source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2605

Last change on this file since 2605 was 2605, checked in by fielding@…, 7 years ago

(editorial) remove redundant ought to receive unbounded lengths that is covered by 2.5; note the security consideration regarding ignored header fields; see #531

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 238.4 KB
1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns=''>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns=''>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns=''>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns=''>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns=''>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns=''>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns=''>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns=''>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns=''>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns=''>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "January">
16  <!ENTITY ID-YEAR "2014">
17  <!ENTITY mdash "&#8212;">
18  <!ENTITY Note "<x:h xmlns:x=''>Note:</x:h>">
19  <!ENTITY caching-overview       "<xref target='Part6' x:rel='#caching.overview' xmlns:x=''/>">
20  <!ENTITY cache-incomplete       "<xref target='Part6' x:rel='#response.cacheability' xmlns:x=''/>">
21  <!ENTITY payload                "<xref target='Part2' x:rel='#payload' xmlns:x=''/>">
22  <!ENTITY media-type            "<xref target='Part2' x:rel='#media.type' xmlns:x=''/>">
23  <!ENTITY content-codings        "<xref target='Part2' x:rel='#content.codings' xmlns:x=''/>">
24  <!ENTITY CONNECT                "<xref target='Part2' x:rel='#CONNECT' xmlns:x=''/>">
25  <!ENTITY content.negotiation    "<xref target='Part2' x:rel='#content.negotiation' xmlns:x=''/>">
26  <!ENTITY diff-mime              "<xref target='Part2' x:rel='#differences.between.http.and.mime' xmlns:x=''/>">
27  <!ENTITY representation         "<xref target='Part2' x:rel='#representations' xmlns:x=''/>">
28  <!ENTITY GET                    "<xref target='Part2' x:rel='#GET' xmlns:x=''/>">
29  <!ENTITY HEAD                   "<xref target='Part2' x:rel='#HEAD' xmlns:x=''/>">
30  <!ENTITY header-allow           "<xref target='Part2' x:rel='#header.allow' xmlns:x=''/>">
31  <!ENTITY header-cache-control   "<xref target='Part6' x:rel='#header.cache-control' xmlns:x=''/>">
32  <!ENTITY header-content-encoding    "<xref target='Part2' x:rel='#header.content-encoding' xmlns:x=''/>">
33  <!ENTITY header-content-location    "<xref target='Part2' x:rel='#header.content-location' xmlns:x=''/>">
34  <!ENTITY header-content-range   "<xref target='Part5' x:rel='#header.content-range' xmlns:x=''/>">
35  <!ENTITY header-content-type    "<xref target='Part2' x:rel='#header.content-type' xmlns:x=''/>">
36  <!ENTITY header-date            "<xref target='Part2' x:rel='' xmlns:x=''/>">
37  <!ENTITY header-etag            "<xref target='Part4' x:rel='#header.etag' xmlns:x=''/>">
38  <!ENTITY header-expect          "<xref target='Part2' x:rel='#header.expect' xmlns:x=''/>">
39  <!ENTITY header-expires         "<xref target='Part6' x:rel='#header.expires' xmlns:x=''/>">
40  <!ENTITY header-last-modified   "<xref target='Part4' x:rel='#header.last-modified' xmlns:x=''/>">
41  <!ENTITY header-mime-version    "<xref target='Part2' x:rel='#mime-version' xmlns:x=''/>">
42  <!ENTITY header-pragma          "<xref target='Part6' x:rel='#header.pragma' xmlns:x=''/>">
43  <!ENTITY header-proxy-authenticate  "<xref target='Part7' x:rel='#header.proxy-authenticate' xmlns:x=''/>">
44  <!ENTITY header-proxy-authorization "<xref target='Part7' x:rel='#header.proxy-authorization' xmlns:x=''/>">
45  <!ENTITY header-server          "<xref target='Part2' x:rel='#header.server' xmlns:x=''/>">
46  <!ENTITY header-warning         "<xref target='Part6' x:rel='#header.warning' xmlns:x=''/>">
47  <!ENTITY idempotent-methods     "<xref target='Part2' x:rel='#idempotent.methods' xmlns:x=''/>">
48  <!ENTITY safe-methods           "<xref target='Part2' x:rel='#safe.methods' xmlns:x=''/>">
49  <!ENTITY methods                "<xref target='Part2' x:rel='#methods' xmlns:x=''/>">
50  <!ENTITY OPTIONS                "<xref target='Part2' x:rel='#OPTIONS' xmlns:x=''/>">
51  <!ENTITY qvalue                 "<xref target='Part2' x:rel='#quality.values' xmlns:x=''/>">
52  <!ENTITY request-header-fields  "<xref target='Part2' x:rel='#request.header.fields' xmlns:x=''/>">
53  <!ENTITY response-control-data  "<xref target='Part2' x:rel='' xmlns:x=''/>">
54  <!ENTITY resource               "<xref target='Part2' x:rel='#resources' xmlns:x=''/>">
55  <!ENTITY semantics              "<xref target='Part2' xmlns:x=''/>">
56  <!ENTITY status-codes           "<xref target='Part2' x:rel='' xmlns:x=''/>">
57  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x=''/>">
58  <!ENTITY status-203             "<xref target='Part2' x:rel='#status.203' xmlns:x=''/>">
59  <!ENTITY status-3xx             "<xref target='Part2' x:rel='#status.3xx' xmlns:x=''/>">
60  <!ENTITY status-304             "<xref target='Part4' x:rel='#status.304' xmlns:x=''/>">
61  <!ENTITY status-4xx             "<xref target='Part2' x:rel='#status.4xx' xmlns:x=''/>">
62  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x=''/>">
63  <!ENTITY iana-header-registry   "<xref target='Part2' x:rel='#header.field.registry' xmlns:x=''/>">
65<?rfc toc="yes" ?>
66<?rfc symrefs="yes" ?>
67<?rfc sortrefs="yes" ?>
68<?rfc compact="yes"?>
69<?rfc subcompact="no" ?>
70<?rfc linkmailto="no" ?>
71<?rfc editing="no" ?>
72<?rfc comments="yes"?>
73<?rfc inline="yes"?>
74<?rfc rfcedstyle="yes"?>
75<?rfc-ext allow-markup-in-artwork="yes" ?>
76<?rfc-ext include-references-in-index="yes" ?>
77<rfc obsoletes="2145,2616" updates="2817,2818" category="std" x:maturity-level="proposed"
78     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
79     xmlns:x=''>
80<x:link rel="next" basename="p2-semantics"/>
81<x:feedback template="{docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
84  <title abbrev="HTTP/1.1 Message Syntax and Routing">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
86  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
87    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
88    <address>
89      <postal>
90        <street>345 Park Ave</street>
91        <city>San Jose</city>
92        <region>CA</region>
93        <code>95110</code>
94        <country>USA</country>
95      </postal>
96      <email></email>
97      <uri></uri>
98    </address>
99  </author>
101  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
102    <organization abbrev="greenbytes">greenbytes GmbH</organization>
103    <address>
104      <postal>
105        <street>Hafenweg 16</street>
106        <city>Muenster</city><region>NW</region><code>48155</code>
107        <country>Germany</country>
108      </postal>
109      <email></email>
110      <uri></uri>
111    </address>
112  </author>
114  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
115  <workgroup>HTTPbis Working Group</workgroup>
119   The Hypertext Transfer Protocol (HTTP) is a stateless application-level
120   protocol for distributed, collaborative, hypertext information systems.
121   This document provides an overview of HTTP architecture and its associated
122   terminology, defines the "http" and "https" Uniform Resource Identifier
123   (URI) schemes, defines the HTTP/1.1 message syntax and parsing
124   requirements, and describes related security concerns for implementations.
128<note title="Editorial Note (To be removed by RFC Editor)">
129  <t>
130    Discussion of this draft takes place on the HTTPBIS working group
131    mailing list (, which is archived at
132    <eref target=""/>.
133  </t>
134  <t>
135    The current issues list is at
136    <eref target=""/> and related
137    documents (including fancy diffs) can be found at
138    <eref target=""/>.
139  </t>
140  <t>
141    The changes in this draft are summarized in <xref target="changes.since.25"/>.
142  </t>
146<section title="Introduction" anchor="introduction">
148   The Hypertext Transfer Protocol (HTTP) is a stateless application-level
149   request/response protocol that uses extensible semantics and
150   self-descriptive message payloads for flexible interaction with
151   network-based hypertext information systems. This document is the first in
152   a series of documents that collectively form the HTTP/1.1 specification:
153   <list style="empty">
154    <t>RFC xxx1: Message Syntax and Routing</t>
155    <t><xref target="Part2" x:fmt="none">RFC xxx2</xref>: Semantics and Content</t>
156    <t><xref target="Part4" x:fmt="none">RFC xxx3</xref>: Conditional Requests</t>
157    <t><xref target="Part5" x:fmt="none">RFC xxx4</xref>: Range Requests</t>
158    <t><xref target="Part6" x:fmt="none">RFC xxx5</xref>: Caching</t>
159    <t><xref target="Part7" x:fmt="none">RFC xxx6</xref>: Authentication</t>
160   </list>
163   This HTTP/1.1 specification obsoletes
164   <xref target="RFC2616" x:fmt="none">RFC 2616</xref> and
165   <xref target="RFC2145" x:fmt="none">RFC 2145</xref> (on HTTP versioning).
166   This specification also updates the use of CONNECT to establish a tunnel,
167   previously defined in <xref target="RFC2817" x:fmt="none">RFC 2817</xref>,
168   and defines the "https" URI scheme that was described informally in
169   <xref target="RFC2818" x:fmt="none">RFC 2818</xref>.
172   HTTP is a generic interface protocol for information systems. It is
173   designed to hide the details of how a service is implemented by presenting
174   a uniform interface to clients that is independent of the types of
175   resources provided. Likewise, servers do not need to be aware of each
176   client's purpose: an HTTP request can be considered in isolation rather
177   than being associated with a specific type of client or a predetermined
178   sequence of application steps. The result is a protocol that can be used
179   effectively in many different contexts and for which implementations can
180   evolve independently over time.
183   HTTP is also designed for use as an intermediation protocol for translating
184   communication to and from non-HTTP information systems.
185   HTTP proxies and gateways can provide access to alternative information
186   services by translating their diverse protocols into a hypertext
187   format that can be viewed and manipulated by clients in the same way
188   as HTTP services.
191   One consequence of this flexibility is that the protocol cannot be
192   defined in terms of what occurs behind the interface. Instead, we
193   are limited to defining the syntax of communication, the intent
194   of received communication, and the expected behavior of recipients.
195   If the communication is considered in isolation, then successful
196   actions ought to be reflected in corresponding changes to the
197   observable interface provided by servers. However, since multiple
198   clients might act in parallel and perhaps at cross-purposes, we
199   cannot require that such changes be observable beyond the scope
200   of a single response.
203   This document describes the architectural elements that are used or
204   referred to in HTTP, defines the "http" and "https" URI schemes,
205   describes overall network operation and connection management,
206   and defines HTTP message framing and forwarding requirements.
207   Our goal is to define all of the mechanisms necessary for HTTP message
208   handling that are independent of message semantics, thereby defining the
209   complete set of requirements for message parsers and
210   message-forwarding intermediaries.
214<section title="Requirement Notation" anchor="intro.requirements">
216   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
217   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
218   document are to be interpreted as described in <xref target="RFC2119"/>.
221   Conformance criteria and considerations regarding error handling
222   are defined in <xref target="conformance"/>.
226<section title="Syntax Notation" anchor="notation">
227<iref primary="true" item="Grammar" subitem="ALPHA"/>
228<iref primary="true" item="Grammar" subitem="CR"/>
229<iref primary="true" item="Grammar" subitem="CRLF"/>
230<iref primary="true" item="Grammar" subitem="CTL"/>
231<iref primary="true" item="Grammar" subitem="DIGIT"/>
232<iref primary="true" item="Grammar" subitem="DQUOTE"/>
233<iref primary="true" item="Grammar" subitem="HEXDIG"/>
234<iref primary="true" item="Grammar" subitem="HTAB"/>
235<iref primary="true" item="Grammar" subitem="LF"/>
236<iref primary="true" item="Grammar" subitem="OCTET"/>
237<iref primary="true" item="Grammar" subitem="SP"/>
238<iref primary="true" item="Grammar" subitem="VCHAR"/>
240   This specification uses the Augmented Backus-Naur Form (ABNF) notation of
241   <xref target="RFC5234"/> with a list extension, defined in
242   <xref target="abnf.extension"/>, that allows for compact definition of
243   comma-separated lists using a '#' operator (similar to how the '*' operator
244   indicates repetition).
245   <xref target="collected.abnf"/> shows the collected grammar with all list
246   operators expanded to standard ABNF notation.
248<t anchor="core.rules">
249  <x:anchor-alias value="ALPHA"/>
250  <x:anchor-alias value="CTL"/>
251  <x:anchor-alias value="CR"/>
252  <x:anchor-alias value="CRLF"/>
253  <x:anchor-alias value="DIGIT"/>
254  <x:anchor-alias value="DQUOTE"/>
255  <x:anchor-alias value="HEXDIG"/>
256  <x:anchor-alias value="HTAB"/>
257  <x:anchor-alias value="LF"/>
258  <x:anchor-alias value="OCTET"/>
259  <x:anchor-alias value="SP"/>
260  <x:anchor-alias value="VCHAR"/>
261   The following core rules are included by
262   reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
263   ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
264   DIGIT (decimal 0-9), DQUOTE (double quote),
265   HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed),
266   OCTET (any 8-bit sequence of data), SP (space), and
267   VCHAR (any visible <xref target="USASCII"/> character).
270   As a convention, ABNF rule names prefixed with "obs-" denote
271   "obsolete" grammar rules that appear for historical reasons.
276<section title="Architecture" anchor="architecture">
278   HTTP was created for the World Wide Web (WWW) architecture
279   and has evolved over time to support the scalability needs of a worldwide
280   hypertext system. Much of that architecture is reflected in the terminology
281   and syntax productions used to define HTTP.
284<section title="Client/Server Messaging" anchor="operation">
285<iref primary="true" item="client"/>
286<iref primary="true" item="server"/>
287<iref primary="true" item="connection"/>
289   HTTP is a stateless request/response protocol that operates by exchanging
290   <x:dfn>messages</x:dfn> (<xref target="http.message"/>) across a reliable
291   transport or session-layer
292   "<x:dfn>connection</x:dfn>" (<xref target=""/>).
293   An HTTP "<x:dfn>client</x:dfn>" is a program that establishes a connection
294   to a server for the purpose of sending one or more HTTP requests.
295   An HTTP "<x:dfn>server</x:dfn>" is a program that accepts connections
296   in order to service HTTP requests by sending HTTP responses.
298<iref primary="true" item="user agent"/>
299<iref primary="true" item="origin server"/>
300<iref primary="true" item="browser"/>
301<iref primary="true" item="spider"/>
302<iref primary="true" item="sender"/>
303<iref primary="true" item="recipient"/>
305   The terms client and server refer only to the roles that
306   these programs perform for a particular connection.  The same program
307   might act as a client on some connections and a server on others.
308   The term "<x:dfn>user agent</x:dfn>" refers to any of the various
309   client programs that initiate a request, including (but not limited to)
310   browsers, spiders (web-based robots), command-line tools, custom
311   applications, and mobile apps.
312   The term "<x:dfn>origin server</x:dfn>" refers to the program that can
313   originate authoritative responses for a given target resource.
314   The terms "<x:dfn>sender</x:dfn>" and "<x:dfn>recipient</x:dfn>" refer to
315   any implementation that sends or receives a given message, respectively.
318   HTTP relies upon the Uniform Resource Identifier (URI)
319   standard <xref target="RFC3986"/> to indicate the target resource
320   (<xref target="target-resource"/>) and relationships between resources.
321   Messages are passed in a format similar to that used by Internet mail
322   <xref target="RFC5322"/> and the Multipurpose Internet Mail Extensions
323   (MIME) <xref target="RFC2045"/> (see &diff-mime; for the differences
324   between HTTP and MIME messages).
327   Most HTTP communication consists of a retrieval request (GET) for
328   a representation of some resource identified by a URI.  In the
329   simplest case, this might be accomplished via a single bidirectional
330   connection (===) between the user agent (UA) and the origin server (O).
332<figure><artwork type="drawing">
333         request   &gt;
334    <x:highlight>UA</x:highlight> ======================================= <x:highlight>O</x:highlight>
335                                &lt;   response
337<iref primary="true" item="message"/>
338<iref primary="true" item="request"/>
339<iref primary="true" item="response"/>
341   A client sends an HTTP request to a server in the form of a <x:dfn>request</x:dfn>
342   message, beginning with a request-line that includes a method, URI, and
343   protocol version (<xref target="request.line"/>),
344   followed by header fields containing
345   request modifiers, client information, and representation metadata
346   (<xref target="header.fields"/>),
347   an empty line to indicate the end of the header section, and finally
348   a message body containing the payload body (if any,
349   <xref target="message.body"/>).
352   A server responds to a client's request by sending one or more HTTP
353   <x:dfn>response</x:dfn>
354   messages, each beginning with a status line that
355   includes the protocol version, a success or error code, and textual
356   reason phrase (<xref target="status.line"/>),
357   possibly followed by header fields containing server
358   information, resource metadata, and representation metadata
359   (<xref target="header.fields"/>),
360   an empty line to indicate the end of the header section, and finally
361   a message body containing the payload body (if any,
362   <xref target="message.body"/>).
365   A connection might be used for multiple request/response exchanges,
366   as defined in <xref target="persistent.connections"/>.
369   The following example illustrates a typical message exchange for a
370   GET request (&GET;) on the URI "":
373Client request:
374</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
375GET /hello.txt HTTP/1.1
376User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
378Accept-Language: en, mi
382Server response:
383</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
384HTTP/1.1 200 OK
385Date: Mon, 27 Jul 2009 12:28:53 GMT
386Server: Apache
387Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
388ETag: "34aa387-d-1568eb00"
389Accept-Ranges: bytes
390Content-Length: <x:length-of target="exbody"/>
391Vary: Accept-Encoding
392Content-Type: text/plain
394<x:span anchor="exbody">Hello World! My payload includes a trailing CRLF.
399<section title="Implementation Diversity" anchor="implementation-diversity">
401   When considering the design of HTTP, it is easy to fall into a trap of
402   thinking that all user agents are general-purpose browsers and all origin
403   servers are large public websites. That is not the case in practice.
404   Common HTTP user agents include household appliances, stereos, scales,
405   firmware update scripts, command-line programs, mobile apps,
406   and communication devices in a multitude of shapes and sizes.  Likewise,
407   common HTTP origin servers include home automation units, configurable
408   networking components, office machines, autonomous robots, news feeds,
409   traffic cameras, ad selectors, and video delivery platforms.
412   The term "user agent" does not imply that there is a human user directly
413   interacting with the software agent at the time of a request. In many
414   cases, a user agent is installed or configured to run in the background
415   and save its results for later inspection (or save only a subset of those
416   results that might be interesting or erroneous). Spiders, for example, are
417   typically given a start URI and configured to follow certain behavior while
418   crawling the Web as a hypertext graph.
421   The implementation diversity of HTTP means that not all user agents can
422   make interactive suggestions to their user or provide adequate warning for
423   security or privacy concerns. In the few cases where this
424   specification requires reporting of errors to the user, it is acceptable
425   for such reporting to only be observable in an error console or log file.
426   Likewise, requirements that an automated action be confirmed by the user
427   before proceeding might be met via advance configuration choices,
428   run-time options, or simple avoidance of the unsafe action; confirmation
429   does not imply any specific user interface or interruption of normal
430   processing if the user has already made that choice.
434<section title="Intermediaries" anchor="intermediaries">
435<iref primary="true" item="intermediary"/>
437   HTTP enables the use of intermediaries to satisfy requests through
438   a chain of connections.  There are three common forms of HTTP
439   <x:dfn>intermediary</x:dfn>: proxy, gateway, and tunnel.  In some cases,
440   a single intermediary might act as an origin server, proxy, gateway,
441   or tunnel, switching behavior based on the nature of each request.
443<figure><artwork type="drawing">
444         &gt;             &gt;             &gt;             &gt;
445    <x:highlight>UA</x:highlight> =========== <x:highlight>A</x:highlight> =========== <x:highlight>B</x:highlight> =========== <x:highlight>C</x:highlight> =========== <x:highlight>O</x:highlight>
446               &lt;             &lt;             &lt;             &lt;
449   The figure above shows three intermediaries (A, B, and C) between the
450   user agent and origin server. A request or response message that
451   travels the whole chain will pass through four separate connections.
452   Some HTTP communication options
453   might apply only to the connection with the nearest, non-tunnel
454   neighbor, only to the end-points of the chain, or to all connections
455   along the chain. Although the diagram is linear, each participant might
456   be engaged in multiple, simultaneous communications. For example, B
457   might be receiving requests from many clients other than A, and/or
458   forwarding requests to servers other than C, at the same time that it
459   is handling A's request. Likewise, later requests might be sent through a
460   different path of connections, often based on dynamic configuration for
461   load balancing.   
464<iref primary="true" item="upstream"/><iref primary="true" item="downstream"/>
465<iref primary="true" item="inbound"/><iref primary="true" item="outbound"/>
466   The terms "<x:dfn>upstream</x:dfn>" and "<x:dfn>downstream</x:dfn>" are
467   used to describe directional requirements in relation to the message flow:
468   all messages flow from upstream to downstream.
469   The terms inbound and outbound are used to describe directional
470   requirements in relation to the request route:
471   "<x:dfn>inbound</x:dfn>" means toward the origin server and
472   "<x:dfn>outbound</x:dfn>" means toward the user agent.
474<t><iref primary="true" item="proxy"/>
475   A "<x:dfn>proxy</x:dfn>" is a message forwarding agent that is selected by the
476   client, usually via local configuration rules, to receive requests
477   for some type(s) of absolute URI and attempt to satisfy those
478   requests via translation through the HTTP interface.  Some translations
479   are minimal, such as for proxy requests for "http" URIs, whereas
480   other requests might require translation to and from entirely different
481   application-level protocols. Proxies are often used to group an
482   organization's HTTP requests through a common intermediary for the
483   sake of security, annotation services, or shared caching. Some proxies
484   are designed to apply transformations to selected messages or payloads
485   while they are being forwarded, as described in
486   <xref target="message.transformations"/>.
488<t><iref primary="true" item="gateway"/><iref primary="true" item="reverse proxy"/>
489<iref primary="true" item="accelerator"/>
490   A "<x:dfn>gateway</x:dfn>" (a.k.a., "<x:dfn>reverse proxy</x:dfn>") is an
491   intermediary that acts as an origin server for the outbound connection, but
492   translates received requests and forwards them inbound to another server or
493   servers. Gateways are often used to encapsulate legacy or untrusted
494   information services, to improve server performance through
495   "<x:dfn>accelerator</x:dfn>" caching, and to enable partitioning or load
496   balancing of HTTP services across multiple machines.
499   All HTTP requirements applicable to an origin server
500   also apply to the outbound communication of a gateway.
501   A gateway communicates with inbound servers using any protocol that
502   it desires, including private extensions to HTTP that are outside
503   the scope of this specification.  However, an HTTP-to-HTTP gateway
504   that wishes to interoperate with third-party HTTP servers ought to conform
505   to user agent requirements on the gateway's inbound connection.
507<t><iref primary="true" item="tunnel"/>
508   A "<x:dfn>tunnel</x:dfn>" acts as a blind relay between two connections
509   without changing the messages. Once active, a tunnel is not
510   considered a party to the HTTP communication, though the tunnel might
511   have been initiated by an HTTP request. A tunnel ceases to exist when
512   both ends of the relayed connection are closed. Tunnels are used to
513   extend a virtual connection through an intermediary, such as when
514   Transport Layer Security (TLS, <xref target="RFC5246"/>) is used to
515   establish confidential communication through a shared firewall proxy.
