,
4111James H. Manger,
4112James Lacey,
4113James M. Snell,
4114Jamie Lokier,
4115Jan Algermissen,
4116Jari Arkko,
4117Jeff Hodges (who came up with the term 'effective Request-URI'),
4118Jeff Pinner,
4119Jeff Walden,
4120Jim Luther,
4121Jitu Padhye,
4122Joe D. Williams,
4123Joe Gregorio,
4124Joe Orton,
4125Joel Jaeggli,
4126John C. Klensin,
4127John C. Mallery,
4128John Cowan,
4129John Kemp,
4130John Panzer,
4131John Schneider,
4132John Stracke,
4133John Sullivan,
4134Jonas Sicking,
4135Jonathan A. Rees,
4136Jonathan Billington,
4137Jonathan Moore,
4138Jonathan Silvera,
4139Jordi Ros,
4140Joris Dobbelsteen,
4141Josh Cohen,
4142Julien Pierre,
4143Jungshik Shin,
4144Justin Chapweske,
4145Justin Erenkrantz,
4146Justin James,
4147Kalvinder Singh,
4148Karl Dubost,
4149Kathleen Moriarty,
4150Keith Hoffman,
4151Keith Moore,
4152Ken Murchison,
4153Koen Holtman,
4154Konstantin Voronkov,
4155Kris Zyp,
4156Leif Hedstrom,
4157Lionel Morand,
4158Lisa Dusseault,
4159Maciej Stachowiak,
4160Manu Sporny,
4161Marc Schneider,
4162Marc Slemko,
4163Mark Baker,
4164Mark Pauley,
4165Mark Watson,
4166Markus Isomaki,
4167Markus Lanthaler,
4168Martin J. Duerst,
4169Martin Musatov,
4170Martin Nilsson,
4171Martin Thomson,
4172Matt Lynch,
4173Matthew Cox,
4174Matthew Kerwin,
4175Max Clark,
4176Menachem Dodge,
4177Meral Shirazipour,
4178Michael Burrows,
4179Michael Hausenblas,
4180Michael Scharf,
4181Michael Sweet,
4182Michael Tuexen,
4183Michael Welzl,
4184Mike Amundsen,
4185Mike Belshe,
4186Mike Bishop,
4187Mike Kelly,
4188Mike Schinkel,
4189Miles Sabin,
4190Murray S. Kucherawy,
4191Mykyta Yevstifeyev,
4192Nathan Rixham,
4193Nicholas Shanks,
4194Nico Williams,
4195Nicolas Alvarez,
4196Nicolas Mailhot,
4197Noah Slater,
4198Osama Mazahir,
4199Pablo Castro,
4200Pat Hayes,
4201Patrick R. McManus,
4202Paul E. Jones,
4203Paul Hoffman,
4204Paul Marquess,
4205Pete Resnick,
4206Peter Lepeska,
4207Peter Occil,
4208Peter Saint-Andre,
4209Peter Watkins,
4210Phil Archer,
4211Phil Hunt,
4212Philippe Mougin,
4213Phillip Hallam-Baker,
4214Piotr Dobrogost,
4215Poul-Henning Kamp,
4216Preethi Natarajan,
4217Rajeev Bector,
4218Ray Polk,
4219Reto Bachmann-Gmuer,
4220Richard Barnes,
4221Richard Cyganiak,
4222Rob Trace,
4223Robby Simpson,
4224Robert Brewer,
4225Robert Collins,
4226Robert Mattson,
4227Robert O'Callahan,
4228Robert Olofsson,
4229Robert Sayre,
4230Robert Siemer,
4231Robert de Wilde,
4232Roberto Javier Godoy,
4233Roberto Peon,
4234Roland Zink,
4235Ronny Widjaja,
4236Ryan Hamilton,
4237S. Mike Dierken,
4238Salvatore Loreto,
4239Sam Johnston,
4240Sam Pullara,
4241Sam Ruby,
4242Saurabh Kulkarni,
4243Scott Lawrence (who maintained the original issues list),
4244Sean B. Palmer,
4245Sean Turner,
4246Sebastien Barnoud,
4247Shane McCarron,
4248Shigeki Ohtsu,
4249Simon Yarde,
4250Stefan Eissing,
4251Stefan Tilkov,
4252Stefanos Harhalakis,
4253Stephane Bortzmeyer,
4254Stephen Farrell,
4255Stephen Kent,
4256Stephen Ludin,
4257Stuart Williams,
4258Subbu Allamaraju,
4259Subramanian Moonesamy,
4260Susan Hares,
4261Sylvain Hellegouarch,
4262Tapan Divekar,
4263Tatsuhiro Tsujikawa,
4264Tatsuya Hayashi,
4265Ted Hardie,
4266Ted Lemon,
4267Thomas Broyer,
4268Thomas Fossati,
4269Thomas Maslen,
4270Thomas Nadeau,
4271Thomas Nordin,
4272Thomas Roessler,
4273Tim Bray,
4274Tim Morgan,
4275Tim Olsen,
4276Tom Zhou,
4277Travis Snoozy,
4278Tyler Close,
4279Vincent Murphy,
4280Wenbo Zhu,
4281Werner Baumann,
4282Wilbur Streett,
4283Wilfredo Sanchez Vega,
4284William A. Rowe Jr.,
4285William Chan,
4286Willy Tarreau,
4287Xiaoshu Wang,
4288Yaron Goland,
4289Yngve Nysaeter Pettersen,
4290Yoav Nir,
4291Yogesh Bang,
4292Yuchung Cheng,
4293Yutaka Oiwa,
4294Yves Lafon (long-time member of the editor team),
4295Zed A. Shaw, and
4296Zhong Yu.
