source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2591

Last change on this file since 2591 was 2591, checked in by fielding@…, 8 years ago

(editorial) remove poorly worded requirements on persistent connections that are redundant to a carefully worded requirement above them; see #531

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 237.5 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 request-header-fields  "<xref target='Part2' x:rel='#request.header.fields' xmlns:x=''/>">
53  <!ENTITY response-control-data  "<xref target='Part2' x:rel='' xmlns:x=''/>">
54  <!ENTITY resource               "<xref target='Part2' x:rel='#resources' xmlns:x=''/>">
55  <!ENTITY semantics              "<xref target='Part2' xmlns:x=''/>">
56  <!ENTITY status-codes           "<xref target='Part2' x:rel='' xmlns:x=''/>">
57  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x=''/>">
58  <!ENTITY status-203             "<xref target='Part2' x:rel='#status.203' xmlns:x=''/>">
59  <!ENTITY status-3xx             "<xref target='Part2' x:rel='#status.3xx' xmlns:x=''/>">
60  <!ENTITY status-304             "<xref target='Part4' x:rel='#status.304' xmlns:x=''/>">
61  <!ENTITY status-4xx             "<xref target='Part2' x:rel='#status.4xx' xmlns:x=''/>">
62  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x=''/>">
63  <!ENTITY iana-header-registry   "<xref target='Part2' x:rel='#header.field.registry' xmlns:x=''/>">
65<?rfc toc="yes" ?>
66<?rfc symrefs="yes" ?>
67<?rfc sortrefs="yes" ?>
68<?rfc compact="yes"?>
69<?rfc subcompact="no" ?>
70<?rfc linkmailto="no" ?>
71<?rfc editing="no" ?>
72<?rfc comments="yes"?>
73<?rfc inline="yes"?>
74<?rfc rfcedstyle="yes"?>
75<?rfc-ext allow-markup-in-artwork="yes" ?>
76<?rfc-ext include-references-in-index="yes" ?>
77<rfc obsoletes="2145,2616" updates="2817,2818" category="std" x:maturity-level="proposed"
78     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
79     xmlns:x=''>
80<x:link rel="next" basename="p2-semantics"/>
81<x:feedback template="{docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
84  <title abbrev="HTTP/1.1 Message Syntax and Routing">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
86  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
87    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
88    <address>
89      <postal>
90        <street>345 Park Ave</street>
91        <city>San Jose</city>
92        <region>CA</region>
93        <code>95110</code>
94        <country>USA</country>
95      </postal>
96      <email></email>
97      <uri></uri>
98    </address>
99  </author>
101  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
102    <organization abbrev="greenbytes">greenbytes GmbH</organization>
103    <address>
104      <postal>
105        <street>Hafenweg 16</street>
106        <city>Muenster</city><region>NW</region><code>48155</code>
107        <country>Germany</country>
108      </postal>
109      <email></email>
110      <uri></uri>
111    </address>
112  </author>
114  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
115  <workgroup>HTTPbis Working Group</workgroup>
119   The Hypertext Transfer Protocol (HTTP) is a stateless application-level
120   protocol for distributed, collaborative, hypertext information systems.
121   This document provides an overview of HTTP architecture and its associated
122   terminology, defines the "http" and "https" Uniform Resource Identifier
123   (URI) schemes, defines the HTTP/1.1 message syntax and parsing
124   requirements, and describes related security concerns for implementations.
128<note title="Editorial Note (To be removed by RFC Editor)">
129  <t>
130    Discussion of this draft takes place on the HTTPBIS working group
131    mailing list (, which is archived at
132    <eref target=""/>.
133  </t>
134  <t>
135    The current issues list is at
136    <eref target=""/> and related
137    documents (including fancy diffs) can be found at
138    <eref target=""/>.
139  </t>
140  <t>
141    The changes in this draft are summarized in <xref target="changes.since.25"/>.
142  </t>
146<section title="Introduction" anchor="introduction">
148   The Hypertext Transfer Protocol (HTTP) is a stateless application-level
149   request/response protocol that uses extensible semantics and
150   self-descriptive message payloads for flexible interaction with
151   network-based hypertext information systems. This document is the first in
152   a series of documents that collectively form the HTTP/1.1 specification:
153   <list style="empty">
154    <t>RFC xxx1: Message Syntax and Routing</t>
155    <t><xref target="Part2" x:fmt="none">RFC xxx2</xref>: Semantics and Content</t>
156    <t><xref target="Part4" x:fmt="none">RFC xxx3</xref>: Conditional Requests</t>
157    <t><xref target="Part5" x:fmt="none">RFC xxx4</xref>: Range Requests</t>
158    <t><xref target="Part6" x:fmt="none">RFC xxx5</xref>: Caching</t>
159    <t><xref target="Part7" x:fmt="none">RFC xxx6</xref>: Authentication</t>
160   </list>
163   This HTTP/1.1 specification obsoletes
164   <xref target="RFC2616" x:fmt="none">RFC 2616</xref> and
165   <xref target="RFC2145" x:fmt="none">RFC 2145</xref> (on HTTP versioning).
166   This specification also updates the use of CONNECT to establish a tunnel,
167   previously defined in <xref target="RFC2817" x:fmt="none">RFC 2817</xref>,
168   and defines the "https" URI scheme that was described informally in
169   <xref target="RFC2818" x:fmt="none">RFC 2818</xref>.
172   HTTP is a generic interface protocol for information systems. It is
173   designed to hide the details of how a service is implemented by presenting
174   a uniform interface to clients that is independent of the types of
175   resources provided. Likewise, servers do not need to be aware of each
176   client's purpose: an HTTP request can be considered in isolation rather
177   than being associated with a specific type of client or a predetermined
178   sequence of application steps. The result is a protocol that can be used
179   effectively in many different contexts and for which implementations can
180   evolve independently over time.
183   HTTP is also designed for use as an intermediation protocol for translating
184   communication to and from non-HTTP information systems.
185   HTTP proxies and gateways can provide access to alternative information
186   services by translating their diverse protocols into a hypertext
187   format that can be viewed and manipulated by clients in the same way
188   as HTTP services.
191   One consequence of this flexibility is that the protocol cannot be
192   defined in terms of what occurs behind the interface. Instead, we
193   are limited to defining the syntax of communication, the intent
194   of received communication, and the expected behavior of recipients.
195   If the communication is considered in isolation, then successful
196   actions ought to be reflected in corresponding changes to the
197   observable interface provided by servers. However, since multiple
198   clients might act in parallel and perhaps at cross-purposes, we
199   cannot require that such changes be observable beyond the scope
200   of a single response.
203   This document describes the architectural elements that are used or
204   referred to in HTTP, defines the "http" and "https" URI schemes,
205   describes overall network operation and connection management,
206   and defines HTTP message framing and forwarding requirements.
207   Our goal is to define all of the mechanisms necessary for HTTP message
208   handling that are independent of message semantics, thereby defining the
209   complete set of requirements for message parsers and
210   message-forwarding intermediaries.
214<section title="Requirement Notation" anchor="intro.requirements">
216   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
217   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
218   document are to be interpreted as described in <xref target="RFC2119"/>.
221   Conformance criteria and considerations regarding error handling
222   are defined in <xref target="conformance"/>.
226<section title="Syntax Notation" anchor="notation">
227<iref primary="true" item="Grammar" subitem="ALPHA"/>
228<iref primary="true" item="Grammar" subitem="CR"/>
229<iref primary="true" item="Grammar" subitem="CRLF"/>
230<iref primary="true" item="Grammar" subitem="CTL"/>
231<iref primary="true" item="Grammar" subitem="DIGIT"/>
232<iref primary="true" item="Grammar" subitem="DQUOTE"/>
233<iref primary="true" item="Grammar" subitem="HEXDIG"/>
234<iref primary="true" item="Grammar" subitem="HTAB"/>
235<iref primary="true" item="Grammar" subitem="LF"/>
236<iref primary="true" item="Grammar" subitem="OCTET"/>
237<iref primary="true" item="Grammar" subitem="SP"/>
238<iref primary="true" item="Grammar" subitem="VCHAR"/>
240   This specification uses the Augmented Backus-Naur Form (ABNF) notation of
241   <xref target="RFC5234"/> with a list extension, defined in
242   <xref target="abnf.extension"/>, that allows for compact definition of
243   comma-separated lists using a '#' operator (similar to how the '*' operator
244   indicates repetition).
245   <xref target="collected.abnf"/> shows the collected grammar with all list
246   operators expanded to standard ABNF notation.
248<t anchor="core.rules">
249  <x:anchor-alias value="ALPHA"/>
250  <x:anchor-alias value="CTL"/>
251  <x:anchor-alias value="CR"/>
252  <x:anchor-alias value="CRLF"/>
253  <x:anchor-alias value="DIGIT"/>
254  <x:anchor-alias value="DQUOTE"/>
255  <x:anchor-alias value="HEXDIG"/>
256  <x:anchor-alias value="HTAB"/>
257  <x:anchor-alias value="LF"/>
258  <x:anchor-alias value="OCTET"/>
259  <x:anchor-alias value="SP"/>
260  <x:anchor-alias value="VCHAR"/>
261   The following core rules are included by
262   reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
263   ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
264   DIGIT (decimal 0-9), DQUOTE (double quote),
265   HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed),
266   OCTET (any 8-bit sequence of data), SP (space), and
267   VCHAR (any visible <xref target="USASCII"/> character).
270   As a convention, ABNF rule names prefixed with "obs-" denote
271   "obsolete" grammar rules that appear for historical reasons.
276<section title="Architecture" anchor="architecture">
278   HTTP was created for the World Wide Web (WWW) architecture
279   and has evolved over time to support the scalability needs of a worldwide
280   hypertext system. Much of that architecture is reflected in the terminology
281   and syntax productions used to define HTTP.
284<section title="Client/Server Messaging" anchor="operation">
285<iref primary="true" item="client"/>
286<iref primary="true" item="server"/>
287<iref primary="true" item="connection"/>
289   HTTP is a stateless request/response protocol that operates by exchanging
290   <x:dfn>messages</x:dfn> (<xref target="http.message"/>) across a reliable
291   transport or session-layer
292   "<x:dfn>connection</x:dfn>" (<xref target=""/>).
293   An HTTP "<x:dfn>client</x:dfn>" is a program that establishes a connection
294   to a server for the purpose of sending one or more HTTP requests.
295   An HTTP "<x:dfn>server</x:dfn>" is a program that accepts connections
296   in order to service HTTP requests by sending HTTP responses.
298<iref primary="true" item="user agent"/>
299<iref primary="true" item="origin server"/>
300<iref primary="true" item="browser"/>
301<iref primary="true" item="spider"/>
302<iref primary="true" item="sender"/>
303<iref primary="true" item="recipient"/>
305   The terms client and server refer only to the roles that
306   these programs perform for a particular connection.  The same program
307   might act as a client on some connections and a server on others.
308   The term "<x:dfn>user agent</x:dfn>" refers to any of the various
309   client programs that initiate a request, including (but not limited to)
310   browsers, spiders (web-based robots), command-line tools, custom
311   applications, and mobile apps.
312   The term "<x:dfn>origin server</x:dfn>" refers to the program that can
313   originate authoritative responses for a given target resource.
314   The terms "<x:dfn>sender</x:dfn>" and "<x:dfn>recipient</x:dfn>" refer to
315   any implementation that sends or receives a given message, respectively.
318   HTTP relies upon the Uniform Resource Identifier (URI)
319   standard <xref target="RFC3986"/> to indicate the target resource
320   (<xref target="target-resource"/>) and relationships between resources.
321   Messages are passed in a format similar to that used by Internet mail
322   <xref target="RFC5322"/> and the Multipurpose Internet Mail Extensions
323   (MIME) <xref target="RFC2045"/> (see &diff-mime; for the differences
324   between HTTP and MIME messages).
327   Most HTTP communication consists of a retrieval request (GET) for
328   a representation of some resource identified by a URI.  In the
329   simplest case, this might be accomplished via a single bidirectional
330   connection (===) between the user agent (UA) and the origin server (O).
332<figure><artwork type="drawing">
333         request   &gt;
334    <x:highlight>UA</x:highlight> ======================================= <x:highlight>O</x:highlight>
335                                &lt;   response
337<iref primary="true" item="message"/>
338<iref primary="true" item="request"/>
339<iref primary="true" item="response"/>
341   A client sends an HTTP request to a server in the form of a <x:dfn>request</x:dfn>
342   message, beginning with a request-line that includes a method, URI, and
343   protocol version (<xref target="request.line"/>),
344   followed by header fields containing
345   request modifiers, client information, and representation metadata
346   (<xref target="header.fields"/>),
347   an empty line to indicate the end of the header section, and finally
348   a message body containing the payload body (if any,
349   <xref target="message.body"/>).
352   A server responds to a client's request by sending one or more HTTP
353   <x:dfn>response</x:dfn>
354   messages, each beginning with a status line that
355   includes the protocol version, a success or error code, and textual
356   reason phrase (<xref target="status.line"/>),
357   possibly followed by header fields containing server
358   information, resource metadata, and representation metadata
359   (<xref target="header.fields"/>),
360   an empty line to indicate the end of the header section, and finally
361   a message body containing the payload body (if any,
362   <xref target="message.body"/>).
365   A connection might be used for multiple request/response exchanges,
366   as defined in <xref target="persistent.connections"/>.
369   The following example illustrates a typical message exchange for a
370   GET request (&GET;) on the URI "":
373Client request:
374</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
375GET /hello.txt HTTP/1.1
376User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
378Accept-Language: en, mi
382Server response:
383</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
384HTTP/1.1 200 OK
385Date: Mon, 27 Jul 2009 12:28:53 GMT
386Server: Apache
387Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
388ETag: "34aa387-d-1568eb00"
389Accept-Ranges: bytes
390Content-Length: <x:length-of target="exbody"/>
391Vary: Accept-Encoding
392Content-Type: text/plain
394<x:span anchor="exbody">Hello World! My payload includes a trailing CRLF.
399<section title="Implementation Diversity" anchor="implementation-diversity">
401   When considering the design of HTTP, it is easy to fall into a trap of
402   thinking that all user agents are general-purpose browsers and all origin
403   servers are large public websites. That is not the case in practice.
404   Common HTTP user agents include household appliances, stereos, scales,
405   firmware update scripts, command-line programs, mobile apps,
406   and communication devices in a multitude of shapes and sizes.  Likewise,
407   common HTTP origin servers include home automation units, configurable
408   networking components, office machines, autonomous robots, news feeds,
409   traffic cameras, ad selectors, and video delivery platforms.
412   The term "user agent" does not imply that there is a human user directly
413   interacting with the software agent at the time of a request. In many
414   cases, a user agent is installed or configured to run in the background
415   and save its results for later inspection (or save only a subset of those
416   results that might be interesting or erroneous). Spiders, for example, are
417   typically given a start URI and configured to follow certain behavior while
418   crawling the Web as a hypertext graph.
421   The implementation diversity of HTTP means that not all user agents can
422   make interactive suggestions to their user or provide adequate warning for
423   security or privacy concerns. In the few cases where this
424   specification requires reporting of errors to the user, it is acceptable
425   for such reporting to only be observable in an error console or log file.
426   Likewise, requirements that an automated action be confirmed by the user
427   before proceeding might be met via advance configuration choices,
428   run-time options, or simple avoidance of the unsafe action; confirmation
429   does not imply any specific user interface or interruption of normal
430   processing if the user has already made that choice.
434<section title="Intermediaries" anchor="intermediaries">
435<iref primary="true" item="intermediary"/>
437   HTTP enables the use of intermediaries to satisfy requests through
438   a chain of connections.  There are three common forms of HTTP
439   <x:dfn>intermediary</x:dfn>: proxy, gateway, and tunnel.  In some cases,
440   a single intermediary might act as an origin server, proxy, gateway,
441   or tunnel, switching behavior based on the nature of each request.
443<figure><artwork type="drawing">
444         &gt;             &gt;             &gt;             &gt;
445    <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>
446               &lt;             &lt;             &lt;             &lt;
449   The figure above shows three intermediaries (A, B, and C) between the
450   user agent and origin server. A request or response message that
451   travels the whole chain will pass through four separate connections.
452   Some HTTP communication options
453   might apply only to the connection with the nearest, non-tunnel
454   neighbor, only to the end-points of the chain, or to all connections
455   along the chain. Although the diagram is linear, each participant might
456   be engaged in multiple, simultaneous communications. For example, B
457   might be receiving requests from many clients other than A, and/or
458   forwarding requests to servers other than C, at the same time that it
459   is handling A's request. Likewise, later requests might be sent through a
460   different path of connections, often based on dynamic configuration for
461   load balancing.   
464<iref primary="true" item="upstream"/><iref primary="true" item="downstream"/>
465<iref primary="true" item="inbound"/><iref primary="true" item="outbound"/>
466   The terms "<x:dfn>upstream</x:dfn>" and "<x:dfn>downstream</x:dfn>" are
467   used to describe directional requirements in relation to the message flow:
468   all messages flow from upstream to downstream.
469   The terms inbound and outbound are used to describe directional
470   requirements in relation to the request route:
471   "<x:dfn>inbound</x:dfn>" means toward the origin server and
472   "<x:dfn>outbound</x:dfn>" means toward the user agent.
474<t><iref primary="true" item="proxy"/>
475   A "<x:dfn>proxy</x:dfn>" is a message forwarding agent that is selected by the
476   client, usually via local configuration rules, to receive requests
477   for some type(s) of absolute URI and attempt to satisfy those
478   requests via translation through the HTTP interface.  Some translations
479   are minimal, such as for proxy requests for "http" URIs, whereas
480   other requests might require translation to and from entirely different
481   application-level protocols. Proxies are often used to group an
482   organization's HTTP requests through a common intermediary for the
483   sake of security, annotation services, or shared caching. Some proxies
484   are designed to apply transformations to selected messages or payloads
485   while they are being forwarded, as described in
486   <xref target="message.transformations"/>.
488<t><iref primary="true" item="gateway"/><iref primary="true" item="reverse proxy"/>
489<iref primary="true" item="accelerator"/>
490   A "<x:dfn>gateway</x:dfn>" (a.k.a., "<x:dfn>reverse proxy</x:dfn>") is an
491   intermediary that acts as an origin server for the outbound connection, but
492   translates received requests and forwards them inbound to another server or
493   servers. Gateways are often used to encapsulate legacy or untrusted
494   information services, to improve server performance through
495   "<x:dfn>accelerator</x:dfn>" caching, and to enable partitioning or load
496   balancing of HTTP services across multiple machines.
499   All HTTP requirements applicable to an origin server
500   also apply to the outbound communication of a gateway.
501   A gateway communicates with inbound servers using any protocol that
502   it desires, including private extensions to HTTP that are outside
503   the scope of this specification.  However, an HTTP-to-HTTP gateway
504   that wishes to interoperate with third-party HTTP servers ought to conform
505   to user agent requirements on the gateway's inbound connection.
507<t><iref primary="true" item="tunnel"/>
508   A "<x:dfn>tunnel</x:dfn>" acts as a blind relay between two connections
509   without changing the messages. Once active, a tunnel is not
510   considered a party to the HTTP communication, though the tunnel might
511   have been initiated by an HTTP request. A tunnel ceases to exist when
512   both ends of the relayed connection are closed. Tunnels are used to
513   extend a virtual connection through an intermediary, such as when
514   Transport Layer Security (TLS, <xref target="RFC5246"/>) is used to
515   establish confidential communication through a shared firewall proxy.
517<t><iref primary="true" item="interception proxy"/>
518<iref primary="true" item="transparent proxy"/>
519<iref primary="true" item="captive portal"/>
520   The above categories for intermediary only consider those acting as
521   participants in the HTTP communication.  There are also intermediaries
522   that can act on lower layers of the network protocol stack, filtering or
523   redirecting HTTP traffic without the knowledge or permission of message
524   senders. Network intermediaries often introduce security flaws or
525   interoperability problems by violating HTTP semantics.  For example, an
526   "<x:dfn>interception proxy</x:dfn>" <xref target="RFC3040"/> (also commonly
527   known as a "<x:dfn>transparent proxy</x:dfn>" <xref target="RFC1919"/> or
528   "<x:dfn>captive portal</x:dfn>")
529   differs from an HTTP proxy because it is not selected by the client.
530   Instead, an interception proxy filters or redirects outgoing TCP port 80
531   packets (and occasionally other common port traffic).
532   Interception proxies are commonly found on public network access points,
533   as a means of enforcing account subscription prior to allowing use of
534   non-local Internet services, and within corporate firewalls to enforce
535   network usage policies.
536   They are indistinguishable from a man-in-the-middle attack.
539   HTTP is defined as a stateless protocol, meaning that each request message
540   can be understood in isolation.  Many implementations depend on HTTP's
541   stateless design in order to reuse proxied connections or dynamically
542   load-balance requests across multiple servers.  Hence, a server &MUST-NOT;
543   assume that two requests on the same connection are from the same user
544   agent unless the connection is secured and specific to that agent.
545   Some non-standard HTTP extensions (e.g., <xref target="RFC4559"/>) have
546   been known to violate this requirement, resulting in security and
547   interoperability problems.
551<section title="Caches" anchor="caches">
552<iref primary="true" item="cache"/>
554   A "<x:dfn>cache</x:dfn>" is a local store of previous response messages and the
555   subsystem that controls its message storage, retrieval, and deletion.
556   A cache stores cacheable responses in order to reduce the response
557   time and network bandwidth consumption on future, equivalent
558   requests. Any client or server &MAY; employ a cache, though a cache
559   cannot be used by a server while it is acting as a tunnel.
562   The effect of a cache is that the request/response chain is shortened
563   if one of the participants along the chain has a cached response
564   applicable to that request. The following illustrates the resulting
565   chain if B has a cached copy of an earlier response from O (via C)
566   for a request that has not been cached by UA or A.
568<figure><artwork type="drawing">
569            &gt;             &gt;
570       <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>
571                  &lt;             &lt;
573<t><iref primary="true" item="cacheable"/>
574   A response is "<x:dfn>cacheable</x:dfn>" if a cache is allowed to store a copy of
575   the response message for use in answering subsequent requests.
576   Even when a response is cacheable, there might be additional
577   constraints placed by the client or by the origin server on when
578   that cached response can be used for a particular request. HTTP
579   requirements for cache behavior and cacheable responses are
580   defined in &caching-overview;. 
583   There are a wide variety of architectures and configurations
584   of caches deployed across the World Wide Web and
585   inside large organizations. These include national hierarchies
586   of proxy caches to save transoceanic bandwidth, collaborative systems that
587   broadcast or multicast cache entries, archives of pre-fetched cache
588   entries for use in off-line or high-latency environments, and so on.
592<section title="Conformance and Error Handling" anchor="conformance">
594   This specification targets conformance criteria according to the role of
595   a participant in HTTP communication.  Hence, HTTP requirements are placed
596   on senders, recipients, clients, servers, user agents, intermediaries,
597   origin servers, proxies, gateways, or caches, depending on what behavior
598   is being constrained by the requirement. Additional (social) requirements
599   are placed on implementations, resource owners, and protocol element
600   registrations when they apply beyond the scope of a single communication.
603   The verb "generate" is used instead of "send" where a requirement
604   differentiates between creating a protocol element and merely forwarding a
605   received element downstream.
608   An implementation is considered conformant if it complies with all of the
609   requirements associated with the roles it partakes in HTTP.
