source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2568

Last change on this file since 2568 was 2566, checked in by julian.reschke@…, 7 years ago

remove unneeded empty tags

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 236.4 KB
1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns=''>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns=''>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns=''>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns=''>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns=''>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns=''>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns=''>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns=''>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns=''>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns=''>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "January">
16  <!ENTITY ID-YEAR "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 status-codes           "<xref target='Part2' x:rel='' xmlns:x=''/>">
54  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x=''/>">
55  <!ENTITY status-203             "<xref target='Part2' x:rel='#status.203' xmlns:x=''/>">
56  <!ENTITY status-3xx             "<xref target='Part2' x:rel='#status.3xx' xmlns:x=''/>">
57  <!ENTITY status-304             "<xref target='Part4' x:rel='#status.304' xmlns:x=''/>">
58  <!ENTITY status-4xx             "<xref target='Part2' x:rel='#status.4xx' xmlns:x=''/>">
59  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x=''/>">
60  <!ENTITY iana-header-registry   "<xref target='Part2' x:rel='#header.field.registry' xmlns:x=''/>">
62<?rfc toc="yes" ?>
63<?rfc symrefs="yes" ?>
64<?rfc sortrefs="yes" ?>
65<?rfc compact="yes"?>
66<?rfc subcompact="no" ?>
67<?rfc linkmailto="no" ?>
68<?rfc editing="no" ?>
69<?rfc comments="yes"?>
70<?rfc inline="yes"?>
71<?rfc rfcedstyle="yes"?>
72<?rfc-ext allow-markup-in-artwork="yes" ?>
73<?rfc-ext include-references-in-index="yes" ?>
74<rfc obsoletes="2145,2616" updates="2817,2818" category="std" x:maturity-level="proposed"
75     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
76     xmlns:x=''>
77<x:link rel="next" basename="p2-semantics"/>
78<x:feedback template="{docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
81  <title abbrev="HTTP/1.1 Message Syntax and Routing">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
83  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
84    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
85    <address>
86      <postal>
87        <street>345 Park Ave</street>
88        <city>San Jose</city>
89        <region>CA</region>
90        <code>95110</code>
91        <country>USA</country>
92      </postal>
93      <email></email>
94      <uri></uri>
95    </address>
96  </author>
98  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
99    <organization abbrev="greenbytes">greenbytes GmbH</organization>
100    <address>
101      <postal>
102        <street>Hafenweg 16</street>
103        <city>Muenster</city><region>NW</region><code>48155</code>
104        <country>Germany</country>
105      </postal>
106      <email></email>
107      <uri></uri>
108    </address>
109  </author>
111  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
112  <workgroup>HTTPbis Working Group</workgroup>
116   The Hypertext Transfer Protocol (HTTP) is a stateless application-level
117   protocol for distributed, collaborative, hypertext information systems.
118   This document provides an overview of HTTP architecture and its associated
119   terminology, defines the "http" and "https" Uniform Resource Identifier
120   (URI) schemes, defines the HTTP/1.1 message syntax and parsing
121   requirements, and describes related security concerns for implementations.
125<note title="Editorial Note (To be removed by RFC Editor)">
126  <t>
127    Discussion of this draft takes place on the HTTPBIS working group
128    mailing list (, which is archived at
129    <eref target=""/>.
130  </t>
131  <t>
132    The current issues list is at
133    <eref target=""/> and related
134    documents (including fancy diffs) can be found at
135    <eref target=""/>.
136  </t>
137  <t>
138    The changes in this draft are summarized in <xref target="changes.since.25"/>.
139  </t>
143<section title="Introduction" anchor="introduction">
145   The Hypertext Transfer Protocol (HTTP) is a stateless application-level
146   request/response protocol that uses extensible semantics and
147   self-descriptive message payloads for flexible interaction with
148   network-based hypertext information systems. This document is the first in
149   a series of documents that collectively form the HTTP/1.1 specification:
150   <list style="empty">
151    <t>RFC xxx1: Message Syntax and Routing</t>
152    <t><xref target="Part2" x:fmt="none">RFC xxx2</xref>: Semantics and Content</t>
153    <t><xref target="Part4" x:fmt="none">RFC xxx3</xref>: Conditional Requests</t>
154    <t><xref target="Part5" x:fmt="none">RFC xxx4</xref>: Range Requests</t>
155    <t><xref target="Part6" x:fmt="none">RFC xxx5</xref>: Caching</t>
156    <t><xref target="Part7" x:fmt="none">RFC xxx6</xref>: Authentication</t>
157   </list>
160   This HTTP/1.1 specification obsoletes
161   <xref target="RFC2616" x:fmt="none">RFC 2616</xref> and
162   <xref target="RFC2145" x:fmt="none">RFC 2145</xref> (on HTTP versioning).
163   This specification also updates the use of CONNECT to establish a tunnel,
164   previously defined in <xref target="RFC2817" x:fmt="none">RFC 2817</xref>,
165   and defines the "https" URI scheme that was described informally in
166   <xref target="RFC2818" x:fmt="none">RFC 2818</xref>.
169   HTTP is a generic interface protocol for information systems. It is
170   designed to hide the details of how a service is implemented by presenting
171   a uniform interface to clients that is independent of the types of
172   resources provided. Likewise, servers do not need to be aware of each
173   client's purpose: an HTTP request can be considered in isolation rather
174   than being associated with a specific type of client or a predetermined
175   sequence of application steps. The result is a protocol that can be used
176   effectively in many different contexts and for which implementations can
177   evolve independently over time.
180   HTTP is also designed for use as an intermediation protocol for translating
181   communication to and from non-HTTP information systems.
182   HTTP proxies and gateways can provide access to alternative information
183   services by translating their diverse protocols into a hypertext
184   format that can be viewed and manipulated by clients in the same way
185   as HTTP services.
188   One consequence of this flexibility is that the protocol cannot be
189   defined in terms of what occurs behind the interface. Instead, we
190   are limited to defining the syntax of communication, the intent
191   of received communication, and the expected behavior of recipients.
192   If the communication is considered in isolation, then successful
193   actions ought to be reflected in corresponding changes to the
194   observable interface provided by servers. However, since multiple
195   clients might act in parallel and perhaps at cross-purposes, we
196   cannot require that such changes be observable beyond the scope
197   of a single response.
200   This document describes the architectural elements that are used or
201   referred to in HTTP, defines the "http" and "https" URI schemes,
202   describes overall network operation and connection management,
203   and defines HTTP message framing and forwarding requirements.
204   Our goal is to define all of the mechanisms necessary for HTTP message
205   handling that are independent of message semantics, thereby defining the
206   complete set of requirements for message parsers and
207   message-forwarding intermediaries.
211<section title="Requirement Notation" anchor="intro.requirements">
213   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
214   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
215   document are to be interpreted as described in <xref target="RFC2119"/>.
218   Conformance criteria and considerations regarding error handling
219   are defined in <xref target="conformance"/>.
223<section title="Syntax Notation" anchor="notation">
224<iref primary="true" item="Grammar" subitem="ALPHA"/>
225<iref primary="true" item="Grammar" subitem="CR"/>
226<iref primary="true" item="Grammar" subitem="CRLF"/>
227<iref primary="true" item="Grammar" subitem="CTL"/>
228<iref primary="true" item="Grammar" subitem="DIGIT"/>
229<iref primary="true" item="Grammar" subitem="DQUOTE"/>
230<iref primary="true" item="Grammar" subitem="HEXDIG"/>
231<iref primary="true" item="Grammar" subitem="HTAB"/>
232<iref primary="true" item="Grammar" subitem="LF"/>
233<iref primary="true" item="Grammar" subitem="OCTET"/>
234<iref primary="true" item="Grammar" subitem="SP"/>
235<iref primary="true" item="Grammar" subitem="VCHAR"/>
237   This specification uses the Augmented Backus-Naur Form (ABNF) notation of
238   <xref target="RFC5234"/> with a list extension, defined in
239   <xref target="abnf.extension"/>, that allows for compact definition of
240   comma-separated lists using a '#' operator (similar to how the '*' operator
241   indicates repetition).
242   <xref target="collected.abnf"/> shows the collected grammar with all list
243   operators expanded to standard ABNF notation.
245<t anchor="core.rules">
246  <x:anchor-alias value="ALPHA"/>
247  <x:anchor-alias value="CTL"/>
248  <x:anchor-alias value="CR"/>
249  <x:anchor-alias value="CRLF"/>
250  <x:anchor-alias value="DIGIT"/>
251  <x:anchor-alias value="DQUOTE"/>
252  <x:anchor-alias value="HEXDIG"/>
253  <x:anchor-alias value="HTAB"/>
254  <x:anchor-alias value="LF"/>
255  <x:anchor-alias value="OCTET"/>
256  <x:anchor-alias value="SP"/>
257  <x:anchor-alias value="VCHAR"/>
258   The following core rules are included by
259   reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
260   ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
261   DIGIT (decimal 0-9), DQUOTE (double quote),
262   HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed),
263   OCTET (any 8-bit sequence of data), SP (space), and
264   VCHAR (any visible <xref target="USASCII"/> character).
267   As a convention, ABNF rule names prefixed with "obs-" denote
268   "obsolete" grammar rules that appear for historical reasons.
273<section title="Architecture" anchor="architecture">
275   HTTP was created for the World Wide Web (WWW) architecture
276   and has evolved over time to support the scalability needs of a worldwide
277   hypertext system. Much of that architecture is reflected in the terminology
278   and syntax productions used to define HTTP.
281<section title="Client/Server Messaging" anchor="operation">
282<iref primary="true" item="client"/>
283<iref primary="true" item="server"/>
284<iref primary="true" item="connection"/>
286   HTTP is a stateless request/response protocol that operates by exchanging
287   <x:dfn>messages</x:dfn> (<xref target="http.message"/>) across a reliable
288   transport or session-layer
289   "<x:dfn>connection</x:dfn>" (<xref target=""/>).
290   An HTTP "<x:dfn>client</x:dfn>" is a program that establishes a connection
291   to a server for the purpose of sending one or more HTTP requests.
292   An HTTP "<x:dfn>server</x:dfn>" is a program that accepts connections
293   in order to service HTTP requests by sending HTTP responses.
295<iref primary="true" item="user agent"/>
296<iref primary="true" item="origin server"/>
297<iref primary="true" item="browser"/>
298<iref primary="true" item="spider"/>
299<iref primary="true" item="sender"/>
300<iref primary="true" item="recipient"/>
302   The terms client and server refer only to the roles that
303   these programs perform for a particular connection.  The same program
304   might act as a client on some connections and a server on others.
305   The term "<x:dfn>user agent</x:dfn>" refers to any of the various
306   client programs that initiate a request, including (but not limited to)
307   browsers, spiders (web-based robots), command-line tools, custom
308   applications, and mobile apps.
309   The term "<x:dfn>origin server</x:dfn>" refers to the program that can
310   originate authoritative responses for a given target resource.
311   The terms "<x:dfn>sender</x:dfn>" and "<x:dfn>recipient</x:dfn>" refer to
312   any implementation that sends or receives a given message, respectively.
315   HTTP relies upon the Uniform Resource Identifier (URI)
316   standard <xref target="RFC3986"/> to indicate the target resource
317   (<xref target="target-resource"/>) and relationships between resources.
318   Messages are passed in a format similar to that used by Internet mail
319   <xref target="RFC5322"/> and the Multipurpose Internet Mail Extensions
320   (MIME) <xref target="RFC2045"/> (see &diff-mime; for the differences
321   between HTTP and MIME messages).
324   Most HTTP communication consists of a retrieval request (GET) for
325   a representation of some resource identified by a URI.  In the
326   simplest case, this might be accomplished via a single bidirectional
327   connection (===) between the user agent (UA) and the origin server (O).
329<figure><artwork type="drawing">
330         request   &gt;
331    <x:highlight>UA</x:highlight> ======================================= <x:highlight>O</x:highlight>
332                                &lt;   response
334<iref primary="true" item="message"/>
335<iref primary="true" item="request"/>
336<iref primary="true" item="response"/>
338   A client sends an HTTP request to a server in the form of a <x:dfn>request</x:dfn>
339   message, beginning with a request-line that includes a method, URI, and
340   protocol version (<xref target="request.line"/>),
341   followed by header fields containing
342   request modifiers, client information, and representation metadata
343   (<xref target="header.fields"/>),
344   an empty line to indicate the end of the header section, and finally
345   a message body containing the payload body (if any,
346   <xref target="message.body"/>).
349   A server responds to a client's request by sending one or more HTTP
350   <x:dfn>response</x:dfn>
351   messages, each beginning with a status line that
352   includes the protocol version, a success or error code, and textual
353   reason phrase (<xref target="status.line"/>),
354   possibly followed by header fields containing server
355   information, resource metadata, and representation metadata
356   (<xref target="header.fields"/>),
357   an empty line to indicate the end of the header section, and finally
358   a message body containing the payload body (if any,
359   <xref target="message.body"/>).
362   A connection might be used for multiple request/response exchanges,
363   as defined in <xref target="persistent.connections"/>.
366   The following example illustrates a typical message exchange for a
367   GET request (&GET;) on the URI "":
370Client request:
371</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
372GET /hello.txt HTTP/1.1
373User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
375Accept-Language: en, mi
379Server response:
380</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
381HTTP/1.1 200 OK
382Date: Mon, 27 Jul 2009 12:28:53 GMT
383Server: Apache
384Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
385ETag: "34aa387-d-1568eb00"
386Accept-Ranges: bytes
387Content-Length: <x:length-of target="exbody"/>
388Vary: Accept-Encoding
389Content-Type: text/plain
391<x:span anchor="exbody">Hello World! My payload includes a trailing CRLF.
396<section title="Implementation Diversity" anchor="implementation-diversity">
398   When considering the design of HTTP, it is easy to fall into a trap of
399   thinking that all user agents are general-purpose browsers and all origin
400   servers are large public websites. That is not the case in practice.
401   Common HTTP user agents include household appliances, stereos, scales,
402   firmware update scripts, command-line programs, mobile apps,
403   and communication devices in a multitude of shapes and sizes.  Likewise,
404   common HTTP origin servers include home automation units, configurable
405   networking components, office machines, autonomous robots, news feeds,
406   traffic cameras, ad selectors, and video delivery platforms.
409   The term "user agent" does not imply that there is a human user directly
410   interacting with the software agent at the time of a request. In many
411   cases, a user agent is installed or configured to run in the background
412   and save its results for later inspection (or save only a subset of those
413   results that might be interesting or erroneous). Spiders, for example, are
414   typically given a start URI and configured to follow certain behavior while
415   crawling the Web as a hypertext graph.
418   The implementation diversity of HTTP means that not all user agents can
419   make interactive suggestions to their user or provide adequate warning for
420   security or privacy concerns. In the few cases where this
421   specification requires reporting of errors to the user, it is acceptable
422   for such reporting to only be observable in an error console or log file.
423   Likewise, requirements that an automated action be confirmed by the user
424   before proceeding might be met via advance configuration choices,
425   run-time options, or simple avoidance of the unsafe action; confirmation
426   does not imply any specific user interface or interruption of normal
427   processing if the user has already made that choice.
431<section title="Intermediaries" anchor="intermediaries">
432<iref primary="true" item="intermediary"/>
434   HTTP enables the use of intermediaries to satisfy requests through
435   a chain of connections.  There are three common forms of HTTP
436   <x:dfn>intermediary</x:dfn>: proxy, gateway, and tunnel.  In some cases,
437   a single intermediary might act as an origin server, proxy, gateway,
438   or tunnel, switching behavior based on the nature of each request.
440<figure><artwork type="drawing">
441         &gt;             &gt;             &gt;             &gt;
442    <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>
443               &lt;             &lt;             &lt;             &lt;
446   The figure above shows three intermediaries (A, B, and C) between the
447   user agent and origin server. A request or response message that
448   travels the whole chain will pass through four separate connections.
449   Some HTTP communication options
450   might apply only to the connection with the nearest, non-tunnel
451   neighbor, only to the end-points of the chain, or to all connections
452   along the chain. Although the diagram is linear, each participant might
453   be engaged in multiple, simultaneous communications. For example, B
454   might be receiving requests from many clients other than A, and/or
455   forwarding requests to servers other than C, at the same time that it
456   is handling A's request. Likewise, later requests might be sent through a
457   different path of connections, often based on dynamic configuration for
458   load balancing.   
461<iref primary="true" item="upstream"/><iref primary="true" item="downstream"/>
462<iref primary="true" item="inbound"/><iref primary="true" item="outbound"/>
463   The terms "<x:dfn>upstream</x:dfn>" and "<x:dfn>downstream</x:dfn>" are
464   used to describe directional requirements in relation to the message flow:
465   all messages flow from upstream to downstream.
466   The terms inbound and outbound are used to describe directional
467   requirements in relation to the request route:
468   "<x:dfn>inbound</x:dfn>" means toward the origin server and
469   "<x:dfn>outbound</x:dfn>" means toward the user agent.
471<t><iref primary="true" item="proxy"/>
472   A "<x:dfn>proxy</x:dfn>" is a message forwarding agent that is selected by the
473   client, usually via local configuration rules, to receive requests
474   for some type(s) of absolute URI and attempt to satisfy those
475   requests via translation through the HTTP interface.  Some translations
476   are minimal, such as for proxy requests for "http" URIs, whereas
477   other requests might require translation to and from entirely different
478   application-level protocols. Proxies are often used to group an
479   organization's HTTP requests through a common intermediary for the
480   sake of security, annotation services, or shared caching. Some proxies
481   are designed to apply transformations to selected messages or payloads
482   while they are being forwarded, as described in
483   <xref target="message.transformations"/>.
485<t><iref primary="true" item="gateway"/><iref primary="true" item="reverse proxy"/>
486<iref primary="true" item="accelerator"/>
487   A "<x:dfn>gateway</x:dfn>" (a.k.a., "<x:dfn>reverse proxy</x:dfn>") is an
488   intermediary that acts as an origin server for the outbound connection, but
489   translates received requests and forwards them inbound to another server or
490   servers. Gateways are often used to encapsulate legacy or untrusted
491   information services, to improve server performance through
492   "<x:dfn>accelerator</x:dfn>" caching, and to enable partitioning or load
493   balancing of HTTP services across multiple machines.
496   All HTTP requirements applicable to an origin server
497   also apply to the outbound communication of a gateway.
498   A gateway communicates with inbound servers using any protocol that
499   it desires, including private extensions to HTTP that are outside
500   the scope of this specification.  However, an HTTP-to-HTTP gateway
501   that wishes to interoperate with third-party HTTP servers ought to conform
502   to user agent requirements on the gateway's inbound connection.
504<t><iref primary="true" item="tunnel"/>
505   A "<x:dfn>tunnel</x:dfn>" acts as a blind relay between two connections
506   without changing the messages. Once active, a tunnel is not
507   considered a party to the HTTP communication, though the tunnel might
508   have been initiated by an HTTP request. A tunnel ceases to exist when
509   both ends of the relayed connection are closed. Tunnels are used to
510   extend a virtual connection through an intermediary, such as when
511   Transport Layer Security (TLS, <xref target="RFC5246"/>) is used to
512   establish confidential communication through a shared firewall proxy.
514<t><iref primary="true" item="interception proxy"/>
515<iref primary="true" item="transparent proxy"/>
516<iref primary="true" item="captive portal"/>
517   The above categories for intermediary only consider those acting as
518   participants in the HTTP communication.  There are also intermediaries
519   that can act on lower layers of the network protocol stack, filtering or
520   redirecting HTTP traffic without the knowledge or permission of message
521   senders. Network intermediaries often introduce security flaws or
522   interoperability problems by violating HTTP semantics.  For example, an
523   "<x:dfn>interception proxy</x:dfn>" <xref target="RFC3040"/> (also commonly
524   known as a "<x:dfn>transparent proxy</x:dfn>" <xref target="RFC1919"/> or
525   "<x:dfn>captive portal</x:dfn>")
526   differs from an HTTP proxy because it is not selected by the client.
527   Instead, an interception proxy filters or redirects outgoing TCP port 80
528   packets (and occasionally other common port traffic).
529   Interception proxies are commonly found on public network access points,
530   as a means of enforcing account subscription prior to allowing use of
531   non-local Internet services, and within corporate firewalls to enforce
532   network usage policies.
533   They are indistinguishable from a man-in-the-middle attack.
536   HTTP is defined as a stateless protocol, meaning that each request message
537   can be understood in isolation.  Many implementations depend on HTTP's
538   stateless design in order to reuse proxied connections or dynamically
539   load-balance requests across multiple servers.  Hence, a server &MUST-NOT;
540   assume that two requests on the same connection are from the same user
541   agent unless the connection is secured and specific to that agent.
542   Some non-standard HTTP extensions (e.g., <xref target="RFC4559"/>) have
543   been known to violate this requirement, resulting in security and
544   interoperability problems.
548<section title="Caches" anchor="caches">
549<iref primary="true" item="cache"/>
551   A "<x:dfn>cache</x:dfn>" is a local store of previous response messages and the
552   subsystem that controls its message storage, retrieval, and deletion.
553   A cache stores cacheable responses in order to reduce the response
554   time and network bandwidth consumption on future, equivalent
555   requests. Any client or server &MAY; employ a cache, though a cache
556   cannot be used by a server while it is acting as a tunnel.
559   The effect of a cache is that the request/response chain is shortened
560   if one of the participants along the chain has a cached response
561   applicable to that request. The following illustrates the resulting
562   chain if B has a cached copy of an earlier response from O (via C)
563   for a request that has not been cached by UA or A.
565<figure><artwork type="drawing">
566            &gt;             &gt;
567       <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>
568                  &lt;             &lt;
570<t><iref primary="true" item="cacheable"/>
571   A response is "<x:dfn>cacheable</x:dfn>" if a cache is allowed to store a copy of
572   the response message for use in answering subsequent requests.
573   Even when a response is cacheable, there might be additional
574   constraints placed by the client or by the origin server on when
575   that cached response can be used for a particular request. HTTP
576   requirements for cache behavior and cacheable responses are
577   defined in &caching-overview;. 
580   There are a wide variety of architectures and configurations
581   of caches deployed across the World Wide Web and
582   inside large organizations. These include national hierarchies
583   of proxy caches to save transoceanic bandwidth, collaborative systems that
584   broadcast or multicast cache entries, archives of pre-fetched cache
585   entries for use in off-line or high-latency environments, and so on.
589<section title="Conformance and Error Handling" anchor="conformance">
591   This specification targets conformance criteria according to the role of
592   a participant in HTTP communication.  Hence, HTTP requirements are placed
593   on senders, recipients, clients, servers, user agents, intermediaries,
594   origin servers, proxies, gateways, or caches, depending on what behavior
595   is being constrained by the requirement. Additional (social) requirements
596   are placed on implementations, resource owners, and protocol element
597   registrations when they apply beyond the scope of a single communication.
600   The verb "generate" is used instead of "send" where a requirement
601   differentiates between creating a protocol element and merely forwarding a
602   received element downstream.
605   An implementation is considered conformant if it complies with all of the
606   requirements associated with the roles it partakes in HTTP.
