source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2610

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bump up document dates, update to latest version of rfc2629.xslt

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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 "February">
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 cache-poisoning        "<xref target='Part6' x:rel='#security.considerations' xmlns:x=''/>">
22  <!ENTITY payload                "<xref target='Part2' x:rel='#payload' xmlns:x=''/>">
23  <!ENTITY media-type             "<xref target='Part2' x:rel='#media.type' xmlns:x=''/>">
24  <!ENTITY content-codings        "<xref target='Part2' x:rel='#content.codings' xmlns:x=''/>">
25  <!ENTITY CONNECT                "<xref target='Part2' x:rel='#CONNECT' xmlns:x=''/>">
26  <!ENTITY content.negotiation    "<xref target='Part2' x:rel='#content.negotiation' xmlns:x=''/>">
27  <!ENTITY diff-mime              "<xref target='Part2' x:rel='#differences.between.http.and.mime' xmlns:x=''/>">
28  <!ENTITY representation         "<xref target='Part2' x:rel='#representations' xmlns:x=''/>">
29  <!ENTITY GET                    "<xref target='Part2' x:rel='#GET' xmlns:x=''/>">
30  <!ENTITY HEAD                   "<xref target='Part2' x:rel='#HEAD' xmlns:x=''/>">
31  <!ENTITY header-allow           "<xref target='Part2' x:rel='#header.allow' xmlns:x=''/>">
32  <!ENTITY header-cache-control   "<xref target='Part6' x:rel='#header.cache-control' xmlns:x=''/>">
33  <!ENTITY header-content-encoding    "<xref target='Part2' x:rel='#header.content-encoding' xmlns:x=''/>">
34  <!ENTITY header-content-location    "<xref target='Part2' x:rel='#header.content-location' xmlns:x=''/>">
35  <!ENTITY header-content-range   "<xref target='Part5' x:rel='#header.content-range' xmlns:x=''/>">
36  <!ENTITY header-content-type    "<xref target='Part2' x:rel='#header.content-type' xmlns:x=''/>">
37  <!ENTITY header-date            "<xref target='Part2' x:rel='' xmlns:x=''/>">
38  <!ENTITY header-etag            "<xref target='Part4' x:rel='#header.etag' xmlns:x=''/>">
39  <!ENTITY header-expect          "<xref target='Part2' x:rel='#header.expect' xmlns:x=''/>">
40  <!ENTITY header-expires         "<xref target='Part6' x:rel='#header.expires' xmlns:x=''/>">
41  <!ENTITY header-last-modified   "<xref target='Part4' x:rel='#header.last-modified' xmlns:x=''/>">
42  <!ENTITY header-mime-version    "<xref target='Part2' x:rel='#mime-version' xmlns:x=''/>">
43  <!ENTITY header-pragma          "<xref target='Part6' x:rel='#header.pragma' xmlns:x=''/>">
44  <!ENTITY header-proxy-authenticate  "<xref target='Part7' x:rel='#header.proxy-authenticate' xmlns:x=''/>">
45  <!ENTITY header-proxy-authorization "<xref target='Part7' x:rel='#header.proxy-authorization' xmlns:x=''/>">
46  <!ENTITY header-server          "<xref target='Part2' x:rel='#header.server' xmlns:x=''/>">
47  <!ENTITY header-warning         "<xref target='Part6' x:rel='#header.warning' xmlns:x=''/>">
48  <!ENTITY idempotent-methods     "<xref target='Part2' x:rel='#idempotent.methods' xmlns:x=''/>">
49  <!ENTITY safe-methods           "<xref target='Part2' x:rel='#safe.methods' xmlns:x=''/>">
50  <!ENTITY methods                "<xref target='Part2' x:rel='#methods' xmlns:x=''/>">
51  <!ENTITY OPTIONS                "<xref target='Part2' x:rel='#OPTIONS' xmlns:x=''/>">
52  <!ENTITY qvalue                 "<xref target='Part2' x:rel='#quality.values' xmlns:x=''/>">
53  <!ENTITY request-header-fields  "<xref target='Part2' x:rel='#request.header.fields' xmlns:x=''/>">
54  <!ENTITY response-control-data  "<xref target='Part2' x:rel='' xmlns:x=''/>">
55  <!ENTITY resource               "<xref target='Part2' x:rel='#resources' xmlns:x=''/>">
56  <!ENTITY semantics              "<xref target='Part2' xmlns:x=''/>">
57  <!ENTITY status-codes           "<xref target='Part2' x:rel='' xmlns:x=''/>">
58  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x=''/>">
59  <!ENTITY status-203             "<xref target='Part2' x:rel='#status.203' xmlns:x=''/>">
60  <!ENTITY status-3xx             "<xref target='Part2' x:rel='#status.3xx' xmlns:x=''/>">
61  <!ENTITY status-304             "<xref target='Part4' x:rel='#status.304' xmlns:x=''/>">
62  <!ENTITY status-4xx             "<xref target='Part2' x:rel='#status.4xx' xmlns:x=''/>">
63  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x=''/>">
64  <!ENTITY iana-header-registry   "<xref target='Part2' x:rel='#header.field.registry' xmlns:x=''/>">
66<?rfc toc="yes" ?>
67<?rfc symrefs="yes" ?>
68<?rfc sortrefs="yes" ?>
69<?rfc compact="yes"?>
70<?rfc subcompact="no" ?>
71<?rfc linkmailto="no" ?>
72<?rfc editing="no" ?>
73<?rfc comments="yes"?>
74<?rfc inline="yes"?>
75<?rfc rfcedstyle="yes"?>
76<?rfc-ext allow-markup-in-artwork="yes" ?>
77<?rfc-ext include-references-in-index="yes" ?>
78<rfc obsoletes="2145,2616" updates="2817,2818" category="std" x:maturity-level="proposed"
79     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
80     xmlns:x=''>
81<x:link rel="next" basename="p2-semantics"/>
82<x:feedback template="{docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
85  <title abbrev="HTTP/1.1 Message Syntax and Routing">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
87  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
88    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
89    <address>
90      <postal>
91        <street>345 Park Ave</street>
92        <city>San Jose</city>
93        <region>CA</region>
94        <code>95110</code>
95        <country>USA</country>
96      </postal>
97      <email></email>
98      <uri></uri>
99    </address>
100  </author>
102  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
103    <organization abbrev="greenbytes">greenbytes GmbH</organization>
104    <address>
105      <postal>
106        <street>Hafenweg 16</street>
107        <city>Muenster</city><region>NW</region><code>48155</code>
108        <country>Germany</country>
109      </postal>
110      <email></email>
111      <uri></uri>
112    </address>
113  </author>
115  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
117  <area>Applications</area>
118  <workgroup>HTTPbis</workgroup>
122   The Hypertext Transfer Protocol (HTTP) is a stateless application-level
123   protocol for distributed, collaborative, hypertext information systems.
124   This document provides an overview of HTTP architecture and its associated
125   terminology, defines the "http" and "https" Uniform Resource Identifier
126   (URI) schemes, defines the HTTP/1.1 message syntax and parsing
127   requirements, and describes related security concerns for implementations.
131<note title="Editorial Note (To be removed by RFC Editor)">
132  <t>
133    Discussion of this draft takes place on the HTTPBIS working group
134    mailing list (, which is archived at
135    <eref target=""/>.
136  </t>
137  <t>
138    The current issues list is at
139    <eref target=""/> and related
140    documents (including fancy diffs) can be found at
141    <eref target=""/>.
142  </t>
143  <t>
144    The changes in this draft are summarized in <xref target="changes.since.25"/>.
145  </t>
149<section title="Introduction" anchor="introduction">
151   The Hypertext Transfer Protocol (HTTP) is a stateless application-level
152   request/response protocol that uses extensible semantics and
153   self-descriptive message payloads for flexible interaction with
154   network-based hypertext information systems. This document is the first in
155   a series of documents that collectively form the HTTP/1.1 specification:
156   <list style="empty">
157    <t>RFC xxx1: Message Syntax and Routing</t>
158    <t><xref target="Part2" x:fmt="none">RFC xxx2</xref>: Semantics and Content</t>
159    <t><xref target="Part4" x:fmt="none">RFC xxx3</xref>: Conditional Requests</t>
160    <t><xref target="Part5" x:fmt="none">RFC xxx4</xref>: Range Requests</t>
161    <t><xref target="Part6" x:fmt="none">RFC xxx5</xref>: Caching</t>
162    <t><xref target="Part7" x:fmt="none">RFC xxx6</xref>: Authentication</t>
163   </list>
166   This HTTP/1.1 specification obsoletes
167   <xref target="RFC2616" x:fmt="none">RFC 2616</xref> and
168   <xref target="RFC2145" x:fmt="none">RFC 2145</xref> (on HTTP versioning).
169   This specification also updates the use of CONNECT to establish a tunnel,
170   previously defined in <xref target="RFC2817" x:fmt="none">RFC 2817</xref>,
171   and defines the "https" URI scheme that was described informally in
172   <xref target="RFC2818" x:fmt="none">RFC 2818</xref>.
175   HTTP is a generic interface protocol for information systems. It is
176   designed to hide the details of how a service is implemented by presenting
177   a uniform interface to clients that is independent of the types of
178   resources provided. Likewise, servers do not need to be aware of each
179   client's purpose: an HTTP request can be considered in isolation rather
180   than being associated with a specific type of client or a predetermined
181   sequence of application steps. The result is a protocol that can be used
182   effectively in many different contexts and for which implementations can
183   evolve independently over time.
186   HTTP is also designed for use as an intermediation protocol for translating
187   communication to and from non-HTTP information systems.
188   HTTP proxies and gateways can provide access to alternative information
189   services by translating their diverse protocols into a hypertext
190   format that can be viewed and manipulated by clients in the same way
191   as HTTP services.
194   One consequence of this flexibility is that the protocol cannot be
195   defined in terms of what occurs behind the interface. Instead, we
196   are limited to defining the syntax of communication, the intent
197   of received communication, and the expected behavior of recipients.
198   If the communication is considered in isolation, then successful
199   actions ought to be reflected in corresponding changes to the
200   observable interface provided by servers. However, since multiple
201   clients might act in parallel and perhaps at cross-purposes, we
202   cannot require that such changes be observable beyond the scope
203   of a single response.
206   This document describes the architectural elements that are used or
207   referred to in HTTP, defines the "http" and "https" URI schemes,
208   describes overall network operation and connection management,
209   and defines HTTP message framing and forwarding requirements.
210   Our goal is to define all of the mechanisms necessary for HTTP message
211   handling that are independent of message semantics, thereby defining the
212   complete set of requirements for message parsers and
213   message-forwarding intermediaries.
217<section title="Requirement Notation" anchor="intro.requirements">
219   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
220   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
221   document are to be interpreted as described in <xref target="RFC2119"/>.
224   Conformance criteria and considerations regarding error handling
225   are defined in <xref target="conformance"/>.
229<section title="Syntax Notation" anchor="notation">
230<iref primary="true" item="Grammar" subitem="ALPHA"/>
231<iref primary="true" item="Grammar" subitem="CR"/>
232<iref primary="true" item="Grammar" subitem="CRLF"/>
233<iref primary="true" item="Grammar" subitem="CTL"/>
234<iref primary="true" item="Grammar" subitem="DIGIT"/>
235<iref primary="true" item="Grammar" subitem="DQUOTE"/>
236<iref primary="true" item="Grammar" subitem="HEXDIG"/>
237<iref primary="true" item="Grammar" subitem="HTAB"/>
238<iref primary="true" item="Grammar" subitem="LF"/>
239<iref primary="true" item="Grammar" subitem="OCTET"/>
240<iref primary="true" item="Grammar" subitem="SP"/>
241<iref primary="true" item="Grammar" subitem="VCHAR"/>
243   This specification uses the Augmented Backus-Naur Form (ABNF) notation of
244   <xref target="RFC5234"/> with a list extension, defined in
245   <xref target="abnf.extension"/>, that allows for compact definition of
246   comma-separated lists using a '#' operator (similar to how the '*' operator
247   indicates repetition).
248   <xref target="collected.abnf"/> shows the collected grammar with all list
249   operators expanded to standard ABNF notation.
251<t anchor="core.rules">
252  <x:anchor-alias value="ALPHA"/>
253  <x:anchor-alias value="CTL"/>
254  <x:anchor-alias value="CR"/>
255  <x:anchor-alias value="CRLF"/>
256  <x:anchor-alias value="DIGIT"/>
257  <x:anchor-alias value="DQUOTE"/>
258  <x:anchor-alias value="HEXDIG"/>
259  <x:anchor-alias value="HTAB"/>
260  <x:anchor-alias value="LF"/>
261  <x:anchor-alias value="OCTET"/>
262  <x:anchor-alias value="SP"/>
263  <x:anchor-alias value="VCHAR"/>
264   The following core rules are included by
265   reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
266   ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
267   DIGIT (decimal 0-9), DQUOTE (double quote),
268   HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed),
269   OCTET (any 8-bit sequence of data), SP (space), and
270   VCHAR (any visible <xref target="USASCII"/> character).
273   As a convention, ABNF rule names prefixed with "obs-" denote
274   "obsolete" grammar rules that appear for historical reasons.
279<section title="Architecture" anchor="architecture">
281   HTTP was created for the World Wide Web (WWW) architecture
282   and has evolved over time to support the scalability needs of a worldwide
283   hypertext system. Much of that architecture is reflected in the terminology
284   and syntax productions used to define HTTP.
287<section title="Client/Server Messaging" anchor="operation">
288<iref primary="true" item="client"/>
289<iref primary="true" item="server"/>
290<iref primary="true" item="connection"/>
292   HTTP is a stateless request/response protocol that operates by exchanging
293   <x:dfn>messages</x:dfn> (<xref target="http.message"/>) across a reliable
294   transport or session-layer
295   "<x:dfn>connection</x:dfn>" (<xref target=""/>).
296   An HTTP "<x:dfn>client</x:dfn>" is a program that establishes a connection
297   to a server for the purpose of sending one or more HTTP requests.
298   An HTTP "<x:dfn>server</x:dfn>" is a program that accepts connections
299   in order to service HTTP requests by sending HTTP responses.
301<iref primary="true" item="user agent"/>
302<iref primary="true" item="origin server"/>
303<iref primary="true" item="browser"/>
304<iref primary="true" item="spider"/>
305<iref primary="true" item="sender"/>
306<iref primary="true" item="recipient"/>
308   The terms client and server refer only to the roles that
309   these programs perform for a particular connection.  The same program
310   might act as a client on some connections and a server on others.
311   The term "<x:dfn>user agent</x:dfn>" refers to any of the various
312   client programs that initiate a request, including (but not limited to)
313   browsers, spiders (web-based robots), command-line tools, custom
314   applications, and mobile apps.
315   The term "<x:dfn>origin server</x:dfn>" refers to the program that can
316   originate authoritative responses for a given target resource.
317   The terms "<x:dfn>sender</x:dfn>" and "<x:dfn>recipient</x:dfn>" refer to
318   any implementation that sends or receives a given message, respectively.
321   HTTP relies upon the Uniform Resource Identifier (URI)
322   standard <xref target="RFC3986"/> to indicate the target resource
323   (<xref target="target-resource"/>) and relationships between resources.
324   Messages are passed in a format similar to that used by Internet mail
325   <xref target="RFC5322"/> and the Multipurpose Internet Mail Extensions
326   (MIME) <xref target="RFC2045"/> (see &diff-mime; for the differences
327   between HTTP and MIME messages).
330   Most HTTP communication consists of a retrieval request (GET) for
331   a representation of some resource identified by a URI.  In the
332   simplest case, this might be accomplished via a single bidirectional
333   connection (===) between the user agent (UA) and the origin server (O).
335<figure><artwork type="drawing">
336         request   &gt;
337    <x:highlight>UA</x:highlight> ======================================= <x:highlight>O</x:highlight>
338                                &lt;   response
340<iref primary="true" item="message"/>
341<iref primary="true" item="request"/>
342<iref primary="true" item="response"/>
344   A client sends an HTTP request to a server in the form of a <x:dfn>request</x:dfn>
345   message, beginning with a request-line that includes a method, URI, and
346   protocol version (<xref target="request.line"/>),
347   followed by header fields containing
348   request modifiers, client information, and representation metadata
349   (<xref target="header.fields"/>),
350   an empty line to indicate the end of the header section, and finally
351   a message body containing the payload body (if any,
352   <xref target="message.body"/>).
355   A server responds to a client's request by sending one or more HTTP
356   <x:dfn>response</x:dfn>
357   messages, each beginning with a status line that
358   includes the protocol version, a success or error code, and textual
359   reason phrase (<xref target="status.line"/>),
360   possibly followed by header fields containing server
361   information, resource metadata, and representation metadata
362   (<xref target="header.fields"/>),
363   an empty line to indicate the end of the header section, and finally
364   a message body containing the payload body (if any,
365   <xref target="message.body"/>).
368   A connection might be used for multiple request/response exchanges,
369   as defined in <xref target="persistent.connections"/>.
372   The following example illustrates a typical message exchange for a
373   GET request (&GET;) on the URI "":
376Client request:
377</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
378GET /hello.txt HTTP/1.1
379User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
381Accept-Language: en, mi
385Server response:
386</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
387HTTP/1.1 200 OK
388Date: Mon, 27 Jul 2009 12:28:53 GMT
389Server: Apache
390Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
391ETag: "34aa387-d-1568eb00"
392Accept-Ranges: bytes
393Content-Length: <x:length-of target="exbody"/>
394Vary: Accept-Encoding
395Content-Type: text/plain
397<x:span anchor="exbody">Hello World! My payload includes a trailing CRLF.
402<section title="Implementation Diversity" anchor="implementation-diversity">
404   When considering the design of HTTP, it is easy to fall into a trap of
405   thinking that all user agents are general-purpose browsers and all origin
406   servers are large public websites. That is not the case in practice.
407   Common HTTP user agents include household appliances, stereos, scales,
408   firmware update scripts, command-line programs, mobile apps,
409   and communication devices in a multitude of shapes and sizes.  Likewise,
410   common HTTP origin servers include home automation units, configurable
411   networking components, office machines, autonomous robots, news feeds,
412   traffic cameras, ad selectors, and video delivery platforms.
415   The term "user agent" does not imply that there is a human user directly
416   interacting with the software agent at the time of a request. In many
417   cases, a user agent is installed or configured to run in the background
418   and save its results for later inspection (or save only a subset of those
419   results that might be interesting or erroneous). Spiders, for example, are
420   typically given a start URI and configured to follow certain behavior while
421   crawling the Web as a hypertext graph.
424   The implementation diversity of HTTP means that not all user agents can
425   make interactive suggestions to their user or provide adequate warning for
426   security or privacy concerns. In the few cases where this
427   specification requires reporting of errors to the user, it is acceptable
428   for such reporting to only be observable in an error console or log file.
429   Likewise, requirements that an automated action be confirmed by the user
430   before proceeding might be met via advance configuration choices,
431   run-time options, or simple avoidance of the unsafe action; confirmation
432   does not imply any specific user interface or interruption of normal
433   processing if the user has already made that choice.
437<section title="Intermediaries" anchor="intermediaries">
438<iref primary="true" item="intermediary"/>
440   HTTP enables the use of intermediaries to satisfy requests through
441   a chain of connections.  There are three common forms of HTTP
442   <x:dfn>intermediary</x:dfn>: proxy, gateway, and tunnel.  In some cases,
443   a single intermediary might act as an origin server, proxy, gateway,
444   or tunnel, switching behavior based on the nature of each request.
446<figure><artwork type="drawing">
447         &gt;             &gt;             &gt;             &gt;
448    <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>
449               &lt;             &lt;             &lt;             &lt;
452   The figure above shows three intermediaries (A, B, and C) between the
453   user agent and origin server. A request or response message that
454   travels the whole chain will pass through four separate connections.
455   Some HTTP communication options
456   might apply only to the connection with the nearest, non-tunnel
457   neighbor, only to the end-points of the chain, or to all connections
458   along the chain. Although the diagram is linear, each participant might
459   be engaged in multiple, simultaneous communications. For example, B
460   might be receiving requests from many clients other than A, and/or
461   forwarding requests to servers other than C, at the same time that it
462   is handling A's request. Likewise, later requests might be sent through a
463   different path of connections, often based on dynamic configuration for
464   load balancing.   
467<iref primary="true" item="upstream"/><iref primary="true" item="downstream"/>
468<iref primary="true" item="inbound"/><iref primary="true" item="outbound"/>
469   The terms "<x:dfn>upstream</x:dfn>" and "<x:dfn>downstream</x:dfn>" are
470   used to describe directional requirements in relation to the message flow:
471   all messages flow from upstream to downstream.
472   The terms inbound and outbound are used to describe directional
473   requirements in relation to the request route:
474   "<x:dfn>inbound</x:dfn>" means toward the origin server and
475   "<x:dfn>outbound</x:dfn>" means toward the user agent.
477<t><iref primary="true" item="proxy"/>
478   A "<x:dfn>proxy</x:dfn>" is a message forwarding agent that is selected by the
479   client, usually via local configuration rules, to receive requests
480   for some type(s) of absolute URI and attempt to satisfy those
481   requests via translation through the HTTP interface.  Some translations
482   are minimal, such as for proxy requests for "http" URIs, whereas
483   other requests might require translation to and from entirely different
484   application-level protocols. Proxies are often used to group an
485   organization's HTTP requests through a common intermediary for the
486   sake of security, annotation services, or shared caching. Some proxies
487   are designed to apply transformations to selected messages or payloads
488   while they are being forwarded, as described in
489   <xref target="message.transformations"/>.
491<t><iref primary="true" item="gateway"/><iref primary="true" item="reverse proxy"/>
492<iref primary="true" item="accelerator"/>
493   A "<x:dfn>gateway</x:dfn>" (a.k.a., "<x:dfn>reverse proxy</x:dfn>") is an
494   intermediary that acts as an origin server for the outbound connection, but
495   translates received requests and forwards them inbound to another server or
496   servers. Gateways are often used to encapsulate legacy or untrusted
497   information services, to improve server performance through
498   "<x:dfn>accelerator</x:dfn>" caching, and to enable partitioning or load
499   balancing of HTTP services across multiple machines.
502   All HTTP requirements applicable to an origin server
503   also apply to the outbound communication of a gateway.
504   A gateway communicates with inbound servers using any protocol that
505   it desires, including private extensions to HTTP that are outside
506   the scope of this specification.  However, an HTTP-to-HTTP gateway
507   that wishes to interoperate with third-party HTTP servers ought to conform
508   to user agent requirements on the gateway's inbound connection.
510<t><iref primary="true" item="tunnel"/>
511   A "<x:dfn>tunnel</x:dfn>" acts as a blind relay between two connections
512   without changing the messages. Once active, a tunnel is not
513   considered a party to the HTTP communication, though the tunnel might
514   have been initiated by an HTTP request. A tunnel ceases to exist when
515   both ends of the relayed connection are closed. Tunnels are used to
516   extend a virtual connection through an intermediary, such as when
517   Transport Layer Security (TLS, <xref target="RFC5246"/>) is used to
518   establish confidential communication through a shared firewall proxy.
521   The above categories for intermediary only consider those acting as
522   participants in the HTTP communication.  There are also intermediaries
523   that can act on lower layers of the network protocol stack, filtering or
524   redirecting HTTP traffic without the knowledge or permission of message
525   senders. Network intermediaries are indistinguishable (at a protocol level)
526   from a man-in-the-middle attack, often introducing security flaws or
527   interoperability problems due to mistakenly violating HTTP semantics.
529<t><iref primary="true" item="interception proxy"/>
530<iref primary="true" item="transparent proxy"/>
531<iref primary="true" item="captive portal"/>
532   For example, an
533   "<x:dfn>interception proxy</x:dfn>" <xref target="RFC3040"/> (also commonly
534   known as a "<x:dfn>transparent proxy</x:dfn>" <xref target="RFC1919"/> or
535   "<x:dfn>captive portal</x:dfn>")
536   differs from an HTTP proxy because it is not selected by the client.
537   Instead, an interception proxy filters or redirects outgoing TCP port 80
538   packets (and occasionally other common port traffic).
539   Interception proxies are commonly found on public network access points,
540   as a means of enforcing account subscription prior to allowing use of
541   non-local Internet services, and within corporate firewalls to enforce
542   network usage policies.
545   HTTP is defined as a stateless protocol, meaning that each request message
546   can be understood in isolation.  Many implementations depend on HTTP's
547   stateless design in order to reuse proxied connections or dynamically
548   load-balance requests across multiple servers.  Hence, a server &MUST-NOT;
549   assume that two requests on the same connection are from the same user
550   agent unless the connection is secured and specific to that agent.
551   Some non-standard HTTP extensions (e.g., <xref target="RFC4559"/>) have
552   been known to violate this requirement, resulting in security and
553   interoperability problems.
557<section title="Caches" anchor="caches">
558<iref primary="true" item="cache"/>
560   A "<x:dfn>cache</x:dfn>" is a local store of previous response messages and the
561   subsystem that controls its message storage, retrieval, and deletion.