518   The above categories for intermediary only consider those acting as
519   participants in the HTTP communication.  There are also intermediaries
520   that can act on lower layers of the network protocol stack, filtering or
521   redirecting HTTP traffic without the knowledge or permission of message
522   senders. Network intermediaries are indistinguishable (at a protocol level)
523   from a man-in-the-middle attack, often introducing security flaws or
524   interoperability problems due to mistakenly violating HTTP semantics.
526<t><iref primary="true" item="interception proxy"/>
527<iref primary="true" item="transparent proxy"/>
528<iref primary="true" item="captive portal"/>
529   For example, an
530   "<x:dfn>interception proxy</x:dfn>" <xref target="RFC3040"/> (also commonly
531   known as a "<x:dfn>transparent proxy</x:dfn>" <xref target="RFC1919"/> or
532   "<x:dfn>captive portal</x:dfn>")
533   differs from an HTTP proxy because it is not selected by the client.
534   Instead, an interception proxy filters or redirects outgoing TCP port 80
535   packets (and occasionally other common port traffic).
536   Interception proxies are commonly found on public network access points,
537   as a means of enforcing account subscription prior to allowing use of
538   non-local Internet services, and within corporate firewalls to enforce
539   network usage policies.
542   HTTP is defined as a stateless protocol, meaning that each request message
543   can be understood in isolation.  Many implementations depend on HTTP's
544   stateless design in order to reuse proxied connections or dynamically
545   load-balance requests across multiple servers.  Hence, a server &MUST-NOT;
546   assume that two requests on the same connection are from the same user
547   agent unless the connection is secured and specific to that agent.
548   Some non-standard HTTP extensions (e.g., <xref target="RFC4559"/>) have
549   been known to violate this requirement, resulting in security and
550   interoperability problems.
554<section title="Caches" anchor="caches">
555<iref primary="true" item="cache"/>
557   A "<x:dfn>cache</x:dfn>" is a local store of previous response messages and the
558   subsystem that controls its message storage, retrieval, and deletion.
559   A cache stores cacheable responses in order to reduce the response
560   time and network bandwidth consumption on future, equivalent
561   requests. Any client or server &MAY; employ a cache, though a cache
562   cannot be used by a server while it is acting as a tunnel.
565   The effect of a cache is that the request/response chain is shortened
566   if one of the participants along the chain has a cached response
567   applicable to that request. The following illustrates the resulting
568   chain if B has a cached copy of an earlier response from O (via C)
569   for a request that has not been cached by UA or A.
571<figure><artwork type="drawing">
572            &gt;             &gt;
573       <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>
574                  &lt;             &lt;
576<t><iref primary="true" item="cacheable"/>
577   A response is "<x:dfn>cacheable</x:dfn>" if a cache is allowed to store a copy of
578   the response message for use in answering subsequent requests.
579   Even when a response is cacheable, there might be additional
580   constraints placed by the client or by the origin server on when
581   that cached response can be used for a particular request. HTTP
582   requirements for cache behavior and cacheable responses are
583   defined in &caching-overview;. 
586   There are a wide variety of architectures and configurations
587   of caches deployed across the World Wide Web and
588   inside large organizations. These include national hierarchies
589   of proxy caches to save transoceanic bandwidth, collaborative systems that
590   broadcast or multicast cache entries, archives of pre-fetched cache
591   entries for use in off-line or high-latency environments, and so on.
595<section title="Conformance and Error Handling" anchor="conformance">
597   This specification targets conformance criteria according to the role of
598   a participant in HTTP communication.  Hence, HTTP requirements are placed
599   on senders, recipients, clients, servers, user agents, intermediaries,
600   origin servers, proxies, gateways, or caches, depending on what behavior
601   is being constrained by the requirement. Additional (social) requirements
602   are placed on implementations, resource owners, and protocol element
603   registrations when they apply beyond the scope of a single communication.
606   The verb "generate" is used instead of "send" where a requirement
607   differentiates between creating a protocol element and merely forwarding a
608   received element downstream.
611   An implementation is considered conformant if it complies with all of the
612   requirements associated with the roles it partakes in HTTP.
615   Conformance includes both the syntax and semantics of protocol
616   elements. A sender &MUST-NOT; generate protocol elements that convey a
617   meaning that is known by that sender to be false. A sender &MUST-NOT;
618   generate protocol elements that do not match the grammar defined by the
619   corresponding ABNF rules. Within a given message, a sender &MUST-NOT;
620   generate protocol elements or syntax alternatives that are only allowed to
621   be generated by participants in other roles (i.e., a role that the sender
622   does not have for that message).
625   When a received protocol element is parsed, the recipient &MUST; be able to
626   parse any value of reasonable length that is applicable to the recipient's
627   role and matches the grammar defined by the corresponding ABNF rules.
628   Note, however, that some received protocol elements might not be parsed.
629   For example, an intermediary forwarding a message might parse a
630   header-field into generic field-name and field-value components, but then
631   forward the header field without further parsing inside the field-value.
634   HTTP does not have specific length limitations for many of its protocol
635   elements because the lengths that might be appropriate will vary widely,
636   depending on the deployment context and purpose of the implementation.
637   Hence, interoperability between senders and recipients depends on shared
638   expectations regarding what is a reasonable length for each protocol
639   element. Furthermore, what is commonly understood to be a reasonable length
640   for some protocol elements has changed over the course of the past two
641   decades of HTTP use, and is expected to continue changing in the future.
644   At a minimum, a recipient &MUST; be able to parse and process protocol
645   element lengths that are at least as long as the values that it generates
646   for those same protocol elements in other messages. For example, an origin
647   server that publishes very long URI references to its own resources needs
648   to be able to parse and process those same references when received as a
649   request target.
652   A recipient &MUST; interpret a received protocol element according to the
653   semantics defined for it by this specification, including extensions to
654   this specification, unless the recipient has determined (through experience
655   or configuration) that the sender incorrectly implements what is implied by
656   those semantics.
657   For example, an origin server might disregard the contents of a received
658   <x:ref>Accept-Encoding</x:ref> header field if inspection of the
659   <x:ref>User-Agent</x:ref> header field indicates a specific implementation
660   version that is known to fail on receipt of certain content codings.
663   Unless noted otherwise, a recipient &MAY; attempt to recover a usable
664   protocol element from an invalid construct.  HTTP does not define
665   specific error handling mechanisms except when they have a direct impact
666   on security, since different applications of the protocol require
667   different error handling strategies.  For example, a Web browser might
668   wish to transparently recover from a response where the
669   <x:ref>Location</x:ref> header field doesn't parse according to the ABNF,
670   whereas a systems control client might consider any form of error recovery
671   to be dangerous.
675<section title="Protocol Versioning" anchor="http.version">
676  <x:anchor-alias value="HTTP-version"/>
677  <x:anchor-alias value="HTTP-name"/>
679   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate
680   versions of the protocol. This specification defines version "1.1".
681   The protocol version as a whole indicates the sender's conformance
682   with the set of requirements laid out in that version's corresponding
683   specification of HTTP.
686   The version of an HTTP message is indicated by an HTTP-version field
687   in the first line of the message. HTTP-version is case-sensitive.
689<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-version"/><iref primary="true" item="Grammar" subitem="HTTP-name"/>
690  <x:ref>HTTP-version</x:ref>  = <x:ref>HTTP-name</x:ref> "/" <x:ref>DIGIT</x:ref> "." <x:ref>DIGIT</x:ref>
691  <x:ref>HTTP-name</x:ref>     = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
694   The HTTP version number consists of two decimal digits separated by a "."
695   (period or decimal point).  The first digit ("major version") indicates the
696   HTTP messaging syntax, whereas the second digit ("minor version") indicates
697   the highest minor version within that major version to which the sender is
698   conformant and able to understand for future communication.  The minor
699   version advertises the sender's communication capabilities even when the
700   sender is only using a backwards-compatible subset of the protocol,
701   thereby letting the recipient know that more advanced features can
702   be used in response (by servers) or in future requests (by clients).
705   When an HTTP/1.1 message is sent to an HTTP/1.0 recipient
706   <xref target="RFC1945"/> or a recipient whose version is unknown,
707   the HTTP/1.1 message is constructed such that it can be interpreted
708   as a valid HTTP/1.0 message if all of the newer features are ignored.
709   This specification places recipient-version requirements on some
710   new features so that a conformant sender will only use compatible
711   features until it has determined, through configuration or the
712   receipt of a message, that the recipient supports HTTP/1.1.
715   The interpretation of a header field does not change between minor
716   versions of the same major HTTP version, though the default
717   behavior of a recipient in the absence of such a field can change.
718   Unless specified otherwise, header fields defined in HTTP/1.1 are
719   defined for all versions of HTTP/1.x.  In particular, the <x:ref>Host</x:ref>
720   and <x:ref>Connection</x:ref> header fields ought to be implemented by all
721   HTTP/1.x implementations whether or not they advertise conformance with
722   HTTP/1.1.
725   New header fields can be introduced without changing the protocol version
726   if their defined semantics allow them to be safely ignored by recipients
727   that do not recognize them. Header field extensibility is discussed in
728   <xref target="field.extensibility"/>.
731   Intermediaries that process HTTP messages (i.e., all intermediaries
732   other than those acting as tunnels) &MUST; send their own HTTP-version
733   in forwarded messages.  In other words, they are not allowed to blindly
734   forward the first line of an HTTP message without ensuring that the
735   protocol version in that message matches a version to which that
736   intermediary is conformant for both the receiving and
737   sending of messages.  Forwarding an HTTP message without rewriting
738   the HTTP-version might result in communication errors when downstream
739   recipients use the message sender's version to determine what features
740   are safe to use for later communication with that sender.
743   A client &SHOULD; send a request version equal to the highest
744   version to which the client is conformant and
745   whose major version is no higher than the highest version supported
746   by the server, if this is known.  A client &MUST-NOT; send a
747   version to which it is not conformant.
750   A client &MAY; send a lower request version if it is known that
751   the server incorrectly implements the HTTP specification, but only
752   after the client has attempted at least one normal request and determined
753   from the response status code or header fields (e.g., <x:ref>Server</x:ref>) that
754   the server improperly handles higher request versions.
757   A server &SHOULD; send a response version equal to the highest version to
758   which the server is conformant that has a major version less than or equal
759   to the one received in the request.
760   A server &MUST-NOT; send a version to which it is not conformant.
761   A server can send a <x:ref>505 (HTTP Version Not Supported)</x:ref>
762   response if it wishes, for any reason, to refuse service of the client's
763   major protocol version.
766   A server &MAY; send an HTTP/1.0 response to a request
767   if it is known or suspected that the client incorrectly implements the
768   HTTP specification and is incapable of correctly processing later
769   version responses, such as when a client fails to parse the version
770   number correctly or when an intermediary is known to blindly forward
771   the HTTP-version even when it doesn't conform to the given minor
772   version of the protocol. Such protocol downgrades &SHOULD-NOT; be
773   performed unless triggered by specific client attributes, such as when
774   one or more of the request header fields (e.g., <x:ref>User-Agent</x:ref>)
775   uniquely match the values sent by a client known to be in error.
778   The intention of HTTP's versioning design is that the major number
779   will only be incremented if an incompatible message syntax is
780   introduced, and that the minor number will only be incremented when
781   changes made to the protocol have the effect of adding to the message
782   semantics or implying additional capabilities of the sender.  However,
783   the minor version was not incremented for the changes introduced between
784   <xref target="RFC2068"/> and <xref target="RFC2616"/>, and this revision
785   has specifically avoided any such changes to the protocol.
788   When an HTTP message is received with a major version number that the
789   recipient implements, but a higher minor version number than what the
790   recipient implements, the recipient &SHOULD; process the message as if it
791   were in the highest minor version within that major version to which the
792   recipient is conformant. A recipient can assume that a message with a
793   higher minor version, when sent to a recipient that has not yet indicated
794   support for that higher version, is sufficiently backwards-compatible to be
795   safely processed by any implementation of the same major version.
799<section title="Uniform Resource Identifiers" anchor="uri">
800<iref primary="true" item="resource"/>
802   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used
803   throughout HTTP as the means for identifying resources (&resource;).
804   URI references are used to target requests, indicate redirects, and define
805   relationships.
807  <x:anchor-alias value="URI-reference"/>
808  <x:anchor-alias value="absolute-URI"/>
809  <x:anchor-alias value="relative-part"/>
810  <x:anchor-alias value="scheme"/>
811  <x:anchor-alias value="authority"/>
812  <x:anchor-alias value="uri-host"/>
813  <x:anchor-alias value="port"/>
814  <x:anchor-alias value="path"/>
815  <x:anchor-alias value="path-abempty"/>
816  <x:anchor-alias value="segment"/>
817  <x:anchor-alias value="query"/>
818  <x:anchor-alias value="fragment"/>
819  <x:anchor-alias value="absolute-path"/>
820  <x:anchor-alias value="partial-URI"/>
822   The definitions of "URI-reference",
823   "absolute-URI", "relative-part", "scheme", "authority", "port", "host",
824   "path-abempty", "segment", "query", and "fragment" are adopted from the
825   URI generic syntax.
826   An "absolute-path" rule is defined for protocol elements that can contain a
827   non-empty path component. (This rule differs slightly from RFC 3986's
828   path-abempty rule, which allows for an empty path to be used in references,
829   and path-absolute rule, which does not allow paths that begin with "//".)
830   A "partial-URI" rule is defined for protocol elements
831   that can contain a relative URI but not a fragment component.
833<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>
834  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
835  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
836  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
837  <x:ref>scheme</x:ref>        = &lt;scheme, defined in <xref target="RFC3986" x:fmt="," x:sec="3.1"/>&gt;
838  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
839  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
840  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
841  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
842  <x:ref>segment</x:ref>       = &lt;segment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
843  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
844  <x:ref>fragment</x:ref>      = &lt;fragment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.5"/>&gt;
846  <x:ref>absolute-path</x:ref> = 1*( "/" segment )
847  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
850   Each protocol element in HTTP that allows a URI reference will indicate
851   in its ABNF production whether the element allows any form of reference
852   (URI-reference), only a URI in absolute form (absolute-URI), only the
853   path and optional query components, or some combination of the above.
854   Unless otherwise indicated, URI references are parsed
855   relative to the effective request URI
856   (<xref target="effective.request.uri"/>).
859<section title="http URI scheme" anchor="http.uri">
860  <x:anchor-alias value="http-URI"/>
861  <iref item="http URI scheme" primary="true"/>
862  <iref item="URI scheme" subitem="http" primary="true"/>
864   The "http" URI scheme is hereby defined for the purpose of minting
865   identifiers according to their association with the hierarchical
866   namespace governed by a potential HTTP origin server listening for
867   TCP (<xref target="RFC0793"/>) connections on a given port.
869<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"><!--terminal production--></iref>
870  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
871             [ "#" <x:ref>fragment</x:ref> ]
874   The origin server for an "http" URI is identified by the
875   <x:ref>authority</x:ref> component, which includes a host identifier
876   and optional TCP port (<xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>).
877   The hierarchical path component and optional query component serve as an
878   identifier for a potential target resource within that origin server's name
879   space. The optional fragment component allows for indirect identification
880   of a secondary resource, independent of the URI scheme, as defined in
881   <xref target="RFC3986" x:fmt="of" x:sec="3.5"/>.
884   A sender &MUST-NOT; generate an "http" URI with an empty host identifier.
885   A recipient that processes such a URI reference &MUST; reject it as invalid.
888   If the host identifier is provided as an IP address, the origin server is
889   the listener (if any) on the indicated TCP port at that IP address.
890   If host is a registered name, the registered name is an indirect identifier
891   for use with a name resolution service, such as DNS, to find an address for
892   that origin server.
893   If the port subcomponent is empty or not given, TCP port 80 (the
894   reserved port for WWW services) is the default.
897   Note that the presence of a URI with a given authority component does not
898   imply that there is always an HTTP server listening for connections on
899   that host and port. Anyone can mint a URI. What the authority component
900   determines is who has the right to respond authoritatively to requests that
901   target the identified resource. The delegated nature of registered names
902   and IP addresses creates a federated namespace, based on control over the
903   indicated host and port, whether or not an HTTP server is present.
906   When an "http" URI is used within a context that calls for access to the
907   indicated resource, a client &MAY; attempt access by resolving
908   the host to an IP address, establishing a TCP connection to that address
909   on the indicated port, and sending an HTTP request message
910   (<xref target="http.message"/>) containing the URI's identifying data
911   (<xref target="message.routing"/>) to the server.
912   If the server responds to that request with a non-interim HTTP response
913   message, as described in &status-codes;, then that response
914   is considered an authoritative answer to the client's request.
917   Although HTTP is independent of the transport protocol, the "http"
918   scheme is specific to TCP-based services because the name delegation
919   process depends on TCP for establishing authority.
920   An HTTP service based on some other underlying connection protocol
921   would presumably be identified using a different URI scheme, just as
922   the "https" scheme (below) is used for resources that require an
923   end-to-end secured connection. Other protocols might also be used to
924   provide access to "http" identified resources &mdash; it is only the
925   authoritative interface that is specific to TCP.
928   The URI generic syntax for authority also includes a deprecated
929   userinfo subcomponent (<xref target="RFC3986" x:fmt="," x:sec="3.2.1"/>)
930   for including user authentication information in the URI.  Some
931   implementations make use of the userinfo component for internal
932   configuration of authentication information, such as within command
933   invocation options, configuration files, or bookmark lists, even
934   though such usage might expose a user identifier or password.
935   A sender &MUST-NOT; generate the userinfo subcomponent (and its "@"
936   delimiter) when an "http" URI reference is generated within a message as a
937   request target or header field value.
938   Before making use of an "http" URI reference received from an untrusted
939   source, a recipient &SHOULD; parse for userinfo and treat its presence as
940   an error; it is likely being used to obscure the authority for the sake of
941   phishing attacks.
945<section title="https URI scheme" anchor="https.uri">
946   <x:anchor-alias value="https-URI"/>
947   <iref item="https URI scheme"/>
948   <iref item="URI scheme" subitem="https"/>
950   The "https" URI scheme is hereby defined for the purpose of minting
951   identifiers according to their association with the hierarchical
952   namespace governed by a potential HTTP origin server listening to a
953   given TCP port for TLS-secured connections (<xref target="RFC5246"/>).
956   All of the requirements listed above for the "http" scheme are also
957   requirements for the "https" scheme, except that TCP port 443 is the
958   default if the port subcomponent is empty or not given,
959   and the user agent &MUST; ensure that its connection to the origin
960   server is secured through the use of strong encryption, end-to-end,
961   prior to sending the first HTTP request.
963<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="https-URI"><!--terminal production--></iref>
964  <x:ref>https-URI</x:ref> = "https:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
965              [ "#" <x:ref>fragment</x:ref> ]
968   Note that the "https" URI scheme depends on both TLS and TCP for
969   establishing authority.
970   Resources made available via the "https" scheme have no shared
971   identity with the "http" scheme even if their resource identifiers
972   indicate the same authority (the same host listening to the same
973   TCP port).  They are distinct name spaces and are considered to be
974   distinct origin servers.  However, an extension to HTTP that is
975   defined to apply to entire host domains, such as the Cookie protocol
976   <xref target="RFC6265"/>, can allow information
977   set by one service to impact communication with other services
978   within a matching group of host domains.
981   The process for authoritative access to an "https" identified
982   resource is defined in <xref target="RFC2818"/>.
986<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
988   Since the "http" and "https" schemes conform to the URI generic syntax,
989   such URIs are normalized and compared according to the algorithm defined
990   in <xref target="RFC3986" x:fmt="of" x:sec="6"/>, using the defaults
991   described above for each scheme.
994   If the port is equal to the default port for a scheme, the normal form is
995   to omit the port subcomponent. When not being used in absolute form as the
996   request target of an OPTIONS request, an empty path component is equivalent
997   to an absolute path of "/", so the normal form is to provide a path of "/"
998   instead. The scheme and host are case-insensitive and normally provided in
999   lowercase; all other components are compared in a case-sensitive manner.
1000   Characters other than those in the "reserved" set are equivalent to their
1001   percent-encoded octets: the normal form is to not encode them
1002   (see Sections <xref target="RFC3986" x:fmt="number" x:sec="2.1"/> and
1003   <xref target="RFC3986" x:fmt="number" x:sec="2.2"/> of
1004   <xref target="RFC3986"/>).
1007   For example, the following three URIs are equivalent:
1009<figure><artwork type="example">
1018<section title="Message Format" anchor="http.message">
1019<x:anchor-alias value="generic-message"/>
1020<x:anchor-alias value="message.types"/>
1021<x:anchor-alias value="HTTP-message"/>
1022<x:anchor-alias value="start-line"/>
1023<iref item="header section"/>
1024<iref item="headers"/>
1025<iref item="header field"/>
1027   All HTTP/1.1 messages consist of a start-line followed by a sequence of
1028   octets in a format similar to the Internet Message Format
1029   <xref target="RFC5322"/>: zero or more header fields (collectively
1030   referred to as the "headers" or the "header section"), an empty line
1031   indicating the end of the header section, and an optional message body.
1033<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"><!--terminal production--></iref>
1034  <x:ref>HTTP-message</x:ref>   = <x:ref>start-line</x:ref>
1035                   *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1036                   <x:ref>CRLF</x:ref>
1037                   [ <x:ref>message-body</x:ref> ]
1040   The normal procedure for parsing an HTTP message is to read the
1041   start-line into a structure, read each header field into a hash
1042   table by field name until the empty line, and then use the parsed
1043   data to determine if a message body is expected.  If a message body
1044   has been indicated, then it is read as a stream until an amount
1045   of octets equal to the message body length is read or the connection
1046   is closed.
1049   A recipient &MUST; parse an HTTP message as a sequence of octets in an
1050   encoding that is a superset of US-ASCII <xref target="USASCII"/>.
1051   Parsing an HTTP message as a stream of Unicode characters, without regard
1052   for the specific encoding, creates security vulnerabilities due to the
1053   varying ways that string processing libraries handle invalid multibyte
1054   character sequences that contain the octet LF (%x0A).  String-based
1055   parsers can only be safely used within protocol elements after the element
1056   has been extracted from the message, such as within a header field-value
1057   after message parsing has delineated the individual fields.
1060   An HTTP message can be parsed as a stream for incremental processing or
1061   forwarding downstream.  However, recipients cannot rely on incremental
1062   delivery of partial messages, since some implementations will buffer or
1063   delay message forwarding for the sake of network efficiency, security
1064   checks, or payload transformations.
1067   A sender &MUST-NOT; send whitespace between the start-line and
1068   the first header field.
1069   A recipient that receives whitespace between the start-line and
1070   the first header field &MUST; either reject the message as invalid or
1071   consume each whitespace-preceded line without further processing of it
1072   (i.e., ignore the entire line, along with any subsequent lines preceded
1073   by whitespace, until a properly formed header field is received or the
1074   header section is terminated).
1077   The presence of such whitespace in a request
1078   might be an attempt to trick a server into ignoring that field or
1079   processing the line after it as a new request, either of which might
1080   result in a security vulnerability if other implementations within
1081   the request chain interpret the same message differently.
1082   Likewise, the presence of such whitespace in a response might be
1083   ignored by some clients or cause others to cease parsing.
1086<section title="Start Line" anchor="start.line">
1087  <x:anchor-alias value="Start-Line"/>
1089   An HTTP message can either be a request from client to server or a
1090   response from server to client.  Syntactically, the two types of message
1091   differ only in the start-line, which is either a request-line (for requests)
1092   or a status-line (for responses), and in the algorithm for determining
1093   the length of the message body (<xref target="message.body"/>).
1096   In theory, a client could receive requests and a server could receive
1097   responses, distinguishing them by their different start-line formats,
1098   but in practice servers are implemented to only expect a request
1099   (a response is interpreted as an unknown or invalid request method)
1100   and clients are implemented to only expect a response.
1102<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="start-line"/>
1103  <x:ref>start-line</x:ref>     = <x:ref>request-line</x:ref> / <x:ref>status-line</x:ref>
1106<section title="Request Line" anchor="request.line">
1107  <x:anchor-alias value="Request"/>
1108  <x:anchor-alias value="request-line"/>
1110   A request-line begins with a method token, followed by a single
1111   space (SP), the request-target, another single space (SP), the
1112   protocol version, and ending with CRLF.