609   Conformance includes both the syntax and semantics of protocol
610   elements. A sender &MUST-NOT; generate protocol elements that convey a
611   meaning that is known by that sender to be false. A sender &MUST-NOT;
612   generate protocol elements that do not match the grammar defined by the
613   corresponding ABNF rules. Within a given message, a sender &MUST-NOT;
614   generate protocol elements or syntax alternatives that are only allowed to
615   be generated by participants in other roles (i.e., a role that the sender
616   does not have for that message).
619   When a received protocol element is parsed, the recipient &MUST; be able to
620   parse any value of reasonable length that is applicable to the recipient's
621   role and matches the grammar defined by the corresponding ABNF rules.
622   Note, however, that some received protocol elements might not be parsed.
623   For example, an intermediary forwarding a message might parse a
624   header-field into generic field-name and field-value components, but then
625   forward the header field without further parsing inside the field-value.
628   HTTP does not have specific length limitations for many of its protocol
629   elements because the lengths that might be appropriate will vary widely,
630   depending on the deployment context and purpose of the implementation.
631   Hence, interoperability between senders and recipients depends on shared
632   expectations regarding what is a reasonable length for each protocol
633   element. Furthermore, what is commonly understood to be a reasonable length
634   for some protocol elements has changed over the course of the past two
635   decades of HTTP use, and is expected to continue changing in the future.
638   At a minimum, a recipient &MUST; be able to parse and process protocol
639   element lengths that are at least as long as the values that it generates
640   for those same protocol elements in other messages. For example, an origin
641   server that publishes very long URI references to its own resources needs
642   to be able to parse and process those same references when received as a
643   request target.
646   A recipient &MUST; interpret a received protocol element according to the
647   semantics defined for it by this specification, including extensions to
648   this specification, unless the recipient has determined (through experience
649   or configuration) that the sender incorrectly implements what is implied by
650   those semantics.
651   For example, an origin server might disregard the contents of a received
652   <x:ref>Accept-Encoding</x:ref> header field if inspection of the
653   <x:ref>User-Agent</x:ref> header field indicates a specific implementation
654   version that is known to fail on receipt of certain content codings.
657   Unless noted otherwise, a recipient &MAY; attempt to recover a usable
658   protocol element from an invalid construct.  HTTP does not define
659   specific error handling mechanisms except when they have a direct impact
660   on security, since different applications of the protocol require
661   different error handling strategies.  For example, a Web browser might
662   wish to transparently recover from a response where the
663   <x:ref>Location</x:ref> header field doesn't parse according to the ABNF,
664   whereas a systems control client might consider any form of error recovery
665   to be dangerous.
669<section title="Protocol Versioning" anchor="http.version">
670  <x:anchor-alias value="HTTP-version"/>
671  <x:anchor-alias value="HTTP-name"/>
673   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate
674   versions of the protocol. This specification defines version "1.1".
675   The protocol version as a whole indicates the sender's conformance
676   with the set of requirements laid out in that version's corresponding
677   specification of HTTP.
680   The version of an HTTP message is indicated by an HTTP-version field
681   in the first line of the message. HTTP-version is case-sensitive.
683<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-version"/><iref primary="true" item="Grammar" subitem="HTTP-name"/>
684  <x:ref>HTTP-version</x:ref>  = <x:ref>HTTP-name</x:ref> "/" <x:ref>DIGIT</x:ref> "." <x:ref>DIGIT</x:ref>
685  <x:ref>HTTP-name</x:ref>     = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
688   The HTTP version number consists of two decimal digits separated by a "."
689   (period or decimal point).  The first digit ("major version") indicates the
690   HTTP messaging syntax, whereas the second digit ("minor version") indicates
691   the highest minor version within that major version to which the sender is
692   conformant and able to understand for future communication.  The minor
693   version advertises the sender's communication capabilities even when the
694   sender is only using a backwards-compatible subset of the protocol,
695   thereby letting the recipient know that more advanced features can
696   be used in response (by servers) or in future requests (by clients).
699   When an HTTP/1.1 message is sent to an HTTP/1.0 recipient
700   <xref target="RFC1945"/> or a recipient whose version is unknown,
701   the HTTP/1.1 message is constructed such that it can be interpreted
702   as a valid HTTP/1.0 message if all of the newer features are ignored.
703   This specification places recipient-version requirements on some
704   new features so that a conformant sender will only use compatible
705   features until it has determined, through configuration or the
706   receipt of a message, that the recipient supports HTTP/1.1.
709   The interpretation of a header field does not change between minor
710   versions of the same major HTTP version, though the default
711   behavior of a recipient in the absence of such a field can change.
712   Unless specified otherwise, header fields defined in HTTP/1.1 are
713   defined for all versions of HTTP/1.x.  In particular, the <x:ref>Host</x:ref>
714   and <x:ref>Connection</x:ref> header fields ought to be implemented by all
715   HTTP/1.x implementations whether or not they advertise conformance with
716   HTTP/1.1.
719   New header fields can be introduced without changing the protocol version
720   if their defined semantics allow them to be safely ignored by recipients
721   that do not recognize them. Header field extensibility is discussed in
722   <xref target="field.extensibility"/>.
725   Intermediaries that process HTTP messages (i.e., all intermediaries
726   other than those acting as tunnels) &MUST; send their own HTTP-version
727   in forwarded messages.  In other words, they are not allowed to blindly
728   forward the first line of an HTTP message without ensuring that the
729   protocol version in that message matches a version to which that
730   intermediary is conformant for both the receiving and
731   sending of messages.  Forwarding an HTTP message without rewriting
732   the HTTP-version might result in communication errors when downstream
733   recipients use the message sender's version to determine what features
734   are safe to use for later communication with that sender.
737   A client &SHOULD; send a request version equal to the highest
738   version to which the client is conformant and
739   whose major version is no higher than the highest version supported
740   by the server, if this is known.  A client &MUST-NOT; send a
741   version to which it is not conformant.
744   A client &MAY; send a lower request version if it is known that
745   the server incorrectly implements the HTTP specification, but only
746   after the client has attempted at least one normal request and determined
747   from the response status code or header fields (e.g., <x:ref>Server</x:ref>) that
748   the server improperly handles higher request versions.
751   A server &SHOULD; send a response version equal to the highest version to
752   which the server is conformant that has a major version less than or equal
753   to the one received in the request.
754   A server &MUST-NOT; send a version to which it is not conformant.
755   A server can send a <x:ref>505 (HTTP Version Not Supported)</x:ref>
756   response if it wishes, for any reason, to refuse service of the client's
757   major protocol version.
760   A server &MAY; send an HTTP/1.0 response to a request
761   if it is known or suspected that the client incorrectly implements the
762   HTTP specification and is incapable of correctly processing later
763   version responses, such as when a client fails to parse the version
764   number correctly or when an intermediary is known to blindly forward
765   the HTTP-version even when it doesn't conform to the given minor
766   version of the protocol. Such protocol downgrades &SHOULD-NOT; be
767   performed unless triggered by specific client attributes, such as when
768   one or more of the request header fields (e.g., <x:ref>User-Agent</x:ref>)
769   uniquely match the values sent by a client known to be in error.
772   The intention of HTTP's versioning design is that the major number
773   will only be incremented if an incompatible message syntax is
774   introduced, and that the minor number will only be incremented when
775   changes made to the protocol have the effect of adding to the message
776   semantics or implying additional capabilities of the sender.  However,
777   the minor version was not incremented for the changes introduced between
778   <xref target="RFC2068"/> and <xref target="RFC2616"/>, and this revision
779   has specifically avoided any such changes to the protocol.
782   When an HTTP message is received with a major version number that the
783   recipient implements, but a higher minor version number than what the
784   recipient implements, the recipient &SHOULD; process the message as if it
785   were in the highest minor version within that major version to which the
786   recipient is conformant. A recipient can assume that a message with a
787   higher minor version, when sent to a recipient that has not yet indicated
788   support for that higher version, is sufficiently backwards-compatible to be
789   safely processed by any implementation of the same major version.
793<section title="Uniform Resource Identifiers" anchor="uri">
794<iref primary="true" item="resource"/>
796   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used
797   throughout HTTP as the means for identifying resources (&resource;).
798   URI references are used to target requests, indicate redirects, and define
799   relationships.
801  <x:anchor-alias value="URI-reference"/>
802  <x:anchor-alias value="absolute-URI"/>
803  <x:anchor-alias value="relative-part"/>
804  <x:anchor-alias value="authority"/>
805  <x:anchor-alias value="uri-host"/>
806  <x:anchor-alias value="port"/>
807  <x:anchor-alias value="path-abempty"/>
808  <x:anchor-alias value="segment"/>
809  <x:anchor-alias value="query"/>
810  <x:anchor-alias value="fragment"/>
811  <x:anchor-alias value="absolute-path"/>
812  <x:anchor-alias value="partial-URI"/>
814   The definitions of "URI-reference",
815   "absolute-URI", "relative-part", "authority", "port", "host",
816   "path-abempty", "segment", "query", and "fragment" are adopted from the
817   URI generic syntax.
818   An "absolute-path" rule is defined, differing slightly from
819   RFC 3986's "path-absolute" in that it allows a leading "//".
820   A "partial-URI" rule is defined for protocol elements
821   that allow a relative URI but not a fragment.
823<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>
824  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
825  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
826  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
827  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
828  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
829  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
830  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
831  <x:ref>segment</x:ref>       = &lt;segment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
832  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
833  <x:ref>fragment</x:ref>      = &lt;fragment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.5"/>&gt;
835  <x:ref>absolute-path</x:ref> = 1*( "/" segment )
836  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
839   Each protocol element in HTTP that allows a URI reference will indicate
840   in its ABNF production whether the element allows any form of reference
841   (URI-reference), only a URI in absolute form (absolute-URI), only the
842   path and optional query components, or some combination of the above.
843   Unless otherwise indicated, URI references are parsed
844   relative to the effective request URI
845   (<xref target="effective.request.uri"/>).
848<section title="http URI scheme" anchor="http.uri">
849  <x:anchor-alias value="http-URI"/>
850  <iref item="http URI scheme" primary="true"/>
851  <iref item="URI scheme" subitem="http" primary="true"/>
853   The "http" URI scheme is hereby defined for the purpose of minting
854   identifiers according to their association with the hierarchical
855   namespace governed by a potential HTTP origin server listening for
856   TCP (<xref target="RFC0793"/>) connections on a given port.
858<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"><!--terminal production--></iref>
859  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
860             [ "#" <x:ref>fragment</x:ref> ]
863   The HTTP origin server is identified by the generic syntax's
864   <x:ref>authority</x:ref> component, which includes a host identifier
865   and optional TCP port (<xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>).
866   The remainder of the URI, consisting of both the hierarchical path
867   component and optional query component, serves as an identifier for
868   a potential resource within that origin server's name space.
871   A sender &MUST-NOT; generate an "http" URI with an empty host identifier.
872   A recipient that processes such a URI reference &MUST; reject it as invalid.
875   If the host identifier is provided as an IP address,
876   then the origin server is any listener on the indicated TCP port at
877   that IP address. If host is a registered name, then that name is
878   considered an indirect identifier and the recipient might use a name
879   resolution service, such as DNS, to find the address of a listener
880   for that host.
881   If the port subcomponent is empty or not given, then TCP port 80 is
882   assumed (the default reserved port for WWW services).
885   Regardless of the form of host identifier, access to that host is not
886   implied by the mere presence of its name or address. The host might or might
887   not exist and, even when it does exist, might or might not be running an
888   HTTP server or listening to the indicated port. The "http" URI scheme
889   makes use of the delegated nature of Internet names and addresses to
890   establish a naming authority (whatever entity has the ability to place
891   an HTTP server at that Internet name or address) and allows that
892   authority to determine which names are valid and how they might be used.
895   When an "http" URI is used within a context that calls for access to the
896   indicated resource, a client &MAY; attempt access by resolving
897   the host to an IP address, establishing a TCP connection to that address
898   on the indicated port, and sending an HTTP request message
899   (<xref target="http.message"/>) containing the URI's identifying data
900   (<xref target="message.routing"/>) to the server.
901   If the server responds to that request with a non-interim HTTP response
902   message, as described in &status-codes;, then that response
903   is considered an authoritative answer to the client's request.
906   Although HTTP is independent of the transport protocol, the "http"
907   scheme is specific to TCP-based services because the name delegation
908   process depends on TCP for establishing authority.
909   An HTTP service based on some other underlying connection protocol
910   would presumably be identified using a different URI scheme, just as
911   the "https" scheme (below) is used for resources that require an
912   end-to-end secured connection. Other protocols might also be used to
913   provide access to "http" identified resources &mdash; it is only the
914   authoritative interface that is specific to TCP.
917   The URI generic syntax for authority also includes a deprecated
918   userinfo subcomponent (<xref target="RFC3986" x:fmt="," x:sec="3.2.1"/>)
919   for including user authentication information in the URI.  Some
920   implementations make use of the userinfo component for internal
921   configuration of authentication information, such as within command
922   invocation options, configuration files, or bookmark lists, even
923   though such usage might expose a user identifier or password.
924   A sender &MUST-NOT; generate the userinfo subcomponent (and its "@"
925   delimiter) when an "http" URI reference is generated within a message as a
926   request target or header field value.
927   Before making use of an "http" URI reference received from an untrusted
928   source, a recipient ought to parse for userinfo and treat its presence as
929   an error; it is likely being used to obscure the authority for the sake of
930   phishing attacks.
934<section title="https URI scheme" anchor="https.uri">
935   <x:anchor-alias value="https-URI"/>
936   <iref item="https URI scheme"/>
937   <iref item="URI scheme" subitem="https"/>
939   The "https" URI scheme is hereby defined for the purpose of minting
940   identifiers according to their association with the hierarchical
941   namespace governed by a potential HTTP origin server listening to a
942   given TCP port for TLS-secured connections
943   (<xref target="RFC0793"/>, <xref target="RFC5246"/>).
946   All of the requirements listed above for the "http" scheme are also
947   requirements for the "https" scheme, except that a default TCP port
948   of 443 is assumed if the port subcomponent is empty or not given,
949   and the user agent &MUST; ensure that its connection to the origin
950   server is secured through the use of strong encryption, end-to-end,
951   prior to sending the first HTTP request.
953<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="https-URI"><!--terminal production--></iref>
954  <x:ref>https-URI</x:ref> = "https:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
955              [ "#" <x:ref>fragment</x:ref> ]
958   Note that the "https" URI scheme depends on both TLS and TCP for
959   establishing authority.
960   Resources made available via the "https" scheme have no shared
961   identity with the "http" scheme even if their resource identifiers
962   indicate the same authority (the same host listening to the same
963   TCP port).  They are distinct name spaces and are considered to be
964   distinct origin servers.  However, an extension to HTTP that is
965   defined to apply to entire host domains, such as the Cookie protocol
966   <xref target="RFC6265"/>, can allow information
967   set by one service to impact communication with other services
968   within a matching group of host domains.
971   The process for authoritative access to an "https" identified
972   resource is defined in <xref target="RFC2818"/>.
976<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
978   Since the "http" and "https" schemes conform to the URI generic syntax,
979   such URIs are normalized and compared according to the algorithm defined
980   in <xref target="RFC3986" x:fmt="of" x:sec="6"/>, using the defaults
981   described above for each scheme.
984   If the port is equal to the default port for a scheme, the normal form is
985   to omit the port subcomponent. When not being used in absolute form as the
986   request target of an OPTIONS request, an empty path component is equivalent
987   to an absolute path of "/", so the normal form is to provide a path of "/"
988   instead. The scheme and host are case-insensitive and normally provided in
989   lowercase; all other components are compared in a case-sensitive manner.
990   Characters other than those in the "reserved" set are equivalent to their
991   percent-encoded octets: the normal form is to not encode them
992   (see Sections <xref target="RFC3986" x:fmt="number" x:sec="2.1"/> and
993   <xref target="RFC3986" x:fmt="number" x:sec="2.2"/> of
994   <xref target="RFC3986"/>).
997   For example, the following three URIs are equivalent:
999<figure><artwork type="example">
1008<section title="Message Format" anchor="http.message">
1009<x:anchor-alias value="generic-message"/>
1010<x:anchor-alias value="message.types"/>
1011<x:anchor-alias value="HTTP-message"/>
1012<x:anchor-alias value="start-line"/>
1013<iref item="header section"/>
1014<iref item="headers"/>
1015<iref item="header field"/>
1017   All HTTP/1.1 messages consist of a start-line followed by a sequence of
1018   octets in a format similar to the Internet Message Format
1019   <xref target="RFC5322"/>: zero or more header fields (collectively
1020   referred to as the "headers" or the "header section"), an empty line
1021   indicating the end of the header section, and an optional message body.
1023<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"><!--terminal production--></iref>
1024  <x:ref>HTTP-message</x:ref>   = <x:ref>start-line</x:ref>
1025                   *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1026                   <x:ref>CRLF</x:ref>
1027                   [ <x:ref>message-body</x:ref> ]
1030   The normal procedure for parsing an HTTP message is to read the
1031   start-line into a structure, read each header field into a hash
1032   table by field name until the empty line, and then use the parsed
1033   data to determine if a message body is expected.  If a message body
1034   has been indicated, then it is read as a stream until an amount
1035   of octets equal to the message body length is read or the connection
1036   is closed.
1039   A recipient &MUST; parse an HTTP message as a sequence of octets in an
1040   encoding that is a superset of US-ASCII <xref target="USASCII"/>.
1041   Parsing an HTTP message as a stream of Unicode characters, without regard
1042   for the specific encoding, creates security vulnerabilities due to the
1043   varying ways that string processing libraries handle invalid multibyte
1044   character sequences that contain the octet LF (%x0A).  String-based
1045   parsers can only be safely used within protocol elements after the element
1046   has been extracted from the message, such as within a header field-value
1047   after message parsing has delineated the individual fields.
1050   An HTTP message can be parsed as a stream for incremental processing or
1051   forwarding downstream.  However, recipients cannot rely on incremental
1052   delivery of partial messages, since some implementations will buffer or
1053   delay message forwarding for the sake of network efficiency, security
1054   checks, or payload transformations.
1057   A sender &MUST-NOT; send whitespace between the start-line and
1058   the first header field.
1059   A recipient that receives whitespace between the start-line and
1060   the first header field &MUST; either reject the message as invalid or
1061   consume each whitespace-preceded line without further processing of it
1062   (i.e., ignore the entire line, along with any subsequent lines preceded
1063   by whitespace, until a properly formed header field is received or the
1064   header section is terminated).
1067   The presence of such whitespace in a request
1068   might be an attempt to trick a server into ignoring that field or
1069   processing the line after it as a new request, either of which might
1070   result in a security vulnerability if other implementations within
1071   the request chain interpret the same message differently.
1072   Likewise, the presence of such whitespace in a response might be
1073   ignored by some clients or cause others to cease parsing.
1076<section title="Start Line" anchor="start.line">
1077  <x:anchor-alias value="Start-Line"/>
1079   An HTTP message can either be a request from client to server or a
1080   response from server to client.  Syntactically, the two types of message
1081   differ only in the start-line, which is either a request-line (for requests)
1082   or a status-line (for responses), and in the algorithm for determining
1083   the length of the message body (<xref target="message.body"/>).
1086   In theory, a client could receive requests and a server could receive
1087   responses, distinguishing them by their different start-line formats,
1088   but in practice servers are implemented to only expect a request
1089   (a response is interpreted as an unknown or invalid request method)
1090   and clients are implemented to only expect a response.
1092<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="start-line"/>
1093  <x:ref>start-line</x:ref>     = <x:ref>request-line</x:ref> / <x:ref>status-line</x:ref>
1096<section title="Request Line" anchor="request.line">
1097  <x:anchor-alias value="Request"/>
1098  <x:anchor-alias value="request-line"/>
1100   A request-line begins with a method token, followed by a single
1101   space (SP), the request-target, another single space (SP), the
1102   protocol version, and ending with CRLF.
1104<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1105  <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>
1107<iref primary="true" item="method"/>
1108<t anchor="method">
1109   The method token indicates the request method to be performed on the
1110   target resource. The request method is case-sensitive.
1112<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1113  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1116   The request methods defined by this specification can be found in
1117   &methods;, along with information regarding the HTTP method registry
1118   and considerations for defining new methods.
1120<iref item="request-target"/>
1122   The request-target identifies the target resource upon which to apply
1123   the request, as defined in <xref target="request-target"/>.
1126   Recipients typically parse the request-line into its component parts by
1127   splitting on whitespace (see <xref target="message.robustness"/>), since
1128   no whitespace is allowed in the three components.
1129   Unfortunately, some user agents fail to properly encode or exclude
1130   whitespace found in hypertext references, resulting in those disallowed
1131   characters being sent in a request-target.
1134   Recipients of an invalid request-line &SHOULD; respond with either a
1135   <x:ref>400 (Bad Request)</x:ref> error or a <x:ref>301 (Moved Permanently)</x:ref>
1136   redirect with the request-target properly encoded.  A recipient &SHOULD-NOT;
1137   attempt to autocorrect and then process the request without a redirect,
1138   since the invalid request-line might be deliberately crafted to bypass
1139   security filters along the request chain.
1142   HTTP does not place a pre-defined limit on the length of a request-line.
1143   A server that receives a method longer than any that it implements
1144   &SHOULD; respond with a <x:ref>501 (Not Implemented)</x:ref> status code.
1145   A server ought to be prepared to receive URIs of unbounded length, as
1146   described in <xref target="conformance"/>, and &MUST; respond with a
1147   <x:ref>414 (URI Too Long)</x:ref> status code if the received
1148   request-target is longer than the server wishes to parse (see &status-414;).
1151   Various ad-hoc limitations on request-line length are found in practice.
1152   It is &RECOMMENDED; that all HTTP senders and recipients support, at a
1153   minimum, request-line lengths of 8000 octets.
1157<section title="Status Line" anchor="status.line">
1158  <x:anchor-alias value="response"/>
1159  <x:anchor-alias value="status-line"/>
1160  <x:anchor-alias value="status-code"/>
1161  <x:anchor-alias value="reason-phrase"/>
1163   The first line of a response message is the status-line, consisting
1164   of the protocol version, a space (SP), the status code, another space,
1165   a possibly-empty textual phrase describing the status code, and
1166   ending with CRLF.
1168<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-line"/>
1169  <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>
1172   The status-code element is a 3-digit integer code describing the
1173   result of the server's attempt to understand and satisfy the client's
1174   corresponding request. The rest of the response message is to be
1175   interpreted in light of the semantics defined for that status code.
1176   See &status-codes; for information about the semantics of status codes,
1177   including the classes of status code (indicated by the first digit),
1178   the status codes defined by this specification, considerations for the
1179   definition of new status codes, and the IANA registry.
1181<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-code"/>
1182  <x:ref>status-code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1185   The reason-phrase element exists for the sole purpose of providing a
1186   textual description associated with the numeric status code, mostly
1187   out of deference to earlier Internet application protocols that were more
1188   frequently used with interactive text clients. A client &SHOULD; ignore
1189   the reason-phrase content.
1191<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
1192  <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> )
1197<section title="Header Fields" anchor="header.fields">
1198  <x:anchor-alias value="header-field"/>
1199  <x:anchor-alias value="field-content"/>
1200  <x:anchor-alias value="field-name"/>
1201  <x:anchor-alias value="field-value"/>
1202  <x:anchor-alias value="obs-fold"/>
1204   Each header field consists of a case-insensitive field name
1205   followed by a colon (":"), optional leading whitespace, the field value,
1206   and optional trailing whitespace.