562   A cache stores cacheable responses in order to reduce the response
563   time and network bandwidth consumption on future, equivalent
564   requests. Any client or server &MAY; employ a cache, though a cache
565   cannot be used by a server while it is acting as a tunnel.
568   The effect of a cache is that the request/response chain is shortened
569   if one of the participants along the chain has a cached response
570   applicable to that request. The following illustrates the resulting
571   chain if B has a cached copy of an earlier response from O (via C)
572   for a request that has not been cached by UA or A.
574<figure><artwork type="drawing">
575            &gt;             &gt;
576       <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>
577                  &lt;             &lt;
579<t><iref primary="true" item="cacheable"/>
580   A response is "<x:dfn>cacheable</x:dfn>" if a cache is allowed to store a copy of
581   the response message for use in answering subsequent requests.
582   Even when a response is cacheable, there might be additional
583   constraints placed by the client or by the origin server on when
584   that cached response can be used for a particular request. HTTP
585   requirements for cache behavior and cacheable responses are
586   defined in &caching-overview;. 
589   There are a wide variety of architectures and configurations
590   of caches deployed across the World Wide Web and
591   inside large organizations. These include national hierarchies
592   of proxy caches to save transoceanic bandwidth, collaborative systems that
593   broadcast or multicast cache entries, archives of pre-fetched cache
594   entries for use in off-line or high-latency environments, and so on.
598<section title="Conformance and Error Handling" anchor="conformance">
600   This specification targets conformance criteria according to the role of
601   a participant in HTTP communication.  Hence, HTTP requirements are placed
602   on senders, recipients, clients, servers, user agents, intermediaries,
603   origin servers, proxies, gateways, or caches, depending on what behavior
604   is being constrained by the requirement. Additional (social) requirements
605   are placed on implementations, resource owners, and protocol element
606   registrations when they apply beyond the scope of a single communication.
609   The verb "generate" is used instead of "send" where a requirement
610   differentiates between creating a protocol element and merely forwarding a
611   received element downstream.
614   An implementation is considered conformant if it complies with all of the
615   requirements associated with the roles it partakes in HTTP.
618   Conformance includes both the syntax and semantics of protocol
619   elements. A sender &MUST-NOT; generate protocol elements that convey a
620   meaning that is known by that sender to be false. A sender &MUST-NOT;
621   generate protocol elements that do not match the grammar defined by the
622   corresponding ABNF rules. Within a given message, a sender &MUST-NOT;
623   generate protocol elements or syntax alternatives that are only allowed to
624   be generated by participants in other roles (i.e., a role that the sender
625   does not have for that message).
628   When a received protocol element is parsed, the recipient &MUST; be able to
629   parse any value of reasonable length that is applicable to the recipient's
630   role and matches the grammar defined by the corresponding ABNF rules.
631   Note, however, that some received protocol elements might not be parsed.
632   For example, an intermediary forwarding a message might parse a
633   header-field into generic field-name and field-value components, but then
634   forward the header field without further parsing inside the field-value.
637   HTTP does not have specific length limitations for many of its protocol
638   elements because the lengths that might be appropriate will vary widely,
639   depending on the deployment context and purpose of the implementation.
640   Hence, interoperability between senders and recipients depends on shared
641   expectations regarding what is a reasonable length for each protocol
642   element. Furthermore, what is commonly understood to be a reasonable length
643   for some protocol elements has changed over the course of the past two
644   decades of HTTP use, and is expected to continue changing in the future.
647   At a minimum, a recipient &MUST; be able to parse and process protocol
648   element lengths that are at least as long as the values that it generates
649   for those same protocol elements in other messages. For example, an origin
650   server that publishes very long URI references to its own resources needs
651   to be able to parse and process those same references when received as a
652   request target.
655   A recipient &MUST; interpret a received protocol element according to the
656   semantics defined for it by this specification, including extensions to
657   this specification, unless the recipient has determined (through experience
658   or configuration) that the sender incorrectly implements what is implied by
659   those semantics.
660   For example, an origin server might disregard the contents of a received
661   <x:ref>Accept-Encoding</x:ref> header field if inspection of the
662   <x:ref>User-Agent</x:ref> header field indicates a specific implementation
663   version that is known to fail on receipt of certain content codings.
666   Unless noted otherwise, a recipient &MAY; attempt to recover a usable
667   protocol element from an invalid construct.  HTTP does not define
668   specific error handling mechanisms except when they have a direct impact
669   on security, since different applications of the protocol require
670   different error handling strategies.  For example, a Web browser might
671   wish to transparently recover from a response where the
672   <x:ref>Location</x:ref> header field doesn't parse according to the ABNF,
673   whereas a systems control client might consider any form of error recovery
674   to be dangerous.
678<section title="Protocol Versioning" anchor="http.version">
679  <x:anchor-alias value="HTTP-version"/>
680  <x:anchor-alias value="HTTP-name"/>
682   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate
683   versions of the protocol. This specification defines version "1.1".
684   The protocol version as a whole indicates the sender's conformance
685   with the set of requirements laid out in that version's corresponding
686   specification of HTTP.
689   The version of an HTTP message is indicated by an HTTP-version field
690   in the first line of the message. HTTP-version is case-sensitive.
692<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-version"/><iref primary="true" item="Grammar" subitem="HTTP-name"/>
693  <x:ref>HTTP-version</x:ref>  = <x:ref>HTTP-name</x:ref> "/" <x:ref>DIGIT</x:ref> "." <x:ref>DIGIT</x:ref>
694  <x:ref>HTTP-name</x:ref>     = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
697   The HTTP version number consists of two decimal digits separated by a "."
698   (period or decimal point).  The first digit ("major version") indicates the
699   HTTP messaging syntax, whereas the second digit ("minor version") indicates
700   the highest minor version within that major version to which the sender is
701   conformant and able to understand for future communication.  The minor
702   version advertises the sender's communication capabilities even when the
703   sender is only using a backwards-compatible subset of the protocol,
704   thereby letting the recipient know that more advanced features can
705   be used in response (by servers) or in future requests (by clients).
708   When an HTTP/1.1 message is sent to an HTTP/1.0 recipient
709   <xref target="RFC1945"/> or a recipient whose version is unknown,
710   the HTTP/1.1 message is constructed such that it can be interpreted
711   as a valid HTTP/1.0 message if all of the newer features are ignored.
712   This specification places recipient-version requirements on some
713   new features so that a conformant sender will only use compatible
714   features until it has determined, through configuration or the
715   receipt of a message, that the recipient supports HTTP/1.1.
718   The interpretation of a header field does not change between minor
719   versions of the same major HTTP version, though the default
720   behavior of a recipient in the absence of such a field can change.
721   Unless specified otherwise, header fields defined in HTTP/1.1 are
722   defined for all versions of HTTP/1.x.  In particular, the <x:ref>Host</x:ref>
723   and <x:ref>Connection</x:ref> header fields ought to be implemented by all
724   HTTP/1.x implementations whether or not they advertise conformance with
725   HTTP/1.1.
728   New header fields can be introduced without changing the protocol version
729   if their defined semantics allow them to be safely ignored by recipients
730   that do not recognize them. Header field extensibility is discussed in
731   <xref target="field.extensibility"/>.
734   Intermediaries that process HTTP messages (i.e., all intermediaries
735   other than those acting as tunnels) &MUST; send their own HTTP-version
736   in forwarded messages.  In other words, they are not allowed to blindly
737   forward the first line of an HTTP message without ensuring that the
738   protocol version in that message matches a version to which that
739   intermediary is conformant for both the receiving and
740   sending of messages.  Forwarding an HTTP message without rewriting
741   the HTTP-version might result in communication errors when downstream
742   recipients use the message sender's version to determine what features
743   are safe to use for later communication with that sender.
746   A client &SHOULD; send a request version equal to the highest
747   version to which the client is conformant and
748   whose major version is no higher than the highest version supported
749   by the server, if this is known.  A client &MUST-NOT; send a
750   version to which it is not conformant.
753   A client &MAY; send a lower request version if it is known that
754   the server incorrectly implements the HTTP specification, but only
755   after the client has attempted at least one normal request and determined
756   from the response status code or header fields (e.g., <x:ref>Server</x:ref>) that
757   the server improperly handles higher request versions.
760   A server &SHOULD; send a response version equal to the highest version to
761   which the server is conformant that has a major version less than or equal
762   to the one received in the request.
763   A server &MUST-NOT; send a version to which it is not conformant.
764   A server can send a <x:ref>505 (HTTP Version Not Supported)</x:ref>
765   response if it wishes, for any reason, to refuse service of the client's
766   major protocol version.
769   A server &MAY; send an HTTP/1.0 response to a request
770   if it is known or suspected that the client incorrectly implements the
771   HTTP specification and is incapable of correctly processing later
772   version responses, such as when a client fails to parse the version
773   number correctly or when an intermediary is known to blindly forward
774   the HTTP-version even when it doesn't conform to the given minor
775   version of the protocol. Such protocol downgrades &SHOULD-NOT; be
776   performed unless triggered by specific client attributes, such as when
777   one or more of the request header fields (e.g., <x:ref>User-Agent</x:ref>)
778   uniquely match the values sent by a client known to be in error.
781   The intention of HTTP's versioning design is that the major number
782   will only be incremented if an incompatible message syntax is
783   introduced, and that the minor number will only be incremented when
784   changes made to the protocol have the effect of adding to the message
785   semantics or implying additional capabilities of the sender.  However,
786   the minor version was not incremented for the changes introduced between
787   <xref target="RFC2068"/> and <xref target="RFC2616"/>, and this revision
788   has specifically avoided any such changes to the protocol.
791   When an HTTP message is received with a major version number that the
792   recipient implements, but a higher minor version number than what the
793   recipient implements, the recipient &SHOULD; process the message as if it
794   were in the highest minor version within that major version to which the
795   recipient is conformant. A recipient can assume that a message with a
796   higher minor version, when sent to a recipient that has not yet indicated
797   support for that higher version, is sufficiently backwards-compatible to be
798   safely processed by any implementation of the same major version.
802<section title="Uniform Resource Identifiers" anchor="uri">
803<iref primary="true" item="resource"/>
805   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used
806   throughout HTTP as the means for identifying resources (&resource;).
807   URI references are used to target requests, indicate redirects, and define
808   relationships.
810  <x:anchor-alias value="URI-reference"/>
811  <x:anchor-alias value="absolute-URI"/>
812  <x:anchor-alias value="relative-part"/>
813  <x:anchor-alias value="scheme"/>
814  <x:anchor-alias value="authority"/>
815  <x:anchor-alias value="uri-host"/>
816  <x:anchor-alias value="port"/>
817  <x:anchor-alias value="path"/>
818  <x:anchor-alias value="path-abempty"/>
819  <x:anchor-alias value="segment"/>
820  <x:anchor-alias value="query"/>
821  <x:anchor-alias value="fragment"/>
822  <x:anchor-alias value="absolute-path"/>
823  <x:anchor-alias value="partial-URI"/>
825   The definitions of "URI-reference",
826   "absolute-URI", "relative-part", "scheme", "authority", "port", "host",
827   "path-abempty", "segment", "query", and "fragment" are adopted from the
828   URI generic syntax.
829   An "absolute-path" rule is defined for protocol elements that can contain a
830   non-empty path component. (This rule differs slightly from RFC 3986's
831   path-abempty rule, which allows for an empty path to be used in references,
832   and path-absolute rule, which does not allow paths that begin with "//".)
833   A "partial-URI" rule is defined for protocol elements
834   that can contain a relative URI but not a fragment component.
836<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>
837  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
838  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
839  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
840  <x:ref>scheme</x:ref>        = &lt;scheme, defined in <xref target="RFC3986" x:fmt="," x:sec="3.1"/>&gt;
841  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
842  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
843  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
844  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
845  <x:ref>segment</x:ref>       = &lt;segment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
846  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
847  <x:ref>fragment</x:ref>      = &lt;fragment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.5"/>&gt;
849  <x:ref>absolute-path</x:ref> = 1*( "/" segment )
850  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
853   Each protocol element in HTTP that allows a URI reference will indicate
854   in its ABNF production whether the element allows any form of reference
855   (URI-reference), only a URI in absolute form (absolute-URI), only the
856   path and optional query components, or some combination of the above.
857   Unless otherwise indicated, URI references are parsed
858   relative to the effective request URI
859   (<xref target="effective.request.uri"/>).
862<section title="http URI scheme" anchor="http.uri">
863  <x:anchor-alias value="http-URI"/>
864  <iref item="http URI scheme" primary="true"/>
865  <iref item="URI scheme" subitem="http" primary="true"/>
867   The "http" URI scheme is hereby defined for the purpose of minting
868   identifiers according to their association with the hierarchical
869   namespace governed by a potential HTTP origin server listening for
870   TCP (<xref target="RFC0793"/>) connections on a given port.
872<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"><!--terminal production--></iref>
873  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
874             [ "#" <x:ref>fragment</x:ref> ]
877   The origin server for an "http" URI is identified by the
878   <x:ref>authority</x:ref> component, which includes a host identifier
879   and optional TCP port (<xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>).
880   The hierarchical path component and optional query component serve as an
881   identifier for a potential target resource within that origin server's name
882   space. The optional fragment component allows for indirect identification
883   of a secondary resource, independent of the URI scheme, as defined in
884   <xref target="RFC3986" x:fmt="of" x:sec="3.5"/>.
887   A sender &MUST-NOT; generate an "http" URI with an empty host identifier.
888   A recipient that processes such a URI reference &MUST; reject it as invalid.
891   If the host identifier is provided as an IP address, the origin server is
892   the listener (if any) on the indicated TCP port at that IP address.
893   If host is a registered name, the registered name is an indirect identifier
894   for use with a name resolution service, such as DNS, to find an address for
895   that origin server.
896   If the port subcomponent is empty or not given, TCP port 80 (the
897   reserved port for WWW services) is the default.
900   Note that the presence of a URI with a given authority component does not
901   imply that there is always an HTTP server listening for connections on
902   that host and port. Anyone can mint a URI. What the authority component
903   determines is who has the right to respond authoritatively to requests that
904   target the identified resource. The delegated nature of registered names
905   and IP addresses creates a federated namespace, based on control over the
906   indicated host and port, whether or not an HTTP server is present.
907   See <xref target="establishing.authority"/> for security considerations
908   related to establishing authority.
911   When an "http" URI is used within a context that calls for access to the
912   indicated resource, a client &MAY; attempt access by resolving
913   the host to an IP address, establishing a TCP connection to that address
914   on the indicated port, and sending an HTTP request message
915   (<xref target="http.message"/>) containing the URI's identifying data
916   (<xref target="message.routing"/>) to the server.
917   If the server responds to that request with a non-interim HTTP response
918   message, as described in &status-codes;, then that response
919   is considered an authoritative answer to the client's request.
922   Although HTTP is independent of the transport protocol, the "http"
923   scheme is specific to TCP-based services because the name delegation
924   process depends on TCP for establishing authority.
925   An HTTP service based on some other underlying connection protocol
926   would presumably be identified using a different URI scheme, just as
927   the "https" scheme (below) is used for resources that require an
928   end-to-end secured connection. Other protocols might also be used to
929   provide access to "http" identified resources &mdash; it is only the
930   authoritative interface that is specific to TCP.
933   The URI generic syntax for authority also includes a deprecated
934   userinfo subcomponent (<xref target="RFC3986" x:fmt="," x:sec="3.2.1"/>)
935   for including user authentication information in the URI.  Some
936   implementations make use of the userinfo component for internal
937   configuration of authentication information, such as within command
938   invocation options, configuration files, or bookmark lists, even
939   though such usage might expose a user identifier or password.
940   A sender &MUST-NOT; generate the userinfo subcomponent (and its "@"
941   delimiter) when an "http" URI reference is generated within a message as a
942   request target or header field value.
943   Before making use of an "http" URI reference received from an untrusted
944   source, a recipient &SHOULD; parse for userinfo and treat its presence as
945   an error; it is likely being used to obscure the authority for the sake of
946   phishing attacks.
950<section title="https URI scheme" anchor="https.uri">
951   <x:anchor-alias value="https-URI"/>
952   <iref item="https URI scheme"/>
953   <iref item="URI scheme" subitem="https"/>
955   The "https" URI scheme is hereby defined for the purpose of minting
956   identifiers according to their association with the hierarchical
957   namespace governed by a potential HTTP origin server listening to a
958   given TCP port for TLS-secured connections (<xref target="RFC5246"/>).
961   All of the requirements listed above for the "http" scheme are also
962   requirements for the "https" scheme, except that TCP port 443 is the
963   default if the port subcomponent is empty or not given,
964   and the user agent &MUST; ensure that its connection to the origin
965   server is secured through the use of strong encryption, end-to-end,
966   prior to sending the first HTTP request.
968<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="https-URI"><!--terminal production--></iref>
969  <x:ref>https-URI</x:ref> = "https:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
970              [ "#" <x:ref>fragment</x:ref> ]
973   Note that the "https" URI scheme depends on both TLS and TCP for
974   establishing authority.
975   Resources made available via the "https" scheme have no shared
976   identity with the "http" scheme even if their resource identifiers
977   indicate the same authority (the same host listening to the same
978   TCP port).  They are distinct name spaces and are considered to be
979   distinct origin servers.  However, an extension to HTTP that is
980   defined to apply to entire host domains, such as the Cookie protocol
981   <xref target="RFC6265"/>, can allow information
982   set by one service to impact communication with other services
983   within a matching group of host domains.
986   The process for authoritative access to an "https" identified
987   resource is defined in <xref target="RFC2818"/>.
991<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
993   Since the "http" and "https" schemes conform to the URI generic syntax,
994   such URIs are normalized and compared according to the algorithm defined
995   in <xref target="RFC3986" x:fmt="of" x:sec="6"/>, using the defaults
996   described above for each scheme.
999   If the port is equal to the default port for a scheme, the normal form is
1000   to omit the port subcomponent. When not being used in absolute form as the
1001   request target of an OPTIONS request, an empty path component is equivalent
1002   to an absolute path of "/", so the normal form is to provide a path of "/"
1003   instead. The scheme and host are case-insensitive and normally provided in
1004   lowercase; all other components are compared in a case-sensitive manner.
1005   Characters other than those in the "reserved" set are equivalent to their
1006   percent-encoded octets: the normal form is to not encode them
1007   (see Sections <xref target="RFC3986" x:fmt="number" x:sec="2.1"/> and
1008   <xref target="RFC3986" x:fmt="number" x:sec="2.2"/> of
1009   <xref target="RFC3986"/>).
1012   For example, the following three URIs are equivalent:
1014<figure><artwork type="example">
1023<section title="Message Format" anchor="http.message">
1024<x:anchor-alias value="generic-message"/>
1025<x:anchor-alias value="message.types"/>
1026<x:anchor-alias value="HTTP-message"/>
1027<x:anchor-alias value="start-line"/>
1028<iref item="header section"/>
1029<iref item="headers"/>
1030<iref item="header field"/>
1032   All HTTP/1.1 messages consist of a start-line followed by a sequence of
1033   octets in a format similar to the Internet Message Format
1034   <xref target="RFC5322"/>: zero or more header fields (collectively
1035   referred to as the "headers" or the "header section"), an empty line
1036   indicating the end of the header section, and an optional message body.
1038<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"><!--terminal production--></iref>
1039  <x:ref>HTTP-message</x:ref>   = <x:ref>start-line</x:ref>
1040                   *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1041                   <x:ref>CRLF</x:ref>
1042                   [ <x:ref>message-body</x:ref> ]
1045   The normal procedure for parsing an HTTP message is to read the
1046   start-line into a structure, read each header field into a hash
1047   table by field name until the empty line, and then use the parsed
1048   data to determine if a message body is expected.  If a message body
1049   has been indicated, then it is read as a stream until an amount
1050   of octets equal to the message body length is read or the connection
1051   is closed.
1054   A recipient &MUST; parse an HTTP message as a sequence of octets in an
1055   encoding that is a superset of US-ASCII <xref target="USASCII"/>.
1056   Parsing an HTTP message as a stream of Unicode characters, without regard
1057   for the specific encoding, creates security vulnerabilities due to the
1058   varying ways that string processing libraries handle invalid multibyte
1059   character sequences that contain the octet LF (%x0A).  String-based
1060   parsers can only be safely used within protocol elements after the element
1061   has been extracted from the message, such as within a header field-value
1062   after message parsing has delineated the individual fields.
1065   An HTTP message can be parsed as a stream for incremental processing or
1066   forwarding downstream.  However, recipients cannot rely on incremental
1067   delivery of partial messages, since some implementations will buffer or
1068   delay message forwarding for the sake of network efficiency, security
1069   checks, or payload transformations.
1072   A sender &MUST-NOT; send whitespace between the start-line and
1073   the first header field.
1074   A recipient that receives whitespace between the start-line and
1075   the first header field &MUST; either reject the message as invalid or
1076   consume each whitespace-preceded line without further processing of it
1077   (i.e., ignore the entire line, along with any subsequent lines preceded
1078   by whitespace, until a properly formed header field is received or the
1079   header section is terminated).
1082   The presence of such whitespace in a request
1083   might be an attempt to trick a server into ignoring that field or
1084   processing the line after it as a new request, either of which might
1085   result in a security vulnerability if other implementations within
1086   the request chain interpret the same message differently.
1087   Likewise, the presence of such whitespace in a response might be
1088   ignored by some clients or cause others to cease parsing.
1091<section title="Start Line" anchor="start.line">
1092  <x:anchor-alias value="Start-Line"/>
1094   An HTTP message can either be a request from client to server or a
1095   response from server to client.  Syntactically, the two types of message
1096   differ only in the start-line, which is either a request-line (for requests)
1097   or a status-line (for responses), and in the algorithm for determining
1098   the length of the message body (<xref target="message.body"/>).
1101   In theory, a client could receive requests and a server could receive
1102   responses, distinguishing them by their different start-line formats,
1103   but in practice servers are implemented to only expect a request
1104   (a response is interpreted as an unknown or invalid request method)
1105   and clients are implemented to only expect a response.
1107<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="start-line"/>
1108  <x:ref>start-line</x:ref>     = <x:ref>request-line</x:ref> / <x:ref>status-line</x:ref>
1111<section title="Request Line" anchor="request.line">
1112  <x:anchor-alias value="Request"/>
1113  <x:anchor-alias value="request-line"/>
1115   A request-line begins with a method token, followed by a single
1116   space (SP), the request-target, another single space (SP), the
1117   protocol version, and ending with CRLF.
1119<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1120  <x:ref>request-line</x:ref>   = <x:ref>method</x:ref> <x:ref>SP</x:ref> <x:ref>request-target</x:ref> <x:ref>SP</x:ref> <x:ref>HTTP-version</x:ref> <x:ref>CRLF</x:ref>
1122<iref primary="true" item="method"/>
1123<t anchor="method">
1124   The method token indicates the request method to be performed on the
1125   target resource. The request method is case-sensitive.
1127<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1128  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1131   The request methods defined by this specification can be found in
1132   &methods;, along with information regarding the HTTP method registry
1133   and considerations for defining new methods.
1135<iref item="request-target"/>
1137   The request-target identifies the target resource upon which to apply
1138   the request, as defined in <xref target="request-target"/>.
1141   Recipients typically parse the request-line into its component parts by
1142   splitting on whitespace (see <xref target="message.robustness"/>), since
1143   no whitespace is allowed in the three components.
1144   Unfortunately, some user agents fail to properly encode or exclude
1145   whitespace found in hypertext references, resulting in those disallowed
1146   characters being sent in a request-target.
1149   Recipients of an invalid request-line &SHOULD; respond with either a
1150   <x:ref>400 (Bad Request)</x:ref> error or a <x:ref>301 (Moved Permanently)</x:ref>
1151   redirect with the request-target properly encoded.  A recipient &SHOULD-NOT;
1152   attempt to autocorrect and then process the request without a redirect,
1153   since the invalid request-line might be deliberately crafted to bypass
1154   security filters along the request chain.
1157   HTTP does not place a pre-defined limit on the length of a request-line,
1158   as described in <xref target="conformance"/>.
1159   A server that receives a method longer than any that it implements
1160   &SHOULD; respond with a <x:ref>501 (Not Implemented)</x:ref> status code.
1161   A server that receives a request-target longer than any URI it wishes to
1162   parse &MUST; respond with a
1163   <x:ref>414 (URI Too Long)</x:ref> status code (see &status-414;).
1166   Various ad-hoc limitations on request-line length are found in practice.
1167   It is &RECOMMENDED; that all HTTP senders and recipients support, at a
1168   minimum, request-line lengths of 8000 octets.
1172<section title="Status Line" anchor="status.line">
1173  <x:anchor-alias value="response"/>
1174  <x:anchor-alias value="status-line"/>
1175  <x:anchor-alias value="status-code"/>
1176  <x:anchor-alias value="reason-phrase"/>
1178   The first line of a response message is the status-line, consisting
1179   of the protocol version, a space (SP), the status code, another space,
1180   a possibly-empty textual phrase describing the status code, and
1181   ending with CRLF.