1114<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1115  <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>
1117<iref primary="true" item="method"/>
1118<t anchor="method">
1119   The method token indicates the request method to be performed on the
1120   target resource. The request method is case-sensitive.
1122<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1123  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1126   The request methods defined by this specification can be found in
1127   &methods;, along with information regarding the HTTP method registry
1128   and considerations for defining new methods.
1130<iref item="request-target"/>
1132   The request-target identifies the target resource upon which to apply
1133   the request, as defined in <xref target="request-target"/>.
1136   Recipients typically parse the request-line into its component parts by
1137   splitting on whitespace (see <xref target="message.robustness"/>), since
1138   no whitespace is allowed in the three components.
1139   Unfortunately, some user agents fail to properly encode or exclude
1140   whitespace found in hypertext references, resulting in those disallowed
1141   characters being sent in a request-target.
1144   Recipients of an invalid request-line &SHOULD; respond with either a
1145   <x:ref>400 (Bad Request)</x:ref> error or a <x:ref>301 (Moved Permanently)</x:ref>
1146   redirect with the request-target properly encoded.  A recipient &SHOULD-NOT;
1147   attempt to autocorrect and then process the request without a redirect,
1148   since the invalid request-line might be deliberately crafted to bypass
1149   security filters along the request chain.
1152   HTTP does not place a pre-defined limit on the length of a request-line,
1153   as described in <xref target="conformance"/>.
1154   A server that receives a method longer than any that it implements
1155   &SHOULD; respond with a <x:ref>501 (Not Implemented)</x:ref> status code.
1156   A server that receives a request-target longer than any URI it wishes to
1157   parse &MUST; respond with a
1158   <x:ref>414 (URI Too Long)</x:ref> status code (see &status-414;).
1161   Various ad-hoc limitations on request-line length are found in practice.
1162   It is &RECOMMENDED; that all HTTP senders and recipients support, at a
1163   minimum, request-line lengths of 8000 octets.
1167<section title="Status Line" anchor="status.line">
1168  <x:anchor-alias value="response"/>
1169  <x:anchor-alias value="status-line"/>
1170  <x:anchor-alias value="status-code"/>
1171  <x:anchor-alias value="reason-phrase"/>
1173   The first line of a response message is the status-line, consisting
1174   of the protocol version, a space (SP), the status code, another space,
1175   a possibly-empty textual phrase describing the status code, and
1176   ending with CRLF.
1178<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-line"/>
1179  <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>
1182   The status-code element is a 3-digit integer code describing the
1183   result of the server's attempt to understand and satisfy the client's
1184   corresponding request. The rest of the response message is to be
1185   interpreted in light of the semantics defined for that status code.
1186   See &status-codes; for information about the semantics of status codes,
1187   including the classes of status code (indicated by the first digit),
1188   the status codes defined by this specification, considerations for the
1189   definition of new status codes, and the IANA registry.
1191<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-code"/>
1192  <x:ref>status-code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1195   The reason-phrase element exists for the sole purpose of providing a
1196   textual description associated with the numeric status code, mostly
1197   out of deference to earlier Internet application protocols that were more
1198   frequently used with interactive text clients. A client &SHOULD; ignore
1199   the reason-phrase content.
1201<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
1202  <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> )
1207<section title="Header Fields" anchor="header.fields">
1208  <x:anchor-alias value="header-field"/>
1209  <x:anchor-alias value="field-content"/>
1210  <x:anchor-alias value="field-name"/>
1211  <x:anchor-alias value="field-value"/>
1212  <x:anchor-alias value="field-vchar"/>
1213  <x:anchor-alias value="obs-fold"/>
1215   Each header field consists of a case-insensitive field name
1216   followed by a colon (":"), optional leading whitespace, the field value,
1217   and optional trailing whitespace.
1219<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"/>
1220  <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>
1222  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1223  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>obs-fold</x:ref> )
1224  <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> ]
1225  <x:ref>field-vchar</x:ref>    = <x:ref>VCHAR</x:ref> / <x:ref>obs-text</x:ref>
1227  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref> 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1228                 ; obsolete line folding
1229                 ; see <xref target="field.parsing"/>
1232   The field-name token labels the corresponding field-value as having the
1233   semantics defined by that header field.  For example, the <x:ref>Date</x:ref>
1234   header field is defined in &header-date; as containing the origination
1235   timestamp for the message in which it appears.
1238<section title="Field Extensibility" anchor="field.extensibility">
1240   Header fields are fully extensible: there is no limit on the
1241   introduction of new field names, each presumably defining new semantics,
1242   nor on the number of header fields used in a given message.  Existing
1243   fields are defined in each part of this specification and in many other
1244   specifications outside the core standard.
1247   New header fields can be defined such that, when they are understood by a
1248   recipient, they might override or enhance the interpretation of previously
1249   defined header fields, define preconditions on request evaluation, or
1250   refine the meaning of responses.
1253   A proxy &MUST; forward unrecognized header fields unless the
1254   field-name is listed in the <x:ref>Connection</x:ref> header field
1255   (<xref target="header.connection"/>) or the proxy is specifically
1256   configured to block, or otherwise transform, such fields.
1257   Other recipients &SHOULD; ignore unrecognized header fields.
1258   These requirements allow HTTP's functionality to be enhanced without
1259   requiring prior update of deployed intermediaries.
1262   All defined header fields ought to be registered with IANA in the
1263   Message Header Field Registry, as described in &iana-header-registry;.
1267<section title="Field Order" anchor="field.order">
1269   The order in which header fields with differing field names are
1270   received is not significant. However, it is good practice to send
1271   header fields that contain control data first, such as <x:ref>Host</x:ref>
1272   on requests and <x:ref>Date</x:ref> on responses, so that implementations
1273   can decide when not to handle a message as early as possible.  A server
1274   &MUST; wait until the entire header section is received before interpreting
1275   a request message, since later header fields might include conditionals,
1276   authentication credentials, or deliberately misleading duplicate
1277   header fields that would impact request processing.
1280   A sender &MUST-NOT; generate multiple header fields with the same field
1281   name in a message unless either the entire field value for that
1282   header field is defined as a comma-separated list [i.e., #(values)]
1283   or the header field is a well-known exception (as noted below).
1286   A recipient &MAY; combine multiple header fields with the same field name
1287   into one "field-name: field-value" pair, without changing the semantics of
1288   the message, by appending each subsequent field value to the combined
1289   field value in order, separated by a comma. The order in which
1290   header fields with the same field name are received is therefore
1291   significant to the interpretation of the combined field value;
1292   a proxy &MUST-NOT; change the order of these field values when
1293   forwarding a message.
1296  <t>
1297   &Note; In practice, the "Set-Cookie" header field (<xref target="RFC6265"/>)
1298   often appears multiple times in a response message and does not use the
1299   list syntax, violating the above requirements on multiple header fields
1300   with the same name. Since it cannot be combined into a single field-value,
1301   recipients ought to handle "Set-Cookie" as a special case while processing
1302   header fields. (See Appendix A.2.3 of <xref target="Kri2001"/> for details.)
1303  </t>
1307<section title="Whitespace" anchor="whitespace">
1308<t anchor="rule.LWS">
1309   This specification uses three rules to denote the use of linear
1310   whitespace: OWS (optional whitespace), RWS (required whitespace), and
1311   BWS ("bad" whitespace).
1313<t anchor="rule.OWS">
1314   The OWS rule is used where zero or more linear whitespace octets might
1315   appear. For protocol elements where optional whitespace is preferred to
1316   improve readability, a sender &SHOULD; generate the optional whitespace
1317   as a single SP; otherwise, a sender &SHOULD-NOT; generate optional
1318   whitespace except as needed to white-out invalid or unwanted protocol
1319   elements during in-place message filtering.
1321<t anchor="rule.RWS">
1322   The RWS rule is used when at least one linear whitespace octet is required
1323   to separate field tokens. A sender &SHOULD; generate RWS as a single SP.
1325<t anchor="rule.BWS">
1326   The BWS rule is used where the grammar allows optional whitespace only for
1327   historical reasons. A sender &MUST-NOT; generate BWS in messages.
1328   A recipient &MUST; parse for such bad whitespace and remove it before
1329   interpreting the protocol element.
1331<t anchor="rule.whitespace">
1332  <x:anchor-alias value="BWS"/>
1333  <x:anchor-alias value="OWS"/>
1334  <x:anchor-alias value="RWS"/>
1336<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"/>
1337  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1338                 ; optional whitespace
1339  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1340                 ; required whitespace
1341  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1342                 ; "bad" whitespace
1346<section title="Field Parsing" anchor="field.parsing">
1348   Messages are parsed using a generic algorithm, independent of the
1349   individual header field names. The contents within a given field value are
1350   not parsed until a later stage of message interpretation (usually after the
1351   message's entire header section has been processed).
1352   Consequently, this specification does not use ABNF rules to define each
1353   "Field-Name: Field Value" pair, as was done in previous editions.
1354   Instead, this specification uses ABNF rules which are named according to
1355   each registered field name, wherein the rule defines the valid grammar for
1356   that field's corresponding field values (i.e., after the field-value
1357   has been extracted from the header section by a generic field parser).
1360   No whitespace is allowed between the header field-name and colon.
1361   In the past, differences in the handling of such whitespace have led to
1362   security vulnerabilities in request routing and response handling.
1363   A server &MUST; reject any received request message that contains
1364   whitespace between a header field-name and colon with a response code of
1365   <x:ref>400 (Bad Request)</x:ref>. A proxy &MUST; remove any such whitespace
1366   from a response message before forwarding the message downstream.
1369   A field value might be preceded and/or followed by optional whitespace
1370   (OWS); a single SP preceding the field-value is preferred for consistent
1371   readability by humans.
1372   The field value does not include any leading or trailing white space: OWS
1373   occurring before the first non-whitespace octet of the field value or after
1374   the last non-whitespace octet of the field value ought to be excluded by
1375   parsers when extracting the field value from a header field.
1378   Historically, HTTP header field values could be extended over multiple
1379   lines by preceding each extra line with at least one space or horizontal
1380   tab (obs-fold). This specification deprecates such line folding except
1381   within the message/http media type
1382   (<xref target=""/>).
1383   A sender &MUST-NOT; generate a message that includes line folding
1384   (i.e., that has any field-value that contains a match to the
1385   <x:ref>obs-fold</x:ref> rule) unless the message is intended for packaging
1386   within the message/http media type.
1389   A server that receives an <x:ref>obs-fold</x:ref> in a request message that
1390   is not within a message/http container &MUST; either reject the message by
1391   sending a <x:ref>400 (Bad Request)</x:ref>, preferably with a
1392   representation explaining that obsolete line folding is unacceptable, or
1393   replace each received <x:ref>obs-fold</x:ref> with one or more
1394   <x:ref>SP</x:ref> octets prior to interpreting the field value or
1395   forwarding the message downstream.
1398   A proxy or gateway that receives an <x:ref>obs-fold</x:ref> in a response
1399   message that is not within a message/http container &MUST; either discard
1400   the message and replace it with a <x:ref>502 (Bad Gateway)</x:ref>
1401   response, preferably with a representation explaining that unacceptable
1402   line folding was received, or replace each received <x:ref>obs-fold</x:ref>
1403   with one or more <x:ref>SP</x:ref> octets prior to interpreting the field
1404   value or forwarding the message downstream.
1407   A user agent that receives an <x:ref>obs-fold</x:ref> in a response message
1408   that is not within a message/http container &MUST; replace each received
1409   <x:ref>obs-fold</x:ref> with one or more <x:ref>SP</x:ref> octets prior to
1410   interpreting the field value.
1413   Historically, HTTP has allowed field content with text in the ISO-8859-1
1414   <xref target="ISO-8859-1"/> charset, supporting other charsets only
1415   through use of <xref target="RFC2047"/> encoding.
1416   In practice, most HTTP header field values use only a subset of the
1417   US-ASCII charset <xref target="USASCII"/>. Newly defined
1418   header fields &SHOULD; limit their field values to US-ASCII octets.
1419   A recipient &SHOULD; treat other octets in field content (obs-text) as
1420   opaque data.
1424<section title="Field Limits" anchor="field.limits">
1426   HTTP does not place a pre-defined limit on the length of each header field
1427   or on the length of the header section as a whole, as described in
1428   <xref target="conformance"/>. Various ad-hoc limitations on individual
1429   header field length are found in practice, often depending on the specific
1430   field semantics.
1433   A server that receives a request header field, or set of fields, larger
1434   than it wishes to process &MUST; respond with an appropriate
1435   <x:ref>4xx (Client Error)</x:ref> status code. Ignoring such header fields
1436   would increase the server's vulnerability to request smuggling attacks.
1439   A client &MAY; discard or truncate received header fields that are larger
1440   than the client wishes to process if the field semantics are such that the
1441   dropped value(s) can be safely ignored without changing the
1442   message framing or response semantics.
1446<section title="Field value components" anchor="field.components">
1447<t anchor="rule.token.separators">
1448  <x:anchor-alias value="tchar"/>
1449  <x:anchor-alias value="token"/>
1450  <iref item="Delimiters"/>
1451   Most HTTP header field values are defined using common syntax components
1452   (token, quoted-string, and comment) separated by whitespace or specific
1453   delimiting characters. Delimiters are chosen from the set of US-ASCII
1454   visual characters not allowed in a <x:ref>token</x:ref>
1455   (DQUOTE and "(),/:;&lt;=>?@[\]{}").
1457<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="token"/><iref primary="true" item="Grammar" subitem="tchar"/>
1458  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1460  NOTE: the definition of tchar and the prose above about special characters need to match!
1461 -->
1462  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1463                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1464                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1465                 ; any <x:ref>VCHAR</x:ref>, except delimiters
1467<t anchor="rule.quoted-string">
1468  <x:anchor-alias value="quoted-string"/>
1469  <x:anchor-alias value="qdtext"/>
1470  <x:anchor-alias value="obs-text"/>
1471   A string of text is parsed as a single value if it is quoted using
1472   double-quote marks.
1474<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"/>
1475  <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>
1476  <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>
1477  <x:ref>obs-text</x:ref>       = %x80-FF
1479<t anchor="rule.comment">
1480  <x:anchor-alias value="comment"/>
1481  <x:anchor-alias value="ctext"/>
1482   Comments can be included in some HTTP header fields by surrounding
1483   the comment text with parentheses. Comments are only allowed in
1484   fields containing "comment" as part of their field value definition.
1486<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1487  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-pair</x:ref> / <x:ref>comment</x:ref> ) ")"
1488  <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>
1490<t anchor="rule.quoted-pair">
1491  <x:anchor-alias value="quoted-pair"/>
1492   The backslash octet ("\") can be used as a single-octet
1493   quoting mechanism within quoted-string and comment constructs.
1494   Recipients that process the value of a quoted-string &MUST; handle a
1495   quoted-pair as if it were replaced by the octet following the backslash.
1497<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1498  <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> )
1501   A sender &SHOULD-NOT; generate a quoted-pair in a quoted-string except
1502   where necessary to quote DQUOTE and backslash octets occurring within that
1503   string.
1504   A sender &SHOULD-NOT; generate a quoted-pair in a comment except
1505   where necessary to quote parentheses ["(" and ")"] and backslash octets
1506   occurring within that comment.
1512<section title="Message Body" anchor="message.body">
1513  <x:anchor-alias value="message-body"/>
1515   The message body (if any) of an HTTP message is used to carry the
1516   payload body of that request or response.  The message body is
1517   identical to the payload body unless a transfer coding has been
1518   applied, as described in <xref target="header.transfer-encoding"/>.
1520<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1521  <x:ref>message-body</x:ref> = *OCTET
1524   The rules for when a message body is allowed in a message differ for
1525   requests and responses.
1528   The presence of a message body in a request is signaled by a
1529   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1530   field. Request message framing is independent of method semantics,
1531   even if the method does not define any use for a message body.
1534   The presence of a message body in a response depends on both
1535   the request method to which it is responding and the response
1536   status code (<xref target="status.line"/>).
1537   Responses to the HEAD request method (&HEAD;) never include a message body
1538   because the associated response header fields (e.g.,
1539   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1540   if present, indicate only what their values would have been if the request
1541   method had been GET (&GET;).
1542   <x:ref>2xx (Successful)</x:ref> responses to a CONNECT request method
1543   (&CONNECT;) switch to tunnel mode instead of having a message body.
1544   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1545   <x:ref>304 (Not Modified)</x:ref> responses do not include a message body.
1546   All other responses do include a message body, although the body
1547   might be of zero length.
1550<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1551  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1552  <iref item="chunked (Coding Format)"/>
1553  <x:anchor-alias value="Transfer-Encoding"/>
1555   The Transfer-Encoding header field lists the transfer coding names
1556   corresponding to the sequence of transfer codings that have been
1557   (or will be) applied to the payload body in order to form the message body.
1558   Transfer codings are defined in <xref target="transfer.codings"/>.
1560<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1561  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1564   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1565   MIME, which was designed to enable safe transport of binary data over a
1566   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1567   However, safe transport has a different focus for an 8bit-clean transfer
1568   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1569   accurately delimit a dynamically generated payload and to distinguish
1570   payload encodings that are only applied for transport efficiency or
1571   security from those that are characteristics of the selected resource.
1574   A recipient &MUST; be able to parse the chunked transfer coding
1575   (<xref target="chunked.encoding"/>) because it plays a crucial role in
1576   framing messages when the payload body size is not known in advance.
1577   A sender &MUST-NOT; apply chunked more than once to a message body
1578   (i.e., chunking an already chunked message is not allowed).
1579   If any transfer coding other than chunked is applied to a request payload
1580   body, the sender &MUST; apply chunked as the final transfer coding to
1581   ensure that the message is properly framed.
1582   If any transfer coding other than chunked is applied to a response payload
1583   body, the sender &MUST; either apply chunked as the final transfer coding
1584   or terminate the message by closing the connection.
1587   For example,
1588</preamble><artwork type="example">
1589  Transfer-Encoding: gzip, chunked
1591   indicates that the payload body has been compressed using the gzip
1592   coding and then chunked using the chunked coding while forming the
1593   message body.
1596   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1597   Transfer-Encoding is a property of the message, not of the representation, and
1598   any recipient along the request/response chain &MAY; decode the received
1599   transfer coding(s) or apply additional transfer coding(s) to the message
1600   body, assuming that corresponding changes are made to the Transfer-Encoding
1601   field-value. Additional information about the encoding parameters can be
1602   provided by other header fields not defined by this specification.
1605   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1606   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1607   neither of which includes a message body,
1608   to indicate that the origin server would have applied a transfer coding
1609   to the message body if the request had been an unconditional GET.
1610   This indication is not required, however, because any recipient on
1611   the response chain (including the origin server) can remove transfer
1612   codings when they are not needed.
1615   A server &MUST-NOT; send a Transfer-Encoding header field in any response
1616   with a status code of
1617   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1618   A server &MUST-NOT; send a Transfer-Encoding header field in any
1619   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1622   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1623   implementations advertising only HTTP/1.0 support will not understand
1624   how to process a transfer-encoded payload.
1625   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1626   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1627   might be in the form of specific user configuration or by remembering the
1628   version of a prior received response.
1629   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1630   the corresponding request indicates HTTP/1.1 (or later).
1633   A server that receives a request message with a transfer coding it does
1634   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref>.
1638<section title="Content-Length" anchor="header.content-length">
1639  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1640  <x:anchor-alias value="Content-Length"/>
1642   When a message does not have a <x:ref>Transfer-Encoding</x:ref> header
1643   field, a Content-Length header field can provide the anticipated size,
1644   as a decimal number of octets, for a potential payload body.
1645   For messages that do include a payload body, the Content-Length field-value
1646   provides the framing information necessary for determining where the body
1647   (and message) ends.  For messages that do not include a payload body, the
1648   Content-Length indicates the size of the selected representation
1649   (&representation;).
1651<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1652  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1655   An example is
1657<figure><artwork type="example">
1658  Content-Length: 3495
1661   A sender &MUST-NOT; send a Content-Length header field in any message that
1662   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1665   A user agent &SHOULD; send a Content-Length in a request message when no
1666   <x:ref>Transfer-Encoding</x:ref> is sent and the request method defines
1667   a meaning for an enclosed payload body. For example, a Content-Length
1668   header field is normally sent in a POST request even when the value is
1669   0 (indicating an empty payload body).  A user agent &SHOULD-NOT; send a
1670   Content-Length header field when the request message does not contain a
1671   payload body and the method semantics do not anticipate such a body.
1674   A server &MAY; send a Content-Length header field in a response to a HEAD
1675   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1676   response unless its field-value equals the decimal number of octets that
1677   would have been sent in the payload body of a response if the same
1678   request had used the GET method.
1681   A server &MAY; send a Content-Length header field in a
1682   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1683   (&status-304;); a server &MUST-NOT; send Content-Length in such a
1684   response unless its field-value equals the decimal number of octets that
1685   would have been sent in the payload body of a <x:ref>200 (OK)</x:ref>
1686   response to the same request.
1689   A server &MUST-NOT; send a Content-Length header field in any response
1690   with a status code of
1691   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1692   A server &MUST-NOT; send a Content-Length header field in any
1693   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1696   Aside from the cases defined above, in the absence of Transfer-Encoding,
1697   an origin server &SHOULD; send a Content-Length header field when the
1698   payload body size is known prior to sending the complete header section.
1699   This will allow downstream recipients to measure transfer progress,
1700   know when a received message is complete, and potentially reuse the
1701   connection for additional requests.
1704   Any Content-Length field value greater than or equal to zero is valid.
1705   Since there is no predefined limit to the length of a payload, a
1706   recipient &MUST; anticipate potentially large decimal numerals and
1707   prevent parsing errors due to integer conversion overflows
1708   (<xref target="attack.protocol.element.size.overflows"/>).
1711   If a message is received that has multiple Content-Length header fields
1712   with field-values consisting of the same decimal value, or a single
1713   Content-Length header field with a field value containing a list of
1714   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1715   duplicate Content-Length header fields have been generated or combined by an
1716   upstream message processor, then the recipient &MUST; either reject the
1717   message as invalid or replace the duplicated field-values with a single
1718   valid Content-Length field containing that decimal value prior to
1719   determining the message body length or forwarding the message.
1722  <t>
1723   &Note; HTTP's use of Content-Length for message framing differs
1724   significantly from the same field's use in MIME, where it is an optional
1725   field used only within the "message/external-body" media-type.
1726  </t>
1730<section title="Message Body Length" anchor="message.body.length">
1731  <iref item="chunked (Coding Format)"/>
1733   The length of a message body is determined by one of the following
1734   (in order of precedence):
1737  <list style="numbers">
1738    <x:lt><t>
1739     Any response to a HEAD request and any response with a
1740     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1741     <x:ref>304 (Not Modified)</x:ref> status code is always
1742     terminated by the first empty line after the header fields, regardless of
1743     the header fields present in the message, and thus cannot contain a
1744     message body.
1745    </t></x:lt>
1746    <x:lt><t>
1747     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1748     connection will become a tunnel immediately after the empty line that
1749     concludes the header fields.  A client &MUST; ignore any
1750     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1751     fields received in such a message.
1752    </t></x:lt>
1753    <x:lt><t>
1754     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1755     and the chunked transfer coding (<xref target="chunked.encoding"/>)
1756     is the final encoding, the message body length is determined by reading
1757     and decoding the chunked data until the transfer coding indicates the
1758     data is complete.
1759    </t>
1760    <t>
1761     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1762     response and the chunked transfer coding is not the final encoding, the
1763     message body length is determined by reading the connection until it is
1764     closed by the server.
1765     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a request and the
1766     chunked transfer coding is not the final encoding, the message body
1767     length cannot be determined reliably; the server &MUST; respond with
1768     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1769    </t>
1770    <t>
1771     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1772     and a <x:ref>Content-Length</x:ref> header field, the Transfer-Encoding
1773     overrides the Content-Length. Such a message might indicate an attempt
1774     to perform request or response smuggling (bypass of security-related
1775     checks on message routing or content) and thus ought to be handled as
1776     an error.  A sender &MUST; remove the received Content-Length field
1777     prior to forwarding such a message downstream.