1208<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"/>
1209  <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>
1210  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1211  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>obs-fold</x:ref> )
1212  <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> )
1213  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref> ( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1214                 ; obsolete line folding
1215                 ; see <xref target="field.parsing"/>
1218   The field-name token labels the corresponding field-value as having the
1219   semantics defined by that header field.  For example, the <x:ref>Date</x:ref>
1220   header field is defined in &header-date; as containing the origination
1221   timestamp for the message in which it appears.
1224<section title="Field Extensibility" anchor="field.extensibility">
1226   Header fields are fully extensible: there is no limit on the
1227   introduction of new field names, each presumably defining new semantics,
1228   nor on the number of header fields used in a given message.  Existing
1229   fields are defined in each part of this specification and in many other
1230   specifications outside the core standard.
1233   New header fields can be defined such that, when they are understood by a
1234   recipient, they might override or enhance the interpretation of previously
1235   defined header fields, define preconditions on request evaluation, or
1236   refine the meaning of responses.
1239   A proxy &MUST; forward unrecognized header fields unless the
1240   field-name is listed in the <x:ref>Connection</x:ref> header field
1241   (<xref target="header.connection"/>) or the proxy is specifically
1242   configured to block, or otherwise transform, such fields.
1243   Other recipients &SHOULD; ignore unrecognized header fields.
1244   These requirements allow HTTP's functionality to be enhanced without
1245   requiring prior update of deployed intermediaries.
1248   All defined header fields ought to be registered with IANA in the
1249   Message Header Field Registry, as described in &iana-header-registry;.
1253<section title="Field Order" anchor="field.order">
1255   The order in which header fields with differing field names are
1256   received is not significant. However, it is "good practice" to send
1257   header fields that contain control data first, such as <x:ref>Host</x:ref>
1258   on requests and <x:ref>Date</x:ref> on responses, so that implementations
1259   can decide when not to handle a message as early as possible.  A server
1260   &MUST; wait until the entire header section is received before interpreting
1261   a request message, since later header fields might include conditionals,
1262   authentication credentials, or deliberately misleading duplicate
1263   header fields that would impact request processing.
1266   A sender &MUST-NOT; generate multiple header fields with the same field
1267   name in a message unless either the entire field value for that
1268   header field is defined as a comma-separated list [i.e., #(values)]
1269   or the header field is a well-known exception (as noted below).
1272   A recipient &MAY; combine multiple header fields with the same field name
1273   into one "field-name: field-value" pair, without changing the semantics of
1274   the message, by appending each subsequent field value to the combined
1275   field value in order, separated by a comma. The order in which
1276   header fields with the same field name are received is therefore
1277   significant to the interpretation of the combined field value;
1278   a proxy &MUST-NOT; change the order of these field values when
1279   forwarding a message.
1282  <t>
1283   &Note; In practice, the "Set-Cookie" header field (<xref target="RFC6265"/>)
1284   often appears multiple times in a response message and does not use the
1285   list syntax, violating the above requirements on multiple header fields
1286   with the same name. Since it cannot be combined into a single field-value,
1287   recipients ought to handle "Set-Cookie" as a special case while processing
1288   header fields. (See Appendix A.2.3 of <xref target="Kri2001"/> for details.)
1289  </t>
1293<section title="Whitespace" anchor="whitespace">
1294<t anchor="rule.LWS">
1295   This specification uses three rules to denote the use of linear
1296   whitespace: OWS (optional whitespace), RWS (required whitespace), and
1297   BWS ("bad" whitespace).
1299<t anchor="rule.OWS">
1300   The OWS rule is used where zero or more linear whitespace octets might
1301   appear. For protocol elements where optional whitespace is preferred to
1302   improve readability, a sender &SHOULD; generate the optional whitespace
1303   as a single SP; otherwise, a sender &SHOULD-NOT; generate optional
1304   whitespace except as needed to white-out invalid or unwanted protocol
1305   elements during in-place message filtering.
1307<t anchor="rule.RWS">
1308   The RWS rule is used when at least one linear whitespace octet is required
1309   to separate field tokens. A sender &SHOULD; generate RWS as a single SP.
1311<t anchor="rule.BWS">
1312   The BWS rule is used where the grammar allows optional whitespace only for
1313   historical reasons. A sender &MUST-NOT; generate BWS in messages.
1314   A recipient &MUST; parse for such bad whitespace and remove it before
1315   interpreting the protocol element.
1317<t anchor="rule.whitespace">
1318  <x:anchor-alias value="BWS"/>
1319  <x:anchor-alias value="OWS"/>
1320  <x:anchor-alias value="RWS"/>
1322<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"/>
1323  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1324                 ; optional whitespace
1325  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1326                 ; required whitespace
1327  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1328                 ; "bad" whitespace
1332<section title="Field Parsing" anchor="field.parsing">
1334   Messages are parsed using a generic algorithm, independent of the
1335   individual header field names. The contents within a given field value are
1336   not parsed until a later stage of message interpretation (usually after the
1337   message's entire header section has been processed).
1338   Consequently, this specification does not use ABNF rules to define each
1339   "Field-Name: Field Value" pair, as was done in previous editions.
1340   Instead, this specification uses ABNF rules which are named according to
1341   each registered field name, wherein the rule defines the valid grammar for
1342   that field's corresponding field values (i.e., after the field-value
1343   has been extracted from the header section by a generic field parser).
1346   No whitespace is allowed between the header field-name and colon.
1347   In the past, differences in the handling of such whitespace have led to
1348   security vulnerabilities in request routing and response handling.
1349   A server &MUST; reject any received request message that contains
1350   whitespace between a header field-name and colon with a response code of
1351   <x:ref>400 (Bad Request)</x:ref>. A proxy &MUST; remove any such whitespace
1352   from a response message before forwarding the message downstream.
1355   A field value is preceded by optional whitespace (OWS); a single SP is
1356   preferred. The field value does not include any leading or trailing white
1357   space: OWS occurring before the first non-whitespace octet of the field
1358   value or after the last non-whitespace octet of the field value ought to be
1359   excluded by parsers when extracting the field value from a header field.
1362   A recipient of field-content containing multiple sequential octets of
1363   optional (OWS) or required (RWS) whitespace &SHOULD; either replace the
1364   sequence with a single SP or transform any non-SP octets in the sequence to
1365   SP octets before interpreting the field value or forwarding the message
1366   downstream.
1369   Historically, HTTP header field values could be extended over multiple
1370   lines by preceding each extra line with at least one space or horizontal
1371   tab (obs-fold). This specification deprecates such line folding except
1372   within the message/http media type
1373   (<xref target=""/>).
1374   A sender &MUST-NOT; generate a message that includes line folding
1375   (i.e., that has any field-value that contains a match to the
1376   <x:ref>obs-fold</x:ref> rule) unless the message is intended for packaging
1377   within the message/http media type.
1380   A server that receives an <x:ref>obs-fold</x:ref> in a request message that
1381   is not within a message/http container &MUST; either reject the message by
1382   sending a <x:ref>400 (Bad Request)</x:ref>, preferably with a
1383   representation explaining that obsolete line folding is unacceptable, or
1384   replace each received <x:ref>obs-fold</x:ref> with one or more
1385   <x:ref>SP</x:ref> octets prior to interpreting the field value or
1386   forwarding the message downstream.
1389   A proxy or gateway that receives an <x:ref>obs-fold</x:ref> in a response
1390   message that is not within a message/http container &MUST; either discard
1391   the message and replace it with a <x:ref>502 (Bad Gateway)</x:ref>
1392   response, preferably with a representation explaining that unacceptable
1393   line folding was received, or replace each received <x:ref>obs-fold</x:ref>
1394   with one or more <x:ref>SP</x:ref> octets prior to interpreting the field
1395   value or forwarding the message downstream.
1398   A user agent that receives an <x:ref>obs-fold</x:ref> in a response message
1399   that is not within a message/http container &MUST; replace each received
1400   <x:ref>obs-fold</x:ref> with one or more <x:ref>SP</x:ref> octets prior to
1401   interpreting the field value.
1404   Historically, HTTP has allowed field content with text in the ISO-8859-1
1405   <xref target="ISO-8859-1"/> charset, supporting other charsets only
1406   through use of <xref target="RFC2047"/> encoding.
1407   In practice, most HTTP header field values use only a subset of the
1408   US-ASCII charset <xref target="USASCII"/>. Newly defined
1409   header fields &SHOULD; limit their field values to US-ASCII octets.
1410   A recipient &SHOULD; treat other octets in field content (obs-text) as
1411   opaque data.
1415<section title="Field Limits" anchor="field.limits">
1417   HTTP does not place a pre-defined limit on the length of each header field
1418   or on the length of the header section as a whole, as described in
1419   <xref target="conformance"/>. Various ad-hoc limitations on individual
1420   header field length are found in practice, often depending on the specific
1421   field semantics.
1424   A server ought to be prepared to receive request header fields of unbounded
1425   length and &MUST; respond with an appropriate
1426   <x:ref>4xx (Client Error)</x:ref> status code if the received header
1427   field(s) are larger than the server wishes to process.
1430   A client ought to be prepared to receive response header fields of
1431   unbounded length.
1432   A client &MAY; discard or truncate received header fields that are larger
1433   than the client wishes to process if the field semantics are such that the
1434   dropped value(s) can be safely ignored without changing the
1435   message framing or response semantics.
1439<section title="Field value components" anchor="field.components">
1440<t anchor="rule.token.separators">
1441  <x:anchor-alias value="tchar"/>
1442  <x:anchor-alias value="token"/>
1443  <iref item="Delimiters"/>
1444   Most HTTP header field values are defined using common syntax components
1445   (token, quoted-string, and comment) separated by whitespace or specific
1446   delimiting characters. Delimiters are chosen from the set of US-ASCII
1447   visual characters not allowed in a <x:ref>token</x:ref>
1448   (DQUOTE and "(),/:;&lt;=>?@[\]{}").
1450<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="token"/><iref primary="true" item="Grammar" subitem="tchar"/>
1451  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1453  NOTE: the definition of tchar and the prose above about special characters need to match!
1454 -->
1455  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1456                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1457                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1458                 ; any <x:ref>VCHAR</x:ref>, except delimiters
1460<t anchor="rule.quoted-string">
1461  <x:anchor-alias value="quoted-string"/>
1462  <x:anchor-alias value="qdtext"/>
1463  <x:anchor-alias value="obs-text"/>
1464   A string of text is parsed as a single value if it is quoted using
1465   double-quote marks.
1467<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"/>
1468  <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>
1469  <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>
1470  <x:ref>obs-text</x:ref>       = %x80-FF
1472<t anchor="rule.comment">
1473  <x:anchor-alias value="comment"/>
1474  <x:anchor-alias value="ctext"/>
1475   Comments can be included in some HTTP header fields by surrounding
1476   the comment text with parentheses. Comments are only allowed in
1477   fields containing "comment" as part of their field value definition.
1479<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1480  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-pair</x:ref> / <x:ref>comment</x:ref> ) ")"
1481  <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>
1483<t anchor="rule.quoted-pair">
1484  <x:anchor-alias value="quoted-pair"/>
1485   The backslash octet ("\") can be used as a single-octet
1486   quoting mechanism within quoted-string and comment constructs.
1487   Recipients that process the value of a quoted-string &MUST; handle a
1488   quoted-pair as if it were replaced by the octet following the backslash.
1490<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1491  <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> )
1494   A sender &SHOULD-NOT; generate a quoted-pair in a quoted-string except
1495   where necessary to quote DQUOTE and backslash octets occurring within that
1496   string.
1497   A sender &SHOULD-NOT; generate a quoted-pair in a comment except
1498   where necessary to quote parentheses ["(" and ")"] and backslash octets
1499   occurring within that comment.
1505<section title="Message Body" anchor="message.body">
1506  <x:anchor-alias value="message-body"/>
1508   The message body (if any) of an HTTP message is used to carry the
1509   payload body of that request or response.  The message body is
1510   identical to the payload body unless a transfer coding has been
1511   applied, as described in <xref target="header.transfer-encoding"/>.
1513<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1514  <x:ref>message-body</x:ref> = *OCTET
1517   The rules for when a message body is allowed in a message differ for
1518   requests and responses.
1521   The presence of a message body in a request is signaled by a
1522   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1523   field. Request message framing is independent of method semantics,
1524   even if the method does not define any use for a message body.
1527   The presence of a message body in a response depends on both
1528   the request method to which it is responding and the response
1529   status code (<xref target="status.line"/>).
1530   Responses to the HEAD request method (&HEAD;) never include a message body
1531   because the associated response header fields (e.g.,
1532   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1533   if present, indicate only what their values would have been if the request
1534   method had been GET (&GET;).
1535   <x:ref>2xx (Successful)</x:ref> responses to a CONNECT request method
1536   (&CONNECT;) switch to tunnel mode instead of having a message body.
1537   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1538   <x:ref>304 (Not Modified)</x:ref> responses do not include a message body.
1539   All other responses do include a message body, although the body
1540   might be of zero length.
1543<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1544  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1545  <iref item="chunked (Coding Format)"/>
1546  <x:anchor-alias value="Transfer-Encoding"/>
1548   The Transfer-Encoding header field lists the transfer coding names
1549   corresponding to the sequence of transfer codings that have been
1550   (or will be) applied to the payload body in order to form the message body.
1551   Transfer codings are defined in <xref target="transfer.codings"/>.
1553<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1554  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1557   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1558   MIME, which was designed to enable safe transport of binary data over a
1559   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1560   However, safe transport has a different focus for an 8bit-clean transfer
1561   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1562   accurately delimit a dynamically generated payload and to distinguish
1563   payload encodings that are only applied for transport efficiency or
1564   security from those that are characteristics of the selected resource.
1567   A recipient &MUST; be able to parse the chunked transfer coding
1568   (<xref target="chunked.encoding"/>) because it plays a crucial role in
1569   framing messages when the payload body size is not known in advance.
1570   A sender &MUST-NOT; apply chunked more than once to a message body
1571   (i.e., chunking an already chunked message is not allowed).
1572   If any transfer coding other than chunked is applied to a request payload
1573   body, the sender &MUST; apply chunked as the final transfer coding to
1574   ensure that the message is properly framed.
1575   If any transfer coding other than chunked is applied to a response payload
1576   body, the sender &MUST; either apply chunked as the final transfer coding
1577   or terminate the message by closing the connection.
1580   For example,
1581</preamble><artwork type="example">
1582  Transfer-Encoding: gzip, chunked
1584   indicates that the payload body has been compressed using the gzip
1585   coding and then chunked using the chunked coding while forming the
1586   message body.
1589   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1590   Transfer-Encoding is a property of the message, not of the representation, and
1591   any recipient along the request/response chain &MAY; decode the received
1592   transfer coding(s) or apply additional transfer coding(s) to the message
1593   body, assuming that corresponding changes are made to the Transfer-Encoding
1594   field-value. Additional information about the encoding parameters &MAY; be
1595   provided by other header fields not defined by this specification.
1598   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1599   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1600   neither of which includes a message body,
1601   to indicate that the origin server would have applied a transfer coding
1602   to the message body if the request had been an unconditional GET.
1603   This indication is not required, however, because any recipient on
1604   the response chain (including the origin server) can remove transfer
1605   codings when they are not needed.
1608   A server &MUST-NOT; send a Transfer-Encoding header field in any response
1609   with a status code of
1610   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1611   A server &MUST-NOT; send a Transfer-Encoding header field in any
1612   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1615   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1616   implementations advertising only HTTP/1.0 support will not understand
1617   how to process a transfer-encoded payload.
1618   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1619   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1620   might be in the form of specific user configuration or by remembering the
1621   version of a prior received response.
1622   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1623   the corresponding request indicates HTTP/1.1 (or later).
1626   A server that receives a request message with a transfer coding it does
1627   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref>.
1631<section title="Content-Length" anchor="header.content-length">
1632  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1633  <x:anchor-alias value="Content-Length"/>
1635   When a message does not have a <x:ref>Transfer-Encoding</x:ref> header
1636   field, a Content-Length header field can provide the anticipated size,
1637   as a decimal number of octets, for a potential payload body.
1638   For messages that do include a payload body, the Content-Length field-value
1639   provides the framing information necessary for determining where the body
1640   (and message) ends.  For messages that do not include a payload body, the
1641   Content-Length indicates the size of the selected representation
1642   (&representation;).
1644<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1645  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1648   An example is
1650<figure><artwork type="example">
1651  Content-Length: 3495
1654   A sender &MUST-NOT; send a Content-Length header field in any message that
1655   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1658   A user agent &SHOULD; send a Content-Length in a request message when no
1659   <x:ref>Transfer-Encoding</x:ref> is sent and the request method defines
1660   a meaning for an enclosed payload body. For example, a Content-Length
1661   header field is normally sent in a POST request even when the value is
1662   0 (indicating an empty payload body).  A user agent &SHOULD-NOT; send a
1663   Content-Length header field when the request message does not contain a
1664   payload body and the method semantics do not anticipate such a body.
1667   A server &MAY; send a Content-Length header field in a response to a HEAD
1668   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1669   response unless its field-value equals the decimal number of octets that
1670   would have been sent in the payload body of a response if the same
1671   request had used the GET method.
1674   A server &MAY; send a Content-Length header field in a
1675   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1676   (&status-304;); a server &MUST-NOT; send Content-Length in such a
1677   response unless its field-value equals the decimal number of octets that
1678   would have been sent in the payload body of a <x:ref>200 (OK)</x:ref>
1679   response to the same request.
1682   A server &MUST-NOT; send a Content-Length header field in any response
1683   with a status code of
1684   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1685   A server &MUST-NOT; send a Content-Length header field in any
1686   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1689   Aside from the cases defined above, in the absence of Transfer-Encoding,
1690   an origin server &SHOULD; send a Content-Length header field when the
1691   payload body size is known prior to sending the complete header section.
1692   This will allow downstream recipients to measure transfer progress,
1693   know when a received message is complete, and potentially reuse the
1694   connection for additional requests.
1697   Any Content-Length field value greater than or equal to zero is valid.
1698   Since there is no predefined limit to the length of a payload, a
1699   recipient &MUST; anticipate potentially large decimal numerals and
1700   prevent parsing errors due to integer conversion overflows
1701   (<xref target="attack.protocol.element.size.overflows"/>).
1704   If a message is received that has multiple Content-Length header fields
1705   with field-values consisting of the same decimal value, or a single
1706   Content-Length header field with a field value containing a list of
1707   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1708   duplicate Content-Length header fields have been generated or combined by an
1709   upstream message processor, then the recipient &MUST; either reject the
1710   message as invalid or replace the duplicated field-values with a single
1711   valid Content-Length field containing that decimal value prior to
1712   determining the message body length or forwarding the message.
1715  <t>
1716   &Note; HTTP's use of Content-Length for message framing differs
1717   significantly from the same field's use in MIME, where it is an optional
1718   field used only within the "message/external-body" media-type.
1719  </t>
1723<section title="Message Body Length" anchor="message.body.length">
1724  <iref item="chunked (Coding Format)"/>
1726   The length of a message body is determined by one of the following
1727   (in order of precedence):
1730  <list style="numbers">
1731    <x:lt><t>
1732     Any response to a HEAD request and any response with a
1733     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1734     <x:ref>304 (Not Modified)</x:ref> status code is always
1735     terminated by the first empty line after the header fields, regardless of
1736     the header fields present in the message, and thus cannot contain a
1737     message body.
1738    </t></x:lt>
1739    <x:lt><t>
1740     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1741     connection will become a tunnel immediately after the empty line that
1742     concludes the header fields.  A client &MUST; ignore any
1743     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1744     fields received in such a message.
1745    </t></x:lt>
1746    <x:lt><t>
1747     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1748     and the chunked transfer coding (<xref target="chunked.encoding"/>)
1749     is the final encoding, the message body length is determined by reading
1750     and decoding the chunked data until the transfer coding indicates the
1751     data is complete.
1752    </t>
1753    <t>
1754     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1755     response and the chunked transfer coding is not the final encoding, the
1756     message body length is determined by reading the connection until it is
1757     closed by the server.
1758     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a request and the
1759     chunked transfer coding is not the final encoding, the message body
1760     length cannot be determined reliably; the server &MUST; respond with
1761     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1762    </t>
1763    <t>
1764     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1765     and a <x:ref>Content-Length</x:ref> header field, the Transfer-Encoding
1766     overrides the Content-Length. Such a message might indicate an attempt
1767     to perform request or response smuggling (bypass of security-related
1768     checks on message routing or content) and thus ought to be handled as
1769     an error.  A sender &MUST; remove the received Content-Length field
1770     prior to forwarding such a message downstream.
1771    </t></x:lt>
1772    <x:lt><t>
1773     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1774     either multiple <x:ref>Content-Length</x:ref> header fields having
1775     differing field-values or a single Content-Length header field having an
1776     invalid value, then the message framing is invalid and
1777     the recipient &MUST; treat it as an unrecoverable error to prevent
1778     request or response smuggling.
1779     If this is a request message, the server &MUST; respond with
1780     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1781     If this is a response message received by a proxy,
1782     the proxy &MUST; close the connection to the server, discard the received
1783     response, and send a <x:ref>502 (Bad Gateway)</x:ref> response to the
1784     client.
1785     If this is a response message received by a user agent,
1786     the user agent &MUST; close the connection to the server and discard the
1787     received response.
1788    </t></x:lt>
1789    <x:lt><t>
1790     If a valid <x:ref>Content-Length</x:ref> header field is present without
1791     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1792     expected message body length in octets.
1793     If the sender closes the connection or the recipient times out before the
1794     indicated number of octets are received, the recipient &MUST; consider
1795     the message to be incomplete and close the connection.
1796    </t></x:lt>
1797    <x:lt><t>
1798     If this is a request message and none of the above are true, then the
1799     message body length is zero (no message body is present).
1800    </t></x:lt>
1801    <x:lt><t>
1802     Otherwise, this is a response message without a declared message body
1803     length, so the message body length is determined by the number of octets
1804     received prior to the server closing the connection.
1805    </t></x:lt>
1806  </list>
1809   Since there is no way to distinguish a successfully completed,
1810   close-delimited message from a partially-received message interrupted
1811   by network failure, a server &SHOULD; generate encoding or
1812   length-delimited messages whenever possible.  The close-delimiting
1813   feature exists primarily for backwards compatibility with HTTP/1.0.
1816   A server &MAY; reject a request that contains a message body but
1817   not a <x:ref>Content-Length</x:ref> by responding with
1818   <x:ref>411 (Length Required)</x:ref>.
1821   Unless a transfer coding other than chunked has been applied,
1822   a client that sends a request containing a message body &SHOULD;
1823   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1824   length is known in advance, rather than the chunked transfer coding, since some
1825   existing services respond to chunked with a <x:ref>411 (Length Required)</x:ref>
1826   status code even though they understand the chunked transfer coding.  This
1827   is typically because such services are implemented via a gateway that
1828   requires a content-length in advance of being called and the server
1829   is unable or unwilling to buffer the entire request before processing.
1832   A user agent that sends a request containing a message body &MUST; send a
1833   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1834   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1835   the form of specific user configuration or by remembering the version of a
1836   prior received response.