1183<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-line"/>
1184  <x:ref>status-line</x:ref> = <x:ref>HTTP-version</x:ref> <x:ref>SP</x:ref> <x:ref>status-code</x:ref> <x:ref>SP</x:ref> <x:ref>reason-phrase</x:ref> <x:ref>CRLF</x:ref>
1187   The status-code element is a 3-digit integer code describing the
1188   result of the server's attempt to understand and satisfy the client's
1189   corresponding request. The rest of the response message is to be
1190   interpreted in light of the semantics defined for that status code.
1191   See &status-codes; for information about the semantics of status codes,
1192   including the classes of status code (indicated by the first digit),
1193   the status codes defined by this specification, considerations for the
1194   definition of new status codes, and the IANA registry.
1196<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-code"/>
1197  <x:ref>status-code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1200   The reason-phrase element exists for the sole purpose of providing a
1201   textual description associated with the numeric status code, mostly
1202   out of deference to earlier Internet application protocols that were more
1203   frequently used with interactive text clients. A client &SHOULD; ignore
1204   the reason-phrase content.
1206<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
1207  <x:ref>reason-phrase</x:ref>  = *( <x:ref>HTAB</x:ref> / <x:ref>SP</x:ref> / <x:ref>VCHAR</x:ref> / <x:ref>obs-text</x:ref> )
1212<section title="Header Fields" anchor="header.fields">
1213  <x:anchor-alias value="header-field"/>
1214  <x:anchor-alias value="field-content"/>
1215  <x:anchor-alias value="field-name"/>
1216  <x:anchor-alias value="field-value"/>
1217  <x:anchor-alias value="field-vchar"/>
1218  <x:anchor-alias value="obs-fold"/>
1220   Each header field consists of a case-insensitive field name
1221   followed by a colon (":"), optional leading whitespace, the field value,
1222   and optional trailing whitespace.
1224<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"/>
1225  <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>
1227  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1228  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>obs-fold</x:ref> )
1229  <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> ]
1230  <x:ref>field-vchar</x:ref>    = <x:ref>VCHAR</x:ref> / <x:ref>obs-text</x:ref>
1232  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref> 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1233                 ; obsolete line folding
1234                 ; see <xref target="field.parsing"/>
1237   The field-name token labels the corresponding field-value as having the
1238   semantics defined by that header field.  For example, the <x:ref>Date</x:ref>
1239   header field is defined in &header-date; as containing the origination
1240   timestamp for the message in which it appears.
1243<section title="Field Extensibility" anchor="field.extensibility">
1245   Header fields are fully extensible: there is no limit on the
1246   introduction of new field names, each presumably defining new semantics,
1247   nor on the number of header fields used in a given message.  Existing
1248   fields are defined in each part of this specification and in many other
1249   specifications outside this document set.
1252   New header fields can be defined such that, when they are understood by a
1253   recipient, they might override or enhance the interpretation of previously
1254   defined header fields, define preconditions on request evaluation, or
1255   refine the meaning of responses.
1258   A proxy &MUST; forward unrecognized header fields unless the
1259   field-name is listed in the <x:ref>Connection</x:ref> header field
1260   (<xref target="header.connection"/>) or the proxy is specifically
1261   configured to block, or otherwise transform, such fields.
1262   Other recipients &SHOULD; ignore unrecognized header fields.
1263   These requirements allow HTTP's functionality to be enhanced without
1264   requiring prior update of deployed intermediaries.
1267   All defined header fields ought to be registered with IANA in the
1268   Message Header Field Registry, as described in &iana-header-registry;.
1272<section title="Field Order" anchor="field.order">
1274   The order in which header fields with differing field names are
1275   received is not significant. However, it is good practice to send
1276   header fields that contain control data first, such as <x:ref>Host</x:ref>
1277   on requests and <x:ref>Date</x:ref> on responses, so that implementations
1278   can decide when not to handle a message as early as possible.
1279   A server &MUST-NOT; apply a request to the target resource until the entire
1280   request header section is received, since later header fields might include
1281   conditionals, authentication credentials, or deliberately misleading
1282   duplicate header fields that would impact request processing.
1285   A sender &MUST-NOT; generate multiple header fields with the same field
1286   name in a message unless either the entire field value for that
1287   header field is defined as a comma-separated list [i.e., #(values)]
1288   or the header field is a well-known exception (as noted below).
1291   A recipient &MAY; combine multiple header fields with the same field name
1292   into one "field-name: field-value" pair, without changing the semantics of
1293   the message, by appending each subsequent field value to the combined
1294   field value in order, separated by a comma. The order in which
1295   header fields with the same field name are received is therefore
1296   significant to the interpretation of the combined field value;
1297   a proxy &MUST-NOT; change the order of these field values when
1298   forwarding a message.
1301  <t>
1302   &Note; In practice, the "Set-Cookie" header field (<xref target="RFC6265"/>)
1303   often appears multiple times in a response message and does not use the
1304   list syntax, violating the above requirements on multiple header fields
1305   with the same name. Since it cannot be combined into a single field-value,
1306   recipients ought to handle "Set-Cookie" as a special case while processing
1307   header fields. (See Appendix A.2.3 of <xref target="Kri2001"/> for details.)
1308  </t>
1312<section title="Whitespace" anchor="whitespace">
1313<t anchor="rule.LWS">
1314   This specification uses three rules to denote the use of linear
1315   whitespace: OWS (optional whitespace), RWS (required whitespace), and
1316   BWS ("bad" whitespace).
1318<t anchor="rule.OWS">
1319   The OWS rule is used where zero or more linear whitespace octets might
1320   appear. For protocol elements where optional whitespace is preferred to
1321   improve readability, a sender &SHOULD; generate the optional whitespace
1322   as a single SP; otherwise, a sender &SHOULD-NOT; generate optional
1323   whitespace except as needed to white-out invalid or unwanted protocol
1324   elements during in-place message filtering.
1326<t anchor="rule.RWS">
1327   The RWS rule is used when at least one linear whitespace octet is required
1328   to separate field tokens. A sender &SHOULD; generate RWS as a single SP.
1330<t anchor="rule.BWS">
1331   The BWS rule is used where the grammar allows optional whitespace only for
1332   historical reasons. A sender &MUST-NOT; generate BWS in messages.
1333   A recipient &MUST; parse for such bad whitespace and remove it before
1334   interpreting the protocol element.
1336<t anchor="rule.whitespace">
1337  <x:anchor-alias value="BWS"/>
1338  <x:anchor-alias value="OWS"/>
1339  <x:anchor-alias value="RWS"/>
1341<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"/>
1342  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1343                 ; optional whitespace
1344  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1345                 ; required whitespace
1346  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1347                 ; "bad" whitespace
1351<section title="Field Parsing" anchor="field.parsing">
1353   Messages are parsed using a generic algorithm, independent of the
1354   individual header field names. The contents within a given field value are
1355   not parsed until a later stage of message interpretation (usually after the
1356   message's entire header section has been processed).
1357   Consequently, this specification does not use ABNF rules to define each
1358   "Field-Name: Field Value" pair, as was done in previous editions.
1359   Instead, this specification uses ABNF rules which are named according to
1360   each registered field name, wherein the rule defines the valid grammar for
1361   that field's corresponding field values (i.e., after the field-value
1362   has been extracted from the header section by a generic field parser).
1365   No whitespace is allowed between the header field-name and colon.
1366   In the past, differences in the handling of such whitespace have led to
1367   security vulnerabilities in request routing and response handling.
1368   A server &MUST; reject any received request message that contains
1369   whitespace between a header field-name and colon with a response code of
1370   <x:ref>400 (Bad Request)</x:ref>. A proxy &MUST; remove any such whitespace
1371   from a response message before forwarding the message downstream.
1374   A field value might be preceded and/or followed by optional whitespace
1375   (OWS); a single SP preceding the field-value is preferred for consistent
1376   readability by humans.
1377   The field value does not include any leading or trailing white space: OWS
1378   occurring before the first non-whitespace octet of the field value or after
1379   the last non-whitespace octet of the field value ought to be excluded by
1380   parsers when extracting the field value from a header field.
1383   Historically, HTTP header field values could be extended over multiple
1384   lines by preceding each extra line with at least one space or horizontal
1385   tab (obs-fold). This specification deprecates such line folding except
1386   within the message/http media type
1387   (<xref target=""/>).
1388   A sender &MUST-NOT; generate a message that includes line folding
1389   (i.e., that has any field-value that contains a match to the
1390   <x:ref>obs-fold</x:ref> rule) unless the message is intended for packaging
1391   within the message/http media type.
1394   A server that receives an <x:ref>obs-fold</x:ref> in a request message that
1395   is not within a message/http container &MUST; either reject the message by
1396   sending a <x:ref>400 (Bad Request)</x:ref>, preferably with a
1397   representation explaining that obsolete line folding is unacceptable, or
1398   replace each received <x:ref>obs-fold</x:ref> with one or more
1399   <x:ref>SP</x:ref> octets prior to interpreting the field value or
1400   forwarding the message downstream.
1403   A proxy or gateway that receives an <x:ref>obs-fold</x:ref> in a response
1404   message that is not within a message/http container &MUST; either discard
1405   the message and replace it with a <x:ref>502 (Bad Gateway)</x:ref>
1406   response, preferably with a representation explaining that unacceptable
1407   line folding was received, or replace each received <x:ref>obs-fold</x:ref>
1408   with one or more <x:ref>SP</x:ref> octets prior to interpreting the field
1409   value or forwarding the message downstream.
1412   A user agent that receives an <x:ref>obs-fold</x:ref> in a response message
1413   that is not within a message/http container &MUST; replace each received
1414   <x:ref>obs-fold</x:ref> with one or more <x:ref>SP</x:ref> octets prior to
1415   interpreting the field value.
1418   Historically, HTTP has allowed field content with text in the ISO-8859-1
1419   <xref target="ISO-8859-1"/> charset, supporting other charsets only
1420   through use of <xref target="RFC2047"/> encoding.
1421   In practice, most HTTP header field values use only a subset of the
1422   US-ASCII charset <xref target="USASCII"/>. Newly defined
1423   header fields &SHOULD; limit their field values to US-ASCII octets.
1424   A recipient &SHOULD; treat other octets in field content (obs-text) as
1425   opaque data.
1429<section title="Field Limits" anchor="field.limits">
1431   HTTP does not place a pre-defined limit on the length of each header field
1432   or on the length of the header section as a whole, as described in
1433   <xref target="conformance"/>. Various ad-hoc limitations on individual
1434   header field length are found in practice, often depending on the specific
1435   field semantics.
1438   A server that receives a request header field, or set of fields, larger
1439   than it wishes to process &MUST; respond with an appropriate
1440   <x:ref>4xx (Client Error)</x:ref> status code. Ignoring such header fields
1441   would increase the server's vulnerability to request smuggling attacks
1442   (<xref target="request.smuggling"/>).
1445   A client &MAY; discard or truncate received header fields that are larger
1446   than the client wishes to process if the field semantics are such that the
1447   dropped value(s) can be safely ignored without changing the
1448   message framing or response semantics.
1452<section title="Field value components" anchor="field.components">
1453<t anchor="rule.token.separators">
1454  <x:anchor-alias value="tchar"/>
1455  <x:anchor-alias value="token"/>
1456  <iref item="Delimiters"/>
1457   Most HTTP header field values are defined using common syntax components
1458   (token, quoted-string, and comment) separated by whitespace or specific
1459   delimiting characters. Delimiters are chosen from the set of US-ASCII
1460   visual characters not allowed in a <x:ref>token</x:ref>
1461   (DQUOTE and "(),/:;&lt;=>?@[\]{}").
1463<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="token"/><iref primary="true" item="Grammar" subitem="tchar"/>
1464  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1466  NOTE: the definition of tchar and the prose above about special characters need to match!
1467 -->
1468  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1469                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1470                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1471                 ; any <x:ref>VCHAR</x:ref>, except delimiters
1473<t anchor="rule.quoted-string">
1474  <x:anchor-alias value="quoted-string"/>
1475  <x:anchor-alias value="qdtext"/>
1476  <x:anchor-alias value="obs-text"/>
1477   A string of text is parsed as a single value if it is quoted using
1478   double-quote marks.
1480<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"/>
1481  <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>
1482  <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>
1483  <x:ref>obs-text</x:ref>       = %x80-FF
1485<t anchor="rule.comment">
1486  <x:anchor-alias value="comment"/>
1487  <x:anchor-alias value="ctext"/>
1488   Comments can be included in some HTTP header fields by surrounding
1489   the comment text with parentheses. Comments are only allowed in
1490   fields containing "comment" as part of their field value definition.
1492<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1493  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-pair</x:ref> / <x:ref>comment</x:ref> ) ")"
1494  <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>
1496<t anchor="rule.quoted-pair">
1497  <x:anchor-alias value="quoted-pair"/>
1498   The backslash octet ("\") can be used as a single-octet
1499   quoting mechanism within quoted-string and comment constructs.
1500   Recipients that process the value of a quoted-string &MUST; handle a
1501   quoted-pair as if it were replaced by the octet following the backslash.
1503<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1504  <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> )
1507   A sender &SHOULD-NOT; generate a quoted-pair in a quoted-string except
1508   where necessary to quote DQUOTE and backslash octets occurring within that
1509   string.
1510   A sender &SHOULD-NOT; generate a quoted-pair in a comment except
1511   where necessary to quote parentheses ["(" and ")"] and backslash octets
1512   occurring within that comment.
1518<section title="Message Body" anchor="message.body">
1519  <x:anchor-alias value="message-body"/>
1521   The message body (if any) of an HTTP message is used to carry the
1522   payload body of that request or response.  The message body is
1523   identical to the payload body unless a transfer coding has been
1524   applied, as described in <xref target="header.transfer-encoding"/>.
1526<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1527  <x:ref>message-body</x:ref> = *OCTET
1530   The rules for when a message body is allowed in a message differ for
1531   requests and responses.
1534   The presence of a message body in a request is signaled by a
1535   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1536   field. Request message framing is independent of method semantics,
1537   even if the method does not define any use for a message body.
1540   The presence of a message body in a response depends on both
1541   the request method to which it is responding and the response
1542   status code (<xref target="status.line"/>).
1543   Responses to the HEAD request method (&HEAD;) never include a message body
1544   because the associated response header fields (e.g.,
1545   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1546   if present, indicate only what their values would have been if the request
1547   method had been GET (&GET;).
1548   <x:ref>2xx (Successful)</x:ref> responses to a CONNECT request method
1549   (&CONNECT;) switch to tunnel mode instead of having a message body.
1550   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1551   <x:ref>304 (Not Modified)</x:ref> responses do not include a message body.
1552   All other responses do include a message body, although the body
1553   might be of zero length.
1556<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1557  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1558  <iref item="chunked (Coding Format)"/>
1559  <x:anchor-alias value="Transfer-Encoding"/>
1561   The Transfer-Encoding header field lists the transfer coding names
1562   corresponding to the sequence of transfer codings that have been
1563   (or will be) applied to the payload body in order to form the message body.
1564   Transfer codings are defined in <xref target="transfer.codings"/>.
1566<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1567  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1570   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1571   MIME, which was designed to enable safe transport of binary data over a
1572   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1573   However, safe transport has a different focus for an 8bit-clean transfer
1574   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1575   accurately delimit a dynamically generated payload and to distinguish
1576   payload encodings that are only applied for transport efficiency or
1577   security from those that are characteristics of the selected resource.
1580   A recipient &MUST; be able to parse the chunked transfer coding
1581   (<xref target="chunked.encoding"/>) because it plays a crucial role in
1582   framing messages when the payload body size is not known in advance.
1583   A sender &MUST-NOT; apply chunked more than once to a message body
1584   (i.e., chunking an already chunked message is not allowed).
1585   If any transfer coding other than chunked is applied to a request payload
1586   body, the sender &MUST; apply chunked as the final transfer coding to
1587   ensure that the message is properly framed.
1588   If any transfer coding other than chunked is applied to a response payload
1589   body, the sender &MUST; either apply chunked as the final transfer coding
1590   or terminate the message by closing the connection.
1593   For example,
1594</preamble><artwork type="example">
1595  Transfer-Encoding: gzip, chunked
1597   indicates that the payload body has been compressed using the gzip
1598   coding and then chunked using the chunked coding while forming the
1599   message body.
1602   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1603   Transfer-Encoding is a property of the message, not of the representation, and
1604   any recipient along the request/response chain &MAY; decode the received
1605   transfer coding(s) or apply additional transfer coding(s) to the message
1606   body, assuming that corresponding changes are made to the Transfer-Encoding
1607   field-value. Additional information about the encoding parameters can be
1608   provided by other header fields not defined by this specification.
1611   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1612   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1613   neither of which includes a message body,
1614   to indicate that the origin server would have applied a transfer coding
1615   to the message body if the request had been an unconditional GET.
1616   This indication is not required, however, because any recipient on
1617   the response chain (including the origin server) can remove transfer
1618   codings when they are not needed.
1621   A server &MUST-NOT; send a Transfer-Encoding header field in any response
1622   with a status code of
1623   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1624   A server &MUST-NOT; send a Transfer-Encoding header field in any
1625   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1628   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1629   implementations advertising only HTTP/1.0 support will not understand
1630   how to process a transfer-encoded payload.
1631   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1632   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1633   might be in the form of specific user configuration or by remembering the
1634   version of a prior received response.
1635   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1636   the corresponding request indicates HTTP/1.1 (or later).
1639   A server that receives a request message with a transfer coding it does
1640   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref>.
1644<section title="Content-Length" anchor="header.content-length">
1645  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1646  <x:anchor-alias value="Content-Length"/>
1648   When a message does not have a <x:ref>Transfer-Encoding</x:ref> header
1649   field, a Content-Length header field can provide the anticipated size,
1650   as a decimal number of octets, for a potential payload body.
1651   For messages that do include a payload body, the Content-Length field-value
1652   provides the framing information necessary for determining where the body
1653   (and message) ends.  For messages that do not include a payload body, the
1654   Content-Length indicates the size of the selected representation
1655   (&representation;).
1657<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1658  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1661   An example is
1663<figure><artwork type="example">
1664  Content-Length: 3495
1667   A sender &MUST-NOT; send a Content-Length header field in any message that
1668   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1671   A user agent &SHOULD; send a Content-Length in a request message when no
1672   <x:ref>Transfer-Encoding</x:ref> is sent and the request method defines
1673   a meaning for an enclosed payload body. For example, a Content-Length
1674   header field is normally sent in a POST request even when the value is
1675   0 (indicating an empty payload body).  A user agent &SHOULD-NOT; send a
1676   Content-Length header field when the request message does not contain a
1677   payload body and the method semantics do not anticipate such a body.
1680   A server &MAY; send a Content-Length header field in a response to a HEAD
1681   request (&HEAD;); 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 response if the same
1684   request had used the GET method.
1687   A server &MAY; send a Content-Length header field in a
1688   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1689   (&status-304;); a server &MUST-NOT; send Content-Length in such a
1690   response unless its field-value equals the decimal number of octets that
1691   would have been sent in the payload body of a <x:ref>200 (OK)</x:ref>
1692   response to the same request.
1695   A server &MUST-NOT; send a Content-Length header field in any response
1696   with a status code of
1697   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1698   A server &MUST-NOT; send a Content-Length header field in any
1699   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1702   Aside from the cases defined above, in the absence of Transfer-Encoding,
1703   an origin server &SHOULD; send a Content-Length header field when the
1704   payload body size is known prior to sending the complete header section.
1705   This will allow downstream recipients to measure transfer progress,
1706   know when a received message is complete, and potentially reuse the
1707   connection for additional requests.
1710   Any Content-Length field value greater than or equal to zero is valid.
1711   Since there is no predefined limit to the length of a payload, a
1712   recipient &MUST; anticipate potentially large decimal numerals and
1713   prevent parsing errors due to integer conversion overflows
1714   (<xref target="attack.protocol.element.length"/>).
1717   If a message is received that has multiple Content-Length header fields
1718   with field-values consisting of the same decimal value, or a single
1719   Content-Length header field with a field value containing a list of
1720   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1721   duplicate Content-Length header fields have been generated or combined by an
1722   upstream message processor, then the recipient &MUST; either reject the
1723   message as invalid or replace the duplicated field-values with a single
1724   valid Content-Length field containing that decimal value prior to
1725   determining the message body length or forwarding the message.
1728  <t>
1729   &Note; HTTP's use of Content-Length for message framing differs
1730   significantly from the same field's use in MIME, where it is an optional
1731   field used only within the "message/external-body" media-type.
1732  </t>
1736<section title="Message Body Length" anchor="message.body.length">
1737  <iref item="chunked (Coding Format)"/>
1739   The length of a message body is determined by one of the following
1740   (in order of precedence):
1743  <list style="numbers">
1744    <x:lt><t>
1745     Any response to a HEAD request and any response with a
1746     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1747     <x:ref>304 (Not Modified)</x:ref> status code is always
1748     terminated by the first empty line after the header fields, regardless of
1749     the header fields present in the message, and thus cannot contain a
1750     message body.
1751    </t></x:lt>
1752    <x:lt><t>
1753     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1754     connection will become a tunnel immediately after the empty line that
1755     concludes the header fields.  A client &MUST; ignore any
1756     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1757     fields received in such a message.
1758    </t></x:lt>
1759    <x:lt><t>
1760     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1761     and the chunked transfer coding (<xref target="chunked.encoding"/>)
1762     is the final encoding, the message body length is determined by reading
1763     and decoding the chunked data until the transfer coding indicates the
1764     data is complete.
1765    </t>
1766    <t>
1767     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1768     response and the chunked transfer coding is not the final encoding, the
1769     message body length is determined by reading the connection until it is
1770     closed by the server.
1771     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a request and the
1772     chunked transfer coding is not the final encoding, the message body
1773     length cannot be determined reliably; the server &MUST; respond with
1774     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1775    </t>
1776    <t>
1777     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1778     and a <x:ref>Content-Length</x:ref> header field, the Transfer-Encoding
1779     overrides the Content-Length. Such a message might indicate an attempt to
1780     perform request smuggling (<xref target="request.smuggling"/>) or
1781     response splitting (<xref target="response.splitting"/>) and ought to be
1782     handled as an error. A sender &MUST; remove the received Content-Length
1783     field prior to forwarding such a message downstream.
1784    </t></x:lt>
1785    <x:lt><t>
1786     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1787     either multiple <x:ref>Content-Length</x:ref> header fields having
1788     differing field-values or a single Content-Length header field having an
1789     invalid value, then the message framing is invalid and
1790     the recipient &MUST; treat it as an unrecoverable error.
1791     If this is a request message, the server &MUST; respond with
1792     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1793     If this is a response message received by a proxy,
1794     the proxy &MUST; close the connection to the server, discard the received
1795     response, and send a <x:ref>502 (Bad Gateway)</x:ref> response to the
1796     client.
1797     If this is a response message received by a user agent,
1798     the user agent &MUST; close the connection to the server and discard the
1799     received response.
1800    </t></x:lt>
1801    <x:lt><t>
1802     If a valid <x:ref>Content-Length</x:ref> header field is present without
1803     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1804     expected message body length in octets.
1805     If the sender closes the connection or the recipient times out before the
1806     indicated number of octets are received, the recipient &MUST; consider
1807     the message to be incomplete and close the connection.
1808    </t></x:lt>
1809    <x:lt><t>
1810     If this is a request message and none of the above are true, then the
1811     message body length is zero (no message body is present).
1812    </t></x:lt>
1813    <x:lt><t>
1814     Otherwise, this is a response message without a declared message body
1815     length, so the message body length is determined by the number of octets
1816     received prior to the server closing the connection.
1817    </t></x:lt>
1818  </list>
1821   Since there is no way to distinguish a successfully completed,
1822   close-delimited message from a partially-received message interrupted
1823   by network failure, a server &SHOULD; generate encoding or
1824   length-delimited messages whenever possible.  The close-delimiting
1825   feature exists primarily for backwards compatibility with HTTP/1.0.
1828   A server &MAY; reject a request that contains a message body but
1829   not a <x:ref>Content-Length</x:ref> by responding with
1830   <x:ref>411 (Length Required)</x:ref>.
1833   Unless a transfer coding other than chunked has been applied,
1834   a client that sends a request containing a message body &SHOULD;
1835   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1836   length is known in advance, rather than the chunked transfer coding, since some
1837   existing services respond to chunked with a <x:ref>411 (Length Required)</x:ref>
1838   status code even though they understand the chunked transfer coding.  This
1839   is typically because such services are implemented via a gateway that
1840   requires a content-length in advance of being called and the server
1841   is unable or unwilling to buffer the entire request before processing.
1844   A user agent that sends a request containing a message body &MUST; send a
1845   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1846   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1847   the form of specific user configuration or by remembering the version of a
1848   prior received response.
1851   If the final response to the last request on a connection has been
1852   completely received and there remains additional data to read, a user agent
1853   &MAY; discard the remaining data or attempt to determine if that data
1854   belongs as part of the prior response body, which might be the case if the
1855   prior message's Content-Length value is incorrect. A client &MUST-NOT;
1856   process, cache, or forward such extra data as a separate response, since
1857   such behavior would be vulnerable to cache poisoning.
1862<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1864   A server that receives an incomplete request message, usually due to a
1865   canceled request or a triggered time-out exception, &MAY; send an error
1866   response prior to closing the connection.