4298<?ENDINC acks ?>
4300   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
4301   acknowledgements from prior revisions.
4308<references title="Normative References">
4310<reference anchor="Part2">
4311  <front>
4312    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
4313    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4314      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4315      <address><email></email></address>
4316    </author>
4317    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4318      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4319      <address><email></email></address>
4320    </author>
4321    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4322  </front>
4323  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
4324  <x:source href="p2-semantics.xml" basename="p2-semantics">
4325    <x:defines>1xx (Informational)</x:defines>
4326    <x:defines>1xx</x:defines>
4327    <x:defines>100 (Continue)</x:defines>
4328    <x:defines>101 (Switching Protocols)</x:defines>
4329    <x:defines>2xx (Successful)</x:defines>
4330    <x:defines>2xx</x:defines>
4331    <x:defines>200 (OK)</x:defines>
4332    <x:defines>203 (Non-Authoritative Information)</x:defines>
4333    <x:defines>204 (No Content)</x:defines>
4334    <x:defines>3xx (Redirection)</x:defines>
4335    <x:defines>3xx</x:defines>
4336    <x:defines>301 (Moved Permanently)</x:defines>
4337    <x:defines>4xx (Client Error)</x:defines>
4338    <x:defines>4xx</x:defines>
4339    <x:defines>400 (Bad Request)</x:defines>
4340    <x:defines>411 (Length Required)</x:defines>
4341    <x:defines>414 (URI Too Long)</x:defines>
4342    <x:defines>417 (Expectation Failed)</x:defines>
4343    <x:defines>426 (Upgrade Required)</x:defines>
4344    <x:defines>501 (Not Implemented)</x:defines>
4345    <x:defines>502 (Bad Gateway)</x:defines>
4346    <x:defines>505 (HTTP Version Not Supported)</x:defines>
4347    <x:defines>Accept-Encoding</x:defines>
4348    <x:defines>Allow</x:defines>
4349    <x:defines>Content-Encoding</x:defines>
4350    <x:defines>Content-Location</x:defines>
4351    <x:defines>Content-Type</x:defines>
4352    <x:defines>Date</x:defines>
4353    <x:defines>Expect</x:defines>
4354    <x:defines>Location</x:defines>
4355    <x:defines>Server</x:defines>
4356    <x:defines>User-Agent</x:defines>
4357  </x:source>
4360<reference anchor="Part4">
4361  <front>
4362    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
4363    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
4364      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4365      <address><email></email></address>
4366    </author>
4367    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
4368      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4369      <address><email></email></address>
4370    </author>
4371    <date month="&ID-MONTH;" year="&ID-YEAR;" />
4372  </front>
4373  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
4374  <x:source basename="p4-conditional" href="p4-conditional.xml">
4375    <x:defines>304 (Not Modified)</x:defines>
4376    <x:defines>ETag</x:defines>
4377    <x:defines>Last-Modified</x:defines>
4378  </x:source>
4381<reference anchor="Part5">
4382  <front>
4383    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
4384    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4385      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4386      <address><email></email></address>
4387    </author>
4388    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
4389      <organization abbrev="W3C">World Wide Web Consortium</organization>
4390      <address><email></email></address>
4391    </author>
4392    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4393      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4394      <address><email></email></address>
4395    </author>
4396    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4397  </front>
4398  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4399  <x:source href="p5-range.xml" basename="p5-range">
4400    <x:defines>Content-Range</x:defines>
4401  </x:source>
4404<reference anchor="Part6">
4405  <front>
4406    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4407    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4408      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4409      <address><email></email></address>
4410    </author>
4411    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4412      <organization>Akamai</organization>
4413      <address><email></email></address>
4414    </author>
4415    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4416      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4417      <address><email></email></address>
4418    </author>
4419    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4420  </front>
4421  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4422  <x:source href="p6-cache.xml" basename="p6-cache">
4423    <x:defines>Cache-Control</x:defines>
4424    <x:defines>Expires</x:defines>
4425  </x:source>
4428<reference anchor="Part7">
4429  <front>
4430    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4431    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4432      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4433      <address><email></email></address>
4434    </author>
4435    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4436      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4437      <address><email></email></address>
4438    </author>
4439    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4440  </front>
4441  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4442  <x:source href="p7-auth.xml" basename="p7-auth">