612   Conformance includes both the syntax and semantics of protocol
613   elements. A sender &MUST-NOT; generate protocol elements that convey a
614   meaning that is known by that sender to be false. A sender &MUST-NOT;
615   generate protocol elements that do not match the grammar defined by the
616   corresponding ABNF rules. Within a given message, a sender &MUST-NOT;
617   generate protocol elements or syntax alternatives that are only allowed to
618   be generated by participants in other roles (i.e., a role that the sender
619   does not have for that message).
622   When a received protocol element is parsed, the recipient &MUST; be able to
623   parse any value of reasonable length that is applicable to the recipient's
624   role and matches the grammar defined by the corresponding ABNF rules.
625   Note, however, that some received protocol elements might not be parsed.
626   For example, an intermediary forwarding a message might parse a
627   header-field into generic field-name and field-value components, but then
628   forward the header field without further parsing inside the field-value.
631   HTTP does not have specific length limitations for many of its protocol
632   elements because the lengths that might be appropriate will vary widely,
633   depending on the deployment context and purpose of the implementation.
634   Hence, interoperability between senders and recipients depends on shared
635   expectations regarding what is a reasonable length for each protocol
636   element. Furthermore, what is commonly understood to be a reasonable length
637   for some protocol elements has changed over the course of the past two
638   decades of HTTP use, and is expected to continue changing in the future.
641   At a minimum, a recipient &MUST; be able to parse and process protocol
642   element lengths that are at least as long as the values that it generates
643   for those same protocol elements in other messages. For example, an origin
644   server that publishes very long URI references to its own resources needs
645   to be able to parse and process those same references when received as a
646   request target.
649   A recipient &MUST; interpret a received protocol element according to the
650   semantics defined for it by this specification, including extensions to
651   this specification, unless the recipient has determined (through experience
652   or configuration) that the sender incorrectly implements what is implied by
653   those semantics.
654   For example, an origin server might disregard the contents of a received
655   <x:ref>Accept-Encoding</x:ref> header field if inspection of the
656   <x:ref>User-Agent</x:ref> header field indicates a specific implementation
657   version that is known to fail on receipt of certain content codings.
660   Unless noted otherwise, a recipient &MAY; attempt to recover a usable
661   protocol element from an invalid construct.  HTTP does not define
662   specific error handling mechanisms except when they have a direct impact
663   on security, since different applications of the protocol require
664   different error handling strategies.  For example, a Web browser might
665   wish to transparently recover from a response where the
666   <x:ref>Location</x:ref> header field doesn't parse according to the ABNF,
667   whereas a systems control client might consider any form of error recovery
668   to be dangerous.
672<section title="Protocol Versioning" anchor="http.version">
673  <x:anchor-alias value="HTTP-version"/>
674  <x:anchor-alias value="HTTP-name"/>
676   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate
677   versions of the protocol. This specification defines version "1.1".
678   The protocol version as a whole indicates the sender's conformance
679   with the set of requirements laid out in that version's corresponding
680   specification of HTTP.
683   The version of an HTTP message is indicated by an HTTP-version field
684   in the first line of the message. HTTP-version is case-sensitive.
686<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-version"/><iref primary="true" item="Grammar" subitem="HTTP-name"/>
687  <x:ref>HTTP-version</x:ref>  = <x:ref>HTTP-name</x:ref> "/" <x:ref>DIGIT</x:ref> "." <x:ref>DIGIT</x:ref>
688  <x:ref>HTTP-name</x:ref>     = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
691   The HTTP version number consists of two decimal digits separated by a "."
692   (period or decimal point).  The first digit ("major version") indicates the
693   HTTP messaging syntax, whereas the second digit ("minor version") indicates
694   the highest minor version within that major version to which the sender is
695   conformant and able to understand for future communication.  The minor
696   version advertises the sender's communication capabilities even when the
697   sender is only using a backwards-compatible subset of the protocol,
698   thereby letting the recipient know that more advanced features can
699   be used in response (by servers) or in future requests (by clients).
702   When an HTTP/1.1 message is sent to an HTTP/1.0 recipient
703   <xref target="RFC1945"/> or a recipient whose version is unknown,
704   the HTTP/1.1 message is constructed such that it can be interpreted
705   as a valid HTTP/1.0 message if all of the newer features are ignored.
706   This specification places recipient-version requirements on some
707   new features so that a conformant sender will only use compatible
708   features until it has determined, through configuration or the
709   receipt of a message, that the recipient supports HTTP/1.1.
712   The interpretation of a header field does not change between minor
713   versions of the same major HTTP version, though the default
714   behavior of a recipient in the absence of such a field can change.
715   Unless specified otherwise, header fields defined in HTTP/1.1 are
716   defined for all versions of HTTP/1.x.  In particular, the <x:ref>Host</x:ref>
717   and <x:ref>Connection</x:ref> header fields ought to be implemented by all
718   HTTP/1.x implementations whether or not they advertise conformance with
719   HTTP/1.1.
722   New header fields can be introduced without changing the protocol version
723   if their defined semantics allow them to be safely ignored by recipients
724   that do not recognize them. Header field extensibility is discussed in
725   <xref target="field.extensibility"/>.
728   Intermediaries that process HTTP messages (i.e., all intermediaries
729   other than those acting as tunnels) &MUST; send their own HTTP-version
730   in forwarded messages.  In other words, they are not allowed to blindly
731   forward the first line of an HTTP message without ensuring that the
732   protocol version in that message matches a version to which that
733   intermediary is conformant for both the receiving and
734   sending of messages.  Forwarding an HTTP message without rewriting
735   the HTTP-version might result in communication errors when downstream
736   recipients use the message sender's version to determine what features
737   are safe to use for later communication with that sender.
740   A client &SHOULD; send a request version equal to the highest
741   version to which the client is conformant and
742   whose major version is no higher than the highest version supported
743   by the server, if this is known.  A client &MUST-NOT; send a
744   version to which it is not conformant.
747   A client &MAY; send a lower request version if it is known that
748   the server incorrectly implements the HTTP specification, but only
749   after the client has attempted at least one normal request and determined
750   from the response status code or header fields (e.g., <x:ref>Server</x:ref>) that
751   the server improperly handles higher request versions.
754   A server &SHOULD; send a response version equal to the highest version to
755   which the server is conformant that has a major version less than or equal
756   to the one received in the request.
757   A server &MUST-NOT; send a version to which it is not conformant.
758   A server can send a <x:ref>505 (HTTP Version Not Supported)</x:ref>
759   response if it wishes, for any reason, to refuse service of the client's
760   major protocol version.
763   A server &MAY; send an HTTP/1.0 response to a request
764   if it is known or suspected that the client incorrectly implements the
765   HTTP specification and is incapable of correctly processing later
766   version responses, such as when a client fails to parse the version
767   number correctly or when an intermediary is known to blindly forward
768   the HTTP-version even when it doesn't conform to the given minor
769   version of the protocol. Such protocol downgrades &SHOULD-NOT; be
770   performed unless triggered by specific client attributes, such as when
771   one or more of the request header fields (e.g., <x:ref>User-Agent</x:ref>)
772   uniquely match the values sent by a client known to be in error.
775   The intention of HTTP's versioning design is that the major number
776   will only be incremented if an incompatible message syntax is
777   introduced, and that the minor number will only be incremented when
778   changes made to the protocol have the effect of adding to the message
779   semantics or implying additional capabilities of the sender.  However,
780   the minor version was not incremented for the changes introduced between
781   <xref target="RFC2068"/> and <xref target="RFC2616"/>, and this revision
782   has specifically avoided any such changes to the protocol.
785   When an HTTP message is received with a major version number that the
786   recipient implements, but a higher minor version number than what the
787   recipient implements, the recipient &SHOULD; process the message as if it
788   were in the highest minor version within that major version to which the
789   recipient is conformant. A recipient can assume that a message with a
790   higher minor version, when sent to a recipient that has not yet indicated
791   support for that higher version, is sufficiently backwards-compatible to be
792   safely processed by any implementation of the same major version.
796<section title="Uniform Resource Identifiers" anchor="uri">
797<iref primary="true" item="resource"/>
799   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used
800   throughout HTTP as the means for identifying resources (&resource;).
801   URI references are used to target requests, indicate redirects, and define
802   relationships.
804  <x:anchor-alias value="URI-reference"/>
805  <x:anchor-alias value="absolute-URI"/>
806  <x:anchor-alias value="relative-part"/>
807  <x:anchor-alias value="scheme"/>
808  <x:anchor-alias value="authority"/>
809  <x:anchor-alias value="uri-host"/>
810  <x:anchor-alias value="port"/>
811  <x:anchor-alias value="path"/>
812  <x:anchor-alias value="path-abempty"/>
813  <x:anchor-alias value="segment"/>
814  <x:anchor-alias value="query"/>
815  <x:anchor-alias value="fragment"/>
816  <x:anchor-alias value="absolute-path"/>
817  <x:anchor-alias value="partial-URI"/>
819   The definitions of "URI-reference",
820   "absolute-URI", "relative-part", "scheme", "authority", "port", "host",
821   "path-abempty", "segment", "query", and "fragment" are adopted from the
822   URI generic syntax.
823   An "absolute-path" rule is defined, differing slightly from
824   RFC 3986's "path-absolute" in that it allows a leading "//".
825   A "partial-URI" rule is defined for protocol elements
826   that allow a relative URI but not a fragment.
828<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="scheme"/><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>
829  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
830  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
831  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
832  <x:ref>scheme</x:ref>        = &lt;scheme, defined in <xref target="RFC3986" x:fmt="," x:sec="3.1"/>&gt;
833  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
834  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
835  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
836  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
837  <x:ref>segment</x:ref>       = &lt;segment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
838  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
839  <x:ref>fragment</x:ref>      = &lt;fragment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.5"/>&gt;
841  <x:ref>absolute-path</x:ref> = 1*( "/" segment )
842  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
845   Each protocol element in HTTP that allows a URI reference will indicate
846   in its ABNF production whether the element allows any form of reference
847   (URI-reference), only a URI in absolute form (absolute-URI), only the
848   path and optional query components, or some combination of the above.
849   Unless otherwise indicated, URI references are parsed
850   relative to the effective request URI
851   (<xref target="effective.request.uri"/>).
854<section title="http URI scheme" anchor="http.uri">
855  <x:anchor-alias value="http-URI"/>
856  <iref item="http URI scheme" primary="true"/>
857  <iref item="URI scheme" subitem="http" primary="true"/>
859   The "http" URI scheme is hereby defined for the purpose of minting
860   identifiers according to their association with the hierarchical
861   namespace governed by a potential HTTP origin server listening for
862   TCP (<xref target="RFC0793"/>) connections on a given port.
864<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"><!--terminal production--></iref>
865  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
866             [ "#" <x:ref>fragment</x:ref> ]
869   The HTTP origin server is identified by the generic syntax's
870   <x:ref>authority</x:ref> component, which includes a host identifier
871   and optional TCP port (<xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>).
872   The remainder of the URI, consisting of both the hierarchical path
873   component and optional query component, serves as an identifier for
874   a potential resource within that origin server's name space.
877   A sender &MUST-NOT; generate an "http" URI with an empty host identifier.
878   A recipient that processes such a URI reference &MUST; reject it as invalid.
881   If the host identifier is provided as an IP address,
882   then the origin server is any listener on the indicated TCP port at
883   that IP address. If host is a registered name, then that name is
884   considered an indirect identifier and the recipient might use a name
885   resolution service, such as DNS, to find the address of a listener
886   for that host.
887   If the port subcomponent is empty or not given, then TCP port 80 is
888   assumed (the default reserved port for WWW services).
891   Regardless of the form of host identifier, access to that host is not
892   implied by the mere presence of its name or address. The host might or might
893   not exist and, even when it does exist, might or might not be running an
894   HTTP server or listening to the indicated port. The "http" URI scheme
895   makes use of the delegated nature of Internet names and addresses to
896   establish a naming authority (whatever entity has the ability to place
897   an HTTP server at that Internet name or address) and allows that
898   authority to determine which names are valid and how they might be used.
901   When an "http" URI is used within a context that calls for access to the
902   indicated resource, a client &MAY; attempt access by resolving
903   the host to an IP address, establishing a TCP connection to that address
904   on the indicated port, and sending an HTTP request message
905   (<xref target="http.message"/>) containing the URI's identifying data
906   (<xref target="message.routing"/>) to the server.
907   If the server responds to that request with a non-interim HTTP response
908   message, as described in &status-codes;, then that response
909   is considered an authoritative answer to the client's request.
912   Although HTTP is independent of the transport protocol, the "http"
913   scheme is specific to TCP-based services because the name delegation
914   process depends on TCP for establishing authority.
915   An HTTP service based on some other underlying connection protocol
916   would presumably be identified using a different URI scheme, just as
917   the "https" scheme (below) is used for resources that require an
918   end-to-end secured connection. Other protocols might also be used to
919   provide access to "http" identified resources &mdash; it is only the
920   authoritative interface that is specific to TCP.
923   The URI generic syntax for authority also includes a deprecated
924   userinfo subcomponent (<xref target="RFC3986" x:fmt="," x:sec="3.2.1"/>)
925   for including user authentication information in the URI.  Some
926   implementations make use of the userinfo component for internal
927   configuration of authentication information, such as within command
928   invocation options, configuration files, or bookmark lists, even
929   though such usage might expose a user identifier or password.
930   A sender &MUST-NOT; generate the userinfo subcomponent (and its "@"
931   delimiter) when an "http" URI reference is generated within a message as a
932   request target or header field value.
933   Before making use of an "http" URI reference received from an untrusted
934   source, a recipient &SHOULD; parse for userinfo and treat its presence as
935   an error; it is likely being used to obscure the authority for the sake of
936   phishing attacks.
940<section title="https URI scheme" anchor="https.uri">
941   <x:anchor-alias value="https-URI"/>
942   <iref item="https URI scheme"/>
943   <iref item="URI scheme" subitem="https"/>
945   The "https" URI scheme is hereby defined for the purpose of minting
946   identifiers according to their association with the hierarchical
947   namespace governed by a potential HTTP origin server listening to a
948   given TCP port for TLS-secured connections
949   (<xref target="RFC0793"/>, <xref target="RFC5246"/>).
952   All of the requirements listed above for the "http" scheme are also
953   requirements for the "https" scheme, except that a default TCP port
954   of 443 is assumed if the port subcomponent is empty or not given,
955   and the user agent &MUST; ensure that its connection to the origin
956   server is secured through the use of strong encryption, end-to-end,
957   prior to sending the first HTTP request.
959<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="https-URI"><!--terminal production--></iref>
960  <x:ref>https-URI</x:ref> = "https:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
961              [ "#" <x:ref>fragment</x:ref> ]
964   Note that the "https" URI scheme depends on both TLS and TCP for
965   establishing authority.
966   Resources made available via the "https" scheme have no shared
967   identity with the "http" scheme even if their resource identifiers
968   indicate the same authority (the same host listening to the same
969   TCP port).  They are distinct name spaces and are considered to be
970   distinct origin servers.  However, an extension to HTTP that is
971   defined to apply to entire host domains, such as the Cookie protocol
972   <xref target="RFC6265"/>, can allow information
973   set by one service to impact communication with other services
974   within a matching group of host domains.
977   The process for authoritative access to an "https" identified
978   resource is defined in <xref target="RFC2818"/>.
982<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
984   Since the "http" and "https" schemes conform to the URI generic syntax,
985   such URIs are normalized and compared according to the algorithm defined
986   in <xref target="RFC3986" x:fmt="of" x:sec="6"/>, using the defaults
987   described above for each scheme.
990   If the port is equal to the default port for a scheme, the normal form is
991   to omit the port subcomponent. When not being used in absolute form as the
992   request target of an OPTIONS request, an empty path component is equivalent
993   to an absolute path of "/", so the normal form is to provide a path of "/"
994   instead. The scheme and host are case-insensitive and normally provided in
995   lowercase; all other components are compared in a case-sensitive manner.
996   Characters other than those in the "reserved" set are equivalent to their
997   percent-encoded octets: the normal form is to not encode them
998   (see Sections <xref target="RFC3986" x:fmt="number" x:sec="2.1"/> and
999   <xref target="RFC3986" x:fmt="number" x:sec="2.2"/> of
1000   <xref target="RFC3986"/>).
1003   For example, the following three URIs are equivalent:
1005<figure><artwork type="example">
1014<section title="Message Format" anchor="http.message">
1015<x:anchor-alias value="generic-message"/>
1016<x:anchor-alias value="message.types"/>
1017<x:anchor-alias value="HTTP-message"/>
1018<x:anchor-alias value="start-line"/>
1019<iref item="header section"/>
1020<iref item="headers"/>
1021<iref item="header field"/>
1023   All HTTP/1.1 messages consist of a start-line followed by a sequence of
1024   octets in a format similar to the Internet Message Format
1025   <xref target="RFC5322"/>: zero or more header fields (collectively
1026   referred to as the "headers" or the "header section"), an empty line
1027   indicating the end of the header section, and an optional message body.
1029<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"><!--terminal production--></iref>
1030  <x:ref>HTTP-message</x:ref>   = <x:ref>start-line</x:ref>
1031                   *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1032                   <x:ref>CRLF</x:ref>
1033                   [ <x:ref>message-body</x:ref> ]
1036   The normal procedure for parsing an HTTP message is to read the
1037   start-line into a structure, read each header field into a hash
1038   table by field name until the empty line, and then use the parsed
1039   data to determine if a message body is expected.  If a message body
1040   has been indicated, then it is read as a stream until an amount
1041   of octets equal to the message body length is read or the connection
1042   is closed.
1045   A recipient &MUST; parse an HTTP message as a sequence of octets in an
1046   encoding that is a superset of US-ASCII <xref target="USASCII"/>.
1047   Parsing an HTTP message as a stream of Unicode characters, without regard
1048   for the specific encoding, creates security vulnerabilities due to the
1049   varying ways that string processing libraries handle invalid multibyte
1050   character sequences that contain the octet LF (%x0A).  String-based
1051   parsers can only be safely used within protocol elements after the element
1052   has been extracted from the message, such as within a header field-value
1053   after message parsing has delineated the individual fields.
1056   An HTTP message can be parsed as a stream for incremental processing or
1057   forwarding downstream.  However, recipients cannot rely on incremental
1058   delivery of partial messages, since some implementations will buffer or
1059   delay message forwarding for the sake of network efficiency, security
1060   checks, or payload transformations.
1063   A sender &MUST-NOT; send whitespace between the start-line and
1064   the first header field.
1065   A recipient that receives whitespace between the start-line and
1066   the first header field &MUST; either reject the message as invalid or
1067   consume each whitespace-preceded line without further processing of it
1068   (i.e., ignore the entire line, along with any subsequent lines preceded
1069   by whitespace, until a properly formed header field is received or the
1070   header section is terminated).
1073   The presence of such whitespace in a request
1074   might be an attempt to trick a server into ignoring that field or
1075   processing the line after it as a new request, either of which might
1076   result in a security vulnerability if other implementations within
1077   the request chain interpret the same message differently.
1078   Likewise, the presence of such whitespace in a response might be
1079   ignored by some clients or cause others to cease parsing.
1082<section title="Start Line" anchor="start.line">
1083  <x:anchor-alias value="Start-Line"/>
1085   An HTTP message can either be a request from client to server or a
1086   response from server to client.  Syntactically, the two types of message
1087   differ only in the start-line, which is either a request-line (for requests)
1088   or a status-line (for responses), and in the algorithm for determining
1089   the length of the message body (<xref target="message.body"/>).
1092   In theory, a client could receive requests and a server could receive
1093   responses, distinguishing them by their different start-line formats,
1094   but in practice servers are implemented to only expect a request
1095   (a response is interpreted as an unknown or invalid request method)
1096   and clients are implemented to only expect a response.
1098<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="start-line"/>
1099  <x:ref>start-line</x:ref>     = <x:ref>request-line</x:ref> / <x:ref>status-line</x:ref>
1102<section title="Request Line" anchor="request.line">
1103  <x:anchor-alias value="Request"/>
1104  <x:anchor-alias value="request-line"/>
1106   A request-line begins with a method token, followed by a single
1107   space (SP), the request-target, another single space (SP), the
1108   protocol version, and ending with CRLF.
1110<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1111  <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>
1113<iref primary="true" item="method"/>
1114<t anchor="method">
1115   The method token indicates the request method to be performed on the
1116   target resource. The request method is case-sensitive.
1118<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1119  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1122   The request methods defined by this specification can be found in
1123   &methods;, along with information regarding the HTTP method registry
1124   and considerations for defining new methods.
1126<iref item="request-target"/>
1128   The request-target identifies the target resource upon which to apply
1129   the request, as defined in <xref target="request-target"/>.
1132   Recipients typically parse the request-line into its component parts by
1133   splitting on whitespace (see <xref target="message.robustness"/>), since
1134   no whitespace is allowed in the three components.
1135   Unfortunately, some user agents fail to properly encode or exclude
1136   whitespace found in hypertext references, resulting in those disallowed
1137   characters being sent in a request-target.
1140   Recipients of an invalid request-line &SHOULD; respond with either a
1141   <x:ref>400 (Bad Request)</x:ref> error or a <x:ref>301 (Moved Permanently)</x:ref>
1142   redirect with the request-target properly encoded.  A recipient &SHOULD-NOT;
1143   attempt to autocorrect and then process the request without a redirect,
1144   since the invalid request-line might be deliberately crafted to bypass
1145   security filters along the request chain.
1148   HTTP does not place a pre-defined limit on the length of a request-line.
1149   A server that receives a method longer than any that it implements
1150   &SHOULD; respond with a <x:ref>501 (Not Implemented)</x:ref> status code.
1151   A server ought to be prepared to receive URIs of unbounded length, as
1152   described in <xref target="conformance"/>, and &MUST; respond with a
1153   <x:ref>414 (URI Too Long)</x:ref> status code if the received
1154   request-target is longer than the server wishes to parse (see &status-414;).
1157   Various ad-hoc limitations on request-line length are found in practice.
1158   It is &RECOMMENDED; that all HTTP senders and recipients support, at a
1159   minimum, request-line lengths of 8000 octets.
1163<section title="Status Line" anchor="status.line">
1164  <x:anchor-alias value="response"/>
1165  <x:anchor-alias value="status-line"/>
1166  <x:anchor-alias value="status-code"/>
1167  <x:anchor-alias value="reason-phrase"/>
1169   The first line of a response message is the status-line, consisting
1170   of the protocol version, a space (SP), the status code, another space,
1171   a possibly-empty textual phrase describing the status code, and
1172   ending with CRLF.
1174<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-line"/>
1175  <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>
1178   The status-code element is a 3-digit integer code describing the
1179   result of the server's attempt to understand and satisfy the client's
1180   corresponding request. The rest of the response message is to be
1181   interpreted in light of the semantics defined for that status code.
1182   See &status-codes; for information about the semantics of status codes,
1183   including the classes of status code (indicated by the first digit),
1184   the status codes defined by this specification, considerations for the
1185   definition of new status codes, and the IANA registry.
1187<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-code"/>
1188  <x:ref>status-code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1191   The reason-phrase element exists for the sole purpose of providing a
1192   textual description associated with the numeric status code, mostly
1193   out of deference to earlier Internet application protocols that were more
1194   frequently used with interactive text clients. A client &SHOULD; ignore
1195   the reason-phrase content.
1197<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
1198  <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> )
1203<section title="Header Fields" anchor="header.fields">
1204  <x:anchor-alias value="header-field"/>
1205  <x:anchor-alias value="field-content"/>
1206  <x:anchor-alias value="field-name"/>
1207  <x:anchor-alias value="field-value"/>
1208  <x:anchor-alias value="field-vchar"/>
1209  <x:anchor-alias value="obs-fold"/>
1211   Each header field consists of a case-insensitive field name
1212   followed by a colon (":"), optional leading whitespace, the field value,
1213   and optional trailing whitespace.