1778    </t></x:lt>
1779    <x:lt><t>
1780     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1781     either multiple <x:ref>Content-Length</x:ref> header fields having
1782     differing field-values or a single Content-Length header field having an
1783     invalid value, then the message framing is invalid and
1784     the recipient &MUST; treat it as an unrecoverable error to prevent
1785     request or response smuggling.
1786     If this is a request message, the server &MUST; respond with
1787     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1788     If this is a response message received by a proxy,
1789     the proxy &MUST; close the connection to the server, discard the received
1790     response, and send a <x:ref>502 (Bad Gateway)</x:ref> response to the
1791     client.
1792     If this is a response message received by a user agent,
1793     the user agent &MUST; close the connection to the server and discard the
1794     received response.
1795    </t></x:lt>
1796    <x:lt><t>
1797     If a valid <x:ref>Content-Length</x:ref> header field is present without
1798     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1799     expected message body length in octets.
1800     If the sender closes the connection or the recipient times out before the
1801     indicated number of octets are received, the recipient &MUST; consider
1802     the message to be incomplete and close the connection.
1803    </t></x:lt>
1804    <x:lt><t>
1805     If this is a request message and none of the above are true, then the
1806     message body length is zero (no message body is present).
1807    </t></x:lt>
1808    <x:lt><t>
1809     Otherwise, this is a response message without a declared message body
1810     length, so the message body length is determined by the number of octets
1811     received prior to the server closing the connection.
1812    </t></x:lt>
1813  </list>
1816   Since there is no way to distinguish a successfully completed,
1817   close-delimited message from a partially-received message interrupted
1818   by network failure, a server &SHOULD; generate encoding or
1819   length-delimited messages whenever possible.  The close-delimiting
1820   feature exists primarily for backwards compatibility with HTTP/1.0.
1823   A server &MAY; reject a request that contains a message body but
1824   not a <x:ref>Content-Length</x:ref> by responding with
1825   <x:ref>411 (Length Required)</x:ref>.
1828   Unless a transfer coding other than chunked has been applied,
1829   a client that sends a request containing a message body &SHOULD;
1830   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1831   length is known in advance, rather than the chunked transfer coding, since some
1832   existing services respond to chunked with a <x:ref>411 (Length Required)</x:ref>
1833   status code even though they understand the chunked transfer coding.  This
1834   is typically because such services are implemented via a gateway that
1835   requires a content-length in advance of being called and the server
1836   is unable or unwilling to buffer the entire request before processing.
1839   A user agent that sends a request containing a message body &MUST; send a
1840   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1841   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1842   the form of specific user configuration or by remembering the version of a
1843   prior received response.
1846   If the final response to the last request on a connection has been
1847   completely received and there remains additional data to read, a user agent
1848   &MAY; discard the remaining data or attempt to determine if that data
1849   belongs as part of the prior response body, which might be the case if the
1850   prior message's Content-Length value is incorrect. A client &MUST-NOT;
1851   process, cache, or forward such extra data as a separate response, since
1852   such behavior would be vulnerable to cache poisoning.
1857<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1859   A server that receives an incomplete request message, usually due to a
1860   canceled request or a triggered time-out exception, &MAY; send an error
1861   response prior to closing the connection.
1864   A client that receives an incomplete response message, which can occur
1865   when a connection is closed prematurely or when decoding a supposedly
1866   chunked transfer coding fails, &MUST; record the message as incomplete.
1867   Cache requirements for incomplete responses are defined in
1868   &cache-incomplete;.
1871   If a response terminates in the middle of the header section (before the
1872   empty line is received) and the status code might rely on header fields to
1873   convey the full meaning of the response, then the client cannot assume
1874   that meaning has been conveyed; the client might need to repeat the
1875   request in order to determine what action to take next.
1878   A message body that uses the chunked transfer coding is
1879   incomplete if the zero-sized chunk that terminates the encoding has not
1880   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1881   incomplete if the size of the message body received (in octets) is less than
1882   the value given by Content-Length.  A response that has neither chunked
1883   transfer coding nor Content-Length is terminated by closure of the
1884   connection, and thus is considered complete regardless of the number of
1885   message body octets received, provided that the header section was received
1886   intact.
1890<section title="Message Parsing Robustness" anchor="message.robustness">
1892   Older HTTP/1.0 user agent implementations might send an extra CRLF
1893   after a POST request as a workaround for some early server
1894   applications that failed to read message body content that was
1895   not terminated by a line-ending. An HTTP/1.1 user agent &MUST-NOT;
1896   preface or follow a request with an extra CRLF.  If terminating
1897   the request message body with a line-ending is desired, then the
1898   user agent &MUST; count the terminating CRLF octets as part of the
1899   message body length.
1902   In the interest of robustness, a server that is expecting to receive and
1903   parse a request-line &SHOULD; ignore at least one empty line (CRLF)
1904   received prior to the request-line.
1907   Although the line terminator for the start-line and header
1908   fields is the sequence CRLF, a recipient &MAY; recognize a
1909   single LF as a line terminator and ignore any preceding CR.
1912   Although the request-line and status-line grammar rules require that each
1913   of the component elements be separated by a single SP octet, recipients
1914   &MAY; instead parse on whitespace-delimited word boundaries and, aside
1915   from the CRLF terminator, treat any form of whitespace as the SP separator
1916   while ignoring preceding or trailing whitespace;
1917   such whitespace includes one or more of the following octets:
1918   SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
1921   When a server listening only for HTTP request messages, or processing
1922   what appears from the start-line to be an HTTP request message,
1923   receives a sequence of octets that does not match the HTTP-message
1924   grammar aside from the robustness exceptions listed above, the
1925   server &SHOULD; respond with a <x:ref>400 (Bad Request)</x:ref> response. 
1930<section title="Transfer Codings" anchor="transfer.codings">
1931  <x:anchor-alias value="transfer-coding"/>
1932  <x:anchor-alias value="transfer-extension"/>
1934   Transfer coding names are used to indicate an encoding
1935   transformation that has been, can be, or might need to be applied to a
1936   payload body in order to ensure "safe transport" through the network.
1937   This differs from a content coding in that the transfer coding is a
1938   property of the message rather than a property of the representation
1939   that is being transferred.
1941<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1942  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1943                     / "compress" ; <xref target="compress.coding"/>
1944                     / "deflate" ; <xref target="deflate.coding"/>
1945                     / "gzip" ; <xref target="gzip.coding"/>
1946                     / <x:ref>transfer-extension</x:ref>
1947  <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> )
1949<t anchor="rule.parameter">
1950  <x:anchor-alias value="transfer-parameter"/>
1951   Parameters are in the form of a name or name=value pair.
1953<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-parameter"/>
1954  <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> )
1957   All transfer-coding names are case-insensitive and ought to be registered
1958   within the HTTP Transfer Coding registry, as defined in
1959   <xref target="transfer.coding.registry"/>.
1960   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1961   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1962   header fields.
1965<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1966  <iref primary="true" item="chunked (Coding Format)"/>
1967  <x:anchor-alias value="chunk"/>
1968  <x:anchor-alias value="chunked-body"/>
1969  <x:anchor-alias value="chunk-data"/>
1970  <x:anchor-alias value="chunk-size"/>
1971  <x:anchor-alias value="last-chunk"/>
1973   The chunked transfer coding wraps the payload body in order to transfer it
1974   as a series of chunks, each with its own size indicator, followed by an
1975   &OPTIONAL; trailer containing header fields. Chunked enables content
1976   streams of unknown size to be transferred as a sequence of length-delimited
1977   buffers, which enables the sender to retain connection persistence and the
1978   recipient to know when it has received the entire message.
1980<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"/>
1981  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1982                   <x:ref>last-chunk</x:ref>
1983                   <x:ref>trailer-part</x:ref>
1984                   <x:ref>CRLF</x:ref>
1986  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1987                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1988  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1989  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1991  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1994   The chunk-size field is a string of hex digits indicating the size of
1995   the chunk-data in octets. The chunked transfer coding is complete when a
1996   chunk with a chunk-size of zero is received, possibly followed by a
1997   trailer, and finally terminated by an empty line.
2000   A recipient &MUST; be able to parse and decode the chunked transfer coding.
2003<section title="Chunk Extensions" anchor="chunked.extension">
2004  <x:anchor-alias value="chunk-ext"/>
2005  <x:anchor-alias value="chunk-ext-name"/>
2006  <x:anchor-alias value="chunk-ext-val"/>
2008   The chunked encoding allows each chunk to include zero or more chunk
2009   extensions, immediately following the <x:ref>chunk-size</x:ref>, for the
2010   sake of supplying per-chunk metadata (such as a signature or hash),
2011   mid-message control information, or randomization of message body size.
2013<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"/>
2014  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
2016  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
2017  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
2020   The chunked encoding is specific to each connection and is likely to be
2021   removed or recoded by each recipient (including intermediaries) before any
2022   higher-level application would have a chance to inspect the extensions.
2023   Hence, use of chunk extensions is generally limited to specialized HTTP
2024   services such as "long polling" (where client and server can have shared
2025   expectations regarding the use of chunk extensions) or for padding within
2026   an end-to-end secured connection.
2029   A recipient &MUST; ignore unrecognized chunk extensions.
2030   A server ought to limit the total length of chunk extensions received in a
2031   request to an amount reasonable for the services provided, in the same way
2032   that it applies length limitations and timeouts for other parts of a
2033   message, and generate an appropriate <x:ref>4xx (Client Error)</x:ref>
2034   response if that amount is exceeded.
2038<section title="Chunked Trailer Part" anchor="chunked.trailer.part">
2039  <x:anchor-alias value="trailer-part"/>
2041   A trailer allows the sender to include additional fields at the end of a
2042   chunked message in order to supply metadata that might be dynamically
2043   generated while the message body is sent, such as a message integrity
2044   check, digital signature, or post-processing status. The trailer fields are
2045   identical to header fields, except they are sent in a chunked trailer
2046   instead of the message's header section.
2048<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="trailer-part"/><iref primary="false" item="Grammar" subitem="header-field"/>
2049  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
2052   A sender &MUST-NOT; generate a trailer that contains a field necessary for
2053   message framing (e.g., <x:ref>Transfer-Encoding</x:ref> and
2054   <x:ref>Content-Length</x:ref>), routing (e.g., <x:ref>Host</x:ref>),
2055   request modifiers (e.g., controls and conditionals in
2056   &request-header-fields;), authentication (e.g., see <xref target="Part7"/>
2057   and <xref target="RFC6265"/>), response control data (e.g., see
2058   &response-control-data;), or determining how to process the payload
2059   (e.g., <x:ref>Content-Encoding</x:ref>, <x:ref>Content-Type</x:ref>,
2060   <x:ref>Content-Range</x:ref>, and <x:ref>Trailer</x:ref>).
2063   When a chunked message containing a non-empty trailer is received, the
2064   recipient &MAY; process the fields (aside from those forbidden above)
2065   as if they were appended to the message's header section.
2066   A recipient &MUST; ignore (or consider as an error) any fields that are
2067   forbidden to be sent in a trailer, since processing them as if they were
2068   present in the header section might bypass external security filters.
2071   Unless the request includes a <x:ref>TE</x:ref> header field indicating
2072   "trailers" is acceptable, as described in <xref target="header.te"/>, a
2073   server &SHOULD-NOT; generate trailer fields that it believes are necessary
2074   for the user agent to receive. Without a TE containing "trailers", the
2075   server ought to assume that the trailer fields might be silently discarded
2076   along the path to the user agent. This requirement allows intermediaries to
2077   forward a de-chunked message to an HTTP/1.0 recipient without buffering the
2078   entire response.
2082<section title="Decoding Chunked" anchor="decoding.chunked">
2084   A process for decoding the chunked transfer coding
2085   can be represented in pseudo-code as:
2087<figure><artwork type="code">
2088  length := 0
2089  read chunk-size, chunk-ext (if any), and CRLF
2090  while (chunk-size &gt; 0) {
2091     read chunk-data and CRLF
2092     append chunk-data to decoded-body
2093     length := length + chunk-size
2094     read chunk-size, chunk-ext (if any), and CRLF
2095  }
2096  read trailer field
2097  while (trailer field is not empty) {
2098     if trailer field is allowed to be sent in a trailer,
2099         append trailer field to existing header fields
2100     read trailer-field
2101  }
2102  Content-Length := length
2103  Remove "chunked" from Transfer-Encoding
2104  Remove Trailer from existing header fields
2109<section title="Compression Codings" anchor="compression.codings">
2111   The codings defined below can be used to compress the payload of a
2112   message.
2115<section title="Compress Coding" anchor="compress.coding">
2116<iref item="compress (Coding Format)"/>
2118   The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding
2119   <xref target="Welch"/> that is commonly produced by the UNIX file
2120   compression program "compress".
2121   A recipient &SHOULD; consider "x-compress" to be equivalent to "compress".
2125<section title="Deflate Coding" anchor="deflate.coding">
2126<iref item="deflate (Coding Format)"/>
2128   The "deflate" coding is a "zlib" data format <xref target="RFC1950"/>
2129   containing a "deflate" compressed data stream <xref target="RFC1951"/>
2130   that uses a combination of the Lempel-Ziv (LZ77) compression algorithm and
2131   Huffman coding.
2134  <t>
2135    &Note; Some non-conformant implementations send the "deflate"
2136    compressed data without the zlib wrapper.
2137   </t>
2141<section title="Gzip Coding" anchor="gzip.coding">
2142<iref item="gzip (Coding Format)"/>
2144   The "gzip" coding is an LZ77 coding with a 32 bit CRC that is commonly
2145   produced by the gzip file compression program <xref target="RFC1952"/>.
2146   A recipient &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2152<section title="TE" anchor="header.te">
2153  <iref primary="true" item="TE header field" x:for-anchor=""/>
2154  <x:anchor-alias value="TE"/>
2155  <x:anchor-alias value="t-codings"/>
2156  <x:anchor-alias value="t-ranking"/>
2157  <x:anchor-alias value="rank"/>
2159   The "TE" header field in a request indicates what transfer codings,
2160   besides chunked, the client is willing to accept in response, and
2161   whether or not the client is willing to accept trailer fields in a
2162   chunked transfer coding.
2165   The TE field-value consists of a comma-separated list of transfer coding
2166   names, each allowing for optional parameters (as described in
2167   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2168   A client &MUST-NOT; send the chunked transfer coding name in TE;
2169   chunked is always acceptable for HTTP/1.1 recipients.
2171<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"/>
2172  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2173  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2174  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2175  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2176             / ( "1" [ "." 0*3("0") ] )
2179   Three examples of TE use are below.
2181<figure><artwork type="example">
2182  TE: deflate
2183  TE:
2184  TE: trailers, deflate;q=0.5
2187   The presence of the keyword "trailers" indicates that the client is willing
2188   to accept trailer fields in a chunked transfer coding, as defined in
2189   <xref target="chunked.trailer.part"/>, on behalf of itself and any downstream
2190   clients. For requests from an intermediary, this implies that either:
2191   (a) all downstream clients are willing to accept trailer fields in the
2192   forwarded response; or,
2193   (b) the intermediary will attempt to buffer the response on behalf of
2194   downstream recipients.
2195   Note that HTTP/1.1 does not define any means to limit the size of a
2196   chunked response such that an intermediary can be assured of buffering the
2197   entire response.
2200   When multiple transfer codings are acceptable, the client &MAY; rank the
2201   codings by preference using a case-insensitive "q" parameter (similar to
2202   the qvalues used in content negotiation fields, &qvalue;). The rank value
2203   is a real number in the range 0 through 1, where 0.001 is the least
2204   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2207   If the TE field-value is empty or if no TE field is present, the only
2208   acceptable transfer coding is chunked. A message with no transfer coding
2209   is always acceptable.
2212   Since the TE header field only applies to the immediate connection,
2213   a sender of TE &MUST; also send a "TE" connection option within the
2214   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2215   in order to prevent the TE field from being forwarded by intermediaries
2216   that do not support its semantics.
2220<section title="Trailer" anchor="header.trailer">
2221  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
2222  <x:anchor-alias value="Trailer"/>
2224   When a message includes a message body encoded with the chunked
2225   transfer coding and the sender desires to send metadata in the form of
2226   trailer fields at the end of the message, the sender &SHOULD; generate a
2227   <x:ref>Trailer</x:ref> header field before the message body to indicate
2228   which fields will be present in the trailers. This allows the recipient
2229   to prepare for receipt of that metadata before it starts processing the body,
2230   which is useful if the message is being streamed and the recipient wishes
2231   to confirm an integrity check on the fly.
2233<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/><iref primary="false" item="Grammar" subitem="field-name"/>
2234  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
2239<section title="Message Routing" anchor="message.routing">
2241   HTTP request message routing is determined by each client based on the
2242   target resource, the client's proxy configuration, and
2243   establishment or reuse of an inbound connection.  The corresponding
2244   response routing follows the same connection chain back to the client.
2247<section title="Identifying a Target Resource" anchor="target-resource">
2248  <iref primary="true" item="target resource"/>
2249  <iref primary="true" item="target URI"/>
2250  <x:anchor-alias value="target resource"/>
2251  <x:anchor-alias value="target URI"/>
2253   HTTP is used in a wide variety of applications, ranging from
2254   general-purpose computers to home appliances.  In some cases,
2255   communication options are hard-coded in a client's configuration.
2256   However, most HTTP clients rely on the same resource identification
2257   mechanism and configuration techniques as general-purpose Web browsers.
2260   HTTP communication is initiated by a user agent for some purpose.
2261   The purpose is a combination of request semantics, which are defined in
2262   <xref target="Part2"/>, and a target resource upon which to apply those
2263   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2264   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2265   would resolve to its absolute form in order to obtain the
2266   "<x:dfn>target URI</x:dfn>".  The target URI
2267   excludes the reference's fragment component, if any,
2268   since fragment identifiers are reserved for client-side processing
2269   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2273<section title="Connecting Inbound" anchor="connecting.inbound">
2275   Once the target URI is determined, a client needs to decide whether
2276   a network request is necessary to accomplish the desired semantics and,
2277   if so, where that request is to be directed.
2280   If the client has a cache <xref target="Part6"/> and the request can be
2281   satisfied by it, then the request is
2282   usually directed there first.
2285   If the request is not satisfied by a cache, then a typical client will
2286   check its configuration to determine whether a proxy is to be used to
2287   satisfy the request.  Proxy configuration is implementation-dependent,
2288   but is often based on URI prefix matching, selective authority matching,
2289   or both, and the proxy itself is usually identified by an "http" or
2290   "https" URI.  If a proxy is applicable, the client connects inbound by
2291   establishing (or reusing) a connection to that proxy.
2294   If no proxy is applicable, a typical client will invoke a handler routine,
2295   usually specific to the target URI's scheme, to connect directly
2296   to an authority for the target resource.  How that is accomplished is
2297   dependent on the target URI scheme and defined by its associated
2298   specification, similar to how this specification defines origin server
2299   access for resolution of the "http" (<xref target="http.uri"/>) and
2300   "https" (<xref target="https.uri"/>) schemes.
2303   HTTP requirements regarding connection management are defined in
2304   <xref target=""/>.
2308<section title="Request Target" anchor="request-target">
2310   Once an inbound connection is obtained,
2311   the client sends an HTTP request message (<xref target="http.message"/>)
2312   with a request-target derived from the target URI.
2313   There are four distinct formats for the request-target, depending on both
2314   the method being requested and whether the request is to a proxy.
2316<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"/>
2317  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2318                 / <x:ref>absolute-form</x:ref>
2319                 / <x:ref>authority-form</x:ref>
2320                 / <x:ref>asterisk-form</x:ref>
2323<section title="origin-form" anchor="origin-form">
2324   <iref item="origin-form (of request-target)"/>
2326   The most common form of request-target is the <x:dfn>origin-form</x:dfn>.
2328<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="origin-form"/>
2329  <x:ref>origin-form</x:ref>    = <x:ref>absolute-path</x:ref> [ "?" <x:ref>query</x:ref> ]
2332   When making a request directly to an origin server, other than a CONNECT
2333   or server-wide OPTIONS request (as detailed below),
2334   a client &MUST; send only the absolute path and query components of
2335   the target URI as the request-target.
2336   If the target URI's path component is empty, the client &MUST; send
2337   "/" as the path within the origin-form of request-target.
2338   A <x:ref>Host</x:ref> header field is also sent, as defined in
2339   <xref target=""/>.
2342   For example, a client wishing to retrieve a representation of the resource
2343   identified as
2345<figure><artwork x:indent-with="  " type="example">
2349   directly from the origin server would open (or reuse) a TCP connection
2350   to port 80 of the host "" and send the lines:
2352<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2353GET /where?q=now HTTP/1.1
2357   followed by the remainder of the request message.
2361<section title="absolute-form" anchor="absolute-form">
2362   <iref item="absolute-form (of request-target)"/>
2364   When making a request to a proxy, other than a CONNECT or server-wide
2365   OPTIONS request (as detailed below), a client &MUST; send the target URI
2366   in <x:dfn>absolute-form</x:dfn> as the request-target.
2368<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="absolute-form"/>
2369  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2372   The proxy is requested to either service that request from a valid cache,
2373   if possible, or make the same request on the client's behalf to either
2374   the next inbound proxy server or directly to the origin server indicated
2375   by the request-target.  Requirements on such "forwarding" of messages are
2376   defined in <xref target="message.forwarding"/>.
2379   An example absolute-form of request-line would be:
2381<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2382GET HTTP/1.1
2385   To allow for transition to the absolute-form for all requests in some
2386   future version of HTTP, a server &MUST; accept the absolute-form
2387   in requests, even though HTTP/1.1 clients will only send them in requests
2388   to proxies.
2392<section title="authority-form" anchor="authority-form">
2393   <iref item="authority-form (of request-target)"/>
2395   The <x:dfn>authority-form</x:dfn> of request-target is only used for
2396   CONNECT requests (&CONNECT;).
2398<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="authority-form"/>
2399  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2402   When making a CONNECT request to establish a
2403   tunnel through one or more proxies, a client &MUST; send only the target
2404   URI's authority component (excluding any userinfo and its "@" delimiter) as
2405   the request-target. For example,
2407<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2412<section title="asterisk-form" anchor="asterisk-form">
2413   <iref item="asterisk-form (of request-target)"/>
2415   The <x:dfn>asterisk-form</x:dfn> of request-target is only used for a server-wide
2416   OPTIONS request (&OPTIONS;).
2418<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="asterisk-form"/>
2419  <x:ref>asterisk-form</x:ref>  = "*"
2422   When a client wishes to request OPTIONS
2423   for the server as a whole, as opposed to a specific named resource of
2424   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2425   For example,
2427<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2428OPTIONS * HTTP/1.1
2431   If a proxy receives an OPTIONS request with an absolute-form of
2432   request-target in which the URI has an empty path and no query component,
2433   then the last proxy on the request chain &MUST; send a request-target
2434   of "*" when it forwards the request to the indicated origin server.
2437   For example, the request
2438</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2442  would be forwarded by the final proxy as
2443</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2444OPTIONS * HTTP/1.1
2448   after connecting to port 8001 of host "".
2454<section title="Host" anchor="">
2455  <iref primary="true" item="Host header field" x:for-anchor=""/>
2456  <x:anchor-alias value="Host"/>
2458   The "Host" header field in a request provides the host and port
2459   information from the target URI, enabling the origin
2460   server to distinguish among resources while servicing requests
2461   for multiple host names on a single IP address.
2463<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2464  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2467   A client &MUST; send a Host header field in all HTTP/1.1 request messages.
2468   If the target URI includes an authority component, then a client &MUST;
2469   send a field-value for Host that is identical to that authority
2470   component, excluding any userinfo subcomponent and its "@" delimiter
2471   (<xref target="http.uri"/>).
2472   If the authority component is missing or undefined for the target URI,
2473   then a client &MUST; send a Host header field with an empty field-value.
2476   Since the Host field-value is critical information for handling a request,
2477   a user agent &SHOULD; generate Host as the first header field following the
2478   request-line.