4443    <x:defines>Proxy-Authenticate</x:defines>
4444    <x:defines>Proxy-Authorization</x:defines>
4445  </x:source>
4448<reference anchor="RFC5234">
4449  <front>
4450    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4451    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4452      <organization>Brandenburg InternetWorking</organization>
4453      <address>
4454        <email></email>
4455      </address> 
4456    </author>
4457    <author initials="P." surname="Overell" fullname="Paul Overell">
4458      <organization>THUS plc.</organization>
4459      <address>
4460        <email></email>
4461      </address>
4462    </author>
4463    <date month="January" year="2008"/>
4464  </front>
4465  <seriesInfo name="STD" value="68"/>
4466  <seriesInfo name="RFC" value="5234"/>
4469<reference anchor="RFC2119">
4470  <front>
4471    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4472    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4473      <organization>Harvard University</organization>
4474      <address><email></email></address>
4475    </author>
4476    <date month="March" year="1997"/>
4477  </front>
4478  <seriesInfo name="BCP" value="14"/>
4479  <seriesInfo name="RFC" value="2119"/>
4482<reference anchor="RFC3986">
4483 <front>
4484  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4485  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4486    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4487    <address>
4488       <email></email>
4489       <uri></uri>
4490    </address>
4491  </author>
4492  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4493    <organization abbrev="Day Software">Day Software</organization>
4494    <address>
4495      <email></email>
4496      <uri></uri>
4497    </address>
4498  </author>
4499  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4500    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4501    <address>
4502      <email></email>
4503      <uri></uri>
4504    </address>
4505  </author>
4506  <date month='January' year='2005'></date>
4507 </front>
4508 <seriesInfo name="STD" value="66"/>
4509 <seriesInfo name="RFC" value="3986"/>
4512<reference anchor="RFC0793">
4513  <front>
4514    <title>Transmission Control Protocol</title>
4515    <author initials='J.' surname='Postel' fullname='Jon Postel'>
4516      <organization>University of Southern California (USC)/Information Sciences Institute</organization>
4517    </author>
4518    <date year='1981' month='September' />
4519  </front>
4520  <seriesInfo name='STD' value='7' />
4521  <seriesInfo name='RFC' value='793' />
4524<reference anchor="USASCII">
4525  <front>
4526    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4527    <author>
4528      <organization>American National Standards Institute</organization>
4529    </author>
4530    <date year="1986"/>
4531  </front>
4532  <seriesInfo name="ANSI" value="X3.4"/>
4535<reference anchor="RFC1950">
4536  <front>
4537    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4538    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4539      <organization>Aladdin Enterprises</organization>
4540      <address><email></email></address>
4541    </author>
4542    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4543    <date month="May" year="1996"/>
4544  </front>
4545  <seriesInfo name="RFC" value="1950"/>
4546  <!--<annotation>
4547    RFC 1950 is an Informational RFC, thus it might be less stable than
4548    this specification. On the other hand, this downward reference was
4549    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4550    therefore it is unlikely to cause problems in practice. See also
4551    <xref target="BCP97"/>.
4552  </annotation>-->
4555<reference anchor="RFC1951">
4556  <front>
4557    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4558    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4559      <organization>Aladdin Enterprises</organization>
4560      <address><email></email></address>
4561    </author>
4562    <date month="May" year="1996"/>
4563  </front>
4564  <seriesInfo name="RFC" value="1951"/>
4565  <!--<annotation>
4566    RFC 1951 is an Informational RFC, thus it might be less stable than
4567    this specification. On the other hand, this downward reference was
4568    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4569    therefore it is unlikely to cause problems in practice. See also
4570    <xref target="BCP97"/>.
4571  </annotation>-->
4574<reference anchor="RFC1952">
4575  <front>
4576    <title>GZIP file format specification version 4.3</title>
4577    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4578      <organization>Aladdin Enterprises</organization>
4579      <address><email></email></address>
4580    </author>
4581    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4582      <address><email></email></address>
4583    </author>
4584    <author initials="M." surname="Adler" fullname="Mark Adler">
4585      <address><email></email></address>
4586    </author>
4587    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4588      <address><email></email></address>
4589    </author>
4590    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4591      <address><email></email></address>
4592    </author>
4593    <date month="May" year="1996"/>
4594  </front>
4595  <seriesInfo name="RFC" value="1952"/>
4596  <!--<annotation>
4597    RFC 1952 is an Informational RFC, thus it might be less stable than
4598    this specification. On the other hand, this downward reference was
4599    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4600    therefore it is unlikely to cause problems in practice. See also
4601    <xref target="BCP97"/>.