1215<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-vchar"/><iref primary="true" item="Grammar" subitem="field-content"/><iref primary="true" item="Grammar" subitem="obs-fold"/>
1216  <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>
1218  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1219  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>obs-fold</x:ref> )
1220  <x:ref>field-content</x:ref>  = <x:ref>field-vchar</x:ref> [ 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> ) <x:ref>field-vchar</x:ref> ]
1221  <x:ref>field-vchar</x:ref>    = <x:ref>VCHAR</x:ref> / <x:ref>obs-text</x:ref>
1223  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref> 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1224                 ; obsolete line folding
1225                 ; see <xref target="field.parsing"/>
1228   The field-name token labels the corresponding field-value as having the
1229   semantics defined by that header field.  For example, the <x:ref>Date</x:ref>
1230   header field is defined in &header-date; as containing the origination
1231   timestamp for the message in which it appears.
1234<section title="Field Extensibility" anchor="field.extensibility">
1236   Header fields are fully extensible: there is no limit on the
1237   introduction of new field names, each presumably defining new semantics,
1238   nor on the number of header fields used in a given message.  Existing
1239   fields are defined in each part of this specification and in many other
1240   specifications outside the core standard.
1243   New header fields can be defined such that, when they are understood by a
1244   recipient, they might override or enhance the interpretation of previously
1245   defined header fields, define preconditions on request evaluation, or
1246   refine the meaning of responses.
1249   A proxy &MUST; forward unrecognized header fields unless the
1250   field-name is listed in the <x:ref>Connection</x:ref> header field
1251   (<xref target="header.connection"/>) or the proxy is specifically
1252   configured to block, or otherwise transform, such fields.
1253   Other recipients &SHOULD; ignore unrecognized header fields.
1254   These requirements allow HTTP's functionality to be enhanced without
1255   requiring prior update of deployed intermediaries.
1258   All defined header fields ought to be registered with IANA in the
1259   Message Header Field Registry, as described in &iana-header-registry;.
1263<section title="Field Order" anchor="field.order">
1265   The order in which header fields with differing field names are
1266   received is not significant. However, it is "good practice" to send
1267   header fields that contain control data first, such as <x:ref>Host</x:ref>
1268   on requests and <x:ref>Date</x:ref> on responses, so that implementations
1269   can decide when not to handle a message as early as possible.  A server
1270   &MUST; wait until the entire header section is received before interpreting
1271   a request message, since later header fields might include conditionals,
1272   authentication credentials, or deliberately misleading duplicate
1273   header fields that would impact request processing.
1276   A sender &MUST-NOT; generate multiple header fields with the same field
1277   name in a message unless either the entire field value for that
1278   header field is defined as a comma-separated list [i.e., #(values)]
1279   or the header field is a well-known exception (as noted below).
1282   A recipient &MAY; combine multiple header fields with the same field name
1283   into one "field-name: field-value" pair, without changing the semantics of
1284   the message, by appending each subsequent field value to the combined
1285   field value in order, separated by a comma. The order in which
1286   header fields with the same field name are received is therefore
1287   significant to the interpretation of the combined field value;
1288   a proxy &MUST-NOT; change the order of these field values when
1289   forwarding a message.
1292  <t>
1293   &Note; In practice, the "Set-Cookie" header field (<xref target="RFC6265"/>)
1294   often appears multiple times in a response message and does not use the
1295   list syntax, violating the above requirements on multiple header fields
1296   with the same name. Since it cannot be combined into a single field-value,
1297   recipients ought to handle "Set-Cookie" as a special case while processing
1298   header fields. (See Appendix A.2.3 of <xref target="Kri2001"/> for details.)
1299  </t>
1303<section title="Whitespace" anchor="whitespace">
1304<t anchor="rule.LWS">
1305   This specification uses three rules to denote the use of linear
1306   whitespace: OWS (optional whitespace), RWS (required whitespace), and
1307   BWS ("bad" whitespace).
1309<t anchor="rule.OWS">
1310   The OWS rule is used where zero or more linear whitespace octets might
1311   appear. For protocol elements where optional whitespace is preferred to
1312   improve readability, a sender &SHOULD; generate the optional whitespace
1313   as a single SP; otherwise, a sender &SHOULD-NOT; generate optional
1314   whitespace except as needed to white-out invalid or unwanted protocol
1315   elements during in-place message filtering.
1317<t anchor="rule.RWS">
1318   The RWS rule is used when at least one linear whitespace octet is required
1319   to separate field tokens. A sender &SHOULD; generate RWS as a single SP.
1321<t anchor="rule.BWS">
1322   The BWS rule is used where the grammar allows optional whitespace only for
1323   historical reasons. A sender &MUST-NOT; generate BWS in messages.
1324   A recipient &MUST; parse for such bad whitespace and remove it before
1325   interpreting the protocol element.
1327<t anchor="rule.whitespace">
1328  <x:anchor-alias value="BWS"/>
1329  <x:anchor-alias value="OWS"/>
1330  <x:anchor-alias value="RWS"/>
1332<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"/>
1333  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1334                 ; optional whitespace
1335  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1336                 ; required whitespace
1337  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1338                 ; "bad" whitespace
1342<section title="Field Parsing" anchor="field.parsing">
1344   Messages are parsed using a generic algorithm, independent of the
1345   individual header field names. The contents within a given field value are
1346   not parsed until a later stage of message interpretation (usually after the
1347   message's entire header section has been processed).
1348   Consequently, this specification does not use ABNF rules to define each
1349   "Field-Name: Field Value" pair, as was done in previous editions.
1350   Instead, this specification uses ABNF rules which are named according to
1351   each registered field name, wherein the rule defines the valid grammar for
1352   that field's corresponding field values (i.e., after the field-value
1353   has been extracted from the header section by a generic field parser).
1356   No whitespace is allowed between the header field-name and colon.
1357   In the past, differences in the handling of such whitespace have led to
1358   security vulnerabilities in request routing and response handling.
1359   A server &MUST; reject any received request message that contains
1360   whitespace between a header field-name and colon with a response code of
1361   <x:ref>400 (Bad Request)</x:ref>. A proxy &MUST; remove any such whitespace
1362   from a response message before forwarding the message downstream.
1365   A field value might be preceded and/or followed by optional whitespace
1366   (OWS); a single SP preceding the field-value is preferred for consistent
1367   readability by humans.
1368   The field value does not include any leading or trailing white space: OWS
1369   occurring before the first non-whitespace octet of the field value or after
1370   the last non-whitespace octet of the field value ought to be excluded by
1371   parsers when extracting the field value from a header field.
1374   Historically, HTTP header field values could be extended over multiple
1375   lines by preceding each extra line with at least one space or horizontal
1376   tab (obs-fold). This specification deprecates such line folding except
1377   within the message/http media type
1378   (<xref target=""/>).
1379   A sender &MUST-NOT; generate a message that includes line folding
1380   (i.e., that has any field-value that contains a match to the
1381   <x:ref>obs-fold</x:ref> rule) unless the message is intended for packaging
1382   within the message/http media type.
1385   A server that receives an <x:ref>obs-fold</x:ref> in a request message that
1386   is not within a message/http container &MUST; either reject the message by
1387   sending a <x:ref>400 (Bad Request)</x:ref>, preferably with a
1388   representation explaining that obsolete line folding is unacceptable, or
1389   replace each received <x:ref>obs-fold</x:ref> with one or more
1390   <x:ref>SP</x:ref> octets prior to interpreting the field value or
1391   forwarding the message downstream.
1394   A proxy or gateway that receives an <x:ref>obs-fold</x:ref> in a response
1395   message that is not within a message/http container &MUST; either discard
1396   the message and replace it with a <x:ref>502 (Bad Gateway)</x:ref>
1397   response, preferably with a representation explaining that unacceptable
1398   line folding was received, or replace each received <x:ref>obs-fold</x:ref>
1399   with one or more <x:ref>SP</x:ref> octets prior to interpreting the field
1400   value or forwarding the message downstream.
1403   A user agent that receives an <x:ref>obs-fold</x:ref> in a response message
1404   that is not within a message/http container &MUST; replace each received
1405   <x:ref>obs-fold</x:ref> with one or more <x:ref>SP</x:ref> octets prior to
1406   interpreting the field value.
1409   Historically, HTTP has allowed field content with text in the ISO-8859-1
1410   <xref target="ISO-8859-1"/> charset, supporting other charsets only
1411   through use of <xref target="RFC2047"/> encoding.
1412   In practice, most HTTP header field values use only a subset of the
1413   US-ASCII charset <xref target="USASCII"/>. Newly defined
1414   header fields &SHOULD; limit their field values to US-ASCII octets.
1415   A recipient &SHOULD; treat other octets in field content (obs-text) as
1416   opaque data.
1420<section title="Field Limits" anchor="field.limits">
1422   HTTP does not place a pre-defined limit on the length of each header field
1423   or on the length of the header section as a whole, as described in
1424   <xref target="conformance"/>. Various ad-hoc limitations on individual
1425   header field length are found in practice, often depending on the specific
1426   field semantics.
1429   A server ought to be prepared to receive request header fields of unbounded
1430   length and &MUST; respond with an appropriate
1431   <x:ref>4xx (Client Error)</x:ref> status code if the received header
1432   field(s) are larger than the server wishes to process.
1435   A client ought to be prepared to receive response header fields of
1436   unbounded length.
1437   A client &MAY; discard or truncate received header fields that are larger
1438   than the client wishes to process if the field semantics are such that the
1439   dropped value(s) can be safely ignored without changing the
1440   message framing or response semantics.
1444<section title="Field value components" anchor="field.components">
1445<t anchor="rule.token.separators">
1446  <x:anchor-alias value="tchar"/>
1447  <x:anchor-alias value="token"/>
1448  <iref item="Delimiters"/>
1449   Most HTTP header field values are defined using common syntax components
1450   (token, quoted-string, and comment) separated by whitespace or specific
1451   delimiting characters. Delimiters are chosen from the set of US-ASCII
1452   visual characters not allowed in a <x:ref>token</x:ref>
1453   (DQUOTE and "(),/:;&lt;=>?@[\]{}").
1455<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="token"/><iref primary="true" item="Grammar" subitem="tchar"/>
1456  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1458  NOTE: the definition of tchar and the prose above about special characters need to match!
1459 -->
1460  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1461                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1462                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1463                 ; any <x:ref>VCHAR</x:ref>, except delimiters
1465<t anchor="rule.quoted-string">
1466  <x:anchor-alias value="quoted-string"/>
1467  <x:anchor-alias value="qdtext"/>
1468  <x:anchor-alias value="obs-text"/>
1469   A string of text is parsed as a single value if it is quoted using
1470   double-quote marks.
1472<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"/>
1473  <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>
1474  <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>
1475  <x:ref>obs-text</x:ref>       = %x80-FF
1477<t anchor="rule.comment">
1478  <x:anchor-alias value="comment"/>
1479  <x:anchor-alias value="ctext"/>
1480   Comments can be included in some HTTP header fields by surrounding
1481   the comment text with parentheses. Comments are only allowed in
1482   fields containing "comment" as part of their field value definition.
1484<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1485  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-pair</x:ref> / <x:ref>comment</x:ref> ) ")"
1486  <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>
1488<t anchor="rule.quoted-pair">
1489  <x:anchor-alias value="quoted-pair"/>
1490   The backslash octet ("\") can be used as a single-octet
1491   quoting mechanism within quoted-string and comment constructs.
1492   Recipients that process the value of a quoted-string &MUST; handle a
1493   quoted-pair as if it were replaced by the octet following the backslash.
1495<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1496  <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> )
1499   A sender &SHOULD-NOT; generate a quoted-pair in a quoted-string except
1500   where necessary to quote DQUOTE and backslash octets occurring within that
1501   string.
1502   A sender &SHOULD-NOT; generate a quoted-pair in a comment except
1503   where necessary to quote parentheses ["(" and ")"] and backslash octets
1504   occurring within that comment.
1510<section title="Message Body" anchor="message.body">
1511  <x:anchor-alias value="message-body"/>
1513   The message body (if any) of an HTTP message is used to carry the
1514   payload body of that request or response.  The message body is
1515   identical to the payload body unless a transfer coding has been
1516   applied, as described in <xref target="header.transfer-encoding"/>.
1518<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1519  <x:ref>message-body</x:ref> = *OCTET
1522   The rules for when a message body is allowed in a message differ for
1523   requests and responses.
1526   The presence of a message body in a request is signaled by a
1527   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1528   field. Request message framing is independent of method semantics,
1529   even if the method does not define any use for a message body.
1532   The presence of a message body in a response depends on both
1533   the request method to which it is responding and the response
1534   status code (<xref target="status.line"/>).
1535   Responses to the HEAD request method (&HEAD;) never include a message body
1536   because the associated response header fields (e.g.,
1537   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1538   if present, indicate only what their values would have been if the request
1539   method had been GET (&GET;).
1540   <x:ref>2xx (Successful)</x:ref> responses to a CONNECT request method
1541   (&CONNECT;) switch to tunnel mode instead of having a message body.
1542   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1543   <x:ref>304 (Not Modified)</x:ref> responses do not include a message body.
1544   All other responses do include a message body, although the body
1545   might be of zero length.
1548<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1549  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1550  <iref item="chunked (Coding Format)"/>
1551  <x:anchor-alias value="Transfer-Encoding"/>
1553   The Transfer-Encoding header field lists the transfer coding names
1554   corresponding to the sequence of transfer codings that have been
1555   (or will be) applied to the payload body in order to form the message body.
1556   Transfer codings are defined in <xref target="transfer.codings"/>.
1558<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1559  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1562   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1563   MIME, which was designed to enable safe transport of binary data over a
1564   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1565   However, safe transport has a different focus for an 8bit-clean transfer
1566   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1567   accurately delimit a dynamically generated payload and to distinguish
1568   payload encodings that are only applied for transport efficiency or
1569   security from those that are characteristics of the selected resource.
1572   A recipient &MUST; be able to parse the chunked transfer coding
1573   (<xref target="chunked.encoding"/>) because it plays a crucial role in
1574   framing messages when the payload body size is not known in advance.
1575   A sender &MUST-NOT; apply chunked more than once to a message body
1576   (i.e., chunking an already chunked message is not allowed).
1577   If any transfer coding other than chunked is applied to a request payload
1578   body, the sender &MUST; apply chunked as the final transfer coding to
1579   ensure that the message is properly framed.
1580   If any transfer coding other than chunked is applied to a response payload
1581   body, the sender &MUST; either apply chunked as the final transfer coding
1582   or terminate the message by closing the connection.
1585   For example,
1586</preamble><artwork type="example">
1587  Transfer-Encoding: gzip, chunked
1589   indicates that the payload body has been compressed using the gzip
1590   coding and then chunked using the chunked coding while forming the
1591   message body.
1594   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1595   Transfer-Encoding is a property of the message, not of the representation, and
1596   any recipient along the request/response chain &MAY; decode the received
1597   transfer coding(s) or apply additional transfer coding(s) to the message
1598   body, assuming that corresponding changes are made to the Transfer-Encoding
1599   field-value. Additional information about the encoding parameters can be
1600   provided by other header fields not defined by this specification.
1603   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1604   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1605   neither of which includes a message body,
1606   to indicate that the origin server would have applied a transfer coding
1607   to the message body if the request had been an unconditional GET.
1608   This indication is not required, however, because any recipient on
1609   the response chain (including the origin server) can remove transfer
1610   codings when they are not needed.
1613   A server &MUST-NOT; send a Transfer-Encoding header field in any response
1614   with a status code of
1615   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1616   A server &MUST-NOT; send a Transfer-Encoding header field in any
1617   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1620   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1621   implementations advertising only HTTP/1.0 support will not understand
1622   how to process a transfer-encoded payload.
1623   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1624   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1625   might be in the form of specific user configuration or by remembering the
1626   version of a prior received response.
1627   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1628   the corresponding request indicates HTTP/1.1 (or later).
1631   A server that receives a request message with a transfer coding it does
1632   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref>.
1636<section title="Content-Length" anchor="header.content-length">
1637  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1638  <x:anchor-alias value="Content-Length"/>
1640   When a message does not have a <x:ref>Transfer-Encoding</x:ref> header
1641   field, a Content-Length header field can provide the anticipated size,
1642   as a decimal number of octets, for a potential payload body.
1643   For messages that do include a payload body, the Content-Length field-value
1644   provides the framing information necessary for determining where the body
1645   (and message) ends.  For messages that do not include a payload body, the
1646   Content-Length indicates the size of the selected representation
1647   (&representation;).
1649<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1650  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1653   An example is
1655<figure><artwork type="example">
1656  Content-Length: 3495
1659   A sender &MUST-NOT; send a Content-Length header field in any message that
1660   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1663   A user agent &SHOULD; send a Content-Length in a request message when no
1664   <x:ref>Transfer-Encoding</x:ref> is sent and the request method defines
1665   a meaning for an enclosed payload body. For example, a Content-Length
1666   header field is normally sent in a POST request even when the value is
1667   0 (indicating an empty payload body).  A user agent &SHOULD-NOT; send a
1668   Content-Length header field when the request message does not contain a
1669   payload body and the method semantics do not anticipate such a body.
1672   A server &MAY; send a Content-Length header field in a response to a HEAD
1673   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1674   response unless its field-value equals the decimal number of octets that
1675   would have been sent in the payload body of a response if the same
1676   request had used the GET method.
1679   A server &MAY; send a Content-Length header field in a
1680   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1681   (&status-304;); a server &MUST-NOT; send Content-Length in such a
1682   response unless its field-value equals the decimal number of octets that
1683   would have been sent in the payload body of a <x:ref>200 (OK)</x:ref>
1684   response to the same request.
1687   A server &MUST-NOT; send a Content-Length header field in any response
1688   with a status code of
1689   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1690   A server &MUST-NOT; send a Content-Length header field in any
1691   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1694   Aside from the cases defined above, in the absence of Transfer-Encoding,
1695   an origin server &SHOULD; send a Content-Length header field when the
1696   payload body size is known prior to sending the complete header section.
1697   This will allow downstream recipients to measure transfer progress,
1698   know when a received message is complete, and potentially reuse the
1699   connection for additional requests.
1702   Any Content-Length field value greater than or equal to zero is valid.
1703   Since there is no predefined limit to the length of a payload, a
1704   recipient &MUST; anticipate potentially large decimal numerals and
1705   prevent parsing errors due to integer conversion overflows
1706   (<xref target="attack.protocol.element.size.overflows"/>).
1709   If a message is received that has multiple Content-Length header fields
1710   with field-values consisting of the same decimal value, or a single
1711   Content-Length header field with a field value containing a list of
1712   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1713   duplicate Content-Length header fields have been generated or combined by an
1714   upstream message processor, then the recipient &MUST; either reject the
1715   message as invalid or replace the duplicated field-values with a single
1716   valid Content-Length field containing that decimal value prior to
1717   determining the message body length or forwarding the message.
1720  <t>
1721   &Note; HTTP's use of Content-Length for message framing differs
1722   significantly from the same field's use in MIME, where it is an optional
1723   field used only within the "message/external-body" media-type.
1724  </t>
1728<section title="Message Body Length" anchor="message.body.length">
1729  <iref item="chunked (Coding Format)"/>
1731   The length of a message body is determined by one of the following
1732   (in order of precedence):
1735  <list style="numbers">
1736    <x:lt><t>
1737     Any response to a HEAD request and any response with a
1738     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1739     <x:ref>304 (Not Modified)</x:ref> status code is always
1740     terminated by the first empty line after the header fields, regardless of
1741     the header fields present in the message, and thus cannot contain a
1742     message body.
1743    </t></x:lt>
1744    <x:lt><t>
1745     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1746     connection will become a tunnel immediately after the empty line that
1747     concludes the header fields.  A client &MUST; ignore any
1748     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1749     fields received in such a message.
1750    </t></x:lt>
1751    <x:lt><t>
1752     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1753     and the chunked transfer coding (<xref target="chunked.encoding"/>)
1754     is the final encoding, the message body length is determined by reading
1755     and decoding the chunked data until the transfer coding indicates the
1756     data is complete.
1757    </t>
1758    <t>
1759     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1760     response and the chunked transfer coding is not the final encoding, the
1761     message body length is determined by reading the connection until it is
1762     closed by the server.
1763     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a request and the
1764     chunked transfer coding is not the final encoding, the message body
1765     length cannot be determined reliably; the server &MUST; respond with
1766     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1767    </t>
1768    <t>
1769     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1770     and a <x:ref>Content-Length</x:ref> header field, the Transfer-Encoding
1771     overrides the Content-Length. Such a message might indicate an attempt
1772     to perform request or response smuggling (bypass of security-related
1773     checks on message routing or content) and thus ought to be handled as
1774     an error.  A sender &MUST; remove the received Content-Length field
1775     prior to forwarding such a message downstream.
1776    </t></x:lt>
1777    <x:lt><t>
1778     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1779     either multiple <x:ref>Content-Length</x:ref> header fields having
1780     differing field-values or a single Content-Length header field having an
1781     invalid value, then the message framing is invalid and
1782     the recipient &MUST; treat it as an unrecoverable error to prevent
1783     request or response smuggling.
1784     If this is a request message, the server &MUST; respond with
1785     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1786     If this is a response message received by a proxy,
1787     the proxy &MUST; close the connection to the server, discard the received
1788     response, and send a <x:ref>502 (Bad Gateway)</x:ref> response to the
1789     client.
1790     If this is a response message received by a user agent,
1791     the user agent &MUST; close the connection to the server and discard the
1792     received response.
1793    </t></x:lt>
1794    <x:lt><t>
1795     If a valid <x:ref>Content-Length</x:ref> header field is present without
1796     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1797     expected message body length in octets.
1798     If the sender closes the connection or the recipient times out before the
1799     indicated number of octets are received, the recipient &MUST; consider
1800     the message to be incomplete and close the connection.
1801    </t></x:lt>
1802    <x:lt><t>
1803     If this is a request message and none of the above are true, then the
1804     message body length is zero (no message body is present).
1805    </t></x:lt>
1806    <x:lt><t>
1807     Otherwise, this is a response message without a declared message body
1808     length, so the message body length is determined by the number of octets
1809     received prior to the server closing the connection.
1810    </t></x:lt>
1811  </list>
1814   Since there is no way to distinguish a successfully completed,
1815   close-delimited message from a partially-received message interrupted
1816   by network failure, a server &SHOULD; generate encoding or
1817   length-delimited messages whenever possible.  The close-delimiting
1818   feature exists primarily for backwards compatibility with HTTP/1.0.
1821   A server &MAY; reject a request that contains a message body but
1822   not a <x:ref>Content-Length</x:ref> by responding with
1823   <x:ref>411 (Length Required)</x:ref>.
1826   Unless a transfer coding other than chunked has been applied,
1827   a client that sends a request containing a message body &SHOULD;
1828   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1829   length is known in advance, rather than the chunked transfer coding, since some
1830   existing services respond to chunked with a <x:ref>411 (Length Required)</x:ref>
1831   status code even though they understand the chunked transfer coding.  This
1832   is typically because such services are implemented via a gateway that
1833   requires a content-length in advance of being called and the server
1834   is unable or unwilling to buffer the entire request before processing.
1837   A user agent that sends a request containing a message body &MUST; send a
1838   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1839   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1840   the form of specific user configuration or by remembering the version of a
1841   prior received response.