2481   For example, a GET request to the origin server for
2482   &lt;; would begin with:
2484<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2485GET /pub/WWW/ HTTP/1.1
2489   A client &MUST; send a Host header field in an HTTP/1.1 request even
2490   if the request-target is in the absolute-form, since this
2491   allows the Host information to be forwarded through ancient HTTP/1.0
2492   proxies that might not have implemented Host.
2495   When a proxy receives a request with an absolute-form of
2496   request-target, the proxy &MUST; ignore the received
2497   Host header field (if any) and instead replace it with the host
2498   information of the request-target.  A proxy that forwards such a request
2499   &MUST; generate a new Host field-value based on the received
2500   request-target rather than forward the received Host field-value.
2503   Since the Host header field acts as an application-level routing
2504   mechanism, it is a frequent target for malware seeking to poison
2505   a shared cache or redirect a request to an unintended server.
2506   An interception proxy is particularly vulnerable if it relies on
2507   the Host field-value for redirecting requests to internal
2508   servers, or for use as a cache key in a shared cache, without
2509   first verifying that the intercepted connection is targeting a
2510   valid IP address for that host.
2513   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2514   to any HTTP/1.1 request message that lacks a Host header field and
2515   to any request message that contains more than one Host header field
2516   or a Host header field with an invalid field-value.
2520<section title="Effective Request URI" anchor="effective.request.uri">
2521  <iref primary="true" item="effective request URI"/>
2522  <x:anchor-alias value="effective request URI"/>
2524   Since the request-target often contains only part of the user agent's
2525   target URI, a server reconstructs the intended target as an
2526   "<x:dfn>effective request URI</x:dfn>" to properly service the request.
2527   This reconstruction involves both the server's local configuration and
2528   information communicated in the <x:ref>request-target</x:ref>,
2529   <x:ref>Host</x:ref> header field, and connection context.
2532   For a user agent, the effective request URI is the target URI.
2535   If the <x:ref>request-target</x:ref> is in <x:ref>absolute-form</x:ref>,
2536   the effective request URI is the same as the request-target. Otherwise, the
2537   effective request URI is constructed as follows:
2538<list style="empty">
2540   If the server's configuration (or outbound gateway) provides a fixed URI
2541   <x:ref>scheme</x:ref>, that scheme is used for the effective request URI.
2542   Otherwise, if the request is received over a TLS-secured TCP connection,
2543   the effective request URI's scheme is "https"; if not, the scheme is "http".
2546   If the server's configuration (or outbound gateway) provides a fixed URI
2547   <x:ref>authority</x:ref> component, that authority is used for the
2548   effective request URI. If not, then if the request-target is in
2549   <x:ref>authority-form</x:ref>, the effective request URI's authority
2550   component is the same as the request-target.
2551   If not, then if a <x:ref>Host</x:ref> header field is supplied with a
2552   non-empty field-value, the authority component is the same as the
2553   Host field-value. Otherwise, the authority component is assigned
2554   the default name configured for the server and, if the connection's
2555   incoming TCP port number differs from the default port for the effective
2556   request URI's scheme, then a colon (":") and the incoming port number (in
2557   decimal form) are appended to the authority component.
2560   If the request-target is in <x:ref>authority-form</x:ref> or
2561   <x:ref>asterisk-form</x:ref>, the effective request URI's combined
2562   <x:ref>path</x:ref> and <x:ref>query</x:ref> component is empty. Otherwise,
2563   the combined <x:ref>path</x:ref> and <x:ref>query</x:ref> component is the
2564   same as the request-target.
2567   The components of the effective request URI, once determined as above, can
2568   be combined into <x:ref>absolute-URI</x:ref> form by concatenating the
2569   scheme, "://", authority, and combined path and query component.
2575   Example 1: the following message received over an insecure TCP connection
2577<artwork type="example" x:indent-with="  ">
2578GET /pub/WWW/TheProject.html HTTP/1.1
2584  has an effective request URI of
2586<artwork type="example" x:indent-with="  ">
2592   Example 2: the following message received over a TLS-secured TCP connection
2594<artwork type="example" x:indent-with="  ">
2595OPTIONS * HTTP/1.1
2601  has an effective request URI of
2603<artwork type="example" x:indent-with="  ">
2608   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2609   field might need to use heuristics (e.g., examination of the URI path for
2610   something unique to a particular host) in order to guess the
2611   effective request URI's authority component.
2614   Once the effective request URI has been constructed, an origin server needs
2615   to decide whether or not to provide service for that URI via the connection
2616   in which the request was received. For example, the request might have been
2617   misdirected, deliberately or accidentally, such that the information within
2618   a received <x:ref>request-target</x:ref> or <x:ref>Host</x:ref> header
2619   field differs from the host or port upon which the connection has been
2620   made. If the connection is from a trusted gateway, that inconsistency might
2621   be expected; otherwise, it might indicate an attempt to bypass security
2622   filters, trick the server into delivering non-public content, or poison a
2623   cache. See <xref target="security.considerations"/> for security
2624   considerations regarding message routing.
2628<section title="Associating a Response to a Request" anchor="">
2630   HTTP does not include a request identifier for associating a given
2631   request message with its corresponding one or more response messages.
2632   Hence, it relies on the order of response arrival to correspond exactly
2633   to the order in which requests are made on the same connection.
2634   More than one response message per request only occurs when one or more
2635   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a
2636   final response to the same request.
2639   A client that has more than one outstanding request on a connection &MUST;
2640   maintain a list of outstanding requests in the order sent and &MUST;
2641   associate each received response message on that connection to the highest
2642   ordered request that has not yet received a final (non-<x:ref>1xx</x:ref>)
2643   response.
2647<section title="Message Forwarding" anchor="message.forwarding">
2649   As described in <xref target="intermediaries"/>, intermediaries can serve
2650   a variety of roles in the processing of HTTP requests and responses.
2651   Some intermediaries are used to improve performance or availability.
2652   Others are used for access control or to filter content.
2653   Since an HTTP stream has characteristics similar to a pipe-and-filter
2654   architecture, there are no inherent limits to the extent an intermediary
2655   can enhance (or interfere) with either direction of the stream.
2658   An intermediary not acting as a tunnel &MUST; implement the
2659   <x:ref>Connection</x:ref> header field, as specified in
2660   <xref target="header.connection"/>, and exclude fields from being forwarded
2661   that are only intended for the incoming connection.
2664   An intermediary &MUST-NOT; forward a message to itself unless it is
2665   protected from an infinite request loop. In general, an intermediary ought
2666   to recognize its own server names, including any aliases, local variations,
2667   or literal IP addresses, and respond to such requests directly.
2670<section title="Via" anchor="header.via">
2671  <iref primary="true" item="Via header field" x:for-anchor=""/>
2672  <x:anchor-alias value="pseudonym"/>
2673  <x:anchor-alias value="received-by"/>
2674  <x:anchor-alias value="received-protocol"/>
2675  <x:anchor-alias value="Via"/>
2677   The "Via" header field indicates the presence of intermediate protocols and
2678   recipients between the user agent and the server (on requests) or between
2679   the origin server and the client (on responses), similar to the
2680   "Received" header field in email
2681   (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2682   Via can be used for tracking message forwards,
2683   avoiding request loops, and identifying the protocol capabilities of
2684   senders along the request/response chain.
2686<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"/>
2687  <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> ] )
2689  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2690                      ; see <xref target="header.upgrade"/>
2691  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2692  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2695   Multiple Via field values represent each proxy or gateway that has
2696   forwarded the message. Each intermediary appends its own information
2697   about how the message was received, such that the end result is ordered
2698   according to the sequence of forwarding recipients.
2701   A proxy &MUST; send an appropriate Via header field, as described below, in
2702   each message that it forwards.
2703   An HTTP-to-HTTP gateway &MUST; send an appropriate Via header field in
2704   each inbound request message and &MAY; send a Via header field in
2705   forwarded response messages.
2708   For each intermediary, the received-protocol indicates the protocol and
2709   protocol version used by the upstream sender of the message. Hence, the
2710   Via field value records the advertised protocol capabilities of the
2711   request/response chain such that they remain visible to downstream
2712   recipients; this can be useful for determining what backwards-incompatible
2713   features might be safe to use in response, or within a later request, as
2714   described in <xref target="http.version"/>. For brevity, the protocol-name
2715   is omitted when the received protocol is HTTP.
2718   The received-by portion of the field value is normally the host and optional
2719   port number of a recipient server or client that subsequently forwarded the
2720   message.
2721   However, if the real host is considered to be sensitive information, a
2722   sender &MAY; replace it with a pseudonym. If a port is not provided,
2723   a recipient &MAY; interpret that as meaning it was received on the default
2724   TCP port, if any, for the received-protocol.
2727   A sender &MAY; generate comments in the Via header field to identify the
2728   software of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2729   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2730   are optional and a recipient &MAY; remove them prior to forwarding the
2731   message.
2734   For example, a request message could be sent from an HTTP/1.0 user
2735   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2736   forward the request to a public proxy at, which completes
2737   the request by forwarding it to the origin server at
2738   The request received by would then have the following
2739   Via header field:
2741<figure><artwork type="example">
2742  Via: 1.0 fred, 1.1
2745   An intermediary used as a portal through a network firewall
2746   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2747   region unless it is explicitly enabled to do so. If not enabled, such an
2748   intermediary &SHOULD; replace each received-by host of any host behind the
2749   firewall by an appropriate pseudonym for that host.
2752   An intermediary &MAY; combine an ordered subsequence of Via header
2753   field entries into a single such entry if the entries have identical
2754   received-protocol values. For example,
2756<figure><artwork type="example">
2757  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2760  could be collapsed to
2762<figure><artwork type="example">
2763  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2766   A sender &SHOULD-NOT; combine multiple entries unless they are all
2767   under the same organizational control and the hosts have already been
2768   replaced by pseudonyms. A sender &MUST-NOT; combine entries that
2769   have different received-protocol values.
2773<section title="Transformations" anchor="message.transformations">
2774   <iref primary="true" item="transforming proxy"/>
2775   <iref primary="true" item="non-transforming proxy"/>
2777   Some intermediaries include features for transforming messages and their
2778   payloads. A proxy might, for example, convert between image formats in
2779   order to save cache space or to reduce the amount of traffic on a slow
2780   link. However, operational problems might occur when these transformations
2781   are applied to payloads intended for critical applications, such as medical
2782   imaging or scientific data analysis, particularly when integrity checks or
2783   digital signatures are used to ensure that the payload received is
2784   identical to the original.
2787   An HTTP-to-HTTP proxy is called a "<x:dfn>transforming proxy</x:dfn>"
2788   if it is designed or configured to modify messages in a semantically
2789   meaningful way (i.e., modifications, beyond those required by normal
2790   HTTP processing, that change the message in a way that would be
2791   significant to the original sender or potentially significant to
2792   downstream recipients).  For example, a transforming proxy might be
2793   acting as a shared annotation server (modifying responses to include
2794   references to a local annotation database), a malware filter, a
2795   format transcoder, or a privacy filter. Such transformations are presumed
2796   to be desired by whichever client (or client organization) selected the
2797   proxy.
2800   If a proxy receives a request-target with a host name that is not a
2801   fully qualified domain name, it &MAY; add its own domain to the host name
2802   it received when forwarding the request.  A proxy &MUST-NOT; change the
2803   host name if the request-target contains a fully qualified domain name.
2806   A proxy &MUST-NOT; modify the "absolute-path" and "query" parts of the
2807   received request-target when forwarding it to the next inbound server,
2808   except as noted above to replace an empty path with "/" or "*".
2811   A proxy &MAY; modify the message body through application
2812   or removal of a transfer coding (<xref target="transfer.codings"/>).
2815   A proxy &MUST-NOT; transform the payload (&payload;) of a message that
2816   contains a no-transform cache-control directive (&header-cache-control;).
2819   A proxy &MAY; transform the payload of a message
2820   that does not contain a no-transform cache-control directive.
2821   A proxy that transforms a payload &MUST; add a <x:ref>Warning</x:ref>
2822   header field with the warn-code of 214 ("Transformation Applied")
2823   if one is not already in the message (see &header-warning;).
2824   A proxy that transforms the payload of a <x:ref>200 (OK)</x:ref> response
2825   can further inform downstream recipients that a transformation has been
2826   applied by changing the response status code to
2827   <x:ref>203 (Non-Authoritative Information)</x:ref> (&status-203;).
2830   A proxy &MUST-NOT; modify header fields that provide information about the
2831   end points of the communication chain, the resource state, or the selected
2832   representation.
2838<section title="Connection Management" anchor="">
2840   HTTP messaging is independent of the underlying transport or
2841   session-layer connection protocol(s).  HTTP only presumes a reliable
2842   transport with in-order delivery of requests and the corresponding
2843   in-order delivery of responses.  The mapping of HTTP request and
2844   response structures onto the data units of an underlying transport
2845   protocol is outside the scope of this specification.
2848   As described in <xref target="connecting.inbound"/>, the specific
2849   connection protocols to be used for an HTTP interaction are determined by
2850   client configuration and the <x:ref>target URI</x:ref>.
2851   For example, the "http" URI scheme
2852   (<xref target="http.uri"/>) indicates a default connection of TCP
2853   over IP, with a default TCP port of 80, but the client might be
2854   configured to use a proxy via some other connection, port, or protocol.
2857   HTTP implementations are expected to engage in connection management,
2858   which includes maintaining the state of current connections,
2859   establishing a new connection or reusing an existing connection,
2860   processing messages received on a connection, detecting connection
2861   failures, and closing each connection.
2862   Most clients maintain multiple connections in parallel, including
2863   more than one connection per server endpoint.
2864   Most servers are designed to maintain thousands of concurrent connections,
2865   while controlling request queues to enable fair use and detect
2866   denial of service attacks.
2869<section title="Connection" anchor="header.connection">
2870  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2871  <iref primary="true" item="close" x:for-anchor=""/>
2872  <x:anchor-alias value="Connection"/>
2873  <x:anchor-alias value="connection-option"/>
2874  <x:anchor-alias value="close"/>
2876   The "Connection" header field allows the sender to indicate desired
2877   control options for the current connection.  In order to avoid confusing
2878   downstream recipients, a proxy or gateway &MUST; remove or replace any
2879   received connection options before forwarding the message.
2882   When a header field aside from Connection is used to supply control
2883   information for or about the current connection, the sender &MUST; list
2884   the corresponding field-name within the "Connection" header field.
2885   A proxy or gateway &MUST; parse a received Connection
2886   header field before a message is forwarded and, for each
2887   connection-option in this field, remove any header field(s) from
2888   the message with the same name as the connection-option, and then
2889   remove the Connection header field itself (or replace it with the
2890   intermediary's own connection options for the forwarded message).
2893   Hence, the Connection header field provides a declarative way of
2894   distinguishing header fields that are only intended for the
2895   immediate recipient ("hop-by-hop") from those fields that are
2896   intended for all recipients on the chain ("end-to-end"), enabling the
2897   message to be self-descriptive and allowing future connection-specific
2898   extensions to be deployed without fear that they will be blindly
2899   forwarded by older intermediaries.
2902   The Connection header field's value has the following grammar:
2904<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2905  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2906  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2909   Connection options are case-insensitive.
2912   A sender &MUST-NOT; send a connection option corresponding to a header
2913   field that is intended for all recipients of the payload.
2914   For example, <x:ref>Cache-Control</x:ref> is never appropriate as a
2915   connection option (&header-cache-control;).
2918   The connection options do not always correspond to a header field
2919   present in the message, since a connection-specific header field
2920   might not be needed if there are no parameters associated with a
2921   connection option. In contrast, a connection-specific header field that
2922   is received without a corresponding connection option usually indicates
2923   that the field has been improperly forwarded by an intermediary and
2924   ought to be ignored by the recipient.
2927   When defining new connection options, specification authors ought to survey
2928   existing header field names and ensure that the new connection option does
2929   not share the same name as an already deployed header field.
2930   Defining a new connection option essentially reserves that potential
2931   field-name for carrying additional information related to the
2932   connection option, since it would be unwise for senders to use
2933   that field-name for anything else.
2936   The "<x:dfn>close</x:dfn>" connection option is defined for a
2937   sender to signal that this connection will be closed after completion of
2938   the response. For example,
2940<figure><artwork type="example">
2941  Connection: close
2944   in either the request or the response header fields indicates that the
2945   sender is going to close the connection after the current request/response
2946   is complete (<xref target="persistent.tear-down"/>).
2949   A client that does not support <x:ref>persistent connections</x:ref> &MUST;
2950   send the "close" connection option in every request message.
2953   A server that does not support <x:ref>persistent connections</x:ref> &MUST;
2954   send the "close" connection option in every response message that
2955   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2959<section title="Establishment" anchor="persistent.establishment">
2961   It is beyond the scope of this specification to describe how connections
2962   are established via various transport or session-layer protocols.
2963   Each connection applies to only one transport link.
2967<section title="Persistence" anchor="persistent.connections">
2968   <x:anchor-alias value="persistent connections"/>
2970   HTTP/1.1 defaults to the use of "<x:dfn>persistent connections</x:dfn>",
2971   allowing multiple requests and responses to be carried over a single
2972   connection. The "<x:ref>close</x:ref>" connection-option is used to signal
2973   that a connection will not persist after the current request/response.
2974   HTTP implementations &SHOULD; support persistent connections.
2977   A recipient determines whether a connection is persistent or not based on
2978   the most recently received message's protocol version and
2979   <x:ref>Connection</x:ref> header field (if any):
2980   <list style="symbols">
2981     <t>If the <x:ref>close</x:ref> connection option is present, the
2982        connection will not persist after the current response; else,</t>
2983     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2984        persist after the current response; else,</t>
2985     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2986        connection option is present, the recipient is not a proxy, and
2987        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2988        the connection will persist after the current response; otherwise,</t>
2989     <t>The connection will close after the current response.</t>
2990   </list>
2993   A client &MAY; send additional requests on a persistent connection until it
2994   sends or receives a <x:ref>close</x:ref> connection option or receives an
2995   HTTP/1.0 response without a "keep-alive" connection option.
2998   In order to remain persistent, all messages on a connection need to
2999   have a self-defined message length (i.e., one not defined by closure
3000   of the connection), as described in <xref target="message.body"/>.
3001   A server &MUST; read the entire request message body or close
3002   the connection after sending its response, since otherwise the
3003   remaining data on a persistent connection would be misinterpreted
3004   as the next request.  Likewise,
3005   a client &MUST; read the entire response message body if it intends
3006   to reuse the same connection for a subsequent request.
3009   A proxy server &MUST-NOT; maintain a persistent connection with an
3010   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
3011   information and discussion of the problems with the Keep-Alive header field
3012   implemented by many HTTP/1.0 clients).
3015   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
3016   for more information on backward compatibility with HTTP/1.0 clients.
3019<section title="Retrying Requests" anchor="persistent.retrying.requests">
3021   Connections can be closed at any time, with or without intention.
3022   Implementations ought to anticipate the need to recover
3023   from asynchronous close events.
3026   When an inbound connection is closed prematurely, a client &MAY; open a new
3027   connection and automatically retransmit an aborted sequence of requests if
3028   all of those requests have idempotent methods (&idempotent-methods;).
3029   A proxy &MUST-NOT; automatically retry non-idempotent requests.
3032   A user agent &MUST-NOT; automatically retry a request with a non-idempotent
3033   method unless it has some means to know that the request semantics are
3034   actually idempotent, regardless of the method, or some means to detect that
3035   the original request was never applied. For example, a user agent that
3036   knows (through design or configuration) that a POST request to a given
3037   resource is safe can repeat that request automatically.
3038   Likewise, a user agent designed specifically to operate on a version
3039   control repository might be able to recover from partial failure conditions
3040   by checking the target resource revision(s) after a failed connection,
3041   reverting or fixing any changes that were partially applied, and then
3042   automatically retrying the requests that failed.
3045   A client &SHOULD-NOT; automatically retry a failed automatic retry.
3049<section title="Pipelining" anchor="pipelining">
3050   <x:anchor-alias value="pipeline"/>
3052   A client that supports persistent connections &MAY; "<x:dfn>pipeline</x:dfn>"
3053   its requests (i.e., send multiple requests without waiting for each
3054   response). A server &MAY; process a sequence of pipelined requests in
3055   parallel if they all have safe methods (&safe-methods;), but &MUST; send
3056   the corresponding responses in the same order that the requests were
3057   received.
3060   A client that pipelines requests &SHOULD; retry unanswered requests if the
3061   connection closes before it receives all of the corresponding responses.
3062   When retrying pipelined requests after a failed connection (a connection
3063   not explicitly closed by the server in its last complete response), a
3064   client &MUST-NOT; pipeline immediately after connection establishment,
3065   since the first remaining request in the prior pipeline might have caused
3066   an error response that can be lost again if multiple requests are sent on a
3067   prematurely closed connection (see the TCP reset problem described in
3068   <xref target="persistent.tear-down"/>).
3071   Idempotent methods (&idempotent-methods;) are significant to pipelining
3072   because they can be automatically retried after a connection failure.
3073   A user agent &SHOULD-NOT; pipeline requests after a non-idempotent method,
3074   until the final response status code for that method has been received,
3075   unless the user agent has a means to detect and recover from partial
3076   failure conditions involving the pipelined sequence.
3079   An intermediary that receives pipelined requests &MAY; pipeline those
3080   requests when forwarding them inbound, since it can rely on the outbound
3081   user agent(s) to determine what requests can be safely pipelined. If the
3082   inbound connection fails before receiving a response, the pipelining
3083   intermediary &MAY; attempt to retry a sequence of requests that have yet
3084   to receive a response if the requests all have idempotent methods;
3085   otherwise, the pipelining intermediary &SHOULD; forward any received
3086   responses and then close the corresponding outbound connection(s) so that
3087   the outbound user agent(s) can recover accordingly.
3092<section title="Concurrency" anchor="persistent.concurrency">
3094   A client ought to limit the number of simultaneous open
3095   connections that it maintains to a given server.
3098   Previous revisions of HTTP gave a specific number of connections as a
3099   ceiling, but this was found to be impractical for many applications. As a
3100   result, this specification does not mandate a particular maximum number of
3101   connections, but instead encourages clients to be conservative when opening
3102   multiple connections.
3105   Multiple connections are typically used to avoid the "head-of-line
3106   blocking" problem, wherein a request that takes significant server-side
3107   processing and/or has a large payload blocks subsequent requests on the
3108   same connection. However, each connection consumes server resources.
3109   Furthermore, using multiple connections can cause undesirable side effects
3110   in congested networks.
3113   Note that a server might reject traffic that it deems abusive or
3114   characteristic of a denial of service attack, such as an excessive number
3115   of open connections from a single client.
3119<section title="Failures and Time-outs" anchor="persistent.failures">
3121   Servers will usually have some time-out value beyond which they will
3122   no longer maintain an inactive connection. Proxy servers might make
3123   this a higher value since it is likely that the client will be making
3124   more connections through the same proxy server. The use of persistent
3125   connections places no requirements on the length (or existence) of
3126   this time-out for either the client or the server.
3129   A client or server that wishes to time-out &SHOULD; issue a graceful close
3130   on the connection. Implementations &SHOULD; constantly monitor open
3131   connections for a received closure signal and respond to it as appropriate,
3132   since prompt closure of both sides of a connection enables allocated system
3133   resources to be reclaimed.
3136   A client, server, or proxy &MAY; close the transport connection at any
3137   time. For example, a client might have started to send a new request
3138   at the same time that the server has decided to close the "idle"
3139   connection. From the server's point of view, the connection is being
3140   closed while it was idle, but from the client's point of view, a
3141   request is in progress.
3144   A server &SHOULD; sustain persistent connections, when possible, and allow
3145   the underlying
3146   transport's flow control mechanisms to resolve temporary overloads, rather
3147   than terminate connections with the expectation that clients will retry.
3148   The latter technique can exacerbate network congestion.
3151   A client sending a message body &SHOULD; monitor
3152   the network connection for an error response while it is transmitting
3153   the request. If the client sees a response that indicates the server does
3154   not wish to receive the message body and is closing the connection, the
3155   client &SHOULD; immediately cease transmitting the body and close its side
3156   of the connection.