1869   A client that receives an incomplete response message, which can occur
1870   when a connection is closed prematurely or when decoding a supposedly
1871   chunked transfer coding fails, &MUST; record the message as incomplete.
1872   Cache requirements for incomplete responses are defined in
1873   &cache-incomplete;.
1876   If a response terminates in the middle of the header section (before the
1877   empty line is received) and the status code might rely on header fields to
1878   convey the full meaning of the response, then the client cannot assume
1879   that meaning has been conveyed; the client might need to repeat the
1880   request in order to determine what action to take next.
1883   A message body that uses the chunked transfer coding is
1884   incomplete if the zero-sized chunk that terminates the encoding has not
1885   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1886   incomplete if the size of the message body received (in octets) is less than
1887   the value given by Content-Length.  A response that has neither chunked
1888   transfer coding nor Content-Length is terminated by closure of the
1889   connection, and thus is considered complete regardless of the number of
1890   message body octets received, provided that the header section was received
1891   intact.
1895<section title="Message Parsing Robustness" anchor="message.robustness">
1897   Older HTTP/1.0 user agent implementations might send an extra CRLF
1898   after a POST request as a workaround for some early server
1899   applications that failed to read message body content that was
1900   not terminated by a line-ending. An HTTP/1.1 user agent &MUST-NOT;
1901   preface or follow a request with an extra CRLF.  If terminating
1902   the request message body with a line-ending is desired, then the
1903   user agent &MUST; count the terminating CRLF octets as part of the
1904   message body length.
1907   In the interest of robustness, a server that is expecting to receive and
1908   parse a request-line &SHOULD; ignore at least one empty line (CRLF)
1909   received prior to the request-line.
1912   Although the line terminator for the start-line and header
1913   fields is the sequence CRLF, a recipient &MAY; recognize a
1914   single LF as a line terminator and ignore any preceding CR.
1917   Although the request-line and status-line grammar rules require that each
1918   of the component elements be separated by a single SP octet, recipients
1919   &MAY; instead parse on whitespace-delimited word boundaries and, aside
1920   from the CRLF terminator, treat any form of whitespace as the SP separator
1921   while ignoring preceding or trailing whitespace;
1922   such whitespace includes one or more of the following octets:
1923   SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
1924   However, lenient parsing can result in security vulnerabilities if there
1925   are multiple recipients of the message and each has its own unique
1926   interpretation of robustness (see <xref target="request.smuggling"/>).
1929   When a server listening only for HTTP request messages, or processing
1930   what appears from the start-line to be an HTTP request message,
1931   receives a sequence of octets that does not match the HTTP-message
1932   grammar aside from the robustness exceptions listed above, the
1933   server &SHOULD; respond with a <x:ref>400 (Bad Request)</x:ref> response. 
1938<section title="Transfer Codings" anchor="transfer.codings">
1939  <x:anchor-alias value="transfer-coding"/>
1940  <x:anchor-alias value="transfer-extension"/>
1942   Transfer coding names are used to indicate an encoding
1943   transformation that has been, can be, or might need to be applied to a
1944   payload body in order to ensure "safe transport" through the network.
1945   This differs from a content coding in that the transfer coding is a
1946   property of the message rather than a property of the representation
1947   that is being transferred.
1949<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1950  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1951                     / "compress" ; <xref target="compress.coding"/>
1952                     / "deflate" ; <xref target="deflate.coding"/>
1953                     / "gzip" ; <xref target="gzip.coding"/>
1954                     / <x:ref>transfer-extension</x:ref>
1955  <x:ref>transfer-extension</x:ref> = <x:ref>token</x:ref> *( <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> <x:ref>transfer-parameter</x:ref> )
1957<t anchor="rule.parameter">
1958  <x:anchor-alias value="transfer-parameter"/>
1959   Parameters are in the form of a name or name=value pair.
1961<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-parameter"/>
1962  <x:ref>transfer-parameter</x:ref> = <x:ref>token</x:ref> <x:ref>BWS</x:ref> "=" <x:ref>BWS</x:ref> ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> )
1965   All transfer-coding names are case-insensitive and ought to be registered
1966   within the HTTP Transfer Coding registry, as defined in
1967   <xref target="transfer.coding.registry"/>.
1968   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1969   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1970   header fields.
1973<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1974  <iref primary="true" item="chunked (Coding Format)"/>
1975  <x:anchor-alias value="chunk"/>
1976  <x:anchor-alias value="chunked-body"/>
1977  <x:anchor-alias value="chunk-data"/>
1978  <x:anchor-alias value="chunk-size"/>
1979  <x:anchor-alias value="last-chunk"/>
1981   The chunked transfer coding wraps the payload body in order to transfer it
1982   as a series of chunks, each with its own size indicator, followed by an
1983   &OPTIONAL; trailer containing header fields. Chunked enables content
1984   streams of unknown size to be transferred as a sequence of length-delimited
1985   buffers, which enables the sender to retain connection persistence and the
1986   recipient to know when it has received the entire message.
1988<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="chunked-body"><!--terminal production--></iref><iref primary="true" item="Grammar" subitem="chunk"/><iref primary="true" item="Grammar" subitem="chunk-size"/><iref primary="true" item="Grammar" subitem="last-chunk"/><iref primary="false" item="Grammar" subitem="trailer-part"/><iref primary="false" item="Grammar" subitem="chunk-ext"/><iref primary="true" item="Grammar" subitem="chunk-data"/>
1989  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1990                   <x:ref>last-chunk</x:ref>
1991                   <x:ref>trailer-part</x:ref>
1992                   <x:ref>CRLF</x:ref>
1994  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1995                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1996  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1997  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1999  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
2002   The chunk-size field is a string of hex digits indicating the size of
2003   the chunk-data in octets. The chunked transfer coding is complete when a
2004   chunk with a chunk-size of zero is received, possibly followed by a
2005   trailer, and finally terminated by an empty line.
2008   A recipient &MUST; be able to parse and decode the chunked transfer coding.
2011<section title="Chunk Extensions" anchor="chunked.extension">
2012  <x:anchor-alias value="chunk-ext"/>
2013  <x:anchor-alias value="chunk-ext-name"/>
2014  <x:anchor-alias value="chunk-ext-val"/>
2016   The chunked encoding allows each chunk to include zero or more chunk
2017   extensions, immediately following the <x:ref>chunk-size</x:ref>, for the
2018   sake of supplying per-chunk metadata (such as a signature or hash),
2019   mid-message control information, or randomization of message body size.
2021<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="chunked-body"><!--terminal production--></iref><iref primary="true" item="Grammar" subitem="chunk-ext"/><iref primary="true" item="Grammar" subitem="chunk-ext-name"/><iref primary="true" item="Grammar" subitem="chunk-ext-val"/>
2022  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
2024  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
2025  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
2028   The chunked encoding is specific to each connection and is likely to be
2029   removed or recoded by each recipient (including intermediaries) before any
2030   higher-level application would have a chance to inspect the extensions.
2031   Hence, use of chunk extensions is generally limited to specialized HTTP
2032   services such as "long polling" (where client and server can have shared
2033   expectations regarding the use of chunk extensions) or for padding within
2034   an end-to-end secured connection.
2037   A recipient &MUST; ignore unrecognized chunk extensions.
2038   A server ought to limit the total length of chunk extensions received in a
2039   request to an amount reasonable for the services provided, in the same way
2040   that it applies length limitations and timeouts for other parts of a
2041   message, and generate an appropriate <x:ref>4xx (Client Error)</x:ref>
2042   response if that amount is exceeded.
2046<section title="Chunked Trailer Part" anchor="chunked.trailer.part">
2047  <x:anchor-alias value="trailer-part"/>
2049   A trailer allows the sender to include additional fields at the end of a
2050   chunked message in order to supply metadata that might be dynamically
2051   generated while the message body is sent, such as a message integrity
2052   check, digital signature, or post-processing status. The trailer fields are
2053   identical to header fields, except they are sent in a chunked trailer
2054   instead of the message's header section.
2056<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="trailer-part"/><iref primary="false" item="Grammar" subitem="header-field"/>
2057  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
2060   A sender &MUST-NOT; generate a trailer that contains a field necessary for
2061   message framing (e.g., <x:ref>Transfer-Encoding</x:ref> and
2062   <x:ref>Content-Length</x:ref>), routing (e.g., <x:ref>Host</x:ref>),
2063   request modifiers (e.g., controls and conditionals in
2064   &request-header-fields;), authentication (e.g., see <xref target="Part7"/>
2065   and <xref target="RFC6265"/>), response control data (e.g., see
2066   &response-control-data;), or determining how to process the payload
2067   (e.g., <x:ref>Content-Encoding</x:ref>, <x:ref>Content-Type</x:ref>,
2068   <x:ref>Content-Range</x:ref>, and <x:ref>Trailer</x:ref>).
2071   When a chunked message containing a non-empty trailer is received, the
2072   recipient &MAY; process the fields (aside from those forbidden above)
2073   as if they were appended to the message's header section.
2074   A recipient &MUST; ignore (or consider as an error) any fields that are
2075   forbidden to be sent in a trailer, since processing them as if they were
2076   present in the header section might bypass external security filters.
2079   Unless the request includes a <x:ref>TE</x:ref> header field indicating
2080   "trailers" is acceptable, as described in <xref target="header.te"/>, a
2081   server &SHOULD-NOT; generate trailer fields that it believes are necessary
2082   for the user agent to receive. Without a TE containing "trailers", the
2083   server ought to assume that the trailer fields might be silently discarded
2084   along the path to the user agent. This requirement allows intermediaries to
2085   forward a de-chunked message to an HTTP/1.0 recipient without buffering the
2086   entire response.
2090<section title="Decoding Chunked" anchor="decoding.chunked">
2092   A process for decoding the chunked transfer coding
2093   can be represented in pseudo-code as:
2095<figure><artwork type="code">
2096  length := 0
2097  read chunk-size, chunk-ext (if any), and CRLF
2098  while (chunk-size &gt; 0) {
2099     read chunk-data and CRLF
2100     append chunk-data to decoded-body
2101     length := length + chunk-size
2102     read chunk-size, chunk-ext (if any), and CRLF
2103  }
2104  read trailer field
2105  while (trailer field is not empty) {
2106     if trailer field is allowed to be sent in a trailer,
2107         append trailer field to existing header fields
2108     read trailer-field
2109  }
2110  Content-Length := length
2111  Remove "chunked" from Transfer-Encoding
2112  Remove Trailer from existing header fields
2117<section title="Compression Codings" anchor="compression.codings">
2119   The codings defined below can be used to compress the payload of a
2120   message.
2123<section title="Compress Coding" anchor="compress.coding">
2124<iref item="compress (Coding Format)"/>
2126   The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding
2127   <xref target="Welch"/> that is commonly produced by the UNIX file
2128   compression program "compress".
2129   A recipient &SHOULD; consider "x-compress" to be equivalent to "compress".
2133<section title="Deflate Coding" anchor="deflate.coding">
2134<iref item="deflate (Coding Format)"/>
2136   The "deflate" coding is a "zlib" data format <xref target="RFC1950"/>
2137   containing a "deflate" compressed data stream <xref target="RFC1951"/>
2138   that uses a combination of the Lempel-Ziv (LZ77) compression algorithm and
2139   Huffman coding.
2142  <t>
2143    &Note; Some non-conformant implementations send the "deflate"
2144    compressed data without the zlib wrapper.
2145   </t>
2149<section title="Gzip Coding" anchor="gzip.coding">
2150<iref item="gzip (Coding Format)"/>
2152   The "gzip" coding is an LZ77 coding with a 32 bit CRC that is commonly
2153   produced by the gzip file compression program <xref target="RFC1952"/>.
2154   A recipient &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2160<section title="TE" anchor="header.te">
2161  <iref primary="true" item="TE header field" x:for-anchor=""/>
2162  <x:anchor-alias value="TE"/>
2163  <x:anchor-alias value="t-codings"/>
2164  <x:anchor-alias value="t-ranking"/>
2165  <x:anchor-alias value="rank"/>
2167   The "TE" header field in a request indicates what transfer codings,
2168   besides chunked, the client is willing to accept in response, and
2169   whether or not the client is willing to accept trailer fields in a
2170   chunked transfer coding.
2173   The TE field-value consists of a comma-separated list of transfer coding
2174   names, each allowing for optional parameters (as described in
2175   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2176   A client &MUST-NOT; send the chunked transfer coding name in TE;
2177   chunked is always acceptable for HTTP/1.1 recipients.
2179<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"/>
2180  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2181  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2182  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2183  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2184             / ( "1" [ "." 0*3("0") ] )
2187   Three examples of TE use are below.
2189<figure><artwork type="example">
2190  TE: deflate
2191  TE:
2192  TE: trailers, deflate;q=0.5
2195   The presence of the keyword "trailers" indicates that the client is willing
2196   to accept trailer fields in a chunked transfer coding, as defined in
2197   <xref target="chunked.trailer.part"/>, on behalf of itself and any downstream
2198   clients. For requests from an intermediary, this implies that either:
2199   (a) all downstream clients are willing to accept trailer fields in the
2200   forwarded response; or,
2201   (b) the intermediary will attempt to buffer the response on behalf of
2202   downstream recipients.
2203   Note that HTTP/1.1 does not define any means to limit the size of a
2204   chunked response such that an intermediary can be assured of buffering the
2205   entire response.
2208   When multiple transfer codings are acceptable, the client &MAY; rank the
2209   codings by preference using a case-insensitive "q" parameter (similar to
2210   the qvalues used in content negotiation fields, &qvalue;). The rank value
2211   is a real number in the range 0 through 1, where 0.001 is the least
2212   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2215   If the TE field-value is empty or if no TE field is present, the only
2216   acceptable transfer coding is chunked. A message with no transfer coding
2217   is always acceptable.
2220   Since the TE header field only applies to the immediate connection,
2221   a sender of TE &MUST; also send a "TE" connection option within the
2222   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2223   in order to prevent the TE field from being forwarded by intermediaries
2224   that do not support its semantics.
2228<section title="Trailer" anchor="header.trailer">
2229  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
2230  <x:anchor-alias value="Trailer"/>
2232   When a message includes a message body encoded with the chunked
2233   transfer coding and the sender desires to send metadata in the form of
2234   trailer fields at the end of the message, the sender &SHOULD; generate a
2235   <x:ref>Trailer</x:ref> header field before the message body to indicate
2236   which fields will be present in the trailers. This allows the recipient
2237   to prepare for receipt of that metadata before it starts processing the body,
2238   which is useful if the message is being streamed and the recipient wishes
2239   to confirm an integrity check on the fly.
2241<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/><iref primary="false" item="Grammar" subitem="field-name"/>
2242  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
2247<section title="Message Routing" anchor="message.routing">
2249   HTTP request message routing is determined by each client based on the
2250   target resource, the client's proxy configuration, and
2251   establishment or reuse of an inbound connection.  The corresponding
2252   response routing follows the same connection chain back to the client.
2255<section title="Identifying a Target Resource" anchor="target-resource">
2256  <iref primary="true" item="target resource"/>
2257  <iref primary="true" item="target URI"/>
2258  <x:anchor-alias value="target resource"/>
2259  <x:anchor-alias value="target URI"/>
2261   HTTP is used in a wide variety of applications, ranging from
2262   general-purpose computers to home appliances.  In some cases,
2263   communication options are hard-coded in a client's configuration.
2264   However, most HTTP clients rely on the same resource identification
2265   mechanism and configuration techniques as general-purpose Web browsers.
2268   HTTP communication is initiated by a user agent for some purpose.
2269   The purpose is a combination of request semantics, which are defined in
2270   <xref target="Part2"/>, and a target resource upon which to apply those
2271   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2272   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2273   would resolve to its absolute form in order to obtain the
2274   "<x:dfn>target URI</x:dfn>".  The target URI
2275   excludes the reference's fragment component, if any,
2276   since fragment identifiers are reserved for client-side processing
2277   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2281<section title="Connecting Inbound" anchor="connecting.inbound">
2283   Once the target URI is determined, a client needs to decide whether
2284   a network request is necessary to accomplish the desired semantics and,
2285   if so, where that request is to be directed.
2288   If the client has a cache <xref target="Part6"/> and the request can be
2289   satisfied by it, then the request is
2290   usually directed there first.
2293   If the request is not satisfied by a cache, then a typical client will
2294   check its configuration to determine whether a proxy is to be used to
2295   satisfy the request.  Proxy configuration is implementation-dependent,
2296   but is often based on URI prefix matching, selective authority matching,
2297   or both, and the proxy itself is usually identified by an "http" or
2298   "https" URI.  If a proxy is applicable, the client connects inbound by
2299   establishing (or reusing) a connection to that proxy.
2302   If no proxy is applicable, a typical client will invoke a handler routine,
2303   usually specific to the target URI's scheme, to connect directly
2304   to an authority for the target resource.  How that is accomplished is
2305   dependent on the target URI scheme and defined by its associated
2306   specification, similar to how this specification defines origin server
2307   access for resolution of the "http" (<xref target="http.uri"/>) and
2308   "https" (<xref target="https.uri"/>) schemes.
2311   HTTP requirements regarding connection management are defined in
2312   <xref target=""/>.
2316<section title="Request Target" anchor="request-target">
2318   Once an inbound connection is obtained,
2319   the client sends an HTTP request message (<xref target="http.message"/>)
2320   with a request-target derived from the target URI.
2321   There are four distinct formats for the request-target, depending on both
2322   the method being requested and whether the request is to a proxy.
2324<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-target"/><iref primary="false" item="Grammar" subitem="origin-form"/><iref primary="false" item="Grammar" subitem="absolute-form"/><iref primary="false" item="Grammar" subitem="authority-form"/><iref primary="false" item="Grammar" subitem="asterisk-form"/>
2325  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2326                 / <x:ref>absolute-form</x:ref>
2327                 / <x:ref>authority-form</x:ref>
2328                 / <x:ref>asterisk-form</x:ref>
2331<section title="origin-form" anchor="origin-form">
2332   <iref item="origin-form (of request-target)"/>
2334   The most common form of request-target is the <x:dfn>origin-form</x:dfn>.
2336<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="origin-form"/>
2337  <x:ref>origin-form</x:ref>    = <x:ref>absolute-path</x:ref> [ "?" <x:ref>query</x:ref> ]
2340   When making a request directly to an origin server, other than a CONNECT
2341   or server-wide OPTIONS request (as detailed below),
2342   a client &MUST; send only the absolute path and query components of
2343   the target URI as the request-target.
2344   If the target URI's path component is empty, the client &MUST; send
2345   "/" as the path within the origin-form of request-target.
2346   A <x:ref>Host</x:ref> header field is also sent, as defined in
2347   <xref target=""/>.
2350   For example, a client wishing to retrieve a representation of the resource
2351   identified as
2353<figure><artwork x:indent-with="  " type="example">
2357   directly from the origin server would open (or reuse) a TCP connection
2358   to port 80 of the host "" and send the lines:
2360<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2361GET /where?q=now HTTP/1.1
2365   followed by the remainder of the request message.
2369<section title="absolute-form" anchor="absolute-form">
2370   <iref item="absolute-form (of request-target)"/>
2372   When making a request to a proxy, other than a CONNECT or server-wide
2373   OPTIONS request (as detailed below), a client &MUST; send the target URI
2374   in <x:dfn>absolute-form</x:dfn> as the request-target.
2376<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="absolute-form"/>
2377  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2380   The proxy is requested to either service that request from a valid cache,
2381   if possible, or make the same request on the client's behalf to either
2382   the next inbound proxy server or directly to the origin server indicated
2383   by the request-target.  Requirements on such "forwarding" of messages are
2384   defined in <xref target="message.forwarding"/>.
2387   An example absolute-form of request-line would be:
2389<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2390GET HTTP/1.1
2393   To allow for transition to the absolute-form for all requests in some
2394   future version of HTTP, a server &MUST; accept the absolute-form
2395   in requests, even though HTTP/1.1 clients will only send them in requests
2396   to proxies.
2400<section title="authority-form" anchor="authority-form">
2401   <iref item="authority-form (of request-target)"/>
2403   The <x:dfn>authority-form</x:dfn> of request-target is only used for
2404   CONNECT requests (&CONNECT;).
2406<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="authority-form"/>
2407  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2410   When making a CONNECT request to establish a
2411   tunnel through one or more proxies, a client &MUST; send only the target
2412   URI's authority component (excluding any userinfo and its "@" delimiter) as
2413   the request-target. For example,
2415<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2420<section title="asterisk-form" anchor="asterisk-form">
2421   <iref item="asterisk-form (of request-target)"/>
2423   The <x:dfn>asterisk-form</x:dfn> of request-target is only used for a server-wide
2424   OPTIONS request (&OPTIONS;).
2426<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="asterisk-form"/>
2427  <x:ref>asterisk-form</x:ref>  = "*"
2430   When a client wishes to request OPTIONS
2431   for the server as a whole, as opposed to a specific named resource of
2432   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2433   For example,
2435<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2436OPTIONS * HTTP/1.1
2439   If a proxy receives an OPTIONS request with an absolute-form of
2440   request-target in which the URI has an empty path and no query component,
2441   then the last proxy on the request chain &MUST; send a request-target
2442   of "*" when it forwards the request to the indicated origin server.
2445   For example, the request
2446</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2450  would be forwarded by the final proxy as
2451</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2452OPTIONS * HTTP/1.1
2456   after connecting to port 8001 of host "".
2462<section title="Host" anchor="">
2463  <iref primary="true" item="Host header field" x:for-anchor=""/>
2464  <x:anchor-alias value="Host"/>
2466   The "Host" header field in a request provides the host and port
2467   information from the target URI, enabling the origin
2468   server to distinguish among resources while servicing requests
2469   for multiple host names on a single IP address.
2471<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2472  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2475   A client &MUST; send a Host header field in all HTTP/1.1 request messages.
2476   If the target URI includes an authority component, then a client &MUST;
2477   send a field-value for Host that is identical to that authority
2478   component, excluding any userinfo subcomponent and its "@" delimiter
2479   (<xref target="http.uri"/>).
2480   If the authority component is missing or undefined for the target URI,
2481   then a client &MUST; send a Host header field with an empty field-value.
2484   Since the Host field-value is critical information for handling a request,
2485   a user agent &SHOULD; generate Host as the first header field following the
2486   request-line.
2489   For example, a GET request to the origin server for
2490   &lt;; would begin with:
2492<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2493GET /pub/WWW/ HTTP/1.1
2497   A client &MUST; send a Host header field in an HTTP/1.1 request even
2498   if the request-target is in the absolute-form, since this
2499   allows the Host information to be forwarded through ancient HTTP/1.0
2500   proxies that might not have implemented Host.
2503   When a proxy receives a request with an absolute-form of
2504   request-target, the proxy &MUST; ignore the received
2505   Host header field (if any) and instead replace it with the host
2506   information of the request-target.  A proxy that forwards such a request
2507   &MUST; generate a new Host field-value based on the received
2508   request-target rather than forward the received Host field-value.
2511   Since the Host header field acts as an application-level routing
2512   mechanism, it is a frequent target for malware seeking to poison
2513   a shared cache or redirect a request to an unintended server.
2514   An interception proxy is particularly vulnerable if it relies on
2515   the Host field-value for redirecting requests to internal
2516   servers, or for use as a cache key in a shared cache, without
2517   first verifying that the intercepted connection is targeting a
2518   valid IP address for that host.
2521   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2522   to any HTTP/1.1 request message that lacks a Host header field and
2523   to any request message that contains more than one Host header field
2524   or a Host header field with an invalid field-value.
2528<section title="Effective Request URI" anchor="effective.request.uri">
2529  <iref primary="true" item="effective request URI"/>
2530  <x:anchor-alias value="effective request URI"/>
2532   Since the request-target often contains only part of the user agent's
2533   target URI, a server reconstructs the intended target as an
2534   "<x:dfn>effective request URI</x:dfn>" to properly service the request.
2535   This reconstruction involves both the server's local configuration and
2536   information communicated in the <x:ref>request-target</x:ref>,
2537   <x:ref>Host</x:ref> header field, and connection context.
2540   For a user agent, the effective request URI is the target URI.
2543   If the <x:ref>request-target</x:ref> is in <x:ref>absolute-form</x:ref>,
2544   the effective request URI is the same as the request-target. Otherwise, the
2545   effective request URI is constructed as follows:
2546<list style="empty">
2548   If the server's configuration (or outbound gateway) provides a fixed URI
2549   <x:ref>scheme</x:ref>, that scheme is used for the effective request URI.
2550   Otherwise, if the request is received over a TLS-secured TCP connection,
2551   the effective request URI's scheme is "https"; if not, the scheme is "http".
2554   If the server's configuration (or outbound gateway) provides a fixed URI
2555   <x:ref>authority</x:ref> component, that authority is used for the
2556   effective request URI. If not, then if the request-target is in
2557   <x:ref>authority-form</x:ref>, the effective request URI's authority
2558   component is the same as the request-target.
2559   If not, then if a <x:ref>Host</x:ref> header field is supplied with a
2560   non-empty field-value, the authority component is the same as the
2561   Host field-value. Otherwise, the authority component is assigned
2562   the default name configured for the server and, if the connection's
2563   incoming TCP port number differs from the default port for the effective
2564   request URI's scheme, then a colon (":") and the incoming port number (in
2565   decimal form) are appended to the authority component.