1844   If the final response to the last request on a connection has been
1845   completely received and there remains additional data to read, a user agent
1846   &MAY; discard the remaining data or attempt to determine if that data
1847   belongs as part of the prior response body, which might be the case if the
1848   prior message's Content-Length value is incorrect. A client &MUST-NOT;
1849   process, cache, or forward such extra data as a separate response, since
1850   such behavior would be vulnerable to cache poisoning.
1855<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1857   A server that receives an incomplete request message, usually due to a
1858   canceled request or a triggered time-out exception, &MAY; send an error
1859   response prior to closing the connection.
1862   A client that receives an incomplete response message, which can occur
1863   when a connection is closed prematurely or when decoding a supposedly
1864   chunked transfer coding fails, &MUST; record the message as incomplete.
1865   Cache requirements for incomplete responses are defined in
1866   &cache-incomplete;.
1869   If a response terminates in the middle of the header section (before the
1870   empty line is received) and the status code might rely on header fields to
1871   convey the full meaning of the response, then the client cannot assume
1872   that meaning has been conveyed; the client might need to repeat the
1873   request in order to determine what action to take next.
1876   A message body that uses the chunked transfer coding is
1877   incomplete if the zero-sized chunk that terminates the encoding has not
1878   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1879   incomplete if the size of the message body received (in octets) is less than
1880   the value given by Content-Length.  A response that has neither chunked
1881   transfer coding nor Content-Length is terminated by closure of the
1882   connection, and thus is considered complete regardless of the number of
1883   message body octets received, provided that the header section was received
1884   intact.
1888<section title="Message Parsing Robustness" anchor="message.robustness">
1890   Older HTTP/1.0 user agent implementations might send an extra CRLF
1891   after a POST request as a workaround for some early server
1892   applications that failed to read message body content that was
1893   not terminated by a line-ending. An HTTP/1.1 user agent &MUST-NOT;
1894   preface or follow a request with an extra CRLF.  If terminating
1895   the request message body with a line-ending is desired, then the
1896   user agent &MUST; count the terminating CRLF octets as part of the
1897   message body length.
1900   In the interest of robustness, a server that is expecting to receive and
1901   parse a request-line &SHOULD; ignore at least one empty line (CRLF)
1902   received prior to the request-line.
1905   Although the line terminator for the start-line and header
1906   fields is the sequence CRLF, a recipient &MAY; recognize a
1907   single LF as a line terminator and ignore any preceding CR.
1910   Although the request-line and status-line grammar rules require that each
1911   of the component elements be separated by a single SP octet, recipients
1912   &MAY; instead parse on whitespace-delimited word boundaries and, aside
1913   from the CRLF terminator, treat any form of whitespace as the SP separator
1914   while ignoring preceding or trailing whitespace;
1915   such whitespace includes one or more of the following octets:
1916   SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
1919   When a server listening only for HTTP request messages, or processing
1920   what appears from the start-line to be an HTTP request message,
1921   receives a sequence of octets that does not match the HTTP-message
1922   grammar aside from the robustness exceptions listed above, the
1923   server &SHOULD; respond with a <x:ref>400 (Bad Request)</x:ref> response. 
1928<section title="Transfer Codings" anchor="transfer.codings">
1929  <x:anchor-alias value="transfer-coding"/>
1930  <x:anchor-alias value="transfer-extension"/>
1932   Transfer coding names are used to indicate an encoding
1933   transformation that has been, can be, or might need to be applied to a
1934   payload body in order to ensure "safe transport" through the network.
1935   This differs from a content coding in that the transfer coding is a
1936   property of the message rather than a property of the representation
1937   that is being transferred.
1939<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1940  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1941                     / "compress" ; <xref target="compress.coding"/>
1942                     / "deflate" ; <xref target="deflate.coding"/>
1943                     / "gzip" ; <xref target="gzip.coding"/>
1944                     / <x:ref>transfer-extension</x:ref>
1945  <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> )
1947<t anchor="rule.parameter">
1948  <x:anchor-alias value="transfer-parameter"/>
1949   Parameters are in the form of a name or name=value pair.
1951<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-parameter"/>
1952  <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> )
1955   All transfer-coding names are case-insensitive and ought to be registered
1956   within the HTTP Transfer Coding registry, as defined in
1957   <xref target="transfer.coding.registry"/>.
1958   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1959   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1960   header fields.
1963<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1964  <iref primary="true" item="chunked (Coding Format)"/>
1965  <x:anchor-alias value="chunk"/>
1966  <x:anchor-alias value="chunked-body"/>
1967  <x:anchor-alias value="chunk-data"/>
1968  <x:anchor-alias value="chunk-size"/>
1969  <x:anchor-alias value="last-chunk"/>
1971   The chunked transfer coding wraps the payload body in order to transfer it
1972   as a series of chunks, each with its own size indicator, followed by an
1973   &OPTIONAL; trailer containing header fields. Chunked enables content
1974   streams of unknown size to be transferred as a sequence of length-delimited
1975   buffers, which enables the sender to retain connection persistence and the
1976   recipient to know when it has received the entire message.
1978<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="false" item="Grammar" subitem="trailer-part"/><iref primary="false" item="Grammar" subitem="chunk-ext"/><iref primary="true" item="Grammar" subitem="chunk-data"/>
1979  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1980                   <x:ref>last-chunk</x:ref>
1981                   <x:ref>trailer-part</x:ref>
1982                   <x:ref>CRLF</x:ref>
1984  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1985                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1986  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1987  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1989  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1992   The chunk-size field is a string of hex digits indicating the size of
1993   the chunk-data in octets. The chunked transfer coding is complete when a
1994   chunk with a chunk-size of zero is received, possibly followed by a
1995   trailer, and finally terminated by an empty line.
1998   A recipient &MUST; be able to parse and decode the chunked transfer coding.
2001<section title="Chunk Extensions" anchor="chunked.extension">
2002  <x:anchor-alias value="chunk-ext"/>
2003  <x:anchor-alias value="chunk-ext-name"/>
2004  <x:anchor-alias value="chunk-ext-val"/>
2006   The chunked encoding allows each chunk to include zero or more chunk
2007   extensions, immediately following the <x:ref>chunk-size</x:ref>, for the
2008   sake of supplying per-chunk metadata (such as a signature or hash),
2009   mid-message control information, or randomization of message body size.
2011<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="chunked-body"><!--terminal production--></iref><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"/>
2012  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
2014  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
2015  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
2018   The chunked encoding is specific to each connection and is likely to be
2019   removed or recoded by each recipient (including intermediaries) before any
2020   higher-level application would have a chance to inspect the extensions.
2021   Hence, use of chunk extensions is generally limited to specialized HTTP
2022   services such as "long polling" (where client and server can have shared
2023   expectations regarding the use of chunk extensions) or for padding within
2024   an end-to-end secured connection.
2027   A recipient &MUST; ignore unrecognized chunk extensions.
2028   A server ought to limit the total length of chunk extensions received in a
2029   request to an amount reasonable for the services provided, in the same way
2030   that it applies length limitations and timeouts for other parts of a
2031   message, and generate an appropriate <x:ref>4xx (Client Error)</x:ref>
2032   response if that amount is exceeded.
2036<section title="Chunked Trailer Part" anchor="chunked.trailer.part">
2037  <x:anchor-alias value="trailer-part"/>
2039   A trailer allows the sender to include additional fields at the end of a
2040   chunked message in order to supply metadata that might be dynamically
2041   generated while the message body is sent, such as a message integrity
2042   check, digital signature, or post-processing status. The trailer fields are
2043   identical to header fields, except they are sent in a chunked trailer
2044   instead of the message's header section.
2046<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="trailer-part"/><iref primary="false" item="Grammar" subitem="header-field"/>
2047  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
2050   A sender &MUST-NOT; generate a trailer that contains a field necessary for
2051   message framing (e.g., <x:ref>Transfer-Encoding</x:ref> and
2052   <x:ref>Content-Length</x:ref>), routing (e.g., <x:ref>Host</x:ref>),
2053   request modifiers (e.g., controls and conditionals in
2054   &request-header-fields;), authentication (e.g., see <xref target="Part7"/>
2055   and <xref target="RFC6265"/>), response control data (e.g., see
2056   &response-control-data;), or determining how to process the payload
2057   (e.g., <x:ref>Content-Encoding</x:ref>, <x:ref>Content-Type</x:ref>,
2058   <x:ref>Content-Range</x:ref>, and <x:ref>Trailer</x:ref>).
2061   When a chunked message containing a non-empty trailer is received, the
2062   recipient &MAY; process the fields (aside from those forbidden above)
2063   as if they were appended to the message's header section.
2064   A recipient &MUST; ignore (or consider as an error) any fields that are
2065   forbidden to be sent in a trailer, since processing them as if they were
2066   present in the header section might bypass external security filters.
2069   Unless the request includes a <x:ref>TE</x:ref> header field indicating
2070   "trailers" is acceptable, as described in <xref target="header.te"/>, a
2071   server &SHOULD-NOT; generate trailer fields that it believes are necessary
2072   for the user agent to receive. Without a TE containing "trailers", the
2073   server ought to assume that the trailer fields might be silently discarded
2074   along the path to the user agent. This requirement allows intermediaries to
2075   forward a de-chunked message to an HTTP/1.0 recipient without buffering the
2076   entire response.
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 trailer field
2095  while (trailer field is not empty) {
2096     if trailer field is allowed to be sent in a trailer,
2097         append trailer field to existing header fields
2098     read trailer-field
2099  }
2100  Content-Length := length
2101  Remove "chunked" from Transfer-Encoding
2102  Remove Trailer from existing header fields
2107<section title="Compression Codings" anchor="compression.codings">
2109   The codings defined below can be used to compress the payload of a
2110   message.
2113<section title="Compress Coding" anchor="compress.coding">
2114<iref item="compress (Coding Format)"/>
2116   The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding
2117   <xref target="Welch"/> that is commonly produced by the UNIX file
2118   compression program "compress".
2119   A recipient &SHOULD; consider "x-compress" to be equivalent to "compress".
2123<section title="Deflate Coding" anchor="deflate.coding">
2124<iref item="deflate (Coding Format)"/>
2126   The "deflate" coding is a "zlib" data format <xref target="RFC1950"/>
2127   containing a "deflate" compressed data stream <xref target="RFC1951"/>
2128   that uses a combination of the Lempel-Ziv (LZ77) compression algorithm and
2129   Huffman coding.
2132  <t>
2133    &Note; Some incorrect implementations send the "deflate"
2134    compressed data without the zlib wrapper.
2135   </t>
2139<section title="Gzip Coding" anchor="gzip.coding">
2140<iref item="gzip (Coding Format)"/>
2142   The "gzip" coding is an LZ77 coding with a 32 bit CRC that is commonly
2143   produced by the gzip file compression program <xref target="RFC1952"/>.
2144   A recipient &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2150<section title="TE" anchor="header.te">
2151  <iref primary="true" item="TE header field" x:for-anchor=""/>
2152  <x:anchor-alias value="TE"/>
2153  <x:anchor-alias value="t-codings"/>
2154  <x:anchor-alias value="t-ranking"/>
2155  <x:anchor-alias value="rank"/>
2157   The "TE" header field in a request indicates what transfer codings,
2158   besides chunked, the client is willing to accept in response, and
2159   whether or not the client is willing to accept trailer fields in a
2160   chunked transfer coding.
2163   The TE field-value consists of a comma-separated list of transfer coding
2164   names, each allowing for optional parameters (as described in
2165   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2166   A client &MUST-NOT; send the chunked transfer coding name in TE;
2167   chunked is always acceptable for HTTP/1.1 recipients.
2169<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="TE"/><iref primary="true" item="Grammar" subitem="t-codings"/><iref primary="true" item="Grammar" subitem="t-ranking"/><iref primary="true" item="Grammar" subitem="rank"/>
2170  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2171  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2172  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2173  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2174             / ( "1" [ "." 0*3("0") ] )
2177   Three examples of TE use are below.
2179<figure><artwork type="example">
2180  TE: deflate
2181  TE:
2182  TE: trailers, deflate;q=0.5
2185   The presence of the keyword "trailers" indicates that the client is willing
2186   to accept trailer fields in a chunked transfer coding, as defined in
2187   <xref target="chunked.trailer.part"/>, on behalf of itself and any downstream
2188   clients. For requests from an intermediary, this implies that either:
2189   (a) all downstream clients are willing to accept trailer fields in the
2190   forwarded response; or,
2191   (b) the intermediary will attempt to buffer the response on behalf of
2192   downstream recipients.
2193   Note that HTTP/1.1 does not define any means to limit the size of a
2194   chunked response such that an intermediary can be assured of buffering the
2195   entire response.
2198   When multiple transfer codings are acceptable, the client &MAY; rank the
2199   codings by preference using a case-insensitive "q" parameter (similar to
2200   the qvalues used in content negotiation fields, &qvalue;). The rank value
2201   is a real number in the range 0 through 1, where 0.001 is the least
2202   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2205   If the TE field-value is empty or if no TE field is present, the only
2206   acceptable transfer coding is chunked. A message with no transfer coding
2207   is always acceptable.
2210   Since the TE header field only applies to the immediate connection,
2211   a sender of TE &MUST; also send a "TE" connection option within the
2212   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2213   in order to prevent the TE field from being forwarded by intermediaries
2214   that do not support its semantics.
2218<section title="Trailer" anchor="header.trailer">
2219  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
2220  <x:anchor-alias value="Trailer"/>
2222   When a message includes a message body encoded with the chunked
2223   transfer coding and the sender desires to send metadata in the form of
2224   trailer fields at the end of the message, the sender &SHOULD; generate a
2225   <x:ref>Trailer</x:ref> header field before the message body to indicate
2226   which fields will be present in the trailers. This allows the recipient
2227   to prepare for receipt of that metadata before it starts processing the body,
2228   which is useful if the message is being streamed and the recipient wishes
2229   to confirm an integrity check on the fly.
2231<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/><iref primary="false" item="Grammar" subitem="field-name"/>
2232  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
2237<section title="Message Routing" anchor="message.routing">
2239   HTTP request message routing is determined by each client based on the
2240   target resource, the client's proxy configuration, and
2241   establishment or reuse of an inbound connection.  The corresponding
2242   response routing follows the same connection chain back to the client.
2245<section title="Identifying a Target Resource" anchor="target-resource">
2246  <iref primary="true" item="target resource"/>
2247  <iref primary="true" item="target URI"/>
2248  <x:anchor-alias value="target resource"/>
2249  <x:anchor-alias value="target URI"/>
2251   HTTP is used in a wide variety of applications, ranging from
2252   general-purpose computers to home appliances.  In some cases,
2253   communication options are hard-coded in a client's configuration.
2254   However, most HTTP clients rely on the same resource identification
2255   mechanism and configuration techniques as general-purpose Web browsers.
2258   HTTP communication is initiated by a user agent for some purpose.
2259   The purpose is a combination of request semantics, which are defined in
2260   <xref target="Part2"/>, and a target resource upon which to apply those
2261   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2262   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2263   would resolve to its absolute form in order to obtain the
2264   "<x:dfn>target URI</x:dfn>".  The target URI
2265   excludes the reference's fragment component, if any,
2266   since fragment identifiers are reserved for client-side processing
2267   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2271<section title="Connecting Inbound" anchor="connecting.inbound">
2273   Once the target URI is determined, a client needs to decide whether
2274   a network request is necessary to accomplish the desired semantics and,
2275   if so, where that request is to be directed.
2278   If the client has a cache <xref target="Part6"/> and the request can be
2279   satisfied by it, then the request is
2280   usually directed there first.
2283   If the request is not satisfied by a cache, then a typical client will
2284   check its configuration to determine whether a proxy is to be used to
2285   satisfy the request.  Proxy configuration is implementation-dependent,
2286   but is often based on URI prefix matching, selective authority matching,
2287   or both, and the proxy itself is usually identified by an "http" or
2288   "https" URI.  If a proxy is applicable, the client connects inbound by
2289   establishing (or reusing) a connection to that proxy.
2292   If no proxy is applicable, a typical client will invoke a handler routine,
2293   usually specific to the target URI's scheme, to connect directly
2294   to an authority for the target resource.  How that is accomplished is
2295   dependent on the target URI scheme and defined by its associated
2296   specification, similar to how this specification defines origin server
2297   access for resolution of the "http" (<xref target="http.uri"/>) and
2298   "https" (<xref target="https.uri"/>) schemes.
2301   HTTP requirements regarding connection management are defined in
2302   <xref target=""/>.
2306<section title="Request Target" anchor="request-target">
2308   Once an inbound connection is obtained,
2309   the client sends an HTTP request message (<xref target="http.message"/>)
2310   with a request-target derived from the target URI.
2311   There are four distinct formats for the request-target, depending on both
2312   the method being requested and whether the request is to a proxy.
2314<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-target"/><iref primary="true" item="Grammar" subitem="origin-form"/><iref primary="true" item="Grammar" subitem="absolute-form"/><iref primary="true" item="Grammar" subitem="authority-form"/><iref primary="true" item="Grammar" subitem="asterisk-form"/>
2315  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2316                 / <x:ref>absolute-form</x:ref>
2317                 / <x:ref>authority-form</x:ref>
2318                 / <x:ref>asterisk-form</x:ref>
2320  <x:ref>origin-form</x:ref>    = <x:ref>absolute-path</x:ref> [ "?" <x:ref>query</x:ref> ]
2321  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2322  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2323  <x:ref>asterisk-form</x:ref>  = "*"
2325<t anchor="origin-form"><iref item="origin-form (of request-target)"/>
2326  <x:h>origin-form</x:h>
2329   The most common form of request-target is the <x:dfn>origin-form</x:dfn>.
2330   When making a request directly to an origin server, other than a CONNECT
2331   or server-wide OPTIONS request (as detailed below),
2332   a client &MUST; send only the absolute path and query components of
2333   the target URI as the request-target.
2334   If the target URI's path component is empty, then the client &MUST; send
2335   "/" as the path within the origin-form of request-target.
2336   A <x:ref>Host</x:ref> header field is also sent, as defined in
2337   <xref target=""/>.
2340   For example, a client wishing to retrieve a representation of the resource
2341   identified as
2343<figure><artwork x:indent-with="  " type="example">
2347   directly from the origin server would open (or reuse) a TCP connection
2348   to port 80 of the host "" and send the lines:
2350<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2351GET /where?q=now HTTP/1.1
2355   followed by the remainder of the request message.
2357<t anchor="absolute-form"><iref item="absolute-form (of request-target)"/>
2358  <x:h>absolute-form</x:h>
2361   When making a request to a proxy, other than a CONNECT or server-wide
2362   OPTIONS request (as detailed below), a client &MUST; send the target URI
2363   in <x:dfn>absolute-form</x:dfn> as the request-target.
2364   The proxy is requested to either service that request from a valid cache,
2365   if possible, or make the same request on the client's behalf to either
2366   the next inbound proxy server or directly to the origin server indicated
2367   by the request-target.  Requirements on such "forwarding" of messages are
2368   defined in <xref target="message.forwarding"/>.
2371   An example absolute-form of request-line would be:
2373<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2374GET HTTP/1.1
2377   To allow for transition to the absolute-form for all requests in some
2378   future version of HTTP, a server &MUST; accept the absolute-form
2379   in requests, even though HTTP/1.1 clients will only send them in requests
2380   to proxies.
2382<t anchor="authority-form"><iref item="authority-form (of request-target)"/>
2383  <x:h>authority-form</x:h>
2386   The <x:dfn>authority-form</x:dfn> of request-target is only used for
2387   CONNECT requests (&CONNECT;). When making a CONNECT request to establish a
2388   tunnel through one or more proxies, a client &MUST; send only the target
2389   URI's authority component (excluding any userinfo and its "@" delimiter) as
2390   the request-target. For example,
2392<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2395<t anchor="asterisk-form"><iref item="asterisk-form (of request-target)"/>
2396  <x:h>asterisk-form</x:h>
2399   The <x:dfn>asterisk-form</x:dfn> of request-target is only used for a server-wide
2400   OPTIONS request (&OPTIONS;).  When a client wishes to request OPTIONS
2401   for the server as a whole, as opposed to a specific named resource of
2402   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2403   For example,
2405<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2406OPTIONS * HTTP/1.1
2409   If a proxy receives an OPTIONS request with an absolute-form of
2410   request-target in which the URI has an empty path and no query component,
2411   then the last proxy on the request chain &MUST; send a request-target
2412   of "*" when it forwards the request to the indicated origin server.
2415   For example, the request
2416</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2420  would be forwarded by the final proxy as
2421</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2422OPTIONS * HTTP/1.1
2426   after connecting to port 8001 of host "".
2431<section title="Host" anchor="">
2432  <iref primary="true" item="Host header field" x:for-anchor=""/>
2433  <x:anchor-alias value="Host"/>
2435   The "Host" header field in a request provides the host and port
2436   information from the target URI, enabling the origin
2437   server to distinguish among resources while servicing requests
2438   for multiple host names on a single IP address.
2440<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2441  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2444   A client &MUST; send a Host header field in all HTTP/1.1 request messages.
2445   If the target URI includes an authority component, then a client &MUST;
2446   send a field-value for Host that is identical to that authority
2447   component, excluding any userinfo subcomponent and its "@" delimiter
2448   (<xref target="http.uri"/>).
2449   If the authority component is missing or undefined for the target URI,
2450   then a client &MUST; send a Host header field with an empty field-value.
2453   Since the Host field-value is critical information for handling a request,
2454   a user agent &SHOULD; generate Host as the first header field following the
2455   request-line.
2458   For example, a GET request to the origin server for
2459   &lt;; would begin with:
2461<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2462GET /pub/WWW/ HTTP/1.1
2466   A client &MUST; send a Host header field in an HTTP/1.1 request even
2467   if the request-target is in the absolute-form, since this
2468   allows the Host information to be forwarded through ancient HTTP/1.0
2469   proxies that might not have implemented Host.
2472   When a proxy receives a request with an absolute-form of
2473   request-target, the proxy &MUST; ignore the received
2474   Host header field (if any) and instead replace it with the host
2475   information of the request-target.  A proxy that forwards such a request
2476   &MUST; generate a new Host field-value based on the received
2477   request-target rather than forward the received Host field-value.
2480   Since the Host header field acts as an application-level routing
2481   mechanism, it is a frequent target for malware seeking to poison
2482   a shared cache or redirect a request to an unintended server.
2483   An interception proxy is particularly vulnerable if it relies on
2484   the Host field-value for redirecting requests to internal
2485   servers, or for use as a cache key in a shared cache, without
2486   first verifying that the intercepted connection is targeting a
2487   valid IP address for that host.
2490   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2491   to any HTTP/1.1 request message that lacks a Host header field and
2492   to any request message that contains more than one Host header field
2493   or a Host header field with an invalid field-value.
2497<section title="Effective Request URI" anchor="effective.request.uri">
2498  <iref primary="true" item="effective request URI"/>
2499  <x:anchor-alias value="effective request URI"/>
2501   Since the request-target often contains only part of the user agent's
2502   target URI, a server reconstructs the intended target as an
2503   "<x:dfn>effective request URI</x:dfn>" to properly service the request.
2504   This reconstruction involves both the server's local configuration and
2505   information communicated in the <x:ref>request-target</x:ref>,
2506   <x:ref>Host</x:ref> header field, and connection context.