1839   If the final response to the last request on a connection has been
1840   completely received and there remains additional data to read, a user agent
1841   &MAY; discard the remaining data or attempt to determine if that data
1842   belongs as part of the prior response body, which might be the case if the
1843   prior message's Content-Length value is incorrect. A client &MUST-NOT;
1844   process, cache, or forward such extra data as a separate response, since
1845   such behavior would be vulnerable to cache poisoning.
1850<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1852   A server that receives an incomplete request message, usually due to a
1853   canceled request or a triggered time-out exception, &MAY; send an error
1854   response prior to closing the connection.
1857   A client that receives an incomplete response message, which can occur
1858   when a connection is closed prematurely or when decoding a supposedly
1859   chunked transfer coding fails, &MUST; record the message as incomplete.
1860   Cache requirements for incomplete responses are defined in
1861   &cache-incomplete;.
1864   If a response terminates in the middle of the header section (before the
1865   empty line is received) and the status code might rely on header fields to
1866   convey the full meaning of the response, then the client cannot assume
1867   that meaning has been conveyed; the client might need to repeat the
1868   request in order to determine what action to take next.
1871   A message body that uses the chunked transfer coding is
1872   incomplete if the zero-sized chunk that terminates the encoding has not
1873   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1874   incomplete if the size of the message body received (in octets) is less than
1875   the value given by Content-Length.  A response that has neither chunked
1876   transfer coding nor Content-Length is terminated by closure of the
1877   connection, and thus is considered complete regardless of the number of
1878   message body octets received, provided that the header section was received
1879   intact.
1883<section title="Message Parsing Robustness" anchor="message.robustness">
1885   Older HTTP/1.0 user agent implementations might send an extra CRLF
1886   after a POST request as a workaround for some early server
1887   applications that failed to read message body content that was
1888   not terminated by a line-ending. An HTTP/1.1 user agent &MUST-NOT;
1889   preface or follow a request with an extra CRLF.  If terminating
1890   the request message body with a line-ending is desired, then the
1891   user agent &MUST; count the terminating CRLF octets as part of the
1892   message body length.
1895   In the interest of robustness, a server that is expecting to receive and
1896   parse a request-line &SHOULD; ignore at least one empty line (CRLF)
1897   received prior to the request-line.
1900   Although the line terminator for the start-line and header
1901   fields is the sequence CRLF, a recipient &MAY; recognize a
1902   single LF as a line terminator and ignore any preceding CR.
1905   Although the request-line and status-line grammar rules require that each
1906   of the component elements be separated by a single SP octet, recipients
1907   &MAY; instead parse on whitespace-delimited word boundaries and, aside
1908   from the CRLF terminator, treat any form of whitespace as the SP separator
1909   while ignoring preceding or trailing whitespace;
1910   such whitespace includes one or more of the following octets:
1911   SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
1914   When a server listening only for HTTP request messages, or processing
1915   what appears from the start-line to be an HTTP request message,
1916   receives a sequence of octets that does not match the HTTP-message
1917   grammar aside from the robustness exceptions listed above, the
1918   server &SHOULD; respond with a <x:ref>400 (Bad Request)</x:ref> response. 
1923<section title="Transfer Codings" anchor="transfer.codings">
1924  <x:anchor-alias value="transfer-coding"/>
1925  <x:anchor-alias value="transfer-extension"/>
1927   Transfer coding names are used to indicate an encoding
1928   transformation that has been, can be, or might need to be applied to a
1929   payload body in order to ensure "safe transport" through the network.
1930   This differs from a content coding in that the transfer coding is a
1931   property of the message rather than a property of the representation
1932   that is being transferred.
1934<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1935  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1936                     / "compress" ; <xref target="compress.coding"/>
1937                     / "deflate" ; <xref target="deflate.coding"/>
1938                     / "gzip" ; <xref target="gzip.coding"/>
1939                     / <x:ref>transfer-extension</x:ref>
1940  <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> )
1942<t anchor="rule.parameter">
1943  <x:anchor-alias value="transfer-parameter"/>
1944   Parameters are in the form of a name or name=value pair.
1946<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-parameter"/>
1947  <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> )
1950   All transfer-coding names are case-insensitive and ought to be registered
1951   within the HTTP Transfer Coding registry, as defined in
1952   <xref target="transfer.coding.registry"/>.
1953   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1954   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1955   header fields.
1958<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1959  <iref primary="true" item="chunked (Coding Format)"/>
1960  <x:anchor-alias value="chunk"/>
1961  <x:anchor-alias value="chunked-body"/>
1962  <x:anchor-alias value="chunk-data"/>
1963  <x:anchor-alias value="chunk-size"/>
1964  <x:anchor-alias value="last-chunk"/>
1966   The chunked transfer coding wraps the payload body in order to transfer it
1967   as a series of chunks, each with its own size indicator, followed by an
1968   &OPTIONAL; trailer containing header fields. Chunked enables content
1969   streams of unknown size to be transferred as a sequence of length-delimited
1970   buffers, which enables the sender to retain connection persistence and the
1971   recipient to know when it has received the entire message.
1973<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"/>
1974  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1975                   <x:ref>last-chunk</x:ref>
1976                   <x:ref>trailer-part</x:ref>
1977                   <x:ref>CRLF</x:ref>
1979  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1980                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1981  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1982  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1984  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1987   The chunk-size field is a string of hex digits indicating the size of
1988   the chunk-data in octets. The chunked transfer coding is complete when a
1989   chunk with a chunk-size of zero is received, possibly followed by a
1990   trailer, and finally terminated by an empty line.
1993   A recipient &MUST; be able to parse and decode the chunked transfer coding.
1996<section title="Chunk Extensions" anchor="chunked.extension">
1997  <x:anchor-alias value="chunk-ext"/>
1998  <x:anchor-alias value="chunk-ext-name"/>
1999  <x:anchor-alias value="chunk-ext-val"/>
2001   The chunked encoding allows each chunk to include zero or more chunk
2002   extensions, immediately following the <x:ref>chunk-size</x:ref>, for the
2003   sake of supplying per-chunk metadata (such as a signature or hash),
2004   mid-message control information, or randomization of message body size.
2006<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"/>
2007  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
2009  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
2010  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
2013   The chunked encoding is specific to each connection and is likely to be
2014   removed or recoded by each recipient (including intermediaries) before any
2015   higher-level application would have a chance to inspect the extensions.
2016   Hence, use of chunk extensions is generally limited to specialized HTTP
2017   services such as "long polling" (where client and server can have shared
2018   expectations regarding the use of chunk extensions) or for padding within
2019   an end-to-end secured connection.
2022   A recipient &MUST; ignore unrecognized chunk extensions.
2023   A server ought to limit the total length of chunk extensions received in a
2024   request to an amount reasonable for the services provided, in the same way
2025   that it applies length limitations and timeouts for other parts of a
2026   message, and generate an appropriate <x:ref>4xx (Client Error)</x:ref>
2027   response if that amount is exceeded.
2031<section title="Chunked Trailer Part" anchor="chunked.trailer.part">
2032  <x:anchor-alias value="trailer-part"/>
2034   A trailer allows the sender to include additional fields at the end of a
2035   chunked message in order to supply metadata that might be dynamically
2036   generated while the message body is sent, such as a message integrity
2037   check, digital signature, or post-processing status. The trailer fields are
2038   identical to header fields, except they are sent in a chunked trailer
2039   instead of the message's header section.
2041<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="trailer-part"/>
2042  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
2045   A sender &MUST-NOT; generate a trailer that contains a field which needs to
2046   be known by the recipient before it can begin processing the message body.
2047   For example, most recipients need to know the values of
2048   <x:ref>Content-Encoding</x:ref> and <x:ref>Content-Type</x:ref> in order to
2049   select a content handler, so placing those fields in a trailer would force
2050   the recipient to buffer the entire body before it could begin, greatly
2051   increasing user-perceived latency and defeating one of the main advantages
2052   of using chunked to send data streams of unknown length.
2053   A sender &MUST-NOT; generate a trailer containing a
2054   <x:ref>Transfer-Encoding</x:ref>,
2055   <x:ref>Content-Length</x:ref>, or
2056   <x:ref>Trailer</x:ref> field.
2059   A server &MUST; generate an empty trailer with the chunked transfer coding
2060   unless at least one of the following is true:
2061  <list style="numbers">
2062    <t>the request included a <x:ref>TE</x:ref> header field that indicates
2063    "trailers" is acceptable in the transfer coding of the response, as
2064    described in <xref target="header.te"/>; or,</t>
2066    <t>the trailer fields consist entirely of optional metadata and the
2067    recipient could use the message (in a manner acceptable to the generating
2068    server) without receiving that metadata. In other words, the generating
2069    server is willing to accept the possibility that the trailer fields might
2070    be silently discarded along the path to the client.</t>
2071  </list>
2074   The above requirement prevents the need for an infinite buffer when a
2075   message is being received by an HTTP/1.1 (or later) proxy and forwarded to
2076   an HTTP/1.0 recipient.
2080<section title="Decoding Chunked" anchor="decoding.chunked">
2082   A process for decoding the chunked transfer coding
2083   can be represented in pseudo-code as:
2085<figure><artwork type="code">
2086  length := 0
2087  read chunk-size, chunk-ext (if any), and CRLF
2088  while (chunk-size &gt; 0) {
2089     read chunk-data and CRLF
2090     append chunk-data to decoded-body
2091     length := length + chunk-size
2092     read chunk-size, chunk-ext (if any), and CRLF
2093  }
2094  read header-field
2095  while (header-field not empty) {
2096     append header-field to existing header fields
2097     read header-field
2098  }
2099  Content-Length := length
2100  Remove "chunked" from Transfer-Encoding
2101  Remove Trailer from existing header fields
2106<section title="Compression Codings" anchor="compression.codings">
2108   The codings defined below can be used to compress the payload of a
2109   message.
2112<section title="Compress Coding" anchor="compress.coding">
2113<iref item="compress (Coding Format)"/>
2115   The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding
2116   <xref target="Welch"/> that is commonly produced by the UNIX file
2117   compression program "compress".
2118   A recipient &SHOULD; consider "x-compress" to be equivalent to "compress".
2122<section title="Deflate Coding" anchor="deflate.coding">
2123<iref item="deflate (Coding Format)"/>
2125   The "deflate" coding is a "zlib" data format <xref target="RFC1950"/>
2126   containing a "deflate" compressed data stream <xref target="RFC1951"/>
2127   that uses a combination of the Lempel-Ziv (LZ77) compression algorithm and
2128   Huffman coding.
2131  <t>
2132    &Note; Some incorrect implementations send the "deflate"
2133    compressed data without the zlib wrapper.
2134   </t>
2138<section title="Gzip Coding" anchor="gzip.coding">
2139<iref item="gzip (Coding Format)"/>
2141   The "gzip" coding is an LZ77 coding with a 32 bit CRC that is commonly
2142   produced by the gzip file compression program <xref target="RFC1952"/>.
2143   A recipient &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2149<section title="TE" anchor="header.te">
2150  <iref primary="true" item="TE header field" x:for-anchor=""/>
2151  <x:anchor-alias value="TE"/>
2152  <x:anchor-alias value="t-codings"/>
2153  <x:anchor-alias value="t-ranking"/>
2154  <x:anchor-alias value="rank"/>
2156   The "TE" header field in a request indicates what transfer codings,
2157   besides chunked, the client is willing to accept in response, and
2158   whether or not the client is willing to accept trailer fields in a
2159   chunked transfer coding.
2162   The TE field-value consists of a comma-separated list of transfer coding
2163   names, each allowing for optional parameters (as described in
2164   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2165   A client &MUST-NOT; send the chunked transfer coding name in TE;
2166   chunked is always acceptable for HTTP/1.1 recipients.
2168<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"/>
2169  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2170  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2171  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2172  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2173             / ( "1" [ "." 0*3("0") ] )
2176   Three examples of TE use are below.
2178<figure><artwork type="example">
2179  TE: deflate
2180  TE:
2181  TE: trailers, deflate;q=0.5
2184   The presence of the keyword "trailers" indicates that the client is willing
2185   to accept trailer fields in a chunked transfer coding, as defined in
2186   <xref target="chunked.trailer.part"/>, on behalf of itself and any downstream
2187   clients. For requests from an intermediary, this implies that either:
2188   (a) all downstream clients are willing to accept trailer fields in the
2189   forwarded response; or,
2190   (b) the intermediary will attempt to buffer the response on behalf of
2191   downstream recipients.
2192   Note that HTTP/1.1 does not define any means to limit the size of a
2193   chunked response such that an intermediary can be assured of buffering the
2194   entire response.
2197   When multiple transfer codings are acceptable, the client &MAY; rank the
2198   codings by preference using a case-insensitive "q" parameter (similar to
2199   the qvalues used in content negotiation fields, &qvalue;). The rank value
2200   is a real number in the range 0 through 1, where 0.001 is the least
2201   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2204   If the TE field-value is empty or if no TE field is present, the only
2205   acceptable transfer coding is chunked. A message with no transfer coding
2206   is always acceptable.
2209   Since the TE header field only applies to the immediate connection,
2210   a sender of TE &MUST; also send a "TE" connection option within the
2211   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2212   in order to prevent the TE field from being forwarded by intermediaries
2213   that do not support its semantics.
2217<section title="Trailer" anchor="header.trailer">
2218  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
2219  <x:anchor-alias value="Trailer"/>
2221   When a message includes a message body encoded with the chunked
2222   transfer coding and the sender desires to send metadata in the form of
2223   trailer fields at the end of the message, the sender &SHOULD; generate a
2224   <x:ref>Trailer</x:ref> header field before the message body to indicate
2225   which fields will be present in the trailers. This allows the recipient
2226   to prepare for receipt of that metadata before it starts processing the body,
2227   which is useful if the message is being streamed and the recipient wishes
2228   to confirm an integrity check on the fly.
2230<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/>
2231  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
2236<section title="Message Routing" anchor="message.routing">
2238   HTTP request message routing is determined by each client based on the
2239   target resource, the client's proxy configuration, and
2240   establishment or reuse of an inbound connection.  The corresponding
2241   response routing follows the same connection chain back to the client.
2244<section title="Identifying a Target Resource" anchor="target-resource">
2245  <iref primary="true" item="target resource"/>
2246  <iref primary="true" item="target URI"/>
2247  <x:anchor-alias value="target resource"/>
2248  <x:anchor-alias value="target URI"/>
2250   HTTP is used in a wide variety of applications, ranging from
2251   general-purpose computers to home appliances.  In some cases,
2252   communication options are hard-coded in a client's configuration.
2253   However, most HTTP clients rely on the same resource identification
2254   mechanism and configuration techniques as general-purpose Web browsers.
2257   HTTP communication is initiated by a user agent for some purpose.
2258   The purpose is a combination of request semantics, which are defined in
2259   <xref target="Part2"/>, and a target resource upon which to apply those
2260   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2261   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2262   would resolve to its absolute form in order to obtain the
2263   "<x:dfn>target URI</x:dfn>".  The target URI
2264   excludes the reference's fragment component, if any,
2265   since fragment identifiers are reserved for client-side processing
2266   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2270<section title="Connecting Inbound" anchor="connecting.inbound">
2272   Once the target URI is determined, a client needs to decide whether
2273   a network request is necessary to accomplish the desired semantics and,
2274   if so, where that request is to be directed.
2277   If the client has a cache <xref target="Part6"/> and the request can be
2278   satisfied by it, then the request is
2279   usually directed there first.
2282   If the request is not satisfied by a cache, then a typical client will
2283   check its configuration to determine whether a proxy is to be used to
2284   satisfy the request.  Proxy configuration is implementation-dependent,
2285   but is often based on URI prefix matching, selective authority matching,
2286   or both, and the proxy itself is usually identified by an "http" or
2287   "https" URI.  If a proxy is applicable, the client connects inbound by
2288   establishing (or reusing) a connection to that proxy.
2291   If no proxy is applicable, a typical client will invoke a handler routine,
2292   usually specific to the target URI's scheme, to connect directly
2293   to an authority for the target resource.  How that is accomplished is
2294   dependent on the target URI scheme and defined by its associated
2295   specification, similar to how this specification defines origin server
2296   access for resolution of the "http" (<xref target="http.uri"/>) and
2297   "https" (<xref target="https.uri"/>) schemes.
2300   HTTP requirements regarding connection management are defined in
2301   <xref target=""/>.
2305<section title="Request Target" anchor="request-target">
2307   Once an inbound connection is obtained,
2308   the client sends an HTTP request message (<xref target="http.message"/>)
2309   with a request-target derived from the target URI.
2310   There are four distinct formats for the request-target, depending on both
2311   the method being requested and whether the request is to a proxy.
2313<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"/>
2314  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2315                 / <x:ref>absolute-form</x:ref>
2316                 / <x:ref>authority-form</x:ref>
2317                 / <x:ref>asterisk-form</x:ref>
2319  <x:ref>origin-form</x:ref>    = <x:ref>absolute-path</x:ref> [ "?" <x:ref>query</x:ref> ]
2320  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2321  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2322  <x:ref>asterisk-form</x:ref>  = "*"
2324<t anchor="origin-form"><iref item="origin-form (of request-target)"/>
2325  <x:h>origin-form</x:h>
2328   The most common form of request-target is the <x:dfn>origin-form</x:dfn>.
2329   When making a request directly to an origin server, other than a CONNECT
2330   or server-wide OPTIONS request (as detailed below),
2331   a client &MUST; send only the absolute path and query components of
2332   the target URI as the request-target.
2333   If the target URI's path component is empty, then the client &MUST; send
2334   "/" as the path within the origin-form of request-target.
2335   A <x:ref>Host</x:ref> header field is also sent, as defined in
2336   <xref target=""/>.
2339   For example, a client wishing to retrieve a representation of the resource
2340   identified as
2342<figure><artwork x:indent-with="  " type="example">
2346   directly from the origin server would open (or reuse) a TCP connection
2347   to port 80 of the host "" and send the lines:
2349<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2350GET /where?q=now HTTP/1.1
2354   followed by the remainder of the request message.
2356<t anchor="absolute-form"><iref item="absolute-form (of request-target)"/>
2357  <x:h>absolute-form</x:h>
2360   When making a request to a proxy, other than a CONNECT or server-wide
2361   OPTIONS request (as detailed below), a client &MUST; send the target URI
2362   in <x:dfn>absolute-form</x:dfn> as the request-target.
2363   The proxy is requested to either service that request from a valid cache,
2364   if possible, or make the same request on the client's behalf to either
2365   the next inbound proxy server or directly to the origin server indicated
2366   by the request-target.  Requirements on such "forwarding" of messages are
2367   defined in <xref target="message.forwarding"/>.
2370   An example absolute-form of request-line would be:
2372<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2373GET HTTP/1.1
2376   To allow for transition to the absolute-form for all requests in some
2377   future version of HTTP, a server &MUST; accept the absolute-form
2378   in requests, even though HTTP/1.1 clients will only send them in requests
2379   to proxies.
2381<t anchor="authority-form"><iref item="authority-form (of request-target)"/>
2382  <x:h>authority-form</x:h>
2385   The <x:dfn>authority-form</x:dfn> of request-target is only used for
2386   CONNECT requests (&CONNECT;). When making a CONNECT request to establish a
2387   tunnel through one or more proxies, a client &MUST; send only the target
2388   URI's authority component (excluding any userinfo and its "@" delimiter) as
2389   the request-target. For example,
2391<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2394<t anchor="asterisk-form"><iref item="asterisk-form (of request-target)"/>
2395  <x:h>asterisk-form</x:h>
2398   The <x:dfn>asterisk-form</x:dfn> of request-target is only used for a server-wide
2399   OPTIONS request (&OPTIONS;).  When a client wishes to request OPTIONS
2400   for the server as a whole, as opposed to a specific named resource of
2401   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2402   For example,
2404<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2405OPTIONS * HTTP/1.1
2408   If a proxy receives an OPTIONS request with an absolute-form of
2409   request-target in which the URI has an empty path and no query component,
2410   then the last proxy on the request chain &MUST; send a request-target
2411   of "*" when it forwards the request to the indicated origin server.
2414   For example, the request
2415</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2419  would be forwarded by the final proxy as
2420</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2421OPTIONS * HTTP/1.1
2425   after connecting to port 8001 of host "".
2430<section title="Host" anchor="">
2431  <iref primary="true" item="Host header field" x:for-anchor=""/>
2432  <x:anchor-alias value="Host"/>
2434   The "Host" header field in a request provides the host and port
2435   information from the target URI, enabling the origin
2436   server to distinguish among resources while servicing requests
2437   for multiple host names on a single IP address.
2439<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2440  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2443   A client &MUST; send a Host header field in all HTTP/1.1 request messages.
2444   If the target URI includes an authority component, then a client &MUST;
2445   send a field-value for Host that is identical to that authority
2446   component, excluding any userinfo subcomponent and its "@" delimiter
2447   (<xref target="http.uri"/>).
2448   If the authority component is missing or undefined for the target URI,
2449   then a client &MUST; send a Host header field with an empty field-value.
2452   Since the Host field-value is critical information for handling a request,
2453   a user agent &SHOULD; generate Host as the first header field following the
2454   request-line.
2457   For example, a GET request to the origin server for
2458   &lt;; would begin with:
2460<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2461GET /pub/WWW/ HTTP/1.1
2465   A client &MUST; send a Host header field in an HTTP/1.1 request even
2466   if the request-target is in the absolute-form, since this
2467   allows the Host information to be forwarded through ancient HTTP/1.0
2468   proxies that might not have implemented Host.
2471   When a proxy receives a request with an absolute-form of
2472   request-target, the proxy &MUST; ignore the received
2473   Host header field (if any) and instead replace it with the host
2474   information of the request-target.  A proxy that forwards such a request
2475   &MUST; generate a new Host field-value based on the received
2476   request-target rather than forward the received Host field-value.
2479   Since the Host header field acts as an application-level routing
2480   mechanism, it is a frequent target for malware seeking to poison
2481   a shared cache or redirect a request to an unintended server.
2482   An interception proxy is particularly vulnerable if it relies on
2483   the Host field-value for redirecting requests to internal
2484   servers, or for use as a cache key in a shared cache, without
2485   first verifying that the intercepted connection is targeting a
2486   valid IP address for that host.
2489   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2490   to any HTTP/1.1 request message that lacks a Host header field and
2491   to any request message that contains more than one Host header field
2492   or a Host header field with an invalid field-value.
2496<section title="Effective Request URI" anchor="effective.request.uri">
2497  <iref primary="true" item="effective request URI"/>
2498  <x:anchor-alias value="effective request URI"/>
2500   A server that receives an HTTP request message &MUST; reconstruct
2501   the user agent's original target URI, based on the pieces of information
2502   learned from the request-target, <x:ref>Host</x:ref> header field, and
2503   connection context, in order to identify the intended target resource and
2504   properly service the request. The URI derived from this reconstruction
2505   process is referred to as the "<x:dfn>effective request URI</x:dfn>".
2508   For a user agent, the effective request URI is the target URI.
2511   If the request-target is in absolute-form, then the effective request URI
2512   is the same as the request-target.  Otherwise, the effective request URI
2513   is constructed as follows.