4602  </annotation>-->
4605<reference anchor="Welch">
4606  <front>
4607    <title>A Technique for High Performance Data Compression</title>
4608    <author initials="T.A." surname="Welch" fullname="Terry A. Welch"/>
4609    <date month="June" year="1984"/>
4610  </front>
4611  <seriesInfo name="IEEE Computer" value="17(6)"/>
4616<references title="Informative References">
4618<reference anchor="ISO-8859-1">
4619  <front>
4620    <title>
4621     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4622    </title>
4623    <author>
4624      <organization>International Organization for Standardization</organization>
4625    </author>
4626    <date year="1998"/>
4627  </front>
4628  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4631<reference anchor='RFC1919'>
4632  <front>
4633    <title>Classical versus Transparent IP Proxies</title>
4634    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4635      <address><email></email></address>
4636    </author>
4637    <date year='1996' month='March' />
4638  </front>
4639  <seriesInfo name='RFC' value='1919' />
4642<reference anchor="RFC1945">
4643  <front>
4644    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4645    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4646      <organization>MIT, Laboratory for Computer Science</organization>
4647      <address><email></email></address>
4648    </author>
4649    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4650      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4651      <address><email></email></address>
4652    </author>
4653    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4654      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4655      <address><email></email></address>
4656    </author>
4657    <date month="May" year="1996"/>
4658  </front>
4659  <seriesInfo name="RFC" value="1945"/>
4662<reference anchor="RFC2045">
4663  <front>
4664    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4665    <author initials="N." surname="Freed" fullname="Ned Freed">
4666      <organization>Innosoft International, Inc.</organization>
4667      <address><email></email></address>
4668    </author>
4669    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4670      <organization>First Virtual Holdings</organization>
4671      <address><email></email></address>
4672    </author>
4673    <date month="November" year="1996"/>
4674  </front>
4675  <seriesInfo name="RFC" value="2045"/>
4678<reference anchor="RFC2047">
4679  <front>
4680    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4681    <author initials="K." surname="Moore" fullname="Keith Moore">
4682      <organization>University of Tennessee</organization>
4683      <address><email></email></address>
4684    </author>
4685    <date month="November" year="1996"/>
4686  </front>
4687  <seriesInfo name="RFC" value="2047"/>
4690<reference anchor="RFC2068">
4691  <front>
4692    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4693    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
4694      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4695      <address><email></email></address>
4696    </author>
4697    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4698      <organization>MIT Laboratory for Computer Science</organization>
4699      <address><email></email></address>
4700    </author>
4701    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4702      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4703      <address><email></email></address>
4704    </author>
4705    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4706      <organization>MIT Laboratory for Computer Science</organization>
4707      <address><email></email></address>
4708    </author>
4709    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4710      <organization>MIT Laboratory for Computer Science</organization>
4711      <address><email></email></address>
4712    </author>
4713    <date month="January" year="1997"/>
4714  </front>
4715  <seriesInfo name="RFC" value="2068"/>
4718<reference anchor="RFC2145">
4719  <front>
4720    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4721    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4722      <organization>Western Research Laboratory</organization>
4723      <address><email></email></address>
4724    </author>
4725    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4726      <organization>Department of Information and Computer Science</organization>
4727      <address><email></email></address>
4728    </author>
4729    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4730      <organization>MIT Laboratory for Computer Science</organization>
4731      <address><email></email></address>
4732    </author>
4733    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4734      <organization>W3 Consortium</organization>
4735      <address><email></email></address>
4736    </author>
4737    <date month="May" year="1997"/>
4738  </front>
4739  <seriesInfo name="RFC" value="2145"/>
4742<reference anchor="RFC2616">
4743  <front>
4744    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4745    <author initials="R." surname="Fielding" fullname="R. Fielding">
4746      <organization>University of California, Irvine</organization>
4747      <address><email></email></address>
4748    </author>
4749    <author initials="J." surname="Gettys" fullname="J. Gettys">
4750      <organization>W3C</organization>
4751      <address><email></email></address>
4752    </author>
4753    <author initials="J." surname="Mogul" fullname="J. Mogul">
4754      <organization>Compaq Computer Corporation</organization>
4755      <address><email></email></address>
4756    </author>
4757    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4758      <organization>MIT Laboratory for Computer Science</organization>
4759      <address><email></email></address>
4760    </author>
4761    <author initials="L." surname="Masinter" fullname="L. Masinter">
4762      <organization>Xerox Corporation</organization>
4763      <address><email></email></address>
4764    </author>
4765    <author initials="P." surname="Leach" fullname="P. Leach">
4766      <organization>Microsoft Corporation</organization>
4767      <address><email></email></address>
4768    </author>
4769    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4770      <organization>W3C</organization>
4771      <address><email></email></address>
4772    </author>
4773    <date month="June" year="1999"/>
4774  </front>
4775  <seriesInfo name="RFC" value="2616"/>
4778<reference anchor='RFC2817'>
4779  <front>
4780    <title>Upgrading to TLS Within HTTP/1.1</title>
4781    <author initials='R.' surname='Khare' fullname='R. Khare'>
4782      <organization>4K Associates / UC Irvine</organization>
4783      <address><email></email></address>
4784    </author>
4785    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4786      <organization>Agranat Systems, Inc.</organization>