2509   For a user agent, the effective request URI is the target URI.
2512   If the <x:ref>request-target</x:ref> is in <x:ref>absolute-form</x:ref>,
2513   the effective request URI is the same as the request-target. Otherwise, the
2514   effective request URI is constructed as follows:
2515<list style="empty">
2517   If the server's configuration (or outbound gateway) provides a fixed URI
2518   <x:ref>scheme</x:ref>, that scheme is used for the effective request URI.
2519   Otherwise, if the request is received over a TLS-secured TCP connection,
2520   the effective request URI's scheme is "https"; if not, the scheme is "http".
2523   If the server's configuration (or outbound gateway) provides a fixed URI
2524   <x:ref>authority</x:ref> component, that authority is used for the
2525   effective request URI. If not, then if the request-target is in
2526   <x:ref>authority-form</x:ref>, the effective request URI's authority
2527   component is the same as the request-target.
2528   If not, then if a <x:ref>Host</x:ref> header field is supplied with a
2529   non-empty field-value, the authority component is the same as the
2530   Host field-value. Otherwise, the authority component is assigned
2531   the default name configured for the server and, if the connection's
2532   incoming TCP port number differs from the default port for the effective
2533   request URI's scheme, then a colon (":") and the incoming port number (in
2534   decimal form) are appended to the authority component.
2537   If the request-target is in <x:ref>authority-form</x:ref> or
2538   <x:ref>asterisk-form</x:ref>, the effective request URI's combined
2539   <x:ref>path</x:ref> and <x:ref>query</x:ref> component is empty. Otherwise,
2540   the combined <x:ref>path</x:ref> and <x:ref>query</x:ref> component is the
2541   same as the request-target.
2544   The components of the effective request URI, once determined as above, can
2545   be combined into <x:ref>absolute-URI</x:ref> form by concatenating the
2546   scheme, "://", authority, and combined path and query component.
2552   Example 1: the following message received over an insecure TCP connection
2554<artwork type="example" x:indent-with="  ">
2555GET /pub/WWW/TheProject.html HTTP/1.1
2561  has an effective request URI of
2563<artwork type="example" x:indent-with="  ">
2569   Example 2: the following message received over a TLS-secured TCP connection
2571<artwork type="example" x:indent-with="  ">
2572OPTIONS * HTTP/1.1
2578  has an effective request URI of
2580<artwork type="example" x:indent-with="  ">
2585   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2586   field might need to use heuristics (e.g., examination of the URI path for
2587   something unique to a particular host) in order to guess the
2588   effective request URI's authority component.
2591   Once the effective request URI has been constructed, an origin server needs
2592   to decide whether or not to provide service for that URI via the connection
2593   in which the request was received. For example, the request might have been
2594   misdirected, deliberately or accidentally, such that the information within
2595   a received <x:ref>request-target</x:ref> or <x:ref>Host</x:ref> header
2596   field differs from the host or port upon which the connection has been
2597   made. If the connection is from a trusted gateway, that inconsistency might
2598   be expected; otherwise, it might indicate an attempt to bypass security
2599   filters, trick the server into delivering non-public content, or poison a
2600   cache. See <xref target="security.considerations"/> for security
2601   considerations regarding message routing.
2605<section title="Associating a Response to a Request" anchor="">
2607   HTTP does not include a request identifier for associating a given
2608   request message with its corresponding one or more response messages.
2609   Hence, it relies on the order of response arrival to correspond exactly
2610   to the order in which requests are made on the same connection.
2611   More than one response message per request only occurs when one or more
2612   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a
2613   final response to the same request.
2616   A client that has more than one outstanding request on a connection &MUST;
2617   maintain a list of outstanding requests in the order sent and &MUST;
2618   associate each received response message on that connection to the highest
2619   ordered request that has not yet received a final (non-<x:ref>1xx</x:ref>)
2620   response.
2624<section title="Message Forwarding" anchor="message.forwarding">
2626   As described in <xref target="intermediaries"/>, intermediaries can serve
2627   a variety of roles in the processing of HTTP requests and responses.
2628   Some intermediaries are used to improve performance or availability.
2629   Others are used for access control or to filter content.
2630   Since an HTTP stream has characteristics similar to a pipe-and-filter
2631   architecture, there are no inherent limits to the extent an intermediary
2632   can enhance (or interfere) with either direction of the stream.
2635   An intermediary not acting as a tunnel &MUST; implement the
2636   <x:ref>Connection</x:ref> header field, as specified in
2637   <xref target="header.connection"/>, and exclude fields from being forwarded
2638   that are only intended for the incoming connection.
2641   An intermediary &MUST-NOT; forward a message to itself unless it is
2642   protected from an infinite request loop. In general, an intermediary ought
2643   to recognize its own server names, including any aliases, local variations,
2644   or literal IP addresses, and respond to such requests directly.
2647<section title="Via" anchor="header.via">
2648  <iref primary="true" item="Via header field" x:for-anchor=""/>
2649  <x:anchor-alias value="pseudonym"/>
2650  <x:anchor-alias value="received-by"/>
2651  <x:anchor-alias value="received-protocol"/>
2652  <x:anchor-alias value="Via"/>
2654   The "Via" header field indicates the presence of intermediate protocols and
2655   recipients between the user agent and the server (on requests) or between
2656   the origin server and the client (on responses), similar to the
2657   "Received" header field in email
2658   (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2659   Via can be used for tracking message forwards,
2660   avoiding request loops, and identifying the protocol capabilities of
2661   senders along the request/response chain.
2663<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"/>
2664  <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> ] )
2666  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2667                      ; see <xref target="header.upgrade"/>
2668  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2669  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2672   Multiple Via field values represent each proxy or gateway that has
2673   forwarded the message. Each intermediary appends its own information
2674   about how the message was received, such that the end result is ordered
2675   according to the sequence of forwarding recipients.
2678   A proxy &MUST; send an appropriate Via header field, as described below, in
2679   each message that it forwards.
2680   An HTTP-to-HTTP gateway &MUST; send an appropriate Via header field in
2681   each inbound request message and &MAY; send a Via header field in
2682   forwarded response messages.
2685   For each intermediary, the received-protocol indicates the protocol and
2686   protocol version used by the upstream sender of the message. Hence, the
2687   Via field value records the advertised protocol capabilities of the
2688   request/response chain such that they remain visible to downstream
2689   recipients; this can be useful for determining what backwards-incompatible
2690   features might be safe to use in response, or within a later request, as
2691   described in <xref target="http.version"/>. For brevity, the protocol-name
2692   is omitted when the received protocol is HTTP.
2695   The received-by portion of the field value is normally the host and optional
2696   port number of a recipient server or client that subsequently forwarded the
2697   message.
2698   However, if the real host is considered to be sensitive information, a
2699   sender &MAY; replace it with a pseudonym. If a port is not provided,
2700   a recipient &MAY; interpret that as meaning it was received on the default
2701   TCP port, if any, for the received-protocol.
2704   A sender &MAY; generate comments in the Via header field to identify the
2705   software of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2706   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2707   are optional and a recipient &MAY; remove them prior to forwarding the
2708   message.
2711   For example, a request message could be sent from an HTTP/1.0 user
2712   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2713   forward the request to a public proxy at, which completes
2714   the request by forwarding it to the origin server at
2715   The request received by would then have the following
2716   Via header field:
2718<figure><artwork type="example">
2719  Via: 1.0 fred, 1.1
2722   An intermediary used as a portal through a network firewall
2723   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2724   region unless it is explicitly enabled to do so. If not enabled, such an
2725   intermediary &SHOULD; replace each received-by host of any host behind the
2726   firewall by an appropriate pseudonym for that host.
2729   An intermediary &MAY; combine an ordered subsequence of Via header
2730   field entries into a single such entry if the entries have identical
2731   received-protocol values. For example,
2733<figure><artwork type="example">
2734  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2737  could be collapsed to
2739<figure><artwork type="example">
2740  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2743   A sender &SHOULD-NOT; combine multiple entries unless they are all
2744   under the same organizational control and the hosts have already been
2745   replaced by pseudonyms. A sender &MUST-NOT; combine entries that
2746   have different received-protocol values.
2750<section title="Transformations" anchor="message.transformations">
2751   <iref primary="true" item="transforming proxy"/>
2752   <iref primary="true" item="non-transforming proxy"/>
2754   Some intermediaries include features for transforming messages and their
2755   payloads. A proxy might, for example, convert between image formats in
2756   order to save cache space or to reduce the amount of traffic on a slow
2757   link. However, operational problems might occur when these transformations
2758   are applied to payloads intended for critical applications, such as medical
2759   imaging or scientific data analysis, particularly when integrity checks or
2760   digital signatures are used to ensure that the payload received is
2761   identical to the original.
2764   An HTTP-to-HTTP proxy is called a "<x:dfn>transforming proxy</x:dfn>"
2765   if it is designed or configured to modify messages in a semantically
2766   meaningful way (i.e., modifications, beyond those required by normal
2767   HTTP processing, that change the message in a way that would be
2768   significant to the original sender or potentially significant to
2769   downstream recipients).  For example, a transforming proxy might be
2770   acting as a shared annotation server (modifying responses to include
2771   references to a local annotation database), a malware filter, a
2772   format transcoder, or a privacy filter. Such transformations are presumed
2773   to be desired by whichever client (or client organization) selected the
2774   proxy.
2777   If a proxy receives a request-target with a host name that is not a
2778   fully qualified domain name, it &MAY; add its own domain to the host name
2779   it received when forwarding the request.  A proxy &MUST-NOT; change the
2780   host name if it is a fully qualified domain name.
2783   A proxy &MUST-NOT; modify the "absolute-path" and "query" parts of the
2784   received request-target when forwarding it to the next inbound server,
2785   except as noted above to replace an empty path with "/" or "*".
2788   A proxy &MUST-NOT; modify header fields that provide information about the
2789   end points of the communication chain, the resource state, or the selected
2790   representation. A proxy &MAY; change the message body through application
2791   or removal of a transfer coding (<xref target="transfer.codings"/>).
2794   A proxy &MUST-NOT; modify the payload (&payload;) of a message that
2795   contains a no-transform cache-control directive (&header-cache-control;).
2798   A proxy &MAY; transform the payload of a message
2799   that does not contain a no-transform cache-control directive.
2800   A proxy that transforms a payload &MUST; add a
2801   Warning header field with the warn-code of 214 ("Transformation Applied")
2802   if one is not already in the message (see &header-warning;).
2803   A proxy that transforms the payload of a <x:ref>200 (OK)</x:ref> response
2804   can further inform downstream recipients that a transformation has been
2805   applied by changing the response status code to
2806   <x:ref>203 (Non-Authoritative Information)</x:ref> (&status-203;).
2812<section title="Connection Management" anchor="">
2814   HTTP messaging is independent of the underlying transport or
2815   session-layer connection protocol(s).  HTTP only presumes a reliable
2816   transport with in-order delivery of requests and the corresponding
2817   in-order delivery of responses.  The mapping of HTTP request and
2818   response structures onto the data units of an underlying transport
2819   protocol is outside the scope of this specification.
2822   As described in <xref target="connecting.inbound"/>, the specific
2823   connection protocols to be used for an HTTP interaction are determined by
2824   client configuration and the <x:ref>target URI</x:ref>.
2825   For example, the "http" URI scheme
2826   (<xref target="http.uri"/>) indicates a default connection of TCP
2827   over IP, with a default TCP port of 80, but the client might be
2828   configured to use a proxy via some other connection, port, or protocol.
2831   HTTP implementations are expected to engage in connection management,
2832   which includes maintaining the state of current connections,
2833   establishing a new connection or reusing an existing connection,
2834   processing messages received on a connection, detecting connection
2835   failures, and closing each connection.
2836   Most clients maintain multiple connections in parallel, including
2837   more than one connection per server endpoint.
2838   Most servers are designed to maintain thousands of concurrent connections,
2839   while controlling request queues to enable fair use and detect
2840   denial of service attacks.
2843<section title="Connection" anchor="header.connection">
2844  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2845  <iref primary="true" item="close" x:for-anchor=""/>
2846  <x:anchor-alias value="Connection"/>
2847  <x:anchor-alias value="connection-option"/>
2848  <x:anchor-alias value="close"/>
2850   The "Connection" header field allows the sender to indicate desired
2851   control options for the current connection.  In order to avoid confusing
2852   downstream recipients, a proxy or gateway &MUST; remove or replace any
2853   received connection options before forwarding the message.
2856   When a header field aside from Connection is used to supply control
2857   information for or about the current connection, the sender &MUST; list
2858   the corresponding field-name within the "Connection" header field.
2859   A proxy or gateway &MUST; parse a received Connection
2860   header field before a message is forwarded and, for each
2861   connection-option in this field, remove any header field(s) from
2862   the message with the same name as the connection-option, and then
2863   remove the Connection header field itself (or replace it with the
2864   intermediary's own connection options for the forwarded message).
2867   Hence, the Connection header field provides a declarative way of
2868   distinguishing header fields that are only intended for the
2869   immediate recipient ("hop-by-hop") from those fields that are
2870   intended for all recipients on the chain ("end-to-end"), enabling the
2871   message to be self-descriptive and allowing future connection-specific
2872   extensions to be deployed without fear that they will be blindly
2873   forwarded by older intermediaries.
2876   The Connection header field's value has the following grammar:
2878<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2879  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2880  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2883   Connection options are case-insensitive.
2886   A sender &MUST-NOT; send a connection option corresponding to a header
2887   field that is intended for all recipients of the payload.
2888   For example, <x:ref>Cache-Control</x:ref> is never appropriate as a
2889   connection option (&header-cache-control;).
2892   The connection options do not always correspond to a header field
2893   present in the message, since a connection-specific header field
2894   might not be needed if there are no parameters associated with a
2895   connection option. In contrast, a connection-specific header field that
2896   is received without a corresponding connection option usually indicates
2897   that the field has been improperly forwarded by an intermediary and
2898   ought to be ignored by the recipient.
2901   When defining new connection options, specification authors ought to survey
2902   existing header field names and ensure that the new connection option does
2903   not share the same name as an already deployed header field.
2904   Defining a new connection option essentially reserves that potential
2905   field-name for carrying additional information related to the
2906   connection option, since it would be unwise for senders to use
2907   that field-name for anything else.
2910   The "<x:dfn>close</x:dfn>" connection option is defined for a
2911   sender to signal that this connection will be closed after completion of
2912   the response. For example,
2914<figure><artwork type="example">
2915  Connection: close
2918   in either the request or the response header fields indicates that the
2919   sender is going to close the connection after the current request/response
2920   is complete (<xref target="persistent.tear-down"/>).
2923   A client that does not support <x:ref>persistent connections</x:ref> &MUST;
2924   send the "close" connection option in every request message.
2927   A server that does not support <x:ref>persistent connections</x:ref> &MUST;
2928   send the "close" connection option in every response message that
2929   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2933<section title="Establishment" anchor="persistent.establishment">
2935   It is beyond the scope of this specification to describe how connections
2936   are established via various transport or session-layer protocols.
2937   Each connection applies to only one transport link.
2941<section title="Persistence" anchor="persistent.connections">
2942   <x:anchor-alias value="persistent connections"/>
2944   HTTP/1.1 defaults to the use of "<x:dfn>persistent connections</x:dfn>",
2945   allowing multiple requests and responses to be carried over a single
2946   connection. The "<x:ref>close</x:ref>" connection-option is used to signal
2947   that a connection will not persist after the current request/response.
2948   HTTP implementations &SHOULD; support persistent connections.
2951   A recipient determines whether a connection is persistent or not based on
2952   the most recently received message's protocol version and
2953   <x:ref>Connection</x:ref> header field (if any):
2954   <list style="symbols">
2955     <t>If the <x:ref>close</x:ref> connection option is present, the
2956        connection will not persist after the current response; else,</t>
2957     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2958        persist after the current response; else,</t>
2959     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2960        connection option is present, the recipient is not a proxy, and
2961        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2962        the connection will persist after the current response; otherwise,</t>
2963     <t>The connection will close after the current response.</t>
2964   </list>
2967   A client &MAY; send additional requests on a persistent connection until it
2968   sends or receives a <x:ref>close</x:ref> connection option or receives an
2969   HTTP/1.0 response without a "keep-alive" connection option.
2972   In order to remain persistent, all messages on a connection need to
2973   have a self-defined message length (i.e., one not defined by closure
2974   of the connection), as described in <xref target="message.body"/>.
2975   A server &MUST; read the entire request message body or close
2976   the connection after sending its response, since otherwise the
2977   remaining data on a persistent connection would be misinterpreted
2978   as the next request.  Likewise,
2979   a client &MUST; read the entire response message body if it intends
2980   to reuse the same connection for a subsequent request.
2983   A proxy server &MUST-NOT; maintain a persistent connection with an
2984   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
2985   information and discussion of the problems with the Keep-Alive header field
2986   implemented by many HTTP/1.0 clients).
2989   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
2990   for more information on backward compatibility with HTTP/1.0 clients.
2993<section title="Retrying Requests" anchor="persistent.retrying.requests">
2995   Connections can be closed at any time, with or without intention.
2996   Implementations ought to anticipate the need to recover
2997   from asynchronous close events.
3000   When an inbound connection is closed prematurely, a client &MAY; open a new
3001   connection and automatically retransmit an aborted sequence of requests if
3002   all of those requests have idempotent methods (&idempotent-methods;).
3003   A proxy &MUST-NOT; automatically retry non-idempotent requests.
3006   A user agent &MUST-NOT; automatically retry a request with a non-idempotent
3007   method unless it has some means to know that the request semantics are
3008   actually idempotent, regardless of the method, or some means to detect that
3009   the original request was never applied. For example, a user agent that
3010   knows (through design or configuration) that a POST request to a given
3011   resource is safe can repeat that request automatically.
3012   Likewise, a user agent designed specifically to operate on a version
3013   control repository might be able to recover from partial failure conditions
3014   by checking the target resource revision(s) after a failed connection,
3015   reverting or fixing any changes that were partially applied, and then
3016   automatically retrying the requests that failed.
3019   A client &SHOULD-NOT; automatically retry a failed automatic retry.
3023<section title="Pipelining" anchor="pipelining">
3024   <x:anchor-alias value="pipeline"/>
3026   A client that supports persistent connections &MAY; "<x:dfn>pipeline</x:dfn>"
3027   its requests (i.e., send multiple requests without waiting for each
3028   response). A server &MAY; process a sequence of pipelined requests in
3029   parallel if they all have safe methods (&safe-methods;), but &MUST; send
3030   the corresponding responses in the same order that the requests were
3031   received.
3034   A client that pipelines requests &SHOULD; retry unanswered requests if the
3035   connection closes before it receives all of the corresponding responses.
3036   When retrying pipelined requests after a failed connection (a connection
3037   not explicitly closed by the server in its last complete response), a
3038   client &MUST-NOT; pipeline immediately after connection establishment,
3039   since the first remaining request in the prior pipeline might have caused
3040   an error response that can be lost again if multiple requests are sent on a
3041   prematurely closed connection (see the TCP reset problem described in
3042   <xref target="persistent.tear-down"/>).
3045   Idempotent methods (&idempotent-methods;) are significant to pipelining
3046   because they can be automatically retried after a connection failure.
3047   A user agent &SHOULD-NOT; pipeline requests after a non-idempotent method,
3048   until the final response status code for that method has been received,
3049   unless the user agent has a means to detect and recover from partial
3050   failure conditions involving the pipelined sequence.
3053   An intermediary that receives pipelined requests &MAY; pipeline those
3054   requests when forwarding them inbound, since it can rely on the outbound
3055   user agent(s) to determine what requests can be safely pipelined. If the
3056   inbound connection fails before receiving a response, the pipelining
3057   intermediary &MAY; attempt to retry a sequence of requests that have yet
3058   to receive a response if the requests all have idempotent methods;
3059   otherwise, the pipelining intermediary &SHOULD; forward any received
3060   responses and then close the corresponding outbound connection(s) so that
3061   the outbound user agent(s) can recover accordingly.
3066<section title="Concurrency" anchor="persistent.concurrency">
3068   A client &SHOULD; limit the number of simultaneous open
3069   connections that it maintains to a given server.
3072   Previous revisions of HTTP gave a specific number of connections as a
3073   ceiling, but this was found to be impractical for many applications. As a
3074   result, this specification does not mandate a particular maximum number of
3075   connections, but instead encourages clients to be conservative when opening
3076   multiple connections.
3079   Multiple connections are typically used to avoid the "head-of-line
3080   blocking" problem, wherein a request that takes significant server-side
3081   processing and/or has a large payload blocks subsequent requests on the
3082   same connection. However, each connection consumes server resources.
3083   Furthermore, using multiple connections can cause undesirable side effects
3084   in congested networks.
3087   Note that servers might reject traffic that they deem abusive, including an
3088   excessive number of connections from a client.
3092<section title="Failures and Time-outs" anchor="persistent.failures">
3094   Servers will usually have some time-out value beyond which they will
3095   no longer maintain an inactive connection. Proxy servers might make
3096   this a higher value since it is likely that the client will be making
3097   more connections through the same proxy server. The use of persistent
3098   connections places no requirements on the length (or existence) of
3099   this time-out for either the client or the server.
3102   A client or server that wishes to time-out &SHOULD; issue a graceful close
3103   on the connection. Implementations &SHOULD; constantly monitor open
3104   connections for a received closure signal and respond to it as appropriate,
3105   since prompt closure of both sides of a connection enables allocated system
3106   resources to be reclaimed.
3109   A client, server, or proxy &MAY; close the transport connection at any
3110   time. For example, a client might have started to send a new request
3111   at the same time that the server has decided to close the "idle"
3112   connection. From the server's point of view, the connection is being
3113   closed while it was idle, but from the client's point of view, a
3114   request is in progress.
3117   A server &SHOULD; sustain persistent connections, when possible, and allow
3118   the underlying
3119   transport's flow control mechanisms to resolve temporary overloads, rather
3120   than terminate connections with the expectation that clients will retry.
3121   The latter technique can exacerbate network congestion.
3124   A client sending a message body &SHOULD; monitor
3125   the network connection for an error response while it is transmitting
3126   the request. If the client sees a response that indicates the server does
3127   not wish to receive the message body and is closing the connection, the
3128   client &SHOULD; immediately cease transmitting the body and close its side
3129   of the connection.
3133<section title="Tear-down" anchor="persistent.tear-down">
3134  <iref primary="false" item="Connection header field" x:for-anchor=""/>
3135  <iref primary="false" item="close" x:for-anchor=""/>
3137   The <x:ref>Connection</x:ref> header field
3138   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
3139   connection option that a sender &SHOULD; send when it wishes to close
3140   the connection after the current request/response pair.
3143   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
3144   send further requests on that connection (after the one containing
3145   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
3146   final response message corresponding to this request.
3149   A server that receives a <x:ref>close</x:ref> connection option &MUST;
3150   initiate a close of the connection (see below) after it sends the
3151   final response to the request that contained <x:ref>close</x:ref>.
3152   The server &SHOULD; send a <x:ref>close</x:ref> connection option
3153   in its final response on that connection. The server &MUST-NOT; process
3154   any further requests received on that connection.
3157   A server that sends a <x:ref>close</x:ref> connection option &MUST;
3158   initiate a close of the connection (see below) after it sends the
3159   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
3160   any further requests received on that connection.
3163   A client that receives a <x:ref>close</x:ref> connection option &MUST;
3164   cease sending requests on that connection and close the connection
3165   after reading the response message containing the close; if additional
3166   pipelined requests had been sent on the connection, the client &SHOULD-NOT;
3167   assume that they will be processed by the server.