2568   If the request-target is in <x:ref>authority-form</x:ref> or
2569   <x:ref>asterisk-form</x:ref>, the effective request URI's combined
2570   <x:ref>path</x:ref> and <x:ref>query</x:ref> component is empty. Otherwise,
2571   the combined <x:ref>path</x:ref> and <x:ref>query</x:ref> component is the
2572   same as the request-target.
2575   The components of the effective request URI, once determined as above, can
2576   be combined into <x:ref>absolute-URI</x:ref> form by concatenating the
2577   scheme, "://", authority, and combined path and query component.
2583   Example 1: the following message received over an insecure TCP connection
2585<artwork type="example" x:indent-with="  ">
2586GET /pub/WWW/TheProject.html HTTP/1.1
2592  has an effective request URI of
2594<artwork type="example" x:indent-with="  ">
2600   Example 2: the following message received over a TLS-secured TCP connection
2602<artwork type="example" x:indent-with="  ">
2603OPTIONS * HTTP/1.1
2609  has an effective request URI of
2611<artwork type="example" x:indent-with="  ">
2616   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2617   field might need to use heuristics (e.g., examination of the URI path for
2618   something unique to a particular host) in order to guess the
2619   effective request URI's authority component.
2622   Once the effective request URI has been constructed, an origin server needs
2623   to decide whether or not to provide service for that URI via the connection
2624   in which the request was received. For example, the request might have been
2625   misdirected, deliberately or accidentally, such that the information within
2626   a received <x:ref>request-target</x:ref> or <x:ref>Host</x:ref> header
2627   field differs from the host or port upon which the connection has been
2628   made. If the connection is from a trusted gateway, that inconsistency might
2629   be expected; otherwise, it might indicate an attempt to bypass security
2630   filters, trick the server into delivering non-public content, or poison a
2631   cache. See <xref target="security.considerations"/> for security
2632   considerations regarding message routing.
2636<section title="Associating a Response to a Request" anchor="">
2638   HTTP does not include a request identifier for associating a given
2639   request message with its corresponding one or more response messages.
2640   Hence, it relies on the order of response arrival to correspond exactly
2641   to the order in which requests are made on the same connection.
2642   More than one response message per request only occurs when one or more
2643   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a
2644   final response to the same request.
2647   A client that has more than one outstanding request on a connection &MUST;
2648   maintain a list of outstanding requests in the order sent and &MUST;
2649   associate each received response message on that connection to the highest
2650   ordered request that has not yet received a final (non-<x:ref>1xx</x:ref>)
2651   response.
2655<section title="Message Forwarding" anchor="message.forwarding">
2657   As described in <xref target="intermediaries"/>, intermediaries can serve
2658   a variety of roles in the processing of HTTP requests and responses.
2659   Some intermediaries are used to improve performance or availability.
2660   Others are used for access control or to filter content.
2661   Since an HTTP stream has characteristics similar to a pipe-and-filter
2662   architecture, there are no inherent limits to the extent an intermediary
2663   can enhance (or interfere) with either direction of the stream.
2666   An intermediary not acting as a tunnel &MUST; implement the
2667   <x:ref>Connection</x:ref> header field, as specified in
2668   <xref target="header.connection"/>, and exclude fields from being forwarded
2669   that are only intended for the incoming connection.
2672   An intermediary &MUST-NOT; forward a message to itself unless it is
2673   protected from an infinite request loop. In general, an intermediary ought
2674   to recognize its own server names, including any aliases, local variations,
2675   or literal IP addresses, and respond to such requests directly.
2678<section title="Via" anchor="header.via">
2679  <iref primary="true" item="Via header field" x:for-anchor=""/>
2680  <x:anchor-alias value="pseudonym"/>
2681  <x:anchor-alias value="received-by"/>
2682  <x:anchor-alias value="received-protocol"/>
2683  <x:anchor-alias value="Via"/>
2685   The "Via" header field indicates the presence of intermediate protocols and
2686   recipients between the user agent and the server (on requests) or between
2687   the origin server and the client (on responses), similar to the
2688   "Received" header field in email
2689   (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2690   Via can be used for tracking message forwards,
2691   avoiding request loops, and identifying the protocol capabilities of
2692   senders along the request/response chain.
2694<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"/>
2695  <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> ] )
2697  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2698                      ; see <xref target="header.upgrade"/>
2699  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2700  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2703   Multiple Via field values represent each proxy or gateway that has
2704   forwarded the message. Each intermediary appends its own information
2705   about how the message was received, such that the end result is ordered
2706   according to the sequence of forwarding recipients.
2709   A proxy &MUST; send an appropriate Via header field, as described below, in
2710   each message that it forwards.
2711   An HTTP-to-HTTP gateway &MUST; send an appropriate Via header field in
2712   each inbound request message and &MAY; send a Via header field in
2713   forwarded response messages.
2716   For each intermediary, the received-protocol indicates the protocol and
2717   protocol version used by the upstream sender of the message. Hence, the
2718   Via field value records the advertised protocol capabilities of the
2719   request/response chain such that they remain visible to downstream
2720   recipients; this can be useful for determining what backwards-incompatible
2721   features might be safe to use in response, or within a later request, as
2722   described in <xref target="http.version"/>. For brevity, the protocol-name
2723   is omitted when the received protocol is HTTP.
2726   The received-by portion of the field value is normally the host and optional
2727   port number of a recipient server or client that subsequently forwarded the
2728   message.
2729   However, if the real host is considered to be sensitive information, a
2730   sender &MAY; replace it with a pseudonym. If a port is not provided,
2731   a recipient &MAY; interpret that as meaning it was received on the default
2732   TCP port, if any, for the received-protocol.
2735   A sender &MAY; generate comments in the Via header field to identify the
2736   software of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2737   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2738   are optional and a recipient &MAY; remove them prior to forwarding the
2739   message.
2742   For example, a request message could be sent from an HTTP/1.0 user
2743   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2744   forward the request to a public proxy at, which completes
2745   the request by forwarding it to the origin server at
2746   The request received by would then have the following
2747   Via header field:
2749<figure><artwork type="example">
2750  Via: 1.0 fred, 1.1
2753   An intermediary used as a portal through a network firewall
2754   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2755   region unless it is explicitly enabled to do so. If not enabled, such an
2756   intermediary &SHOULD; replace each received-by host of any host behind the
2757   firewall by an appropriate pseudonym for that host.
2760   An intermediary &MAY; combine an ordered subsequence of Via header
2761   field entries into a single such entry if the entries have identical
2762   received-protocol values. For example,
2764<figure><artwork type="example">
2765  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2768  could be collapsed to
2770<figure><artwork type="example">
2771  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2774   A sender &SHOULD-NOT; combine multiple entries unless they are all
2775   under the same organizational control and the hosts have already been
2776   replaced by pseudonyms. A sender &MUST-NOT; combine entries that
2777   have different received-protocol values.
2781<section title="Transformations" anchor="message.transformations">
2782   <iref primary="true" item="transforming proxy"/>
2783   <iref primary="true" item="non-transforming proxy"/>
2785   Some intermediaries include features for transforming messages and their
2786   payloads. A proxy might, for example, convert between image formats in
2787   order to save cache space or to reduce the amount of traffic on a slow
2788   link. However, operational problems might occur when these transformations
2789   are applied to payloads intended for critical applications, such as medical
2790   imaging or scientific data analysis, particularly when integrity checks or
2791   digital signatures are used to ensure that the payload received is
2792   identical to the original.
2795   An HTTP-to-HTTP proxy is called a "<x:dfn>transforming proxy</x:dfn>"
2796   if it is designed or configured to modify messages in a semantically
2797   meaningful way (i.e., modifications, beyond those required by normal
2798   HTTP processing, that change the message in a way that would be
2799   significant to the original sender or potentially significant to
2800   downstream recipients).  For example, a transforming proxy might be
2801   acting as a shared annotation server (modifying responses to include
2802   references to a local annotation database), a malware filter, a
2803   format transcoder, or a privacy filter. Such transformations are presumed
2804   to be desired by whichever client (or client organization) selected the
2805   proxy.
2808   If a proxy receives a request-target with a host name that is not a
2809   fully qualified domain name, it &MAY; add its own domain to the host name
2810   it received when forwarding the request.  A proxy &MUST-NOT; change the
2811   host name if the request-target contains a fully qualified domain name.
2814   A proxy &MUST-NOT; modify the "absolute-path" and "query" parts of the
2815   received request-target when forwarding it to the next inbound server,
2816   except as noted above to replace an empty path with "/" or "*".
2819   A proxy &MAY; modify the message body through application
2820   or removal of a transfer coding (<xref target="transfer.codings"/>).
2823   A proxy &MUST-NOT; transform the payload (&payload;) of a message that
2824   contains a no-transform cache-control directive (&header-cache-control;).
2827   A proxy &MAY; transform the payload of a message
2828   that does not contain a no-transform cache-control directive.
2829   A proxy that transforms a payload &MUST; add a <x:ref>Warning</x:ref>
2830   header field with the warn-code of 214 ("Transformation Applied")
2831   if one is not already in the message (see &header-warning;).
2832   A proxy that transforms the payload of a <x:ref>200 (OK)</x:ref> response
2833   can further inform downstream recipients that a transformation has been
2834   applied by changing the response status code to
2835   <x:ref>203 (Non-Authoritative Information)</x:ref> (&status-203;).
2838   A proxy &MUST-NOT; modify header fields that provide information about the
2839   end points of the communication chain, the resource state, or the selected
2840   representation.
2846<section title="Connection Management" anchor="">
2848   HTTP messaging is independent of the underlying transport or
2849   session-layer connection protocol(s).  HTTP only presumes a reliable
2850   transport with in-order delivery of requests and the corresponding
2851   in-order delivery of responses.  The mapping of HTTP request and
2852   response structures onto the data units of an underlying transport
2853   protocol is outside the scope of this specification.
2856   As described in <xref target="connecting.inbound"/>, the specific
2857   connection protocols to be used for an HTTP interaction are determined by
2858   client configuration and the <x:ref>target URI</x:ref>.
2859   For example, the "http" URI scheme
2860   (<xref target="http.uri"/>) indicates a default connection of TCP
2861   over IP, with a default TCP port of 80, but the client might be
2862   configured to use a proxy via some other connection, port, or protocol.
2865   HTTP implementations are expected to engage in connection management,
2866   which includes maintaining the state of current connections,
2867   establishing a new connection or reusing an existing connection,
2868   processing messages received on a connection, detecting connection
2869   failures, and closing each connection.
2870   Most clients maintain multiple connections in parallel, including
2871   more than one connection per server endpoint.
2872   Most servers are designed to maintain thousands of concurrent connections,
2873   while controlling request queues to enable fair use and detect
2874   denial of service attacks.
2877<section title="Connection" anchor="header.connection">
2878  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2879  <iref primary="true" item="close" x:for-anchor=""/>
2880  <x:anchor-alias value="Connection"/>
2881  <x:anchor-alias value="connection-option"/>
2882  <x:anchor-alias value="close"/>
2884   The "Connection" header field allows the sender to indicate desired
2885   control options for the current connection.  In order to avoid confusing
2886   downstream recipients, a proxy or gateway &MUST; remove or replace any
2887   received connection options before forwarding the message.
2890   When a header field aside from Connection is used to supply control
2891   information for or about the current connection, the sender &MUST; list
2892   the corresponding field-name within the "Connection" header field.
2893   A proxy or gateway &MUST; parse a received Connection
2894   header field before a message is forwarded and, for each
2895   connection-option in this field, remove any header field(s) from
2896   the message with the same name as the connection-option, and then
2897   remove the Connection header field itself (or replace it with the
2898   intermediary's own connection options for the forwarded message).
2901   Hence, the Connection header field provides a declarative way of
2902   distinguishing header fields that are only intended for the
2903   immediate recipient ("hop-by-hop") from those fields that are
2904   intended for all recipients on the chain ("end-to-end"), enabling the
2905   message to be self-descriptive and allowing future connection-specific
2906   extensions to be deployed without fear that they will be blindly
2907   forwarded by older intermediaries.
2910   The Connection header field's value has the following grammar:
2912<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2913  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2914  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2917   Connection options are case-insensitive.
2920   A sender &MUST-NOT; send a connection option corresponding to a header
2921   field that is intended for all recipients of the payload.
2922   For example, <x:ref>Cache-Control</x:ref> is never appropriate as a
2923   connection option (&header-cache-control;).
2926   The connection options do not always correspond to a header field
2927   present in the message, since a connection-specific header field
2928   might not be needed if there are no parameters associated with a
2929   connection option. In contrast, a connection-specific header field that
2930   is received without a corresponding connection option usually indicates
2931   that the field has been improperly forwarded by an intermediary and
2932   ought to be ignored by the recipient.
2935   When defining new connection options, specification authors ought to survey
2936   existing header field names and ensure that the new connection option does
2937   not share the same name as an already deployed header field.
2938   Defining a new connection option essentially reserves that potential
2939   field-name for carrying additional information related to the
2940   connection option, since it would be unwise for senders to use
2941   that field-name for anything else.
2944   The "<x:dfn>close</x:dfn>" connection option is defined for a
2945   sender to signal that this connection will be closed after completion of
2946   the response. For example,
2948<figure><artwork type="example">
2949  Connection: close
2952   in either the request or the response header fields indicates that the
2953   sender is going to close the connection after the current request/response
2954   is complete (<xref target="persistent.tear-down"/>).
2957   A client that does not support <x:ref>persistent connections</x:ref> &MUST;
2958   send the "close" connection option in every request message.
2961   A server that does not support <x:ref>persistent connections</x:ref> &MUST;
2962   send the "close" connection option in every response message that
2963   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2967<section title="Establishment" anchor="persistent.establishment">
2969   It is beyond the scope of this specification to describe how connections
2970   are established via various transport or session-layer protocols.
2971   Each connection applies to only one transport link.
2975<section title="Persistence" anchor="persistent.connections">
2976   <x:anchor-alias value="persistent connections"/>
2978   HTTP/1.1 defaults to the use of "<x:dfn>persistent connections</x:dfn>",
2979   allowing multiple requests and responses to be carried over a single
2980   connection. The "<x:ref>close</x:ref>" connection-option is used to signal
2981   that a connection will not persist after the current request/response.
2982   HTTP implementations &SHOULD; support persistent connections.
2985   A recipient determines whether a connection is persistent or not based on
2986   the most recently received message's protocol version and
2987   <x:ref>Connection</x:ref> header field (if any):
2988   <list style="symbols">
2989     <t>If the <x:ref>close</x:ref> connection option is present, the
2990        connection will not persist after the current response; else,</t>
2991     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2992        persist after the current response; else,</t>
2993     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2994        connection option is present, the recipient is not a proxy, and
2995        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2996        the connection will persist after the current response; otherwise,</t>
2997     <t>The connection will close after the current response.</t>
2998   </list>
3001   A client &MAY; send additional requests on a persistent connection until it
3002   sends or receives a <x:ref>close</x:ref> connection option or receives an
3003   HTTP/1.0 response without a "keep-alive" connection option.
3006   In order to remain persistent, all messages on a connection need to
3007   have a self-defined message length (i.e., one not defined by closure
3008   of the connection), as described in <xref target="message.body"/>.
3009   A server &MUST; read the entire request message body or close
3010   the connection after sending its response, since otherwise the
3011   remaining data on a persistent connection would be misinterpreted
3012   as the next request.  Likewise,
3013   a client &MUST; read the entire response message body if it intends
3014   to reuse the same connection for a subsequent request.
3017   A proxy server &MUST-NOT; maintain a persistent connection with an
3018   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
3019   information and discussion of the problems with the Keep-Alive header field
3020   implemented by many HTTP/1.0 clients).
3023   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
3024   for more information on backward compatibility with HTTP/1.0 clients.
3027<section title="Retrying Requests" anchor="persistent.retrying.requests">
3029   Connections can be closed at any time, with or without intention.
3030   Implementations ought to anticipate the need to recover
3031   from asynchronous close events.
3034   When an inbound connection is closed prematurely, a client &MAY; open a new
3035   connection and automatically retransmit an aborted sequence of requests if
3036   all of those requests have idempotent methods (&idempotent-methods;).
3037   A proxy &MUST-NOT; automatically retry non-idempotent requests.
3040   A user agent &MUST-NOT; automatically retry a request with a non-idempotent
3041   method unless it has some means to know that the request semantics are
3042   actually idempotent, regardless of the method, or some means to detect that
3043   the original request was never applied. For example, a user agent that
3044   knows (through design or configuration) that a POST request to a given
3045   resource is safe can repeat that request automatically.
3046   Likewise, a user agent designed specifically to operate on a version
3047   control repository might be able to recover from partial failure conditions
3048   by checking the target resource revision(s) after a failed connection,
3049   reverting or fixing any changes that were partially applied, and then
3050   automatically retrying the requests that failed.
3053   A client &SHOULD-NOT; automatically retry a failed automatic retry.
3057<section title="Pipelining" anchor="pipelining">
3058   <x:anchor-alias value="pipeline"/>
3060   A client that supports persistent connections &MAY; "<x:dfn>pipeline</x:dfn>"
3061   its requests (i.e., send multiple requests without waiting for each
3062   response). A server &MAY; process a sequence of pipelined requests in
3063   parallel if they all have safe methods (&safe-methods;), but &MUST; send
3064   the corresponding responses in the same order that the requests were
3065   received.
3068   A client that pipelines requests &SHOULD; retry unanswered requests if the
3069   connection closes before it receives all of the corresponding responses.
3070   When retrying pipelined requests after a failed connection (a connection
3071   not explicitly closed by the server in its last complete response), a
3072   client &MUST-NOT; pipeline immediately after connection establishment,
3073   since the first remaining request in the prior pipeline might have caused
3074   an error response that can be lost again if multiple requests are sent on a
3075   prematurely closed connection (see the TCP reset problem described in
3076   <xref target="persistent.tear-down"/>).
3079   Idempotent methods (&idempotent-methods;) are significant to pipelining
3080   because they can be automatically retried after a connection failure.
3081   A user agent &SHOULD-NOT; pipeline requests after a non-idempotent method,
3082   until the final response status code for that method has been received,
3083   unless the user agent has a means to detect and recover from partial
3084   failure conditions involving the pipelined sequence.
3087   An intermediary that receives pipelined requests &MAY; pipeline those
3088   requests when forwarding them inbound, since it can rely on the outbound
3089   user agent(s) to determine what requests can be safely pipelined. If the
3090   inbound connection fails before receiving a response, the pipelining
3091   intermediary &MAY; attempt to retry a sequence of requests that have yet
3092   to receive a response if the requests all have idempotent methods;
3093   otherwise, the pipelining intermediary &SHOULD; forward any received
3094   responses and then close the corresponding outbound connection(s) so that
3095   the outbound user agent(s) can recover accordingly.
3100<section title="Concurrency" anchor="persistent.concurrency">
3102   A client ought to limit the number of simultaneous open
3103   connections that it maintains to a given server.
3106   Previous revisions of HTTP gave a specific number of connections as a
3107   ceiling, but this was found to be impractical for many applications. As a
3108   result, this specification does not mandate a particular maximum number of
3109   connections, but instead encourages clients to be conservative when opening
3110   multiple connections.
3113   Multiple connections are typically used to avoid the "head-of-line
3114   blocking" problem, wherein a request that takes significant server-side
3115   processing and/or has a large payload blocks subsequent requests on the
3116   same connection. However, each connection consumes server resources.
3117   Furthermore, using multiple connections can cause undesirable side effects
3118   in congested networks.
3121   Note that a server might reject traffic that it deems abusive or
3122   characteristic of a denial of service attack, such as an excessive number
3123   of open connections from a single client.
3127<section title="Failures and Time-outs" anchor="persistent.failures">
3129   Servers will usually have some time-out value beyond which they will
3130   no longer maintain an inactive connection. Proxy servers might make
3131   this a higher value since it is likely that the client will be making
3132   more connections through the same proxy server. The use of persistent
3133   connections places no requirements on the length (or existence) of
3134   this time-out for either the client or the server.
3137   A client or server that wishes to time-out &SHOULD; issue a graceful close
3138   on the connection. Implementations &SHOULD; constantly monitor open
3139   connections for a received closure signal and respond to it as appropriate,
3140   since prompt closure of both sides of a connection enables allocated system
3141   resources to be reclaimed.
3144   A client, server, or proxy &MAY; close the transport connection at any
3145   time. For example, a client might have started to send a new request
3146   at the same time that the server has decided to close the "idle"
3147   connection. From the server's point of view, the connection is being
3148   closed while it was idle, but from the client's point of view, a
3149   request is in progress.
3152   A server &SHOULD; sustain persistent connections, when possible, and allow
3153   the underlying
3154   transport's flow control mechanisms to resolve temporary overloads, rather
3155   than terminate connections with the expectation that clients will retry.
3156   The latter technique can exacerbate network congestion.
3159   A client sending a message body &SHOULD; monitor
3160   the network connection for an error response while it is transmitting
3161   the request. If the client sees a response that indicates the server does
3162   not wish to receive the message body and is closing the connection, the
3163   client &SHOULD; immediately cease transmitting the body and close its side
3164   of the connection.
3168<section title="Tear-down" anchor="persistent.tear-down">
3169  <iref primary="false" item="Connection header field" x:for-anchor=""/>
3170  <iref primary="false" item="close" x:for-anchor=""/>
3172   The <x:ref>Connection</x:ref> header field
3173   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
3174   connection option that a sender &SHOULD; send when it wishes to close
3175   the connection after the current request/response pair.
3178   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
3179   send further requests on that connection (after the one containing
3180   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
3181   final response message corresponding to this request.
3184   A server that receives a <x:ref>close</x:ref> connection option &MUST;
3185   initiate a close of the connection (see below) after it sends the
3186   final response to the request that contained <x:ref>close</x:ref>.
3187   The server &SHOULD; send a <x:ref>close</x:ref> connection option
3188   in its final response on that connection. The server &MUST-NOT; process
3189   any further requests received on that connection.
3192   A server that sends a <x:ref>close</x:ref> connection option &MUST;
3193   initiate a close of the connection (see below) after it sends the
3194   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
3195   any further requests received on that connection.
3198   A client that receives a <x:ref>close</x:ref> connection option &MUST;
3199   cease sending requests on that connection and close the connection
3200   after reading the response message containing the close; if additional
3201   pipelined requests had been sent on the connection, the client &SHOULD-NOT;
3202   assume that they will be processed by the server.
3205   If a server performs an immediate close of a TCP connection, there is a
3206   significant risk that the client will not be able to read the last HTTP
3207   response.  If the server receives additional data from the client on a
3208   fully-closed connection, such as another request that was sent by the
3209   client before receiving the server's response, the server's TCP stack will
3210   send a reset packet to the client; unfortunately, the reset packet might
3211   erase the client's unacknowledged input buffers before they can be read
3212   and interpreted by the client's HTTP parser.
3215   To avoid the TCP reset problem, servers typically close a connection in
3216   stages. First, the server performs a half-close by closing only the write
3217   side of the read/write connection. The server then continues to read from
3218   the connection until it receives a corresponding close by the client, or
3219   until the server is reasonably certain that its own TCP stack has received
3220   the client's acknowledgement of the packet(s) containing the server's last
3221   response. Finally, the server fully closes the connection.
3224   It is unknown whether the reset problem is exclusive to TCP or might also
3225   be found in other transport connection protocols.
3229<section title="Upgrade" anchor="header.upgrade">
3230  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3231  <x:anchor-alias value="Upgrade"/>
3232  <x:anchor-alias value="protocol"/>
3233  <x:anchor-alias value="protocol-name"/>
3234  <x:anchor-alias value="protocol-version"/>
3236   The "Upgrade" header field is intended to provide a simple mechanism
3237   for transitioning from HTTP/1.1 to some other protocol on the same
3238   connection.  A client &MAY; send a list of protocols in the Upgrade
3239   header field of a request to invite the server to switch to one or
3240   more of those protocols, in order of descending preference, before sending
3241   the final response. A server &MAY; ignore a received Upgrade header field
3242   if it wishes to continue using the current protocol on that connection.
3243   Upgrade cannot be used to insist on a protocol change.
3245<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3246  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3248  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3249  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3250  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3253   A server that sends a <x:ref>101 (Switching Protocols)</x:ref> response
3254   &MUST; send an Upgrade header field to indicate the new protocol(s) to
3255   which the connection is being switched; if multiple protocol layers are
3256   being switched, the sender &MUST; list the protocols in layer-ascending
3257   order. A server &MUST-NOT; switch to a protocol that was not indicated by
3258   the client in the corresponding request's Upgrade header field.
3259   A server &MAY; choose to ignore the order of preference indicated by the
3260   client and select the new protocol(s) based on other factors, such as the
3261   nature of the request or the current load on the server.
3264   A server that sends a <x:ref>426 (Upgrade Required)</x:ref> response
3265   &MUST; send an Upgrade header field to indicate the acceptable protocols,
3266   in order of descending preference.
3269   A server &MAY; send an Upgrade header field in any other response to
3270   advertise that it implements support for upgrading to the listed protocols,
3271   in order of descending preference, when appropriate for a future request.