2516   If the request is received over a TLS-secured TCP connection,
2517   then the effective request URI's scheme is "https"; otherwise, the
2518   scheme is "http".
2521   If the request-target is in authority-form, then the effective
2522   request URI's authority component is the same as the request-target.
2523   Otherwise, if a <x:ref>Host</x:ref> header field is supplied with a
2524   non-empty field-value, then the authority component is the same as the
2525   Host field-value. Otherwise, the authority component is the concatenation of
2526   the default host name configured for the server, a colon (":"), and the
2527   connection's incoming TCP port number in decimal form.
2530   If the request-target is in authority-form or asterisk-form, then the
2531   effective request URI's combined path and query component is empty.
2532   Otherwise, the combined path and query component is the same as the
2533   request-target.
2536   The components of the effective request URI, once determined as above,
2537   can be combined into absolute-URI form by concatenating the scheme,
2538   "://", authority, and combined path and query component.
2542   Example 1: the following message received over an insecure TCP connection
2544<artwork type="example" x:indent-with="  ">
2545GET /pub/WWW/TheProject.html HTTP/1.1
2551  has an effective request URI of
2553<artwork type="example" x:indent-with="  ">
2559   Example 2: the following message received over a TLS-secured TCP connection
2561<artwork type="example" x:indent-with="  ">
2562OPTIONS * HTTP/1.1
2568  has an effective request URI of
2570<artwork type="example" x:indent-with="  ">
2575   An origin server that does not allow resources to differ by requested
2576   host &MAY; ignore the <x:ref>Host</x:ref> field-value and instead replace it
2577   with a configured server name when constructing the effective request URI.
2580   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2581   field &MAY; attempt to use heuristics (e.g., examination of the URI path for
2582   something unique to a particular host) in order to guess the
2583   effective request URI's authority component.
2587<section title="Associating a Response to a Request" anchor="">
2589   HTTP does not include a request identifier for associating a given
2590   request message with its corresponding one or more response messages.
2591   Hence, it relies on the order of response arrival to correspond exactly
2592   to the order in which requests are made on the same connection.
2593   More than one response message per request only occurs when one or more
2594   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a
2595   final response to the same request.
2598   A client that has more than one outstanding request on a connection &MUST;
2599   maintain a list of outstanding requests in the order sent and &MUST;
2600   associate each received response message on that connection to the highest
2601   ordered request that has not yet received a final (non-<x:ref>1xx</x:ref>)
2602   response.
2606<section title="Message Forwarding" anchor="message.forwarding">
2608   As described in <xref target="intermediaries"/>, intermediaries can serve
2609   a variety of roles in the processing of HTTP requests and responses.
2610   Some intermediaries are used to improve performance or availability.
2611   Others are used for access control or to filter content.
2612   Since an HTTP stream has characteristics similar to a pipe-and-filter
2613   architecture, there are no inherent limits to the extent an intermediary
2614   can enhance (or interfere) with either direction of the stream.
2617   An intermediary not acting as a tunnel &MUST; implement the
2618   <x:ref>Connection</x:ref> header field, as specified in
2619   <xref target="header.connection"/>, and exclude fields from being forwarded
2620   that are only intended for the incoming connection.
2623   An intermediary &MUST-NOT; forward a message to itself unless it is
2624   protected from an infinite request loop. In general, an intermediary ought
2625   to recognize its own server names, including any aliases, local variations,
2626   or literal IP addresses, and respond to such requests directly.
2629<section title="Via" anchor="header.via">
2630  <iref primary="true" item="Via header field" x:for-anchor=""/>
2631  <x:anchor-alias value="pseudonym"/>
2632  <x:anchor-alias value="received-by"/>
2633  <x:anchor-alias value="received-protocol"/>
2634  <x:anchor-alias value="Via"/>
2636   The "Via" header field indicates the presence of intermediate protocols and
2637   recipients between the user agent and the server (on requests) or between
2638   the origin server and the client (on responses), similar to the
2639   "Received" header field in email
2640   (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2641   Via can be used for tracking message forwards,
2642   avoiding request loops, and identifying the protocol capabilities of
2643   senders along the request/response chain.
2645<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"/>
2646  <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> ] )
2648  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2649                      ; see <xref target="header.upgrade"/>
2650  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2651  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2654   Multiple Via field values represent each proxy or gateway that has
2655   forwarded the message. Each intermediary appends its own information
2656   about how the message was received, such that the end result is ordered
2657   according to the sequence of forwarding recipients.
2660   A proxy &MUST; send an appropriate Via header field, as described below, in
2661   each message that it forwards.
2662   An HTTP-to-HTTP gateway &MUST; send an appropriate Via header field in
2663   each inbound request message and &MAY; send a Via header field in
2664   forwarded response messages.
2667   For each intermediary, the received-protocol indicates the protocol and
2668   protocol version used by the upstream sender of the message. Hence, the
2669   Via field value records the advertised protocol capabilities of the
2670   request/response chain such that they remain visible to downstream
2671   recipients; this can be useful for determining what backwards-incompatible
2672   features might be safe to use in response, or within a later request, as
2673   described in <xref target="http.version"/>. For brevity, the protocol-name
2674   is omitted when the received protocol is HTTP.
2677   The received-by portion of the field value is normally the host and optional
2678   port number of a recipient server or client that subsequently forwarded the
2679   message.
2680   However, if the real host is considered to be sensitive information, a
2681   sender &MAY; replace it with a pseudonym. If a port is not provided,
2682   a recipient &MAY; interpret that as meaning it was received on the default
2683   TCP port, if any, for the received-protocol.
2686   A sender &MAY; generate comments in the Via header field to identify the
2687   software of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2688   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2689   are optional and a recipient &MAY; remove them prior to forwarding the
2690   message.
2693   For example, a request message could be sent from an HTTP/1.0 user
2694   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2695   forward the request to a public proxy at, which completes
2696   the request by forwarding it to the origin server at
2697   The request received by would then have the following
2698   Via header field:
2700<figure><artwork type="example">
2701  Via: 1.0 fred, 1.1
2704   An intermediary used as a portal through a network firewall
2705   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2706   region unless it is explicitly enabled to do so. If not enabled, such an
2707   intermediary &SHOULD; replace each received-by host of any host behind the
2708   firewall by an appropriate pseudonym for that host.
2711   An intermediary &MAY; combine an ordered subsequence of Via header
2712   field entries into a single such entry if the entries have identical
2713   received-protocol values. For example,
2715<figure><artwork type="example">
2716  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2719  could be collapsed to
2721<figure><artwork type="example">
2722  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2725   A sender &SHOULD-NOT; combine multiple entries unless they are all
2726   under the same organizational control and the hosts have already been
2727   replaced by pseudonyms. A sender &MUST-NOT; combine entries that
2728   have different received-protocol values.
2732<section title="Transformations" anchor="message.transformations">
2733   <iref primary="true" item="transforming proxy"/>
2734   <iref primary="true" item="non-transforming proxy"/>
2736   Some intermediaries include features for transforming messages and their
2737   payloads. A proxy might, for example, convert between image formats in
2738   order to save cache space or to reduce the amount of traffic on a slow
2739   link. However, operational problems might occur when these transformations
2740   are applied to payloads intended for critical applications, such as medical
2741   imaging or scientific data analysis, particularly when integrity checks or
2742   digital signatures are used to ensure that the payload received is
2743   identical to the original.
2746   An HTTP-to-HTTP proxy is called a "<x:dfn>transforming proxy</x:dfn>"
2747   if it is designed or configured to modify messages in a semantically
2748   meaningful way (i.e., modifications, beyond those required by normal
2749   HTTP processing, that change the message in a way that would be
2750   significant to the original sender or potentially significant to
2751   downstream recipients).  For example, a transforming proxy might be
2752   acting as a shared annotation server (modifying responses to include
2753   references to a local annotation database), a malware filter, a
2754   format transcoder, or a privacy filter. Such transformations are presumed
2755   to be desired by whichever client (or client organization) selected the
2756   proxy.
2759   If a proxy receives a request-target with a host name that is not a
2760   fully qualified domain name, it &MAY; add its own domain to the host name
2761   it received when forwarding the request.  A proxy &MUST-NOT; change the
2762   host name if it is a fully qualified domain name.
2765   A proxy &MUST-NOT; modify the "absolute-path" and "query" parts of the
2766   received request-target when forwarding it to the next inbound server,
2767   except as noted above to replace an empty path with "/" or "*".
2770   A proxy &MUST-NOT; modify header fields that provide information about the
2771   end points of the communication chain, the resource state, or the selected
2772   representation. A proxy &MAY; change the message body through application
2773   or removal of a transfer coding (<xref target="transfer.codings"/>).
2776   A proxy &MUST-NOT; modify the payload (&payload;) of a message that
2777   contains a no-transform cache-control directive (&header-cache-control;).
2780   A proxy &MAY; transform the payload of a message
2781   that does not contain a no-transform cache-control directive.
2782   A proxy that transforms a payload &MUST; add a
2783   Warning header field with the warn-code of 214 ("Transformation Applied")
2784   if one is not already in the message (see &header-warning;).
2785   A proxy that transforms the payload of a <x:ref>200 (OK)</x:ref> response
2786   can further inform downstream recipients that a transformation has been
2787   applied by changing the response status code to
2788   <x:ref>203 (Non-Authoritative Information)</x:ref> (&status-203;).
2794<section title="Connection Management" anchor="">
2796   HTTP messaging is independent of the underlying transport or
2797   session-layer connection protocol(s).  HTTP only presumes a reliable
2798   transport with in-order delivery of requests and the corresponding
2799   in-order delivery of responses.  The mapping of HTTP request and
2800   response structures onto the data units of an underlying transport
2801   protocol is outside the scope of this specification.
2804   As described in <xref target="connecting.inbound"/>, the specific
2805   connection protocols to be used for an HTTP interaction are determined by
2806   client configuration and the <x:ref>target URI</x:ref>.
2807   For example, the "http" URI scheme
2808   (<xref target="http.uri"/>) indicates a default connection of TCP
2809   over IP, with a default TCP port of 80, but the client might be
2810   configured to use a proxy via some other connection, port, or protocol.
2813   HTTP implementations are expected to engage in connection management,
2814   which includes maintaining the state of current connections,
2815   establishing a new connection or reusing an existing connection,
2816   processing messages received on a connection, detecting connection
2817   failures, and closing each connection.
2818   Most clients maintain multiple connections in parallel, including
2819   more than one connection per server endpoint.
2820   Most servers are designed to maintain thousands of concurrent connections,
2821   while controlling request queues to enable fair use and detect
2822   denial of service attacks.
2825<section title="Connection" anchor="header.connection">
2826  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2827  <iref primary="true" item="close" x:for-anchor=""/>
2828  <x:anchor-alias value="Connection"/>
2829  <x:anchor-alias value="connection-option"/>
2830  <x:anchor-alias value="close"/>
2832   The "Connection" header field allows the sender to indicate desired
2833   control options for the current connection.  In order to avoid confusing
2834   downstream recipients, a proxy or gateway &MUST; remove or replace any
2835   received connection options before forwarding the message.
2838   When a header field aside from Connection is used to supply control
2839   information for or about the current connection, the sender &MUST; list
2840   the corresponding field-name within the "Connection" header field.
2841   A proxy or gateway &MUST; parse a received Connection
2842   header field before a message is forwarded and, for each
2843   connection-option in this field, remove any header field(s) from
2844   the message with the same name as the connection-option, and then
2845   remove the Connection header field itself (or replace it with the
2846   intermediary's own connection options for the forwarded message).
2849   Hence, the Connection header field provides a declarative way of
2850   distinguishing header fields that are only intended for the
2851   immediate recipient ("hop-by-hop") from those fields that are
2852   intended for all recipients on the chain ("end-to-end"), enabling the
2853   message to be self-descriptive and allowing future connection-specific
2854   extensions to be deployed without fear that they will be blindly
2855   forwarded by older intermediaries.
2858   The Connection header field's value has the following grammar:
2860<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2861  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2862  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2865   Connection options are case-insensitive.
2868   A sender &MUST-NOT; send a connection option corresponding to a header
2869   field that is intended for all recipients of the payload.
2870   For example, <x:ref>Cache-Control</x:ref> is never appropriate as a
2871   connection option (&header-cache-control;).
2874   The connection options do not always correspond to a header field
2875   present in the message, since a connection-specific header field
2876   might not be needed if there are no parameters associated with a
2877   connection option. In contrast, a connection-specific header field that
2878   is received without a corresponding connection option usually indicates
2879   that the field has been improperly forwarded by an intermediary and
2880   ought to be ignored by the recipient.
2883   When defining new connection options, specification authors ought to survey
2884   existing header field names and ensure that the new connection option does
2885   not share the same name as an already deployed header field.
2886   Defining a new connection option essentially reserves that potential
2887   field-name for carrying additional information related to the
2888   connection option, since it would be unwise for senders to use
2889   that field-name for anything else.
2892   The "<x:dfn>close</x:dfn>" connection option is defined for a
2893   sender to signal that this connection will be closed after completion of
2894   the response. For example,
2896<figure><artwork type="example">
2897  Connection: close
2900   in either the request or the response header fields indicates that the
2901   sender is going to close the connection after the current request/response
2902   is complete (<xref target="persistent.tear-down"/>).
2905   A client that does not support <x:ref>persistent connections</x:ref> &MUST;
2906   send the "close" connection option in every request message.
2909   A server that does not support <x:ref>persistent connections</x:ref> &MUST;
2910   send the "close" connection option in every response message that
2911   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2915<section title="Establishment" anchor="persistent.establishment">
2917   It is beyond the scope of this specification to describe how connections
2918   are established via various transport or session-layer protocols.
2919   Each connection applies to only one transport link.
2923<section title="Persistence" anchor="persistent.connections">
2924   <x:anchor-alias value="persistent connections"/>
2926   HTTP/1.1 defaults to the use of "<x:dfn>persistent connections</x:dfn>",
2927   allowing multiple requests and responses to be carried over a single
2928   connection. The "<x:ref>close</x:ref>" connection-option is used to signal
2929   that a connection will not persist after the current request/response.
2930   HTTP implementations &SHOULD; support persistent connections.
2933   A recipient determines whether a connection is persistent or not based on
2934   the most recently received message's protocol version and
2935   <x:ref>Connection</x:ref> header field (if any):
2936   <list style="symbols">
2937     <t>If the <x:ref>close</x:ref> connection option is present, the
2938        connection will not persist after the current response; else,</t>
2939     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2940        persist after the current response; else,</t>
2941     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2942        connection option is present, the recipient is not a proxy, and
2943        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2944        the connection will persist after the current response; otherwise,</t>
2945     <t>The connection will close after the current response.</t>
2946   </list>
2949   A server &MAY; assume that an HTTP/1.1 client intends to maintain a
2950   persistent connection until a <x:ref>close</x:ref> connection option
2951   is received in a request.
2954   A client &MAY; reuse a persistent connection until it sends or receives
2955   a <x:ref>close</x:ref> connection option or receives an HTTP/1.0 response
2956   without a "keep-alive" connection option.
2959   In order to remain persistent, all messages on a connection need to
2960   have a self-defined message length (i.e., one not defined by closure
2961   of the connection), as described in <xref target="message.body"/>.
2962   A server &MUST; read the entire request message body or close
2963   the connection after sending its response, since otherwise the
2964   remaining data on a persistent connection would be misinterpreted
2965   as the next request.  Likewise,
2966   a client &MUST; read the entire response message body if it intends
2967   to reuse the same connection for a subsequent request.
2970   A proxy server &MUST-NOT; maintain a persistent connection with an
2971   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
2972   information and discussion of the problems with the Keep-Alive header field
2973   implemented by many HTTP/1.0 clients).
2976   Clients and servers &SHOULD-NOT; assume that a persistent connection is
2977   maintained for HTTP versions less than 1.1 unless it is explicitly
2978   signaled.
2979   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
2980   for more information on backward compatibility with HTTP/1.0 clients.
2983<section title="Retrying Requests" anchor="persistent.retrying.requests">
2985   Connections can be closed at any time, with or without intention.
2986   Implementations ought to anticipate the need to recover
2987   from asynchronous close events.
2990   When an inbound connection is closed prematurely, a client &MAY; open a new
2991   connection and automatically retransmit an aborted sequence of requests if
2992   all of those requests have idempotent methods (&idempotent-methods;).
2993   A proxy &MUST-NOT; automatically retry non-idempotent requests.
2996   A user agent &MUST-NOT; automatically retry a request with a non-idempotent
2997   method unless it has some means to know that the request semantics are
2998   actually idempotent, regardless of the method, or some means to detect that
2999   the original request was never applied. For example, a user agent that
3000   knows (through design or configuration) that a POST request to a given
3001   resource is safe can repeat that request automatically.
3002   Likewise, a user agent designed specifically to operate on a version
3003   control repository might be able to recover from partial failure conditions
3004   by checking the target resource revision(s) after a failed connection,
3005   reverting or fixing any changes that were partially applied, and then
3006   automatically retrying the requests that failed.
3009   A client &SHOULD-NOT; automatically retry a failed automatic retry.
3013<section title="Pipelining" anchor="pipelining">
3014   <x:anchor-alias value="pipeline"/>
3016   A client that supports persistent connections &MAY; "<x:dfn>pipeline</x:dfn>"
3017   its requests (i.e., send multiple requests without waiting for each
3018   response). A server &MAY; process a sequence of pipelined requests in
3019   parallel if they all have safe methods (&safe-methods;), but &MUST; send
3020   the corresponding responses in the same order that the requests were
3021   received.
3024   A client that pipelines requests &SHOULD; retry unanswered requests if the
3025   connection closes before it receives all of the corresponding responses.
3026   When retrying pipelined requests after a failed connection (a connection
3027   not explicitly closed by the server in its last complete response), a
3028   client &MUST-NOT; pipeline immediately after connection establishment,
3029   since the first remaining request in the prior pipeline might have caused
3030   an error response that can be lost again if multiple requests are sent on a
3031   prematurely closed connection (see the TCP reset problem described in
3032   <xref target="persistent.tear-down"/>).
3035   Idempotent methods (&idempotent-methods;) are significant to pipelining
3036   because they can be automatically retried after a connection failure.
3037   A user agent &SHOULD-NOT; pipeline requests after a non-idempotent method,
3038   until the final response status code for that method has been received,
3039   unless the user agent has a means to detect and recover from partial
3040   failure conditions involving the pipelined sequence.
3043   An intermediary that receives pipelined requests &MAY; pipeline those
3044   requests when forwarding them inbound, since it can rely on the outbound
3045   user agent(s) to determine what requests can be safely pipelined. If the
3046   inbound connection fails before receiving a response, the pipelining
3047   intermediary &MAY; attempt to retry a sequence of requests that have yet
3048   to receive a response if the requests all have idempotent methods;
3049   otherwise, the pipelining intermediary &SHOULD; forward any received
3050   responses and then close the corresponding outbound connection(s) so that
3051   the outbound user agent(s) can recover accordingly.
3056<section title="Concurrency" anchor="persistent.concurrency">
3058   A client &SHOULD; limit the number of simultaneous open
3059   connections that it maintains to a given server.
3062   Previous revisions of HTTP gave a specific number of connections as a
3063   ceiling, but this was found to be impractical for many applications. As a
3064   result, this specification does not mandate a particular maximum number of
3065   connections, but instead encourages clients to be conservative when opening
3066   multiple connections.
3069   Multiple connections are typically used to avoid the "head-of-line
3070   blocking" problem, wherein a request that takes significant server-side
3071   processing and/or has a large payload blocks subsequent requests on the
3072   same connection. However, each connection consumes server resources.
3073   Furthermore, using multiple connections can cause undesirable side effects
3074   in congested networks.
3077   Note that servers might reject traffic that they deem abusive, including an
3078   excessive number of connections from a client.
3082<section title="Failures and Time-outs" anchor="persistent.failures">
3084   Servers will usually have some time-out value beyond which they will
3085   no longer maintain an inactive connection. Proxy servers might make
3086   this a higher value since it is likely that the client will be making
3087   more connections through the same proxy server. The use of persistent
3088   connections places no requirements on the length (or existence) of
3089   this time-out for either the client or the server.
3092   A client or server that wishes to time-out &SHOULD; issue a graceful close
3093   on the connection. Implementations &SHOULD; constantly monitor open
3094   connections for a received closure signal and respond to it as appropriate,
3095   since prompt closure of both sides of a connection enables allocated system
3096   resources to be reclaimed.
3099   A client, server, or proxy &MAY; close the transport connection at any
3100   time. For example, a client might have started to send a new request
3101   at the same time that the server has decided to close the "idle"
3102   connection. From the server's point of view, the connection is being
3103   closed while it was idle, but from the client's point of view, a
3104   request is in progress.
3107   A server &SHOULD; sustain persistent connections, when possible, and allow
3108   the underlying
3109   transport's flow control mechanisms to resolve temporary overloads, rather
3110   than terminate connections with the expectation that clients will retry.
3111   The latter technique can exacerbate network congestion.
3114   A client sending a message body &SHOULD; monitor
3115   the network connection for an error response while it is transmitting
3116   the request. If the client sees a response that indicates the server does
3117   not wish to receive the message body and is closing the connection, the
3118   client &SHOULD; immediately cease transmitting the body and close its side
3119   of the connection.
3123<section title="Tear-down" anchor="persistent.tear-down">
3124  <iref primary="false" item="Connection header field" x:for-anchor=""/>
3125  <iref primary="false" item="close" x:for-anchor=""/>
3127   The <x:ref>Connection</x:ref> header field
3128   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
3129   connection option that a sender &SHOULD; send when it wishes to close
3130   the connection after the current request/response pair.
3133   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
3134   send further requests on that connection (after the one containing
3135   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
3136   final response message corresponding to this request.
3139   A server that receives a <x:ref>close</x:ref> connection option &MUST;
3140   initiate a close of the connection (see below) after it sends the
3141   final response to the request that contained <x:ref>close</x:ref>.
3142   The server &SHOULD; send a <x:ref>close</x:ref> connection option
3143   in its final response on that connection. The server &MUST-NOT; process
3144   any further requests received on that connection.
3147   A server that sends a <x:ref>close</x:ref> connection option &MUST;
3148   initiate a close of the connection (see below) after it sends the
3149   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
3150   any further requests received on that connection.
3153   A client that receives a <x:ref>close</x:ref> connection option &MUST;
3154   cease sending requests on that connection and close the connection
3155   after reading the response message containing the close; if additional
3156   pipelined requests had been sent on the connection, the client &SHOULD-NOT;
3157   assume that they will be processed by the server.
3160   If a server performs an immediate close of a TCP connection, there is a
3161   significant risk that the client will not be able to read the last HTTP
3162   response.  If the server receives additional data from the client on a
3163   fully-closed connection, such as another request that was sent by the
3164   client before receiving the server's response, the server's TCP stack will
3165   send a reset packet to the client; unfortunately, the reset packet might
3166   erase the client's unacknowledged input buffers before they can be read
3167   and interpreted by the client's HTTP parser.
3170   To avoid the TCP reset problem, servers typically close a connection in
3171   stages. First, the server performs a half-close by closing only the write
3172   side of the read/write connection. The server then continues to read from
3173   the connection until it receives a corresponding close by the client, or
3174   until the server is reasonably certain that its own TCP stack has received
3175   the client's acknowledgement of the packet(s) containing the server's last
3176   response. Finally, the server fully closes the connection.