4787      <address><email></email></address>
4788    </author>
4789    <date year='2000' month='May' />
4790  </front>
4791  <seriesInfo name='RFC' value='2817' />
4794<reference anchor='RFC2818'>
4795  <front>
4796    <title>HTTP Over TLS</title>
4797    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4798      <organization>RTFM, Inc.</organization>
4799      <address><email></email></address>
4800    </author>
4801    <date year='2000' month='May' />
4802  </front>
4803  <seriesInfo name='RFC' value='2818' />
4806<reference anchor='RFC3040'>
4807  <front>
4808    <title>Internet Web Replication and Caching Taxonomy</title>
4809    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4810      <organization>Equinix, Inc.</organization>
4811    </author>
4812    <author initials='I.' surname='Melve' fullname='I. Melve'>
4813      <organization>UNINETT</organization>
4814    </author>
4815    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4816      <organization>CacheFlow Inc.</organization>
4817    </author>
4818    <date year='2001' month='January' />
4819  </front>
4820  <seriesInfo name='RFC' value='3040' />
4823<reference anchor='BCP90'>
4824  <front>
4825    <title>Registration Procedures for Message Header Fields</title>
4826    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4827      <organization>Nine by Nine</organization>
4828      <address><email></email></address>
4829    </author>
4830    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4831      <organization>BEA Systems</organization>
4832      <address><email></email></address>
4833    </author>
4834    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4835      <organization>HP Labs</organization>
4836      <address><email></email></address>
4837    </author>
4838    <date year='2004' month='September' />
4839  </front>
4840  <seriesInfo name='BCP' value='90' />
4841  <seriesInfo name='RFC' value='3864' />
4844<reference anchor='RFC4033'>
4845  <front>
4846    <title>DNS Security Introduction and Requirements</title>
4847    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4848    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4849    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4850    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4851    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4852    <date year='2005' month='March' />
4853  </front>
4854  <seriesInfo name='RFC' value='4033' />
4857<reference anchor="BCP13">
4858  <front>
4859    <title>Media Type Specifications and Registration Procedures</title>
4860    <author initials="N." surname="Freed" fullname="Ned Freed">
4861      <organization>Oracle</organization>
4862      <address>
4863        <email></email>
4864      </address>
4865    </author>
4866    <author initials="J." surname="Klensin" fullname="John C. Klensin">
4867      <address>
4868        <email></email>
4869      </address>
4870    </author>
4871    <author initials="T." surname="Hansen" fullname="Tony Hansen">
4872      <organization>AT&amp;T Laboratories</organization>
4873      <address>
4874        <email></email>
4875      </address>
4876    </author>
4877    <date year="2013" month="January"/>
4878  </front>
4879  <seriesInfo name="BCP" value="13"/>
4880  <seriesInfo name="RFC" value="6838"/>
4883<reference anchor='BCP115'>
4884  <front>
4885    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4886    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4887      <organization>AT&amp;T Laboratories</organization>
4888      <address>
4889        <email></email>
4890      </address>
4891    </author>
4892    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4893      <organization>Qualcomm, Inc.</organization>
4894      <address>
4895        <email></email>
4896      </address>
4897    </author>
4898    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4899      <organization>Adobe Systems</organization>
4900      <address>
4901        <email></email>
4902      </address>
4903    </author>
4904    <date year='2006' month='February' />
4905  </front>
4906  <seriesInfo name='BCP' value='115' />
4907  <seriesInfo name='RFC' value='4395' />
4910<reference anchor='RFC4559'>
4911  <front>
4912    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4913    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4914    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4915    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4916    <date year='2006' month='June' />
4917  </front>
4918  <seriesInfo name='RFC' value='4559' />
4921<reference anchor='RFC5226'>
4922  <front>
4923    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4924    <author initials='T.' surname='Narten' fullname='T. Narten'>
4925      <organization>IBM</organization>
4926      <address><email></email></address>
4927    </author>
4928    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4929      <organization>Google</organization>
4930      <address><email></email></address>
4931    </author>
4932    <date year='2008' month='May' />
4933  </front>
4934  <seriesInfo name='BCP' value='26' />
4935  <seriesInfo name='RFC' value='5226' />
4938<reference anchor='RFC5246'>
4939   <front>
4940      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4941      <author initials='T.' surname='Dierks' fullname='T. Dierks'/>
4942      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4943         <organization>RTFM, Inc.</organization>
4944      </author>
4945      <date year='2008' month='August' />
4946   </front>
4947   <seriesInfo name='RFC' value='5246' />
4950<reference anchor="RFC5322">
4951  <front>
4952    <title>Internet Message Format</title>
4953    <author initials="P." surname="Resnick" fullname="P. Resnick">
4954      <organization>Qualcomm Incorporated</organization>
4955    </author>
4956    <date year="2008" month="October"/>
4957  </front>
4958  <seriesInfo name="RFC" value="5322"/>
4961<reference anchor="RFC6265">
4962  <front>
4963    <title>HTTP State Management Mechanism</title>
4964    <author initials="A." surname="Barth" fullname="Adam Barth">
4965      <organization abbrev="U.C. Berkeley">
4966        University of California, Berkeley
4967      </organization>
4968      <address><email></email></address>
4969    </author>
4970    <date year="2011" month="April" />
4971  </front>
4972  <seriesInfo name="RFC" value="6265"/>
4975<reference anchor='RFC6585'>
4976  <front>
4977    <title>Additional HTTP Status Codes</title>
4978    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4979      <organization>Rackspace</organization>
4980    </author>
4981    <author initials='R.' surname='Fielding' fullname='R. Fielding'>
4982      <organization>Adobe</organization>
4983    </author>
4984    <date year='2012' month='April' />
4985   </front>
4986   <seriesInfo name='RFC' value='6585' />
4989<!--<reference anchor='BCP97'>
4990  <front>
4991    <title>Handling Normative References to Standards-Track Documents</title>