3274   The following is a hypothetical example sent by a client:
3275</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
3276GET /hello.txt HTTP/1.1
3278Connection: upgrade
3279Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3283   The capabilities and nature of the
3284   application-level communication after the protocol change is entirely
3285   dependent upon the new protocol(s) chosen. However, immediately after
3286   sending the 101 response, the server is expected to continue responding to
3287   the original request as if it had received its equivalent within the new
3288   protocol (i.e., the server still has an outstanding request to satisfy
3289   after the protocol has been changed, and is expected to do so without
3290   requiring the request to be repeated).
3293   For example, if the Upgrade header field is received in a GET request
3294   and the server decides to switch protocols, it first responds
3295   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3296   then immediately follows that with the new protocol's equivalent of a
3297   response to a GET on the target resource.  This allows a connection to be
3298   upgraded to protocols with the same semantics as HTTP without the
3299   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3300   protocols unless the received message semantics can be honored by the new
3301   protocol; an OPTIONS request can be honored by any protocol.
3304   The following is an example response to the above hypothetical request:
3305</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
3306HTTP/1.1 101 Switching Protocols
3307Connection: upgrade
3308Upgrade: HTTP/2.0
3310[... data stream switches to HTTP/2.0 with an appropriate response
3311(as defined by new protocol) to the "GET /hello.txt" request ...]
3314   When Upgrade is sent, the sender &MUST; also send a
3315   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3316   that contains an "upgrade" connection option, in order to prevent Upgrade
3317   from being accidentally forwarded by intermediaries that might not implement
3318   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3319   is received in an HTTP/1.0 request.
3322   A client cannot begin using an upgraded protocol on the connection until
3323   it has completely sent the request message (i.e., the client can't change
3324   the protocol it is sending in the middle of a message).
3325   If a server receives both Upgrade and an <x:ref>Expect</x:ref> header field
3326   with the "100-continue" expectation (&header-expect;), the
3327   server &MUST; send a <x:ref>100 (Continue)</x:ref> response before sending
3328   a <x:ref>101 (Switching Protocols)</x:ref> response.
3331   The Upgrade header field only applies to switching protocols on top of the
3332   existing connection; it cannot be used to switch the underlying connection
3333   (transport) protocol, nor to switch the existing communication to a
3334   different connection. For those purposes, it is more appropriate to use a
3335   <x:ref>3xx (Redirection)</x:ref> response (&status-3xx;).
3338   This specification only defines the protocol name "HTTP" for use by
3339   the family of Hypertext Transfer Protocols, as defined by the HTTP
3340   version rules of <xref target="http.version"/> and future updates to this
3341   specification. Additional tokens ought to be registered with IANA using the
3342   registration procedure defined in <xref target="upgrade.token.registry"/>.
3347<section title="ABNF list extension: #rule" anchor="abnf.extension">
3349   A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
3350   improve readability in the definitions of some header field values.
3353   A construct "#" is defined, similar to "*", for defining comma-delimited
3354   lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
3355   at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
3356   comma (",") and optional whitespace (OWS).   
3359   In any production that uses the list construct, a sender &MUST-NOT;
3360   generate empty list elements. In other words, a sender &MUST; generate
3361   lists that satisfy the following syntax:
3362</preamble><artwork type="example">
3363  1#element =&gt; element *( OWS "," OWS element )
3366   and:
3367</preamble><artwork type="example">
3368  #element =&gt; [ 1#element ]
3371   and for n &gt;= 1 and m &gt; 1:
3372</preamble><artwork type="example">
3373  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
3376   For compatibility with legacy list rules, a recipient &MUST; parse and ignore
3377   a reasonable number of empty list elements: enough to handle common mistakes
3378   by senders that merge values, but not so much that they could be used as a
3379   denial of service mechanism. In other words, a recipient &MUST; accept lists
3380   that satisfy the following syntax:
3382<figure><artwork type="example">
3383  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
3385  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
3388   Empty elements do not contribute to the count of elements present.
3389   For example, given these ABNF productions:
3391<figure><artwork type="example">
3392  example-list      = 1#example-list-elmt
3393  example-list-elmt = token ; see <xref target="field.components"/>
3396   Then the following are valid values for example-list (not including the
3397   double quotes, which are present for delimitation only):
3399<figure><artwork type="example">
3400  "foo,bar"
3401  "foo ,bar,"
3402  "foo , ,bar,charlie   "
3405   In contrast, the following values would be invalid, since at least one
3406   non-empty element is required by the example-list production:
3408<figure><artwork type="example">
3409  ""
3410  ","
3411  ",   ,"
3414   <xref target="collected.abnf"/> shows the collected ABNF for recipients
3415   after the list constructs have been expanded.
3419<section title="IANA Considerations" anchor="IANA.considerations">
3421<section title="Header Field Registration" anchor="header.field.registration">
3423   HTTP header fields are registered within the Message Header Field Registry
3424   maintained at
3425   <eref target=""/>.
3428   This document defines the following HTTP header fields, so their
3429   associated registry entries shall be updated according to the permanent
3430   registrations below (see <xref target="BCP90"/>):
3432<?BEGININC p1-messaging.iana-headers ?>
3433<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3434<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3435   <ttcol>Header Field Name</ttcol>
3436   <ttcol>Protocol</ttcol>
3437   <ttcol>Status</ttcol>
3438   <ttcol>Reference</ttcol>
3440   <c>Connection</c>
3441   <c>http</c>
3442   <c>standard</c>
3443   <c>
3444      <xref target="header.connection"/>
3445   </c>
3446   <c>Content-Length</c>
3447   <c>http</c>
3448   <c>standard</c>
3449   <c>
3450      <xref target="header.content-length"/>
3451   </c>
3452   <c>Host</c>
3453   <c>http</c>
3454   <c>standard</c>
3455   <c>
3456      <xref target=""/>
3457   </c>
3458   <c>TE</c>
3459   <c>http</c>
3460   <c>standard</c>
3461   <c>
3462      <xref target="header.te"/>
3463   </c>
3464   <c>Trailer</c>
3465   <c>http</c>
3466   <c>standard</c>
3467   <c>
3468      <xref target="header.trailer"/>
3469   </c>
3470   <c>Transfer-Encoding</c>
3471   <c>http</c>
3472   <c>standard</c>
3473   <c>
3474      <xref target="header.transfer-encoding"/>
3475   </c>
3476   <c>Upgrade</c>
3477   <c>http</c>
3478   <c>standard</c>
3479   <c>
3480      <xref target="header.upgrade"/>
3481   </c>
3482   <c>Via</c>
3483   <c>http</c>
3484   <c>standard</c>
3485   <c>
3486      <xref target="header.via"/>
3487   </c>
3490<?ENDINC p1-messaging.iana-headers ?>
3492   Furthermore, the header field-name "Close" shall be registered as
3493   "reserved", since using that name as an HTTP header field might
3494   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3495   header field (<xref target="header.connection"/>).
3497<texttable align="left" suppress-title="true">
3498   <ttcol>Header Field Name</ttcol>
3499   <ttcol>Protocol</ttcol>
3500   <ttcol>Status</ttcol>
3501   <ttcol>Reference</ttcol>
3503   <c>Close</c>
3504   <c>http</c>
3505   <c>reserved</c>
3506   <c>
3507      <xref target="header.field.registration"/>
3508   </c>
3511   The change controller is: "IETF ( - Internet Engineering Task Force".
3515<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3517   IANA maintains the registry of URI Schemes <xref target="BCP115"/> at
3518   <eref target=""/>.
3521   This document defines the following URI schemes, so their
3522   associated registry entries shall be updated according to the permanent
3523   registrations below:
3525<texttable align="left" suppress-title="true">
3526   <ttcol>URI Scheme</ttcol>
3527   <ttcol>Description</ttcol>
3528   <ttcol>Reference</ttcol>
3530   <c>http</c>
3531   <c>Hypertext Transfer Protocol</c>
3532   <c><xref target="http.uri"/></c>
3534   <c>https</c>
3535   <c>Hypertext Transfer Protocol Secure</c>
3536   <c><xref target="https.uri"/></c>
3540<section title="Internet Media Type Registration" anchor="">
3542   IANA maintains the registry of Internet media types <xref target="BCP13"/> at
3543   <eref target=""/>.
3546   This document serves as the specification for the Internet media types
3547   "message/http" and "application/http". The following is to be registered with
3548   IANA.
3550<section title="Internet Media Type message/http" anchor="">
3551<iref item="Media Type" subitem="message/http" primary="true"/>
3552<iref item="message/http Media Type" primary="true"/>
3554   The message/http type can be used to enclose a single HTTP request or
3555   response message, provided that it obeys the MIME restrictions for all
3556   "message" types regarding line length and encodings.
3559  <list style="hanging" x:indent="12em">
3560    <t hangText="Type name:">
3561      message
3562    </t>
3563    <t hangText="Subtype name:">
3564      http
3565    </t>
3566    <t hangText="Required parameters:">
3567      N/A
3568    </t>
3569    <t hangText="Optional parameters:">
3570      version, msgtype
3571      <list style="hanging">
3572        <t hangText="version:">
3573          The HTTP-version number of the enclosed message
3574          (e.g., "1.1"). If not present, the version can be
3575          determined from the first line of the body.
3576        </t>
3577        <t hangText="msgtype:">
3578          The message type &mdash; "request" or "response". If not
3579          present, the type can be determined from the first
3580          line of the body.
3581        </t>
3582      </list>
3583    </t>
3584    <t hangText="Encoding considerations:">
3585      only "7bit", "8bit", or "binary" are permitted
3586    </t>
3587    <t hangText="Security considerations:">
3588      see <xref target="security.considerations"/>
3589    </t>
3590    <t hangText="Interoperability considerations:">
3591      N/A
3592    </t>
3593    <t hangText="Published specification:">
3594      This specification (see <xref target=""/>).
3595    </t>
3596    <t hangText="Applications that use this media type:">
3597      N/A
3598    </t>
3599    <t hangText="Fragment identifier considerations:">
3600      N/A
3601    </t>
3602    <t hangText="Additional information:">
3603      <list style="hanging">
3604        <t hangText="Magic number(s):">N/A</t>
3605        <t hangText="Deprecated alias names for this type:">N/A</t>
3606        <t hangText="File extension(s):">N/A</t>
3607        <t hangText="Macintosh file type code(s):">N/A</t>
3608      </list>
3609    </t>
3610    <t hangText="Person and email address to contact for further information:">
3611      See Authors Section.
3612    </t>
3613    <t hangText="Intended usage:">
3614      COMMON
3615    </t>
3616    <t hangText="Restrictions on usage:">
3617      N/A
3618    </t>
3619    <t hangText="Author:">
3620      See Authors Section.
3621    </t>
3622    <t hangText="Change controller:">
3623      IESG
3624    </t>
3625  </list>
3628<section title="Internet Media Type application/http" anchor="">
3629<iref item="Media Type" subitem="application/http" primary="true"/>
3630<iref item="application/http Media Type" primary="true"/>
3632   The application/http type can be used to enclose a pipeline of one or more
3633   HTTP request or response messages (not intermixed).
3636  <list style="hanging" x:indent="12em">
3637    <t hangText="Type name:">
3638      application
3639    </t>
3640    <t hangText="Subtype name:">
3641      http
3642    </t>
3643    <t hangText="Required parameters:">
3644      N/A
3645    </t>
3646    <t hangText="Optional parameters:">
3647      version, msgtype
3648      <list style="hanging">
3649        <t hangText="version:">
3650          The HTTP-version number of the enclosed messages
3651          (e.g., "1.1"). If not present, the version can be
3652          determined from the first line of the body.
3653        </t>
3654        <t hangText="msgtype:">
3655          The message type &mdash; "request" or "response". If not
3656          present, the type can be determined from the first
3657          line of the body.
3658        </t>
3659      </list>
3660    </t>
3661    <t hangText="Encoding considerations:">
3662      HTTP messages enclosed by this type
3663      are in "binary" format; use of an appropriate
3664      Content-Transfer-Encoding is required when
3665      transmitted via E-mail.
3666    </t>
3667    <t hangText="Security considerations:">
3668      see <xref target="security.considerations"/>
3669    </t>
3670    <t hangText="Interoperability considerations:">
3671      N/A
3672    </t>
3673    <t hangText="Published specification:">
3674      This specification (see <xref target=""/>).
3675    </t>
3676    <t hangText="Applications that use this media type:">
3677      N/A
3678    </t>
3679    <t hangText="Fragment identifier considerations:">
3680      N/A
3681    </t>
3682    <t hangText="Additional information:">
3683      <list style="hanging">
3684        <t hangText="Deprecated alias names for this type:">N/A</t>
3685        <t hangText="Magic number(s):">N/A</t>
3686        <t hangText="File extension(s):">N/A</t>
3687        <t hangText="Macintosh file type code(s):">N/A</t>
3688      </list>
3689    </t>
3690    <t hangText="Person and email address to contact for further information:">
3691      See Authors Section.
3692    </t>
3693    <t hangText="Intended usage:">
3694      COMMON
3695    </t>
3696    <t hangText="Restrictions on usage:">
3697      N/A
3698    </t>
3699    <t hangText="Author:">
3700      See Authors Section.
3701    </t>
3702    <t hangText="Change controller:">
3703      IESG
3704    </t>
3705  </list>
3710<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3712   The HTTP Transfer Coding Registry defines the name space for transfer
3713   coding names. It is maintained at <eref target=""/>.
3716<section title="Procedure" anchor="transfer.coding.registry.procedure">
3718   Registrations &MUST; include the following fields:
3719   <list style="symbols">
3720     <t>Name</t>
3721     <t>Description</t>
3722     <t>Pointer to specification text</t>
3723   </list>
3726   Names of transfer codings &MUST-NOT; overlap with names of content codings
3727   (&content-codings;) unless the encoding transformation is identical, as
3728   is the case for the compression codings defined in
3729   <xref target="compression.codings"/>.
3732   Values to be added to this name space require IETF Review (see
3733   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3734   conform to the purpose of transfer coding defined in this specification.
3737   Use of program names for the identification of encoding formats
3738   is not desirable and is discouraged for future encodings.
3742<section title="Registration" anchor="transfer.coding.registration">
3744   The HTTP Transfer Coding Registry shall be updated with the registrations
3745   below:
3747<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3748   <ttcol>Name</ttcol>
3749   <ttcol>Description</ttcol>
3750   <ttcol>Reference</ttcol>
3751   <c>chunked</c>
3752   <c>Transfer in a series of chunks</c>
3753   <c>
3754      <xref target="chunked.encoding"/>
3755   </c>
3756   <c>compress</c>
3757   <c>UNIX "compress" data format <xref target="Welch"/></c>
3758   <c>
3759      <xref target="compress.coding"/>
3760   </c>
3761   <c>deflate</c>
3762   <c>"deflate" compressed data (<xref target="RFC1951"/>) inside
3763   the "zlib" data format (<xref target="RFC1950"/>)
3764   </c>
3765   <c>
3766      <xref target="deflate.coding"/>
3767   </c>
3768   <c>gzip</c>
3769   <c>GZIP file format <xref target="RFC1952"/></c>
3770   <c>
3771      <xref target="gzip.coding"/>
3772   </c>
3773   <c>x-compress</c>
3774   <c>Deprecated (alias for compress)</c>
3775   <c>
3776      <xref target="compress.coding"/>
3777   </c>
3778   <c>x-gzip</c>
3779   <c>Deprecated (alias for gzip)</c>
3780   <c>
3781      <xref target="gzip.coding"/>
3782   </c>
3787<section title="Content Coding Registration" anchor="content.coding.registration">
3789   IANA maintains the registry of HTTP Content Codings at
3790   <eref target=""/>.
3793   The HTTP Content Codings Registry shall be updated with the registrations
3794   below:
3796<texttable align="left" suppress-title="true" anchor="iana.content.coding.registration.table">
3797   <ttcol>Name</ttcol>
3798   <ttcol>Description</ttcol>
3799   <ttcol>Reference</ttcol>
3800   <c>compress</c>
3801   <c>UNIX "compress" data format <xref target="Welch"/></c>
3802   <c>
3803      <xref target="compress.coding"/>
3804   </c>
3805   <c>deflate</c>
3806   <c>"deflate" compressed data (<xref target="RFC1951"/>) inside
3807   the "zlib" data format (<xref target="RFC1950"/>)</c>
3808   <c>
3809      <xref target="deflate.coding"/>
3810   </c>
3811   <c>gzip</c>
3812   <c>GZIP file format <xref target="RFC1952"/></c>
3813   <c>
3814      <xref target="gzip.coding"/>
3815   </c>
3816   <c>x-compress</c>
3817   <c>Deprecated (alias for compress)</c>
3818   <c>
3819      <xref target="compress.coding"/>
3820   </c>
3821   <c>x-gzip</c>
3822   <c>Deprecated (alias for gzip)</c>
3823   <c>
3824      <xref target="gzip.coding"/>
3825   </c>
3829<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3831   The HTTP Upgrade Token Registry defines the name space for protocol-name
3832   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3833   field. The registry is maintained at <eref target=""/>.
3836<section title="Procedure" anchor="upgrade.token.registry.procedure">  
3838   Each registered protocol name is associated with contact information
3839   and an optional set of specifications that details how the connection
3840   will be processed after it has been upgraded.
3843   Registrations happen on a "First Come First Served" basis (see
3844   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3845   following rules:
3846  <list style="numbers">
3847    <t>A protocol-name token, once registered, stays registered forever.</t>
3848    <t>The registration &MUST; name a responsible party for the
3849       registration.</t>
3850    <t>The registration &MUST; name a point of contact.</t>
3851    <t>The registration &MAY; name a set of specifications associated with
3852       that token. Such specifications need not be publicly available.</t>
3853    <t>The registration &SHOULD; name a set of expected "protocol-version"
3854       tokens associated with that token at the time of registration.</t>
3855    <t>The responsible party &MAY; change the registration at any time.
3856       The IANA will keep a record of all such changes, and make them
3857       available upon request.</t>
3858    <t>The IESG &MAY; reassign responsibility for a protocol token.
3859       This will normally only be used in the case when a
3860       responsible party cannot be contacted.</t>
3861  </list>
3864   This registration procedure for HTTP Upgrade Tokens replaces that
3865   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3869<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3871   The "HTTP" entry in the HTTP Upgrade Token Registry shall be updated with
3872   the registration below:
3874<texttable align="left" suppress-title="true">
3875   <ttcol>Value</ttcol>
3876   <ttcol>Description</ttcol>
3877   <ttcol>Expected Version Tokens</ttcol>
3878   <ttcol>Reference</ttcol>
3880   <c>HTTP</c>
3881   <c>Hypertext Transfer Protocol</c>
3882   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3883   <c><xref target="http.version"/></c>
3886   The responsible party is: "IETF ( - Internet Engineering Task Force".
3893<section title="Security Considerations" anchor="security.considerations">
3895   This section is meant to inform developers, information providers, and
3896   users of known security considerations relevant to HTTP message syntax,
3897   parsing, and routing. Security considerations about HTTP semantics and
3898   payloads are addressed in &semantics;.
3901<section title="Establishing Authority" anchor="establishing.authority">
3902  <iref item="authoritative response" primary="true"/>
3903  <iref item="phishing" primary="true"/>
3905   HTTP relies on the notion of an <x:dfn>authoritative response</x:dfn>: a
3906   response that has been determined by (or at the direction of) the authority
3907   identified within the target URI to be the most appropriate response for
3908   that request given the state of the target resource at the time of
3909   response message origination. That is the essence of what a user is looking
3910   for when they make any HTTP request, especially when it is a request to an
3911   authority trusted by that user to provide a specific service. Providing a
3912   response from a non-authoritative source, such as a shared cache, is often
3913   useful to improve performance and availability, but only to the extent that
3914   the source can be trusted or that a distrusted response can be safely used.
3917   Unfortunately, establishing authority can be difficult.
3918   For example, <x:dfn>phishing</x:dfn> is an attack on the user's perception
3919   of authority, where that perception can be misled by presenting similar
3920   branding in hypertext, possibly aided by userinfo obfuscating the authority
3921   component (see <xref target="http.uri"/>).
3922   User agents can reduce the impact of phishing attacks by enabling users to
3923   easily inspect a target URI prior to making an action, by prominently
3924   distinguishing (or rejecting) userinfo when present, and by not sending
3925   stored credentials and cookies when the referring document is from an
3926   unknown or untrusted source.
3929   When a registered name is used in the authority component, the "http" URI
3930   scheme (<xref target="http.uri"/>) relies on the user's local name
3931   resolution service to determine where it can find authoritative responses.
3932   This means that any attack on a user's network host table, cached names, or
3933   name resolution libraries becomes an avenue for attack on establishing
3934   authority. Likewise, the user's choice of server for Domain Name Service
3935   (DNS), and the hierarchy of servers from which it obtains resolution
3936   results, could impact the authenticity of address mappings;
3937   DNSSEC (<xref target="RFC4033"/>) is one way to improve authenticity.
3940   Furthermore, after an IP address is obtained, establishing authority is
3941   vulnerable to attacks on Internet Protocol routing.
3944   The "https" scheme (<xref target="https.uri"/>) is intended to prevent
3945   (or at least reveal) many of these potential attacks on establishing
3946   authority, assuming the negotiated TLS connection is secured in a way that
3947   verifies the communicating server's identity matches the target URI's
3948   authority component (see <xref target="RFC2818"/> and
3949   <xref target="Georgiev"/>).
3953<section title="Risks of Intermediaries" anchor="risks.intermediaries">
3955   By their very nature, HTTP intermediaries are men-in-the-middle, and thus
3956   represent an opportunity for man-in-the-middle attacks. Compromise of
3957   the systems on which the intermediaries run can result in serious security
3958   and privacy problems. Intermediaries might have access to security-related
3959   information, personal information about individual users and
3960   organizations, and proprietary information belonging to users and
3961   content providers. A compromised intermediary, or an intermediary
3962   implemented or configured without regard to security and privacy
3963   considerations, might be used in the commission of a wide range of
3964   potential attacks.
3967   Intermediaries that contain a shared cache are especially vulnerable
3968   to cache poisoning attacks, as described in &cache-poisoning;.
3971   Implementers need to consider the privacy and security
3972   implications of their design and coding decisions, and of the
3973   configuration options they provide to operators (especially the
3974   default configuration).
3977   Users need to be aware that intermediaries are no more trustworthy than
3978   the people who run them; HTTP itself cannot solve this problem.
3982<section title="Attacks via Protocol Element Length" anchor="attack.protocol.element.length">
3984   Because HTTP uses mostly textual, character-delimited fields, parsers are
3985   often vulnerable to attacks based on sending very long (or very slow)
3986   streams of data, particularly where an implementation is expecting a
3987   protocol element with no predefined length.
3990   To promote interoperability, specific recommendations are made for minimum
3991   size limits on request-line (<xref target="request.line"/>)
3992   and header fields (<xref target="header.fields"/>). These are
3993   minimum recommendations, chosen to be supportable even by implementations
3994   with limited resources; it is expected that most implementations will
3995   choose substantially higher limits.
3998   A server can reject messages that
3999   have request-targets that are too long (&status-414;) or request entities
4000   that are too large (&status-4xx;). Additional status codes related to
4001   capacity limits have been defined by extensions to HTTP
4002   <xref target="RFC6585"/>.
4005   Recipients ought to carefully limit the extent to which they process other
4006   protocol elements, including (but not limited to) request methods, response
4007   status phrases, header field-names, numeric values, and body chunks.
4008   Failure to limit such processing can result in buffer overflows, arithmetic
4009   overflows, or increased vulnerability to denial of service attacks.
4013<section title="Response Splitting" anchor="response.splitting">
4015   Response splitting (a.k.a, CRLF injection) is a common technique, used in
4016   various attacks on Web usage, that exploits the line-based nature of HTTP
4017   message framing and the ordered association of requests to responses on
4018   persistent connections <xref target="Klein"/>. This technique can be
4019   particularly damaging when the requests pass through a shared cache.
4022   Response splitting exploits a vulnerability in servers (usually within an
4023   application server) where an attacker can send encoded data within some
4024   parameter of the request that is later decoded and echoed within any of the
4025   response header fields of the response. If the decoded data is crafted to
4026   look like the response has ended and a subsequent response has begun, the
4027   response has been split and the content within the apparent second response
4028   is controlled by the attacker. The attacker can then make any other request
4029   on the same persistent connection and trick the recipients (including
4030   intermediaries) into believing that the second half of the split is an
4031   authoritative answer to the second request.
4034   For example, a parameter within the request-target might be read by an
4035   application server and reused within a redirect, resulting in the same
4036   parameter being echoed in the <x:ref>Location</x:ref> header field of the
4037   response. If the parameter is decoded by the application and not properly
4038   encoded when placed in the response field, the attacker can send encoded
4039   CRLF octets and other content that will make the application's single
4040   response look like two or more responses.