3160<section title="Tear-down" anchor="persistent.tear-down">
3161  <iref primary="false" item="Connection header field" x:for-anchor=""/>
3162  <iref primary="false" item="close" x:for-anchor=""/>
3164   The <x:ref>Connection</x:ref> header field
3165   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
3166   connection option that a sender &SHOULD; send when it wishes to close
3167   the connection after the current request/response pair.
3170   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
3171   send further requests on that connection (after the one containing
3172   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
3173   final response message corresponding to this request.
3176   A server that receives a <x:ref>close</x:ref> connection option &MUST;
3177   initiate a close of the connection (see below) after it sends the
3178   final response to the request that contained <x:ref>close</x:ref>.
3179   The server &SHOULD; send a <x:ref>close</x:ref> connection option
3180   in its final response on that connection. The server &MUST-NOT; process
3181   any further requests received on that connection.
3184   A server that sends a <x:ref>close</x:ref> connection option &MUST;
3185   initiate a close of the connection (see below) after it sends the
3186   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
3187   any further requests received on that connection.
3190   A client that receives a <x:ref>close</x:ref> connection option &MUST;
3191   cease sending requests on that connection and close the connection
3192   after reading the response message containing the close; if additional
3193   pipelined requests had been sent on the connection, the client &SHOULD-NOT;
3194   assume that they will be processed by the server.
3197   If a server performs an immediate close of a TCP connection, there is a
3198   significant risk that the client will not be able to read the last HTTP
3199   response.  If the server receives additional data from the client on a
3200   fully-closed connection, such as another request that was sent by the
3201   client before receiving the server's response, the server's TCP stack will
3202   send a reset packet to the client; unfortunately, the reset packet might
3203   erase the client's unacknowledged input buffers before they can be read
3204   and interpreted by the client's HTTP parser.
3207   To avoid the TCP reset problem, servers typically close a connection in
3208   stages. First, the server performs a half-close by closing only the write
3209   side of the read/write connection. The server then continues to read from
3210   the connection until it receives a corresponding close by the client, or
3211   until the server is reasonably certain that its own TCP stack has received
3212   the client's acknowledgement of the packet(s) containing the server's last
3213   response. Finally, the server fully closes the connection.
3216   It is unknown whether the reset problem is exclusive to TCP or might also
3217   be found in other transport connection protocols.
3221<section title="Upgrade" anchor="header.upgrade">
3222  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3223  <x:anchor-alias value="Upgrade"/>
3224  <x:anchor-alias value="protocol"/>
3225  <x:anchor-alias value="protocol-name"/>
3226  <x:anchor-alias value="protocol-version"/>
3228   The "Upgrade" header field is intended to provide a simple mechanism
3229   for transitioning from HTTP/1.1 to some other protocol on the same
3230   connection.  A client &MAY; send a list of protocols in the Upgrade
3231   header field of a request to invite the server to switch to one or
3232   more of those protocols, in order of descending preference, before sending
3233   the final response. A server &MAY; ignore a received Upgrade header field
3234   if it wishes to continue using the current protocol on that connection.
3235   Upgrade cannot be used to insist on a protocol change.
3237<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3238  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3240  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3241  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3242  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3245   A server that sends a <x:ref>101 (Switching Protocols)</x:ref> response
3246   &MUST; send an Upgrade header field to indicate the new protocol(s) to
3247   which the connection is being switched; if multiple protocol layers are
3248   being switched, the sender &MUST; list the protocols in layer-ascending
3249   order. A server &MUST-NOT; switch to a protocol that was not indicated by
3250   the client in the corresponding request's Upgrade header field.
3251   A server &MAY; choose to ignore the order of preference indicated by the
3252   client and select the new protocol(s) based on other factors, such as the
3253   nature of the request or the current load on the server.
3256   A server that sends a <x:ref>426 (Upgrade Required)</x:ref> response
3257   &MUST; send an Upgrade header field to indicate the acceptable protocols,
3258   in order of descending preference.
3261   A server &MAY; send an Upgrade header field in any other response to
3262   advertise that it implements support for upgrading to the listed protocols,
3263   in order of descending preference, when appropriate for a future request.
3266   The following is a hypothetical example sent by a client:
3267</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
3268GET /hello.txt HTTP/1.1
3270Connection: upgrade
3271Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3275   The capabilities and nature of the
3276   application-level communication after the protocol change is entirely
3277   dependent upon the new protocol(s) chosen. However, immediately after
3278   sending the 101 response, the server is expected to continue responding to
3279   the original request as if it had received its equivalent within the new
3280   protocol (i.e., the server still has an outstanding request to satisfy
3281   after the protocol has been changed, and is expected to do so without
3282   requiring the request to be repeated).
3285   For example, if the Upgrade header field is received in a GET request
3286   and the server decides to switch protocols, it first responds
3287   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3288   then immediately follows that with the new protocol's equivalent of a
3289   response to a GET on the target resource.  This allows a connection to be
3290   upgraded to protocols with the same semantics as HTTP without the
3291   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3292   protocols unless the received message semantics can be honored by the new
3293   protocol; an OPTIONS request can be honored by any protocol.
3296   The following is an example response to the above hypothetical request:
3297</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
3298HTTP/1.1 101 Switching Protocols
3299Connection: upgrade
3300Upgrade: HTTP/2.0
3302[... data stream switches to HTTP/2.0 with an appropriate response
3303(as defined by new protocol) to the "GET /hello.txt" request ...]
3306   When Upgrade is sent, the sender &MUST; also send a
3307   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3308   that contains an "upgrade" connection option, in order to prevent Upgrade
3309   from being accidentally forwarded by intermediaries that might not implement
3310   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3311   is received in an HTTP/1.0 request.
3314   A client cannot begin using an upgraded protocol on the connection until
3315   it has completely sent the request message (i.e., the client can't change
3316   the protocol it is sending in the middle of a message).
3317   If a server receives both Upgrade and an <x:ref>Expect</x:ref> header field
3318   with the "100-continue" expectation (&header-expect;), the
3319   server &MUST; send a <x:ref>100 (Continue)</x:ref> response before sending
3320   a <x:ref>101 (Switching Protocols)</x:ref> response.
3323   The Upgrade header field only applies to switching protocols on top of the
3324   existing connection; it cannot be used to switch the underlying connection
3325   (transport) protocol, nor to switch the existing communication to a
3326   different connection. For those purposes, it is more appropriate to use a
3327   <x:ref>3xx (Redirection)</x:ref> response (&status-3xx;).
3330   This specification only defines the protocol name "HTTP" for use by
3331   the family of Hypertext Transfer Protocols, as defined by the HTTP
3332   version rules of <xref target="http.version"/> and future updates to this
3333   specification. Additional tokens ought to be registered with IANA using the
3334   registration procedure defined in <xref target="upgrade.token.registry"/>.
3339<section title="ABNF list extension: #rule" anchor="abnf.extension">
3341   A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
3342   improve readability in the definitions of some header field values.
3345   A construct "#" is defined, similar to "*", for defining comma-delimited
3346   lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
3347   at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
3348   comma (",") and optional whitespace (OWS).   
3351   In any production that uses the list construct, a sender &MUST-NOT;
3352   generate empty list elements. In other words, a sender &MUST; generate
3353   lists that satisfy the following syntax:
3354</preamble><artwork type="example">
3355  1#element =&gt; element *( OWS "," OWS element )
3358   and:
3359</preamble><artwork type="example">
3360  #element =&gt; [ 1#element ]
3363   and for n &gt;= 1 and m &gt; 1:
3364</preamble><artwork type="example">
3365  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
3368   For compatibility with legacy list rules, a recipient &MUST; parse and ignore
3369   a reasonable number of empty list elements: enough to handle common mistakes
3370   by senders that merge values, but not so much that they could be used as a
3371   denial of service mechanism. In other words, a recipient &MUST; accept lists
3372   that satisfy the following syntax:
3374<figure><artwork type="example">
3375  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
3377  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
3380   Empty elements do not contribute to the count of elements present.
3381   For example, given these ABNF productions:
3383<figure><artwork type="example">
3384  example-list      = 1#example-list-elmt
3385  example-list-elmt = token ; see <xref target="field.components"/>
3388   Then the following are valid values for example-list (not including the
3389   double quotes, which are present for delimitation only):
3391<figure><artwork type="example">
3392  "foo,bar"
3393  "foo ,bar,"
3394  "foo , ,bar,charlie   "
3397   In contrast, the following values would be invalid, since at least one
3398   non-empty element is required by the example-list production:
3400<figure><artwork type="example">
3401  ""
3402  ","
3403  ",   ,"
3406   <xref target="collected.abnf"/> shows the collected ABNF for recipients
3407   after the list constructs have been expanded.
3411<section title="IANA Considerations" anchor="IANA.considerations">
3413<section title="Header Field Registration" anchor="header.field.registration">
3415   HTTP header fields are registered within the Message Header Field Registry
3416   maintained at
3417   <eref target=""/>.
3420   This document defines the following HTTP header fields, so their
3421   associated registry entries shall be updated according to the permanent
3422   registrations below (see <xref target="BCP90"/>):
3424<?BEGININC p1-messaging.iana-headers ?>
3425<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3426<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3427   <ttcol>Header Field Name</ttcol>
3428   <ttcol>Protocol</ttcol>
3429   <ttcol>Status</ttcol>
3430   <ttcol>Reference</ttcol>
3432   <c>Connection</c>
3433   <c>http</c>
3434   <c>standard</c>
3435   <c>
3436      <xref target="header.connection"/>
3437   </c>
3438   <c>Content-Length</c>
3439   <c>http</c>
3440   <c>standard</c>
3441   <c>
3442      <xref target="header.content-length"/>
3443   </c>
3444   <c>Host</c>
3445   <c>http</c>
3446   <c>standard</c>
3447   <c>
3448      <xref target=""/>
3449   </c>
3450   <c>TE</c>
3451   <c>http</c>
3452   <c>standard</c>
3453   <c>
3454      <xref target="header.te"/>
3455   </c>
3456   <c>Trailer</c>
3457   <c>http</c>
3458   <c>standard</c>
3459   <c>
3460      <xref target="header.trailer"/>
3461   </c>
3462   <c>Transfer-Encoding</c>
3463   <c>http</c>
3464   <c>standard</c>
3465   <c>
3466      <xref target="header.transfer-encoding"/>
3467   </c>
3468   <c>Upgrade</c>
3469   <c>http</c>
3470   <c>standard</c>
3471   <c>
3472      <xref target="header.upgrade"/>
3473   </c>
3474   <c>Via</c>
3475   <c>http</c>
3476   <c>standard</c>
3477   <c>
3478      <xref target="header.via"/>
3479   </c>
3482<?ENDINC p1-messaging.iana-headers ?>
3484   Furthermore, the header field-name "Close" shall be registered as
3485   "reserved", since using that name as an HTTP header field might
3486   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3487   header field (<xref target="header.connection"/>).
3489<texttable align="left" suppress-title="true">
3490   <ttcol>Header Field Name</ttcol>
3491   <ttcol>Protocol</ttcol>
3492   <ttcol>Status</ttcol>
3493   <ttcol>Reference</ttcol>
3495   <c>Close</c>
3496   <c>http</c>
3497   <c>reserved</c>
3498   <c>
3499      <xref target="header.field.registration"/>
3500   </c>
3503   The change controller is: "IETF ( - Internet Engineering Task Force".
3507<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3509   IANA maintains the registry of URI Schemes <xref target="BCP115"/> at
3510   <eref target=""/>.
3513   This document defines the following URI schemes, so their
3514   associated registry entries shall be updated according to the permanent
3515   registrations below:
3517<texttable align="left" suppress-title="true">
3518   <ttcol>URI Scheme</ttcol>
3519   <ttcol>Description</ttcol>
3520   <ttcol>Reference</ttcol>
3522   <c>http</c>
3523   <c>Hypertext Transfer Protocol</c>
3524   <c><xref target="http.uri"/></c>
3526   <c>https</c>
3527   <c>Hypertext Transfer Protocol Secure</c>
3528   <c><xref target="https.uri"/></c>
3532<section title="Internet Media Type Registration" anchor="">
3534   IANA maintains the registry of Internet media types <xref target="BCP13"/> at
3535   <eref target=""/>.
3538   This document serves as the specification for the Internet media types
3539   "message/http" and "application/http". The following is to be registered with
3540   IANA.
3542<section title="Internet Media Type message/http" anchor="">
3543<iref item="Media Type" subitem="message/http" primary="true"/>
3544<iref item="message/http Media Type" primary="true"/>
3546   The message/http type can be used to enclose a single HTTP request or
3547   response message, provided that it obeys the MIME restrictions for all
3548   "message" types regarding line length and encodings.
3551  <list style="hanging" x:indent="12em">
3552    <t hangText="Type name:">
3553      message
3554    </t>
3555    <t hangText="Subtype name:">
3556      http
3557    </t>
3558    <t hangText="Required parameters:">
3559      N/A
3560    </t>
3561    <t hangText="Optional parameters:">
3562      version, msgtype
3563      <list style="hanging">
3564        <t hangText="version:">
3565          The HTTP-version number of the enclosed message
3566          (e.g., "1.1"). If not present, the version can be
3567          determined from the first line of the body.
3568        </t>
3569        <t hangText="msgtype:">
3570          The message type &mdash; "request" or "response". If not
3571          present, the type can be determined from the first
3572          line of the body.
3573        </t>
3574      </list>
3575    </t>
3576    <t hangText="Encoding considerations:">
3577      only "7bit", "8bit", or "binary" are permitted
3578    </t>
3579    <t hangText="Security considerations:">
3580      see <xref target="security.considerations"/>
3581    </t>
3582    <t hangText="Interoperability considerations:">
3583      N/A
3584    </t>
3585    <t hangText="Published specification:">
3586      This specification (see <xref target=""/>).
3587    </t>
3588    <t hangText="Applications that use this media type:">
3589      N/A
3590    </t>
3591    <t hangText="Fragment identifier considerations:">
3592      N/A
3593    </t>
3594    <t hangText="Additional information:">
3595      <list style="hanging">
3596        <t hangText="Magic number(s):">N/A</t>
3597        <t hangText="Deprecated alias names for this type:">N/A</t>
3598        <t hangText="File extension(s):">N/A</t>
3599        <t hangText="Macintosh file type code(s):">N/A</t>
3600      </list>
3601    </t>
3602    <t hangText="Person and email address to contact for further information:">
3603      See Authors Section.
3604    </t>
3605    <t hangText="Intended usage:">
3606      COMMON
3607    </t>
3608    <t hangText="Restrictions on usage:">
3609      N/A
3610    </t>
3611    <t hangText="Author:">
3612      See Authors Section.
3613    </t>
3614    <t hangText="Change controller:">
3615      IESG
3616    </t>
3617  </list>
3620<section title="Internet Media Type application/http" anchor="">
3621<iref item="Media Type" subitem="application/http" primary="true"/>
3622<iref item="application/http Media Type" primary="true"/>
3624   The application/http type can be used to enclose a pipeline of one or more
3625   HTTP request or response messages (not intermixed).
3628  <list style="hanging" x:indent="12em">
3629    <t hangText="Type name:">
3630      application
3631    </t>
3632    <t hangText="Subtype name:">
3633      http
3634    </t>
3635    <t hangText="Required parameters:">
3636      N/A
3637    </t>
3638    <t hangText="Optional parameters:">
3639      version, msgtype
3640      <list style="hanging">
3641        <t hangText="version:">
3642          The HTTP-version number of the enclosed messages
3643          (e.g., "1.1"). If not present, the version can be
3644          determined from the first line of the body.
3645        </t>
3646        <t hangText="msgtype:">
3647          The message type &mdash; "request" or "response". If not
3648          present, the type can be determined from the first
3649          line of the body.
3650        </t>
3651      </list>
3652    </t>
3653    <t hangText="Encoding considerations:">
3654      HTTP messages enclosed by this type
3655      are in "binary" format; use of an appropriate
3656      Content-Transfer-Encoding is required when
3657      transmitted via E-mail.
3658    </t>
3659    <t hangText="Security considerations:">
3660      see <xref target="security.considerations"/>
3661    </t>
3662    <t hangText="Interoperability considerations:">
3663      N/A
3664    </t>
3665    <t hangText="Published specification:">
3666      This specification (see <xref target=""/>).
3667    </t>
3668    <t hangText="Applications that use this media type:">
3669      N/A
3670    </t>
3671    <t hangText="Fragment identifier considerations:">
3672      N/A
3673    </t>
3674    <t hangText="Additional information:">
3675      <list style="hanging">
3676        <t hangText="Deprecated alias names for this type:">N/A</t>
3677        <t hangText="Magic number(s):">N/A</t>
3678        <t hangText="File extension(s):">N/A</t>
3679        <t hangText="Macintosh file type code(s):">N/A</t>
3680      </list>
3681    </t>
3682    <t hangText="Person and email address to contact for further information:">
3683      See Authors Section.
3684    </t>
3685    <t hangText="Intended usage:">
3686      COMMON
3687    </t>
3688    <t hangText="Restrictions on usage:">
3689      N/A
3690    </t>
3691    <t hangText="Author:">
3692      See Authors Section.
3693    </t>
3694    <t hangText="Change controller:">
3695      IESG
3696    </t>
3697  </list>
3702<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3704   The HTTP Transfer Coding Registry defines the name space for transfer
3705   coding names. It is maintained at <eref target=""/>.
3708<section title="Procedure" anchor="transfer.coding.registry.procedure">
3710   Registrations &MUST; include the following fields:
3711   <list style="symbols">
3712     <t>Name</t>
3713     <t>Description</t>
3714     <t>Pointer to specification text</t>
3715   </list>
3718   Names of transfer codings &MUST-NOT; overlap with names of content codings
3719   (&content-codings;) unless the encoding transformation is identical, as
3720   is the case for the compression codings defined in
3721   <xref target="compression.codings"/>.
3724   Values to be added to this name space require IETF Review (see
3725   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3726   conform to the purpose of transfer coding defined in this specification.
3729   Use of program names for the identification of encoding formats
3730   is not desirable and is discouraged for future encodings.
3734<section title="Registration" anchor="transfer.coding.registration">
3736   The HTTP Transfer Coding Registry shall be updated with the registrations
3737   below:
3739<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3740   <ttcol>Name</ttcol>
3741   <ttcol>Description</ttcol>
3742   <ttcol>Reference</ttcol>
3743   <c>chunked</c>
3744   <c>Transfer in a series of chunks</c>
3745   <c>
3746      <xref target="chunked.encoding"/>
3747   </c>
3748   <c>compress</c>
3749   <c>UNIX "compress" data format <xref target="Welch"/></c>
3750   <c>
3751      <xref target="compress.coding"/>
3752   </c>
3753   <c>deflate</c>
3754   <c>"deflate" compressed data (<xref target="RFC1951"/>) inside
3755   the "zlib" data format (<xref target="RFC1950"/>)
3756   </c>
3757   <c>
3758      <xref target="deflate.coding"/>
3759   </c>
3760   <c>gzip</c>
3761   <c>GZIP file format <xref target="RFC1952"/></c>
3762   <c>
3763      <xref target="gzip.coding"/>
3764   </c>
3765   <c>x-compress</c>
3766   <c>Deprecated (alias for compress)</c>
3767   <c>
3768      <xref target="compress.coding"/>
3769   </c>
3770   <c>x-gzip</c>
3771   <c>Deprecated (alias for gzip)</c>
3772   <c>
3773      <xref target="gzip.coding"/>
3774   </c>
3779<section title="Content Coding Registration" anchor="content.coding.registration">
3781   IANA maintains the registry of HTTP Content Codings at
3782   <eref target=""/>.
3785   The HTTP Content Codings Registry shall be updated with the registrations
3786   below:
3788<texttable align="left" suppress-title="true" anchor="iana.content.coding.registration.table">
3789   <ttcol>Name</ttcol>
3790   <ttcol>Description</ttcol>
3791   <ttcol>Reference</ttcol>
3792   <c>compress</c>
3793   <c>UNIX "compress" data format <xref target="Welch"/></c>
3794   <c>
3795      <xref target="compress.coding"/>
3796   </c>
3797   <c>deflate</c>
3798   <c>"deflate" compressed data (<xref target="RFC1951"/>) inside
3799   the "zlib" data format (<xref target="RFC1950"/>)</c>
3800   <c>
3801      <xref target="deflate.coding"/>
3802   </c>
3803   <c>gzip</c>
3804   <c>GZIP file format <xref target="RFC1952"/></c>
3805   <c>
3806      <xref target="gzip.coding"/>
3807   </c>
3808   <c>x-compress</c>
3809   <c>Deprecated (alias for compress)</c>
3810   <c>
3811      <xref target="compress.coding"/>
3812   </c>
3813   <c>x-gzip</c>
3814   <c>Deprecated (alias for gzip)</c>
3815   <c>
3816      <xref target="gzip.coding"/>
3817   </c>
3821<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3823   The HTTP Upgrade Token Registry defines the name space for protocol-name
3824   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3825   field. The registry is maintained at <eref target=""/>.
3828<section title="Procedure" anchor="upgrade.token.registry.procedure">  
3830   Each registered protocol name is associated with contact information
3831   and an optional set of specifications that details how the connection
3832   will be processed after it has been upgraded.
3835   Registrations happen on a "First Come First Served" basis (see
3836   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3837   following rules:
3838  <list style="numbers">
3839    <t>A protocol-name token, once registered, stays registered forever.</t>
3840    <t>The registration &MUST; name a responsible party for the
3841       registration.</t>
3842    <t>The registration &MUST; name a point of contact.</t>
3843    <t>The registration &MAY; name a set of specifications associated with
3844       that token. Such specifications need not be publicly available.</t>
3845    <t>The registration &SHOULD; name a set of expected "protocol-version"
3846       tokens associated with that token at the time of registration.</t>
3847    <t>The responsible party &MAY; change the registration at any time.
3848       The IANA will keep a record of all such changes, and make them
3849       available upon request.</t>
3850    <t>The IESG &MAY; reassign responsibility for a protocol token.
3851       This will normally only be used in the case when a
3852       responsible party cannot be contacted.</t>
3853  </list>
3856   This registration procedure for HTTP Upgrade Tokens replaces that
3857   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3861<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3863   The "HTTP" entry in the HTTP Upgrade Token Registry shall be updated with
3864   the registration below:
3866<texttable align="left" suppress-title="true">
3867   <ttcol>Value</ttcol>
3868   <ttcol>Description</ttcol>
3869   <ttcol>Expected Version Tokens</ttcol>
3870   <ttcol>Reference</ttcol>
3872   <c>HTTP</c>
3873   <c>Hypertext Transfer Protocol</c>
3874   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3875   <c><xref target="http.version"/></c>
3878   The responsible party is: "IETF ( - Internet Engineering Task Force".
3885<section title="Security Considerations" anchor="security.considerations">
3887   This section is meant to inform developers, information providers, and
3888   users of known security considerations relevant to HTTP message syntax,
3889   parsing, and routing. Security considerations about HTTP semantics and
3890   payloads are addressed in &semantics;.
3893<section title="DNS-related Attacks" anchor="dns.related.attacks">
3895   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3896   generally prone to security attacks based on the deliberate misassociation
3897   of IP addresses and DNS names not protected by DNSSEC. Clients need to be
3898   cautious in assuming the validity of an IP number/DNS name association unless
3899   the response is protected by DNSSEC (<xref target="RFC4033"/>).
3903<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3905   By their very nature, HTTP intermediaries are men-in-the-middle, and
3906   represent an opportunity for man-in-the-middle attacks. Compromise of
3907   the systems on which the intermediaries run can result in serious security
3908   and privacy problems. Intermediaries have access to security-related
3909   information, personal information about individual users and
3910   organizations, and proprietary information belonging to users and
3911   content providers. A compromised intermediary, or an intermediary
3912   implemented or configured without regard to security and privacy
3913   considerations, might be used in the commission of a wide range of
3914   potential attacks.
3917   Intermediaries that contain a shared cache are especially vulnerable
3918   to cache poisoning attacks.