3179   It is unknown whether the reset problem is exclusive to TCP or might also
3180   be found in other transport connection protocols.
3184<section title="Upgrade" anchor="header.upgrade">
3185  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3186  <x:anchor-alias value="Upgrade"/>
3187  <x:anchor-alias value="protocol"/>
3188  <x:anchor-alias value="protocol-name"/>
3189  <x:anchor-alias value="protocol-version"/>
3191   The "Upgrade" header field is intended to provide a simple mechanism
3192   for transitioning from HTTP/1.1 to some other protocol on the same
3193   connection.  A client &MAY; send a list of protocols in the Upgrade
3194   header field of a request to invite the server to switch to one or
3195   more of those protocols, in order of descending preference, before sending
3196   the final response. A server &MAY; ignore a received Upgrade header field
3197   if it wishes to continue using the current protocol on that connection.
3199<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3200  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3202  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3203  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3204  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3207   A server that sends a <x:ref>101 (Switching Protocols)</x:ref> response
3208   &MUST; send an Upgrade header field to indicate the new protocol(s) to
3209   which the connection is being switched; if multiple protocol layers are
3210   being switched, the sender &MUST; list the protocols in layer-ascending
3211   order. A server &MUST-NOT; switch to a protocol that was not indicated by
3212   the client in the corresponding request's Upgrade header field.
3213   A server &MAY; choose to ignore the order of preference indicated by the
3214   client and select the new protocol(s) based on other factors, such as the
3215   nature of the request or the current load on the server.
3218   A server that sends a <x:ref>426 (Upgrade Required)</x:ref> response
3219   &MUST; send an Upgrade header field to indicate the acceptable protocols,
3220   in order of descending preference.
3223   A server &MAY; send an Upgrade header field in any other response to
3224   advertise that it implements support for upgrading to the listed protocols,
3225   in order of descending preference, when appropriate for a future request.
3228   The following is a hypothetical example sent by a client:
3229</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
3230GET /hello.txt HTTP/1.1
3232Connection: upgrade
3233Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3237   Upgrade cannot be used to insist on a protocol change; its acceptance and
3238   use by the server is optional. The capabilities and nature of the
3239   application-level communication after the protocol change is entirely
3240   dependent upon the new protocol(s) chosen. However, immediately after
3241   sending the 101 response, the server is expected to continue responding to
3242   the original request as if it had received its equivalent within the new
3243   protocol (i.e., the server still has an outstanding request to satisfy
3244   after the protocol has been changed, and is expected to do so without
3245   requiring the request to be repeated).
3248   For example, if the Upgrade header field is received in a GET request
3249   and the server decides to switch protocols, it first responds
3250   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3251   then immediately follows that with the new protocol's equivalent of a
3252   response to a GET on the target resource.  This allows a connection to be
3253   upgraded to protocols with the same semantics as HTTP without the
3254   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3255   protocols unless the received message semantics can be honored by the new
3256   protocol; an OPTIONS request can be honored by any protocol.
3259   The following is an example response to the above hypothetical request:
3260</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
3261HTTP/1.1 101 Switching Protocols
3262Connection: upgrade
3263Upgrade: HTTP/2.0
3265[... data stream switches to HTTP/2.0 with an appropriate response
3266(as defined by new protocol) to the "GET /hello.txt" request ...]
3269   When Upgrade is sent, the sender &MUST; also send a
3270   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3271   that contains an "upgrade" connection option, in order to prevent Upgrade
3272   from being accidentally forwarded by intermediaries that might not implement
3273   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3274   is received in an HTTP/1.0 request.
3277   A client cannot begin using an upgraded protocol on the connection until
3278   it has completely sent the request message (i.e., the client can't change
3279   the protocol it is sending in the middle of a message).
3280   If a server receives both Upgrade and an <x:ref>Expect</x:ref> header field
3281   with the "100-continue" expectation (&header-expect;), the
3282   server &MUST; send a <x:ref>100 (Continue)</x:ref> response before sending
3283   a <x:ref>101 (Switching Protocols)</x:ref> response.
3286   The Upgrade header field only applies to switching protocols on top of the
3287   existing connection; it cannot be used to switch the underlying connection
3288   (transport) protocol, nor to switch the existing communication to a
3289   different connection. For those purposes, it is more appropriate to use a
3290   <x:ref>3xx (Redirection)</x:ref> response (&status-3xx;).
3293   This specification only defines the protocol name "HTTP" for use by
3294   the family of Hypertext Transfer Protocols, as defined by the HTTP
3295   version rules of <xref target="http.version"/> and future updates to this
3296   specification. Additional tokens ought to be registered with IANA using the
3297   registration procedure defined in <xref target="upgrade.token.registry"/>.
3302<section title="ABNF list extension: #rule" anchor="abnf.extension">
3304  A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
3305  improve readability in the definitions of some header field values.
3308  A construct "#" is defined, similar to "*", for defining comma-delimited
3309  lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
3310  at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
3311  comma (",") and optional whitespace (OWS).   
3314  Thus, a sender &MUST; expand the list construct as follows:
3315</preamble><artwork type="example">
3316  1#element =&gt; element *( OWS "," OWS element )
3319  and:
3320</preamble><artwork type="example">
3321  #element =&gt; [ 1#element ]
3324  and for n &gt;= 1 and m &gt; 1:
3325</preamble><artwork type="example">
3326  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
3329  For compatibility with legacy list rules, a recipient &MUST; parse and ignore
3330  a reasonable number of empty list elements: enough to handle common mistakes
3331  by senders that merge values, but not so much that they could be used as a
3332  denial of service mechanism. In other words, a recipient &MUST; expand the
3333  list construct as follows:
3335<figure><artwork type="example">
3336  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
3338  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
3341  Empty elements do not contribute to the count of elements present.
3342  For example, given these ABNF productions:
3344<figure><artwork type="example">
3345  example-list      = 1#example-list-elmt
3346  example-list-elmt = token ; see <xref target="field.components"/>
3349  Then the following are valid values for example-list (not including the
3350  double quotes, which are present for delimitation only):
3352<figure><artwork type="example">
3353  "foo,bar"
3354  "foo ,bar,"
3355  "foo , ,bar,charlie   "
3358  In contrast, the following values would be invalid, since at least one
3359  non-empty element is required by the example-list production:
3361<figure><artwork type="example">
3362  ""
3363  ","
3364  ",   ,"
3367  <xref target="collected.abnf"/> shows the collected ABNF after the list
3368  constructs have been expanded, as described above, for recipients.
3372<section title="IANA Considerations" anchor="IANA.considerations">
3374<section title="Header Field Registration" anchor="header.field.registration">
3376   HTTP header fields are registered within the Message Header Field Registry
3377   maintained at
3378   <eref target=""/>.
3381   This document defines the following HTTP header fields, so their
3382   associated registry entries shall be updated according to the permanent
3383   registrations below (see <xref target="BCP90"/>):
3385<?BEGININC p1-messaging.iana-headers ?>
3386<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3387<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3388   <ttcol>Header Field Name</ttcol>
3389   <ttcol>Protocol</ttcol>
3390   <ttcol>Status</ttcol>
3391   <ttcol>Reference</ttcol>
3393   <c>Connection</c>
3394   <c>http</c>
3395   <c>standard</c>
3396   <c>
3397      <xref target="header.connection"/>
3398   </c>
3399   <c>Content-Length</c>
3400   <c>http</c>
3401   <c>standard</c>
3402   <c>
3403      <xref target="header.content-length"/>
3404   </c>
3405   <c>Host</c>
3406   <c>http</c>
3407   <c>standard</c>
3408   <c>
3409      <xref target=""/>
3410   </c>
3411   <c>TE</c>
3412   <c>http</c>
3413   <c>standard</c>
3414   <c>
3415      <xref target="header.te"/>
3416   </c>
3417   <c>Trailer</c>
3418   <c>http</c>
3419   <c>standard</c>
3420   <c>
3421      <xref target="header.trailer"/>
3422   </c>
3423   <c>Transfer-Encoding</c>
3424   <c>http</c>
3425   <c>standard</c>
3426   <c>
3427      <xref target="header.transfer-encoding"/>
3428   </c>
3429   <c>Upgrade</c>
3430   <c>http</c>
3431   <c>standard</c>
3432   <c>
3433      <xref target="header.upgrade"/>
3434   </c>
3435   <c>Via</c>
3436   <c>http</c>
3437   <c>standard</c>
3438   <c>
3439      <xref target="header.via"/>
3440   </c>
3443<?ENDINC p1-messaging.iana-headers ?>
3445   Furthermore, the header field-name "Close" shall be registered as
3446   "reserved", since using that name as an HTTP header field might
3447   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3448   header field (<xref target="header.connection"/>).
3450<texttable align="left" suppress-title="true">
3451   <ttcol>Header Field Name</ttcol>
3452   <ttcol>Protocol</ttcol>
3453   <ttcol>Status</ttcol>
3454   <ttcol>Reference</ttcol>
3456   <c>Close</c>
3457   <c>http</c>
3458   <c>reserved</c>
3459   <c>
3460      <xref target="header.field.registration"/>
3461   </c>
3464   The change controller is: "IETF ( - Internet Engineering Task Force".
3468<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3470   IANA maintains the registry of URI Schemes <xref target="BCP115"/> at
3471   <eref target=""/>.
3474   This document defines the following URI schemes, so their
3475   associated registry entries shall be updated according to the permanent
3476   registrations below:
3478<texttable align="left" suppress-title="true">
3479   <ttcol>URI Scheme</ttcol>
3480   <ttcol>Description</ttcol>
3481   <ttcol>Reference</ttcol>
3483   <c>http</c>
3484   <c>Hypertext Transfer Protocol</c>
3485   <c><xref target="http.uri"/></c>
3487   <c>https</c>
3488   <c>Hypertext Transfer Protocol Secure</c>
3489   <c><xref target="https.uri"/></c>
3493<section title="Internet Media Type Registration" anchor="">
3495   IANA maintains the registry of Internet media types <xref target="BCP13"/> at
3496   <eref target=""/>.
3499   This document serves as the specification for the Internet media types
3500   "message/http" and "application/http". The following is to be registered with
3501   IANA.
3503<section title="Internet Media Type message/http" anchor="">
3504<iref item="Media Type" subitem="message/http" primary="true"/>
3505<iref item="message/http Media Type" primary="true"/>
3507   The message/http type can be used to enclose a single HTTP request or
3508   response message, provided that it obeys the MIME restrictions for all
3509   "message" types regarding line length and encodings.
3512  <list style="hanging" x:indent="12em">
3513    <t hangText="Type name:">
3514      message
3515    </t>
3516    <t hangText="Subtype name:">
3517      http
3518    </t>
3519    <t hangText="Required parameters:">
3520      N/A
3521    </t>
3522    <t hangText="Optional parameters:">
3523      version, msgtype
3524      <list style="hanging">
3525        <t hangText="version:">
3526          The HTTP-version number of the enclosed message
3527          (e.g., "1.1"). If not present, the version can be
3528          determined from the first line of the body.
3529        </t>
3530        <t hangText="msgtype:">
3531          The message type &mdash; "request" or "response". If not
3532          present, the type can be determined from the first
3533          line of the body.
3534        </t>
3535      </list>
3536    </t>
3537    <t hangText="Encoding considerations:">
3538      only "7bit", "8bit", or "binary" are permitted
3539    </t>
3540    <t hangText="Security considerations:">
3541      see <xref target="security.considerations"/>
3542    </t>
3543    <t hangText="Interoperability considerations:">
3544      N/A
3545    </t>
3546    <t hangText="Published specification:">
3547      This specification (see <xref target=""/>).
3548    </t>
3549    <t hangText="Applications that use this media type:">
3550      N/A
3551    </t>
3552    <t hangText="Fragment identifier considerations:">
3553      N/A
3554    </t>
3555    <t hangText="Additional information:">
3556      <list style="hanging">
3557        <t hangText="Magic number(s):">N/A</t>
3558        <t hangText="Deprecated alias names for this type:">N/A</t>
3559        <t hangText="File extension(s):">N/A</t>
3560        <t hangText="Macintosh file type code(s):">N/A</t>
3561      </list>
3562    </t>
3563    <t hangText="Person and email address to contact for further information:">
3564      See Authors Section.
3565    </t>
3566    <t hangText="Intended usage:">
3567      COMMON
3568    </t>
3569    <t hangText="Restrictions on usage:">
3570      N/A
3571    </t>
3572    <t hangText="Author:">
3573      See Authors Section.
3574    </t>
3575    <t hangText="Change controller:">
3576      IESG
3577    </t>
3578  </list>
3581<section title="Internet Media Type application/http" anchor="">
3582<iref item="Media Type" subitem="application/http" primary="true"/>
3583<iref item="application/http Media Type" primary="true"/>
3585   The application/http type can be used to enclose a pipeline of one or more
3586   HTTP request or response messages (not intermixed).
3589  <list style="hanging" x:indent="12em">
3590    <t hangText="Type name:">
3591      application
3592    </t>
3593    <t hangText="Subtype name:">
3594      http
3595    </t>
3596    <t hangText="Required parameters:">
3597      N/A
3598    </t>
3599    <t hangText="Optional parameters:">
3600      version, msgtype
3601      <list style="hanging">
3602        <t hangText="version:">
3603          The HTTP-version number of the enclosed messages
3604          (e.g., "1.1"). If not present, the version can be
3605          determined from the first line of the body.
3606        </t>
3607        <t hangText="msgtype:">
3608          The message type &mdash; "request" or "response". If not
3609          present, the type can be determined from the first
3610          line of the body.
3611        </t>
3612      </list>
3613    </t>
3614    <t hangText="Encoding considerations:">
3615      HTTP messages enclosed by this type
3616      are in "binary" format; use of an appropriate
3617      Content-Transfer-Encoding is required when
3618      transmitted via E-mail.
3619    </t>
3620    <t hangText="Security considerations:">
3621      see <xref target="security.considerations"/>
3622    </t>
3623    <t hangText="Interoperability considerations:">
3624      N/A
3625    </t>
3626    <t hangText="Published specification:">
3627      This specification (see <xref target=""/>).
3628    </t>
3629    <t hangText="Applications that use this media type:">
3630      N/A
3631    </t>
3632    <t hangText="Fragment identifier considerations:">
3633      N/A
3634    </t>
3635    <t hangText="Additional information:">
3636      <list style="hanging">
3637        <t hangText="Deprecated alias names for this type:">N/A</t>
3638        <t hangText="Magic number(s):">N/A</t>
3639        <t hangText="File extension(s):">N/A</t>
3640        <t hangText="Macintosh file type code(s):">N/A</t>
3641      </list>
3642    </t>
3643    <t hangText="Person and email address to contact for further information:">
3644      See Authors Section.
3645    </t>
3646    <t hangText="Intended usage:">
3647      COMMON
3648    </t>
3649    <t hangText="Restrictions on usage:">
3650      N/A
3651    </t>
3652    <t hangText="Author:">
3653      See Authors Section.
3654    </t>
3655    <t hangText="Change controller:">
3656      IESG
3657    </t>
3658  </list>
3663<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3665   The HTTP Transfer Coding Registry defines the name space for transfer
3666   coding names. It is maintained at <eref target=""/>.
3669<section title="Procedure" anchor="transfer.coding.registry.procedure">
3671   Registrations &MUST; include the following fields:
3672   <list style="symbols">
3673     <t>Name</t>
3674     <t>Description</t>
3675     <t>Pointer to specification text</t>
3676   </list>
3679   Names of transfer codings &MUST-NOT; overlap with names of content codings
3680   (&content-codings;) unless the encoding transformation is identical, as
3681   is the case for the compression codings defined in
3682   <xref target="compression.codings"/>.
3685   Values to be added to this name space require IETF Review (see
3686   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3687   conform to the purpose of transfer coding defined in this specification.
3690   Use of program names for the identification of encoding formats
3691   is not desirable and is discouraged for future encodings.
3695<section title="Registration" anchor="transfer.coding.registration">
3697   The HTTP Transfer Coding Registry shall be updated with the registrations
3698   below:
3700<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3701   <ttcol>Name</ttcol>
3702   <ttcol>Description</ttcol>
3703   <ttcol>Reference</ttcol>
3704   <c>chunked</c>
3705   <c>Transfer in a series of chunks</c>
3706   <c>
3707      <xref target="chunked.encoding"/>
3708   </c>
3709   <c>compress</c>
3710   <c>UNIX "compress" data format <xref target="Welch"/></c>
3711   <c>
3712      <xref target="compress.coding"/>
3713   </c>
3714   <c>deflate</c>
3715   <c>"deflate" compressed data (<xref target="RFC1951"/>) inside
3716   the "zlib" data format (<xref target="RFC1950"/>)
3717   </c>
3718   <c>
3719      <xref target="deflate.coding"/>
3720   </c>
3721   <c>gzip</c>
3722   <c>GZIP file format <xref target="RFC1952"/></c>
3723   <c>
3724      <xref target="gzip.coding"/>
3725   </c>
3726   <c>x-compress</c>
3727   <c>Deprecated (alias for compress)</c>
3728   <c>
3729      <xref target="compress.coding"/>
3730   </c>
3731   <c>x-gzip</c>
3732   <c>Deprecated (alias for gzip)</c>
3733   <c>
3734      <xref target="gzip.coding"/>
3735   </c>
3740<section title="Content Coding Registration" anchor="content.coding.registration">
3742   IANA maintains the registry of HTTP Content Codings at
3743   <eref target=""/>.
3746   The HTTP Content Codings Registry shall be updated with the registrations
3747   below:
3749<texttable align="left" suppress-title="true" anchor="iana.content.coding.registration.table">
3750   <ttcol>Name</ttcol>
3751   <ttcol>Description</ttcol>
3752   <ttcol>Reference</ttcol>
3753   <c>compress</c>
3754   <c>UNIX "compress" data format <xref target="Welch"/></c>
3755   <c>
3756      <xref target="compress.coding"/>
3757   </c>
3758   <c>deflate</c>
3759   <c>"deflate" compressed data (<xref target="RFC1951"/>) inside
3760   the "zlib" data format (<xref target="RFC1950"/>)</c>
3761   <c>
3762      <xref target="deflate.coding"/>
3763   </c>
3764   <c>gzip</c>
3765   <c>GZIP file format <xref target="RFC1952"/></c>
3766   <c>
3767      <xref target="gzip.coding"/>
3768   </c>
3769   <c>x-compress</c>
3770   <c>Deprecated (alias for compress)</c>
3771   <c>
3772      <xref target="compress.coding"/>
3773   </c>
3774   <c>x-gzip</c>
3775   <c>Deprecated (alias for gzip)</c>
3776   <c>
3777      <xref target="gzip.coding"/>
3778   </c>
3782<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3784   The HTTP Upgrade Token Registry defines the name space for protocol-name
3785   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3786   field. The registry is maintained at <eref target=""/>.
3789<section title="Procedure" anchor="upgrade.token.registry.procedure">  
3791   Each registered protocol name is associated with contact information
3792   and an optional set of specifications that details how the connection
3793   will be processed after it has been upgraded.
3796   Registrations happen on a "First Come First Served" basis (see
3797   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3798   following rules:
3799  <list style="numbers">
3800    <t>A protocol-name token, once registered, stays registered forever.</t>
3801    <t>The registration &MUST; name a responsible party for the
3802       registration.</t>
3803    <t>The registration &MUST; name a point of contact.</t>
3804    <t>The registration &MAY; name a set of specifications associated with
3805       that token. Such specifications need not be publicly available.</t>
3806    <t>The registration &SHOULD; name a set of expected "protocol-version"
3807       tokens associated with that token at the time of registration.</t>
3808    <t>The responsible party &MAY; change the registration at any time.
3809       The IANA will keep a record of all such changes, and make them
3810       available upon request.</t>
3811    <t>The IESG &MAY; reassign responsibility for a protocol token.
3812       This will normally only be used in the case when a
3813       responsible party cannot be contacted.</t>
3814  </list>
3817   This registration procedure for HTTP Upgrade Tokens replaces that
3818   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3822<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3824   The "HTTP" entry in the HTTP Upgrade Token Registry shall be updated with
3825   the registration below:
3827<texttable align="left" suppress-title="true">
3828   <ttcol>Value</ttcol>
3829   <ttcol>Description</ttcol>
3830   <ttcol>Expected Version Tokens</ttcol>
3831   <ttcol>Reference</ttcol>
3833   <c>HTTP</c>
3834   <c>Hypertext Transfer Protocol</c>
3835   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3836   <c><xref target="http.version"/></c>
3839   The responsible party is: "IETF ( - Internet Engineering Task Force".
3846<section title="Security Considerations" anchor="security.considerations">
3848   This section is meant to inform developers, information providers, and
3849   users of known security concerns relevant to HTTP/1.1 message syntax,
3850   parsing, and routing.
3853   The list of considerations below is not exhaustive &mdash; security
3854   analysis in an ongoing activity. Various organizations, such as the
3855   "Open Web Application Security Project" (OWASP,
3856   <eref target=""/>), provide information about current
3857   research.
3860<section title="DNS-related Attacks" anchor="dns.related.attacks">
3862   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3863   generally prone to security attacks based on the deliberate misassociation
3864   of IP addresses and DNS names not protected by DNSSEC. Clients need to be
3865   cautious in assuming the validity of an IP number/DNS name association unless
3866   the response is protected by DNSSEC (<xref target="RFC4033"/>).
3870<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3872   By their very nature, HTTP intermediaries are men-in-the-middle, and
3873   represent an opportunity for man-in-the-middle attacks. Compromise of
3874   the systems on which the intermediaries run can result in serious security
3875   and privacy problems. Intermediaries have access to security-related
3876   information, personal information about individual users and
3877   organizations, and proprietary information belonging to users and
3878   content providers. A compromised intermediary, or an intermediary
3879   implemented or configured without regard to security and privacy
3880   considerations, might be used in the commission of a wide range of
3881   potential attacks.
3884   Intermediaries that contain a shared cache are especially vulnerable
3885   to cache poisoning attacks.
3888   Implementers need to consider the privacy and security
3889   implications of their design and coding decisions, and of the
3890   configuration options they provide to operators (especially the
3891   default configuration).
3894   Users need to be aware that intermediaries are no more trustworthy than
3895   the people who run them; HTTP itself cannot solve this problem.
3899<section title="Buffer Overflows" anchor="attack.protocol.element.size.overflows">
3901   Because HTTP uses mostly textual, character-delimited fields, attackers can
3902   overflow buffers in implementations, and/or perform a Denial of Service
3903   against implementations that accept fields with unlimited lengths.
3906   To promote interoperability, this specification makes specific
3907   recommendations for minimum size limits on request-line
3908   (<xref target="request.line"/>)
3909   and header fields (<xref target="header.fields"/>). These are
3910   minimum recommendations, chosen to be supportable even by implementations
3911   with limited resources; it is expected that most implementations will
3912   choose substantially higher limits.
3915   This specification also provides a way for servers to reject messages that
3916   have request-targets that are too long (&status-414;) or request entities
3917   that are too large (&status-4xx;). Additional status codes related to
3918   capacity limits have been defined by extensions to HTTP
3919   <xref target="RFC6585"/>.