4043   A common defense against response splitting is to filter requests for data
4044   that looks like encoded CR and LF (e.g., "%0D" and "%0A"). However, that
4045   assumes the application server is only performing URI decoding, rather
4046   than more obscure data transformations like charset transcoding, XML entity
4047   translation, base64 decoding, sprintf reformatting, etc.  A more effective
4048   mitigation is to prevent anything other than the server's core protocol
4049   libraries from sending a CR or LF within the header section, which means
4050   restricting the output of header fields to APIs that filter for bad octets
4051   and not allowing application servers to write directly to the protocol
4052   stream.
4056<section title="Request Smuggling" anchor="request.smuggling">
4058   Request smuggling (<xref target="Linhart"/>) is a technique that exploits
4059   differences in protocol parsing among various recipients to hide additional
4060   requests (which might otherwise be blocked or disabled by policy) within an
4061   apparently harmless request.  Like response splitting, request smuggling
4062   can lead to a variety of attacks on HTTP usage.
4065   This specification has introduced new requirements on request parsing,
4066   particularly with regard to message framing in
4067   <xref target="message.body.length"/>, to reduce the effectiveness of
4068   request smuggling.
4072<section title="Message Integrity" anchor="message.integrity">
4074   HTTP does not define a specific mechanism for ensuring message integrity,
4075   instead relying on the error-detection ability of underlying transport
4076   protocols and the use of length or chunk-delimited framing to detect
4077   completeness. Additional integrity mechanisms, such as hash functions or
4078   digital signatures applied to the content, can be selectively added to
4079   messages via extensible metadata header fields. Historically, the lack of
4080   a single integrity mechanism has been justified by the informal nature of
4081   most HTTP communication.  However, the prevalence of HTTP as an information
4082   access mechanism has resulted in its increasing use within environments
4083   where verification of message integrity is crucial.
4086   User agents are encouraged to implement configurable means for detecting
4087   and reporting failures of message integrity such that those means can be
4088   enabled within environments for which integrity is necessary. For example,
4089   a browser being used to view medical history or drug interaction
4090   information needs to indicate to the user when such information is detected
4091   by the protocol to be incomplete, expired, or corrupted during transfer.
4092   Such mechanisms might be selectively enabled via user agent extensions or
4093   the presence of message integrity metadata in a response.
4094   At a minimum, user agents ought to provide some indication that allows a
4095   user to distinguish between a complete and incomplete response message
4096   (<xref target="incomplete.messages"/>) when such verification is desired.
4100<section title="Message Confidentiality" anchor="message.confidentiality">
4102   HTTP relies on underlying transport protocols to provide message
4103   confidentiality when that is desired. HTTP has been specifically designed
4104   to be independent of the transport protocol, such that it can be used
4105   over many different forms of encrypted connection, with the selection of
4106   such transports being identified by the choice of URI scheme or within
4107   user agent configuration.
4110   The "https" scheme can be used to identify resources that require a
4111   confidential connection, as described in <xref target="https.uri"/>.
4115<section title="Privacy of Server Log Information" anchor="privacy.of.server.log.information">
4117   A server is in the position to save personal data about a user's requests
4118   over time, which might identify their reading patterns or subjects of
4119   interest.  In particular, log information gathered at an intermediary
4120   often contains a history of user agent interaction, across a multitude
4121   of sites, that can be traced to individual users.
4124   HTTP log information is confidential in nature; its handling is often
4125   constrained by laws and regulations.  Log information needs to be securely
4126   stored and appropriate guidelines followed for its analysis.
4127   Anonymization of personal information within individual entries helps,
4128   but is generally not sufficient to prevent real log traces from being
4129   re-identified based on correlation with other access characteristics.
4130   As such, access traces that are keyed to a specific client are unsafe to
4131   publish even if the key is pseudonymous.
4134   To minimize the risk of theft or accidental publication, log information
4135   ought to be purged of personally identifiable information, including
4136   user identifiers, IP addresses, and user-provided query parameters,
4137   as soon as that information is no longer necessary to support operational
4138   needs for security, auditing, or fraud control.
4143<section title="Acknowledgments" anchor="acks">
4145   This edition of HTTP/1.1 builds on the many contributions that went into
4146   <xref target="RFC1945" format="none">RFC 1945</xref>,
4147   <xref target="RFC2068" format="none">RFC 2068</xref>,
4148   <xref target="RFC2145" format="none">RFC 2145</xref>, and
4149   <xref target="RFC2616" format="none">RFC 2616</xref>, including
4150   substantial contributions made by the previous authors, editors, and
4151   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
4152   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
4153   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
4156   Since 1999, the following contributors have helped improve the HTTP
4157   specification by reporting bugs, asking smart questions, drafting or
4158   reviewing text, and evaluating open issues:
4160<?BEGININC acks ?>
4161<t>Adam Barth,
4162Adam Roach,
4163Addison Phillips,
4164Adrian Chadd,
4165Adrian Cole,
4166Adrien W. de Croy,
4167Alan Ford,
4168Alan Ruttenberg,
4169Albert Lunde,
4170Alek Storm,
4171Alex Rousskov,
4172Alexandre Morgaut,
4173Alexey Melnikov,
4174Alisha Smith,
4175Amichai Rothman,
4176Amit Klein,
4177Amos Jeffries,
4178Andreas Maier,
4179Andreas Petersson,
4180Andrei Popov,
4181Anil Sharma,
4182Anne van Kesteren,
4183Anthony Bryan,
4184Asbjorn Ulsberg,
4185Ashok Kumar,
4186Balachander Krishnamurthy,
4187Barry Leiba,
4188Ben Laurie,
4189Benjamin Carlyle,
4190Benjamin Niven-Jenkins,
4191Benoit Claise,
4192Bil Corry,
4193Bill Burke,
4194Bjoern Hoehrmann,
4195Bob Scheifler,
4196Boris Zbarsky,
4197Brett Slatkin,
4198Brian Kell,
4199Brian McBarron,
4200Brian Pane,
4201Brian Raymor,
4202Brian Smith,
4203Bruce Perens,
4204Bryce Nesbitt,
4205Cameron Heavon-Jones,
4206Carl Kugler,
4207Carsten Bormann,
4208Charles Fry,
4209Chris Burdess,
4210Chris Newman,
4211Christian Huitema,
4212Cyrus Daboo,
4213Dale Robert Anderson,
4214Dan Wing,
4215Dan Winship,
4216Daniel Stenberg,
4217Darrel Miller,
4218Dave Cridland,
4219Dave Crocker,
4220Dave Kristol,
4221Dave Thaler,
4222David Booth,
4223David Singer,
4224David W. Morris,
4225Diwakar Shetty,
4226Dmitry Kurochkin,
4227Drummond Reed,
4228Duane Wessels,
4229Edward Lee,
4230Eitan Adler,
4231Eliot Lear,
4232Emile Stephan,
4233Eran Hammer-Lahav,
4234Eric D. Williams,
4235Eric J. Bowman,
4236Eric Lawrence,
4237Eric Rescorla,
4238Erik Aronesty,
4239EungJun Yi,
4240Evan Prodromou,
4241Felix Geisendoerfer,
4242Florian Weimer,
4243Frank Ellermann,
4244Fred Akalin,
4245Fred Bohle,
4246Frederic Kayser,
4247Gabor Molnar,
4248Gabriel Montenegro,
4249Geoffrey Sneddon,
4250Gervase Markham,
4251Gili Tzabari,
4252Grahame Grieve,
4253Greg Slepak,
4254Greg Wilkins,
4255Grzegorz Calkowski,
4256Harald Tveit Alvestrand,
4257Harry Halpin,
4258Helge Hess,
4259Henrik Nordstrom,
4260Henry S. Thompson,
4261Henry Story,
4262Herbert van de Sompel,
4263Herve Ruellan,
4264Howard Melman,
4265Hugo Haas,
4266Ian Fette,
4267Ian Hickson,
4268Ido Safruti,
4269Ilari Liusvaara,
4270Ilya Grigorik,
4271Ingo Struck,
4272J. Ross Nicoll,
4273James Cloos,
4274James H. Manger,
4275James Lacey,
4276James M. Snell,
4277Jamie Lokier,
4278Jan Algermissen,
4279Jari Arkko,
4280Jeff Hodges (who came up with the term 'effective Request-URI'),
4281Jeff Pinner,
4282Jeff Walden,
4283Jim Luther,
4284Jitu Padhye,
4285Joe D. Williams,
4286Joe Gregorio,
4287Joe Orton,
4288Joel Jaeggli,
4289John C. Klensin,
4290John C. Mallery,
4291John Cowan,
4292John Kemp,
4293John Panzer,
4294John Schneider,
4295John Stracke,
4296John Sullivan,
4297Jonas Sicking,
4298Jonathan A. Rees,
4299Jonathan Billington,
4300Jonathan Moore,
4301Jonathan Silvera,
4302Jordi Ros,
4303Joris Dobbelsteen,
4304Josh Cohen,
4305Julien Pierre,
4306Jungshik Shin,
4307Justin Chapweske,
4308Justin Erenkrantz,
4309Justin James,
4310Kalvinder Singh,
4311Karl Dubost,
4312Kathleen Moriarty,
4313Keith Hoffman,
4314Keith Moore,
4315Ken Murchison,
4316Koen Holtman,
4317Konstantin Voronkov,
4318Kris Zyp,
4319Leif Hedstrom,
4320Lionel Morand,
4321Lisa Dusseault,
4322Maciej Stachowiak,
4323Manu Sporny,
4324Marc Schneider,
4325Marc Slemko,
4326Mark Baker,
4327Mark Pauley,
4328Mark Watson,
4329Markus Isomaki,
4330Markus Lanthaler,
4331Martin J. Duerst,
4332Martin Musatov,
4333Martin Nilsson,
4334Martin Thomson,
4335Matt Lynch,
4336Matthew Cox,
4337Matthew Kerwin,
4338Max Clark,
4339Menachem Dodge,
4340Meral Shirazipour,
4341Michael Burrows,
4342Michael Hausenblas,
4343Michael Scharf,
4344Michael Sweet,
4345Michael Tuexen,
4346Michael Welzl,
4347Mike Amundsen,
4348Mike Belshe,
4349Mike Bishop,
4350Mike Kelly,
4351Mike Schinkel,
4352Miles Sabin,
4353Murray S. Kucherawy,
4354Mykyta Yevstifeyev,
4355Nathan Rixham,
4356Nicholas Shanks,
4357Nico Williams,
4358Nicolas Alvarez,
4359Nicolas Mailhot,
4360Noah Slater,
4361Osama Mazahir,
4362Pablo Castro,
4363Pat Hayes,
4364Patrick R. McManus,
4365Paul E. Jones,
4366Paul Hoffman,
4367Paul Marquess,
4368Pete Resnick,
4369Peter Lepeska,
4370Peter Occil,
4371Peter Saint-Andre,
4372Peter Watkins,
4373Phil Archer,
4374Phil Hunt,
4375Philippe Mougin,
4376Phillip Hallam-Baker,
4377Piotr Dobrogost,
4378Poul-Henning Kamp,
4379Preethi Natarajan,
4380Rajeev Bector,
4381Ray Polk,
4382Reto Bachmann-Gmuer,
4383Richard Barnes,
4384Richard Cyganiak,
4385Rob Trace,
4386Robby Simpson,
4387Robert Brewer,
4388Robert Collins,
4389Robert Mattson,
4390Robert O'Callahan,
4391Robert Olofsson,
4392Robert Sayre,
4393Robert Siemer,
4394Robert de Wilde,
4395Roberto Javier Godoy,
4396Roberto Peon,
4397Roland Zink,
4398Ronny Widjaja,
4399Ryan Hamilton,
4400S. Mike Dierken,
4401Salvatore Loreto,
4402Sam Johnston,
4403Sam Pullara,
4404Sam Ruby,
4405Saurabh Kulkarni,
4406Scott Lawrence (who maintained the original issues list),
4407Sean B. Palmer,
4408Sean Turner,
4409Sebastien Barnoud,
4410Shane McCarron,
4411Shigeki Ohtsu,
4412Simon Yarde,
4413Stefan Eissing,
4414Stefan Tilkov,
4415Stefanos Harhalakis,
4416Stephane Bortzmeyer,
4417Stephen Farrell,
4418Stephen Kent,
4419Stephen Ludin,
4420Stuart Williams,
4421Subbu Allamaraju,
4422Subramanian Moonesamy,
4423Susan Hares,
4424Sylvain Hellegouarch,
4425Tapan Divekar,
4426Tatsuhiro Tsujikawa,
4427Tatsuya Hayashi,
4428Ted Hardie,
4429Ted Lemon,
4430Thomas Broyer,
4431Thomas Fossati,
4432Thomas Maslen,
4433Thomas Nadeau,
4434Thomas Nordin,
4435Thomas Roessler,
4436Tim Bray,
4437Tim Morgan,
4438Tim Olsen,
4439Tom Zhou,
4440Travis Snoozy,
4441Tyler Close,
4442Vincent Murphy,
4443Wenbo Zhu,
4444Werner Baumann,
4445Wilbur Streett,
4446Wilfredo Sanchez Vega,
4447William A. Rowe Jr.,
4448William Chan,
4449Willy Tarreau,
4450Xiaoshu Wang,
4451Yaron Goland,
4452Yngve Nysaeter Pettersen,
4453Yoav Nir,
4454Yogesh Bang,
4455Yuchung Cheng,
4456Yutaka Oiwa,
4457Yves Lafon (long-time member of the editor team),
4458Zed A. Shaw, and
4459Zhong Yu.
4461<?ENDINC acks ?>
4463   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
4464   acknowledgements from prior revisions.
4471<references title="Normative References">
4473<reference anchor="Part2">
4474  <front>
4475    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
4476    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4477      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4478      <address><email></email></address>
4479    </author>
4480    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4481      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4482      <address><email></email></address>
4483    </author>
4484    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4485  </front>
4486  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
4487  <x:source href="p2-semantics.xml" basename="p2-semantics">
4488    <x:defines>1xx (Informational)</x:defines>
4489    <x:defines>1xx</x:defines>
4490    <x:defines>100 (Continue)</x:defines>
4491    <x:defines>101 (Switching Protocols)</x:defines>
4492    <x:defines>2xx (Successful)</x:defines>
4493    <x:defines>2xx</x:defines>
4494    <x:defines>200 (OK)</x:defines>
4495    <x:defines>203 (Non-Authoritative Information)</x:defines>
4496    <x:defines>204 (No Content)</x:defines>
4497    <x:defines>3xx (Redirection)</x:defines>
4498    <x:defines>3xx</x:defines>
4499    <x:defines>301 (Moved Permanently)</x:defines>
4500    <x:defines>4xx (Client Error)</x:defines>
4501    <x:defines>4xx</x:defines>
4502    <x:defines>400 (Bad Request)</x:defines>
4503    <x:defines>411 (Length Required)</x:defines>
4504    <x:defines>414 (URI Too Long)</x:defines>
4505    <x:defines>417 (Expectation Failed)</x:defines>
4506    <x:defines>426 (Upgrade Required)</x:defines>
4507    <x:defines>501 (Not Implemented)</x:defines>
4508    <x:defines>502 (Bad Gateway)</x:defines>
4509    <x:defines>505 (HTTP Version Not Supported)</x:defines>
4510    <x:defines>Accept-Encoding</x:defines>
4511    <x:defines>Allow</x:defines>
4512    <x:defines>Content-Encoding</x:defines>
4513    <x:defines>Content-Location</x:defines>
4514    <x:defines>Content-Type</x:defines>
4515    <x:defines>Date</x:defines>
4516    <x:defines>Expect</x:defines>
4517    <x:defines>Location</x:defines>
4518    <x:defines>Server</x:defines>
4519    <x:defines>User-Agent</x:defines>
4520  </x:source>
4523<reference anchor="Part4">
4524  <front>
4525    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
4526    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
4527      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4528      <address><email></email></address>
4529    </author>
4530    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
4531      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4532      <address><email></email></address>
4533    </author>
4534    <date month="&ID-MONTH;" year="&ID-YEAR;" />
4535  </front>
4536  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
4537  <x:source basename="p4-conditional" href="p4-conditional.xml">
4538    <x:defines>304 (Not Modified)</x:defines>
4539    <x:defines>ETag</x:defines>
4540    <x:defines>Last-Modified</x:defines>
4541  </x:source>
4544<reference anchor="Part5">
4545  <front>
4546    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
4547    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4548      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4549      <address><email></email></address>
4550    </author>
4551    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
4552      <organization abbrev="W3C">World Wide Web Consortium</organization>
4553      <address><email></email></address>
4554    </author>
4555    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4556      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4557      <address><email></email></address>
4558    </author>
4559    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4560  </front>
4561  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4562  <x:source href="p5-range.xml" basename="p5-range">
4563    <x:defines>Content-Range</x:defines>
4564  </x:source>
4567<reference anchor="Part6">
4568  <front>
4569    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4570    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4571      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4572      <address><email></email></address>
4573    </author>
4574    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4575      <organization>Akamai</organization>
4576      <address><email></email></address>
4577    </author>
4578    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4579      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4580      <address><email></email></address>
4581    </author>
4582    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4583  </front>
4584  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4585  <x:source href="p6-cache.xml" basename="p6-cache">
4586    <x:defines>Cache-Control</x:defines>
4587    <x:defines>Expires</x:defines>
4588    <x:defines>Warning</x:defines>
4589  </x:source>
4592<reference anchor="Part7">
4593  <front>
4594    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4595    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4596      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4597      <address><email></email></address>
4598    </author>
4599    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4600      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4601      <address><email></email></address>
4602    </author>
4603    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4604  </front>
4605  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4606  <x:source href="p7-auth.xml" basename="p7-auth">
4607    <x:defines>Proxy-Authenticate</x:defines>
4608    <x:defines>Proxy-Authorization</x:defines>
4609  </x:source>
4612<reference anchor="RFC5234">
4613  <front>
4614    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4615    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4616      <organization>Brandenburg InternetWorking</organization>
4617      <address>
4618        <email></email>
4619      </address> 
4620    </author>
4621    <author initials="P." surname="Overell" fullname="Paul Overell">
4622      <organization>THUS plc.</organization>
4623      <address>
4624        <email></email>
4625      </address>
4626    </author>
4627    <date month="January" year="2008"/>
4628  </front>
4629  <seriesInfo name="STD" value="68"/>
4630  <seriesInfo name="RFC" value="5234"/>
4633<reference anchor="RFC2119">
4634  <front>
4635    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4636    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4637      <organization>Harvard University</organization>
4638      <address><email></email></address>
4639    </author>
4640    <date month="March" year="1997"/>
4641  </front>
4642  <seriesInfo name="BCP" value="14"/>
4643  <seriesInfo name="RFC" value="2119"/>
4646<reference anchor="RFC3986">
4647 <front>
4648  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4649  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4650    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4651    <address>
4652       <email></email>
4653       <uri></uri>
4654    </address>
4655  </author>
4656  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4657    <organization abbrev="Day Software">Day Software</organization>
4658    <address>
4659      <email></email>
4660      <uri></uri>
4661    </address>
4662  </author>
4663  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4664    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4665    <address>
4666      <email></email>
4667      <uri></uri>
4668    </address>
4669  </author>
4670  <date month='January' year='2005'></date>
4671 </front>
4672 <seriesInfo name="STD" value="66"/>
4673 <seriesInfo name="RFC" value="3986"/>
4676<reference anchor="RFC0793">
4677  <front>
4678    <title>Transmission Control Protocol</title>
4679    <author initials='J.' surname='Postel' fullname='Jon Postel'>
4680      <organization>University of Southern California (USC)/Information Sciences Institute</organization>
4681    </author>
4682    <date year='1981' month='September' />
4683  </front>
4684  <seriesInfo name='STD' value='7' />
4685  <seriesInfo name='RFC' value='793' />
4688<reference anchor="USASCII">
4689  <front>
4690    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4691    <author>
4692      <organization>American National Standards Institute</organization>
4693    </author>
4694    <date year="1986"/>
4695  </front>
4696  <seriesInfo name="ANSI" value="X3.4"/>
4699<reference anchor="RFC1950">
4700  <front>
4701    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4702    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4703      <organization>Aladdin Enterprises</organization>
4704      <address><email></email></address>
4705    </author>
4706    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4707    <date month="May" year="1996"/>
4708  </front>
4709  <seriesInfo name="RFC" value="1950"/>
4710  <!--<annotation>
4711    RFC 1950 is an Informational RFC, thus it might be less stable than
4712    this specification. On the other hand, this downward reference was
4713    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4714    therefore it is unlikely to cause problems in practice. See also
4715    <xref target="BCP97"/>.
4716  </annotation>-->
4719<reference anchor="RFC1951">
4720  <front>
4721    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4722    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4723      <organization>Aladdin Enterprises</organization>
4724      <address><email></email></address>
4725    </author>
4726    <date month="May" year="1996"/>
4727  </front>
4728  <seriesInfo name="RFC" value="1951"/>
4729  <!--<annotation>
4730    RFC 1951 is an Informational RFC, thus it might be less stable than
4731    this specification. On the other hand, this downward reference was
4732    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4733    therefore it is unlikely to cause problems in practice. See also
4734    <xref target="BCP97"/>.
4735  </annotation>-->
4738<reference anchor="RFC1952">
4739  <front>
4740    <title>GZIP file format specification version 4.3</title>
4741    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4742      <organization>Aladdin Enterprises</organization>
4743      <address><email></email></address>
4744    </author>
4745    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4746      <address><email></email></address>
4747    </author>
4748    <author initials="M." surname="Adler" fullname="Mark Adler">
4749      <address><email></email></address>
4750    </author>
4751    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4752      <address><email></email></address>
4753    </author>
4754    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4755      <address><email></email></address>
4756    </author>
4757    <date month="May" year="1996"/>
4758  </front>
4759  <seriesInfo name="RFC" value="1952"/>
4760  <!--<annotation>
4761    RFC 1952 is an Informational RFC, thus it might be less stable than
4762    this specification. On the other hand, this downward reference was
4763    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4764    therefore it is unlikely to cause problems in practice. See also
4765    <xref target="BCP97"/>.