4992    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
4993      <address>
4994        <email></email>
4995      </address>
4996    </author>
4997    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
4998      <organization>MIT</organization>
4999      <address>
5000        <email></email>
5001      </address>
5002    </author>
5003    <date year='2007' month='June' />
5004  </front>
5005  <seriesInfo name='BCP' value='97' />
5006  <seriesInfo name='RFC' value='4897' />
5009<reference anchor="Kri2001" target="">
5010  <front>
5011    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
5012    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
5013    <date year="2001" month="November"/>
5014  </front>
5015  <seriesInfo name="ACM Transactions on Internet Technology" value="1(2)"/>
5021<section title="HTTP Version History" anchor="compatibility">
5023   HTTP has been in use since 1990. The first version, later referred to as
5024   HTTP/0.9, was a simple protocol for hypertext data transfer across the
5025   Internet, using only a single request method (GET) and no metadata.
5026   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
5027   methods and MIME-like messaging, allowing for metadata to be transferred
5028   and modifiers placed on the request/response semantics. However,
5029   HTTP/1.0 did not sufficiently take into consideration the effects of
5030   hierarchical proxies, caching, the need for persistent connections, or
5031   name-based virtual hosts. The proliferation of incompletely-implemented
5032   applications calling themselves "HTTP/1.0" further necessitated a
5033   protocol version change in order for two communicating applications
5034   to determine each other's true capabilities.
5037   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
5038   requirements that enable reliable implementations, adding only
5039   those features that can either be safely ignored by an HTTP/1.0
5040   recipient or only sent when communicating with a party advertising
5041   conformance with HTTP/1.1.
5044   HTTP/1.1 has been designed to make supporting previous versions easy.
5045   A general-purpose HTTP/1.1 server ought to be able to understand any valid
5046   request in the format of HTTP/1.0, responding appropriately with an
5047   HTTP/1.1 message that only uses features understood (or safely ignored) by
5048   HTTP/1.0 clients. Likewise, an HTTP/1.1 client can be expected to
5049   understand any valid HTTP/1.0 response.
5052   Since HTTP/0.9 did not support header fields in a request, there is no
5053   mechanism for it to support name-based virtual hosts (selection of resource
5054   by inspection of the <x:ref>Host</x:ref> header field).
5055   Any server that implements name-based virtual hosts ought to disable
5056   support for HTTP/0.9. Most requests that appear to be HTTP/0.9 are, in
5057   fact, badly constructed HTTP/1.x requests caused by a client failing to
5058   properly encode the request-target.
5061<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
5063   This section summarizes major differences between versions HTTP/1.0
5064   and HTTP/1.1.
5067<section title="Multi-homed Web Servers" anchor="">
5069   The requirements that clients and servers support the <x:ref>Host</x:ref>
5070   header field (<xref target=""/>), report an error if it is
5071   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
5072   are among the most important changes defined by HTTP/1.1.
5075   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
5076   addresses and servers; there was no other established mechanism for
5077   distinguishing the intended server of a request than the IP address
5078   to which that request was directed. The <x:ref>Host</x:ref> header field was
5079   introduced during the development of HTTP/1.1 and, though it was
5080   quickly implemented by most HTTP/1.0 browsers, additional requirements
5081   were placed on all HTTP/1.1 requests in order to ensure complete
5082   adoption.  At the time of this writing, most HTTP-based services
5083   are dependent upon the Host header field for targeting requests.
5087<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
5089   In HTTP/1.0, each connection is established by the client prior to the
5090   request and closed by the server after sending the response. However, some
5091   implementations implement the explicitly negotiated ("Keep-Alive") version
5092   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
5093   target="RFC2068"/>.
5096   Some clients and servers might wish to be compatible with these previous
5097   approaches to persistent connections, by explicitly negotiating for them
5098   with a "Connection: keep-alive" request header field. However, some
5099   experimental implementations of HTTP/1.0 persistent connections are faulty;
5100   for example, if an HTTP/1.0 proxy server doesn't understand
5101   <x:ref>Connection</x:ref>, it will erroneously forward that header field
5102   to the next inbound server, which would result in a hung connection.
5105   One attempted solution was the introduction of a Proxy-Connection header
5106   field, targeted specifically at proxies. In practice, this was also
5107   unworkable, because proxies are often deployed in multiple layers, bringing
5108   about the same problem discussed above.
5111   As a result, clients are encouraged not to send the Proxy-Connection header
5112   field in any requests.
5115   Clients are also encouraged to consider the use of Connection: keep-alive
5116   in requests carefully; while they can enable persistent connections with
5117   HTTP/1.0 servers, clients using them will need to monitor the
5118   connection for "hung" requests (which indicate that the client ought stop
5119   sending the header field), and this mechanism ought not be used by clients
5120   at all when a proxy is being used.
5124<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
5126   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
5127   (<xref target="header.transfer-encoding"/>).
5128   Transfer codings need to be decoded prior to forwarding an HTTP message
5129   over a MIME-compliant protocol.
5135<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
5137  HTTP's approach to error handling has been explained.
5138  (<xref target="conformance" />)
5141  The HTTP-version ABNF production has been clarified to be case-sensitive.
5142  Additionally, version numbers has been restricted to single digits, due
5143  to the fact that implementations are known to handle multi-digit version
5144  numbers incorrectly.