3170   If a server performs an immediate close of a TCP connection, there is a
3171   significant risk that the client will not be able to read the last HTTP
3172   response.  If the server receives additional data from the client on a
3173   fully-closed connection, such as another request that was sent by the
3174   client before receiving the server's response, the server's TCP stack will
3175   send a reset packet to the client; unfortunately, the reset packet might
3176   erase the client's unacknowledged input buffers before they can be read
3177   and interpreted by the client's HTTP parser.
3180   To avoid the TCP reset problem, servers typically close a connection in
3181   stages. First, the server performs a half-close by closing only the write
3182   side of the read/write connection. The server then continues to read from
3183   the connection until it receives a corresponding close by the client, or
3184   until the server is reasonably certain that its own TCP stack has received
3185   the client's acknowledgement of the packet(s) containing the server's last
3186   response. Finally, the server fully closes the connection.
3189   It is unknown whether the reset problem is exclusive to TCP or might also
3190   be found in other transport connection protocols.
3194<section title="Upgrade" anchor="header.upgrade">
3195  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3196  <x:anchor-alias value="Upgrade"/>
3197  <x:anchor-alias value="protocol"/>
3198  <x:anchor-alias value="protocol-name"/>
3199  <x:anchor-alias value="protocol-version"/>
3201   The "Upgrade" header field is intended to provide a simple mechanism
3202   for transitioning from HTTP/1.1 to some other protocol on the same
3203   connection.  A client &MAY; send a list of protocols in the Upgrade
3204   header field of a request to invite the server to switch to one or
3205   more of those protocols, in order of descending preference, before sending
3206   the final response. A server &MAY; ignore a received Upgrade header field
3207   if it wishes to continue using the current protocol on that connection.
3209<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3210  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3212  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3213  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3214  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3217   A server that sends a <x:ref>101 (Switching Protocols)</x:ref> response
3218   &MUST; send an Upgrade header field to indicate the new protocol(s) to
3219   which the connection is being switched; if multiple protocol layers are
3220   being switched, the sender &MUST; list the protocols in layer-ascending
3221   order. A server &MUST-NOT; switch to a protocol that was not indicated by
3222   the client in the corresponding request's Upgrade header field.
3223   A server &MAY; choose to ignore the order of preference indicated by the
3224   client and select the new protocol(s) based on other factors, such as the
3225   nature of the request or the current load on the server.
3228   A server that sends a <x:ref>426 (Upgrade Required)</x:ref> response
3229   &MUST; send an Upgrade header field to indicate the acceptable protocols,
3230   in order of descending preference.
3233   A server &MAY; send an Upgrade header field in any other response to
3234   advertise that it implements support for upgrading to the listed protocols,
3235   in order of descending preference, when appropriate for a future request.
3238   The following is a hypothetical example sent by a client:
3239</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
3240GET /hello.txt HTTP/1.1
3242Connection: upgrade
3243Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3247   Upgrade cannot be used to insist on a protocol change; its acceptance and
3248   use by the server is optional. The capabilities and nature of the
3249   application-level communication after the protocol change is entirely
3250   dependent upon the new protocol(s) chosen. However, immediately after
3251   sending the 101 response, the server is expected to continue responding to
3252   the original request as if it had received its equivalent within the new
3253   protocol (i.e., the server still has an outstanding request to satisfy
3254   after the protocol has been changed, and is expected to do so without
3255   requiring the request to be repeated).
3258   For example, if the Upgrade header field is received in a GET request
3259   and the server decides to switch protocols, it first responds
3260   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3261   then immediately follows that with the new protocol's equivalent of a
3262   response to a GET on the target resource.  This allows a connection to be
3263   upgraded to protocols with the same semantics as HTTP without the
3264   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3265   protocols unless the received message semantics can be honored by the new
3266   protocol; an OPTIONS request can be honored by any protocol.
3269   The following is an example response to the above hypothetical request:
3270</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
3271HTTP/1.1 101 Switching Protocols
3272Connection: upgrade
3273Upgrade: HTTP/2.0
3275[... data stream switches to HTTP/2.0 with an appropriate response
3276(as defined by new protocol) to the "GET /hello.txt" request ...]
3279   When Upgrade is sent, the sender &MUST; also send a
3280   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3281   that contains an "upgrade" connection option, in order to prevent Upgrade
3282   from being accidentally forwarded by intermediaries that might not implement
3283   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3284   is received in an HTTP/1.0 request.
3287   A client cannot begin using an upgraded protocol on the connection until
3288   it has completely sent the request message (i.e., the client can't change
3289   the protocol it is sending in the middle of a message).
3290   If a server receives both Upgrade and an <x:ref>Expect</x:ref> header field
3291   with the "100-continue" expectation (&header-expect;), the
3292   server &MUST; send a <x:ref>100 (Continue)</x:ref> response before sending
3293   a <x:ref>101 (Switching Protocols)</x:ref> response.
3296   The Upgrade header field only applies to switching protocols on top of the
3297   existing connection; it cannot be used to switch the underlying connection
3298   (transport) protocol, nor to switch the existing communication to a
3299   different connection. For those purposes, it is more appropriate to use a
3300   <x:ref>3xx (Redirection)</x:ref> response (&status-3xx;).
3303   This specification only defines the protocol name "HTTP" for use by
3304   the family of Hypertext Transfer Protocols, as defined by the HTTP
3305   version rules of <xref target="http.version"/> and future updates to this
3306   specification. Additional tokens ought to be registered with IANA using the
3307   registration procedure defined in <xref target="upgrade.token.registry"/>.
3312<section title="ABNF list extension: #rule" anchor="abnf.extension">
3314  A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
3315  improve readability in the definitions of some header field values.
3318  A construct "#" is defined, similar to "*", for defining comma-delimited
3319  lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
3320  at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
3321  comma (",") and optional whitespace (OWS).   
3324  Thus, a sender &MUST; expand the list construct as follows:
3325</preamble><artwork type="example">
3326  1#element =&gt; element *( OWS "," OWS element )
3329  and:
3330</preamble><artwork type="example">
3331  #element =&gt; [ 1#element ]
3334  and for n &gt;= 1 and m &gt; 1:
3335</preamble><artwork type="example">
3336  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
3339  For compatibility with legacy list rules, a recipient &MUST; parse and ignore
3340  a reasonable number of empty list elements: enough to handle common mistakes
3341  by senders that merge values, but not so much that they could be used as a
3342  denial of service mechanism. In other words, a recipient &MUST; expand the
3343  list construct as follows:
3345<figure><artwork type="example">
3346  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
3348  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
3351  Empty elements do not contribute to the count of elements present.
3352  For example, given these ABNF productions:
3354<figure><artwork type="example">
3355  example-list      = 1#example-list-elmt
3356  example-list-elmt = token ; see <xref target="field.components"/>
3359  Then the following are valid values for example-list (not including the
3360  double quotes, which are present for delimitation only):
3362<figure><artwork type="example">
3363  "foo,bar"
3364  "foo ,bar,"
3365  "foo , ,bar,charlie   "
3368  In contrast, the following values would be invalid, since at least one
3369  non-empty element is required by the example-list production:
3371<figure><artwork type="example">
3372  ""
3373  ","
3374  ",   ,"
3377  <xref target="collected.abnf"/> shows the collected ABNF after the list
3378  constructs have been expanded, as described above, for recipients.
3382<section title="IANA Considerations" anchor="IANA.considerations">
3384<section title="Header Field Registration" anchor="header.field.registration">
3386   HTTP header fields are registered within the Message Header Field Registry
3387   maintained at
3388   <eref target=""/>.
3391   This document defines the following HTTP header fields, so their
3392   associated registry entries shall be updated according to the permanent
3393   registrations below (see <xref target="BCP90"/>):
3395<?BEGININC p1-messaging.iana-headers ?>
3396<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3397<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3398   <ttcol>Header Field Name</ttcol>
3399   <ttcol>Protocol</ttcol>
3400   <ttcol>Status</ttcol>
3401   <ttcol>Reference</ttcol>
3403   <c>Connection</c>
3404   <c>http</c>
3405   <c>standard</c>
3406   <c>
3407      <xref target="header.connection"/>
3408   </c>
3409   <c>Content-Length</c>
3410   <c>http</c>
3411   <c>standard</c>
3412   <c>
3413      <xref target="header.content-length"/>
3414   </c>
3415   <c>Host</c>
3416   <c>http</c>
3417   <c>standard</c>
3418   <c>
3419      <xref target=""/>
3420   </c>
3421   <c>TE</c>
3422   <c>http</c>
3423   <c>standard</c>
3424   <c>
3425      <xref target="header.te"/>
3426   </c>
3427   <c>Trailer</c>
3428   <c>http</c>
3429   <c>standard</c>
3430   <c>
3431      <xref target="header.trailer"/>
3432   </c>
3433   <c>Transfer-Encoding</c>
3434   <c>http</c>
3435   <c>standard</c>
3436   <c>
3437      <xref target="header.transfer-encoding"/>
3438   </c>
3439   <c>Upgrade</c>
3440   <c>http</c>
3441   <c>standard</c>
3442   <c>
3443      <xref target="header.upgrade"/>
3444   </c>
3445   <c>Via</c>
3446   <c>http</c>
3447   <c>standard</c>
3448   <c>
3449      <xref target="header.via"/>
3450   </c>
3453<?ENDINC p1-messaging.iana-headers ?>
3455   Furthermore, the header field-name "Close" shall be registered as
3456   "reserved", since using that name as an HTTP header field might
3457   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3458   header field (<xref target="header.connection"/>).
3460<texttable align="left" suppress-title="true">
3461   <ttcol>Header Field Name</ttcol>
3462   <ttcol>Protocol</ttcol>
3463   <ttcol>Status</ttcol>
3464   <ttcol>Reference</ttcol>
3466   <c>Close</c>
3467   <c>http</c>
3468   <c>reserved</c>
3469   <c>
3470      <xref target="header.field.registration"/>
3471   </c>
3474   The change controller is: "IETF ( - Internet Engineering Task Force".
3478<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3480   IANA maintains the registry of URI Schemes <xref target="BCP115"/> at
3481   <eref target=""/>.
3484   This document defines the following URI schemes, so their
3485   associated registry entries shall be updated according to the permanent
3486   registrations below:
3488<texttable align="left" suppress-title="true">
3489   <ttcol>URI Scheme</ttcol>
3490   <ttcol>Description</ttcol>
3491   <ttcol>Reference</ttcol>
3493   <c>http</c>
3494   <c>Hypertext Transfer Protocol</c>
3495   <c><xref target="http.uri"/></c>
3497   <c>https</c>
3498   <c>Hypertext Transfer Protocol Secure</c>
3499   <c><xref target="https.uri"/></c>
3503<section title="Internet Media Type Registration" anchor="">
3505   IANA maintains the registry of Internet media types <xref target="BCP13"/> at
3506   <eref target=""/>.
3509   This document serves as the specification for the Internet media types
3510   "message/http" and "application/http". The following is to be registered with
3511   IANA.
3513<section title="Internet Media Type message/http" anchor="">
3514<iref item="Media Type" subitem="message/http" primary="true"/>
3515<iref item="message/http Media Type" primary="true"/>
3517   The message/http type can be used to enclose a single HTTP request or
3518   response message, provided that it obeys the MIME restrictions for all
3519   "message" types regarding line length and encodings.
3522  <list style="hanging" x:indent="12em">
3523    <t hangText="Type name:">
3524      message
3525    </t>
3526    <t hangText="Subtype name:">
3527      http
3528    </t>
3529    <t hangText="Required parameters:">
3530      N/A
3531    </t>
3532    <t hangText="Optional parameters:">
3533      version, msgtype
3534      <list style="hanging">
3535        <t hangText="version:">
3536          The HTTP-version number of the enclosed message
3537          (e.g., "1.1"). If not present, the version can be
3538          determined from the first line of the body.
3539        </t>
3540        <t hangText="msgtype:">
3541          The message type &mdash; "request" or "response". If not
3542          present, the type can be determined from the first
3543          line of the body.
3544        </t>
3545      </list>
3546    </t>
3547    <t hangText="Encoding considerations:">
3548      only "7bit", "8bit", or "binary" are permitted
3549    </t>
3550    <t hangText="Security considerations:">
3551      see <xref target="security.considerations"/>
3552    </t>
3553    <t hangText="Interoperability considerations:">
3554      N/A
3555    </t>
3556    <t hangText="Published specification:">
3557      This specification (see <xref target=""/>).
3558    </t>
3559    <t hangText="Applications that use this media type:">
3560      N/A
3561    </t>
3562    <t hangText="Fragment identifier considerations:">
3563      N/A
3564    </t>
3565    <t hangText="Additional information:">
3566      <list style="hanging">
3567        <t hangText="Magic number(s):">N/A</t>
3568        <t hangText="Deprecated alias names for this type:">N/A</t>
3569        <t hangText="File extension(s):">N/A</t>
3570        <t hangText="Macintosh file type code(s):">N/A</t>
3571      </list>
3572    </t>
3573    <t hangText="Person and email address to contact for further information:">
3574      See Authors Section.
3575    </t>
3576    <t hangText="Intended usage:">
3577      COMMON
3578    </t>
3579    <t hangText="Restrictions on usage:">
3580      N/A
3581    </t>
3582    <t hangText="Author:">
3583      See Authors Section.
3584    </t>
3585    <t hangText="Change controller:">
3586      IESG
3587    </t>
3588  </list>
3591<section title="Internet Media Type application/http" anchor="">
3592<iref item="Media Type" subitem="application/http" primary="true"/>
3593<iref item="application/http Media Type" primary="true"/>
3595   The application/http type can be used to enclose a pipeline of one or more
3596   HTTP request or response messages (not intermixed).
3599  <list style="hanging" x:indent="12em">
3600    <t hangText="Type name:">
3601      application
3602    </t>
3603    <t hangText="Subtype name:">
3604      http
3605    </t>
3606    <t hangText="Required parameters:">
3607      N/A
3608    </t>
3609    <t hangText="Optional parameters:">
3610      version, msgtype
3611      <list style="hanging">
3612        <t hangText="version:">
3613          The HTTP-version number of the enclosed messages
3614          (e.g., "1.1"). If not present, the version can be
3615          determined from the first line of the body.
3616        </t>
3617        <t hangText="msgtype:">
3618          The message type &mdash; "request" or "response". If not
3619          present, the type can be determined from the first
3620          line of the body.
3621        </t>
3622      </list>
3623    </t>
3624    <t hangText="Encoding considerations:">
3625      HTTP messages enclosed by this type
3626      are in "binary" format; use of an appropriate
3627      Content-Transfer-Encoding is required when
3628      transmitted via E-mail.
3629    </t>
3630    <t hangText="Security considerations:">
3631      see <xref target="security.considerations"/>
3632    </t>
3633    <t hangText="Interoperability considerations:">
3634      N/A
3635    </t>
3636    <t hangText="Published specification:">
3637      This specification (see <xref target=""/>).
3638    </t>
3639    <t hangText="Applications that use this media type:">
3640      N/A
3641    </t>
3642    <t hangText="Fragment identifier considerations:">
3643      N/A
3644    </t>
3645    <t hangText="Additional information:">
3646      <list style="hanging">
3647        <t hangText="Deprecated alias names for this type:">N/A</t>
3648        <t hangText="Magic number(s):">N/A</t>
3649        <t hangText="File extension(s):">N/A</t>
3650        <t hangText="Macintosh file type code(s):">N/A</t>
3651      </list>
3652    </t>
3653    <t hangText="Person and email address to contact for further information:">
3654      See Authors Section.
3655    </t>
3656    <t hangText="Intended usage:">
3657      COMMON
3658    </t>
3659    <t hangText="Restrictions on usage:">
3660      N/A
3661    </t>
3662    <t hangText="Author:">
3663      See Authors Section.
3664    </t>
3665    <t hangText="Change controller:">
3666      IESG
3667    </t>
3668  </list>
3673<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3675   The HTTP Transfer Coding Registry defines the name space for transfer
3676   coding names. It is maintained at <eref target=""/>.
3679<section title="Procedure" anchor="transfer.coding.registry.procedure">
3681   Registrations &MUST; include the following fields:
3682   <list style="symbols">
3683     <t>Name</t>
3684     <t>Description</t>
3685     <t>Pointer to specification text</t>
3686   </list>
3689   Names of transfer codings &MUST-NOT; overlap with names of content codings
3690   (&content-codings;) unless the encoding transformation is identical, as
3691   is the case for the compression codings defined in
3692   <xref target="compression.codings"/>.
3695   Values to be added to this name space require IETF Review (see
3696   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3697   conform to the purpose of transfer coding defined in this specification.
3700   Use of program names for the identification of encoding formats
3701   is not desirable and is discouraged for future encodings.
3705<section title="Registration" anchor="transfer.coding.registration">
3707   The HTTP Transfer Coding Registry shall be updated with the registrations
3708   below:
3710<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3711   <ttcol>Name</ttcol>
3712   <ttcol>Description</ttcol>
3713   <ttcol>Reference</ttcol>
3714   <c>chunked</c>
3715   <c>Transfer in a series of chunks</c>
3716   <c>
3717      <xref target="chunked.encoding"/>
3718   </c>
3719   <c>compress</c>
3720   <c>UNIX "compress" data format <xref target="Welch"/></c>
3721   <c>
3722      <xref target="compress.coding"/>
3723   </c>
3724   <c>deflate</c>
3725   <c>"deflate" compressed data (<xref target="RFC1951"/>) inside
3726   the "zlib" data format (<xref target="RFC1950"/>)
3727   </c>
3728   <c>
3729      <xref target="deflate.coding"/>
3730   </c>
3731   <c>gzip</c>
3732   <c>GZIP file format <xref target="RFC1952"/></c>
3733   <c>
3734      <xref target="gzip.coding"/>
3735   </c>
3736   <c>x-compress</c>
3737   <c>Deprecated (alias for compress)</c>
3738   <c>
3739      <xref target="compress.coding"/>
3740   </c>
3741   <c>x-gzip</c>
3742   <c>Deprecated (alias for gzip)</c>
3743   <c>
3744      <xref target="gzip.coding"/>
3745   </c>
3750<section title="Content Coding Registration" anchor="content.coding.registration">
3752   IANA maintains the registry of HTTP Content Codings at
3753   <eref target=""/>.
3756   The HTTP Content Codings Registry shall be updated with the registrations
3757   below:
3759<texttable align="left" suppress-title="true" anchor="iana.content.coding.registration.table">
3760   <ttcol>Name</ttcol>
3761   <ttcol>Description</ttcol>
3762   <ttcol>Reference</ttcol>
3763   <c>compress</c>
3764   <c>UNIX "compress" data format <xref target="Welch"/></c>
3765   <c>
3766      <xref target="compress.coding"/>
3767   </c>
3768   <c>deflate</c>
3769   <c>"deflate" compressed data (<xref target="RFC1951"/>) inside
3770   the "zlib" data format (<xref target="RFC1950"/>)</c>
3771   <c>
3772      <xref target="deflate.coding"/>
3773   </c>
3774   <c>gzip</c>
3775   <c>GZIP file format <xref target="RFC1952"/></c>
3776   <c>
3777      <xref target="gzip.coding"/>
3778   </c>
3779   <c>x-compress</c>
3780   <c>Deprecated (alias for compress)</c>
3781   <c>
3782      <xref target="compress.coding"/>
3783   </c>
3784   <c>x-gzip</c>
3785   <c>Deprecated (alias for gzip)</c>
3786   <c>
3787      <xref target="gzip.coding"/>
3788   </c>
3792<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3794   The HTTP Upgrade Token Registry defines the name space for protocol-name
3795   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3796   field. The registry is maintained at <eref target=""/>.
3799<section title="Procedure" anchor="upgrade.token.registry.procedure">  
3801   Each registered protocol name is associated with contact information
3802   and an optional set of specifications that details how the connection
3803   will be processed after it has been upgraded.
3806   Registrations happen on a "First Come First Served" basis (see
3807   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3808   following rules:
3809  <list style="numbers">
3810    <t>A protocol-name token, once registered, stays registered forever.</t>
3811    <t>The registration &MUST; name a responsible party for the
3812       registration.</t>
3813    <t>The registration &MUST; name a point of contact.</t>
3814    <t>The registration &MAY; name a set of specifications associated with
3815       that token. Such specifications need not be publicly available.</t>
3816    <t>The registration &SHOULD; name a set of expected "protocol-version"
3817       tokens associated with that token at the time of registration.</t>
3818    <t>The responsible party &MAY; change the registration at any time.
3819       The IANA will keep a record of all such changes, and make them
3820       available upon request.</t>
3821    <t>The IESG &MAY; reassign responsibility for a protocol token.
3822       This will normally only be used in the case when a
3823       responsible party cannot be contacted.</t>
3824  </list>
3827   This registration procedure for HTTP Upgrade Tokens replaces that
3828   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3832<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3834   The "HTTP" entry in the HTTP Upgrade Token Registry shall be updated with
3835   the registration below:
3837<texttable align="left" suppress-title="true">
3838   <ttcol>Value</ttcol>
3839   <ttcol>Description</ttcol>
3840   <ttcol>Expected Version Tokens</ttcol>
3841   <ttcol>Reference</ttcol>
3843   <c>HTTP</c>
3844   <c>Hypertext Transfer Protocol</c>
3845   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3846   <c><xref target="http.version"/></c>
3849   The responsible party is: "IETF ( - Internet Engineering Task Force".
3856<section title="Security Considerations" anchor="security.considerations">
3858   This section is meant to inform developers, information providers, and
3859   users of known security considerations relevant to HTTP message syntax,
3860   parsing, and routing. Security considerations about HTTP semantics and
3861   payloads are addressed in &semantics;.
3864<section title="DNS-related Attacks" anchor="dns.related.attacks">
3866   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3867   generally prone to security attacks based on the deliberate misassociation
3868   of IP addresses and DNS names not protected by DNSSEC. Clients need to be
3869   cautious in assuming the validity of an IP number/DNS name association unless
3870   the response is protected by DNSSEC (<xref target="RFC4033"/>).
3874<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3876   By their very nature, HTTP intermediaries are men-in-the-middle, and
3877   represent an opportunity for man-in-the-middle attacks. Compromise of
3878   the systems on which the intermediaries run can result in serious security
3879   and privacy problems. Intermediaries have access to security-related
3880   information, personal information about individual users and
3881   organizations, and proprietary information belonging to users and
3882   content providers. A compromised intermediary, or an intermediary
3883   implemented or configured without regard to security and privacy
3884   considerations, might be used in the commission of a wide range of
3885   potential attacks.
3888   Intermediaries that contain a shared cache are especially vulnerable
3889   to cache poisoning attacks.
3892   Implementers need to consider the privacy and security
3893   implications of their design and coding decisions, and of the
3894   configuration options they provide to operators (especially the
3895   default configuration).
3898   Users need to be aware that intermediaries are no more trustworthy than
3899   the people who run them; HTTP itself cannot solve this problem.
3903<section title="Buffer Overflows" anchor="attack.protocol.element.size.overflows">
3905   Because HTTP uses mostly textual, character-delimited fields, attackers can
3906   overflow buffers in implementations, and/or perform a Denial of Service
3907   against implementations that accept fields with unlimited lengths.
3910   To promote interoperability, this specification makes specific
3911   recommendations for minimum size limits on request-line
3912   (<xref target="request.line"/>)
3913   and header fields (<xref target="header.fields"/>). These are
3914   minimum recommendations, chosen to be supportable even by implementations
3915   with limited resources; it is expected that most implementations will
3916   choose substantially higher limits.
3919   This specification also provides a way for servers to reject messages that
3920   have request-targets that are too long (&status-414;) or request entities
3921   that are too large (&status-4xx;). Additional status codes related to
3922   capacity limits have been defined by extensions to HTTP
3923   <xref target="RFC6585"/>.