3921   Implementers need to consider the privacy and security
3922   implications of their design and coding decisions, and of the
3923   configuration options they provide to operators (especially the
3924   default configuration).
3927   Users need to be aware that intermediaries are no more trustworthy than
3928   the people who run them; HTTP itself cannot solve this problem.
3932<section title="Buffer Overflows" anchor="attack.protocol.element.size.overflows">
3934   Because HTTP uses mostly textual, character-delimited fields, attackers can
3935   overflow buffers in implementations, and/or perform a Denial of Service
3936   against implementations that accept fields with unlimited lengths.
3939   To promote interoperability, this specification makes specific
3940   recommendations for minimum size limits on request-line
3941   (<xref target="request.line"/>)
3942   and header fields (<xref target="header.fields"/>). These are
3943   minimum recommendations, chosen to be supportable even by implementations
3944   with limited resources; it is expected that most implementations will
3945   choose substantially higher limits.
3948   This specification also provides a way for servers to reject messages that
3949   have request-targets that are too long (&status-414;) or request entities
3950   that are too large (&status-4xx;). Additional status codes related to
3951   capacity limits have been defined by extensions to HTTP
3952   <xref target="RFC6585"/>.
3955   Recipients ought to carefully limit the extent to which they read other
3956   fields, including (but not limited to) request methods, response status
3957   phrases, header field-names, and body chunks, so as to avoid denial of
3958   service attacks without impeding interoperability.
3962<section title="Message Integrity" anchor="message.integrity">
3964   HTTP does not define a specific mechanism for ensuring message integrity,
3965   instead relying on the error-detection ability of underlying transport
3966   protocols and the use of length or chunk-delimited framing to detect
3967   completeness. Additional integrity mechanisms, such as hash functions or
3968   digital signatures applied to the content, can be selectively added to
3969   messages via extensible metadata header fields. Historically, the lack of
3970   a single integrity mechanism has been justified by the informal nature of
3971   most HTTP communication.  However, the prevalence of HTTP as an information
3972   access mechanism has resulted in its increasing use within environments
3973   where verification of message integrity is crucial.
3976   User agents are encouraged to implement configurable means for detecting
3977   and reporting failures of message integrity such that those means can be
3978   enabled within environments for which integrity is necessary. For example,
3979   a browser being used to view medical history or drug interaction
3980   information needs to indicate to the user when such information is detected
3981   by the protocol to be incomplete, expired, or corrupted during transfer.
3982   Such mechanisms might be selectively enabled via user agent extensions or
3983   the presence of message integrity metadata in a response.
3984   At a minimum, user agents ought to provide some indication that allows a
3985   user to distinguish between a complete and incomplete response message
3986   (<xref target="incomplete.messages"/>) when such verification is desired.
3990<section title="Server Log Information" anchor="abuse.of.server.log.information">
3992   A server is in the position to save personal data about a user's requests
3993   over time, which might identify their reading patterns or subjects of
3994   interest.  In particular, log information gathered at an intermediary
3995   often contains a history of user agent interaction, across a multitude
3996   of sites, that can be traced to individual users.
3999   HTTP log information is confidential in nature; its handling is often
4000   constrained by laws and regulations.  Log information needs to be securely
4001   stored and appropriate guidelines followed for its analysis.
4002   Anonymization of personal information within individual entries helps,
4003   but is generally not sufficient to prevent real log traces from being
4004   re-identified based on correlation with other access characteristics.
4005   As such, access traces that are keyed to a specific client are unsafe to
4006   publish even if the key is pseudonymous.
4009   To minimize the risk of theft or accidental publication, log information
4010   ought to be purged of personally identifiable information, including
4011   user identifiers, IP addresses, and user-provided query parameters,
4012   as soon as that information is no longer necessary to support operational
4013   needs for security, auditing, or fraud control.
4018<section title="Acknowledgments" anchor="acks">
4020   This edition of HTTP/1.1 builds on the many contributions that went into
4021   <xref target="RFC1945" format="none">RFC 1945</xref>,
4022   <xref target="RFC2068" format="none">RFC 2068</xref>,
4023   <xref target="RFC2145" format="none">RFC 2145</xref>, and
4024   <xref target="RFC2616" format="none">RFC 2616</xref>, including
4025   substantial contributions made by the previous authors, editors, and
4026   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
4027   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
4028   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
4031   Since 1999, the following contributors have helped improve the HTTP
4032   specification by reporting bugs, asking smart questions, drafting or
4033   reviewing text, and evaluating open issues:
4035<?BEGININC acks ?>
4036<t>Adam Barth,
4037Adam Roach,
4038Addison Phillips,
4039Adrian Chadd,
4040Adrian Cole,
4041Adrien W. de Croy,
4042Alan Ford,
4043Alan Ruttenberg,
4044Albert Lunde,
4045Alek Storm,
4046Alex Rousskov,
4047Alexandre Morgaut,
4048Alexey Melnikov,
4049Alisha Smith,
4050Amichai Rothman,
4051Amit Klein,
4052Amos Jeffries,
4053Andreas Maier,
4054Andreas Petersson,
4055Andrei Popov,
4056Anil Sharma,
4057Anne van Kesteren,
4058Anthony Bryan,
4059Asbjorn Ulsberg,
4060Ashok Kumar,
4061Balachander Krishnamurthy,
4062Barry Leiba,
4063Ben Laurie,
4064Benjamin Carlyle,
4065Benjamin Niven-Jenkins,
4066Benoit Claise,
4067Bil Corry,
4068Bill Burke,
4069Bjoern Hoehrmann,
4070Bob Scheifler,
4071Boris Zbarsky,
4072Brett Slatkin,
4073Brian Kell,
4074Brian McBarron,
4075Brian Pane,
4076Brian Raymor,
4077Brian Smith,
4078Bruce Perens,
4079Bryce Nesbitt,
4080Cameron Heavon-Jones,
4081Carl Kugler,
4082Carsten Bormann,
4083Charles Fry,
4084Chris Burdess,
4085Chris Newman,
4086Christian Huitema,
4087Cyrus Daboo,
4088Dale Robert Anderson,
4089Dan Wing,
4090Dan Winship,
4091Daniel Stenberg,
4092Darrel Miller,
4093Dave Cridland,
4094Dave Crocker,
4095Dave Kristol,
4096Dave Thaler,
4097David Booth,
4098David Singer,
4099David W. Morris,
4100Diwakar Shetty,
4101Dmitry Kurochkin,
4102Drummond Reed,
4103Duane Wessels,
4104Edward Lee,
4105Eitan Adler,
4106Eliot Lear,
4107Emile Stephan,
4108Eran Hammer-Lahav,
4109Eric D. Williams,
4110Eric J. Bowman,
4111Eric Lawrence,
4112Eric Rescorla,
4113Erik Aronesty,
4114EungJun Yi,
4115Evan Prodromou,
4116Felix Geisendoerfer,
4117Florian Weimer,
4118Frank Ellermann,
4119Fred Akalin,
4120Fred Bohle,
4121Frederic Kayser,
4122Gabor Molnar,
4123Gabriel Montenegro,
4124Geoffrey Sneddon,
4125Gervase Markham,
4126Gili Tzabari,
4127Grahame Grieve,
4128Greg Slepak,
4129Greg Wilkins,
4130Grzegorz Calkowski,
4131Harald Tveit Alvestrand,
4132Harry Halpin,
4133Helge Hess,
4134Henrik Nordstrom,
4135Henry S. Thompson,
4136Henry Story,
4137Herbert van de Sompel,
4138Herve Ruellan,
4139Howard Melman,
4140Hugo Haas,
4141Ian Fette,
4142Ian Hickson,
4143Ido Safruti,
4144Ilari Liusvaara,
4145Ilya Grigorik,
4146Ingo Struck,
4147J. Ross Nicoll,
4148James Cloos,
4149James H. Manger,
4150James Lacey,
4151James M. Snell,
4152Jamie Lokier,
4153Jan Algermissen,
4154Jari Arkko,
4155Jeff Hodges (who came up with the term 'effective Request-URI'),
4156Jeff Pinner,
4157Jeff Walden,
4158Jim Luther,
4159Jitu Padhye,
4160Joe D. Williams,
4161Joe Gregorio,
4162Joe Orton,
4163Joel Jaeggli,
4164John C. Klensin,
4165John C. Mallery,
4166John Cowan,
4167John Kemp,
4168John Panzer,
4169John Schneider,
4170John Stracke,
4171John Sullivan,
4172Jonas Sicking,
4173Jonathan A. Rees,
4174Jonathan Billington,
4175Jonathan Moore,
4176Jonathan Silvera,
4177Jordi Ros,
4178Joris Dobbelsteen,
4179Josh Cohen,
4180Julien Pierre,
4181Jungshik Shin,
4182Justin Chapweske,
4183Justin Erenkrantz,
4184Justin James,
4185Kalvinder Singh,
4186Karl Dubost,
4187Kathleen Moriarty,
4188Keith Hoffman,
4189Keith Moore,
4190Ken Murchison,
4191Koen Holtman,
4192Konstantin Voronkov,
4193Kris Zyp,
4194Leif Hedstrom,
4195Lionel Morand,
4196Lisa Dusseault,
4197Maciej Stachowiak,
4198Manu Sporny,
4199Marc Schneider,
4200Marc Slemko,
4201Mark Baker,
4202Mark Pauley,
4203Mark Watson,
4204Markus Isomaki,
4205Markus Lanthaler,
4206Martin J. Duerst,
4207Martin Musatov,
4208Martin Nilsson,
4209Martin Thomson,
4210Matt Lynch,
4211Matthew Cox,
4212Matthew Kerwin,
4213Max Clark,
4214Menachem Dodge,
4215Meral Shirazipour,
4216Michael Burrows,
4217Michael Hausenblas,
4218Michael Scharf,
4219Michael Sweet,
4220Michael Tuexen,
4221Michael Welzl,
4222Mike Amundsen,
4223Mike Belshe,
4224Mike Bishop,
4225Mike Kelly,
4226Mike Schinkel,
4227Miles Sabin,
4228Murray S. Kucherawy,
4229Mykyta Yevstifeyev,
4230Nathan Rixham,
4231Nicholas Shanks,
4232Nico Williams,
4233Nicolas Alvarez,
4234Nicolas Mailhot,
4235Noah Slater,
4236Osama Mazahir,
4237Pablo Castro,
4238Pat Hayes,
4239Patrick R. McManus,
4240Paul E. Jones,
4241Paul Hoffman,
4242Paul Marquess,
4243Pete Resnick,
4244Peter Lepeska,
4245Peter Occil,
4246Peter Saint-Andre,
4247Peter Watkins,
4248Phil Archer,
4249Phil Hunt,
4250Philippe Mougin,
4251Phillip Hallam-Baker,
4252Piotr Dobrogost,
4253Poul-Henning Kamp,
4254Preethi Natarajan,
4255Rajeev Bector,
4256Ray Polk,
4257Reto Bachmann-Gmuer,
4258Richard Barnes,
4259Richard Cyganiak,
4260Rob Trace,
4261Robby Simpson,
4262Robert Brewer,
4263Robert Collins,
4264Robert Mattson,
4265Robert O'Callahan,
4266Robert Olofsson,
4267Robert Sayre,
4268Robert Siemer,
4269Robert de Wilde,
4270Roberto Javier Godoy,
4271Roberto Peon,
4272Roland Zink,
4273Ronny Widjaja,
4274Ryan Hamilton,
4275S. Mike Dierken,
4276Salvatore Loreto,
4277Sam Johnston,
4278Sam Pullara,
4279Sam Ruby,
4280Saurabh Kulkarni,
4281Scott Lawrence (who maintained the original issues list),
4282Sean B. Palmer,
4283Sean Turner,
4284Sebastien Barnoud,
4285Shane McCarron,
4286Shigeki Ohtsu,
4287Simon Yarde,
4288Stefan Eissing,
4289Stefan Tilkov,
4290Stefanos Harhalakis,
4291Stephane Bortzmeyer,
4292Stephen Farrell,
4293Stephen Kent,
4294Stephen Ludin,
4295Stuart Williams,
4296Subbu Allamaraju,
4297Subramanian Moonesamy,
4298Susan Hares,
4299Sylvain Hellegouarch,
4300Tapan Divekar,
4301Tatsuhiro Tsujikawa,
4302Tatsuya Hayashi,
4303Ted Hardie,
4304Ted Lemon,
4305Thomas Broyer,
4306Thomas Fossati,
4307Thomas Maslen,
4308Thomas Nadeau,
4309Thomas Nordin,
4310Thomas Roessler,
4311Tim Bray,
4312Tim Morgan,
4313Tim Olsen,
4314Tom Zhou,
4315Travis Snoozy,
4316Tyler Close,
4317Vincent Murphy,
4318Wenbo Zhu,
4319Werner Baumann,
4320Wilbur Streett,
4321Wilfredo Sanchez Vega,
4322William A. Rowe Jr.,
4323William Chan,
4324Willy Tarreau,
4325Xiaoshu Wang,
4326Yaron Goland,
4327Yngve Nysaeter Pettersen,
4328Yoav Nir,
4329Yogesh Bang,
4330Yuchung Cheng,
4331Yutaka Oiwa,
4332Yves Lafon (long-time member of the editor team),
4333Zed A. Shaw, and
4334Zhong Yu.
4336<?ENDINC acks ?>
4338   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
4339   acknowledgements from prior revisions.
4346<references title="Normative References">
4348<reference anchor="Part2">
4349  <front>
4350    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
4351    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4352      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4353      <address><email></email></address>
4354    </author>
4355    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4356      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4357      <address><email></email></address>
4358    </author>
4359    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4360  </front>
4361  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
4362  <x:source href="p2-semantics.xml" basename="p2-semantics">
4363    <x:defines>1xx (Informational)</x:defines>
4364    <x:defines>1xx</x:defines>
4365    <x:defines>100 (Continue)</x:defines>
4366    <x:defines>101 (Switching Protocols)</x:defines>
4367    <x:defines>2xx (Successful)</x:defines>
4368    <x:defines>2xx</x:defines>
4369    <x:defines>200 (OK)</x:defines>
4370    <x:defines>203 (Non-Authoritative Information)</x:defines>
4371    <x:defines>204 (No Content)</x:defines>
4372    <x:defines>3xx (Redirection)</x:defines>
4373    <x:defines>3xx</x:defines>
4374    <x:defines>301 (Moved Permanently)</x:defines>
4375    <x:defines>4xx (Client Error)</x:defines>
4376    <x:defines>4xx</x:defines>
4377    <x:defines>400 (Bad Request)</x:defines>
4378    <x:defines>411 (Length Required)</x:defines>
4379    <x:defines>414 (URI Too Long)</x:defines>
4380    <x:defines>417 (Expectation Failed)</x:defines>
4381    <x:defines>426 (Upgrade Required)</x:defines>
4382    <x:defines>501 (Not Implemented)</x:defines>
4383    <x:defines>502 (Bad Gateway)</x:defines>
4384    <x:defines>505 (HTTP Version Not Supported)</x:defines>
4385    <x:defines>Accept-Encoding</x:defines>
4386    <x:defines>Allow</x:defines>
4387    <x:defines>Content-Encoding</x:defines>
4388    <x:defines>Content-Location</x:defines>
4389    <x:defines>Content-Type</x:defines>
4390    <x:defines>Date</x:defines>
4391    <x:defines>Expect</x:defines>
4392    <x:defines>Location</x:defines>
4393    <x:defines>Server</x:defines>
4394    <x:defines>User-Agent</x:defines>
4395  </x:source>
4398<reference anchor="Part4">
4399  <front>
4400    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
4401    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
4402      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4403      <address><email></email></address>
4404    </author>
4405    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
4406      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4407      <address><email></email></address>
4408    </author>
4409    <date month="&ID-MONTH;" year="&ID-YEAR;" />
4410  </front>
4411  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
4412  <x:source basename="p4-conditional" href="p4-conditional.xml">
4413    <x:defines>304 (Not Modified)</x:defines>
4414    <x:defines>ETag</x:defines>
4415    <x:defines>Last-Modified</x:defines>
4416  </x:source>
4419<reference anchor="Part5">
4420  <front>
4421    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
4422    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4423      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4424      <address><email></email></address>
4425    </author>
4426    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
4427      <organization abbrev="W3C">World Wide Web Consortium</organization>
4428      <address><email></email></address>
4429    </author>
4430    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4431      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4432      <address><email></email></address>
4433    </author>
4434    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4435  </front>
4436  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4437  <x:source href="p5-range.xml" basename="p5-range">
4438    <x:defines>Content-Range</x:defines>
4439  </x:source>
4442<reference anchor="Part6">
4443  <front>
4444    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4445    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4446      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4447      <address><email></email></address>
4448    </author>
4449    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4450      <organization>Akamai</organization>
4451      <address><email></email></address>
4452    </author>
4453    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4454      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4455      <address><email></email></address>
4456    </author>
4457    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4458  </front>
4459  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4460  <x:source href="p6-cache.xml" basename="p6-cache">
4461    <x:defines>Cache-Control</x:defines>
4462    <x:defines>Expires</x:defines>
4463    <x:defines>Warning</x:defines>
4464  </x:source>
4467<reference anchor="Part7">
4468  <front>
4469    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4470    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4471      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4472      <address><email></email></address>
4473    </author>
4474    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4475      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4476      <address><email></email></address>
4477    </author>
4478    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4479  </front>
4480  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4481  <x:source href="p7-auth.xml" basename="p7-auth">
4482    <x:defines>Proxy-Authenticate</x:defines>
4483    <x:defines>Proxy-Authorization</x:defines>
4484  </x:source>
4487<reference anchor="RFC5234">
4488  <front>
4489    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4490    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4491      <organization>Brandenburg InternetWorking</organization>
4492      <address>
4493        <email></email>
4494      </address> 
4495    </author>
4496    <author initials="P." surname="Overell" fullname="Paul Overell">
4497      <organization>THUS plc.</organization>
4498      <address>
4499        <email></email>
4500      </address>
4501    </author>
4502    <date month="January" year="2008"/>
4503  </front>
4504  <seriesInfo name="STD" value="68"/>
4505  <seriesInfo name="RFC" value="5234"/>
4508<reference anchor="RFC2119">
4509  <front>
4510    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4511    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4512      <organization>Harvard University</organization>
4513      <address><email></email></address>
4514    </author>
4515    <date month="March" year="1997"/>
4516  </front>
4517  <seriesInfo name="BCP" value="14"/>
4518  <seriesInfo name="RFC" value="2119"/>
4521<reference anchor="RFC3986">
4522 <front>
4523  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4524  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4525    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4526    <address>
4527       <email></email>
4528       <uri></uri>
4529    </address>
4530  </author>
4531  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4532    <organization abbrev="Day Software">Day Software</organization>
4533    <address>
4534      <email></email>
4535      <uri></uri>
4536    </address>
4537  </author>
4538  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4539    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4540    <address>
4541      <email></email>
4542      <uri></uri>
4543    </address>
4544  </author>
4545  <date month='January' year='2005'></date>
4546 </front>
4547 <seriesInfo name="STD" value="66"/>
4548 <seriesInfo name="RFC" value="3986"/>
4551<reference anchor="RFC0793">
4552  <front>
4553    <title>Transmission Control Protocol</title>
4554    <author initials='J.' surname='Postel' fullname='Jon Postel'>
4555      <organization>University of Southern California (USC)/Information Sciences Institute</organization>
4556    </author>
4557    <date year='1981' month='September' />
4558  </front>
4559  <seriesInfo name='STD' value='7' />
4560  <seriesInfo name='RFC' value='793' />
4563<reference anchor="USASCII">
4564  <front>
4565    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4566    <author>
4567      <organization>American National Standards Institute</organization>
4568    </author>
4569    <date year="1986"/>
4570  </front>
4571  <seriesInfo name="ANSI" value="X3.4"/>
4574<reference anchor="RFC1950">
4575  <front>
4576    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4577    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4578      <organization>Aladdin Enterprises</organization>
4579      <address><email></email></address>
4580    </author>
4581    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4582    <date month="May" year="1996"/>
4583  </front>
4584  <seriesInfo name="RFC" value="1950"/>
4585  <!--<annotation>
4586    RFC 1950 is an Informational RFC, thus it might be less stable than
4587    this specification. On the other hand, this downward reference was
4588    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4589    therefore it is unlikely to cause problems in practice. See also
4590    <xref target="BCP97"/>.
4591  </annotation>-->
4594<reference anchor="RFC1951">
4595  <front>
4596    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4597    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4598      <organization>Aladdin Enterprises</organization>
4599      <address><email></email></address>
4600    </author>
4601    <date month="May" year="1996"/>
4602  </front>
4603  <seriesInfo name="RFC" value="1951"/>
4604  <!--<annotation>
4605    RFC 1951 is an Informational RFC, thus it might be less stable than
4606    this specification. On the other hand, this downward reference was
4607    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4608    therefore it is unlikely to cause problems in practice. See also
4609    <xref target="BCP97"/>.
4610  </annotation>-->
4613<reference anchor="RFC1952">
4614  <front>
4615    <title>GZIP file format specification version 4.3</title>
4616    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4617      <organization>Aladdin Enterprises</organization>
4618      <address><email></email></address>
4619    </author>
4620    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4621      <address><email></email></address>
4622    </author>
4623    <author initials="M." surname="Adler" fullname="Mark Adler">
4624      <address><email></email></address>
4625    </author>
4626    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4627      <address><email></email></address>
4628    </author>
4629    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4630      <address><email></email></address>
4631    </author>
4632    <date month="May" year="1996"/>
4633  </front>
4634  <seriesInfo name="RFC" value="1952"/>
4635  <!--<annotation>
4636    RFC 1952 is an Informational RFC, thus it might be less stable than
4637    this specification. On the other hand, this downward reference was
4638    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4639    therefore it is unlikely to cause problems in practice. See also
4640    <xref target="BCP97"/>.