5145  (<xref target="http.version"/>)
5148  Userinfo (i.e., username and password) are now disallowed in HTTP and
5149  HTTPS URIs, because of security issues related to their transmission on the
5150  wire.
5151  (<xref target="http.uri" />)
5154  The HTTPS URI scheme is now defined by this specification; previously,
5155  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
5156  Furthermore, it implies end-to-end security.
5157  (<xref target="https.uri"/>)
5160  HTTP messages can be (and often are) buffered by implementations; despite
5161  it sometimes being available as a stream, HTTP is fundamentally a
5162  message-oriented protocol.
5163  Minimum supported sizes for various protocol elements have been
5164  suggested, to improve interoperability.
5165  (<xref target="http.message" />)
5168  Invalid whitespace around field-names is now required to be rejected,
5169  because accepting it represents a security vulnerability.
5170  The ABNF productions defining header fields now only list the field value.
5171  (<xref target="header.fields"/>)
5174  Rules about implicit linear whitespace between certain grammar productions
5175  have been removed; now whitespace is only allowed where specifically
5176  defined in the ABNF.
5177  (<xref target="whitespace"/>)
5180  Header fields that span multiple lines ("line folding") are deprecated.
5181  (<xref target="field.parsing" />)
5184  The NUL octet is no longer allowed in comment and quoted-string text, and
5185  handling of backslash-escaping in them has been clarified.
5186  The quoted-pair rule no longer allows escaping control characters other than
5187  HTAB.
5188  Non-ASCII content in header fields and the reason phrase has been obsoleted
5189  and made opaque (the TEXT rule was removed).
5190  (<xref target="field.components"/>)
5193  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
5194  handled as errors by recipients.
5195  (<xref target="header.content-length"/>)
5198  The algorithm for determining the message body length has been clarified
5199  to indicate all of the special cases (e.g., driven by methods or status
5200  codes) that affect it, and that new protocol elements cannot define such
5201  special cases.
5202  CONNECT is a new, special case in determining message body length.
5203  "multipart/byteranges" is no longer a way of determining message body length
5204  detection.
5205  (<xref target="message.body.length"/>)
5208  The "identity" transfer coding token has been removed.
5209  (Sections <xref format="counter" target="message.body"/> and
5210  <xref format="counter" target="transfer.codings"/>)
5213  Chunk length does not include the count of the octets in the
5214  chunk header and trailer.
5215  Line folding in chunk extensions is  disallowed.
5216  (<xref target="chunked.encoding"/>)
5219  The meaning of the "deflate" content coding has been clarified.
5220  (<xref target="deflate.coding" />)
5223  The segment + query components of RFC 3986 have been used to define the
5224  request-target, instead of abs_path from RFC 1808.
5225  The asterisk-form of the request-target is only allowed with the OPTIONS
5226  method.
5227  (<xref target="request-target"/>)
5230  The term "Effective Request URI" has been introduced.
5231  (<xref target="effective.request.uri" />)
5234  Gateways do not need to generate <x:ref>Via</x:ref> header fields anymore.
5235  (<xref target="header.via"/>)
5238  Exactly when "close" connection options have to be sent has been clarified.
5239  Also, "hop-by-hop" header fields are required to appear in the Connection header
5240  field; just because they're defined as hop-by-hop in this specification
5241  doesn't exempt them.
5242  (<xref target="header.connection"/>)
5245  The limit of two connections per server has been removed.
5246  An idempotent sequence of requests is no longer required to be retried.
5247  The requirement to retry requests under certain circumstances when the
5248  server prematurely closes the connection has been removed.
5249  Also, some extraneous requirements about when servers are allowed to close
5250  connections prematurely have been removed.
5251  (<xref target="persistent.connections"/>)
5254  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
5255  responses other than 101 (this was incorporated from <xref
5256  target="RFC2817"/>). Furthermore, the ordering in the field value is now
5257  significant.
5258  (<xref target="header.upgrade"/>)
5261  Empty list elements in list productions (e.g., a list header field containing
5262  ", ,") have been deprecated.
5263  (<xref target="abnf.extension"/>)
5266  Registration of Transfer Codings now requires IETF Review
5267  (<xref target="transfer.coding.registry"/>)
5270  This specification now defines the Upgrade Token Registry, previously
5271  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
5272  (<xref target="upgrade.token.registry"/>)
5275  The expectation to support HTTP/0.9 requests has been removed.
5276  (<xref target="compatibility"/>)
5279  Issues with the Keep-Alive and Proxy-Connection header fields in requests
5280  are pointed out, with use of the latter being discouraged altogether.