3926   Recipients ought to carefully limit the extent to which they read other
3927   fields, including (but not limited to) request methods, response status
3928   phrases, header field-names, and body chunks, so as to avoid denial of
3929   service attacks without impeding interoperability.
3933<section title="Message Integrity" anchor="message.integrity">
3935   HTTP does not define a specific mechanism for ensuring message integrity,
3936   instead relying on the error-detection ability of underlying transport
3937   protocols and the use of length or chunk-delimited framing to detect
3938   completeness. Additional integrity mechanisms, such as hash functions or
3939   digital signatures applied to the content, can be selectively added to
3940   messages via extensible metadata header fields. Historically, the lack of
3941   a single integrity mechanism has been justified by the informal nature of
3942   most HTTP communication.  However, the prevalence of HTTP as an information
3943   access mechanism has resulted in its increasing use within environments
3944   where verification of message integrity is crucial.
3947   User agents are encouraged to implement configurable means for detecting
3948   and reporting failures of message integrity such that those means can be
3949   enabled within environments for which integrity is necessary. For example,
3950   a browser being used to view medical history or drug interaction
3951   information needs to indicate to the user when such information is detected
3952   by the protocol to be incomplete, expired, or corrupted during transfer.
3953   Such mechanisms might be selectively enabled via user agent extensions or
3954   the presence of message integrity metadata in a response.
3955   At a minimum, user agents ought to provide some indication that allows a
3956   user to distinguish between a complete and incomplete response message
3957   (<xref target="incomplete.messages"/>) when such verification is desired.
3961<section title="Server Log Information" anchor="abuse.of.server.log.information">
3963   A server is in the position to save personal data about a user's requests
3964   over time, which might identify their reading patterns or subjects of
3965   interest.  In particular, log information gathered at an intermediary
3966   often contains a history of user agent interaction, across a multitude
3967   of sites, that can be traced to individual users.
3970   HTTP log information is confidential in nature; its handling is often
3971   constrained by laws and regulations.  Log information needs to be securely
3972   stored and appropriate guidelines followed for its analysis.
3973   Anonymization of personal information within individual entries helps,
3974   but is generally not sufficient to prevent real log traces from being
3975   re-identified based on correlation with other access characteristics.
3976   As such, access traces that are keyed to a specific client are unsafe to
3977   publish even if the key is pseudonymous.
3980   To minimize the risk of theft or accidental publication, log information
3981   ought to be purged of personally identifiable information, including
3982   user identifiers, IP addresses, and user-provided query parameters,
3983   as soon as that information is no longer necessary to support operational
3984   needs for security, auditing, or fraud control.
3989<section title="Acknowledgments" anchor="acks">
3991   This edition of HTTP/1.1 builds on the many contributions that went into
3992   <xref target="RFC1945" format="none">RFC 1945</xref>,
3993   <xref target="RFC2068" format="none">RFC 2068</xref>,
3994   <xref target="RFC2145" format="none">RFC 2145</xref>, and
3995   <xref target="RFC2616" format="none">RFC 2616</xref>, including
3996   substantial contributions made by the previous authors, editors, and
3997   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
3998   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
3999   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
4002   Since 1999, the following contributors have helped improve the HTTP
4003   specification by reporting bugs, asking smart questions, drafting or
4004   reviewing text, and evaluating open issues:
4006<?BEGININC acks ?>
4007<t>Adam Barth,
4008Adam Roach,
4009Addison Phillips,
4010Adrian Chadd,
4011Adrian Cole,
4012Adrien W. de Croy,
4013Alan Ford,
4014Alan Ruttenberg,
4015Albert Lunde,
4016Alek Storm,
4017Alex Rousskov,
4018Alexandre Morgaut,
4019Alexey Melnikov,
4020Alisha Smith,
4021Amichai Rothman,
4022Amit Klein,
4023Amos Jeffries,
4024Andreas Maier,
4025Andreas Petersson,
4026Andrei Popov,
4027Anil Sharma,
4028Anne van Kesteren,
4029Anthony Bryan,
4030Asbjorn Ulsberg,
4031Ashok Kumar,
4032Balachander Krishnamurthy,
4033Barry Leiba,
4034Ben Laurie,
4035Benjamin Carlyle,
4036Benjamin Niven-Jenkins,
4037Benoit Claise,
4038Bil Corry,
4039Bill Burke,
4040Bjoern Hoehrmann,
4041Bob Scheifler,
4042Boris Zbarsky,
4043Brett Slatkin,
4044Brian Kell,
4045Brian McBarron,
4046Brian Pane,
4047Brian Raymor,
4048Brian Smith,
4049Bruce Perens,
4050Bryce Nesbitt,
4051Cameron Heavon-Jones,
4052Carl Kugler,
4053Carsten Bormann,
4054Charles Fry,
4055Chris Burdess,
4056Chris Newman,
4057Christian Huitema,
4058Cyrus Daboo,
4059Dale Robert Anderson,
4060Dan Wing,
4061Dan Winship,
4062Daniel Stenberg,
4063Darrel Miller,
4064Dave Cridland,
4065Dave Crocker,
4066Dave Kristol,
4067Dave Thaler,
4068David Booth,
4069David Singer,
4070David W. Morris,
4071Diwakar Shetty,
4072Dmitry Kurochkin,
4073Drummond Reed,
4074Duane Wessels,
4075Edward Lee,
4076Eitan Adler,
4077Eliot Lear,
4078Emile Stephan,
4079Eran Hammer-Lahav,
4080Eric D. Williams,
4081Eric J. Bowman,
4082Eric Lawrence,
4083Eric Rescorla,
4084Erik Aronesty,
4085EungJun Yi,
4086Evan Prodromou,
4087Felix Geisendoerfer,
4088Florian Weimer,
4089Frank Ellermann,
4090Fred Akalin,
4091Fred Bohle,
4092Frederic Kayser,
4093Gabor Molnar,
4094Gabriel Montenegro,
4095Geoffrey Sneddon,
4096Gervase Markham,
4097Gili Tzabari,
4098Grahame Grieve,
4099Greg Slepak,
4100Greg Wilkins,
4101Grzegorz Calkowski,
4102Harald Tveit Alvestrand,
4103Harry Halpin,
4104Helge Hess,
4105Henrik Nordstrom,
4106Henry S. Thompson,
4107Henry Story,
4108Herbert van de Sompel,
4109Herve Ruellan,
4110Howard Melman,
4111Hugo Haas,
4112Ian Fette,
4113Ian Hickson,
4114Ido Safruti,
4115Ilari Liusvaara,
4116Ilya Grigorik,
4117Ingo Struck,
4118J. Ross Nicoll,
4119James Cloos,
4120James H. Manger,
4121James Lacey,
4122James M. Snell,
4123Jamie Lokier,
4124Jan Algermissen,
4125Jari Arkko,
4126Jeff Hodges (who came up with the term 'effective Request-URI'),
4127Jeff Pinner,
4128Jeff Walden,
4129Jim Luther,
4130Jitu Padhye,
4131Joe D. Williams,
4132Joe Gregorio,
4133Joe Orton,
4134Joel Jaeggli,
4135John C. Klensin,
4136John C. Mallery,
4137John Cowan,
4138John Kemp,
4139John Panzer,
4140John Schneider,
4141John Stracke,
4142John Sullivan,
4143Jonas Sicking,
4144Jonathan A. Rees,
4145Jonathan Billington,
4146Jonathan Moore,
4147Jonathan Silvera,
4148Jordi Ros,
4149Joris Dobbelsteen,
4150Josh Cohen,
4151Julien Pierre,
4152Jungshik Shin,
4153Justin Chapweske,
4154Justin Erenkrantz,
4155Justin James,
4156Kalvinder Singh,
4157Karl Dubost,
4158Kathleen Moriarty,
4159Keith Hoffman,
4160Keith Moore,
4161Ken Murchison,
4162Koen Holtman,
4163Konstantin Voronkov,
4164Kris Zyp,
4165Leif Hedstrom,
4166Lionel Morand,
4167Lisa Dusseault,
4168Maciej Stachowiak,
4169Manu Sporny,
4170Marc Schneider,
4171Marc Slemko,
4172Mark Baker,
4173Mark Pauley,
4174Mark Watson,
4175Markus Isomaki,
4176Markus Lanthaler,
4177Martin J. Duerst,
4178Martin Musatov,
4179Martin Nilsson,
4180Martin Thomson,
4181Matt Lynch,
4182Matthew Cox,
4183Matthew Kerwin,
4184Max Clark,
4185Menachem Dodge,
4186Meral Shirazipour,
4187Michael Burrows,
4188Michael Hausenblas,
4189Michael Scharf,
4190Michael Sweet,
4191Michael Tuexen,
4192Michael Welzl,
4193Mike Amundsen,
4194Mike Belshe,
4195Mike Bishop,
4196Mike Kelly,
4197Mike Schinkel,
4198Miles Sabin,
4199Murray S. Kucherawy,
4200Mykyta Yevstifeyev,
4201Nathan Rixham,
4202Nicholas Shanks,
4203Nico Williams,
4204Nicolas Alvarez,
4205Nicolas Mailhot,
4206Noah Slater,
4207Osama Mazahir,
4208Pablo Castro,
4209Pat Hayes,
4210Patrick R. McManus,
4211Paul E. Jones,
4212Paul Hoffman,
4213Paul Marquess,
4214Pete Resnick,
4215Peter Lepeska,
4216Peter Occil,
4217Peter Saint-Andre,
4218Peter Watkins,
4219Phil Archer,
4220Phil Hunt,
4221Philippe Mougin,
4222Phillip Hallam-Baker,
4223Piotr Dobrogost,
4224Poul-Henning Kamp,
4225Preethi Natarajan,
4226Rajeev Bector,
4227Ray Polk,
4228Reto Bachmann-Gmuer,
4229Richard Barnes,
4230Richard Cyganiak,
4231Rob Trace,
4232Robby Simpson,
4233Robert Brewer,
4234Robert Collins,
4235Robert Mattson,
4236Robert O'Callahan,
4237Robert Olofsson,
4238Robert Sayre,
4239Robert Siemer,
4240Robert de Wilde,
4241Roberto Javier Godoy,
4242Roberto Peon,
4243Roland Zink,
4244Ronny Widjaja,
4245Ryan Hamilton,
4246S. Mike Dierken,
4247Salvatore Loreto,
4248Sam Johnston,
4249Sam Pullara,
4250Sam Ruby,
4251Saurabh Kulkarni,
4252Scott Lawrence (who maintained the original issues list),
4253Sean B. Palmer,
4254Sean Turner,
4255Sebastien Barnoud,
4256Shane McCarron,
4257Shigeki Ohtsu,
4258Simon Yarde,
4259Stefan Eissing,
4260Stefan Tilkov,
4261Stefanos Harhalakis,
4262Stephane Bortzmeyer,
4263Stephen Farrell,
4264Stephen Kent,
4265Stephen Ludin,
4266Stuart Williams,
4267Subbu Allamaraju,
4268Subramanian Moonesamy,
4269Susan Hares,
4270Sylvain Hellegouarch,
4271Tapan Divekar,
4272Tatsuhiro Tsujikawa,
4273Tatsuya Hayashi,
4274Ted Hardie,
4275Ted Lemon,
4276Thomas Broyer,
4277Thomas Fossati,
4278Thomas Maslen,
4279Thomas Nadeau,
4280Thomas Nordin,
4281Thomas Roessler,
4282Tim Bray,
4283Tim Morgan,
4284Tim Olsen,
4285Tom Zhou,
4286Travis Snoozy,
4287Tyler Close,
4288Vincent Murphy,
4289Wenbo Zhu,
4290Werner Baumann,
4291Wilbur Streett,
4292Wilfredo Sanchez Vega,
4293William A. Rowe Jr.,
4294William Chan,
4295Willy Tarreau,
4296Xiaoshu Wang,
4297Yaron Goland,
4298Yngve Nysaeter Pettersen,
4299Yoav Nir,
4300Yogesh Bang,
4301Yuchung Cheng,
4302Yutaka Oiwa,
4303Yves Lafon (long-time member of the editor team),
4304Zed A. Shaw, and
4305Zhong Yu.
4307<?ENDINC acks ?>
4309   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
4310   acknowledgements from prior revisions.
4317<references title="Normative References">
4319<reference anchor="Part2">
4320  <front>
4321    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
4322    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4323      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4324      <address><email></email></address>
4325    </author>
4326    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4327      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4328      <address><email></email></address>
4329    </author>
4330    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4331  </front>
4332  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
4333  <x:source href="p2-semantics.xml" basename="p2-semantics">
4334    <x:defines>1xx (Informational)</x:defines>
4335    <x:defines>1xx</x:defines>
4336    <x:defines>100 (Continue)</x:defines>
4337    <x:defines>101 (Switching Protocols)</x:defines>
4338    <x:defines>2xx (Successful)</x:defines>
4339    <x:defines>2xx</x:defines>
4340    <x:defines>200 (OK)</x:defines>
4341    <x:defines>203 (Non-Authoritative Information)</x:defines>
4342    <x:defines>204 (No Content)</x:defines>
4343    <x:defines>3xx (Redirection)</x:defines>
4344    <x:defines>3xx</x:defines>
4345    <x:defines>301 (Moved Permanently)</x:defines>
4346    <x:defines>4xx (Client Error)</x:defines>
4347    <x:defines>4xx</x:defines>
4348    <x:defines>400 (Bad Request)</x:defines>
4349    <x:defines>411 (Length Required)</x:defines>
4350    <x:defines>414 (URI Too Long)</x:defines>
4351    <x:defines>417 (Expectation Failed)</x:defines>
4352    <x:defines>426 (Upgrade Required)</x:defines>
4353    <x:defines>501 (Not Implemented)</x:defines>
4354    <x:defines>502 (Bad Gateway)</x:defines>
4355    <x:defines>505 (HTTP Version Not Supported)</x:defines>
4356    <x:defines>Accept-Encoding</x:defines>
4357    <x:defines>Allow</x:defines>
4358    <x:defines>Content-Encoding</x:defines>
4359    <x:defines>Content-Location</x:defines>
4360    <x:defines>Content-Type</x:defines>
4361    <x:defines>Date</x:defines>
4362    <x:defines>Expect</x:defines>
4363    <x:defines>Location</x:defines>
4364    <x:defines>Server</x:defines>
4365    <x:defines>User-Agent</x:defines>
4366  </x:source>
4369<reference anchor="Part4">
4370  <front>
4371    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
4372    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
4373      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4374      <address><email></email></address>
4375    </author>
4376    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
4377      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4378      <address><email></email></address>
4379    </author>
4380    <date month="&ID-MONTH;" year="&ID-YEAR;" />
4381  </front>
4382  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
4383  <x:source basename="p4-conditional" href="p4-conditional.xml">
4384    <x:defines>304 (Not Modified)</x:defines>
4385    <x:defines>ETag</x:defines>
4386    <x:defines>Last-Modified</x:defines>
4387  </x:source>
4390<reference anchor="Part5">
4391  <front>
4392    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
4393    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4394      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4395      <address><email></email></address>
4396    </author>
4397    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
4398      <organization abbrev="W3C">World Wide Web Consortium</organization>
4399      <address><email></email></address>
4400    </author>
4401    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4402      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4403      <address><email></email></address>
4404    </author>
4405    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4406  </front>
4407  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4408  <x:source href="p5-range.xml" basename="p5-range">
4409    <x:defines>Content-Range</x:defines>
4410  </x:source>
4413<reference anchor="Part6">
4414  <front>
4415    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4416    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4417      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4418      <address><email></email></address>
4419    </author>
4420    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4421      <organization>Akamai</organization>
4422      <address><email></email></address>
4423    </author>
4424    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4425      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4426      <address><email></email></address>
4427    </author>
4428    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4429  </front>
4430  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4431  <x:source href="p6-cache.xml" basename="p6-cache">
4432    <x:defines>Cache-Control</x:defines>
4433    <x:defines>Expires</x:defines>
4434  </x:source>
4437<reference anchor="Part7">
4438  <front>
4439    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4440    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4441      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4442      <address><email></email></address>
4443    </author>
4444    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4445      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4446      <address><email></email></address>
4447    </author>
4448    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4449  </front>
4450  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4451  <x:source href="p7-auth.xml" basename="p7-auth">
4452    <x:defines>Proxy-Authenticate</x:defines>
4453    <x:defines>Proxy-Authorization</x:defines>
4454  </x:source>
4457<reference anchor="RFC5234">
4458  <front>
4459    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4460    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4461      <organization>Brandenburg InternetWorking</organization>
4462      <address>
4463        <email></email>
4464      </address> 
4465    </author>
4466    <author initials="P." surname="Overell" fullname="Paul Overell">
4467      <organization>THUS plc.</organization>
4468      <address>
4469        <email></email>
4470      </address>
4471    </author>
4472    <date month="January" year="2008"/>
4473  </front>
4474  <seriesInfo name="STD" value="68"/>
4475  <seriesInfo name="RFC" value="5234"/>
4478<reference anchor="RFC2119">
4479  <front>
4480    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4481    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4482      <organization>Harvard University</organization>
4483      <address><email></email></address>
4484    </author>
4485    <date month="March" year="1997"/>
4486  </front>
4487  <seriesInfo name="BCP" value="14"/>
4488  <seriesInfo name="RFC" value="2119"/>
4491<reference anchor="RFC3986">
4492 <front>
4493  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4494  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4495    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4496    <address>
4497       <email></email>
4498       <uri></uri>
4499    </address>
4500  </author>
4501  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4502    <organization abbrev="Day Software">Day Software</organization>
4503    <address>
4504      <email></email>
4505      <uri></uri>
4506    </address>
4507  </author>
4508  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4509    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4510    <address>
4511      <email></email>
4512      <uri></uri>
4513    </address>
4514  </author>
4515  <date month='January' year='2005'></date>
4516 </front>
4517 <seriesInfo name="STD" value="66"/>
4518 <seriesInfo name="RFC" value="3986"/>
4521<reference anchor="RFC0793">
4522  <front>
4523    <title>Transmission Control Protocol</title>
4524    <author initials='J.' surname='Postel' fullname='Jon Postel'>
4525      <organization>University of Southern California (USC)/Information Sciences Institute</organization>
4526    </author>
4527    <date year='1981' month='September' />
4528  </front>
4529  <seriesInfo name='STD' value='7' />
4530  <seriesInfo name='RFC' value='793' />
4533<reference anchor="USASCII">
4534  <front>
4535    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4536    <author>
4537      <organization>American National Standards Institute</organization>
4538    </author>
4539    <date year="1986"/>
4540  </front>
4541  <seriesInfo name="ANSI" value="X3.4"/>
4544<reference anchor="RFC1950">
4545  <front>
4546    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4547    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4548      <organization>Aladdin Enterprises</organization>
4549      <address><email></email></address>
4550    </author>
4551    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4552    <date month="May" year="1996"/>
4553  </front>
4554  <seriesInfo name="RFC" value="1950"/>
4555  <!--<annotation>
4556    RFC 1950 is an Informational RFC, thus it might be less stable than
4557    this specification. On the other hand, this downward reference was
4558    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4559    therefore it is unlikely to cause problems in practice. See also
4560    <xref target="BCP97"/>.
4561  </annotation>-->
4564<reference anchor="RFC1951">
4565  <front>
4566    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4567    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4568      <organization>Aladdin Enterprises</organization>
4569      <address><email></email></address>
4570    </author>
4571    <date month="May" year="1996"/>
4572  </front>
4573  <seriesInfo name="RFC" value="1951"/>
4574  <!--<annotation>
4575    RFC 1951 is an Informational RFC, thus it might be less stable than
4576    this specification. On the other hand, this downward reference was
4577    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4578    therefore it is unlikely to cause problems in practice. See also
4579    <xref target="BCP97"/>.
4580  </annotation>-->
4583<reference anchor="RFC1952">
4584  <front>
4585    <title>GZIP file format specification version 4.3</title>
4586    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4587      <organization>Aladdin Enterprises</organization>
4588      <address><email></email></address>
4589    </author>
4590    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4591      <address><email></email></address>
4592    </author>
4593    <author initials="M." surname="Adler" fullname="Mark Adler">
4594      <address><email></email></address>
4595    </author>
4596    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4597      <address><email></email></address>
4598    </author>
4599    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4600      <address><email></email></address>
4601    </author>
4602    <date month="May" year="1996"/>
4603  </front>
4604  <seriesInfo name="RFC" value="1952"/>
4605  <!--<annotation>
4606    RFC 1952 is an Informational RFC, thus it might be less stable than
4607    this specification. On the other hand, this downward reference was
4608    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4609    therefore it is unlikely to cause problems in practice. See also
4610    <xref target="BCP97"/>.