3922   Recipients ought to carefully limit the extent to which they read other
3923   fields, including (but not limited to) request methods, response status
3924   phrases, header field-names, and body chunks, so as to avoid denial of
3925   service attacks without impeding interoperability.
3929<section title="Message Integrity" anchor="message.integrity">
3931   HTTP does not define a specific mechanism for ensuring message integrity,
3932   instead relying on the error-detection ability of underlying transport
3933   protocols and the use of length or chunk-delimited framing to detect
3934   completeness. Additional integrity mechanisms, such as hash functions or
3935   digital signatures applied to the content, can be selectively added to
3936   messages via extensible metadata header fields. Historically, the lack of
3937   a single integrity mechanism has been justified by the informal nature of
3938   most HTTP communication.  However, the prevalence of HTTP as an information
3939   access mechanism has resulted in its increasing use within environments
3940   where verification of message integrity is crucial.
3943   User agents are encouraged to implement configurable means for detecting
3944   and reporting failures of message integrity such that those means can be
3945   enabled within environments for which integrity is necessary. For example,
3946   a browser being used to view medical history or drug interaction
3947   information needs to indicate to the user when such information is detected
3948   by the protocol to be incomplete, expired, or corrupted during transfer.
3949   Such mechanisms might be selectively enabled via user agent extensions or
3950   the presence of message integrity metadata in a response.
3951   At a minimum, user agents ought to provide some indication that allows a
3952   user to distinguish between a complete and incomplete response message
3953   (<xref target="incomplete.messages"/>) when such verification is desired.
3957<section title="Server Log Information" anchor="abuse.of.server.log.information">
3959   A server is in the position to save personal data about a user's requests
3960   over time, which might identify their reading patterns or subjects of
3961   interest.  In particular, log information gathered at an intermediary
3962   often contains a history of user agent interaction, across a multitude
3963   of sites, that can be traced to individual users.
3966   HTTP log information is confidential in nature; its handling is often
3967   constrained by laws and regulations.  Log information needs to be securely
3968   stored and appropriate guidelines followed for its analysis.
3969   Anonymization of personal information within individual entries helps,
3970   but is generally not sufficient to prevent real log traces from being
3971   re-identified based on correlation with other access characteristics.
3972   As such, access traces that are keyed to a specific client are unsafe to
3973   publish even if the key is pseudonymous.
3976   To minimize the risk of theft or accidental publication, log information
3977   ought to be purged of personally identifiable information, including
3978   user identifiers, IP addresses, and user-provided query parameters,
3979   as soon as that information is no longer necessary to support operational
3980   needs for security, auditing, or fraud control.
3985<section title="Acknowledgments" anchor="acks">
3987   This edition of HTTP/1.1 builds on the many contributions that went into
3988   <xref target="RFC1945" format="none">RFC 1945</xref>,
3989   <xref target="RFC2068" format="none">RFC 2068</xref>,
3990   <xref target="RFC2145" format="none">RFC 2145</xref>, and
3991   <xref target="RFC2616" format="none">RFC 2616</xref>, including
3992   substantial contributions made by the previous authors, editors, and
3993   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
3994   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
3995   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
3998   Since 1999, the following contributors have helped improve the HTTP
3999   specification by reporting bugs, asking smart questions, drafting or
4000   reviewing text, and evaluating open issues:
4002<?BEGININC acks ?>
4003<t>Adam Barth,
4004Adam Roach,
4005Addison Phillips,
4006Adrian Chadd,
4007Adrian Cole,
4008Adrien W. de Croy,
4009Alan Ford,
4010Alan Ruttenberg,
4011Albert Lunde,
4012Alek Storm,
4013Alex Rousskov,
4014Alexandre Morgaut,
4015Alexey Melnikov,
4016Alisha Smith,
4017Amichai Rothman,
4018Amit Klein,
4019Amos Jeffries,
4020Andreas Maier,
4021Andreas Petersson,
4022Andrei Popov,
4023Anil Sharma,
4024Anne van Kesteren,
4025Anthony Bryan,
4026Asbjorn Ulsberg,
4027Ashok Kumar,
4028Balachander Krishnamurthy,
4029Barry Leiba,
4030Ben Laurie,
4031Benjamin Carlyle,
4032Benjamin Niven-Jenkins,
4033Benoit Claise,
4034Bil Corry,
4035Bill Burke,
4036Bjoern Hoehrmann,
4037Bob Scheifler,
4038Boris Zbarsky,
4039Brett Slatkin,
4040Brian Kell,
4041Brian McBarron,
4042Brian Pane,
4043Brian Raymor,
4044Brian Smith,
4045Bruce Perens,
4046Bryce Nesbitt,
4047Cameron Heavon-Jones,
4048Carl Kugler,
4049Carsten Bormann,
4050Charles Fry,
4051Chris Burdess,
4052Chris Newman,
4053Christian Huitema,
4054Cyrus Daboo,
4055Dale Robert Anderson,
4056Dan Wing,
4057Dan Winship,
4058Daniel Stenberg,
4059Darrel Miller,
4060Dave Cridland,
4061Dave Crocker,
4062Dave Kristol,
4063Dave Thaler,
4064David Booth,
4065David Singer,
4066David W. Morris,
4067Diwakar Shetty,
4068Dmitry Kurochkin,
4069Drummond Reed,
4070Duane Wessels,
4071Edward Lee,
4072Eitan Adler,
4073Eliot Lear,
4074Emile Stephan,
4075Eran Hammer-Lahav,
4076Eric D. Williams,
4077Eric J. Bowman,
4078Eric Lawrence,
4079Eric Rescorla,
4080Erik Aronesty,
4081EungJun Yi,
4082Evan Prodromou,
4083Felix Geisendoerfer,
4084Florian Weimer,
4085Frank Ellermann,
4086Fred Akalin,
4087Fred Bohle,
4088Frederic Kayser,
4089Gabor Molnar,
4090Gabriel Montenegro,
4091Geoffrey Sneddon,
4092Gervase Markham,
4093Gili Tzabari,
4094Grahame Grieve,
4095Greg Slepak,
4096Greg Wilkins,
4097Grzegorz Calkowski,
4098Harald Tveit Alvestrand,
4099Harry Halpin,
4100Helge Hess,
4101Henrik Nordstrom,
4102Henry S. Thompson,
4103Henry Story,
4104Herbert van de Sompel,
4105Herve Ruellan,
4106Howard Melman,
4107Hugo Haas,
4108Ian Fette,
4109Ian Hickson,
4110Ido Safruti,
4111Ilari Liusvaara,
4112Ilya Grigorik,
4113Ingo Struck,
4114J. Ross Nicoll,
4115James Cloos,
4116James H. Manger,
4117James Lacey,
4118James M. Snell,
4119Jamie Lokier,
4120Jan Algermissen,
4121Jari Arkko,
4122Jeff Hodges (who came up with the term 'effective Request-URI'),
4123Jeff Pinner,
4124Jeff Walden,
4125Jim Luther,
4126Jitu Padhye,
4127Joe D. Williams,
4128Joe Gregorio,
4129Joe Orton,
4130Joel Jaeggli,
4131John C. Klensin,
4132John C. Mallery,
4133John Cowan,
4134John Kemp,
4135John Panzer,
4136John Schneider,
4137John Stracke,
4138John Sullivan,
4139Jonas Sicking,
4140Jonathan A. Rees,
4141Jonathan Billington,
4142Jonathan Moore,
4143Jonathan Silvera,
4144Jordi Ros,
4145Joris Dobbelsteen,
4146Josh Cohen,
4147Julien Pierre,
4148Jungshik Shin,
4149Justin Chapweske,
4150Justin Erenkrantz,
4151Justin James,
4152Kalvinder Singh,
4153Karl Dubost,
4154Kathleen Moriarty,
4155Keith Hoffman,
4156Keith Moore,
4157Ken Murchison,
4158Koen Holtman,
4159Konstantin Voronkov,
4160Kris Zyp,
4161Leif Hedstrom,
4162Lionel Morand,
4163Lisa Dusseault,
4164Maciej Stachowiak,
4165Manu Sporny,
4166Marc Schneider,
4167Marc Slemko,
4168Mark Baker,
4169Mark Pauley,
4170Mark Watson,
4171Markus Isomaki,
4172Markus Lanthaler,
4173Martin J. Duerst,
4174Martin Musatov,
4175Martin Nilsson,
4176Martin Thomson,
4177Matt Lynch,
4178Matthew Cox,
4179Matthew Kerwin,
4180Max Clark,
4181Menachem Dodge,
4182Meral Shirazipour,
4183Michael Burrows,
4184Michael Hausenblas,
4185Michael Scharf,
4186Michael Sweet,
4187Michael Tuexen,
4188Michael Welzl,
4189Mike Amundsen,
4190Mike Belshe,
4191Mike Bishop,
4192Mike Kelly,
4193Mike Schinkel,
4194Miles Sabin,
4195Murray S. Kucherawy,
4196Mykyta Yevstifeyev,
4197Nathan Rixham,
4198Nicholas Shanks,
4199Nico Williams,
4200Nicolas Alvarez,
4201Nicolas Mailhot,
4202Noah Slater,
4203Osama Mazahir,
4204Pablo Castro,
4205Pat Hayes,
4206Patrick R. McManus,
4207Paul E. Jones,
4208Paul Hoffman,
4209Paul Marquess,
4210Pete Resnick,
4211Peter Lepeska,
4212Peter Occil,
4213Peter Saint-Andre,
4214Peter Watkins,
4215Phil Archer,
4216Phil Hunt,
4217Philippe Mougin,
4218Phillip Hallam-Baker,
4219Piotr Dobrogost,
4220Poul-Henning Kamp,
4221Preethi Natarajan,
4222Rajeev Bector,
4223Ray Polk,
4224Reto Bachmann-Gmuer,
4225Richard Barnes,
4226Richard Cyganiak,
4227Rob Trace,
4228Robby Simpson,
4229Robert Brewer,
4230Robert Collins,
4231Robert Mattson,
4232Robert O'Callahan,
4233Robert Olofsson,
4234Robert Sayre,
4235Robert Siemer,
4236Robert de Wilde,
4237Roberto Javier Godoy,
4238Roberto Peon,
4239Roland Zink,
4240Ronny Widjaja,
4241Ryan Hamilton,
4242S. Mike Dierken,
4243Salvatore Loreto,
4244Sam Johnston,
4245Sam Pullara,
4246Sam Ruby,
4247Saurabh Kulkarni,
4248Scott Lawrence (who maintained the original issues list),
4249Sean B. Palmer,
4250Sean Turner,
4251Sebastien Barnoud,
4252Shane McCarron,
4253Shigeki Ohtsu,
4254Simon Yarde,
4255Stefan Eissing,
4256Stefan Tilkov,
4257Stefanos Harhalakis,
4258Stephane Bortzmeyer,
4259Stephen Farrell,
4260Stephen Kent,
4261Stephen Ludin,
4262Stuart Williams,
4263Subbu Allamaraju,
4264Subramanian Moonesamy,
4265Susan Hares,
4266Sylvain Hellegouarch,
4267Tapan Divekar,
4268Tatsuhiro Tsujikawa,
4269Tatsuya Hayashi,
4270Ted Hardie,
4271Ted Lemon,
4272Thomas Broyer,
4273Thomas Fossati,
4274Thomas Maslen,
4275Thomas Nadeau,
4276Thomas Nordin,
4277Thomas Roessler,
4278Tim Bray,
4279Tim Morgan,
4280Tim Olsen,
4281Tom Zhou,
4282Travis Snoozy,
4283Tyler Close,
4284Vincent Murphy,
4285Wenbo Zhu,
4286Werner Baumann,
4287Wilbur Streett,
4288Wilfredo Sanchez Vega,
4289William A. Rowe Jr.,
4290William Chan,
4291Willy Tarreau,
4292Xiaoshu Wang,
4293Yaron Goland,
4294Yngve Nysaeter Pettersen,
4295Yoav Nir,
4296Yogesh Bang,
4297Yuchung Cheng,
4298Yutaka Oiwa,
4299Yves Lafon (long-time member of the editor team),
4300Zed A. Shaw, and
4301Zhong Yu.
4303<?ENDINC acks ?>
4305   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
4306   acknowledgements from prior revisions.
4313<references title="Normative References">
4315<reference anchor="Part2">
4316  <front>
4317    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
4318    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4319      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4320      <address><email></email></address>
4321    </author>
4322    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4323      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4324      <address><email></email></address>
4325    </author>
4326    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4327  </front>
4328  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
4329  <x:source href="p2-semantics.xml" basename="p2-semantics">
4330    <x:defines>1xx (Informational)</x:defines>
4331    <x:defines>1xx</x:defines>
4332    <x:defines>100 (Continue)</x:defines>
4333    <x:defines>101 (Switching Protocols)</x:defines>
4334    <x:defines>2xx (Successful)</x:defines>
4335    <x:defines>2xx</x:defines>
4336    <x:defines>200 (OK)</x:defines>
4337    <x:defines>203 (Non-Authoritative Information)</x:defines>
4338    <x:defines>204 (No Content)</x:defines>
4339    <x:defines>3xx (Redirection)</x:defines>
4340    <x:defines>3xx</x:defines>
4341    <x:defines>301 (Moved Permanently)</x:defines>
4342    <x:defines>4xx (Client Error)</x:defines>
4343    <x:defines>4xx</x:defines>
4344    <x:defines>400 (Bad Request)</x:defines>
4345    <x:defines>411 (Length Required)</x:defines>
4346    <x:defines>414 (URI Too Long)</x:defines>
4347    <x:defines>417 (Expectation Failed)</x:defines>
4348    <x:defines>426 (Upgrade Required)</x:defines>
4349    <x:defines>501 (Not Implemented)</x:defines>
4350    <x:defines>502 (Bad Gateway)</x:defines>
4351    <x:defines>505 (HTTP Version Not Supported)</x:defines>
4352    <x:defines>Accept-Encoding</x:defines>
4353    <x:defines>Allow</x:defines>
4354    <x:defines>Content-Encoding</x:defines>
4355    <x:defines>Content-Location</x:defines>
4356    <x:defines>Content-Type</x:defines>
4357    <x:defines>Date</x:defines>
4358    <x:defines>Expect</x:defines>
4359    <x:defines>Location</x:defines>
4360    <x:defines>Server</x:defines>
4361    <x:defines>User-Agent</x:defines>
4362  </x:source>
4365<reference anchor="Part4">
4366  <front>
4367    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
4368    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
4369      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4370      <address><email></email></address>
4371    </author>
4372    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
4373      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4374      <address><email></email></address>
4375    </author>
4376    <date month="&ID-MONTH;" year="&ID-YEAR;" />
4377  </front>
4378  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
4379  <x:source basename="p4-conditional" href="p4-conditional.xml">
4380    <x:defines>304 (Not Modified)</x:defines>
4381    <x:defines>ETag</x:defines>
4382    <x:defines>Last-Modified</x:defines>
4383  </x:source>
4386<reference anchor="Part5">
4387  <front>
4388    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
4389    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4390      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4391      <address><email></email></address>
4392    </author>
4393    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
4394      <organization abbrev="W3C">World Wide Web Consortium</organization>
4395      <address><email></email></address>
4396    </author>
4397    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4398      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4399      <address><email></email></address>
4400    </author>
4401    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4402  </front>
4403  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4404  <x:source href="p5-range.xml" basename="p5-range">
4405    <x:defines>Content-Range</x:defines>
4406  </x:source>
4409<reference anchor="Part6">
4410  <front>
4411    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4412    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4413      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4414      <address><email></email></address>
4415    </author>
4416    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4417      <organization>Akamai</organization>
4418      <address><email></email></address>
4419    </author>
4420    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4421      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4422      <address><email></email></address>
4423    </author>
4424    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4425  </front>
4426  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4427  <x:source href="p6-cache.xml" basename="p6-cache">
4428    <x:defines>Cache-Control</x:defines>
4429    <x:defines>Expires</x:defines>
4430  </x:source>
4433<reference anchor="Part7">
4434  <front>
4435    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4436    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4437      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4438      <address><email></email></address>
4439    </author>
4440    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4441      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4442      <address><email></email></address>
4443    </author>
4444    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4445  </front>
4446  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4447  <x:source href="p7-auth.xml" basename="p7-auth">
4448    <x:defines>Proxy-Authenticate</x:defines>
4449    <x:defines>Proxy-Authorization</x:defines>
4450  </x:source>
4453<reference anchor="RFC5234">
4454  <front>
4455    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4456    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4457      <organization>Brandenburg InternetWorking</organization>
4458      <address>
4459        <email></email>
4460      </address> 
4461    </author>
4462    <author initials="P." surname="Overell" fullname="Paul Overell">
4463      <organization>THUS plc.</organization>
4464      <address>
4465        <email></email>
4466      </address>
4467    </author>
4468    <date month="January" year="2008"/>
4469  </front>
4470  <seriesInfo name="STD" value="68"/>
4471  <seriesInfo name="RFC" value="5234"/>
4474<reference anchor="RFC2119">
4475  <front>
4476    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4477    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4478      <organization>Harvard University</organization>
4479      <address><email></email></address>
4480    </author>
4481    <date month="March" year="1997"/>
4482  </front>
4483  <seriesInfo name="BCP" value="14"/>
4484  <seriesInfo name="RFC" value="2119"/>
4487<reference anchor="RFC3986">
4488 <front>
4489  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4490  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4491    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4492    <address>
4493       <email></email>
4494       <uri></uri>
4495    </address>
4496  </author>
4497  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4498    <organization abbrev="Day Software">Day Software</organization>
4499    <address>
4500      <email></email>
4501      <uri></uri>
4502    </address>
4503  </author>
4504  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4505    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4506    <address>
4507      <email></email>
4508      <uri></uri>
4509    </address>
4510  </author>
4511  <date month='January' year='2005'></date>
4512 </front>
4513 <seriesInfo name="STD" value="66"/>
4514 <seriesInfo name="RFC" value="3986"/>
4517<reference anchor="RFC0793">
4518  <front>
4519    <title>Transmission Control Protocol</title>
4520    <author initials='J.' surname='Postel' fullname='Jon Postel'>
4521      <organization>University of Southern California (USC)/Information Sciences Institute</organization>
4522    </author>
4523    <date year='1981' month='September' />
4524  </front>
4525  <seriesInfo name='STD' value='7' />
4526  <seriesInfo name='RFC' value='793' />
4529<reference anchor="USASCII">
4530  <front>
4531    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4532    <author>
4533      <organization>American National Standards Institute</organization>
4534    </author>
4535    <date year="1986"/>
4536  </front>
4537  <seriesInfo name="ANSI" value="X3.4"/>
4540<reference anchor="RFC1950">
4541  <front>
4542    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4543    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4544      <organization>Aladdin Enterprises</organization>
4545      <address><email></email></address>
4546    </author>
4547    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4548    <date month="May" year="1996"/>
4549  </front>
4550  <seriesInfo name="RFC" value="1950"/>
4551  <!--<annotation>
4552    RFC 1950 is an Informational RFC, thus it might be less stable than
4553    this specification. On the other hand, this downward reference was
4554    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4555    therefore it is unlikely to cause problems in practice. See also
4556    <xref target="BCP97"/>.
4557  </annotation>-->
4560<reference anchor="RFC1951">
4561  <front>
4562    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4563    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4564      <organization>Aladdin Enterprises</organization>
4565      <address><email></email></address>
4566    </author>
4567    <date month="May" year="1996"/>
4568  </front>
4569  <seriesInfo name="RFC" value="1951"/>
4570  <!--<annotation>
4571    RFC 1951 is an Informational RFC, thus it might be less stable than
4572    this specification. On the other hand, this downward reference was
4573    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4574    therefore it is unlikely to cause problems in practice. See also
4575    <xref target="BCP97"/>.
4576  </annotation>-->
4579<reference anchor="RFC1952">
4580  <front>
4581    <title>GZIP file format specification version 4.3</title>
4582    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4583      <organization>Aladdin Enterprises</organization>
4584      <address><email></email></address>
4585    </author>
4586    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4587      <address><email></email></address>
4588    </author>
4589    <author initials="M." surname="Adler" fullname="Mark Adler">
4590      <address><email></email></address>
4591    </author>
4592    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4593      <address><email></email></address>
4594    </author>
4595    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4596      <address><email></email></address>
4597    </author>
4598    <date month="May" year="1996"/>
4599  </front>
4600  <seriesInfo name="RFC" value="1952"/>
4601  <!--<annotation>
4602    RFC 1952 is an Informational RFC, thus it might be less stable than
4603    this specification. On the other hand, this downward reference was
4604    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4605    therefore it is unlikely to cause problems in practice. See also
4606    <xref target="BCP97"/>.