5281  (<xref target="compatibility.with.http.1.0.persistent.connections" />)
5286<?BEGININC p1-messaging.abnf-appendix ?>
5287<section xmlns:x="" title="Collected ABNF" anchor="collected.abnf">
5289<artwork type="abnf" name="p1-messaging.parsed-abnf">
5290<x:ref>BWS</x:ref> = OWS
5292<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
5293 connection-option ] )
5294<x:ref>Content-Length</x:ref> = 1*DIGIT
5296<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
5297 ]
5298<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
5299<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
5300<x:ref>Host</x:ref> = uri-host [ ":" port ]
5302<x:ref>OWS</x:ref> = *( SP / HTAB )
5304<x:ref>RWS</x:ref> = 1*( SP / HTAB )
5306<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
5307<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
5308<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
5309 transfer-coding ] )
5311<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
5312<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
5314<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
5315 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
5316 comment ] ) ] )
5318<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
5319<x:ref>absolute-form</x:ref> = absolute-URI
5320<x:ref>absolute-path</x:ref> = 1*( "/" segment )
5321<x:ref>asterisk-form</x:ref> = "*"
5322<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
5323<x:ref>authority-form</x:ref> = authority
5325<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
5326<x:ref>chunk-data</x:ref> = 1*OCTET
5327<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
5328<x:ref>chunk-ext-name</x:ref> = token
5329<x:ref>chunk-ext-val</x:ref> = token / quoted-string
5330<x:ref>chunk-size</x:ref> = 1*HEXDIG
5331<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
5332<x:ref>comment</x:ref> = "(" *( ctext / quoted-pair / comment ) ")"
5333<x:ref>connection-option</x:ref> = token
5334<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
5335 / %x2A-5B ; '*'-'['
5336 / %x5D-7E ; ']'-'~'
5337 / obs-text
5339<x:ref>field-content</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5340<x:ref>field-name</x:ref> = token
5341<x:ref>field-value</x:ref> = *( field-content / obs-fold )
5342<x:ref>fragment</x:ref> = &lt;fragment, defined in [RFC3986], Section 3.5&gt;
5344<x:ref>header-field</x:ref> = field-name ":" OWS field-value OWS
5345<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ] [ "#"
5346 fragment ]
5347<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ] [ "#"
5348 fragment ]
5350<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
5352<x:ref>message-body</x:ref> = *OCTET
5353<x:ref>method</x:ref> = token
5355<x:ref>obs-fold</x:ref> = CRLF ( SP / HTAB )
5356<x:ref>obs-text</x:ref> = %x80-FF
5357<x:ref>origin-form</x:ref> = absolute-path [ "?" query ]
5359<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
5360<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
5361<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
5362<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
5363<x:ref>protocol-name</x:ref> = token
5364<x:ref>protocol-version</x:ref> = token
5365<x:ref>pseudonym</x:ref> = token
5367<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5368 / %x5D-7E ; ']'-'~'
5369 / obs-text
5370<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5371<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5372<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5374<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5375<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5376<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5377<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5378<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5379<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5380<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5381 asterisk-form
5383<x:ref>segment</x:ref> = &lt;segment, defined in [RFC3986], Section 3.3&gt;
5384<x:ref>start-line</x:ref> = request-line / status-line
5385<x:ref>status-code</x:ref> = 3DIGIT
5386<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5388<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5389<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5390<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5391 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5392<x:ref>token</x:ref> = 1*tchar
5393<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5394<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5395 transfer-extension
5396<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5397<x:ref>transfer-parameter</x:ref> = token BWS "=" BWS ( token / quoted-string )
5399<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5403<?ENDINC p1-messaging.abnf-appendix ?>
5405<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5407<section title="Since RFC 2616">
5409  Changes up to the IETF Last Call draft are summarized
5410  in <eref target=""/>.
5414<section title="Since draft-ietf-httpbis-p1-messaging-24" anchor="changes.since.24">
5416  Closed issues:
5417  <list style="symbols">
5418    <t>
5419      <eref target=""/>:
5420      "APPSDIR review of draft-ietf-httpbis-p1-messaging-24"
5421    </t>
5422    <t>
5423      <eref target=""/>:
5424      "integer value parsing"
5425    </t>
5426    <t>
5427      <eref target=""/>:
5428      "move IANA registrations to correct draft"
5429    </t>
5430  </list>
5434<section title="Since draft-ietf-httpbis-p1-messaging-25" anchor="changes.since.25">
5436  Closed issues:
5437  <list style="symbols">
5438    <t>
5439      <eref target=""/>:
5440      "check media type registration templates"
5441    </t>
5442    <t>
5443      <eref target=""/>:
5444      "Redundant rule quoted-str-nf"
5445    </t>
5446    <t>
5447      <eref target=""/>:
5448      "add 'stateless' to Abstract"
5449    </t>
5450    <t>
5451      <eref target=""/>:
5452      "clarify ABNF layering"
5453    </t>
5454    <t>
5455      <eref target=""/>:
5456      "use of 'word' ABNF production"
5457    </t>
5458    <t>
5459      <eref target=""/>:
5460      "improve introduction of list rule"
5461    </t>
5462    <t>
5463      <eref target=""/>:
5464      "moving 2616/2068/2145 to historic"
5465    </t>
5466    <t>
5467      <eref target=""/>:
5468      "augment security considerations with pointers to current research"
5469    </t>
5470  </list>
5473  Partly resolved issues:
5474  <list style="symbols">
5475    <t>
5476      <eref target=""/>:
5477      "IESG ballot on draft-ietf-httpbis-p1-messaging-25"
5478    </t>
5479  </list>
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