4611  </annotation>-->
4614<reference anchor="Welch">
4615  <front>
4616    <title>A Technique for High Performance Data Compression</title>
4617    <author initials="T.A." surname="Welch" fullname="Terry A. Welch"/>
4618    <date month="June" year="1984"/>
4619  </front>
4620  <seriesInfo name="IEEE Computer" value="17(6)"/>
4625<references title="Informative References">
4627<reference anchor="ISO-8859-1">
4628  <front>
4629    <title>
4630     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4631    </title>
4632    <author>
4633      <organization>International Organization for Standardization</organization>
4634    </author>
4635    <date year="1998"/>
4636  </front>
4637  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4640<reference anchor='RFC1919'>
4641  <front>
4642    <title>Classical versus Transparent IP Proxies</title>
4643    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4644      <address><email></email></address>
4645    </author>
4646    <date year='1996' month='March' />
4647  </front>
4648  <seriesInfo name='RFC' value='1919' />
4651<reference anchor="RFC1945">
4652  <front>
4653    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4654    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4655      <organization>MIT, Laboratory for Computer Science</organization>
4656      <address><email></email></address>
4657    </author>
4658    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4659      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4660      <address><email></email></address>
4661    </author>
4662    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4663      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4664      <address><email></email></address>
4665    </author>
4666    <date month="May" year="1996"/>
4667  </front>
4668  <seriesInfo name="RFC" value="1945"/>
4671<reference anchor="RFC2045">
4672  <front>
4673    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4674    <author initials="N." surname="Freed" fullname="Ned Freed">
4675      <organization>Innosoft International, Inc.</organization>
4676      <address><email></email></address>
4677    </author>
4678    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4679      <organization>First Virtual Holdings</organization>
4680      <address><email></email></address>
4681    </author>
4682    <date month="November" year="1996"/>
4683  </front>
4684  <seriesInfo name="RFC" value="2045"/>
4687<reference anchor="RFC2047">
4688  <front>
4689    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4690    <author initials="K." surname="Moore" fullname="Keith Moore">
4691      <organization>University of Tennessee</organization>
4692      <address><email></email></address>
4693    </author>
4694    <date month="November" year="1996"/>
4695  </front>
4696  <seriesInfo name="RFC" value="2047"/>
4699<reference anchor="RFC2068">
4700  <front>
4701    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4702    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
4703      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4704      <address><email></email></address>
4705    </author>
4706    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4707      <organization>MIT Laboratory for Computer Science</organization>
4708      <address><email></email></address>
4709    </author>
4710    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4711      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4712      <address><email></email></address>
4713    </author>
4714    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4715      <organization>MIT Laboratory for Computer Science</organization>
4716      <address><email></email></address>
4717    </author>
4718    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4719      <organization>MIT Laboratory for Computer Science</organization>
4720      <address><email></email></address>
4721    </author>
4722    <date month="January" year="1997"/>
4723  </front>
4724  <seriesInfo name="RFC" value="2068"/>
4727<reference anchor="RFC2145">
4728  <front>
4729    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4730    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4731      <organization>Western Research Laboratory</organization>
4732      <address><email></email></address>
4733    </author>
4734    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4735      <organization>Department of Information and Computer Science</organization>
4736      <address><email></email></address>
4737    </author>
4738    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4739      <organization>MIT Laboratory for Computer Science</organization>
4740      <address><email></email></address>
4741    </author>
4742    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4743      <organization>W3 Consortium</organization>
4744      <address><email></email></address>
4745    </author>
4746    <date month="May" year="1997"/>
4747  </front>
4748  <seriesInfo name="RFC" value="2145"/>
4751<reference anchor="RFC2616">
4752  <front>
4753    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4754    <author initials="R." surname="Fielding" fullname="R. Fielding">
4755      <organization>University of California, Irvine</organization>
4756      <address><email></email></address>
4757    </author>
4758    <author initials="J." surname="Gettys" fullname="J. Gettys">
4759      <organization>W3C</organization>
4760      <address><email></email></address>
4761    </author>
4762    <author initials="J." surname="Mogul" fullname="J. Mogul">
4763      <organization>Compaq Computer Corporation</organization>
4764      <address><email></email></address>
4765    </author>
4766    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4767      <organization>MIT Laboratory for Computer Science</organization>
4768      <address><email></email></address>
4769    </author>
4770    <author initials="L." surname="Masinter" fullname="L. Masinter">
4771      <organization>Xerox Corporation</organization>
4772      <address><email></email></address>
4773    </author>
4774    <author initials="P." surname="Leach" fullname="P. Leach">
4775      <organization>Microsoft Corporation</organization>
4776      <address><email></email></address>
4777    </author>
4778    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4779      <organization>W3C</organization>
4780      <address><email></email></address>
4781    </author>
4782    <date month="June" year="1999"/>
4783  </front>
4784  <seriesInfo name="RFC" value="2616"/>
4787<reference anchor='RFC2817'>
4788  <front>
4789    <title>Upgrading to TLS Within HTTP/1.1</title>
4790    <author initials='R.' surname='Khare' fullname='R. Khare'>
4791      <organization>4K Associates / UC Irvine</organization>
4792      <address><email></email></address>
4793    </author>
4794    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4795      <organization>Agranat Systems, Inc.</organization>
4796      <address><email></email></address>
4797    </author>
4798    <date year='2000' month='May' />
4799  </front>
4800  <seriesInfo name='RFC' value='2817' />
4803<reference anchor='RFC2818'>
4804  <front>
4805    <title>HTTP Over TLS</title>
4806    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4807      <organization>RTFM, Inc.</organization>
4808      <address><email></email></address>
4809    </author>
4810    <date year='2000' month='May' />
4811  </front>
4812  <seriesInfo name='RFC' value='2818' />
4815<reference anchor='RFC3040'>
4816  <front>
4817    <title>Internet Web Replication and Caching Taxonomy</title>
4818    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4819      <organization>Equinix, Inc.</organization>
4820    </author>
4821    <author initials='I.' surname='Melve' fullname='I. Melve'>
4822      <organization>UNINETT</organization>
4823    </author>
4824    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4825      <organization>CacheFlow Inc.</organization>
4826    </author>
4827    <date year='2001' month='January' />
4828  </front>
4829  <seriesInfo name='RFC' value='3040' />
4832<reference anchor='BCP90'>
4833  <front>
4834    <title>Registration Procedures for Message Header Fields</title>
4835    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4836      <organization>Nine by Nine</organization>
4837      <address><email></email></address>
4838    </author>
4839    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4840      <organization>BEA Systems</organization>
4841      <address><email></email></address>
4842    </author>
4843    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4844      <organization>HP Labs</organization>
4845      <address><email></email></address>
4846    </author>
4847    <date year='2004' month='September' />
4848  </front>
4849  <seriesInfo name='BCP' value='90' />
4850  <seriesInfo name='RFC' value='3864' />
4853<reference anchor='RFC4033'>
4854  <front>
4855    <title>DNS Security Introduction and Requirements</title>
4856    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4857    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4858    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4859    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4860    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4861    <date year='2005' month='March' />
4862  </front>
4863  <seriesInfo name='RFC' value='4033' />
4866<reference anchor="BCP13">
4867  <front>
4868    <title>Media Type Specifications and Registration Procedures</title>
4869    <author initials="N." surname="Freed" fullname="Ned Freed">
4870      <organization>Oracle</organization>
4871      <address>
4872        <email></email>
4873      </address>
4874    </author>
4875    <author initials="J." surname="Klensin" fullname="John C. Klensin">
4876      <address>
4877        <email></email>
4878      </address>
4879    </author>
4880    <author initials="T." surname="Hansen" fullname="Tony Hansen">
4881      <organization>AT&amp;T Laboratories</organization>
4882      <address>
4883        <email></email>
4884      </address>
4885    </author>
4886    <date year="2013" month="January"/>
4887  </front>
4888  <seriesInfo name="BCP" value="13"/>
4889  <seriesInfo name="RFC" value="6838"/>
4892<reference anchor='BCP115'>
4893  <front>
4894    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4895    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4896      <organization>AT&amp;T Laboratories</organization>
4897      <address>
4898        <email></email>
4899      </address>
4900    </author>
4901    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4902      <organization>Qualcomm, Inc.</organization>
4903      <address>
4904        <email></email>
4905      </address>
4906    </author>
4907    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4908      <organization>Adobe Systems</organization>
4909      <address>
4910        <email></email>
4911      </address>
4912    </author>
4913    <date year='2006' month='February' />
4914  </front>
4915  <seriesInfo name='BCP' value='115' />
4916  <seriesInfo name='RFC' value='4395' />
4919<reference anchor='RFC4559'>
4920  <front>
4921    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4922    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4923    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4924    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4925    <date year='2006' month='June' />
4926  </front>
4927  <seriesInfo name='RFC' value='4559' />
4930<reference anchor='RFC5226'>
4931  <front>
4932    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4933    <author initials='T.' surname='Narten' fullname='T. Narten'>
4934      <organization>IBM</organization>
4935      <address><email></email></address>
4936    </author>
4937    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4938      <organization>Google</organization>
4939      <address><email></email></address>
4940    </author>
4941    <date year='2008' month='May' />
4942  </front>
4943  <seriesInfo name='BCP' value='26' />
4944  <seriesInfo name='RFC' value='5226' />
4947<reference anchor='RFC5246'>
4948   <front>
4949      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4950      <author initials='T.' surname='Dierks' fullname='T. Dierks'/>
4951      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4952         <organization>RTFM, Inc.</organization>
4953      </author>
4954      <date year='2008' month='August' />
4955   </front>
4956   <seriesInfo name='RFC' value='5246' />
4959<reference anchor="RFC5322">
4960  <front>
4961    <title>Internet Message Format</title>
4962    <author initials="P." surname="Resnick" fullname="P. Resnick">
4963      <organization>Qualcomm Incorporated</organization>
4964    </author>
4965    <date year="2008" month="October"/>
4966  </front>
4967  <seriesInfo name="RFC" value="5322"/>
4970<reference anchor="RFC6265">
4971  <front>
4972    <title>HTTP State Management Mechanism</title>
4973    <author initials="A." surname="Barth" fullname="Adam Barth">
4974      <organization abbrev="U.C. Berkeley">
4975        University of California, Berkeley
4976      </organization>
4977      <address><email></email></address>
4978    </author>
4979    <date year="2011" month="April" />
4980  </front>
4981  <seriesInfo name="RFC" value="6265"/>
4984<reference anchor='RFC6585'>
4985  <front>
4986    <title>Additional HTTP Status Codes</title>
4987    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4988      <organization>Rackspace</organization>
4989    </author>
4990    <author initials='R.' surname='Fielding' fullname='R. Fielding'>
4991      <organization>Adobe</organization>
4992    </author>
4993    <date year='2012' month='April' />
4994   </front>
4995   <seriesInfo name='RFC' value='6585' />
4998<!--<reference anchor='BCP97'>
4999  <front>
5000    <title>Handling Normative References to Standards-Track Documents</title>
5001    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
5002      <address>
5003        <email></email>
5004      </address>
5005    </author>
5006    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
5007      <organization>MIT</organization>
5008      <address>
5009        <email></email>
5010      </address>
5011    </author>
5012    <date year='2007' month='June' />
5013  </front>
5014  <seriesInfo name='BCP' value='97' />
5015  <seriesInfo name='RFC' value='4897' />
5018<reference anchor="Kri2001" target="">
5019  <front>
5020    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
5021    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
5022    <date year="2001" month="November"/>
5023  </front>
5024  <seriesInfo name="ACM Transactions on Internet Technology" value="1(2)"/>
5030<section title="HTTP Version History" anchor="compatibility">
5032   HTTP has been in use since 1990. The first version, later referred to as
5033   HTTP/0.9, was a simple protocol for hypertext data transfer across the
5034   Internet, using only a single request method (GET) and no metadata.
5035   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
5036   methods and MIME-like messaging, allowing for metadata to be transferred
5037   and modifiers placed on the request/response semantics. However,
5038   HTTP/1.0 did not sufficiently take into consideration the effects of
5039   hierarchical proxies, caching, the need for persistent connections, or
5040   name-based virtual hosts. The proliferation of incompletely-implemented
5041   applications calling themselves "HTTP/1.0" further necessitated a
5042   protocol version change in order for two communicating applications
5043   to determine each other's true capabilities.
5046   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
5047   requirements that enable reliable implementations, adding only
5048   those features that can either be safely ignored by an HTTP/1.0
5049   recipient or only sent when communicating with a party advertising
5050   conformance with HTTP/1.1.
5053   HTTP/1.1 has been designed to make supporting previous versions easy.
5054   A general-purpose HTTP/1.1 server ought to be able to understand any valid
5055   request in the format of HTTP/1.0, responding appropriately with an
5056   HTTP/1.1 message that only uses features understood (or safely ignored) by
5057   HTTP/1.0 clients. Likewise, an HTTP/1.1 client can be expected to
5058   understand any valid HTTP/1.0 response.
5061   Since HTTP/0.9 did not support header fields in a request, there is no
5062   mechanism for it to support name-based virtual hosts (selection of resource
5063   by inspection of the <x:ref>Host</x:ref> header field).
5064   Any server that implements name-based virtual hosts ought to disable
5065   support for HTTP/0.9. Most requests that appear to be HTTP/0.9 are, in
5066   fact, badly constructed HTTP/1.x requests caused by a client failing to
5067   properly encode the request-target.
5070<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
5072   This section summarizes major differences between versions HTTP/1.0
5073   and HTTP/1.1.
5076<section title="Multi-homed Web Servers" anchor="">
5078   The requirements that clients and servers support the <x:ref>Host</x:ref>
5079   header field (<xref target=""/>), report an error if it is
5080   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
5081   are among the most important changes defined by HTTP/1.1.
5084   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
5085   addresses and servers; there was no other established mechanism for
5086   distinguishing the intended server of a request than the IP address
5087   to which that request was directed. The <x:ref>Host</x:ref> header field was
5088   introduced during the development of HTTP/1.1 and, though it was
5089   quickly implemented by most HTTP/1.0 browsers, additional requirements
5090   were placed on all HTTP/1.1 requests in order to ensure complete
5091   adoption.  At the time of this writing, most HTTP-based services
5092   are dependent upon the Host header field for targeting requests.
5096<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
5098   In HTTP/1.0, each connection is established by the client prior to the
5099   request and closed by the server after sending the response. However, some
5100   implementations implement the explicitly negotiated ("Keep-Alive") version
5101   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
5102   target="RFC2068"/>.
5105   Some clients and servers might wish to be compatible with these previous
5106   approaches to persistent connections, by explicitly negotiating for them
5107   with a "Connection: keep-alive" request header field. However, some
5108   experimental implementations of HTTP/1.0 persistent connections are faulty;
5109   for example, if an HTTP/1.0 proxy server doesn't understand
5110   <x:ref>Connection</x:ref>, it will erroneously forward that header field
5111   to the next inbound server, which would result in a hung connection.
5114   One attempted solution was the introduction of a Proxy-Connection header
5115   field, targeted specifically at proxies. In practice, this was also
5116   unworkable, because proxies are often deployed in multiple layers, bringing
5117   about the same problem discussed above.
5120   As a result, clients are encouraged not to send the Proxy-Connection header
5121   field in any requests.
5124   Clients are also encouraged to consider the use of Connection: keep-alive
5125   in requests carefully; while they can enable persistent connections with
5126   HTTP/1.0 servers, clients using them will need to monitor the
5127   connection for "hung" requests (which indicate that the client ought stop
5128   sending the header field), and this mechanism ought not be used by clients
5129   at all when a proxy is being used.
5133<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
5135   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
5136   (<xref target="header.transfer-encoding"/>).
5137   Transfer codings need to be decoded prior to forwarding an HTTP message
5138   over a MIME-compliant protocol.
5144<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
5146  HTTP's approach to error handling has been explained.
5147  (<xref target="conformance" />)
5150  The HTTP-version ABNF production has been clarified to be case-sensitive.
5151  Additionally, version numbers has been restricted to single digits, due
5152  to the fact that implementations are known to handle multi-digit version
5153  numbers incorrectly.
5154  (<xref target="http.version"/>)
5157  Userinfo (i.e., username and password) are now disallowed in HTTP and
5158  HTTPS URIs, because of security issues related to their transmission on the
5159  wire.
5160  (<xref target="http.uri" />)
5163  The HTTPS URI scheme is now defined by this specification; previously,
5164  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
5165  Furthermore, it implies end-to-end security.
5166  (<xref target="https.uri"/>)
5169  HTTP messages can be (and often are) buffered by implementations; despite
5170  it sometimes being available as a stream, HTTP is fundamentally a
5171  message-oriented protocol.
5172  Minimum supported sizes for various protocol elements have been
5173  suggested, to improve interoperability.
5174  (<xref target="http.message" />)
5177  Invalid whitespace around field-names is now required to be rejected,
5178  because accepting it represents a security vulnerability.
5179  The ABNF productions defining header fields now only list the field value.
5180  (<xref target="header.fields"/>)
5183  Rules about implicit linear whitespace between certain grammar productions
5184  have been removed; now whitespace is only allowed where specifically
5185  defined in the ABNF.
5186  (<xref target="whitespace"/>)
5189  Header fields that span multiple lines ("line folding") are deprecated.
5190  (<xref target="field.parsing" />)
5193  The NUL octet is no longer allowed in comment and quoted-string text, and
5194  handling of backslash-escaping in them has been clarified.
5195  The quoted-pair rule no longer allows escaping control characters other than
5196  HTAB.
5197  Non-ASCII content in header fields and the reason phrase has been obsoleted
5198  and made opaque (the TEXT rule was removed).
5199  (<xref target="field.components"/>)
5202  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
5203  handled as errors by recipients.
5204  (<xref target="header.content-length"/>)
5207  The algorithm for determining the message body length has been clarified
5208  to indicate all of the special cases (e.g., driven by methods or status
5209  codes) that affect it, and that new protocol elements cannot define such
5210  special cases.
5211  CONNECT is a new, special case in determining message body length.
5212  "multipart/byteranges" is no longer a way of determining message body length
5213  detection.
5214  (<xref target="message.body.length"/>)
5217  The "identity" transfer coding token has been removed.
5218  (Sections <xref format="counter" target="message.body"/> and
5219  <xref format="counter" target="transfer.codings"/>)
5222  Chunk length does not include the count of the octets in the
5223  chunk header and trailer.
5224  Line folding in chunk extensions is  disallowed.
5225  (<xref target="chunked.encoding"/>)
5228  The meaning of the "deflate" content coding has been clarified.
5229  (<xref target="deflate.coding" />)
5232  The segment + query components of RFC 3986 have been used to define the
5233  request-target, instead of abs_path from RFC 1808.
5234  The asterisk-form of the request-target is only allowed with the OPTIONS
5235  method.
5236  (<xref target="request-target"/>)
5239  The term "Effective Request URI" has been introduced.
5240  (<xref target="effective.request.uri" />)
5243  Gateways do not need to generate <x:ref>Via</x:ref> header fields anymore.
5244  (<xref target="header.via"/>)
5247  Exactly when "close" connection options have to be sent has been clarified.
5248  Also, "hop-by-hop" header fields are required to appear in the Connection header
5249  field; just because they're defined as hop-by-hop in this specification
5250  doesn't exempt them.
5251  (<xref target="header.connection"/>)
5254  The limit of two connections per server has been removed.
5255  An idempotent sequence of requests is no longer required to be retried.
5256  The requirement to retry requests under certain circumstances when the
5257  server prematurely closes the connection has been removed.
5258  Also, some extraneous requirements about when servers are allowed to close
5259  connections prematurely have been removed.
5260  (<xref target="persistent.connections"/>)
5263  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
5264  responses other than 101 (this was incorporated from <xref
5265  target="RFC2817"/>). Furthermore, the ordering in the field value is now
5266  significant.
5267  (<xref target="header.upgrade"/>)
5270  Empty list elements in list productions (e.g., a list header field containing
5271  ", ,") have been deprecated.
5272  (<xref target="abnf.extension"/>)
5275  Registration of Transfer Codings now requires IETF Review
5276  (<xref target="transfer.coding.registry"/>)
5279  This specification now defines the Upgrade Token Registry, previously
5280  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
5281  (<xref target="upgrade.token.registry"/>)
5284  The expectation to support HTTP/0.9 requests has been removed.
5285  (<xref target="compatibility"/>)
5288  Issues with the Keep-Alive and Proxy-Connection header fields in requests
5289  are pointed out, with use of the latter being discouraged altogether.
5290  (<xref target="compatibility.with.http.1.0.persistent.connections" />)
5295<?BEGININC p1-messaging.abnf-appendix ?>
5296<section xmlns:x="" title="Collected ABNF" anchor="collected.abnf">
5298<artwork type="abnf" name="p1-messaging.parsed-abnf">
5299<x:ref>BWS</x:ref> = OWS
5301<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
5302 connection-option ] )
5303<x:ref>Content-Length</x:ref> = 1*DIGIT
5305<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
5306 ]
5307<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
5308<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
5309<x:ref>Host</x:ref> = uri-host [ ":" port ]
5311<x:ref>OWS</x:ref> = *( SP / HTAB )
5313<x:ref>RWS</x:ref> = 1*( SP / HTAB )
5315<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
5316<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
5317<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
5318 transfer-coding ] )
5320<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
5321<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
5323<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
5324 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
5325 comment ] ) ] )
5327<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
5328<x:ref>absolute-form</x:ref> = absolute-URI
5329<x:ref>absolute-path</x:ref> = 1*( "/" segment )
5330<x:ref>asterisk-form</x:ref> = "*"
5331<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
5332<x:ref>authority-form</x:ref> = authority
5334<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
5335<x:ref>chunk-data</x:ref> = 1*OCTET
5336<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
5337<x:ref>chunk-ext-name</x:ref> = token
5338<x:ref>chunk-ext-val</x:ref> = token / quoted-string
5339<x:ref>chunk-size</x:ref> = 1*HEXDIG
5340<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
5341<x:ref>comment</x:ref> = "(" *( ctext / quoted-pair / comment ) ")"
5342<x:ref>connection-option</x:ref> = token
5343<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
5344 / %x2A-5B ; '*'-'['
5345 / %x5D-7E ; ']'-'~'
5346 / obs-text
5348<x:ref>field-content</x:ref> = field-vchar [ 1*( SP / HTAB ) field-vchar ]
5349<x:ref>field-name</x:ref> = token
5350<x:ref>field-value</x:ref> = *( field-content / obs-fold )
5351<x:ref>field-vchar</x:ref> = VCHAR / obs-text
5352<x:ref>fragment</x:ref> = &lt;fragment, defined in [RFC3986], Section 3.5&gt;
5354<x:ref>header-field</x:ref> = field-name ":" OWS field-value OWS
5355<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ] [ "#"
5356 fragment ]
5357<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ] [ "#"
5358 fragment ]
5360<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
5362<x:ref>message-body</x:ref> = *OCTET
5363<x:ref>method</x:ref> = token
5365<x:ref>obs-fold</x:ref> = CRLF 1*( SP / HTAB )
5366<x:ref>obs-text</x:ref> = %x80-FF
5367<x:ref>origin-form</x:ref> = absolute-path [ "?" query ]
5369<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
5370<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
5371<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
5372<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
5373<x:ref>protocol-name</x:ref> = token
5374<x:ref>protocol-version</x:ref> = token
5375<x:ref>pseudonym</x:ref> = token
5377<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5378 / %x5D-7E ; ']'-'~'
5379 / obs-text
5380<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5381<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5382<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5384<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5385<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5386<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5387<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5388<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5389<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5390<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5391 asterisk-form
5393<x:ref>scheme</x:ref> = &lt;scheme, defined in [RFC3986], Section 3.1&gt;
5394<x:ref>segment</x:ref> = &lt;segment, defined in [RFC3986], Section 3.3&gt;
5395<x:ref>start-line</x:ref> = request-line / status-line
5396<x:ref>status-code</x:ref> = 3DIGIT
5397<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5399<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5400<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5401<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5402 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5403<x:ref>token</x:ref> = 1*tchar
5404<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5405<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5406 transfer-extension
5407<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5408<x:ref>transfer-parameter</x:ref> = token BWS "=" BWS ( token / quoted-string )
5410<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5414<?ENDINC p1-messaging.abnf-appendix ?>
5416<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5418<section title="Since RFC 2616">
5420  Changes up to the IETF Last Call draft are summarized
5421  in <eref target=""/>.
5425<section title="Since draft-ietf-httpbis-p1-messaging-24" anchor="changes.since.24">
5427  Closed issues:
5428  <list style="symbols">
5429    <t>
5430      <eref target=""/>:
5431      "APPSDIR review of draft-ietf-httpbis-p1-messaging-24"
5432    </t>
5433    <t>
5434      <eref target=""/>:
5435      "integer value parsing"
5436    </t>
5437    <t>
5438      <eref target=""/>:
5439      "move IANA registrations to correct draft"
5440    </t>
5441  </list>
5445<section title="Since draft-ietf-httpbis-p1-messaging-25" anchor="changes.since.25">
5447  Closed issues:
5448  <list style="symbols">
5449    <t>
5450      <eref target=""/>:
5451      "check media type registration templates"
5452    </t>
5453    <t>
5454      <eref target=""/>:
5455      "Redundant rule quoted-str-nf"
5456    </t>
5457    <t>
5458      <eref target=""/>:
5459      "add 'stateless' to Abstract"
5460    </t>
5461    <t>
5462      <eref target=""/>:
5463      "clarify ABNF layering"
5464    </t>
5465    <t>
5466      <eref target=""/>:
5467      "use of 'word' ABNF production"
5468    </t>
5469    <t>
5470      <eref target=""/>:
5471      "improve introduction of list rule"
5472    </t>
5473    <t>
5474      <eref target=""/>:
5475      "moving 2616/2068/2145 to historic"
5476    </t>
5477    <t>
5478      <eref target=""/>:
5479      "augment security considerations with pointers to current research"
5480    </t>
5481    <t>
5482      <eref target=""/>:
5483      "intermediaries handling trailers"
5484    </t>
5485  </list>
5488  Partly resolved issues:
5489  <list style="symbols">
5490    <t>
5491      <eref target=""/>:
5492      "IESG ballot on draft-ietf-httpbis-p1-messaging-25"
5493    </t>
5494  </list>
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