4641  </annotation>-->
4644<reference anchor="Welch">
4645  <front>
4646    <title>A Technique for High Performance Data Compression</title>
4647    <author initials="T.A." surname="Welch" fullname="Terry A. Welch"/>
4648    <date month="June" year="1984"/>
4649  </front>
4650  <seriesInfo name="IEEE Computer" value="17(6)"/>
4655<references title="Informative References">
4657<reference anchor="ISO-8859-1">
4658  <front>
4659    <title>
4660     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4661    </title>
4662    <author>
4663      <organization>International Organization for Standardization</organization>
4664    </author>
4665    <date year="1998"/>
4666  </front>
4667  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4670<reference anchor='RFC1919'>
4671  <front>
4672    <title>Classical versus Transparent IP Proxies</title>
4673    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4674      <address><email></email></address>
4675    </author>
4676    <date year='1996' month='March' />
4677  </front>
4678  <seriesInfo name='RFC' value='1919' />
4681<reference anchor="RFC1945">
4682  <front>
4683    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4684    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4685      <organization>MIT, Laboratory for Computer Science</organization>
4686      <address><email></email></address>
4687    </author>
4688    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4689      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4690      <address><email></email></address>
4691    </author>
4692    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4693      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4694      <address><email></email></address>
4695    </author>
4696    <date month="May" year="1996"/>
4697  </front>
4698  <seriesInfo name="RFC" value="1945"/>
4701<reference anchor="RFC2045">
4702  <front>
4703    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4704    <author initials="N." surname="Freed" fullname="Ned Freed">
4705      <organization>Innosoft International, Inc.</organization>
4706      <address><email></email></address>
4707    </author>
4708    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4709      <organization>First Virtual Holdings</organization>
4710      <address><email></email></address>
4711    </author>
4712    <date month="November" year="1996"/>
4713  </front>
4714  <seriesInfo name="RFC" value="2045"/>
4717<reference anchor="RFC2047">
4718  <front>
4719    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4720    <author initials="K." surname="Moore" fullname="Keith Moore">
4721      <organization>University of Tennessee</organization>
4722      <address><email></email></address>
4723    </author>
4724    <date month="November" year="1996"/>
4725  </front>
4726  <seriesInfo name="RFC" value="2047"/>
4729<reference anchor="RFC2068">
4730  <front>
4731    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4732    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
4733      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4734      <address><email></email></address>
4735    </author>
4736    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4737      <organization>MIT Laboratory for Computer Science</organization>
4738      <address><email></email></address>
4739    </author>
4740    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4741      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4742      <address><email></email></address>
4743    </author>
4744    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4745      <organization>MIT Laboratory for Computer Science</organization>
4746      <address><email></email></address>
4747    </author>
4748    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4749      <organization>MIT Laboratory for Computer Science</organization>
4750      <address><email></email></address>
4751    </author>
4752    <date month="January" year="1997"/>
4753  </front>
4754  <seriesInfo name="RFC" value="2068"/>
4757<reference anchor="RFC2145">
4758  <front>
4759    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4760    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4761      <organization>Western Research Laboratory</organization>
4762      <address><email></email></address>
4763    </author>
4764    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4765      <organization>Department of Information and Computer Science</organization>
4766      <address><email></email></address>
4767    </author>
4768    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4769      <organization>MIT Laboratory for Computer Science</organization>
4770      <address><email></email></address>
4771    </author>
4772    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4773      <organization>W3 Consortium</organization>
4774      <address><email></email></address>
4775    </author>
4776    <date month="May" year="1997"/>
4777  </front>
4778  <seriesInfo name="RFC" value="2145"/>
4781<reference anchor="RFC2616">
4782  <front>
4783    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4784    <author initials="R." surname="Fielding" fullname="R. Fielding">
4785      <organization>University of California, Irvine</organization>
4786      <address><email></email></address>
4787    </author>
4788    <author initials="J." surname="Gettys" fullname="J. Gettys">
4789      <organization>W3C</organization>
4790      <address><email></email></address>
4791    </author>
4792    <author initials="J." surname="Mogul" fullname="J. Mogul">
4793      <organization>Compaq Computer Corporation</organization>
4794      <address><email></email></address>
4795    </author>
4796    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4797      <organization>MIT Laboratory for Computer Science</organization>
4798      <address><email></email></address>
4799    </author>
4800    <author initials="L." surname="Masinter" fullname="L. Masinter">
4801      <organization>Xerox Corporation</organization>
4802      <address><email></email></address>
4803    </author>
4804    <author initials="P." surname="Leach" fullname="P. Leach">
4805      <organization>Microsoft Corporation</organization>
4806      <address><email></email></address>
4807    </author>
4808    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4809      <organization>W3C</organization>
4810      <address><email></email></address>
4811    </author>
4812    <date month="June" year="1999"/>
4813  </front>
4814  <seriesInfo name="RFC" value="2616"/>
4817<reference anchor='RFC2817'>
4818  <front>
4819    <title>Upgrading to TLS Within HTTP/1.1</title>
4820    <author initials='R.' surname='Khare' fullname='R. Khare'>
4821      <organization>4K Associates / UC Irvine</organization>
4822      <address><email></email></address>
4823    </author>
4824    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4825      <organization>Agranat Systems, Inc.</organization>
4826      <address><email></email></address>
4827    </author>
4828    <date year='2000' month='May' />
4829  </front>
4830  <seriesInfo name='RFC' value='2817' />
4833<reference anchor='RFC2818'>
4834  <front>
4835    <title>HTTP Over TLS</title>
4836    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4837      <organization>RTFM, Inc.</organization>
4838      <address><email></email></address>
4839    </author>
4840    <date year='2000' month='May' />
4841  </front>
4842  <seriesInfo name='RFC' value='2818' />
4845<reference anchor='RFC3040'>
4846  <front>
4847    <title>Internet Web Replication and Caching Taxonomy</title>
4848    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4849      <organization>Equinix, Inc.</organization>
4850    </author>
4851    <author initials='I.' surname='Melve' fullname='I. Melve'>
4852      <organization>UNINETT</organization>
4853    </author>
4854    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4855      <organization>CacheFlow Inc.</organization>
4856    </author>
4857    <date year='2001' month='January' />
4858  </front>
4859  <seriesInfo name='RFC' value='3040' />
4862<reference anchor='BCP90'>
4863  <front>
4864    <title>Registration Procedures for Message Header Fields</title>
4865    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4866      <organization>Nine by Nine</organization>
4867      <address><email></email></address>
4868    </author>
4869    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4870      <organization>BEA Systems</organization>
4871      <address><email></email></address>
4872    </author>
4873    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4874      <organization>HP Labs</organization>
4875      <address><email></email></address>
4876    </author>
4877    <date year='2004' month='September' />
4878  </front>
4879  <seriesInfo name='BCP' value='90' />
4880  <seriesInfo name='RFC' value='3864' />
4883<reference anchor='RFC4033'>
4884  <front>
4885    <title>DNS Security Introduction and Requirements</title>
4886    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4887    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4888    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4889    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4890    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4891    <date year='2005' month='March' />
4892  </front>
4893  <seriesInfo name='RFC' value='4033' />
4896<reference anchor="BCP13">
4897  <front>
4898    <title>Media Type Specifications and Registration Procedures</title>
4899    <author initials="N." surname="Freed" fullname="Ned Freed">
4900      <organization>Oracle</organization>
4901      <address>
4902        <email></email>
4903      </address>
4904    </author>
4905    <author initials="J." surname="Klensin" fullname="John C. Klensin">
4906      <address>
4907        <email></email>
4908      </address>
4909    </author>
4910    <author initials="T." surname="Hansen" fullname="Tony Hansen">
4911      <organization>AT&amp;T Laboratories</organization>
4912      <address>
4913        <email></email>
4914      </address>
4915    </author>
4916    <date year="2013" month="January"/>
4917  </front>
4918  <seriesInfo name="BCP" value="13"/>
4919  <seriesInfo name="RFC" value="6838"/>
4922<reference anchor='BCP115'>
4923  <front>
4924    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4925    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4926      <organization>AT&amp;T Laboratories</organization>
4927      <address>
4928        <email></email>
4929      </address>
4930    </author>
4931    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4932      <organization>Qualcomm, Inc.</organization>
4933      <address>
4934        <email></email>
4935      </address>
4936    </author>
4937    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4938      <organization>Adobe Systems</organization>
4939      <address>
4940        <email></email>
4941      </address>
4942    </author>
4943    <date year='2006' month='February' />
4944  </front>
4945  <seriesInfo name='BCP' value='115' />
4946  <seriesInfo name='RFC' value='4395' />
4949<reference anchor='RFC4559'>
4950  <front>
4951    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4952    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4953    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4954    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4955    <date year='2006' month='June' />
4956  </front>
4957  <seriesInfo name='RFC' value='4559' />
4960<reference anchor='RFC5226'>
4961  <front>
4962    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4963    <author initials='T.' surname='Narten' fullname='T. Narten'>
4964      <organization>IBM</organization>
4965      <address><email></email></address>
4966    </author>
4967    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4968      <organization>Google</organization>
4969      <address><email></email></address>
4970    </author>
4971    <date year='2008' month='May' />
4972  </front>
4973  <seriesInfo name='BCP' value='26' />
4974  <seriesInfo name='RFC' value='5226' />
4977<reference anchor='RFC5246'>
4978   <front>
4979      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4980      <author initials='T.' surname='Dierks' fullname='T. Dierks'/>
4981      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4982         <organization>RTFM, Inc.</organization>
4983      </author>
4984      <date year='2008' month='August' />
4985   </front>
4986   <seriesInfo name='RFC' value='5246' />
4989<reference anchor="RFC5322">
4990  <front>
4991    <title>Internet Message Format</title>
4992    <author initials="P." surname="Resnick" fullname="P. Resnick">
4993      <organization>Qualcomm Incorporated</organization>
4994    </author>
4995    <date year="2008" month="October"/>
4996  </front>
4997  <seriesInfo name="RFC" value="5322"/>
5000<reference anchor="RFC6265">
5001  <front>
5002    <title>HTTP State Management Mechanism</title>
5003    <author initials="A." surname="Barth" fullname="Adam Barth">
5004      <organization abbrev="U.C. Berkeley">
5005        University of California, Berkeley
5006      </organization>
5007      <address><email></email></address>
5008    </author>
5009    <date year="2011" month="April" />
5010  </front>
5011  <seriesInfo name="RFC" value="6265"/>
5014<reference anchor='RFC6585'>
5015  <front>
5016    <title>Additional HTTP Status Codes</title>
5017    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
5018      <organization>Rackspace</organization>
5019    </author>
5020    <author initials='R.' surname='Fielding' fullname='R. Fielding'>
5021      <organization>Adobe</organization>
5022    </author>
5023    <date year='2012' month='April' />
5024   </front>
5025   <seriesInfo name='RFC' value='6585' />
5028<!--<reference anchor='BCP97'>
5029  <front>
5030    <title>Handling Normative References to Standards-Track Documents</title>
5031    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
5032      <address>
5033        <email></email>
5034      </address>
5035    </author>
5036    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
5037      <organization>MIT</organization>
5038      <address>
5039        <email></email>
5040      </address>
5041    </author>
5042    <date year='2007' month='June' />
5043  </front>
5044  <seriesInfo name='BCP' value='97' />
5045  <seriesInfo name='RFC' value='4897' />
5048<reference anchor="Kri2001" target="">
5049  <front>
5050    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
5051    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
5052    <date year="2001" month="November"/>
5053  </front>
5054  <seriesInfo name="ACM Transactions on Internet Technology" value="1(2)"/>
5060<section title="HTTP Version History" anchor="compatibility">
5062   HTTP has been in use since 1990. The first version, later referred to as
5063   HTTP/0.9, was a simple protocol for hypertext data transfer across the
5064   Internet, using only a single request method (GET) and no metadata.
5065   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
5066   methods and MIME-like messaging, allowing for metadata to be transferred
5067   and modifiers placed on the request/response semantics. However,
5068   HTTP/1.0 did not sufficiently take into consideration the effects of
5069   hierarchical proxies, caching, the need for persistent connections, or
5070   name-based virtual hosts. The proliferation of incompletely-implemented
5071   applications calling themselves "HTTP/1.0" further necessitated a
5072   protocol version change in order for two communicating applications
5073   to determine each other's true capabilities.
5076   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
5077   requirements that enable reliable implementations, adding only
5078   those features that can either be safely ignored by an HTTP/1.0
5079   recipient or only sent when communicating with a party advertising
5080   conformance with HTTP/1.1.
5083   HTTP/1.1 has been designed to make supporting previous versions easy.
5084   A general-purpose HTTP/1.1 server ought to be able to understand any valid
5085   request in the format of HTTP/1.0, responding appropriately with an
5086   HTTP/1.1 message that only uses features understood (or safely ignored) by
5087   HTTP/1.0 clients. Likewise, an HTTP/1.1 client can be expected to
5088   understand any valid HTTP/1.0 response.
5091   Since HTTP/0.9 did not support header fields in a request, there is no
5092   mechanism for it to support name-based virtual hosts (selection of resource
5093   by inspection of the <x:ref>Host</x:ref> header field).
5094   Any server that implements name-based virtual hosts ought to disable
5095   support for HTTP/0.9. Most requests that appear to be HTTP/0.9 are, in
5096   fact, badly constructed HTTP/1.x requests caused by a client failing to
5097   properly encode the request-target.
5100<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
5102   This section summarizes major differences between versions HTTP/1.0
5103   and HTTP/1.1.
5106<section title="Multi-homed Web Servers" anchor="">
5108   The requirements that clients and servers support the <x:ref>Host</x:ref>
5109   header field (<xref target=""/>), report an error if it is
5110   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
5111   are among the most important changes defined by HTTP/1.1.
5114   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
5115   addresses and servers; there was no other established mechanism for
5116   distinguishing the intended server of a request than the IP address
5117   to which that request was directed. The <x:ref>Host</x:ref> header field was
5118   introduced during the development of HTTP/1.1 and, though it was
5119   quickly implemented by most HTTP/1.0 browsers, additional requirements
5120   were placed on all HTTP/1.1 requests in order to ensure complete
5121   adoption.  At the time of this writing, most HTTP-based services
5122   are dependent upon the Host header field for targeting requests.
5126<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
5128   In HTTP/1.0, each connection is established by the client prior to the
5129   request and closed by the server after sending the response. However, some
5130   implementations implement the explicitly negotiated ("Keep-Alive") version
5131   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
5132   target="RFC2068"/>.
5135   Some clients and servers might wish to be compatible with these previous
5136   approaches to persistent connections, by explicitly negotiating for them
5137   with a "Connection: keep-alive" request header field. However, some
5138   experimental implementations of HTTP/1.0 persistent connections are faulty;
5139   for example, if an HTTP/1.0 proxy server doesn't understand
5140   <x:ref>Connection</x:ref>, it will erroneously forward that header field
5141   to the next inbound server, which would result in a hung connection.
5144   One attempted solution was the introduction of a Proxy-Connection header
5145   field, targeted specifically at proxies. In practice, this was also
5146   unworkable, because proxies are often deployed in multiple layers, bringing
5147   about the same problem discussed above.
5150   As a result, clients are encouraged not to send the Proxy-Connection header
5151   field in any requests.
5154   Clients are also encouraged to consider the use of Connection: keep-alive
5155   in requests carefully; while they can enable persistent connections with
5156   HTTP/1.0 servers, clients using them will need to monitor the
5157   connection for "hung" requests (which indicate that the client ought stop
5158   sending the header field), and this mechanism ought not be used by clients
5159   at all when a proxy is being used.
5163<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
5165   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
5166   (<xref target="header.transfer-encoding"/>).
5167   Transfer codings need to be decoded prior to forwarding an HTTP message
5168   over a MIME-compliant protocol.
5174<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
5176  HTTP's approach to error handling has been explained.
5177  (<xref target="conformance" />)
5180  The HTTP-version ABNF production has been clarified to be case-sensitive.
5181  Additionally, version numbers has been restricted to single digits, due
5182  to the fact that implementations are known to handle multi-digit version
5183  numbers incorrectly.
5184  (<xref target="http.version"/>)
5187  Userinfo (i.e., username and password) are now disallowed in HTTP and
5188  HTTPS URIs, because of security issues related to their transmission on the
5189  wire.
5190  (<xref target="http.uri" />)
5193  The HTTPS URI scheme is now defined by this specification; previously,
5194  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
5195  Furthermore, it implies end-to-end security.
5196  (<xref target="https.uri"/>)
5199  HTTP messages can be (and often are) buffered by implementations; despite
5200  it sometimes being available as a stream, HTTP is fundamentally a
5201  message-oriented protocol.
5202  Minimum supported sizes for various protocol elements have been
5203  suggested, to improve interoperability.
5204  (<xref target="http.message" />)
5207  Invalid whitespace around field-names is now required to be rejected,
5208  because accepting it represents a security vulnerability.
5209  The ABNF productions defining header fields now only list the field value.
5210  (<xref target="header.fields"/>)
5213  Rules about implicit linear whitespace between certain grammar productions
5214  have been removed; now whitespace is only allowed where specifically
5215  defined in the ABNF.
5216  (<xref target="whitespace"/>)
5219  Header fields that span multiple lines ("line folding") are deprecated.
5220  (<xref target="field.parsing" />)
5223  The NUL octet is no longer allowed in comment and quoted-string text, and
5224  handling of backslash-escaping in them has been clarified.
5225  The quoted-pair rule no longer allows escaping control characters other than
5226  HTAB.
5227  Non-ASCII content in header fields and the reason phrase has been obsoleted
5228  and made opaque (the TEXT rule was removed).
5229  (<xref target="field.components"/>)
5232  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
5233  handled as errors by recipients.
5234  (<xref target="header.content-length"/>)
5237  The algorithm for determining the message body length has been clarified
5238  to indicate all of the special cases (e.g., driven by methods or status
5239  codes) that affect it, and that new protocol elements cannot define such
5240  special cases.
5241  CONNECT is a new, special case in determining message body length.
5242  "multipart/byteranges" is no longer a way of determining message body length
5243  detection.
5244  (<xref target="message.body.length"/>)
5247  The "identity" transfer coding token has been removed.
5248  (Sections <xref format="counter" target="message.body"/> and
5249  <xref format="counter" target="transfer.codings"/>)
5252  Chunk length does not include the count of the octets in the
5253  chunk header and trailer.
5254  Line folding in chunk extensions is  disallowed.
5255  (<xref target="chunked.encoding"/>)
5258  The meaning of the "deflate" content coding has been clarified.
5259  (<xref target="deflate.coding" />)
5262  The segment + query components of RFC 3986 have been used to define the
5263  request-target, instead of abs_path from RFC 1808.
5264  The asterisk-form of the request-target is only allowed with the OPTIONS
5265  method.
5266  (<xref target="request-target"/>)
5269  The term "Effective Request URI" has been introduced.
5270  (<xref target="effective.request.uri" />)
5273  Gateways do not need to generate <x:ref>Via</x:ref> header fields anymore.
5274  (<xref target="header.via"/>)
5277  Exactly when "close" connection options have to be sent has been clarified.
5278  Also, "hop-by-hop" header fields are required to appear in the Connection header
5279  field; just because they're defined as hop-by-hop in this specification
5280  doesn't exempt them.
5281  (<xref target="header.connection"/>)
5284  The limit of two connections per server has been removed.
5285  An idempotent sequence of requests is no longer required to be retried.
5286  The requirement to retry requests under certain circumstances when the
5287  server prematurely closes the connection has been removed.
5288  Also, some extraneous requirements about when servers are allowed to close
5289  connections prematurely have been removed.
5290  (<xref target="persistent.connections"/>)
5293  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
5294  responses other than 101 (this was incorporated from <xref
5295  target="RFC2817"/>). Furthermore, the ordering in the field value is now
5296  significant.
5297  (<xref target="header.upgrade"/>)
5300  Empty list elements in list productions (e.g., a list header field containing
5301  ", ,") have been deprecated.
5302  (<xref target="abnf.extension"/>)
5305  Registration of Transfer Codings now requires IETF Review
5306  (<xref target="transfer.coding.registry"/>)
5309  This specification now defines the Upgrade Token Registry, previously
5310  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
5311  (<xref target="upgrade.token.registry"/>)
5314  The expectation to support HTTP/0.9 requests has been removed.
5315  (<xref target="compatibility"/>)
5318  Issues with the Keep-Alive and Proxy-Connection header fields in requests
5319  are pointed out, with use of the latter being discouraged altogether.
5320  (<xref target="compatibility.with.http.1.0.persistent.connections" />)
5325<?BEGININC p1-messaging.abnf-appendix ?>
5326<section xmlns:x="" title="Collected ABNF" anchor="collected.abnf">
5328<artwork type="abnf" name="p1-messaging.parsed-abnf">
5329<x:ref>BWS</x:ref> = OWS
5331<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
5332 connection-option ] )
5333<x:ref>Content-Length</x:ref> = 1*DIGIT
5335<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
5336 ]
5337<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
5338<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
5339<x:ref>Host</x:ref> = uri-host [ ":" port ]
5341<x:ref>OWS</x:ref> = *( SP / HTAB )
5343<x:ref>RWS</x:ref> = 1*( SP / HTAB )
5345<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
5346<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
5347<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
5348 transfer-coding ] )
5350<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
5351<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
5353<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
5354 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
5355 comment ] ) ] )
5357<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
5358<x:ref>absolute-form</x:ref> = absolute-URI
5359<x:ref>absolute-path</x:ref> = 1*( "/" segment )
5360<x:ref>asterisk-form</x:ref> = "*"
5361<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
5362<x:ref>authority-form</x:ref> = authority
5364<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
5365<x:ref>chunk-data</x:ref> = 1*OCTET
5366<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
5367<x:ref>chunk-ext-name</x:ref> = token
5368<x:ref>chunk-ext-val</x:ref> = token / quoted-string
5369<x:ref>chunk-size</x:ref> = 1*HEXDIG
5370<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
5371<x:ref>comment</x:ref> = "(" *( ctext / quoted-pair / comment ) ")"
5372<x:ref>connection-option</x:ref> = token
5373<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
5374 / %x2A-5B ; '*'-'['
5375 / %x5D-7E ; ']'-'~'
5376 / obs-text
5378<x:ref>field-content</x:ref> = field-vchar [ 1*( SP / HTAB ) field-vchar ]
5379<x:ref>field-name</x:ref> = token
5380<x:ref>field-value</x:ref> = *( field-content / obs-fold )
5381<x:ref>field-vchar</x:ref> = VCHAR / obs-text
5382<x:ref>fragment</x:ref> = &lt;fragment, defined in [RFC3986], Section 3.5&gt;
5384<x:ref>header-field</x:ref> = field-name ":" OWS field-value OWS
5385<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ] [ "#"
5386 fragment ]
5387<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ] [ "#"
5388 fragment ]
5390<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
5392<x:ref>message-body</x:ref> = *OCTET
5393<x:ref>method</x:ref> = token
5395<x:ref>obs-fold</x:ref> = CRLF 1*( SP / HTAB )
5396<x:ref>obs-text</x:ref> = %x80-FF
5397<x:ref>origin-form</x:ref> = absolute-path [ "?" query ]
5399<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
5400<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
5401<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
5402<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
5403<x:ref>protocol-name</x:ref> = token
5404<x:ref>protocol-version</x:ref> = token
5405<x:ref>pseudonym</x:ref> = token
5407<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5408 / %x5D-7E ; ']'-'~'
5409 / obs-text
5410<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5411<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5412<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5414<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5415<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5416<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5417<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5418<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5419<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5420<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5421 asterisk-form
5423<x:ref>scheme</x:ref> = &lt;scheme, defined in [RFC3986], Section 3.1&gt;
5424<x:ref>segment</x:ref> = &lt;segment, defined in [RFC3986], Section 3.3&gt;
5425<x:ref>start-line</x:ref> = request-line / status-line
5426<x:ref>status-code</x:ref> = 3DIGIT
5427<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5429<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5430<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5431<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5432 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5433<x:ref>token</x:ref> = 1*tchar
5434<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5435<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5436 transfer-extension
5437<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5438<x:ref>transfer-parameter</x:ref> = token BWS "=" BWS ( token / quoted-string )
5440<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5444<?ENDINC p1-messaging.abnf-appendix ?>
5446<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5448<section title="Since RFC 2616">
5450  Changes up to the IETF Last Call draft are summarized
5451  in <eref target=""/>.
5455<section title="Since draft-ietf-httpbis-p1-messaging-24" anchor="changes.since.24">
5457  Closed issues:
5458  <list style="symbols">
5459    <t>
5460      <eref target=""/>:
5461      "APPSDIR review of draft-ietf-httpbis-p1-messaging-24"
5462    </t>
5463    <t>
5464      <eref target=""/>:
5465      "integer value parsing"
5466    </t>
5467    <t>
5468      <eref target=""/>:
5469      "move IANA registrations to correct draft"
5470    </t>
5471  </list>
5475<section title="Since draft-ietf-httpbis-p1-messaging-25" anchor="changes.since.25">
5477  Closed issues:
5478  <list style="symbols">
5479    <t>
5480      <eref target=""/>:
5481      "check media type registration templates"
5482    </t>
5483    <t>
5484      <eref target=""/>:
5485      "Redundant rule quoted-str-nf"
5486    </t>
5487    <t>
5488      <eref target=""/>:
5489      "add 'stateless' to Abstract"
5490    </t>
5491    <t>
5492      <eref target=""/>:
5493      "clarify ABNF layering"
5494    </t>
5495    <t>
5496      <eref target=""/>:
5497      "use of 'word' ABNF production"
5498    </t>
5499    <t>
5500      <eref target=""/>:
5501      "improve introduction of list rule"
5502    </t>
5503    <t>
5504      <eref target=""/>:
5505      "moving 2616/2068/2145 to historic"
5506    </t>
5507    <t>
5508      <eref target=""/>:
5509      "augment security considerations with pointers to current research"
5510    </t>
5511    <t>
5512      <eref target=""/>:
5513      "intermediaries handling trailers"
5514    </t>
5515  </list>
5518  Partly resolved issues:
5519  <list style="symbols">
5520    <t>
5521      <eref target=""/>:
5522      "IESG ballot on draft-ietf-httpbis-p1-messaging-25"
5523    </t>
5524  </list>
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