4607  </annotation>-->
4610<reference anchor="Welch">
4611  <front>
4612    <title>A Technique for High Performance Data Compression</title>
4613    <author initials="T.A." surname="Welch" fullname="Terry A. Welch"/>
4614    <date month="June" year="1984"/>
4615  </front>
4616  <seriesInfo name="IEEE Computer" value="17(6)"/>
4621<references title="Informative References">
4623<reference anchor="ISO-8859-1">
4624  <front>
4625    <title>
4626     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4627    </title>
4628    <author>
4629      <organization>International Organization for Standardization</organization>
4630    </author>
4631    <date year="1998"/>
4632  </front>
4633  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4636<reference anchor='RFC1919'>
4637  <front>
4638    <title>Classical versus Transparent IP Proxies</title>
4639    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4640      <address><email></email></address>
4641    </author>
4642    <date year='1996' month='March' />
4643  </front>
4644  <seriesInfo name='RFC' value='1919' />
4647<reference anchor="RFC1945">
4648  <front>
4649    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4650    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4651      <organization>MIT, Laboratory for Computer Science</organization>
4652      <address><email></email></address>
4653    </author>
4654    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4655      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4656      <address><email></email></address>
4657    </author>
4658    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4659      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4660      <address><email></email></address>
4661    </author>
4662    <date month="May" year="1996"/>
4663  </front>
4664  <seriesInfo name="RFC" value="1945"/>
4667<reference anchor="RFC2045">
4668  <front>
4669    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4670    <author initials="N." surname="Freed" fullname="Ned Freed">
4671      <organization>Innosoft International, Inc.</organization>
4672      <address><email></email></address>
4673    </author>
4674    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4675      <organization>First Virtual Holdings</organization>
4676      <address><email></email></address>
4677    </author>
4678    <date month="November" year="1996"/>
4679  </front>
4680  <seriesInfo name="RFC" value="2045"/>
4683<reference anchor="RFC2047">
4684  <front>
4685    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4686    <author initials="K." surname="Moore" fullname="Keith Moore">
4687      <organization>University of Tennessee</organization>
4688      <address><email></email></address>
4689    </author>
4690    <date month="November" year="1996"/>
4691  </front>
4692  <seriesInfo name="RFC" value="2047"/>
4695<reference anchor="RFC2068">
4696  <front>
4697    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4698    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
4699      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4700      <address><email></email></address>
4701    </author>
4702    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4703      <organization>MIT Laboratory for Computer Science</organization>
4704      <address><email></email></address>
4705    </author>
4706    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4707      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4708      <address><email></email></address>
4709    </author>
4710    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4711      <organization>MIT Laboratory for Computer Science</organization>
4712      <address><email></email></address>
4713    </author>
4714    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4715      <organization>MIT Laboratory for Computer Science</organization>
4716      <address><email></email></address>
4717    </author>
4718    <date month="January" year="1997"/>
4719  </front>
4720  <seriesInfo name="RFC" value="2068"/>
4723<reference anchor="RFC2145">
4724  <front>
4725    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4726    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4727      <organization>Western Research Laboratory</organization>
4728      <address><email></email></address>
4729    </author>
4730    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4731      <organization>Department of Information and Computer Science</organization>
4732      <address><email></email></address>
4733    </author>
4734    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4735      <organization>MIT Laboratory for Computer Science</organization>
4736      <address><email></email></address>
4737    </author>
4738    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4739      <organization>W3 Consortium</organization>
4740      <address><email></email></address>
4741    </author>
4742    <date month="May" year="1997"/>
4743  </front>
4744  <seriesInfo name="RFC" value="2145"/>
4747<reference anchor="RFC2616">
4748  <front>
4749    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4750    <author initials="R." surname="Fielding" fullname="R. Fielding">
4751      <organization>University of California, Irvine</organization>
4752      <address><email></email></address>
4753    </author>
4754    <author initials="J." surname="Gettys" fullname="J. Gettys">
4755      <organization>W3C</organization>
4756      <address><email></email></address>
4757    </author>
4758    <author initials="J." surname="Mogul" fullname="J. Mogul">
4759      <organization>Compaq Computer Corporation</organization>
4760      <address><email></email></address>
4761    </author>
4762    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4763      <organization>MIT Laboratory for Computer Science</organization>
4764      <address><email></email></address>
4765    </author>
4766    <author initials="L." surname="Masinter" fullname="L. Masinter">
4767      <organization>Xerox Corporation</organization>
4768      <address><email></email></address>
4769    </author>
4770    <author initials="P." surname="Leach" fullname="P. Leach">
4771      <organization>Microsoft Corporation</organization>
4772      <address><email></email></address>
4773    </author>
4774    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4775      <organization>W3C</organization>
4776      <address><email></email></address>
4777    </author>
4778    <date month="June" year="1999"/>
4779  </front>
4780  <seriesInfo name="RFC" value="2616"/>
4783<reference anchor='RFC2817'>
4784  <front>
4785    <title>Upgrading to TLS Within HTTP/1.1</title>
4786    <author initials='R.' surname='Khare' fullname='R. Khare'>
4787      <organization>4K Associates / UC Irvine</organization>
4788      <address><email></email></address>
4789    </author>
4790    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4791      <organization>Agranat Systems, Inc.</organization>
4792      <address><email></email></address>
4793    </author>
4794    <date year='2000' month='May' />
4795  </front>
4796  <seriesInfo name='RFC' value='2817' />
4799<reference anchor='RFC2818'>
4800  <front>
4801    <title>HTTP Over TLS</title>
4802    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4803      <organization>RTFM, Inc.</organization>
4804      <address><email></email></address>
4805    </author>
4806    <date year='2000' month='May' />
4807  </front>
4808  <seriesInfo name='RFC' value='2818' />
4811<reference anchor='RFC3040'>
4812  <front>
4813    <title>Internet Web Replication and Caching Taxonomy</title>
4814    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4815      <organization>Equinix, Inc.</organization>
4816    </author>
4817    <author initials='I.' surname='Melve' fullname='I. Melve'>
4818      <organization>UNINETT</organization>
4819    </author>
4820    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4821      <organization>CacheFlow Inc.</organization>
4822    </author>
4823    <date year='2001' month='January' />
4824  </front>
4825  <seriesInfo name='RFC' value='3040' />
4828<reference anchor='BCP90'>
4829  <front>
4830    <title>Registration Procedures for Message Header Fields</title>
4831    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4832      <organization>Nine by Nine</organization>
4833      <address><email></email></address>
4834    </author>
4835    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4836      <organization>BEA Systems</organization>
4837      <address><email></email></address>
4838    </author>
4839    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4840      <organization>HP Labs</organization>
4841      <address><email></email></address>
4842    </author>
4843    <date year='2004' month='September' />
4844  </front>
4845  <seriesInfo name='BCP' value='90' />
4846  <seriesInfo name='RFC' value='3864' />
4849<reference anchor='RFC4033'>
4850  <front>
4851    <title>DNS Security Introduction and Requirements</title>
4852    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4853    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4854    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4855    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4856    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4857    <date year='2005' month='March' />
4858  </front>
4859  <seriesInfo name='RFC' value='4033' />
4862<reference anchor="BCP13">
4863  <front>
4864    <title>Media Type Specifications and Registration Procedures</title>
4865    <author initials="N." surname="Freed" fullname="Ned Freed">
4866      <organization>Oracle</organization>
4867      <address>
4868        <email></email>
4869      </address>
4870    </author>
4871    <author initials="J." surname="Klensin" fullname="John C. Klensin">
4872      <address>
4873        <email></email>
4874      </address>
4875    </author>
4876    <author initials="T." surname="Hansen" fullname="Tony Hansen">
4877      <organization>AT&amp;T Laboratories</organization>
4878      <address>
4879        <email></email>
4880      </address>
4881    </author>
4882    <date year="2013" month="January"/>
4883  </front>
4884  <seriesInfo name="BCP" value="13"/>
4885  <seriesInfo name="RFC" value="6838"/>
4888<reference anchor='BCP115'>
4889  <front>
4890    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4891    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4892      <organization>AT&amp;T Laboratories</organization>
4893      <address>
4894        <email></email>
4895      </address>
4896    </author>
4897    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4898      <organization>Qualcomm, Inc.</organization>
4899      <address>
4900        <email></email>
4901      </address>
4902    </author>
4903    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4904      <organization>Adobe Systems</organization>
4905      <address>
4906        <email></email>
4907      </address>
4908    </author>
4909    <date year='2006' month='February' />
4910  </front>
4911  <seriesInfo name='BCP' value='115' />
4912  <seriesInfo name='RFC' value='4395' />
4915<reference anchor='RFC4559'>
4916  <front>
4917    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4918    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4919    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4920    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4921    <date year='2006' month='June' />
4922  </front>
4923  <seriesInfo name='RFC' value='4559' />
4926<reference anchor='RFC5226'>
4927  <front>
4928    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4929    <author initials='T.' surname='Narten' fullname='T. Narten'>
4930      <organization>IBM</organization>
4931      <address><email></email></address>
4932    </author>
4933    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4934      <organization>Google</organization>
4935      <address><email></email></address>
4936    </author>
4937    <date year='2008' month='May' />
4938  </front>
4939  <seriesInfo name='BCP' value='26' />
4940  <seriesInfo name='RFC' value='5226' />
4943<reference anchor='RFC5246'>
4944   <front>
4945      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4946      <author initials='T.' surname='Dierks' fullname='T. Dierks'/>
4947      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4948         <organization>RTFM, Inc.</organization>
4949      </author>
4950      <date year='2008' month='August' />
4951   </front>
4952   <seriesInfo name='RFC' value='5246' />
4955<reference anchor="RFC5322">
4956  <front>
4957    <title>Internet Message Format</title>
4958    <author initials="P." surname="Resnick" fullname="P. Resnick">
4959      <organization>Qualcomm Incorporated</organization>
4960    </author>
4961    <date year="2008" month="October"/>
4962  </front>
4963  <seriesInfo name="RFC" value="5322"/>
4966<reference anchor="RFC6265">
4967  <front>
4968    <title>HTTP State Management Mechanism</title>
4969    <author initials="A." surname="Barth" fullname="Adam Barth">
4970      <organization abbrev="U.C. Berkeley">
4971        University of California, Berkeley
4972      </organization>
4973      <address><email></email></address>
4974    </author>
4975    <date year="2011" month="April" />
4976  </front>
4977  <seriesInfo name="RFC" value="6265"/>
4980<reference anchor='RFC6585'>
4981  <front>
4982    <title>Additional HTTP Status Codes</title>
4983    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4984      <organization>Rackspace</organization>
4985    </author>
4986    <author initials='R.' surname='Fielding' fullname='R. Fielding'>
4987      <organization>Adobe</organization>
4988    </author>
4989    <date year='2012' month='April' />
4990   </front>
4991   <seriesInfo name='RFC' value='6585' />
4994<!--<reference anchor='BCP97'>
4995  <front>
4996    <title>Handling Normative References to Standards-Track Documents</title>
4997    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
4998      <address>
4999        <email></email>
5000      </address>
5001    </author>
5002    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
5003      <organization>MIT</organization>
5004      <address>
5005        <email></email>
5006      </address>
5007    </author>
5008    <date year='2007' month='June' />
5009  </front>
5010  <seriesInfo name='BCP' value='97' />
5011  <seriesInfo name='RFC' value='4897' />
5014<reference anchor="Kri2001" target="">
5015  <front>
5016    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
5017    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
5018    <date year="2001" month="November"/>
5019  </front>
5020  <seriesInfo name="ACM Transactions on Internet Technology" value="1(2)"/>
5026<section title="HTTP Version History" anchor="compatibility">
5028   HTTP has been in use since 1990. The first version, later referred to as
5029   HTTP/0.9, was a simple protocol for hypertext data transfer across the
5030   Internet, using only a single request method (GET) and no metadata.
5031   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
5032   methods and MIME-like messaging, allowing for metadata to be transferred
5033   and modifiers placed on the request/response semantics. However,
5034   HTTP/1.0 did not sufficiently take into consideration the effects of
5035   hierarchical proxies, caching, the need for persistent connections, or
5036   name-based virtual hosts. The proliferation of incompletely-implemented
5037   applications calling themselves "HTTP/1.0" further necessitated a
5038   protocol version change in order for two communicating applications
5039   to determine each other's true capabilities.
5042   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
5043   requirements that enable reliable implementations, adding only
5044   those features that can either be safely ignored by an HTTP/1.0
5045   recipient or only sent when communicating with a party advertising
5046   conformance with HTTP/1.1.
5049   HTTP/1.1 has been designed to make supporting previous versions easy.
5050   A general-purpose HTTP/1.1 server ought to be able to understand any valid
5051   request in the format of HTTP/1.0, responding appropriately with an
5052   HTTP/1.1 message that only uses features understood (or safely ignored) by
5053   HTTP/1.0 clients. Likewise, an HTTP/1.1 client can be expected to
5054   understand any valid HTTP/1.0 response.
5057   Since HTTP/0.9 did not support header fields in a request, there is no
5058   mechanism for it to support name-based virtual hosts (selection of resource
5059   by inspection of the <x:ref>Host</x:ref> header field).
5060   Any server that implements name-based virtual hosts ought to disable
5061   support for HTTP/0.9. Most requests that appear to be HTTP/0.9 are, in
5062   fact, badly constructed HTTP/1.x requests caused by a client failing to
5063   properly encode the request-target.
5066<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
5068   This section summarizes major differences between versions HTTP/1.0
5069   and HTTP/1.1.
5072<section title="Multi-homed Web Servers" anchor="">
5074   The requirements that clients and servers support the <x:ref>Host</x:ref>
5075   header field (<xref target=""/>), report an error if it is
5076   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
5077   are among the most important changes defined by HTTP/1.1.
5080   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
5081   addresses and servers; there was no other established mechanism for
5082   distinguishing the intended server of a request than the IP address
5083   to which that request was directed. The <x:ref>Host</x:ref> header field was
5084   introduced during the development of HTTP/1.1 and, though it was
5085   quickly implemented by most HTTP/1.0 browsers, additional requirements
5086   were placed on all HTTP/1.1 requests in order to ensure complete
5087   adoption.  At the time of this writing, most HTTP-based services
5088   are dependent upon the Host header field for targeting requests.
5092<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
5094   In HTTP/1.0, each connection is established by the client prior to the
5095   request and closed by the server after sending the response. However, some
5096   implementations implement the explicitly negotiated ("Keep-Alive") version
5097   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
5098   target="RFC2068"/>.
5101   Some clients and servers might wish to be compatible with these previous
5102   approaches to persistent connections, by explicitly negotiating for them
5103   with a "Connection: keep-alive" request header field. However, some
5104   experimental implementations of HTTP/1.0 persistent connections are faulty;
5105   for example, if an HTTP/1.0 proxy server doesn't understand
5106   <x:ref>Connection</x:ref>, it will erroneously forward that header field
5107   to the next inbound server, which would result in a hung connection.
5110   One attempted solution was the introduction of a Proxy-Connection header
5111   field, targeted specifically at proxies. In practice, this was also
5112   unworkable, because proxies are often deployed in multiple layers, bringing
5113   about the same problem discussed above.
5116   As a result, clients are encouraged not to send the Proxy-Connection header
5117   field in any requests.
5120   Clients are also encouraged to consider the use of Connection: keep-alive
5121   in requests carefully; while they can enable persistent connections with
5122   HTTP/1.0 servers, clients using them will need to monitor the
5123   connection for "hung" requests (which indicate that the client ought stop
5124   sending the header field), and this mechanism ought not be used by clients
5125   at all when a proxy is being used.
5129<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
5131   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
5132   (<xref target="header.transfer-encoding"/>).
5133   Transfer codings need to be decoded prior to forwarding an HTTP message
5134   over a MIME-compliant protocol.
5140<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
5142  HTTP's approach to error handling has been explained.
5143  (<xref target="conformance" />)
5146  The HTTP-version ABNF production has been clarified to be case-sensitive.
5147  Additionally, version numbers has been restricted to single digits, due
5148  to the fact that implementations are known to handle multi-digit version
5149  numbers incorrectly.
5150  (<xref target="http.version"/>)
5153  Userinfo (i.e., username and password) are now disallowed in HTTP and
5154  HTTPS URIs, because of security issues related to their transmission on the
5155  wire.
5156  (<xref target="http.uri" />)
5159  The HTTPS URI scheme is now defined by this specification; previously,
5160  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
5161  Furthermore, it implies end-to-end security.
5162  (<xref target="https.uri"/>)
5165  HTTP messages can be (and often are) buffered by implementations; despite
5166  it sometimes being available as a stream, HTTP is fundamentally a
5167  message-oriented protocol.
5168  Minimum supported sizes for various protocol elements have been
5169  suggested, to improve interoperability.
5170  (<xref target="http.message" />)
5173  Invalid whitespace around field-names is now required to be rejected,
5174  because accepting it represents a security vulnerability.
5175  The ABNF productions defining header fields now only list the field value.
5176  (<xref target="header.fields"/>)
5179  Rules about implicit linear whitespace between certain grammar productions
5180  have been removed; now whitespace is only allowed where specifically
5181  defined in the ABNF.
5182  (<xref target="whitespace"/>)
5185  Header fields that span multiple lines ("line folding") are deprecated.
5186  (<xref target="field.parsing" />)
5189  The NUL octet is no longer allowed in comment and quoted-string text, and
5190  handling of backslash-escaping in them has been clarified.
5191  The quoted-pair rule no longer allows escaping control characters other than
5192  HTAB.
5193  Non-ASCII content in header fields and the reason phrase has been obsoleted
5194  and made opaque (the TEXT rule was removed).
5195  (<xref target="field.components"/>)
5198  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
5199  handled as errors by recipients.
5200  (<xref target="header.content-length"/>)
5203  The algorithm for determining the message body length has been clarified
5204  to indicate all of the special cases (e.g., driven by methods or status
5205  codes) that affect it, and that new protocol elements cannot define such
5206  special cases.
5207  CONNECT is a new, special case in determining message body length.
5208  "multipart/byteranges" is no longer a way of determining message body length
5209  detection.
5210  (<xref target="message.body.length"/>)
5213  The "identity" transfer coding token has been removed.
5214  (Sections <xref format="counter" target="message.body"/> and
5215  <xref format="counter" target="transfer.codings"/>)
5218  Chunk length does not include the count of the octets in the
5219  chunk header and trailer.
5220  Line folding in chunk extensions is  disallowed.
5221  (<xref target="chunked.encoding"/>)
5224  The meaning of the "deflate" content coding has been clarified.
5225  (<xref target="deflate.coding" />)
5228  The segment + query components of RFC 3986 have been used to define the
5229  request-target, instead of abs_path from RFC 1808.
5230  The asterisk-form of the request-target is only allowed with the OPTIONS
5231  method.
5232  (<xref target="request-target"/>)
5235  The term "Effective Request URI" has been introduced.
5236  (<xref target="effective.request.uri" />)
5239  Gateways do not need to generate <x:ref>Via</x:ref> header fields anymore.
5240  (<xref target="header.via"/>)
5243  Exactly when "close" connection options have to be sent has been clarified.
5244  Also, "hop-by-hop" header fields are required to appear in the Connection header
5245  field; just because they're defined as hop-by-hop in this specification
5246  doesn't exempt them.
5247  (<xref target="header.connection"/>)
5250  The limit of two connections per server has been removed.
5251  An idempotent sequence of requests is no longer required to be retried.
5252  The requirement to retry requests under certain circumstances when the
5253  server prematurely closes the connection has been removed.
5254  Also, some extraneous requirements about when servers are allowed to close
5255  connections prematurely have been removed.
5256  (<xref target="persistent.connections"/>)
5259  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
5260  responses other than 101 (this was incorporated from <xref
5261  target="RFC2817"/>). Furthermore, the ordering in the field value is now
5262  significant.
5263  (<xref target="header.upgrade"/>)
5266  Empty list elements in list productions (e.g., a list header field containing
5267  ", ,") have been deprecated.
5268  (<xref target="abnf.extension"/>)
5271  Registration of Transfer Codings now requires IETF Review
5272  (<xref target="transfer.coding.registry"/>)
5275  This specification now defines the Upgrade Token Registry, previously
5276  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
5277  (<xref target="upgrade.token.registry"/>)
5280  The expectation to support HTTP/0.9 requests has been removed.
5281  (<xref target="compatibility"/>)
5284  Issues with the Keep-Alive and Proxy-Connection header fields in requests
5285  are pointed out, with use of the latter being discouraged altogether.
5286  (<xref target="compatibility.with.http.1.0.persistent.connections" />)
5291<?BEGININC p1-messaging.abnf-appendix ?>
5292<section xmlns:x="" title="Collected ABNF" anchor="collected.abnf">
5294<artwork type="abnf" name="p1-messaging.parsed-abnf">
5295<x:ref>BWS</x:ref> = OWS
5297<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
5298 connection-option ] )
5299<x:ref>Content-Length</x:ref> = 1*DIGIT
5301<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
5302 ]
5303<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
5304<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
5305<x:ref>Host</x:ref> = uri-host [ ":" port ]
5307<x:ref>OWS</x:ref> = *( SP / HTAB )
5309<x:ref>RWS</x:ref> = 1*( SP / HTAB )
5311<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
5312<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
5313<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
5314 transfer-coding ] )
5316<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
5317<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
5319<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
5320 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
5321 comment ] ) ] )
5323<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
5324<x:ref>absolute-form</x:ref> = absolute-URI
5325<x:ref>absolute-path</x:ref> = 1*( "/" segment )
5326<x:ref>asterisk-form</x:ref> = "*"
5327<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
5328<x:ref>authority-form</x:ref> = authority
5330<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
5331<x:ref>chunk-data</x:ref> = 1*OCTET
5332<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
5333<x:ref>chunk-ext-name</x:ref> = token
5334<x:ref>chunk-ext-val</x:ref> = token / quoted-string
5335<x:ref>chunk-size</x:ref> = 1*HEXDIG
5336<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
5337<x:ref>comment</x:ref> = "(" *( ctext / quoted-pair / comment ) ")"
5338<x:ref>connection-option</x:ref> = token
5339<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
5340 / %x2A-5B ; '*'-'['
5341 / %x5D-7E ; ']'-'~'
5342 / obs-text
5344<x:ref>field-content</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5345<x:ref>field-name</x:ref> = token
5346<x:ref>field-value</x:ref> = *( field-content / obs-fold )
5347<x:ref>fragment</x:ref> = &lt;fragment, defined in [RFC3986], Section 3.5&gt;
5349<x:ref>header-field</x:ref> = field-name ":" OWS field-value OWS
5350<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ] [ "#"
5351 fragment ]
5352<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ] [ "#"
5353 fragment ]
5355<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
5357<x:ref>message-body</x:ref> = *OCTET
5358<x:ref>method</x:ref> = token
5360<x:ref>obs-fold</x:ref> = CRLF ( SP / HTAB )
5361<x:ref>obs-text</x:ref> = %x80-FF
5362<x:ref>origin-form</x:ref> = absolute-path [ "?" query ]
5364<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
5365<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
5366<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
5367<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
5368<x:ref>protocol-name</x:ref> = token
5369<x:ref>protocol-version</x:ref> = token
5370<x:ref>pseudonym</x:ref> = token
5372<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5373 / %x5D-7E ; ']'-'~'
5374 / obs-text
5375<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5376<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5377<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5379<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5380<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5381<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5382<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5383<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5384<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5385<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5386 asterisk-form
5388<x:ref>segment</x:ref> = &lt;segment, defined in [RFC3986], Section 3.3&gt;
5389<x:ref>start-line</x:ref> = request-line / status-line
5390<x:ref>status-code</x:ref> = 3DIGIT
5391<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5393<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5394<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5395<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5396 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5397<x:ref>token</x:ref> = 1*tchar
5398<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5399<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5400 transfer-extension
5401<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5402<x:ref>transfer-parameter</x:ref> = token BWS "=" BWS ( token / quoted-string )
5404<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5408<?ENDINC p1-messaging.abnf-appendix ?>
5410<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5412<section title="Since RFC 2616">
5414  Changes up to the IETF Last Call draft are summarized
5415  in <eref target=""/>.
5419<section title="Since draft-ietf-httpbis-p1-messaging-24" anchor="changes.since.24">
5421  Closed issues:
5422  <list style="symbols">
5423    <t>
5424      <eref target=""/>:
5425      "APPSDIR review of draft-ietf-httpbis-p1-messaging-24"
5426    </t>
5427    <t>
5428      <eref target=""/>:
5429      "integer value parsing"
5430    </t>
5431    <t>
5432      <eref target=""/>:
5433      "move IANA registrations to correct draft"
5434    </t>
5435  </list>
5439<section title="Since draft-ietf-httpbis-p1-messaging-25" anchor="changes.since.25">
5441  Closed issues:
5442  <list style="symbols">
5443    <t>
5444      <eref target=""/>:
5445      "check media type registration templates"
5446    </t>
5447    <t>
5448      <eref target=""/>:
5449      "Redundant rule quoted-str-nf"
5450    </t>
5451    <t>
5452      <eref target=""/>:
5453      "add 'stateless' to Abstract"
5454    </t>
5455    <t>
5456      <eref target=""/>:
5457      "clarify ABNF layering"
5458    </t>
5459    <t>
5460      <eref target=""/>:
5461      "use of 'word' ABNF production"
5462    </t>
5463    <t>
5464      <eref target=""/>:
5465      "improve introduction of list rule"
5466    </t>
5467    <t>
5468      <eref target=""/>:
5469      "moving 2616/2068/2145 to historic"
5470    </t>
5471    <t>
5472      <eref target=""/>:
5473      "augment security considerations with pointers to current research"
5474    </t>
5475  </list>
5478  Partly resolved issues:
5479  <list style="symbols">
5480    <t>
5481      <eref target=""/>:
5482      "IESG ballot on draft-ietf-httpbis-p1-messaging-25"
5483    </t>
5484  </list>
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