4766  </annotation>-->
4769<reference anchor="Welch">
4770  <front>
4771    <title>A Technique for High Performance Data Compression</title>
4772    <author initials="T.A." surname="Welch" fullname="Terry A. Welch"/>
4773    <date month="June" year="1984"/>
4774  </front>
4775  <seriesInfo name="IEEE Computer" value="17(6)"/>
4780<references title="Informative References">
4782<reference anchor="ISO-8859-1">
4783  <front>
4784    <title>
4785     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4786    </title>
4787    <author>
4788      <organization>International Organization for Standardization</organization>
4789    </author>
4790    <date year="1998"/>
4791  </front>
4792  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4795<reference anchor='RFC1919'>
4796  <front>
4797    <title>Classical versus Transparent IP Proxies</title>
4798    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4799      <address><email></email></address>
4800    </author>
4801    <date year='1996' month='March' />
4802  </front>
4803  <seriesInfo name='RFC' value='1919' />
4806<reference anchor="RFC1945">
4807  <front>
4808    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4809    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4810      <organization>MIT, Laboratory for Computer Science</organization>
4811      <address><email></email></address>
4812    </author>
4813    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4814      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4815      <address><email></email></address>
4816    </author>
4817    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4818      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4819      <address><email></email></address>
4820    </author>
4821    <date month="May" year="1996"/>
4822  </front>
4823  <seriesInfo name="RFC" value="1945"/>
4826<reference anchor="RFC2045">
4827  <front>
4828    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4829    <author initials="N." surname="Freed" fullname="Ned Freed">
4830      <organization>Innosoft International, Inc.</organization>
4831      <address><email></email></address>
4832    </author>
4833    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4834      <organization>First Virtual Holdings</organization>
4835      <address><email></email></address>
4836    </author>
4837    <date month="November" year="1996"/>
4838  </front>
4839  <seriesInfo name="RFC" value="2045"/>
4842<reference anchor="RFC2047">
4843  <front>
4844    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4845    <author initials="K." surname="Moore" fullname="Keith Moore">
4846      <organization>University of Tennessee</organization>
4847      <address><email></email></address>
4848    </author>
4849    <date month="November" year="1996"/>
4850  </front>
4851  <seriesInfo name="RFC" value="2047"/>
4854<reference anchor="RFC2068">
4855  <front>
4856    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4857    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
4858      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4859      <address><email></email></address>
4860    </author>
4861    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4862      <organization>MIT Laboratory for Computer Science</organization>
4863      <address><email></email></address>
4864    </author>
4865    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4866      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4867      <address><email></email></address>
4868    </author>
4869    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4870      <organization>MIT Laboratory for Computer Science</organization>
4871      <address><email></email></address>
4872    </author>
4873    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4874      <organization>MIT Laboratory for Computer Science</organization>
4875      <address><email></email></address>
4876    </author>
4877    <date month="January" year="1997"/>
4878  </front>
4879  <seriesInfo name="RFC" value="2068"/>
4882<reference anchor="RFC2145">
4883  <front>
4884    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4885    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4886      <organization>Western Research Laboratory</organization>
4887      <address><email></email></address>
4888    </author>
4889    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4890      <organization>Department of Information and Computer Science</organization>
4891      <address><email></email></address>
4892    </author>
4893    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4894      <organization>MIT Laboratory for Computer Science</organization>
4895      <address><email></email></address>
4896    </author>
4897    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4898      <organization>W3 Consortium</organization>
4899      <address><email></email></address>
4900    </author>
4901    <date month="May" year="1997"/>
4902  </front>
4903  <seriesInfo name="RFC" value="2145"/>
4906<reference anchor="RFC2616">
4907  <front>
4908    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4909    <author initials="R." surname="Fielding" fullname="R. Fielding">
4910      <organization>University of California, Irvine</organization>
4911      <address><email></email></address>
4912    </author>
4913    <author initials="J." surname="Gettys" fullname="J. Gettys">
4914      <organization>W3C</organization>
4915      <address><email></email></address>
4916    </author>
4917    <author initials="J." surname="Mogul" fullname="J. Mogul">
4918      <organization>Compaq Computer Corporation</organization>
4919      <address><email></email></address>
4920    </author>
4921    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4922      <organization>MIT Laboratory for Computer Science</organization>
4923      <address><email></email></address>
4924    </author>
4925    <author initials="L." surname="Masinter" fullname="L. Masinter">
4926      <organization>Xerox Corporation</organization>
4927      <address><email></email></address>
4928    </author>
4929    <author initials="P." surname="Leach" fullname="P. Leach">
4930      <organization>Microsoft Corporation</organization>
4931      <address><email></email></address>
4932    </author>
4933    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4934      <organization>W3C</organization>
4935      <address><email></email></address>
4936    </author>
4937    <date month="June" year="1999"/>
4938  </front>
4939  <seriesInfo name="RFC" value="2616"/>
4942<reference anchor='RFC2817'>
4943  <front>
4944    <title>Upgrading to TLS Within HTTP/1.1</title>
4945    <author initials='R.' surname='Khare' fullname='R. Khare'>
4946      <organization>4K Associates / UC Irvine</organization>
4947      <address><email></email></address>
4948    </author>
4949    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4950      <organization>Agranat Systems, Inc.</organization>
4951      <address><email></email></address>
4952    </author>
4953    <date year='2000' month='May' />
4954  </front>
4955  <seriesInfo name='RFC' value='2817' />
4958<reference anchor='RFC2818'>
4959  <front>
4960    <title>HTTP Over TLS</title>
4961    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4962      <organization>RTFM, Inc.</organization>
4963      <address><email></email></address>
4964    </author>
4965    <date year='2000' month='May' />
4966  </front>
4967  <seriesInfo name='RFC' value='2818' />
4970<reference anchor='RFC3040'>
4971  <front>
4972    <title>Internet Web Replication and Caching Taxonomy</title>
4973    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4974      <organization>Equinix, Inc.</organization>
4975    </author>
4976    <author initials='I.' surname='Melve' fullname='I. Melve'>
4977      <organization>UNINETT</organization>
4978    </author>
4979    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4980      <organization>CacheFlow Inc.</organization>
4981    </author>
4982    <date year='2001' month='January' />
4983  </front>
4984  <seriesInfo name='RFC' value='3040' />
4987<reference anchor='BCP90'>
4988  <front>
4989    <title>Registration Procedures for Message Header Fields</title>
4990    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4991      <organization>Nine by Nine</organization>
4992      <address><email></email></address>
4993    </author>
4994    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4995      <organization>BEA Systems</organization>
4996      <address><email></email></address>
4997    </author>
4998    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4999      <organization>HP Labs</organization>
5000      <address><email></email></address>
5001    </author>
5002    <date year='2004' month='September' />
5003  </front>
5004  <seriesInfo name='BCP' value='90' />
5005  <seriesInfo name='RFC' value='3864' />
5008<reference anchor='RFC4033'>
5009  <front>
5010    <title>DNS Security Introduction and Requirements</title>
5011    <author initials='R.' surname='Arends' fullname='R. Arends'/>
5012    <author initials='R.' surname='Austein' fullname='R. Austein'/>
5013    <author initials='M.' surname='Larson' fullname='M. Larson'/>
5014    <author initials='D.' surname='Massey' fullname='D. Massey'/>
5015    <author initials='S.' surname='Rose' fullname='S. Rose'/>
5016    <date year='2005' month='March' />
5017  </front>
5018  <seriesInfo name='RFC' value='4033' />
5021<reference anchor="BCP13">
5022  <front>
5023    <title>Media Type Specifications and Registration Procedures</title>
5024    <author initials="N." surname="Freed" fullname="Ned Freed">
5025      <organization>Oracle</organization>
5026      <address>
5027        <email></email>
5028      </address>
5029    </author>
5030    <author initials="J." surname="Klensin" fullname="John C. Klensin">
5031      <address>
5032        <email></email>
5033      </address>
5034    </author>
5035    <author initials="T." surname="Hansen" fullname="Tony Hansen">
5036      <organization>AT&amp;T Laboratories</organization>
5037      <address>
5038        <email></email>
5039      </address>
5040    </author>
5041    <date year="2013" month="January"/>
5042  </front>
5043  <seriesInfo name="BCP" value="13"/>
5044  <seriesInfo name="RFC" value="6838"/>
5047<reference anchor='BCP115'>
5048  <front>
5049    <title>Guidelines and Registration Procedures for New URI Schemes</title>
5050    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
5051      <organization>AT&amp;T Laboratories</organization>
5052      <address>
5053        <email></email>
5054      </address>
5055    </author>
5056    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
5057      <organization>Qualcomm, Inc.</organization>
5058      <address>
5059        <email></email>
5060      </address>
5061    </author>
5062    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
5063      <organization>Adobe Systems</organization>
5064      <address>
5065        <email></email>
5066      </address>
5067    </author>
5068    <date year='2006' month='February' />
5069  </front>
5070  <seriesInfo name='BCP' value='115' />
5071  <seriesInfo name='RFC' value='4395' />
5074<reference anchor='RFC4559'>
5075  <front>
5076    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
5077    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
5078    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
5079    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
5080    <date year='2006' month='June' />
5081  </front>
5082  <seriesInfo name='RFC' value='4559' />
5085<reference anchor='RFC5226'>
5086  <front>
5087    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
5088    <author initials='T.' surname='Narten' fullname='T. Narten'>
5089      <organization>IBM</organization>
5090      <address><email></email></address>
5091    </author>
5092    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
5093      <organization>Google</organization>
5094      <address><email></email></address>
5095    </author>
5096    <date year='2008' month='May' />
5097  </front>
5098  <seriesInfo name='BCP' value='26' />
5099  <seriesInfo name='RFC' value='5226' />
5102<reference anchor='RFC5246'>
5103   <front>
5104      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
5105      <author initials='T.' surname='Dierks' fullname='T. Dierks'/>
5106      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
5107         <organization>RTFM, Inc.</organization>
5108      </author>
5109      <date year='2008' month='August' />
5110   </front>
5111   <seriesInfo name='RFC' value='5246' />
5114<reference anchor="RFC5322">
5115  <front>
5116    <title>Internet Message Format</title>
5117    <author initials="P." surname="Resnick" fullname="P. Resnick">
5118      <organization>Qualcomm Incorporated</organization>
5119    </author>
5120    <date year="2008" month="October"/>
5121  </front>
5122  <seriesInfo name="RFC" value="5322"/>
5125<reference anchor="RFC6265">
5126  <front>
5127    <title>HTTP State Management Mechanism</title>
5128    <author initials="A." surname="Barth" fullname="Adam Barth">
5129      <organization abbrev="U.C. Berkeley">
5130        University of California, Berkeley
5131      </organization>
5132      <address><email></email></address>
5133    </author>
5134    <date year="2011" month="April" />
5135  </front>
5136  <seriesInfo name="RFC" value="6265"/>
5139<reference anchor='RFC6585'>
5140  <front>
5141    <title>Additional HTTP Status Codes</title>
5142    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
5143      <organization>Rackspace</organization>
5144    </author>
5145    <author initials='R.' surname='Fielding' fullname='R. Fielding'>
5146      <organization>Adobe</organization>
5147    </author>
5148    <date year='2012' month='April' />
5149   </front>
5150   <seriesInfo name='RFC' value='6585' />
5153<!--<reference anchor='BCP97'>
5154  <front>
5155    <title>Handling Normative References to Standards-Track Documents</title>
5156    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
5157      <address>
5158        <email></email>
5159      </address>
5160    </author>
5161    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
5162      <organization>MIT</organization>
5163      <address>
5164        <email></email>
5165      </address>
5166    </author>
5167    <date year='2007' month='June' />
5168  </front>
5169  <seriesInfo name='BCP' value='97' />
5170  <seriesInfo name='RFC' value='4897' />
5173<reference anchor="Kri2001" target="">
5174  <front>
5175    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
5176    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
5177    <date year="2001" month="November"/>
5178  </front>
5179  <seriesInfo name="ACM Transactions on Internet Technology" value="1(2)"/>
5182<reference anchor="Klein" target="">
5183  <front>
5184    <title>Divide and Conquer - HTTP Response Splitting, Web Cache Poisoning Attacks, and Related Topics</title>
5185    <author initials="A." surname="Klein" fullname="Amit Klein">
5186      <organization>Sanctum, Inc.</organization>
5187    </author>
5188    <date year="2004" month="March"/>
5189  </front>
5192<reference anchor="Georgiev" target="">
5193  <front>
5194    <title>The Most Dangerous Code in the World: Validating SSL Certificates in Non-browser Software</title>
5195    <author initials="M." surname="Georgiev" fullname="Martin Georgiev"/>
5196    <author initials="S." surname="Iyengar" fullname="Subodh Iyengar"/>
5197    <author initials="S." surname="Jana" fullname="Suman Jana"/>
5198    <author initials="R." surname="Anubhai" fullname="Rishita Anubhai"/>
5199    <author initials="D." surname="Boneh" fullname="Dan Boneh"/>
5200    <author initials="V." surname="Shmatikov" fullname="Vitaly Shmatikov"/>
5201    <date year="2012" month="October"/>
5202  </front>
5203  <x:prose>In Proceedings of the 2012 ACM Conference on Computer and Communications Security (CCS '12), pp. 38-49</x:prose>
5206<reference anchor="Linhart" target="">
5207  <front>
5208    <title>HTTP Request Smuggling</title>
5209    <author initials="C." surname="Linhart" fullname="Chaim Linhart"/>
5210    <author initials="A." surname="Klein" fullname="Amit Klein"/>
5211    <author initials="R." surname="Heled" fullname="Ronen Heled"/>
5212    <author initials="S." surname="Orrin" fullname="Steve Orrin"/>
5213    <date year="2005" month="June"/>
5214  </front>
5220<section title="HTTP Version History" anchor="compatibility">
5222   HTTP has been in use since 1990. The first version, later referred to as
5223   HTTP/0.9, was a simple protocol for hypertext data transfer across the
5224   Internet, using only a single request method (GET) and no metadata.
5225   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
5226   methods and MIME-like messaging, allowing for metadata to be transferred
5227   and modifiers placed on the request/response semantics. However,
5228   HTTP/1.0 did not sufficiently take into consideration the effects of
5229   hierarchical proxies, caching, the need for persistent connections, or
5230   name-based virtual hosts. The proliferation of incompletely-implemented
5231   applications calling themselves "HTTP/1.0" further necessitated a
5232   protocol version change in order for two communicating applications
5233   to determine each other's true capabilities.
5236   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
5237   requirements that enable reliable implementations, adding only
5238   those features that can either be safely ignored by an HTTP/1.0
5239   recipient or only sent when communicating with a party advertising
5240   conformance with HTTP/1.1.
5243   HTTP/1.1 has been designed to make supporting previous versions easy.
5244   A general-purpose HTTP/1.1 server ought to be able to understand any valid
5245   request in the format of HTTP/1.0, responding appropriately with an
5246   HTTP/1.1 message that only uses features understood (or safely ignored) by
5247   HTTP/1.0 clients. Likewise, an HTTP/1.1 client can be expected to
5248   understand any valid HTTP/1.0 response.
5251   Since HTTP/0.9 did not support header fields in a request, there is no
5252   mechanism for it to support name-based virtual hosts (selection of resource
5253   by inspection of the <x:ref>Host</x:ref> header field).
5254   Any server that implements name-based virtual hosts ought to disable
5255   support for HTTP/0.9. Most requests that appear to be HTTP/0.9 are, in
5256   fact, badly constructed HTTP/1.x requests caused by a client failing to
5257   properly encode the request-target.
5260<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
5262   This section summarizes major differences between versions HTTP/1.0
5263   and HTTP/1.1.
5266<section title="Multi-homed Web Servers" anchor="">
5268   The requirements that clients and servers support the <x:ref>Host</x:ref>
5269   header field (<xref target=""/>), report an error if it is
5270   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
5271   are among the most important changes defined by HTTP/1.1.
5274   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
5275   addresses and servers; there was no other established mechanism for
5276   distinguishing the intended server of a request than the IP address
5277   to which that request was directed. The <x:ref>Host</x:ref> header field was
5278   introduced during the development of HTTP/1.1 and, though it was
5279   quickly implemented by most HTTP/1.0 browsers, additional requirements
5280   were placed on all HTTP/1.1 requests in order to ensure complete
5281   adoption.  At the time of this writing, most HTTP-based services
5282   are dependent upon the Host header field for targeting requests.
5286<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
5288   In HTTP/1.0, each connection is established by the client prior to the
5289   request and closed by the server after sending the response. However, some
5290   implementations implement the explicitly negotiated ("Keep-Alive") version
5291   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
5292   target="RFC2068"/>.
5295   Some clients and servers might wish to be compatible with these previous
5296   approaches to persistent connections, by explicitly negotiating for them
5297   with a "Connection: keep-alive" request header field. However, some
5298   experimental implementations of HTTP/1.0 persistent connections are faulty;
5299   for example, if an HTTP/1.0 proxy server doesn't understand
5300   <x:ref>Connection</x:ref>, it will erroneously forward that header field
5301   to the next inbound server, which would result in a hung connection.
5304   One attempted solution was the introduction of a Proxy-Connection header
5305   field, targeted specifically at proxies. In practice, this was also
5306   unworkable, because proxies are often deployed in multiple layers, bringing
5307   about the same problem discussed above.
5310   As a result, clients are encouraged not to send the Proxy-Connection header
5311   field in any requests.
5314   Clients are also encouraged to consider the use of Connection: keep-alive
5315   in requests carefully; while they can enable persistent connections with
5316   HTTP/1.0 servers, clients using them will need to monitor the
5317   connection for "hung" requests (which indicate that the client ought stop
5318   sending the header field), and this mechanism ought not be used by clients
5319   at all when a proxy is being used.
5323<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
5325   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
5326   (<xref target="header.transfer-encoding"/>).
5327   Transfer codings need to be decoded prior to forwarding an HTTP message
5328   over a MIME-compliant protocol.
5334<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
5336  HTTP's approach to error handling has been explained.
5337  (<xref target="conformance" />)
5340  The HTTP-version ABNF production has been clarified to be case-sensitive.
5341  Additionally, version numbers has been restricted to single digits, due
5342  to the fact that implementations are known to handle multi-digit version
5343  numbers incorrectly.
5344  (<xref target="http.version"/>)
5347  Userinfo (i.e., username and password) are now disallowed in HTTP and
5348  HTTPS URIs, because of security issues related to their transmission on the
5349  wire.
5350  (<xref target="http.uri" />)
5353  The HTTPS URI scheme is now defined by this specification; previously,
5354  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
5355  Furthermore, it implies end-to-end security.
5356  (<xref target="https.uri"/>)
5359  HTTP messages can be (and often are) buffered by implementations; despite
5360  it sometimes being available as a stream, HTTP is fundamentally a
5361  message-oriented protocol.
5362  Minimum supported sizes for various protocol elements have been
5363  suggested, to improve interoperability.
5364  (<xref target="http.message" />)
5367  Invalid whitespace around field-names is now required to be rejected,
5368  because accepting it represents a security vulnerability.
5369  The ABNF productions defining header fields now only list the field value.
5370  (<xref target="header.fields"/>)
5373  Rules about implicit linear whitespace between certain grammar productions
5374  have been removed; now whitespace is only allowed where specifically
5375  defined in the ABNF.
5376  (<xref target="whitespace"/>)
5379  Header fields that span multiple lines ("line folding") are deprecated.
5380  (<xref target="field.parsing" />)
5383  The NUL octet is no longer allowed in comment and quoted-string text, and
5384  handling of backslash-escaping in them has been clarified.
5385  The quoted-pair rule no longer allows escaping control characters other than
5386  HTAB.
5387  Non-ASCII content in header fields and the reason phrase has been obsoleted
5388  and made opaque (the TEXT rule was removed).
5389  (<xref target="field.components"/>)
5392  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
5393  handled as errors by recipients.
5394  (<xref target="header.content-length"/>)
5397  The algorithm for determining the message body length has been clarified
5398  to indicate all of the special cases (e.g., driven by methods or status
5399  codes) that affect it, and that new protocol elements cannot define such
5400  special cases.
5401  CONNECT is a new, special case in determining message body length.
5402  "multipart/byteranges" is no longer a way of determining message body length
5403  detection.
5404  (<xref target="message.body.length"/>)
5407  The "identity" transfer coding token has been removed.
5408  (Sections <xref format="counter" target="message.body"/> and
5409  <xref format="counter" target="transfer.codings"/>)
5412  Chunk length does not include the count of the octets in the
5413  chunk header and trailer.
5414  Line folding in chunk extensions is  disallowed.
5415  (<xref target="chunked.encoding"/>)
5418  The meaning of the "deflate" content coding has been clarified.
5419  (<xref target="deflate.coding" />)
5422  The segment + query components of RFC 3986 have been used to define the
5423  request-target, instead of abs_path from RFC 1808.
5424  The asterisk-form of the request-target is only allowed with the OPTIONS
5425  method.
5426  (<xref target="request-target"/>)
5429  The term "Effective Request URI" has been introduced.
5430  (<xref target="effective.request.uri" />)
5433  Gateways do not need to generate <x:ref>Via</x:ref> header fields anymore.
5434  (<xref target="header.via"/>)
5437  Exactly when "close" connection options have to be sent has been clarified.
5438  Also, "hop-by-hop" header fields are required to appear in the Connection header
5439  field; just because they're defined as hop-by-hop in this specification
5440  doesn't exempt them.
5441  (<xref target="header.connection"/>)
5444  The limit of two connections per server has been removed.
5445  An idempotent sequence of requests is no longer required to be retried.
5446  The requirement to retry requests under certain circumstances when the
5447  server prematurely closes the connection has been removed.
5448  Also, some extraneous requirements about when servers are allowed to close
5449  connections prematurely have been removed.
5450  (<xref target="persistent.connections"/>)
5453  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
5454  responses other than 101 (this was incorporated from <xref
5455  target="RFC2817"/>). Furthermore, the ordering in the field value is now
5456  significant.
5457  (<xref target="header.upgrade"/>)
5460  Empty list elements in list productions (e.g., a list header field containing
5461  ", ,") have been deprecated.
5462  (<xref target="abnf.extension"/>)
5465  Registration of Transfer Codings now requires IETF Review
5466  (<xref target="transfer.coding.registry"/>)
5469  This specification now defines the Upgrade Token Registry, previously
5470  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
5471  (<xref target="upgrade.token.registry"/>)
5474  The expectation to support HTTP/0.9 requests has been removed.
5475  (<xref target="compatibility"/>)
5478  Issues with the Keep-Alive and Proxy-Connection header fields in requests
5479  are pointed out, with use of the latter being discouraged altogether.
5480  (<xref target="compatibility.with.http.1.0.persistent.connections" />)
5485<?BEGININC p1-messaging.abnf-appendix ?>
5486<section xmlns:x="" title="Collected ABNF" anchor="collected.abnf">
5488<artwork type="abnf" name="p1-messaging.parsed-abnf">
5489<x:ref>BWS</x:ref> = OWS
5491<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
5492 connection-option ] )
5493<x:ref>Content-Length</x:ref> = 1*DIGIT
5495<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
5496 ]
5497<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
5498<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
5499<x:ref>Host</x:ref> = uri-host [ ":" port ]
5501<x:ref>OWS</x:ref> = *( SP / HTAB )
5503<x:ref>RWS</x:ref> = 1*( SP / HTAB )
5505<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
5506<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
5507<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
5508 transfer-coding ] )
5510<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
5511<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
5513<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
5514 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
5515 comment ] ) ] )
5517<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
5518<x:ref>absolute-form</x:ref> = absolute-URI
5519<x:ref>absolute-path</x:ref> = 1*( "/" segment )
5520<x:ref>asterisk-form</x:ref> = "*"
5521<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
5522<x:ref>authority-form</x:ref> = authority
5524<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
5525<x:ref>chunk-data</x:ref> = 1*OCTET
5526<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
5527<x:ref>chunk-ext-name</x:ref> = token
5528<x:ref>chunk-ext-val</x:ref> = token / quoted-string
5529<x:ref>chunk-size</x:ref> = 1*HEXDIG
5530<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
5531<x:ref>comment</x:ref> = "(" *( ctext / quoted-pair / comment ) ")"
5532<x:ref>connection-option</x:ref> = token
5533<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
5534 / %x2A-5B ; '*'-'['
5535 / %x5D-7E ; ']'-'~'
5536 / obs-text
5538<x:ref>field-content</x:ref> = field-vchar [ 1*( SP / HTAB ) field-vchar ]
5539<x:ref>field-name</x:ref> = token
5540<x:ref>field-value</x:ref> = *( field-content / obs-fold )
5541<x:ref>field-vchar</x:ref> = VCHAR / obs-text
5542<x:ref>fragment</x:ref> = &lt;fragment, defined in [RFC3986], Section 3.5&gt;
5544<x:ref>header-field</x:ref> = field-name ":" OWS field-value OWS
5545<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ] [ "#"
5546 fragment ]
5547<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ] [ "#"
5548 fragment ]
5550<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
5552<x:ref>message-body</x:ref> = *OCTET
5553<x:ref>method</x:ref> = token
5555<x:ref>obs-fold</x:ref> = CRLF 1*( SP / HTAB )
5556<x:ref>obs-text</x:ref> = %x80-FF
5557<x:ref>origin-form</x:ref> = absolute-path [ "?" query ]
5559<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
5560<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
5561<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
5562<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
5563<x:ref>protocol-name</x:ref> = token
5564<x:ref>protocol-version</x:ref> = token
5565<x:ref>pseudonym</x:ref> = token
5567<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5568 / %x5D-7E ; ']'-'~'
5569 / obs-text
5570<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5571<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5572<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5574<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5575<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5576<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5577<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5578<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5579<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5580<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5581 asterisk-form
5583<x:ref>scheme</x:ref> = &lt;scheme, defined in [RFC3986], Section 3.1&gt;
5584<x:ref>segment</x:ref> = &lt;segment, defined in [RFC3986], Section 3.3&gt;
5585<x:ref>start-line</x:ref> = request-line / status-line
5586<x:ref>status-code</x:ref> = 3DIGIT
5587<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5589<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5590<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5591<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5592 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5593<x:ref>token</x:ref> = 1*tchar
5594<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5595<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5596 transfer-extension
5597<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5598<x:ref>transfer-parameter</x:ref> = token BWS "=" BWS ( token / quoted-string )
5600<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5604<?ENDINC p1-messaging.abnf-appendix ?>
5606<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5608<section title="Since RFC 2616">
5610  Changes up to the IETF Last Call draft are summarized
5611  in <eref target=""/>.
5615<section title="Since draft-ietf-httpbis-p1-messaging-24" anchor="changes.since.24">
5617  Closed issues:
5618  <list style="symbols">
5619    <t>
5620      <eref target=""/>:
5621      "APPSDIR review of draft-ietf-httpbis-p1-messaging-24"
5622    </t>
5623    <t>
5624      <eref target=""/>:
5625      "integer value parsing"
5626    </t>
5627    <t>
5628      <eref target=""/>:
5629      "move IANA registrations to correct draft"
5630    </t>
5631  </list>
5635<section title="Since draft-ietf-httpbis-p1-messaging-25" anchor="changes.since.25">
5637  Closed issues:
5638  <list style="symbols">
5639    <t>
5640      <eref target=""/>:
5641      "check media type registration templates"
5642    </t>
5643    <t>
5644      <eref target=""/>:
5645      "Redundant rule quoted-str-nf"
5646    </t>
5647    <t>
5648      <eref target=""/>:
5649      "add 'stateless' to Abstract"
5650    </t>
5651    <t>
5652      <eref target=""/>:
5653      "clarify ABNF layering"
5654    </t>
5655    <t>
5656      <eref target=""/>:
5657      "use of 'word' ABNF production"
5658    </t>
5659    <t>
5660      <eref target=""/>:
5661      "improve introduction of list rule"
5662    </t>
5663    <t>
5664      <eref target=""/>:
5665      "moving 2616/2068/2145 to historic"
5666    </t>
5667    <t>
5668      <eref target=""/>:
5669      "augment security considerations with pointers to current research"
5670    </t>
5671    <t>
5672      <eref target=""/>:
5673      "intermediaries handling trailers"
5674    </t>
5675  </list>
5678  Partly resolved issues:
5679  <list style="symbols">
5680    <t>
5681      <eref target=""/>:
5682      "IESG ballot on draft-ietf-httpbis-p1-messaging-25"
5683    </t>
5684  </list>
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